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

The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/NBu4B(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