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A cost-efficient method for green synthesis of novel derivatives lower-rim-connected bisresorcinarene macrocycles in large-scale by sodium p-styrenesulfonate

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Supramolecular chemistry is an interdisciplinary scientific field, including chemical, physical and biological properties of more complex chemical species than the molecules themselves. Calixarenes/calixresorcinarenes are macrocyclic compounds, consisting of ‘n’ phenolic/resorcinolic units linked together by methylene bridges; these macrocycles are often used for molecular recognition. Thus, different modifications can be made to both the lower and upper rim, allowing the construction of well-defined multivalent buildings. In this work, three calix[4]resorcinarene macrocycles were synthesized, namely C-dec-9-en-1-ylcalix[4]resorcinarene (CAL 11U), C-trans-2, cis-6-octa-1,5-dien-1-ylcalix[4]resorcinarene (CAL 9U) and C-nonylcalix[4]resorcinarene (CAL 10) by a simple condensation reaction. The compounds CAL 11U and CAL 10 have been already synthesized by researchers, while the CAL 9U has been synthesized for the first time. Their structures were confirmed using ATR-FTIR, 1 H NMR and 13C NMR. Thermal analysis combined with mass spectrometric evolved analysis of the vapors was used to study the thermal behavior of the different synthesized molecules, and they were the subject of characterization by X-ray powder diffraction in order to analyze their degree of crystallinity.
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This perspective provides an overview of contemporary developments in the vibrant field of macrocycles, particularly peptide-based molecules. The complex structures of these molecules continue to inspire the development of novel cyclization technologies with improved efficiency. While the discovery of new methods to make ring structures continues to be a significant goal, site-selective structural modification of existing macrocycle cores is equally important. The presence of partially rotatable bonds in macrocycles contributes to their complex three-dimensional structures, elucidation of which is both formidable and exciting. The most challenging questions in the area of macrocycles will likely relate to reconciling the synthetic and biological approaches to synthesis. The domain of chemical synthesis is fertile with methods that enable the development of molecules with optimal pharmacological profiles. Unfortunately, the accessible molecular diversity is still a challenging proposition. In contrast, biological synthesis appears to readily provide enormous molecular diversity, albeit at the expense of offering only a limited palette of useful building blocks. In addition, the molecules that are accessible with biological methods are rarely attractive drug candidates, particularly when it comes to intracellular targets. The inherent friction between the synthetic and biological domains of synthesis is likely to result in exciting innovations in the years to come.
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The macrocyclization of resorcinol and odd-numbered bisdioxolanes under acidic conditions produced feet-to-feet connected trisresorcinarenes possessing three resorcinarene units linked with odd-numbered alkyl chains. The formation of trisresorcinarenes was confirmed using high-resolution mass spectrometry and NMR spectroscopy. The trisresorcinarenes were isolated as protected forms in moderate yields. The protected trisresorcinarenes exhibited D3h symmetry in conformation in solution. Crystal structure analysis revealed that the protected trisresorcinarenes possess large inner spaces surrounded by three resorcinarene units.
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The host-guest complex of C-tetra(methyl)calix[4]resorcinarene (1), C-tetra(pentyl)calix[4]resorcinarene (2), and C-tetra(nonyl)calix[4]resorcinarene (3) with choline in DMSO was examined via dynamic NMR. Under these conditions, only C-tetra(methyl)calix[4]resorcinarene formed a complex with choline. In order to establish the role of DMSO during the solubilization/complexation process, titration was carried out with resorcinarenes and DMSO in CDCl3. Our careful NMR analysis was based on the assignment of ¹H NMR signals of the resorcinarenes in CDCl3 after the addition of variable amounts of DMSO, which showed an interesting host-guest interaction with C-tetra(pentyl)calix[4]resorcinarene. With both solvents, it is possible to define stable cone conformational arrangements based on the signals shown in the spectra of all the experiments. The results show the formation of a 1:1 inclusion complex between DMSO and C-tetra(pentyl)calix[4]resorcinarene. Suitable crystals of C-tetra(pentyl)resorcinarene in DMSO were characterized through an X-ray crystal structure determination and showed the inclusion of a molecule of DMSO in the cavity of the resorcinarene. The asymmetric unit contains one molecule of water and five molecules of DMSO, and analysis indicated that resorcinarene prefers a cone configuration (rccc conformation) in the solid state. In the crystal array, classical hydrogen bond O–H⋯O interactions and intermolecular contacts were observed.
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Calix[n]arenes (n = 4, 5, 6, 8) are "chalicelike" phenol-based macrocycles that are among the most fascinating and highly studied scaffolds in supramolecular chemistry. This stems from the functional and tunable diversity at both their upper and lower rims, their preorganized nonpolar cavities and preorganized ion-binding sites, and their well-defined conformations. Conjugation of calixarene scaffolds with various fluorogenic groups has led to the development of smart fluorescent probes that have been utilized as molecular sensors, in bioimaging, for drug and gene delivery, in self-assembly/aggregation, and as smart materials. The fine-tuning and incorporation of different ligating sites in the calix[4]arene scaffold have produced numerous molecular sensors for cations, anions, and biomolecules. Moreover, the aqueous solubility of p-sulfonatocalix[4]arenes has engendered their potential use in drug/gene delivery and enzymatic assays. In addition, because of their strong optical properties, fluorescent calix[4]arenes have been used to develop smart materials, including gels as well as nonlinear optical, organic light-emitting diode, and multiphoton materials. Finally, significant developments in the utility of fluorescent higher calixarenes have been made for bioapplications. This review critically summarizes the recent advances made in all of these different areas.
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In this Account, we outline our investigation into the supramolecular resorcinarene capsule as a catalyst. Molecular capsules not only are of interest due to the similarities of their binding pockets with those of natural enzymes but also feature potential advantages for catalysis. Due to the restricted internal volume of the binding pockets, substrate selectivities are commonly observed. Substrates that are encapsulated more efficiently will be converted selectively in the presence of less suitable substrates. This size selectivity cannot be obtained in a regular solution experiment. In addition, because of the distinct chemical environment inside the capsule, different product selectivities may be observed. Furthermore, the encapsulation of reactive catalysts inside confined environments may improve catalyst compatibility for multicatalyst tandem reactions.
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Bis-resorcin[4]arene 2 connected by its lower rims was synthesized via click chemistry and found, by diffusion NMR, to self-assemble quantitatively, despite the different alternatives, into hexameric capsules reminiscent of the well-characterized hexameric capsule of resorcin[4]arene 1. The hexameric capsules of the flexible bis-resorcin[4]arene 2 prevail in apolar organic solvents and can, as expected, encapsulate quite well ammonium salts, demonstrating that spontaneous self-assembly into hexameric capsules is a general characteristic of such systems.
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The development of synthetic host–guest chemistry to investigate and quantify weak, non-covalent interactions has been key to unraveling the complexity of molecular recognition in chemical and biological systems. Macrocycles have shown great utility in the design of receptors, enabling the development of highly preorganized structures. Among macrocycles, resorcin[4]arene-based cavitands have become privileged scaffolds due to their synthetic tunability, which allows access to structures with precisely defined geometries, as well as receptors that display conformational switching between two distinct states with a large difference in guest-binding properties. Here, we highlight three case studies demonstrating redox- and photoredox-controlled switching of molecular recognition properties, the formation of guest-binding supramolecular capsules based solely on halogen-bonding interactions, and enantioselective encapsulation of chiral, substituted cyclohexanes by enantiopure cage compounds as a result of perfect shape complementarity, dispersion interactions, and halogen bonding. The high geometrical and conformational control that can be achieved with resorcin[4]arene-derived host systems will continue to be a powerful resource in future molecular recognition studies.
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Bisresorcinarenes 1a–d were obtained in excellent yields, and 1e was finally obtained in 50% yield. X-ray diffraction analysis showed that 1a and 1b adopted helical conformations, whereas the two resorcinarenes of 1c–e were in parallel orientations in which the clefts of the aliphatic chains entrapped one or two solvent molecules. The conformational study revealed that the helix interconversion between the (P)- and (M)-helical conformers depended on the length of the aliphatic chains. 1a had the largest energetic barrier to helix interconversion, while in 1b, its more flexible aliphatic chains lowered its energetic barriers. The P/M interconversion of 1a was coupled with the clockwise/anticlockwise interconversion of the interannular hydrogen bonding of the two resorcinarenes. The large negative entropic contributions indicate that the transition state is most likely more ordered than the ground states, suggesting that the transition state is most likely symmetric and is solvated by water molecules. Calculations at the M06-2X/6-31G(d,p) level revealed that the more stable (P)-conformation has clockwise interannular hydrogen bonding between the two resorcinarenes.
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In 2008, we reported a new class of pillar-shaped macrocyclic hosts, known as "pillar[n]arenes". Today, pillar[n]arenes are recognized as key players in supramolecular chemistry because of their facile synthesis, unique pillar shape, versatile functionality, interesting host-guest properties, and original supramolecular assembly characteristics, which have resulted in numerous electrochemical and biomedical material applications. In this Review, we have provided historical background to macrocyclic chemistry, followed by a detailed discussion of the fundamental properties of pillar[n]arenes, including their synthesis, structure, and host-guest properties. Furthermore, we have discussed the applications of pillar[n]arenes to materials science, as well as their applications in supramolecular chemistry, in terms of their fundamental properties. Finally, we have described the future perspectives of pillar[n]arene chemistry. We hope that this Review will provide a useful reference for researchers working in the field and inspire discoveries concerning pillar[n]arene chemistry.
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In this article, a multifunctional and environment-friendly agent for industry circulation cooling water, maleic acid and sodium ρ-styrenesulfonate copolymer containing low-phosphorus was synthesized in aqueous solution with hypophosphorous acid/hydrogen peroxide as redox initiators. The effects of monomer feed ratio, temperature, dose of hypophosphorous acid, and drop speed of hydrogen peroxide on calcium carbonate inhibition were investigated, and the experimental conditions were optimized using orthogonal method. The composition and structure of the copolymer were analyzed by enery-dispersive X-ray analysis, fourier transform infrared spectroscopy, and nuclear magnetic resonance spectroscopy. The results of the static scale inhibition tests showed that the copolymer was effective in inhibiting the formation of calcium carbonate scale. The calcium carbonate precipitates formed in the static tests were examined by scanning electronic microscopy and X-ray diffraction spectra. The results indicate that in the presence of the copolymer the calcium carbonate precipitate is the mixture of calcite and vaterite that could be stabilized kinetically in the presence of the copolymer. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
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A novel terpolymer 2-acrylamide-2-methylpropanesulfonic acid/acrylamide/sodium styrene sulfonate (AMPS/AM/SSS), used as fluid loss agent for oil well cement under high temperature, was prepared with initiator of azobisisobutryamide chloride. The optimum reaction conditions of polymerization were obtained from the orthogonal experiments, reaction temperature 40°C, initiator 0.1 wt%, molar ratio of monomers AMPS/AM/SSS 20/5/1. The structure and performance of terpolymer was characterized with Fourier transform infrared spectrometer, nuclear magnetic resonance hydrogen spectrum, thermogravimetric and simultaneous differential thermal analysis, high-temperature and high-pressure water loss meter, and rotational viscometer, indicating that the terpolymer was perfectly synthesized and started to decompose at 350°C and showed good performance of fluid loss control up to 160°C. The ζ-potential instrument and scanning electron microscopy were used to investigate the mechanisms of fluid loss control, indicating that the terpolymer can prevent the generation of flocculated structure in the cement and reduce the porosity of the filter cake. These results have important referential value for developing new high-temperature-resisting fluid loss agents. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers
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p-tert-Butylcalix[6]arene hexaacetic acid is in a symmetric cone conformation in CHCl3, but it becomes conformationally flexible in CHCl3/CH3CN (1 : 1). In this mixture the host has a strong binding affinity towards Pb(ii) and instantly forms a complex of low symmetry - shortly thereafter structural reorganization occurs resulting in a high symmetry complex of Pb(ii) in an octahedral cage of carboxylates. Sr(ii) and Ba(ii) display similar behavior over a longer period of time.
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Although quite common in nature, notably in metalloenzymes, the coordination of transition metals within a confined three dimensional environment has long been hampered by the lack of molecular receptors that may force ligand binding to take place internally. However, an increasing number of appropriate cavity-shaped molecules has become available over the years. Their modification with donor atoms has led to metalloreceptors that can not only bind substrates inside a well-defined hollow space, but which can also promote transformations of the confined guests. This article gives an overview of transition metal complexes capable of directing the binding of exogeneous ligands inside a molecular cavity. Special emphasis is placed on the stabilisation of unusual coordination modes and geometries as well as the role of the cavity in catalysis and metal-assisted molecular recognition.
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A systematic study of water absorbency, thermal, and rheological properties was performed on nanocomposite hydrogels of poly(sodium 4-styrene sulfonate) (PSSNa) and poly(2-acrylamide glycolic acid) (PAAG). Montmorillonite was used as clay filler and was previously modified to hydrogel synthesis by addition of (3-acrylamide propyl)trimethylammonium chloride. Syntheses were carried out by in situ radical polymerization, using N,N-methylen-bis-acrylamide as crosslinker reagent. Nanocomposites showed an exfoliated morphology, confirmed by transmission electron microscopy and X-ray diffraction. The water absorption capacity (WAC) of unloaded PSSNa hydrogel was three times higher than for PAAG; due to clay addition, absorption capacity increased for PSSNa nanocomposites and decreased for PAAG. Finally, rheological properties of nanocomposite hydrogels were studied by both dynamic oscillatory test and shear creep analysis. Results showed improvements on mechanical properties, such as yield point, elastic recovery, and storage modulus as consequence of montmorillonite addition.
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The long known condensation of resorcinol with acetaldehyde yields the tetraphenoxide H (RCH3). This bowl‐shaped host compound is characterized by cyclic hydrogen bridges and delocalized negative charge, which leads to a high symmetry and stability and to the highest bonding constants known so far for methylammonium ions of the choline type and similar derivatives. (Figure Presented.)
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A convenient methodology for introducing single functional groups to the lower rim of resorcinarenes is described. The methodology allows for very convenient differential protection or derivatisation of the upper and lower rims, and a wide range of functional groups (alcohol, carboxylic acid, thiol, amine, carbamate, alkyl halide) can be incorporated as a single unit at the lower rim, opening up the way to further modification at this point and generally widening the scope for further utilising resorcinarenes. Furthermore, our approach has enabled us to link two resorcinarenes together to form novel resorcinarene dimers.
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We describe the synthesis and photochemical behavior of open-ended container modules connected by a 4,4'-azobiphenyl spacer. Both trans and cis azo configurations of the host can be accessed and their binding of guest molecules was characterized by NMR methods.
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Spontaneous self-assembly processes that lead to discrete spherical molecular structures are common in nature. Spherical viruses (such as hepatitis B) and fullerenes are well-known examples in which non-covalent and covalent forces, respectively, direct the assembly of smaller subunits into larger superstructures. A common feature of these shell-like architectures is their ability to encapsulate neutral and/or charged guests whose size, shape and chemical exteriors complement those of the host's inner surface. Their interiors can often be regarded as a new phase of matter, capable of controlling the flow of reactants, transients and products, and of catalysing reactions of both chemical and biological relevance. Such properties have inspired the recent emergence of monomolecular and supramolecular dimeric molecular capsules, many of which have been based on the head-to-head alignment of bowl-shaped polyaromatic macro-cycles such as calix[4]arenes. But true structural mimicry of frameworks akin to viruses and fullerenes, which are based on the self-assembly of n > 3 subunits, and where surface curvature is supplied by edge sharing of regular polygons, has remained elusive. Here we present an example of such a system: a chiral spherical molecular assembly held together by 60 hydrogen bonds (1) (Fig. 1). We demonstrate the ability of 1, which consists of six calix[4]resorcinarenes 2 and eight water molecules, to self-assemble and maintain its structure in apolar media and to encapsulate guest species within a well-defined cavity that possesses an internal volume of about 1.375 Å3. Single crystal X-ray analysis shows that its topology resembles that of a spherical virus and conforms to the structure of a snub cube, one of the 13 Archimedean solids.
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Polyampholyte hydrogels were synthesized by copolymerizing acrylamide with the cationic monomer methacrylamidopropyl trimethylammonium chloride and the anionic monomer sodium styrene sulfonate. The total nominal charge density of the hydrogels was held constant at 4.7 +/- 0.1 mol % (dry basis), while the molar ratio of anionic to cationic moieties within the hydrogels was varied. Swelling equilibria were measured in water and in aqueous sodium chloride solutions ranging in ionic strength from 10(-5) to 1.0 M. The hydrogels showed increasing insensitivity to ionic strength as the molar ratio of anionic to cationic moieties in the hydrogel approached unity. Donnan membrane equilibria qualitatively explain the experimental results.
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The products obtained from the base-catalyzed condensation of formaldehyde with several para-substituted phenols have been shown to be mixtures of two or more components which appear to be cyclic oligomers with five or more aromatic units in the cyclic array.
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The condensation of p-tert-butylphenol with formaldehyde to form cyclic oligomers has been investigated as part of a program concerned with the synthesis of enzyme model catalysts. The product of this condensation has been shown to comprise four identifiable materials which, because of their shape, have been named calixarenes (Greek calix, chalice; arene, indicating the incorporation of aromatic rings). The distribution of these materials in the product depends on the conditions of the condensation. From a one-step process involving p-tert-butylphenol, paraformaldehyde, and a trace of base in refluxing xylene a cyclic octamer, i.e., p-tert-butylcalix[8]arene (2), can be isolated in yields as high as 64%. With a larger amount of base a cyclic hexamer, i.e., p-tert-butylcalix[6]arene (3), becomes the major product, isolable in 74% yield when rubidium hydroxide is used. From a three-step process involving p-tert-butylphenol, aqueous formaldehyde, and base followed, at a later stage, by strenuous heating, a cyclic tetramer, i.e., p-tert-butylcalix[4]arene (4), can be isolated in 20-25% yield. From the mixture produced in the one-step process it is also possible to isolate, in varying amounts, a p-tert-butylbishomooxacalix[4]arene (14). The structures of these materials have been established by elemental analyses, osmometric and mass spectral molecular weight determinations, and 1H NMR and 13C NMR measurements. In addition, the p-tert-butylcalix[4]arene has been synthesized by a stepwise procedure involving the preparation of a hydroxymethylated linear tetramer which undergoes acid-catalyzed converstion to the cyclic compound. The influence of reaction conditions, including concentration, reactant ratios, solvents, bases, and heating rates, on the composition of the product has been investigated. The mechanism of formation, the 1H NMR spectral characteristics, and the complex-forming ability of the calixarenes are briefly discussed.
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The resorcinol-aldehyde cyclotetramer 1a as a lipophilic polar host solubilizes glycerol and water (neat liquids) and ribose (in an aqueous solution) as polar guests in CCl4 upon formation of monomeric complexes 1a-4(glycerol), 1a-4H2O, and 1a-(ribose)·2H2O, where ribose is bound highly selectively in the α-pyranose form. The extraction of 1,4-cyclohexanediol is also stereoselective, the cis isomer being extracted readily. A pair of hydrogen-bonded OH groups on adjacent benzene rings in 1a provide the essential binding site for a guest OH group. While glycerol and H2O are singly bound with such a binding site via hydrogen bonding, α-ribopyranose and CM-1,4-cyclohexanediol are doubly bonded with two binding sites separated by a metaphenylene bridge. Examination of CPK molecular models indicates that such a two-point 1a-guest interaction is possible when the six-membered ring of a guest has cis OH groups on 1-C and 4-C, as in the case of α-ribopyranose and cis-1,4-cyclohexanediol. The extractabilities, or affinities to 1a, of various aldopentoses, aldohexoses, and their deoxy derivatives decrease in the following order: fucose (6-deoxygalactose) > 2-deoxyribose > ribose > arabinose ≅ rhamnose (6-deoxymannose) ≫ galactose ≅ xylose as lyxose ≅ mannose ≅ glucose. The affinities of sugars are governed by three factors: (1) the stereochemistry of the OH groups on 3-C and 4-C (cis ≫ trans), (2) the lipophilicity of the substituent on 5-C (CH3 ≫ H ≫ CH2OH), and (3) the nature of the substituent on 2-C (H > cis-OH > trans-OH, where cis and trans are with respect to the OH group on 3-C). Structural requirements for the formation of stable 1a-sugar complexes are discussed in terms of maximization of favorable hydrogen-bonding interaction and minimization of unfavorable exposure of the sugar OH groups to bulk solvent.
Article
Based on a first example of an allosteric hemicarcerand (1) we prepared four new 2,2′-bipyridines that carry resorcinarene moieties in a highly convergent manner. Upon coordination to suitable transition metal ions or their complexes these compounds undergo conformational changes in a way that they switch between “open” and “closed” forms (2, 3, and 4) or vice versa (5), thus, bringing together or separating the two functional moieties on the central bipyridine. Among the transition metal complexes that act as effectors for the conformational switching, [Re(CO)5Cl] and monomeric copper(I) complexes of sterically hindered 2,9-arylated 1,10-phenanthrolines proved to be very effective. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Article
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.
Article
A foot-to-foot or `back-to-back' connected biscavitand is prepared directly from a hexadecol resorcinarene precursor. The axial orientation of the biphenyl linker and hence the crown conformation of the hexadecol was established by an X-ray crystal study of the biscavitand. Each cavitand bowl is filled in the crystal by an alkyl `foot' from the next molecule, a self-inclusion which results in polymeric host–guest chains. The new biscavitand differs from previously prepared Z and C isomers of a bowl-to-bowl or `front-to-front' connected host, which crystallize as chains of carcerand-like, solvent-filled cages or as distinct molecules of hemicarceplex, respectively.
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The interaction of poly(sodium acrylate) (NaPAA) and poly(sodium styrenesulfonate) (NaPSS) with water over a broad range of concentrations was followed by isothermal titration calorimetry (ITC) and compared to the corresponding low molecular weight salts, sodium acetate and sodium sulfate. Astonishingly, the backbone of NaPSS, known to be more hydrophobic, remains unseen in the dilution enthalpy, and NaPAA and NaPSS show very similar and close to ideal dilution curves. The NaPSS dilution is even more exothermic than Na2SO4. This is related to the specific structure of water screening the hydrophobic polystyrene backbone. Counterintuitively, binding of Ca2+ ions to PAA, a spontaneous process, shows to be highly endothermic; i.e., the binding is solely driven by entropy. This suggests that not Coulomb interactions, but liberation of water molecules from the hydration shells of the components is the driving energy source for the binding of multivalent ions onto polyelectrolytes. Analysis of binding titrations of Ca2+ to three different industrial scale inhibitors (poly(acrylic acid), poly(aspartic acid), poly(acrylic-co-maleic acid) (Sokolan)) allows to qualify their performance. The experiments underline that the understanding of polyelectrolytes and counterion condensation on the basis of Coulomb descriptions, i.e., ions as point charges and water as a homogeneous dielectric media, is at least misleading, as it disregards the predominant thermodynamic effects, the chemical specificity of the components in response to the structure of water.
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The encapsulation of a Au(I) catalyst within a self-assembled, hydrogen bonded, hexameric capsule dramatically changes its catalytic activity, leading to unusual products due to the steric requirements of the host's cavity.
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The outstanding complexing properties of tetraphosphonate cavitands towards N-methylpyridinium salts were exploited to realise a new class of linear and cyclic AABB supramolecular polymers through host-guest interactions. The effectiveness of the selected self-association processes was tested by (1)H NMR studies, whereas microcalorimetric analyses clarified the binding thermodynamics and revealed the possibility of tuning entropic contributions by acting on the flexibility of the guest linker. Although the formation of linear polymeric chains for a rigid system was demonstrated by X-ray analysis, the presence of a concentration-dependent ring-chain equilibrium was indicated by solution viscosity measurements in the case of a very flexible ditopic BB guest co-monomer.
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Irradiation of dimethylbenzil within a cylindrical capsule gives bright green phosphorescence, while irradiation of benzil and dimethoxybenzil in the same capsule results in high energy blue fluorescence. This difference is likely due to the geometric restrictions imposed by the capsule's space on its excited guests, forcing a trans-planar conformation in some cases and cis-skewed in others.
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A 1,3-dipolar cycloaddition takes place within a reversibly formed, self-assembled capsule. The reaction proceeds through an unsymmetrically loaded encapsulation complex with absolute regioselectivity.
Article
Resorcinarenes assemble in wet chloroform or benzene to form hexameric capsules, resembling inflated cubes or volleyballs. NMR methods are used to determine the number of solvent molecules detained inside; eight molecules of benzene are encapsulated.
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A cyclophane-based resorcinarene trimer (3) bearing a dansyl moiety as an environmentally sensitive fluorophore was prepared by stepwise condensation of a tetraaza[6.1.6.1]paracyclophane skeleton with a dansyl moiety and three resorcinarene derivatives having heptacarboxylic acid residues in this sequence. The dansyl-appended cyclophane exhibited the following fluorescence properties regarding solvent polarity dependency and histone surface recognition: With increasing dioxane contents in dioxane/water solvents, the fluorescence intensity originating from the dansyl moiety of 3 increased along with a concomitant blue shift of the fluorescence maximum (lambdaem). The microenvironmentally sensitive fluorescence properties of dansyl fluorophore were maintained, even when the dansyl moiety was covalently attached to a cyclophane. Most interestingly, the cyclophane-based resorcinarene trimer exhibited recognition and fluorescence sensing capabilities toward histone, a small basic protein of eukaryotic chromatins. The fluorescence intensity originating from 3 increased along with a concomitant blue shift of lambdaem upon the addition of histone, reflecting the formation of 3-histone complexes. A relatively large fluorescence polarization (P) value was obtained for the 3-histone complexes (0.15), reflecting highly restricted conformations of 3, and the obtained P value was much larger than that of 3 alone in aqueous medium (0.07). The binding constant (K) of 3 with histone (unit basis) was estimated to be 2.1 x 106 M-1. On the other hand, upon the addition of acetylated histone (Ac-histone) to an aqueous solution containing 3, the extent of change in fluorescence intensity originating from the dansyl group of 3 was almost negligible, indicating that the electrostatic interactions between 3 and Ac-histone were weak. In addition, the fluorescence spectral changes were also small or negligible upon the addition of other proteins such as albumin, ovalbumin, peanut agglutinin, myoglobin, concanavalin A, cytochrome c, and lysozyme, having isoelectric points of 4.7, 4.8, 5.7-6.7, 6.8, 7.1, 9, and 11.0, respectively, to an aqueous solution containing 3.