Dynamic exchange of heterocyclic subunits during halogen substitution in chloroheptamethinecyanine dyes by benzoazolium salts

The chlorine atom present in the exocyclic conjugated bridge incorporated in the polymethine chain of heptamethinecyanine dyes can be easily replaced by a substituted 2-methylbenzoazolium salt. This reaction is promoted by residual water present in the reaction medium. This strategy was applied to the synthesis of a heptamethinecyanine dye possessing a vinyl group in the substituting heterocycle. Surprisingly, the expected monovinyl substituted dye was obtained along with di- and trisubstituted vinyl dyes. Such mixture can be, in principle, incorporated into an organic polymer monolith and used for affinity chromatography. This substitution reaction was investigated varying the nature of the benzoazole moiety of the chloroheptamethinecyanine dye and the substituting 2-methylbenzoazolium salt. A mechanism rationalizing the substitution pattern observed is proposed.