Photochemistry of N-hydroxypyridine-2-thione derivatives: Involvement of the 2-pyridylthiyl radical in the radical chain reaction mechanism

Article abstract of DOI:10.1021/ja00143a013

The primary and subsequent photochemistry of four N-hydroxypyridine-2-thione esters has been investigated by laser flash photolysis (λ_exc = 355 nm). A simple, high-yield synthetic method is given for their preparation with high purity. UV irradiation of these ester compounds was shown to lead specifically to the formation of benzyl, diphenylmethyl, tert-butyl, and benzoyloxyl radicals in addition to the 2-pyridylthiyl radical. In all cases, the initial photoinduced nitrogen-oxygen bond cleavage was found to occur in high quantum yield (Φ_N-O ≈ 0.5). The radical species generated by this process (2-pyridylthiyl radical and carbon-centered or oxygen-centered radicals) were characterized and their reactivity toward several radical scavengers has been studied. An efficient delayed depletion of the N-hydroxypyridine-2-thione esters was also observed, leading to overall bleaching quantum yields, Φ_Bl, close to unity. We have demonstrated that the delayed consumption of ground-state ester was due to the reaction of the 2-pyridylthiyl radical with its precursor, occurring with a rate constant, k_r, of 3-4 × 10⁹ M^-1 s^-1. This reaction, hitherto never proposed, leads to the formation of 2,2′-dipyridyl disulfide and further release of propagating radicals.