A computational study of the origin of stereoinduction in NHC-catalyzed annulation reactions of alpha,beta-unsaturated acyl azoliums

The origin of stereoselectivity of NHC-catalyzed annulation reactions of ynals and stable enols was studied with Density Functional Theory. The data suggest that the C-C bond formation is the stereo-determining step. Only the deprotonated pathway (containing an oxy-anion and overall neutral species) was found to give rise to discrimination of the competing stereoisomers. This is due predominantly to electrostatic repulsion of the beta-stabilizing enolate functionality with the pi-cloud of the aryl group in the NHC-catalyst.