Electrochemical reduction of 1,2,5,6,9,10-hexabromocyclododecane at carbon and silver cathodes in dimethylformamide

Electrochemical reduction of the flame retardant, 1,2,5,6,9,10-hexabromocyclododecane (HBCD), at carbon and silver cathodes has been investigated with the aid of cyclic voltammetry and controlled-potential (bulk) electrolysis in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4). Cyclic voltammograms for reduction of HBCD at a glassy carbon electrode exhibit one prominent cathodic peak and a smaller shoulder at a more negative potential. Cyclic voltammograms obtained with a silver cathode show only one peak that occurs at significantly more positive potentials than the reduction processes recorded at glassy carbon. Bulk electrolyses of HBCD at a reticulated vitreous carbon electrode held at a potential corresponding to the first cathodic peak cause complete conversion of HBCD to 1,5,9-cyclododecatriene (CDT) in a six-electron process, whereas electrolysis at the more negative peak potential affords predominantly cyclododeca-1,5-dien-9-yne (CDY, 66%), along with CDT (21%). Addition of a proton donor (diethyl malonate) alters the product distribution to favor CDT (79%) over CDY (11%). Bulk electrolysis of HBCD at a silver gauze cathode yields 97% CDT with a coulometric n value of approximately six; however, unlike the results for carbon cathodes, the efficiency of debromination decreases with increased concentrations of HBCD. Pentabromocyclododecene, tetrabromocyclododecene, tribromocyclododecadiene, dibromocyclododecadiene, and bromocyclododecatriene have been identified as intermediates in the reduction of HBCD.