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

摘要

Treatment of complex trans-[RuCl_2(eta~2-C_2H_4){k~3-N,N,N-(R,R)-Phpybox}][R,R)-Ph-pybox=2,6-bis{4'-(R)-phenyloxazolin-2'-yl}pyridine] with phosphines or phosphites in dichloromethane at 50 deg C leads to the formation of novel ruthenium(II)-pybox complexes trans-[RuCl_2(L){k~3-N,N,N-(R,R)-Ph-pybox}][L=PPh_3 (1a),PPh_2Me (2a),PPh_2(C_3H_5) (3a),PPh_2(C_4H_7) (4a),PMe_3(5a),PiPr_3 (6a),P(OMe)_3 (7a) and P(OPh)_3 (8a)].Likewise,reaction of trans-[RuCl_2(eta~5-C_2H_4){k~3-N,N,N-(R,R)-Ph-pybox}] with PPh_3 or PiPr_3 in refluxing methanol leads to the complexes cis-[RuCl_2(L)(k~3-N,N,N-(R,R)-Ph-pybox][L=PPh_3(1b),PiPr_3 (6a)].No trans-cis isomerisation of complexes 1a-8a has been observed.Comoplexes 1a-8a,1b,6b together with the analogues trans-[RuCl_2{P(O-Me)_3}{K~3-N,N,N-(S,S)-iPr-pybox}] (10a) and the previously reported trans- and cis-[RuCl_2(PPh_3)(k~3-N,N,N-(S,S)-iPr-pybox}] (9a and 9b,respectively) are active catalyst 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.