| Abstract: | The spontaneous decarboxylation of 4-nitrobenzisoxazole-3-carboxylic acid (4-NB-3-CA) was studied kinetically in dimethyl sulfoxide (DMSO) in the presence and absence of the naturally occurring bile salt surfactant, sodium cholate (NaC). The observed rate constants exhibited a sigmoidal dependence followed by a plateau over the NaC concentration range of 1.5 x 10⁻³ to 0.2 M. The observed rate constants for the decarboxylation of 4-NB-3-CA in the presence of aggregated sodium cholate in DMSO are enhanced by a factor of ca. 5 x 10⁵-fold as compared to that in pure DMSO solution. The sigmoidal dependence is explicable in terms of solubilization of 4-NB-3-CA by NaC via H-bonding and abstraction of the acidic carboxylic proton of 4-NB-3-CA by the NaC anion. Mechanisms for the NaC catalysis are postulated and discussed, in the light of previous work, in terms of both physical and chemical interactions. |
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