Laser flash photolysis study of bis(1,3-benzoxazol-2-yl) disulfide and bis(1,3-benzothiazol-2-yl) disulfide; reactivities of benzoxazol-2-ylsulfanyl and benzothiazol-2-ylsulfanyl radicals

Abstract

Photo-cleavage of the S–S bond of bis(1,3-benzoxazol-2-yl) disulfide and bis(1,3-benzothiazol-2-yl) disulfide have been studied by nanosecond laser flash photolysis. The transient absorption bands at ca. 590 nm were attributed to the radical species formed by S–S bond fission. Radical addition to conjugated dienes takes place forming the S–C bond, but not the N–C bond, as evidenced by the absence of the thione group in the product, suggesting that unpaired electron density is greater on the S atom of the primary radicals formed. From the decay rates of the radicals, the addition reaction rate constants for 2-methylbuta-1,3-diene are evaluated to be 3.0 × 10⁸ and 5.0 × 10⁸ dm³ mol^–1 s^–1 in THF at 23 °C, respectively, for the benzoxazol-2-ylsulfanyl radical and the benzothiazol-2-ylsulfanyl radical. The reactivity of the latter radical is slightly higher than that of the former radical for all dienes studied. The low reactivity of these sulfanyl radicals with O₂, which is one of the characteristics of the S-centred radical, was confirmed. The rate constants for hydrogen abstraction from cyclohexa-1,4-diene are ca. 10⁶ dm³ mol^–1 s^–1, which are ca. 1/100 compared with the addition reaction to conjugated dienes. MO calculations have been performed for these radicals to reveal the factors controlling the reactivity of the radicals.