Radical addition to alkynes: electron spin resonance studies of the formation and reactions of vinyl radicals

Abstract

The e.s.r. spectra are described of a series of vinyl radicals XRCCR·(R = CO₂H, CO₂–) formed by the addition of a variety of alkyl, hydroxyalkyl, and aryl radicals (X) to butynedioic acid and the corresponding dianion. For the reaction between Prⁱ˙ and HO₂CCCCO₂H, k is estimated as ca. 2 × 10⁶ dm³ mol^–1 S^–1. In acid solution the first formed vinyl radicals react with more alkyne to give further vinyl radicals XRCCR–CRCR· which, if structurally suited, undergo a 1,5-shift to give pentadienyl radicals. The spectra of the latter [e.g. for CH₂C(CO₂H)–C(CO₂H)C(CO₂H)–ĊHCO₂H from ·CH₃ and butynedioic acid] indicate there to be considerable twisting around the C(3)–C(4) bonds so that conjugation is reduced compared to that in the fully conjugated unsubstituted pentadienyl radical.