Asymmetric transfer hydrogenation of prochiral ketones catalyzed by chiral ruthenium complexes with aminophosphine ligands

The condensation of (S)-propane-1,2-diamine with two equivalents of o-(diphenylphosphino)benzaldehyde gives (S)-N,N'-bis[o-(diphenylphosphino)benzylidene]propane-1,2-diamine [(S)-1] ligand. The reduction of (S)-1 with excess NaBH(4) is carried out in refluxing ethanol to afford corresponding (S)-N, N'-bis[o-(diphenylphosphino)benzyl]propane-1,2-diamine [(S)-2]. The interaction of trans-RuCl(2)(DMSO)(4) with one equivalent of (S)-1 or (S)-2 in refluxing toluene gives (S)-3 or (S)-4 in good yield, respectively. (S)-1, (S)-2, (S)-3 and (S)-4 have been fully characterized by analytical and spectroscopic methods. The structure of (R)-3 has been also established by an X-ray diffraction study. Catalytic studies showed that (S)-4 as an excellent catalyst precursor for the asymmetric transfer hydrogenation of acetophenone with 90% yield and up to 91% enantiomeric excess. (C) 1999 Elsevier Science B.V. All rights reserved.