NMR studies of the reversible dimerization and guest exchange processes of tetra urea calix[4]arenes using a derivative with lower symmetry

The calix[4]arene derivative 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 C-2v-symmetrical, and the hydrogen bonded dimers 3.3 formed in benzene have C-2-symmetry. Thus, the H-1 NMR spectrum not only gives an unambiguous proof for the dimerization but also allows the determination of the exchange rates for four sets of protons by NOESY experiments. The rate constant for the dissociation/dimerization k(d) = 0.26 +/- 0.06 s(-1) is in reasonable agreement with the rate constant for the exchange of included and free benzene k(e) = 0.47 +/- 0.1 s(-1). It was also shown that the formation of dimers is induced by the presence of suitable guest molecules like benzene.