Mechanistic Investigations and Secondary Coordination Sphere Effects in the Hydration of Nitriles with [Ru(η6-arene)Cl2PR3] Complexes

Abstract

The mechanism of the nitrile-to-amide hydration reaction using [Ru(η6-arene)Cl2(PR3)] complexes as catalysts was investigated (η6-arene = C6H6, p-cymene, C6Me6; R = NMe2, OMe, OEt, Et, iPr). Experiments showed that the mechanism involves the following general sequence of reactions: substitution of a chloride ligand by the nitrile substrate, intermolecular nucleophilic attack by water to form an amidate intermediate, and dissociation of the resulting amide. The effects of secondary coordination sphere interactions on the rates and yields of the hydration reaction were investigated. Ligands that are capable of acting as hydrogen bond acceptors with the entering water molecule result in faster rates and higher yields than non-hydrogen-bonding ligands. The faster rates are attributable to the H-bonding-facilitated deprotonation of the water as the oxygen of the water bonds to the coordinated nitrile. DFT calculations on the proposed H-bonding intermediates support this interpretation. Most homogeneous catalys...