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

Abstract

Herein, the mechanism and origin of stereoselectivity for the asymmetric [4+2] cycloaddition between (E)-3-(p-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 19.48 kcal mol⁻¹ (Re-TS₂BA) and is the rate-determining step. The [4+2] cycloaddition reaction is the stereoselectivity-determining step forming the chiral C–C bond with RR, RS, SR and SS configurations, among which RS is the most desirable configuration. The origin of stereoselectivity was investigated using distortion energy analysis. The first and fourth steps helped in investigating the effects of electron-donating (Me) and electron-withdrawing (Cl) groups on cinnamaldehyde. Conceptual DFT (CDFT) analysis was carried out to confirm the critical role of the NHC catalyst as a Lewis base during the reaction processes.