Interconversion between multicomponent slider-on-deck and palladium capsule: regulation of catalysis and encapsulation

When the slider-on-deck [Cu3(1)(2)]3+ and guest G were treated with palladium(II) ions, the biped 2 was released from [Cu3(1)(2)]3+ generating the nanocage [Pd2(2)4(G)]4+ with guest G being encapsulated (NetState-II). This transformation that was reversed by the addition of DMAP enabled modulation of both the overall fluorescence and the activity of copper(I) catalyzing an aza Hopf cyclization.

Picking the lock of coordination cage catalysis

The design principles of metallo-organic assembly reactions have facilitated access to hundreds of coordination cages of varying size and shape. Many of these assemblies possess a well-defined cavity capable of hosting a guest, pictorially mimicking the action of a substrate binding to the active site of an enzyme. While there are now a growing collection of coordination cages that show highly proficient catalysis, exhibiting both excellent activity and efficient turnover, this number is still small compared to the vast library of metal-organic structures that are known. In this review, we will attempt to unpick and discuss the key features that make an effective coordination cage catalyst, linking structure to activity (and selectivity) using lessons learnt from both experimental and computational analysis of the most notable exemplars. We will also provide an outlook for this area, reasoning why coordination cages have the potential to become the gold-standard in (synthetic) non-covalent catalysis.

Control of reactivity and selectivity in isomerization and rearrangement reactions inside confined spaces

In the confined space of supramolecular systems, the substrate can be forced into a reactive conformation and labile intermediates may be stabilized while isolated from the bulk solution. In this highlight, unusual processes mediated by supramolecular hosts are described. These include unfavourable conformational equilibria, unusual product selectivities in bond and ring-chain isomerizations, accelerated rearrangement reactions through labile intermediates, and encapsulated oxidations. In the host, controlled or altered isomerization of the guests can occur via hydrophobic, photochemical and thermal interventions. The inner spaces of the hosts resemble enzyme cavities that stabilize labile intermediates not accessible in the bulk solvent. The effects of confinement and the binding forces involved are discussed and further applications are suggested.

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.

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.

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.

Adhesion of Grafted-to Polyelectrolyte Brushes Functionalized with Calix[4]resorcinarene and Deposited as a Monolayer

Polyelectrolyte adhesives, either poly[2-(dimethylamino)ethyl methacrylate] or poly(methacrylic acid), functionalized with a surface-active calix[4]resorcinarene were grafted onto silicon wafers. Adhesion studies on these grafted-to brushes using polyelectrolyte hydrogels of opposite charge showed that it is the calix[4]resorcinarene, rather than adsorption of polyelectrolyte monomers, that adheres the brush to the silicon substrate. The adhesion measured was similar to that measured using polymers grafted from the surface, and was stronger than a control layer of poly(vinyl acetate) under the same test conditions. The limiting factor was determined to be adhesive failure at the hydrogel-brush interface, rather than the brush-silicon interface. Therefore, the adhesion has not been adversely affected by changing from a grafted-from to a grafted-to brush, demonstrating the possibility of a one-pot approach to creating switchable adhesives.

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.

One-Step Synthesis of C2v-Symmetric Resorcin[4]arene Tetraethers

The three-component reaction between a resorcinol, 1,3-dimethoxybenzene, and an alkyl aldehyde (R = C₁-C₁₁) along with BF₃·OEt₂ affords a C_2v-symmetric resorcin[4]arene tetraether in one step; in most cases, the single isomer can be precipitated from the reaction mixture in moderate to excellent yields (up to 89%). The reaction is tolerant of 2-substituted resorcinols (R' = OH, Cl, Br, Me), allowing a third type of functionality to be regioselectively incorporated during the macrocyclization.

Chalcogen Bonding and Hydrophobic Effects Force Molecules into Small Spaces

Supramolecular capsules are desirable containers for the study of molecular behavior in small spaces and offer applications in transport, catalysis, and material science. We report here the use of chalcogen bonding to form container assemblies that are stable in water. Cavitands 1-3 functionalized with 2,1,3-benzoselenadiazole walls were synthesized in good yield from resorcin[4]arenes. The solid-state single-crystal X-ray structure of 3 showed a dimeric assembly cemented together through multiple Se···N chalcogen bonds. Binding of hydrophobic and amphiphilic guests in D₂O was investigated by ¹H NMR methods and revealed host-guest assemblies of 1:1, 2:1, and 2:2 stoichiometries. Small guests such as n-hexane or cyclohexane assembled as 2:2 capsular complexes, larger guests like cyclohexane carboxylic acid or cyclodecane formed 1:1 cavitand complexes, and longer linear guests like n-dodecane, cyclohexane carboxylic acid anhydride, and amides created 2:1 capsular complexes. The 2:1 complex of the capsule with cyclohexane carboxylic acid anhydride was stable over 2 weeks, showing that the seam of chalcogen bonds is "waterproof". Selective uptake of cyclohexane over benzene and methyl cyclohexane over toluene was observed in aqueous solution with the capsule. Hydrophobic forces and hydrogen-bonding attractions between guest molecules such as 3-methylbutanoic acid stabilized the assemblies in the presence of the competing effects of water. The high polarizability and modest electronegativity of Se provide a capsule lining complementary to guest C-H bonds. The 2,1,3-benzoselenadiazole walls impart an unusually high magnetic anisotropy to the capsule environment, which is supported by density functional theory calculations.

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.

Nature-driven approaches to non-natural terpene analogues

The reactions catalysed by terpene synthases belong to the most complex and fascinating cascade-type transformations in Nature.

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.

Reversing Chemoselectivity: Simultaneous Positive and Negative Catalysis by Chemically Equivalent Rims of a Cucurbit[7]uril Host

Enzyme catalysis has always been an inspiration and an unattainable goal for chemists due to features such as high specificity, selectivity, and efficiency. Here, we disclose a feature neither common in enzymes nor ever described for enzyme mimics, but one that could prove crucial for the catalytic performance of the latter, namely the ability to catalyze and inhibit two different reactions at the same time. Remarkably, this can be realized by two identical, spatially resolved catalytic sites. In the future, such a synchronized catalyst action could be used not only for controlling chemoselectivity, as in the present case, but also for regulating other types of chemical reactivity.