ABSTRACT
The complexes trans-[OsCl2(L){(S,S)-(i)Pr-pybox}] ((S,S)-(i)Pr-pybox = 2,6-bis[4'-(S)-isopropyloxazolin-2'-yl]pyridine, L = P(OMe)3 (1a), P(OEt)3 (2a), P(O(i)Pr)3 (3a), P(OPh)3 (4a), and cis-[OsCl2(L){(S,S)-(i)Pr-pybox}] (L = PPh3 (5a), P(i)Pr3 (6a), and PCy3 (7a)) have been synthesized from the complex trans-[OsCl2(η(2)-C2H4){(S,S)-(i)Pr-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), P(i)Pr3 (6b), and PCy3 (7b)) has been achieved from the complex trans-[OsCl2(η(2)-C2H4){(R,R)-Ph-pybox}] ((R,R)-Ph-pybox = 2,6-bis[4'-(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 (i)Pr-pybox complexes trans-[OsCl2(L){(S,S)-(i)Pr-pybox}] (L = P(OMe)3 (1a), P(OEt)3 (2a), P(O(i)Pr)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.
