Polyionic polymers--heterogeneous media for metal nanoparticles as catalyst in Suzuki-Miyaura and Heck-Mizoroki reactions under flow conditions

Preparation of Metal-Immobilized Methacrylate-Based Monolithic Columns for Flow-Through Cross-Coupling Reactions

With the aim of developing efficient flow-through microreactors for high-throughput organic synthesis, in this work, microreactors were fabricated by chemically immobilizing palladium-, nickel-, iron-, and copper-based catalysts onto ligand-modified poly(glycidyl methacrylate-co-ethylene dimethacrylate) [poly(GMA-co-EDMA)] monoliths, which were prepared inside a silicosteel tubing (10 cm long with an inner diameter of 1.0 mm) and modified with several ligands including 5-amino-1,10-phenanthroline (APHEN), iminodiacetic acid (IDA), and iminodimethyl phosphonic acid (IDP). The performance of the resulting microreactors in Suzuki-Miyaura cross-coupling reactions was evaluated, finding that the poly(GMA-co-EDMA) monolith chemically modified with 5-amino-1,10-phenanthroline as a binding site for the palladium catalyst provided an excellent flow-through performance, enabling highly efficient and rapid reactions with high product yields. Moreover, this monolithic microreactor maintained its good activity and efficiency during prolonged use.

Development of Facile and Simple Processes for the Heterogeneous Pd-Catalyzed Ligand-Free Continuous-Flow Suzuki–Miyaura Coupling

The Suzuki–Miyaura coupling reaction is one of the most widely utilized C–C bond forming methods to create (hetero)biaryl scaffolds. The continuous-flow reaction using heterogeneous catalyst-packed cartridges is a practical and efficient synthetic method to replace batch-type reactions. A continuous-flow ligand-free Suzuki–Miyaura coupling reaction of (hetero)aryl iodides, bromides, and chlorides with (hetero)aryl boronic acids was developed using cartridges packed with spherical resin (tertiary amine-based chelate resin: WA30)-supported palladium catalysts (7% Pd/WA30). The void space in the cartridge caused by the spherical catalyst structures enables the smooth flow of a homogeneously dissolved reaction solution that consists of a mixture of organic and aqueous solvents and is delivered by the use of a single syringe pump. Clogging or serious backpressure was not observed.

Recent Developments in the Immobilization of Palladium Complexes on Renewable Polysaccharides for Suzuki–Miyaura Cross-Coupling of Halobenzenes and Phenylboronic Acids

Polysaccharides derived from natural sources exhibit unique structures and functional groups, which have recently garnered them increased attention for their potential applicability as supports for metal catalysts. Renewable polysaccharide matrices were employed as supports for palladium complexes, with or without previous modification of the support, and were used in Suzuki cross-coupling of halobenzenes and phenylboronic acid derivatives. In this review, recent developments in the immobilization of palladium-based complexes are reported, including descriptions of the preparation procedures and catalytic activity of each system. In addition, the effects of the nature of the polymeric support and of the reaction conditions on catalytic performance are discussed.

Metallopolymers for advanced sustainable applications

Since the development of metallopolymers, there has been tremendous interest in the applications of this type of materials. The interest in these materials stems from their potential use in industry as catalysts, biomedical agents in healthcare, energy storage and production as well as climate change mitigation. The past two decades have clearly shown exponential growth in the development of many new classes of metallopolymers that address these issues. Today, metallopolymers are considered to be at the forefront for discovering new and sustainable heterogeneous catalysts, therapeutics for drug-resistant diseases, energy storage and photovoltaics, molecular barometers and thermometers, as well as carbon dioxide sequesters. The focus of this review is to highlight the advances in design of metallopolymers with specific sustainable applications.

Scope and limitation of propylene carbonate as a sustainable solvent in the Suzuki–Miyaura reaction

The Suzuki–Miyaura reaction is one of the most used transformations in drug research. Thus making this reaction more sustainable is of considerable current interest. Here we show that propylene carbonate (PC) can be used as a solvent for the Suzuki–Miyaura reaction. PC is one of the greenest solvents since it is synthesized under green conditions by the use of carbon dioxide in the air. All reactions proceeded well and good or excellent yields were observed for the biaryl products. Nonetheless in the case of pyridazinones, 2-hydroxypropyl- chain containing side-products were observed. Importantly, this fact allowed the isolation of several novel compounds which were generated under prominently green conditions.

The Suzuki–Miyaura reaction was carried out in propylene carbonate yielding an interesting side-product besides the biphenyl derivative.

Continuous-Flow Suzuki-Miyaura and Mizoroki-Heck Reactions under Microwave Heating Conditions

Microwave-assisted continuous-flow reactions have attracted significant interest from synthetic organic chemists, especially process chemists from practical points of view, due to a less complicated shift to large-scale synthesis based on simple and continuous access to products with low energy requirements. In this personal account, we focused on the Suzuki-Miyaura and Mizoroki-Heck reactions, both of which are significantly important cross-coupling reactions for the synthesis of various functional materials. Microwave power is effective for heating. Typical homogeneous palladium catalysts, such as PdCl₂ (PPh₃ )₂ , Pd(PPh₃ )₄ , and Pd(OAc)₂ , as well as heterogeneous palladium catalysts, such as Pd-film, Pd/Al₂ O₃ , Pd/SiO₂ , and Pd supported on polymers, can be used for these reactions.

Amphipathic monolith-supported palladium catalysts for chemoselective hydrogenation and cross-coupling reactions

Monolithic polymer-supported, effective, and reusable palladium catalysts were developed for cross-coupling reactions and hydrogenation.

Hybrid single-chain nanoparticles via the metal induced crosslinking of N-donor functionalized polymer chains

A set of single-chain nanoparticles was prepared via the intramolecular crosslinking of functionalized copolymers with various metal salts.

Nanocatalysis in Flow

Nanocatalysis in flow is catalysis by metallic nanoparticles (NPs; 1-50 nm) performed in microstructured reactors. These catalytic processes make use of the enhanced catalytic activity and selectivity of NPs and fulfill the requirements of green chemistry. Anchoring catalytically active metal NPs within a microfluidic reactor enhances the reagent/catalyst interaction, while avoiding diffusion limitations experienced in classical approaches. Different strategies for supporting NPs are reviewed herein, namely, packed-bed reactors, monolithic flow-through reactors, wall catalysts, and a selection of novel approaches (NPs embedded on nanotubes, nanowires, catalytic membranes, and magnetic NPs). Through a number of catalytic reactions, such as hydrogenations, oxidations, and cross-coupling reactions, the advantages and possible drawbacks of each approach are illustrated.

Miniaturized catalysis: monolithic, highly porous, large surface area capillary flow reactors constructed in situ from polyhedral oligomeric silsesquioxanes (POSS)

A single-step molding process utilizing free-radical cross-linking reaction of vinyl POSS in microliter-sized dimensions leads to hierarchically-structured, mechanically robust, porous hybrid structures. Functional variants show excellent performance in Suzuki-type coupling reactions. Due to their small volume, long-term operational robustness, and potential chemical diversity, these materials are promising candidates for catalyst screening applications.

Heterogenization of Pd–NHC complexes onto a silica support and their application in Suzuki–Miyaura coupling under batch and continuous flow conditions

Immobilised Pd–NHC catalysts were successfully applied in Suzuki–Miyaura reaction under batch and flow conditions.

Dendrimer-encapsulated Pd nanoparticles as catalysts for C–C cross-couplings in flow microreactors

Dendrimer-encapsulated Pd nanoparticles anchored within continuous flow microreactors are efficient for C–C cross-coupling reactions. This is witnessed by the good catalytic performance for the Heck–Cassar and Suzuki–Miyaura couplings.

Bridging homogeneous and heterogeneous catalysis with CAN·SiO2as a solid catalyst for four-component reactions for the synthesis of tetrasubstituted pyrroles

The attractive features of this methodology are newer and greener methods, simple procedures, reusability and high efficiency of the catalyst and easy workability.

The application of a monolithic triphenylphosphine reagent for conducting Ramirez gem-dibromoolefination reactions in flow

The application of a monolithic form of triphenylphosphine to the Ramirez gem-dibromoolefination reaction using flow chemistry techniques is reported. A variety of gem-dibromides were synthesised in high purity and excellent yield following only removal of solvent and no further off-line purification. It is also possible to perform the Appel reaction using the same monolith and the relationship between the mechanisms of the two reactions is discussed.

Selective hydrogenation over Pd nanoparticles supported on a pore-flow-through silica monolith microreactor with hierarchical porosity

Well-dispersed Pd nanoparticles have been synthesized inside the mesoporosity of a silica monolith featuring hierarchical porosity of homogeneous interconnected macropores (4 microns) and mesopores (11 nm). These monoliths have been implemented as microreactors for selective hydrogenation reactions. Conversion and selectivity can be tuned by adjusting the flow rates of hydrogen and substrates. In the selective hydrogenation of cyclooctadiene, a conversion of 95% and a selectivity of 90% in the monohydrogenated product, constant over a period of 70 h, have been reached. These figures correspond to a productivity of 4.2 mmol s(-1) g(-1)(MonoSil) (or 0.32 mol s(-1) g(-1)(Pd)). In the stereoselective hydrogenation of 3-hexyn-1-ol a constant conversion of 85% was observed, with however moderate selectivity into the cis isomer, over a test period of 7 h. These results open the route to the synthesis of important chemicals and intermediates via safe and green processes.

Continuous-flow hydration-condensation reaction: Synthesis of α,β-unsaturated ketones from alkynes and aldehydes by using a heterogeneous solid acid catalyst

A simple, practical and efficient continuous-flow hydration-condensation protocol was developed for the synthesis of α,β-unsaturated ketones starting from alkynes and aldehydes by employing a heterogeneous catalyst in a flow microwave. The procedure presents a straightforward and convenient access to valuable differently substituted chalcones and can be applied on multigram scale.

The application of a monolithic triphenylphosphine reagent for conducting Appel reactions in flow microreactors

Herein we describe the application of a monolithic triphenylphosphine reagent to the Appel reaction in flow-chemistry processing, to generate various brominated products with high purity and in excellent yields, and with no requirement for further off-line purification.

Triple-channel microreactor for biphasic gas-liquid reactions: Photosensitized oxygenations

A triple-channel microreactor fabricated by means of a soft-lithography technique was devised for efficient biphasic gas–liquid reactions. The excellent performance of the microreactor was demonstrated by carrying out photosensitized oxygenations of α-terpinene, citronellol, and allyl alcohols.

Scaling up of continuous-flow, microwave-assisted, organic reactions by varying the size of Pd-functionalized catalytic monoliths

A product-scalable, catalytically mediated flow system has been developed to perform Suzuki-Miyaura reactions under a microwave heating regime, in which the volumetric throughput of a Pd-supported silica monolith can be used to increase the quantity of the product without changing the optimal operating conditions. Two silica monoliths (both 3 cm long), with comparable pore diameters and surface areas, were fabricated with diameters of 3.2 and 6.4 mm to give volumetric capacities of 0.205 and 0.790 mL, respectively. The two monoliths were functionalized with a loading of 4.5 wt % Pd and then sealed in heat-shrinkable Teflon(®) tubing to form a monolithic flow reactor. The Pd-supported silica monolith flow reactor was then placed into the microwave cavity and connected to an HPLC pump and a backpressure regulator to minimize the formation of gas bubbles. The flow rate and microwave power were varied to optimize the reactant contact time and temperature, respectively. Under optimal reaction conditions the quantity of product could be increased from 31 mg per hour to 340 mg per hour simply by changing the volumetric capacity of the monolith.

A gram-scale batch and flow total synthesis of perhydrohistrionicotoxin

The total synthesis of the spiropiperidine alkaloid (-)-perhydrohistrionicotoxin (perhydro-HTX) 2 has been accomplished on a gram scale by employing both conventional batch chemistry as well as microreactor techniques. (S)-(-)-6-Pentyltetrahydro-pyran-2-one 8 underwent nucleophilic ring opening to afford the alcohol 10, which was elaborated to the nitrone 13. Protection of the nitrone as the 1,3-adduct of styrene and side-chain extension to the unsaturated nitrile afforded a precursor 17, which underwent dipolar cycloreversion and 1,3-dipolar cycloaddition to give the core spirocyclic precursor 18 that was converted into perhydro-HTX 2. The principal steps to the spirocycle 18 have successfully been transferred into flow mode by using different types of microreactors and in a telescoped fashion, allowing for a more rapid access to the histrionicotoxins and their analogues by continuous processing.

Recent advances in noble metal nanocatalysts for Suzuki and Heck cross-coupling reactions

Since metal nanoparticles have a high surface-to-volume ratio and very active surface atoms, they are very attractive catalysts for a wide variety of organic and inorganic reactions, compared to bulk catalysts. Metal nanoparticles suspended in colloidal solutions and those adsorbed onto bulk supports have been used as catalysts for a wide variety of carbon-carbon bond formation reactions such as the Suzuki and Heck cross-coupling reactions. This review article highlights some of the latest advances in the application of noble metal nanoparticles as catalysts for these two industrially important classes of cross-coupling reactions. We will discuss several important advances in using metal nanocatalysts in Suzuki and Heck cross-coupling reactions such as investigations on the nanoparticle shape dependence on the catalytic activity, novel types of supported metal nanoparticles as nanocatalysts, and the use of bi-metallic, tri-metallic and multi-metallic nanoparticles as catalysts for the Suzuki and Heck cross-coupling reactions.

Continuous flow enantioselective arylation of aldehydes with ArZnEt using triarylboroxins as the ultimate source of aryl groups

A continuous flow system for the synthesis of enantioenriched diarylmethanols from aldehydes is described. The system uses an amino alcohol-functionalized polystyrene resin as the catalyst, and the arylating agent is conveniently prepared by transmetallation of triarylboroxins with diethylzinc.