title: Guest exchange in dimeric capsules formed by tetra-urea calix[4]arenes.
abstract: Ten tetra-urea calix[4]arene derivatives with different ether residues (methyl, pentyl, benzyl, all combinations of methyl and pentyl, 1,3-dibenzyl-2,4-dipentyl), including also the tetrahydroxy compound and the 1,3-dipentyl ether, were synthesised. Their urea groups were substituted with a lipophilic residue to ensure sufficient solubility in cyclohexane. Thus, kinetics for the exchange of the included guest (benzene) against the solvent (cyclohexane) could be followed by 1H NMR spectroscopy. The apparent first order rate constants decrease with increasing size of the ether residues from methyl to benzyl by more than three orders of magnitude. This can be understood by a decreasing flexibility/mobility of the calixarene skeleton. In line with this explanation is the rather slow exchange for the tetrahydroxy compound, where the cone conformation is stabilised by a cyclic array of intramolecular OH...OH hydrogen bonds.
authors.affiliation: Fachbereich Chemie, Pharmazie und Geowissenschaften, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, D-55099, Mainz, Germany.
authors.forename: Ivan
authors.initials: I
authors.lastname: Vatsouro
authors.forename: Ellen
authors.initials: E
authors.lastname: Alt
authors.forename: Myroslav
authors.initials: M
authors.lastname: Vysotsky
authors.forename: Volker
authors.initials: V
authors.lastname: Böhmer
mesh_terms: D001554:Benzene / Q000737:chemistry; D047250:Calixarenes / Q000138:chemical synthesis* / Q000737:chemistry*; D003510:Cyclohexanes / Q000737:chemistry; D019281:Dimerization; D004987:Ethers / Q000737:chemistry; D006860:Hydrogen Bonding; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010636:Phenols / Q000138:chemical synthesis* / Q000737:chemistry*; D014508:Urea / Q000737:chemistry*
chemical_list: D003510:Cyclohexanes; D004987:Ethers; D010636:Phenols; C121325:calix(4)arene; D047250:Calixarenes; C506365:Cyclohexane; D014508:Urea; D001554:Benzene