Hydroelementation of diynes†

Jędrzej Walkowiak *^a, Jakub Szyling ^ab, Adrian Franczyk ^a and Rebecca L. Melen *^c
aAdam Mickiewicz University in Poznan, Center for Advanced Technology, Uniwersytetu Poznanskiego 10, 61-614, Poznan. E-mail: jedrzej.walkowiak@amu.edu.pl
^bAdam Mickiewicz University in Poznan, Faculty of Chemistry, Uniwersytetu Poznanskiego 8, 61-614, Poznan, Poland
^cCardiff Catalysis Institute, Cardiff University, School of Chemistry, Park Place, Main Building, Cardiff CF10 3AT, Cymru/Wales, UK. E-mail: MelenR@cardiff.ac.uk

First published on 10th January 2022

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Jędrzej Walkowiak

Jędrzej Walkowiak received his PhD degree (maxima cum laude) in 2009 from Adam Mickiewicz University in Poznań (Poland) with Prof. B. Marciniec. He completed postdoctoral research in Prof. W. Leitner group at the RWTH Aachen in Germany. In 2019, he obtained habilitation in chemical sciences. From 2011 he has been employed at the Center for Advanced Technology, at Adam Mickiewicz University in Poznan, first as an Assistant Professor and from 2020 as Associate Professor. He also received an MBA degree in 2019. His main research concerns homogeneous catalysis, organoboron, and organosilicon chemistry as well as sustainable and green processes.

Jakub Szyling

Jakub Szyling received his PhD degree with honours in 2018 from Adam Mickiewicz University in Poznań, Poland under the supervision of Professor H. Maciejewski and Dr J. Walkowiak. Since 2019, he has been employed as an Assistant Professor at the Faculty of Chemistry, Adam Mickiewicz University and since 2021 at the Center for Advanced Technology. His research is currently focused on the synthesis and application of organoboron compounds in organic synthesis with a great emphasis on green chemistry principles. He is a scientist in the Laboratory of Applied and Sustainable Catalysis working on hydroboration of conjugated compounds.

Adrian Franczyk

Adrian Franczyk received his PhD degree with honors in 2014 from Adam Mickiewicz University in Poznań (Poland), under the supervision of Professor B. Marciniec and Professor K. Matyjaszewski. During his education, he did internships at Mitsubishi Chemical Corporation (Yokohama, Japan), Universidade de Lisboa (Portugal), and Carnegie Mellon University (Pittsburgh, US). Since 2015 he has been employed at the Center for Advanced Technology, at Adam Mickiewicz University in Poznan as an Assistant Professor. His research is currently focused on the synthesis, characterisation, and application of molecular and macromolecular organosilicon compounds.

Rebecca Melen

Rebecca Melen studied for her PhD degree at the University of Cambridge (UK) with Professor D. S. Wright. Following Postdoctoral studies in Toronto (Canada) with Professor D. W. Stephan and Heidelberg (Germany) with Professor L. H. Gade, she took up a position at Cardiff University (UK) in 2014 where she is now a Professor in Inorganic Chemistry. In 2018, she was awarded an EPSRC early career fellowship and she was the 2019 recipient of the RSC Harrison Meldola Memorial Prize. Her research interests include diverse aspects of main group reactivity and catalysis, including the applications of main group chemistry in organic synthesis.

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1. Introduction

Hydroelementation reactions are one of the most prominent transformations in organic and organometallic chemistry, to obtain functionalised compounds from the addition of E–H bonds (E = Mg, B, Al, Si, Ge, Sn, N, P, O, S, Se, Te) to unsaturated C–C bonds in olefins (CC) or alkynes (CC),^1–31 C–N bonds in imines (CN)^2,32–36 or nitriles (CN),^2,33,34,37 and CO bonds in carbonyl compounds.^{2,32–35,38–47} The processes are mostly catalytic but may also occur as uncatalysed. In both cases, the stereo- and regioselectivity of the reaction depends upon the catalyst, reagent, and reaction conditions.

Hydroelementation of alkynes is perhaps the simplest, most straightforward, and atom economic method for the synthesis of unsaturated organometallic or organometalloid compounds. Over the last few decades, several reviews have been published focusing on this subject.^{14,17,18,22,25,30,48–61} Particularly useful are hydroboration, hydrosilylation, hydroamination, hydrophosphination, and hydrostannation processes, which lead to important building blocks in organic and materials chemistry. Although many different terminal and internal alkynes have been used in these transformations, literature focused on the hydroelementation of conjugated or separated diynes is much rarer and has never been collated in a review before. The more complex structure of diynes together with the possibility to obtain various isomers or different products (e.g., enynes, dienes, allenes, heterocyclic compounds, polymers), as well as the problems with carrying out monohydroelementation or bishydroelementation selectively, define the complexity of these processes (Scheme 1). For example, as was described by Perry et al., the hydrosilylation of conjugated 1,3-diynes may lead to the formation of nine different products.⁶² The difficulties in distinction in the reactivity of both CC bonds and the potential for overreduction are the most problematic issues reported. Actually, because of the synthetic potential of diyne hydrometallative products in the production of natural compounds, pharmaceuticals, or highly conjugated materials, within the last two decades, the subject is getting more explored.^63–71

Working on the hydroboration and hydrosilylation of various unsaturated compounds and especially on the reactivity of conjugated 1,3-diynes in these processes, we have found that literature information is often scattered, with no detailed procedures or much discussion on the process optimisation or methodology.^72–87 Therefore, we have decided to build a comprehensive and critical compendium focused on this subject, which will systemise the existing knowledge on the hydroelementation of diynes in relation to the formation of different products. We will also show the possible applications of the obtained products in the synthesis of fine chemicals and materials. The review is divided into subchapters according to the type of hydroelementation reactions according to the element group of the periodic table of elements: hydromagnesation, hydroboration, hydroalumination, hydrosilylation, hydrogermylation, hydrostannation, hydroamination, hydrophosphination, hydration, hydrothiolation, hydroselenation, and hydrotelluration. Each hydroelementation reaction type is then subdivided into conjugated and separated diynes, and the type of product formed (enynes, dienes, heterocyclic compounds, and polymers). This comprehensive review will be helpful for all advanced researchers and newcomers working on the synthesis of organometallic compounds and their further applications in organic chemistry, synthesis of natural compounds, pharmaceuticals, with the emphasis placed on the process regio- and stereoselectivity.

2. Hydromagnesation

2.1. Hydromagnesation of conjugated 1,4-diaryl-1,3-diynes

Hydromagnesation of alkynes is just limited to a few examples, which were performed in the presence of nickel, titanium, or iron complexes.^88–95 Hydromagnesation of conjugated and separated diynes was reported only by Nakamura et al., who synthesised alkenylmagnesium compounds using EtMgBr 2 as a hydrogen source and FeCl₂3 as a catalyst.⁹⁶ The reaction occurred with 1,2-diarylalkynes and 1,3-diynes 1a–e with high (Z)-selectivity, in short reaction time (15 min), at room temperature. For the reaction 5 mol% of an iron catalyst was used. Under the applied reaction conditions, the alkenes were unreactive, so no overreduction was observed. Moreover, only one CC bond in the diynes 1a–e was converted to the magnesium derivative 4 or 5 (regioisomers), which then was treated with an electrophile (HCl, D₂O, allyl bromide, or DMF) giving products with H, D, allyl or CHO groups respectively 6, 7 (Scheme 2 and Table 1). Other iron complexes (e.g., FeCl₃8, Fe(acac)₂9, Fe(acac)₃10) were also active in this transformation but gave products with lower yields. Primary alkyl magnesium derivatives were also active in this reaction (except for bulky iso-butylmagnesium bromide), as well as secondary alkyl Grignard compounds (with cyclohexyl or cyclopentyl groups). The magnesium compound had to be used in a 2.0 to 2.5-fold excess to obtain high conversion of alkyne or diyne, but the necessity of this excess was not clear. The source of hydrogen was from the magnesium compound, which was determined from the reaction of 1,2-diphenylethyne with deuterated d₅-ethylmagnesium bromide. Both diynes with electron-donating and electron-withdrawing groups attached to aryl rings were transformed into enynes with high regio- and stereoselectivity. The stereoselectivity was slightly lower for electron-rich reagents 1c and 1d (Table 1, entries 4 and 5). When the unsymmetrically substituted diyne with phenyl and trimethylsilyl groups 1e was used, the addition of magnesium compound 2 occurred at the CC bond to which the phenyl ring was attached (Table 1, entry 6). Moreover, the lack of reactivity of alkyl-substituted alkynes permitted the selective functionalisation of 1-(3,3-dimethylbut-1-yn-1-yl)-4-(phenylethynyl)benzene 11, which reacted at the diarylalkyne site. Only a small amount (7%) of diene was formed in the reaction mixture as a side product (Scheme 3). As it was shown for hydromagnesation of alkynes, the catalytic system tolerates many functional groups (e.g., halogens, amines, phenoxide, alkenes).

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3. Hydroboration

Organoboron compounds constitute important building blocks in the synthesis of structurally advanced organic and organometallic compounds due to their versatile reactivity in many catalytic and noncatalytic couplings and deborylation reactions, together with their low toxicity and moderate stability. There are numerous papers focused on the synthesis and applications of organoboron compounds, especially arylboronic acids and vinyl boranes.^97–110 The hydroboration reaction is still the most important and useful transformation in the synthesis of boranes because of its straightforward procedure, 100% atom economy, the possibility to control process regio- and stereoselectivity by the application of a catalyst or modification of the reagent steric properties or process conditions. The hydroboration of monoalkynes furnishing important alkenylborane building blocks is well established in the literature and has been discussed in several reviews.^{1,2,4,7,10,12,48} Hydroboration of conjugated and separated diynes, because of the increased complexity of their structure, is much more challenging in the case of selectivity control. Moreover, the possibility for carrying out mono-, bishydroboration, polyaddition reactions and cyclisation reactions with these reagents creates the possibility to obtain various products, which have been used in the synthesis of natural compounds, pharmaceuticals (e.g., anticancer rizoxin D, cytotoxic nannocystin Ax, ivorenolides),^111–119 dyes,¹²⁰ π-conjugated compounds, or heterocycles.^121–123 The information in this section is divided according to the type of reagent used: conjugated or separated diynes, as well as the formation of different products: enynes, dienes, heterocyclic compounds, and polymers.

3.1. Hydroboration of conjugated 1,3-diynes

Hydroboration of conjugated 1,3-diynes is the simplest procedure for the synthesis of boryl-substituted 1,3-enynes or bisboryl-substituted 1,3-dienes, but the transition metal-catalysed selective addition of the B–H bond to the CC bond is limited only to three recently published examples.^72,124,125

The first selective hydroboration of 1,3-diynes was reported by Zweifel and Ponso. Noncatatalytic reduction of alkyl-substituted buta-1,3-diynes 13a–c was carried out using disiamylborane (bis(3-methyl-2-butyl)borane) 14 or less bulky dicyclohexylborane 15. The monohydroboration of dodeca-5,7-diyne 13a, 2,7-dimethylocta-3,5-diyne 13b and 2,2,7,7-tetramethyl-octa-3,5-diyne 13c with disiamylborane 14 (used in 1.1-fold excess) occurred with high regio- and stereoselectivity at 0–5 °C within 3 h. Protonolysis of the enynylborane intermediate 16a (obtained in the reaction of 13a with 14) with acetic acid at 55–60 °C for 5 h furnished (Z)-5-dodecen-7-yne 17a (83%), (Z,Z)-dodeca-5,7-diene (7%), and only traces of unreacted diyne 13a. Bulkier diyne 13c yielded the bishydroboration products only in trace amounts. Oxidation of the monohydroboration products with NaOH/H₂O₂ (30%) afforded the acetylenic ketones 18a–c in high yields >70% (Scheme 4).

The boryl group was attached to the external carbon atom, which was proved by protonolysis with deuterated acetic acid. NMR analysis indicated that deuterium was attached to the internal carbon atom (95(±3%)). Synthesis of (Z,Z)-bisborylated dienes was more effective when less hindered borane 15 was used and could also be reacted with the non-symmetrically substituted diyne e.g., 2-methyldodeca-3,5-diyne 13d. In the case of hindered diynes 13b and 13c, the reduction was carried stepwise using borane 14 to obtain (Z,Z)-diene, by the hydroboration protonolysis of the first triple bond and the subsequent repetition of these processes. The application of 14 instead of 15 was caused due to the fact that dienes 19 formed in the reaction with 15 have a similar boiling point to the side product cyclohexanol, making the distillation method ineffective for their separation (Scheme 5). The analogous experiment with deuterium labeling proved that the second boryl group is attached to the external carbon atom of the second CC bond, showing the directing induction effect of the firstly attached boryl group.¹²⁶

The same group reported that for unsymmetrically substituted 1,3-diynes with silyl and alkyl or cycloalkyl groups attached to the opposite sides of 20a–d, monohydroboration predominantly occurred at the CC bond without the silyl group. Application of the more hindered borane 14, as well as the structure of silyl groups (trimethyl, (tert-butyl)dimethyl, dimethylthexyl) influenced the reaction selectivity. For symmetrical 1,4-bis(trimethylsilyl)-1,3-butadiyne, the addition of sterically hindered 14 occurred at positions C¹ and C² in the ratio 26:74. The high regioselectivity of the reaction towards the product with borane at C² position was obtained when the bulky dimethylthexylsilyl substituent was attached in 20c and 20d.¹²⁷ The obtained products were further transformed into silyl-functionalised ketones 24 or enynes 22–23 with the above-described procedure (Scheme 6).¹²⁶

These alkylboryl-substituted enynes (16, 21) are difficult to handle due to their low stability, therefore the hydroboration of diynes with alkoxyboranes (e.g., pinacolborane 25 or cateholborane 26) is much more desirable. Moreover, alkoxyboranes are easy to use and non-flammable. However, due to the lower acidity of the B–H bond in comparison to alkylboranes, the addition of alkoxyboranes to unsaturated CC bonds requires the application of a catalyst to accelerate the process. Relating to this, our recent paper focused on the selective monohydroboration of 1,4-diaryl-buta-1,3-diynes 1a–d, 27a–c with pinacolborane 25 in the presence of Ru(CO)Cl(H)(PPh₃)₃28. Compound 28 has previously been described as an active catalyst in hydroboration of terminal- or internal monoalkynes in conventional and novel, green reaction media (supercritical CO₂ (scCO₂), ionic liquids (ILs), polyethylene glycol, (PEG)).^{72,75,76,128,129} The reaction proceeded effectively for various diynes possessing electron-withdrawing or electron-donating substituents on the aryl ring, as well as for heterocyclic 1,4-di(thiophen-3-yl)buta-1,3-diyne 27c. Alkyl-substituted diynes yielded boryl-substituted enynes by cis-addition of borane to the CC bond, but the postreaction mixture also consisted of other monoborylated enynes, bisborylfunctionalised dienes, and some undefined products. Thus, the electronic properties of diynes have an important influence on the process regio- and stereoselectivity. Under the optimised reaction conditions (3 mol% of Ru(CO)Cl(H)(PPh₃)₃28, 60 °C, 24 h, with a small excess of borane 25 (1.2 equiv.)), the boryl-substituted enynes 29 were obtained with high yields (85–97%). Due to their instability during purification by column chromatography, the products were directly transformed to the corresponding stable trifluoroborane salts 31 with KHF₂30 furnishing the desired products with 75–84% yield (Scheme 7).

The regioselectivity of the process was confirmed by NOESY and X-ray diffraction analysis for product 29 obtained in the hydroboration of 27b with 25 (Fig. 1). The borane 25 was added to the CC bond in a syn-manner according to the anti-Markownikow rule, with the boron group attached to the less shielded internal carbon atom in 29. We have also proposed the mechanism of the process according to the stoichiometric reactions monitored by ¹H NMR and 1D selective gradient NOESY. The process initiates from the insertion of diyne 1a–d or 27a–c into the Ru–H bond of catalyst 28 forming but-3-en-1-yn-3-yl complex 32. The addition of borane 25 then leads to a σ-bond metathesis between Ru–C and B–H (33), followed by the elimination of the product 29 and regeneration of the initial Ru-hydride complex 28 (Scheme 8 and Fig. 1).

The utility of the resulting boryl-substituted 1,4-diaryl-but-1-en-3-ynes was presented in the Suzuki coupling reaction of pinacoloborane derivative 29a and trifluoroborate salt 31a with iodobenzene 34 using 5 mol% of Pd(PPh₃)₄35. The reaction occurred with the retention of the configuration and (Z)-1,2,4-triphenylbut-1-en-3-yne 36a–b was formed with high yields 71% and 78%, respectively (Scheme 9).⁷²

Ge et al. reported an interesting method for the synthesis of boryl-substituted enynes from unsymmetrical and symmetrical 1,3-diynes 1c, 1e, 37a–t in the presence of a cobalt catalyst generated from inexpensive and stable Co(acac)₂38 and bidentate phosphine ligands 39a–c. The authors showed that the regioselectivity of the process was dependent on the bidentate phosphine ligand used. When Co(acac)₂/xantphos 38/39a was used, enynes with boron groups attached to the internal carbon atom were formed 40. However, when applying dppf 39b as a ligand, the opposite regioselectivity was observed, furnishing the product functionalised with borane at the external position 41. The pinacolborane derivatives were further transformed to more stable 1,8-diaminonaphthalene boronates 43 and 44 respectively, with 1,8-diaminonaphthalene 42, which were easier to isolate (Scheme 10).¹²⁵ To find the answer to this different reactivity pathway, the authors carried out a reaction with deuterated DBpin 45 (Scheme 11a). The formation of products with different regioselectivity was also confirmed using 1D NOE and 2D HMBC NMR correlations. For the Co(acac)₂/xantphos 38/39a catalyst, the reaction occurred through the formation of Co–H intermediate 48, while for Co(acac)₂/dppf 38/39b the process proceeded through the Co–borane species 50 (Scheme 11b). For both catalytic systems, several boryl-substituted enynes 40 and 41 were formed using an equimolar amount of reagents. When silyl groups were attached to one alkyne, the borane was added to the second triple bond with different (aryl, heteroaryl, or alkyl) substituents. The products were obtained with excellent regioselectivity and yield. For alkyl-substituted diynes, better selectivity was obtained when L-N₃39c was used as a ligand instead of xphos 39a. The catalytic systems tolerate a lot of functional groups in the diyne structure. No significant changes in their reactivity were observed (Scheme 10).¹²⁵ The utility of the resulting boryl-functionalised 1,3-enynes was presented in the bromodeborylation reaction with CuBr₂52, as well as the Pd-catalysed Suzuki–Miyaura and Hiyama coupling reactions (Scheme 12).

Applying a CuCl 55/P(p-Tol)₃/NaOt–Bu catalytic system, it was possible to carry out formal hydroboration of symmetrical and unsymmetrical 1,3-diynes 1a–b, 1d–e, 13a, 13c, 27a, and 60a–e with bis(pinacolato)diboron 61 and methanol as a proton source. The process was carried out under strictly assigned conditions (11 °C for 6 h) with 5 mol% of CuCl 55, 6 mol% of phosphine, and 10 mol% of the base. Reactions with weaker donating phosphines (P(OEt)₃ or PPh₃) resulted in lower yields and longer reaction times, with some exceptions. For diynes with tert-butyl 13c, 4-methoxyphenyl 1d, cyclohexyl 60a groups, P(OEt)₃ was applied, while for 4-fluorophenyl-substituted diyne 1b, PPh₃ was successfully used. The boryl group was attached at the external carbon position of the 1,3-diyne and only for sterically hindered 2,2,7,7-tetramethyl-octa-3,5-diyne 13c the regioselectivity was reversed and the borane was bonded to the less shielded internal carbon atom. When hydroboration of unsymmetrically functionalised diynes with aryl and alkyl groups in the terminal position was carried out, the reaction occurred at a more accessible CC bond. Diynes bearing a silyl group were characterised by their strong directing effect, where the functionalisation proceeded at the triple bond situated further from the silyl group.¹²⁴ The same observation was noticed in the noncatalytic hydroboration of silyl-substituted diynes.¹²⁷ The resulting pinacolborane derivatives 62 were then transformed to their potassium trifluoroborate analogs 63 with KHF₂30. When unsymmetrical 1,4-diaryl-diyne was used, a 1:1 mixture of regioisomers was formed due to the similar reactivity of both CC bonds (Scheme 13). Using this catalytic system, different regioisomers were formed with the boryl group attached to the internal carbon bond in comparison to the previously described works on noncatalytic or Ru–H catalysed hydroboration.^72,126,130 The obtained products were subsequently derivatised by Suzuki–Miyaura coupling with iodobenzene 34 using PdCl₂64/dppf 39b as a catalyst and KOH as a base, as well as deborylated to enynes with acetic acids. Moreover, it was possible to carry out selective desilylation of silylboryl-substituted enynes with K₂CO₃/MeOH while Bpin remained unreactive under applied process conditions.

Very recently, Taniguchi and co-workers reported the first trans-hydroboration of 1,3-diyne derivatives 60a and 65a–l under radical conditions in the presence of AIBN (2,2′-azobis(isobutyronitrile)) 67 or ACCN ((1,1′-azobis(cyclohexane-1-carbonitrile))) 68 as an azo initiator and tert-dodecanethiol 69 (TDT) as a polarity-reversal catalyst. The addition of an N-heterocyclic carbene borane 66 to symmetrical and unsymmetrical 1,3-diynes gave (E)-alkynylalkenyl boranes 70 in high selectivity (E/Z = 95/5) and good isolated yields (51–77%). Interestingly, a 4-fold excess of the borane 66 with diynes 65a–l and 60a caused the formation of bisadducts in inconsiderable amounts. The protocol was suitable for 1,3-diynes with different substituents (n-alkyl, c-alkyl, propargyl ether, silyl) however, hydroboration of 1,4-diphenylbuta-1,3-diyne 1a under the standard conditions gave a mixture of undefined products due to different rate of hydrogen atom transfer for aryl and alkyl or silyl substituted 1,3-diynes. It is worth noting that obtained NHC-based boryl functionalised enynes 70, in contrast to pinacolborane-based enynes, are bench-stable compounds and can be easily purified by silica-column chromatography (Scheme 14). The authors proposed the mechanism of this anti-selective hydroboration of 1,3-diynes which in the first step involved the thermal decomposition of azo initiator AIBN 67 or ACCN 68 and formation of thiyl radical 71 from thiol 69. The abstraction of the hydrogen atom from NHC–borane 66 by the radical 71 yielded NHC–boryl radical 72. Subsequently, the regioselective addition of boryl-radical 72 to 1,3-diyne 60a, 65a–l gave alkenyl radical 73 conjugated to the alkyne moiety. The presence of thiol TDT 69 promoted the hydrogen atom transfer step and formation of thiyl radical 71 which abstracted hydrogen atom from NHC–borane 66 and closed radical chain. The authors suggested that the bisadducts 74 would not be efficiently formed since the presence of electron-rich NHC–borylalkenyl group of 73 would cause polarity mismatching during the addition of nucleophilic NHC–boryl radical 72 (Scheme 15). 70a could be easily converted to the corresponding (Z)-1-aryl-1,3-enyne derivatives 77a and 77b through a one-pot procedure involving chlorination and hydrolysis of the boron moiety with N-chlorosuccinimide (NCS) 78 and water followed by a Suzuki–Miyaura coupling with aryl iodides 75 and 76 (Scheme 16).¹³¹

3.2. Hydroboration of separated 1,n-diynes

The hydroboration of separated 1,n-diynes leads to the formation of various products from boryl-substituted enynes and bisboryl-substituted dienes, towards cyclic products, which are important building blocks in the synthesis of natural compounds, that can be used in e.g., Diels–Alder reactions. The selectivity of the hydroboration processes depends on the catalyst type and reagent structure. Their choice is essential to direct the desired course of the reaction. In this section, the synthesis of molecular boryl-derivatives is presented, while the formation of macromolecular compounds is described in Section 3.3.

Selective hydroboration of the separated 1,6-diynes, 1-pinacolboryl-hepta-1,6-diyne 79a or 1-pinacolboryl-octa-1,7-diyne 79b, occurred at the terminal CC bond with the syn-addition of pinacolborane 25 in the presence of 5 mol% Cp₂ZrHCl 80 as a catalyst (according to Wang's procedure)¹³² or using HBBr₂–SMe₂81, which was further transformed with pinacol 82 to generate stable boryl derivatives 83a–b in 94–95% yield. The obtained diborylenynes 83a–b were further cyclised using Cp₂ZrCl₂84 and n-BuLi 85 followed by treatment with anhydrous HCl in diethyl ether. The resulting products 87a–b possessing boryl groups attached to C_sp² and C_sp³ were used in Suzuki coupling reactions with iodoarenes in typical conditions, applying the commonly used Pd(PPh₃)₄35 as a catalyst. Here the reaction occurred exclusively on C_sp²–B bond, because of its much higher reactivity in this coupling reaction (Scheme 17).¹³⁰

Wang's procedure was also used for the bishydroboration of aminodiyne 90 with pinacolborane 25. (E,E)-Bis(vinylboronate ester) 91 was obtained in 54% yield. When the modified Srebrnik procedure was used, the same product was formed but with opposite (Z,Z)-stereoselectivity 93.¹³³ Here reaction of diyne 90 with n-BuLi 85 (2 equiv.) in Et₂O at −78 °C, which was then transferred to a solution of 2 equiv. of PINBOP 92, followed by the addition of HCl. The crude bis-alkynyl-Bpin was then added to a solution of Schwartz reagent Cp₂ZrHCl 80, and the obtained zirconocene was then hydrolysed to the bisborylated diene 93 in 49% yield (Scheme 18).¹³⁴ Wang's and Srebrnik's procedures were also applied for the formation of other (E,E)- or (Z,Z)-vinyl boronate esters with moderate yields, which were further cyclised using PdCl₂(PPh₃)₂94 to various cyclic polyenes with controlled (E,E), (Z,Z) or (E,Z) selectivities 95–104 (Schemes 18 and 19).¹³⁴

Non-catalytic hydroboration of diyne 105 with sterically hindered di(iso-pinocampheyl)borane (^lIpc₂BH) 106 was carried out in THF at 0 °C yielded the desired selective hydroboration of the Me-substituted alkyne. Addition of bromodienoate 107, Pd(PPh₃)₄35, and TlOEt to the product provided the targeted cross-coupling product 109 in 83% yield with excellent regioselectivity (>95:5) (Scheme 20). This method was developed and used as a part of the synthesis of the natural compound Apoptolidin An isolated from actinomycete identified as Nocardiopsis sp, which possesses cytotoxic properties. Interestingly, the application of the less hindered pinacolborane 25, cateholborane 26, or dicyclohexylborane 15, led to the mixture of isomers in the reaction.

Hydroboration of chiral binol derived diynes 110a–e was carried out with Piers borane (HB(C₆F₅)₂) 111 in mesitylene to 112a–e. After 5 min, tri(tert-butyl)phosphine 113 was added to 112 to generate a frustrated Lewis pair in situ. This system was used for the enantioselective reduction of enol silyl ethers 114a–u under 40 bars of H₂. After workup with TBAF (tetrabutylammonium fluoride) 108, chiral secondary alcohols 115a–u, with excellent yields and enantioselectivities (87–99% ee) were obtained. The catalytic alkenylborane activity 112a–e (Lewis acidity) was tuned by conjugation of the system as well as the type of electron-rich or deficient substituents attached to the binaphthyl ring (Scheme 21).¹³⁵

Ruthenium catalysts have not only been used in the hydroboration of conjugated 1,3-diynes⁷² but also in the hydroboration of separated diynes 116a–c.¹³⁶ In this case, application of ruthenium hydride pincer complex [Ru(t-BuPNP)(H)₂(H₂)] 117 (PNP = 1,3-bis(di-tert-butyl-phosphinomethyl)pyridine) permitted anti-Markownikow trans-hydroboration leading to (Z)-vinyl boranates 118 under mild reaction conditions (r.t., 24 h, toluene). Generally, the system was active in the reaction with various terminal alkynes, but very good yields and selectivities were also obtained in bishydroboration of hepta-1,6-diyne 116a, deca-1,9-diyne 116b and 1,4-diethynylbenzene 116c (Scheme 22).

The complex [Ru(PNP)(H){(µ-H)₂Bpin}] 119, which is formed in the reaction of 117 with pinacolborane 25, with the simultaneous evolution of H₂, was found to be the catalyst for this transformation which was structuraly characterised (Scheme 23). Based on stoichiometric reactions, DFT calculations, and catalytic transformation with deuterated d₁-phenylacetylene, the mechanism of this transformation was determined. 119 is generated by the formation of the ruthenium hydride complex with a covalent bond Ru–B through σ-bond metathesis. 119 is subsequently further substituted with an alkyne generating complex 120. Then dihydrogen migration led to η¹-vinylidene complex 121. Complex 122 is then formed by the coupling between borane and vinylidene ligands. Coordination of pinacolborane 25 followed by σ-bond metathesis releases product 124 and generates complex 125. The addition of the next alkyne molecule regenerates complex 120 closing the catalytic cycle. The (Z)-stereochemistry of the product is determined in the reaction sequence from 120 to 122, presumably reflecting steric interactions in the formation of complex 121.¹³⁶

Applying the low-valent Co catalyst 126, generated in situ from CoCl₂/phenanthroline, TBAF 108, and pinacolborane 25, it was possible to carry out the cyclisation/hydroboration of 1,6-diynes 116a, 127a–r, and 129 yielding cyclic 1,3-dienylborons 128. No other work has been reported on this type of cyclisation of diynes, although analogous systems with enynes and dienes have been published.^137–142 This reaction was observed to be more effective in dilute solutions. Different ligands and activators, e.g., TMSCH₂Li, KOAc, t-BuOK could be used, but TBAF 108 and 126 were the most efficient. It was found that the system was active using various 1,6-diynes 116a and 127a–r with different substituents, e.g., ketone, amide, nitrile, or sulfone. Not only C-tethered but also N- and O-tethered 1,6-diynes were reactive in this transformation furnishing heterocyclic compounds 128a–s. Interestingly the reaction with 1,7-diyne failed in most cases with only one example using 4,4,5,5-tetraester 129 which underwent cyclisation/hydroboration to the six-membered ring product 130 (Scheme 24).

The mechanism of this transformation was proposed according to the stoichiometric reactions and experiments with DBpin 45. The reaction is initiated by the formation of low-valent Co complex 131 in the reaction of L-CoCl₂126 with HBpin 25 (DBpin 45) and TBAF 108, which reduces the Co(II) to Co(0). In the next step coordination of diyne 116a, or 127a–r to 132, occurs followed by the oxidative cyclisation to form a five-membered cobalt-containing cyclic intermediate 133. 133 then undergos σ-bond metathesis with 25 (transition state 134) to give intermediate 135. Reductive elimination of the product 128a–s from 135 regenerates the low-valent cobalt species 131 (Scheme 25).¹²³ The utility of 1,3-dienylborones 128a–s as building blocks was tested for 128a in Diels–Alder, oxidation, chlorodeborylation, and Suzuki–Miyaura coupling reactions (Scheme 26).

Taniguchi et al. reported another type of hydroboration of benzo[3,4]cyclo-dec-3-ene-1,5-diynes 144a–m which, following a borylative radical cyclisation, permitted the formation of 5-borylated 6,7,8,9-tetra-hydrobenzo[a]azulenes products 145a–m.^143,144 The boryl radicals were formed from N-heterocyclic carbene–boranes 146 with radical initiators, of which di-tert-butyl hyponitrite (TBHN) 147 was the most effective. The homolytic bond dissociation energies of N-heterocyclic carbene boranes are much lower than those which possess typical boryl hydrides, and therefore these compounds might be used as precursors for rather stable boryl radicals 148. The reaction occurred according to a radical chain mechanism by the intramolecular addition of the boryl radical followed by the cyclisation process. Under the optimised reaction conditions, 100 °C, trifluoromethylbenzene, 0.4 equiv. of TBHN 147, and 5-fold excess of NHC–borane 146 products were obtained with moderate yields, which slightly varies depending on the substituents in both reagents: borane 146 and diyne 144 (Scheme 27). The obtained borylated compound 145a was subsequently transformed in a one-pot cascade reaction into deborylated products 150a–c and 151a–c in the following reactions with N-chlorosuccinimide (NCS) 78 and then a Suzuki–Miyaura coupling with aryl iodides. Depending on the reaction conditions (path A or B), a different distribution of products was observed (Scheme 28).^143,144 The mechanism of this transformation was discussed based on the formation of borepin 156 from cyclic diyne 152 and NHC–boryl radical 148, whose formation was initiated by the homolysis of the di-tert-butyl peroxide activator. The obtained radical 148 was added to diyne 152 to form alkenyl radical 153, which was transformed to the hydroboration product by a hydrogen atom transfer from NHC–borane 148. Alkenyl borane 154 has two remaining B–H bonds and can undergo a second hydroboration to give product 156 (Scheme 29).¹⁴⁵

3.3. Hydroboration of separated 1,n-diynes in the synthesis of macromolecular compounds

Non-catalytic hydroboration of internal and terminal diynes with alkyl or aryl spacers between ethynyl groups with dihydroboranes can lead to the formation of polymers, which possess a boryl group attached to C_sp² determining their further reactivity. In most cases, sterically hindered thexylborane 157, mesitylborane 158, or tripylborane 159 have been used as hydroboration agents.

The foundation research carried out by Chujo et al., used thexylborane 157 in the polyaddition process to terminal octa-1,7-diyne 160, as well as internal 3,9-dodecadiyne 161a, 3,8-undecadiyne 161b, 3,10-tridecadiyne 161c, 3,9-dodecadiyne 161d, which occurred in THF, at 0 °C. When terminal diyne 160 was used, the polymer 162, possessing 20% of the branched structure was obtained, which was visible by the gelation of the reaction mixture. The cross-linking structure occurred because of the easy access of the second borane molecule to the unshielded vinylborane bond in the subsequent hydroboration process. This branched structure caused the broadening of the molecular weight distribution of the polymer. When internal diynes 161 were used, the linear polymers 163 were mostly formed, which was determined by the observation of vinylic groups in ¹H NMR spectra (Scheme 30).¹⁴⁵

When mesitylborane 158 was used as a hydroborating agent, no gelation and crosslinking were observed due to its high steric hindrance. The linear polymers 165 were formed from terminal separated diynes 116b, c, 160, and 164a–i with good or moderate yields (35–95%) and moderate molecular weights (Scheme 31 and Table 2). The solvent type was observed to have a big influence on the products yields (Table 2, entries 1–4). Moreover, the temperature and the diyne type are also important factors for the reaction course. The best results were obtained for polymerisation of 160 in deuterated CDCl₃, but for most of the examples, THF was used. The polymers were more stable to air-oxidation than the products obtained by hydroboration with thexylborane 157. The application of diynes with chromophores permitted the synthesis of polymers with optoelectronic properties.¹⁴⁶ The organoboron polymer prepared from diethynylbenzene 116c and mesitylborane 158 was subsequently subjected to reaction with iodine to form poly(phenylene–butadienylene) 166 (Scheme 32).^146–150

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The same authors described the application of hydroboration with tripylborane 159 as a method for producing optoelectronic polymers 167a–d using various 1,4-diethynylbenzenes as starting monomers (Scheme 33). The obtained polymers with chromophores 167a–d emitted green or blue light, while their photostability depended on the electron density of the substituents in the comonomers. Moreover, conjugated polymers containing boron atoms in their backbone are known to extend π-conjugation through the vacant p-orbital of the boron atom. The authors used Gaussian 03 and theoretical calculations using DFT methods at the B3LYP/6-31G(d,p)/B3LYP/6-31G(d,p) level to calculate the bandgap in the polymers. The results showed that the bandgap decreased significantly with increasing the number of repeating mers in the polymer, showing that conjugation length was extended in polymer via the vacant p-orbital of the boron atoms.^147–150

Using the same theoretical methods, it was possible to calculate the reactivity of CC bonds depending on the R-groups attached to the aryl ring in the polymer by calculating the bond order. The bond order was found to be OCH₃ < CH₃ < H, while the stability was in the opposite order OCH₃ > CH₃ > H.¹⁵⁰ The polymerisation was also carried out for optically active diyne 168 with tripylborane 159. A chiroptical activity was induced to the polymer 169via the chiral side chain (Scheme 34).¹⁵¹

4. Hydroalumination of conjugated and separated diynes

The addition of an Al–H bond to the CC bond of a diyne may proceed via mono- or bishydroalumination for the synthesis of metallated enynes or dienes, which can be further used in the chemical transformation towards the synthesis of natural products or fine chemicals. In comparison to boranes, there is a limited availability of organoaluminium hydrides which is responsible for just a small number of papers focused on the hydroalumination of alkynes and diynes.^152,153

In 1977, Zewifel described the hydroalumination of 1,3-diynes with lithium di(iso-butyl)methylaluminium hydride 172, which was formed in the reaction of di(iso-butyl)aluminium hydride 170 with methyllithium 171. The reaction occurred in diglyme at room temperature furnishing lithium enynylaluminate 173. The rate of hydroalumination was found to be dependant upon the solvent and, when 1,2-dimethoxyethane or THF were used, the yields were much lower. The hydroalumination of the second CC bond in 13a–c, 65d was not observed even when a 50% excess of aluminum hydride 172 was used (Scheme 35).¹⁵⁴ The reaction was highly stereoselective, which was confirmed with the exclusive formation of (E)-enynes 174a–d after hydrolysis of obtained aluminate.¹⁵⁴ Deuterolisys of the aluminate in D₂O was used to prove the reaction regioselectivity. More than 98% deuterium was placed at the less shielded internal carbon bond 175. The products 173 were additionally transformed to enyonic acids 176 in the reaction with CO₂. The reaction was only selective for symmetrical diynes 13a–c, 65d (Scheme 35). In the case of 2,2-dimethyldeca-3,5-diyne 177, two regioisomers 178 and 179 were obtained with comparable yields (Scheme 36). The opposite (Z)-isomer 16 to 173 was obtained by the same authors using hydroboration reaction.¹²⁶ To apply this transformation to unsymmetrically substituted diynes, reagents with electronically different substituents attached to CC bonds were used 180a–c. The trimethylsilyl group was attached to one alkyne, while an alkyl or cycloalkyl group was included on the other alkyne. The presence of the silyl protecting group made the second triple bond more susceptible to nucleophilic attack by the aluminum hydride 181 (Scheme 37). This strong activating effect from the silyl group was proved by the reaction of equimolar amounts of two different diynes: silyl-substituted 180a and alkyl-substituted deca-4,6-diyne 65b. Within the process, the silyl-substituted diyne 180a was converted to enyne 182a, while diyne 65b was unreactive.¹⁵² The obtained enynes following deprotonation 182a–d were subjected to a second hydroalumination reaction with i-Bu₂AlH 170. Here, the aluminate was attached to the C₁ atom with silyl group 183a–d and then hydrolysed to 184a–d. This regiochemistry was analysed according to the deuterolysis reaction, indicating more than 95% of D atoms at the C₁ position (Scheme 37).

The hydroalumination and hydroboration reactions were applied in the synthesis of insect pheromone Bombykol187 applying a desilylation procedure with KF × 2H₂O in DMF. The product 187 was obtained in high trans-selectivity in 81% yield (Scheme 38).¹⁵²

Hydroalumination of 1,4-bis(trimethylsilyl)-1,3-butadiyne 180c and 1,4-bis(trimethylsilylethynyl)benzene 188 with di(tert-butyl)aluminium hydride 189 proceeded via cis-addition of the Al–H bond to both CC bonds in the diyne. Due to the directing effect of the silyl group, both organoaluminium groups were attached to the carbon atoms possessing the silicon atom. Within this reaction, the kinetic dienes 190 and 191 with (Z,Z)-stereoselectivity were formed. Increasing the temperature to 60 °C degrees caused the rearrangement of diene towards the thermodynamic product with (E,E)-configuration 193. The exclusive formation of this isomer occurred when 1,4-bis(trimethylsilylethynyl)benzene 188 was used as an initial reagent. In the case of 1,4-bis(trimethylsilyl)-1,3-butadiyne 180c upon heating, a mixture of different products was obtained. The formation of both products: kinetic and thermodynamic were confirmed using NMR spectroscopy and X-ray analysis. The rearrangement of isomers from (Z,Z) 191 to (E,E) 192 took 7 days, while the total consumption of initial diynes in the first hydroalumination step was carried out for 15 or 3 hours respectively (Scheme 39). The formation of products 190, 191, 193 was confirmed by X-ray analyses (Fig. 2).¹⁵⁵

5. Hydrosilylation

The subject focused on the hydrosilylation of diynes is the most documented of all hydroelementation processes discussed in this review. This is owing to the fact that the products are useful synthons in organic chemistry. The presence of the silyl group in the product structures, as well as other functional groups (hydrosilylation is a highly tolerant reaction), these compounds might be applied in various transformations leading to fine organic and organometallic compounds and materials.^{67,68,156–162} This broad applicability is a result of the ease of substitution of the silyl group with a broad range of functional groups, as well as the formation of different silylated products: 1,3-enynes, allenes, polymers, or cyclic compounds, depending on the type of diyne starting material (conjugated or separated with alkyl or aryl spacers). Moreover, silyl-substituted compounds are easy to handle, simple for isolation, stable in air, and active in many chemical transformations. Additionally, the hydrosilylation of diynes, when an appropriate catalyst is chosen, might be carried out in a 100% atom economic way yielding a single product. Such an approach is especially important owing to the simplification of separation steps. Therefore, considering the reaction methodology, conditions, and application of a specific type of catalyst (often tailored-made) is of prior importance, especially when such complex diyne molecules are used as reagents.^{68,73,74,156,163} The simplicity of the hydrosilylation process, its high tolerance towards various functional groups present in the reagent structures, as well as the diversity of the selectivities, which can be tuned by the proper choice of the catalyst, has rendered this transformation the first choice for the synthesis of organosilicon compounds. The reactivity of the silyl group in coupling reactions or desilylation processes has allowed the application of the resulting compounds (e.g., 1,3-enyne or 1,3-diene fragments), in the synthesis of natural or biologically active compounds.^{67,68,156–160}

To systemise the results in this section, the information is ordered according to the hydrosilylation of conjugated 1,3-diynes, 1,n-diynes, as well as the formation of various products, 1,3-enynes, allenes, polymers, or cyclic compounds.

5.1. Hydrosilylation of conjugated 1,3-diynes towards molecular and macromolecular unsaturated organosilicon compounds

The hydrosilylation of conjugated 1,3-diynes is a straightforward and 100% atom economic method, which occurs via the addition of the Si–H bond to the CC bond, but due to the presence of two such alkyne groups, the formation of a specific single product with high selectivity is a challenging task. Depending on the type of the catalyst, reagents, their concentration, ratio, and process conditions, silylated 1,3-enynes, 1,3-dienes, or allenes can be formed, frequently as a complex mixture of products (up to nine different compounds can potentially be formed).^{62,73,74,156,164} Many papers describe the hydrosilylation of monoalkynes,^{14,41,48,49,51} but the addition of the Si–H bond to 1,3-diynes is much more demanding and limited only to a few papers. The hydrosilylation of 1,3-diynes occurs mainly in the presence of noble metal complexes (Rh, Pt, Pd, Ru). There are also some examples of the application of less expensive Ni or Co catalysts. However, in the majority of examples there was little discussion of the influence of the reagent structure, reaction conditions, or the nature of the catalyst on the reaction outcome. Our recent papers focused on the hydrosilylation of 1,3-diynes with silanes or silsesquioxanes in the presence of commercially available platinum complexes give the first detailed research which discusses the influence of several parameters on the hydrosilylation selectivity.^73,74,165

In a report by Perry et al. the synthesis of conjugated polymers from 1,3-diynes and bis(silylhydrides) were described. The products were obtained via hydrosilylation reactions in the presence of Karstedt's catalyst 194. To check whether the double Si–H addition to the 1,3-diyne had occurred, the authors carried out model reactions using monohydrosilanes 195a–f with methyl, phenyl, or trimethylsiloxy groups with 1,4-diphenylbuta-1,3-diyne 1a, dodeca-5,7-diyne 13a, and 2,2,7,7-tetramethyl-octa-3,5-diyne 13c. Bissilyl adducts 197 were obtained under harsh reaction conditions (120–145 °C) in xylene. The silyl-substituted but-3-en-1-ynes 196 were obtained for less bulky silanes 195a and 195b under lower temperature (80 °C) in toluene. Not only were the steric properties of silanes important but also the 1,3-diyne used influenced the formation of monosilyl or bissilylated adducts. The functionalisation of bulky 13c gave exclusively the silyl-substituted enyne 196 (75–99%) (Scheme 40). The hydrosilylation occurred according to the syn-addition with the silyl group attached to the most internal carbon atom.

Furthermore, the polymerisations were carried out with dihydrosilanes 198a–c which furnished polymers 199 with a (Z,Z)-2,3-disubstituted-1,3-butadiene mers. The main products were linear polymers 199, but a few percent of cyclic oligomers were formed as well. Most of the cyclic products and low molecular weight linear polymers were separated by precipitation in MeOH:acetone = 3:1 solution, which presence was confirmed by MALDI and SEC analysis. The authors reported also that the rate of polymerisation depended on the catalyst 194 concentration, but did not influence the molecular weight of the polymers and polydispersity, which varies from 2.0 to 2.61 for specific reagents (Scheme 41 and Table 3).⁶²

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The hydrosilylation of poly[(dimethylsilylene, methylphenylsilylene, and diethylsilylene)but-1,3-diyne] 200a–c with 1,4-bis(methylphenylsilyl)benzene 201 was selectively carried out at 80 °C in the presence of 0.5 mol% of Rh₆(CO)₁₆202, with a 200:201 ratio = 1:0.3–0.42 (Scheme 42). Applying the catalyst 202, the addition to only one alkynyl group in the polymer occurred, while other Rh complexes, e.g., Rh(acac)(CO)₂204 and RhCl(PPh₃)₃205 also yielded allenes. H₂PtCl₆206 additionally catalysed depolymerisation reaction. The polymers 203 were obtained with 49–79% yield, the M_w = 112000–424000, and M_w/M_n = 2.1. The higher the molecular weight of 203, the longer reaction time was needed. The catalyst activity was checked in the model reaction of poly[(dimethyl-silylene)buta-1,3-diyne] 200a with triethylsilane 207a.¹⁶⁶

Escribano's group used a heterogeneous monometallic or bimetallic catalyst with active calcinated or non-calcinated platinum supported on titania in the hydrosilylation of symmetrical 1,4-diaryl 1a, 1c–d, 208a, or 1,4-dialkyl-substituted-1,3-diynes 60a, 65f, 208b, and one unsymmetrical diyne 208c with silanes 195e and 207a–b. The best activity occurred using Pt/TiO₂ catalyst 209 (Scheme 43). Under the optimised reaction conditions (0.25 mol% of Pt/TiO₂209, 70 °C, solvent-free conditions), depending on the reagent structures, monohydrosilylation or bishydrosilylation resulted. Electronically different diaryl-1,3-diynes underwent hydrosilylation using three silanes (Et₃SiH 207a, Ph₃SiH 195e, (MeO)₃SiH 207b) giving silylated 1,3-enynes 210a–g with high yields (85–98%). The hydrosilylation of electron-rich reagents was much faster than for electron-poor diynes and occurred with higher yields. Bishydrosilylation of diynes was possible only for the dialkyl-substituted reagents 60a, 65f, and 208b using 2.5 equiv. of silane 207a. Additionally, the hydrosilylation of unsymmetrical 5-phenylpenta-2,4-diyn-1-ol 208c with Et₃SiH 207a gave silylated 1,3-enyne 210g as a product with 75% isolated yield. For the reaction of 1,4-diphenylbuta-1,3-diyne 1a with triethylsilane 207a, catalyst 209 was filtered and three times recycled, giving the product 210a with 100, 70, and 15% in the following cycles. The significant decrease of the product yield was caused by the Pt leaching, which was confirmed by ICP-MS analysis (the catalyst 209 contained 22% of initial Pt loading after the third cycle). The products 210 and 211 were obtained with (E)-stereochemistry and with the silicon atom bonded to the internal carbon atom of the conjugated system.¹⁶⁷

Another heterogeneous catalyst, which was used in the hydrosilylation of alkynes and 1,3-diynes was based on Rh nanoparticles 214 synthesised by the reduction of RhCl₃213 with NaBH₄212 and their further stabilisation in a nitrogen-rich poly(oxyethylate) derivative. The catalyst was used for hydrosilylation of dodeca-5,7-diyne 13a and 2,2,7,7-tetramethylocta-3,5-diyne 13c with triethylsilane 207a used in a 4.0–6.0 fold excess. The more hindered diyne 13c gave monosilylated enyne 215, while the less shielded 13a gave bissilylated diene 216 with excellent isolated yields (95% and 98% respectively) (Scheme 44).¹⁶⁸ Palladium nanoparticles were also tested for single alkyne examples.^169,170

The hydrosilylation of conjugated symmetrical 1,3-diynes 1a, 13c, and 180c with mono- or dihydrosilanes 195a, 207a, 217 was carried out in the presence of various Ni(0) 218–220 or Rh(I) 221 complexes with the addition of different chiral or non-stereoselective ligands 222a–i. In all experiments, the silane was used in a 3.5 to 4.0-fold excess with respect to the diyne. The bishydrosilylated allene 224 was obtained for 1,4-bis(trimethylsilyl)buta-1,3-diyne 180c, while for diynes substituted with tert-butyl 13c and phenyl 1a groups, a mixture of silyl-substituted enyne 223 and allene 224 was formed (Scheme 45). The synthetic procedure was quite enigmatic, and the silane was used in high excess with no equimolar reagent ratios tested (Table 4).^164,171,172

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A recent publication from our group details the application of commercially available catalysts: Pt₂(dvs)₃194, Pt(PPh₃)₄225, or PtO₂226 in the hydrosilylation of various symmetrical 1,4-disubstituted buta-1,3-diynes (1a, 13c, 65a, 227a–b) with sterically and electronically different triethyl-207a and triphenylsilane 195e. Comprehensive optimisation studies were carried out to find the most suitable conditions that permitted obtaining either the monosilylated enynes or bissilyated dienes with high stereo- and regioselectivity. The application of a Pt catalyst led to the syn-addition of silane to the CC bond and the formation of the alkenyl silane with the silyl group attached to the internal carbon atom. This was confirmed by the crystal structures of the products 228h and 228i (Fig. 3), as well as with ¹H–¹³C HSQC and NOESY 2D NMR. Within the study, an equimolar ratio of reagents was reported for the first time, which is in agreement with the atom economy policy and simplifies the separation procedure, additionally reducing the process costs. Pt Karstedt's catalyst 194 was used for the synthesis of bisadducts 229, whereas the less active PtO₂226 and Pt(PPh₃)₄225 were capable of the synthesis of monosilylated enynes 228 (Scheme 46). Moreover, the influence of the reaction temperature on reaction selectivity was noticeable. For monohydrosilylation, 40 °C or lower temperature gave better selectivity. The structure of the reagents has also an important role in reaction selectivity. For sterically hindered diynes 13c and 227b and triphenylsilane 195e, only monoadducts 228h and 228e were obtained. The products 228a–j and 229a–d were isolated with 82–98% yield and were fully characterised.⁷³

The same catalytic systems 194, 225, 226, and heterogeneous Pt/SDB 232, and the equimolar ratio of reagents were used in the hydrosilylation of 1,4-symmetrically substituted 1,3-diynes 1a, 13c, 65a, 227a–b, 230a–d with 1-dimethylsiloxy-3,5,7,9,11,13,15-hepta-iso-butylpentacyclo-[9.5.1.1.^3,91.^5,151^7,13] octasiloxane ((HSiMe₂O)(i-Bu)₇Si₈O₁₂) 231 yielding silsesquioxane products with several functionalities attached to the enyne 233a–m, 234a–m, or diene moieties 235a–m, 236a–m, e.g., 4-boronic acid pinacol ester, 4-bromophenyl, hydroxyl groups, making them potentially useful nanobuilding blocks in polymerisation or Suzuki–Miyaura, Sonogashira, Heck, and Hiyama coupling reactions.⁷⁴ The process selectivity depended on the catalyst type and concentration, as well as the structure of the reagent. For hindered 1,3-diynes as 13c or 227b, only silsesquioxyl-substituted enynes 235 were formed (Scheme 47 and Table 5). Alkenylsilsesquioxanes have already been used in materials chemistry, in the synthesis of OLEDs, liquid crystals, or porous biocompatible materials. The attachment of silsesquioxanes as pendant groups to the conjugated molecular or macromolecular compounds is known to increase material brightness, color stability, and their solubility in organic solvents, or to improve the mechanical or thermal properties of the final products.^173–176 Similar systems were obtained by the use of incompletely condensed silsesquioxanes 238a–b. In these cases, a stoichiometric amount of diyne was used for unsymmetrical diynes with Si(i-Pr)₃ groups 239a–c. An excess of diyne (6–12 mol) compared to silsesquioxanes was necessary when symmetrically substituted diynes 1a–b, 13c, 27c, and 227b were tested. All the target products 240a–i were formed with a very high selectivity of 99% (Scheme 48).¹⁶⁵

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The hydrosilylation of 1,4-bis(trimethylsilyl)buta-1,3-diyne 180c with triethylsilane 207a using RhCl(PPh₃)₃205, H₂PtCl₆206, Pt(PPh₃)₄225, and Pd(PPh₃)₄35 was also described in 1984 by Hiyama et al., but complex mixtures of bissilylated allenes and monosilyl-substituted enynes were obtained, regardless of the catalyst used.¹⁷⁷ Better selectivity was observed when silyl-substituted butenynes 241 in analogous reactions were used.¹⁷⁸

Recent work published by Ge et al. focused on the hydrosilylation of symmetrically or unsymmetrically substituted 1,3-diynes 1a–b, 1d–e, 13a, 27c, 37a, 37c–n, 37s, and 242a–c catalysed by inexpensive Co(acac)₂ complex 38 with xantphos 39a, dppf 39b, or dppp 243 ligands with dihydrosilanes (Scheme 49). The authors previously reported the effectiveness of this system in the hydrosilylation of alkynes.^179,180 Moreover, other Co-catalysed systems for terminal and internal alkynes hydrosilylation have been recently published.^181–185 Under the optimised conditions of 2 mol% of Co(acac)₂38, 2 mol% of dppp 243, 50 °C, toluene, after 24 h, several silylated enynes 246a–z were obtained with high yields and selectivity as confirmed by GC-MS and NMR analyses. The electronic effects of substituents attached to the aryl ring were not noticeable, and the catalyst was tolerant towards many functional groups. Mainly diphenylsilane 217, but also diethylsilane 244 and methylphenylsilane 245 were used as silylating agents.¹⁵⁶

The authors proposed the mechanism of this transformation, which started from the generation of Co-hydride complex 247 in the reaction with H₂SiPh₂217, in the presence of dppp 243. The insertion of 1,3-diyne 37 or 242 into the Co–H bond generated the vinylcobalt intermediate 248, which directly reacted with dihydrosilane 217 with the elimination of the desired product – (E)-1-en-3-yn-2-ylsilane 246 and regeneration of initial catalyst 247 (Scheme 50).¹⁵⁶ The utility of silyl-substituted 1,3-enynes 246e and 246p as building blocks in organic synthesis was presented in the desilylation reaction by oxidation to ketone 250 and silanols (251, 253), protodesilylation to enynes 249, and Hiyama and Sonogashira coupling reactions (Scheme 51) furnishing products 252 and 255 respectively.¹⁵⁶

Another example of Co-catalysed hydrosilylation of 1,3-diynes (symmetrical and one nonsymmetrical reagent) was recently reported by Chen et al.¹⁶³ Cobalt tridentate complexes N^CNN–CoX₂ were previously reported as effective systems for hydrosilylation of alkynes.¹⁸⁶ The catalyst obtained from CoBr₂256 and tridentate ligand 257 transpired to be highly active in the hydrosilylation of various 1,3-diynes 1a–e, 27b–c, 37t, 208a, 208c, 230d, 242b–c, and 258a–q with electron-donating and electron-withdrawing groups (Scheme 52). Several ligands were tested, but the best results were obtained when 257 was used. The high conversion of diynes was obtained within 5 minutes at room temperature. A longer reaction time was required for fluoro, chloro, bromo, trifluoromethyl, and cyano electron-withdrawing groups to obtain satisfying yields of 260a–ad. The alkenyl-substituted diyne reacted in the hydrosilylation process under the applied conditions without addition to the CC bond. Excellent regioselectivity was observed in the hydrosilylation of an unsymmetrical diyne. The mechanism of this transformation started from the formation of active complex 261 in the reaction with NaHBEt₃259 and Ph₂SiH₂217 followed by the coordination of 1,3-diyne 1a–e, 27b–c, 37t, 208a, 208c, 230d, 242b–c, 258a–q to form intermediate 262. The 1,3-diyne inserts to the Co–Si bond yielding the vinylcobalt species 263. The reaction with the second molecule of Ph₂SiH₂217 causes the catalyst 261 regeneration and evolution of the silylated enyne product 260a–ad (Scheme 53).

The enyne 260a was desilylated according to the procedure described by Ge,¹⁵⁶ and then hydrosilylated again with diphenylsilane 217 to silylated 1,3-diene 264. Two regioisomers with silyl groups attached to the internal and external C bond were formed in the ratio 15:85 with a high yield of 95%. The double bond in enyne was unreactive under the applied reaction conditions.¹⁶³

Chen et al. tested several cobalt complexes in hydrosilylation of 1,3-diynes 1a–e, 27b–c, 37t, 208a, 208c, 230d, 242b–c, 258a–i, 258k–q, 265a–c (Scheme 54). Among tested catalytic systems synthesised from the commercially available materials, CoCl₂-dppp (1 mol%, 266) exhibited the best regio- and stereoselectivity (in the presence of 3 mol% NaHBEt₃259). A variety of (E)-2-silyl-1,3-enynes 268a–ah were obtained in high yields through monohydrosilylation at the internal carbon of the 1,3-diyne unit via syn-addition. Good functional (alkoxy, amine, halides, esters, heterocyclics) tolerance was achieved by testing more than thirty substrates. A mechanism of 1,3-diyne hydrosilylation was proposed (Scheme 55) in which CoCl₂-dppp 266 initially reacts with NaHBEt₃259 to afford the low-valent cobalt(I) hydride intermediate 269. Subsequently, the coordination of 1,3-diyne with 269 is followed by the migratory insertion of one of the alkynyl groups into the Co–H bond and forms the intermediate 270. In the end, Ph₂SiH₂217 reacts with 270 and as a result, the alkenylsilanes (268a–ah) are obtained, accompanied by the regeneration of 269.¹⁸⁷

Zhan et al. studied the hydrosilylation of 1,3-diynes catalysed by Ni(acac)₂273 with a series of organophosphine ligands screened in THF at room temperature. First, the use of xantphos 39a as the ligand showed moderate regioselectivity and yield. Several other commercially available phosphorus ligands were examined, however, the results were less than satisfactory. Therefore, from vinyl-functionalised xantphos monomer, through solvothermal polymerisation, POL–xantphos 274 was obtained and employed as a heterogeneous ligand for nickel catalysed 1,3-diyne hydrosilylation. Unsymmetrical and symmetrical 1,3-diynes 1a, 1e, 37f, 37h, 180a, 239a, and 271a–b were reacted with silanes 217, 245, 272 yielding the corresponding silyl-functionalised 1,3-enynes 275a–x (Scheme 56). The authors claimed that, due to the microporous structure of immobilised system Ni(acac)₂/POL–xantphos 273/274, the selectivity of the process increased compared to the system based on the monomeric xantphos ligand. Based on the experimental results a hydrometalation pathway with a Ni(0) intermediate for this Ni-catalysed hydrosilylation of 1,3-diynes was proposed (Scheme 57). In the mechanism, the nickel precursor is reduced in situ by phenylsilane 272 to form Ni(0) 276, and then oxidative addition of the silane generates 277. Reaction with the 1,3-diyne generates 278 which then leads to the alkenyl nickel intermediate 279 after the insertion of the alkyne into the Ni–Si bond. The final product 275a–x is obtained by C–H reductive elimination with the return of the Ni(0) active species 276 into the catalytic cycle. The recyclability of the catalytic system based on Ni(acac)₂/POL–xantphos 273/274 was examined for the hydrosilylation of 1,4-diphenylbuta-1,3-diyne 1a and PhSiH₃272. After five runs, the Ni/POL–xantphos 273/274 reacted with nearly no loss of activity and selectivity demonstrating the good reusability of this catalytic system.¹⁸⁸

When the hydrosilylation occurs as a trans-addition, cyclic siloles 283 and 284 are formed. The reaction proceeded in the presence of 20 mol% of [Cp*Ru(MeCN)₃]PF₆281, which was described as an effective trans-hydrosilylation catalyst of alkynes.¹⁸⁹ The symmetrical and nonsymmetrical 1,4-disubstituted-buta-1,3-diynes 1b–d, 27c, 60e, 258i, 258o, and 280a–f reacted with 9-silafluorene 282 or diphenylsilane 217 to the corresponding 2,5-diarylsiloles 283 and 284 with moderate to good yields. It was found that electron-donating groups attached to the aryl ring facilitated the process, while electron-withdrawing functions, e.g., acetyl group 280b, rendered the process more sluggish (Scheme 58).

The process occurred stepwise. After the first trans-hydrosilylation, the intramolecular second trans-addition proceeds. The obtained siloles were characterised by high fluorescence maxima.¹⁹⁰ The same catalytic system ([Cp*Ru(MeCN)₃]PF₆281) was used by Trost et al. in the hydrosilylation of diynols which was one of the stages in the total synthesis of biologically important natural products.¹⁹¹ In this study, diynols 285 and 289 were reacted with dimethylethoxysilane 286 and benzyldimethylsilane 290, respectively. It was observed that the partial reduction of CC to trans CC led to the enynols 287 and 291, and is directed by the propargyl alcohol fragments of the diynols (Scheme 59).¹⁹¹

5.2. Hydrosilylation of separated diynes – synthesis of molecular unsaturated linear compounds

Selective hydrosilylation of α,ω-diynes with CH₂OCH₂127p, C₄H₈160, and CH₂NHCH₂293 spacers were successfully carried out using the [Pt(IPr*OMe)(dvs)] 294 catalyst with bulky NHC ligand (where IPr*OMe = 1,3-bisimidazol-2-ylidene) and dimethylphenylsilane 195a under the typical anti-Markownikow manner. The hydrosilylation of the CC bonds leads to (E)-products, while the formation of monosilylated enyne 295 or bissilylated diene 296 can be distinguished with different reagents stoichiometry:diyne:silane = 1:1 or 1:2. The exclusive formation of silylated enyne was furnished for diyne 160, while in the case of diynes with heteroatoms 127p and 293 a small amount (up to 4%) of bissilylated diene 296 was observed when equimolar reagents ratio were used. Bishydrosilylation was carried out quantitatively for 127p and 293, while for octa-1,7-diyne 160 a complex reaction mixture consisting of (β-E)/(β-Z)/α isomers in 77:20:3 ratio were formed (Scheme 60).¹⁹² The single example of hydrosilylation of deca-1,9-diyne 116b with diphenylsilane 217 was also carried out by Leitner et al., who used ruthenium pincer complex [Ru(t-BuPNP)(H₂)(H)₂] 117 [t-BuPNP = 2,6-bis(di(tert-butyl)phosphinomethyl)pyridine], which was also active in the addition of B–H bonds to alkynes and diynes.¹³⁶ The bissilylate diene 297 (E):(Z) = 91:9 was obtained using an equimolar ratio of the neat reagents and 0.2 mol% of Ru 117 within 16 h at 50 °C. No dehydrogenative coupling reaction occurred, which was visible in the case of sterically hindered terminal alkynes (Scheme 61).¹⁹³

The thermal hydrosilylation of α,ω-diynes was also used for the modification of silica surface Si(100) 298 used in the preparation of monolayers for electrodes in the reaction of diynes (e.g., nona-1,8-diyne 164a) with silica enriched with the Si–H bonds 299. The products 300 were next modified in “click” chemistry with azides 301a–d by Husigen-type cycloaddition (Scheme 62).^194–197

A broad range of diyne and triyne π-electron bridges arenes 116c, 304a–f (aryl cores = phenyl, azulene, fluorene, carbazole, 9,9′-spirobi[fluorene], 1,1′-binaphtalene) were hydrosilylated with chlorodimethylsilane 305 in the presence of Karstedt's catalyst 194 in a syn-addition manner towards (E)-silylated products 306 with high yields and selectivities. The reactions occurred with the best yield in Et₂O, at room temperature for 6–12 h with 3 mol% of a Pt-catalyst 194 (Scheme 63). The influence of the solvent on reagents conversion was visible and when THF or toluene was used a much lower conversion was observed. Good results were also obtained for the hydrosilylation of 1,3,5-triethynyllbenezene 304a with H₂PtCl₆206 and Pt/C 307 in toluene. The rhodium catalysts on the other hand were less active.

The product 306a possessing halogen attached to silicon atom was further functionalised by a substitution reaction with lithiated chromophores 312, 314, or LiAlH₄308 in stoichiometric reactions, followed by the subsequent hydrosilylation reaction with 4-ethynylbenzonitrile 310.¹⁹⁸ The presence of the SiMe₂ bridge between chromophores facilitates intramolecular photoinduced charge transfer process and interrupts the π-conjugated chains (Scheme 64).^199–201

5.3. Hydrosilylation of separated diynes – synthesis of conjugated polymers

In 2008, Trogler and Sanchez published a review regarding the synthesis and application of functionalised polymers delocalised through silicon, which can be obtained by a hydrosilylation reactions of diynes. The authors discussed the properties and applications of such polymers in detail, therefore within this review we focus only on the synthetic aspects in the formation of such macromolecular compounds.⁶⁸

Luh et al. reported the RhCl(PPh₃)₃205 catalysed hydrosilylation of 1,4-diethynylarenes 116c, 164k, and 316a–d with disilanes 317a–b obtained in the NiCl₂(PPh₃)₂319 catalysed reaction of dithiolano-substituted arenes 320 with Me₂(i-PrO)SiCH₂MgCl 321, followed by the reduction of alkoxy group with LiAlH₄308. The process was carried out with 0.5 mol% RhCl(PPh₃)₃205 with an equimolar ratio of reagents. The molecular weight of the obtained polymers was a function of reagents concentration and reaction time. Increasing both parameters led to a higher molecular weight of 318 being obtained (Scheme 65).^202,203

The synthesis of poly(silyl-vinylenes) was also carried out in the presence of Pt-catalysts, in particular Speier's catalyst H₂PtCl₆206, with the predominant formation of polymer 322 with (E)-β-regioselectivity, sometimes in addition to α-silylated mers in residual amounts. The formation of both isomers was due to the steric freedom of the monomers, which did not possess any bulky substituents responsible for preventing α-silylation. The products were obtained with low M_w (Schemes 66, 67 and Table 6).^204–206

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Platinum-based Karstedt's catalyst 194 was used in the polymerisation of aromatic diynes 327a–d with two different types of silsesquioxanes: octakis(hydrodosilsesqioxane) T^H₈328²⁰⁷ and double-decker-shaped silsesquioxane (DDSQ) with two hydrido functions 329.²⁰⁸ Hydrosilylation with T^H₈328 was carried out with the use of 2.1–2.4 mol% of 194. The total consumption of reagents was observed in 2 h, but the reaction mixture was homogeneous even after 24 h, with the M_w ranging from 10000 to 34000, depending on the diyne 327a–d used. The polymerisation was carried out in an equimolar ratio of the reagents, at room temperature. When the diyne 327:T^H₈328 ratio was increased to 1:1.55, the M_w of polymer 330a–e increased to 87000, but crosslinking with T^H₈328 was observed. ¹H NMR spectra did not detect CH–sp³ bonds in the post-reaction mixture, which indicated that the CC bonds are much more reactive than the CC bonds (Scheme 68 and Table 7).²⁰⁷

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The same conclusions were obtained for the polymerisation with DDSQ 329. The linear polymers 331a–d were obtained for internal diynes 327 (yield 90–97%, M_n = 11900–29100, M_w/M_n = 2.9–4.9) after 24 h at 100 °C with 0.2 mol% of Karstedt's catalyst 194. When 1,4-diethynylbenzene 116c was used as a monomer, the insoluble polymer was achieved within 30 minutes, due to the subsequent crosslinking reaction of the less shielded vinyl bonds (Scheme 69 and Table 8).²⁰⁸

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The same group synthesised polymer 333 by the hydrosilylation of (1,4-bis(4-(tetrahydroxypyranyloxy)phenyl)ethenyl)benzene 332 with dihydrido-DDSQ 329 used in equal amounts with Pt₂(dvs)₃194 as the catalyst. The reaction was carried out in toluene, at 100 °C for 7 h. The THP groups in 333 were further hydrolysed to give 334, and the polymer with 10 wt% MBHP 335 and 1.5 wt% PTMA 336 was applied as a chemically amplified negative-working photoresist system 337 (Scheme 70).²⁰⁹ Incorporation of silsesquioxane into the polymer chain or as a pendant group enhances many properties of the final materials: mechanical, thermal, fire or oxygen resistance.^173,176

Pd₂(dba)₃338/PCy₃ was also used as an effective catalyst in the polymerisation of diethynylarenes with dihydrosilanes.^210–212 Yamashita et al. used 0.1 mol% Pd₂(dba)₃338/0.2 mol% PCy₃ and an equimolar ratio of silane (Ph₂SiH₂217, MePhSiH₂245 and Ph(H₂CCH)SiH₂339) in the hydrosilylation of p-diethylnybenzene 116c and m-diethynylbenzene 324 at 70–110 °C for 0.5–9 h yielding polymers 340a–f with 84–91% yield and M_w = 12000 to 49000. The reaction occurred mostly through the syn-addition with the silyl group attached to β-carbon atom, but some other possible isomers (β,α) and (α,α) were also presented in the reaction mixture what was distinguished with ¹H and ²⁹Si NMR. The signals at δ = −15.7 and −15.6 ppm for 340 seemed to arise from the –CHCH–Si–CHCH–(β,β) linkages, while the signal at δ = −13.4 ppm in the ²⁹Si NMR spectrum could be signed to the –CHCH–Si–C(CH₂)–(β,α) linkages. The coupling constants J_H–H in the ¹H NMR spectrum also confirmed the predominant formation of the (E)-product. Since the Pd-complex 338 only catalysed the addition of the Si–H bond to the CC bond it was possible to prepare polymers with vinyl function attached to Si-atom 340e–f, which can be then transformed in further reactions (Scheme 71 and Table 9).²¹⁰

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Rao et al. used the same strategy for the formation of macromolecular compounds, but to build a crosslinked matrix. They used 1,3,5-triethynylbenzene 304a or 1,3,5-triethynyl-2,4,6-trimethylborazine 342 in different ratios 245:(116c or 324):(304a or 342) = 100:95:5, 100:90:10, and 100:80:20. The reaction time was 4.5 h and the polymers 343a–i were precipitated in propan-1-ol before gelation. Depending on the ratio of momers and crosslinking agents, different molecular weights of 343a–i were obtained, with the highest M_w = 110000–130000 for a 100:80:20 ratio. The degree of crosslinking influenced the thermal stability of polymers (Scheme 72 and Table 10).²¹¹

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The same group also used phenylsilane 272 as a monomer possessing three Si–H bonds in an equimolar hydrosilylation reaction with 1,4-diethynyl-116c or 1,3-diethynylbenzene 324 with the Pd₂(dba)₃338/PCy₃ catalytic system to furnish polymers 344 with different ratios of regioisomers (β,β):(β,α):(α,α) = 60:35:<5. The presence of a free Si–H bond in the polymer structure 344 allowed its further modification with different ethynylarenes 345a–b, 346 in the next step towards functional polymers 347a–d. Therefore, some additional functional groups or chromophores were included in the polymer 347a–d as pendant groups (Scheme 73 and Table 11).²¹²

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RhI(PPh₃)₃351 was reported as the selective catalyst for hydrosilylation of alkynes, in which the stereoselectivity can be tuned by altering the reaction temperature. When the reaction was carried out at 80 °C, the formation of (E)-alkenylsilane occurred, while at 0 °C the (Z)-isomer was predominantly formed.²¹³ The same observations were visible for hydrosilylative polyaddition of dihydrosilanes with diethynylarenes. The products were obtained with high yields and stereoselectivity depended on the reaction conditions. The coupling constants for ethenyl hydrogens were typical for (Z)- or (E)-isomers. At elevated temperatures (80 °C), polymers 353 with (E) regioselectivity were mainly formed ((E) > 93%), while at 0 °C (Z)-isomers 354 were synthesised ((Z) > 91%) With more sterically hindered silanes 1,4-bis[methyl(3,3,3-trifluoropropylsilyl)]benzene 349 and 1,3-bis[methyl(3,3,3-trifluoroprorylsilyl)]benzene 350, the polymerisation was carried out in the presence of [RhI(cod)]₂352. All polymers 353–354 were obtained with high yields 54–96% and M_n = 5000–22000 (Scheme 74).^213–215

The same catalyst was used in the synthesis of hyperbranched polymers by the homopolymerisation of bis(4-ethynylphenyl)methylsilane 355. The process resulted in polymer 356 with the (E)-regularity in 95% yield.²¹⁶ Dendrimeric structures were also obtained in the reaction of bis(1-ethynylphenyl)dimethylsilane 327c with dichloromethylsilane 357 in the presence of a Pt/C catalyst 307 (10% of Pt).

Bishydrosilylation leading to saturated products and α-hydrosilylation was not visible. The further substitution of halogen in 358 with 1,4-bislithium-1,2,3,4-tetraphenylbuta-1,3-diene 359 led to siloles 360, while the reaction with lithiumethynylbenzene 361 led to the product 362 that can be further hydrosilylated with dichloromethylsilane 357 to build a more branched product (Scheme 75).²¹⁷

Sanchez et al. reported a special type of hydrosilylation of diynes using dihydrosiloles 282 and 368. The reaction was carried out with different transition metal catalysts based upon Rh, Pt, and Pd catalysts (e.g., Pt₂(dvs)₂194, RhCl(PPh₃)₃205, Pd(PPh₃)₄35). The best results according to polymer molecular weight, yield, and selectivity were obtained when heterogeneous H₂PtCl₆206 was used in boiling toluene. The reactions were carried out for 10 min–12 h. Very bulky 2,3,4,5-tetraphenylsilole 368, as well as silafluorene 282, were used in these polyaddition reactions. The process occurred by cis-addition of the Si–H bond to the CC bond of diyne forming exclusively (E)-products. In the case of other complexes, α-hydrosilylation or desilylative coupling was also observed. During the reaction, the selectivity was controlled sterically and kinetically. Less bulky groups such as silafluorene 282 required more accurate temperature control. At lower temperatures, (E)-products were obtained, while at higher temperature complex mixture of β- and α-hydrosilylation was observed. Bulky reagents such as siliptycene (1,1-dihydrido-4,5,8,9-bis(triptycene)silafluorene) 369d remained completely unreactive towards polyaddition. The structure of diyne also influenced the polydispersity and molecular weight of the polymers 370 and 371 (Scheme 76). Obtained polymers were used as luminescence chemosensors for explosives. Cyclic siloles increase the efficiency of application of these vinylene-silole polymers as light-emitting diodes (LEDs), luminescent sensors, or organic charge carrier materials.^68,218–220

5.4. Cyclisation of 1,n-diynes by hydrosilylation reactions

Silanes and 1,n-diynes were also used in the hydrosilylation/cyclisation to silylated (Z)-1,2-dialkylidenecyclohexanes, useful synthons in the synthesis of fine chemicals. The reactions occurred in the presence of metal complexes including Ni, Rh, Ru, Pd, Pt, but there are several limitations of each catalytic system. Described by Tamao et al., Ni(0) complexes catalysed the cyclisation of 1,7-diynes 160, 372–375, while 1,6-diynes were not active in this transformation.^157,221 Different types of trialkyl, trialkoxysilanes, and silazanes 376a–e were applied as reagents. The reactions were carried out in the presence of Ni(acac)₂ (1 mol%) 273/DIBAH (2 mol%) 170, at 50–100 °C, for 6–24 h. The higher the temperature and the longer the reaction time, the lower yield of exocyclic diene was obtained, due to the subsequent polymerisation process. The cyclisation of terminal diynes occurred with moderate or good yield (47–73%) with the exclusive or predominant formation of (Z)-product. The process was effective also for optically active diyne 372. The asymmetric diyne 373 containing nitrogen led to the silyl-substituted tricyclic alkaloid-type dienes 379 with lower selectivity (Z)/(E) (379a/379b) = 79:21, suggesting a directing effect of the nitrogen atom, which can easily coordinate to the metal centre. The internal diyne 374 was less reactive and reacted only under higher reaction temperatures and using accelerating triphenylphosphine as an additive. Unsymmetrical diyne 375 containing one terminal and one internal CC bond reacted with the silane from the less shielded terminal acetylene furnishing a single regioisomer 381 in the post-reaction mixture (Scheme 77).^157,221

The mechanism of this transformation (Scheme 78) started from the insertion of one of the CC bonds (less shielded) to the Ni–Si bond, generated by the oxidative addition of silane 376a–e to metal centre 382. The insertion of the second acetylene group to the Ni–vinyl bond 384 and reductive elimination of the exocyclic diene 381 closed the catalytic cycle. The insertion of acetylene to the Ni–H bond can be eliminated, because of the lack of other isomers in the post-reaction mixture.^157,221 This Ni-catalysed reaction was possible only for 1,7-diynes. 1,6- or 1,8-diynes in the hydrosilylation process gave only polymeric products. To cyclise 1,6-diynes with hydrodisilanes, the reaction was catalysed with 5 mol% of Ni(acac)₂273/DIBAH 170/PEt₃ and the mechanism proceeded with the formation of Ni–silylene intermediate.¹⁵⁷ The obtained exocyclic dienes with (Z)-selectivity were used as reagents in Diels–Alder reactions, or the silyl groups were reacted in C–C bond forming reactions with aryl halides in Hiyama coupling reactions (Scheme 79).^157,221

Widenhoefer et al. developed cationic Pt-complex, formed in situ from (phen)PtMe₂399 (phen = phenanthroline) and B(C₆F₅)₃401 that was highly active and selective in the cyclisation/hydrosilylation reactions of 1,6- and 1,7-diynes 127a, 127k, 127p, and 394a–l leading to silylated 1,2-dialkylidenecyclopentanes 402–410 and 1,2-dialkylidenecyclohexane 411, with high (Z)-selectivity ((Z)/(E) > 8:1). The catalyst was found to be inactive in the cyclisation/hydrosilylation of separated dienes (for which palladium analogs were active), making this process highly selective.^222–225 The reactions were carried out for 10 min–3 h at 110 °C in toluene for different silanes 207a, 395–398 (Scheme 80).¹⁵⁸

The same group developed a diimine cationic Pt complex, [PhNC(Me)C(Me)NPh]PtMe₂400, which was much more active and selective than the complex with phenanthroline 399. The products 402–411 were obtained in 15 min at 110 °C or 85 min at 70 °C with higher selectivity towards (Z)-isomer (Z):(E) >30:1. The electronic and steric properties of the diimine ligands were found to have an important influence on the cyclisation/hydrosilylation reaction. The rate of the process decreased with the increase of the electron density and steric bulk of the ligand. The structure of silane and diyne also influenced the reaction rate. When HSi(i-Pr)₃398 was used instead of HSiEt₃207a, the reaction was 10 times slower. The catalytic system was tolerant towards many functional groups including inter alia sulfones, amides, ketones (Scheme 80).¹⁵⁹

The authors proposed the mechanism of this transformation (Scheme 81). Initially CO or B(C₆F₅)₃401 abstracts the methyl group from the pre-catalyst 400 forming the Pt-cationic complex 412 upon coordination to the diyne substrate. Oxidative addition of the silane, which occurred readily, even at −30 °C leads to complex 413. Loss of CH₄ leads to 414. Next, the insertion of the alkyne into the Pt–Si bond occurs leading to 415, followed by the β-migratory insertion of the coordinated second alkyne group and formation of the platinum dienyl intermediate 416. The oxidative addition of silane 207a or 395–398 formed 417. Elimination of the product 402–411 and the coordination of diyne regenerates the initial catalyst 414. The obtained cyclic products were used in protodesilylation and Diels–Alder transformations. Examples of these processes are presented in Scheme 82 using 402a as a reagent.^158,159

Several papers discussed the application of Rh complexes in the synthesis of 1,2-dialkylidenecyclopentanes. The use of the popular Wilkinson's complex 205 in this transformation was reported by Matsuda et al.^226,227 The exact catalyst, which facilitated the formation of cyclic compounds was the complex Rh(H)(SiR₃)Cl(PPh₃)₂438, which was obtained by oxidative addition of silane to the metal centre. The order and time of addition of silane and diyne were important for the reaction course. When reagents 127a, 127k, 127p, 195a and 394a–l were added 1,2-dialkilidenecyclopentane 434 was formed immediately. In other cases, indane 435 was formed as the main product (Scheme 83). Scheme 84 shows various dialkilidenecyclopentanes 434 obtained within this transformation in the presence of 438. Depending on the catalyst structure (E)- or (Z)-cyclic isomers were obtained. When mono- or bidentate electron-donating phosphine ligands are coordinated to the metal centre RhCl(PPh₃)₃205 or [Rh(cod)(dppb)][PF₆] 439, the insertion of the second alkyne group is slowed down giving time to convert the (Z)-isomer 434 into (E)-product 434. For Rh₄(CO)₁₂440 with electron-withdrawing CO ligands, the insertion process is much faster, and there is no time for the formation of (E)-isomer (Scheme 85).^226,227 The obtained products were used in Diels–Alder transformations with different dienophiles, as well as in the hydrogenation process catalysed by the Pd/C 446 system, followed by the homologation reaction (Scheme 86).

Ojima et al. reported several papers based on the cyclisation/hydrosilylation of 1,6-diynes 127o–p, 450a–f in the presence of rhodium complexes 204, 451–452 and different pressures of CO.^{66,67,228–230} The course of the reaction strictly depended on the reagents (silane, diyne) structure, the type of the catalyst, as well as the pressure of CO. When Rh₂Co₂(C0)₁₂451, Rh(acac)(CO)₂204 or Rh(t-BuNC)₄Co(CO)₄452 were used, the corresponding bicyclo[3.3.0]octenones 453–455 were obtained in 82–93% yield via carbobicyclisation with the incorporation of CO (15–50 bar) (Scheme 87 and Table 12). Under lower CO pressures (1–2 bar), no reaction with CO was observed and typical dialkilidenecycloalkanes were formed.²³⁰ Moreover, the steric hindrance of silane or diyne influenced the formation of a specific product. Additionally, the C₄ position in 1,6-diynes 450a–f exerts marked influence on the product distribution. When the heteroatom is at the C₄ position 1,2-hydrosilylation is the main process, while 1,4-hydrosilylation is favoured with 4,4-gem-disubstitution with ester groups.²²⁸ Products 453 can easily isomerise quantitatively to 454 in the presence of RhCl₃·3H₂O 213 as a catalyst in ethanol under 50 °C.

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A detailed mechanism of this transformation was also presented which explained the formation of various cyclic products. The product outcome was found to be dependent on the further transformations of complex 459 formed in the carbocylisation of 457 to 458, followed by CO insertion and subsequent carbocyclization to bicyclic 460. When a 1,3-[Rh]-shift occurred from 460, the complex 461 is formed, which then after reductive elimination furnishes product 453. When β-hydride elimination occurred from 460, the dienone–M–H complex 462 is formed or/and bicyclic diene 464. The addition of the M–H species leads to intermediates 463 or 465, which next (via addition of the next silane and reductive elimination of R₃Si–[Rh]) accomplishes the products 454 or 455. The formation of 455 was observed only for the product which was able to form a stable aromatic pyrrole product (Scheme 88).²²⁹

Ojima discussed also that endiynes 466 (dodec-11-ene-1,6-diyne or their heteroatom analogs) reacted with silanes (PhMe₂SiH 195a, Et₃SiH 207a, or (t-Bu)Me₂SiH 395) in the presence of Rh(acac)(CO)₂204 in unique silylative cascade carbonylative carbotricyclisation process, at room temperature and under ambient pressure of CO. The reaction yielding fused 5-7-5 tricyclic products 5-oxo-1,3a,4,5,7,9-hexahydro-3H-cyclopenta[e]azulenes 467 or their heteroatom congeners. Within this process, functionalised polycyclic compounds were obtained that are useful synthons in the synthesis of natural products (Scheme 89).^66,67

Using the same [Rh(acac)(CO)₂] complex 204, it was possible to carry out hydrosilylative cyclisation with carbonylation of various 1,5-diynes with aromatic, olefinic, and ethylene tethered spacers 468 under the ambient pressure of CO (1 atm.). The reaction furnished various 2,5-dialkylidenecyclopentanones 469 in good yields. In this example, the insertion of CO was favoured to build a five-membered ring and avoid high strains. The products 469a–m were obtained with moderate or high yields 30–92%, which varies with both reagent structures (Scheme 90).²³¹ The mechanism of carbonylative cyclisation of 1,5-diynes 468 using [Rh(acac)(CO)₂] 204 started from silylrhodation, followed by the insertion of CO to 471 to form acylrhodium species 472, then acylrhodation to the second alkynyl group forms the 5-membered ring 473 (Scheme 91).²³¹

Cyclisation/hydrosilylation of 1,6-, 1,7- and 1,8-diynes 127a, 127o–p, 160, 164a, 474a–d was carried out in the presence of ionic Pd complex [(η³-C₃H₅)Pd(cod)][PF₆] 476 with chlorodimethyl-305, dichloromethyl-357, and trichlorosilane 475. The reaction occurred at room temperature in CH₂Cl₂ and products (Z)-1-methylene-2-silylmethylenecycloalkanes 477 were obtained in good yields, which were further transformed to their ethoxy analogs 478 (Scheme 92). For unsymmetrical diyne 2-butynyl propargyl ether 474b, it was found by NOE analysis that the silyl group is attached to the internal CC bond, suggesting that the formation of the regioisomer 478f was due to the fact, that the reaction started from the hydropalladation at the terminal alkyne site to 481, instead of the insertion of the alkyne into the metal–Si bond. The further steps in the plausible mechanism are: intramolecular carbopalladation, the formation of cyclised (Z)-alkenylpalladium intermediate 482, and finally σ-metathesis with a hydrosilane, followed by the product release 477 and regeneration of the initial catalyst 479 (Scheme 93).^232–234

Liu and Wiedenhoefer reported that the cationic rhodium complex [Rh(BINAP)(cod)][BF₄] 485 (BINAP = ((±)-2,2-bis(diphenylphosphino)binaphthyl)) is as an effective catalyst (10 mol%) in the cyclisation/hydrosilylation reaction of terminal and internal 1,n-diynes 127a, 483a–m with silanes 195d, 207a, and 484 leading to 1,2-dialkilidynecycloalkanes 486a–p with high yield and high diastereoselectivity (Scheme 94). The higher the concertation of silane the lower the selectivity. The best results were obtained when triethylsilane 207a was used (1.0–1.7 M) and was added slowly to the reaction mixture. The mechanism of this transformation is similar to that previously described for Pt-complexes (Scheme 81).¹⁶⁰ The obtained silylated 1,2-dialkilidynecycloalkanes 486 were further used in the Diels–Alder reaction with 4-phenyl[1,2,4]triazole-3,5-dione 430 at 0 °C for 30 min (Scheme 95).

Another example that involved the reaction of diynes 116a, 127c, 127o–p, 488a–c, tert-butyldimethylsilane 395, and CO, which furnished two different catechol derivatives 490 and 491, was carried out in the presence of [Ru₃(CO)₁₂] 489/PCy₃. Product 490 was the primary product, which can be readily hydrosilylated to 491 when 6 equiv. of silane 395 was used. In the case of a lower excess of 395, product 490 was visible in the reaction mixture. The best results were obtained using acetonitrile as a solvent. Various terminal and internal diynes were used as reagents giving products with moderate yields (Scheme 96).²³⁵ The mechanism of the process involves the formation of an oxycarbyne complex 493 as an intermediate and the process requires the introduction of two CO molecules into the diyne structure. A carbyne/CO coupling yields intermediate 493 was previously tested for tungsten.²³⁵ Analog complexes to 495 for alkynes were determined for other metals, e.g., Nb, Ta, V, Ta. Katz et al. reported the formation of a similar product in the reaction of (CO)₄BrMCCH₃ (M = Cr, W) with diyne.²³⁶ This proved that the proposed mechanism is plausible (Scheme 97).

Lewis acids such as AlCl₃496 and EtAlCl₂497 were successfully applied for the hydrosilylation of alkynes with trialklylsilanes, which occurred as a syn-addition of the Si–H bond to the CC bond with the formation of the trans-product.^57,237 The mechanism of this transformation assumes the formation of a zwitterionic intermediate by the coordination of 496 or 497 to the acetylenic bond. Next, the hydride of silane attacks the electron-deficient carbon atom from the opposite site to AlX₃ with the formation of ate-complex. The coupling between the silyl cation and vinyl group furnishes the silylated olefin with retention of configuration. The same catalysts 496 and 497 were also used in the hydrosilylation of hepta-1,6-diyne 116a and octa-1,7-diyne 160, using 4 equiv. of triethylsilane 207a. For a shorter chain of terminal diyne 116a, the cyclic product 498 was obtained in 60% yield, while for octa-1,7-diyne 160, 1,8-bistriethylsilyl-octa-1,7-diene 499 was formed predominantly (Scheme 98).²³⁷ Formation of bissilylated diene using this Lewis catalyst contrasts with the cyclization process via hydrosilylation, which occurred in the presence of Ni or Rh catalysts.^157,221

6. Hydrogermylation of conjugated and separated diynes

Hydrogermylation of diynes is limited only to two examples, which describe the formation of 2,5-disubstituted germoles²³⁸ or germylene–divinylene polymers.²³⁹

Murakami et al. developed a trans-hydrogermylation of conjugated symmetrical and nonsymmetrical 1,3-diynes 1a–b, 1d, 27c, 60e, 258i, 258o, 500a–d with diphenylgermane 501 in the presence of [Cp*Ru(MeCN)₃][PF₆] 281 wich yielded cyclic germoles 502a–o with good or moderate yields (Scheme 99).

The same complex 281 was previously used by Trost et al. in the trans-hydrosilylation of alkynes,^{56,189,240,241} but its activity in the reaction with conjugated diynes was much lower than for hydrogermylation (Table 13). The double addition of diphenylgermane 501 to 1,4-diphenyl-buta-1,3-diyne 1a occurred with a much higher yield in comparison to the hydrosilylation reaction (93% vs. 29%) (Scheme 58).¹⁵⁶ The hydrogermylation reaction was carried out with 3 equiv. of germane 501 and 10 mol% of Ru catalyst 281, but a lower excess of reagent 501 was also possible (1.2 equiv.).

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The reaction was efficient for diaryl-substituted diynes with different functional groups (silyl, boryl, methoxy, bromo, fluoro) or compounds with heteroaryl substituents 27c, 500b. The presence of strongly electron-withdrawing nitro groups in the para position 500a was responsible for the lower product yield (40%, 502i). No reactions were observed for diynes with alkyl groups (hexa-2,4-diyne 65a) and silyl functionalities (1,4-bis(trimethylsilyl)buta-1,3-diyne 180c). When dibutylgermane 503 was used as a reagent, the reaction was less effective, even with 20 mol% of 281. Applying conjugated 1,8-diphenylocta-1,3,5,7-tetrayne 504, diphenylgermane 501 (6 equiv.), and 20 mol% of Ru complex 281 it was also possible to obtain 2,2′-bigermole 505 with 56% yield (Scheme 100).²³⁸

Diphenylgermane 501 was also used as a reagent in the hydrogermylation of various diynes, with aryl or alkyl spacers between the CC bonds leading to germylene–divinylene polymers. The polymerisation was effectively catalysed in the presence of 0.9 mol% of Pd catalysts (Pd₂(dba)₃338/2PCy₃, PdCl₂(PCy₃)₂341). The reactions were carried out at 50–90 °C and the polymers 506 were obtained with M_w = 12000–83000 and M_w/M_n = 3.3–12.0 (Scheme 101). They were isolated by precipitation in benzene/propan-2-ol solution. Due to the high conjugation, the germylene–divinylene polymers 506a–d indicated intense light emission depending on the structures of the monomers. The best results were obtained for anthrylene polymer which gave intense and broad UV-Vis spectra from 420 to ≥600 nm with λ_max peaks at 440, 464, and 534 nm.²³⁹

7. Hydrostannation

Alkenylstannanes are useful building blocks in the synthesis of various organic compounds (also complex molecules as pharmaceuticals or natural compounds) due to their ability to the formation of the new C–C bonds in Stille coupling reactions.^242–247 The hydrostannation of alkynes, which can occur under a free radical manner, in the presence of a transition–metal catalyst or via a hydrogen atom transfer reaction (with trialkyltin hydride used as a nucleophilic species), is the most convenient and popular method for the synthesis of alkenylstannanes.^18,54–56 Despite several papers focused on the hydrostannation of alkynes, the literature concerning the addition of the Sn–H bond to the CC bonds in diynes is limited to a few examples based on radical or transition metal-catalyzed transformations.

7.1. Radical hydrostannation of conjugated and separated diynes

Radical hydrostannation was successfully applied in the reaction with 1,3-diynes,²⁴⁸ as well as diynes possessing an aryl spacer between the CC bonds.²⁴⁹

Konno et al. reported selective radical hydrostannation of 5-benzyloxy-1-trifluoromethyl-5-methyl-hexa-1,3-diyne 507 with tributyltin hydride 508. The radical is generated from HSn(n-Bu)₃508 in the presence of Et₃B 509 and oxygen (Scheme 102).

Despite the fact that even eight different products might be obtained in hydrostannation due to the presence of double CC bonds and different substituents in terminal positions, some of the products might be eliminated. The attack of the radical on the carbon in position β- or γ-can be excluded because of the lack of resonance of the vinyltin radical. The radical, which has a nucleophilic character attacks the more electrophilic α-carbon atom in 507 with a strong electron-withdrawing CF₃ group. Moreover, the bulky group in δ-position limits the access of the organotin group, therefore product 510 is formed with high regio- and stereoselectivity (Scheme 103). The obtained enyne 510 was generated in 75% yield and was further used in the synthesis of CF₃-substituted (Z)-enediyne 517 compounds in iododestannylation/Sonogashira coupling reactions (Scheme 104).²⁴⁸

The radical hydrostannation of various diynes and triynes 102c and 264a–e was carried out stereoselectively with tributyltin hydride 508. In the two cases, the (E)-products 521a and 521c were exclusively formed. The hydrostannation of other diynes 264a–c, and 264e occurred with lower selectivity, but still with an excess of the (E)-products 521 (Scheme 105). To obtain high selectivity, an elevated temperature (80 °C) has to be maintained. Under lower temperatures, the conversion was not complete and other isomers were also formed. The authors proved the (E)-selectivity of the products through ¹H NMR spectroscopy by the large coupling constant of the vinyl group (J_H–H = 18–19 Hz) and the characteristic values for tin hydrogen coupling (J_Sn–H = 124–138 Hz). For 1,3,5-tris[(E)-2-(tributylstannyl)vinyl]benzene 521f, the authors carried out Stille coupling with various bromo-substituted chromophores 522 in the presence of PdCl₂(PPh₃)₂94 catalyst and CsF 523 or CuI 519 as additives. The products were obtained with moderate or good yield with the retention of the configuration, showing the utility of organotin compounds (Scheme 106).²⁴⁹ Previously published papers described that radical hydrostannation of diynes led to the mixture of various isomers, which is in opposition to the above-reported results.²⁵⁰

The non-catalysed addition of organotin compounds to the CC bond in elevated temperatures occurs relatively easily due to the weak Sn–H bond. The application of dihydrides in the hydrostannation of diynes may lead to cyclic or polymeric products which can be controlled by appropriate selection of the substrates and reaction conditions.²⁵¹ The addition of Bu₂SnH₂525 to penta-1,4-diyne 526a in refluxing heptane followed by heating the reaction mixture to 200 °C gave a six-membered heterocycle 528a with 43% yield. The product was distilled from a viscous polymeric residue together with the small amount of five-membered by-product 530a. Generally, the terminal addition (path A) of the Sn–H bond to the CC bonds yielded a six-membered heterocycles 528, whereas the non-terminal addition (path B) led to five-membered adducts 530 (Scheme 107). The regioselectivity could be controlled by the proper selection of the substituents attached to C_sp carbon. When the hepta-2,5-diyne 526b or 1-phenyl-1,4-pendadiyne 526f were used the five-membered heterocycles 530b and 530f were formed as the major regioisomers. The application of monoalkyl-substituted 1,4-diynes 526c–e on the other hand gave in an excess stannabenzene derivatives 528c–e. The authors suggested that radical-stabilising substituents in 1,4-diynes mainly led to stannoles 530, whereas 6-substituted hexa-1,4-diynes 526g–i gave the six-membered adducts 528g–i (Scheme 107).^252,253 The substitution of CH₂ spacer between alkynyl groups, in the case of 3-organyl-substituted 1,5-diynes, did not influence process regioselectivity leading mainly to the six-membered products.^254–256

The hydrostannation of diynes possessing the p-block element as a linker between alkynyl groups gave in major an attractive six-membered rings with two heteroatoms, which are useful synthons in organic synthesis. For instance, the hydrostannation of (dialkylamino)dialkynylboranes 531a–c with dimethylstannane 532 yielded 1,1-dimethyl-1-stanna-4-bora-2,5-cyclohexadies 533, which could be further converted via trans-amination to 4-amino derivatives 534 or via solvolysis of 533 to alkoxy derivatives 535. These latter were precursors for 4-alkyl-1,1-dimethyl-1-stanna-4-boracyclohexadienes 537 or lithium-1,1,2,4,4,6-hexamethyl-1-stanna-4-borata-2,5-cyclohexadiene which were obtained in high yield 536 (Scheme 108).²⁵⁷ Analogous ring systems with different heteroatoms could also be obtained for the hydrostannation of diynes containing Si, Sn, or P atoms as spacers between alkynyl groups.^258–260

The diyne structure, as well as reaction conditions, have a crucial influence on the product formed. The hydrostannation of α,ω-diynes, such as 1,4-diethynylbenzene 116c, nona-1,8-diyne 164a, and hexa-1,5-diyne 541 with diorganotin dihydrides 525, 532, 538–540 at high temperatures gave rubber-like polymers. However, for hexa-1,5-diyne 541 small amounts of 1-stanna-2,6-cycloheptadiene 544 derivatives were isolated as well. The polymer formation occurred via intermolecular poly-addition of alkenyldiorganoltin hydride 542 whereas, the cyclic product is obtained through its intramolecular cyclisation (Scheme 109). The molecular weight of the polymers depended on both the α,ω-diynes, and organotin compounds (Table 14).²⁶¹ Similar observations were made when p-phenylene-bis(dimethyltin hydride) 545 was used in the poly-addition to α,ω-diynes.²⁶²

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The appropriate selection of the reaction condition was also crucial for the synthesis of tin-containing seven-membered heterocycles (stannepines) by the hydrostannation of (Z)-endiynes. Mild reaction conditions and the presence of base led to the desired heterocycles instead of polymeric material.²⁶³

The hydrostannation of o-diethynylbenzene 545 with diorganotin hydrides 532, 538, 540, and 546 yielded, in addition to polymers 549, the seven and fourteen-membered tin-containing heterocycles (547 and 548) with low or moderate yields.^264,265 The highest yield of the fourteen-membered ring system was observed for 548d when ethylphenyltin dihydride 546 was used, whereas the benzostannepin 547b was formed in 22% yield when diethyltin dihydride 538 was applied. Nevertheless, in all cases, the polymers were the main products (Scheme 110 and Table 15). The obtained heterocyclic compounds and polymeric materials could be readily transformed with the retention of configuration into alkenyl iodides by the reaction with I₂418. Polymer degradation with iodine 418 revealed that the polymer product contained Z,Z-, Z,E- and E,E-units.²⁶⁵

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The tin-containing six-membered heterocycles are attractive precursors in the preparation of various 15 group heterobenzenes.^266,267 In 1971, Ashe reported the synthesis of arsabenzene 555 based on the arsenic/tin exchange. The l,4-dihydro-l,l-dibutylstannobenzene 528a was converted in a one-step procedure to desired product 555 through the reaction with the arsenic trichloride 553. Similarly, 528a reacted with phosphorus tribromide 550 to give phosphabenzene 552.²⁶⁸ The same research group extended the scope of 15 group heterobenzenes to stibabenzenes 558²⁶⁹ and bismabenzene 561²⁷⁰ in an analogous manner. However, treatment of l,4-dihydro-l,l-dibutylstannobenzene 528a with SbCl₃556 or BiCl₃559 gave 1-chloro-1-stibacyclohexa-2,5-diene 557 or 1-chloro-1-bismacyclohexa-2,5-diene 560, respectively. The group 15 heterobenzenes underwent Diels–Alder reactions with hexafluorobutyne 562 to give 563. The reactivity of heterobenzenes increased with the higher atomic number of heteroatom. For instance, stilabenzene 558 reacted rapidly with hexafluorobutyne 562 at 0 °C, arsabenzene 555 at room temperature, whereas phospabenzene 552 was converted to Diels–Alder adduct at 100 °C (Scheme 111).²⁷⁰ The 2- and 4-subtituted heterobenzenes could be also synthesised through hydrostannation of appropriate 1,4-diynes. Further transformation of the stannabeznes to the phospha- or arsabeznene derivatives could also be achieved.^271–273 The same procedure was adopted to the formation of borabenzenes.²⁵³

The hydrostannation of penta-1-4-diyne 526a with Bu₂SnH₂525 was used in the synthesis of 13-thiaarachidonic acid 573. Compounds 573 is a time- and O₂-dependent irreversible inhibitor of soybean lipoxygenase and was prepared in reaction sequence presented in Scheme 112.²⁷⁴ The process was characterised by excellent stereoselectivity and satisfactory yields of each of the individual reaction steps.

7.2. Transition metal-catalysed hydrostannation of diynes

The phosphine-free palladium Pearlman's catalyst Pd(OH)₂/C 575 was found to be effective in the hydrostannation of 1,6-diynes 127a, 127j, 127m, 127p, 394d and 574a–c with HSn(n-Bu)₃508, which generated 1,2-dialkylidenecyclopentenes with the tributylstannyl group 576. This stannylative coupling was effective for various 1,6-diynes, including those possessing hydroxyl groups or protected alcohols, as well as reagents with heteroatoms in the propargylic position. The reactions occurred with high yields of the products 576 (58–95% yield) (Scheme 113). Several other complexes such as Pd₂(dba)₃338, Pd/C 446, Pd(acac)₂577 gave the desired cyclised product 576a with the yield >75%. Adding 1 or 2 equiv. of PPh₃ or dppb to Pd₂(dba)₃338, gave a complex postreaction mixture with less than 15% of 576a. The authors suggested that the phosphine coordinates to the metal centre, blocking the possibility chelate formation with the 1,6-diynes 127a, 127j, 127m, 127p, 394d, and 574a–c. The mechanism of the process begins with the oxidative addition of HSn(n-Bu)₃508 and chelation of the 1,6-diyne to give 579. The formation of the product might occur within two possible pathways based on stannylpalladation (path A) or hydropalladation/carbopalladation (path B) (Scheme 114). There was no information on which cycle is more probable. Terminally substituted 1,6-diynes 584a–c were also reactive in this cyclisation reaction, but the electronic properties of the diyne substituents strongly influenced the selectivity, and a mixture of cyclised 585 and 586 and linear 587 vinylstannanes were generated. The linear product was predominantly formed (587c, 59%) in the case of hydrostannylation of the silyl-substituted reagent 584c (Scheme 115).^275,276 The obtained dialkylidenecyclopentenes functionalised with stannyl group 576a–c were used in several destannylation reactions: Diels–Alder with N-phenyl maleimide 387 to 588 followed by protodestannylation to 589, Stille coupling with p-iodoanisole 58, and homocoupling of 576a, showing the high utility of this reagent in organic synthesis (Scheme 116).

Furstner et al. reported that conjugated 1,3-diynes 594, as well as non-conjugated 1,n-diynes 595 (with an unprotected hydroxyl group in the propargyl position), underwent double or site-selective trans-monohydrostannation depending on the reaction conditions in the presence of catalytic [Cp*RuCl]₄596. The process was found to be temperature-dependent. When the reaction was carried out in boiling 1,2-dichloroethane (at 80 °C), the site-selective reaction is favoured, while at a lower temperature (especially at −40 °C) bishydrostannylation occurred in a large amount. Irrespective of the alcohol type (primary, secondary, or tertiary) the trans-hydrostannylation occurred with high selectivity (Scheme 117). Additionally, the type of substituent attached to the second alkyne influence the process selectivity with bulkier groups giving better selectivity towards trans-hydrostannation. The selectivity of the stannylation of diyne 594 from the propargylic side resulted from the hydrogen bonding of OH with the polarised [Ru–Cl] bond of 596. The propargylic alcohol readily forms an adduct with the Ru-complex under room or higher temperature, while binding the alkyne occurs only at a lower temperature. The reaction was also effective for trans-monohydrostannation of 1,n-diynes 595 to give products 599a–e (Scheme 118). The strong directing effect of the hydroxyl group in the propargylic position was responsible for the high process selectivity. The stannyl-substituted products might be directly transformed to (E)-conjugated enynes by the protodestannation reaction with copper diphenylphosphinate CuOP(O)Ph₂601 in DMF. The site-selective trans-hydrostannation was applied in the total synthesis of typhonoside series of glycolipids 608 and 614, which have neuroprotective properties (Scheme 119). Moreover, the application of this transformation permitted for late-stage modification of the bioactive compound, which was illustrated by the synthesis of the fluoroalkene sphingosine analog. The replacement of tin with fluorine was carried out with F-TEDA-PF₆615 in the presence of silver phosphinate AgOP(O)Ph₂616.⁶³

In 1990 Zhang et al. reported the palladium- and molybdenum-catalysed addition of Sn–H to CC bonds leading to vinylstannanes in high regio- and stereoselectivity. Although the authors described in detail the hydrostannation of mono alkynes, a few examples of diyne reactivity was also reported. The readily available and air-stable catalyst PdCl₂(PPh₃)₂94 was applied in the hydrostannation of symmetrical and unsymmetrical 1,3-diynes. The addition of HSn(n-Bu)₃508 to symmetrically substituted dodeca-3,5-diyne 13a under mild reaction conditions and a short reaction time (10 min) gave (E)-enyne 618a in 78% yield. The n-Bu₃Sn moiety was attached to the carbon atom contiguous to the CC unit. The further addition of Sn–H bond to the unreacted triple CC bond was not possible and led to the decomposition of 618a. Similar regio- and stereoselectivity was observed when unsymmetrically substituted diyne 617 terminated with ethynyl group was used. In turn, the hydrostannation of deca-1,3-diyn-1-yltrimethylsilane 180a gave monohydrostannation product 618b in 86% yield. The presence of trimethylsilyl moiety caused the addition of Sn–H to CC bond adjacent to the alkyl substituents. Intriguingly, the hydrostannation of 1,2-bis(trimethylsilyl)ethyne did not occur at all, thus the SiMe₃ moiety in 180a could be considered as a directing group (Scheme 120).²⁷⁷

The same palladium catalyst PdCl₂(PPh₃)₂94 was applied for the tin-functionalised dienynes by the hydrostannation of (Z)- or (E)-endiynes with HSn(n-Bu₃) 508 in just 20 minutes at room temperature. The protocol was suitable for the symmetrical and unsymmetrical (Z)-endiynes 619a–j with various (aryl, alkyl, alkoxy, silyl) substituents. Among many possible isomers only α-products with the tin atom located on the carbon atom adjacent to CC bond, were formed. Nevertheless, very high selectivity was noticed only for symmetrical (Z)-endiynes. In the case of unsymmetrical (Z)-trideca-5-en-3,7-diyn-1-ol 619e an equimolar mixture of α-isomers and α′-isomers was observed since, the HSn(n-Bu₃) 508 did not distinguish in its addition between the two triple bonds. Intriguingly, the hydrostannation of SiMe₃ substituted unsymmetrical diynes with HSn(n-Bu)₃508 gave exclusively α-isomers, thus the silyl moiety acted as a directing group. The addition of Sn–H bond occurred on the silyl-unsubstituted CC bond with tin moiety attached at C_α (Scheme 121).²⁷⁸

Notably, the geometry of endiynes double bond had a crucial influence on reaction regioselectivity. The hydrostannation of unsymmetrical (E)-endiynes 621a–d in the same reaction conditions gave a mixture of α- and β-isomers even in the presence of the directing SiMe₃ group. However, the silyl-substituted CC bond, similar to hydrostannation of (Z)-endiynes, remained unreactive. The ratio of α- and β-isomers was dependent on the second substituent and ranging from 64:36 to 94:6 (Scheme 122).²⁷⁸

(Z,E)-Stannylated dienynes 620 were also found to be attractive building blocks in organic synthesis. Bujard et al. reported the synthesis of (Z,E)-dienediynes through the iododestannylation of 620d and 620i with NIS 624 and subsequent Pd/Cu catalysed coupling of vinyl iodide 625 with terminal alkyne 626a–c. The process was highly stereoselective and gave desired products 627a–c in good isolated yields (47–51%) (Scheme 123). The authors suggested that the obtained acyclic dienediynes are promising substrates for the synthesis of more complex molecules such as neocarzinostatin chromophore which was found to be an antitumor antibiotic.^279,280

Kazmaier et al. described the Mo(CO)₃(NC-t-Bu)₃629 catalysed hydrostannation of CC bonds. Although in the report a detailed research on hydrostannation of alkynes was presented, a single example of hydrostannation of a diyne was presented. The hydrostannation of diynoic ester 628, possessing internal and terminal triple CC bonds, with HSn(n-Bu)₃508 occurred preferentially at the internal CC bond bearing electron-withdrawing group. The reaction was relatively selective leading to a mixture of α- and β-isomers (630/631 = 82/18) in 74% isolation yield (Scheme 124).²⁸¹

8. Hydroamination

Compounds (acyclic and heterocyclic) possessing carbon–nitrogen bonds are omnipresent in an array of chemicals, especially in natural compounds, agrochemicals, pharmaceuticals, or cosmetics.^22,282–288 They are produced on a gram scale as fine chemicals, as well as in feedstock in tonnage scale in the industry. A limited number of chemical transformations leading to the formation of the C–N bonds in stoichiometric reactions led to the intensive development of hydroamination reactions, which simply introduce the N-atom to the compound structure, and occurs by the addition of the N–H bond to the unsaturated C–C bonds in olefins and alkynes.^{19–22,29,289–293} This 100% atom economic method mostly requires the application of a catalyst to (i) overcome the repulsion electrostatic effect between the high electron-dense unsaturated CC bond and the strong Lewis base (electron-rich amine 1° or 2°, ammonia, or hydrazine), and (ii) to facilitate this addition reaction due to the high energy difference between both types of bonds.²⁹⁴ The hydroamination of (non)conjugated diynes leads to various products, but intramolecular cyclisation is of utmost importance to produce N-heterocyclic compounds, e.g., indoles, pyrroles, pyrazoles, pyrimidines.

8.1. Noncatalytic hydroamination of conjugated 1,3-diynes

The origins of noncatalytic hydroamination of 1,3-diynes date back to the 1960s and 1970s, which was briefly described in the review published in 2002, which focused on the heterocyclisation of diynes.²⁹⁵ Different hydroamination agents (e.g., ammonia 632, hydrazine 633, substituted hydrazines 634, amines 635, diamines 636–637, hydroxylamine 638, 2-aminoethan-1-ol 639, guanidine 640) were used in this transformation. Depending on the type of reagents and reaction conditions various heterocyclic products (e.g., pyrazoles 645–647, pyridines 649, diazepines 651, pyrymidines 650, isoxazole 648) were obtained (Scheme 125).^295–298

The Cope-type hydroamination of conjugated 1,3-diynes occurs under noncatalytic and relatively mild conditions, while the reactivity of the substrates depends on the electronic structure of the 1,3-diyne. A reduction of the electronic density on the CC bond has a positive influence on the reaction yield and formation of the hydroaminated product. Therefore, the electron-withdrawing groups attached to the benzene ring in 1,4-diphenyl-buta-1,3-diyne permitted the desired products to be isolated in higher yields, while electron-donating groups caused the opposite effect. Bao et al. have reported the synthesis of 3,5-disubstituted isoxazoles or pyrroles (Scheme 126) by the Cope-type intramolecular hydroamination of 1,3-diynes 1a, 1c–d, 27a, 60a, 60e, 208b, 230d, 500a, 500d, 655a–g with hydroxylamine 639 or hydrazine 633 respectively.^299,300 Both reactions occurred at elevated temperatures (110 °C or 60 °C) in DMSO and using an excess of hydroaminating reagent 633 or 639 (1.5–4.0 equiv.) to provide the full conversion of 1,3-diynes. Triethylamine (Et₃N) was used as the most effective base in the synthesis of isoxazoles 656–657. The reactions yielded isoxazoles 656 and 657 in 61–92% or pyrazoles 658 and 659 in 60–93% isolated yields (Table 16). The high selectivity for unsymmetrical diynes was obtained when the reagent was substituted with groups with a distinct difference in electronic properties (e.g., hexyl- and 4-nitrophenyl).^299,300 The mechanism of intermolecular Cope-type hydroamination of 1,3-diynes occurred via the formation of intermediate 661 in a proton-transfer process, which further undergoes isomerisation to the allenyl oxime intermediate 662, followed by the electrophilic cycloaddition towards 3,5-disubstituted isoxazoles 656 or pyrazoles 658 (Scheme 127).^299,300 Moreover, the same group developed a one-pot procedure for the synthesis of heterocycles via Glasser coupling of alkynes followed by intramolecular hydroamination. The final products were obtained in comparable yields.^299,300

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The hydroamination with hydroxylamine 638 or hydrazine 633 was carried out also for symmetrical 663a–f (Scheme 128) and nonsymmetrical 1,3-diyne indole derivatives 668 (Scheme 129). The reactions were conducted in eco-friendly PEG-400 as a solvent, which facilitates a proton transfer to the allenyl intermediate, which according to the DFT calculations is the rate-determining step of the process. Applying PEG-400 as a solvent, it was possible to shorten the reaction time from 20 h to 2–6 h, and to carry out the reactions under milder conditions.^301,302 Additionally, N-substituted products were obtained by the application of arylhydrazines 665a–d (Schemes 128 and 129).³⁰²

3,5-Disubstituted pyrazoles 674a–d were synthesised using the Cope-type hydroamination in a sustainable manner by the application of a continuous flow process, starting from terminal alkynes 518a and 672a–c and hydrazine 633. Two coil reactors were combined, the temperature, the volume of the coils, and reagent flow rates, which influence the residence times, which were carefully chosen to obtain high product yields of 1a, 1c, 27c, and 258a in the Glasser coupling of alkynes and hydroamination process. For the homocoupling of alkynes 518a, 672a–c a 3.5 mL coil, alkyne concentration 0.75 M in DMSO, 120 °C, and 0.1 mL min⁻¹ flow (residence time: 35 minutes) were used. After the reactor outlet, the thiourea 673 scavenger column was applied to trap the copper (CuBr₂52) used as a catalyst for the Glaser reaction. The hydroamination was carried out in a 17.5 mL coil with hydrazine 633 in DMSO (0.1–0.2 mL min⁻¹) at 140 °C. The 87.5 min residence time was sufficient for the total conversion of 1,3-diynes 1a, 1c, 27c, and 258a yielding the appropriate pyrazoles 674a–d in 90–98% (isolated yields: 84–90%). The system was active for 16 hours for subsequent continuous flow Glaser coupling/hydroamination of 3-ethynylthiophene 27c with hydrazine 633, leading to 0.52 g of pure 3-(thiophen-3-yl)-5-(thiophen-3-ylmethyl)-1H-pyrazole 674d in 81% isolated yield. The ICP analysis detected only residue amounts of Cu (3 ppm), showing a high efficiency of the in-line Cu-scavenger (Scheme 130).³⁰³

This noncatalytic hydroamination was also used in the synthesis of 2,4,6-pyrimidines 677a–o, which possess biological activities (e.g., antitumor, antifungal, anticancer, anticonvulsant), luminescence properties, or are the component of nucleic acids. They can be effectively synthesised from diaryl or monoaryl-substituted 1,3-diynes (1a–d, 27b, 258a, 258e, 258h, and 675), and amidines 676 (acetamide hydrochloride 676a, benzamidine hydrochloride 676b or formamidine acetate 676c), which are used as bidentate nucleophiles in the presence of Et₃N as a base. The reaction occurred effectively in DMSO under a high temperature (160 °C), with the reagent ratio [diyne]:[676]:[Et₃N] = 1:3:3. The products 677a–o were obtained with 46–88% isolated yields, with the highest efficiency for electron-poor 1,3-diynes with electron-withdrawing groups (Scheme 131).³⁰⁴

8.2. Catalytic hydroamination of conjugated 1,3-diynes and separated diynes

The hydroamination of conjugated, as well as nonconjugated diynes, is often catalysed by homogeneous transition metal catalysts (Au, Ag, Cu, Pd) as well as non-noble metal or main group element complexes (Ti, Ni, Co, Ca). Among them, Au complexes have found a prominent position in their application for catalytic hydroamination. The hydroamination reaction constitutes one of the steps in the synthesis of natural products as e.g., indolizidine alkaloid (±)-Monomorine, pharmaceuticals or agrochemicals.^64,305–307

Skrydstrup et al. reported the synthesis of electron-rich 2,5-diamidopyrroles, 1,2,5-trisubstituted pyrroles, as well as pyrazoles using a hydroamination reaction in the presence of Au(I)-complexes. These products are difficult to synthesise according to other methods. Using (Ph₃P)AuNTf₂680 and only a slight excess of aniline 679a (1.05 equiv.), appropriate 2,5-diamidopyrroles were obtained in 30 min, under low temperature (30 °C) in CH₂Cl₂. The anilines with electron-withdrawing groups in the para position required a longer reaction time (60 min). The trisubstituted products were obtained after 24 h using different Au 680 and 682 catalysts in toluene and at elevated temperatures (80 °C), (Scheme 132 and Table 17).³⁰⁸

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The cationic gold(I) catalyst supported by a cyclic(alkyl)(amino)carbene (CAAC) generated in situ from an equimolar mixture of AuCl(CAAC) 684/KB(C₆F₅)₄ was able to activate NH₃632 and NH₂NH₂633 in the hydroamination reactions of alkynes, and conjugated and non-conjugated diynes 1a, 65a, 541, 690 (Scheme 133). These hydroaminating reagents are powerful reductive agents, which can form saturated products, as well as metal nanoparticles, therefore their use in the formation of the new C–N bonds is problematic. The gold centre is capable of NH₃632 or NH₂NH₂633 addition if it is coordinated by the CAAC ligand and rendered cationic by Cl⁻ abstraction. The same Ag complex: (CAAC)AgCl 692/KB(C₆F₅)₄ or NH₄B(C₆F₅)₄685 did not cause the activation of NH₃632. The coordination of NH₃632 or NH₂NH₂633 led to a typical Werner complex immediately. The same happened when the alkyne was added to the initial catalyst, η-2 bounded to the gold atom. The reaction occurred according to the insertion mechanism. The addition of NH₃632 to 1,4-diphenyl-buta-1,3-diyne 1a or hexa-1,5-diyne 541, occurred according to the Markovnikow rule, followed by the ring-closing hydroamination to give pyrroles with high yields: 87% for 688 and 96% for 689a. The same reaction with 3,3-dimethyl-1,5-diphenylpenta-1,4-diyne 690 formed two products: Markovnikov six-membered ring and anti-Markovnikov five-membered heterocycles 691 and 691′ in a 2:3 ratio. Similar activity was observed for the reaction with hydrazine 633 where pyrroles or pyrazoles were formed (Scheme 133).^309,310

Amphiphilic gold nanoparticles: Au-HS/SO₃H-PMO(Et) 693, obtained with a narrow particle distribution 1–2 nm (which is important for their high catalytic activity) permitted the reactions to be carried out with organic reagents in an aqueous solution without using any organic solvents. The intramolecular hydroamination of hexa-2,4-diyne 65a in water occurred with high yields of the product 676 (87%). Moreover, the addition of catalytic amount of H₂SO₄ to AuCl(PPh₃) 694 (used as a homogeneous catalyst) was also successful, but the yields were much lower than for the reaction catalysed by nanoparticles 693.³¹¹

Nolan et al. described the application of 5 mol% of [Au(IPr)OH] 695 (IPr = 1,3-bis-(2,6-di-iso-propylphenyl)imidazol-2-ylidene 696) as a precatalyst for the hydroamination and hydration of conjugated 1,3-diynes 1a and 258o to pyrroles 697a–d and furans respectively. The active cationic form of the catalyst is formed in the presence of 7.5 mol% of HNTf₂. Microwave irradiation was used as a heating source, and the reaction was carried out at 120 °C for 90 min (Scheme 134).³¹² The results were similar to those obtained by Skrydstrup (see Scheme 132).³⁰⁸

Ohno et al. developed a method for the formation of various fused indoles and indolines using gold catalysts. Depending on the catalyst and ligands type different products were selectively formed with very good yields.^313–315 The first paper focused on the synthesis of aryl-annulated[α]carbazoles 700via gold-catalysed 5-endo-dig hydroamination of diynes followed by 6-endo-dig hydroarylation. The type of phosphine ligand attached to the gold atom 699a–c has a strong impact on the diyne 698a–s conversion. Particularly, when bulky biarylphosphine ligands were used, the dissociation of the catalyst from a substrate is accelerated, improving the possibility for activation of the appropriate CC bond for hydroamination, even in the case of reagents with electron-rich aryl groups (p-MeC₆H₄, 698ap-MeOC₆H₄698b). The reaction was sluggish when o-CNC₆H₄698e was used probably due to the interaction of CN group with the catalyst. The process was carried out in the presence of R₃PAuCl 699/AgOTf systems yielding aryl-annulated[α]carbazoles 700 with very good yields (Scheme 135).³¹³ Carbazoles 700c and 700n showed good antifungal activity against T. metagrophytes and modest activity against T. rubrum.³¹³

Applying this method it was also possible to synthesize dihydrobenzoindole 702 and 703 and azepino-705a oxepino[3,4-b]indole 705b and cyclohepta[b]indole 705c derivatives with moderate to good yields (Scheme 136). The authors proposed the mechanism of this transformation, which started from the activation of diyne 698t by gold catalysts 699 to 706. Next the 5-endo-dig cyclisation furnishes the indolylgold intermediate 707. After proto-deuaration the cyclised product 708 is formed. It is activated by the gold catalyst, which promotes 6-endo-dig cyclisation at the C-3 position of the indole, followed by the rearomatisation to arylgold species 709. The cycle is finished with the proto-deauration of 709 and production of fused carbazole 700t, with the subsequent regeneration of the initial catalyst 699 (Scheme 137).

The same group developed a method for the synthesis of fused indolines 716 and indoles 712 from anilines functionalised with diyne group 710 catalysed by gold complexes. The formation of both products is controlled by the reagent, ligand, and solvent. When IPr 696 ligands and protic solvents were used the fused indoles 712 were predominantly formed. While Buchwald's type ligands (e.g., JohnPhos 714 and BrettPhos 715) and nonpolar solvents (e.g., toluene) promoted the synthesis of indolines 716 as the main products. The most active catalyst for the preparation of indoles was IPrAuNTf₂ (5 mol%) 711, while for the synthesis of indolines John-PhosAuNTf₂713 was applied (Scheme 138 and Table 18).

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The catalytic systems 711 and 713 were tolerant to many electron-donating and electron-withdrawing functional groups in the diyne structures. For 710g (with highly electron-withdrawing CN group R¹) the indole 712g was formed with very low yield (17%), while propellane type indoline 716g was not formed regardless of the method A or B. Moreover the influence of the position of substituents in phenyl ring of aniline influences the products yields. When the ring was substituted in the ortho position to alkyne, 710h propellane type indoline 716h was formed in moderate yield (44%), while oxocine-fused indole was not detected at all. The steric repulsion between o-Br and phenyl groups interferes with the formation of indole 712h. When the length of the chain between both CC bonds was shorter as in 710u, the propellane type indoline 716u is not obtained due to the higher ring strain, whlie for longer chains as in 710v no nine-membered ring fused indole 712v was produced, while 716v was obtained in 28% (Table 18).

The mechanism of this transformation, in which hydroamination is a crucial step was proposed according to the experiments. Activation of alkyne with gold 717 is responsible for 5-endo-dig cyclisation followed by the protodeauratiom towards the indole. Next, the activation of the second alkyne group promotes the 8-endo-dig hydroarylation of 719 to intermediate 720. The subsequent protodeuaration of 720 furnishes oxocine fused indole 712. The intermediate 720 can be easily opened to cationic intermediate 721. Elimination of the gold from 721 leads to allene 722, which is essential for obtaining propellane-type indoline 716 (Scheme 139).³¹⁵ Protic solvents accelerate the protodeauration of vinyl-gold intermediate 720 yielding oxocine-fused indoles 712. The same influence is observed for electron-donating IPr ligand 696. When allene 722 is formed the mechanism is favoured to obtained propellane-type indolines 716. The DFT calculations for this transformation was also used to help underpin the reaction mechanism.³¹⁶ A detailed discussion on the influence of substituents attached to the nitrogen atom in aniline, in the diyne, and the aryl ring on process selectivity and product yields and mechanism of the process were comprehensively discussed by the authors in several papers.^313–315

Wiest, Helquist et al. applied a hydroamination reaction for the desymmetrisation of diynes 727a–c, 730a–c, 732 in the presence of Ag(phen)OTf 728 yielding to 1-pyrrolines 729a–c, 731a–c, or 733 with two entirely different, orthogonal functional groups, which are capable of further functionalisation. The reaction occurred under mild reaction conditions (25–50 °C), with the low catalyst 728 loadings (0.5–2.0 mol%) (Scheme 140).⁶⁴ Additionally, this method was applied to the synthesis of natural indolizidine alkaloid (±)-monomorine 743, which started from the hydroamination/cyclisation of diyne 734 synthesised from 4-bromo-1-butyne followed by the several steps illustrated in Scheme 141.⁶⁴

In the hydroaminative cyclisation of diynes was used also AgSbF₆746 as a catalyst. The process was developed for the synthesis of naphthol-indole derivatives 750a–l from 1,3-diynes 745a–l and sulfoxonium ylides 744 in a one-pot cascade reaction (i) intramolecular hydroamination/cyclisation of diyne-substituted anilines 745a–l to 750a–l, and (ii) [RhCp*Cl₂]₂748 catalysed arene ortho-C–H bond activation. Indoles functionalised in the C2 position 750 were obtained with good yields with high functional groups tolerance (Scheme 142).³¹⁷

The system generated in situ from TiCl₄448 by the addition of t-BuNH₂751 in toluene is an active catalyst for the hydroamination of alkynes and 1,3-diynes with hydrazine 633 leading to indole or pyrrole derivatives respectively. Using 20 mol% of TiCl₄448, at 105 °C for 18 h, the reagents (anilines 679a–b and dodeca-5,7-diyne 13a) are quantitatively converted to the products mixture of mono- and bishydroamination of diyne 13a. The pyrroles 752 were obtained as the main products in 30% yield (Scheme 143).³¹⁸

CpCo(C₂H₄)₂755 was applied in the hydroaminative coupling of α,ω-diynes 79b, 160, 753a–j with various amides 754a–f, which resulted in the formation of dienamides 757–758 with high regio- and stereoselectivity (Scheme 144 and Table 19). Such compounds can be used as reagents in Diels–Alder reaction, in the synthesis of polycyclic compounds as well as natural product derivatives. They can be also synthesised from alkynes by a co-oligomerisation reaction with N-vinyl amides or Ti-catalysed coupling with ynamides.^319,320 The mechanism of Co-mediated reaction started from the oxidative addition of diyne 79b, 160, or 753a–j to the metal centre of 755 with the formation of cobalt–cyclopentadiene 760, followed by the formation of 18 electron N-coordinated complex 761. Proton transfer from nitrogen to carbon then takes place to generate intermediate 762, which subsequently rearranges to N-coordinated cobaltcyclopentene 763, that tautomerises to product 764. The regioselectivity is controlled by the proton transfer step to the least hindered carbon atom in cobaltcyclopentadiene (Scheme 145). The reaction occurring according to this mechanism permitted several amidated 1,2-dimethylenecycloalkanes to be obtained in moderate to good yields (24–81%) (Scheme 144 and Table 19).³²¹

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Shimada and Yamamoto have developed a different approach applying hydroamination reaction in the C–C bond cleavage in diynes 1a, 13a, 617, and 765a–f with o-aminophenols 766a–h. The transformation leads to 2-substituted benzoxazoles 769 and 771 and ketones 770 and 772. The reaction occurred according to two possible pathways in the presence of Ru₃(CO)₁₂489 with NH₄PF₆767 by the CC (path A) or C–C single (path B) bond cleavage. The formation of more sterically hindered benzoxazoles 769 is favourable. Additionally, the bulky groups in the diyne (e.g., t-butyl 765c) led to the almost exclusive formation of product (769, 769:771 = 30:1) with 81% yield. For substituted o-amimophenols 766d–f in positions 4 and 5 with strong electron-donating or electron-withdrawing groups, the reactions were sluggish. Ru-catalyst 489 was found to be more effective in the reaction with terminal diynes 765a–f, while internal diynes 1a and 13a proceeded better with Pd(NO₃)₂768 (Scheme 146). The key step in the bond cleavage is the hydroamination of one of the CC bonds of 765a–f with 766, followed by the tautomerisation leading to α,β-unsaturated imines 774. The addition of the second molecule of o-aminophenols 766 to 765 yielded β-aminoimines 775 and their tautomers 777, which further undergoes intramolecular cyclisation to ketals 776 and 778. The final step leading to benzoxazoles 769 and 771 occurred by the C–C bond cleavage through a retro-Mannich-type reaction (Scheme 147).³²²

The copper-catalysed synthesis of pyrroles via hydroamination of diynes was first published in 1965.³²³ 0.1 mol% of CuCl 55 was used for the hydroamination/cyclisation of 1,3-diynes 1a, 1d, 65a, 208b, 258n, 781a–c with aromatic and aliphatic primary amines 679a, 679c, 679g, 782a–i and ammonia 632. The reaction was carried out in MeOH, EtOH, 1,4-dioxane, or DMF for 1 h, at 150–180 °C, furnishing pyrroles 783a–w in moderate yields. Increasing the catalyst 55 concentration to 10 mol%, under solvent-free conditions, and with 10 equiv. of amine 679 and 782 it was possible to obtain almost quantitative yields of pyrroles 783 in 24 h (Scheme 148).^324,325 The same catalytic system was applied in the synthesis of 2,2′-bipyrolle derivatives possessing four aryl groups in 1, 1′, 5, 5′ positions. The reaction was carried out with CuCl 55 as a catalyst, in DMF at 90–150 °C.^326,327

Modified Ullmann conditions (CuI 519/L/Cs₂CO₃, where L = 1,10-phenatrholine, L-proline, (E)-4-hydroxy-L-proline) were used for the synthesis of N-alkenynes in hydroamination/amidation reaction of 1,4-diaryl-1,3-diynes 1a, 1c–d, 27b with heterocyclic indoles 784a–d, azoles 784e–h, pyrazole 784i and cyclic or acyclic amides 754b, 785a–c. The reaction yielded a mixture of (Z)- and (E)-N-alkenynes 786–796 with an excess of the (Z)-isomer in the range of 60–95%, and 75–95% yields for cyclic reagents 784a–i, 754b, and exclusive formation of (E)-isomer for acyclic amides 785a–c. In the latter case, the yield was reduced to low to moderate values 10–41% (Scheme 149). The authors assumed that the hydroamination reaction occurred via an oxidative addition/reductive elimination mechanism with the addition of the N–H bond to Cu^I519 as an initial step of the mechanism. The insertion of diyne 1a, 1c–d, 27b to the Cu–N bond, followed by the reductive elimination of N-alkenyne 786–796 is postulated as the next stage of the mechanism. The system was not efficient for alkyl-substituted 1,3-diynes and unsymmetrical reagents.³²⁸

CuCl 55 was used also for the hydroamination of meso,meso’-1,3-butadiyne-bridged Zn(II) diporphyrin 797 with various amines 635, 679a, 782b, 798a–c to meso,meso′-pyrrole-bridged Zn(II) diporphyrins 799a–f. The structure of diporphyrin 799 was confirmed by the single-crystal X-ray diffraction method. The bulky mesitylamine 798b and octylamine 798c were less active in the hydroamination reaction (Scheme 150). Moreover, it was possible to modify in Suzuki–Miyaura coupling reaction of diporphyrin with 4-bromophenyl substituted pyrrole 799c with porphyrin possessing Bpin 800 groups to 801 with 15% yield. As a catalyst PdCl₂64/dppf 39b was used. (Scheme 151).³²⁹

3- or 4-Aminomethylpyrroles 806a–k and aminomethylfurans 807a–i bearing a sulfur group were obtained by the hydoamination/cyclisation reaction of N- or O-tethered 1,6-diynes 802 and 803a–d with a sulfur substituent attached to one of the alkynyl group using two catalytic systems Ni(hfa)₂ hydrate 805 (10 mol%)/DBU (Method A) or Ni(hfa)₂ hydrate 805/PdCl₂(PPh₃)₂94/DBU (Method B) in DMSO at room temperature (Scheme 152). The products were obtained with good yields (50–92%) using cyclic and acyclic amines 804a–n between 2–72 h (Table 20). The possible mechanism of this transformation started from the isomerisation of diyne (802, 803) to alkyne-allene 809 or allene-allene 810 intermediates via a carbanion 808, followed by its coordination to the Ni atom 805 with a sulfur ligand. This activates the alkyne moiety 812 towards intermolecular attack by the amine 804. This leads to the diamino metal intermediate 815 through intermediates 813 and 814. Next, the second intramolecular cyclisation towards 816 occurred, followed by the formation of 817. Its protonolysis and isomerisation yields the product 806 or 807 and regenerates the catalyst. Less nucleophilic amines might react with water according to path II with the formation of side product 811 (Scheme 153). The presence of the sulfur group in the products 806 permitted their further functionalisation such as the introduction of formyl or acetyl groups (819a–c, 820) which are then susceptible to subsequent modification in other chemical transformations, or the reaction with the strong base leading to 1H-pyrrole 821 (Scheme 154).³³⁰

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Another approach to N-heterocyclic 1,2,5-trisubstituted pyrroles 826a–e and 829a–c was based on the hydroamination reaction of separated 1,4- or 1,5-diynes 822a–b, 541, or 827 with primary amines (aniline 679a, benzylamine 635, or 4-methoxybenzylaniline 782b) with the subsequent 5-endo dig or 5-exo dig cyclisation in the presence of Ti(NMe₂)₂(dpma) 823 or Ti(NMe₂)₂(dmpm) 827 as a catalyst. The addition of amine 679a or 635 occurred according to the Markovnikov rule. The hydroamination of unsymmetrical 1,4-diynes with aryl and alkyl substituents led exclusively to the product with amine attached to the β-carbon to aryl substituent 826a–e, while nonsubstituted 1,4-diynes led to the dihydroamination product, since the second hydroamination of the terminal alkyne is faster than the intermolecular cyclisation reaction. In the case of internal or terminal 1,5-diynes 541, 827, the cyclisation was faster than the hydroamination of the second CC bond, and the cyclic pyrroles 829a–c were formed exclusively (Scheme 155). This method is an alternative of Paal–Knorr synthesis to pyrroles, especially when unsymmetrical 1,4-diketones are used as reagents.³³¹

Wasterhausen et al. published several papers on the hydroamination reactions of alkynes and conjugated 1,4-diphenylbuta-1,3-diyne 1a in the presence of heterobimetallic complexes constructed from Ca- and K-complexes 830 and 839. The homometallic Ca- or K-catalysts were inactive in the hydroamination reactions. Depending on the structure of the complex and amine, various products were formed such as cyclic cyclohepta-1,2,4,6-tetraenes 832a–b and 833, pyrroles 834a–d, aminated enynes 836a–c, 840–841, or bisaminated dienes 837a–b. The reactions under room temperature lead to thermodynamic products, e.g., cyclohepta-1,2,4,6-tetraenes 832a–b or 833, while at higher temperatures, kinetic pyrrole products 834a–d were formed. The structure of amines has a significant influence on the product type (Scheme 156.). The authors discussed in detail the mechanisms of these transformations, which differ according to the hydroamination reagent.²⁹⁴

9. Hydrophosphination

Unsaturated organophosphorus compounds have found several applications as building blocks in organic synthesis, (chiral) ligands for catalyst formation, biologically active compounds, or in the preparation of flame retardant materials.^332–337 They are also used in medicinal- or agrochemistry, as components of drugs, which are used in e.g., bone, calcium-metabolism or neurological diseases, antiviral and antibacterial systems, enzymes inhibitors.^337–345 They are commonly applied as monodentante as well as chelating ligands in various chemical transformations.^{332–334,339} The synthesis of vinylicphoshpines can be carried out using the hydrophosphination reaction.^30,59,346 This type of addition reaction was also used in the reaction with conjugated and separated diynes, but, unlike hydroamination, the examples are limited only to a few papers.

9.1. Hydrophosphination of conjugated and non-conjugated diynes

Hydrophosphination of conjugated 1,3-diynes 1a, 13a, 13c, 60a, 208b, 617, 655c, and 842a–c with Ph₂PH 843 was carried out in the presence of ytterbium complexes [Yb(η²-Ph₂CNPh)(hmpa)₃] 844 or [Yb[N(SiMe₃)₂]₃(hmpa)₂] 845. The reaction occurred according to the double addition of two diphenylphosphine 843 molecules to the CC bonds of diyne, even at low temperatures −35 to (−78) °C, and the formation of bis(diphenylphosphinyl)-dienes 846–849 with high yields but relatively low selectivities (Scheme 157 and Table 21). The stereochemistry of the process was kinetically and thermodynamically controlled and the formation of the specific isomer depended on the structure of diyne. Hydrophosphination of disubstituted diynes predominantly formed (Z,Z)-846 isomers with a minor amount of (Z,E)-847 butadiene. Terminal diynes yielded (E,Z)-butadiene 848 as the main product, while the sterically hindered 1,4-ditertbutyl-buta-1,3-diyene 13c was quantitatively converted to allenic product 850. The reaction started from the formation of the [Yb]–PPh₂ complex 851, which underwent anti-addition to diyne to form enynylyterrbium complex 852. Protonation of 852 with Ph₂PH 843 yields diphenylphosphine-substituted enyne 853 and regenerates ytterbium–phosphide active complex 851. Repetition of this process provided the bishydrophosphination product 846. The products were easily oxidised with H₂O₂ to phosphine oxides, which were easier to isolate (Scheme 158).^347,348 The formation of diphenylphosphine-substituted enyne 853 in the reaction was also possible using an equimolar ratio of reagents and a shorter the reaction time of up to 30 minutes. After oxidation with H₂O₂, the (Z)-products were predominantly formed.

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Tanaka et al. developed the hydrophospinylation reaction of alkynes in the presence of Pd(PPh₃)₄35 or PdMe₂(PR₃)₂855 (PR₃PPh₃ or PPhMe₂, PPh₂Me, PEt₃, PMe₃) catalysts. All these complexes catalysed the synthesis of alkenyldiphenylphosphine oxides from alkynes and Ph₂P(O)H 856. Less basic phosphines (e.g., PPh₃, PPh₂Me) accelerate the formation of anti-Markovnikov products, whereas application of more basic phosphines e.g., PMe₃ or PEt₃ increases the amount of the geminal regioisomer. The best results were obtained when 5 mol% of Pd(PPh₃)₄35 was used at 35 °C. This method was used also for hydrophosphinylation of nona-1,8-diyne 164a derivatives towards 1,9-diphosphinyl-nona-1,8-diene 857 when 2.2 equiv. of Ph₂P(O)H 856 and Pd(PPh₃)₄35 was used (Scheme 159).³⁴⁹

The same authors reported Pd-catalysed hydrophosphinylative carbocyclisation of α,ω-diynes. The reaction occurred in the presence of 5 mol% Pd(OAc)₂861 and chelating phosphine ligands such as ethylenebis(diphenylphosphine) (dppe) or 1,2-bis(diphenylphosphino)benzene (dppben) at 70 °C in chlorobenzene, toluene, ethylbenzene, or dioxane. The carbocyclisation was the most effective for 1,7-heptadiyne derivatives, while longer or shorter α,ω-diynes were less susceptible to cyclisation, and linear hydrophophinylatve products were mainly obtained. Scheme 160 presents the formation of various products, which depends on the reagents used (diynes 116a or 127o, and phosphorus compounds 856, or 858–860).³⁵⁰ For diyne 116a product 862 is mainly formed, while for 127o product 864 is predominantly obtained (Scheme 160).

Hydrophosphinylative carbocyclisation was also reported by Yamamoto et al. but, instead of Pd-complexes, ruthenium catalysts 281a–c with cyclopentadiene ligands (responsible for the formation of the active ruthenacyclopentatriene intermediate) were used. The best results were obtained for [Cp*Ru(MeCN)₃]PF₆281a for which exocyclic 1,3-dienylphosphine oxides 866a–j were exclusively formed, under the optimised reaction conditions using HP(O)Ph₂856. In the case of complexes 281b–c, the hydrative cyclisation product 867 was formed as well. To suppress the formation of these by-products molecular sieves 4 Å were added to the reaction mixture. The [Cp*Ru(MeCN)₃]PF₆281a was used in 5–10 mol%, depending on the reactivity of diynes 865a–j. The substituents in the tether position have a significant influence on the product yields. The more hindered these groups, the lower the yields of the desired products 866a–j observed. To obtain the desired cyclic product, 1,6-heptadiyne derivatives need to be used with the aryl substituents in the terminal position (Scheme 161). When shorter chain diynes such as 1,5-hexadiyne, or reagents with alkyl substituents in terminal positions were used, the reaction did not occur or the products were formed in low yield. The aryl terminal groups accelerate the formation of active ruthenacyclopentatriene intermediate, which is essential for this transformation.

The mechanism of this transformation was proposed based on DFT calculations. The reaction started with the oxidative cyclisation of bis(alkyne) complex 868. The rate-determining step was found to be H-atom transfer, which leads to the monocarbenoid with a phosphinate ligand 869. The intramolecular attack of phosphorus on the remaining carbenoid carbon generates oxaphospharuthenatricycle 870 followed by the formation of (Z)-4-diene complex 871. The addition of diyne 865 and HP(O)Ph₂856 generates product 866 and regenerates the active catalytic intermediate 868 (Scheme 162).³⁵¹

Hydrophosphinylation of symmetrical 1,4-diphenyl or 1,4-tert-butyl-buta-1,3-diynes (1a or 13c) was also carried out in the presence of a main group element catalyst [(thf)₄Ca(PPh₂)₂] 872. The composition of the postreaction-mixture depended on the phosphorus reagent. When diphenylphosphane oxide HP(O)Ph₂856 was used as reagent, 1,4-diphenyl-2,3-bis(diphenylphosphoryl)-buta-1,3-diene 873 or 2,2,7,7-tetramethyl-3,6-bis(diphenyl-phosphoryl)-4-octyne 874 were selectively formed in the reaction with 1,4-diphenylbuta-1,3-diyne 1a or 1,4-di-tert-butylbuta-1,3-diyne 13c respectively in very good yields (80–82%). The reaction with Ph₂PH 843 yielded different products in 1,4- or 1,3 phoshponylation (875–876). These differences in process selectivity are due to the different base-acid interactions between calcium catalyst 872 and Ph₂PH 843 or HP(O)Ph₂856. Rather, strong Ca–O interactions are responsible for the closeness of the alkali metal to reactive multiple C–C bonds (Scheme 163).³⁵²

10. Hydration of conjugated 1,3- and separated 1,n-diynes

Hydration of 1,3- and 1,n-diynes is limited to several examples, which are focused on the catalytic activation of the water molecule and diyne with various catalysts mostly based on transition metals. This transformation leads to many important building blocks, especially in cyclisation reactions to furanes, 3-(2H)-furanones, or γ-pyrones. The obtained products are used in the synthesis of antitumor agents, antibiotics, natural and bioactive compounds.^353–355 The addition of water to the CC triple bond may also yield carbonyl compounds via tautomerisation of the hydroxylated enyne.

Hydration or hydration/cyclisation reactions are simple and 100% atom economic transformations, which can provide the desired products in a straightforward procedure, without (or with a small number of) side-products. Therefore, they are a useful alternative to common methods, that require the application of complex reagents and multi-step procedures. Most of the catalytic systems for selective hydration reactions of diynes, which are based on predominately gold, ruthenium and palladium complexes, were developed within the last two decades.

The first papers on the hydration of diynes were published in the 1960s and apply mercuric salts. The addition of water to undeca-1,7-diyne 877 provided a mixture of two diketones, undecane-2,7-dione 878 and undecane-2,8-dione 879 with moderate yields and low selectivity.^356,357 A modified procedure was used by Constantino et al. in the preparation of natural marine compound 1-(2,6,6-trimethyl-4-hydroxy-cyclohexenyl)-1,3-butanedione 880, which possess antibiotic activity. They used HgSO₄ and formic acid (85%) in the hydration step. The compound was formed in 50% crude yield. The same system was applied for hydration of other cyclohexyl-substituted diynes. The terminal CC group was hydrated at first, followed by the reduction of the second alkynyl group.³⁵⁸

Ruthenium catalysed hydrative cyclisation of various diynes was studied in detail by Trost et al.^359–364 They have found that simple cationic [Cp*Ru(CH₃CN)₃]PF₆281a complex, which catalyses many different transformations such as alkyne–alkyne coupling reactions (e.g., dimerisation, trimerisation)^365–368 or cycloaddition reactions with dienes,³⁶⁹ isocyanates,³⁷⁰ nitriles,³⁷¹ can be effectively used in diyne hydrative cyclisation or cycloisomerisation reactions (Scheme 164).^359–364 Depending on the structure of the diyne, different mechanisms for the reaction occur. Internal diynes can directly react with water in the presence of catalytic amounts of the Ru complex 281a (3–10 mol%) producing five- or six-membered enones with moderate or excellent yields. The same catalyst was used for the dimerisation of propargylic alcohol and a further intramolecular cycloisomerisation reaction (Scheme 164). Tertiary or secondary propargylic alcohols cycloisomerise to α,β,γ,δ-unsaturated aldehydes and ketones, while primary propargylic alcohols also gave the hydrated cyclised product. The key step in both paths of mechanism (Cycle A and Cycle B) starts from the reseonance invocation to ruthenacyclopentatriene 904. For primary propargylic alcohol diynes, the addition of water might occur to two carbene carbons yielding intermediates 905 and 910. The hydrative cyclisation process leads to the rearrangement of 905 to 907, followed by a hydride shift and protontion to the product 909. In the case of cyclodimerisation, compound 910 is rearranged to 911 which, after hydride shift and β-hydroxide elimination or protonation and water elimination, leads to product 913 (Scheme 165). The mechanism common for secondary and tertiary propargylic alcohols (which possess better-leaving groups) occurs mainly via Cycle B. The detailed mechanistic studies on the activation of a water molecule by ruthenacyclopentatriene 904 were studied by Yamamoto et al. Using DFT calculation, they postulated the formation of half-open oxaruthenocene as an initial step of the mechanism.³⁷² The methodology was used in the cyclisation of various diynes (Scheme 164). Moreover, the directing effect of carbonyl group attached to the CC in the δ- or ε-position was observed, by the coordination of CO to ruthenacyclopentadiene complex 914.³⁶⁰ Hydrative cyclisation was a step in the formation of natural compounds: tricyclic alkaloids Cylindricine C920 (Scheme 166), while cycloisomerisation was used in the synthesis of (+)-α-kainic acid 933 (Scheme 167).^362,364 Moreover, the cyclised products were applied in both intra- and intermolecular Diels–Alder reactions.^360,361

Another example of the application of hydration process is the formation of functionalised benzene derivatives 936a–e in the aromatisation of enediynes 934a–e catalysed by 10 mol% [TpRu(PPh₃)(CH₃CN)₂][PF₆] 935 (Tp = tris(1-pyrazolyl)borate) (Scheme 168).³⁷³ The process is also possible for the addition of other nucleophiles than H₂O (e.g., aniline, acetylacetone, pyrroles, and dimethyl malonate) to non-functionalised enediynes 934. The addition is highly selective and the attack occurs at the more electron-rich alkyne carbon yielding various functionalised aromatic compounds 936a–e depending on the nucleophile. The mechanism was proposed according to the reactions with D₂O. These experiments proved that the catalytically active species is a ruthenium-π-alkyne complex instead of the ruthenium-vinylidene intermediate, which is a characteristic step in Saito-Myers cyclisation (Scheme 169).³⁷³

Gold complexes are another big class of catalysts, which have been used in the hydration of conjugated and separated diynes. The presence of water was essential for the hydrative cyclisation. In 2010, Skrydstrup et al. published that 1,3-diynes can be converted in a hydration reaction towards 2,5-disubstituted furans 946 (Scheme 170) or in a hydroamination process to 1,2,5-trisubstituted pyrroles 681 (Scheme 132). Au(I) complexes such as (Ph₃P)AuNTf₂680 and SPhosAuNTf₂945 were able to catalyse these two reactions under mild conditions. Complex 945 was more active in hydration reaction since H₂O is a better nucleophile when 1,4-diaryl or dialkylbuta-1,3-diynes were used. Within this methodology, it was possible to furnish a selection of 2,5-diamidofurans 946k–m in 45 minutes with good to moderate yields (Table 22, entries 14–17). To obtain high yields in the case of the hydration of symmetrical 1a, 1c–d, and non-symmetrical diraryl-944b or dialkyl-substitued 208b diynes using complex 945, 24 hour reaction times were necessary. Moreover, increasing the polarisation of the diyne by the introduction of electron-donating OMe groups led to a small amount of side products (Scheme 170). When D₂O was used instead of H₂O, furans 946e with deuterium atom at 3,4-position were synthesied.³⁰⁸

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The same products were formed, when [Au(IPr)OH] 695 was used as a precursor. The reaction proceeded only in the presence of Brønsted acid HX, which generated the active complex [Au(IPr)]X 947in situ. HNTf₂ was the most effective in the model reaction with 1,4-diphenybuta-1,3-diyne 1a. Poorer results were observed when HBF₄948 and HPF₆949 were used (77% vs. 37–39%). No catalyst activity was noticed for the complex with SbF₆⁻ or OTf⁻ groups. Elevated temperatures are needed to perform the reaction with the [Au(IPr)OH] 695/HX system. Additionally, the type of substituents attached to buta-1,3-diyne skeleton is important for the reaction. Diynes with aryl groups in the terminal positions were the most active in the formation of 2,5-disubstituted furanes 946. For the diyne with cyclohexene groups (258o) the reaction was less effective, while reagents with dialkyl sidechains in positions 1,4 did not lead to the desired products. When one of these group was substituted with an aryl ring, the reaction occurred with good yield (Scheme 170 and Table 22). According to stoichiometric experiments and DFT calculations, it was proved that the reaction proceeded via hydration of the one CC bond. Two pathways are possible through the keto or enolate form. It was determined that the keto-pathway is favoured by 9.6 kcal mol⁻¹ (Scheme 171).³¹²

Hydration of conjugated diynes was used for the synthesis of 6,5,6-trioxabispiroacetal moieties, the spacers between the steroid cores. Steroid diynediols were used as reagents, while JohnPhos-Au(MeCN)SbF₆682 was applied as a catalyst.^374,375

Sanz et al. reported the Au-catalysed hydration-oxacyclisation reactions of 1,4-diyn-3-ones 959a–s, which were obtained from ethyl lactate as carbonyl source, a feedstock derived from biomass. Depending on the catalytic system composition it was possible to carry out the selective synthesis of 4-pyrones 960a–s or 3(2H)-furanones 961–962. Such compounds possess many biological activities, e.g., phenoxans, funicones and rapicones indicate anti-HIV activity.^376,377 The reaction can be tuned by the ligand attached to the gold complex, the presence or absence of silver salts, and the counteranion. When 5 mol% of IPrAuNTf₂711 was used 4-pyrones 960a–s were predominantly formed (5:1–20:1), while applying 5 mol% of AuCl(PPh₃) 694/AgSbF₆746 3(2H)-furanones 961–962 were obtained (1:11–1:20). Both products were formed in moderate yields of 65–86%. The lowest yield of 3(2H)-furanones 961 was obtained for alkyl-substituted diynones. This pathway was much more effective for aryl- or heteroaryl-functionalised diynones, while 4-pyrones 960a–s were furnished with similar yields regardless of the type of substituents (Scheme 172 and Table 23). This is an alternative method towards 4-pyrones and furanones, which are typically made by multistep condensation cyclisation reactions of carbonyl compounds.³⁷⁸ The mechanism of this transformation was demonstrated from the reaction with D₂O. The key step in the formation of 4-pyrones 960 or 3(2H)-furanones 961 is the hydration of diynone 959, which might proceed according to Michael or anti-Michael addition. Both pathways are possible and depending on the catalytic system. Next, the intramolecular oxacyclisation occurred leading to Au-intermediates 965 or 966. Finally, protodeauration affords the final products with the elimination of the catalytic species. The regioselectivity is controlled by hydration step, not by a 6-endo vs. 5-exo oxacyclisation reaction (Scheme 173). The Michael or anti-Michael addition of water also had an influence on the synthesis of furanones and pyrones when unsymmetrically substituted diynones 959o–s were used. Anti-Michael addition was favoured with the more electron-poor alkyne group causing the synthesis of furanones in a higher amount.³⁷⁸ This methodology was used in the preparation of Polyporapyranone B969, which is naturally occurring γ-pyrone in sea-grass derived fungi Polyporales (Scheme 174). The hydration–oxacyclisation reaction is the final step in the synthesis of this bioactive compound, proceeded by Sonogashira coupling of 2,4-dimethoxyiodobenzene 967 with propargylic alcohol, oxidation, the addition of ethynylmagnesium bromide, and the next oxidation step. Finally, both products, which could be prepared on a gram scale, were utilised in further transformations leading to pyrylium salts, that can be used as photoredox catalysts or in the reaction with N-nucleophilic reagents to functionalised N-heterocycles (Scheme 175).³⁷⁸

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Diynones 959 were also converted to 4-pyranones 960 in the presence of TfOH 974, which promotes the hydration reaction, followed by cyclisation. The reaction occurred under metal-free conditions, making the process more legitimate in the case of the process economy (no expensive gold catalysts) and sustainability. Under the optimised conditions (1 equiv. TfOH 974, 100 °C, 36 h) various symmetrically and non-symmetrically 2,6-substituted 4-pyranonens 960 were obtained with good yields (57–82%). Other acids as e.g., p-TSA or PhCOOH were much less active than TfOH 974. Diynones 959 substituted in the terminal position with aryl groups bearing electron-donating groups (e.g., Me, t-Bu, OMe) gave products with slightly better yields, than those with electron-withdrawing groups (e.g., F, Cl). The mechanism of the reaction starts from the activation of carbonyl group in diynone 959 by TfOH 974 and nucleophilic addition of water to the CC triple bond followed by a keto–enol tautomerisation towards intermediate 963. Subsequently the protonation and C–C bond rotation, which occurred under elevated temperature leads to species 975. The cycle is subsequently closed by the intramolecular nucleophilic attack of the oxohydryl group to the second CC to give cyclic intermediate 976, which furnished 4-pyrorone 960 after deprotonation (Scheme 176).³⁷⁹

MeAuPPh₃978 or (PPh₃)AuNO₃979 with trifluoromethanesulfonic acid (TfOH) 974 are active catalysts in the hydrative cyclisation of 1,6-heptadiynes 116a, 127a, 127e, 127j, 394b, 394g, 450a, 450c, and 977a–o functionalised with various different groups, e.g., alkoxy, esters, carboxyl, carbonyl, phenyl, or nitrile (Scheme 177).^380,381 Other acids as mineral H₂SO₄ or heteropolyacids H₃PW₁₂O₄₀982a, H₃PMo₁₂O₄₀982b, and H₄SiW₁₂O₄₀982c were also used as co-catalysts and permitted the isolation of the corresponding 3-methyl-hex-2-enone but with lower yields. The proposed mechanism of this transformation assumes the formation of an active Au⁺ species in the first step. The coordination of diyne, followed by the H₂O attack then leads to intermediate 983, which further isomerises to gold cyclohexanone complex 984 by the intramolecular attack of enolic ion to the gold cation binding through the CC bond. Product 981 is then released through a tandem double bond isomerisation process and gold catalyst elimination (Scheme 178).³⁸⁰

Moreover, ILs were used as solvents and immobilisation media for (PPh₃)AuNO₃979. The best results were obtained for [BMIM][BF₄], allowing to obtain stable products with yields of 72–78% in six cycles of hydrative cyclisation of 127a. Such strategy permitted the recycling of the expensive gold catalyst. No information about catalyst leaching was presented.³⁸¹

The same authors discovered that the Pt(cod)Cl₂980 catalyst with TfOH 974 as a co-catalyst is active in the hydrative cyclisation of the same reagents (1,6-heptadiynes): 116a, 127a, 127e, 127j, 394b, 394g, 450a, 450c, 977a–o functionalised in position 4. This catalytic system furnishes 3-methyl-hex-2-enone 981 with good yields (Scheme 177). The mechanism of the reaction was similar to that presented for Au-catalyst in Scheme 178. Interestingly Ru- or Pd-complexes were not active in this transformation.³⁸²

Liu et al. reported platinum and gold-catalysed hydrative cyclisation or carbocyclisation of oxo diynes or triynes, which led to benzopyrones and bicyclic spiro ketones.^383–385 As a model reagent, diynone 986a was used which gave products 987a–991a depending on the catalyst used (Scheme 179 and Table 24). Simple AuCl 992 led to diketone product 987a, when the hydration step was carried out at room temperature. Increasing the temperature to 100 °C provided 1-H-inden-1-one 988a as the main product, while spiroketone 989a was obtained using PPh₃AuCl 694/AgOTf as a catalyst. Product 987a is an intermediate in the synthesis of spiro ketone 989a. Switching from gold to a platinum catalyst, by application of PtCl₂/CO 994, the chemoselectivity was directed to benzoisochromene 990a. Triketone 991a was formed when 10 mol% of lutidine was added to the catalytic system. The yield towards 990a was improved by the application of 1 atm of CO, which role is to increase the nucleophilcity of Pt(II) by the formation of PtCl₂(CO)_n. Moreover, CO was essential to increase the process selectivity to 990a. PPh₃AuCl 694/AgOTf produced spiro ketones 989b–j with a very good yields (63–88%), depending on the substrate 986b–j (Scheme 180). The authors postulated that the ketone group accelerates the hydration of proximate C(1)–carbon of the neighboring alkyne (according to intermediate 996). The obtained diketone 987a undergoes a Conia–ene transformation³⁸⁶ based on the attack of its enol form 997 at the π-alkyne group to form indenyl ketone 998. A subsequent gold-or proto-catalysed aldol reaction formed spiro ketone 989a (Scheme 181). Whereas PtCl₂/CO 994 catalyses the transformation of various diynones 986 to isochromenes 990, hydrative cyclisation of diynones 986 and 999 or diynals 1002 catalysed by PtCl₂1000 furnishes benzoisochromenes 1001 or primary lactol derivatives 1003 (Schemes 182 and 183). The mechanism of this transformation was proposed on the basis of the reaction with D₂O. The diynone 986a leads to the formation of benzopyriliums 1004, which is transformed to triketone 1005. Next, the aldol condensation of 1005 catalysed by a Brønsted acid or PtCl₂1000 produces 1-naphtol 1007via enol intermediate 1006. Finally, the tetracyclic ketal 990′ is formed, which is reduced by D₂O. Oxonium intermediate 1010 then undergoes hydride addition by DPtCl₂. Its formation from CO and HOPtCl₂⁻ was reported in the literature (Scheme 184).³⁸⁷

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In the case of triynes 1012a–g and 1015a–m, applying PtCl₂/CO 994 (or more active PtI₂/CO 1016) as a catalyst led to nucleophilic hydration of the alkyne moiety, followed by the cyclisation led to tetracyclic 1013a–g and 1014a–g, or bicyclic spiro ketones 1017a–m with excellent selectivity (Scheme 185). The authors postulated that the formation of products occurred according to two hydration processes, further alkyne insertion, and aldol condensation. The type of product which is formed depends on the order of the hydration process. Spiro ketones 1017a–m are synthesised when the initial hydration occurred at the central diphenyl alkynes. When the outer alkyne is hydrated at first, tetracyclic ketones 1013a–g and 1014a–g were effectively synthesised. Both types of products were formed in good yields (Scheme 185).^383,384

Conjugated 1,3-diynes can be converted to 2,5-disubstituted furans using a simple and cheap copper(I) catalyst 519, which constitutes an alternative to reactions catalysed by much more expensive Au(I) complexes.^308,312,388 The formation of furans can occur directly from haloalkynes 1018a–n and 1019a–nvia preliminary Glaser coupling to 1,3-diynes 1a–d, 27c, 37t, 230d, 258a, 258g–h, 265a, 500b, 1020a–b or direct hydration of diynes 60e, 655a, 1020c–e, followed by cyclisation. As a base, KOH was used, and CuI 519/1,10-phen was much more active than CuCl 55 or CuBr 1022 (Scheme 186). The mechanism of both subsequent processes: Glasser coupling and hydration is presented in Scheme 187.

The hydration of 1,3-diyne alcohols was also catalysed by non-metal systems, based on base-functionalised ionic liquids under an atmosphere of CO₂. The best results were obtained using [HDBU][BenIm] which possess moderate basicity. DFT calculations proved that the process started from the reaction of 2-methyl-6-phenylhexa-3,5-diyn-2-ol 1028 with CO₂, followed by intramolecular cyclisation, which was estimated to be the rate-determining step in this reaction. Then the cyclic carbonate is hydrolysed and CO₂ is released by the base [BenIm]. Finally 3(2H)-furanone is formed through isomerisation with the base catalyst and the intramolecular cyclisation. Much better results were obtained when protic ILs were used.³⁸⁹

Performing the hydration/cyclisation process with InI₃1030 as a catalyst and para-toluene sulfonic acid (p-TSA) 1031 as a co-catalyst, it was possible to obtain 2-disubstituted tetrahydrofurans 1032a–h and 1034a–b from 1,7- or 1,8-diynyl ethers 1029a–h and 1033a–b with moderate yields. The products contain an exocyclic enone part. The best yields were obtained for reagents bearing with nucleophilic aryl groups in the terminal positions (Scheme 188).³⁹⁰ The authors also postulated the mechanism of the reaction, which started from the activation of homopropargylic alkyne by chelation of InI₃1030, with the ether oxygen atom. This accelerates the initial 7-endo-dig cyclisation with the nucleophilic aryl alkyne. The presence of such an aryl ring is necessary for the desired reaction course. Next, hydration occurred, which furnishes enol 1037, which via elimination process leads to acyclic cross-conjugated dienone 1038. The mechanism is concluded by the protonation of 1038 to tertiary carbocation 1039 and its cyclisation to the desired 2-disubstituted tetrahydrofurans (Scheme 189).³⁹⁰

In addition to the application of metal based catalysts in the hydration reaction of diynes, there are also some examples focused on photocatalytic processes. Photohydration of nonsymmetricaly substituted conjugated 1,3-diynes 781c, 1040a–g with an aryl (naphthyl, phenyl, 4-MeOC₆H₄, 4-CH₃COOC₆H₄ 4- or 3-CF₃C₆H₄, 4- or 3-NO₂C₆H₄) or alkyl groups (tert-butyl, methyl) occurred in an aqueous sulphuric acid solution. The acidity influences the ration of products 1041 and 1042, which differs in the hydration of a specific CC bond. A medium-acidity gives quantitative yields of hydration, and azulene-quenching postulating that the singlet excited state furnishes both 1041 and 1042 photoadducts. The triplet excited state yields only 1041 photoadducts when R is an alkyl group. Moreover, the type of the substituent attached to the aryl group has an influence on the photohydration process is in the order of 3-NO₂ > 4-NO₂ > 3-CF₃ > 4-CF₃ > 4-CO₂CH₃. Depending on the reagent, various products with a carbonyl group attached to the CC or allenic structures were obtained, which are presented in the mechanism shown in Scheme 190.^391,392

11. Hydrothiolation of conjugated 1,3-diynes

Hydrothiolation of conjugated diynes is carried out mostly according to two pathways: (i) nucleophilic addition of thiols to unsaturated carbon–carbon bonds in the presence of various alkaline metal bases or, (ii) according to radical processes. Vinyl sulfides obtained in the hydrothiolation reactions are the components of several drugs used in the Alzheimer's, Parkinson's, cancer, or AIDS diseases.^393,394 They are also important building blocks in organic synthesis, which might be easily converted to carboxylic acids, ketones, or aldehydes in a thio-Claisen rearrangement. They can also be used in Michael, Peterson, or Diels–Alder transformations as well being easily reductively cleaved.^395–401 Vinylsulfides were isolated in the biologically active compounds e.g., Griseoviridin from Streptomyces graminofaciens or benzylthiocrellidone from Crella spinulata.^402–404

Nucleophilic addition of thiolate anions to CC bonds in alkynes and diynes occurs mainly according to a trans-addition reaction with the generation of (Z)-vinylic isomers. The formation of these nucleophilic species occurred predominantly in the reactions with strong bases (e.g., hydroxides: KOH, NaOH, or alkoxides: NaOR or KOR). In most cases, the other possible isomers are accomplished by the post-reaction mixture. The addition of thiols 1046a–d to conjugated 1,3-diynes 1a, 180c, 1045 led to the formation of 1,4-dithiol-1,4-disubstituted dienes 1047. The reaction occurred in a stepwise process. First, the monothiolate 1,3-enyne is formed, which then is hydrothiolated to 1,4-dithiol-1,4-disubstituted dienes 1047. The obtained products can be cyclised to dithiins 1048 by deprotection of the thiol group with Li in liquid NH₃632, followed by the oxidation of thiolate anions with I₂418 in KF. These cyclic compounds 1048 can be potentially used as antiviral compounds or antibiotics. During the hydrothiolation of 1,4-TMS-substituted buta-1,3-diyne 180c, the desilylation reaction occurred (Scheme 191).^405–408 Changing the reaction conditions, by applying a different solvent (DMSO), led to the formation of biologically active thiophenes 1051a–f with moderate isolated yields (51–66%), instead of thio-substituted buta-1,3-dienes 1047 when EtOH or DMF were used.^406,409 The 1,2-addition product of arylmethanethiol 1046a or 1049a–c led to the corresponding enyne 1050. The thiophene was formed by the cyclisation of enenyne thiol 1050, which occurs from the nucleophilic attack of benzyl anion on C_sp bond in the second alkynyl group of 1050 (Scheme 192).^410,411

The synthesis of thiophenes and other cyclic compounds from diynes was briefly reviewed by Maretina and Trofimov.²⁹⁵ The paper presented the procedures that were published mostly in 1960–1980 and are focused on the addition of sulfide ions to conjugated diynes yielding thiols. Very good results were obtained in the case of the formation of thiols when Na₂S 1052 was used as a reagent in KOH/DMSO. When a quantitative amount of Na₂S 1052 and KOH was applied thiophene 1053 was formed from buta-1,3-diyne with excellent yield up to 99%. The process occurred via hydrothiolation of diyne followed by cyclisation. Nonhydroxylic polar solvents e.g., DMSO or N-methylpyrrolidone should be used in this transformation, because these solvents did not decrease the activity of anions by their solvation.²⁹⁵ By replacing KOH with TBAOH 1056 and benzylthiol 1046a with butyl analog 1055 it was possible to shorten the reaction time to just 5 minutes and to increase the product yields and selectivity. (Z)-Thiobutenynes 1057a-k were obtained exclusively for symmetrical and unsymmetrical diynes 1a, 1d, 13a, 208b, 258g, 617, c, 1028, 1054a–b with sterically different substituents in positions C₁ and C₄. Hydrothiolation using the reductive system n-C₄H₉SH 1055/TBAOH 1056 is more efficient because it is a stronger base which, due to its phase-transfer ability, increases the solubility of reagents in the organic phase and accelerates the formation of the butylthiolate anion (Scheme 193).⁴¹² In the presence of iodine 418, the obtained (Z)-organylthioenynes underwent electrophilic cyclisation towards 3-iodothiophenes 1058. The reaction was tested using 1.0 equiv. of 1057c and 1.1 equiv. of I₂418 (Scheme 194).

Thiophenes 1061a–f were obtained also from haloalkynes 1018 and 1059 from a Glaser reaction to conjugated 1,3-diynes 1a–c, 258g, 258n, and 1060 followed by the hydrothiolation to sulfanyl substituted enynes, which further cyclises to thiophenes possessing different aryl or heteroaryl groups in positions 2- and 5- 1061a–f. The reactions were catalysed by a CuI 519/1,10-phen system and proceeded with high products yields. As a hydrothiolation agent, Na₂S·9H₂O 1052 was used (Scheme 195). The mechanism of this transformation was previously described for the analgous hydration process (Scheme 187).³⁸⁸

The synthesis of 3-halosubstituted thiophenes 1066 from simple aryl or alkyl-functionalised conjugated buta-1,3-diynes 1a, 1d, 13a, 27c, 65b, 208b, 258g, 258o, 617, 842b–c, 1028, and 1054a–b was described by Kesharwani et al. They proposed a two-step procedure yielding 3-chloro, 3-bromo and 3-iodothiophenes 1066 based on hydrothiolation reaction of 1,4-diaryl or 1,4-dialkyl-substituted diynes with methyl disulfide 1062 in the presence of NaBH₄212 as a hydrogen source and, electrophilic cyclisation of the obtained sulfanyl modified enynes 1063 with natrium halides 1064a–c (NaCl, NaBr, NaI) in the presence of CuSO₄·5H₂O 1065 (Scheme 196).⁴¹³ This methodology has a positive impact on the environment, because it uses the green solvent ethanol and simple inorganic salts. In many cases it also gave better results than typical methods used for the preparation of halothiophenes (Scheme 196).^413–415 The mechanism of the cyclisation proposed the formation of CuCl₂ in the first step from CuSO₄1065 and NaCl 1064a, which can easily coordinate to the CC triple bond in the enyne to 1067. Nucleophilic attack of sulfur provided intermediate 1068 that eliminates the methyl group attached to the sulfur atom via an S_N2 substitution reaction by the chloride anion yielding intermediate 1069. Reductive elimination furnished the desired halothiophene 1066a, while the Cu(0) is oxidised to CuCl 55 by CuCl₂ (Scheme 197). In the case of the application of NaBr 1064b and NaI 1064c, CuBr₂52 and CuI₂ can easily release I₂418 and Br₂, and by applying these as electrophiles forms of bromo- and iodothiophenes.⁴¹³ 3,4-Dichloro-substituted thiophenes 1071 can be formed in the reaction of 1,4-diarylsubstituted buta-1,3-diynes with sulfur chloride 1070. Products 1071 were obtained with 18–80% yields.^295,416 Excellent yields and selectivities of (Z)-thioenynes 1074a–l were obtained when disulfides 1073 (PhSSPh 1073a, BuSSBu 1073b) were used as reagents. Oganylthiolate anions were generated in situ with NaBH₄212. The application of disulfide 1073a–b may constitute an alternative towards the use of toxic and bad-smelling thiols (Scheme 198).⁴¹⁷

A sustainable and clean method for obtaining thiobutenynes 1074/1075 was carried out in the presence of KF/Al₂O₃ as a catalyst, using glycerol or poly(ethylene glycol) (M_w = 400, PEG400) as a green solvent. Applying this system, it was possible to decrease the amount of the catalyst employed, and the generation of inorganic products is reduced to a minimum. The products were extracted in hexane/ethyl acetate and KF/Al₂O₃ was directly used in following cycles. The reaction was effective for various diynes substituted in the terminal position with electron-withdrawing or electron-donating groups. The reaction was optimised for the use of an equimolar ratio of reagents. The best results were accomplished when 90 °C was used and the reaction was carried out for 6 hours. A lower temperature led to lower yields, while higher temperatures reduced the selectivity. Under the optimal conditions’ product 1074 with (Z)-geometry was obtained in excess, in the ratio 90:10 to 100:0 depending on the reagent structure. The yields of the (Z)-1,4-diphenyl-2-(phenylthio)but-1-en-3-yne 1074a in the three repetitive batches reached 93%, 89%, 80% respectively (64%, 55%, 48% isolated yield).⁴¹⁸

Perin et al. proved that the addition of phenyldisulfide 1073a to 1,4-diphenylbuta-1,3-diyne 1a using the same conditions (NaBH₄212, PEG400, 30 °C) may be accelerated applying microwave irradiation as a heating source. It was possible to reduce the reaction time to 85 minutes from 24 hours under traditional conditions with a slightly better yield of 1074a (96% vs. 82%). Moreover, by increasing the temperature to 90 °C, 1,4-diphenyl-1,4-di(phenylthio)buta-1,3-diene 1076 was selectively formed in good yield (65% vs. 69%).⁴¹⁹

The hydrothiolation of 1,3-butadiynes 1a, 208b, 655c, 1028, 1045, 1072a, and 1072c were carried also using various diaryl disulfides 1073a or 1077a–e, sodium hydroxymethanesulfinate 1078 (rongalite 1078, HOCH₂SO₂Na), and potassium carbonate. Rongalite 1078 was applied as a reducing agent cleaving the bond of disulfide. When disulfide was used in 0.5 equiv. to the diyne, (Z)-1-sulfanyl-but-1-en-3-ynes 1079a–o were obtained with isolated yields in the range 45–86%. Increasing the temperature to 70 °C and an equimolar ratio of diyne and disulfide, it was possible to obtain a mixture monothiolation 1079a–o and bisthiolation 1080 products with moderate yields. Moreover, the introduction of two different arylthiol groups to the product was possible by subsequent hydrothiolation of diyne with two others disulfides. The reaction did not occur for benzyl and alkyl-substituted disulfides (Scheme 199). The mechanism of this transformation started by the decomposition of rongalite 1078 to formaldehyde 1081 and HSO₂⁻1082 in the presence of the base. Next, the single-electron transfer to disulfide 1073a leads to anionic 1084 and radical species 1085. The radical thiolate 1085 is then reduced to its ionic form 1084 by another single electron transfer from radical HSO₂˙. 1083. Addition of thiolate 1084 to diyne 1a followed by the protonation of the intermediate 1086 yields the desired product 1079a (Scheme 200).

Moreover, bishydrothiolation was also carried out in a sequence of one-pot reactions. Sonogashira coupling of 1,4-bis(trimethylsilyl)buta-1,3-diyne 180c with aryl halides 1087 followed by bishydrothilation of the obtained 1,4-bisarylbuta-1,3-diyne with various thiols 1046a and 1089a to (Z,Z)-1,4-diaryl-buta-1,3-dienes 1090. The formation of the new C–C bonds was catalysed by Pd(OAc)₂ (1 mol%) 861 and Cu(xantphos)I (1 mol%) 1088, while double hydrothiolation was promoted by basic Cs₂CO₃. The whole process occurred with moderate or good yields, with high regio- and stereoselectivity (Scheme 201).⁴²⁰ The mechanism of this transformation started with the generation of the sulfur anion from thiol 1046a or 1089 in the presence of base a (BH). The hydrothiolation step then occurs with the formation of intermediate 1091. The (Z)-isomer 1093 is formed in the presence of thiol 1089a or base according to the protonation step. Subsequent hydrothiolation of obtained enyne 1093 furnished (1Z,4Z)-bissulfanylbuta-1,3-diene 1090a (Scheme 202).⁴²⁰

The addition of aminothiols 1094a–e to buta-1,3-diyne 641 was carried out in ammonia 632, which was used as a solvent and base at the same time. HS⁻ anions are 280 times more reactive than NH₂⁻, therefore the addition of aminothiols 1094a–e to the CC bonds occurred from the S-side. The enyne sulfides 1095a–e were obtained in 78–98% yield (Scheme 203).⁴²¹

12. Hydroselenation of 1,3-diynes

Alkenyl selenides are an important class of organoselenium compounds, which have broad applications in organic chemistry leading to a vast spectrum of valuable products.^422–426 These molecules can be prepared by various synthetic pathways however, the most frequently employed method for their preparation is the hydroselenation of the CC bond by nucleophilic organoselenolate anions. The synthesis and application of organoselenium compounds, especially selenophenes, has been summarised in many books and reviews,^{422,423,425,427–434} nevertheless the hydroselenation of diynes have not been comprehensively reviewed.

The first example addition of a Se–H bond to diynes was reported by Taylor et al. in 1968.⁴³⁵ The addition of H₂Se 1097 to symmetrical and unsymmetrical 1,3-diynes 1a, 208c, 1045, 1072a, 1096a–b was catalysed by the Ag⁺ cations and led to 2,5-disubstituted selenophenes 1098a–f in good yields (Scheme 204).

Dabdoub et al. developed an alternative and efficient synthetic protocol employing the phenylselenolate anion which was generated in situ by the reaction of Ph₂Se₂1099 with sodium borohydride 212 in ethanol, instead of using toxic hydrogen selenide. Hydroselenation of 1,4-substituted-1,3-butadiynes 1a, 613a, 842b–c, 1072a–b, 1072d occurred with excellent regio-, stereo- and chemoselectivity smoothly leading to (Z)-1-phenylseleno-1,4-diorganyl-1-buten-3-ynes 1100a–h in high yields. However, reacting 2-hydroxy-2-methyl-3,5-dodecadiyne 1072a with Ph₂Se₂1099 and NaBH₄212 in ethanol under reflux, gave a mixture of regioisomers (1100e/1100f = 58/42) which was confirmed by ¹H NOESY experiments (Scheme 205).⁴³⁶

A similar strategy was applied by Zeni et al. who used various diorganodiselenides 1112a–e in the preparation of (Z)-selenoenynes 1113a–o through the hydroselenation of symmetrical and unsymmetrical 1,3-diynes 1a, 1c, 13a, 37s, 842b–c, 1028, 1045, 1072a, and 1111. The obtained products were further cyclised with different electrophiles such as I₂418, ICl 1114a, PhSeBr 1114b, or PhSeCl 1114c to 3-substituted selenophenes 1115a–i in good yields. The electrophilic cyclisation did not occur for (Z)-1-(phenylseleno)-1,4-diphenyl-but-1-en-3-yne 1113f even when using harsh reaction conditions. The authors also presented the versatility of these compounds for many transformations such as Sonogashira coupling, halogen-metal exchange reaction, or Ullmann-type C–O bond forming reactions (Scheme 206).⁴³⁷

An analogous protocol was applied by Kesharwan et al. who used sodium halides 1064a–c as a source of electrophilic halogens for the synthesis of halogenated selenophenes 1115a, 1116. The hydroselenation of 1,4-diphenyl-1,3-diyne 1a by Me₂Se₂1112a in the presence of NaBH₄212 in ethanol gave for (Z)-1-(benzylseleno)-1,4-diphenyl-but-1-en-3-yne 1113a in 60% yield which was transformed to halogenated selenophenes 1115a and 1116 (Scheme 207a). The protocol was also suitable for the preparation of halogenated thiophenes (Scheme 196).⁴¹³ The replacement of electrophiles to a base such as t-BuOK in the cyclisation step is also efficient for the synthesis of selenophenes 1119 through selenoenynes 1113e and 1118 intermediates. The hydroselenation of 1,3-diynes 1a, 1c–d, 27a, 258a, or 280a with Bn₂Se₂1112e in the presence of NaBH₄212 in ethanol was limited to its symmetrical derivatives, and the reaction yields were significantly lower for bulky diynes (Scheme 207b). The authors suggested that 3-benzyl-substituted selenophenes 1119 could be highly promising building blocks for the preparation of polysubstituted selenophenes.⁴³⁸

The hydroselenation of diynes was also adopted to synthesise more complex structures such as tetrapyrrolic macrocycles. Chauhan et al. presented the preparation of selenium core-modified porphyrinogens based on the hydroselenation of diynediols 1072b, 1121a–b with in situ generated sodium selenide 1122 in the presence of MeOH and CH₃COOAg in the first step. The obtained selephones 1123 were further transformed to selenophene dipyrranes and used in 3 + 1 condensation reactions with the corresponding diols in the presence of boron trifluoride 565. It was proved by UV-Vis, fluorescence and ¹H NMR spectroscopy that the selenium core-modified porphyrinogens have a coordination ability to detect of Hg²⁺ cations.⁴³⁹ The same group reported the preparation of porphomethenes 1128, porphodimethenes 1127, and porphotrimethenes 1129 using the same methodology including diynediol hydroselenation followed by 3 + 1 condensation of the selenatripyrranes 1125 with selenophene-2,5-diols 1123 and subsequent oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone 1126 (DDQ). Similar to porphyrinogens, the obtained macrocycles showed the binding affinity with Hg²⁺ anions (Scheme 208).⁴⁴⁰

Lopes et al. described the application of deep eutectic solvents (DES), commonly considered as the third generation of ionic liquids, in the preparation of vinyl selenides. Although the report concerns mainly the synthesis of (E)-1,2-bisorganylseleno alkenes through the hydroselenation of alkynes, the utilisation of symmetrical buta-1,3-diynes was also presented. Diphenyl selenide 1099 reacted with 1,4-diphenyl-1,3-diyne 1a or hexa-2,4-diyne-1,6-diol 1045 in the presence of NaBH₄212 in a choline chloride/urea (1/2) mixture at 90 °C to give corresponding (Z)-selenoenynes 1100g and 1130 with excellent selectivity and high yields (Scheme 209a).⁴⁴¹ A similar strategy based on the application of green solvents was used by Lara et al. who applied a poly(ethylene glycol) (PEG 400) as an alternative for DES. Depending on the reaction temperature, the (Z)-selenoenynes 1100g–h and 1132a–b or (Z,Z)-1,4-bisselenobuta-1,3-dienes 1133a–c were obtained in the reaction of diorganodiselenides 1099, 1112c, and 1131 with the symmetrical 1,3-diynes 1a, 1045, 1072a in the presence of NaBH₄212. The process is highly stereoselective exclusively leading to the corresponding products in high yields. The reaction time was reduced from 24 to 1.25 h by the application of microwave radiation (MW) as a heating method (Scheme 209b).⁴¹⁹

An interesting protocol for the synthesis of (Z)-1-(organoselanyl or sulfanyl)enynes was developed by Venkateswarlu et al. who utilised sodium hydroxymethanesulfinate (rongalite) 1078 as a reducing agent instead of commonly used NaBH₄212. The hydroselenation of 1,3-diynes 1a, 208b, 655c, 1028, 1045, 1072a–b was carried out in the presence of potassium carbonate in a DMF–H₂O (20:1) mixture and under mild reaction conditions leading exclusively to (Z)-isomers 1100g–h, 1130 and 1134a–d. It is worth noting that the discussed procedure was suitable for symmetrical and unsymmetrical 1,3-diynes, however, it failed when an aliphatic diacetylene such as hexadeca-7,9-diyne 208b was applied. The low reactivity of 208b in this transformation can be explained in the same manner as hydrothiolation,⁴⁴² by weak stabilisation of transition state which is dependent on steric and electronic factors. Depending on reaction conditions it was possible to obtain mono- or bishydroselenation products. The hydroselenation of 1,3-diynes performed at a lower temperature (50 °C) and with 0.5 eq. of Ph₂Se₂1099 furnished (Z)-1-(organoselanyl)enynes 1100g–h, 1130 and 1134a–d, whereas application of 1.0 equiv. of Ph₂Se₂1099 and a higher temperature (70 °C) yielded a mixture of mono- and bishydroselenation products (Scheme 210). The presented protocol is also adequate for sulfanyl derivatives (Scheme 199).⁴⁴³ The mechanistic studies for the reaction were performed based on hydrothiolation of diynes (Scheme 200), however, it also can be extended to hydroselenation.⁴⁴⁴ It involves the reduction of Ph₂Se₂1099 with the generation of the PhSe⁻ anion 1137 followed by hydroselenation of the 1,3-diyne. In the initial step, the rongalite 1078 is decomposed in the presence of K₂CO₃ to formaldehyde 1081 and HSO₂⁻Na⁺1082. Single electron transfer (SET) gives anion 1137 and radical 1136. Another SET reduces 1136 to 1137. The trans-addition of the benzeneselenolate anion 1137 to the 1,3-diyne gives intermediate 1138 which is protonated to yields (Z)-1-(organoselanyl)enynes 1100g–h, 1130 and 1134a–d (Scheme 211).⁴⁴³

The hydroselenation of 1,3-diynes was utilised by Męcik et al. for the synthesis of di(selenophen-3-yl)diselenides 1143a–e and 3-methylene-3H-1,2-diselenoles 1144a–b. These uncommon selenium heterocycles were only formed when 1-amino-4-aryl-buta-1,3-diynes or 1-amino-4-ester-buta-1,3-diynes 1141 (synthesised from 1-bromobutadiynes 1139 and secondary amine 1140) were used. The reaction of 1-aminobutadiynes 1141 with generated in situ sodium selenide led to desired products 1143a–e and 1144a–b, instead of excepted selenophenes, in moderate to high yields.

Simple diaryl or dialkyl-1,3-butadiynes led to classical selenophenes or did not react at all thus, the presence of amine group in diyne structure was crucial for the synthesis of di(selenophen-3-yl)diselenides 1143a–e and 3-methylene-3H-1,2-diselenoles 1144a–b. It is worth noting that this protocol could be also adopted in a one-step strategy without the isolation of 1-aminobutadiynes 1141, by the addition of sodium selenide solution to the reaction mixture directly after the amination step (Scheme 212). The authors proposed the mechanism of this transformation, which started from the hydroselenation of 1,3-butadiyne by the nucleophilic attack of generated in situ SeH⁻ anion 1145 to CC bond. Subsequently, the bisselenide 1147 is formed and transformed to selenirenium ion 1148, which undergos nucleophilic attack to a carbon atom (for aryl-substituted diynes) 1149 and its dimerisation with the generation of di(selenophen-3-yl)diselenides 1143a–e. For ester-substituted diynes occurs an internal nucleophilic attack of Se⁻ to Se⁺1151 and further rearrangement to 3-methylene-3H-1,2-diselenoles 1144a–b (Scheme 213).⁴⁴⁵

13. Hydrotelluration of 1,3-diynes

Unsaturated organotellurium compounds, especially vinylic tellurides have found numerous applications in organic synthesis due to their high reactivity, tolerance towards many functional groups and the possibility for carbon–carbon bond formation. This has been covered in previous reviews.^{431,433,434,446–448} This versatile class of tellurium compounds is also an important intermediate in the synthesis of tellurophenes which have applications in material chemistry^{430,447,449–455} and biological chemistry.^433,456 Among many synthetic strategies towards unsaturated organotellurium compounds, the hydrotelluration of diynes is a highly efficient and stereoselective method for enynyl tellurides which are useful building blocks in modern chemistry. Since the first synthesis of tellurophene by the interaction of 1,3-butadiyne 1a and 1152 with Na₂Te 1153 in methanol developed by Mack's in 1966,⁴⁵⁷ the several papers describing hydrotelluration of diynes appeared^458–468 which was also covered by Detty⁴³¹ and Zeni.^446,469 In this chapter the recent development of functionalisation of diynes by tellurium compounds will be presented.

Seferos et al. reported π-conjugated 2,5-substituted tellurophene 1154 compounds which were synthesised via ring-closing reactions of 1,4-substituted butadiynes 1a and 1152 in the presence of Na₂Te 1153 and a protic solvent (Scheme 214). This synthetic procedure avoids harsh reaction conditions and degradation of the tellurophene ring. The oxidation of tellurophene 1154 through Br₂ addition to 1155 changed the measured optical absorption spectrum and oxidation potential which was confirmed by absorption spectroscopy and DFT calculations. The authors suggested that this class of compounds might have potential applications as semiconducting materials or as transition metal-free catalysts for energy storage reactions.^453,470,471

A similar approach was applied by Chauhan et al. who synthesised a series of calixpyrroles and calixphyrins by the interaction of diynediols 1045, 1072a, 1121a–b, and AgOAc in MeOH with an aqueous solution of Na₂Te 1153. Subsequent 3 + 1 condensation of telluratdipyrranes 1157 or telluratripyrranes 1158 with corresponding tellurophene-2,5-diols 1156 in the presence of BF₃–etherate 565 gave desired products (Scheme 215). The obtained compounds had a binding affinity with Hg²⁺ cations which was confirmed by spectroscopy studies. A described synthetic protocol could also be applied for selenophenoediols (see Section 11.2).^439,440

Application of environmentally-friendly poly(ethylene glycol, M_w = 400) in the selective synthesis of (Z)-telluroenynes 1161 and (Z,Z)-1,4-bis-tellurobuta-1,3-dienes 1162 in the reaction of symmetrical diynes with diphenyl telluride 1160 and NaBH₄212 as a reducing agent was reported by Perin et al. The process was found to be temperature-dependent. When the reaction was carried out at 30 °C, the (Z)-telluroenynes 1162 were obtained with excellent selectivity while higher temperatures (90 °C) led to (Z,Z)-1,4-bis-tellurobuta-1,3-diene 1162 (Scheme 216). The use of microwave radiation as an alternative heating source furnished desired products in a few minutes instead of several hours. The protocol is also suitable for selenium and sulfur derivatives.⁴¹⁹

Deep eutectic solvents (DES) composed with the choline chloride (ChCl) and urea mixture (1:2) could be applied as another green solvent in the synthesis of organoseleno alkenes and mono-chalcogenated (Z)-alkenynes. The hydrotelluration of 1a with diphenyl telluride 1160 in the presence of NaBH₄212 led to the (Z)-1-phenyltelluro-1,4-diphenyl-but-1-en-3-yne 1161a with excellent regio- and stereoselectivity and moderate isolated yield (42%) (Scheme 217).⁴⁴¹

Męcik et al. reported the synthesis of tellurophenes 1164a–d by the reaction of 1-aminobutadiynes 1141 with sodium telluride 1153 which was generated in situ from Te 1163 and NaBH₄212. The 1-aminobutadiynes 1141 were prepared from 1-bromobutadiynes 1139 and used without purification step leading to 2-aminotellurophenes 1164a–d in good yields. Intriguingly, the application of sodium selenide 1122 gave di(selenophen-3-yl)diselenides 1143 and methylene-3H-1,2-diselenoles 1144 instead of simple selenophenes (Scheme 212). The authors suggested that it might be caused by the lower stability of Te–Te bond compared with the Se–Se bond (Scheme 218).⁴⁴⁵

14. Conclusions and outlook

Conjugated and separated diynes constitute a special class of compounds, which due to their structural and electronic versatility that can be tuned by the presence of various functional groups attached to the CC bonds, as well as by the spacer between both CC bonds, create a “chemical mine” for developing fine organometallic and organic chemicals. The combination of these compounds with hydroelementation reagents, chosen from main group elements, permits an incredibly diverse array of products (enynes, dienes, allenes, cyclic compounds, heterocycles, polymers) to be obtained. Due to the presence of the unsaturated C–C bonds, as well as main group elements in their structures, these are extremely useful building blocks in the synthesis of tailor-made materials, pharmaceuticals, natural compounds analogs, and structurally advanced organic molecules. The review presents the library of the reactions, reagents, products, and catalysts for the hydroelementation of conjugated and separated diynes and can be used as a guidebook for planning the synthesis of advanced compounds via hydroelementation processes. The problems with the selective activation of the one or two CC bonds, possible overreduction, and the stereo- and regioselectivity of hydroelementation processes are the biggest challenges that need to be overcome during the reduction of diynes. This can be achieved by the proper choice of reagents (steric and electronic properties have a significant influence on the process selectivity) catalyst, and reaction conditions. The number of examples that produce only one product and one isomer is however limited. Therefore, there is still a lot of scope for developing catalytic systems that might be highly selective, active, and stable as well as being straightforward from a synthetic perspective. Bearing in mind that the hydroelementation reaction is a 100% atom economic process, searching for highly effective and selective methods, which might be applied using an equimolar ratio of reagents is of prior importance. The products obtained in the hydroelementation reactions of conjugated and separated diynes can be used as important synthons in organic synthesis. Several demetallation, coupling, and addition reactions were presented in this review to demonstrate the power of the products obtained in the hydrometallation of diynes. In the future, improvements in the catalyst effectiveness and availability, selectivity, and productivity need to be undertaken to make the hydroelementation process straightforward for the formation of different products. Control of the process regio- and stereoselectivity is the biggest task for all chemists, which are focused on the synthesis of fine chemicals.

Abbreviations

acac	Acetylacetone
ACCN	((1,1-Azobis(cyclohexane-1-carbonitrile)))
BINAP	2,2′-Bis(diphenylphosphino)-1,1′-binaphthalene
bmin	1-Butyl-3-methylimidazolium
Bn	Benzyl
BPPM	4-(Diphenylphosphino)-2-[di(phenylphosphino)methyl]pyrrolidine
CAAC	Cyclic(alkyl)(amino)carbene
CAN	Ammonium cerium(IV) nitrate
Cbz	Benzyloxycarbonyl
cod	1,5-Cyclooctadiene
m-CPBA	3-Chlorobenzene-1-carboperoxoic acid
Cy	Cyclohexyl
t-BuPNP	2,6-Bis(di(tert-butyl)phosphinomethyl)pyridine
dba	Dibenzylideneacetone
DBN	1,5-Diazabicyclo[4.3.0]non-5-ene
DBU	1,8-Diazabicyclo[5.4.0]undec-7-ene
DCC	Dicyclohexylcarbodiimide
DDSQ	Double-decker-shaped silsesquioxane
DDQ	2,3-Dichloro-5,6-dicyanobenzoquinone
DES	Deep eutectic solvent
DIBAH	Di(iso-butyl)aluminum hydride
Dipp	2,6-Di(iso-propyl)phenyl
DMAP	4-(Dimethylamino)pyridine
dmpm	5,5-Dimethyldipyrrolylmethane
dpma	N,N-Di(pyrrolyl-alpha-methyl)-N-methylamine
DP	Polymerisation degree
dppb	1,4-Bis(diphenylphosphino)butane
dppben	1,2-Bis(diphenylphosphino)benzene
dppe	Ethylenebis(diphenylphosphine)
dppf	1,1′-Ferrocenediyl-bis(diphenylphosphine)
dppp	1,3-Bis(diphenylphosphino)propane
dvs	1,1,3,3-Tetramethyl-1,3-divinyldisiloxane
DIEA	N,N-Di-iso-propylethylamine
DIOP	2,3-O-Iso-propylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane
F-TEDA	1-Chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate)
hfa	Hexafluoroacetone
hmpa	Hexamethylphosphoramide
IL	Ionic liquid
lIpc2BH	Di-iso-pinocampheylborane
IPr*OMe	1,3-Bisimidazol-2-ylidene
LDA	Lithium diisopropylamide
MBPH	4,4′-Methylenebis[2,6-bis(hydroxymentyl)]phenol
Ms	Methanesulfonyl
MS	Molecular sieves
M w	Molecular weight
MW	Microwave radiation
NCS	N-Chlorosuccinimide
NHC	N-Heterocyclic carbene
NIS	N-Iodosuccinimide
NMDPP	Neomenthyldiphenylphosphine
Norphos	2,3-Bis(diphenylphosphino)bicyclo[2.2.1]hept-5-ene
NTf2	Bis(trifluoromethane)sulfonimide
OTf	Trifluoromethanesulfonate
PEG	Poly(ethylene glycol)
pin	Pinacol
PINBOP	2-Iso-propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolan
pivOH	Pivalic acid
PNP	1,3-Bis(di-tert-butyl-phosphinomethyl)pyridine
POP-BZ	1-Benzyl-3,4-bis((diphenylphosphaneyl)oxy)pyrrolidine
PPM	4-(Diphenylphosphaneyl)-2-((diphenylphosphaneyl)methyl)pyrrolidine
PTMA	(5-Propylsulfonyloxyimino-5H-thiophen-2-ylidene)-2(methylphenyl)acetonitrile
pv	Pivaldehyde
PyrPhos	3,4-Bis-diphenylphosphino-pyrrolidine
QUINAP	1-(2-Diphenylphosphino-1-naphthyl)isoquinoline
scCO2	Supercritical CO2
SET	Single electron transfer
TBAF	Tetrabutylammonium fluoride
TBHN	Di-tert-butyl hyponitrite
TBDMS	Tert-butyldimethylsilyl
TC	Thiophene 2-carboxylate
TDT	Tert-dodecanethiol
THP	Tetrahydropyranyl
TIPS	Tri(iso-propyl)silyl
TMEDA	N,N,N′,N′-Tetramethylethylenediamine
TMS	Trimethylsilyl
Tos	Toluenesulfonyl
TP	Tris(1-pyrazolyl)borate
p-TSA	para-Toluenesulfonic acid
xantphos	4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We are grateful for the financial support of the National Science Centre (Poland) grants no. UMO-2018/31/G/ST4/04012, UMO-2019/34/E/ST4/00068 and UMO-2019/32/C/ST4/00235, as well as the EPSRC grant no. EP/R026912/1.

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Footnote

† This article is dedicated to Prof. Bogdan Marciniec from Adam Mickiewicz University in Poznań (Poland), expert in hydrosilylation reactions, on the occasion of his 80th birthday.

This journal is © The Royal Society of Chemistry 2022