Alkylammonium cation complexation into the narrow cavity of dihomooxacalix[4]arene macrocycle

How big should a calixarene macrocycle be for endo-cavity complexation to occur or to allow through-the-annulus threading? To answer these questions, a complete study on the complexation of primary and secondary (di)alkylammonium cations by 18-membered p-tert-butyldihomooxacalix[4]arene macroring has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that this macrocycle is currently the smallest calixarene able to host linear and branched alkylammonium guests inside its aromatic cavity. Molecular mechanics calculations revealed that this recognition event is mainly driven by a H-bonding interaction between the guest ammonium group and the host CH(2)OCH(2) bridge. The endo-cavity complexation of chiral s-BuNH(3)(+) guest results in an asymmetric complex in the NMR time scale. The chirality transfer from guest to host is likely due to a restricted guest motion inside the tight cavity. The complexation study with secondary di-n-alkylammonium·TFPB salts revealed that the 18-membered dihomooxacalix[4]arene macroring cannot give the through-the-annulus threading with them because of its small dimension. However, the macrocycle is able to complex such ions, which can only be accommodated in an hook-like conformation characterized by two unfavorable gauche interactions around the CH(2)-NH(2)(+) bonds. The strain generated by this unfavorable folding is very likely compensated by stronger H-bonds and more favorable CH/π interactions between guest and host.