Influence of the nature of the substrate and of operative parameters in the electrocarboxylation of halogenated acetophenones and benzophenones

The electrocarboxylation of halogenated acetophenones and benzophenones to the corresponding hydroxycarboxylic acids has been carried out in undivided cell equipped with aluminium sacrificial anode and using 1-methyl-2-pyrrolidinone (NMP) as the solvent. The radical anion generated by the electro-reduction of the aromatic ketone is involved in several competitive reactions which lead to the formation of the target hydroxycarboxylic acid, the corresponding alcohol and pinacol and the de-halogenated parent ketone. If sufficiently negative working potentials are imposed, the latter is reduced to the corresponding carboxylate, pinacol and alcohol. Very different results in terms of selectivity and faradic efficiency in the target hydroxycarboxylic acids were obtained when the substrate was changed. Higher selectivities were obtained with chlorobenzophenones with respect to the homo-substituted chloroacetophenones as a result of a less relevant formation of dehalogenated compounds, alcohols and pinacols. The cleavage of the carbon–halogen bond was, moreover, more favoured by changing from meta to para to ortho isomer and from fluoro to chloro to bromo derivatives. The performances of the process have been found to be strongly dependent on the adopted values of temperature, substrate concentration and working potential.