CO2 as a C1-organic building block: electrocarboxylation of aromatic ketones. A quantitative study of the effect of the concentration of substrate and of carbon dioxide on the selectivity of the process

The purpose of this work is to establish and discuss quantitative relationships between the selectivity in preparative scale electrolysis and intrinsic and operational parameters for the electrocarboxylation of aromatic ketones. For the investigated ketones, under appopriate experimental conditions the selectivity of the process is mainly determined by the competition between carboxylation and protonation, de-halogenation reactions (when an halogenated ketone is involved), and possibly dimerizations involving the electrogenerated radical anion. A simple model was proposed to account for these unwanted side paths which allows to predict the dependence of the selectivity as a function of the [CO2]/[ketone] ratio and of the proton donor concentrations. Theoretical predictions fit excellently with experimental results. In particular, a dramatic increase of the selectivity of the target hydroxy acid can be achieved by performing the electrolysis at moderately high carbon dioxide pressure.