Alkylammonium Cation Complexation into the Narrow Cavity of Dihomooxacalix[4]arene Macrocycle

摘要

How big should a calixarene macrocycle be fornendo-cavity complexation to occur or to allow through-theannulusnthreading? To answer these questions, a completenstudy on the complexation of primary and secondaryn(di)alkylammonium cations by 18-membered p-tertbutyldihomooxacalix[n4]arene macroring has been performednin the presence of the “superweak” TFPB counterion. Thus, itnwas found that this macrocycle is currently the smallest calixarene able to host linear and branched alkylammonium guests insidenits aromatic cavity. Molecular mechanics calculations revealed that this recognition event is mainly driven by a H-bondingninteraction between the guest ammonium group and the host CH2OCH2 bridge. The endo-cavity complexation of chiral s-nBuNH3n+ guest results in an asymmetric complex in the NMR time scale. The chirality transfer from guest to host is likely due to anrestricted guest motion inside the tight cavity. The complexation study with secondary di-n-alkylammonium·TFPB salts revealednthat the 18-membered dihomooxacalix[4]arene macroring cannot give the through-the-annulus threading with them because of itsnsmall dimension. However, the macrocycle is able to complex such ions, which can only be accommodated in an hook-likenconformation characterized by two unfavorable gauche interactions around the CH2−NH2n+ bonds. The strain generated by thisnunfavorable folding is very likely compensated by stronger H-bonds and more favorable CH/π interactions between guest andnhost.