SYNTHESIS AND CONFORMATIONAL PROPERTIES OF CALIX[6]ARENES BRIDGED ON THE LOWER RIM - SELF-ANCHORED ROTAXANES

The 1,4-di-p-tolyl ether of p-tert-butylcalix[6]arene can be converted to transannularly bridged esters by treatment with diacid chlorides ranging from succinoyl to suberoyl and including terephthaloyl. The parent compounds p-tert-butylcalix[6]arene and p-H-calix[6]arene can also be transannularly bridged by treatment with bis-halomethylaryl compounds by using KOSiMe3 as the base. Methylation of the resulting calix[6]arenes containing 1,4-CH2ArCH2 bridging moieties produces the corresponding tetramethyl ethers. When Ar is anthrylene the conformation of the starting material is retained, but when Ar is phenylene the conformation changes; when Ar is duryl a mixture of conformers is obtained. H-1 NMR analysis and molecular modeling studies suggest that, when Ar is phenyl or durylene, the system is capable of undergoing a conformational transformation in which the bridging moiety becomes threaded through the annulus of the macrocyclic ring to produce a ''self-anchored rotaxane''. Employing a new scheme for denoting calixarene conformations, this is designated as a (u,u,u,u,u,u) to (u,u,d,d,d,u) transformation. The apparent driving force for the conformational change is the greater spatial separation of the OMe groups in the (u,u,d,d,d,u) conformation as compared with the (u,u,u,u,u,u) conformation, resulting in a more favorable electrostatic energy contribution to the total energy of the former.