Elucidation of the Reaction Mechanisms of Hydrosilylations with Cationic Transition-Metal Catalysts

对化学反应的机理理解是化学反应进一步发展的基本要求。我们已经从事了相当长的一段时间的实验阐明离子硅氢化羰基化合物催化的主基团路易斯酸和过渡金属配合物。我们初步工作的关键见解是，那些由阳离子过渡金属催化剂促进的硅氢化反应并不按照其假设的反应机制进行。我们的实验室在两个突出的案例中表明，在催化过程中作为中间体形成的中性过渡金属氢化物的水合性不足以还原硅碳鎓离子。需要另一个氢硅烷分子来激活过渡金属氢化物，要么是通过配位，要么是氧化添加Si-H键（双硅而不是单硅循环）。因此，需要一个以上的氢硅烷当量来加速和驱动这些氢硅化完成。这一发现的结果是深远的，因为它意味着相关的硅氢化反应的报道机制也可能需要部分修订。在这个项目中，我们通过系统地研究两个额外的代表性羰基氢化硅的机制来验证我们假设的普遍性。我们希望，一幅普遍有效的机械图景将由此演化而来。

The mechanistic understanding of chemical reactions is a fundamental requirement for their further development. We have been engaged for quite a while in the experimental elucidation of the ionic hydrosilylation of carbonyl compounds catalyzed by main-group Lewis acids and transition-metal complexes, respectively. The key insight of our preliminary work is that those hydrosilylation reactions that are promoted by cationic transition-metal catalysts do not proceed by their postulated reaction mechanisms. Our laboratory showed for two prominent cases that the neutral transition-metal hydride formed as an intermediate during the catalysis is not sufficiently hydridic to reduce the silylcarboxonium ion. Another molecule of the hydrosilane is needed to activate that transition-metal hydride either by coordination or even oxidative addition of the Si‒H bond (two-silicon rather than one-silicon cycle). Hence, more than one equivalent of the hydrosilane is required to accelerate and drive these hydrosilylations to completion. The consequences of this finding are far-reaching as it means that reported mechanisms of related hydrosilylation reactions are also likely to require partial revision. Within this project, we verify the generality of our hypothesis by systematically looking into the mechanisms of two additional representative carbonyl hydrosilylations. We hope that a universally valid mechanistic picture will evolve from this.