3-Hydroxy pyrroles and 1H-Pyrrol-3(2H)-ones. Part 4. Oxidation of 2-Monosubstituted and 2-Unsubstituted pyrrolones, with an electron spin resonance study of a dimeric intermediate

Oxidation of 2-substituted 1 H-pyrrol-3(2H)-ones in air gives 2-hydroxy derivatives, which can exist in solvent-dependent equilibrium with open-chain acyl enaminones. Oxidation of 2-unsubstitued analogues by irradiation in the presence of di-t-butyl peroxide gives rise to persistent radicals, identified as pyrrolone dimers [e.g. (21)] by e.s.r. spectroscopy. The formation of the various products is rationalised by a single mechanism (Scheme 3) in which a capto-dative radical (22) is a key intermediate.