Electroreduction of 1‐Methyl‐2‐, 3‐, and 4‐Carbomethoxypyridinium Ions

Abstract

The character of both polarographic waves of the three isomers 1‐methyl‐2‐, 3‐, and 4‐carbomethoxypyridinium ions (2+, 3+ , and 4+, respectively) has been studied in acetonitrile and in methanol. The first wave is due to a one‐electron reduction under DC and normal pulse polarographic (NPP) conditions. Whereas a stable free radical 4ċ is reversibly formed in the reduction of 4+ and a dimer is irreversibly formed in the reduction of 3+, the 2+ reversibly forms radical 2ċ at low concentration and short pulse width and a dimer under DC conditions. The equilibrium constant and the rate constants for the dimerization reaction dimer have been determined by the reverse‐pulse polarographic method (RPP). The radicals 2ċ and 4ċ have been identified by RPP, and the formal potentials of the couples 2+/2ċ and 4+/4ċ are reported. The second reduction wave of the three compounds is diffusion controlled in protic media (methanol) and yields dihydropyridine as a product. In acetonitrile, the pyridinium salts (n+) exhibit quite complicated behavior, which is dependent on the nature of the pyridinium ion (position of the carbomethoxy group on the ring) and its concentration, the nature of the cations present in the solution, and the time scale in which the electrochemical method is applied. The 3+ isomer does not undergo a second reduction step in this solvent. The anions 2− and 4− react with the cations to form (dihydropyridines), , , or the corresponding dimers n‐n (reaction with the starting material n+). The latter reaction causes a decrease in the amplitude of the second wave and is the most favored reaction in the presence of the indifferent bulky tetrabutylammonium cation, under which conditions the anions (2− and 4−) are most stable and are easily detected. The formal potential of the couples 2ċ/2− and 4ċ/4− are reported.