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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1997-04, Vol.101 (16), p.2979-2987
• Main Authors: Ioele, Marcella, Steenken, Steen, Baciocchi, Enrico
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp970035u

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 k C - H = 1.3 × 103 s-1 and k C - S = 1.3 × 103 s-1 for 3 • + and k C - H = 0.95 × 103 s-1 and k C - S = 2.65 × 103 s-1 for 4 • + . In the presence of OH- or HPO4 2-, 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).