The basicities of simple organic bases – aliphatic and aromatic amines, amidines, phosphazenes, as well as saturated and unsaturated nitrogen heterocycles – are examined in acetonitrile, dimethyl sulfoxide, tetrahydrofuran, water and the gas phase. The basicities (pKaH values) of conjugate acids of a large variety of bases in these media are presented and discussed. Equations employing easily usable structural descriptors have been derived for approximately converting basicities from acetonitrile to other solvents. Recommendations are given on their practical use and a number of pKaH values that are experimentally unavailable are estimated from these relationships. An important part of the minireview is a large compilation of pKaH and GB values of the compounds in solvents and the gas phase, respectively, as well as the revised basicity scale in acetonitrile, now containing more than 270 pKaH values.

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In this work we explored the relationship between the structure and solvent effects on the basicity of a large selection of conjugated N‐heterocyclic nitrogen bases in different media: the polar aprotic solvent acetonitrile, the polar protic solvent water and the gas phase. Altogether, 58 previously unpublished basicity values in different media for 39 compounds are presented, including 30 experimentally determined pKa values in acetonitrile. We present the pKa and gas‐phase basicity values for quino[7,8‐h]quinoline, which is one of the most basic conjugated nitrogen heterocyclic compounds without basicity‐enhancing substituents. The trends in basicity are rationalized by comparing the basicity data of related compounds in different solvents, as well as by using isodesmic reactions. The gas‐phase basicity is predominantly determined by the ability of a molecule to disperse the excess positive charge over a large number of atoms. In solution the situation is less clear and smaller systems with localized charge often lead to higher basicities because of solvent effects. In particular, it was found that the fusion of an additional benzene ring does not always lead to an increase in basicity in solution: its effect can be either basicity‐increasing or ‐decreasing, depending on the ring size, number and position of nitrogen atoms and medium. A correlation between the measured pKa values in MeCN and in water suggests that these two different solvents exert a similar effect on the basicity of the studied heterocycles.

Experimental basicities of some of the strongest superbases ever measured (phosphonium ylides) are reported, and by employing these compounds, the experimental self-consistent basicity scale of superbases in THF, reaching a pKα (estimate of pKa) of 35 and spanning more than 30 pKa units, has been compiled. Basicities of 47 compounds (around half of which are newly synthesized) are included. The solution basicity of the well-known t-Bu-N═P4(dma)9 phosphazene superbase is now rigorously linked to the scale. The compiled scale is a useful tool for further basicity studies in THF as well as in other solvents, in particular, in acetonitrile. A good correlation between basicities in THF and acetonitrile spanning 25 orders of magnitude gives access to experimentally supported very high (pKa > 40) basicities in acetonitrile, which cannot be directly measured. Analysis of structure-basicity trends is presented.

The equilibrium acidity scale (pKa scale) in acetonitrile has been supplemented by numerous new compounds and new ΔpKa measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 ΔpKa measurements and spans between the pKa values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. Measurement results acquired over the last 15 years were added to the scale and new least‐squares treatment was carried out. The treatment yielded revised pKa values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale. Correlation equations were developed for estimating pKa values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of pKa values in acetonitrile. These equations enable predicting pKa values with an average error around or less than 1 pKa unit, which is a sufficient accuracy for many applications. The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc.

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.

Biomolecular systems are able to respond to their chemical environment through reversible, selective, noncovalent intermolecular interactions. Typically, these interactions induce conformational changes that initiate a signaling cascade, allowing the regulation of biochemical pathways. In this work, we describe an artificial molecular system that mimics this ability to translate selective noncovalent interactions into reversible conformational changes. An achiral but helical foldamer carrying a basic binding site interacts selectively with the most acidic member of a suite of chiral ligands. As a consequence of this noncovalent interaction, a global absolute screw sense preference, detectable by 13C NMR, is induced in the foldamer. Addition of base, or acid, to the mixture of ligands competitively modulates their interaction with the binding site, and reversibly switches the foldamer chain between its left and right-handed conformations. As a result, the foldamer–ligand mixture behaves as a biomimetic chemical system with emergent properties, functioning as a “proton-counting” molecular device capable of providing a tunable, pH-dependent conformational response to its environment.

The potential limits of superbasicity achievable with different families of neutral bases by expanding the molecular framework are explored using DFT computations.Anumber of different core structures of non-ionic organosuperbases are considered (such as phosphazenes,g uanidinophosphazenes, guanidino phosphorus ylides). As imple 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 À1 , 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,w hich is significantly higher than previously expected.Non-ionic organosuperbases [1] are attracting significant interest because of their practical importance as catalysts [1,2] and auxiliary reagents [3] in synthesis and technology as well as for fundamental challenges. [4,5] Design and synthesis of new superbases has been aflourishing field of research during the last decades. [1,4] Numerous families of superbases (such as phosphazenes, [3] phosphatranes, [6] bisphosphazene proton sponges, [7,8] imidazolidines, [9] imidazolidino-phosphazenes [10] and -guanidines, [11] bis-guanidines [12] )h ave been created and potentially superbasic compound families have been proposed, for example carbenes. [13] More recently different innovative bases have been proposed, such as cyclopropeneimines [14] and silylene bases. [15] Despite av ast range of potential applications, [1] many classes of superbases (guanidino proton sponges, [7] phosphorus ylides [16] )a re still almost unexplored.An early Minireview by Schwesinger [17] summarizes the principal approaches to enhancing the basicity of organosuperbases:1 )the "battery cell" principle, [3] that is,s tepwise expansion of the molecular scaffold by forming alternating formal single and double bonds and thereby enhancing the structure where the positive charge can be delocalized (Scheme 1);2 )stabilization of the protonated form by intramolecular hydrogen bond (chelation);a nd 3) structures that become aromatic on protonation. It was concluded that the most fruitful approach to design of superbases is the battery cell principle.T he best known practical example is the phosphazene family of superbases. [3] Creating as tructure where the protonated form is stabilized by one or more hydrogen bonds is also frequently used [4,7,8] and often remarkable basicity enhancement is seen, especially in the gas phase.H owever,i np ractical usage,n ot only thermodynamic but also kinetic basicity is important and here is the drawback of the chelating bases,a st hey generally are kinetically slow. [7] In this work, we computationally explore (DFT B3LYP 6-311 + G** and DFT BP TZVP) five families of potentially extreme...