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

摘要

Four new chiral bis(amino alcohol)oxalamides (1-4:amino alcohol=leucinol,valinol,phenylglycinol,and henylalaninol,respectively)have been prepared as low-moelcular-weight organic gelators.Their gelation properties towards various organic solvents and mixtures were detemined and these were then compared to related bis(amino acid)oxalamide gelators.Spectroscopic (FTIR,~1H NMR)and X-ray diffraction studies revealed tha tthe 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-2reveals meso bilayers of hydrogen-bonded aggregates.Within the bilayers formed,the gelator molecules are connected by cooperative hydrogen bonding betwene oxalamide units and OH groups,while the interbilayer interactions are realized through lipophilic interactions between the iBu groups of leucinol.ZOxalamide meso-1 lacks any gelation ability and crystallizes inmonolayers.In dichloromethane rac-1 forms an unstable gel;this is prone to crystallization as a reslt 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 te presence of thinner fibers in the former gel,and,hence,a more compact network that is capable of immobilizing a larger volme of the solvent.The self-assembly of these types of gelator moelcules into bilayers and subsequent formation of fibrous aggregates can be exlained by considering the strength and direction of aggregate forces (supramolecular vectors)in three-dimensional space.