2- -5-methylpyrazol-3-ones

Complex (NBu4)2[Pd(QCl5)2(/x.-OH)]2 behaves differently in the presence of an alkali with respect to pyrazole and 3-methylpyrazole, on the one hand, and... 

The above method can also be used to simultaneously transform two acetyl groups into acetylenic ones in positions 3 and 5 of the pyrazole ring. This is demonstrated by the synthesis of 3,5-diethynyl- 1-methylpyrazole (yield 62%) from 3,5-diacetyl-1-methylpyrazole (Scheme 30). 

The cyclization of l-alkoxybut-l-en-3-ynes with hydrazine was first achieved by Franke and Kraft (55AG395). By heating 1-methoxybut- l-en-3-yne with hydrazine sulfate in an aqueous alcohol medium they obtained 3(5)-methylpyrazole (13) in high yield. Winter (63HCA1754) used the cyclization of 1-methoxybut-l-en-3-yne with hydrazine hydrate and phenylhydrazine to establish the structure of the initial enyne ether [in this case a mixture of l-phenyl-3(5)-propylpyrazoles was obtained]. The reaction with hydrazine sulfate gives only one product, 3(5)-propyl-pyr azole. 

The reaction of 4-diethylaminobut-3-en-2-one with monosubstituted hydrazines (95°C, H+, MeOH, HjO, 10 h) leads to pyrazoles 261 and 263 (yield 54%) at a ratio depending on the acidity of the medium. In an acidic medium the isomers 263 are predominantly obtained (76ZOR2063). Aminobutenones with hydrazine give only 5-methylpyrazoles (263) (69ZOR223). Enhydrazones 260 and 262 appear to be the synthesis intermediates. 

The triselenadiborolanes 3,5-R2-l,2,4,3,5-Se3B2 R=Et (33), Pr readily formed coordination adducts with two equivalents of pyridine, 3,5-dime-thylpyridine, and 3-chloropyridine.168 With one equivalent of base, only one of the B atoms became coordinated, and surprisingly, the system was not fluxional at room temperature.168 The addition of two equivalents of pyrazole to 33 (Scheme 7) resulted in a brown suspension and a yellow solution. Crystals of a B2N4Se2-bicyclo[2.2.2]octane were formed upon cooling this solution to —80 °C. With bulkier pyrazole derivatives (phenyl-pyrazole), the B2N4Se-bicyclo[2.2.1] heptanes were formed.169... 

One example was reported by Tolman and coworkers (78) who found that the copper(I) complex C Tp112 (TpR2=tris(3-(R2)-5-methylpyrazol-l-yl)hydroborate) promotes NO disproportionation via a weakly bound CuITpR2(NO) intermediate (formally a MNO 11 species). The products are N20 and a copper(II) nitrito complex (Eq. (36)). The rate law established the reaction to be first-order in copper complex concentration and second-order in [NO], and this was interpreted in terms of establishment of a pre-equilibrium between NO and the Cu(I) precursor and the Cux(NO) adduct, followed by rate-limiting electrophilic attack of a second NO molecule (mechanism B of Scheme 5) (78b). 

The last group of metal-free azo pigments are the pyrazolone compounds, the most commonly used examples being made from the coupling component l-phenyl-3-methylpyrazol-5-one, also used in making azo dyes. The two most important pigments are... 

For orange colours, simple 2-naphthol derivatives are the most commonly used coupling components as, for instance, in 2,4-dinitroaniline—>2-naphthol (4.118 Cl Pigment Orange 5). As in the yellow series, superior disazo pigments can be prepared using 3,3 -dichlorobenzidine as tetrazo component with derivatives of 1 -phenyl-3-methylpyrazol-5-one as couplers. 

When only one heteroatom of the dinucleophile possesses a hydrogen substituent, the reactions lead instead to alkenyl complexes rather than carbene compounds. Effectively, treatment of diphenylallenylidenes 1 and 6 with pyrazoles yields the heterocyclic derivatives 61 (Scheme 2.25) [76]. Interestingly, the dissymmetric 3-methylpyrazole (R=H, R = Me) provides only one regioisomer, in which the methyl group points towards the metal. This process, which formally corresponds to the addition of two nitrogen nuclei at C and Cy and a hydrogen atom at Cp, is assumed to take place through an initial nucleophilic attack at the Ca position. 

Azoles with bulky substituents near the donor site, such as 1,2-dimethylimidazole and 3,5-dimethylpyrazole, result in lower coordination numbers.36,37 When a ligand such as 3(5)-methylpyrazole is used, in which the methyl group is adjacent to the donor site in one tautomer and distant in the other tautomer, coordination takes place exclusively through the non-sterically-hindered form of the ligand, i.e. the 5-methylpyrazole. This has been proved by X-ray studies for this ligand38 and for indazole (4).39... 

The introduction of other electron-withdrawing or -donating groups enables nitration of one or another of the rings to be accomplished selectively. Thus, l-phenyl-3-methylpyrazole is nitrated only in the pyrazole ring, whereas 1 -phenylpyrazole-3-carboxylic acid nitrates only in the para position of the benzene ring.586 The only dinitro-pyrazole recorded was obtained from 3-nitro-5-(pyrid-3-yl)-pyrazole.249,587 In the nitration product (54) the nitro group at... 

Based on the assumptions about the reaction mechanism, one can predict that this technique will be applicable to other binucleophiles for the synthesis of perfluoroalkylated heterocyclic compounds. For example, the reaction of arylhydrazine with perfluoro-2-methylpent-2-ene in the presence of triethylamine led to N-arylperfluoro-3-ethyl-4-methylpyrazole 58 and N-arylperfluoro-4-methyl-5-ethylpyrazole 59 in different ratios depending on the reaction conditions (89RP1456419, 87RP1456418, 90IZY2583 89JAP(K)01 22855 99JFC(98)29). Syn- and /-aminoimines are intermediates in syntheses of pyrazoles they were isolated individually. On heating in the presence of triethylamine they are transformed into mixtures of 58 and 59. 

In order to study the 1-methylpyrazole to 1-methylimidazole phototransposition process with minimum substituent perturbation, the phototransposition chemistry of 3,4-dideuterio-l-methylpyrazole (1-3,4d2) has been studied. This labelling pattern allows distinction between the three pathways since Scheme 17 shows that the conversion of 1 to 2 via the three pathways is accompanied by transposition of C-5 of the reactant to ring position 5 by the N-2-C-3 interchange pathway or to ring positions 2 or 4 if the transposition occurs by electrocyclic ring closure followed by one or two nitrogen migrations respectively. 

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