Syn addition hydrogenation

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. 

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