Enantioselective transfer hydrogenation of aryl ketones: synthesis and 2D-NMR characterization of new 8-amino-5,6,7,8-tetrahydroquinoline Ru(II)-complexes

Chiral (R)-(-) and (S)-(+)-8-amino-5,6,7,8-tetrahydroquinoline, hereafter defined (R)-CAMPY and (S)-CAMPY, have proved to be valuable chiral ligands in Ru(II) precatalysts for the reduction of prochiral ketones by asymmetric hydrogen transfer AHT. These ligands bear chiral information on an alkyl ring fused to a pyridine and therefore have a flat nature, characterized by a high conformational rigidity. It represent the first application in AHT of a member of a family of ancillary ligands already known as starting material for medicinal compouds. The reaction with [RuCl2PPh3] furnished the (OC-6-14)-[RuCl2(PPh3)2(CAMPY)] which evolved to (OC-6-42-C)- [RuCl2(PPh3)2(CAMPY)] in refluxing toluene. The absence of a suitable crystal for X-ray structure was easily overcame by a combination of routines 2D-NMR experiments based on an unprecedent 4-bond P-H coupling. The formation of (OC-6-42-C)- [RuCl2(PPh3)2(CAMPY)] is totally stereoselective at the chiral metal center; the same behavior is followed when triphenyl phosphines are displaced by chelating diphosphines like Ph2P(CH2)4PPh2 (DPPB) and Ph2P(CH2)3PPh2 (DPPP). The flat and rigid nature of CAMPY alone gave catalyst able to reduce acetophenone with moderate e.e.; substitution of triphenylphosphines in the complex with an achiral chelating diphosphines produce Ru-catalysts witch gave enantioselectivities up to 96%.