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

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

1,3-Dipolar Cycloadditions of Ruthenium(II) Azido Complexes with Alkynes and Nitriles

The diazido complex [Na]­[Ru­(N₃)₂{κ³(<i>N</i>,<i>N</i>,<i>N</i>)-Tpms}­(PPh₃)] (<b>1</b>) (Tpms = tris­(pyrazolyl)­methanesulfonate) has been synthesized, and its reactivity toward dipolarophiles has been investigated. Thus, the reaction of <b>1</b> with alkynes leads to complexes with one or two triazolate ligands depending on the alkyne and the reaction conditions. Complex <b>1</b> also reacts with nitriles. Thus, the reaction with RCN (R = Me, Ph) leads to the substitution products [Ru­(N₃)­(NCR)­{κ³(<i>N</i>,<i>N</i>,<i>N</i>)-Tpms}­(PPh₃)]. However, when fumaronitrile is used, a complex containing a new κ²(<i>N</i>¹,<i>N</i>³)-5-(1,2,3-triazol-4-yl)-1,2,3,4-tetrazolate ligand is obtained as the result of two consecutive cycloaddition reactions. The mechanism for this unusual reaction has been unambiguously established through the isolation of the intermediate complex resulting from a first cycloaddition between a coordinated azide and the CC double bond