Thermoresponsive chiral salen Mn(III) complexes as efficient and reusable catalysts for the oxidative kinetic resolution of secondary alcohols in water

A series of novel chiral salen Mn(III) complexes possessing a thermoregulated phase-transfer function was prepared for the first time. The preparation involved the introduction of the "smart" poly(Nisopropylacrylamide) (PNIPAAm) group to the framework of the chiral salen Mn(III) complex by covalently connecting to one side of the 5-position in the salen ligand (complex I) or by axially grafting onto the metal center of the complex (complex 2). Characterization results suggested the presence of the PNIPAAm moiety and active sites in the complexes. The thermoresponsive PNIPAAm groups imparted inverse temperature-dependent water solubility to the PNIPAAm-based complexes, which allowed them to undergo thermoregulated phase-separable catalysis during oxidative kinetic resolution (OKR) in water. Excellent enantioselectivity (up to 96%) with high kinetic resolution efficacy (13.1) was achieved over complex 1 in the aqueous OKR of a-methylbenzyl alcohols, and this efficacy was significantly higher than that obtained over neat complex. The PNIPAAm-based complexes can also be easily recovered by simply regulating the temperature and be reused for four times without obvious loss of enantioselectivity and activity. (C) 2013 Elsevier B.V. All rights reserved.