Derivatives of Azacalix[3](2,6)pyridine are Strong Neutral Organic Superbases:  A DFT Study

Abstract

The gas phase basicities and pKa values in acetonitrile of azacalix[3](2,6)pyridine and its derivatives are determined by the B3LYP DFT method. It is found that all compounds of this series are neutral organic superbases. The proton attacks the inner pyridine N(sp2) atom, thus forming a bifurcated intramolecular hydrogen bond. The most powerful superbase is provided by the hexakis(dimethylamino) derivative of the title compound. Its gas phase proton affinity is 296.6 kcal mol-1, its basicity is 291.3 kcal mol-1, and its pKa(MeCN) is 30.9 units. [structure: see text]

Perspectives

“…Following these concepts, N-substituted azacalix[n](2,6)pyridines have been synthesized and N,N 0 ,N 00 -tris (p-tolyl)azacalix [3](2,6)pyridine 10 has been found to be the size of the cavity appropriate for a high proton affinity [16] (Figure 11.2). These findings are corroborated by theoretical study by Despotovi c et al [17]. A closely related macrocycle 11, with an additional pyridine ring incorporated, also showed respectable superbasicity [18].…”

Cover Picture: Laves Phases, γ‐Brass, and 2×2×2 Superstructures: A New Class of Quasicrystal Approximants and the Suggestion of a New Quasicrystal (Chem. Eur. J. 22/2008)

“…These extraordinary basicity values approaching 300 kcal mol –1 are comparable to those calculated by Maksić for azacalix[3](2,6)pyridines {calcd. APA B3LYP(2) [ GB ] are 296.6 and [291.3] kcal mol –1 }31 or calculated and measured by Koppel and Schwesinger of P 1 ‐triguanidinophosphazenes {[(H 2 N) 2 C=N] 2 C=N} 3 P=NH ( GB = 296.2, PA = 302.3 kcal mol –1 , B3LYP/6‐311+G**),32 and [(H 2 N) 3 P=N] 2 P(NH 2 )=CH 2 ( GB = 288.6, PA = 295.0 kcal mol –1 , B3LYP/6‐311+G**) 33…”

“…42.7)],36 and azacalix[3](2,6)pyridines [calcd. : APA MeCN = 304.8 p K a (MeCN) = 30.9] 37. In addition, absolute proton affinities, which are calculated in MeCN [ APA (MeCN)], could also be used as an indicator of relative basicity.…”

Exceptional Superbasicity of Bis(guanidine) Proton Sponges Imposed by the Bis(secododecahedrane) Molecular Scaffold: A Computational Study

“…These extraordinary basicity values approaching 300 kcal mol -1 are comparable to those calculated by Maksić for azacalix [3](2,6)pyridines {calcd. APA B3LYP (2) [GB] are 296.6 and [291.3] kcal mol -1 } [31] or calculated and measured by Koppel and Schwesinger of P 1 -triguanidinophosphazenes {[(H 2 N) 2 C=N] 2 C=N} 3 P=NH (GB = 296.2, PA = 302.3 kcal mol -1 , B3LYP/6-311+G**), [32] and [(H 2 N) 3 P=N] 2 P(NH 2 )=CH 2 (GB = 288.6, PA = 295.0 kcal mol -1 , B3LYP/6-311+G**). [33] Calculation of APA values by using the M052X/6-31G* method [APA M052X (1) ] gives similar basicity trends as obtained with the B3LYP/6-31G* method (Table 1).…”

“…: APA MeCN = 304.8 pK a -(MeCN) = 30.9]. [37] In addition, absolute proton affinities, which are calculated in MeCN [APA(MeCN)], could also be used as an indicator of relative basicity. These values are obtained for the pK a (MeCN) calculations (at 0 K) and are generally larger by 30 kcal mol -1 than gas-phase calculations (exceeding 300 kcal mol -1 ), whereas the relative basicity order remained almost unchanged (see Table S3 in the Supporting Information).…”