Multiple reaction pathways in rhodium-catalyzed hydrosilylations of ketones

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium-catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L(n)Rh(I)]+ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh-silyl intermediates [L(n)Rh(III)(H)SiHMe2]+. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the Rh-Si bond occurs, as previously proposed by Ojima et al., or into the Si-H bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the Si-H bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This "silylene mechanism" was found to have the lowest activation barrier for the rate-determining step, the migration of a rhodium-bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R2SiH2 compared to R3SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, C-D versus M-D.