Some new information on the formation of substituted 4-amino-1-substituted phenyl-1H-pyrazoles from β-enaminones and diazonium tetrafluoroborates

Upon reaction of 2-methyl-, 3-ethoxycarbonyl, and 4-ethoxycarbonylbenzenediazonium tetrafluoroborate with 1-cyclopropyl-3-phenylaminohex-2-en-1-one 3-cyclopropylcarbonyl-1-(substituted phenyl)-5-ethyl-4-phenylamino-1H-pyrazoles are formed. On the other hand, the reaction of 1-cyclopropyl-3-phenylaminohex-2-en-1-one and 5-methylaminohept-4-en-3-one with sterically more demanding 2-ethoxycarbonylbenzenediazonium tetrafluoroborate does not give the corresponding pyrazoles but the probable intermediates on the route to the pyrazoles: 1-cyclopropyl-3-phenyliminohexane-1,2,4-trione 2,4-bis(2-ethoxycarbonylphenylhydrazone) and 3-methyliminoheptane-2,4,5-trione 2,4-bis(2-ethoxycarbonylphenylhydrazone), respectively. All the compounds were identified on the basis of 1H- and 13C-NMR spectra. The structure of 1-cyclopropyl-3-phenyliminohexane-1,2,4-trione 2,4-bis(2-ethoxycarbonylphenylhydrazone) was confirmed by means of 15N-NMR spectra and X-ray. The bis(2-ethoxycarbonylphenylhydrazones) were found to show atropoisomerism due to a hindered rotation around the bond between the carbons of imino group and the hydrazono group next to carbonyl. In the case of the crystalline cyclopropyl derivative, the unit cell was found out to contain two molecules of opposite chirality. J. Heterocyclic Chem., (2011).