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

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journal contribution
posted on 20.05.2013, 00:00 authored by Esmeralda Vega, E. Lastra, M. Pilar Gamasa
The complexes trans-[OsCl2(L)­{(S,S)-iPr-pybox}] ((S,S)-iPr-pybox = 2,6-bis­[4′-(S)-isopropyloxazolin-2′-yl]­pyridine, L = P­(OMe)3 (1a), P­(OEt)3 (2a), P­(OiPr)3 (3a), P­(OPh)3 (4a), and cis-[OsCl2(L)­{(S,S)-iPr-pybox}] (L = PPh3 (5a), PiPr3 (6a), and PCy3 (7a)) have been synthesized from the complex trans-[OsCl22-C2H4)­{(S,S)-iPr-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), PiPr3 (6b), and PCy3 (7b)) has been achieved from the complex trans-[OsCl22-C2H4)­{(R,R)-Ph-pybox}] ((R,R)-Ph-pybox = 2,6-bis­[4′-(R)-phenyloxazolin-2′-yl]­pyridine under microwave irradiation. Complexes 1a6a, 1b, 5b, and 6b have been assayed as catalysts for the asymmetric transfer hydrogenation (ATH) of ketones. Among the catalysts tested, the iPr-pybox complexes trans-[OsCl2(L)­{(S,S)-iPr-pybox}] (L = P­(OMe)3 (1a), P­(OEt)3 (2a), P­(OiPr)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.

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