Studies of reactions within molecular complexes: alkaline hydrolysis of substituted phenyl benzoates in the presence of xanthines

Abstract

Complexation with caffeine and theophylline-7-acetate depresses the rate of alkaline hydrolysis of substituted phenyl benzoates and is consistent with the formation of molecular complexes with 1∶1 stoichiometry between the hosts and esters; stacking of the xanthines is excluded as an explanation in the range of concentrations studied. Brønsted-type correlations have been determined for the rate and complexation constants and for the transition-state binding constants. Development of effective charge in the transition state of the reactions in bulk solvent is slightly less than that in the host–ester complex, consistent with a similar electronic environment in both states. The negative Brønsted β values for K_S indicate that the interactions between ester and hosts involve electron donation to the host from the ester. Inhibition of hydrolysis is attributed to repulsion of the hydroxide ion from the host–ester complex by the extra hydrophobicity engendered by the xanthine host, as well as by the weaker binding of the transition state to the host compared with that in the host–ester complex.

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