Voltammetric oxidation of Hantzsch 1,4-dihydropyridines in protic media: substituent effect on positions 3,4,5 of the heterocyclic ring

A detailed study was done of the electrochemical oxidation of some 1,4-dihydropyridine (1,4-DHP) derivatives in order to determine the influence of the substituents in the heterocyclic ring. Two types of derivatives were synthesized, namely, 3,5-(substituted)-4-(5′-nitro-2′-furyl)-1,4-DHP for series A, and 3,5-dicarboethoxy-4-(substituted or non-substituted)-1,4-DHP for series B. Voltammetry, coulometry, controlled potential electrolysis, UV-vis spectroscopy and GC-MS techniques were employed to collect data that permitted to postulate oxidation mechanisms in a protic medium. In acid media, at pH < 4, all derivatives follow oxidation mechanisms obeying the ECE sequence. However, at pH > 4, series A derivatives follow an ECEC sequence, while series B derivatives obey a DISP1 mechanism. In both cases, the uptake of proton at N-1 by the OH⁻ ion of the media was the rate-determining step.