Dianions, proton sponges

It was already mentioned (Section III.A.2) that 4,5-dihydroxy-l,8-bis(dimethylamino) naphthalene actually exists in the form of the internal salt 11139. Somewhat similar is the situation with 2,7-dihydroxynaphthalene 140, also behaving in solution as a betaine165. The latter, on treatment with acids, gives the cation 140-H+, while the action of alkali results in protonated dianion 141 (Scheme 15). All attempts to deprotonate 141 (even with the KOH-DMSO system at 100 °C or with LiH-DMSO) failed. This implies that the basicity of still unisolated dianion 142 lies in the limit of pKa = 25-26 (H2O scale). Thus, dianion 142 can be presently considered as the strongest base among all proton sponge and aniline derivatives. 

---