Pyrazole dimers

We have published some papers concerning some less common heterocycles such as 1,2,4-diazaphospholes 257 and 258 [203], Using X-ray crystallography, CPMAS NMR, and GIAO-type calculations we have found that 257 is a cyclic dimer with localized N-H protons (similar to pyrazole dimers) while 258 is probably a tetramer (similar to pyrazole tetramers) showing ISSPT (intramolelecular solid state proton transfer). This prediction was only partly true because 258 crystallizes in two cyclic dimers, both presenting proton disorder [204] (Fig. 8). 

An early example is the cyclic trimer of AfAf-bis(pentadeuterophenyl)-l-amino-3-iminopropene 15, the dianil of malonaldehyde (Figure 14.8) dissolved in CS2. This trimer dominates at low temperature whereas at room temperature a monomer is observed [40,41]. In the trimer a triple proton transfer is observed. With respect to a single molecule, the two other molecules constitute catalysts that accept a proton at one nitrogen site and transfer another proton to the other nitrogen site. Thus, the tautomerism is Unked to an intermolecular proton transfer. The double proton transfers in solid cyclic carboxylic acids dimers 16 [42], bis-arylformamidine dimers 17 [43], substituted pyrazole dimers 18 [44], and the triple proton transfer in cyclic substituted solid pyrazole trimers 19 [44] constitute similar processes. 

Imidazoles and pyrazoles with free NH groups form hydrogen-bonded dimers and oligomers (66AHC(6)347). 

Experimental values are collected in the McClellan book (B-63MI40400) and in a review on dipole moments and structure of azoles (71KGS867). Some selected values are reported in Table 3. The old controversy about the dipole moment of pyrazole in solution has been settled by studying its permittivity over a large range of concentrations (75BSF1675). These measurements show that pyrazole forms non-polar cyclic dimers (39) when concentration increases and, in consequence, the permittivity value decreases. 

Pyrazoles with free NH groups form hydrogen-bonded cyclic dimers (195) and trimers (196) as well as linear polymers, depending on the substituents at positions 3 and 5. For R = H, Me or Et, the oligomers are preferred, but for R = Ph, the cyclic dimer and the linear polymers exist. The cyclic trimer (196 R = Ph) is) is not formed because of steric hindrance (B-76MI40402). 

Analogously, pyrazolyl-aluminate and -indate ligands have been prepared <75JCS(D)749) and their chelating properties evaluated with cobalt, nickel, copper and zinc. Gallyl derivatives of pyrazoles and indazoles have been extensively studied by Storr and Trotter e.g. 75CJC2944) who determined several X-ray structures of these compounds. These derivatives exist in the solid state as dimers, such as (212) and (288). A NMR study in acetone solution showed the existence of a slow equilibrium between the dimer (212) and two identical tautomers (289) and (290) (Section 4.04.1.5.1) (81JOM(215)157). 

Many reagents are able to chlorinate aromatic pyrazole derivatives chlorine-water, chlorine in carbon tetrachloride, hypochlorous acid, chlorine in acetic acid (one of the best experimental procedures), hydrochloric acid and hydrogen peroxide in acetic acid, sulfuryl chloride (another useful procedure), etc. iV-Unsubstituted pyrazoles are often used as silver salts. When methyl groups are present they are sometimes chlorinated yielding CCI3 groups. Formation of dimers and trimers (308 R = C1) has also been observed. 

A -Pyrazolines such as (410) are oxidized by iodine, mercury(II) acetate and trityl chloride to pyrazolium salts (411), and compound (410) even reduces silver nitrate to Ag° (69JOU1480). Electrochemical oxidation of l,3,5-triaryl-2-pyrazolines has been studied in detail (74BSF768, 79CHE115). They Undergo oxidative dimerization and subsequent transformation into the pyrazole derivative (412). 

Acid chlorides are useful reagents, but when the pyrazole is N- unsubstituted a dimerization occurs and the diketopiperazine (254) is isolated (Section 4.04.2.3.3(x)). However, (254) reacts with many compounds as an acid chloride would, for example with amines to yield amides (67HC(22)l). The difunctional pyrazole derivative (441) affords polymers by reaction with diphenols (69RRC763). Cyanopyrazoles can be hydrolyzed to the corresponding carboxylic acids (68CB829). 

Examples of dynamic processes involving two, three, or four identical tautomers (degenerate or autotrope annular tautomerism) have been found in pyrazoles (type 2 of Table VII). Thus, 3,5-diphenyl-4-bromopyr-azole and 3,5-di-ferf-butylpyrazole (dimers), 3,5-dimethyl-pyrazole (trimer). 

Lifnbach et al. [92JA9657 97BBPG889] made an exhaustive study of proton transfer in solid pyrazoles. For instance, the activation barriers, isotope and tunneling effects of the dimer 67, the trimer 68, and the tetramer 69 were determined. Catemers, like pyrazole itself, do not show dynamic behavior. 

After these findings, evidence was presented that pyrazoles which crystalline as cyclic dimers 9a/9b and tetramers 11a are subject to double and quadruple proton transfers respectively (92JA9657 95JOC1965). An im-... 

Tetraazafulvalenes bearing two pyrazole subunits could be prepared by an original way. Tlius, treatment of benzylidene acetophenone with iso-pentylnitrite leads to an A, A -dihydroxy-bipyrazolyl-A, A -oxide, which in turn can be oxidized to TAF of type 100 (72CC961, 79JOC3211). Another type of oxidative dimerization was observed by the reaction of the electron-rich l-methyl-2,4-bis(dimethylamino)imidazole with silver salts (83TL3563). A bis-cation was isolated in 30% yield in the presence of sodium tetrafluo-roborate an unsymmetrical structure 101 was predicted from its NMR data (Scheme 40). 

A series of pyrazoles [pyrazole, 3,5-dimefliyl-, 3(5)-ferf-butyl, 3-methyt-5-ferf-butyl-, 3(5)-phenyl-,3-methyl-5-phenyl-, 3(5)-p-meflioxyphenylpyrazole] reacts wifli [Rh(/r-Cl)( 7 -nbd)]2 in die presence of potassium hydroxide to give [Rh(/r-Pz)(jj -nbd)]2 [96JOM(511)l 15]. For the 3(5)-tm-butylpyrazolato dimer. 

Reaction of the cyclometallated complexes 244-246 with pyrazole and excess sodium hydride affords cyclic dimers 247 (990M3991), where C N denotes the corresponding cyclometallated ligand in accordance with structures 244-246. The [Pt2(thienylpyridine)2(/j.-pz)](C104)3] is known as well. 

Au-C bonds, mixed metal gold-silver dimers of planar, trinuclear complexes are readily formed by mixing gold(I) carbeniates and gold(I) benzylimidazolates with silver(I) pyrazolates in stoichiometric ratios. The complexes retain the ligands associated with the metal atoms of the starting materials. 

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