Calix[5]arene Through-the-Annulus Threading of Dialkylammonium Guests Weakly Paired to the TFPB Anion

5 years ago

Calix[5]arene Through-the-Annulus Threading of Dialkylammonium Guests Weakly Paired to the TFPB Anion

Placido Neri, Melchiorre F. Parisi, Ilenia Pisagatti, Carmine Gaeta, Margherita De Rosa, Annunziata Soriente, Carmen Talotta, Anna Notti, Sebastiano Pappalardo, Giuseppe Gattuso

Through-the-annulus threading of calix[5]arene penta-O-ethers by dialkylammonium cations coupled to the loosely coordinating superweak TFPB^– anion has been achieved. ¹H NMR titration data show that preorganization of the calix[5]arene scaffold leads to great thermodynamic stability of the pseudorotaxane complexes as well as to a favorable threading kinetic. Thus, calix[5]arene 1c, bearing tert-butyl groups at the wide rim, was threaded by all of the cations under study (with the exception of the dibenzylammonium 2b⁺) more tightly than the other derivatives under investigation (K_a’s up to 2.02 ± 0.2 × 10⁵ M^–1) because of its preorganized cone conformation. According to DFT calculations, van der Waals interactions between the tert-butyl groups of 1c and the alkyl chain of the cationic axle are likely responsible for the remarkable stability observed. The threading of the calix[5]arene wheels with the asymmetric pentylbenzylammonium axle 2c⁺ led to the toposelective formation of the endo-pentylpseudorotaxane stereoisomer in agreement with the known “endo-alkyl rule”. Owing to the steric hindrance of the axle phenyl group, the threading of the guest was seen to occur in a unidirectional fashion through the calixarene narrow rim.

Publisher URL: http://dx.doi.org/10.1021/acs.joc.7b00406

DOI: 10.1021/acs.joc.7b00406