3-Methyl-1-phenylpyrazole

In addition to phototransposition, N-substituted pyrazoles have also been observed to undergo two photocleavage reactions to yield enaminonitrile and enaminoisocyanide photoproducts. Thus, 1-methyl-4-phenylpyrazole 24 phototransposes regiospecifically to the P4 product l-methyl-4-phenylimidazole 25 and to the two photocleavage products ( ,Z)-3-( N-methylamino)-2-phenylpropenenitrile 26 and (E,Z)-2-( N-methylamino)-l-phenylethenylisocyanide 27 in the chemical and quantum yields shown in Scheme 13. Furthermore, these photocleavage pathways are not limited to the reactions of 24, they are involved in all other pyrazole to imidazole photoreactions investigated. 

Hydroxy-6-methyl-2-phenylpyridazin-3(2Fr)-one and 4-hydroxy-5-nitropyridazin-3(2FT)-one rearrange in acidic medium to 3-methyl-l-phenylpyrazole-5-carboxylic acid and 4-nitropyrazole-5-carboxylic acid. 4-Hydroxypyridazin-3(2FT)-ones with a hydroxy group or other group at positions 5 or 6, which is easily replaced in alkaline medium, are transformed into 5-(or 3-)pyrazolones with hot alkali. An interesting example is ring contraction of 5-chloro-4-(methylthio)-l-phenylpyridazin-6(lFT)-one which gives, besides pyrazole derivative (127), 4-hydroxy-5-methylthio-l-phenylpyridazin-6(lFf)-one (128 Scheme 41). 

Hi) Electrochemical reactions and reactions with free electrons Electrochemical oxidation of 3-methyl-l-phenylpyrazole gave the 3-carboxylic acid whereas electrochemical reduction (Section 4.04.2.1.6(i)) of l,5-diphenyl-3-styrylpyrazole produced the A -pyrazoline (B-76MI40402) with concomitant reduction of the exocyclic double bond (343). 

In 1973 two papers appeared almost simultaneously (73T101, 73CPB2026) describing the formation, as a minor product, of 3,4,5-trimethoxycarbonyl-l-phenylpyrazole (346) in the reaction between benzaldehyde phenylhydrazone and DMAD (EC=CE). To account for the formation of (346) George et al. (73T101) proposed a tentative mechanism (Scheme 29) involving a Diels-Alder reaction of type (a Figure 25), followed by a retro-Diels-Alder elimination of methyl phenylpropiolate (347). 

Further evidence showed this mechanism to be incorrect, especially the fact that it was methyl cinnamate and not (347) which was isolated from the reaction (73CPB2026). Also 1-phenylpyrazoles did not react with DMAD under the reaction conditions (74BSF2547). The origin of (346) remains obscure, but in no circumstances does it imply a Diels-Alder reaction of a pyrazole. For Ogura et al., it has its origin in an intermediate A -pyrazoline (73CPB2026). 

Fong (80AJC1763) assumes that (o-r°)o (the substituent eonstant for the eonformation where the azole is eoplanar with the benzene ring) is the same for all the pyrazoles. From the experimental value of o-r° he ealeulated the dihedral angle (Seetion 4.04.1.4.3) assuming that (o-R°)a = (o-R°)oeos 6. If instead of -0.165 we assume that (o-r°)o is equal to -0.20 then 6 for 1-phenylpyrazole and 5-methyl-l-phenylpyrazole is 25° and 57°, respeetively. 

Hagenbach used 5-hydroxy-3-methyl-1-phenylpyrazole as a coupling component and made Eriochrome Red B (732), an important chrome dye (B-70MI40403). 

The study of tautomerism using H NMR spectroscopy is simple when the tautomers give separate signals (84B2906) otherwise, interpolation methods need to be applied, which entail several sources of imprecision [83JPR(325)238]. A paper reports the observation of two NH signals for the N-labeled tautomers of 3(5)-methyl-5(3)-phenylpyrazole (45) in toluene-dg at 190 K (Scheme 15) [92JCS(P2)1737]. 

Ring-opening products were observed in the photoisomerization of l-methyl-4-phenylpyrazole (73) (95JOC8138 97JOC8325) and in the reaction of 1-phenylpyrazole (74) (Scheme 27) (93JA7645). 

The mechanism depicted in Scheme 28 accounts for the formation of the ringopening products (97JOC8325). It is a variation of the RCRE mechanism. By contrast, on irradiation l-methyl-5-phenylpyrazole (75) gave 76, 77, and 78 (Scheme 29) (97JOC8325). Compounds 76 and 77 can be obtained via an ICI... 

Biquard and Grammaticakis demonstrated in 1941 that the ultraviolet spectra of 3-methyl- and 3,4-dimethyl-l-phenylpyrazol-5-one were intermediate between those of methylated derivatives of the CH (i.e., 55, R = R — Me) and NH forms (i.e., 56, R = Me, R — H or Me) of the compounds, concluding that the potentially tautomeric compounds exist as mixtures of these forms. However, they did not consider the possible occurrence of an OH form. Other ultraviolet... 

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... 

Methyl-5-oxo-l-phenyl-4,5-dihydropyrazol-4-ylidene)-2,3-dihydro-l//-azepine (3), formed in 62% yield by the condensation of 2-amino- or 2-butoxy-37/-azepine with 3-methyl-l-phenylpyrazol-5(4//)-one, with Meerwein s reagent yields the tetrafluoroborate salt 4 which, on treatment with sodium dihydrogen phosphate, liberates the free base 5.64... 

Methyl-3-(2,3,4-trihydroxy- l-phenyUiydrazonobutyl)-2( 1 //)-quinoxalinone (186) gave 3-(5-acetoxymethyl-1 -phenylpyrazol-3-yl)-1 -methyl-2( 1 //)-qui-noxahnone (187) (neat AC2O, reflux, 3 min 80% note additional acetyla-tion) ° also analogous cyclizations. ... 

The bispyrazolodihydropyran 435 was obtained directly as a rapidly formed insoluble by-product in the reaction of 3-methyl-l-phenylpyrazol-5-one 433 with activated nitriles 434 in the presence of catalytic piperidine (Equation 117) <2000MOL746, 2000JCCS937>. It is proposed that the reaction proceeds by loss of the active methylene moiety from the initial Michael adduct, allowing attack by a second molecule of pyrazol-5-one. 

The ring system in 541 was synthesized (76JHC1249) by interaction of 4-hydrazino-7-phenylpyrazolo[l,5-a][l,3,5]triazine 540 and ethyl pyruvate. The hydrazino derivative 540 was prepared on cyclocondensation of 5-amino-l-thioamido-3-phenylpyrazole 537 with triethyl ortho-formate to give the pyrazolotriazinethione 538 followed by methylation to give 539 and hydrazinolysis to give 540. 

Compound 197 has been treated with carbonyl-containing derivatives such as cyclohexanone and 3-methyl-l-phenylpyrazol-5-one, in refluxing ethanol containing some drops of piperidine as catalyst, in order to promote Michael additions leading to spiro derivatives 198 and 199, where an acetyl group has been eliminated during the process (Scheme 8) <2000FES641>. 

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). 

To carry out the similar MCR involving 5-amino-3-methyl-l-phenylpyrazole, aromatic aldehydes, and 3-cyanoacetyl indoles, Zhu et al. [70] used microwave-assisted synthesis in glygol at 150°C. Application of other solvents was less effective and gave either no reaction products (water medium) or led to the sufficient yield decreasing (EtOH, HOAc, DME). Microwave irradiation was also used by Quiroga et al. [71] to synthesize ether 4-aryl-5-cyano-6-phenylpyrazolo[3,4-b]pyridine-3-ones or their dihydroderivatives under argone atmosphere. 

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