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.