Using Soluble Polymers to Enforce Catalyst‐Phase‐Selective Solubility and as Antileaching Agents to Facilitate Homogeneous Catalysis

The enforced phase‐selective solubility of polyisobutylene (PIB)‐bound Rh^II catalysts in biphasic heptane/acetonitrile mixtures can be used not only to recycle these catalysts but also to minimize bimolecular reactions with ethyl diazoacetate. When cyclopropanation and O? H insertion reactions are carried out with PIB‐bound Rh^II catalysts either with or without addition of an unfunctionalized hydrocarbon polymer cosolvent, dimer by‐product formation is suppressed even without slow syringe pump addition of the ethyl diazoacetate. This suppression of by‐product formation is shown to be due to increased phase segregation of the soluble polymer‐bound catalyst and the ethyl diazoacetate reactant. These studies also reveal that added hydrocarbon polymer cosolvents can function as antileaching agents, decreasing the already small amount of a soluble polymer‐bound species that leaches into a polar phase in a biphasic mixture during a liquid/liquid separation step.     

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

Pickering emulsions are surfactant‐free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant‐stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.     

Direct Arylation of N‐Heteroarenes with Aryldiazonium Salts by Photoredox Catalysis in Water

A highly effective visible light‐promoted “radical‐type” coupling of N‐heteroarenes with aryldiazonium salts in water has been developed. The reaction proceeds at room temperature with [Ru(bpy)₃]Cl₂ ? 6 H₂O as a photosensitizer and a commercial household light bulb as a light source. Pyridine and a variety of substituted pyridines are effective substrates under these reaction conditions, and only monosubstituted products are formed with different regioselectivities. Using aqueous formic acid as solvent, an array of xanthenes, thiazole, pyrazine, and pyridazine are compatible with this new arylation approach. The broad substrate scope, mild reaction conditions, and use of water as reaction solvent make this procedure a practical and environmentally friendly method for the synthesis of compounds containing aryl‐heteroaryl motifs.     

“On‐Water,” Microwave‐Assisted,Pd‐Catalyzed Synthesis of Indoles from Imines and o‐Difunctionalized Arenes

Regioselectively substituted indoles are prepared by a Pd‐catalyzed C? C/C? N bond‐forming sequence from imines and o‐dihaloarenes or o‐haloarene sulfonates. The heterogeneous reaction as a suspension in water and under microwave heating offers important advantages in comparison with the conventional reaction in an organic solvent, among them, operational simplicity, the employment of KOH solutions instead of alkoxides, and a dramatic reduction of reaction times.     

Carbocatalysis: The State of “Metal‐Free” Catalysis

The rise in global demand for crucial chemical compounds has driven immense research in the fundamental science of catalysis. Graphene and its derivatives (chemically modified graphene, CMGs) have recently emerged as a new class of heterogeneous catalyst that promises economically viable and greener routes to these compounds. Although CMGs possess unique catalytic properties, the actual active sites are often points of discussion. Current minimal understanding on the possible effects of metallic impurities on the electrocatalytic performances of these CMGs calls forth the need to raise awareness on possible metallic impurities misrepresenting the actual chemical catalytic performances of the CMGs. This Minireview highlights the latest advances in the application of CMGs as catalysts, with an emphasis on the possible effects of metallic impurities on CMG catalysis.     

Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in Water

Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C−C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis.     

Water‐Assisted Nitrile Oxide Cycloadditions: Synthesis of Isoxazoles and Stereoselective Syntheses of Isoxazolines and 1,2,4‐Oxadiazoles

Conventional methods generate nitrile oxides from oxime halides in organic solvents under basic conditions. However, the present work revealed that water‐assisted generation of nitrile oxides proceeds under mild acidic conditions (pH 4–5). Cycloadditions of nitrile oxides with alkynes and alkenes easily occurred in water without using catalysts, thus yielding isoxazoles and isoxazolines, respectively, with excellent stereoselectivity toward five‐ and six‐membered cyclic alkenes. A double stereoselective cycloaddition of two units of a nitrile oxide with cyclohexene was also achieved, thus yielding 1,2,4‐oxadiazole derivatives having a unique hybrid isoxazoline‐oxadiazole skeleton. Enantiomerically pure isoxazolines were prepared from monoterpenes with a ring strain. In one case, the isoxazoline with a butterfly‐like structure was simply prepared, and it might be used as a ligand in asymmetric catalysis.     

Urea‐Based Porous Organic Frameworks: Effective Supports for Catalysis in Neat Water

Two urea‐based porous organic frameworks, UOF‐1 and UOF‐2, were synthesized through a urea‐forming condensation of 1,3,5‐benzenetriisocyanate with 1,4‐diaminobenzene and benzidine, respectively. UOF‐1 and UOF‐2 possess good hydrophilic properties and high scavenging ability for palladium. Their palladium polymers, Pd^II/UOF‐1 and Pd^II/UOF‐2, exhibit high catalytic activity and selectivity for Suzuki–Miyaura cross‐coupling reactions and selective reduction of nitroarenes in water. The catalytic reactions can be efficiently performed at room temperature. Palladium nanoparticles with narrow size distribution were formed after the catalytic reaction and were well dispersed in UOF‐1 and UOF‐2. XPS analysis confirmed the coordination of the urea oxygen atom with palladium. SEM and TEM images showed that the original network morphology of UOF‐1 and UOF‐2 was maintained after palladium loading and catalytic reactions.     

Control over Electrochemical Water Oxidation Catalysis by Preorganization of Molecular Ruthenium Catalysts in Self‐Assembled Nanospheres

Oxygen formation through water oxidation catalysis is a key reaction in the context of fuel generation from renewable energies. The number of homogeneous catalysts that catalyze water oxidation at high rate with low overpotential is limited. Ruthenium complexes can be particularly active, especially if they facilitate a dinuclear pathway for oxygen bond formation step. A supramolecular encapsulation strategy is reported that involves preorganization of dilute solutions (10^?5 m ) of ruthenium complexes to yield high local catalyst concentrations (up to 0.54 m ). The preorganization strategy enhances the water oxidation rate by two‐orders of magnitude to 125 s^?1, as it facilitates the diffusion‐controlled rate‐limiting dinuclear coupling step. Moreover, it modulates reaction rates, enabling comprehensive elucidation of electrocatalytic reaction mechanisms.     

A Z‐Scheme Strategy that Utilizes ZnIn2S4 and Hierarchical VS2 Microflowers with Improved Charge‐Carrier Dynamics for Superior Photoelectrochemical Water Oxidation

One of the major limiting factors for efficient photoelectrochemical water oxidation is the fast recombination kinetics of photogenerated charge carriers. Herein, we propose a model system that utilizes ZnIn₂S₄ and hierarchical VS₂ microflowers for efficient charge separation through a Z‐scheme pathway, without the need for an electron mediator. An impressive 18‐fold increase in photocurrent was observed for ZnIn₂S₄–VS₂ compared to ZnIn₂S₄ alone. The charge‐transfer dynamics in the composite were found to follow a Z‐scheme pathway, which resulted in decreased charge recombination and greater accumulation of the surface charge. Furthermore, slow kinetics of the surface reaction in the ZnIn₂S₄–VS₂ composite correlated to an increased surface‐charge capacitance. This feature of the composite material facilitated partial storage of the photogenerated charge carriers (e^?/h⁺) under illumination and dark‐current conditions, thus storing and utilizing solar energy more efficiently.     

Heterogeneous Catalysis for Water Oxidation by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Heterogenization of metal‐complex catalysts for water oxidation without loss of their catalytic activity is important for the development of devices simulating photosynthesis. In this study, efficient heterogeneous iridium complexes for water oxidation were prepared using bipyridine‐bridged periodic mesoporous organosilica (BPy‐PMO) as a solid chelating ligand. The BPy‐PMO‐based iridium catalysts (Ir‐BPy‐PMO) were prepared by postsynthetic metalation of BPy‐PMO and characterized through physicochemical analyses. The Ir‐BPy‐PMOs showed high catalytic activity for water oxidation. The turnover frequency (TOF) values for Ir‐BPy‐PMOs were one order of magnitude higher than those of conventional heterogeneous iridium catalysts. The reusability and stability of Ir‐BPy‐PMO were also examined, and detailed characterization was conducted using powder X‐ray diffraction, nitrogen adsorption, ¹³C DD MAS NMR spectroscopy, TEM, and XAFS methods.     

Tuning Interfacial Structures for Better Catalysis of Water Electrolysis

Interface modulation, as an old concept of heterogeneous catalysis, represents an emerging, fast-growing and exciting direction in the field of water electrolysis. Over the past five years, diverse hetero-nanostructures have been synthesised as water electrolysis catalysts by taking advantage of interface modulation. However, it seems that the performance (i.e., efficiency and durability) of these materials needs to be further improved. Therefore, a comprehensive summary of recent achievements and the challenging issues concerning the regulation of material functionalities through interface modulation is necessary and helpful. Herein, firstly, the fundamentals of water electrolysis are outlined, and then the delicate design and fine control of well-defined interfaces, as well as related mechanisms for performance improvement are discussed. Finally, future opportunities and challenges in the everlasting pursuit of highly efficient and robust water electrolysis catalysts are highlighted.     

Water‐ and Organo‐Dispersible Gold Nanoparticles Supported by Using Ammonium Salts of Hyperbranched Polystyrene: Preparation and Catalysis

Gold nanoparticles (1 nm in size) stabilized by ammonium salts of hyperbranched polystyrene are prepared. Selection of the R groups provides access to both water‐ and organo‐dispersible gold nanoparticles. The resulting gold nanoparticles are subjected to studies on catalysis in solution, which include reduction of 4‐nitrophenol with sodium borohydride, aerobic oxidation of alcohols, and homocoupling of phenylboronic acid. In the reduction of 4‐nitrophenol, the catalytic activity is clearly dependent on the size of the gold nanoparticles. For the aerobic oxidation of alcohols, two types of biphasic oxidation are achieved: one is the catalyst dispersing in the aqueous phase, whereas the other is in the organic phase. The catalysts are reusable more than four times without loss of the catalytic activity. Selective synthesis of biphenyl is achieved by the homocoupling of phenylboronic acid catalyzed by organo‐dispersible gold nanoparticles.     

Water—More Than Just a Green Solvent: A Stereoselective One‐Pot Access to All‐Chiral Tetrahydronaphthalenes in Aqueous Media

A facile and highly stereoselective construction of heavily functionalized chiral tetrahydronaphthalene skeletons fused with an oxazolidine moiety has been developed. The process involves an organocatalytic tandem Michael/nitrone formation/intramolecular [3+2] nitrone–olefin cycloaddition in aqueous media. Using rationally designed substrates, the reaction conditions have been optimized and the one‐pot process has been applied to a series of nitroolefin acrylates and aldehydes. The N‐hydroxyphenylamine component used in the second step has also been varied. The stereochemistry of one product has been verified by an X‐ray crystal structure determination. The water used in the strategy not only constitutes an environmentally benign solvent, but also helps to improve the reactivity and stereoselectivity.