Cage-confined catalysis in open-cage solution merging heterogeneity and multifunctionality

In work published in the National Science Review (nwab155), a collaborative team at Sun Yat-Sen University and Université de Montpellier led by Prof. Cheng-Yong Su described a multifunctional supramolecular catalysis protocol based on open-cage solution to achieve versatile and anomalous cage-confined catalysis with the aid of biomimetic supramolecular cage effect. They consider open-cage solution as a combinatorial system of cage-defined nanospaces and liquid-phase fluidity to display solution heterogeneity with the physicochemical properties strikingly different from the inside to the outside of the dissolved cages, enabling formation of local amphoteric environments, molecular transfer via cage surface and phase transfer. Therefore, the cage-confined nanospaces in solution can act as multicellular and multifunctional nanoreactors with enzyme mimicking and synergistic actions to benefit selective mass transfer (substrate inclusion and product release), C-H activation and anionic intermediate stabilization, imidazole-proton equilibrium and pK_a shift, substrates concentration and rate enhancement, phase-transfer of insoluble reactants/products/co-catalysts, and so on.

The nanoscale chemical spaces inherent in various porous solids/liquids are of interest because the molecules confined therein can change their physicochemical properties and behaviors to differentiate from free collision and diffusion. The permeable open-cage solution presents an ideal blueprint to maximize the confined nanopaces and their confinement effect due to its solution heterogeneity and fluidity. The catalytic processes in open-cage solution resemble the heterogeneous one in porous solids like metal-organic frameworks or zeolites, reminiscent of enzymatic catalysis in protein imposed microenvironment but characteristic of fluid homogeneous distribution, also possess phase transfer feature owing to the encapsulation of hardly soluble compounds which is different from dissolution of solutes in solvent. The effectiveness of the enzymatic cage effect depends on the shape rigidity and thermodynamic stability of the open-cages in solution, in striking contrast to the known solvent cage effect which is only short-lived in transient state.

Taking advantages of highly positive charges distributed on the [(Pd/Pt)₆(RuL₃)₈]²⁸⁺ nanocage surface, researchers find that the 24 imidazole-NH protons can partially dissociate to shift pK_a values of imidazole-moieties, thus generating basic cage nanospaces in acid bulky solution to build-up local amphoteric characters. Selective molecular transfer is driven by hydrophobic effect of cage nanocavities and host-guest interactions through 12 cage portals, which even enables phase transfer of hardly soluble compounds to encapsulate substrates and release products. They apply this open-cage solution system to a series of chemical transformations, and find that the deuteration reactions of terminal alkynes can proceed via fast H/D-exchange under acidic conditions without addition of any extraneous bases. The traditionally base-catalyzed Knoevenagel condensation can also be smoothly carried out in acidic open-cage solution, confirming a supramolecular cage effect for C-H activation and anionic intermediate stabilization by the basic and positively charged cage-confined nanospaces. More interestingly, the amphoteric open-cage solution provides an ideal catalytical protocol to promote acid/base-catalyzed cascade reactions in an one-pot fashion，namely, the hydrolysis of acetal derivatives to aldehydes in extrinsic acidic solution in the first step and then the Knoevenagel condensation inside the basic cage nanospaces in the second step, which is hard to accomplish in normal homogenous solution. Such acid/base-catalyzed one-pot reaction can also be applied to hydrolysis−Henry cascade reactions. More complicated three-component A3-coupling reactions can also be achieved in this open-cage solution, where the water-insoluble and sensitive CuOTf co-catalyst are able to be transferred into aqueous solution via binding with the 24 imine-N sites around open portals to be stabilized and concentrated, affording prominent rate acceleration (up to 70-fold), catalytic turnover (TON > 700), good efficiency and recyclability for a wide scope of substrates.

See the article:

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

https://doi.org/10.1093/nsr/nwab155

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