title: Through-the-annulus threading of the larger calix[8]arene macrocycle.
abstract: 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(+).
authors.affiliation: Dipartimento di Chimica e Biologia, Università di Salerno, Via Giovanni Paolo II 132, I-84084 Fisciano, Salerno, Italy. cgaeta@unisa.it
authors.forename: Carmine
authors.initials: C
authors.lastname: Gaeta
authors.forename: Carmen
authors.initials: C
authors.lastname: Talotta
authors.forename: Luigi
authors.initials: L
authors.lastname: Margarucci
authors.forename: Agostino
authors.initials: A
authors.lastname: Casapullo
authors.forename: Placido
authors.initials: P
authors.lastname: Neri
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011789:Quantum Theory
chemical_list: D047028:Macrocyclic Compounds; D047250:Calixarenes