Studies on Paraionic Compounds. Anhydro-1-hydroxy-3-oxopyrazolo[1,2-a]pyrazolium Hydroxides. Formation and Stability of a Novel Series of 4n π Heterocyclic Betaines

Different substituted anhydro-1-hydroxy-3-oxopyrazolo[1,2-a]pyrazolium hydroxides were prepared by the reaction of 1,3-dicarbonyl compounds with derivatives of 4-phenyl-3,5-dihydroxypyrazole. These diazapentalene derivatives belong to a new series of 4nπ cyclic betaines which are named “paraionic” heterocycles. The effects of substituents on the stability of both the anionic and the cationic rings were kinetically studied. Selective cleavage of either the anionic or the cationic ring was achieved by varying the conditions of the reaction with morpholine. Electron releasing groups on the cationic ring and electron attracting groups on the anionic ring enhance the stability of the bicyclic system. They also cause a hypsochromic shift of the visible light absorption.