Through-the-Annulus Threading of the Larger Calix[8]arene Macrocycle

A complete study of the through-the-annulus threading of the larger calix[8]­arene macrocycle with di-n-alkylammonium cations has been performed in the presence of the “superweak” TFPB counterion. Thus, it was found that such threading occurs only upon partial preorganization of the calix[8]­arene macroring by intramolecular bridging. In particular, 1,5-bridged calix[8]­arenes with a meta- or para-xylylene bridge (2 and 3) gave pseudo[2]­rotaxanes in which one dialkylammonium axle (4a–4e +) was threaded into one of the two subcavities of the calix[8]-wheel. Conformational studies by using chemical shift surface maps and DFT calculations evidenced a 3/4-cone geometry for these subcavities. Higher pseudorotaxane K ass values were obtained for calix[8]-wheels 2 and 3, with respect to calix[6]-host 1a, due to the cooperative effect of their two subcavities. Dynamic NMR studies on calix[8]-pseudorotaxanes evidenced a direct correlation between K ass (and ΔG ass) values and energy barriers for calix inversion due to the effectiveness of thread templation. In accordance with DFT calculations, an endo-alkyl preference, over the endo-benzyl one, was observed by threading calix[8]-wheel 3 with the directional n-butylbenzylammonium axle 4d +.