Lactim-lactam tautomeric interconversion mechanism in water-polar aprotic solvent water systems. II. Hydration of 2-hydroxypyridines. Evidence for a bifunctional water-catalyzed proton transfer

In previous work, lactim-lactam tautomeric interconversion of 2-hydroxypyridines has been shown to partly involve in aqueous solution a nondissociative proton-transfer mechanism, whereas, in aprotic solvents, the interconversion occurs totally within a cyclic dimer. Therefore, in order to determine whether water could also catalyze the tautomeric interconversion within a water/substrate association, temperature-jump experiments were performed in the water-propylene carbonate solvent system. Indeed, evidence is presented which suggests that the tautomeric interconversion partly involved a bifunctional water-catalyzed proton transfer. A combination of kinetic and uv spectral data indicates the formation of stoichiometric hydrates which inhibit the substrate dimerization. A deuterium kinetic isotope effect confirms the dimerization step to be rate-encounter controlled. 4 figures, 3 tables