Cationic rhodium complexes with chiral tetradentate ligands as catalysts for enantioselective reduction of simple ketones

Abstract

The interaction of [Rh(COD)Cl](2) with two equivalents of (S)-N,N'-bis[o-(diphenylphosphino)benzylidene]propane-1,2-diamine [(S)-1] or (S)-N,N'-bis[o-(diphenylphosphino)benzyl]propane-1,2-diamine [(S)-2] in benzene/methanol mixture and then precipitation by the addition of a solution of NH,PF, in water afforded cationic rhodium(I) complexes [Rh(S)-MeP2N2][PF6] and [Rh(S)-MeP2(NH)(2)][PF6] in good yield, respectively. Complexes [Rh(R,R)-C6P2N2][PF6] and [Rh(R,R)-C6P2(NH)(2)][PF6] were also prepared by an analogous manner. All these rhodium complexes have been characterized by analytical and spectroscopic methods and their asymmetric catalytic properties for enantioselective transfer hydrogenation of acetophenone have been tested. [Rh(R,R)-C6P2(NH)(2)][PF6] was used as an excellent catalyst precursor for enantioselective transfer reduction of acetophenone in 2-propanol, leading to 2-phenylathanol in 97% yield and in 91% ee after 7 h at 83 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.