1,8-Bis(dimethylamino)naphthalene-2,7-diolate: A Simple Arylamine Nitrogen Base with Hydride-Ion-Comparable Proton Affinity

摘要

Since the discovery by Alder et al. of the anomalously high basicity (pKa=12.1, H2O, 25 ℃) of 1,8-bis(dimethylamino)naphthalene (1) , widely known as a proton sponge, much effort has been made to design even stronger bases of similar type. The synthesis of 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene (2) with pKa=16.1 has become the currently unsurpassed success in this direction. The high basicity of 2 was attributed to the +M effect of the two MeO groups, which in conjunction with their buttressing effect destabilizes the base and leads to a large gain in free energy on protonation. This explanation allows one to expect that further increases in pKa may be achieved by substitution of methoxy groups in 2 by more strongly electron-donating dimethylamino groups in 3. However, due to steric hindrance, the electron-donating ability of the NMe2 groups at C-2 and C-7 is diminished, and the basicity of 3 (pKa=15.8) does not exceed that of 2. We therefore assumed that more promising and more basic candidates are 4 and 5 with less sterically demanding OH and O- substituents in the ortho positions. This assumption has been fully confirmed, and herein we report a novel 1,8-bis(dimethylamino)naphthalene derivative with the highest basicity among all currently studied aminoarene systems.