Hydrosilylation, of alkynes

Alkynes are hydrosilylated more readily than alkenes, offering a simple and direct means of producing vinylsilanes. However, a mixture of three isomeric vinylsilanes including the branched a-isomer as well as the p-( )- and p-(Z)-vinylsilanes may be obtained (Equation (13.5)). 

Marko and co-workers extended their hydrosilylation reactions of alkenes to terminal alkynes. Initially, complexes 80-type were screened in the hydrosilylation of 1-octyne by bis(tiimethylsilyloxy)methylsilane as model reaction and enhanced selectivity was observed when bulky aryl substituted NHCs were employed (IPr and SlPr).  

Poyatos et al. investigated the activity of the N -chelating oxazole-NHC-containing Pt complex 85 with various substrates. From phenylacetylene, the p-( )-isomer and the a-isomer were the only two reaetion produets (60 40). Styrene hydrosilylation led to the formation of the linear produet with 85% 

A series of mixed PCys/NHC Ru complexes, such as [(IMes)-RuCl2(=CHPh)(PCy3)] 86, favored the formation of the Z-isomer, while the a-isomer was only formed in minor amounts.  

Additionally, the first NHC-Rh-catalyzed hydrosilylation/cyclization of enynes was achieved using Rh catalyst 93 by Chung and co-workers (Equation (13.6)).  

The ability of organo-rare-earth metal complexes to undergo alkene or alkyne insertion provides the possibility to perform polyene cyclizations, producing metal-alkyl species which can then undergo o-bond metathesis with an appropriate reagent to produce a cyclic compound. Thus, termination via protonolysis (6) results in cycloalkane derivatives however, termination via silylation is more desirable as a functionalized cyclic framework is formed (Fig. 9). 

Initial studies on the hydrosilylation/carbocyclization of terminal dienes carried out in the early 1990s have demonstrated that five-membered ring formation proceeds smoothly in the presence of lutetocene [40], neodymocene [43], and samarocene [29] catalysts (18). 

Postmetallocene catalysts can also facilitate this transformation [50, 60, 62], although they are typically less selective and a significant amount of acyclic hy-drosilylation product may form if the rate of silylation of the initial alkene insertion product A (Fig. 9) is competitive with the rate of carbocyclization. 

Rare-earth metal catalysts tolerate a variety of aprotic functional groups for example, sterically encumbered mesityl sulfonamides sufficiently shield the oxygen from the metal center in order to allow hydrosilylation/carbocyclization, albeit at a reduced rate (20) [72], 

Diastereoselective cyclizations are possible [73], in particular when the outcome of the stereochemistry is set by the configuration of a cylic ring junction (Fig. 10). The stereochemistry-determining insertion step proceeds via the less encumbered transition state providing a single diastereoisomer. 

Vinylsilanes are also formed by the hydrosilylation of alkynes. Similar to the selectivity of the hydrosilylation of alkenes, the selectivity of the hydrosilylation of alkynes can be controlled by both the catalyst and the silane. Alkyne hydrosilylation can form either of two regioisomeric vinylsilanes and can form products from either cis or trans addition of the silane across the alkyne unit. 

The hydrosilylation of alkynes catalyzed by platinum complexes tends to generate products from cis addition of the silane across the alkyne unit. Thus, the product obtained from the reaction of a terminal acetylene is the trans (E)-vinylsilane, as shown in Equation 16.30. This example also shows that the simple Speier s catalyst does not adequately control the regioselectivity of alkyne hydrosilylation. 

Takeuchi has shown that the stereoselectivity of alkyne hydrosilylation can be controlled by both the ligand and the solvent (Equation 16.31). addition of the Si-H bond across the alkyne has been observed for the hydrosilylation of hexyne by triethyl-silane in ethanol solvent in the presence of a catalyst generated from the combination of [Rh(COD),] and PPhj. In contrast, trans addition occurs for the same reaction in the same medium with a neutral rhodium catalyst lacking added phosphine. Trans addition also occurs in the presence of Wilkinson s catalyst in toluene. (Equation 16.32). As discussed in Section 16.3.5.2.4 below, trans addition likely results from the isomerization of a vinyl intermediate. - 

Although (TMS)3Si has been proven to isomerize alkenes (see above), the post-isomerization of the hydrosilylation adduct could not be observed due to steric hindrance. Only with Ph3Ge radical, the (Z)-( ) interconversion of (TMS)3Si substituted alkenes was achieved [39]. 

The addition of (TMS)3SiH to a number of 1,2-disubstituted acetylenes has also been studied [25]. Yields varied from moderate at room temperature to good at 90 °C (Reaction 5.20). The shielding effect of substituents X is the major contribution for the observed stereoselectivity. 

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Radical-mediated silyldesulfonylation of various vinyl and (a-fluoro)vinyl sulfones 21 with (TMSlsSiH (Reaction 25) provide access to vinyl and (a-fluoro)vinyl silanes 22. These reactions presumably occur via a radical addition of (TMSlsSi radical followed by /)-scission with the ejection of PhS02 radical. Hydrogen abstraction from (TMSlsSiH by PhS02 radical completes the cycle of these chain reactions. Such silyldesulfonylation provides a flexible alternative to the hydrosilylation of alkynes with (TMSlsSiH (see below). On oxidative treatment with hydrogen peroxide in basic aqueous solution, compound 22 undergoes Pd-catalyzed cross-couplings with aryl halides. 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Scheme 2.6 Possible isomeric products in the hydrosilylation of alkynes (disUanes from further hydrosilylation of the alkenyl silanes are excluded)...

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

Scheme 2.8 Rationalisation of the selectivity observed in the hydrosilylation of alkynes by the complexes 61 and 62...

A variety of transition metal complexes catalyze hydrosilylation of alkynes. Catalysis of hydrosilylation by rhodium gives T -alkenylsilanes from 1-alkynes.74... 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

Complexes (65) and (66) result from the reaction of IrCl3 with inah and PPh3 (inah = isonicotinic acid hydrazide).92 Reaction of Troger s base (tb) (67) with IrCl3 yields dark violet tb 2IrCl3 (6S).93 (68) was not catalytically active towards the hydrosilylation of alkynes. 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

C-E Bond Formation through Hydrosilylation of Alkynes and Related Reactions... 

Combining two different hydrosilylation catalysts in sequence with chlorodimethylsilane has allowed the construction of complex dye assemblies and conducting polymers. In the example shown (Scheme 8), Karstedt s catalyst was chosen after a brief screen for the hydrosilylation of an aromatic diyne with chlorodimethylsilane. After reduction of the chlorosilane, an equimolar mixture of disilane B and diyne A was treated with a catalytic amount of Wilkinson s catalyst, resulting in the formation of polymer C.42 Hydrosilylation of alkynes has also been studied as a means of synthesizing oligo(phenylenevinylene) units with pendant alkoxysilanes to create curable, hole-transporting films.43,43 ... 

Within group 8, a bis-dinitrogen complex of an iron(O) tridentate pyridinediimine structure has also recently been shown to catalyze the hydrosilylation of alkyne.60 This discovery is a new example of the utility of low-valent iron in catalysis.61... 

Figure 18.12. Double hydrosilylation of alkynes to form 1,2-diols...

As with alkenes, the addition of silyl radicals to a carbon-carbon triple bond (Reaction 5.16) is also the key step in the hydrosilylation of alkynes [9,10]. 

HYdrosilylation and Dehydrogenative Silylation of Carbon- Carbon Multiple Bonds 351 Table 14.3 Hydrosilylation of alkynes. 

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