Electrocatalytic Activation of Aromatic Carbon-Bromine Bonds toward Carboxylation at Silver and Copper Cathodes

The electrocatalytic activation of the carbon-bromine bond in susbstituted bromobenzenes has been studied in dimethylformamide (DMF) at Ag, Au, Cu, Fe, Ni, Pd, Pt and Zn electrodes. The electrocatalytic properties of these metals have been evaluated by comparing reduction peak potentials, Ep, of C6H5Br at such electrodes with Ep measured for the same compound at glassy carbon (GC). Only Ag, Cu and Au show remarkable electrocatalytic properties with positive shifts of Ep between 0.42 and 0.69 V with respect to GC. Ag, Cu and Au show also good catalytic activities for various substituted bromobenzenes (ArBr). The possibility of exploiting the electrocatalytic activation of ArBr for the synthesis of benzoic acids has also been investigated. In CO2-saturated DMF, Ag and Cu show high conversions of ArBr with moderate to good yields of ArCO2H. In all cases hydrodebromination to ArH is in competition with carboxylation, the relative importance of the two reactions strongly depending on the substituent as well as on the nature of the electrode material. Instead, Au does not show good selectivity for the reduction of ArBr, electroreduction of CO2 being always concomitant with, and sometimes even more favorable than, that of ArBr.