Basicity proton sponges

It should be emphasized that unlike the initial theoretical suggestion143, there is no obvious relationship between the proton sponge basicities and the electrostatic repulsion of the unshared electron pairs of the NMe2 groups. This conclusion follows from comparison of values of the n+/n splitting (Figure 5) whose magnitude, AE (Table 13), could serve as a measure of such repulsion. 

The term proton sponges indicates a series of compounds with unusually high basicity, compared to the basicity of isomers. A recent review106 collects and discusses crystallographic results of the literature on this subject. 

Quino[7,8-/i]quinoline (50) and benzo[l, 2-h 4,3-/r Jdiquinoline (52) belong to the so-called proton sponges and possess abnormally high basicities, pKa= 12.8 and 10.3, respectively (89AG(E)84, 89AG(E)86). This is mostly due to strong destabilization of both bases because of electrostatic repulsion... 

Diketones 29 are not intermediates in the formation of chrysenes because under the reaction conditions they form only a-naphthols 204, which are products of intramolecular cyclization. The real intermediate for chrysene formation is the anhydrobase 267. The key role of this compound is supported by the isolation of the dimeric pseudobase 269 (85KGS910), as well as by trapping of the latter compound with the proton sponge, which possesses pronounced basic properties (68CC723). 

H.A. Staab, T. Saupe, Proton sponges and the geometry of hydrogen bonds aromatic nitrogen bases with exceptional basicities. Angew. Chem. Int. Ed. Engl. 27, 865-1008 (1988)... 

Examples Acid salts, acids, proton sponges, HF complexes Acids, alcohols, hydrates, phenols, biological molecules Weak base, basic salts C-H- O/N O/N-H- ji... 

This section covers primary, secondary, tertiary, and quaternary aminopyrazines (both nuclear and extranuclear) but not (functionally substituted amino)pyrazines such as hydrazino-, hydroxyamino-, or azidopyrazines. General discussions have appeared on the spectra of 2-pyrazinamine,255 257 991 the proton-sponge properties of 2,3,5,6-tetra(pyridin-2-yl)pyrazine in relation to its fine structure,925 the fluorescene properties of 3,6-diamino-2,5-pyrazinedicarboxylic acid derivatives in relation to their fine structures,1646,1659 the basic properties of aminopyrazines and other such azines in relation to their electronic structures,412,928 and the fine structures of 3-amino-2-pyrazinecarboxylic acid1340 and l,4-diacetyl-2,3-diphenylpiperazine.559... 

Proton sponges —heteroaromatic bases with exeptional basicities 88AG(E)865. 

Such compounds are very basic (p/T[, = 1.9 cf. NHj, p T, = 4.74 l-dimethyl-aminonapthalene, p/w, = 4.57). and have been nicknamed proton sponges from their avidity for hydrogen ions." - The strong, symmetric N—H—N hydrogen bond (see Chapter 8) stabilizes the conjugate acid. Note, however, that a second proton cannot be added without incurring the original steric problem Diprotonation is only half complete in 86% sulfuric acid ... 

All known proton sponge analogues of this type contain /V-phenyl, /V-acyl, /V-amino or /V-niii oso groups. Though the majority of these compounds is not characterized by increased basicity, consideration of their structural and physicochemical features gives additional insight into the peri-interaction of the amino groups. 

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