Kinetics of intramolecular alkyl radical attack on sulfur in disulfides and thioesters

Abstract

The 4-(alkyldithio)butyl radicals 8a and c, and the 4-(phenyldithio)butyl radical 8b, generated from the corresponding esters of N-hydroxypyridine-2(1H)-thione, undergo fast exo-cyclisation by S_Hi attack at sulfur. Similarly, the 5-(alkyldithio)pentyl radical 8d undergoes 1,6-ring formation. The rate constants for cyclisation were determined by photolysis of the radical precursors in the presence of appropriate thiols. Butyl and pentyl radicals bearing ω-acetylthio or ω-benzoylthio substituents also undergo ring closure but much more slowly. The kinetics of these intramolecular S_H2 reactions are discussed and compared with those for the intermolecular attack of hexyl radicals on diphenyl disulfide and on dibutyl disulfide.