Masayuki Morita

Electrochemical fluorination of aromatic compounds in liquid R4NF·mHF—Part III. Fluorination of trifluoromethylbenzenes

Trifluoromethylbenzene (1) has been electrochemically fluorinated on a platinum anode at 2.5 or 2.7 V vs. Ag/Ag+ in Me4NF·mHF or Et4NF·mHF (Me = CH3, Et = C2H5, m > 3.5). Approximately equivalent amounts of 2-fluoro-1-trifluoromethylbenzene (2) and 3-fluoro-1-trifluoromethylbenzene (3) were obtained as the primary fluorinated products. The product 2 was further fluorinated to 3,6,6-trifluoro-1-trifluoromethyl-1,4-cyclohexadiene (2a) and 5,6,6-trifluoro-1-trifluoromethyl-1,3-cyclohexadiene (2b). And then, 3 was further fluorinated to 3,3,6-trifluoro-1-trifluoromethyl-1,4-cyclohexadiene (3a) and 5,5,6-trifluoro-1-trifluoromethyl-1,3-cyclohexadiene (3b). Some of the 2a and 3a were converted into 2,5-difluoro-1-trifluoromethylbenzene (4) by dehydrofluorination during the electrolysis. Resulting 4 was further fluorinated to 3,3,6,6-tetrafluoro-1-trifluoromethyl-1,4-cyclohexadiene (4a). The reaction behavior of 2b and 3b was complex, and various kinds of fluorinated products were obtained from them by fluorination to fluoro-cyclohexenes and by dehydrofluorination to 2,3-difluoro-1-trifluoromethylbenzene (5). As a result, the fluorination of 1 formed electrochemically stable cyclohexadienes, 2a, 3a and 4a, as the final major products with 60–70% of the total yields.