Mixture of poly(ethylene glycol) and water as environmentally friendly media for efficient enantioselective transfer hydrogenation and catalyst recycling

The asymmetric transfer hydrogenation of nonfunctionalized aromatic ketones catalyzed by Ru-TsDPEN was performed successfully in a mixture of poly(ethylene glycol) (PEG) and water. High activity and enantioselectivity were obtained, which were better than or comparable to those obtained in conventional organic solvents. The unmodified catalyst could be easily recovered after extraction of the reduced product with a less polar solvent such as hexane, and was reused at least 14 times without obvious loss in enantioselectivity. These results indicate that the inexpensive and non-toxic aqueous PEG can serve as a new means for the immobilization of Ru-TsDPEN catalyst and related variants without calling for catalyst modification. (c) 2007 Elsevier B.V. All rights reserved.