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

Abstract

So 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.

Theoretical studies on the proton affinities of four different series of nano-size diamines and designing strong superbases based on fullerene(C60) molecule

“…1a), is known as ''proton sponge" because of its high thermodynamic basicity combined with a kinetic inactivity to deprotonation that resembles the affinity of a sponge for water [14]. Since the discovery of DMAN as a superbase [13], many proton sponges have been created and they are finding a growing number of interesting applications [15][16][17][18][19][20][21]. One of the strongest superbases designed so far is TMGN (1,8-bis(tetramethylguanidino)naphthalene) shown in Fig.…”

Heterocyclic superbases: retrospective and current trends

“…Recent experience has shown that a much greater effect is obtained by structural organization of the molecule, in particular, through optimization of the directional alignment of the axes of the lone electron pairs of several heteroatoms together. This was first demonstrated by Staab et al [54][55][56] for diazaarenes 37-39, also termed diaza[n]helicenes (Scheme 14). Even the parent compound of this series, quino [7,8:7',8']quinoline 37 is a superbase with somewhat stronger basicity than proton sponge 11.…”

Heteroannulation of 3-Bis(methylthio)acrolein with Aromatic Amines - A Convenient Highly Regioselective Synthesis of 2-(Methylthio)quinolines and their Benzo/Hetero Fused Analogs - A Modified Skraup Quinoline Synthesis

“…These successful results prompted us to extend this process to 1,8-diaminonaphthalene which yielded the novel 2,11-bis(methylthio)quino[7, 8-h]quinoline (10) in moderate yield (Scheme 3). 18 Also the cyclocondensation of 1 with o-phenylenediamine under two steps reaction conditions (method B) provided the 2,9-bis(methylthio)pyrido[3,2-h]quinoline 19 (12) in the moderate yield of 40%. The structures of these condensed quinolines were confirmed with the help of spectral and analytical data.…”