Electrochemical reduction of C-4 nitrosophenyl 1,4-dihydropyridines and their parent C-4 nitrophenyl derivatives in protic media

A comprehensive study of the electrochemical reduction in protic media of parent C-4 nitrophenyl 1,4-DHPs and their corresponding nitrosophenyl 1,4-DHPs was carried out. The reduction potentials of synthesized aromatic nitro compounds were compared with those of the corresponding nitroso compounds, concluding that the latter were reduced at significant more positive potentials. Arylhydroxylamines formed in the 4-electrons reduction of nitrophenyl 1,4-DHPs at acid pH can be protonated and further reduced. At pH 12, in some compounds the 1-electron reduction to give the nitro radical anion could be distinguished from the cyclic voltammograms. Ortho-nitro derivatives were reduced at more negative potentials than other meta- and para-nitro derivatives. Nitrosophenyl 1,4-DHPs were reduced on Hg in a quasi-reversible process, involving 2- electrons, 2-protons to give the hydroxylamine derivative. Reduction potentials of the nitrosophenyl 1,4-DHPs having methoxycarbonyl- and ethoxycarbonyl-groups in the 3- and 5-positions were not significantly affected. However, a bulky substituent in those positions, such as the isopropyloxycarbonyl group, produced an increase in peak potential values, making reduction more difficult than the other derivatives.