DFT investigation of the mechanism and role of N-heterocyclic carbene (NHC) in constructing asymmetric organosilanes using NHC-catalyzed [4+2] cycloaddition reaction.

Herein, the mechanism and origin of stereoselectivity for the asymmetric [4+2] cycloaddition between ()-3-(-tolyl)acrylaldehyde (R1) and phenyl-3-(trimethylsilyl)prop-2-en-1-one (R2) in the presence of an N-heterocyclic carbene (NHC) were theoretically scrutinized. The desirable catalytic cycle is characterized by five steps: (1) the coupling reaction of the NHC catalyst with R1, the formation of the Breslow and enolate intermediates in the second and third steps, (4) the formal [4+2] cycloaddition reaction to form the stereoselective C-C bond, and (5) the regeneration of NHC to obtain asymmetric organosilanes. In the most energetically favorable pathway, the formation of the enolate intermediate exhibits the highest energy barrier of about 19.