4- Nitropyrazol-3-ones, reduction

Dumanovic with colleagues carried out polarographic investigation of various derivatives 3-, 5-nitropyrazoles and 2-, 4-, 5-nitroimidazoles in aqueous buffer solutions (pH 1.8-9.3) and 0.1 M solution NaOH [903-912], On polarograms all the investigated nitroazoles have one or two waves of reduction both in acid and neutral medium. The first four-electron wave corresponds to reduction of nitro group to hydroxylamine one. The second two-electron wave corresponds to further reduction of hydroxylamine derivative to aminoazole (Scheme 3.34). 

The electrochemical reduction of C- and IV-nitroazoles has been investigated in detail in an aprotic media [850-853], The reduction potentials of nitropyrazoles in acetonitrile are presented in Section 3.3 (Table 3.36). iV-substituted 3(5)-nitro- and -4-nitropyrazoles in acetonitrile are reduced in two one-electron stages [850, 851],... 

The presence of N-N02 fragment in 1-nitropyrazole significantly facilitates the process of electrochemical reduction E m=-0.95, It" xn=-. 73 V (acetonitrile). The first wave corresponds to a one-electron irreversible transfer the second wave is approximately 3.5 times higher and shows sharp drop prior to background discharge (Scheme 3.36). Analysis of the dependence of 1-nitropyrazole reduction potentials on pH shows that the second wave corresponds to the reduction of the N02 anion (nitrite ion) formed [851],... 

The polarogram of 1,4-dinitropyrazole is considerably more complex and has five waves (Table 3.36, Section 3.3). This compound is reduced more easily than all the investigated nitroazoles. The first wave corresponds to an irreversible one-electron transfer (Scheme 3.19). As with 1-nitropyrazole, at this stage an unstable anion radical is formed and then breaks up at bond N-N02. The N02 anion is reduced at potential -1.7 V. 4-Nitropyrazolyl radical further is dimerized with subsequent reduction [851],... 

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