Kinetics and Mechanism of Cation-Induced Guest Release from Cucurbit[7]uril

The release of two organic guests from cucurbit[7]uril (CB7) was selectively monitored by the stopped‐flow method in aqueous solutions of inorganic salts to reveal the mechanistic picture in detail. Two contrasting mechanisms were identified: The symmetric dicationic 2,7‐dimethyldiazapyrenium shows a cation‐independent complex dissociation mechanism coupled to deceleration of the ingression in the presence of alkali and alkaline earth cations (Mⁿ⁺) due to competitive formation of CB7–Mⁿ⁺ complexes. A much richer, unprecedented kinetic behaviour was observed for the ingression and egression of the monocationic and non‐symmetric berberine (B⁺). The formation of ternary complex B⁺–CB7–Mⁿ⁺ was unambiguously revealed. A difference of more than two orders of magnitude was found in the equilibrium constants of Mⁿ⁺ binding to B⁺–CB7 inclusion complex. Large cations, such as K⁺ and Ba²⁺, also promoted B⁺ expulsion from the ternary complex in a bimolecular process. This study reveals a previously hidden mechanistic picture and motivates systematic kinetic investigations of other host–guest systems.

A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift

A viologen derivative carrying a benzimidazole group (V-P-I ²⁺ ; viologen-phenylene-imidazole V-P-I) can be dimerized in water using cucurbit[8]uril (CB[8]) in the form of a 2:2 complex resulting in a negative shift of the guest pK_a , by more than 1 pH unit, contrasting with the positive pK_a shift usually observed for CB-based complexes. Whereas 2:2 complex protonation is unclear by NMR, silver cations have been used for probing the accessibility of the imidazole groups of the 2:2 complexes. The protonation capacity of the buried imidazole groups is reduced, suggesting that CB[8] could trigger proton release upon 2:2 complex formation. The addition of CB[8] to a solution containing V-P- I³⁺ indeed released protons as monitored by pH-metry and visualized by a coloured indicator. This property was used to induce a host/guest swapping, accompanied by a proton transfer, between V-P-I ³⁺ ⋅CB[7] and a CB[8] complex of 1-methyl-4-(4-pyridyl)pyridinium. The origin of this negative pK_a shift is proposed to stand in an ideal charge state, and in the position of the two pH-responsive fragments inside the two CB[8] which, alike residues engulfed in proteins, favour the deprotonated form of the guest molecules. Such proton release triggered by a recognition event is reminiscent of several biological processes and may open new avenues toward bioinspired enzyme mimics catalyzing proton transfer or chemical reactions.

Highly Efficient Supramolecular Catalysis by Endowing the Reaction Intermediate with Adaptive Reactivity

A new strategy of highly efficient supramolecular catalysis is developed by endowing the reaction intermediate with adaptive reactivity. The supramolecular catalyst, prepared by host-guest complexation between 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and cucurbit[7]uril (CB[7]), was used for biphasic oxidation of alcohols. Cationic TEMPO⁺ , the key intermediate, was stabilized by the electrostatic effect of CB[7] in aqueous phase, thus promoting the formation of TEMPO⁺ and inhibiting side reactions. Moreover, through the migration into the organic phase, TEMPO⁺ was separated from CB[7] and recovered the high reactivity to drive a fast oxidation of substrates. The adaptive reactivity of TEMPO⁺ induced an integral optimization of the catalytic cycle and greatly improved the conversion of the reaction. This work highlights the unique advantages of dynamic noncovalent interactions on modulating the activity of reaction intermediates, which may open new horizons for supramolecular catalysis.

Five-Component Self-Assembly of Cucurbituril-Based Hetero-pseudorotaxanes

[5]Pseudorotaxanes can be obtained by self-sorting using heteroditopic guests and various cucurbituril homologues as hosts. The assembly and chemically induced disassembly of the pseudorotaxanes can be monitored by measuring the fluorescence of the anthracene guest in solution. Mass spectral evidence for the supramolecular assemblies is obtained in the gas phase. The disassembly in the gas phase can be achieved by collision-induced dissociation leading to the corresponding [2]- and [3]pseudorotaxanes.

From Packed "Sandwich" to "Russian Doll": Assembly by Charge-Transfer Interactions in Cucurbit[10]uril

As the host possessing the largest cavity in the cucurbit[n]uril (CB[n]) family, CB[10] has previously displayed unusual recognition and assembly properties with guests but much remains to be explored. Herein, we present the recognition properties of CB[10] toward a series of bipyridinium guests including the tetracationic cyclophane known as blue box along with electron-rich guests and detail the influence of encapsulation on the charge-transfer interactions between guests. For the mono-bipyridinium guest (methylviologen, MV²⁺ ), CB[10] not only forms 1:1 and 1:2 inclusion complexes, but also enhances the charge-transfer interactions between methylviologen and dihydroxynaphthalene (HN) by mainly forming the 1:2:1 packed "sandwich" complex (CB[10]⋅2 MV²⁺ ⋅HN). For guest 1 with two bipyridinium units, an interesting conformational switching from linear to "U" shape is observed by adding catechol to the solution of CB[10] and the guest. For the tetracationic cyclophane-blue box, CB[10] forms a stable 1:1 inclusion complex; the two bipyridinium units tilt inside the cavity of CB[10] according to the X-ray crystal structure. Finally, a supramolecular "Russian doll" was built up by threading a guest through the cavities of both blue box and CB[10].

Turning Cucurbit[8]uril into a Supramolecular Nanoreactor for Asymmetric Catalysis

Chiral macromolecules have been widely used as synthetic pockets to mimic natural enzymes and promote asymmetric reactions. An achiral host, cucurbit[8]uril (CB[8]), was used for an asymmetric Lewis acid catalyzed Diels-Alder reaction. We achieved a remarkable increase in enantioselectivity and a large rate acceleration in the presence of the nanoreactor by using an amino acid as the chiral source. Mechanistic and computational studies revealed that both the amino acid-Cu2+ complex and the dienophile substrate are included inside the macrocyclic host cavity, suggesting that contiguity and conformational constraints are fundamental to the catalytic process and rate enhancement. These results pave the way towards new studies on asymmetric reactions catalyzed in confined achiral cavities.

Turning Cucurbit[8]uril into a Supramolecular Nanoreactor for Asymmetric Catalysis

Chiral macromolecules have been widely used as synthetic pockets to mimic natural enzymes and promote asymmetric reactions. An achiral host, cucurbit[8]uril (CB[8]), was used for an asymmetric Lewis acid catalyzed Diels–Alder reaction. We achieved a remarkable increase in enantioselectivity and a large rate acceleration in the presence of the nanoreactor by using an amino acid as the chiral source. Mechanistic and computational studies revealed that both the amino acid–Cu²⁺ complex and the dienophile substrate are included inside the macrocyclic host cavity, suggesting that contiguity and conformational constraints are fundamental to the catalytic process and rate enhancement. These results pave the way towards new studies on asymmetric reactions catalyzed in confined achiral cavities.