Trisubstituted vinylsilanes

Under the same conditions, the hydrosilylation of internal alkynes has been made possible with a nonclassical trans-addition of the silane which has been further used for the synthesis of trisubstituted vinylsilanes [114, 115], Another complex, the trihydride Cp RuH3(PPh3), is also able to provide selectively the internal hydrosilylation product. However, the use of chlorosilanes is here necessary in order to obtain clean reactions [116],... 

The synthesis of trisubstituted alkenes has scarcely been disclosed some exanples may be found in the work of Najera and coworkers using electron-deficient aryl sulfones 47 combined with phosphazene tScheme 19.21T The scope is even more limited for tetrasubstituted olefins as the yields are quite low. Acylsilanes afford trisubstituted vinylsilanes such as 48 mainly as the E isomers, but the stereoselectivity can be controlled by modulating the nature of the silyl group tScheme 19.22T E-configured conjugated esters and Weinreb amides have been synthesized successfully as well by Najera and coworkers starting from conveniently substituted arylsulfone 49 (Scheme 19.23). 

The [l,5]-anion relay/[2,3]-Wittig rearrangement of 3,3-bis(silyl) enol allyl ethers to vinyl bissilanes provides vinyl bissilanes, which can be transformed into trisubstituted vinylsilanes through a [l,4]-Brookrearrangement/alkylation protocol with electrophiles (Scheme 79). ... 

The reaction of the ( )-a,/J-disubstituted vinylsilane 97 with the glyoxalate 98 gives exclusively the (E)-trisubstituted product with the 2S configuration 99 (equation 52)124. 

The substrate scope of this epoxidation was subsequently investigated using a variety of olefins with a catalytic amount of ketone 1 (usually 20-30 mol%). A variety of hms-substituted and trisubstituted olefins have been shown to be effective substrates (Table 10.1),39 and the high ee obtained with hms-7-tetradecene suggests that this epoxidation is quite general for simple trans-olefins (Table 10.1, Entry 5). Various functional groups such as ethers, ketals, esters, and so on are compatible with the epoxidation conditions (Table 10.1). A variety of 2,2-disubstituted vinylsilanes... 

The catalytic inactivity of metallacarbene species e.g. Schrock catalyst [32] and Grubbs complex RuCl2(PPh3)(CHPh) in metathesis of vinyl-trisubstituted silanes and siloxanes also supports such a mechanism. This reaction is also called homo(hetero)coupling or trans-silylation of olefins with vinylsilanes. 

A GC-MS study of deuterated styrene in the preliminary stage of its reaction with vinylsilane showed the exclusive formation of a product-dv for the three vinyl-trisubstituted sila nes tested [33]. 

The mechanistic scheme presents the conventional oxidative addition— reductive elimination steps to explain the hydrosilylation. The oxidative addition of trisubstituted silanes HSiRs to a metal alkene complex (usually with d and d ° configuration) is followed by migratory insertion of alkene into the M—H bond, and the resulting metal(silyl)-(alkyl) complex undergoes reductive elimination by the Si—C bond formation and regeneration of metal alkene complex in excess of alkene. As the facile reductive elimination of silylalkane from [alkenyl-M]-SiRs species has not been well established in stoichiometric reaction, a modified Chalk-Harrod mechanism has been proposed to explain the formation of unsaturated (vinylsilane) organosilicon product, involving the alkene insertion into the metal-silyl bond followed by C—H reductive elimination (Scheme 2) (38). 

The effect of substituents at silicon on the reaction is outlined. We discuss advantages and drawbacks of cross-metathesis of vinylsilanes with olefins as a general method for the synthesis of vinyl-trisubstituted silanes and finally present a general mechanistic scheme that explains the processes taking place in the studied systems. Moreover, recent results on the cross-metathesis of a variety of (di)vinyl-substituted silanes, disiloxanes, and monovinyl-substituted siloxanes with olefins are reported [5]. 

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