Asymmetric Henry Reaction Catalyzed by C₂-Symmetric Tridentate Bis(oxazoline) and Bis(thiazoline) Complexes:  Metal-Controlled Reversal of Enantioselectivity

C₂-symmetric tridentate bis(oxazoline) and bis(thiazoline) ligands with a diphenylamine backbone have been investigated in the catalytic asymmetric Henry reaction of α-keto esters with different Lewis acids. Their Cu(OTf)₂ complexes furnished S enantiomers, while Et₂Zn complexes afforded R enantiomers, both of them with higher enantioselectivities (up to 85% ee). Reversal of enantioselectivity in asymmetric Henry reactions was achieved with the same chiral ligand by changing the Lewis acid center from Cu(II) to Zn(II). The results show that the NH group in C₂-symmetric tridentate chiral ligands plays a very important role in controlling both the yields and enantiofacial selectivity of the Henry products.