A Theoretical Study on the Mechanism, Regiochemistry, and Stereochemistry of Hydrosilylation Catalyzed by Cationic Ruthenium Complexes

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Abstract
Density functional calculations have been carried out to understand the anti-addition stereochemistry and Markovnikov regiochemistry of the hydrosilylation of terminal alkynes and the endo-dig product of intramolecular hydrosilylation of homopropargyl alcohols catalyzed by cationic cyclopentadienyl-ruthenium complexes. It has been found that hydride or silyl insertion is concerted with the oxidative addition of the H−Si bond. Hydride insertion is much more favorable than silyl insertion. Such a hydride insertion nicely reproduces the observed regioselectivity, while silyl insertion would predict the opposite result. The hydride insertion leads to the formation of a η2-vinylruthenium intermediate for the reaction of acetylene or a metallacyclopropene intermediate for the reaction of propyne. In the formation of both intermediates, there is a Cα−Cβ bond rotation so that the transferring hydride becomes anti to the silyl group. This is followed by a facile reductive α-silyl migration transition state, which results in the overall anti-addition stereochemistry. The proposed mechanism also rationalizes the observed regio- and stereochemistry of the intramolecular reaction.

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