CONTROL OF CALIX[6]ARENE CONFORMATIONS BY SELF-INCLUSION OF 1,3,5-TRI-O-ALKYL SUBSTITUENTS - SYNTHESIS AND NMR-STUDIES

Hexa-O-alkylated p-tert-butylcalix[6]arenes with an alternate 1,3,5-R( 1)-2,4,6-R(2)-substitution pattern have been prepared and studied by N MR spectroscopy. When R(1) = CH3 and R(2) is a bulky substituent, the calix[6]arene moiety adopts predominantly a flattened cone conformatio n. At room temperature this conformation slowly interconverts with a 1 ,2,3-alternate conformation (Delta G(double dagger) = 70-88 kJ mol(-1) , T = 328 K, CDCl3). The Gibbs free energy between these two conformat ions at T = 303 K in CDCl3 varies from 2 to 7 kJ mol(-1). From quantit ative NOE data (obtained for tris[(3-nitrobenzyl)oxyl-p-tert-butylcali x[6]arene (10) three-dimensional structures of the flattened cone and the 1,2,3-alternate conformers were calculated. In both conformers the methoxy groups are accommodated in the annulus of the calix [6] arene skeleton, indicating specific interactions with the aromatic calixare ne moiety. Compared to the hexasubstituted analogues, this structural feature slightly increases the activation Gibbs free energy for the in terconversion process (13-21 kJ mol(-1)). Even with substituents that are too large to rotate through the annulus, there is still slow inter conversion (Delta G(double dagger) = 88 kJ mol(-1), T = 328 K, CDCl3). This is attributed to a very slow transannular motion of the p-tert-b utyl moiety. Also the ethoxy (16) or propoxy (17) derivatives of is(N, N-diethylacetamido)-p-tert-butylcalix[6]arene predominantly adopt flat tened cone (16) and 1,2,3-alternate (17) conformations, respectively. The self-inclusion of the larger alkyl groups also contributes to the overall conformational freezing of the p-tert-butylcalix[6]arenes.