Structure phenylpyrazoles

Pyridazinones may undergo ring contraction to pyrroles, pyrazoles and indoles, the process being induced either by an acid or base. The structure of the final product is strongly dependent on the reaction conditions. For example, 4,5-dichloro-l-phenylpyridazin-6(lFT)-one rearranges thermally to 4-chloro-l-phenylpyrazole-5-carboxylic acid (12S), while in aqueous base the corresponding 4-hydroxy acid (126) is formed (Scheme 40). 

Italian investigators" have discussed the tautomerism of 5-amino-3-methyl-l-phenylpyrazole in terms of structures 183 and 184 (185 was not considered) and appear to be of the opinion that reaction... 

Recently, a somewhat different synthetic approach has been reported. Halcrow et al. (215) synthesized a series of five-coordinate copper(II) complexes comprising a tridentate tris(pyrazolyl)borate ligand and a bidentate phenol derivative. Neutral complexes [Cun(TpPh)(bidentate phenolate)] were synthesized and structurally characterized [Tpph] = hydrido-tris(3-phenylpyrazol-l-yl)borate. The species [Cun(TpPh)(2-hydroxy-5-methyl-3-methylsulfanylbenzaldehydato)] can electro-chemically be converted to the (phenoxyl)copper(II) monocation, which has been characterized in solution by UV-vis spectroscopy. It displays two intense absorption maxima at 907 nm (e = 1.2 x 103 M 1 cm-1), and 1037 (1.1 x 103 M l cm-1), resembling in this respect the radical cofactor in GO (Fig. 7). 

Benzoyl-3-diazo-4-phenylpyrazole, decomposed in hot sulfuric acid (50%) with evolution of nitrogen but no pure product, could be isolated [60CI(L)659]. By contrast, 4-diazopyrazoles were very stable in strong acids [60CI(L)659], especially the unconventional diazo 22a in which, due to the particular mesomeric structure, protonation at the ipso carbon is not... 

Another type of substituted pyrazole is obtained by refluxing osazones with acetic anhydride during the acetylation, two molecules of water per molecule are removed. The structure of the resulting 5-(D-g/ycero-diacetoxyethyl)-3-formyl-l-phenylpyrazole 2-acetyl-2-phenylhydrazone (71) was determined by deacetylation followed by oxidation with (a) periodate to a pyrazolaldehyde (72), and (b) permanganate to the known l-phenyl-pyrazole-3,5-dicarboxylic acid (73), as well as by a study of its n.m.r. spectrum, which confirmed the pres-... 

Spiro and Fabra (72MII) reinvestigated the reaction of semicarbazide with benzoylacetonitrile. The authors showed that the product that melted at 170°C, previously reported to be 5-phenylpyrazole-l-carboxamide (52G373), is 3-phenyl-3-ketopropionitrilesemicarbazone. The other product (m.p. 278°C), for which no structure was proposed, is 7-amino-2,5-diphenylpyrazolo[ 1,5-a]pyrimidine (85). 

A number of complexes with ligands related to amides, lactams and antipyrine (2,3-dimethyl-l-phenylpyrazol-5-one), as well as complexes with dicarboxylic acid amides are also known (Table 36), but little structural information is available for them. The diantipyrylmethane complex, UI4-4L-2H20, is probably ionic, [UL4]I4-2H20. 

While there appears to have been no general study of the mercuration of azoles, the reaction seems to proceed readily in several systems. Thus pyrazoles are 4-chloromercurated by HgCl2. Mercuration of 1-phenylpyrazole and l-phenyl-3,4,5-trimethylpyrazole affords organomercury derivatives (152) and (153) (93AJC1323), the structures have been established using 199Hg NMR spectroscopy. 

FIGURE 17. Molecular structure of bis[4-(2 -hydroxyphenylazo)-3-methyl-l-phenylpyrazol-5-onato(2-)]tin(IV). Reproduced by permission of Verlag der Zeitschrift fur Naturforschung from... 

In neutral and alkaline media, pyrazoles unsubstituted in the 1-position undergo hydroxymethylation at that position.653,667 Polymeric material of unknown structure is obtained in acid media, but from 3,5-dimethylpyrazole a small yield of l,4-bis(hydroxy-methyl)-3,5-dimethylpyrazole was isolated.653 Hydroxymethylation was used in the synthesis of the naturally occurring pyrazolyl-alanine.657 When the reaction is carried out in hydrochloric acid on 1 -phenylpyrazole,350 1,1, 5,5 -tetraphenyl-3,3 -dipyrazolyl,219 and other N-substituted pyrazoles,656 it is found that W-alkylpyrazoles give hydroxymethyl, and iV-arylpyrazoles chloromethyl derivatives. The 4-position is always the site of substitution. A side reaction is the linking of pyrazoles by methylene bridges.656... 

The ultraviolet spectra of l-phenylpyrazol-5-ones, particularly the bromo derivatives, were studied by Westoo.72 He concluded that the monocations were usually of structure 59, but that the monocation from bromoantipyrine was in equilibrium with 60 under some circumstances. The spectrum of 4-bromo-3-methyl-l-phenylpyrazol-5-one corresponds to that of its 4-methyl derivative in chloroform, sug-... 

Mercuration of 1-phenylpyrazole and 1-phenyl-3,4,5-trimethylpyrazole affords organomercury derivatives (138) and (139) <93AJC1323>, the structures have b n established using Hg NMR spectroscopy. 

Suggest structures for the isomeric products, C9H7N3O2 formed when 1-phenylpyrazole is reacted with (i) c. H2SO4/C. HNO3 or (ii) AC2O/HNO3. Explain the formation of different products under the two conditions. 

When 4-oxo-5-methyl-lH,4,5-dihydro-l,2,5-benzotriazepine is heated with hydrochloric acid, isomerization occurs with the formation of the N-iminoquinoxaline 4. The structure of this product was demonstrated by reaction with diethyl acetylenedicarboxylate which on hydrolysis and methylation gave the pyrazolo[l,5-a]quinoxaline 5, identical with authentic material prepared from the appropriately substituted phenylpyrazole. ... 

Figure 5 Single-crystal X-ray structures of [CuL] (L = 2,6-/ w [3-phenylpyrazol-l-yl]methyl pyridine) (30) (A) and [CuL(OH)2)2] showing the increase in coordination number that typically occurs...

In a study of the tautomerism of 3-(5)-phenylpyrazoles, the X-ray structure of 4-bromo-3-phenylpyrazole showed that a trimeric structure bonded through an H-bond was formed with a considerable twist of the phenyl group about the pyrazole-phenyl bond (92JCS(P2)1737). 

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