Heterocyclic free radicals. Part 9. The single-electron oxidation of phenothiazine and its methylated derivatives by diazonium ions

Abstract

In acidified acetonitrile the reaction of certain phenothiazines with diazonium ions leads to the formation of the corresponding phenothiazine cation-radicals. Kinetic investigation of the reactions between a series of methylphenothiazines and 4-methoxybenzenediazonium ion suggests a mechanism involving the rapid formation, at the heterocyclic nitrogen, of a cationic σ-complex which subsequently undergoes rate-determining homolysis. Correlations of the logarithms of the apparent second-order rate constants with the charge-transfer transition frequencies of the molecular π-complexes of the methylphenothiazines with 1,3,5-trinitrobenzene and with HMO coefficients of phenothiazine cation-radical itself, lead to conclusions concerning the nature of the rate-determining transition state.