V. V. Kuz'menko

1,2,4-Triazolo[1,5-a]benzimidazoles: Tautomerism and alkylation

The position of the tautomeric equilibrium in unsubstituted 1,2,4-triazolo[1,5-a] benzimidazole, as well as in its 2-methyl and 2-phenyl derivatives, was investigated by UV, IR, and PMR spectroscopy and by determination of the ionization constants. In all cases the amount of the 4H tautomer in the equilibrium mixture is two to three orders of magnitude greater than the amount of the 3H tautomer, while signs of the existence of the 1 H form are not observed. The synthesis of unsubstituted triazolo [1,5-a]-benzimidazole was accomplished for the first time. The alkylation of the indicated triazolo [1,5-a]benzimidazoles was studied and a relationship between the regiospecificity of this reaction and the position of the tautomeric equilibrium was established.

Mutual effect of the N-amino group and the heteroring in N-aminobenzazoles

The basicity constants, NH acidities, and anode oxidation potentials of N-amino derivatives of a number of heteroaromatic systems, viz., indole, benzimidazole, indazole, and 1,3-dimethylxanthine, were measured for the first time. The results obtained provide evidence for the inductive character of the interaction of the N-amino group and the benzazole ring vis-á-vis the almost complete absence of π interaction between them; the heteroring has a very strong electron-acceptor effect on the properties of the amino group, while the amino group changes the properties of the heteroring to only a small extent.

The Chichibabin reaction in the Aminopyridine, azaindole, and azaindoline series

Abstract2-Aminopyridine, unlike the 3- and 4-isomers, is animated by sodium amide with the formation of 2,6-diaminopyridine. Among the dimethylaminopyridines, the 3- and 4-isomers are more active in the Chichibabin reaction, while 2-dimethylaminopyridine is converted with great difficulty under the action of sodium amide into 2,6-diaminopyridine with the initial replacement of the dimethylamine residue by an amino group. Azaindoles and azaindolines do not take part in the animation reaction. In 1-phenyl-5-azaindole, under the action of sodium amide, the pyrrole ring opens with the formation of 4-anilino-3-vinylpyridine.

Relative reactivities of the pyridine and benzimidazole systems in the chichibabin reaction

Abstract1-α (orγ)-Pyridyl- and 1-α (orγ)-pyridylethylbenzimidazoles were synthesized, and their behavior with respect to methyl iodide and sodium amide was studied. The results are discussed from the point of view of the basicities, magnitudes of the dipole moments, interaction of the heterorings, and the electron density distribution in them, calculated by the Hückel MO method or estimated on the basis of the relative position of the chemical shifts in the PMR spectra. It was established that the benzimidazole system undergoes the Chichibabin reaction considerably more readily than the pyridine system. On the basis of the PMR spectral data for the pyridine and benzimidazole bases and cations, it is assumed that this is due to the greater polarizability of the C = N bond of benzimidazole as compared with the C = N bond of pyridine during coordination with NaNH2.

Oxidation of 1-aminobenzimidazoles. Synthesis and properties of 1,1′-azobenzimidazoles

Abstract1-Amino-2-R-benzimidazoles are oxidized by lead tetraacetate to give, depending on the substituent in the 2-position, either to 1,1′-azobenzimidazoles (R=H, CH3, C6H5, Cl, N(CH3)2) or 3-R-benzo-1,2,4-triazines (R=NH2, NHCH3, NHC6H5, OH). The factors affecting the course of the reaction are discussed. The physicochemical properties of the 1,1′-azobenzimidiazoles obtained have been examined.

Cyclization of 3-Ethoxycarbonylmethyl-, 3-Cyanomethyl-, and 3-Acetylmethyl-1-amino-2-methylbenzimidazolium Salts Effected by Acetic Anhydride in the Presence of a Base

Abstract3-Ethoxycarbonylmethyl- and 3-cyanomethyl-1-amino-2-methylbenzimidazolium chlorides cyclized at treatment with acetic anhydride in the presence of potassium carbonate to afford a mixture of derivatives of pyrazolo[1,5-a]benzimidazole and 4-aminopyrrolo[1,2-a]benzimidazole. Therewith in the first case prevails the process of pyrazole ring fusion, and in the latter pyrrole ring is fused. 3-Aroyl(thenoyl)methyl-1-amino-2-methylbenzimidazolium bromides under the same conditions are converted into 1-aroyl(thenoyl)-2-methyl- and 2-4-acetamido-3-acetylpyrrolo[1,2-a]benzimidazoles.

1-Amino-3-alkyl-2-(methylthio)benzimidazolium salts: synthesis, reaction with CH-acid anions, and conversion to pyrazolo[1,5-a]benzimidazole derivatives

For the first time, 1-amino-3-alkylbenzimidazolinethiones were obtained by heating 1-amino-3-alkylbenzimidazolium iodides with sulfur in the presence of triethylamine. They were converted to 1-amino-3-alkyl-2-(methylthio)benzimidazolium salts, which, during reaction with CH-acid onions, gave the corresponding 1-amino-3-alkyl-2-methylenebenzimidazoline derivatives. Under conditions of alkaline or acid catalysis, the latter derivatives cyclized to 2-amino- or 2-hydroxy derivatives of pyrazolo[1,5-a]benzimidazole.

3-α-Halocarbonyl derivatives of pyrazolo[1,5-a] benzimidazole and their reactions

Abstract3-(α-haloacyl)-2,4-dialkylpyrazolo[1,5-a]benzimidazoles can be obtained either by brominating 3-acetylpyrazolo[1,5-a]benzimidazoles with bromine in acetic acid, or by acylating the 3-unsubstituted pyrazolobenzimidazoles with haloacetic halides. Halogenation of 3-acetylpyrazolo[1,5-a]benzimidazoles with bromine in acetic acid in the presence of sodium acetate, and bromination with N-bromosuccinimide or 1-chlorobenzotriazole, result in deacylation to give 3,6(7)-dibromo- and 3-chloropyrazolo[1,5-a]benzimidazoles. The mono- and trihaloketones obtained have been used to prepare the corresponding aminoketones, the 3-carboxylic acid, and its derivatives.

1-Amino-2-alkylaminobenzimidazoles and their reactions with carbonyl-containing compounds

Abstract1-Amino-2-alkylaminobenzimidazoles were synthesized by a reaction involving exchange of the sulfo group in 1-aminobenzimidazole-2-sulfonic acid. 3-Alkyl-3H-1, 2,4-triazolo [1,5-a] benzimidazoles and 4-alkyl-4H-1,2,4-triazinol[2,3-a]benzimidazol-3-ones were obtained on the basis of them.

Unusual cleavage of phenacyl-substituted benzimidazolium salts. Synthesis of 1,4-diarylimidazoles

The action of ammonium acetate in acetic acid on 1-alkyl(aralkyl)-3-phenacylbenzimidazolium bromides leads, in addition to the formation of a new imidazole ring, to cleavage of the benzimidazole fragment of the molecule at the 1,2 bond to give 1-alkyl(aralkyl)aminoaryl-4-arylimidazoles. 1,2-Dimethyl-3-(p-nitrophenacyl)benzimidazolium bromide is converted under similar conditions to the corresponding 2-methylimidazole. 4,4′-Dimethyl-2,2′-bis(p-nitrophenyl)-1,1′-dipyrrolo[1,2-a]benzimidazole was isolated as a side product of this reaction.