Synthesis, characterization, and conformational analysis of new perfunctionalized calix[9]arenes

The synthesis of a series of six new per-substituted p-tert-butylcalix[9]arenes derivatives has been achieved. Functions such as ester, nitrile, or carbonate have been grafted on the phenolic oxygens of calix[9]arene: p-tert-butylcalix[9]arene nona-ethyl acetate, p-tert-butylcalix[9]arene nona-carboxylic acid, p-tert-butylcalix[9]arene nona-cyanopropyloxy, p-tert-butylcalix[9]arene nona-tert-butoxycarbonyle, p-tert-butylcalix[9]arene nona-acetyl, and p-tert-butylcalix[9]arene nona-trifluoroacetyl have been obtained in good yields (from 48% to 78%) and fully characterized. The X-ray structures of p-tert-butylcalix[9]arene nona-tert-butoxycarbonyl and p-tert-butylcalix[9]arene nona-ethyl acetate have been determined. A dynamic NMR study has revealed the high conformational mobility of such structures, even at low temperature.     

Endo-cavity Complexes between Calix[6]arene Dianions and Aliphatic Ammonium Cations: Structure of a Hexylammonium Complex by X-ray Crystallography

The interactions of calix[6]arene and p-tert-butylcalix[6]arene with aliphatic amines in acetonitrile solution were studied by spectrophotometric titrations in the UV region and ¹H NMR spectrometry. Calix[6]arenes can undergo two deprotonations by aliphatic amines and the extent of the second proton transfer is mainly governed by the size of the α-substituent of the amine. ¹H NMR spectra show that the macrocycle adopts a 1,2,3-alternate conformation and that the α-hydrogens of the ammonium ions are shielded by the π-clouds of the aromatic rings. The X-ray crystal structure of the dihexylammonium complex of the p-tert-butylcalix[6]arene dianion, reported here, confirms the 1,2,3-alternate conformation and shows one of the two ammonium moieties encapsulated in the inner cavity of the macrocycle.     

Regioselective intramolecular bridging of p-tert-butylcalix[10]arene

The synthesis and characterization of a series of regioselective intramolecular bridging of calix[10]arene are described for the first time. Reacting p-tert-butylcalix[10]arene with tri-ethylene glycol ditosylate using K₂CO₃ as a base in toluene, 1,2-calix[10]crown-4 2a, 1,4-calix[10]crown-4 2b and 1,6-calix[10]crown-4 2c were obtained in yields of 9%, 14% and 7%, respectively. While using Cs₂CO₃/acetone instead of K₂CO₃/toluene, the 1,4-calix[10]crown-4 2b was obtained selectively in good yield up to 50%.     

Azobenzene-bridged calix[8]arenes

An azobenzene bridge was introduced into the lower (or smaller) rim of p-tert-butylcalix[8]arene (1) and 1,5-calix[8]crown-3 (2) to form 1,4-singly bridged (3) and 1,5:3,7-doubly bridged (4) calix[8]arene derivatives, respectively. Trans and cis isomers of conformationally rigid 4 were isolated. The quantum yields of the trans-cis photoisomerisation reactions have been measured.     

p-tert-Butylcalix[4]arene Derivatives Containing Azathiol Receptors and Their Recognition towards Hg(II)

25,27(2,2'-bis(ethoxybenzyl))-1,9-diimine-3,7-dithianonane-p-tert-butylcalix[4]arene (3),25,27(4,4'-bis(ethoxybenzyl))-1,9-diimine-3,7-dithianonane-p-tert-butylcalix[4]arene (4),25,27(2,2'-bis(ethoxybenzyl))-1,8-diimine-3,6-dithianonane-p-tert-butylcalix[4]arene (5) and25,27(4,4'-bis(ethoxybenzyl))-1,8-diimine-3,6-dithianonane-p-tert-butylcalix[4]arene (6) havebeen synthesized by condensation reactions between25,27(2,2'-bis(ethoxybenzaldehyde))-p-tert-butylcalix[4]arene (1) and25,27(4,4'-bis(ethoxybenzaldehyde))-p-tert-butylcalix[4]arene (2) and appropriateamines. Reduction of 3 and 4 withNaBH₄ and subsequent protonation byHCl/CH₃OH resulted in25,27(2,2'-bis(ethoxybenzyl))-1,9-diaza-3,7-dithianonane-p-tert-butylcalix[4]arenedihydrogenchloride (7) and25,27(4,4'-bis(ethoxybenzyl))-1,9-diaza-3,7-dithianonane-p-tert-butylcalix[4]arenedihydrogenchloride (8), respectively. Complexation studies of 7 and 8 withZn(II), Cd(II) and Hg(II) ions were carried out bypotentiometric titration. Compounds 7 and8 selectively form 1 : 1 complexes with Hg(II), andtheir stability constants (log Ks) were estimated tobe 4.47 ± 0.08 and 3.20 ± 0.13, respectively. From spiecies distribution plots, 7 and 8were found to form the highest amount of 1 : 1 complexes with Hg(II) at pH 8.7 and 9.1, respectively.     

Serendipitous one-pot formation of an unusual calix[5]arene-bis-crown-3 receptor

Alkylation of p-tert-butylcalix[5]arene 1c with an excess of 2-(2-chloroethoxy)ethyl tosylate 2 and anhydrous K₂CO₃, has led to the serendipitous regioselective formation of cone-in (1,2-3,5)-calix[5]arene bis-crown-3 derivative 4 possessing a reactive 2-(2-chloroethoxy)ethoxy pendant functionality at the lower rim. The structure and conformation of 4 have firmly been assigned by a combination of NMR analysis and HF/6-31G∗ ab initio calculations, as well as by an independent stepwise synthesis. A preliminary ESI-MS screening of the binding properties of 4 toward alkali metal ions has shown a clear preference for the larger cations.     

Synthesis and Complexation properties of N,N’-bis(calix[4]Arenoxyacetyl)Hydrazine Derivative

Heating of p-tert-butylcalix[4]arene bearing acethydrazide moiety at the lower rim in MeCN or PrⁱOH (EtOH) affords the corresponding N,N’-bis(calix[4]arenoxyacetyl)hydrazine derivative. The hydrazinolysis of mono(methoxycarbonylmethoxy)-p-tert-butylcalix[4]-arene in benzene or refluxing biscalixarene hydrazide in benzene in the presence of hydrazine hydrate produces the mixture of 1,2- and 1,3-(N,N’-diacetylhydrazine)-p-tert-butylcalix[4]arenes.     

Synthesis and Solid-State Conformation of a Calix[8]arene 1,5-Diquinone Derivative

The first example of a calix[8]quinone derivative, hexamethoxy-p-tert-butylcalix[8]-1,5-diquinone 5, has been synthesized from p-tert-butylcalix[8]arene 1by exploiting a protection-deprotection procedure. The structure of the 5·toluene inclusion compound has been determined by a single crystal X-ray diffraction study. The calix[8]arene molecule possesses a crystallographic inversion centre and assumes a ‘pseudo-chair-like’ conformation, with two opposite 3/4-cone moieties, which resembles the previously reported chair-like conformation of p-tert-butylcalix[8]arene. The 4 toluene molecules per unit cell occupy interstitial voids and are released in the temperature range of 30–160 °C.This revised version was published online in July 2005 with a corrected issue number.     

Synthesis and conformation studies of multiple bridged p-tert-butylcalix[7]arene diphosphates

p-tert-Butylcalix[7]arene reacting with phosphorus pentachloride and then with water afforded two stereoisomers p-tert-butylcalix[7]arene diphosphates 2a and 2b, representing the first phosphorus multiple bridged calix[7]arene. The more accurate structure of 2a was investigated with the aid of Gaussian03 calculations.     

Stereochemical control of calixarenes useful as rigid and conformationally diversiform platforms for molecular design

Stereochemical problems and related functions of calix[4]arenes, calix[6]arenes and their chiral derivatives have been reviewed. In p-tert-butylcalix[4]arene (1H₄) and its mono-, di-, tri-, and tetra-O-alkyl derivatives (1H₃R, 1H₂R₂,1HR₃, and 1R₄, respectively), 23 different homologues can exist (including 1H₄). We found that the OH group in the unmodified phenol unit is permeable through the calix[4]arene ring. Thus, several conformational isomers become equivalent after the ‘oxygen-through-the-annulus’ rotation of the OH group and the number of possible homologues is reduced to 13 (including 1H₃). We report in this paper the syntheses of all of these possible conformational isomers using a protection-deprotection method with a benzyl group and metal template effects. On the other hand, all possible chiral isomers that can be derived from calix[4]arene by modification of the OH groups have been systematically classified. Molecular asymmetry can be generated not only by different substituents but also by conformational isomerism. The numbers of chiral isomers are 17 for tetra-O-substituted calix[4]arenes, 9 for tri-O-substituted calix[4]arenes, 3 for di-O-substituted calix[4]arenes, and 0 for mono-O-substituted calix[4]arenes. Chiral calix[4]arenes can also be designed by the introduction of a substituent into the m-position of a phenol unit or by the use of a dissymmetric ‘stapling reaction’ in proximal phenol units. In p-tert-butylcalix[6]arene, the conformational behaviour is totally different from that in p-tert-butylcalix[4]arene. A large degree of conformational freedom remains in the framework, and both ‘oxygen-through-the-annulus rotation’ and ‘para-substituent-through-the-annulus rotation’ can take place. However, when metal cations are bound to calix[6]aryl esters, the conformation is changed to a cone type. Bridging and capping are powerful methods to immobilize the conformation of calix[6]arenes. In addition, definitive evidence for ring immobilization was obtained from the absence of racemization in the chiral calix[6]arene. A successful example of chiral recognition for α-amino acid derivatives was achieved by using chiral homooxacalix[3]arene which has ‘pseudo C₂ symmetry’. These examples indicate that calixarenes serve as rigid and conformationally diversiform platforms for the design of novel functional supramolecules.     

Thermodynamic and electrochemical aspects ofp-tert-butylcalix[n]arenes (n=4, 6, 8) and their interactions with amines

Attention is drawn to the need of detailed thermodynamics in calixarene chemistry. The reasons for increasing efforts in this area are underlined and suggestions for new issues to be addressed are given. The solution thermodynamics ofp-tert-butylcalix[n]arenes (n=4, 6, 8) is discussed with particular reference to transfer Gibbs energies which reflect the selective solvation that the tetramer and the octamer undergo in the various solvents. This is followed by recent solution studies on amine-p-tert-butylcalix[n]arene (n=4, 6, 8) in nitrobenzene and in benzonitrile at 298.15 K which indicate the lower acidic character of the tetramer relative to the hexamer and the octamer in these solvents. As an implication of these results, very low conductivities are observed in studies involving the interaction of the former with amines. Thus, thermodynamic studies suggest thatp-tert-butylcalix[4]arene interacts with triethylamine in benzonitrile and in nitrobenzene through hydrogen bonding or ion-pair formation. A thermodynamic cycle is used to investigate the effect associated with the interaction of the amine with the tetramer in these solvents.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Synthesis of two new p-tert-butylcalix[4]arene β-ketoimin derivatives for extraction of dichromate anion

In this study the selective derivatization of p-tert-butylcalix[4]arene was carried out and two new p-tert-butylcalix[4]arene β-ketoimin, 5,11,17,23-tetra-tert-butyl-25,27-bis-3-methyl-[(β-ketoimine)-ethoxy]-26,28-dihydroxycalix[4]arene (4), and 5,11,17,23-tetra-tert-butyl-25,27-bis-3-chloro-[(β-ketoimine)-ethoxy]-26,28-dihydroxycalix[4]arene (5) have been synthesized. In the synthesis, the lower rim of p-tert-butylcalix[4]arene was modified in order to acquire binding site for the recognition of dichromate anion. It was observed that these ionophores 4 and 5 showed high affinity towards dichromate anion. The protonated Schiff-base forms of the receptors were effective for transferring the HCr₂O₇ ^? anion from aqueous phase to a dichloromethane phase.     

Preparation and biological properties of inclusion compounds of calix[4]arene and 1,4-benzodiazepinone derivatives

The possibility of enhancing the oral bioavailability of certain 1,4-benzodiazepinones by using them as complexes with a series of calix[4]arene derivatives was examined. All the inclusion compounds prepared exhibit anticonvulsant activity by antagonism with Corazol spasmodic agent. Complexes with 25,27-bis[(hydrazinocarbonyl)methoxy]-26,28-dihydroxy-p-tert-butylcalix[4]arene exhibit higher pharmacological activity compared to the starting 7-bromo-5-(o-chloro)phenyl-and 1-hydrazinocarbonylmethyl-7-bromo-5-phenyl-1,2-dihydro-3H-1,4-benzodiazepin-2-ones.     

Synthesis and crystal structures of Ag and Hg complexes of bis(N-heterocyclic carbenes) on p-tert-butylcalix[4]arene platform

A series of p-tert-butylcalix[4]arene 1,3-diimidazolium salts were successfully prepared by the alkylation of p-tert-butylcalix[4]arene with dibromoalkanes, and sequential substitution reaction with 1-alkylimidazole. Furthermore, coordination reactions of p-tert-butylcalix[4]arene 1,3-diimidazolium salts with silver oxide and mercury acetate gave novel Ag and Hg complexes of bis(N-heterocyclic carbenes) on p-tert-butylcalix[4]arene platform. The single-crystal structures of four p-tert-butylcalix[4]arene 1,3-diimidazolium salts and three metal complexes were successfully determined by X-ray diffraction. An Ag–Ag argentophilic interaction (Ag–Ag bond length is 3.1599(6) Å) is formed between the two Ag–NHC complexes and a dimetallic coordination mode exists in Hg–NHC complexes.     

Oxidation with potassium permanganate of <Emphasis Type="Italic">p-tert</Emphasis>-butylcalix[4]arene derivatives containing olefinic substituents at the lower rim

In oxidation with potassium permanganate of p-tert-butylcalix[4]arene derivatives containing olefinic substituents at the lower rim, with the formation of the corresponding carbonyl compounds, calixarene acts as both the substrate and the catalyst.     

Two Stabilized Calix[6]Cryptand Stereo Isomers used as Stationary Phases for Capillary Gas Chromatography

ABSTRACT

A couple of stabilized calix[6]cryptand steroisomers were synthesized by capping 1,3,5-trimethoxy-p-tert-butylcalix[6]arene with 1,1,1-tri(tosyloxyethoxyethoxymethyl)-propane and used as stationary phases for capillary gas chromatography. Their chromatographic characteristics and separation mechanism for aromatic positional isomers and polycyclic aromatic hydrocarbons are discussed.     

Preparation and characterisation of thiol functionalised p-tert-butylcalix[4]arene-embedded polymer inclusion membranes: performance and selectivity

ABSTRACT

Here in we report the syntheses of the thiol functionalised p-tert-butylcalix[4]arene from the reaction of p-tert-butylcalix[4]arene-dialkylbromide derivative with thiourea. The structure of the thiol functionalised p-tert-butylcalix[4]arene was determined by using ¹HNMR, ¹³CNMR and elemental analysis techniques. Furthermore, the thiol functionalised p-tert-butylcalix[4]arene, cellulose triacetate (CTA) and 2-nitrophenyl octyl ether (o-NPPE) in dichloromethane were used to make a new calixarene-embedded polymer inclusion membrane (Bu^t-C@PIM). The surface and structure morphology of Bu^t-C@PIM was detected using thermogravimetric analysis, elemental analysis techniques and scanning electron microscopy. The affinity (the percentage of metal ion transferred from the source solution) of a PIM towards a range of divalent cations was found to follow the order Pb²⁺ > Zn²⁺ > Ni²⁺ > Cu²⁺ > Cd²⁺ > Co²⁺. The results also indicated that the transport efficiency of Bu^t-C@PIM was repeatable and may be beneficial in the development of a simplistic and highly influential trace metal recovery method from aqueous solution.     

Synthesis of Oligomer and Styrene Polymer-supported Calix[4]arene Derivatives and Selective Extraction of Fe3+

Five new polymeric compounds containing more than one calix[4]arene have been synthesized by reacting an oligomer with 5,11,17,23-tetrakis[(diethylamino)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (L1), 25,27-dimethyl-26,28-dihydroxy-p-tert-butylcalix[4]arene diketone (L2), and p-nitrocalix[4]arene (L3), and chloromethylated polystyrene with 25,27-dimethyl-26,28-dihydroxy-p-tert-butylcalix[4]arene diketone (L4) and p-nitrocalix[4]arene (L5). These compounds were studied by the selective extraction of Fe³⁺ cation from the aqueous phase into the organic phase and was carried out by using compounds L1–L5. It was observed that the polymer support attached to the lower rim of p-nitrocalix[4]arene was the most efficient carrier of Fe³⁺ in the extraction process.     

Adsorption of direct black-38 azo dye on p-tert-butylcalix[6]arene immobilized material

The present study demonstrates the adsorption of the direct black-38 (DB-38) azo dye on potential and newly synthesized p-tert-butylcalix[6]arene based silica resin (4). Resin 4 was synthesized via the modification of pure silica 1 as 2 followed by the immobilization of p-tert-butylcalix[6]arene (3) onto modified silica (2). It was characterized by using different analytical techniques such as FT-IR, scanning electron microscopy (SEM) and thermo gravimetric analysis (TGA). The effect of adsorbent dosage, pH and the electrolyte effect on the removal of the DB-38 azo dye were evaluated through batch wise adsorption experiments. Maximum adsorption of 91% was achieved at pH 9.0. The textile wastewater samples were used to ensure the field applicability of the newly synthesized adsorbent 4 for the treatment of dye contaminated effluents. All results regarding the removal of the DB-38 azo dye from the aqueous environment prop up resin 4 as an effective adsorbent and it was found that resin 4 has high adsorption efficiency toward the DB-38 azo dye at a wide range of pH as compared to 1 and 2.     

Inherently chiral biscalixarene cone–cone conformers consisting of calix[4]arene and calix[5]arene subunits

Inherently chiral biscalixarenes with hetero-cavities were synthesized by a covalent assembly of p-tert-butylcalix[5]arene with a 1,3-substituted calix[4]arene via 1,3-alkylation reaction and subsequent desymmetrization. The racemates were resolved by chiral HPLC method. ¹H NMR spectra, VT-NMR spectra, and theoretical calculations support that the calix[5]arene subunit of the inherently chiral calix[4][5]arene ester adopts a cone-in conformation, with the aromatic ring bearing the CH₂CO₂Et group tilting inward the calix[5]arene cavity. By contrast, such a cone-in structural feature of the calix[5]arene subunit disappears for the corresponding inherently chiral calix[4][5]arene carboxylic acid, due to the intramolecular hydrogen bonding between the carboxyl group and an ethereal oxygen of the glycolic chain.