Oxidation of Sulfides with Electrocatalytic P-450 Model System

Controlled potential electrolysis (CPE) of meso-tetraphenylporphyrinatomanganese (III) chloride (1 mM, 1a) at -0.4V (vs. saturated calomel electrode (SCE)) in acetonitrile containing diphenyl sulfide (100mM, 2), 1-methylimidazole (5mM), and tetrabutylammonium perchlorate (0.1 M) as supporting electrolyte with a reticulated vitreous carbon (RVC) cathode and bubbling O 2 gas, gave diphenylsulfoxide (12.6%-16.4%, 3) and diphenylsulfone (0.5%-1.5%, 4) in the presence of acetic acid/or tetramethylammonium hydroxide (5). In the absence of acetic acid or 5, compound 2 was not oxidized. The results of cyclic voltammetry and CPE at -0.4 V (vs. SCE) showed that the oxidant of 2 was an oxo-manganese (V) species which was generated from la and dissolved dioxygen by two-electron transfer and that the presence of H + was essential not only to cleave the O-O bond in the peroxo-manganese species, but also to transfer the second electron. This catalytic cycle is similar to that of P-450. The current efficiency was 79.1%. CPE of dissolved O 2 was carried out at -1.0 V in acetonitrile and superoxide ion was detected by use of an electron spin resonance spectrometer in the frozen electrolyzed solution. Addition of potassium superoxide to acetonitrile containing 1a, 1-methylimidazole and 2 gave 3 (15.6%-26.7%) and 4 (0%-2.7%) in the presence of acetic acid or 5. A similar procedure in the absence of the acid or 5 did not give 3 or 4. When the applied potential was -1.0 V, superoxide ion generated by cathodic reduction of dissolved oxygen in the electrolytic solution containing acetic acid was converted into hydrogen peroxide by the reaction with protons. The reaction of manganese (III) porphyrin with hydrogen peroxide produced an oxo-manganese (V) species, which is a strong oxidant and oxidized 2 and 3. This mechanism is similar to the shunt mechanism in the cytochrome P-450 catalytic cycle