(Keynote) Electrochemical Reduction of Bromobenzenes at Silver Cathodes

In 1959, it was observed that poly-halogenated arenes would undergo a halogen dance under very basic conditions. 1 Later, in a study of the electrochemical reduction of halogenated thiophenes at glassy carbon cathodes, an electrochemically induced halogen dance was discovered. 2 Halobenzenes were studied shortly after and it was found that 1,2,4,5-tetrabromobenzene would undergo an electrochemically induced halogen dance at glassy carbon in dimethylformamide with 0.10 M tetramethylammonium perchlorate as the supporting electrolyte. 3 In the present study, we have investigated a possible halogen dance resulting from the reduction of poly-brominated benzenes at a silver cathode for comparison with known behavior at glassy carbon cathodes. Cyclic voltammograms (CVs) of di-, tri-, and tetrabromobenzenes were recorded at a scan rate of 100 mV/s with a silver cathode in dimethylformamide containing 0.1 M tetramethylammonium perchlorate as a supporting electrolyte. All potentials are reported versus a Cd/Hg amalgam reference electrode with a potential of –0.76 vs SCE. CVs of 2 mM 1,2,4,5-tetrabromobenzene exhibit three peaks, the most positive of which is found at –0.70 V; a small shoulder is also present at –0.65 V. Controlled-potential (bulk) electrolysis of 1,2,4,5-tetrabromobenzene at –0.65 V vs. Cd/Hg yielded 1,2,4- and 1,3,5-tribromobenzene in trace amounts. Products were confirmed by means of GC–MS. The presence of 1,3,5-tribromobenzene among electrolysis products indicates the occurance of a halogen dance following the reduction of the first bromine–carbon bond. Due to similar reduction potentials for multiple carbon–bromine bonds at silver, complete removal of only a single halogen is difficult; to better isolate a single electron-transfer event, other halogenated benzenes were investigated. CVs of 2 mM 1-bromo-2,4,5-trichlorobenzene also exhibited three peaks, with the most positive at –0.50 V, separated by 800 mV from the next reduction peak. This large peak separation allows for more complete electrolysis and quantitation of halogen-dance products, aiding in development of a mechanism for the reduction of halogenated benzenes at a silver cathode. References Wotiz, J.; Huba, F. J. Org. Chem. , 1959 , 24, 595–598. Mubarak, M. S.; Peters, D. G. J. Org. Chem., 1996 , 61, 8074–8078. Mubarak, M. S.; Peters, D. G. J. Electroanal. Chem. , 1997 , 435, 47-53. Figure 1