Iminium Catalysis inside a Self-Assembled Supramolecular Capsule: Modulation of Enantiomeric Excess

Reactivity Inside Molecular Flasks: Acceleration Modes and Types of Selectivity Obtainable

There is increasing interest in the discovery and application of molecular flasks-supramolecular host structures capable of catalyzing organic reactions. Reminiscent of enzymes due to possessing a host cavity akin to an active site, molecular flasks can exhibit complex catalytic mechanisms and in many cases provide selectivity not achievable in bulk solvent. In this Review, we aim to organize the increasingly diverse examples through a two-part structure. In part one, we provide an overview of the different acceleration modes that operate within molecular flasks, while in part two we showcase, through selected examples, the different types of selectivity that are obtainable through the use of molecular flasks. Particular attention is given to examples that are relevant to current challenges in synthetic organic chemistry. We believe that this structure makes the field more approachable and thus will stimulate the development of novel applications of molecular flasks.

A macrocycle-based new organometallic nano-vessel towards sustainable C2-selective arylation of free indole in water

C2-selectivity of unsubstituted indole over facile C3-substitution is attempted by utilizing the π-cavity of a nano-vessel made up of a palladium complex of an amino-ether heteroditopic macrocycle. Functional group tolerance (cyano, nitro, halo, ester, etc.), a broad substrate scope and outstanding selectivities with excellent yields (80-93%) of the desired products have been achieved in 12 h by maintaining all sustainable conditions like aqueous medium, recyclable catalyst, one-pot reaction, no external additives, mild temperature, etc. Interestingly, we observed that electron-deficient indole derivatives underwent the present transformation with marginally superior reactivity in comparison with electron-rich indole derivatives. This approach establishes a green pathway for selective C-C coupling employing a π-cavitand as a nano-reactor.

Nonsilyl Bicyclic Secondary Amine Catalysts for the Asymmetric Transfer Hydrogenation of α,β-Unsaturated Aldehydes

The first chiral synthesis of nonsilyl bicyclic secondary amine organocatalysts and their application to the asymmetric transfer hydrogenation of α,β-unsaturated aldehydes are disclosed. A lower catalytic loading (5 mol %) is demonstrated for the reduction of a wide range of α,β-unsaturated aldehydes (up to 97% yield and up to 99% ee). The application of this scalable methodology is showcased for the asymmetric synthesis of bioactive molecules such as phenoxanol, citronellol, ramelteon, and terikalant.

The Resorcinarene Hexameric Capsule as a Supramolecular Photoacid to Trigger Olefin Hydroarylation in Confined Space

The self-assembled resorcinarene capsule C6 shows remarkable photoacidity upon light irradiation, which is here exploited to catalyze olefin hydroarylation reactions in confined space. An experimental pKa* value range of -3.3--2.8 was estimated for the photo-excited hexameric capsule C6*, and consequently an increase in acidity of 8.8 log units was observed with respect to its ground state (pKa=5.5-6.0). This makes the hexameric capsule the first example of a self-assembled supramolecular photoacid. The photoacid C6* can catalyze hydroarylation reaction of olefins with aromatic substrates inside its cavity, while no reaction occurred between them in the absence of irradiation and/or capsule. DFT calculations corroborated a mechanism in which the photoacidity of C6* plays a crucial role in the protonation step of the aromatic substrate. A further proton transfer to olefin with a concomitant C-C bond formation and a final deprotonation step lead to product releasing.

Catalytic Inhibition of Base-Mediated Reactivity by a Self-Assembled Metal-Ligand Host

Spacious M4 L6 tetrahedra can act as catalytic inhibitors for base-mediated reactions. Upon adding only 5 % of a self-assembled Fe4 L6 cage complex, the conversion of the conjugate addition between ethylcyanoacetate and β-nitrostyrene catalyzed by proton sponge can be reduced from 83 % after 75 mins at ambient temperature to <1 % under identical conditions. The mechanism of the catalytic inhibition is unusual: the octacationic Fe4 L6 cage increases the acidity of exogenous water in the acetonitrile reaction solvent by favorably binding the conjugate acid of the basic catalyst. The inhibition only occurs for Fe4 L6 hosts with spacious internal cavities: minimal inhibition is seen with smaller tetrahedra or Fe2 L3 helicates. The surprising tendency of the cationic cage to preferentially bind protonated, cationic ammonium guests is quantified via the comprehensive modeling of spectrophotometric titration datasets.

Enzyme Grafting with a Cofactor-Decorated Metal-Organic Capsule for Solar-to-Chemical Conversion

Semi-artificial approaches to solar-to-chemical conversion can achieve chemical transformations that are beyond the capability of natural enzymes, but face marked challenges to facilitate in vivo cascades, due to their inevitable need for cofactor shuttling and regeneration. Here, we report on an enzyme grafting strategy to build a metal-organic capsule-docking artificial enzyme (metal-organic-enzyme, MOE) that comprised the self-assembly of a cofactor-decorated capsule and the supramolecular enzyme-recognition features between the enzyme scaffold and the capsule to bypass cofactor shuttling and regeneration. The incorporated NADH mimics within the metal-organic capsule interacted with the imine intermediate that formed from the condensation of the amines and the dehydrogenation of alcohol substrates in the microenvironment to form complexes within the capsule and subsequently served as an in situ-generated photoresponsive cofactor. Upon illumination, the photoresponsive cofactor facilitates efficient proton/electron transport between the inner space (supramolecular hydrogenation) and outer space (enzymatic dehydrogenation) of the capsule to dehydrogenize the alcohols and hydrogenize the imine intermediates, respectively, circumventing the conventionally complex multistep cofactor shuttling and regeneration. The semi-artificial enzyme endows the conversion of diverse types of alcohol to amine products in both aqueous/organic solutions and Escherichia coli with high efficiency, offering a wide range of opportunities for sustainable and environmentally friendly biomanufacturing of commodity and fine chemicals.

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd₁₂ L₂₄ ] and [Pd₂ L₄ ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd₂ L₄ ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd₂ (L^A )₂ (L^B )(L^C )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

Combining iminium and supramolecular catalysis for the [4 + 2] cycloaddition of E-cinnamaldehydes

Herein we describe a method for combining supramolecular catalysis with imininum-based organocatalysis in the Diels-Alder cycloaddition reaction. Both supramolecular host and L-proline are required for the reaction to occur, implying that encapsulation of the substrates and co-catalyst are necessary for the reaction to occur. We explore the substrate scope for a variety of E-cinnamaldehydes and dienes. Finally, we probe the supramolecular assembly processes responsible for the observed catalysis using NMR spectroscopic methods.

Impact of Host Flexibility on Selectivity in a Supramolecular Host-Catalyzed Enantioselective aza-Darzens Reaction

A highly enantioselective aza-Darzens reaction (up to 99% ee) catalyzed by an enantiopure supramolecular host has been discovered. To understand the role of host structure on reaction outcome, nine new gallium(III)-based enantiopure supramolecular assemblies were prepared via substitution of the external chiral amide. Despite the distal nature of the substitution in these catalysts, changes in enantioselectivity (61 to 90% ee) in the aziridine product were observed. The enantioselectivities were correlated to the flexibility of the supramolecular host scaffold as measured by the kinetics of exchange of a model cationic guest. This correlation led to the development of a best-in-class catalyst by substituting the gallium(III)-based host with one based on indium(III), which generated the most flexible and selective catalyst.

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.

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.

Chiral proline-substituted porous organic cages in asymmetric organocatalysis

The efficient preparation of chiral porous organic cages (POCs) with specific functions is challenging, and their application in asymmetric catalysis has not previously been explored. In this work, we have achieved the construction of chiral POCs based on a supramolecular tetraformyl-resorcin[4]arene scaffold with different chiral proline-modified diamine ligands and utilizing dynamic imine chemistry. The incorporation of V-shaped or linear chiral diamines affords the [4 + 8] square prism and [6 + 12] octahedral POCs respectively. The appended chiral proline moieties in such POCs make them highly active supramolecular nanoreactors for asymmetric aldol reactions, delivering up to 92% ee. The spatial distribution of chiral catalytic sites in these two types of POCs greatly affects their catalytic activities and enantioselectivities. This work not only lays a foundation for the asymmetric catalytic application of chiral POCs, but also contributes to our understanding of the catalytic function of biomimetic supramolecular systems.

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.

Secondary-sphere modification in proline catalysis: Old friend, new connection

In this work, we exploit our strategy of in situ secondary-sphere modification of organocatalysts to improve the reactivity and selectivity of amino catalysts. Herein, the carboxylic acid moiety of proline...

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.

Size- and Shape-Selective Catalysis with a Functionalized Self-Assembled Cage Host

A self-assembled Fe₄L₆ cage with internally oriented carboxylic acid functions was shown to catalyze a variety of dissociative nucleophilic substitution reactions that proceed via oxocarbenium ion or carbocation intermediates. The catalytic behavior of the cage was compared to that of other small acid catalysts, which illustrated large differences in reactivity of the cage-catalyzed reactions, dependent on the structure of the substrate. For example, only a 5% cage confers a 1000-fold rate acceleration of the thioetherification of vinyldiphenylmethanol when compared to the rate with free carboxylic acid surrogates but only a 52-fold acceleration in the formation of small thioacetals. Multiple factors control the variable reactivity in the host, including substrate inhibition, binding affinity, and accessibility of reactive groups once bound. Simple effective concentration increases or the overall charge of the cage does not explain the variations in reactivity shown by highly similar reactants in the host: small differences in structure can have large effects on reactivity. Reaction of large spherical guests is highly dependent on substitution, whereas flat guests are almost unaffected by size and shape differences. The cage is a promiscuous catalyst but has strong selectivity for particular substrate shapes, reminiscent of enzymatic activity.

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.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.

Selective aldehyde reductions in neutral water catalysed by encapsulation in a supramolecular cage

The enhancement of reactivity inside supramolecular coordination cages has many analogies to the mode of action of enzymes, and continues to inspire the design of new catalysts for a range of reactions. However, despite being a near-ubiquitous class of reactions in organic chemistry, enhancement of the reduction of carbonyls to their corresponding alcohols remains very much underexplored in supramolecular coordination cages. Herein, we show that encapsulation of small aromatic aldehydes inside a supramolecular coordination cage allows the reduction of these aldehydes with the mild reducing agent sodium cyanoborohydride to proceed with high selectivity (ketones and esters are not reduced) and in good yields. In the absence of the cage, low pH conditions are essential for any appreciable conversion of the aldehydes to the alcohols. In contrast, the specific microenvironment inside the cage allows this reaction to proceed in bulk solution that is pH-neutral, or even basic. We propose that the cage acts to stabilise the protonated oxocarbenium ion reaction intermediates (enhancing aldehyde reactivity) whilst simultaneously favouring the encapsulation and reduction of smaller aldehydes (which fit more easily inside the cage). Such dual action (enhancement of reactivity and size-selectivity) is reminiscent of the mode of operation of natural enzymes and highlights the tremendous promise of cage architectures as selective catalysts.

Kinetic and Thermodynamic Modulation of Dynamic Imine Libraries Driven by the Hexameric Resorcinarene Capsule

The composition of dynamic covalent imine libraries (DCL) adapts to the presence of the hexameric resorcinarene capsule. In the presence of the self-assembled capsule, a kinetic and thermodynamic modulation of the imine constituents of the DCLs was observed, which was induced by an unusual predatory action of the capsule on specific imine constituents. More complex 2 × 2 DCLs also adapt to the presence of the hexameric capsule, showing a thermodynamic and kinetic modulation of the constituents induced by the predatory action of the capsule. By cross-referencing experimental data, a good selectivity (up to 66%) for one constituent can be induced in a 2 × 2 DCL.

Modeling the Reaction of Carboxylic Acids and Isonitriles in a Self-Assembled Capsule

Quantum chemical calculations were used to study the reaction of carboxylic acids with isonitriles inside a resorcinarene-based self-assembled capsule. Experimentally, it has been shown that the reactions between p-tolylacetic acid and n-butyl isonitrile or isopropyl isonitrile behave differently in the presence of the capsule compared both with each other and also with their solution counterparts. Herein, the reasons for these divergent behaviors are addressed by comparing the detailed energy profiles for the reactions of the two isonitriles inside and outside the capsule. An energy decomposition analysis was conducted to quantify the different factors affecting the reactivity. The calculations reproduce the experimental findings very well. Thus, encapsulation leads to lowering of the energy barrier for the first step of the reaction, the concerted α-addition and proton transfer, which in solution is rate-determining, and this explains the rate acceleration observed in the presence of the capsule. The barrier for the final step of the reaction, the 1,3 O→N acyl transfer, is calculated to be higher with the isopropyl substituent inside the capsule compared with n-butyl. With the isopropyl substituent, the transition state and the product of this step are significantly shorter than the preceding intermediate, and this results in energetically unfavorable empty spaces inside the capsule, which cause a higher barrier. With the n-butyl substituent, on the other hand, the carbon chain can untwine and hence uphold an appropriate guest length.

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.

Enabling protein-hosted organocatalytic transformations

In this review, the development of organocatalytic artificial enzymes will be discussed. This area of protein engineering research has underlying importance, as it enhances the biocompatibility of organocatalysis for applications in chemical and synthetic biology research whilst expanding the catalytic repertoire of enzymes. The approaches towards the preparation of organocatalytic artificial enzymes, techniques used to improve their performance (selectivity and reactivity) as well as examples of their applications are presented. Challenges and opportunities are also discussed.

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.

Anion-mediated encapsulation-induced emission enhancement of an IrIII complex within a resorcin[4]arene hexameric capsule

The anions of the Ir complex salts control the thermodynamic stability and photoluminescence properties of the host–guest complex.

Cofactor-Mediated Nucleophilic Substitution Catalyzed by a Self-Assembled Holoenzyme Mimic

A self-assembled Fe₄L₆ cage is capable of co-encapsulating multiple carboxylic acid containing guests in its cavity, and these acids can act as cofactors for cage-catalyzed nucleophilic substitutions. The kinetics of the substitution reaction depend on the size, shape, and binding affinity of each of the components, and small structural changes in guest size can have large effects on the reaction. The host is quite promiscuous and is capable of binding multiple guests with micromolar binding affinities while retaining the ability to effect turnover and catalysis. Substrate binding modes vary widely, from simple 1:1 complexes to 1:2 complexes that can show either negative or positive cooperativity, depending on the guest. The molecularity of the dissociative substitution reaction varies, depending on the electrophile leaving group, acid cofactor, and nucleophile size: small changes in the nature of substrate can have large effects on reaction kinetics, all controlled by selective molecular recognition in the cage interior.

Catalysis inside Supramolecular Capsules: Recent Developments

In the last decades, supramolecular chemists have developed new molecular receptors able to include a wide range of guests. In addition, they have designed synthetic hosts able to form capsules having an internal volume of thousands of Å3. This inner space shows different features from the bulk solution. In particular, this environment has recently been employed to perform chemical reactions, obtaining reaction products different from the “normal” conditions. These supramolecular capsules act as nanoreactors, catalyzing many chemical transformations. This review collects the recent developments (since 2015) in this field, focusing on supramolecular capsules based on resorcinarene hexameric capsules and metal-cage capsules.

Binding Properties and Supramolecular Polymerization of a Water-Soluble Resorcin[4]arene

Controlling the self-assembly of molecules in water is difficult because the small size, polarity, and hydrogen bond donating and accepting properties of water attenuate most non-covalent interactions. Here we describe how resorcinarene 1, with pyridinium pendent groups, assembles in water to form head-to-tail assemblies. These small supramolecular polymers form because they offer greater stabilization than any latent head-to-head assembly of resorcinarenes to form dimeric (or hexameric) containers. Instead, the resorcinarene bowl - particularly if negatively charged - is a good host for the pyridinium pendent groups of a second resorcinarene. Alternatively, resorcinarene 1 is also a good host for complexing anions and cations of any added salt. In combination therefore, host 1 possesses a rich repertoire of supramolecular properties that is dependent on the ionic strength and the nature of salts, pH, and the concentration of the host. These findings provide new information about controlling the self-assembly of resorcinarenes in water.

Discovering Monoterpene Catalysis Inside Nanocapsules with Multiscale Modeling and Experiments

Large-scale production of natural products, such as terpenes, presents a significant scientific and technological challenge. One promising approach to tackle this problem is chemical synthesis inside nanocapsules, although enzyme-like control of such chemistry has not yet been achieved. In order to better understand the complex chemistry inside nanocapsules, we design a multiscale nanoreactor simulation approach. The nanoreactor simulation protocol consists of hybrid quantum mechanics-molecular mechanics-based high temperature Langevin molecular dynamics simulations. Using this approach we model the tail-to-head formation of monoterpenes inside a resorcin[4]arene-based capsule (capsule I). We provide a rationale for the experimentally observed kinetics of monoterpene product formation and product distribution using capsule I, and we explain why additional stable monoterpenes, like camphene, are not observed. On the basis of the in-capsule I simulations, and mechanistic insights, we propose that feeding the capsule with pinene can yield camphene, and this proposal is verified experimentally. This suggests that the capsule may direct the dynamic reaction cascades by virtue of π-cation interactions.

pH-Responsive Pillar[6]arene-based Water-Soluble Supramolecular Hexagonal Boxes

We describe the preparation of the first water-soluble pH-responsive supramolecular hexagonal boxes (SHBs) based on multiple charge-assisted hydrogen bonds between peramino-pillar[6]arenes 2 with the molecular "lid" mellitic acid (1 a). The interaction between 2 and 1 a, as well as the other "lids" pyromellitic and trimesic acids (1 b and 1 c, respecively) were studied by a combination of experimental and computational methods. Interestingly, the addition of 1 a to the complexes of the protonated form of pillar[6]arene 2, that is, 3, with bis-sulfonate 4 a or 4 b, immediately led to guest escape along with the formation of closed 1 a₂ 2 supramolecular boxes. Moreover, the process of the openning and closing of the supramolecular boxes along with threading and escaping of the guests, respectively, was found to be reversible and pH-responsive. This study paves the way for the easy and modular preparation of different SHBs that may have myriad applications.

A hexameric resorcinarene capsule as a hydrogen bonding catalyst in the conjugate addition of pyrroles and indoles to nitroalkenes

A hexameric resorcinarene capsule (C) acts as a hydrogen bonding catalyst for the addition of nitroalkenes to pyrroles and indoles.