A new C2-symmetric azolium compound for Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

A new chiral N-heterocyclic carbene (NHC) ligand was designed. Thus, an efficient synthetic route to C2-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 Bu2Zn at room temperature.

Graphical abstractA new chiral N-heterocyclic carbene ligand precusor such as bis(hydroxyamide)-functionalized benzimidazolium salt was designed and synthesized. The conjugate addition reaction of cyclic enone with dialkylzinc catalyzed by Cu(OTf)2 in the presence of the azolium salt gave the corresponding adduct with good to excellent enantioselectivity.Figure optionsDownload full-size imageDownload as PowerPoint slide