New chiral cationic rhodium–aminophosphine complexes for asymmetric transfer hydrogenation of aromatic ketones

The new chiral ligands (S,S)-N,N′-bis[o-(diphenylphosphino)benzylidene]1,2-diiminocyclohexane, [(S,S)-1] and (S,S)-N,N′-bis[o-diphenylphosphino]benzyl-1,2-diaminocyclohexane, [(S,S)-2] have been prepared. The interaction of [(S,S)-1] and [(S,S)-2] with [Rh(COD)Cl]2 afforded the corresponding cationic rhodium complexes [(S,S)-3][X] and [(S,S)-4][X] (X=PF6−, BF4− or ClO4−), respectively. [(S,S)-1], [(S,S)-2], [(S,S)-3][X] and [(S,S)-4][X] have been fully characterized by elemental analyses and spectroscopic methods. These chiral cationic rhodium complexes serve as catalytst precursors for the asymmetric transfer hydrogenation of acetophenone derivatives in 2-propanol and [(S,S)-4][PF6] acts as an excellent catalyst in the reduction of m-chloroacetophenone, giving the corresponding optical alcohols in 99% yield and up to 94% ee.