Hydrolysis of aryl hydrogen maleate esters mediated by cyclodextrins — Effect on the intramolecular catalysis

Abstract

Kinetic studies of the hydrolysis of Z-aryl hydrogen maleates (Z = H, p-CH3, m-CH3, p-Cl, m-Cl) were carried out in the presence and absence of hydroxypropyl-β-cyclodextrin (HPCD) at variable pH from 1.00 to 3.00. The reaction involves the formation of maleic anhydride as an intermediate and the rate of its formation is strongly dependent on the pH. This is because the neighboring carboxylate group is a better catalyst than the carboxylic group. The rate constant for the formation of maleic anhydride decreases as the HPCD concentration increases in a nonlinear fashion. The results were interpreted in terms of the formation of a 1:1 inclusion complex of the esters with HPCD. The neutral (HA) and anionic (A) species of the substrate have different association constants (K[Formula: see text] and K[Formula: see text]). In all cases studied,K[Formula: see text] is higher than K[Formula: see text] for the same substrate. This difference is responsible for a decrease in the amount of the anionic substrate (reactive species) in the presence of HPCD, which results in a diminution of the observed rate constant. Besides, the rate constant for the reaction of the complexed substrate is smaller than that in the bulk solution indicating that the transition state of the cyclodextrin mediated reaction is less stabilized than the anionic substrate. The values of ΔΔG‡are almost independent of the substituent on the aryl ring and range within 0.48 and 1.05 kcal mol–1(1 cal = 4.184 J). There is no correlation between KTSand the association constant of the substrate indicating that the factors stabilizing the transition state are different from those that stabilize the substrate. Key words: cyclodextrins, intramolecular catalysis, hydrolysis, inhibition.