Amine oxidation. Part IX. The electrochemical oxidation of some tertiary amines: the effect of structure on reactivity

Abstract

The anodic oxidation of a selection of tertiary alkyl- and aralkyl-amines has been studied by iinear sweep voltammetry and controlled potential electrolysis at a glassy-carbon electrode in aqueous alkaline solution. Each amine shows only one oxidation wave. The oxidation peak potentials of a number of substituted NN-dimethylbenzylamines have been correlated with the Hammett σ values of the substituents (ρ–0.94) and the pK_a values of the amines (α 0.82). The size and magnitude of these values agree well with those from oxidations by one-electron chemical oxidants and are consistent with a potential-determining loss of one electron from the nitrogen of the amine. As expected α-deuteriation has little effect on the oxidation peak potential. Structural features in the amines that influence the ease of rehybridisation of the nitrogen (sp³→sp²) in the potential-determining step have been studied using a selection of cyclic and bicyclic amines. These show that the transition state in the potential-determining step resembles the aminium radical. The distribution of products is close to that predicted statistically showing that the product-determining α-C–H cleavage is very unselective. When coupled with a love primary isotope effect for the deprotonation this suggests that in the transition state the α-C–H bond is almost intact and that the transition state resembles the aminium radical more than the α-amino-radical.