Electroreduction of dibromobenzenes on silver electrode in the presence of CO₂

Electroreduction behavior of three dibromobenzenes (1) has been investigated by cyclic voltammetry on glassy carbon and silver electrodes in DMF in the absence and presence of CO₂. For meta- and para-dibromobenzenes, two successive two-electron reduction peaks could be detected, attributed to sequential cleavage of two reductive C-Br bond, while most of ortho-dibromobenzene underwent four-electron transfer reduction simultaneously. Related to distinguished electroreduction behaviors of three dibromobenzenes, distinct carboxylation rules were indicated. Both meta- and para-dibromobenzenes generally gave mono-carboxylation product of methyl bromobenzoate (3), while dehalogenated carboxylation product of methyl benzoate (2) was mainly obtained for the ortho-dibromobenzene. Moderate to good electrocarboxylation yields (49-74%) were obtained by preparative electrolyses in an undivided cell equipped with Mg rod as the sacrificial anode in the presence of CO₂. Among the investigated electrode materials such as Ag, Cu, Ti and Ni, both cyclic voltammetry and confirmative electrolysis revealed the catalytic ability of Ag in the electroreduction and electrocarboxylation of dibromobenzenes.