Redox chemistry of trifluoromethyl sulfonium salts as CF3 radical sources

Abstract

The electrochemical behaviour of Umemoto reagent 5 and the sulfonium-based Shibata reagents 7, 8, 12 and 13 was investigated in anhydrous acetonitrile, N,N-dimethylformamide and methanol using cyclic voltammetry. In MeCN, all reagents are reduced in two distinct steps with the first reduction occurring at low cathodic potentials between −0.49 and −0.72V on a glassy carbon electrode. This event probably corresponds to the cleavage of the SCF3 bond with formation of the trifluoromethyl radical. In contrast to the Umemoto reagent 5, the Shibata reagents 7, 8, 12 and 13 were found to be unstable in N,N-dimethylformamide (in the absence of electrochemical or photochemical activation), giving either the ring-opened alkyne 14 for Shibata reagent 7 or the corresponding 2-substituted benzothiophene derivatives 15–18 for reagents 8, 12 and 13. In MeOH, the Umemoto reagent 5 and Shibata reagent 7 underwent irreversible reductions at more cathodic potentials than observed in MeCN and DMF; the Shibata reagent 7 was unstable in this solvent. The electrochemical data obtained in this study should be of general interest for those interested in the development of trifluoromethylation processes carried out under photoredox and electrochemical conditions.