Chromogenic Oxazines for Cyanide Detection

We have designed two heterocyclic compounds for the colorimetric detection of cyanide. The skeleton
of both molecules fuses a benzooxazine ring to an indoline fragment and can be assembled efficiently in
three synthetic steps starting from commercial precursors. The two compounds differ in the nature of the
substituent on the carbon atom at the junction of the fused heterocycles, which can be either a methyl or
a phenyl group. In the presence of cyanide, both molecules are converted quantitatively into cyanoamines
with the concomitant appearance of an intense band in the visible region of the absorption spectrum. The
developing absorption is a result of the opening of the benzooxazine ring with the formation of a
4-nitrophenylazophenolate chromophore. Nuclear magnetic resonance spectroscopy and X-ray crystallographic analyses demonstrate that the covalent attachment of a cyanide anion to the indoline fragment
is responsible for these transformations. The chromogenic process is particularly fast for the methyl-substituted oxazine and can be exploited to detect micromolar concentrations of cyanide in water.
Furthermore, the colorimetric response of this compound to cyanide does not suffer the interference of
the halide anions, which instead are known to complicate the detection of cyanide in conventional sensing
protocols. Thus, our mechanism and compounds for the colorimetric identification of cyanide can lead
to the development of practical strategies for the convenient determination of this toxic anion in aqueous
environments.