4-Nitropyrazol-3-ones

The best method by far to reduce 4-nitropyrazol-3-ones is by catalytic hydrogena-... 

Elguero et al. (68BSF5019) (Scheme 58) used concentrated nitric acid for the nitration of pyrazol-3-one 177a and obtained 4-nitropyrazol-3-one 178b in 50%... 

Pyrazol-3-one 179 was nitrated by Hirota et al. (82JCS(P1)277) (Scheme 59) in a mixture of fuming nitric acid and concentrated sulfuric acid to give 4-nitropyrazol-3-one 180 in 43% yield. 

Hydroxy-6-methyl-2-phenylpyridazin-3(2Fr)-one and 4-hydroxy-5-nitropyridazin-3(2FT)-one rearrange in acidic medium to 3-methyl-l-phenylpyrazole-5-carboxylic acid and 4-nitropyrazole-5-carboxylic acid. 4-Hydroxypyridazin-3(2FT)-ones with a hydroxy group or other group at positions 5 or 6, which is easily replaced in alkaline medium, are transformed into 5-(or 3-)pyrazolones with hot alkali. An interesting example is ring contraction of 5-chloro-4-(methylthio)-l-phenylpyridazin-6(lFT)-one which gives, besides pyrazole derivative (127), 4-hydroxy-5-methylthio-l-phenylpyridazin-6(lFf)-one (128 Scheme 41). 

One of our rare incursions into the field of microwave spectroscopy concerns 1-nitropyrazole (162, X = N02). MP2/6-31G calculations reproduce very well the experimental geometry [172,173] and the calculated rotational constants of a series of pyrazoles (in MHz) were reported (155, X = H, BH2, BH3 , CH3, CHO, CF3, NH2, N02, OH, A1H2, SiH3, PH2, S02H, A-oxide). Our other contributions deal with simple molecules [174],... 

Nitro-l,2,4-triazole (45%) [451] and l-methyl-4-cyano-5-nitropyrazole (42%) [452] were isolated during the oxidation of corresponding aminoazole derivatives by a solution of hydrogen peroxide in trifluoroacetic acid. One of the amino groups in l-acyl-3,5-diamino-l,2,4-triazole is oxidized by hydrogen peroxide in the presence of sodium tungstate [453] (Scheme 59). 

The construction of a heterocyclic ring from two reagents, one of which contains a nitro group, is widely used in the synthesis of the nitro derivatives of pyrazole, isoxazole, and 1,2,3-triazole. Thus, for example, the reaction of sodionitromalonal-dehyde with substituted hydrazines leads to the corresponding derivatives of 4-nitropyrazole [33, 61, 471 173] (Scheme 63). 

The proton spectra of 1-substituted 3-nitropyrazoles [296], 5-substituted 3-methyl-l-aryl-4-nitropyrazoles [297, 298], 1,3- and l,5-diphenyl-4-nitropyra-zoles [281], 5-iodo-4-nitro-l,3-dimethylpyrazole [299], l-methyl-3-nitro-4- and l,3-methyl-4-nitro-5-phenylethynylpyrazoles [300], l-methyl-3-nitro-5-methoxy-carbonylpyrazole [301], l-methyl-3-nitro- and l-methyl-5-nitro-4-cyanopyrazoles [302], A-(2,4-dinitrophenyl)nitropyrazoles [303], a- and [3-anomers of 3-nitro-and4-nitropyrazolyl-l-ribonucleosides [304, 305], 3-substituted 1,5-dimethyl- [306] and 5-substituted l,3-dimethyl-4-nitropyrazoles [279], l-acetyl-3-anilino-4-nitro-5-dimethylaminopyrazoles [307], 3-substituted 4-nitro-5-carboxylic acid derivatives [308, 309], 4-nitropyrazolo[4,3-e][l, 4]diazepin-5,8-diones showing antimicrobial activity [310], l-heteryl-4-nitropyrazole derivatives [311], 3-nitro- and 5-nitro-l-methylpyrazole [312], 4-nitro-5-(trimethylsilyl)pyrazole [313], 3-methyl-4-nitro-pyrazol-5-ones [298], and some other nitropyrazoles [248, 314-320] have been examined. 

The observation of ESR signals of nitroazoles radical anions during ESR monitoring and the appearance of blue color in the reactions of substrates with 1,1,1-trim-ethylhydrozinium iodide or 4-amino-1,2,4-triazole in f-BuOK/DMSO or MeONa/ DMSO suggests that the reaction includes a one-electron transfer stage. ESR spectra of radical anions of l-methyl-4-nitropyrazole, l-methyl-4-nitroimidazole and... 

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],... 

A- Alky In i tropy razo I cs are also reduced in two stages the first stage corresponds to reversible one-electron transfer (Scheme 3.35). In comparison with nitropyrazoles not substituted on nitrogen atom, the first half-wave potentials of A-alkylnitro-pyrazoles are essentially moved in cathodic region. Using the ESR method the signals of primary radical anions are recorded. 

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],... 

Solvatochromism and specific features of interaction between nitropyrazoles and amphiprotic solvents have been studied in detail by Prof. Turchaninov s team [1119-1123, 1127]. The dependence on acid-base properties of solvents of the electron transition in 4-nitropyrazole connected with intramolecular charge transfer has been analyzed [1121], Amphiprotic solvents with a pronounced acidic function form with 4-nitropyrazole cyclic solvates. The results of ab initio calculations 6-31G show that a cyclic complex of both 4-nitropyrazole and 3-nitropyrazole with one water molecule is thermodynamically more stable than a linear complex of the same composition by 0.38 and 1.0 kcal/mol, accordingly [1120-1123] ... 

The solvatochromism of H-complexes of 5-amino-l-methyl-4-nitropyrazole in aprotic protophilic media has been described by Kamlet-Taft empirical parameters. Specific solvation affects only one of the two long-wave bands, namely that corresponding to an electronic transition involving orbital electron density transfer from the H-bound nitrogen atom [1123],... 

A structural study of some derivatives of 3- and 5-nitropyrazoles [1128], 4-nitropyrazole [315], and 4-nitropyrazole-5-ones [1129] with the help of UV spectroscopy has been reported. 

Mononitration into the imidazole ring causes more significant changes in UV spectra (AA, 100 nm) than the nitro group introduction into the nitropyrazole cycle (A na 50 nm). In the spectra of nitroimidazoles there are usually two maxima, one... 

In one preparation of 5-methylamino-4-nitro-l-phenylpyridazin-6-one from the corresponding S-hydroxy compound and aqueous methylamine, an equal amount of l-phenyl-3-nitropyrazole was obtained. 4-Amino-3-halopyridazines with excess of potassium amide in liquid ammonia are rearranged to 4-cyanopyrazole (265), whereas the 3,6-dihalo analog gives... 

One of the most interesting structures of N/f-pyrazoles is that of 3,5-dimethyl-4-nitropyrazole... 

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