Electronic absorption spectra and protolysis of 4- and 5-azaindan-1,3-diones
The protolytic equilibria in a series of 2-aryl-substituted 4- and 5-azaindan-1,3-diones and the corresponding N-methylbetaines and 5,6-benzo analogs were studied by spectrophotometry. The substantial difference between the 4- and 5-aza derivatives is explained by the different effect of the anionic system on the Tr-electron density and the reactivities of the nitrogen atoms in the 4 and 5 positions. The protonation and deprotonation constants of the dipolar ions correlate linearly with the σ constants of the substituents in the 2-phenyl ring. In the case of 5,6-benzo-4-azaindan-1,3-diones, the keto group of the azaphthaloyl ring is protonated along with the dipolar ion in strongly acidic media.