Diazoles imidazoles

Bibliography h 76EW,76PJB,77ASa,77ERG Co 77ASa 

Other references 52A,53P,60HJ,62HJ,64AR, 66PS.70Z 

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Diazole (imidazole) is a planar molecule with substantial delocalization (resonance) energy. 

Despite the greater acidities of the diazoles and triazoles (pKa = 10-14), fairly vigorous reaction conditions are still required for the alkylation of the unsubstituted systems [26] (Table 5.32) and the effectiveness of the alkylation of pyrazoles and imidazoles is enhanced under solidiliquid conditions [27-30]. Under these conditions, quaternization is avoided if no solvent is added [27],... 

As with the reaction of pyrroles, diazoles and triazoles react with propargyl bromide to yield TV-substituted products and, depending upon the base strength, either TV-prop-2-ynylazoles or allenic derivatives are formed [30]. Generally, with potassium carbonate under soliddiquid two-phase conditions at room temperature in the absence of a solvent, the prop-2-ynyl compounds are formed as sole products, whereas with solid potassium hydroxide at elevated temperatures the allenes are obtained as the major products. Benztriazole produces a mixture of the N1- and N2-prop-2-ynyl, and N2-allenic derivatives, whereas with potassium hydroxide only the N -allenic derivative is obtained. The alkynes readily isomerize to the allenes in the presence of base and the quaternary ammonium salt, or upon treatment with methanolic sodium hydroxide. A series of l-(alk-2-ynyl)imidazoles have been prepared, as intermediates in the synthesis of imidazopyridines [31 ] and the reaction of 3-hydroxymethylpyrazoles with propargyl bromide leads to pyrazolooxazines [32]. 

In general, the 1,3-diazoles do not react by nucleophilic substitution, although imidazole can participate in the Chichibabin reaction with substitution at C-2 the position of substitution is eqmvalent to that noted with pyridine (see Section 11.4.1). Nucleophilic species that are strong bases, like... 

Pyrazole and Imidazole. These are five-membered ring heterocycles containing two nitrogens, one basic and one neutral. Two common forms are pyrazole (1,2-diazole) and imidazole (1,3-diazole) both are prone to aromatic hydroxylation. 

Diazole,Glyoxo1ine or Imidazole (called Imidazol or Glyoxalin in Ger),... 

TT-Electron delocalization in isoxazole seems to be more effective than in oxazole however, isothiazole is less aromatic than thiazole thus it is not a general rule that 1,2-diazoles possess higher aromaticity in comparison with 1,3-diazoles. Oxygen-containing heterocycles are always less aromatic than their sulfur and nitrogen counterparts, e.g. thiazole > imidazole > > oxazole. At the same time, the relative aromaticity of S- and N-containing heterocycles can interchange (pyrazole > isothiazole > isoxazole). 

Van Meeteren and van der Plas100 oxidized several imidazoles to oxa-diazoles. They proposed the formylamidine intermediate shown in Eq. (25) since alkaline oxidation of imidazoles is known101 to produce... 

The ending -ole is systematic and refers to a five-membered heterocyclic ring. All the flve-membered aromatic heterocycles with nitrogen in the ring are sometimes called the azoles. Strictly speaking, pyrrole is "azole, pyrazole is 1,2-diazole", and imidazole is 1,3-diazole. These names are not used but oxazole and thiazole are used for the oxygen and sulfur analogues of imidazole. 

Among radicals from diazoles, in the imidazole series hydantoin, methyl-hydantoin, and thiohydantoin have given various radicals e.g., 159, differing in degree of protonation, upon radiolysis of aqueous solutions.546 Values of... 

Carbon-13 magnetic resonance studies have been made on imidazole and its corresponding anion and cation.241-243 Linear relationships have been found243 between the 13C and proton chemical shifts and the Hiickel 7r-electron densities in diazoles and triazoles. 

As discussed in the General Chapter (Section 4.01.5), imidazoles with potential hydroxy, thiol and amino substituents can exist in a variety of tautomeric forms. In contrast to the hydroxypyrazoles which have been studied in detail, comparatively little is known about the corresponding 1,3-diazoles. Many of the imidazoles are not easily accessible synthetically, and they may not be particularly stable e.g. 4-hydroxyimidazoles). Amino derivatives... 

Usually the 2- or 4-imidazolines that are formed in the cyclization of 3- or 2-aza-l,3-butadienylnitrenes are dehydrogenated under the reaction conditions to imidazoles. There are, however, some cases in which the hydrogenated diazoles can be isolated in moderate yields (79CHE467). 

N-arylimidazoles, important compounds in medicinal research, have been synthesized by nucleophilic aromatic substitution and Ulmann-type coupling. Aromatic substitution is, however, limited by the need for substrates activated by electron-withdrawing groups. The arylation of diazoles and triazoles, e.g. imidazole, by p-tolyllead triacetate compares very favorably with the Ullmann and related methods in that the conditions employed are much milder and the yields are usually excellent and reproducible (Scheme 13.43) [64]. 

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