CONFORMATIONAL DYNAMICS OF CALIXARENES - KINETICS OF CONFORMATIONAL INTERCONVERSION IN 5,11,17,23-TETRA-P-TERT-BUTYL-25,26,27,28-TETRAMETHOXYCALIX[4]ARENE UNDER ENTROPIC CONTROL

The kinetics of interconversions of the four conformers of the title compound (partial cone (pc); cone (c); 1,2-alternate (alt,2); 1,3-alternate (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 results are fully consistent with the coexistence of three one-step processes involving the partial cone conformation (pc reversible arrow alt,2; pc reversible arrow c; pc reversible arrow alt,3). The rate constants for the forward processes are, for chloroform and the binary mixture, 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 (Delta 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 chloroform, indicating that the conformational exchange is under entropy control.