Continuous flow hydrogenation using polysilane-supported palladium/alumina hybrid catalysts

Reductive N-Alkylation of Amines with Ketones Using Heterogeneous Polysilane-Palladium Catalysts under Continuous-Flow Conditions

This work reports a continuous-flow reductive N-alkylation of amines with ketones using molecular hydrogen. The reaction, performed with highly active polysilane-modified heterogeneous palladium catalysts, enables the efficient synthesis of diversely substituted amines under mild flow conditions. The developed catalyst exhibits sustained activity for 5 days (turnover number of >2400). Moreover, the utility of the method is demonstrated by the synthesis of a key intermediate of the active pharmaceutical ingredient teneligliptin.

Hydrogen Generation from Organic Hydrides under Continuous-Flow Conditions Using Polymethylphenylsilane-Aluminum Immobilized Platinum Catalyst

Hydrogen is an important resource for realizing the goal of a hydrogen-based society as well as for synthetic organic chemistry. Catalytic dehydrogenation of organic hydrides such as methyl cyclohexane is attractive for hydrogen storage and transportation in terms of reversibility and selectivity of catalytic reactions and hydrogen storage density. We developed a highly active polymethylphenylsilane-aluminum immobilized platinum catalyst (Pt/MPPSi-Al2O3) for dehydrogenation of organic hydrides. Organic hydrides were fully converted into the corresponding aromatic compounds under reactive distillation conditions at 200 °C or under circulation-flow conditions using the Pt/MPPSi-Al2O3 catalyst packed in a column at 260 °C. The dehydrogenation reaction reached a maximum conversion at equilibrium (ca. 60%) under continuous-flow conditions at 260 °C. This catalytic continuous-flow dehydrogenation was applied to a formal hydrogen transfer from organic hydrides to unsaturated organic substrates under sequential and continuous-flow conditions for practical flow hydrogenation reactions by connecting two different heterogeneous catalysts packed in columns.

Continuous Hydrogenation: Triphasic System Optimization at Kilo Lab Scale Using a Slurry Solution

Despite their widespread use in the chemical industries, hydrogenation reactions remain challenging. Indeed, the nature of reagents and catalysts induce intrinsic safety challenges, in addition to demanding process development involving a 3-phase system. Here, to address common issues, we describe a successful process intensification study using a meso-scale flow reactor applied to a hydrogenation reaction of ethyl cinnamate at kilo lab scale with heterogeneous catalysis. This method relies on the continuous pumping of a catalyst slurry, delivering fresh catalyst through a structured flow reactor in a continuous fashion and a throughput up to 54.7 g/h, complete conversion and yields up to 99%. This article describes the screening of equipment, reactions conditions and uses statistical analysis methods (Monte Carlo/DoE) to improve the system further and to draw conclusions on the key influential parameters (temperature and residence time).

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Development of Robust Heterogeneous Chiral Rhodium Catalysts Utilizing Acid-Base and Electrostatic Interactions for Efficient Continuous-Flow Asymmetric Hydrogenations

Heterogeneous chiral Rh catalysts based on acid-base and electrostatic interactions have been developed. The robust catalysts demonstrate high activity and selectivity in the continuous-flow asymmetric hydrogenation of a wide variety of enamides and dehydroamino acids, providing optically active amides without leaching of metal species. The chiral environments can be easily tuned by changing the chiral ligands, demonstrating the high versatility of the heterogeneous catalysts. By applying these efficient catalysts, continuous synthesis of several active pharmaceutical ingredient intermediates was achieved.

Polysilane-Immobilized Rh-Pt Bimetallic Nanoparticles as Powerful Arene Hydrogenation Catalysts: Synthesis, Reactions under Batch and Flow Conditions and Reaction Mechanism

Hydrogenation of arenes is an important reaction not only for hydrogen storage and transport but also for the synthesis of functional molecules such as pharmaceuticals and biologically active compounds. Here, we describe the development of heterogeneous Rh-Pt bimetallic nanoparticle catalysts for the hydrogenation of arenes with inexpensive polysilane as support. The catalysts could be used in both batch and continuous-flow systems with high performance under mild conditions and showed wide substrate generality. In the continuous-flow system, the product could be obtained by simply passing the substrate and 1 atm H₂ through a column packed with the catalyst. Remarkably, much higher catalytic performance was observed in the flow system than in the batch system, and extremely strong durability under continuous-flow conditions was demonstrated (>50 days continuous run; turnover number >3.4 × 10⁵). Furthermore, details of the reaction mechanisms and the origin of different kinetics in batch and flow were studied, and the obtained knowledge was applied to develop completely selective arene hydrogenation of compounds containing two aromatic rings toward the synthesis of an active pharmaceutical ingredient.

Continuous-flow synthesis using a column reactor packed with heterogeneous catalysts: A convenient production of nitroolefins by using amino-functionalized silicagel

A continuous-flow synthesis of β-nitroolefins by using heterogeneous base catalysts has been developed. Although the use of an excess amount of nitro-donor such as nitromethane is required in conventional methods, nearly equimolar amounts of nitro-donors and carbonyl compounds are sufficient for high-yielding production of nitroolefins. Catalysts for this flow protocol are inexpensive and abundant, and high durability and high productivity were also realized by using an appropriate second support.

Flow "Fine" Synthesis: High Yielding and Selective Organic Synthesis by Flow Methods

The concept of flow "fine" synthesis, that is, high yielding and selective organic synthesis by flow methods, is described. Some examples of flow "fine" synthesis of natural products and APIs are discussed. Flow methods have several advantages over batch methods in terms of environmental compatibility, efficiency, and safety. However, synthesis by flow methods is more difficult than synthesis by batch methods. Indeed, it has been considered that synthesis by flow methods can be applicable for the production of simple gasses but that it is difficult to apply to the synthesis of complex molecules such as natural products and APIs. Therefore, organic synthesis of such complex molecules has been conducted by batch methods. On the other hand, syntheses and reactions that attain high yields and high selectivities by flow methods are increasingly reported. Flow methods are leading candidates for the next generation of manufacturing methods that can mitigate environmental concerns toward sustainable society.

Highly Selective Continuous-Flow Synthesis of Potentially Bioactive Deuterated Chalcone Derivatives

The selective synthesis of various dideuterochalcones as potentially bioactive deuterium-labeled products is presented, by means of the highly controlled partial deuteration of antidiabetic chalcone derivatives. The benefits of continuous-flow processing in combination with on-demand electrolytic D₂ gas generation has been exploited to avoid over-reaction to undesired side products and to achieve selective deuterium addition to the carbon-carbon double bond of the starting enones without the need for unconventional catalysts or expensive special reagents. The roles of pressure, temperature, and residence time proved crucial for the fine-tuning of the sensitive balance between the product selectivity and the reaction rate. The presented flow-chemistry-based deuteration technique lacks most of the drawbacks of the classical batch methods, and is convenient, time- and cost-efficient, and safe.

Continuous-flow hydrogenation of olefins and nitrobenzenes catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

Continuous-flow hydrogenation of olefins and nitrobenzenes with ARP-Pt.

Simple and rapid hydrogenation of p-nitrophenol with aqueous formic acid in catalytic flow reactors

The inner surface of a metallic tube (i.d. 0.5 mm) was coated with a palladium (Pd)-based thin metallic layer by flow electroless plating. Simultaneous plating of Pd and silver (Ag) from their electroless-plating solution produced a mixed distributed bimetallic layer. Preferential acid leaching of Ag from the Pd–Ag layer produced a porous Pd surface. Hydrogenation of p-nitrophenol was examined in the presence of formic acid simply by passing the reaction solution through the catalytic tubular reactors. p-Aminophenol was the sole product of hydrogenation. No side reaction occurred. Reaction conversion with respect to p-nitrophenol was dependent on the catalyst layer type, the temperature, pH, amount of formic acid, and the residence time. A porous and oxidized Pd (PdO) surface gave the best reaction conversion among the catalytic reactors examined. p-Nitrophenol was converted quantitatively to p-aminophenol within 15 s of residence time in the porous PdO reactor at 40 °C. Evolution of carbon dioxide (CO₂) was observed during the reaction, although hydrogen (H₂) was not found in the gas phase. Dehydrogenation of formic acid did not occur to any practical degree in the absence of p-nitrophenol. Consequently, the nitro group was reduced via hydrogen transfer from formic acid to p-nitrophenol and not by hydrogen generated by dehydrogenation of formic acid.

Silica-dendrimer core-shell microspheres with encapsulated ultrasmall palladium nanoparticles: efficient and easily recyclable heterogeneous nanocatalysts

We report the synthesis, characterization, and catalytic properties of novel monodisperse SiO(2)@Pd-PAMAM core-shell microspheres containing SiO(2) microsphere cores and PAMAM dendrimer-encapsulated Pd nanoparticle (Pd-PAMAM) shells. First, SiO(2) microspheres, which were prepared by the Stöber method, were functionalized with vinyl groups by grafting their surfaces with vinyltriethoxysilane (VTS). The vinyl groups were then converted into epoxides by using m-chloroperoxybenzoic acid. Upon treatment with amine-terminated G4 poly(amidoamine) (PAMAM) dendrimers, the SiO(2)-supported epoxides underwent ring-opening and gave SiO(2)@PAMAM core-shell microspheres. Pd nanoparticles within the cores of the SiO(2)-supported PAMAM dendrimers were synthesized by letting Pd(II) ions complex with the amine groups in the cores of the dendrimers and then reducing them into Pd(0) with NaBH(4). This produced the SiO(2)@Pd-PAMAM core-shell microspheres. The presence of the different functional groups on the materials was monitored by following the changes in FTIR spectra, elemental analyses, and weight losses on thermogravimetric traces. Transmission electron microscopy (TEM) images showed the presence of Pd nanoparticles with average size of 1.56 ± 0.67 nm on the surface of the monodisperse SiO(2)@Pd-PAMAM core-shell microspheres. The SiO(2)@Pd-PAMAM core-shell microspheres were successfully used as an easily recyclable catalyst for hydrogenation of various olefins, alkynes, keto, and nitro groups, giving ~100% conversion and high turnover numbers (TONs) under 10 bar H(2) pressure, at room temperature and in times ranging from 10 min to 3 h. In addition, the SiO(2)@Pd-PAMAM core-shell microspheres were proven to be recyclable catalysts up to five times with barely any leaching of palladium into the reaction mixture.