KINETICS AND MECHANISM OF THE HYDROLYSIS OF SUBSTITUTED PHENYL BENZOATES CATALYZED BY THE o-IODOSOBENZOATE ANION

Article abstract of DOI:10.1002/poc.610081003

The o-iodosobenzoate (IBA^-)-catalyzed hydrolysis of the following esters was studied spectrophotometrically: 4-nitrophenyl 4-X-benzoate (series I, a-e, X = CH3, H, Cl, CN, NO2), 4-Y-phenyl 4-nitrobenzoate (series II, a-d, Y = CH3, H, Cl, CN), 2,4-dinitrophenyl 4-X-benzoate (series III, b-e, X = H, Cl, CN, NO2) and 4-Y-phenyl 2,4-dinitrobenzoate (series IV, a-e, Y = CH3, H, Cl, CN, NO2).Direct detection of a reaction intermediate, viz 1-(4-nitrobenzyloxy)-1,2-benzodoxol-3(1H)-one, the absence of (general base) catalysis by the leaving group, the small kinetic solvent isotope effect and the fast catalyst 'turnover' show that the mechanism of catalysis by IBA^- is nucleophilic.Catalytic rate constants, activation parameters and application of the Hammett equation show the following mechanistic features: the first step of the reaction, i.e. that leading to the formation of the above-mentioned intermediate is irreversible, the leaving group is the phenol and the rate-limiting step is the collapse of the tetrahedral intermediate that is formed by attack of IBA^- on the ester.A comparison of the results of hydrolysis by IBA^- and by imidazole (Imz) revealed that IBA^- and Imz differ in their sensitivity toward an ester structure and, for the same ester series, IBA^- is a much more efficient catalyst.The reasons for the higher catalytic activity of IBA^- include nucleophilicity enhancement due to the α-effect, a smaller influence of steric effects and a more favorable entropy of activation.