“Proton Sponges” and the Geometry of Hydrogen Bonds: Aromatic Nitrogen Bases with Exceptional Basicities

Abstract

Certain aromatic diamines (the "proton sponges") are found to have exceptionally high basicity constants: this is due to spatial interaction of the basic centers, which are in close proximity. The two factors which are most important in causing this effect are, on the one hand, the extreme steric strain in these systems and the destabilizing effect of the overlap of the nitrogen lone pairs of the neutral diamines and, on the other, the strong N . . H . . . N hydrogen bonds which are formed on monoprotonation and which lead to a considerable relaxation of the steric strain. By the systematic variation of the structures of such aromatic diamines we have been able to study these effects as a function of steric factors, in particular of the geometry and the bond length of the N -. . H . . -N hydrogen bonds, by means of X-ray structural analysis. The hydrophobic shielding of the basic centers and the N . . . H . . . N hydrogen bonds, which was characteristic of the "proton sponge" compounds studied previously, is indeed responsible for the extremely low rate of protonation and deprotonation of these compounds; however, it apparently has no influence on their high thermodynamic basicity. The recent synthesis and basicity determination of a new type of "proton sponge" with no hydrophobic shielding whatever show that not only very strong but also kinetically active bases are accessible using the "proton sponge" concept. Their unusual properties, which are discussed here as the result of steric interactions between two basic centers, provide examples of the fact that cooperative steric interactions of reactive structural elements can lead to properties which cannot be derived from an isolated consideration of the various functional groups. Such "proximity effects" are certainly of general importance in chemistry and biochemistry; the study of their structure-function relationships is worthy of closer consideration.

Synthesis of an extra-large molecular sieve using proton sponges as organic structure-directing agents

“…The adsorption data observed, both from Ar and N 2 adsorption measurements, would indicate that the framework structure of ITQ-51 may contain 1D extra-large (14-or 16-ring) pores. 27 Al, 29 Si, and 31 P MAS NMR spectra of the calcined ITQ-51 material were recorded to obtain information on the local atomic environments, which also may be very useful for structure elucidation. The 31 P MAS NMR spectrum shows at least four welldefined, distinct crystallographic positions for P atoms (SI Appendix, Fig.…”

“…S6). The 27 Al MAS NMR spectrum shows a broad band between 20 and 45 ppm (SI Appendix, Fig. S7), clearly indicating that all aluminum species in the ITQ-51 molecular sieve are in tetrahedral coordination (no signal is observed close to 0 ppm, characteristic of Al in octahedral coordination).…”

“…1)-are bulky aromatic diamines with amine groups in close proximity (27). These molecules present a highly unusual basicity because of the repulsion of the close electronic lone pairs (pK a ≈12.1) (28).…”

Acid–base-driven matrix-assisted mass spectrometry for targeted metabolomics

“…To explore this concept, we investigated ion pairs of 4 aromatic bases with aliphatic acids monitoring the formation of anions using TOF mass spectrometry. The first experiments were performed with a ''proton sponge'' (15,16), which is a strong base [pK a of its conjugate acids 12.5 (15)], 1,8-bis(dimethylamino)naphthalene (DMAN), never previously used as a matrix. Analytes (fatty acids, fatty acid-amino acid conjugates and other anionic species) were mixed in a 1:1 molar ratio with DMAN in ethanol and allowed to dry on a MALDI metallic target.…”

Synthetic and oxidative studies on 8-(arylamino)-2'-deoxyguanosine and -guanosine derivatives.

“…In the absence of added amine hydrobromide, the reaction is extremely slow, whereas the addition of the proton sponge (1,8-diaminonaphthalene) completely inhibits both the substitution and the depurination reactions (26). Under the conditions, even after 60 hr at 1 10°C, both starting materials were the only substances present.…”