title: New Insights into Mechanism of Molybdenum(VI)-Dioxo Complex Catalyzed Hydrosilylation of Carbonyls: An Alternative Model for Activating Si-H Bond.
abstract: 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.
authors.affiliation: Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Provincial Key Laboratory for NSLSCS, College of Chemistry and Materials Science, Nanjing Normal University , Nanjing 210046, China.
authors.forename: Xiaoshuang
authors.initials: X
authors.lastname: Ning
authors.affiliation: Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Provincial Key Laboratory for NSLSCS, College of Chemistry and Materials Science, Nanjing Normal University , Nanjing 210046, China.
authors.forename: Jiandi
authors.initials: J
authors.lastname: Wang
authors.affiliation: Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Provincial Key Laboratory for NSLSCS, College of Chemistry and Materials Science, Nanjing Normal University , Nanjing 210046, China.
authors.forename: Haiyan
authors.initials: H
authors.lastname: Wei