The Mechanism of Formation of Phenosafranine Dyes

The formation of phenosafranine dyes, from the oxidation of mixtures of p ‐diamines and arylamines, is shown to be a seven‐step process. It entails oxidation of the p ‐diamine to the p ‐benzoquinonedi‐imine, which, as its conjugate acid, reacts with the arylamine to give a diarninodiphenylamine. Rapid oxidation of this gives an indamine that reacts, as its conjugate acid, with a second molecule of the arylamine to give a tetramino compound that is rapidly oxidised to the corresponding arylaminoindamine. The latter compound, in the form of its conjugate acid, undergoes intramolecular cyclisation, followed by rapid oxidation, to give the phenosafranine. In dilute solution (˜10 ‐4 m), all but the four oxidation steps are slow and proceed at comparable rates, whereas, under preparative conditions, the arylamine addition and the intramolecular cyclisation are the slow steps.