Tautomers and polymorphs of pyrazolinone derivatives, generated from reactions of arylhydrazines, ArNHNH₂, with 1,3-dicarbonyl compounds

From reactions of arylhydrazines with R²COCH₂CONHPh or R²COCH₂CO₂Me, different tautomeric pyrazolinone forms, 1-aryl-3-R²-4,5-dihydro-1H-pyrazol-5-ones (IIa), 1-aryl-3-R²-1H-pyrazol-5-ols (IIb) and 2-aryl-5-R²-1,2- dihydro-3H-pyrazol-3-ones (IIc)], can be isolated. The crystal structures of monoclinic- and triclinic-3-phenyl-1-(3-nitrophenyl)-4,5-dihydro-1H-pyrazol-5- one (IIa forms), tautomeric 1-(4-methylphenyl)-3-phenyl-4,5-dihydro-1H-pyrazol- 5-one (IIa form), and 1-(4-methyl-phenyl)-3-phenyl-1H-pyrazol-5-ol (IIb form), as well as1-(4-methoxyphenyl)-3-phenyl-1H-pyrazo-5-ol (IIb form) and 2-(4-methoxyphenyl)-5-methyl-1,2-dihydro-3H-pyrazol-3-one (IIc form) are reported. The data suggest that the most electron withdrawing aryl group [3-nitrophenyl] generates IIa forms, while the most electron releasing aryl group [4-methoxyphenyl] generates IIb and IIc forms and that the aryl group with the intermediate electronic effect [4-methylphenyl] provides both IIa and IIb forms. This trend is supported by the reported isolations of 1-(4-chlorophenyl)-3-phenyl-4,5-dihydro-1H-pyrazol-5-one and 1-(2,4-nitrophenyl)-3-methyl-4,5-dihydro-1H-pyrazol-5-one [both IIa-forms] with electron-withdrawing chloro and nitro groups respectively and by the isolation of IIb and co-crystallised IIb/IIc forms from use of phenylhydrazine as the reactant.