Quino[7,8‐h]quinoline, a New Type of “Proton Sponge”

Abstract

AbstractSo far, “proton sponges” have been defined as bis(dialkylamino)arenes whose dialkylamino groups are in close spatial proximity.[1] The unusual basicity of these compounds is ascribed to the destabilizing overlap of the lone electron pairs on the nitrogen atoms, to the formation of especially strong hydrogen bonds in the monoprotonated diamines, and to the hydrophobic shielding of these hydrogen bonds. In order to differentiate and assess the relative importance of these factors, we were interested in quino[7,8‐h]quinoline 1, whose nitrogen atoms exhibit a mutual orientation similar to that in 1,8‐bis(dimethylamino)naphthalene 2 (“proton sponge”). In contrast to 2, however, 1 lacks the hydrophobic shielding of the hydrogen bonds of its monoprotonated derivative. This shielding is considered to be responsible for the low rates of proton transfer, which make the “proton sponges” reported so far unsuitable as auxiliary bases in chemical reactions.