INFLUENCE OF THE SIZE OF UPPER AND LOWER RIM SUBSTITUENTS ON THE FLUXIONAL AND COMPLEXATION BEHAVIOR OF CALIX[5]ARENES

A comparative study on a series of penta-O-alkylated p-H-calix[5]arene s 1b-e and p-tert-butylcalix[5]arenes 1g-m shows that the former are i nherently mobile and adopt in solution non-cone conformations. The 1,2 -alternate conformation for penta-O-benzyl ether 1c was proven by sing le-crystal X-ray analysis. VT-NMR studies on 1c-e have provided the fi rst experimental evidence that conformational interconversion occurs v ia the p-phenyl-through-the-annulus mechanism with Delta G double dagg er in the range 17.9-18.8 kcal mol(-1). Unlike the corresponding tert- butylated counterparts, pH-calix[5]arenes are unable to form 1:1 endo- calix complexes with primary unbranched alkylammonium cations. (C) 199 8 Elsevier Science Ltd. All rights reserved.