Recent advances in hydrogen-bonded hexameric encapsulation complexes

Tetrafluororesorcin[4]arene Hexameric Capsule Enables the Expansion of the Reactivity Space in Supramolecular Catalysis

This study presents the development and catalytic applications of the tetrafluororesorcin[4]arene hexameric capsule (capsule II) as a novel supramolecular catalyst. It demonstrates unprecedented catalytic activity, enabling the β-selective glycosylation of glycals to 2-deoxy glycosides─a transformation that has not been achieved before in molecular and supramolecular catalysis. Mechanistic investigations, including experimental and computational studies, revealed that the high β-selectivity arises from a proton wire mechanism along the capsule's surface, coupling glycal protonation with nucleophile deprotonation. Control experiments confirmed the unique reactivity of capsule II compared to its nonfluorinated predecessor, capsule I, showcasing its potential to expand the boundaries of supramolecular catalysis.

Unusual Reaction of Isocyanides with Aromatic Aldehydes Catalyzed by a Supramolecular Capsule

The supramolecular resorcinarene hexameric capsule efficiently promotes the unprecedented reaction between isocyanides and electron-deficient aromatic aldehydes leading to the formation of imines and carbon monoxide. The mechanism of the reaction was investigated via isotope labelling, kinetic analysis of the reaction, computational studies and the independent synthesis of a proposed intermediate. Control experiments indicate that the formation of the key aziridinone intermediate is limited to the cavity of the capsule.

Beyond symmetric self-assembly and effective molarity: unlocking functional enzyme mimics with robust organic cages

The bespoke environments in enzyme active sites can selectively accelerate chemical reactions by as much as 1019. Macromolecular and supramolecular chemists have been inspired to understand and mimic these accelerations and selectivities for applications in catalysis for sustainable synthesis. Over the past 60+ years, mimicry strategies have evolved with changing interests, understanding, and synthetic advances but, ubiquitously, research has focused on use of a molecular "cavity". The activities of different cavities vary with the subset of features available to a particular cavity type. Unsurprisingly, without synthetic access to mimics able to encompass more/all of the functional features of enzyme active sites, examples of cavity-catalyzed processes demonstrating enzyme-like rate accelerations remain rare. This perspective will briefly highlight some of the key advances in traditional cavity catalysis, by cavity type, in order to contextualize the recent development of robust organic cage catalysts, which can exploit stability, functionality, and reduced symmetry to enable promising catalytic modes.

Quinoacridane[4]arenes─Very Large Conformationally Restricted Macrocycles

Phenol-based macrocycles play a fundamental role in supramolecular chemistry, but their size has been rather limited. Here we report a novel class of very large, bowl-shaped macrocycles with a diameter of 21.8 Å. These quinoacridane[4]arenes are 150% larger than the current record holders, the acridane[4]arenes, and three times the size of resorcin[4]arene. We expect the quinoacridane[4]arenes to be a useful platform for the construction of molecular containers.

Window[1]resorcin[3]arenes: A Novel Macrocycle Able to Self-Assemble to a Catalytically Active Hexameric Cage

The hexameric resorcin[4]arene capsule has been utilized as one of the most versatile supramolecular capsule catalysts. Enlarging its size would enable expansion of the substrate size scope. However, no larger catalytically active versions have been reported. Herein, we introduce a novel class of macrocycles, named window[1]resorcin[3]arene (wRS), that assemble to a cage-like hexameric host. The new host was studied by NMR, encapsulation experiments, and molecular dynamics simulations. The cage is able to bind tetraalkylammonium ions that are too large for encapsulation inside the hexameric resorcin[4]arene capsule. Most importantly, it retained its catalytic activity, and the accelerated conversion of a large substrate that does not fit the closed hexameric resorcin[4]arene capsule was observed. Thus, it will help to expand the limited substrate size scope of the closed hexameric resorcin[4]arene capsule.

Planar Polymer Membranes Accommodate Functional Self-Assembly of Inserted Resorcinarene Nanocapsules

Solid-supported polymer membranes (SSPMs) offer great potential in material and life sciences due to their increased mechanical stability and robustness compared to solid-supported lipid membranes. However, there is still a need for expanding the functionality of SSPMs by combining them with synthetic molecular assemblies. In this study, SSPMs served as a flexible matrix for the insertion of resorcinarene monomers and their self-assembly into functional hexameric resorcinarene capsules. Resorcinarene capsules provide a large cavity with affinity specifically for cationic and polyhydroxylated molecules. While the capsules are stable in apolar organic solvents, they disassemble when placed in polar solvents, which limits their application. Here, a solvent-assisted approach was used for copolymer membrane deposition on solid support and simultaneous insertion of the resorcinarene monomers. By investigation of the molecular factors and conditions supporting the codeposition of the copolymer and resorcinarene monomers, a stable hybrid membrane was formed. The hydrophobic domain of the membrane played a crucial role by providing a sufficiently thick and apolar layer, allowing for the self-assembly of the capsules. The capsules were functional inside the membranes by encapsulating cationic guests from the aqueous environment. The amount of resorcinarene capsules in the hybrid membranes was quantified by a combination of quartz-crystal microbalance with dissipation and liquid chromatography-mass spectrometry, while the membrane topography and layer composition were analyzed by atomic force microscopy and neutron reflectometry. Functional resorcinarene capsules inside SSPMs can serve as dynamic sensors and potentially as cross-membrane transporters, thus holding great promise for the development of smart surfaces.

Silapillar[n]arenes: Their Enhanced Electronic Conjugation and Conformational Versatility

During recent decades, methylene-bridged macrocyclic arenes have been widely used in supramolecular chemistry. However, their π-conjugations are very weak, as the methylene bridges disrupt the electronic communication between π orbitals of the aromatic units. Herein, we successfully synthesized a series of silapillar[n]arenes (n = 4, 6, and 8) using silylene bridging. These showed enhanced electronic conjugation compared with the parent pillar[n]arenes because of σ*-π* conjugation between σ* (Si-C) orbitals and π* orbitals of the benzenes. Owing to the longer Si-C bond compared with the C-C bond, silylene-bridging provides additional structural flexibility into the pillar[n]arene scaffolds; a strained silapillar[4]arene was formed, which is unavailable in the parent pillar[n]arenes because of the steric requirements. Furthermore, silapillar[n]arenes displayed interesting size-dependent structural and optical properties.

Selective Synthesis and Functionalization of an Acyclic Methylene-Bridged-Arene Trimer in a Cage

Arene-formaldehyde condensation is a versatile reaction for producing various oligomeric/polymeric materials. However, the precise control of oligomerization degree is still challenging because the starting materials and intermediates have similar reactivities. Here, we demonstrate the selective synthesis of a methylene-bridged arene trimer using the confined cavity of a coordination cage. The limited space of the cavity prevents unregulated polymerization. The confinement effect for the kinetic protection is also demonstrated by the subsequent site-selective iodination of the trimer product within the cage.

Biomimetic tail-to-head terpene cyclizations using the resorcin[4]arene capsule catalyst

The tail-to-head terpene (THT) cyclization is a biochemical process that gives rise to many terpene natural product skeletons encountered in nature. Historically, it has been difficult to achieve THT synthetically without using an enzyme. In this protocol, a hexameric resorcin[4]arene capsule acts as an artificial enzyme mimic to carry out biomimetic THT cyclizations and related carbocationic rearrangements. The precursor molecule bears a leaving group (usually an alcohol or acetate group) and undergoes the THT reaction in the presence of the capsule catalyst and HCl as a cocatalyst. Careful control of several parameters (including water content, amount of HCl cocatalyst, temperature and solvent) is crucial to successfully carrying out the reaction. To facilitate the application of this unique capsule-catalysis methodology, we therefore developed a very detailed procedure that includes the preparation and analysis of all reaction components. In this protocol, we describe how to prepare two different terpenes: isolongifolene and presilphiperfolan-1β-ol. The two procedures differ in the water content required for efficient product formation, and thus exemplify the two common use cases of this methodology. The influence of other crucial reaction parameters and means of precisely controlling them are described. A commercially available substrate, nerol, can be used as simple test substrate to validate the reaction setup. Each synthetic procedure requires 5-7 d, including 1-5 h of hands-on time. The protocol applies to the synthesis of many complex terpene natural products that would otherwise be difficult to access in synthetically useful yields.

Exploring the Glycosylation Reaction Inside the Resorcin[4]arene Capsule

In the past decade, there has been an increased interest in applying supramolecular capsule and cage catalysis to the current challenges in synthetic organic chemistry. In this context, we recently reported the resorcin[4]arene capsule-catalyzed conversion of α-glycosyl halides into β-glycosides with high selectivity. Interestingly, this methodology enabled the formation of a wide range of β-pyranosides as well as β-furanosides, although these two donor classes exhibit different reactivities and usually require different reaction conditions and catalysts. Evidence was provided that a proton wire plays a key role in this reaction by enabling dual activation of the glycosyl donor and acceptor. Here, we describe a detailed investigation of several aspects of this reactivity. Besides a mechanistic study, we elucidated the size limitation, the origin of catalytic turnover, and the electrophile scope of nonglycosylic halides. Moreover, a screening of the sensitivity to changes in the reaction conditions provides guidelines to facilitate reproducibility. Furthermore, we demonstrate the compatibility with environmentally benign solvent alternatives, including the renewable solvent limonene.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Supramolecular Capsule Catalysis Enables the Exploration of Terpenoid Chemical Space Untapped by Nature

Terpenes represent the largest and the most diverse class of natural compounds. This is remarkable as the whole variety is accessed from just a handful of highly conserved linear precursors. Modification of the cyclization precursors would enable a dramatic expansion of the accessible chemical space. However, natural enzymes do not enable us to tap into this potential, as they do not tolerate larger deviations from the prototypical substrate structure. Herein we report that supramolecular capsule catalysis enables facile access to diverse and novel terpenoid skeletons that formally can be traced back to C3-phenyl, benzyl, and homoprenyl derivatives of farnesol. Novel skeletons related to the presilphiperfolane core structure, as well as novel neoclovene derivatives were accessed efficiently in only four synthetic steps. Importantly, the products obtained carry functional groups that may be readily derivatized further.

Supramolecular Capsule-Catalyzed Highly β-Selective Furanosylation Independent of the SN1/SN2 Reaction Pathway

The resorcin[4]arene capsule was found to catalyze β-selective furanosylation reactions for a variety of different furanosyl donors: α-d- and α-l-arabinosyl-, α-l-fucosyl-, α-d-ribosyl-, α-d-xylosyl-, and even α-d-lyxosyl fluorides. The scope is only limited by the inherently finite volume inside the closed capsular catalyst. The catalyst is readily available on a multi-100 g scale and can be recycled for at least seven rounds without significant loss in activity, yield, and selectivity. The mechanistic investigations indicated that the furanosylation mechanism is shifted toward an SN1 reaction on the mechanistic continuum between the prototypical SN1 and SN2 substitution types, as compared to the pyranosylation reaction inside the same catalyst. This is especially true for the lyxosyl donor, as indicated by the nucleophile reaction order of 0.26, and supported by metadynamics calculations. The mechanistic shift toward SN1 is of high interest as it indicates that this catalyst not only enables β-selective furanosylations and pyranoslyations independently of the substrate configuration but in addition also independently of the operating mechanism. To our knowledge, there is no alternative catalyst available that displays such properties.

Diffusion NMR Reveals the Structures of the Molecular Aggregates of Resorcin[4]arenes and Pyrogallol[4]arenes in Aromatic and Chlorinated Solvents

The hexameric assemblies of resorcinarenes and pyrogallolarenes are fascinating structures that can serve as nanoreactors in which new chemistry and catalysis occur. Recently, it was suggested based on SANS or SAXS that C₁₁-resorcin[4]arene (1) forms octameric aggregates of a micellar rather than capsular structure in toluene. Here, using NMR spectroscopy, diffusion NMR, and DOSY performed on solutions of C₁₁-resorcin[4]arene (1), C₁₁-pyrogallol[4]arene (2), and mixtures thereof in protonated and deuterated solvents, we found that, in benzene and toluene, 1 primarily formed hexameric capsules accompanied by a minor product with diffusion characteristics consistent with an octameric assembly. In chloroform, 1 formed hexameric capsules. In toluene, 2D NMR revealed two populations of encapsulated toluene molecules in the same capsule of 1. The addition of tetrahexylammonium bromide to the assemblies of 1 in aromatic solvents drove the equilibrium toward the formation of the hexameric capsules. Interestingly, 2 formed only hexameric capsules in all solvents tested.

Mimicry of the proton wire mechanism of enzymes inside a supramolecular capsule enables β-selective O-glycosylations

Enzymes achieve high substrate and product selectivities by orientating and activating the substrate(s) appropriately inside a confined and finely optimized binding pocket. Although some basic aspects of enzymes have already been mimicked successfully with man-made catalysts, substrate activation by proton wires inside enzyme pockets has not been recreated with man-made catalysts so far. A proton wire facilitates the dual activation of a nucleophile and an electrophile via a reciprocal proton transfer, enabling highly stereoselective reactions under mild conditions. Here we present evidence for such an activation mode inside the supramolecular resorcin[4]arene capsule and demonstrate that it enables catalytic and highly β-selective glycosylation reactions-still a major challenge in glycosylation chemistry. Extensive control experiments provide very strong evidence that the reactions take place inside the molecular container. We show that this activation strategy is compatible with a broad scope of glycoside donors and nucleophiles, and is only limited by the cavity size.

Enantioselective Tail-to-Head Terpene Cyclizations by Optically Active Hexameric Resorcin[4]arene Capsule Derivatives

Molecular capsules enable the conversion of substrates inside a closed cavity, mimicking to some extent enzymatic catalysis. Chirality transfer from the molecular capsule onto the encapsulated substrate has been only studied in a few cases. Here we demonstrate that chirality transfer is possible inside a rather large molecular container of approximately 1400 Å3 . Specifically, we present 1) the first examples of optically active hexameric resorcin[4]arene capsules, 2) their ability to enantioselectively catalyze tail-to-head terpene cyclizations, and 3) the surprisingly high sensitivity of enantioselectivity on the structural modifications.

Circular Dichroism Based Chirality Sensing with Supramolecular Host-Guest Chemistry

Optical methods are promising to address the ever-increasing demands for chirality analysis in drug discovery and related fields because they are amenable to high-throughput screening. Circular dichroism-based chiroptical sensing using host-guest chemistry is especially appealing due to the fast equilibrium kinetics, wide substrate scope, and potential for sustainable development. In this Minireview, we give an overview on this emerging field. General aspects of molecular recognition and chirality transfer are analyzed. Chirality sensors are discussed by dividing them into three classes according to their structural features. Applications of these chirality sensors for chirality analysis of the products of asymmetric reactions and for the real-time monitoring of reaction kinetics are demonstrated with selected examples. Moreover, challenges and research directions in this field are also highlighted.

Recent Advances in the Applications of Water-soluble Resorcinarene-based Deep Cavitands

We review here the use of container molecules known as cavitands for performing organic reactions in water. Central to these endeavors are binding forces found in water, and among the strongest of these is the hydrophobic effect. We describe how the hydrophobic effect can be used to drive organic molecule guests into the confined space of cavitand hosts. Other forces participating in guest binding include cation-π interactions, chalcogen bonding and even hydrogen bonding to water involved in the host structure. The reactions of guests take advantage of their contortions in the limited space of the cavitands which enhance macrocyclic and site-selective processes. The cavitands are applied to the removal of organic pollutants from water and to the separation of isomeric guests. Progress is described on maneuvering the containers from stoichiometric participation to roles as catalysts.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

A resorcin[4]arene hexameric capsule as a supramolecular catalyst in elimination and isomerization reactions

The hexameric resorcin[4]arene capsule as a self-assembled organocatalyst promotes a series of reactions like the carbonyl-ene cyclization of (S)-citronellal preferentially to isopulegol, the water elimination from 1,1-diphenylethanol, the isomerization of α-pinene and β-pinene preferentially to limonene and minor amounts of camphene. The role of the supramolecular catalyst consists in promoting the protonation of the substrates leading to the formation of cationic intermediates that are stabilized within the cavity with consequent peculiar features in terms of acceleration and product selectivity. In all cases the catalytic activity displayed by the hexameric capsule is remarkable if compared to many other strong Brønsted or Lewis acids.

Anion-Based Self-assembly of Resorcin[4]arenes and Pyrogallol[4]arenes

Spatial sequestration of molecules is a prerequisite for the complexity of biological systems, enabling the occurrence of numerous, often non-compatible chemical reactions and processes in one cell at the same time. Inspired by this compartmentalization concept, chemists design and synthesize artificial nanocontainers (capsules and cages) and use them to mimic the biological complexity and for new applications in recognition, separation, and catalysis. Here, we report the formation of large closed-shell species by interactions of well-known polyphenolic macrocycles with anions. It has been known since many years that C-alkyl resorcin[4]arenes (R4C) and C-alkyl pyrogallol[4]arenes (P4C) narcissistically self-assemble in nonpolar solvents to form hydrogen-bonded capsules. Here, we show a new interaction model that additionally involves anions as interacting partners and leads to even larger capsular species. Diffusion-ordered spectroscopy and titration experiments indicate that the anion-sealed species have a diameter of >26 Å and suggest stoichiometry (M)₆(X^–)₂₄ and tight ion pairing with cations. This self-assembly is effective in a nonpolar environment (THF and benzene but not in chloroform), however, requires initiation by mechanochemistry (dry milling) in the case of non-compatible solubility. Notably, it is common among various polyphenolic macrocycles (M) having diverse geometries and various conformational lability.

Ionic effects on supramolecular hosts: solvation and counter-ion binding in polar media

For the progress of synthetic supramolecular chemistry in aqueous solution the design of host molecules soluble in this medium is essential. A possible route is the introduction of ionic residues, with the additional advantage that also electrostatic interactions can be used to form supramolecular architectures. In this work we study the effect of different ionic substituents on a resorcin[4]arene host on solvation and counterion binding in water and dimethyl sulfoxide (DMSO). To do so, we combine dielectric relaxation spectroscopy (DRS) at 298.15 K and dilute-solution conductivity measurements covering 278.15-308.15 K. The results indicate that studied substituents lead to a comparable increase in solubility in both water and the dipolar-aprotic DMSO. However, solvation and counterion binding not only depend on the nature of the ionic substituent but also on the solvent. Although intrinsically hydrophobic in nature, resorcin[4]arenes with ionic substituents also show strong hydrophilic hydration in water, with the extent depending on the nature of the ionic group. In contrast to that, solvophobicity apparently dominates the interactions of DMSO with the solute. Counterion binding was found for both solvents and is essentially determined by solvent polarity. It appears that, compared to neat DMSO, the solubility of the cationic resorcin[4]arene with dimethylamine substituents is strongly increased in water-DMSO mixtures due to the formation of hydrogen bonds between two DMSO molecules and one water molecule.

Carbocation catalysis in confined space: activation of trityl chloride inside the hexameric resorcinarene capsule

The hexameric resorcinarene capsule is able to promote carbocation catalysis inside its cavity.

Naphthalene-functionalized resorcinarene as selective, fluorescent self-quenching sensor for kynurenic acid

Naphthalene-functionalized resorcinarene selectively binds kynurenic acid in the presence of excess tryptophan in aqueous media, highlighting the potential of functionalized resorcinarenes as sensory recognition elements for biomolecular analytes.

Expanding the Protecting Group Scope for the Carbonyl Olefin Metathesis Approach to 2,5-Dihydropyrroles

Chiral pyrrolidine derivatives are important building blocks for natural product synthesis. Carbonyl olefin metathesis has recently emerged as a powerful tool for the construction of such building blocks from chiral amino acid derivatives. Here, we demonstrate that the supramolecular resorcinarene catalyst enables access to chiral 2,5-dihydropyrroles under Brønsted acid catalysis. Moreover, this catalytic system even tolerated Lewis-basic-protecting groups like mesylates that are not compatible with alternative catalysts. As expected for conversion inside a closed cavity, the product yield and selectivity depended on the size of the substrates.

Just Add Water: Modulating the Structure-Derived Acidity of Catalytic Hexameric Resorcinarene Capsules

The hexameric undecyl-resorcin[4]arene capsule (C11R6) features eight discrete structural water molecules located at the vertices of its cubic suprastructure. Combining NMR spectroscopy with classical molecular dynamics (MD), we identified and characterized two distinct species of this capsule, C11R6-A and C11R6-B, respectively featuring 8 and 15 water molecules incorporated into their respective hydrogen-bonded networks. Furthermore, we found that the ratio of the C11R6-A and C11R6-B found in solution can be modulated by controlling the water content of the sample. The importance of this supramolecular modulation in C11R6 capsules is highlighted by its ability to perform acid-catalyzed transformations, which is an emergent property arising from the hydrogen bonding within the suprastructure. We show that the conversion of C11R6-A to C11R6-B enhances the catalytic rate of a model Diels-Alder cyclization by 10-fold, demonstrating the cofactor-derived control of a supramolecular catalytic process that emulates natural enzymatic systems.

Solution NMR of synthetic cavity containing supramolecular systems: what have we learned on and from?

NMR has been instrumental in studies of both the structure and dynamics of molecular systems for decades, so it is not surprising that NMR has played a pivotal role in the study of host-guest complexes and supramolecular systems. In this mini-review, selected examples will be used to demonstrate the added value of using (multiparametric) NMR for studying macrocycle-based host-guest and supramolecular systems. We will restrict the discussion to synthetic host systems having a cavity that can engulf their guests thus restricting them into confined spaces. So discussion of selected examples of cavitands, cages, capsules and their complexes, aggregates and polymers as well as organic cages and porous liquids and other porous materials will be used to demonstrate the insights that have been gathered from the extracted NMR parameters when studying such systems emphasizing the information obtained from somewhat less routine NMR methods such as diffusion NMR, diffusion ordered spectroscopy (DOSY) and chemical exchange saturation transfer (CEST) and their variants. These selected examples demonstrate the impact that the results and findings from these NMR studies have had on our understanding of such systems and on the developments in various research fields.

Thioderivatives of Resorcin[4]arene and Pyrogallol[4]arene: Are Thiols Tolerated in the Self-Assembly Process?

Three novel thiol bearing resorcin[4]arene and pyrogallol[4]arene derivatives were synthesized. Their properties were studied with regards to self-assembly, disulfide chemistry, and Brønsted acid catalysis. This work demonstrates that (1) one aromatic thiol on the resorcin[4]arene framework is tolerated in the self-assembly process to a hexameric hydrogen bond-based capsule, (2) thio-derivatized resorcin[4]arene analogs can be covalently linked through disulfides, and (3) the increased acidity of aromatic thio-substituent is not sufficient to replace HCl as cocatalyst for capsule catalyzed terpene cyclizations.

Optimized iminium-catalysed 1,4-reductions inside the resorcinarene capsule: achieving >90% ee with proline as catalyst

In previous work, we demonstrated that iminium-catalysed 1,4-reductions inside the supramolecular resorcinarene capsule display increased enantioselectivities as compared to their regular solution counterparts. Utilizing proline as the chiral catalyst, enantioselectivities remained below 80% ee. In this study, the reaction conditions were optimized by determining the optimal capsule loading and HCl content. Additionally, it was found that alcohol additives increase the enantioselectivity of the capsule-catalysed reaction. As a result, we report enantioselectivities of up to 92% ee for iminium-catalysed 1,4-reductions relying on proline as the sole chiral source. This is of high interest, as proline is unable to deliver high enantioselectivities for 1,4-reductions in a regular solution setting. Investigations into the role of the alcohol additive revealed a dual role: it not only slowed down the background reaction but also increased the capsule-catalysed reaction rate.

A supramolecular container enables highly enantioselective iminium chemistry using simple proline as the chiral source.

Efficient ethylene purification by a robust ethane-trapping porous organic cage

The removal of ethane (C₂H₆) from its analogous ethylene (C₂H₄) is of paramount importance in the petrochemical industry, but highly challenging due to their similar physicochemical properties. The use of emerging porous organic cage (POC) materials for C₂H₆/C₂H₄ separation is still in its infancy. Here, we report the benchmark example of a truncated octahedral calix[4]resorcinarene-based POC adsorbent (CPOC-301), preferring to adsorb C₂H₆ than C₂H₄, and thus can be used as a robust absorbent to directly separate high-purity C₂H₄ from the C₂H₆/C₂H₄ mixture. Molecular modelling studies suggest the exceptional C₂H₆ selectivity is due to the suitable resorcin[4]arene cavities in CPOC-301, which form more multiple C–H···π hydrogen bonds with C₂H₆ than with C₂H₄ guests. This work provides a fresh avenue to utilize POC materials for highly selective separation of industrially important hydrocarbons.

The removal of ethane from ethylene is of importance in the petrochemical industry, but similar physicochemical properties of these molecules makes separation a challenging task. Here, the authors demonstrate that a robust octahedral calix[4]resorcinarene-based porous organic cage can separate high-purity ethylene from ethane/ethylene mixtures.

A Concise Synthesis of a Methyl Ester 2-Resorcinarene: A Chair-Conformation Macrocycle

Anions are important hydrogen bond acceptors in a range of biological, chemical, environmental and medical molecular recognition processes. These interactions have been exploited for the design and synthesis of ditopic resorcinarenes as the hydrogen bond strength can be tuned through the modification of the substituent at the 2-position. However, many potentially useful compounds, especially those incorporating electron-withdrawing functionalities, have not been prepared due to the challenge of their synthesis: their incorporation slows resorcinarene formation that is accessed by electrophilic aromatic substitution. As part of our broader campaign to employ resorcinarenes as selective recognition elements, we need access to these specialized materials. In this article, we report a straightforward synthetic pathway for obtaining a 2-(carboxymethyl)-resorcinarene, and resorcinarene esters in general. We discuss the unusual conformation it adopts and propose that this arises from the electron-withdrawing nature of the ester substituents that renders them better hydrogen bond acceptors than the phenols, ensuring that each of them acts as a donor only. Density Functional Theory (DFT) calculations show that this conformation arises as a consequence of the unusual configurational isomerism of this compound and interruption of the archetypal hydrogen bonding by the ester functionality.

Concentration-Dependent Self-Assembly of an Unusually Large Hexameric Hydrogen-Bonded Molecular Cage

The sizes of available self-assembled hydrogen-bond-based supramolecular capsules and cages are rather limited. The largest systems have volumes of approximately 1400-2300 ų . Herein, we report a large, hexameric cage based on intermolecular amide-amide dimerization. The unusual structure with openings, reminiscent of covalently linked cages, is held together by 24 hydrogen bonds. With a diameter of 2.3 nm and a cavity volume of ∼2800 ų , the assembly is larger than any previously known capsule/cage structure relying exclusively on hydrogen bonds. The self-assembly process in chlorinated, organic solvents was found to be strongly concentration dependent, with the monomeric form prevailing at low concentrations. Additionally, the formation of host-guest complexes with fullerenes (C₆₀ and C₇₀ ) was observed.

Dodecameric Anion-Sealed Capsules based on Pyrogallol[5]arenes and Resorcin[5]arenes

The paper reports formation of exceptionally large capsular species (diameter of c. a. 30 Å) by interactions of polyphenolic macrocycles with 5-fold symmetry with anions. Pyrogallol[5]arenes and resorcin[5]arenes interact with anions via hydrogen bonds involving phenolic OH groups or aromatic CH groups. Based on NMR titration experiments, diffusion coefficients, and geometric requirements, it is postulated that the capsules have (P5H)₁₂ (X^- )₆₀ or (R5H)₁₂ (X^- )₆₀ stoichiometry and a unique geometry of one of the Platonic solids-a dodecahedron. The capsules exist in THF and in benzene, but not in chloroform, reflecting competitive effects in the solvation of anions. It is also demonstrated that mechanochemical pre-treatment (dry-milling) of solid samples is indispensable to initialize self-assembly in benzene.

Solution structure of zinc-seamed C-alkylpyrogallol[4]arene dimeric nanocapsules

The structural stability and solution geometry of zinc-seamed-C-propylpyrogallol[4]arene dimers has been studied in solution using in situ neutron scattering and 2D-DOSY NMR methods. In comparison with the structures of the analogous copper-/nickel-seamed dimeric entities, the spherical geometry of the PgC₃Zn species (R = 9.4 Å; diffusion coefficient = 1.05 × 10^-10 m² s^-1) is larger due to the presence of ligands at the periphery in solution. This enhanced radius in solution due to ligation is also consistent with the findings of model molecular dynamics simulations of the zinc-seamed dimers.

Reticular Chemistry in the Construction of Porous Organic Cages

Reticular chemistry offers the possibility of systematic design of porous materials with different pores by varying the building blocks, while the emerging porous organic cage (POC) system remains generally unexplored. A series of new POCs with dimeric cages with odd-even behaviors, unprecedented trimeric triangular prisms, and the largest recorded hexameric octahedra have been prepared. These POCs are all constructed from the same tetratopic tetraformylresorcin[4]arene cavitand by simply varying the diamine ligands through Schiff-base reactions and are fully characterized by X-ray crystallography, gas sorption measurements, NMR spectroscopy, and mass spectrometry. The odd-even effects in the POC conformation changes of the [2 + 4] dimeric cages have been confirmed by density functional theory calculations, which are the first examples of odd-even effects reported in the cavitand-based cage system. Moreover, the "V" shape phenylenediamine linkers are responsible for the novel [3 + 6] triangular prisms. The window size and environment can be easily functionalized by different groups, providing a promising platform for the construction of multivariate POCs. Use of linear phenylenediamines led to record-breakingly large [6 + 12] truncated octahedral cages, the maximum inner cavity diameters and volumes of which could be readily modulated by increasing the spacer length of the phenylenediamine linkers. This work can lead to an understanding of the self-assembly behaviors of POCs and also sheds light on the rational design of POC materials for practical applications.

Macrocycles and Supramolecules as Antioxidants: Excellent Scaffolds for Development of Potential Therapeutic Agents

Oxidative stress due to the high levels of reactive oxygen species (ROS) that damage biomolecules (lipids, proteins, DNA) results in acute inflammation. However, without proper intervention, acute inflammation progresses to chronic inflammation and then to several chronic diseases, including cancer, myocardial infarction, cardiovascular diseases, chronic inflammation, atherosclerosis, and more. There has been extensive research on the antioxidants of natural origin. However, there are myriad possibilities for the development of synthetic antioxidants for pharmacological applications. There is an increasing interest in the identification of novel synthetic antioxidants for the modulation of biochemical processes related to ROS. In this regard, derivatives of supramolecules, such as calix[n]arene, resorcinarene, calixtyrosol, calixpyrrole, cucurbit[n]uril, porphyrin etc. are gaining attention for their abilities to scavenge the free radicals. Supramolecular chemistry offers excellent scaffolds for the development of novel antioxidants that can be used to modulate free radical reactions and to improve the disorders related to oxidative stress. This review focuses on the interdisciplinary approach for the design and development of novel synthetic antioxidants based on supramolecular scaffolds, with potentially protective effects against oxidative stress.

Catechol[4]arene: The Missing Chiral Member of the Calix[4]arene Family

A missing, inherently chiral member of the calix[4]arene family denoted "catechol[4]arene" was synthesized. Its properties were studied and compared to the ones of its close relatives resorcin[4]arene and pyrogallol[4]arene. This novel supramolecular host exhibits binding capabilities that are superior to its sister molecules in polar media. The enantiomerically pure forms of the macrocycle display modest recognition of chiral ammonium salts.

Kinetic Stabilities and Exchange Dynamics of Water-Soluble Bis-Formamide Caviplexes Studied Using Diffusion-Ordered NMR Spectroscopy (DOSY)

A deep cavitand binds long-chain trans,trans- and trans,cis-bis-formamide isomers in water solution giving a pair of caviplexes in a ca. 60:40 ratio. Both caviplexes display in/out guest exchange dynamics that are slow on the ¹ H NMR chemical shift timescale, but fast on the EXSY timescale. We apply diffusion-ordered NMR spectroscopy (DOSY) to characterize the caviplexes. On the diffusion timescale, the guest in/out exchange processes feature intermediate dynamics allowing the assessment of their kinetic stabilities. We found that the trans,cis-bis-formamide isomers form kinetically more stable caviplexes than the trans,trans-counterparts. We also show that the kinetic stabilities of the bis-formamide caviplexes relate well with their relative thermodynamic stabilities. Fortunately, the tuning of the DOSY parameters allowed the observation of the exchange dynamics as slow processes on the experiment timescale.

Structural Polymorphism of Resorcinarene Assemblies

In 1997, a study based on X-ray crystallography revealed that resorcinarenes adopt a hexameric capsule-like structure. The function of resorcinarenes has been discussed on the basis of this structure; however, our recent study showed that the hexamer may be only one of resorcinarenes' polymorphic members. Here, we present the solvent dependence of the aggregation number of C-undecylresorcinarene in water-saturated toluene and chloroform using small-angle neutron and X-ray scattering and analytical ultracentrifugation measurements. We found that a new octamer was formed in toluene where the eight resorcinarene units were placed at the vertices of a regular cube; this contrasts to the previous structure in chloroform, namely, a hexamer with the six resorcinarenes located at the vertices of a regular octahedron that has a cavity inside where chloroform molecules are pooled.

Bringing a Molecular Plus One: Synergistic Binding Creates Guest-Mediated Three-Component Complexes

C_ethyl-2-methylresorcinarene (A), pyridine (B), and a set of 10 carboxylic acids (Cn) associate to form A·B·Cn ternary assemblies with 1:1:1 stoichiometry, representing a useful class of ternary systems where the guest mediates complex formation between the host and a third component. Although individually weak in solution, the combined strength of the multiple noncovalent interactions organizes the complexes even in a highly hydrogen-bond competing methanol solution, as explored by both experimental and computational methods. The interactions between A·B and Cn are dependent on the pK_a values of carboxylic acids. The weak interactions between A and C further reinforce the interactions between A and B, demonstrating positive cooperativity. Our results reveal that the two-component system such as that formed by A and B can form the basis for the development of specific sensors for the molecular recognition of carboxylic acids.

Water plays a dynamical role in a hydrogen-bonded, hexameric supramolecular assembly

The hexameric resorcin[4]arene supramolecular assembly has attracted significant interest as a self-assembled capsule that exhibits dynamic host-guest chemistry. Many studies have been carried out to investigate the structure and thermodynamics of the assembly, but considerably less is known about its dynamical properties. Here, molecular dynamics simulations are used to investigate the timescales of water encapsulation in this assembly in wet chloroform. We have previously shown [A. Katiyar et al., Chem. Commun. 2019, 55, 6591-6594] that at low water content there are three distinct populations of water molecules present, while at higher water content an additional population, long water chains interacting with the assembly, appears. The relative free energies of these different water positions are calculated and time correlation functions are used to determine the timescales for interconversion between the populations. This analysis demonstrates that the water molecules are in rapid exchange with each other on timescales of tens of ps to a few ns, and suggests that water molecules might be acting as a critical component in the guest exchange mechanism.

Host-Guest Chemistry of Truncated Tetrahedral Imine Cages with Ammonium Ions

Three shape-persistent [4+4] imine cages with truncated tetrahedral geometry with different window sizes were studied as hosts for the encapsulation of tetra-n-alkylammonium salts of various bulkiness. In various solvents the cages behave differently. For instance, in dichloromethane the cage with smallest window size takes up NEt4+ but not NMe4+, which is in contrast to the two cages with larger windows hosting both ions. To find out the reason for this, kinetic experiments were carried out to determine the velocity of uptake but also to deduce the activation barriers for these processes. To support the experimental results, calculations for the guest uptakes have been performed by molecular mechanics' simulations. Finally, the complexation of pharmaceutical interested compounds, such as acetylcholine, muscarine or denatonium have been determined by NMR experiments.