Changing mechanisms in the beta-cyclodextrin-mediated hydrolysis of phenyl esters of perfluoroalkanoic acids.

Abstract

The rate of hydrolysis of esters CF(3)(CF(2))(n)COOPh (1 (n = 1), 2 (n = 2), and 3 (n = 6)) was measured at pH 6.00 and at pH higher than 9.00 in the presence of beta-cyclodextrin (beta-CD). For compounds 1 and 2 the reaction rate decreases as the beta-CD concentration increases, and they show saturation effects at all pH. It is suggested that the substrate forms an inclusion complex with cyclodextrin. Analysis of the rate data allows calculation of the association equilibrium constant, K(CD), the rate constant for the reaction of the included compound, k(c), and K(TS) which is the hypothetical association equilibrium constant for the transition state of the cyclodextrin-mediated reaction. The dependence of log K(CD) and log K(TS) with the number of atoms in the chain is different. We suggest that the reactions of 1 and 2 take place with the perfluorinated alkyl chain included in the cavity, whereas the transition state for the reaction of phenyl trifluoroacetate involves a complex with the aryl ring inside the cavity. At low pH the inhibition comes from the protection of the carbonyl group toward nucleophilic attack by water. In basic pH the reaction of HO(-) as an external nucleophile is also inhibited. The cyclodextrin-mediated reaction involves the ionized OH group at the rim of the cyclodextrin cavity with poor efficiency due to an unfavorable orientation of the substrate in the complex. On the other hand, the reaction of compound 3 is strongly accelerated by cyclodextrin because the association of the substrate with cyclodextrin competes with the monomer-aggregate equilibrium and at high enough cyclodextrin concentration the main species present in solution is the complex between 3 and cyclodextrin.