Pyrazoles, Imidazoles and Triazoles

Table 11 Known Compounds MnX2- L for the Pyrazoles, Imidazoles and Triazoles...

V-Alkyl IT-excessive heterocycles such as pyrazoles, imidazoles and triazoles - j-an be lithiated. In the example shown in Scheme 34, lidiiation occurs selectively on the V-methyl in preference to the C-methyl (azaenolate). ... 

On the other hand, the analogous formation and hydrolysis of the compounds based on the more acidic heterocycles, viz. pyrazole, imidazole, and triazole, proceeds easily. Hydrolysis occurs rapidly at room temperature, and the reverse reaction can be brought about completely or at least to a substantial degree, if much more slowly, by continuous elimination of water above 150° C. 

The high reactivity of the derivatives of pyrazole, imidazole, and triazole is likely to be connected with the presence of the second nitrogen atom. The free electron pair of this nitrogen atom is not involved in the aromatic it system and consequently is very prone to electrophilic attack (46). The probable reaction sequence for these molecules is therefore... 

In azole chemistry the total effect of the several heteroatoms in one ring approximates the superposition of their separate effects. It is found that pyrazole, imidazole and isoxazole undergo nitration and sulfonation about as readily as nitrobenzene thiazole and isothiazole react less readily ica. equal to m-dinitrobenzene), and oxadiazoles, thiadiazoles, triazoles, etc. with great difficulty. In each case, halogenation is easier than the corresponding nitration or sulfonation. Strong electron-donor substituents help the reaction. 

The synthesis and chemistry of pyrazoles, imidazoles, and 1,2,3-triazoles were actively pursued in 2006. A review on the cross-coupling reactions on azoles with two and more heteroatoms for pyrazoles and imidazoles has been published <06EJO3283>. Publications relating to 1,2,4-triazole and tetrazole chemistry were not particularly well represented this year. The solid-phase and combinatorial chemistry of these ring systems have not been investigated compared to past years. No attempt has been made to incorporate all the exciting chemistry or biological applications that have been published this year. 

Major advancements in the chemistry of pyrazoles, imidazoles, triazoles, tetrazoles, and related fused heterocyclic derivatives continued in 2000. Solid-phase combinatorial chemistry of pyrazoles and benzimidazoles has been particularly active. Synthetic routes to all areas continue to be pursued vigorously with improvements and applications. Notably, metal-promoted and cross-coupling reactions of all classes seemed to be a dominant theme in 2000. Applications of pyrazole-, imidazole-, and 1,2,3-benzotriazole-containing reagents to a wide array of synthetic applications remained active. 

Aza analogs of cyclopentadienylmanganese tricarbonyl have also been prepared.158 160,162 Application of this sequence to pyrazole, imidazole, and 1,2,4-triazole, however, led not to aza analogs but to coordination polymers.168 A mixed manganese and chromium carbonyl complex of 2-benzylpyrrole has been prepared.163a... 

This conversion employs a boronic acid with a copper(ll) catalyst. The method was developed for reactions of heterocycles, including the V-arylation of isatin ° and of pyrazole, imidazole and their benzo derivatives (1,2,3-triazole, 1,2,4-triazole and 5-phenyltetrazole gave only very modest yields). These conditions also apply to the V-arylation of 2-pyridone (and various fused derivatives), 3-pyridazinones, indole-2-carboxylates and pyrrole-2-carboxylates. ... 

Poly(glycidyl methacrylate) resins were modified with pyrazole, imidazole, and 1,2,4-triazole as shown in 32 and also with bis(benzimidazole)s [89], All resins selectively remove Cu(II) from a solution also containing Cd(II), Co(II), Ni(II) and Zn(II) at pH >2.5. 

C-N bonds are formed by cross-coupling of nitrogen heterocycles such as pyrazole, imidazole, pyrrole, triazole, or indole with aryl halides. The reaction is catalyzed by tris(acetylacetonato)iron and copper(II) oxide in the presence of cesium carbonate as base. A variety of N-arylated N-heterocycles is available by this method (Scheme 4-262). ... 

Typical amphoteric liquids include phosphoric acid and diverse heterocycles such as imidazole, pyrazole, benzimidazole, and triazole. In the liquid state, they all show relatively high proton conductivities with significant contributions from structure diffusion, that is, the motion of protonic defects (excess or deficient protons) by intermolecular proton transfer, coupled to hydrogen bond breaking and forming processes. A promising approach to eliminate the liquid electrolyte is to attach the protic groups of the liquid electrolyte to... 

The MPM group was used in the preparation of a variety of triazoles, imidazoles, and pyrazoles. It is readily cleaved with CF3COOH at 65° (52-100% yield). It is also cleaved from a pyrido[2,3-Z ]indole with DDQ, 88% yield. ... 

All triazoles, tetrazoles, and unsymmetrically substituted imidazoles and pyrazoles can exist in two tautomeric forms, e.g., 1 2 and 3 4. However, attempts to isolate the individual tautomers have been unsuccessful, always leading to one isomer (for summaries of this aspect of the tautomerism of imidazoles, see references 1 and 2). Although the isolation of both tautomers of a number of com-... 

Carbamoylazoles of pyrazole and triazoles dissociate only at 130 °C and 145 °C, respectively. An activating influence of imidazole (significant shortening of reaction time... 

For five-membered heterocycles other than thiazole, (such as pyrazole [27], imidazole [28], and triazole [29]) the effect of replacement of just one pyridine moiety in 1 is greater and the [Fe N6]2+ derivatives in these instances show crossover behaviour. The [Fe N6]2+ derivative of 2-(pyridin-2-yl)imidazole 19 (Dq(Ni2+) 1150 cm-1 [22]) was shown relatively early on to be a crossover system [28]. In solid salts and in solution the transition is continuous and centred above room temperature. The dynamics for the 5T2— Ai relaxation for this system have been investigated by a number of techniques [30-32] and Beattie and McMahon have shown that in solution there is not only a spin equilibrium but also a ligand dissociation process, very reasonably ascribed to the high spin form of the tris complex [32]. 

This section is divided into eight subsections, covering imidazoles, pyrazoles, isoxazoles, oxazoles, triazoles, tetrazoles, pyridines, and pyrimidines, purines, and nucleic acid bases respectively. 

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