Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability: Part II. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinoline 1-oxides in DMF

The electroreduction of some 4-substituted 2-phenylquinoline 1-oxides in dimethylformamide with and without a protonating agent was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry.In an aprotic medium the 1-oxides studied exhibit three or four subsequent steps. In every case the first is a reversible one-electron transfer producing a stable anion radical characterized by the ESR spectra. Potential sweep voltammetry give evidence of a chemical reaction following the second electron transfer producing the corresponding free bases, which are also electroactive at the same potentials.In a protic medium two subsequent bi-electronic (or only one tetraelectronic step) can be observed, the corresponding free bases and then 1,4-dihydroquinolines (or directly 1,4-dihydroquinolines) being the final reduction products.According to the experimental evidence a multistep scheme is suggested in which the role played by the substituents in determining the electroreduction pattern is considered.