Kinetic and theoretical studies on the mechanism of intramolecular catalysis in phenyl ester hydrolysis

The kinetic study of the hydrolysis reaction of Z-substituted phenyl hydrogen maleates (Z = H, m-CH3, p-CH3, m-Cl, p-Cl and m-CN) was carried out in aqueous solution, and the results were complemented with theoretical studies. Under some experimental conditions, two kinetic processes were observed. One of them was ascribed to maleic anhydride formation and the other to the anhydride hydrolysis. The Bronsted-type plot for the leaving-group dependence was linear with slope beta(lg) = -1. The experimental results are consistent with a mechanism that involves significant bond breaking in the rate-limiting transition state (alpha(lg) = 0.64). Theoretical results for the reaction in the gas phase showed an excellent Bronsted-type dependence with beta(lg) of -1.03. A tetrahedral intermediate (TI) could not be found through DFT gas-phase studies (B3LYP/6-311+G*). Calculations carried out within a continuous solvation model or with discrete water molecules failed to find a stable TI. With both models, a flat region on the potential-energy surface is found and a tight optimization of the structures led back to starting materials. The theoretical results do not discard the possible existence of an unstable intermediate on the free-energy surface, but the analysis of the whole body of results compared with other acyl transfer reactions lead us to suggest that an enforced concerted mechanism is the most appropriate to describe these reactions.