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Generation and Reactivity of Aromatic Thioether Radical Cations in Aqueous Solution As Studied by Pulse Radiolysis

1997; American Chemical Society; Volume: 101; Issue: 16 Linguagem: Inglês

10.1021/jp970035u

1520-5215

Marcella Ioele, Steen Steenken, Enrico Baciocchi,

Pharmaceutical and Antibiotic Environmental Impacts

The radical cations of thioanisole (1), p-methylthioanisole (2), and the benzyl phenyl sulfides 3−5 have been produced by pulse radiolysis in aqueous solutions, using SO4•- or Tl2+ as oxidizing species. The radical cations 1•+−5•+ exhibit very similar UV spectra, with strong absorptions between 300−350 and 500−600 nm. In contrast to aliphatic thioether radical cations, 1•+−5•+ do not undergo dimerization (via formation of a three-electron bond with the parent thioethers). In the absence of bases, 1•+ is a long-lived species with a lifetime >30 ms, whereas 3•+, 4•+, and 5•+ decay rapidly by both C−S bond and C−H bond cleavage with kC-H = 1.3 × 103 s-1 and kC-S = 1.3 × 103 s-1 for 3•+and kC-H = 0.95 × 103 s-1 and kC-S = 2.65 × 103 s-1 for 4•+. In the presence of OH- or HPO42-, also 1•+ undergoes a deprotonation process, with a rate larger than those of the benzyl phenyl sulfide radical cations. For example, the rate constant for the OH--induced deprotonation is 3.4 × 107 M-1 s-1 for 1•+and 9.5 × 106 M-1 s-1 for 3•+. Thioanisole radical cation 1•+ was also produced by reduction of thioanisole sulfoxide. Under these conditions, it was possible to study the reaction of 1•+ with a number of nucleophiles or electron donors. It was found that 1•+ reacts with I-, N3-, PhS-, PhSH, Br-, and SCN- by an electron transfer mechanism, producing the oxidized form of the nucleophile. This reaction is diffusion controlled with the first four nucleophiles, which are more easily oxidized than 1 (E° < 1.45 V); slower reactions were observed with SCN- (E° = 1.62 V) and with Br- (E° = 1.96 V). NO3- (E° = 2.3V) is unreactive (k < 106 M-1 s-1).