Hexameric Resorcinarene Capsule is a Brønsted Acid: Investigation and Application to Synthesis and Catalysis

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.

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.

Substrate Selectivity Imparted by Self-Assembled Molecular Containers and Catalysts

Recent trends in catalysis are devoted to mimicking some peculiar features of enzymes like site selectivity, through functional group recognition, and substrate selectivity, through recognition of the entire surface of the substrate. The latter is a specific feature of enzymes that is seldomly present in homogeneous catalysis. Supramolecular catalysis, thanks to the self-assembly of simple subunits, enables the creation of cavities and surfaces whose confinement effects drive the preferential binding of a substrate among others with consequent substrate selectivity. The topic is an emerging field that exploits recognition phenomena to discriminate the reagents based on their size and shape. This review deals this cutting-edge field of research covering examples of supramolecular self-assembled molecular containers and catalysts operating in organic as well as aqueous media, with special emphasis for catalytic systems dealing with direct competitive experiments involving two or more substrates.

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.

Leveraging Torsional and Steric Strains: A Pre-macrocyclization Strategy Enables Conformation-Specific Fullerene Binding in m-Cyclophanes

Though the chemistry of resorcinarenes is half a century old, the conformationally-locked resorcinarene crowns are generally constructed using hydrogen bonds or covalent tethers. Often, covalent tethering involves extra post-macrocyclization steps involving upper-rim functionalities. We have leveraged the torsional and steric strains through α-substituents of the lower-rim C-alkyl chains and accomplished conformationally-rigid fluorescent m-cyclophane deep-crowns in a predetermined way. The strategy offers a pre-macrocyclization route conserving upper-rim functionalities, an aspect overlooked in resorcinarene chemistry. X-ray structural and computational analyses unveil the cause for conformational rigidity in m-cyclophanes due to α-branching in C-alkyls (linear vs. α-/β-branched). The conformationally-locked fluorescent deep-crown with a preorganized cavity captures hydrophobic spherical guest C60 in both solution and solid states specifically, when compared to conformationally-dynamic boats, enabling conformation-specific binding.

Reactivity and Selectivity of the Diels-Alder Reaction of Anthracene in [Pd6L4]12+ Supramolecular Cages: A Computational Study

The field of supramolecular metal-organic cage catalysis has grown rapidly in recent years. However, theoretical studies regarding the reaction mechanism and reactivity and selectivity controlling factors for supramolecular catalysis are still underdeveloped. Herein, we demonstrate a detailed density functional theory study on the mechanism, catalytic efficiency, and regioselectivity of the Diels-Alder reaction in bulk solution and within two [Pd₆L₄]¹²⁺ supramolecular cages. Our calculations are consistent with experiments. The origins of the catalytic efficiency of the bowl-shaped cage 1 have been elucidated to be the host-guest stabilization of the transition states and the favorable entropy effect. The reasons for the switch of the regioselectivity from 9,10-addition to 1,4-addition within the octahedral cage 2 were attributed to the confinement effect and the noncovalent interactions. This work would shed light on the understanding of [Pd₆L₄]¹²⁺ metallocage-catalyzed reactions and provide a detailed mechanistic profile otherwise difficult to obtain from experiments. The findings of this study could also aid to the improvement and development of more efficient and selective supramolecular catalysis.

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.

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.

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.

Supramolecular catalysis in confined space: making the pyrogallol[4]arene capsule catalytically active in non-competitive solvent

The confined space inside the hexameric pyrogallol[4]arene capsule (CP6) has been exploited for the catalysis of the 1,3-dipolar cycloaddition (1,3-DC) between the proline-based iminium derivative I and nitrone 3, in the presence of the non-competitive benzene solvent.

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.

Creating Dynamic Nanospaces in Solution by Cationic Cages as Multirole Catalytic Platform for Unconventional C(sp)-H Activation Beyond Enzyme Mimics

Herein we demonstrate that, based on the creation of dynamic nanospaces in solution by highly charged positive coordination cage of [Pd₆ (RuL₃ )₈ ]²⁸⁺ , multirole and multi-way cage-confined catalysis is accomplishable for versatile functions and anomalous reactivities with the aid of the biomimetic cage effect. The high cationic-host charges drive partial deprotonation of 24 imidazole-NHs on cage sphere alike imidazole-residuals in proteins, generating amphoteric heterogeneity in solution to enforce effective cavity-basicity against solution-acidity. Synergistic actions arisen from cage hydrophobicity, host-guest electrostatic interactions and imidazole-N coordination facilitate C(sp)-H activation and carbanionic intermediate stabilization of terminal alkynes to achieve unusual H/D-exchange and Glaser coupling under acidic conditions, and enable phase transfers of water-insoluble substrates/products/co-catalysts to make immiscible-phase and bi-phase catalysis feasible, thus providing a useful catalytic protocol to combine merits from homogeneous, heterogeneous, enzymatic and phase transfer catalysis.

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.

Selective Chloride-Mediated Neat Ethanol Oxidation to 1,1-Diethoxyethane via an Electrochemically Generated Ethyl Hypochlorite Intermediate

Selective primary alcohol oxidation to form aldehydes products without overoxidation to carboxylic acids remains a key chemistry challenge. Using simple alkylammonium chloride as the electrolyte with a glassy carbon working electrode in neat ethanol solvent, 1,1-diethoxyethane (DEE) was prepared with >95% faradaic efficiency (FE). DEE serves as a storage platform protecting acetaldehyde from overoxidation and volatilization. UV-vis spectroscopy shows that the reaction proceeds through an ethyl hypochlorite intermediate as the sole chloride oxidation product, and that this intermediate decomposes unimolecularly (rate constant k = (6.896 ± 0.516) × 10^-4 s^-1) to form HCl catalyst and acetaldehyde, which undergoes rapid nucleophilic attack by ethanol solvent to form the DEE product. This indirect oxidation mechanism enables ethanol oxidation at much less positive potentials due to the fast kinetics for chloride anion oxidation.

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.

Acidic open-cage solution containing basic cage-confined nanospaces for multipurpose catalysis

The nanoscale chemical spaces inherent in porous organic/coordination cages or solid/liquid materials have been continuously explored for their nanoconfinement effect on selective adsorption and reaction of small gas or organic molecules. Herein, we aim to rationalize the unconventional chemical reactivities motivated by the cage-confined nanospaces in aqueous solutions, where the robust yet permeable nanospaces defined by the open cages facilitate dynamic guest exchange and unusual chemical reactions. The high positive charges on [(Pd/Pt)₆(RuL₃)₈]²⁸⁺ nanocages drive imidazole–proton equilibrium to display a significantly perturbed pK_a shift, creating cage-defined nanospaces in solution with distinct intrinsic basicity and extrinsic acidity. The supramolecular cage effect plays pivotal roles in elaborating robust solution nanospaces, controlling ingress-and-egress molecular processes through open-cage portals and endowing nanocages with transition-state stabilization, amphoteric reactivities and the phase transfer of insoluble molecules, thus promoting chemical transformations in unconventional ways. Consequently, a wide range of application of cage-confined catalysis with anomalous reactivities may be expected based on this kind of open-cage solution medium, which combines cage nanocavity, solution heterogeneity and liquid-phase fluidity to benefit various potential mass transfer and molecular process options.

Bioinspired Ether Cyclizations within a π-Basic Capsule Compared to Autocatalysis on π-Acidic Surfaces and Pnictogen-Bonding Catalysts

While the integration of supramolecular principles in catalysis attracts increasing attention, a direct comparative assessment of the resulting systems catalysts to work out distinct characteristics is often difficult. Herein is reported how the broad responsiveness of ether cyclizations to diverse inputs promises to fill this gap. Cyclizations in the confined, π-basic and Brønsted acidic interior of supramolecular capsules, for instance, are found to excel with speed (exceeding general Brønsted acid and hydrogen-bonding catalysts by far) and selective violations of the Baldwin rules (as extreme as the so far unique pnictogen-bonding catalysts). The complementary cyclization on π-acidic aromatic surfaces remains unique with regard to autocatalysis, which is shown to be chemo- and diastereoselective with regard to product-like co-catalysts but, so far, not enantioselective.

Electrostatic Potential Field Effects on Amine Macrocyclizations within Yoctoliter Spaces: Supramolecular Electron Withdrawing/Donating Groups

The central role of Coulombic interactions in enzyme catalysis has inspired multiple approaches to sculpting electrostatic potential fields (EPFs) for controlling chemical reactivity, including ion gradients in water microdroplets, the tips of STMs, and precisely engineered crystals. These are powerful tools because EPFs can affect all reactions, even those whose mechanisms do not involve formal charges. For some time now, supramolecular chemists have become increasingly proficient in using encapsulation to control stoichiometric and catalytic reactions. However, the field has not taken advantage of the broad range of nanocontainers available to systematically explore how EPFs can affect reactions within their inner-spaces. With that idea in mind, previously, we reported on how positively and negatively charged supramolecular capsules can modulate the acidity and reactivity of thiol guests bound within their inner, yoctoliter spaces (Cai, X.; Kataria, R.; Gibb, B. C. J. Am. Chem. Soc. 2020, 142, 8291–8298; Wang, K.; Cai, X.; Yao, W.; Tang, D.; Kataria, R.; Ashbaugh, H. S.; Byers, L. D.; Gibb, B. C. J. Am. Chem. Soc.2019, 141, 6740–6747). Building on this, we report here on the cyclization of 14-bromotetradecan-1-amine inside these yoctoliter containers. We examine the rate and activation thermodynamics of cyclization (Eyring analysis), both in the absence and presence of exogenous salts whose complementary ion can bind to the outside of the capsule and hence attenuate its EPF. We find the cyclization rates and activation thermodynamics in the two capsules to be similar, but that for either capsule attenuation of the EPF slows the reaction down considerably. We conclude the capsules behave in a manner akin to covalently attached electron donating/withdrawing groups in a substrate, with each capsule enforcing their own deviations from the idealized S_N2 mechanism by moving electron density and charge in the activated complex and TS, and that the idealized S_N2 mechanism inside the theoretical neutral host is relatively difficult because of the lack of solvation of the TS.

Carbonyl-Olefin Metathesis

This Review describes the development of strategies for carbonyl-olefin metathesis reactions relying on stepwise, stoichiometric, or catalytic approaches. A comprehensive overview of currently available methods is provided starting with Paternò-Büchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid-catalyzed strategies relying on the formation of intermediate oxetanes, and protocols based on initial carbon-carbon bond formation between carbonyls and alkenes and subsequent Grob-fragmentations. The Review concludes with an overview of applications of these currently available methods for carbonyl-olefin metathesis in complex molecule synthesis. Over the past eight years, the field of carbonyl-olefin metathesis has grown significantly and expanded from stoichiometric reaction protocols to efficient catalytic strategies for ring-closing, ring-opening, and cross carbonyl-olefin metathesis. The aim of this Review is to capture the status quo of the field and is expected to contribute to further advancements in carbonyl-olefin metathesis in the coming years.

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.

Photochemistry using a host-guest charge transfer paradigm: DMABN as a dynamical probe of ground and excited states

Photoexciting charge transfer (CT) transitions arising from host-guest interactions in a confined environment can efficiently yield kinetically trapped radicals. In order to predispose these photogenerated radicals for diffusion limited reactions it becomes imperative to understand the nature of the host-guest CT interactions in the ground and excited states. Here we probe the heterogeneity of guest orientations and the ensuing excited state charge transfer dynamics of an electron-rich molecular probe N,N-dimethylaminobenzonitrile (DMABN) incarcerated inside an electron deficient water-soluble cationic Pd6L412+ nanohost. Using a combination of 1H-NMR, resonance Raman spectrosocopy, and pump-probe spectroscopy we highlight the necessary challenges that need to be addressed in order to use molecular cages as photocatalytic reaction vessels.

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.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

A Molecular Capsule with Revolving Doors Partitioning Its Inner Space

Covalent capsule 1 was designed to include two molecular baskets linked with three mobile pyridines tucked into its inner space. On the basis of both theory (DFT) and experiments (NMR and X-ray crystallography), we found that the pyridine "doors" split the chamber (380 ų ) of 1 so that two equally sizeable compartments (190 ų ) became joined through a conformationally flexible aromatic barrier. The compartments of such unique host could be populated with CCl₄ (88 ų ; PC=46 %), CBr₄ (106 ų ; 56 %) or their combination CCl₄ /CBr₄ (PC=51 %), with thermodynamic stabilities ΔG° tracking the values of packing coefficients (PC). Halogen (C-X⋅⋅⋅π) and hydrogen bonding (C-H⋅⋅⋅X) contacts held the haloalkane guests in the cavities of 1. The consecutive complexations were found to occur in a negative allosteric manner, which we propose to result from the induced-fit mode of complexation. Newly designed 1 opens a way for probing the effects of inner conformational dynamics on noncovalent interactions, reactivity and intramolecular translation in confined spaces of hollow molecules.

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.

Size-selective Catalytic Polymer Acylation with a Molecular Tetrahedron

Selective catalysis at the molecular level represents a cornerstone of chemical synthesis. However, it still remains an open question how to elevate tunable catalysis to larger length scales to functionalize whole polymer chains in a selective manner. We now report a hydrazone-linked tetrahedron with wide openings, which acts as a catalyst to size-selectively functionalize polydisperse polymer mixtures. Our experimental and computational evidence supports a dual role of the hydrazone-linked tetrahedron. To accelerate functionalization of the polymer substrates, the tetrahedron (i) unfolds the polymer substrates and/or breaks the polymer aggregates as well as (ii) enables target sites (amino groups) on the polymers to coordinate with catalytic units (triglyme) attached to the tetrahedron. With the tetrahedron as the catalyst, we find that the reactivity of the shorter polymers increases selectively. Our findings enable the possibility to engineer hydrolytically stable molecular polyhedra as organocatalysts for size-selective polymer modification.

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.

Intrinsic and Extrinsic Control of the pKa of Thiol Guests inside Yoctoliter Containers

Despite decades of research, there are still many open questions surrounding the mechanisms by which enzymes catalyze reactions. Understanding all the noncovalent forces involved has the potential to allow de novo catalysis design, and as a step toward this, understanding how to control the charge state of ionizable groups represents a powerful yet straightforward approach to probing complex systems. Here we utilize supramolecular capsules assembled via the hydrophobic effect to encapsulate guests and control their acidity. We find that the greatest influence on the acidity of bound guests is the location of the acidic group within the yoctoliter space. However, the nature of the electrostatic field generated by the (remote) charged solubilizing groups also plays a significant role in acidity, as does counterion complexation to the outer surfaces of the capsules. Taken together, these results suggest new ways by which to affect reactions in confined spaces.

Multicomponent Metallo-Supramolecular Nanocapsules Assembled from Calix[4]resorcinarene-Based Terpyridine Ligands

Tetrafunctionalized calix[4]resorcinarene cavitands commonly serve as supramolecular scaffolds for construction of coordination-driven self-assembled capsules. However, due to the calix-like shape, the structural diversity of assemblies is mostly restricted to dimeric and hexameric capsules. Previously, we reported a spontaneous heteroleptic complexation strategy based on a pair of self-recognizable terpyridine-based ligands and Cd^II ions. Building on this complementary ligand pairing system, herein three types of nanocapsules, including a dimeric capsule, a Sierpiński triangular prism, and a cubic star, could be readily obtained through dynamic complexation reactions between a tetratopic cavitand-based ligand and various multitopic counterparts in the presence of Cd^II ions. The dimeric capsular assemblies display the spacer-length-dependent self-sorting behavior in a four-component system. Moreover, the precise multicomponent self-assembly of a Sierpiński triangular prism and a cubic star possessing three and six cavitand-based motifs, respectively, demonstrates that such self-assembly methodology is able to efficiently enhance architectural complexity for calix[4]resorcinarene-containing metallo-supramolecules.

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.

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.

The Hexameric Resorcinarene Capsule as a Brønsted Acid Catalyst for the Synthesis of Bis(heteroaryl)methanes in a Nanoconfined Space

Herein, we show that the hexameric resorcinarene capsule C is able to catalyze the formation of bis(heteroaryl)methanes by reaction between pyrroles or indoles and carbonyl compounds (α-ketoesters or aldehydes) in excellent yields and selectivity. Our results suggest that the capsule can play a double catalytic role as a H-bond catalyst, for the initial activation of the carbonyl substrate, and as a Brønsted acid catalyst, for the dehydration of the intermediate alcohol.