Basicity Limits of Neutral Organic Superbases

Abstract

The potential limits of superbasicity achievable with different families of neutral bases by expanding the molecular framework are explored using DFT computations. A number of different core structures of non-ionic organosuperbases are considered (such as phosphazenes, guanidinophosphazenes, guanidino phosphorus ylides). A simple model for describing the dependence of basicity on the extent of the molecular framework is proposed, validated, and used for quantitatively predicting the ultimate basicities of different compound families and the rates of substituent effect saturation. Some of the considered bases (guanidino phosphorus carbenes) are expected to reach gas-phase basicity around 370 kcal mol(-1), thus being the most basic neutral bases ever reported. Also, the classical substituted alkylphosphazenes were predicted to reach pK(a) values of around 50 in acetonitrile, which is significantly higher than previously expected.

Electronic nature of zwitterionic alkali metal methanides, silanides and germanides – a combined experimental and computational approach

“…On a final note, the calculated GPBs of M- 1 –M- 3 are fairly similar to those of classical organosuperbases such as t BuNP[NP(NMe 2 ) 3 ] 3 (–288 kcal mol –1 ) 27 , [(Me 2 N) 2 CN] 3 PNBu t (–279 kcal mol –1 ) 28 , HMPN (–270 kcal mol –1 ) 29 and Verkade's base (–261 kcal mol –1 ) 30 , which have found use in organic synthesis and organocatalysis. 31,32 Given the similar basicities, the ease of tuning basicity and the steric protection of the “naked” anion, E – , by proper choice of the metal cation and substituents, these types of zwitterions may offer new opportunities to be used in base and frustrated Lewis pair (FLP) catalysis as well as in the activation of small molecules.…”

On the Basicity of Conjugated Nitrogen Heterocycles in Different Media

“…As an example, the pyridine derivatives quinoline and isoquinoline both have higher basicity than pyridine by more than 5 kcal mol –1 in the gas phase, because of the larger size of the molecular framework into which the cation charge is delocalized . However, in both studied solvents the basicity order is isoquinoline > pyridine > quinoline, despite the very similar structures of the two benzo derivatives.…”

On the Basicity of Organic Bases in Different Media

“…Brønsted acidity is a measure of the propensity of a molecule or ion to donate a proton, whereas Brønsted basicity is the ability of a molecule or ion to accept a proton. In the case of neutral bases – the topic of this review – the basicity is dependent on the ability of the resulting cation to distribute its positive charge over its structure as efficiently as possible . Both acidity and basicity as phenomena have contributed to the understanding of structure‐reactivity relationships throughout the years.…”

“…In the case of neutral bases -the topic of this review -the basicity is dependent on the ability of the resulting cation to distribute its positive charge over its structure as efficiently as possible. [2] Both acidity acetonitrile to other solvents. Recommendations are given on their practical use and a number of pK aH values that are experimentally unavailable are estimated from these relationships.…”