Electron spin resonance studies. Part 59. Radical reactions of thiophens: the formation and reactions of some sulphur-conjugated radical-cations

Abstract

E.s.r. spectroscopy has been employed to study the mechanisms of reaction of thiophen and several of its derivatives with a variety of radicals (·OH, Cl₂^–˙, NH₃⁺˙, and SO₄^–˙) as a function of pH. The radicals detected are mainly hydroxy-adducts but also include 2-thenyl, formed from 2-methylthiophen and 2-thenylacetic acid under certain circumstances, and thenoxyl radicals. Evidence is presented that radical-cations are involved in many of the reactions; with Cl₂^–˙ and SO₄^–˙ they are formed directly, whereas in the reaction with ·OH they are formed only at low pH, via acid-catalysed elimination of OH^– from the first-formed hydroxy-adducts. Although the radical-cations are normally readily hydrated, other fates, including deprotonation and decarboxylation, have been identified. Comparisons between the results obtained for ·OH and for Cl₂^–˙ with certain substrates enable the regioselectivities of both ·OH addition and radical-cation hydration to be determined.