Pyrazoles radicals from

Eq. 5.17 shows ipso substitution of imidazoles (30) at the 2-position by an sp3 carbon-centered radical. Here, the tosyl group plays an important role in both activation of the ipso 2-position and as an effective leaving group, Ts [40-46]. The phosphoryl group also shows the same leaving ability as the tosyl group. Preparation of [1.2-b]-fused bicyclic pyrazoles (33) from l-(phenylselenoalkyl)pyrazoles (32) also works well, as shown in eq. 5.18. 

Substituted 3-anilino-l,5-diphenylpyrazoles were oxidized with lead(iv) tetraacetate in benzene or methylene chloride solution <1997M261>. ESR measurements confirmed the formation of aminyl radicals from /irzra-CFIs-substituted pyrazoles. The radical intermediates from unsubstituted pyrazoles were recognized by their transformations to triarylaminium cation radicals. These were generated by consecutive oxidation of the dimeric products the structures of which were proved by NMR spectroscopy and labeling. 

Since the authors did not succeed in obtaining an ESR spectrum, they were unable to decide whether the IV-pyrazolyl radical is of the a (112a) or the v (112b) type. Ab initio calculations indicate that the radical has Bi (rr) symmetry (76T1555). However, the radical is formed from (111) as a cr radical and is able to react as such in its lifetime. This is in agreement with the experimental results (75JOC915), no C-phenylated pyrazoles being detected. 

To shed light on the reaction pathway, a cyclization with methylhydrazine was carried out (70ZOR439 73ZOR832), since pyrazoles with nonequivalent positions 3 and 5 are obtained in this case. From l-methoxybut-l-en-3-yne and methylhydrazine sulfate a 4 5 mixture of 1,3- (110) and 1,5-dimethylpyrazoles (111) was formed (GLC) (70ZOR439). In the reaction of methoxybutenyne homologs (R = Ft, -Pr) with methylhydrazine (72°C, H", H2O, 2 h), lengthening of the alkyl radical leads to an increase in the content of 1,5-isomer (111), with yields of 50-60%. Thus, when R = Ft, the ratio is 2 3 (73ZOR832). 

Brown has also predicted, from localization energy calculations, that pyrrole and glyoxaline should react with radicals mainly at the 2-position, whereas pyrazole should be most reactive at the 3-position. Browm and Heffernan s calculation that the orientation in pyrimidine substitution should be 4 > 2 > 5 is in agreement with the results from the p-nitrophenylation of pyrimidine. ... 

Although N-halogenated pyrazoles are known, they are quite unstable compounds, increasing in stability from chloro to iodo. They may be implicated in the formation of poly halogenated derivatives (76MI2). There are many examples of competition between nuclear and side-chain halogenation, particularly when radical species can form (90CHE301). 

The use of stable derivatives of leuco dyes has been proposed. These have the advantage over dye developers in that because they are colourless, they can be coated in the appropriate emulsion layers. Examples are the pyrazole (79) and the pyrazolotriazole (80) (75BRP1415537). Processing in alkali results in loss of the acetyl groups from the oxichromic developers and transfer to the mordant from areas where no exposed silver halide is present. The leuco dye can be oxidized in this mordant layer by an incorporated oxidant such as the nitroxyl radical (81) (77BRP1478995). 

The regioselectivity of pyrazole 55 formation is completely the reverse of that of pyrazoline 57 in the reaction of ketones 54 with hydrazines without DPPH. In addition, heterocycles 56 cannot be obtained from 57 by the action of DPPH. These facts show that the radical-initiated treatment of a, 3-unsaturated ketones with hydrazines does not occur via the initial pyrazoline 57 formation. 

An unexpected [1.4] migration of a trifluoromethyl group was observed when azomethine imines were synthesized from hexafluoroacetone azine and alkoxyalkynes. The rearrangement, which occurs at temperatures as low as 0°C, is probably a radical process and results in the formation of A[-(perfluoro-tert-butyl)pyrazoles (79CC792). The formation of... 

In a bid to prove that the ligand is the source of the hydrogen atoms, the reaction was repeated with the analogous Tp -complexes. We reasoned that hydrogen atom abstraction from the isopropyl groups would generated isopropyl radicals, which should disproportionate into isopropyl- and isopropenyl-substituted ligands. Reaction of Tp "Pf Co-02 with half an equivalent of [(Tp -Pf MeCo)2( 4-N2)l produced a mixture of products, one of which was the dinuclear hydroxide complex [(Tp Co)2(ti-OH)2]. Hydrolysis of the whole product mixture, and examination of the recovered pyrazoles indeed showed that 10 % of the pyrazole fraction was 3-isopropenyl-5-methylpyrazole. 

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