Hydrosilylation of phenylacetylenes

Complexes 59 and 60 catalyse the hydrosilylation of phenylacetylene (but not other terminal alkyl alkynes) with HSi(Me)jPh. Generally, the Rh analogue is more active than the relative Ir. Both catalysts gave mixtures of all regioisomers, with a preference for the p-Z-isomer, in contrast to what has been reported with other non-NHC cationic complexes of Rh, where the p-f isomers are predominating. Here also the exact nature of the catalytic species is unclear [48],... 

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

K over the range 298-398 K. Partition eoeffieients for HFeCU between water and 1-octanol or dibutyl ether depend markedly on HCl and LiCl concentrations. For dibutyl ether as organic phase there is also a dependence of partition coefficient on Fe " " concentration as there is significant polynuclear complex formation in the ether layer. [FeCU] , in the form of its Et3BzN" salt, acts as a bifunctional or phase transfer catalyst for hydrosilylation of phenylacetylene. ... 

A successful study of non-phosphine iridium complexes Ir", Ir , and Ir e. g., IrX(cod)2 [60], IrH2(triso)(SiMePh2)2 [61, 62], Ir(triso)(coe)2 (coe = cyclooctene triso = tris(diphenyloxophosphoranyl)methanide), Ir(triso)(C2H4)2 [61], has demonstrated effective hydrosilylation of alkenes and alkynes. Iridium phosphine complexes, e. g., Ir(C=CPh)(CO)2PCy2 [63] and IrCl(CO)(PPh3)2 [64], are also found to be active for hydrosilylation of phenylacetylene and 1-hexyne. 

The hydrosilylation of phenylacetylene with phenyldimethylsilane catalyzed by RhCl(PPh3)3 or RhCl(CO)(PPh3)2 gives j3-silylstyrene with only a small amount of a-silylstyrene in excellent yield65. The stereochemistry of the reaction is found to depend markedly upon the reaction temperature and the amounts of the catalyst (equation 16). 

The intermediacy of the complex [Os(SiEt3)Cl(H2)(CO)(P Pr3)2] in the catalytic hydrosilylation of phenylacetylene was first observed by Oro and co-workers when the neutral hydride [OsHCl(CO)(P Pr3)2] was reacted with triethylsilane and the alkyne [81]. A theoretical study of silane addition across the C-C triple bond has also been reported [82] and the general scope of this reaction has been amply reviewed [17],... 

Hydrosilylation of phenylacetylene with triethylsilane catalysed by rm -chloro(car-bonyl)bis(triphenylphosphine)iridium yields the trans-2idduct 111, accompanied by traces of the cis-isomQT 278 and of the regioisomer 219. ... 

Recent examples that have utilized the hydrosilylation reaction to prepare very imique organosiloxanes are shown in equations 4 and 5. Equation 4 depicts the hydrosilylation of phenylacetylene to provide beta-vinyl silicones (84,85) and is a convenient route to high refractive index silicones. 

To further substantiate the validity of this correlation, we decided to probe the influence of the cr-donating ability of the NHC on the regioselectivity. For this purpose, the hydrosilylation of phenylacetylene by triethylsilane imder the standard conditions was performed with three catalysts possessing the same limited steric hindrance but completely different electronic properties (Figure 5.18) [32]. As can be seen, identical selectivities were obtained, despite the fundamental differences between the catalysts, thereby reinforcing our proposal that the regioselectivity of the hydrosilylation of alkynes is controlled essentially by steric parameters. It is noteworthy that the major isomer obtained in this case is the a-adduct. This is not surprising since arylacetylenes are known to lead... 

Very effective catalysts for the hydrosilylation of alkynes were developed by Lee and coworkers. They prepared a series of rhodium(I) and rhodium (III) complexes containing their PCP pincer 10 (e.g., ter-[(10)Rh(CO)Cl], jer-[(10)RhCl3]), all of which showed high activities in the hydrosilylation of phenylacetylene with HSiMe2Ph, yielding the ( )-p-silylalkenyl products in >80% selectivity at 0.1-0.001 mol% catalyst loadings [21]. 

A variety of NHC Pt complexes 82 were studied by Hor and co-workers, although, the activity and selectivities were considerably lower than for NHC-Pt eomplexes (Figure 13.13). " Complex 82 was highly selective for the formation of the a-isomer in the hydrosilylation of phenylacetylene with bis (trimethylsilyloxy)methylsilane. Bis-carbene derivative 83 selectively afforded the p-(. -isomer (25 41 15 19, p-( )/p-(Z)/a/dehydrogenative), whereas 84 behaved similarly to 82 but less efficiently. 

Peris and co-workers reported dimetallic complexes 87, where the neutral metal fragment was proposed to be the active site in the hydrosilylation of phenylacetylene or 1-hexyne (Figure 13.14). These catalysts produced a mixture of p-( ), P-(Z) and a-isomers, although the p-(Z) isomers were major (up to 5 93 2, p-( )/ i-(Z)/a) in all cases. Related 88 was also found active. In contrast with other cationic rhodium complexes, 89 produced (3-( )-vinylsi-lanes as the major product. ... 

Figure 25 Hydrosilylation of phenylacetylene or trimethylsilylacetylene with different silanes, catalyzed by di-NHC Pt(ll) complexes.

Finally, poly-NHC complexes of other metals have occasionally been employed as catalysts for hydrosilylations as well. The group of Hollis reported in 2012 on dinuclear rhodium(I) complexes with a bridging dicarbene ligand (structure 78, Fig. 26) as catalysts for the hydrosilylation of phenylacetylene with dimethylphenylsilane. Results were comparable to those previously obtained with platinum dicarbene complexes, which is in contrast to previous reports on Rh-catalyzed hydrosilylations, in which the Z-beta product is predominantly formed without production of the alpha isomer. 

Figure 26 Hydrosilylation of phenylacetylene with dimethylphenylsilane catalyzed by the dinuclear Rh(l) complex 78.

Huckaba AJ, Hollis TK, Howell TO, Valle HU, Wu Y. Synthesis and characterization of a 1,3-phenylene-bridged N-alkyl bis(benzimidazole) CCC-NHC pincer ligand precursor homobimetaUic silver and rhodium complexes and the catalytic hydrosilylation of phenylacetylene. Organometallics. 2013 32 63-69. 

Poyatos et al. investigated the activity of the N-C-chelating oxazole-NHC containing Pt complex 142 with various substrates (Figure 13.16). From phenylacetylene, the p-( )-isomer and the a-isomer were the only two reaction products (60 40 respectively). Styrene hydrosilylation led to the formation of the linear product with 85% selectivity (70% conv.). The cationic Pt" complex, 143, of multidentate NHC, was found to be an efficient catalyst precursor for hydrosilylation of alkynes, and only a- and p-( ) regioisomers were produced (Figure 13.16). The water-soluble Pt complex 144, bearing sulfonated NHC, was an active and recyclable catalyst for the hydrosilylation of phenylacetylene and other alkynes at room temperature in water. ... 

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