ELECTROCHEMICAL REDUCTION OF 1,3-DIHALOPROPANES AT CARBON CATHODES IN DIMETHYLFORMAMIDE

Cyclic voltammograms for the reduction of 1,3-dibromo-, 1,3-diiodo-, 1-bromo-3-chloro-, 1-chloro-3-iodo-, and 1-bromo-3-iodopropane at glassy carbon electrodes in dimethylformamide containing tetra-n-butylammonium perchlorate exhibit single irreversible waves corresponding to two-electron cleavage of a carbon-halogen bond. Controlled-potential electrolyses of each of the five starting materials at reticulated vitreous carbon cathodes result in the formation of cyclo-propane as the predominant product; in addition, small amounts of propylene and traces of propane are observed. On the basis of these findings, along with information gained from an investigation of the electrochemical behavior of meso- and dl-2,4-dibromopentane at carbon, we conclude that the reduction of each 1,3-dihalopropane leads to a 3-haloprop-1-yl carbanion intermediate which undergoes rapid intramolecular cyclization to give cyclopropane; propylene can arise by hydroxide-promoted dehydrohalogenation of 1-halopropane or by dehydrohalogenation of starting material to afford 3-halo-1-propene which is subsequently reduced and protonated, while propane is formed via reduction of 1-halopropane followed by protonation of the resulting propyl carbanion.