The condensation reaction between p-tert-butylphenol and formaldehyde leads in a single step to good yields of cyclic oligomers in which, depending on the reaction conditions, either four, six, or eight phenol units are joined by methylene bridges. The beakerlike shape of the most stable conformation of the tetramer has led to their being given the name “calixarenes” (calix = chalice). Resorcinol can undergo condensation in a similar manner with a variety of aldehydes to afford cyclic tetramers with the same basic structure (the resorcarenes). In both cases the reaction does not require the use of dilution techniques, so that large quantities of product can be readily obtained. In addition, the parent compounds can be modified in various ways, in particular at the phenolic hydroxy groups or the phenyl residues; these approaches can be used separately or in combination. Calixarenes are thus ideal starting materials for the synthesis of various types of host molecules and can also act as building blocks for the construction of larger molecular systems with defined structures and functions. Their potential applications range from use as highly specific ligands for analytical chemistry, sensor techniques and medical diagnostics to their use in the decontamination of waste water and the construction of artificial enzymes and the synthesis of new materials for non-linear optics or for ultrathin layers and sieve membranes with molecular pores.

Calixarenes, Macrocycles with (Almost) Unlimited Possibilities

Artificial Organic Host Molecules for Anions

The nature and characteristics of the CH/π interaction are discussed by comparison with other weak molecular forces such as the CH/O and OH/π interaction. The CH/π interaction is a kind of hydrogen bond operating between a soft acid CH and a soft base π-system (double and triple bonds, C6 and C5 aromatic rings, heteroaromatics, convex surfaces of fullerenes and nanotubes). The consequences of CH/π hydrogen bonds in supramolecular chemistry are reviewed on grounds of recent crystallographic findings and database analyses. The topics include intramolecular interactions, crystal packing (organic and organometallic compounds), host/guest complexes (cavity-type inclusion compounds of cyclodextrins and synthetic macrocyclic hosts such as calixarenes, catenanes, rotaxanes and pseudorotaxanes), lattice-inclusion type clathrates (including liquid crystals, porphyrin derivatives, cyclopentadienyl compounds and C60 fullerenes), enantioselective clathrate formation, catalytic enantioface discriminating reactions and solid-state photoreaction. The implications of the CH/π concept for crystal engineering and drug design are evident.

CH/π hydrogen bonds in crystals

Calixarenes are macrocyclic molecules, like crown ethers and cyclodextrins.1-7 Calixarenes made up of phenol and methylene units have many conformational isomers because of two possible rotational modes of the phenol unit: the oxygen-through-theannulus rotation and the para-substituent-throughthe-annulus rotation (Figure 1). The conformational isomers thus yielded afford a great number of unique cavities with the different size and the different shape. Recently, a number of strategies have been exploited by which not only the conformation of calix[4]arenes, but also those of calix[6]arenes and calix[8]arenes, can be immobilized. This means that our group can now design various calixarene-based receptors that show high selectivity for guest molecules and metal cations. In this review article, our group describe novel strategies for cavity design using calix[n]arene skeletons, strategies that are intended to allow complexation of specific molecular targets or metal ions.

Novel Cavity Design Using Calix[n]arene Skeletons: Toward Molecular Recognition and Metal Binding.

The challenge of cyclic and acyclic schiff bases and related derivatives

A novel class of neutral receptors for anions that contain a unique combination of an immobilized Lewis acidic binding site (UO[sub 2][sup 2+]) and additional amide C(O)NH groups, which can form a favorable H-bond with a coordinated anion guest, has been developed. The unique combination of a Lewis acidic UO[sub 2] center and amide C(O)NH groups in one receptor leads to highly specific H[sub 2]PO[sub 4[sup [minus]] anion recognition. The anion complexation has been demonstrated by X-ray analysis, conductometry, cyclic voltametry, [sup 1]H and [sup 31]P NMR spectroscopy, and FAB mass spectrometry. In the case of three pre-organized ligands, strong (K[sub ass] > 10[sup 5] M[minus]1] in MeCN-DMSO, 99:1) and selective complexation of H[sub 2]PO[sub 4]NO[sub 2][sup [minus]], respectively, were obtained for one receptor. [open quotes]Naked[close quotes] UO[sub 2] salophenes in which the Lewis acidic binding center contains two positions for guest coordination are good receptors for dianions. 42 refs., 5 figs., 4 tabs.

/pdf/functionalized-uo2-salenes-neutral-receptors-for-anions-3p2g5842nj.pdf

Functionalized UO2 Salenes: Neutral Receptors for Anions

/pdf/control-of-calix-6-arene-conformations-by-self-inclusion-of-qc8v7qbwm8.pdf

Control of Calix[6]arene Conformations by Self-Inclusion of 1,3,5-Tri-O-alkyl Substituents: Synthesis and NMR Studies

Three new calixspherands (2-4) were synthesized in good yields (>60%) via a new method; p-tert-butylcalix[4]arene (6) is bridged with a m-terphenyl (7-9) and subsequently alkylated. H-1 NMR spectroscopy and X-ray crystallography showed that all the complexes are in a partial cone conformation. All the calixspherands form kinetically stable complexes with Na+, K+, and Rb+. The kinetic stability was determined both by H-1 NMR spectroscopy, in CDCl3 saturated with D2O, and by a new method based on the exchange of radioactive rubidium or sodium in the complexes for nonradioactive sodium in different solvents. Both methods showed that the kinetic stability of the different complexes is strongly increased when the size of the group on the central aromatic ring of the m-terphenyl is increased. This effect is most pronounced for the rubidium complexes. The half-life times for decomplexation, in CDCl3 saturated with D2O, increased from 2.8 h for [1.Rb]+ to 139 h and 180 days for [2.Rb]+ and [3.Rb]+, respectively. The ''exchange method'' shows that the rate of decomplexation is the rate-limiting step in the exchange of rubidium in the complex for sodium present in solution. These results can be explained in terms of increased shielding of the cavity from solvent molecules. The kinetic stabilities of the complex of 3 with Na+, K+, and Rb+ are the highest ever reported.

/pdf/kinetically-stable-complexes-of-alkali-cations-with-117uw957sq.pdf

Kinetically stable complexes of alkali cations with calixspherands : An evaluation of shielding

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Functionalized UO2 Salenes: Neutral Receptors for Anions

Control of Calix[6]arene Conformations by Self-Inclusion of 1,3,5-Tri-O-alkyl Substituents: Synthesis and NMR Studies

Kinetically stable complexes of alkali cations with calixspherands : An evaluation of shielding