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DOI: 10.1055/sos-SD-031-01625

Rück-Braun, K.; Dietrich, S.; Kempa, S.; Priewisch, B.Science of Synthesis, (2007311461.

Several nitroarenes react with arylamines in an aqueous alkaline medium to furnish the corresponding diaryldiazenes. The reaction has found industrial application for the preparation of azo dyes inaccessible by coupling of diazonium salts. For example, the condensation of 2,2-ethene-1,2-diylbis(5-nitrobenzenesulfonic acid) with various aromatic amino compounds has been used for the preparation of stilbene azo dyes.[‌235‌,‌236‌] Another prominent example is the synthesis of 4-[(4-aminophenyl)diazenyl]benzoic acid (119), which has found application in the synthesis of photoswitchable peptides (Scheme 54).[‌237‌‌239‌] The reaction is typically carried out in 35% aqueous sodium hydroxide at temperatures between 40 and 120°C. Electron-withdrawing groups on the nitroarene and electron-donating substituents on the arylamine enhance the reaction, while substituents in the position ortho to the nitro group hamper it. Under the reaction conditions, the amino compound is partly oxidized to the symmetrically substituted diaryldiazene 120; thus, an excess of amine should be used. N-Monoalkylated arylamines also react with nitroarenes to give diaryldiazenes with concomitant loss of the alkyl group.[‌240‌]

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