Judging on Host−Guest Binding Mode Uniqueness:  Association Entropy as an Indicator in Enantioselection

Abstract

[reaction: see text] Supramolecular enantiodifferentiation was studied by isothermal titration calorimetry in an effort to address the order-disorder distinction in the diastereomeric complexes formed from a chiral macrocyclic host and enantiomeric carboxylates. As a result, the association entropy component TDeltaS emerged as an indicator in the enantioselection of tartrate 14 and aspartate 15 by the macrocycle 13 containing two guanidinium anchor groups connected to each other by four urea units. The parent monotopic guanidinium compounds 1 or 2 did not show any enantioselection for chiral carboxylates.

Binding Mechanisms in Supramolecular Complexes

“…[221] The complexation is actually dominated by interaction between the protonated N termini of the amino acids and the phosphoryl groups on the calixarene. [223] Hydrogen bonds can also contribute in the less competitive solvent acetonitrile selected in this study. Pyridinium units on the edges of the calix[4]arenes can serve as selective receptors for a,w-dicarboxylates.…”

“…The complexation of alkali-metal and alkaline-earth cations by crown ethers and related compounds is based on an attraction between the ion and the free electron pairs on a heteroatom in the ligand. [223] H.-J. [226] Complexes of p-tert-butylcalix[4]arene esters with alkalimetal ions show linear Hammett correlations with substituents , which alter the electron density at the C=O group.…”

Bindungsmechanismen in supramolekularen Komplexen

“…[ 220] H. [222] Die Assoziation eines zwei Guanidiniumeinheiten enthaltenden Makrocyclus in Acetonitril (Schema 7) zeigt, dass die bemerkenswerte Enantiodifferenzierung selbst in einem nichtprotischen Solvens weitgehend auf die bevorzugte Entropie mit dem einen der Ionenpaare zurückgehen kann. [223] Beitragen können im hier gewählten weniger kompetitiven Lösungsmittel Acetonitril auch Wasserstoffbrücken.…”

Recent Advances in Macrocyclic and Macrocyclic-Based Anion Receptors

“…Schmidtchen prepared the macrocycle 70, derived from two chiral guanidinium groups connected through four urea units [78]. The enantioselective binding of (L,D) tartrate dianion and (L,D) aspartate monoanion as their tetraethylammonium salts in acetonitrile was investigated by ITC experiments.…”