Reactions of squaric acid with phenylhydrazine

The aim of the present work is to identify the various phenylhydrazones and anilides that are formed when squaric acid is treated with phenylhydrazine. The structures tautomerism, and hydrogen bonding of these derivatives have been analyzed by infra-red and mass spectrometry, and studied by $\sp1$H and $\sp{13}$C NMR, including 2D COSY, 2D J-resolved, and $\sp1$H-detected, single and multiple bond 2D carbon-proton chemical shift correlation techniques. To help in the structure elucidation, asymmetric hydrazines such as N-methyl-N-phenyl- and N,N-diphenylhydrazine were reacted with cyclobutanetetraone to give polyhydrazones incapable of undergoing certain tautomerizations. Finally, a quinoxaline was prepared to determine the structure of cyclobutanetetraone 1,3-bis(phenylhydrazone). The study revealed striking resemblances between the reaction of squaric acid with phenylhydrazine and those occurring during sugar osazone formation. The major products isolated in both reactions are (a) the substrate monophenylhydrazones and (b) the poly(phenylhydrazones) of the substrate oxidation products, (the osazones and the poly(phenylhydrazones) of cyclobutanetetraone, respectively). In addition, the by-products formed during the reaction were similar to those proposed or isolated during osazone formation. For example, 1-anilino-2-phenyl-hydrazino-cyclobutene-3,4-dione is an analog of aldose 1-anilino-1-deoxy-2-phenylhydrazones proposed by Micheel and Dijong, and 3-hydroxy-4-phenylhydrazino-3-cyclobutene-1,2-dione is an analog of aldose phenylhydrazones proposed by Weygand. The intermediates isolated favor a combination of the Weygand mechanism and the Micheel and Dijong mechanism of osazone formation. The reverse reaction to osazone formation was observed for the first time in the present work when cyclobutanetetraone 1,3-bis(phenylhydrazone) was reacted with aniline to afford squaric acid dianilide.