1.5- Dimethylpyrazole, irradiation

Irradiation of l-(2-azidophenyl)-3,5-dimethylpyrazole affords the corresponding nitrene which cyclizes into the corresponding pyrazolo[l,2-u] benzotriazole <91JA6928> (compare with <84CHEC-I(5)167> formulae (341) and (342)). 

Sometimes thermal and photochemical activation can result in the formation of different products. For example, heating the solution of the ethylrhodium carbonyl complex RhHBPz 3(CO)(C2H4) (Pz = 3,5-dimethylpyrazole) in benzene entails the elimination of the ethylene 7t-ligand and the formation of phenyl-rhodium hydride (complex IV-3 in Scheme IV. 11) [20a]. On irradiation of this solution, the hydride ligand adds not to the metal atom but to the ethylene molecule, which results in the appearance of an o-ethyl group in the complex IV-4 [20b]. 

H.A. Stefani, C.M.P. Pereira, R.B. Almeida, R.C. Braga, K.P Guzen, R. Celia, A mild and efficient method for halogenation of 3,5-dimethylpyrazoles by ultrasound irradiation using N-halosuccinimides, Tetrahedron Lett. 46 (2005) 6833-6837. 

Methylbenzoyl chloride added dropwise at 0° to ethereal diazomethane, kept 20 hrs. at 0-5°, then evaporated in vacuo 4-methylhenzoyldiazomethane (Y 81%) and 3,5-dimethylpyrazole in ether irradiated 20 hrs. with a water-cooled immersion lamp -> N -(4-methylphenacetyl)-3,5-dimethylpyrazole (Y 90%) in abs. ether stirred and treated portionwise at 0° with LiAlH4, stirring continued 10 hrs. at 0°, then treated with 2 N H2SO4 4-methylphenylacetaldehyde (Y 76%). F. e., procedures, and methods s. W. Ried, G. Deuschel, and A. Kotelko,, 642, 121 (1961). 

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