Electron spin resonance studies of azasemiquinone free radical intermediates in the oxidation of hydroxypyridones

The oxidation of hydroxy-2-pyridones by a number of one-electron oxidants has been studied, and intermediate free radicals observed by ESR spectroscopy. In alkaline media, azasemiquinones arising from electron transfer, solvation and oxidative coupling processes have been detected. The ESR hyperfine splittings are assigned with the aid of Me substitution, and the spin densities can be understood by considering the perturbation of the N atom within the semiquinone nucleus. In acid solution, the protonation of nitrogen produces structural changes in the semiquinone nucleus, which greatly affect the spin density distributions. The apparent lifetimes of the various radical species can be correlated with their expected tendencies to direct free radical dimerisation.