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Selective Hydration of Nitriles to Amides Promoted by an Os–NHC Catalyst: Formation and X‑ray Characterization of κ2‑Amidate Intermediates

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posted on 08.10.2012, 00:00 by María L. Buil, Victorio Cadierno, Miguel A. Esteruelas, José Gimeno, Juana Herrero, Susana Izquierdo, Enrique Oñate
The complex [Os­(η6-p-cymene)­(OH)­IPr]­OTf (1; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolylidene; OTf = CF3SO3) reacts with benzonitrile and acetonitrile to afford the κ2-amidate derivatives [Os­(η6-p-cymene)­{κ2O,N-NHC­(O)­R}­IPr]­OTf (R = Ph (2), CH3 (3)). Their formation has been investigated by DFT calculations (B3PWP1), starting from the model intermediate [Os­(η6-benzene)­(OH)­(CH3CN)­IMe]+ (IMe = 1,3-bis­(2,6-dimethylphenyl)­imidazolylidene). Complex 2 has been characterized by X-ray diffraction analysis. In the presence of water, the κ2-amidate species release the corresponding amides and regenerate 1. In agreement with this, complex 1 has been found to be an efficient catalyst for the selective hydration of a wide range of aromatic and aliphatic nitriles to amides, including substituted benzonitriles, cyanopyridines, acetonitrile, and 2-(4-isobutylphenyl)­propionitrile among others. The mechanism of the catalysis is also discussed.