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

Abstract

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.