Methyl Ether Derivatives of p-tert-Butyl[3.1.3.1]homooxacalixarene. Formation, Structure, and Complexes with Quaternary Ammonium Ions

2,3-Ethylene-bridged dihomooxacalix[4]arenes: synthesis, X-ray crystal structures and highly selective binding properties with anions

Three novel 2,3-ethylene-bridged p-tert-butyldihomooxacalix[4]arenes 3a–3c were selectively synthesized by direct O-alkylation of p-tert-butyldihomooxacalix[4]arene 1 with excess of 1,2-dibromoethane in controlled basic systems.

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.

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Synthesis and Complexation Behaviour of Tetrakis[(N,N-diethylamino-carbonyl)methoxy]tetrahomodioxacalix[4]arene

O-Alkylation of the flexible tetrahomodioxacalix[4]arene 1 with N,N-diethylchloroacetamide in the presence of NaH, K2CO3 or Cs2CO3 gave only one pure stereoisomer, 1,4- alternate-2, while other possible isomers were not observed.

Complexation of Quaternary Ammonium Ions by Tetraester Derivatives of [3.1.3.1]Homooxacalixarene in Mobile and in Fixed Conformation

In the tetraalkylation of p-tert-butyl[3.1.3.1]homooxacalixarene with BrCH2CO2R and K2CO3 in acetone, the initially formed cone conformer is converted into the more stable 1,4-alternate conformer when R = Me or Et, but not when R = i-Pr or t-Bu. In the case of R = i-Pr, derivatives in fixed 1,4-alternate conformation and in partial cone conformation were also isolated. Compounds in fixed cone conformation are good ligands for tetramethylammonium, acetylcholine, and N-methylpyridinium salts in CDCl3, but the partial cone isomer proved to be somewhat better and even the 1,4-alternate conformer turned out to be active. The possible involvement of the ester functions as additional binding sites is discussed; moreover, an insight into the energetics of the complexation and conformational isomerization processes is given.

CsF-Promoted Acetyl Dance at the Narrow Rim of p-tert-Butyl[3.1.3.1]homooxacalixarene

In the acetylation of an oxygenated calix[4]arene homologue in the presence of CsF as a base, relaxation of the reaction system to the equilibrium composition takes place through several intra- and intermolecular steps that can be easily controlled to obtain the various acetyl derivatives. The effect of different bases is also discussed.