Unusual Chemistry in an Uncatalyzed Bromate–Aniline Oscillator: Ring-Contraction Oxidation of Aniline with Pulsative CO2 Production

The bromate-aniline oscillatory reaction was discovered 4 decades ago, but neither the detailed mechanism nor the key products or intermediates of the reaction were described. We report herein a detailed study of this reaction, which yielded new insights. We found that oscillatory oxidation of aniline by acidic bromate proceeds, to a significant extent, via a novel reaction pathway with the periodic release of carbon dioxide. Several products were isolated, and their structures, not described so far, were justified on the basis of MS and NMR. One of the main products of the reaction associated with the CO2 release route can be assigned to 2,2-dibromo-5-(phenylimino)cyclopent-3-en-1-one. A number of known compounds produced in the studied reaction, including unexpected brominated 1-phenylpyrroles and 1-phenylmaleimides, were identified by comparison with standards. A mechanism is suggested to explain the appearance of the detected compounds, based on coupling of the anilino radical with the produced 1,4-benzoquinone. We assume that the radical adduct reacts with bromine to form a cyclopropanone intermediate that undergoes a Favorskii-type rearrangement. Further oxidation and bromination steps including decarboxylation lead to the found brominated phenyliminocyclopentenones. The detected derivatives of 1-phenylpyrrole could be produced by a one-electron oxidation of a proposed intermediate 2-phenylamino-5-bromocyclopenta-1,3-dien-1-ol followed by β-scission with the abstraction of carbon monoxide. Such a mechanism is known from the combustion chemistry of cyclopentadiene. The proposed mechanism of this reaction provides a framework for understanding the observed oscillatory kinetics.