Complexes of p-t-Butylcalix[4]arene Derivatives with Neutral Molecules: Structures and Stabilities

Complexation of solvents and conformational equilibria in solutions of the simplest calix[4]arenes

Structure optimization and calculation of electronic adsorption spectra of 25,26,27,28-tetrahydroxycalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene conformers have been performed by density-functional theory using hybrid B3LYP functional in cc-p VTZ and cc-p VDZ basis sets in Gaussian 09 package. Analysis of experimental UV-Vis spectra of solutions of 25,26,27,28-tetrahydroxycalix[4]arene, 4-tert-butylcalix[4]arene and 25,27-dimethoxy-26,28-dihydroxycalix[4]arene in various solvents has been carried out. It was shown that the ratio of absorption maxima at characteristic wavelengths at ca. 274 and 283 nm may be used to assess the extent of calix[4]arene/solvent interactions. The conclusion is drawn that spectral characteristics of calix[4]arenes are strongly affected by acid-base interactions.

Extraction and complexation of alkali and alkaline earth metal cations by lower-rim calix[4]arene diethylene glycol amide derivatives

The metal-ion extraction abilities of calix[4]arene derivatives1and2are largely dependent on the type of the amide group forming the cation-binding site. Tertiary-amide derivative2was shown to be an excellent extractant for most alkali and alkaline earth cations.

Synthesis of deep-cavity fluorous calix[4]arenes as molecular recognition scaffolds

Several lower-rim perfluoroalkylated (fluorous) calix[4]arenes have been synthesized by O-alkylation of the parent calix[4]arene. The compounds are formed in the cone conformation. They are soluble in several fluorous solvents and show promise for use in sensing, selective extractions and other applications.

First Synthesis and Structure of β-Ketoimine Calix[4]arenes: Complexation and Extraction Studies

The synthesis of a new series of beta-ketoimine calix[4]arene derivatives is described. The reaction of calix[4]arene or p-tert-butylcalix[4]arene with bromoacetonitrile or bromobutyronitrile afforded di-, tri-, and tetranitrile calixarene derivatives (3-8, 3a), which were then reduced into the corresponding amine (9-13, 3b). The condensation of these aminocalixarenes with acetylacetone led to six beta-ketoimine calix[4]arene derivatives (14-18, 3c) as a class of selective receptors toward transition metals. Molecular structures of 4, 7, and 17 have been determined by X-ray diffraction. The packing of 17 revealed a network of intramolecular and intermolecular hydrogen bonds. The complexation properties of receptors 15, 17, and 3c toward different metal ions have been investigated by UV-vis titrations in organic media. The stoichiometries of complexes with 17 were determined by both the mole ratio method and Job plots. These novel receptors selectively complex Cu2+, Hg2+, and Ag+. Moreover, the extraction properties of 17 toward cations have been studied by liquid-liquid extraction and atomic absorption spectrometry. Compound 17 has good affinity and selectivity toward Pb2+.

Synthesis of Cavity Extended Cyclotriveratrylenes

Aromatic nucleophilic substitution reaction of cyclotriguaiacylene 1 with fluorobenzene derivatives bearing electron-withdrawing groups X (CHO, COCH(3), CN, NO(2)) in the para position gives a series of cyclotriveratrylene derivatives (3a-d), where the X substituents can be transformed to hydrogen-bond donor groups to afford new CTV-based heteroditopic receptors. The substituents of compounds 3a-d favor the facile demethylation reaction of the CTV derivatives. Attempts to perform alkylation reactions on derivatives (8c,d) evidenced the formation of a stereoisomeric mixture of symmetrical and unsymmetrical compounds.

Tuning of the Cavity of Water-Soluble Thiacalix[4]arene for the Control of Inclusion Ability Toward Water-Miscible Organic Molecules

Inclusion abilities of water-soluble thiacalix[4]arenetetrasulfonate (3) and mono-O-carboxymethylated derivatives 5 and 6 toward water-miscible organic molecules such as alcohols, ketones, and nitriles in water were investigated by 1H NMR. The limit chemical shift change of the guest upon inclusion in the host suggested that the hosts regioselectively encapsulate the guests from the side of aliphatic moiety. Large guests such as pentan-1-ol showed folding of the alkyl chain to be included inside the cavity. On the other hand, mono-O-substituted 6 included a guest molecule less deep in the cavity than 3 did. Binding constants (K) of 3 toward guest molecules increased with hydrophobicity of the guest, suggesting that hydrophobic effect plays a main role for the complexation. On the contrary, mono-O-carboxymethyl derivatives 5 and 6 showed the opposite dependency of stability on hydrophobicity; higher affinities toward less hydrophobic guests. Among the guests examined, the smallest entities such as CH3OH and CH3CN were best included in 5 with the highest binding constant ever reported (K > 102 M-1). It was concluded that introduction of the carboxymethyl group into one of the phenolic OH of thiacalix[4]arene led to tuning of the cavity shape for stereospecific inclusion of small guests. The tuning of cavity by the O-alkylation was actually confirmed by X-ray crystal analysis of a complex of 5 with diethyl ketone.

Synthesis and structure of lower rim C-linked N-tosyl peptidocalix[4]arenes

Chiral p-tert-butylcalix[4]arenes functionalized at the lower rim with amino acid residues have been prepared. The (1)H and (13)C NMR spectra indicate that the macrocycles preferably adopt a cone conformation. Calix[4]arenes bearing amino acid moieties were prepared as a class of receptors selective for anions that are bound through hydrogen bonding with the NH group. The association constants are dependent on the nature of the substituents at the lower rim. Derivative 9 shows the strongest complexation and the largest selectivity for N-tosyl-(L)-alaninate. Finally, a preliminary X-ray crystal study of the difunctionalized receptor 6f shows the "flattened cone" conformation in the solid state.

Molecular recognition in methanol: the first example of hydrogen-bond-mediated self-association of a calix[4]arene in polar, protic solvent

Tetraalaninecalixarene was prepared by coupling of tetraaminocalix[4]arene with alanine. It dimerizes in methanol, providing the first example of a substituted calixarene that undergoes self-association through hydrogen bonding in polar, protic solvent. The association constant in 24:1 MeOH:H2O was determined to be 29 000 M-1. Addition of arginine or lysine results in disruption of the dimer and formation of a 1:1 complex between the amino acid and the tetraalaninecalixarene. The preparation of a peptidocalixarene that associates in polar solvent opens new doors for the use of calixarenes for molecular recognition in biologically relevant environments.

Ion channel formation from a calix[4]arene amide that binds HCl

The ion transport activity of calix[4]arene tetrabutylamide 1,3-alt 2 was studied in liposomes, planar lipid bilayers, and HEK-293 cells. These experiments, when considered together with (1)H NMR and X-ray crystallography data, indicate that calix[4]arene tetrabutylamide 2 (1) forms ion channels in bilayer membranes, (2) mediates ion transport across cell membranes at positive holding potential, (3) alters the pH inside liposomes experiencing a Cl(-) gradient, and (4) shows a significant Cl(-)/SO(4)(2)(-) transport selectivity. An analogue, calix[4]arene tetramethylamide 1, self-assembles in the presence of HCl to generate solid-state structures with chloride-filled and water-filled channels. Structureminus signactivity studies indicate that the hydrophobicity, amide substitution, and macrocyclic framework of the calixarene are essential for HCl binding and transport. Calix[4]arene tetrabutylamide 2 is a rare example of an anion-dependent, synthetic ion channel.

Recognition of amides by new rigid Calix

The synthesis of new hosts specifically designed for the recognition of amides, characterized by two binding regions: a rigid calix[4]arene cavity and a sidearm, inserted at its rim, able to form strong hydrogen bonds, is described. The binding abilities of the new receptors toward amides of general structure R(1)CONR(2)R(3) have been investigated in CDCl(3) solution by (1)H NMR spectroscopy. When the additional binding site is the N-phenylureido group spaced by a methylene unit from the apolar cavity, binding constants up to 756 M(-)(1) were measured. Neither the two separate potential binding sites, nor the model host, where the calix[4]arene skeleton is flexible show detectable binding ability toward the series of guests examined. The rigidity of the calix[4]arene apolar cavity is the key control element in determining the efficiency of these molecular recognition processes. The presence of NH groups in the guest controls the efficiency and selectivity of binding.

A Reversed-phase HPLC Study of the Complexation of Benzene Derivatives Guest Molecules with 5,17-bis(N-Tolyliminomethyl)-25,27-dipropoxycalix[4]arene in Acetonitrile–Water Solution

Stability constants of the host–guest complexes of 5,17-bis( N-tolyliminomethyl)-25,27-dipropoxycalix[4]arene with benzene derivatives were determined by reversed-phase HPLC in acetonitrile–water solution.