A New Class of Organosuperbases, N‐Alkyl‐ and N‐Aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene Amines: Synthesis, Structure, pK_BH+ Measurements, and Properties

Abstract

A series of stable organosuperbases, N-alkyl- and N-aryl-1,3-dialkyl-4,5-dimethylimidazol-2-ylidene amines, were efficiently synthesized from N,N'-dialkylthioureas and 3-hydroxy-2-butanone and their basicities were measured in acetonitrile. The derivatives with tert-alkyl groups on the imino nitrogen were found to be more basic than the tBuP(1) (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by (13)C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino-substituents, including electron-acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas-phase basicity becomes more dependent on the field-inductive effect, polarizability, and resonance effects of the substituent.

Basicity Limits of Neutral Organic Superbases

“…[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.…”

Gas phase basicities of polyfunctional molecules. Part 5: Non‐aromatic sp2 nitrogen containing compounds

“…Recently, gas phase basicities of organosuperbases with the imino group as the basic center attached to 1,3‐dialkylimidazoli(di)ne ring was reported (Scheme ) (Kunetskiy et al, ; Polyakova et al, ). Proton affinities of 1,3‐dimethyl‐2‐iminoimidazolidine, 1 , and of 1,3‐dimethyl‐2‐iminoimidazoline, 2 , are quite comparable pointing to a limited effect of the aromatization of the imidazolium ring during protonation (Kunetskiy et al, ). By contrast, introducing two methyl groups in positions 4 and 5 leads to a significant increase in PA (23 kJ/mol, Scheme ) showing that substituent effects (polarizability here) in that positions are more significant than aromatization of the imidazolium moiety.…”

“…The resulting values (Scheme ) are significantly higher than PA of 2‐imino pyrrolidine (1000 kJ/mol, Scheme ) and lie in the range of superbasicity. Introducing an alkyl group at the imino nitrogen atom leads to PA values situated in the range 1090–1115 kJ/mol, whereas electronegative substituents lead to a decrease in basicity (Kunetskiy et al, ).…”

“…Proton affinities (kJ/mol) of selected 1,3‐dialkyl‐imidazolines: computed values for 1 , 2 , and 3 , (B3LYP/6‐311 + G(d,p), (Kunetskiy et al, )), experimental values for 4 (kinetic method, (Polyakova et al, )).…”

Stabilities of Immonium Ions Derived from N‐Heterocyclic Carbenes Probed by Collision‐Induced Dissociation Mass Spectrometry

“…Although the 1,3dialkyl-4,5-dimethylimidazol-2-ylidenimine system is quite new as a structural fragment in synthetic methodology, [8,9] it has already proved itself to be a highly electron-donating group. Although the 1,3dialkyl-4,5-dimethylimidazol-2-ylidenimine system is quite new as a structural fragment in synthetic methodology, [8,9] it has already proved itself to be a highly electron-donating group.…”