Chiral Bis(amino alcohol)oxalamide Gelators—Gelation Properties and Supramolecular Organization: Racemate versus Pure Enantiomer Gelation

Abstract

Four new chiral bis(amino alcohol)oxalamides (1-4: amino alcohol=leucinol, valinol, phenylglycinol, and phenylalaninol, respectively) have been prepared as low-molecular-weight organic gelators. Their gelation properties towards various organic solvents and mixtures were determined and these were then compared to related bis(amino acid) oxalamide gelators. Spectroscopic (FTIR, (1)H NMR) and X-ray diffraction studies revealed that the primary organization motif of (S,S)-1 and racemate 1 (rac-1) in lipophilic solvents involved the formation of inverse bilayers. The X-ray crystal structure of (S,S)-1 also shows this type of bilayer organization. The crystal structure of rac-2 reveals meso bilayers of hydrogen-bonded aggregates. Within the bilayers formed, the gelator molecules are connected by cooperative hydrogen bonding between oxalamide units and OH groups, while the interbilayer interactions are realized through lipophilic interactions between the iBu groups of leucinol. Oxalamide meso-1 lacks any gelation ability and crystallizes in monolayers. In dichloromethane rac-1 forms an unstable gel; this is prone to crystallization as a result of the formation of symmetrical meso bilayers. In contrast, in aromatic solvents rac-1 forms stable gels; this indicates that enantiomeric bilayers are formed. Oxalamide rac-1 is capable of gelling a volume of toluene three times larger than (S,S)-1. A tranmission electron microscopy investigation of rac-1 and (S,S)-1 toluene gels reveals the presence of thinner fibers in the former gel, and, hence, a more compact network that is capable of immobilizing a larger volume of the solvent. The self-assembly of these types of gelator molecules into bilayers and subsequent formation of fibrous aggregates can be explained by considering the strength and direction of aggregate forces (supramolecular vectors) in three-dimensional space.

Ionic Liquids as Precatalysts in the Highly Stereoselective Conjugate Addition of α,β‐Unsaturated Aldehydes to Chalcones

“…[28] This has led us to propose that a pair of enantiomers in contrast to a single enantiomer results in favorable p-p interactions and van der Waals interactions leading to the self-assembly of the supramolecular network of the organogel.…”

A Historical Perspective on Crystallography in Croatia and the Career of Biserka Kojić-Prodić from the Viewpoint of the CSD

“…[11] and UNEGOJ (right). [12] Figure 5. Pie chart of frequency of space groups observed within Dr Kojić-Prodić's CSD entries.…”

“…[10] Some of Biserka's most high-profile structures in the CSD include those of supramolecular organic gelators based on amino acid and amino alcohol oxalamides (Figure 4) -the publications associated with these structures have each been cited over 100 times. [11,12] Other key studies include the structural analysis of interaction behaviour in calixarene amino acid derivatives [13] and the structures of palladium(II) quinolinylaminophosphonate complexes. [14] Roughly two thirds of Biserka's CSD structures are organic, the remaining third containing at least one metal atom with an increasing fraction of polymeric coordination compounds since 2009.…”

Cyclohexane‐Based Low Molecular Weight Hydrogelators: A Chirality Investigation

“…[12] In some cases, as has often been observed with many other gelator systems, very slight changes to the gelator structure can influence the gelation behaviour dramatically. [13] Chirality, for example, often determines whether a compound will function as a gelator or not. Whereas racemic mixtures are sometimes less efficient gelators than the corresponding enantiomerically pure products, [14,15] the opposite effect has been observed in other cases.…”

“…Whereas racemic mixtures are sometimes less efficient gelators than the corresponding enantiomerically pure products, [14,15] the opposite effect has been observed in other cases. [13,16,17] Very recently, Hirst et al have shown that changing the chirality of one amino acid of a dendritic organogelator can lead to changes in the gelation behavior in organic solvents. [18] Herein we report that changes in chirality effect the gelation behaviour of amino acid-substituted 1,3,5-cyclohexyltricarboxamide derivatives in water (Scheme 1), which lead to novel hierarchical self-assembled systems.…”