Electron spin resonance spectra of radicals derived from tropolones and benztropolones

Abstract

The oxidation of tropolones and benztropolones such as purpurogallin by cerium(IV) in a flow system yields radicals which can be characterised by means of e.s.r. spectroscopy. The spin distribution can be related to non-bonding orbital coefficients in model compounds. In alkaline solutions 1′,2′-dihydroxy-3,4-benztropolone and its derivatives can be autoxidised to corresponding semiquinones from which intense e.s.r. spectra can be obtained. Secondary radicals may also be formed when the 4′-position is vacant. The tropolone analogue of p-benzosemiquinone can be observed in hexamethylphosphoramide, but ring contraction takes place too quickly for this to be detected in aqueous solution.