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 (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 [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, 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 [R₃S₃]⁺, consistent with earlier literature reports. A mechanism is proposed in which the trisulfide is formed by reaction of the dimer dication [Ar₄S ₄]²⁺ with neutral Ar₂S₂ to afford the trisulfide in a net 2/3 e^- process. Oxidation of Ar ₂S₂, either anodically or by a strong one-electron oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2-substituted cyclohexyldisulfides.