Disclosing the mechanism and origin of stereoselectivity of the NHC-catalyzed transformation reaction of enals with acyl azolium as a key intermediate†

α,β-Unsaturated acyl azolium has emerged as a central reactive intermediate for reaction discovery using N-heterocyclic carbene catalysis in the past decades, whereas mechanistic studies on transformations of carbonyl compounds involving α,β-unsaturated acyl azolium as a key intermediate are rare. In the present study, the mechanism of the activation/transformation of enals catalyzed by NHC is theoretically studied using the DFT method. Based on the calculations, the Michael-type addition process is the stereoselectivity-determining step with the SR-configured product being generated preferentially. Furthermore, NCI and AIM analyses have been performed to disclose the origin of stereoselectivity, by which the stronger and larger number of noncovalent interactions are key for stabilizing the low-energy transition state, thus inducing the stereoselectivity.