Asymmetric catalysis by chiral lanthanide complexes in water

The development of catalytic, asymmetric transformations in water is a challenging task. The lanthanides are becoming reagents of choice for many Lewis acid-catalyzed reactions in aqueous media as they are water tolerant. However, enantioselective reactions catalyzed by lanthanides are difficult to achieve in water due to the instability of the reported catalysts. Herein we report the development of stable, well-defined chiral lanthanide complexes and their effectiveness in the asymmetric reduction of alpha-keto acids in aqueous solution. This is the first. example of asymmetric reduction by a chiral lanthanide complex in water. Although modest ees are obtained (40-50%) the ytterbium complexes offer a unique advantage as they have the ability to monitor, direct from the reaction mixture, the % ee for the reaction, by H-1 NMR, through a dipolar analysis of the observed paramagnetic shift.