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 ur ea residues was prepared in three steps from the corresponding tert-bu tyl 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 N MR spectrum not only gives an unambiguous proof for the dimerization b ut also allows the determination of the exchange rates for four sets o f protons by NOESY experiments. The rate constant for the dissociation /dimerization k(d) = 0.26 +/- 0.06 s(-1) is in reasonable agreement wi th 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.