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.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...

A Phosphorus Bisylide: Exploring a New Class of Superbases with Two Interacting Carbon Atoms as Basicity Centers

“…[4] In this context, Leito et al have just published ap aper elucidating the basicity limit of nonchelating neutral superbases by means of theoretical calculations. [5] They come to the conclusion that P-ylidic carbon bases should outperform classical phosphazene bases.T his statement is in accord with Koppel et al who demonstrated by theory that the intrinsic basicity of P-ylides distinctly exceeds that of phosphazenes:F or example,t he proton affinity of (Me 2 N) 3 P = CH 2 (274.5 kcal mol À1 )e xceeds that of the corresponding phosphazene (Me 2 N) 3 P=NH (256.3 kcal mol À1 )b yabout 18 kcal mol À1 . [6] Going back to early works by Alder,another approach to obtain extraordinarily basic compounds is accomplished in socalled proton sponges which exhibit two basic entities in close proximity.…”

Phosphorbisylid: Eine neue Klasse von Superbasen mit zwei superbasischen Kohlenstoffatomen in räumlicher Nähe

“…[4] In diesem Zusammenhang verçffentlichten Leito et al kürzlich eine Arbeit, die sich auf Basis theoretischer Berechnungen mit dem Basizitätslimit nicht-chelatisierender Superbasen beschäftigt. [5] Daraus geht hervor,d ass P-ylidische Kohlenstoffbasen eine hçhere Basizitäta ls die entsprechenden Phosphazene aufweisen. Diese Aussage stimmt mit einer Publikation von Koppel et al überein, die auf theoretischer Basis ebenfalls eine hçhere intrinsische Basizitätv on Phosphoryliden gegenüber Phosphazenbasen beschreiben.…”

Sustainable Cyclic Carbonate Production, Utilizing Carbon Dioxide and Azolate Ionic Liquids

“…47 Even though they are more basic than common trialkylamines, the pKa values of their conjugate acids would not warrant them to qualify as superbasic substances. [48][49][50] Nevertheless, their associated anions possess sufficiently high nucleophilicity to attach weakly electrophilic CO2, enabling them to be used in CO2 gas capture and catalysis. 47,[51][52][53][54][55] In this paper, we refer to them as azolate ionic liquids (or salts).…”