Ning Xiaoshuang, Wang Jiandi, Wei Haiyan
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.
J Phys Chem A. 2016 Jun 23;120(24):4167-78. doi: 10.1021/acs.jpca.6b01978. Epub 2016 Jun 13.
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.
最近，一系列氧代/氮化物-Re(V)/Mo(VI)/Ru(VI)/Mn(V)配合物被证明是活化硅烷以及催化不饱和有机底物氢化硅烷化反应的高效催化剂。在本研究中，采用密度泛函理论方法对高价钼(VI) - 二氧代配合物MoO₂Cl₂催化的羰基氢化硅烷化反应进行了重新研究。先前的实验和理论研究表明，MoO₂Cl₂催化酮的氢化硅烷化反应遵循[2 + 2]加成途径。在本研究中，我们提出离子外球机理途径是最有利的途径。离子外球途径中的关键步骤是C═O键的氧原子亲核进攻η(1)-硅烷钼加合物中的硅原子。然后Si - H键发生异裂。这个过程具有一个新颖的SN2@Si过渡态，随后产生一个松散结合的离子对：在溶剂中，阴离子氢化钼与甲硅烷基碳正离子(MoO₂Cl₂H SiR₃(OCR'R″))配对。最后一步是甲硅烷基碳正离子夺取氢化钼上的氢化物生成甲硅烷基醚。对于二苯甲酮(PhC═OPh)和PhMe₂SiH硅烷，速率决定步骤的计算活化自由能垒为24.1 kcal/mol。此外，对于各种硅烷和醛/酮底物，计算得出离子外球途径比先前提出的[2 + 2]加成途径低约10.0 kcal/mol。这种偏好源于高价钼(VI)金属中心对氢化物更强的亲电性。在此，我们强调MoO₂Cl₂在活化Si - H键方面的行为类似于路易斯酸性的三(五氟苯基)硼烷B(C₆F₅)₃。
