Costa Paulo Jorge, Romão Carlos C, Fernandes Ana C, Royo Beatriz, Reis Patrícia M, Calhorda Maria José
Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal.
Chemistry. 2007;13(14):3934-41. doi: 10.1002/chem.200601699.
[MoCl(2)O(2)] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data. We found that the most favorable pathway to the first step, the Si--H activation, is a [2+2] addition to the Mo=O bond, in agreement with previous and related work. The stable intermediate that results is a distorted-square-pyramidal hydride complex. In the following step, the aldehyde approaches this species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged: the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving migration of a hydrogen atom to a carbon atom and of a silyl group to an oxygen atom to generate the silyl ether weakly bound to the molybdenum atom. In this Mo(VI) system, the gas-phase free energies of activation for both approaches are very similar, but if solvent effects are taken into account and HSiMe(3) is used as a source of silicon, the classical mechanism is favored. Several unexpected results led us to search for still another route, namely a radical path. The energy involved in this and the classical pathway are similar, which suggests that hydrosilylation of aldehydes and ketones catalyzed by [MoCl(2)O(2)] in acetonitrile may follow a radical pathway, in agreement with experimental results.
[MoCl₂O₂]催化醛和酮的硅氢加成反应以及其他相关基团的还原反应，这与其作为氧化催化剂的已知行为形成明显对比。在本工作中，借助使用B3LYP泛函的密度泛函理论计算并辅以实验数据，对该反应的机理进行了研究。我们发现，第一步即Si-H活化的最有利途径是对Mo=O键进行[2+2]加成，这与之前的相关工作一致。由此产生的稳定中间体是一个扭曲的四方锥氢化物配合物。在接下来的步骤中，醛接近该物种并通过氧原子进行弱配位。可以设想两条替代途径：经典还原途径，其中氢原子迁移至碳原子形成醇盐，然后继续生成最终的硅醚；或者是一种协同机理，涉及氢原子迁移至碳原子以及硅基迁移至氧原子以生成与钼原子弱结合的硅醚。在这个Mo(VI)体系中，两种途径的气相活化自由能非常相似，但如果考虑溶剂效应并使用HSiMe₃作为硅源，则经典机理更占优势。几个意外的结果促使我们寻找另一条途径，即自由基途径。这条途径和经典途径所涉及的能量相似，这表明在乙腈中由[MoCl₂O₂]催化的醛和酮的硅氢加成反应可能遵循自由基途径，这与实验结果相符。
