In search of enantioselective catalysts for the Henry reaction: are two metal centres better than one?

Abstract

Catalysts for the asymmetric Henry reaction involving copper(II) complexes of the chiral Schiff bases N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(3-hydroxysalicylideneamine) (H₂1) and N,N′-(1R,2R)-(−)-1,2-cyclohexylenebis(3-ethoxysalicylideneamine) (H₂2), and H₂3, which is the reduced analogue of H₂1, have been studied. Whereas [Cu(1)] and [Cu(2)] give poor yields and enantioselectivity, [Cu(3)] produced moderate to high yields and enantioselectivities were optimal when reactions were carried out in toluene rather than a polar solvent. A significant finding is that both yield and enantioselectivity are enhanced when a second equivalent of Cu(OAc)₂ is added to the catalyst. The single-crystal structures of [Cu(3)] and [Cu(1)(H₂O)] are presented, and the host–guest interactions and molecular packing in the latter are compared with those in [Cu(2)(H₂O)].