Merging Bambus[6]uril and Biotin[6]uril into an Enantiomerically Pure Monofunctionalized Hybrid Macrocycle

Bambus[6]urils and biotin[6]urils are macrocycles with an exceptional affinity for inorganic anions. Here, we investigated statistical condensation of 2,4-dibenzylglycoluril and d-biotin, monomers of the corresponding macrocycles, to prepare the enantiomerically pure macrocycle 1 containing a single d-biotin and five glycoluril units. Host-guest properties of 1 in chloroform solution and solid state were investigated. The macrocycle 1 bearing a single functional group was employed in the formation of [1]rotaxane utilizing reversible covalent bonds.

Tuning CH Hydrogen Bond-Based Receptors toward Picomolar Anion Affinity via the Inductive Effect of Distant Substituents

Inspired by nature, artificial hydrogen bond-based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3 )-H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron-withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013  M-1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron-withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

Synthesis of Enantiomerically Pure Bambus[6]urils Utilizing Orthogonal Protection of Glycolurils

A general strategy for the synthesis of 2N,4N'-disubstituted glycoluril enantiomers on a multigram scale using orthogonal protection is reported. The use of these glycolurils is demonstrated in the synthesis of enantiomerically pure bambus[6]uril macrocycles. Moreover, the deprotection of (S)-1-phenylethyl substituents on the macrocycle was achieved, opening access to various chiral bambus[6]urils via post-macrocyclization modification strategy.

Transmembrane transport of fluoride studied by time-resolved emission spectroscopy

Here we present a new method to monitor fluoride transmembrane transport into liposomes using a europium(III) complex. We take advantage of the long emission lifetime of this probe to measure the transport activity of a fluorescent transporter. The high sensitivity, selectivity, and versatility of the assay allowed us to study different types of fluoride transporters and unravel their mechanisms of action.

Monomerization of Phthalocyanines in Water via Their Supramolecular Interactions with Cucurbiturils

Aggregation of phthalocyanines (Pcs) represents a problematic feature that decreases the potential of these macrocycles in a number of applications. In this work, we present a supramolecular approach based on the interaction of aminoadamantyl-substituted Pcs with bulky and hydrophilic cucurbit[7]uril (CB[7]) to increase the levels of Pc monomers in water. A series of zinc(II) Pcs substituted at positions α or β by an aminoadamantyl substituent (with a different level of alkylation of nitrogen) were prepared from the corresponding phthalonitriles. A ¹H nuclear magnetic resonance study of the interaction of phthalonitriles with CB[7] in water confirmed the formation of an inclusion complex with an aminoadamantyl moiety with K_a values of ∼10¹² M^-1. The interaction of CB[7] with Pcs in water substantially weakened H-type aggregation and improved both fluorescence and singlet oxygen production, confirming that this approach is efficient for the monomerization of Pcs. In vitro evaluation of the photodynamic activity of prepared Pcs led to EC₅₀ values in the submicromolar range on HeLa and SK-MEL-28 cells. However, the activity decreased for at least an order of magnitude after host-guest interaction with CB[7] despite better photophysical properties. This was attributed to a much lower uptake by cells due to the very bulky and hydrophilic character of the Pc-CB[7] assembly.

Monofunctionalized Fluorinated Bambusurils and Their Conjugates for Anion Transport and Extraction

Bambusurils are macrocyclic molecules that are known for their high binding affinity and selectivity toward anions. Here, we present the preparation of two bambusurils bearing fluorinated substituents and one carboxylic function. These monofunctionalized bambusurils were conjugated with crown ether and cholesterol units. The resulting conjugates were successfully tested in liquid-liquid extraction of inorganic salts and chloride/bicarbonate transport across lipid bilayers.

Dynamic [1]rotaxanes via a reversible covalent bond and host-guest anion recognition

Bambus[6]uril-based [1]rotaxanes were formed quantitatively, utilizing a bis(acyloxy)iodate(I) reversible covalent bond and host-guest anion recognition. These novel [1]rotaxanes exhibited a dynamic nature facilitating carboxylate component exchange reactions in acetonitrile.

Transmembrane Transport of Bicarbonate Unravelled

Invited for the cover of this issue are Dr. Stephen Butler, Dr. Hennie Valkenier and co-workers at Université Libre de Bruxelles, Loughborough University, Masaryk University, and the University of Bristol. The image depicts the transport of bicarbonate anions versus the spontaneous diffusion of CO₂ across the lipid bilayer of a liposome. Read the full text of the article at 10.1002/chem.202100491.

Transmembrane Transport of Bicarbonate Unravelled*

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Bambusuril Macrocycles as Mediators of Supramolecular Interactions: Application to the Europium Cage Helicate

Herein, it is shown how bambusurils can be used for tuning and/or characterizing supramolecular systems. Indeed, the addition of bambusurils as anion scavengers to metal-mediated self-assemblies allows manipulation of the subtle equilibria in the given system. This is demonstrated for the case of the tetranuclear europium helical cage, which is well suited to different applications. Among the reported results, experimental evidence is provided showing that perchlorate and triflate anions act as a molecular template for the cage assembly. The complexation of inorganic anions with neutral bambusurils resulted in bulky non-coordinating counterions that may trigger the self-assembly process or stimulate specific interactions between components. Moreover, bambusuril was able to selectively remove coordinating nitrates from the mixture with non-coordinating anions, enabling the regeneration of the helical cage.

Functionalized Chiral Bambusurils: Synthesis and Host-Guest Interactions with Chiral Carboxylates

Bambusurils are a class of macrocyclic anion receptors that exhibit notable anion recognition properties, able to bind various inorganic anions as well the carboxylates or sulfonates. Recently, we reported enantioselective recognition of chiral carboxylates using non-functionalized chiral bambusuril derivatives. Herein, we report the synthesis and host-guest properties of two new representatives of chiral bambusuril macrocycles bearing ester functional groups, differing by the substituents attached to their portals. Their supramolecular properties in terms of carboxylate binding were studied by means of NMR in DMSO-d₆ . The reported bambusurils bind selected chiral carboxylates with enantioselectivity factors up to 3.1. The results indicated that the selectivity towards different carboxylates is governed by the steric constraint of the substituents surrounding bambusuril portals. No clear trend in the binding affinities and their enantioselectivities was found.

Monofunctionalized Bambus[6]urils and Their Conjugates with Crown Ethers for Liquid-Liquid Extraction of Inorganic Salts

Bambusurils are a growing family of macrocyclic anion receptors. In this Letter, we present the first syntheses of monofunctionalized bambusurils and their use for the preparation of heteroditopic bambusuril-crown ether conjugates suitable for the extraction of ion pairs from water to chloroform.

Conformationally Mobile Acyclic Cucurbit[n]uril-Type Receptors Derived from an S-shaped Methylene Bridged Glycoluril Pentamer

We report the synthesis of the conformationally mobile S-shaped glycoluril pentamer building block 3a and two new acyclic CB[n]-type receptors P1 and P2. P1 (9 mM) and P2 (11 mM) have moderate aqueous solubility but their host•guest complexes are poorly soluble. Host P1 does not undergo intermolecular self-association whereas P2 does (K_s = 189±27 M^-1). ¹H NMR titrations show that P1 and P2 are poor hosts toward hydrophobic (di)cations 6 - 11 (P1: K_a = 375-1400 M^-1; P2: K_a = 1950-19800 M^-1) compared to Tet1 and Tet2 (Tet1: K_a = 3.09 × 10⁶ to 4.69 × 10⁸ M^-1; Tet2: K_a = 4.59 × 10⁸ to 1.30 × 10¹⁰ M^-1). Molecular modelling shows that P1 and P2 exist as a mixture of three different conformers due to the two S-shaped methylene bridged glycoluril dimer subunits that each possess two different conformations. The lowest energy conformers of P1 and P2 do not feature a well-defined central cavity. In the presence of guests, P2 adapts its conformation to form 1:1 P2•guest complexes; the binding free energy pays the energetic price of conformer selection. This energetically unfavorable conformer selection results in significantly decreased K_a values of P1 and P2 compared to Tet1 and Tet2.

Hypervalent Iodine Based Reversible Covalent Bond in Rotaxane Synthesis

Reversible covalent bonds play a significant role in achieving the high-yielding synthesis of mechanically interlocked molecules. Still, only a handful of such bonds have been successfully employed in synthetic procedures. Herein, we introduce a novel approach for the fast and simple preparation of interlocked molecules, combining the dynamic bond character of bis(acyloxy)iodate(I) anions with macrocyclic bambusuril anion receptors. The proof of principle was demonstrated on rotaxane synthesis, with near-quantitative yields observed in both the classical and "in situ" approach. The rotaxane formation was confirmed in the solid-state and solution by the X-ray and NMR studies. Our novel approach could be utilized in the fields of dynamic combinatorial chemistry, supramolecular polymers, or molecular machines, as well inspire further research on molecules that exhibit dynamic behavior, but owing to their high reactivity, have not been considered as constituents of more elaborate supramolecular structures.

Bambusurils as a mechanistic tool for probing anion effects

Bambusuril macrocycles have high affinity towards anions (X⁻) such as PF₆⁻ and SbF₆⁻ or BF₄⁻ and ClO₄⁻. Therefore, addition of bambusurils to reaction mixtures containing these anions effectively removes the free anions from the reaction process. Hence, comparing reactions with and without addition of bambusurils can demonstrate whether the anions actively participate in the reaction mechanism or not. We show this approach for gold(i) mediated addition of methanol to an alkyne. The reaction mechanism can proceed via monoaurated intermediates (e.g., in catalysis with [(IPr)AuX]) or via diaurated intermediates (e.g., in catalysis with [(PPh₃)AuX]). We show that anions X⁻ slightly affect the reaction rates, however the effect stays almost the same even after their encapsulation in the cavity of bambusurils. We also demonstrate that X⁻ affects the overall reaction rate in the very same way as the reaction rate of the protodeauration step. All results are consistent with the indirect effect of X⁻ by the acidity of the conjugated acid HX on the rate-determining step. There is no evidence that a direct involvement of X⁻ would affect the reaction rate.

We present bambusurils, macrocycles which strongly bind anions of different sizes, as a mechanistic tool to probe counter ion effects in metal catalysis.

Phase-transfer extraction for the fast quantification of perchlorate anions in water

Supramolecular approaches for the quantitative anion analysis in water remain scarce due to the lack of receptors that effectively bind anions in this medium. Herein, we present a novel, fast and easy, supramolecular approach for a selective and quantitative analysis of perchlorate anions in water, coupling the UV-Vis spectroscopic method and phase-transfer extraction of anions by a water-insoluble anion receptor.

Bambusuril analogs based on alternating glycoluril and xylylene units

The glycoluril monomer is a popular building block in supramolecular chemistry as it is used for the synthesis of versatile host molecules which can interact with cationic, anionic or neutral guest molecules. Here we present the design and synthesis of a new hybrid macrocycle containing glycoluril and aromatic units. The reaction afforded a mixture of macrocyclic homologues from which a two-membered macrocycle was isolated as the main product. Two disastereomers of the macrocycle were separated and characterized by means of NMR spectroscopy and X-ray crystallography. Conformational changes of these diastereomers were investigated using DFT models and variable-temperature NMR.

Dynamic Interactions in Synthetic Receptors: A Guest Exchange Saturation Transfer Study

The accumulated knowledge regarding molecular architectures is based on established, reliable, and accessible analytical tools that provide robust structural and functional information on assemblies. However, both the dynamicity and low population of noncovalently interacting moieties within studied molecular systems limit the efficiency and accuracy of traditional methods. Herein, the use of a saturation transfer-based NMR approach to study the dynamic binding characteristics of an anion to a series of synthetic receptors derived from bambusuril macrocycles is demonstrated. The exchange rates of BF₄ ^- are mediated by the side chains on the receptor (100 s^-1 <k_ex <5000 s^-1 ), which play a critical role in receptor-anion binding dynamics. The signal amplification obtained with this approach allows for the identification of different types of intermolecular interactions between the receptor and the anion, something that could not have been detected by techniques hitherto used to study molecular assemblies. These findings, which are supported by a computational molecular dynamic study, demonstrate the uniqueness and added value of this NMR method.

Unsubstituted Bambusurils: Post-Macrocyclization Modification of Versatile Intermediates

A new bambusuril derivative, (H)BU[6], lacking substituents on the ureidic nitrogen atoms, has been isolated and characterized. This macrocycle was prepared by the deprotection of bambusuril (PMB)BU[6]. (H)BU[6] is attractive for use as a starting compound for the preparation of other bambusuril derivatives, which was demonstrated via propargylation and the copper-catalyzed click reaction performed on the macrocycle.

Transport of Gases and Organic Vapours Through Membranes Made of Poly(amide-imide)s Crosslinked with Poly(ethylene adipate)

Transport of permanent gases (N2, O2, CO2) and of saturated organic vapours of benzene, toluene and methanol through membranes based on poly(ethylene adipate)-crosslinked poly(amide-imide)s was studied. The membranes containing more than 50 wt.% of non-polyimides showed higher permeability coefficients (by 2-3 orders) for the organic vapours than for the gases.