Heterocyclic superbases: retrospective and current trends

Abstract

The structural principles known to date serving as the basis for the design of heterocyclic compounds with anomalously high basicity (superbasicity) are reviewed. A general classification is given for such superbases as proton sponges, compounds that are similar to proton sponges but are not sponges (proton-sponge-like), and proton traps.

Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles – An Interpretation by Triadic Analysis

“…We selected the nitrogen atom, since it represents the basic site of a large variety of strongest superbases known today. 28,34,35 Proper insight into the mentioned intrinsic effect is important, since strain relief on protonation can have major impact on the basicity of strained compounds. For example, this was clearly demonstrated by Abboud, Koppel and co-workers, 36 who employed modified G2(MP2) approach to calculate the gas-phase basicity (GB) of cubane, C 8 H 8 , to be as large as GB = 258.8 kcal mol -1 , and attributed pronounced basicity to the formation of the rearrangement product, namely tetracyclo[4.2.0.0 2,4 .0 3,8 ]oct-7-ylium cation, created after strain relief and the opening of the rigid framework of cubane upon protonation.…”

“…He obtained values of SR = 6.4 kcal mol -1 and HB = -16.0 kcal mol -1 , 39 in that way demonstrating that strain energy largely determines the basicity of DMAN and related compounds, and that it cannot be neglected but rather employed as a useful electronic effect to promote the basicity when designing novel superbases. Furthermore, in a recent article, Alder proposed 41 that the key to the design of even stronger bases than either DMAN or related systems having amino groups replaced by more basic functionalities, 34 is to further enhance ring strain in the initial bases by "limiting conformational freedom, especially by preventing nitrogen inversion, through the introduction of additional ring fusions. "…”

Coordination and acid-base properties of meso-nitro derivatives of β-octaethylporphyrin

“…In this case one can assume that porphyrins with a deformed structure of a macrocycle will be easily deprotonated in the presence of strong bases and rate of complex formation process will be increased. The chemical reactions in the solutions between organic compounds one of which is used as a proton acceptor are well known in the literature [32][33][34]. An important advantage of deprotonating organic bases as compared to alkalis is their solubility in…”

1,8-Bis(bromomethyl)naphthalene in the synthesis of 1,5-diazacyclodecane and benz[de]isoquinoline proton sponges

“…1,2 When the latter criterion is not satisfied it is more appropriate to use a term 'proton sponge-like' compound. 3,4 A typical example is 1,8-bis(dimethylaminomethyl)-naphthalene 2, which is kinetically active despite the fact that its basicity (pK a = 18.3 in MeCN) closely resembles that of 1. Some time ago one of us with co-workers tried to prepare naphthalene derivative 3, which can be considered as an interesting combination of bases 1 and 2.…”