New Insights into Mechanism of Molybdenum(VI)-Dioxo Complex Catalyzed Hydrosilylation of Carbonyls: An Alternative Model for Activating Si-H Bond.

Recently, a series of oxo/nitrido-Re(V)/Mo(VI)/Ru(VI)/Mn(V) complexes were demonstrated to be efficient catalysts in activating silanes and catalyzing hydrosilylations of unsaturated organic substrates. In the present study, the high-valent molybdenum(VI)-dioxo complex MoO2Cl2 catalyzed hydrosilylations of carbonyls was reinvestigated using density functional theory method. Previous experimental and theoretical investigations suggested a [2 + 2] addition pathway for MoO2Cl2 catalyzed hydrosilylations of ketones. In the present study, we propose an ionic outer-sphere mechanistic pathway to be the most favorable pathway. The key step in the ionic outer-sphere pathway is oxygen atom of C═O bonds nucleophilically attacking the silicon atom in an η(1)-silane molybdenum adduct. The Si-H bond is then cleaved heterolytically. This process features a novel SN2@Si transition state, which then generates a loosely bound ion pair: anionic molybdenum hydride paired with silylcarbenium ion (MoO2Cl2H SiR3(OCR'R″)) in solvent. The last step is silylcarbenium ion abstracting the hydride on molybdenum hydride to yield silyl ether. The calculated activation free energy barrier of the rate-determing step was 24.1 kcal/mol for diphenylketone (PhC═OPh) and silane of PhMe2SiH. Furthermore, the ionic outer-sphere pathway is calculated to be ∼10.0 kcal/mol lower than the previously proposed [2 + 2] addition pathway for a variety of silanes and aldehyde/ketone substrates. This preference arises from stronger electrophilicity of the high-valent molybdenum(VI) metal center toward a hydride. Here, we emphasize MoO2Cl2 behaves similar to Lewis acidic trispentafluorophenyl borane B(C6F5)3 in activating Si-H bond.