N-heterocyclic carbenes(NHCs) are both a family of strong σ-donor ligands for transition metals and a privileged class of organocatalysts rival the popular amine and phosphoric acid catalysts.Early exploration of NHC catalysis was primarily centered at the biomimetic Breslow intermediate,an umpolung acyl anion equivalent.More recently,research efforts were more oriented towards uncovering hidden activation potential of functional groups beyond aldehydes.Despite numerous reports on the activation of ketenes,anhydrides,acid chlorides,esters and etc.,the key steps of NHC-catalyzed reactions always involve the nucleophilic attack of NHC to an carbonyl to generate a strong,covalently bonded intermediate.Besides nucleophilicity,NHCs are also a strong organic base,with pKa values(of their conjugate acid forms) ranging between 17 and 25 in water.Therefore,new activation mode through non-covalent interactions is also possible.However,asymmetric catalysis using this generic non-covalent mechanism has long been unsuccessful for NHCs,likely due to poor catalyst turnover and reaction reversibility.Herein,we present our recent endeavors on asymmetric nucleophilic addition reactions using NHCs as a new family of Br(o)nsted base catalysts that shows unique reactivity and broad substrate scopes for versatile bond formations.