Synthesis and crystallographic studies of a calix[4]arene with a 1,3-alternate conformation

Abstract

25,27-Bis(ethoxycarbonylmethoxy)-26,28-bis(2-pyridylmethoxy)-5,11,17,23-tetra-tert-butylcalix[4]arene with a 1,3-alternate conformation (1,3-alternate-2) has been synthesized. Dynamic ¹H NMR spectroscopy has established that the conformation is immobilized (i.e., the oxygen-through-the-annulus rotation is inhibited). The single crystal was grown from acetonitrile and analysed by X-ray crystallography. It was found that the structural characteristics of a 1,3-alternate conformation are (i) the four phenyl rings are more or less parallel to each other, the dihedral angles between the four phenyl rings and the mean plane of the four methylene groups being 93.5, 100.4, 101.4 and 106.1°, (ii) the ArCH₂Ar bond angles are more expanded than those for other conformations and (iii) 1,3-alternate-2 in solution adopts C₂ symmetry whereas in the solid state it is less symmetrical because of the unsymmetrical orientation of the ester and the pyridyl substituents. This is a rare example of the X-ray crystallographic analysis of 1,3-alternate calix[4]arenes.