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

Abstract

The influence of chirality in calixarene threading has been studied by exploiting the &ldquo;superweak anion approach&rdquo;. In particular, the formation of chiral pseudo[2]rotaxanes bearing a classical stereogenic center in their axle and/or wheel components has been considered. Two kind of pseudo[2]rotaxane stereoadducts, the &ldquo;<i>endo</i>-chiral&rdquo; and &ldquo;<i>exo</i>-chiral&rdquo; ones, having the stereogenic center of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene were preferentially formed with specifically designed chiral axles by a fine exploitation of the so-called &ldquo;<i>endo</i>-alkyl rule&rdquo; and a newly defined &ldquo;<i>endo</i>-&alpha;-methyl-benzyl rule&rdquo; (<i>threading of a hexaalkoxycalix[6]arene with a directional (&alpha;-methyl-benzyl)benzylammonium axle occurs with an</i> endo<i>-&alpha;-methyl-benzyl preference</i>). The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase by means of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hosts with 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/guest systems. Possible rationales are given to explain the scarce reciprocal influence between the guest and host chiral centers.