title: 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.
authors.affiliation: Laboratory of Supramolecular and Nucleoside Chemistry, Rudjer Bosković Institute, P.O. Box 180, 10002 Zagreb, Croatia.
authors.forename: Janja
authors.initials: J
authors.lastname: Makarević
authors.forename: Milan
authors.initials: M
authors.lastname: Jokić
authors.forename: Zlata
authors.initials: Z
authors.lastname: Raza
authors.forename: Zoran
authors.initials: Z
authors.lastname: Stefanić
authors.forename: Biserka
authors.initials: B
authors.lastname: Kojić-Prodić
authors.forename: Mladen
authors.initials: M
authors.lastname: Zinić
mesh_terms: D000605:Amino Alcohols / Q000138:chemical synthesis / Q000737:chemistry*; D004983:Ethanolamines; D005782:Gels / Q000737:chemistry; D005998:Glycine / Q000031:analogs & derivatives* / Q000737:chemistry; D006860:Hydrogen Bonding; D007930:Leucine / Q000031:analogs & derivatives* / Q000737:chemistry; D046529:Microscopy, Electron, Transmission; D015394:Molecular Structure; D010072:Oxamic Acid / Q000031:analogs & derivatives*; D010649:Phenylalanine / Q000031:analogs & derivatives* / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013237:Stereoisomerism; D014633:Valine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D000605:Amino Alcohols; D004983:Ethanolamines; D005782:Gels; D012997:Solvents; C012481:phenylalaninol; C040994:valinol; D010649:Phenylalanine; C005332:N-phenylethanolamine; C417575:leucinol; D007930:Leucine; D014633:Valine; D010072:Oxamic Acid; D005998:Glycine