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PubMed2006

Kinetic acidity of supramolecular imidazolium salts-effects of substituent, preorientation, and counterions on H/D exchange rates.

The deprotonation of imidazolium salts to N-heterocyclic carbenes is often a decisive step in modern catalytic reactions. Therefore, we studied the H/D exchange of the C2 H of 15 imidazolium-substituted calix[4]arenes and 11 nonmacrocyclic model compounds in methanol/water (97:3). The influence of the counterion, substitution directly on the imidazolium unit or on the preorientating calixarene backbone could be studied. The observed exchange rates might give a rational for the suitability of the imidazolium salts as precursors in the Suzuki coupling.

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PubMed2006

Efficient synthesis and host-guest properties of a new class of calix[6]azacryptands.

Two members of a new class of calix[6]azacryptands, namely, calix[6]tampo and calix[6]tamb, have been synthesized through an efficient [1 + 1] macrocyclization reaction--reduction sequence. One of them has been obtained in a remarkably high overall yield from the known X(6)H(3)Me(3). In comparison to all the other calix[6]azacryptands, they possess unique conformational properties since they present a rigidified cone conformation with a partial filling of the cavity by the methoxy groups. In contrast to calix[6]tampo, the fully protonated derivative of calix[6]tamb behaves as a remarkable molecular receptor toward polar neutral guests. NMR studies have shown that the intracavity binding process is governed by a conformational flip of the aromatic walls of the calixarene core.

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PubMed1999

Effects on Metal Binding of Circular NH- - -O=C Intramolecular Hydrogen Bonding in Hydrophilic Pseudocavities of p-tert-Butylcalix[4]arenes Carrying Amino Acid Moieties.

Calix[4]arenes bearing amino acid moieties were prepared by the reaction of carboxyl-protected amino acids with the acid chlorides of O-carboxymethylcalix[4]arenes. The binding ability toward metal ions was estimated by solvent extraction. These calixarenes showed selective extraction capability toward Na(+) and Ag(+) ions, which increased in the order 2a < 2b < 2d < 2c. The complexing ability was also affected by the nature of the solvents and was higher in ethyl acetate than dichloromethane. (1)H NMR studies showed that the complexation of 2c with Na(+) ion induces a change of the orientation of the amide groups from a network-like pattern with circular N-H- - -O=C intramolecular hydrogen bonding of 2c to a pattern in which carbonyl groups converge on the Na(+) ion. This change was also supported by a measurement of CD spectra.

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PubMed1999

Synthesis and Reactions of Calix[4]arene Bisanhydrides(1).

The tetrabenzyl ether, tetra-p-bromobenzenesulfonate, and tetra-p-methylbenzenesulfonate of p-carboxymethylcalix[4]arene in the 1,3-alternate conformation have been converted to the corresponding bisanhydrides. Reactions of the bisanhydrides with alcohols or amines afford calix[4]arenes carrying two carboxymethyl and two carboalkoxymethyl or two amidomethyl groups on the upper rim positions. The substitution pattern that is established confers molecular chirality on the calixarenes.

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PubMed2006

Alkali cation extraction by calix[4]crown-6 to room-temperature ionic liquids. The effect of solvent anion and humidity investigated by molecular dynamics simulations.

We report a molecular dynamics study on the solvation of M+ (Na+ to Cs+) alkali cations and of their LM+ complexes with a calix[4]arene host (L = 1,3-dimethoxy-calix[4]arene-crown-6 in the 1,3-alternate conformation) in the [BMI][PF6] and [BMI][Tf2N] room-temperature ionic liquids "ILs" based on the BMI+ (1-butyl-3-methylimidazolium) cation. The comparison of the two liquids and the dry versus humid form of the former one (with a 1:1 ratio of H2O and BMI+PF6- species) reveals the importance of humidity: in [BMI][PF6]-dry as in the [BMI][Tf2N] liquid, the first solvation shell of the "naked" M+ ions is composed of solvent anions only (four PF6- anions, and from four to five Tf2N- anions, respectively, quasi-neutralized by a surrounding cage of BMI+ cations), while in the [BMI][PF6]-humid IL, it comprises from one to three solvent anions and about four H2O molecules. In the LM+ complexes, the cation is shielded from solvent, but still somewhat interacts with a solvent anion in the dry ILs and with water in the humid IL. We also report tests on M+ interactions with solvent anions PF6- and Tf2N- in the gas phase, showing that the AMBER results are in satisfactory agreement with QM results obtained at different levels of theory. The question of ion recognition by L is then examined by free energy perturbation studies in the three liquids, predicting a high Cs+/Na+ selectivity upon liquid extraction from an aqueous phase, in agreement with experimental results on a parent calixarene host. A similar Cs+/Na+ selectivity is predicted upon complexation in a homogeneous IL phase, mainly due to the desolvation energy of the free cations. Thus, despite their polar character, ionic liquids qualitatively behave as classical weakly polar organic liquids (e.g., choroform) as far as liquid-liquid extraction is concerned but more like polar liquids (water, alcohols) as far as complexation in a single phase is concerned.

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PubMed2000

Molecular assembly and disassembly: novel photolabile molecular hosts

A new approach to the assembly and photochemical disassembly of molecular hosts is developed. It is based on photoinduced fragmentation in hydroxyalkyl dithianes and utilizes a novel spiro-bis-dithiane as a photolabile molecular tether to link two formylated macromolecular blocks, e.g., formyl calixarenes or formyl dibenzocrown ethers. A key feature of this molecular system is that after an assembly-disassembly cycle the starting macromolecular blocks are recovered intact and can be used again.

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PubMed2004

Differential and substrate-selective reactivity of calix[4]arene derivatives with cyclenyl-Zn(II) modifications at the upper rim.

[structure: see text] Novel "cone conformation" calix[4]arene derivatives, carrying either one or two cyclen (1,4,7,10-tetra-azacyclododecane) moieties at the upper rim, have been synthesized. The hydrolytic activities of the Zn(II) complexes of these calixarenes were studied. A surprising behavior was observed with p-nitrophenylstearate; whereas the bis-cyclenyl-2Zn(II) complex showed negligible hydrolytic activity over the background, the mononuclear complex showed a significant 400-fold rate increase at pH 8.5.

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PubMed2006

Regioselectively N-methylated azacalix[8]arene octamethyl ether prepared by catalytic aryl amination reaction using a temporal N-silylation protocol.

[Structure: see text] A temporal N-silylation protocol in the catalytic aryl amination reaction has been devised to prepare nitrogen-bridged calixarene analogues. The protocol involves a smooth in situ N-silylation before aryl amination reaction, followed by spontaneous cleavage of the N-Si bond in the usual workup process, to furnish secondary aromatic amines as the cross-coupled product with no silyl group on the nitrogen atom. A successful application to the preparation of regioselectively N-methylated azacalix[8]arene is described, together with the crystallographic analysis.

Amination↗
PubMed2000

Synthesis, Structure, and Complexing Property of p-tert-Butylcalix

p-tert-Butylcalix[4]arene 1,3-digallate, which contains a nonbonded close contact between galloyl groups, was synthesized and its structure was determined by dynamic (1)H NMR and X-ray crystallography. The electronic spectra showed that a new absorption band of the complexes appeared at a longer wavelength region upon adding Ag(+) ion. This spectral shift was explained in terms of the interaction between the facing galloyl groups of the lower rim of the calixarene framework.

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PubMed2001

p-Sulfonatocalix[6]arene is an effective coacervator of poly(allylamine hydrochloride).

p-Sulfonatocalix[6]arene is shown to form insoluble complexes with poly(allylamine hydrochloride) when the charge balance between the negative calixarene sulfonate groups matches the positive charge carried by the polyelectrolyte, this makes this glycosylaminoglycan analog an interesting candidate for controlled release systems in the case of proteins encapsulated in mesoscopic complexes with polyelectrolytes.

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PubMed2002

Novel tungsten calix[8]arene complexes.

p-tert-Butylcalix[8]areneH8 reacts with WCl6 forming a number of new metallocalix[8]arenes, one of which is reduced to give the first example of a metal-metal triple bond supported by a large calixarene ligand; the degree of metallation and conformations adopted by the newly generated metallocalix[8]arene systems are determined by single crystal X-ray diffraction.

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PubMed2004

Supramolecular optical chemosensors for organic analytes.

Supramolecular optical chemosensors are abiotic molecular devices that bind analytes by noncovalent interactions, producing a change in light absorption or fluorescence. This review summarizes recent progress in the development of such chemosensors for organic analytes based on artificial receptors. Important design considerations, such as analyte affinity, choice of chromophore or fluorophore, binding selectivity, and optical signaling mechanism are briefly discussed. Chemists have fashioned chemosensors from a wide range of molecular structures, including polyalcohols, crown ethers, calixarenes, helicenes, sterically geared tripods, metal complexes, pinwheels, porphyrins, and fused-ring heterocycles. Analytes of interest include amines, carboxylic acids, amino acids, hydroquinones, alkaloids, carbohydrates, peptides, urea and creatinine.

Biosensing Techniques↗
PubMed2003

A paradigm shift in the construction of heterobimetallic complexes: synthesis of group 2 & 4 metal-calix[6]arene complexes.

Deprotonation of calix[6]arenes with barium in methanol followed by the addition of [Ti(OPr(i))(4)] or [Zr(OBu(n))(4)] is effective in the formation of novel dimeric 2:1 barium-titanium(IV)/zirconium(IV) calix[6]arene complexes. In these complexes a central Ti(IV)/Zr(IV) coordinated in the exo-position connects the two calix[6]arenes in the 1,3-alternate conformation, each with an endo-barium sharing common phenolate groups with the titanium/zirconium centre and participating in cation-pi interactions. A homometallic barium calix[6]arene dimer was also prepared wherein the calix[6]arenes are in the 1,3-alternate conformation with each coordinating one endo- and one exo-barium centre. The exo-barium cations connect the two calix[6]arenes through bridging methanol ligands. In this and the heterometallic complexes, cation-pi complexation of the Ba(2+) ion within the 1,3 alternate conformation of calix[6]arene facilitates the formation of the dimeric complexes in methanol. In contrast, the smaller Sr(2+) ion did not form similar complexes in methanol, and the formation of an analogous 2:1 strontium-titanium calixarene complex required the use of the more sterically demanding donor alcohol, isopropanol, the resulting complex being devoid of cation-pi interaction. The results show (i) that a subtle interplay of solvation strength, coordination array type and cavity/cation size influences the accessibility of heterobimetallic complexes based on calix[6]arenes, and (ii) a synergistic endo-exo binding behaviour.

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