The Introduction of OH and COOH Groups into 4H-Imidazoles: Water-Soluble Functional Dyes and Quinomethides

Abstract

Water-soluble 4H-imidazoles, which represent redox- as well as pH-switchable functional dyes, are synthesized via a novel procedure. In a smooth reaction, phthalic anhydride reacts with oxalic acid amidine 5 yielding 4H-imidazole 1a. Analogously, but in lower yields, naphthalene-1,8-dicarboxylic anhydride and 2-sulfobenzoic anhydride can be transformed into 4H-imidazoles 1b,c. 4-Hydroxybenzoic acids do not form the expected 4H-imidazoles, which possess hydroxyaryl substructures. However, in the course of the cyclization-long-range prototropism sequence, the new quinomethides 7a-c were obtained. This sequence could be adapted successfully for the synthesis of corresponding thioxo derivatives 7d. The water-soluble 4H-imidazoles 1a-c as well as quino­methides of type 7 proved to be multifunctional and switchable dyes. They show acidochromism and in addition, can be transformed into fluorescent leuco forms, which reoxidize when exposed to air.

Key words

heterocycles - carboxylic acids - quinomethides - functional dyes - tautomerism

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