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1,2-Dialkyl-4-nitroimidazoles are reduced between pH 0 and 13 in a four-electron wave. The resulting arylhydroxylamine is stable and in acidic media reduced further to the amine. 1,2-Dialkyll-5nitroimidazoles and 2-alkyl-4( 5)-nitroimidazoles are reduced in alkaline media with the number of electrons approaching six. The hvdroxylamine derivative formed in these reductions can be dehydrated and yields an easily reducible, quinone-like ketiniine. The rate of the dehydration governs the heights of the first step and is base catalyzed. The sequence of electron and proton transfers for 2alkylsubstituted nitroimidazoles is the same as that observed for corresponding desalkyl compounds.l-Alkyl-4-amino-5-nitroimidazoles, as well as 1 -alky-4-nitro-5-aminoiiiiidazoles, are reduced in a single wave, the height of which approaches that of a six-electron process. The dehydration of the hydrolrylamino derivative is favored, probably due to an internal base catalysis. The shifts of half-wave potentials with pH differ principally from those of all other nitroimidazoles and indicate the predominant role of the amino group in the proton transfer.KEY WORDS: Nitroimidazoles, Reduction, Polarography.information useful in development of analytical procedures. EXPERXMENTM ApparatusPolarographic curves were recorded with Polarograph Sargent Model XV. Electrolytic cells enabling the use of three-electrode systems were used. Four capillaries were used: for the study of compounds I-V, VTI-X, and XII-XIV, the capillary characteristics were t, = 2.90 s, m = 2.30 mg s-l; for VI and XI, tl = 3.88 s, m = 2.26 mg s-I; for XV-XXIII, tl = 3.76 s, m = 2.00 mg s-l; and for Xxn:

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.

Reduction of 1-nitropyrazole (I) results in acidic media in a cleavage of the N-N bond and formation of nitrous acid which is further reduced at more negative potentials. At pH > 4 a competitive reduction of the nitramine to nitrosamine occurs. Four-electron reduction of nitro derivatives of pyrazolium (11) and (111) and imidazolium (IV) and (V) salts results in a formation of a hydroxylamine derivative. The reduction follows the scheme: H+, e, e, H + , 2e, 2H+. In the reduction product the azolium ring is reduced at more negative potentials. Whereas the effect of pyrazolium and irnidazolium rings on the reducibility of the nitro group is comparable, the interaction between the nitro group and the azolium ring markedly depends on the position of the nitro group on the ring.