Elucidation of the Reaction Mechanisms of Hydrosilylations with Cationic Transition-Metal Catalysts

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The mechanistic understanding of chemical reactions is a fundamental requirement for their further development. We have been engaged for quite a while in the experimental elucidation of the ionic hydrosilylation of carbonyl compounds catalyzed by main-group Lewis acids and transition-metal complexes, respectively. The key insight of our preliminary work is that those hydrosilylation reactions that are promoted by cationic transition-metal catalysts do not proceed by their postulated reaction mechanisms. Our laboratory showed for two prominent cases that the neutral transition-metal hydride formed as an intermediate during the catalysis is not sufficiently hydridic to reduce the silylcarboxonium ion. Another molecule of the hydrosilane is needed to activate that transition-metal hydride either by coordination or even oxidative addition of the Si‒H bond (two-silicon rather than one-silicon cycle). Hence, more than one equivalent of the hydrosilane is required to accelerate and drive these hydrosilylations to completion. The consequences of this finding are far-reaching as it means that reported mechanisms of related hydrosilylation reactions are also likely to require partial revision. Within this project, we verify the generality of our hypothesis by systematically looking into the mechanisms of two additional representative carbonyl hydrosilylations. We hope that a universally valid mechanistic picture will evolve from this.