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 R k ). 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.