Insights into N-heterocyclic carbene and Lewis acid cooperatively catalyzed oxidative [3 + 3] annulation reactions of α,β-unsaturated aldehyde with 1,3-dicarbonyl compounds†
Unravelling the origin of stereoselectivity and chemoselectivity is one of the most challenging questions in the field of N-heterocyclic carbene (NHC) catalysis. Recently, Lewis acid (LA) additives were found to be key additives for improving the stereoselectivity in many NHC-catalyzed annulation reactions (especially [3 + 3] annulation) of dicarbonyl compounds; however, the reason for this general phenomenon remains unclear so far. In order to solve this issue, a universal model has been suggested for NHC and Lewis acid cooperatively catalyzed oxidative [3 + 3] annulation reactions between α,β-unsaturated aldehyde and 1,3-dicarbonyl compounds to explore the origin of the improved stereoselectivity. The computed results indicate that the Lewis acid can increase the free energy difference (ΔΔG‡) between the transition states involved in the stereoselectivity-determining step to improve stereoselectivity, which is mainly due to the stronger non-covalent interactions between the LA-coordinated substrate and the chiral NHC catalyst in the favorable R-isomer transition state. Furthermore, two new reactivity indices, i.e. electrophilic and nucleophilic atom energies (denoted Ea+ and Ea−), were proposed to measure the electrophilicity and nucleophilicity of possible reactive sites, which could be used to predict the chemoselective products in the [3 + 3] and [3 + 2] annulations. The obtained insights will be helpful for the rational design of organocatalytic reactions with excellent stereoselectivity and special chemoselectivity.