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

Abstract

The complexes <i>trans</i>-[OsCl₂(L)­{(<i>S</i>,<i>S</i>)-^<i>i</i>Pr-pybox}] ((<i>S</i>,<i>S</i>)-^<i>i</i>Pr-pybox = 2,6-bis­[4′-(<i>S</i>)-isopropyloxazolin-2′-yl]­pyridine, L = P­(OMe)₃ (<b>1a</b>), P­(OEt)₃ (<b>2a</b>), P­(O^<i>i</i>Pr)₃ (<b>3a</b>), P­(OPh)₃ (<b>4a</b>), and <i>cis</i>-[OsCl₂(L)­{(<i>S</i>,<i>S</i>)-^<i>i</i>Pr-pybox}] (L = PPh₃ (<b>5a</b>), P^<i>i</i>Pr₃ (<b>6a</b>), and PCy₃ (<b>7a</b>)) have been synthesized from the complex <i>trans</i>-[OsCl₂(η²-C₂H₄)­{(<i>S</i>,<i>S</i>)-^<i>i</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 <i>trans</i>-[OsCl₂(L)­{(<i>R</i>,<i>R</i>)-Ph-pybox}] (L = P­(OMe)₃ (<b>1b</b>) and <i>cis</i>-[OsCl₂(L)­{(<i>R</i>,<i>R</i>)-Ph-pybox}] (L = PPh₃ (<b>5b</b>), P^<i>i</i>Pr₃ (<b>6b</b>), and PCy₃ (<b>7b</b>)) has been achieved from the complex <i>trans</i>-[OsCl₂(η²-C₂H₄)­{(<i>R</i>,<i>R</i>)-Ph-pybox}] ((<i>R</i>,<i>R</i>)-Ph-pybox = 2,6-bis­[4′-(<i>R</i>)-phenyloxazolin-2′-yl]­pyridine under microwave irradiation. Complexes <b>1a</b>–<b>6a</b>, <b>1b</b>, <b>5b</b>, and <b>6b</b> have been assayed as catalysts for the asymmetric transfer hydrogenation (ATH) of ketones. Among the catalysts tested, the ^<i>i</i>Pr-pybox complexes <i>trans</i>-[OsCl₂(L)­{(<i>S</i>,<i>S</i>)-^<i>i</i>Pr-pybox}] (L = P­(OMe)₃ (<b>1a</b>), P­(OEt)₃ (<b>2a</b>), P­(O^<i>i</i>Pr)₃ (<b>3a</b>), P­(OPh)₃ (<b>4a</b>)) 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