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 origin of stereoselectivity and chemoselectivity is one of the most challenging questions in N-heterocyclic carbene (NHC) catalysis field. Recently, Lewis acid (LA) additive was found to be key additive for improving the stereoselectivity in many NHC-catalyzed annulation reactions (especially for [3+3] annulation) of dicarbonyl compounds, however, the reason of this general phenomenon remains to be 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 origin of the improving stereoselectivity. The computed results indicate that Lewis acid can increase the free energy difference (ΔΔG⧧) between the transition states involved in stereoselectivity-determining step to improve stereoselectivity, which is mainly due to the stronger non-covalent interactions between LA-coordinated substrate and chiral NHC catalyst in favorable R-isomer transition state. Furthermore, two new indexes, i.e. nucleophilic and electrophilic atom energies (denoted as Ea+ and Ea-), were proposed to measure the electrophilicity and nucleophilicity of possible reactive sites, which can be used to predict the chemoselective [3+3] and [3+2] annulations. The obtained insights would be helpful for rational design of organocatalytic reactions with excellent stereoselectivity and special chemoselectivity.