Mechanistic Insights into Grubbs-Type Ruthenium-Complex-Catalyzed Intramolecular Alkene Hydrosilylation: Direct σ-Bond Metathesis in the Initial Stage of Hydrosilylation

Grubbs-type ruthenium-complex-mediated intramolecular alkene hydrosilylation of alkenylsilyl ethers has been developed to provide cyclic silyl ethers with high regioselectivity. This non-metathetical use of such ruthenium complexes for alkene hydrosilylation via preferential Si-H bond activation over alkene activation is notable, where the competing alkene metathesis dimerization was not detected. In addition to the synthesis of organosilicon heterocycles from readily available olefins, this study provides fundamental mechanistic insights into the non-metathetical function of Grubbs-type ruthenium catalysts. In the initial stage of hydrosilylation within a ruthenium coordination sphere, evidence for activation of a ruthenium complex by direct sigma-bond metathesis between Si-H and Ru-Cl via a four-centered transition state is presented. This study counters the traditionally accepted Chauvin type mechanism, specifically the addition of R3Si-H across the pi-bond of a Ru-benzylidene.