THE REDUCTION-MECHANISM OF THE GREATER-THAN-C=O GROUP .5. THE ELECTROCHEMICAL REDUCTION OF 1-METHYL-4-CARBOXYPYRIDINIUM IODIDE AND 1-METHYL-4-METHOXYCARBONYLPYRIDINIUM IODIDE IN AN AQUEOUS-MEDIUM

The electrochemical reduction of 1-methyl-4-methoxycarbonylpyridinium iodide (A) and 1-methyl-4-carboxypyridinium iodide (B) has been examin ed in an aqueous medium between H-0 = -3 and -4.5 and pH 9 and 14, res pectively. As in the case of isonicotinic acid and its methyl ester, u p to pH 7, a global 2e(-), 2H(+) reversible transfer is followed by tw o successive first order reactions. The first is a fast internal proto n transfer; the second a demethanolation for A (rate constant k(d) < 5 x 10(-3) s(-1))and a dehydration for B (rate constant k(d) = 0.45 s(- 1)). Comparison with earlier results obtained for isonicotinic acid an d its methyl ester shows that the dehydration rate constant is at leas t two orders of magnitude larger when the pyridine nitrogen bears a pl us charge, and that the rate constant is at least 100 times smaller fo r the demethanolation than for the dehydration. Above pH 7, two 1e(-) waves, of which the first is reversible, are observed; reduction of th e pyridine ring probably occurs. (C) 1997 Elsevier Science Ltd.