Heterogeneous Versus Homogeneous Palladium Catalysts for Ligandless Mizoroki-Heck Reactions: A Comparison of Batch/Microwave and Continuous-Flow Processing

Support Engineering for the Stabilisation of Heterogeneous Pd3 P-Based Catalysts for Heck Coupling Reactions

Herein we report the use of a supported Pd3 P catalyst for Heck coupling reactions. For the stabilisation of Pd3 P and Pd, as reference system, the silica support material was modified via phosphorus doping (0.5 and 1 wt % P). Through this so-called support engineering approach, the catalytic activity of Pd3 P was clearly enhanced. Whereas an iodobenzene conversion of 79 % was witnessed for Pd3 P@SiO2 in the coupling of styrene and iodobenzene in 1 h, 90 % conversion could be achieved using Pd3 P@1P-SiO2 . This improved catalytic activity probably stems from an electronic modulation of the support surface via the introduction of phosphorus. Simultaneously, the recyclability was boosted and the Pd3 P@1P-SiO2 catalyst has shown to maintain its catalytic activity over several recovery tests. Hereby, metal leaching could almost be suppressed completely to 3 % by the use of a P-modified silica support.

Microwave-Assisted Synthesis: Can Transition Metal Complexes Take Advantage of This “Green” Method?

Microwave-assisted synthesis is considered environmental-friendly and, therefore, in agreement with the principles of green chemistry. This form of energy has been employed extensively and successfully in organic synthesis also in the case of metal-catalyzed synthetic procedures. However, it has been less widely exploited in the synthesis of metal complexes. As microwave irradiation has been proving its utility as both a time-saving procedure and an alternative way to carry on tricky transformations, its use can help inorganic chemists, too. This review focuses on the use of microwave irradiation in the preparation of transition metal complexes and organometallic compounds and also includes new, unpublished results. The syntheses of the compounds are described following the group of the periodic table to which the contained metal belongs. A general overview of the results from over 150 papers points out that microwaves can be a useful synthetic tool for inorganic chemists, reducing dramatically the reaction times with respect to traditional heating. This is often accompanied by a more limited risk of decomposition of reagents or products by an increase in yield, purity, and (sometimes) selectivity. In any case, thermal control is operative, whereas nonthermal or specific microwave effects seem to be absent.

Access to hexahydroazepinone heterocycles via palladium-catalysed C(sp3)-H alkenylation/ring-opening of cyclopropanes

In this communication, we describe the synthesis of novel hexahydroazepinone derivatives starting from two simple building blocks in presence of a readily available palladium catalyst. The reaction proceeds through a selective C(sp³)-H alkenylation/ring-opening process to obtain the seven-membered ring products in good to excellent yields on a wide variety of substrates under batch, microwave, and continuous flow conditions.

In situ supported Pd NPs on biodegradable chitosan/agarose modified magnetic nanoparticles as an effective catalyst for the ultrasound assisted oxidation of alcohols and activities against human breast cancer

In this content, a green approach for the ultrasound promoted in situ immobilization of Pd NPs over biodegradable chitosan/agarose modified ferrite NP (Fe₃O₄@CS-Agarose/Pd) is developed. The structural and physicochemical features of the material were estimated using advanced analytical techniques like FT-IR, ICP-OES, FESEM, EDS, XRD, TEM and VSM. The magnetic material was catalytically explored in the oxidation of alcohols under ultrasonic waves. Sonication had a significant role in enhancing the catalytic performance in the alcohol's oxidation as compared to conventional heating. The heterogeneous nanocatalyst was efficiently recycled up to 10 times with nominal loss in catalytic activity. Towards the biological applications, the Fe₃O₄@CS-Agarose/Pd nanocomposite showed high antioxidant activities against DPPH free radicals, comparable to standard butylated hydroxytoluene (BHT). In addition, it exhibited excellent cytotoxicity in terms of % cell viability against breast adenocarcinoma (MCF7), breast carcinoma (Hs 578Bst), infiltrating ductal cell carcinoma (Hs 319.T), and metastatic carcinoma (MDA-MB-453) cell lines. The best anti-breast cancer potential of the nanocomposite was observed in Hs 319.T cell line.

Synthesis and antiproliferative activity of 6-naphthylpterocarpans

The Heck-oxyarylation of racemic 2-(1-naphthyl)- and 2-(2-naphthyl)-2H-chromene derivatives were carried out resulting diastereoselectively in (6S*,6aR*,11aR*)-6-(1-naphthyl)- and 6-(2-naphthyl)-pterocarpans as major products and bridged (6R*,12R*)-6,12-methanodibenzo[d,g][1,3]dioxocine derivatives as minor products. Antiproliferative activity of two 6-naphthylpterocarpans was identified by MTT assay against A2780 and WM35 human cancer cell lines with low micromolar IC50 values. The measured 0.80 and 3.51 μM IC50 values of the (6S*,6aR*,11aR*)-6-(1-naphthyl)pterocarpan derivative with 8,9-methylenedioxy substitution represent the best activities in the pterocarpan family. Enantiomers of the pterocarpan and dioxocine derivatives and their chiral 2-naphthylchroman-4-one and 2-naphthyl-2H-chromene precursors were separated by HPLC using chiral stationary phase. HPLC-ECD spectra were recorded and absolute configuration and low-energy solution conformations were determined by TDDFT-ECD calculations. Characteristic ECD transitions of the separated enantiomers were correlated with their absolute configuration.

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.

Microwave-assisted synthesis of palladium nanoparticles supported on copper oxide in aqueous medium as an efficient catalyst for Suzuki cross-coupling reaction

We report here a reliable green method for the synthesis of palladium nanoparticles supported on copper oxide as a highly active and efficient catalyst for Suzuki cross-coupling reaction. The experimental synthetic approach is based on microwave-assisted chemical reduction of an aqueous mixture of palladium and copper salt simultaneously using hydrazine hydrate as reducing agent. The catalyst was fully characterized using various techniques showing well-dispersed palladium nanoparticles. The catalytic activity and recyclability of the prepared catalyst were experimentally explored in the ligand-free Suzuki cross-coupling reaction with a diverse series of functionalized substrates. The synthesized Pd/CuO catalyst shows many advantages beside its high catalytic efficiency such as the recyclability of up to five times with negligible loss of catalytic activity, short reaction times, use of environmentally benign solvent systems, and mild reaction conditions.

Heterogeneous Pd catalysts as emulsifiers in Pickering emulsions for integrated multistep synthesis in flow chemistry

Within the “compartmentalised smart factory” approach of the ONE-FLOW project the implementation of different catalysts in “compartments” provided by Pickering emulsions and their application in continuous flow is targeted. We present here the development of heterogeneous Pd catalysts that are ready to be used in combination with biocatalysts for catalytic cascade synthesis of active pharmaceutical ingredients (APIs). In particular, we focus on the application of the catalytic systems for Suzuki–Miyaura cross-coupling reactions, which is the key step in the synthesis of the targeted APIs valsartan and sacubitril. An immobilised enzyme will accomplish the final product formation via hydrolysis. In order to create a large interfacial area for the catalytic reactions and to keep the reagents separated until required, the catalyst particles are used to stabilise Pickering emulsions of oil and water. A set of Ce–Sn–Pd oxides with the molecular formula Ce_0.99−xSn_xPd_0.01O_2−δ (x = 0–0.99) has been prepared utilising a simple single-step solution combustion method. The high applicability of the catalysts for different functional groups and their minimal leaching behaviour is demonstrated with various Suzuki–Miyaura cross-coupling reactions in batch as well as in continuous flow employing the so-called “plug & play reactor”. Finally, we demonstrate the use of these particles as the sole emulsifier of oil–water emulsions for a range of oils.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

Investigation of Selective Microwave Heating Phenomena in the Reactions of 2-Substituted Pyridines

Debated selective microwave heating effects were investigated in a rearrangement and a benzylation reaction involving 2-substituted pyridines. An accurate, reproducible comparison technique and simultaneous temperature measurement using both external infrared and internal fibre optic sensors were utilized. The experimental details of the benzylation reaction were thoroughly addressed to resolve the inconsistencies that have been discussed previously in the literature. Hidden inhomogeneities in temperature and concentration were revealed within the reaction mixtures during microwave heating, which could be prevented by the modification of the vessel wall using an inert fluoropolymer liner. Instead of the previously proposed microscopic thermal microwave effect, the enhanced reaction rate could be explained by macroscopic-scale localized heating in the boundary layers close to the vessel surface in the microwave-heated experiment.

Chitosan microspheres-supported palladium species as an efficient and recyclable catalyst for Mizoroki–Heck reaction

Chitosan microsphere supported palladium catalysts prepared by electrospraying were very active and stable to catalyze the Mizoroki–Heck reaction.

Encapsulation of Homogeneous Catalysts in Porous Polymer Nanocapsules Produces Fast-Acting Selective Nanoreactors

Nanoreactors were created by entrapping homogeneous catalysts in hollow nanocapsules with 200 nm diameter and semipermeable nanometer-thin shells. The capsules were produced by the polymerization of hydrophobic monomers in the hydrophobic interior of the bilayers of self-assembled surfactant vesicles. Controlled nanopores in the shells of nanocapsules ensured long-term retention of the catalysts coupled with the rapid flow of substrates and products in and out of nanocapsules. The study evaluated the effect of encapsulation on the catalytic activity and stability of five different catalysts. Comparison of kinetics of five diverse reactions performed in five different solvents revealed the same reaction rates for free and encapsulated catalysts. Identical reaction kinetics confirmed that placement of catalysts in the homogeneous interior of polymer nanocapsules did not compromise catalytic efficiency. Encapsulated organometallic catalysts showed no loss of metal ions from nanocapsules suggesting stabilization of the complexes was provided by nanocapsules. Controlled permeability of the shells of nanocapsules enabled size-selective catalytic reactions.

Immobilized Pd(0) nanoparticles on phosphine-functionalized graphene as a highly active catalyst for Heck, Suzuki and N-arylation reactions

Phosphine functionalized graphene was used for the immobilization of Pd nanoparticles and utilized in C–C and C–N bond formation reactions.

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.

Microwave-promoted solventless Mizoroki–Heck reactions catalysed by Pd nanoparticles supported on laponite clay

Palladium nanoparticles supported on laponite efficiently catalyse solventless Mizoroki–Heck reactions activated by microwaves. High yields are obtained in a few minutes and the catalyst can be efficiently recovered and reused up to thirteen times.

A magnetic porous chitosan-based palladium catalyst: a green, highly efficient and reusable catalyst for Mizoroki–Heck reaction in aqueous media

We have prepared a highly active and recyclable heterogeneous catalyst for the Heck reaction in aqueous media in high yields.

Aerobic flow oxidation of alcohols in water catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

Various alcohols were aerobically oxidized in an aqueous solution in a continuous-flow reactor containing a platinum nanoparticles dispersed in a polystyrene–poly(ethylene glycol) resin to give the corresponding carbonyl compounds in up to 99% yield.

Novel chitosan-based/montmorillonite/palladium hybrid microspheres as heterogeneous catalyst for Sonogashira reactions

The objective of this study was to develop novel chitosan-based/montmorillonite/palladium (CS/MMT/Pd) hybrid microsphere catalysts with improved properties for use in Sonogashira reactions.

Thermoregulated Aqueous Biphasic Catalysis of Sonogashira Reactions

A water-based thermoregulated system for Pd-catalyzed Sonogashira reactions is presented, which allows for not only a highly efficient homogeneous catalytic reaction, but also an easy separation/recovery of the catalyst. The novel catalytic system exhibits high efficiency and excellent reusability. In addition, the Sonogashira reactions are performed with Pd(OAc)2 without a copper co-catalyst.