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The rate of decarboxylation of 6‐nitrobenzisoxazole‐3‐carboxylate ion increases sharply with increasing head group size in a series of cetyltrialkylammonium bromides (C16H33NR3Br: R = Me, CTABr; R = Et, CTEABr; R = n – Pr, CTPABr; R = n – Bu, CTBABr) with rate enhancements of 102 (CTABr) and 2·8 × 103 (CTBABr). Micellized tetradecylquinuclidinium bromide and hexadecyl‐N‐methylmorpholinium bromide are slightly better catalysts than CTABr, as is 1,3‐bis(N‐cetyl‐N,N‐dimethylamino)propane dibromide, but p‐octyloxybenzyltrialkylammonium bromides (alkyl = Me, n – Bu) are less effective than the corresponding CTA+ surfactants. These differences in catalytic efficiency depend on the head group structure and the extent to which the cationic head groups become less accessible to water rather than the overall micellar structure.

Decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion is speeded more b y small assemblies of didodecyl(dimethyl)ammonium chloride (DDDACI) than b y fully formed assemblies or b y normal cationic micelles. The first-order rate constants, FM, 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 = C, , H, , NMe, and Tos = toluene-p-sulphonate), or from cationic to zwitterionic micelles. These changes are ascribed to changes in transfer free energies of the initial-state carboxylate ion and the chargedelocalized transition state so that small assemblies of cationic amphiphiles, e.g., of D D D A or (C,H,,),N +R, are better catalysts than cationic micelles because of less initial-state stabilization. A similar explanation can be applied to catalysis of decarboxylation b y synthetic cationic vesicles.

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