Galvanostatic and potentiostatic fluorination of 2-indanone, 1-indanone and 1,3-indandione in Et3N · 4HF medium. Adsorption effects on yield and product selectivity

Selective electrochemical fluorination (SEF) of 1-indanone, 2-indanone and 1,3-indandione were carried out in Et3N · 4HF ionic liquid. Cyclic voltammetric measurements indicated that the ionic liquid undergoes oxidation below 2 V on platinum electrode while the background limit is extended up to 2.3 V on glassy carbon electrode. Well defined voltammetric peaks were observed for all the three compounds on glassy carbon electrode. Significant absorption effects were also noticed. Preparative electrolysis indicated lower selectivity for all the three compounds under galvanostatic conditions when compared to potentiostatic conditions. Both under galvanostatic and potentiostatic conditions, 2-indanone, which exhibits weak adsorption, gave 1-fluoro-2-indanone with high selectivity. 1-Indanone exhibits higher anodic oxidation potential around 2 V. 1,3-Indandione exhibited strong intermediate or product adsorption during voltammetric measurements. The yield and selectivity were much lower for these two compounds under galvanostatic conditions. Quite interestingly in both these cases, the selectivity close to 70% for the monofluorinated compound could be achieved under potentiostatic conditions by passing up to 6 F/mole of electric charge. Prevention of formation of over oxidation products under potentiostatic conditions may be the main cause for this improvement as suggested by chronoamperometric measurements.