Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in surfactant self assemblies

Decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion is speeded more by small assemblies of didodecyl(dimethyl)ammonium chloride (DDDACl) than by fully formed assemblies or by normal cationic micelles. The first-order rate constants, k′M, of reaction of fully micelle-bound substrate increase with decreasing surface charge density of normal cationic micelles with a change from hydrophilic to less hydrophilic counter-ions, e.g., in going from CTAOH to CTAOTos (CTA = C16H33NMe3 and Tos = toluene-p-suIphonate), or from cationic to zwitterionic micelles. These changes are ascribed to changes in transfer free energies of the initial-state carboxylate ion and the charge-delocalized transition state so that small assemblies of cationic amphiphiles, e.g., of DDDA or (C8H17)3N+R, are better catalysts than cationic micelles because of less initial-state stabilization. A similar explanation can be applied to catalysis of decarboxylation by synthetic cationic vesicles.