Electrochemical evidence for oxo-enol tautomerism of azaanthracene-4,9,10-triones.

The redox properties of a series of 1-aza-, 1,5-diaza- and 1,8-diazaanthrac ene-4,9,10-triones are reported: The results obtained show that the presenc e of electron-withdrawing heterocyclic nitrogen atoms causes an increase of the reduction potentials, and also the expected similar effect of electron -withdrawing substituents. N-substituted derivatives exhibited a reduction at ca. -1.45 V, which is assigned to the alpha,beta -unsaturated carbonyl s ystem. In contrast with their 2-oxo vinylogues, 1,5-diaza- and 1,8-diazaant hracene-4,9,10-triones show three reduction potentials. On the basis of the results obtained with N-substituted derivatives, the first of these potent ials is attributed to the presence of detectable amounts of the 4-hydroxy- tautomers.