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

Catalysts for the asym. Henry reaction involving Cu(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 analog of H21, were 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 2nd equiv. 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 mol. packing in the latter are compared with those in [Cu(2)(H2O)].