Mechanism of the Iron(0)-Catalyzed Hydrosilylation of Aldehydes: A Combined DFT and Experimental Investigation

Hydrosilylation is a fundamental
organometallic transformation
for the reduction of alkene, carbonyl, and imine functionalities.
Herein, we use density functional theory (DFT) calculations, supplemented
by experiments, to propose and explore two possible reaction mechanisms
of iron(0)-catalyzed hydrosilylation of aldehydes: One is initiated
via hydrogen transfer from silane to benzaldehyde, and the other is
initiated via σ-bond metathesis between Si–H and Fe–O
bonds. The former mechanism is favored owing to the much lower calculated
activation energy. In addition, spin crossover is expected to occur
in this low-valent iron(0)-catalyzed reaction and is verified by different
computational methods, which indicates that this reaction should have
two-state reactivity (TSR). Hence, the reaction along the favored
pathway is proposed to take place via quintet intermediates and triplet
transition states before the generation of the final product, silyl
ether. This study will be helpful for understanding reaction mechanisms
involving iron catalysts and may provide a new insight on the design
of new iron catalysts by careful tuning of the electronic structure
of ligands.

硅氢加成反应（Hydrosilylation）是一类用于还原烯烃、羰基与亚胺官能团的基础有机金属转化反应。本文结合密度泛函理论（density functional theory, DFT）计算与实验验证，提出并探究了零价铁催化醛类硅氢加成反应的两种可能反应机制：其一由硅烷向苯甲醛的氢转移引发，其二则通过Si–H与Fe–O键间的σ键复分解启动。由于前者的计算活化能显著更低，因此被认定为更优的反应路径。此外，该低价零价铁催化反应中预计会发生自旋交叉现象，且经多种计算方法验证，表明该反应具备双态反应性（two-state reactivity, TSR）。因此，沿最优路径进行的反应，在生成最终产物硅醚之前，将经历五重态中间体与三重态过渡态阶段。本研究有助于理解铁催化剂参与的反应机制，并可为通过精准调控配体电子结构设计新型铁催化剂提供全新思路。