Catalysis by transition metal complexes of alkene silylation - recent progress and mechanistic implications

摘要

Many efficient stereo- and regio-selective methodologies for synthesis of substituted vinylsilanes have been reported, involving mostly classical stoichiometric routes as well as, more recently, transition metal catalyzed transformations, in particular, hydrosilylation of alkynes and hydrogenation of silylalkynes. In the last two decades, two new reactions based on transformations of silicon derivatives catalyzed by transition metal complexes have been developed to provide universal routes for synthesis of well-defined molecular and macromolecular compounds with vinyl-silicon functionality commonly used in organic and polymer synthesis. These are the reactions of silylation of alkenes by hydrosilanes and by vinylsilanes. Since the synthetic aspects of these reactions have been recently discussed, this review is focused on the mechanism of homogeneous catalysis, i.e., the way of activation of C_(vinyl)—H bond of alkenes as well as ≡Si—H and/or ≡Si—C_(vinyl) of silicon derivatives by late transition metal complexes both leading to vinyl-silicon containing compounds. The two reactions occur via the same active intermediates, i.e., late TM complexes involving M—H and M—Si bonds (where M is representative at iron- and cobalt-triades: for silylation by hydrosilane also nickel-triad). On the basis of experimental and theoretical studies of the reactivity, structure and catalytic activity of late transition metal complexes in the above mentioned reactions, particularly those reported in the last decade, generalized schemes for catalysis of both reactions have been proposed. The common crucial step of the two reactions catalyzed by complexes containing initially M—H and/or M—Si bond is an insertion of alkene into M—Si bonds followed by β-H transfer to metal with elimination of substituted vinylsilane. In this case, a formation of M—Si bond proceeds either via oxidative addition of hydrosilane (silylation by hydrosilane) or via insertion of vinylsilane into M—H bond followed by β-Si transfer with evolution of ethylene (silylation by vinylsilane). Yet, if the catalysts contain initially no M—H and M—Si bond, their formation occurs via oxidative addition of hydrosilane (silylation by hydrosilane) or via preliminary oxidative addition of C_(vinyl)—H of alkene to get M—H bond, followed by insertion of vinylsilane to this bond and next β-Si transfer to metal with elimination of ethylene (silylation by vinylsilane).