4-Methyl/Phenyl-1,2,5,6-Tetraazafluoranthen-3(2H)-Ones Synthesis: Mechanistic Pathway Study and Single-Crystal X-ray Analysis of the Intermediates

The synthesis of 4-methyl/phenyl-1,2,5,6-tetraazafluoranthen-3(2H)-one 4 and 7 has been reported with ninhydrin via a reaction first with ethyl acetoacetate or ethyl benzoylacetate and then a reaction of the resultant esters with hydrazine hydrate. The mechanism of hydrazinolysis and cyclization to obtain tetraazafluoranthen-3(2H)-ones is ambiguous, and the previously proposed mechanism was not based on facts because the actual intermediates were not isolated. Herein, the important intermediates involved in the hydrazinolysis–cyclization mechanistic pathway were isolated and characterized using NMR and X-ray single-crystal analysis. The intermediates demonstrate that the reaction carried out via two hydrazinolysis–cyclization reactions, the first of which includes the condensation of one hydrazine molecule with two ketone groups and the second of which includes the reaction of another hydrazine molecule with the ester and then condensation with the other ketone group. The stability of hydrazide 11 enabled the hydrazine to reduce the carbonyl of the ketone group to form 12 via a Wolff–Kishner-like reduction. The structure of the three intermediates was confirmed using X-ray crystallographic analysis. It was found that the three fused ring systems deviated from planarity to different extents, with their deviation from being coplanar reaching up to 5.3°. The possible non-covalent interactions which control the molecular packing of these intermediates were elucidated with the aid of Hirshfeld analysis.