No abstract

The shaping of a calix[7]arene macrocycle into cone-like structure 3, through exhaustive alkylation of doubly bridged calix[7]arene derivative 2 with bulky groups, has been investigated. Conformational details about the structure adopted by calix[7]arene derivative 3 in solution have been obtained by using chemical shift surface maps, as previously reported by our group. Thus, chemical shift contour plots indicated that 3 adopted a cone-shaped structure in solution analogous to that adopted by the known p-tert-butylcalix[7]arene heptacarboxylic acid derivative 4. Interestingly, the X-ray structure of derivative 3 showed a high degree of similarity to the theoretical structure, which confirmed the validity of the contour plots method. The preorganized calix[7]arene host 3 showed interesting recognition abilities toward both organic and alkali cations. In fact, an unprecedented endo-cavity complexation of linear and branched alkyl ammonium cations with a larger calix[7]arene host was evidenced. A comparable affinity for branched tBuNH(3)(+) and linear nBuNH(3)(+) guests was observed.

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.

Selectively functionalized resorcinarenes were obtained by weak-base-promoted O-alkylation of C-undecylresorcin[4]arene 1. Tetrasubstituted derivatives with C(4)-1,3,5,7, C(2v)-1,2,5,6, C(s)-1,2,4,7, and C(i)-1,2,4,6 pattern were obtained in workable yields by using K(2)CO(3) as the base in acetone at reflux. The good regioselectivity with respect to the statistical distribution was explained in terms of preferential formation of H-bonded monoanions. Heptaaroylated resorcin[4]arenes were also easily obtained by reaction of 1 with aroyl chlorides in pyridine.