Electrochemical reduction of some 2,6-disubstituted pyridine-based esters and thioic S-esters in acetonitrile

Abstract

Voltammetric studies at glassy carbon, gold, mercury, platinum and silver electrodes on 2,6-disubstituted pyridine esters and thioic S-esters in acetonitrile have shown that they may be reduced at negative potentials [–1.95 to –2.27 V vs. Fc/Fc⁺](Fc = ferrocene). The reduction process corresponds to a chemically reversible one-electron reduction step leading to the formation of a radical anion for many of the compounds at moderately high scan rates ( >1–10 V s^–1) but at lower scan rates the process appears chemically irreversible owing to the instability of the radical anion. Reduction is believed to occur in the carbonyl region of the molecules with the thioic S-esters being easier to reduce than the esters by ca. 300 mV and the reduction potential correlating with the strength of the CO bond as measured by IR spectroscopy. Controlled potential electrolysis experiments performed on an ester and the analogous thioic S-ester showed that reduction of an ester on the long time domain experiments resulted in cleavage of the O–R bond while the thioic S-ester cleaved at the C(O)–S bond and underwent further reaction to form interesting and unusual products. The isolated products and mechanism of the reduction, as determined from voltammetric, kinetic, EPR spectroscopy and other spectroscopic data, are discussed in detail. The design and operation of a simple and efficient bulk electrolysis cell used in these experiments is also discussed.