Vinyl-radical rearrangements. E.s.r. evidence for the formation of enol radical cations by the protonation at carbon of β-hydroxyvinyl radicals and for a 1,4-hydrogen shift in some β-sulphur-substituted analogues

Abstract

E.s.r. results reveal the occurrence of acid-catalysed conversion of β-hydroxyvinyl radicals (from the reaction of ˙OH and alkynes) into carbonyl-conjugated radicals in aqueous solution [e.g. RC(OH)CR˙ into RC(O)ĊHR, R = alkyl, hydroxyalkyl]; it is suggested that the reaction involves the rapid protonation of the vinyl radical at carbon (k 5 × 10⁶ dm³ mol^–1 s^–1) followed by deprotonation of the intermediate enol radical cation. The ease of hydrogen-atom abstraction by the vinyl radical itself from parent alkyne (k 1.6 × 10⁷ dm³ mol^–1 s^–1 for but-2-yne-1,4-diol) is also noted.

Further examples of 1,4-hydrogen shifts are reported for β-sulphur-substituted vinyl radicals in which a capto-dative stabilized radical can thereby result.