Chiral Copper Complexes of Phosphino Sulfenyl Ferrocenes as Efficient Catalysts for Enantioselective Formal Aza Diels−Alder Reactions of <i>N</i>-Sulfonyl Imines

Abstract

In the presence of a catalytic amount of silver perchlorate, copper(I) bromide complexes of planar chiral 1-phosphino-2-sulfenylferrocenes behave as very efficient chiral Lewis acids catalysts in the formal Aza Diels-Alder reaction of N-arylsulfonyl aldimines with electron-rich dienes (Danishefsky's and related dienes). Mixing of equimolar amounts of the readily available enantiopure ferrocenyl P,S-bidentate ligand and CuBr quantitatively affords the precatalyst Cu complex as an air-stable solid. This catalytic asymmetric procedure has a broad structural scope: aldimines of aromatic, alpha,beta-unsaturated, and even enolizable aliphatic aldehydes have been successfully used. The corresponding 2,3-dihydro-4-pyridones were obtained in good yields (57-90%) and with homogeneously high enantioselectivity (82-97% ee).