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

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 dm ³ 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.