8-Bromo-4,5-bis(dimethylamino)quinoline: Synthesis, protonation, and the first structural studies of quinoline “proton sponge” as a base

Abstract

Synthesized 8-bromo-4,5-bis(dimethylamino)quinoline is the simplest and at the same time the first representative of quinoline “proton sponges”, existing as a base in a crystalline form, and not as an oil. This allowed to perform the XRD study both for the title compound and its salt with HBF4. The results obtained revealed a significant conjugation of the cyclic nitrogen atom with peri-NMe2 groups, which initiates a noticeable planarization of the latter and lengthening the distance between them. As a result, the mono-protonated salts of the title quinoline, either in solid or in solution, exist completely in the quinolinium form. To compare the effect of peri-NMe2 groups on the stability of naphthalene and quinoline proton sponges, the structural index of their aromaticity was calculated for the first time.