New chiral ruthenium(II) catalysts containing 2,6-bis(4′-(R)-phenyloxazolin-2′-yl)pyridine (Ph-pybox) ligands for highly enantioselective transfer hydrogenation of ketones

Treatment of complex trans-[RuCl2(eta(2)-C2H4){kappa(3)-N,N,N-(R,R)-Ph-pybox}] [(R,R)-Ph-pybox = 2,6-bis{4'-(R)-phenyloxazolin-2'-yl}pyridine] with phosphines or phosphites in dichloromethane at 50degreesC leads to the formation of novel ruthenium (II)-pybox complexes trans-[RuCl2(L){kappa(3)-N,N,N-(R,R)-Ph-pybox}] [L = PPh3 (1a), PPh2Me (2a), PPh2(C3H3) (3a), PPh2(C4H7) (4a) PMe3 (5a), PiPr(3) (6a), P(OMe)(3) (7a) and P(OPh)(3) (8a)]. Likewise, reaction of trans-[RuCl2(eta(2)-C2H4){kappa(3)-N,N,N-(R,R)-Ph-pybox}] with PPh3 or PiPr(3) in refluxing methanol leads to the complexes cis-[RuCl2(L)(kappa(3)-N,N,N-(R,R)-Ph-pybox] [L = PPh3 (1b), PiPr(3) (6b)]. No trans-cis isomerisation complexes 1a-8a has been observed. Complexes 1a-8a, 1b, 6b together with the analogous trans- [RuCl2{P(O-Me)(3)]{kappa(3) -N,N,N-(S,S)-iPr-pybox)] (10a) and the previously reported trans- and cis-[RuCl2(PPh3){kappa(3) -N,N,N-(S,S)-iPr-pybox}] (9a and 9b, respectively) are active catalysts for the transfer hydrogenation of acetophenone in 2-propanol in the presence of NaOH (ketone/cat/NaOH 500:1:6). cis-Ph-pybox derivatives are the most active catalysts. In particular, cis complexes 1b and 6b led to almost quantitative conversions in less than 5 min with a high enantioselectivity (up to 95%). A variety of aromatic ketones have also been reduced to the corresponding secondary alcohols with very high TOF and ee up to 94%. The overall catalytic performance seems to be a subtle combination of the steric and/or electronic properties both the phosphines and the ketones. A high TOF (27300 h(-1)) and excellent ee (94%) have been found for the reduction of 3-bromoacetophenone with catalyst 6b. Reductions of alkyl ketones also proceed with high and rapid conversions but low enantioselectivities are achieved.