Electrochemical behaviour of heterocyclic amine derivatives in media with controlled proton availability: Part III. Reduction mechanism and electron spin resonance study of cyanoquinolines and their N-oxides in DMF

Abstract

The electroreduction of some cyanoquinolines and their N-oxides in dimethylformamide was studied by a variety of techniques, principally dc and ac phase-selective polarography, potential-sweep voltammetry and ESR spectrometry. Quinolines are reduced in two monoelectronic steps. The first is reversible and leads to a stable anion radical investigated by ESR spectroscopy; the second leads to the 1,4-dihydroquinoline di-anion, which is sufficiently basic to capture protons from any source near the electrode. Quinoline-1-oxides exhibit three reduction steps. The first is a reversible one-electron transfer producing a stable radical. The second electron transfer is followed by a very rapid dehydration producing the corresponding free base, which is further reducible at the same potentials. The third step leads to the 1,4-dihydroquinoline di-anion. The results were compared with those for the previously studied 2-phenyl quinolines and their N-oxide derivatives, with emphasis on the influence of substituents in determining the direction of the electrode process.