CONFORMATIONAL DYNAMICS OF CALIXARENES - KINETICS OF CONFORMATIONAL INTERCONVERSION IN P-TERT-BUTYL-25,26,27,28-TETRAMETHOXYCALIX[4]ARENE UNDER ENTROPIC CONTROL

The kinetics of interconversions of the four conformers of the title c ompound (partial cone (pc); cone (c); 1,2-alternate (alt,2); 1,3-alter nate (alt,3)) were studied in chloroform and in a 50:50 binary mixture of chloroform and acetonitrile by 2D-EXSY H-1 NMR spectroscopy. The r esults are fully consistent with the coexistence of three one-step pro cesses involving the partial cone conformation (pc reversible arrow al t,2; pc reversible arrow c; pc reversible arrow alt,3). The rate const ants for the forward processes are, for chloroform and the binary mixt ure, respectively, at 270 K, 0.0040 and 0.12, 1.6 and 3.4, and 2.1 and 4.7 s(-1). All the values of the activation enthalpies are close (Del ta H-double dagger approximate to 60 kJ mol(-1)), while the entropies of activation are, respectively, -70, -16, and -29 J mol(-1) K-1 in ch loroform, indicating that the conformational exchange is under entropy control.