Solvation of 4-nitropyrazoles according to UV spectroscopy data

The dependence of the energies of intramolecular charge-transfer electron transitions in 1,3,5-trimethyl-4-nitropyrazole, 4-nitropyrazole, and its anion on the dielectric and acidbase properties of solvents was analyzed. The dipole moments of these species in the excited state are larger than those in the ground state. The nonspecific solvation is accomplished through dipole-dipole interactions. 4-Nitropyrazole, which possesses acidic properties, is also solvated by the mechanism of hydrogen bonding with the solvent, which acts as a base. The strength of the hydrogen bond increases on going from the ground to the electron-excited state. The 4-nitropyrazole anion is a base, and its hydrogen bonds with the solvent are stronger in the ground state. The mechanism of specific solvation of the molecule and the anion of 4-nitropyrazole during electronic excitation is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 867–869, April, 1997.     

Nitropyrazoles 17. Synthesis of 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole and the study of its chemical transformations

A new one-step method for the synthesis of 4-methyl-3(5)-nitropyrazole by nitration of 4-methylpyrazole is developed. Arylation of 4-methyl-3(5)-nitropyrazole with 1,3,5-trinitrobenzene gives 1-(3,5-dinitrophenyl)-4-methyl-3-nitropyrazole, nitration of which leads to 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole. The action of 1 equiv. of an O- or S-nucleophile (phenolate, p-chlorobenzenethiolate and ethoxide ions; anions of glycolanilide and thioglycolanilide) on 1-(3,5-dinitrophenyl)-4-methyl-3,5-dinitropyrazole led to the substitution of the 5-NO₂ group of the pyrazole ring; under the action of one more equivalent of a nucleophile the NO₂ group of the benzene ring was substituted. The substitution product of the anion of thioglycolanilide for the 5-NO₂ group undergoes the intramolecular cyclization — oxidative nucleophilic hydrogen substitution in the benzene ring under the action of K₂CO₃.     

Amination of isomeric bromo-1-methylnitropyrazoles

A method was developed for the synthesis of 5-bromo-1-methyl-4-nitropyrazole from 1-methylpyrazole. In the reaction with 25% aqueous ammonia at 180–190 °C, 5-bromo-1-methyl-4-nitropyrazole is readily converted to 5-amino-1-methyl-4-nitropyrazole; the production of 4-amino-1-methyl-5-nitropyrazole from 4-bromo-1-methyl-5-nitropyrazole requires the presence of a copper catalyst; under the same conditions in the amination of 4-bromo-1-methyl-3-nitropyrazole, 4-amino-1-methyl-3-nitro- and 1-methyl-3-nitropyrazoles are formed in a 23 ratio.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1672–1675, December, 1983.     

Diazotization of aminonitropyrazoles

3(5)-Diazonia-4-nitropyrazole hydrochloride, which readily undergoes nucleophilic substitution, is formed by diazotization of 3(5)-amino-4-nitropyrazole. Depending on the conditions, in the diazotization of 3,5-diamino-4-nitropyrazole one or both amino groups undergo reaction. Moreover, a nitro group is simultaneously replaced by a hydroxyl group, and the final product is 3,5-bisdiazo-4-pyrazolone.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1649–1653, December, 1976.     

Investigation of UV spectra of isomeric nitropyrazoles by the semiempirical AM1 (CI) method

The absorption bands in the UV spectra of isomeric nitropyrazoles were assigned by the calculations in the semiempirical AMI (CI) approximation. The long-wave absorption of nitropyrazoles is caused by π→π* and η₀→π* transitions. The charge-transfer band is the most intense. The π→π* transitions undergo a considerable bathochromic shift in the deprotonation. The first ionization potential (PI) of the 4-nitropyrazole anion was estimated from the empirical dependence of the energy of the excited π-state on PI of alkyl-substituted 4-nitropyrazoles. The PI of the 4-nitropyrazole anion is 3 eV lower than that of a neutral molecule. This is evidence for a substantial destabilization of the boundary β-orbital in the heterolytic cleavage of the N−H bond. The analysis of the UV and NMR spectra of 3(5)-nitropyrazole confirms the viewpoint that the 3-nitro tautomer predominates in solution. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 2, pp. 310–314, February, 1997.     

Synthesis of 3-amino-4-nitropyrazoles

3-Bromo-1,5-dimethyl-4-nitropyrazole does not react upon heating with aqueous ammonia, while 1,5-dimethyl-3,4-dinitropyrazole under the same conditions yields 3-amino-1,5-dimethyl-4-nitropyrazole, which is formed from 3-bromo-1,5-dimethyl-4-nitropyrazole in the presence of a copper catalyst. The amination of 1-methyl-3,4-dinitropyrazole-5-carboxylic acid is accompanied by decarboxylation, which is characteristic for 4-substituted 1-methylpyrazole-5-carboxylic acids upon heating in aqueous ammonia or water.     

Ring transformations in reactions of heterocyclic compounds with nucleophiles (XX). Conversion of 5-nitropyrimidine into 3,5-dinitropyridine. A novel ring transformation

Heating of 5-nitropyrimidine with acetic acid in water leads to the formation of 3,5-dinitro-pyridine. The mechanism proposed for this novel ring transformation involves a fragmentation of the pyrimidine ring, leading to the formation of a two-atom and a four-atom moiety, which on condensation gives the pyridine ring. With hydrazine hydrate, 5-nitropyrimidine undergoes ring contraction into 4-nitropyrazole.     

Nitropyrazoles. 11. Isomeric 1-methyl-3(5)-nitropyrazole-4-carbonitriles in nucleophilic substitution reactions. Comparative reactivity of the nitro group in positions 3 and 5 of the pyrazole ring

With reactions of isomeric 1-methyl-3-nitro- and 1-methyl-5-nitropyrazole-4-carbonitriles with anionic S-, O-, and N-nucleophiles (RSH, PhOH, and 3,5-dimethyl-4-nitropyrazole in the presence of K₂CO₃ or MeONa), it was shown that for N-substituted 3(5)-nitropyrazoles, the nitro group in position 5 is much more reactive than in position 3.     

Investigation of the Reaction of 3,5-Diamidino-4-nitropyrazole with Amines

The interaction of 3,5-bis(N,N-dimethylaminomethylene)amino-1-methyl-4-nitropyrazole with amines proceeds regioselectively with the formation of 3-amino-5-(N,N-dimethylaminomethylene)amino-1-methyl-4-nitropyrazole. Amines are converted in this way into N,N-dimethylformamidine derivatives. The structures of the compounds obtained were proved by ¹H and ¹³C NMR spectroscopy and by X-ray crystallographic analysis.     

Synthesis of 5-Substituted 1,3-Dimethylpyrazolo[4,3-e][1,2,4]triazines

A novel method for the synthesis of a new series of 5-substituted 1,3-dimethyl pyrazolo[4,3-e][1,2,4]triazines is described. The new synthetic strategy is based on the classical Bischler 1,2,4-benzotriazine synthesis. This approach involves the preparation of 5-hydrazinopyrazole from 5-chloro-1,3-dimethyl-4-nitropyrazole followed by acylation and nitro group reduction to form the corresponding 4-amino-3-(acylhydrazino)pyrazoles. Intramolecular oxidative cyclization of the latter derivatives, using polyphosphoric acid, produced the respective target pyrazolotriazines.     

Synthesis of 5-Substituted 1,3-Dimethylpyrazolo[4,3-<Emphasis Type="Italic">e</Emphasis>][1,2,4]triazines

Summary.  A novel method for the synthesis of a new series of 5-substituted 1,3-dimethyl pyrazolo[4,3-e][1,2,4]triazines is described. The new synthetic strategy is based on the classical Bischler 1,2,4-benzotriazine synthesis. This approach involves the preparation of 5-hydrazinopyrazole from 5-chloro-1,3-dimethyl-4-nitropyrazole followed by acylation and nitro group reduction to form the corresponding 4-amino-3-(acylhydrazino)pyrazoles. Intramolecular oxidative cyclization of the latter derivatives, using polyphosphoric acid, produced the respective target pyrazolotriazines.     

Synthesis of pyrazolo[1,5-a]pyrimidines by the reaction of β-dicarbonyl compounds with 3,5-diamino-4-nitropyrazole

The reaction of 3,5-diamino-4-nitropyrazole with asymmetrical β-dicarbonyl compounds gave polysubstituted pyrazolo[1,5-a]pyrimidines, which are used in reactions with both nucleophilic and electrophilic reagents. The structure of the compounds obtained was demonstrated by x-ray diffraction structural analysis. Center for Pharmacological Chemistry, All-Russian Pharmaceutical Chemistry Institute, 119815 Moscow and M. V. Lomonosov Moscow State University, 119899 Moscow, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1676–1681, December, 1998.     

Efficient iodination of structurally varying pyrazoles in heterophase medium

A synthesis of 4-iodo-substituted pyrazoles by iodination of pyrazole and its derivatives in the heterophase (H₂O/CHCl₃ (CCl₄)) medium with the system KI-KIO₃ in the presence of H₂SO₄ additives was accomplished. The yields of 4-iodo-substituted pyrazoles in the iodination of pyrazole, 3,5-dimethylpyrazole, pyrazole-3(5)-carboxylic acid, 1-methylpyrazole-3-carboxylic acid, 1-methylpyrazole-5-carboxylic acid, 3-nitropyrazole, 1-methyl-3-nitropyrazole, 1-methylpyrazole, 1-ethylpyrazole, and 1-isopropylpyrazole were within 80–97%, whereas in the case of 3-nitropyrazole-5-carboxylic acid it was 32%.     

Synthesis of pyrazolo[1,5-a]-pyrimidines by reaction of 3,5-diamino-4-nitropyrazole with acetoacetic ester in the presence of alkaline agents

Two major compounds are formed in the reaction of 3,5-diamino-4-nitropyrazole with acetoacetic ester: 2-amino-5-methyl-3-nitro-4,7-dihydropyrazolo[1,5-a]pyrimidin-7-one and 3-amino-5-(-hydroxycrotonyl)amino-4-nitropyrazole. The latter is converted to bicyclic dihydropyrazolo[1,5-a]-pyrimidinone in solutions with heating.State Science Center of the Russian Federation NIOPIK (Scientific Research Institute of Organic Intermediates and Dyes), Moscow 103787Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 73–77, January, 2000.     

Dipole moment and tautomeric form of 3(5)-nitropyrazole in dioxane solution

The dipole moments of 3(5)-nitropyrazole, its methyl-substituted derivatives, and H-complexes with dioxane were measured experimentally and estimated byab initio calculations (6-31G^* basis set). Comparison of the experimental and calculated dipole moments suggests a shift of the tautomeric equilibrium toward the 3-nitroisomer. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2202–2203, November, 1999.     

N-Arylation of azoles with low basicity by 1,4-dimethoxybenzene during undivided electrolysis

The reactions of 1,4-dimethoxybenzene with 4-nitropyrazole, 3,4-dinitro-5-methylpyrazole, 1,2,4-triazole, 3-nitro-1,2,4-triazole, and tetrazole were studied during undivided amperostatic electrolysis on a Pt electrode in MeCN, CH₂Cl₂, and MeOH. The main reaction products were 2-azolyl-1,4-dimethoxybenzenes and (or) 1,4-diazolyl-1,4-dimethoxycyclohexa-2,5-dienes. In all cases except 1,2,4-triazole, N-arylation occurs only in the presence of the Alk₄N⁺ salts of azoles or 2,4,6-trimethylpyridine as a base. The mechanism of the reactions is discussed.     

Synthesis of heterocyclic systems with a carbohydrate fragment

Levoglucosenone, the ,ß-unsaturated ketone obtained by the pyrolysis of cellulose, reacts with NH-azoles in the presence of bases. In this process not only normal Michael adducts are formed, but in some cases their hydrates (gem-diols) are also formed. In all cases the addition proceeds stereospecifically and with good yields. 3-Nitro-s-triazole, 3-nitro- and 4-nitropyrazoles, 3-methyl-4-nitropyrazole, 5-nitrotetrazole, 4,5-dicarbomethoxy-v-triazole, 3,4-dinitropyrazole, 3(5)-methylpyrazole, pyrazole, and imidazole have been studied in this reaction. All of the adducts are characterized by NMR- and IR-spectroscopy.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1135–1140, June, 1994.     

New synthetic route to diaminonitropyrazoles as precursors of energetic materials

Treatment of nitropyrimidine derivatives with (N-substituted) hydrazines (2 equiv.) gave 1-(substituted)-3,5-diamino-4-nitropyrazole, providing a very mild conversion of pyrimidines into pyrazoles. This reaction provided a convenient route to precursors for new efficient and insensitive explosives.     

Nitropyrazoles

A preparative method ofN-amination of pyrazoles bearing nitro groups and other electron withdrawing substituents in the pyrazole ring with hydroxylamine-O-sulfonic acid involving control of the pH of the medium has been elaborated. A series of previously unknown pyrazoles has been prepared. Basicities of 1-aminopyrazole and 1-amino-4-nitropyrazole have been measured and differences between the basicities of theC- andN-amino groups for the pyrazole series have been revealed.For part 3, see ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1434–1436, August, 1993.