Anodic oxidation of disulfides: Detection and reactions of disulfide radical cations

摘要

The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/[NBu_4][B(C_6F5)_4] electrolyte. Cyclic voltammetry scans of (p-RC_6H_4) _2S_2 (R = Me, 1a; R = F, 1b; R = OMe, 1c) show modest chemical reversibility for the 1~(0/+) couple (E_(1/2) values vs ferrocene: 1.04 V for 1a, 1.21 V for 1b, 0.92 V for 1c), providing the first voltammetric evidence for the radical cation [Ar_2S_2] ~+. A dimer dication, [Ar_4S_4]~(2+), is proposed as an intermediate in the formation of the electrolysis product, the trisulfide [Ar_3S_3]~+. The chemical reversibility of the one-electron oxidations of Ar_2S_2 vanishes in [PF_6]~--containing electrolytes. The radical cations of the more sterically constrained ortho-substituted analogues dimesityldisulfide (2a, E_(1/2) = 1.01 V) and bis(2,_4,6-triisopropylphenyl)disulfide (2b, E_(1/2) = 0.98 V) show less tendency to dimerize. In all cases except 2b, the bulk electrolysis product is [R_3S_3]~+, consistent with earlier literature reports. A mechanism is proposed in which the trisulfide is formed by reaction of the dimer dication [Ar_4S _4]~(2+) with neutral Ar_2S_2 to afford the trisulfide in a net 2/3 e~- process. Oxidation of Ar _2S_2, either anodically or by a strong one-electron oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2-substituted cyclohexyldisulfides.