Polarographic and electrochemical studies of some aromatic and heterocyclic nitro compounds, part III: Electroreduction of mono‐ and dinitropyrazoles and ‐imidazoles

Abstract

Institute of General arzd Physicul C b m i s q , Studentski wg 12-16, ABSTRlCTThe reduction of mono-and dinitropyrazoles and of nitroimidazoles f o l l o~~ the general pattern of reduction of aromatic nitro compounds: The nitro group is reduced in a four-electron step to a hydroxylamino group and the protonated form of the hydroxylamino group is-in the lower pH range-further reduced to an amine. This reduction differs from that of nitrobenzenes in participation of a second hydrogen ion probably involved in protonation of the heterocyclic ring. This second proton is for nitroimidazoles transferred before the uptake of the first electron, for nitropyrazoles probably after this uptake. The transfer of the second electron is indicated to be the potential determining step. The two sequences are H', H', e, H+, e, 2e, H+ and H+, e, H', H+, e, 2e, Ht, respectively. For nitropyrazoles and nitroimidazoles without an alk~7l substituent on the ring nitrogen, the reduction process is further complicated by the dissociation of the NH-group in the heterocyclic ring. For l-alkyl-5-nitroimidazoles, for 4(5)-nitroimidazole and for N-unsubstituted-4-and 3(5)nitropyrazoles (but not for 2-nitroimidazoles, 1-alkyl-4-nitroimidazoles and 1-alkylnitropyrazoles) the hydroxylamino derivative formed in the first four-electron step undergoes a base catalyzed dehydration yielding a quinone-like ketimine. Easy reduction of this species results in alkaline solutions in a limiting current which is significantly higher than corresponds to a four-electron and limits to a sixelectron reduction. Such dehydration reactions occur considerably faster for dihydroxylamino derivatives formed in the reduction of dinitropyrazoles resulting in two waves with total transfer of up to 12 electrons.

Half a Century of Research Using Polarography

“…Similar stabilization of arylhydroxylamines by dehydration was observed for reductions of some heterocyclic nitro compounds (74,189,190). The incidence of dehydration depends on the mutual position of the nitro group and the ring heteroatom and on the presence or absence of alkylation of this heteroatom.…”

Cyclic voltammetric determination of free radical species from nitroimidazopyran: a new antituberculosis agent

“…The electrochemistry of metronidazole has been reported at different electrodes such as mercury [5][6][7][8][9], glassy carbon, DNA-modified glassy carbon and a mercury thin film electrode [10,11]. According to these studies, the reduction of metronidazole in acid medium is pH-dependent and four electrons are involved in the complete reduction to the hydroxylamine derivative.…”

Voltammetric Behaviour of Metronidazole at Mercury Electrodes

“…Zuman and co-workers 5,7,13 showed that the E1/2 for the first reduction step of nitrocompounds is shifted to more negative potentials with increasing pH. This change in the E1/2 values occurs both in aqueous medium and solutions with different percentages of organic solvents.…”

“…Under anaerobic conditions or low oxygen pressure, the reduction process is similar to that observed for nitrobenzene 4 . The reduction mechanism for several aromatic and heterocyclic nitrocompounds was presented by Zuman and co-workers [5][6][7][8][9][10][11][12][13] . A total of two electrons and two protons is involved in the formation of the nitroso (R-NO) intermediate, two more electrons and protons result in the hydroxylamine (R-NHOH) [5][6][7][8][9][10][11][12][13][14] In voltammetric studies of nitroimidazole derivatives [15][16][17] , two reduction waves were observed in aqueous acid medium.…”