Synthesis of upper rim calix[4]arene carcerands

The synthesis of novel tetra-olefinic calix[4]arene carcerands is described using a synthetic strategy involving palladium catalysed Heck coupling followed by imine formation.     

Lower rim substituted p-tert-butyl calix[4]arene; Part 14. Synthesis, structures and binding studies of calix[4]arene thioamides

A number of p-tert-butylcalix[4]arene thioamides were synthesized and characterized by ¹H-NMR and elemental analysis. Compounds 1–5 are O-substituted derivatives with –CH₂–C(=S)–N–X groups, where NX = morpholidyl, NEt₂, NHC₂H₄Ph, NHCH₂Ph and NHEt, respectively. The X-ray structures of the ligands 1, 3, 5 and of the complex 3·Pb(ClO₄)₂, (compound 6), are presented and their slightly distorted cone conformation is established. The influence of the nature of the thioamide functions (secondary or tertiary) on the extractability of some selected metal cations was investigated. Whereas all these calixarenes show the highest extraction level for Ag⁺, tertiary thioamides are more efficient extractants for Pb²⁺ than secondary thioamides.     

Synthesis and binding properties of calix[4]arene telomers

Two calix[4]arene derivatives (5 and 8) and their telomers are synthesized to estimate selective extraction of alkali and transition metal cations from the aqueous to the organic phase (chloroform). Compound 5 shows selectivity toward Hg²⁺. Compound 8 and telomers 6 and 9 are not selective but are good extractants for all used transition metal ions. The observations suggest that ethylene glycol bridges are efficient for carrying transition metal cations in a two-phase solvent system. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4351–4355, 1999     

Synthesis of calix[4]arene library substituted with peptides at the upper rim

A fluorescence-labeled calix[4]arene library substituted with peptides at the upper rim was synthesized. Screening of the library for binding a dye-labeled oligopeptide indicated that some peptidocalix[4]arenes selectively bind the oligopeptide. The chemosensitivity of the library members for a target peptide was also investigated.     

Synthesis of upper rim calix[4]arene divalent glycoclusters via amide bond conjugation

Synthetic routes for linking two sugar units at the upper rim of cone calix[4]arenes, through the formation of amide bonds, have been explored. Steric effects prevent the coupling of calix[4]arene dicarboxylic acid with simple aminoglycosides, whereas the corresponding reaction with carbohydrates bearing a two or three carbon atoms spacer, terminating with a primary amino group, allows the synthesis of several difunctionalized calix[4]arene neoglycoconjugates, attractive in chemical glycobiology and supramolecular chemistry.     

Functionalisation of the upper rim of calix[4]arene via alcoholysis and hydrosilylation reactions

Metalation of 5,17-dibromo-25,26,27,28-tetra propoxy calix[4]arene (1) with n-BuLi in THF at −78 °C gave organolithium reagent, which reacted with Me₂HSiCl to give 5,17-bis(dimethylsilyl)-25,26,27,28-tetra propoxy calix[4]arene (2). The Si-H groups of calixarene 2 were treated with methanol, ethanol, propanol, butanol, pentanol, hexanol, 2-propanol and 2-methyl propanol in the presence of Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl disiloxane complex, solution in xylene) to give the corresponding 5,17-bis(alkoxydimethylsilyl)-25,26,27,28-tetra propoxy calix[4]arene (3). Moreover, calixarene 2 was easily functionalized with a variety of alkenes using Karstedt catalyst to give the corresponding organosilylated calix[4]arene (4).     

Selectively formylated and bridged calix[6]arene derivatives at the upper rim

The strategy of bridging the anisole units at the upper rim of calix[6]arene has been applied to strain the conformations of calix[6]arene. Based on the selective formylation of the 1,3,5-tri-p-tert-butylcalix[6]arene, several new calix[6]arene derivatives with different 1,3-bridged chains or a 1,3,5-tripod bridge at the upper rim have been prepared with moderate yields. The ¹H NMR spectra indicate that these calix[6]arene derivatives adopt a cone conformation, which has also been confirmed by the theoretical calculation at AM1 level. X-ray crystal structure of 1,3,5-tripod bridged compound 5 discloses that the calix[6]arene host stands in a cone conformation with approximate C_3v symmetry, and that a methanol molecule is enclosed in its hydrophobic cavity and stabilized by multi hydrogen bonds.     

Regioselective upper rim substitution of calix[4]arenes

The electrophilic substitution of 25,27-dipropoxy-26,28-dinosyloxycalix[4]arene leads exclusively to the para-substitution of the alkylated phenol rings, while in the next step, the protecting nosyl group can be easily removed using a basic hydrolysis. The overall process yields dialkoxycalix[4]arenes with the substitution on the alkylated rings—the substitution pattern, which is complementary to the common dialkoxycalix[4]arenes with substituted nonalkylated phenolic units. The usefulness of this protection/deprotection procedure was documented by the synthesis of novel type of calixarene dipropoxy derivatives, and by the preparation of a novel anion receptor based on this substitution pattern.     

Band intensity in the IR spectra and conformations of calix[4]arene and thiacalix[4]arene

The experimental IR and Far IR spectra of the calix[4]arene, p-tert-butylcalix[4]arene, thialcalix[4]arene and p-tert-butylthiacalix[4]arene were examined at different temperatures and interpreted. The band frequencies and intensities in the IR spectra of the calix[4]arene and thialcalix[4]arene were calculated. The absorption curves of the four possible calix[4]arene conformations: cone, partial cone, 1,2- and 1,3-alternate were computed. The bands characteristic for each conformation are defined and assigned. The obtained spectra-structure correlation can be used for the characteristic of calixarenes conformation.     

Synthesis and cation binding properties of fluorescent calix[4]arene derivatives bearing tryptophan units at the lower rim

The fluorescent peptidocalixarenes, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (1) and 5,11,17,23-tetra-tert-butyl-25,27-di(O-methyl)-26,28-bis(O-methyl-l-tryptophanylcarbonylmethoxy)calix[4]arene (2), were prepared by introducing tryptophan subunits at a lower calixarene rim. Coordination abilities of 1 and 2 towards Eu(III) and alkali metal cations were studied by spectrophotometric, spectrofluorimetric, conductometric and potentiometric titrations in acetonitrile at 25°C. Rather strong complexation was observed for smaller alkali metal cations Li⁺ and Na⁺ (log K _Li1 >6, log K _Li2 >6, log K _Na1  = 8.25, log K _Na2  = 6.94), and moderate for K⁺ (log K _K1  = 5.09, log K _K2  = 4.09). Larger Rb⁺ and Cs⁺ cations did not fit in the ion binding site of 1 so no complexation was detected, whereas the more flexible ligand 2 accommodated Rb⁺ cation (log K _Rb2  = 3.44). The fluorescence of 1 (λ_ex = 280 nm, λ_em = 340 nm) was remarkably quenched by Eu(III). Stability constant of 1:1 (Eu³⁺:1) complex determined spectrofluorimetrically amounted to log K _Eu1  = 6.16.     

Anion‐binding properties of two calix[4]arene derivatives containing two ferrocene imine or ferrocene amine units at the upper rim

Calix[4]arene derivatives containing ferrocene units at the upper rim have been synthesized and their anion‐binding and sensing investigated towards anions such as chloride, bromide and hydrogen sulfate by cyclic voltammetry. Electrochemical studies show that these redox‐active ligands electrochemically recognize chloride and bromide anions. These compounds have excellent selectivity for chloride and bromide ions, in the comparison with hydrogen sulfate anion. With ferrocenyl Schiff base calix[4]arene 1 an anodic shift as large as 302 mV is observed on addition of four equivalents of Cl⁻ anion. Also electrochemical and UV–vis spectroscopic titrations ferrocenyl calix[4]arene derivatives 1 with chloride and bromide ions and 2 (only with Cl⁻) indicate a 1:1 binding‐stiochiometry. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of the upper rim binding of triphenylpyrylium cation with p-sulfonatocalix[4]arene

The interaction of 2,4,6-triphenylpyrylium cation with p-sulfonatocalix[4]arene is studied using absorption, emission, NMR and electrochemical techniques. The increase in the absorption is observed with the increase in the concentration of p-sulfonatocalix[4]arene. The emission intensity of 2,4,6-triphenylpyrylium cation is also enhanced in the presence of p-sulfonatocalix[4]arene. The electrochemical titration reveals the presence of host–guest interaction. The NMR analysis explains the upper rim interaction of 2,4,6-triphenypyrylium cation with p-sulfonatocalix[4]arene. The mode of binding is studied using computational methods. The quantum chemical simulations reveal the binding orientation of cationic TPP with p-SC4. The calculated complexation energy (??33.19 kcal mol^?1) indicates the strong binding nature of 2,4,6-triphenylpyrylium cation with p-sulfonatocalix[4]arene.     

Asymmetric calix[4]-thiacalix[4]arene tubes: Synthesis and ionophore properties

Asymmetric p-tert-butylcalix[4]-p-R-thiacalix[4]arene tubes (R = tert-Bu, H, 1-adamantyl) were prepared for the first time by reaction of tosyloxyethoxy derivative of p-tert-butylcalix-[4]arene and the corresponding p-R-thiacalix[4]arenas in acetonitrile in the presence of K₂CO₃. Complex formation of compounds obtained with sodium, potassium, and rubidium iodides in CDCl₃-CD₃OD, 4:1, was studied by means of ¹H NMR. The ionophore properties of the molecule were governed by the character of substituents on the upper rim of the thiacalixarene fragment, and only the molecular tube containing a fragment of the p-(1-adamantyl)-thiacalix[4]arene quantitatively bound potassium ions (quickly) and rubidium ions (slowly).     

Vibrational spectra, co-operative intramolecular hydrogen bonding and conformations of calix[4]arene and thiacalix[4]arene molecules and their para-tert-butyl derivatives

The IR and Raman spectra and conformations of calix[4]arene, thiacalix[4]arene and their p-tert-butyl derivatives have been analysed within the framework of scaled quantum mechanics (SQM). It is shown that the introduction of four p-tert-Bu groups into the calixarene molecules influences the relative energies of their conformers and the enthalpy of the cooperative intramolecular H-bonding (DeltaH(intra)) almost negligibly. DeltaH(intra), evaluated from Iogansen's rule, amounts to approximately 26-28 kcal mol(-1) for the calixarenes and approximately 20-21 kcal mol(-1) for the thiacalixarenes, which essentially exceeds the enthalpies of non-cooperative H-bonds formed by related phenols. As a result of this strong bonding, bands of stretching, bending and torsion vibrations of an eight-membered cyclic system (OH...)4 arise, e.g., two delta(OH)4 bands are observed in the IR spectra of the most highly symmetric C4 cone conformations of calix[4]arene and thiacalix[4]arene. The "duplication" of the number of OH infrared bands is a good new indicator of cooperativity of intramolecular H-bonding of the calixarenes.     

Wide rim urethanes derived from calix[4]arenes: synthesis and self-assembly

[reaction: see text] Calix[4]arenes 4, substituted at the wide rim by four N-tolyl-urethane groups, were synthesized, as well as derivatives 10a,b bearing two or three tolyl-urea groups beside of one or two urethane group(s). In contrast to tetra-tolyl urea 11, the urethane derivatives do not form hydrogen-bonded, dimeric capsules in CDCl3 or benzene-d6, but the dimerization can be induced for the triurea 10b by tetraethylammonium cations as guests. The quantitative formation of heterodimers is observed for all urethanes 4 and 10a,bin benzene-d6 in mixtures with a "tetra-loop" tetraurea 14, while "bisloop" tetraureas 13 require di- or triurea derivatives 10a,b for a clean heterodimerization.     

Photoswitchable calix[4]arenes bearing dihydroacridine substituents at the upper rim

Eight new calix[4]arene host molecules bearing acridinium or the corresponding dihydroacridine substituents have been prepared. ¹H NMR and electrochemical studies reveal that the acridinium substituents block the host cavity. Both photochemical and chemical switching between acridinium and dihydroacridine hosts are demonstrated.     

Synthesis and capsule formation of upper rim substituted tetra-acrylamido calix[4]arenes

Upper rim substituted tetraiodo calix[4]arenes are coupled to a variety of acrylamides using the palladium catalysed Heck reaction. Tetra-acrylamido upper rim substituted calix[4]arenes are obtained in good yields with exceptionally high stereoselectivity, to produce the all-trans isomers. Tetra-acrylamido calix[4]arenes derived from secondary acrylamides are shown to dimerise via eight hydrogen bonds to form dimeric capsules, which are able to include small organic molecules.     

Experimental and computational studies of the binding properties of lower rim tetra- and di-substituted calix[4]arene amide derivatives with metal ions

Abstract

Experimental and theoretical binding studies of representative alkali, alkaline earth, transition, heavy metal and lanthanide cations by tetra- and di-substituted calix[4]arene amide derivatives (diethyl amide 1a–c and morpholide amide 2a–c) in the cone conformation were carried out. Binding was assessed by extraction experiments of the metal picrates from water to dichloromethane and proton NMR titrations. Density functional theory calculations were also performed to determine the binding energy of the complexes formed and to analyse the host–guest interaction modes. In the cases of ligands 1b and 2c with Na⁺ and Ag⁺ picrates, the extraction energy was also determined using the polarisable continuum model. The results are discussed in terms of the nature of the amide residue and the substitution pattern (1,3 vs. 1,2). Both tetra-amide derivatives are good extractants, showing preference for Na⁺, Ca²⁺, Ag⁺ and Pb²⁺ cations, mainly di-ethylamide 1a. Concerning di-amide derivatives, the relative position of the substituents seems to be more important than the nature of the amide group in the extraction process. Proton NMR studies indicate the formation of 1:1 complexes between the amides and the cations studied, and DFT data show that all ligands form the most stable complexes with La³⁺.     

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.