Investigation of the factors controlling the regioselectivity of the hydrosilylation of alkynes catalysed by trans-di-μ-hydridobis(tricylcohexylphosphine)bis(silyl)diplatinum complexes

Abstract

The hydrosilylation of terminal and internal alkynes and also of some hydroxy-alkynes catalysed by trans-di-μ-hydridobis(tricyclohexylphosphine)bis(silyl)diplatinum complexes is described. These diplatinum complexes are very efficient catalysts for such hydrosilylations; high yields (80–98%) can be obtained under mild conditions with a catalysts/reactant ratio of 10 −4–10 −5/1. Addition of hydrosilanes X 3SiH to the alkynes RCCH, RCCR and RCCR′ proceeds in a cis fashion yielding trans-RCHCHSiX 3, cis-RC(SiX 3)CHR and mixtures of the regioisomers E-RC(SiX 3)CHR′ and E-RCHC(SiX 3)R′ respectively. In the case of terminal alkynes, minor amounts of internal adducts corresponding to Markownikow addition of the hydrosilanes are also formed. The effects of variation of the reaction conditions (concentration of reactants or catalyst and temperature) upon the rate and regioselectivity of the addition were assessed. The regioselectivity increases as the asymmetry of the electron density of the acetylenic π-bonds increases; the regioselectivity is explained in terms of a qualitative molecular orbital treatment of the transition state formed in the insertion step of the alkyne into the PtH bond.