Facile synthesis of C₂-symmetric tridentate bis(thiazoline) and bis(oxazoline) ligands and their application in the enantioselective Henry reaction

A series of novel C₂-symmetric bis(thiazoline) ligands with a diphenylamine backbone as a linkage between two thiazoline rings were synthesized by the use of the simple reagent phosphorus pentasulfide. Their application in the catalytic asymmetric Henry reaction of α-keto esters was investigated with comparison to the corresponding bis(oxazoline) ligands. Cu(II)-bis(oxazoline) complexes furnished moderate enantioselectivities (up to 60% ee), while Cu(II)-bis(thiazoline) complexes gave higher enantioselectivities (up to 70% ee) with neat nitromethane. The enantioselectivity was improved when a halogenated solvent, such as CH₂Cl₂ was used (up to 82% ee), but the yield obtained lower than that in neat reactions.