Influence of magnesium(II) ions on cathodic reactions in aprotic solvents: the reduction of benzoate esters

Abstract

The use of an Mg anode in an unidivided cell alters the products from many electrosynthetic reactions. In an attempt to understand this phenomenon, the influence of Mg^II ions on the mechanism for the reduction of benzoate esters in DMF has been investigated. It is confirmed that the presence of Mg^II ions in solution leads to a change in the major reaction pathway from cleavage of the ester linkage (to give benzoate ion) to dimerisation (to diketone). The dimer is then reduced further at the same potential. Cyclic voltammetry shows that in the absence of Mg^II, the also, the ester radical anion is stable, While in the presence of Mg^II, the radical anion dimerises rapidly and the Mg^II also catalyses further reduction. Voltammetry within the synthesis cell, however, shows the much of the Mg^II formed by dissolution of the anode is tied up by anion products and that the dimerisation of the radical anion must occur well from the cathode surface.