Experimental and theoretical evidence for a concerted catalysis by water clusters in the hydrolysis of isocyanates

A kinetic and mechanistic investigation of the catalyzed hydration of isocyanates was undertaken. Both experimental and theoretical results showed that the hydrolysis reaction involves a chain of water molecules. The detailed hydration mechanism by water and water clusters (H-N=C=O + n(H2O) --> H2NCOOH + (n - 1)H2O, n = 1-3) has been modeled by ab initio methods, both in the gas phase and in aqueous solution. While two water molecules in the form of a dimer seem to play the key role in hydrating the isocyanate, a third water molecule may be needed to bridge the gap from the point of attack on the isocyanate to the water dimer and to facilitate further the hydration. In accordance with these facts, experimental results imply a second-order dependence on water during its nucleophilic addition to phenyl isocyanate, over a wide concentration range. In this specific case, water oligomers higher than the dimer seem to make no appreciable contribution to the rate of the hydrolysis reaction. The nucleophilic addition occurs in a concerted way across the N=C bond of the isocyanate rather than across the C=O bond. This preferential reaction mechanism could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks. Although a charge separation occurs in the transition state, electrostatic solvent effects are not quite important in reducing only marginally the energy barriers.