Electrolytic-Reduction of tert-Butyl Bromide at Mercury Cathodes in Dimethylformamide

Article abstract of DOI:10.1021/jo00358a013

A polarogram or cyclic voltammogram for reduction of tert-butyl bromide at mercury in dimethylformamide containing a tetraalkylammonium salt exhibits two waves, signaling stepwise generation of the tert-butyl radical and carbanion.Electrolyses of the alkyl bromide at a mercury pool cathode and at potentials corresponding to the first wave produce isobutane (40-43percent), isobutylene (43-45percent), and 2,2,3,3-tetramethylbutane (9-10percent).Experiments with perdeuterated tert-butyl bromide have revealed that the first two products arise via disproportionation of tert-butyl radicals, whereas the third compound is formed by combination of radicals.At potentials on the second wave for tert-butyl bromide, electrolyses in dimethylformamide containing either tetramethylammonium or tetra-n-butylammonium perchlorate yield equal quantities of isobutane (47-50percent) and isobutylene (47percent), but little 2,2,3,3-tetramethylbutane (<1percent) is formed; interestingly, when tetraethylammonium perchlorate is the electrolyte, the products are isobutane (55percent), isobutylene (30percent), pivalaldehyde (14percent), and 2,2,3,3-tetramethylbutane (<1percent).Electrolyses done in the presence of deuterated reagents have demonstrated that electrogenerated tert-butyl carbanions are protonated by water and, to a lesser extent, tetramethylammonium and tetraethylammonium cations to yield isobutane.Hydroxide ion, formed when water is deprotonated by the alkyl carbanion, attacks unreduced tert-butyl bromide to give isobutylene.