Sodium Binding Effects on Conformational Exchange in a Diquinone Calix[4]arene

Abstract

NMR studies show that a 2,4-diethoxycalix[4]arene-1,3-diquinone undergoes an unusually slow conformational change, from a predominantly anti-aryl conformation to that of a syn-aryl conformation, in the presence of Na+ ion. Dramatic changes are observed throughout the entire NMR spectral region. A novel finding is that the timescale of this conversion is unusually long, taking many hours at room temperature to complete. The kinetics of this conformational change over the range of 5−55 °C were measured in a binary solvent system. The rate constants are on the order of 10-5−10-4 s-1 in the presence or absence of Na+. These results suggest a cation effect on the equilibrium position of the anti and syn complexes rather than on the rate of interconversion. The kinetics of interconversion in the presence and absence of the salt are the same. The rate determining step corresponds to rotation of an aryl moiety through the annulus of the calixarene. Additionally, we note that the initially dominant anti-aryl conformer appears to be a kinetic product of the oxidation step of the synthesis with Tl(CF3CO2)3. Variable-temperature NMR studies in the absence of salt show a stable ∼4:1 ratio of anti to syn conformer, while the compound, as initially prepared in solution, exhibits a ratio of >50:1.