Mechanistic Investigations and Secondary Coordination Sphere Effects in the Hydration of Nitriles with [Ru(eta(6)-arene)Cl2PR3] Complexes

作者：Knapp Spring Melody M; Sherbow Tobias J; Yelle Robert B; Zakharov Lev N; Juliette J Jerrick; Tyler David R^*

来源：Organometallics, 2013, 32(3): 824-834.

DOI：10.1021/om301079u

The mechanism of the nitrile-to-amide hydration reaction using [Ru(eta(6)-arene)Cl-2(PR3)] complexes as catalysts was investigated (eta(6)-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 catalysts will not hydrate cyanohydrins because of the equilibrium amounts of cyanide that are present in solutions of cyanohydrins; the cyanide poisons the catalyst. Because of its increased catalytic reactivity due to secondary coordination sphere effects, the [Ru(eta(6)-arene)Cl-2(P(NMe2)(3))] catalyst gives significant yields of cyanohydrin hydration products with glycolonitrile, lactonitrile, acetone cyanohydrin, and mandelonitrile. A Taft plot showed that an increase in the steric bulk of the nitrile results in a decrease in the hydration rate, and a Hammett plot showed that electron-withdrawing groups facilitate nitrite hydration. The decrease in rate as the size of the cyanohydrin increases is likely due to both increased steric bulk and to the addition of electron-donating groups on the nitrile. The [Ru(eta(6)-arene)Cl-2(PR3)] catalysts are initially less susceptible to cyanide poisoning than other homogeneous nitrile hydration catalysts because [Ru(eta(6)-p-cymene)(CN)(Cl)(P(NMe2)(3))] forms in the presence of cyanide. The electron-withdrawing cyanide ligand facilitates nucleophilic attack of water on a coordinated nitrile in this molecule.

• 出版日期2013-2-11