New Polyheterocyclic 4nπ-Electron Dianions: Paratropicity, Charge Delocalization, and Reactions

A novel class of anions, namely, nitrogen-containing polyheterocyclic dianions, is described. These species show a very interesting spectral and chemical behavior. 2,3-Diphenylquinoxaline (1), 1,4-diphenylphthalazine (2), 2,3,7,8-tetra-phenylpyrzo[2,3-g]quinoxaline (3), tribenzo[a,c,i]phenazine (4), and 2,3-diphenyldibenzo[f,h]quinoxaline (5) were reduced by alkali metals to the corresponding 4mr conjugated dianions. The ¹H and ¹³C NMR spectra as well as theoretical calculations (ωß) are reported and discussed in terms of the mode of charge delocalization and its relevance to paratropicity in these systems. The proton NMR line shapes of these charged species correlate with the energy difference calculated between their ground and first excited states. The striking differences in the ¹H and ¹³C spectra of the two apparently similar dianions 1^2- and 2^2- are rationalized by the calculated HOMO-LUMO energy gaps. A very unusual mode of charge delocalization is detected in the NMR spectrum of the doubly charged system 4^2-, rationalized by a partitioning of the conjugated system into an uncharged phenanthrene-like moiety and a benzo[g]quinoxaline dianion moiety. A thermal ring closure reaction of 1^2- and 5^2- has been observed, forming dibenzo[a,c]phenazine dianion 6^2- and phenanthrazine dianion 7^2-, respectively. This reaction opens up a new pathway to synthetic transformations of heterocycles.