Electrocatalytic synthesis of 6-aminonicotinic acid at silver cathodes under mild conditions

The feasibility of electrosynthesis of 6-aminonicotinic acid by electrochemical reduction of 2-amino-5-bromo and 2-amino-5- chloropyridine in the presence of CO2 has been investigated in DMF and CH3CN at glassy carbon (GC), Hg, Pt and Ag electrodes. Reduction of both halides at GC, Hg and Pt electrodes occurs at potentials only slightly more positive than Ep of CO2. These electrodes are not appropriate for a simple and clean electrocarboxylation of the halides to 6-aminonicotinic acid. The silver electrode shows a remarkable electrocatalytic effect for the reduction of only the bromo derivative. Reduction of 2-amino-5-bromopyridine at such an electrode occurs at potentials 0.5–0.8 V more positive than at the other electrodes. The electrosynthesis process was investigated at Ag using only the bromo derivative as the starting substrate. The process was investigated both in a divided and an undivided cell with Al sacrificial anode under constant potential or current density. In all cases fairly good yields of 6-aminonicotinic acid (48–82%) with moderate quantities of 2-aminopyridine were obtained in DMF. Instead, the principal reduction product in CH3CN is 2-aminopyridine with yields >80% while only very small quantities of the acid are formed.