1,8-Bis(dimethylamino)-4,5-dihydroxy-naphthalene, a Neutral, Intramolecularly Protonated 'Proton Sponge' with Zwitterionic Structure

摘要

The interaction of basic groups in close proximity to each other may lead, as in the case of 1,8-bis(dimethylamino)-naphthalene (1), to unusually high basicities ("proton sponges"). The influences of gradually changed distances and orientations of thebasic centers as well as of inductive, mesomeric, and steric effects on the basicity of such compounds have been thoroughly studied. In comparison to the basicity of 1 [pK_a approx 12.1 (H_2O); 7.5 (DMSO)], that of 2,7-dimethoxy-1,8-bis(dimethylamino)naphthalene (2) is found to be increased by four powers of ten [pK_a approx 16.1 (H_2O); 11.5 (DMSO)]. To separate the mesomeric effect of the two methoxy groups from their steric effect on the dimethylamino groups, we were interested in 3, an isomer of 2in which the two methoxy groups are not in the 2,7-positions but in the opposite peri-positions. In fact, 1,8-bis(dimethylamino)-4,5-dimethoxynaphthalene (3) is considerably less basic [pK_a appprox 13.9 (H_2O); 9.3 (DMSO)] than the isomer 2, indicatingthat the main reason for the high basicity of 2 is the steric effect of the methoxy groups in ortho-positions to the dimethylamino groups. Irrespective of this primarily intended basicity comparison of 2 and 3, the synthesis of 3 should allow the easy preparation of the corresponding 4,5-dihydroxy compound 5, which by intramolecular proton displacement may lead to a new type of neutral, yet zwitterionic "proton sponge" (formula 6).