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.