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

Abstract

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 SO 4 •- or Tl 2+ 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 × 10 3 s -1 and k C - S = 1.3 × 10 3 s -1 for 3 • + and k C - H = 0.95 × 10 3 s -1 and k C - S = 2.65 × 10 3 s -1 for 4 • + . In the presence of OH - or HPO 4 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 × 10 7 M -1 s -1 for 1 • + and 9.5 × 10 6 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 -, N 3 -, 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). NO 3 - ( E ° = 2.3V) is unreactive ( k < 10 6 M -1 s -1 ).