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.
The calix[4]arenederivative 3 substituted at the wider rim by four urea residues was prepared in three steps from the corresponding tert-butyl calix[4]arene 2a. Due to the different ether residues attached to the narrow rim its constitution is C2v-symmetrical, and the hydrogenbonded dimers 3·3 formed in benzene have C2-symmetry. Thus, the 1H NMR spectrum not only gives an unambiguous proof for the