Direct observation of a 1,4-hydrogen shift in vinyl radicals derived from the reaction of alkynes with thiyl radicals

E.s.r. experiments demonstrate that in vinyl radicals generated from thiyl radicals and alkynes, intermolecular abstraction of a thiol hydrogen (k≈ 107 dm3 mol–1 s–1) is in competition with 1,n-hydrogen shifts. A rapid 1,4-shift (k≈ 105 s–1) is shown to occur in cases where the resulting radical is stabilized by the presence of α-sulphur and α-carboxy substituents, whereas in other examples a 1,5-shift predominates.