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 RFC(O) group increases, 5-RF-pyrazoles are isolated. The reaction of 2-trifluoroacetylcyclohexanone with 2-hydrazinopyradine gives 5-hydroxypyrazoline, an intermediate in the formation of 5-CF3-pyrazoles. The regiochemistry of the condensation is probably controlled by the nucleophilicity of the terminal nitrogen in the mono substituted hydrazines and by the steric requirements of the RF group in the 1,3-diketones. Polymethylene chain containing pyrazoles obtained from the 1,3-diketones in question and hydrazine prefer the 3-RF tautomeric form, irrespective of carbocyclic ring size and polyfluoroalkyl group chain length.Key words: polyfluorinated 1,3-diketones, hydrazines, isomeric pyrazoles, molecular structure.