Imidazole 1-oxides tautomerism

Chichibabin reaction, 5, 409-410 UV spectra, 5, 356 Naphthimidazoles, 2-amino-tautomerism, 5, 368 Naphth[2,3-h]imidazoles oxidation, 5, 405 Naphth[l,2-d]imidazolium salts nucleophilic substitution, 5, 412 Naphth[l, 2-h]isoquinolines... 

Imidazole, 2-amino-1 -methyl-4,5-diphenyl-tautomerism, 5, 368 Imidazole, 2-aroyl-mass spectra, 5, 360 synthesis, 5, 391, 402 UV spectra, 5, 356 Imidazole, 4-aroyl-synthesis, 5, 474 Imidazole, C-aroyl-UV spectra, 5, 356 Imidazole, aryl-nitration, 5, 396, 433 oxidation, 5, 433 Imidazole, 1-aryl-dipole moments, 5, 351 dearylation, 5, 449 ethylation, 5, 448 H NMR, 5, 353 hydroxymethylation, 5, 404 rearrangement, 5, 108 synthesis, 5, 390 thermal rearrangement, 5, 363 Imidazole, 2-aryl-chlorosulfonation, 5, 397 synthesis, 5, 475 Imidazole, 4-aryl-bromination, 5, 399 Imidazole, C-aryl-electrophilic substitution, 5, 432-433 nitration, 5, 433 Imidazole, N-aryl-reactions, 5, 448-449 structure, 5, 448-449 Imidazole, arylmercapto-... 

Imidazole-5-thione, 4,4-diphenyl-tautomerism, 5, 368 3 H-Imidazole-2-thione, 1,3-dimethyl-structure, 5, 367 Imidazole-2-thiones acidity, 5, 367 betaines, 5, 372 synthesis, 5, 481 tautomerism, 5, 367 3H-Imidazole-2-thiones synthesis, 5, 473, 6, 992 Imidazolides deacylation, 5, 453 mass spectra, 5, 360 phosphoric acid reactions, 5, 454 reactions, 5, 451-453 Imidazolidine, l-alkyl-3-phenyl-N-oxidation, 5, 427 Imidazolidine, 1,3-benzyl-2-phenyl-oxidation, S, 427... 

The only example of ring-chain tautomerism of imidazoles is 1-hydroxy-4 -imidazoline-3-oxide (69MI86 78MI2, pp. 155,182). 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

The 1,3-dipoles were generated by the addition of Et3N in 20% excess. Only imidazole was basic enough to generate a nitrile oxide in the absence of triethy-lamine. Due to prototropic tautomerism, reactions of triazoles and tetrazoles led to mixtures of two isomers. With unsubstituted pyrazole and imidazole only one hydroximoylazole was formed (117). 

A strictly dehned region of chemical shifts of C2, C4, and C5 atoms in A-oxides of 4A-imidazoles allows to dehne clearly the position of the A-oxide oxygen atom (102). Chemical shifts of the a-C nitrone group in a-N-, O-, and S-substituted nitrones are located in the region of 137 to 150 ppm (388, 413). On the basis of 13C NMR analysis of 3-imidazoline-3-oxide derivatives, the position of tautomeric equilibria in amino-, hydroxy-, and mercapto- nitrones has been estimated. It is shown that tautomeric equilibria in OH- and SH-derivatives are shifted toward the oxo and thioxo forms (approximately 95%), while amino derivatives remain as amino nitrones (413). In the compounds with an intracyclic amino group, an aminonitrone (A) - A-hydroxyaminoimino (B) tautomeric equilibrium was observed (Scheme 2.76), depending on both, the nature of the solvent and the character of the substituent in position 2 of the heterocycle (414). 

Bei Verwendung von Formaldehyd als Aldehyd-Komponente wurden mil mehreren unter-schiedlichen a-Hydroximino-ketonen keine Imidazol-3-oxide, sondern 2-Hydroxy-imidazo-le erhalten, die als tautomere 2-Oxo-2,3-dihydro-imidazole vorliegen71b. 

The aromatic imidazole N-oxides have structure 245. The parent compound 246 displays tautomerism between the N-oxide tautomer 246 and the 1-hydroximidazole tautomer 247 (Scheme 68). 1-Hydroxyimidazole 247 and substituted 1 -hyd roxyimidazoIes 249 belong to a separate group of compounds, which is not discussed in the present review. 

The crystal molecular structure of /V1-hydroxylophine A -oxide (= I -hydroxy-2,4,5-triphenyl-17/-imidazole 3-oxide) (59) has been determined [72], This compound presents a case of degenerate or autotrope tautomerism (59a and 59b are identical), which is very common in annular tautomerism of NH-azoles but very rare in functional tautomerism [40], In the solid state, the tautomeric proton is in the middle between two consecutive monomers of the catemer [59]n [72],... 

The photolysis of imidazole and benzimidazole W-oxides has been described earlier (Section 4.07.1.2) as has their tautomerism (Section 4.06.5.2). 

Electrophilic attack in imidazole is usually most facile at an annular nitrogen, and there are many examples of Af-aUcylation, -protonation, -acylation, cyanation, -arylation and -silylation. A-Nitration is much less common A-oxidation is virtually non-existent. When an annular nitrogen becomes substituted, tautomerism in the molecules is blocked, and mixtures of isomers are usually formed with substituted benzimidazoles and 4(5)-substituled imidazoles. 

Many workers in recent years have been interested in the tautomeric equilibria pertaining to oxygenated imidazoles such as imidazolones, " " N-hydroxyimidazoles, and imidazole iV-oxides. The earlier results have been critically summarized. In the solid state l,2-dimethyl-5-phenylimidazol-4-one exists in the OH form, but insolubility prevented studies in solution. Theoretically l-methylimidazol-5-ones can exist in the forms 93-96 shown in Eq. (22). French workers ... 

Among the species produced upon radiolysis of water, hydroxyl radical ( OH) is the most reactive. Indeed, its reaction rate with the four bases and related nucleosides is diffusion-controlled. The main reactive sites of hydroxyl radicals on nucleobases are the double-bonds of the heterocycles. Accordingly, addition of OH at the C8 position of adenine and guanine yields the corresponding reducing 8-hydroxy-7,8-dihydropurin-7-yl radical (Fig. 2). Oxidation of this intermediate leads to the formation of related 8-hydroxypurines that are in dynamic equilibrium with their more stable 8-oxo-7,8-dihydropurine tautomeric form. Competitive reduction of the latter purine radical gives rise to imidazole ring opened compounds the... 

It was assumed that the mechanism resembles to some extent the mechanism of the Fischer reaction. Undoubtedly, the primary products of the reactions are the Schiff bases 466 and 467. The former can be isolated if the reaction is carried out at a lower temperature. The formation of 467 is supported by the noticeably increased yield of 468 if mesityl oxide or dypnone are used in reaction instead of acetone or acetophenone. Further steps include tautomerization of Schiff base 467, the attack by the terminal methylene group on electron-defficient position 2 in the imidazole ring, the cleavage of an N—N bond, and recyclization followed by aromatization with elimination of a molecule of ammonia. 

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