ELECTROCHEMICAL REDUCTION OF VINYL BROMIDES

The electrochemical reduction of triphenylbromoethylene in dimethylformamide at a mercury cathode has been studied. Isolation and characterization of the cathodic products from a controlled potential electrolysis at - 2.2 V (vs. sce) demonstrated the formation of triphenylethylene and triphenylethane. Gas chromatographic and coulometric analysis showed that at this potential triphenylbromoethylene is cleanly converted into triphenylethylene in a two-electron reduction. This is then followed by conversion of the latter into triphenylethane in a second two-electron step. Reduction at -1.8 V gave only triphenylethylene. Voltammetry with a dropping mercury electrode gave a two-wave polarogram. The half-wave potential of the second wave was the same as that of triphenylethylene, and the data is interpreted as arising from reductive cleavage of the bromide at the first wave followed by reduction of the olefin at the second. A comparison of the first wave, half-wave potentials (vs. sce) for triphenylbromoethylene (-1.60), 1, 1-diphenyl-2-bromoethylene (-1.80), α-bromostyrene (-1.86), trans-β-bromostyrene (-1.98), 1-anisyl-2,2-diphenylbromoethylene (-1.66), 2-anisyl-1,2-diphenylbromoethylene (-1.63), vinyl bromide (-2.46), and 1-methyl-2-bromopropene (>-2.6) is interpreted mechanistically. It is proposed that the rate-determining step is addition of an electron to the lowest vacant π moleclar orbital. This is probably followed by loss of bromide from the radical anion, addition of a second electron, and protonation of the resulting vinyl anion. Alternatively, protonation of the radical anion could precede loss of bromide. © 1969, American Chemical Society. All rights reserved.