New Insights into Electrocatalysis and Dissociative Electron Transfer Mechanisms: The Case of Aromatic Bromides

The electrochem. redn. of arom. bromides including substituted bromobenzenes and polycyclic bromoarenes were studied in MeCN at Ag and glassy C (GC) electrodes. Whereas GC behaves as a noncatalytic electrode, Ag exhibits remarkable electrocatalytic activities for the redn. of all compds. The electrocatalytic effects are strongly influenced by the mol. structure of the arom. bromide, decreasing with increasing electron-withdrawing ability of the substituents as well as with extension of the polycyclic arom. system. Dissociative electron transfer (ET) to an org. halide RX may occur either in a single step (concerted mechanism) or in two distinct steps with the formation of an intermediate radical anion (stepwise mechanism). The mechanism of the redn. process was analyzed at both catalytic and noncatalytic electrodes. Electroredn. of all compds. at GC occurs according to a stepwise mechanism, whereas at Ag both mechanisms were obsd. depending on the structure of the mol. This study reports unprecedented examples of a passage from one dissociative ET mechanism to the other upon a change of the nature of the electrode material. The process at Ag involves adsorption of reagents, intermediates and products and is catalytic regardless of the mechanism of the dissociative ET. Anal. of the data herein reported together with literature data on the electroredn. of different types of org. halides sheds some light on the relation between catalysis and dissociative ET mechanism