Study on the Influence of Chirality in the Threading of Calix[6]arene Hosts with Dialkylammonium Axles

The influence of chirality in calixarene threading has been studied by exploiting the“superweak anion approach”. In particular, the formation of chiral pseudo[2]rotaxanes bearing aclassical stereogenic center in their axle and/or wheel components has been considered. Two kindof pseudo[2]rotaxane stereoadducts, the “endo-chiral” and “exo-chiral” ones, having the stereogeniccenter of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene werepreferentially formed with specifically designed chiral axles by a fine exploitation of the so-called“endo-alkyl rule” and a newly defined “endo-α-methyl-benzyl rule” (threading of a hexaalkoxycalix[6]arenewith a directional (α-methyl-benzyl)benzylammonium axle occurs with anendo-α-methyl-benzyl preference).The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase bymeans of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hostswith pseudoracemates of one deuterated and one unlabeled chiral axle enantiomer. In both instances,there was not a clear enantiodiscrimination in the threading process with the studied host/guestsystems. Possible rationales are given to explain the scarce reciprocal influence between the guestand host chiral centers.