Enantioselective Catalysis. Part 156 [1]. Ruthenium Procatalysts and 2-Pyridinealdehyde/(S)-NOBIN-Derived Cocatalysts in the Transfer Hydrogenation of Acetophenone with 2-Propanol

Abstract

Several ruthenium procatalysts were tested in the transfer hydrogenation of acetophenone with 2-propanol using the chiral imine ligand (S)-2-(2-pyridinylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl and the corresponding amine (S)-2-(2-pyridinylmethylamino)-2′-hydroxy-1,1′-binaphthyl. Ru(PPh3)3Cl2 was the best procatalyst. Its triphenylphosphane ligands were crucial for the catalytic activity and take part in the chirality transfer. Triphenylphosphane removing reagents such as copper(I) chloride, TEMPO, or TMAO improved the catalytic performance to enantioselectivities up to 99% ee. The findings led to a mechanistic proposal including dissociation equilibria of triphenylphosphane and chelate ring opening of the tridentate chiral binaphthyl ligand. New ligands with an additional chiral center were synthesized and tested as cocatalysts. The nature of catalytically active intermediates was examined by MS and NMR spectroscopy.