中国科学院大学 School of Chemistry and Chemical Engineering

Using DFT calculations, Wang’s group gave insight into the catalytic mechanism of the first metal-free NHC(N-Heterocyclic Carbene)-catalyzed conversion of carbon dioxide into methanol. Among the various examined reaction pathways, they identified the most favorable to lead to the experimentally detected intermediates such as formoxysilane (FOS), bis(silyl)acetal (BSA), silylmethoxide (SMO), and disiloxane (DSO). However, the study also reveals that formaldehyde (CH₂O), generated from the dissociation of BSA into DSO and CH₂O via a mechanism somewhat similar to the Brook rearrangement, is an inevitable intermediate but was not reported by the experimentalists. When NHC catalyzes the reactions of CO₂/FOS/CH₂O with silane, there are two activation modes. It has been found that NHC prefers activating Si-H bonds of silane and pushing electron density to the H atoms of the Si-H bonds in favor of transferring a hydridic atom of silane to the electrophilic C center of CO2/FOS/CH2O. This holds true in particular for the NHC-catalyzed reactions of silane with FOS/CH2O to produce BSA/SMO. The preferred activation mode can operate by firstly passing an energetically unfavorable NHC-silane local minimum via Pi-Pi interactions or by directly crossing a transition state involving three components simultaneously. The activation mode via firstly coordinating NHC with the electrophilic C atom of CO₂/FOS/CH₂O is less favorable or inoperable. The predicted catalytic mechanism provides a successful interpretation of the experimental observation that phenylsilane is more efficient than diphenylsilane to perform the conversion. The related results were published by Journal of the American Chemical Society (J. Am. Chem. Soc. 2010, 132, 12388–12396).