Stereodynamics of 4,6,10,12,16,18,22,24-Octamethylcalix[4]arene

The dynamic stereochemistry of 4,6,10,12,16,18,22,24-octamethyl-25,26,27,28-tetrahydroxycalix[4]arene (2) is analyzed. The measured barrier for the coalescence of the diastereotropic methylene protons (10.9 kcal mol^-1) and the coalescence of the methyl signals (10.7 kcal mol^-1) is identical. It is concluded that the dynamic pathway observed by NMR corresponds to a ring-inversion process which exchanges both the methylene protons and the symmetry nonequivalent “perpendicular” and “coplanar” aryl rings. Molecular mechanics calculations satisfactorily reproduce the boat conformation of 2 and indicate that the presumed transition state for the pseudorotation process lies 27 kcal mol^-1 above the ground state. In contrast to the parent p-tert-butylcalix[4]arene (1), changing the solvent from CDCl₃ to pyridine-d₅ raises the inversion barrier of 2. Calixarene 2 crystallizes from pyridine as a 1:1 complex in which the calixarene molecule exists in a boat conformation and the pyridine molecule is partially included in the calix cavity. Fixation of the conformation of 2 was obtained by preparing the 1,3-dimethyl ether derivative 6 by alkylation of 2 under phase-transfer catalysis conditions. The conformation of 6 in the crystal is similar to that of 2, with the two methoxy groups located in the “perpendicular” rings. The inversion barrier of 6 is higher than 24.0 kcal mol^-1 as estimated by saturation transfer experiments.