A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R 2Zn to cyclic enone

Abstract

A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C 2-symmetric bis(hydroxyamide)-functionalized benzimidazolium salts from chiral β-amino alcohols was developed. The combination of Cu(OTf) 2 and the chiral azolium compound efficiently promoted the conjugate addition reaction of cyclic enone with dialkylzinc to give the corresponding adduct in good yield. Among a series of chiral NHC proligands, the functionalized benzimidazolium chloride possessing a tert-butyl group as a stereodirecting group was found to be the best choice of ligand. Under optimized reaction conditions, an excellent enantioselectivity (96% ee) was realized by allowing 2-cyclohepten-1-one to react with Bu 2Zn at room temperature.