Mechanistic Insights into the Reduction of Carbon Dioxide with Silanes over N-Heterocyclic Carbene Catalysts

A metal-free reduction of carbon dioxide to methanol with N-heterocyclic carbenes (NHCs) as organocatalysts was achieved, which was conducted under ambient conditions with hydrosilane as the reducing reagent and the hydrogen source. Experimental results and density functional theory calculations indicated that the overall process involved a three-step cascade reaction. All three steps in the cascade reaction were found to be exothermic and were catalyzed by NHCs. The rate-determining step for the process was the first hydrosilylation step, which produced the formoxysilane intermediate, with an activation energy barrier of 19.6kcalmol^-1. This observation explained the high tendency of the system to form the methoxide end product, with a hydrogen-transfer yield in excess of 95%.