Switchable aqueous catalytic systems for organic transformations

Abstract

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in "nature-like" environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched 'on' or 'off' by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

Fig. 1. Switchable aqueous catalytic system.

A schematic representation of switchable catalytic systems in aqueous environments. Presence or absence of trigger can module the catalytic activity in switch ‘on’/‘off’ mode for organic reactions.

Fig. 2. pH-switchable catalytic systems.

a Schematic representation of the pH-switchable VK2H peptide as artificial hydrolase. The peptide showed conformational transition from unfolded random coil to β-sheet via changing the pH from acidic to alkaline. (Inset: chemical structure of peptide VK2H). The fibril structure can catalyze the hydrolysis of pNPA, whereas the unfolded structure is catalytically inactive. Switching the catalytic activity can be controlled by altering the pH. Adopted with permission from ref. , copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. b pH-switchable organocatalysis with amine-porphyrin hybrid in aqueous solution. Formation of J-aggregates, at acidic pH, suppressed the catalytic activity of Isoindoline moiety of the hybrid, whereas deaggregated state of the hybrid at higher pH resulted efficient aldol reaction. Adopted with permission from ref. , copyright 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. c A schematic of assembly behavior of polymer structure in aqueous solution at different pH (inset: the structure of the polymer), and aggregation-induced aldol reaction that catalyzed by proline moiety. Adopted with permission from ref. , copyright 2020 Elsevier B.V. d Schematic of pH-induced emulsion inversion for styrene hydrogenation. At acidic pH, the catalyst can efficiently convert the substrate to product, whereas the reaction is terminated at basic pH (inset: The structural description of silica microsphere with catalytically active center). Adopted with permission from ref. , copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 3. Thermoresponsive polymer-based switchable catalytic systems.

a Positive thermoresponsive micellar nanoreactor—at lower temperature, the polymer is soluble in water and Aldol reaction is ‘off’ due to the absence of suitable environment (Inset: chemical structure of the polymer). At elevated temperature, micelle is formed and provided suitable environment for L-proline-catalyzed Aldol reaction. Adopted with permission from ref. , copyright 2013 American Chemical Society. b Negative thermoresponsive micellar nanoreactor—at lower temperature, formation of micelle allowed the substrates to access catalyst, whereas at higher temperature polymer chain collapsed to hydrophobic globules, which inhibit the formation of intermediate, resulting poor selectivity. Adopted with permission from ref. , copyright 2021 American Chemical Society. c Self-regulating thermoresponsive catalyst surface—structure tips are coated with catalyst and upright/bent tips corresponding to ‘on/off’ catalysis. Below LCST of PNIPAAm, the catalyst tips enter the reagent layer, resulting an exothermic click reaction. Above the LCST, the PNIPAAm contracts and the structures bend, removing the catalyst from the reagent layer and turning ‘off’ the reaction. Adopted with permission from ref. , 2012, Nature Publishing Group, a division of Macmillan Publishers Limited.

Fig. 4. Temperature-switchable nanoparticle-based catalytic systems.

a PE-based catalytic system—Schematic for MOF-stabilized PE for dehalogenation reactions of chlorobenzene derivatives at 25 °C (Inset: structure of Pd-nanoparticle encapsulated thermoresponsive MOF). By increasing temperature, the system de-emulsified with phase separation and catalytic activity is switched ‘off’. Adopted with permission from ref. , copyright Royal Society of Chemistry. b Ru-NPs-based catalytic system—Ru-NPs embedded in the rotaxane-based gel phase, where the alkene cannot access the nanoparticle for catalytic transformation. When temperature has increased, the gel-sol transition occurred, and α-CD could play the role of mass transfer and bring alkenes into Ru-NPs-containing aqueous phase for their conversion into the corresponding alkanes. Adapted from ref. , copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 5. Light-switchable catalytic systems.

a Photoresponsive PLP mimic—Light-induced isomerization of dithienylethene structure between its “inactive” ring-opened and “active” ring-closed indicating whether the pyridinium and aldehyde are insulated or connected to each other for providing aldimine from reaction of aldehyde derivative and L-alanine. b β-cyclodextrin-based catalyst system—trans-azobenzene makes inclusion complex with the cavity of β-CD, resulting unavailability of catalytic site for the hydrolysis reaction. Light irradiation results cis-azobenzene that excluded from the cavity of β-CD allowing ester molecules to access the catalytic site in the cavity, which resulted faster hydrolysis. Adopted with permission from ref. , copyright 2001 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.

Fig. 6. Light-switchable catalytic systems based on cooperativity effect.

a Peptide-based artificial hydrolase—azobenzene-functionalized peptide molecules can self-assemble into nanofibers, where basicity of imidazole is enhanced for hydrolase-like catalytic activity on pNPA. The activity can be switched ‘off’ under UV light irradiation via trans → cis photo-isomerization of the azobenzene group, leading to the disassembly of nanofibers. Adopted with permission from ref. , copyright 2018 Royal Society of Chemistry. b Photoswitchable glycosidase mimic – deprotonation of the carboxylic acid groups from cis-isomer of azobenzene-functionalized dicarboxylic acid occurs in a stepwise fashion, whereas deprotonation for trans-isomer occurs simultaneously. The monoanionic form of the cis-isomer can act as a glycosidase mimic that proceeds through a general acid-base catalytic mechanism for the hydrolysis of 4-nitrophenyl-β-D–glucopyranoside. Catalysis via the cooperative mechanism is absent for trans-isomer. c Photoswitchable self-assembled catalytic system—the catalysis can be switched between the ‘on’ and ‘off’ states by light irradiation. The trans-isomer of an amphiphile self-assembles into vesicular structures, which show cooperative catalysis for transphosphorylation reaction (Inset: cis- and trans-isomer of the amphiphile). UV light irradiation provides the cis-isomer, resulting disassembly and switching ‘off’ the catalysis. Adopted with permission from ref. , copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 7. Light-switchable nanoparticle-based catalytic systems.

a Light-triggered dynamic formation of ‘nanoflasks’ based on azobenzene-functionalized Au-NPs—upon UV irradiation, cis-isomer aggregated to provide the nanoflask, which provide suitable atmosphere for acetal hydrolysis. Visible-light-triggered trans-isomer disintegrated the nanoflask and consequently switching ‘off’ the catalysis. Adopted with permission from ref. , copyright 2015, Nature Publishing Group. b Light-switchable PE-based system—trans-azobenzene-based catalytic system efficiently catalyzes hydrogenation reaction at ambient condition. UV irradiation results phase separation and switching ‘off’ the catalysis, whereas visible light irradiation and homogenization results emulsification for catalytic reaction. Adopted with permission from ref. , copyright 2020 Wiley-VCH GmbH.

Fig. 8. Small molecule-responsive switchable catalytic systems.

a CO₂-responsive switchable system—amphiphilic molecule, PTC₁₂ is insoluble in water and catalytic aldol reaction is switched ‘off’. In presence of CO₂, PTC₁₂ self-assembles to vesicle nanoreactor, where catalysis is turned ‘on’ for efficient aldol reaction. Adopted with permission from the ref. , copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. b Small molecule/ion-induced supramolecular allosteric catalyst—use of small molecule regulators (CO and Cl⁻) change the size of the macrocycle, the accessibility to the active binuclear Zn site, and thereby the catalysis of HPNP hydrolysis reaction. Adopted with permission from the ref. , copyright 2007, American Chemical Society. c DNA duplex-scaffold functionalized with Cu-bpy and TEMPO for switchable catalysis—the distance between the two cocatalysts in DNA architecture can be altered upon introduction of ssDNA strand (Inset: structures of Cu(I)-bpy and TEMPO cocatalysts). Presence of an ssDNA trigger sequence, the structure holds the cocatalysts apart and turning ‘off’ the catalysis. The original catalyst conformation is restored upon addition of an ssDNA antitrigger strand that catalyze the oxidation of 2-naphthalenemethanol in borate buffer (pH 9.5) environment. Adopted with permission from ref. , copyright 2021, American Chemical Society. d Chemical fuel-induced transient availability of catalyst for a chemical reaction—glycine betaine methyl ester competes for CB[7] binding with aniline, and their hydrolysis controls the release of aniline from CB[7] for hydrazone formation reaction. Glycine betaine methyl ester binds with CB[7], allowing the release of aniline and turning ‘on’ the catalysis. Rebinding of aniline in the cavity of CB[7] due to decay of ester molecules switch ‘off’ the catalysis. Adopted with permission from ref. , copyright 2022 American Chemical Society.

Fig. 9. Other stimuli-switchable catalytic systems.

a Electrochemical-induced switchable catalyst stem—activation and deactivation of Au-NP-based supramolecular nanocatalyst via electrochemical stimulus for the hydrolysis of HPNPP. Adopted with permission from the ref. , copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. b Polymer photocatalyst via switchable hydrophilicity by CO₂/N₂ swing—in presence of CO₂, photocatalyst generates a hydrophilic structure that enables the catalyst to support photo-oxidation of 2-furoic acid in water. On purging in N₂ into the system, CO₂ is released from the system and the photocatalysts clusters back to its original hydrophobic nature, terminating the catalytic activity and the reaction (inset: the structure of photocatalyst in absence/presence of CO₂). Adopted with permission from ref. , copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.