Regiochemical Aspects of the Platinum Oxide Catalyzed Hydrosilylation of Alkynes

Abstract

The platinum-catalyzed hydrosilylation of unsymmetrical substituted arylalkynes with various hydrosilanes was investigated and the reaction selectivity of various para-substituted substrates was compared with that of their corresponding ortho-substituted derivatives. We showed that heterogeneous platinum oxide is a very efficient catalyst for such hydrosilylations and that H-Si bond addition proceeds in a stereoselective cis-fashion. The regioselectivity was found to be under the control of the ortho-substituent rather than due to the nature of the platinum catalyst. Aryl­alkynes with an ortho-substituent provided predominantly to exclusively α-selectivity, regardless of the electronic nature of the substituent. The precise contributions of steric, electronic, and coordinative factors controlling the regioselectivity of the H-Si bond addition are discussed.

Key words

hydrosilylation - platinum oxide - vinylsilane - alkyne

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12

Treatment of 2b with Et₃SiH (1.5 equiv) in the presence of H₂PtCl₆ or Pt/C (5 mol%) at 60 °C for a prolonged time (3 h) did not promote any protodesilylation of 2b, indicating that the reduced side product was formed from starting material 1b.

13

Performing the hydrosilylation of 10a at 60 °C led to a mixture of α- (10%) and β-isomers (23%) together with the protodesilylated product (67%).