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

The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/[NBu4][B(C6F5)4] electrolyte. Cyclic voltammetry scans of (p-RC6H4)2S2 (R = Me, 1a; R = F, 1b; R = OMe, 1c) show modest chemical reversibility for the 1(0/+) couple (E1/2 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 [Ar2S2](+). A dimer dication, [Ar4S4](2+), is proposed as an intermediate in the formation of the electrolysis product, the trisulfide [Ar3S3](+). The chemical reversibility of the one-electron oxidations of Ar2S2 vanishes in [PF6](-)-containing electrolytes. The radical cations of the more sterically constrained ortho-substituted analogues dimesityldisulfide (2a, E1/2 = 1.01 V) and bis(2,4,6-triisopropylphenyl)disulfide (2b, E1/2 = 0.98 V) show less tendency to dimerize. In all cases except 2b, the bulk electrolysis product is [R3S3](+), consistent with earlier literature reports. A mechanism is proposed in which the trisulfide is formed by reaction of the dimer dication [Ar4S4](2+) with neutral Ar2S2 to afford the trisulfide in a net 2/3 e(-) process. Oxidation of Ar2S2, either anodically or by a strong one-electron oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2-substituted cyclohexyldisulfides.