Isolation of the Half-Condensed Pyrazoline Intermediates en Route to N-(1,10-Phenanthroline)-pyrazole Derivatives from the Knorr Reaction

Abstract During the synthesis of 2-pyrazole-1,10-phenanthroline derivatives by the Knorr reaction from benzoylacetone and 2-hydrazine-1,10-phenanthroline, three half-condensed intermediates, the 5-hydroxyl pyrazolines (2b–2d), were isolated and characterized by NMR spectroscopy and x-ray diffraction (for 2b and 2d). These pyrazoline intermediates could be readily dehydrated under the catalysis of acetic acid to afford the corresponding pyrazoles with high regioselectivity.