Tautomerism benzimidazole

Benzimidazole, 2-amino-5-bromo-l-methyl-diazo coupling, 5, 429 Benzimidazole, 2-amino-6-bromo-1 -methyl-diazo coupling, 5, 429 Benzimidazole, 2-amino-1-ethyl-alkylation, 5, 438 Benzimidazole, 2-amino-1-methyl-acylation, 5, 438 bromination, 5, 429 tautomerism, 5, 368 Benzimidazole, 4-amino-2-methyl-diazo coupling, 5, 429 Benzimidazole, 2-aroyl-mass spectra, 5, 360 Benzimidazole, 1-aryl-metallation, 5, 448 reactions... 

Chichibabin reaction, 5, 409-410 sulfonation, 5, 429 synthesis, 5, 160, 161, 457-498, 496 tautomerism, 5, 365 toxicity, 1, 139 UV spectra, 5, 356, 357 2H-Benzimidazoles 3,3-dioxides synthesis, 6, 407 2V,7ST-dioxides reactions, 5, 456 Benzimidazoles, aryl-oxidation, 5, 433 Benzimidazoles, nitro-mass spectra, 5, 359 reduction, 5, 441 Benzimidazole-2-sulfonic acids reactions... 

Benzimidazole-2-thione, l-( -D-ribofuranosyl)-tautomerism, 5, 368 Benzimidazolethiones tautomerism, 5, 368 Benzimidazole-2-thiones alkylation, 5, 444 synthesis, 5, 458... 

Not much information has been added in recent years to the earlier studies of tautomeric equilibria of benzimidazoles based on basicity measurements [76AHC(S1), p. 292]. For 5(6)- and 4(7)-substituted benzimidazoles and 2-methyl-5(6)-substituted benzimidazoles values are very close to 1, which indicates near equivalence in the stability of N1(H) and N3(H) tautomers. The tautomeric equilibria of 2-substituted (H, NH2, OMe, CN) 5-nitrobenzimidazoles and 4-nitrobenzimidazoles were analyzed with the use of semiempirical MINDO/3 and INDO methods. It was predicted that electron-releasing groups in position 2 shifted the equilibria to the 6-NO2 and 4-NO2 tautomers, respectively. 

Stabilization of a metal-substituted derivative of a minor tautomeric form of the Ugand was reported for the complexes of mixed benzothiazo-line-benzimidazole (388 —> 389) (71ZOB1370 98POL381) and benzothia-zoline-pyridine ligands (390 —> 391/392) (Scheme 144) (77JA7704). 

Fyrazoles and indazoles, imidazoles and benzimidazoles/ and benzotriazoles which possess a free NH group are associated and have been considered to exhibit mesohydric tautomerism see discussion in Volume 1, article I, Section I,C. 

By using basicity data, Ridd and Smith- showed that 5-nitro- and 5-chloro-benzimidazole and their 2-methyl analogs exist essentially as mixtures of equivalent amounts of 29 and 30, and, in agreement with this ratio, 5-substituted benzimidazoles form comparable amounts of 1- and 3-derivatives on alkylation,- showing earlier alkylation ratios- to be erroneous. There are, however, other factors which can lead to the predominance of one tautomeric form. Basicity measurements indicate that 31 is preferred to the alternative non-hydro-... 

Most electrophilic substitutions in benzimidazole (31 R = H) occur primarily in the 5-position. In multiple bromination the order followed, 5 > 7 > 6,4 > 2, parallels molecular orbital calculations. In benzimidazole itself the 4(7)- and 5(6)-positions are tautomerically equivalent. Fusion of a benzene ring deactivates C-2 to electrophilic attack to such an extent that it is around 5000 times less reactive than the 2-position of imidazole. Strong electron donors at C-5 direct halogenation to the 4-position, whereas electron-withdrawing groups favor C-4 or C-6 substitution (84MI21). 

Rodrigues Prieto, F., Rios Rodriguez, M. C., Mosquera Gonzalez, M. and Rios Fernandez, M. A. Ground- and excited-state tautomerism in 2-(3 -Hydroxy-2 -pyridyl)benzimidazole, J. Phys. Chem., 98 (1994), 8666-8672... 

Bei einer Rcihe von 2-Alkyl-benzimidazolen mit Elektronen-ziehenden Gruppen im Alkyl-Rest liegt das tautomere Gleichgewicht ganz auf der Seite der 2-Alkyliden-2,3-ditiydro-benzimidazole s. dazu... 

A detailed analysis of substituent effects on the pKa values of imidazoles and tetrazoles as well as benzimidazoles and naphthimidazoles has been made.308 The ortho effect is shown to parallel that of 2-substituted pyridines and quinolines47 190 and application of the Hammett equation to the tautomerism of these systems is also considered. The equation also satisfies the effect of substituents on the basicity of the nitrogen in the 1-position for 1-pyrazolines.309... 

Increasing the electron-donor properties of the R, substituent in the case of dihydro derivatives of 1,2,4-triazolo[l, 5-a]pyrimidines [X, Z are N, Y is C(R)], tetrazolo[l, 5-a]pyrimidines (X, Y, Z are N), and pyrimidofl, 2-a]benzimidazoles (X, Y are o-C() I4, Z is N) tends to stabilize the B form. This can be explained by the conjugation of Rj with the electron-withdrawing azomethine group and the azole moiety. Reduction of the electron-accepting properties of the azole fragment (imidazopyrimidines and pyrazolopyrimidines) decreases the influence of the Ri substituent on tautomeric composition. 

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