Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Enantiopure Osmium(II) Pybox Complexes

The complexes trans-[OsCl2(L){(S,S)-Pr-i-pybox}] ((S,S)-Pr-i-pybox = 2,6-bis[4'-(S)-isopropyloxazolin-2'-yl]pyridine, L = P(OMe)(3) (1a), P(OET)(3) (2a), P((OPr)-Pr-i)(3) (3a), P(OPh)(3) (4a), and cis-[OsCl2(L){(S,S)-Pr-i-pybox}] (L = PPh3 (5a), (PPr3)-Pr-i (6a), and PCy3 (7a)) have been synthesized from the complex trans-[OsCl2(eta(2)-C2H4){(S,S)-Pr-i-pybox}] via substitution of ethylene by phosphites and phosphines, respectively, under toluene reflux conditions. On the other hand, the synthesis of the complexes trans-[OsCl2(L){(R,R)-Ph-pybox}] (L = P(OMe)(3) (1b) and cis-[OsCl2(L){(R,R)-Ph-pybox}] (L = PPh3 (5b), (PPr3)-Pr-i (6b), and PCy3 (7b)) has been achieved from the complex trans(OsCl2(eta(2)-C2H4){(R,R)-Ph-pybox}] ((R,R)-Ph-pybox = 2,6-bis[41-(R)-phenyloxazolin-2'yl]pyridine under microwave irradiation. Complexes 1a-6a, 1b, 5b, and 6b have been assayed as catalysts for the asymmetric transfer hydrogenation (ATH) of ketones. Among the catalysts tested, the 'Pr-pybox complexes trans-[0sC12(L){(S,S'Pr-pybox}] (L = P(OMe)(3) (1a), P(OEt)(3) (2a), P((OPr)-Pr-i)(3) (3a), P(OPh)(3) (4a)) have proven to be the most active catalysts for the reduction of a variety of aromatic ketones as nearly complete conversion and high enantioselectivity (up to 94%) are reached.