Elimination reactions dehydrogenative silane

The amount of each product obtained depends on the catalyst and the nature of R and R, but the linear form generally tends to predominate. The unsaturated vinylsilane, RCH=CHSiR3, is also a product. Although minor in most cases, conditions can be found in which it predominates. The Chalk-Harrod mechanism cannot explain the formation of this dehydrogenative sU-ation product, but the alternate mechanism of Fig. 9.76 in which the alkene inserts into the M—Si bond first does explain it because 3 elimination of the intermediate alkyl leads directly to the vinylsilane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eqs. 7.21 and 7.22, a reaction in which initial insertion of alkyne into the M—Si bond must predominate (>99%). Co2(CO)g also catalyzes a number of other reactions of silanes, as shown in Fig. 9.8. 

Similar experiments have been carried out with the monosilyl substituted cyclohexane, CsSiH 63. Reactions of the silane with Fe+, Co+ and Ni+ in this case, however, did not lead to dehydrogenation, but instead to mainly ethene elimination. Labeling studies indicated a reaction mechanism as depicted in equation 5. Initially, insertion of the metal... 

Besides the reversible processes just mentioned, yS-SiRs ehmination from a silyl substituted metal alkyl or metallacycle is a well-documented process [185,186]. This reaction accounts for an easy loss of a silyl group that can be used to generate a M-S1R3 moiety in catalytic processes such as dehydrogenative silation reactions (see below). This reaction, and the analogous yS-SnRs elimination, may also be involved in the loss of regioselectivity found for some C-C coupling reactions of vinyl silanes or vinyl tin derivatives (cine substitution in Hiyama and Stille... 

Effective disproportionation and co-disproportionation of vinylsilane with ruthenium complexes containing the Ru-H, Ru-Si bond, called subsequently silylative coupling or trans-si y aiion of olefins with vinylsubstituted silanes, was revealed in 1984 as a new synthetic route to substituted vinylsilanes and are commonly used as organic reagents. Subsequent extensive synthetic and catalytic study has shown that silylative coupling of olefins with vinylsubstituted silicon compounds occurs (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing the M-Si (silicometallics) and the M-H bond (where M = Ru, Rh, Ir, Co, Fe). The insertion of olefin into M-Si bond and vinylsilanes into M-H followed by elimination of vinylsilane and ethane respectively, are the key steps in this new process. 

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