A curve-crossing model for oxidative decarboxylation. Kinetics of anilino carboxylate fragmentations

The kinetics of oxidative decarboxylation of a series of anilino carboxylates have been measured. These compounds find use in the recently described two-electron sensitization process for silver halide photography, and control of their fragmentation kinetics is essential to be useful in this application. The measured rate constants vary over 5 orders of magnitude. The decarboxylation rate constants decrease substantially with increasing solvent polarity, an effect that results in misleading Arrhenius analysis of temperature effect measurements. The rate constants also depend on the product radical stability and the oxidation potential of the aniline precursor. A valence bond curve-crossing model is proposed that provides a combined picture of this and the related decarboxylation of acyloxy radicals. The model is able to explain the substitution and solvent polarity effects. Consistent with this model, manipulation of solvation via changes in substituents (hydrophilic vs hydrophobic) around the reaction site is shown to strongly influence the decarboxylation rate constants. In this way, the decarboxylation rate constants can be altered by nearly 3 orders of magnitude at constant solvent polarity and precursor oxidation potential.