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

The complex [Os(eta(6)-p-cymene)(OH)IPr]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO3) reacts with benzonitrile and acetonitrile to afford the kappa(2)-amidate derivatives [Os(eta(6)-p-cymene){kappa O-2,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(eta(6)-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 kappa(2)-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.