Generation and reactivity of aromatic thioether radical cations in aqueous solution as studied by pulse radiolysis

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 x 10(3) s(-1) and k(C-S) = 1.3 x 10(3) s(-1) for 3(.+) and k(C-H) = 0.95 x 10(3) s(-1) and k(C-S) = 2.65 x 10(3) 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 x 10(7) M-1 s(-1) for 1(.+) and 9.5 x 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 degrees < 1.45 V); slower reactions were observed with SCN- (E degrees = 1.62 V) and with Br- (E degrees = 1.96 V). NO3- (E degrees = 2.3 V) is unreactive (k < 10(6) M-1 s(-1)).