Rg. Janssen et al., STUDIES ON THE DYNAMICS OF PHOSPHORYLATED P-TERT-BUTYLCALIX[6]ARENES BY USING 2D NMR-SPECTROSCOPY, Journal of the American Chemical Society, 118(15), 1996, pp. 3666-3675
The overall dynamics of partially phosphorylated and thiophosphorylate
d p-tert-butylcalix[6]arenes has been studied by NMR spectroscopy. Whe
n p-tert-butylcalix[6]arene is monosubstituted with a phosphate or thi
ophosphate group, the calix[6]arene skeleton is remarkably rigidified.
The 1,3- and 1,4-(thio)phosphorylated p-tert-butylcalix[6]arenes are
more flexible. 2D NMR spectroscopy showed that these calix[6]arenes ad
opt syn conformations. A combination of H-1 and P-31 NMR spectroscopy
revealed that the calix[6]arene (thio)phosphates are involved in at le
ast three dynamic processes, viz., macrocyclic ring interconversion, h
ydrogen bond array reversal, and pinched conformer interconversion. Th
e activation barrier (Delta G(h)(double dagger)) for macrocyclic ring
interconversion depends on the type and number of substituents and ran
ges from 67 to 86 kJ . mol(-1). The activation barrier (Delta G(h)(dou
ble dagger)) for hydrogen bond array reversal depends on the number of
hydroxyl groups and ranges from 31 to 45 Id mol-l For the pinched con
former interconversion an activation barrier (Delta G(h)(double dagger
)) ranging from 44 to 55 kJ . mol(-1) was found. Pinched conformations
have been frequently observed in the solid state, for example, for mo
no- and 1,4-bis(thiophosphorylated) p-tert-butylcalix[6]arene. These s
tudies, however, show for the first time that these conformations also
exist in solution and that differently pinched conformers may rapidly
interconvert.