Proton‐Sponge‐Like Superbases Built on the Benzo[h]quinoline Platform

Abstract

6,10‐Bis(dimethylamino)benzo[h]quinoline (6) and its “reverse” counterpart, the 6,7‐isomer 7 (a “pyridine‐extended” proton sponge), have been prepared in a three‐step synthetic protocol starting from 1,5‐bis‐ and 1,8‐bis(dimethylamino)naphthalenes, respectively, through azo coupling and Skraup cyclisation reactions. These novel polyfunctional nitrogen bases demonstrate excellent chelation ability towards protons and even PdII, and behave as kinetically active (6) and kinetically inactive (7) compounds, as demonstrated by NMR transprotonation experiments in DMSO. Single‐crystal X‐ray diffractometry was used to characterise rare types of chelated [NHN]+ hydrogen bonds and to reveal the location of the proton in protonated 6 (H+ between NMe2 and the pyridine nitrogen) and its zwitterionic azo‐dye precursor 8Z (H+ between NMe2 and the azo group). Experimental pKa values measured in DMSO show that 6 and 7 are strong bases, and the strength of the basicity of the 6,10‐isomer 6 (pKa = 8.1) places this benzo[h]quinoline in the category of heterocyclic superbases.

Substitution reactions in the acenaphthene analog of quino[7,8-h]quinoline and an unusual synthesis of the corresponding acenaphthylenes by tele-elimination

“…For comparison, in the protonated cation of the starting diazine 5 in CD 3 CN, the chemical shift of the “acidic” proton is observed at 18.02 ppm [ 15 ]. Next, we evaluated the p K a value of base 14 by a competitive method in acetonitrile (NMR transprotonation involving an equivalent amount of "proton sponge" 1 as a reference compound) [ 6 ]. Additionally, we measured the basicity of unsubstituted compound 5 in acetonitrile for the first time by the same method and the results are given in Figure 7 ; the p K a values for compounds 1 and 3 are taken from references [ 24 – 25 ].…”

“…At the same time, quinoline bases are a popular platform for the molecular design of polycyclic systems with receptor properties; they easily form proton complexes with high stability and selectivity [ 5 – 6 ]. This, in turn, attracts the attention of researchers involved in the study of various types of hydrogen bonds and the problem of superbasicity [ 7 – 8 ].…”