Novel polyfluoroalkylated pyrazoles from 2-polyfluoroacylcycloalkanones and hydrazines: syntheses and unequivocal molecular structure assignment

The reaction between 2-trifluoroacetylcycloalkanones and methylhydrazine or phenylhydrazine exclusively yields 3-CF3-pyrazoles. When the chain length of the polyfluorinated substituent in (RC)-C-F(O) group increases, 5-R-F-py razoles are isolated. The reaction of 2-trifluoroacetylcyclohexanone with 2 -hydrazinopyradine gives 5-hydroxypyrazoline, an intermediate in the format ion of 5-CF3-pyrazoles. The regiochemistry of the condensation is probably controlled by the nucleophilicity of the terminal nitrogen in the mono subs tituted hydrazines and by the steric requirements of the R-F group in the 1 ,3-diketones. Polymethylene chain containing pyrazoles obtained from the 1, 3-diketones in question and hydrazine prefer the 3-R-F tautomeric form, irr espective of carbocyclic ring size and polyfluoroalkyl group chain length.