N-Heterocyclic Carbene/Brønsted Acid Cooperatively Catalyzed Conversions of α, β-Unsaturated Carbonyls: Hydrogen Bond Donor/Acceptor-Electrophile/Nucleophile Combination Models

Great success has been achieved in precisely constructing C–C bonds under N-heterocyclic carbene (NHC)/Brønsted acid (BA) cooperative catalysis. However, how NHC and BA co-catalyst accelerate conversions of α,β-unsaturated carbonyls remains unclear. Herein, we have systematically studied several representative NHC/BA cooperative catalysis models to help chemists better understand the nature of these reactions. The favorable models of NHC/BA cooperatively catalyzed reactions of α,β-unsaturated carbonyls were identified and confirmed by the proposed hydrogen bond donor/acceptor-electrophile/nucleophile combination rules and reactivity index (denoted as Rk). This work should be important for understanding the cooperative catalytic mechanism and also provides valuable insights for designing an efficient cooperative catalytic reaction with special selectivity.