Recent Advances in Continuous-Flow Enantioselective Catalysis

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

Deciphering the immobilization of lipases on hydrophobic wrinkled silica nanoparticles

In this work, the adsorption of Candida antarctica B (CALB) and Rhizomucor miehei (RML) lipases into hydrophobic wrinkled silica nanoparticles (WSNs) is investigated. WSNs are hydrophobized by chemical vapor deposition. Both proteins are homogeneously distributed inside the pores of the nanoparticles, as confirmed by Transmission Electron Microscopy and Energy Dispersive X-ray measurements. The maximum enzyme load of CALB is twice that obtained for RML. Fourier Transform Infrared Spectroscopy confirms the preservation of the enzyme secondary structure after immobilization for both enzymes. Adsorption isotherms fit to a Langmuir model, resulting in a binding constant (KL) for RML 4.5-fold higher than that for CALB, indicating stronger binding for the former. Kinetic analysis reveals a positive correlation between enzyme load and RML activity unlike CALB where activity decreases along the enzyme load increases. Immobilization allows for enhancing the thermal stability of both lipases. Finally, CALB outperforms RML in the hydrolysis of ethyl-3-hydroxybutyrate. However, immobilized CALB yielded 20 % less 3-HBA than free lipase, while immobilized RML increases 3-fold the 3-HBA yield when compared with the free enzyme. The improved performance of immobilized RML can be explained due to the interfacial hyperactivation undergone by this lipase when immobilized on the superhydrophobic surface of WSNs.

Reversal of Regioselectivity in Asymmetric C-H Bond Annulation with Bromoalkynes under Cobalt Catalysis

Metal-catalyzed asymmetric C-H bond annulation strategy offers a versatile platform, allowing the construction of complex P-chiral molecules through atom- and step-economical fashion. However, regioselective insertion of π-coupling partner between M-C bond with high enantio-induction remain elusive. Using commercially available Co(II) salt and chiral-Salox ligands, we demonstrate an unusual protocol for the regio-reversal, enantioselective C-H bond annulation of phosphinamide with bromoalkyne through desymmetrization. The reaction proceeds through ligand-assisted enantiodetermining cyclocobaltation followed by regioselective insertion of bromoalkyne between Co-C, subsequent reductive elimination, and halogen exchange with carboxylate resulted in P-stereogenic compounds in excellent ee (up to >99 %). The isolation of cobaltacycle involved in the catalytic cycle and the outcome of control experiments provide support for a plausible mechanism.

Continuous-Flow Enantioselective Hydroacylations under Heterogeneous Chiral Rhodium Catalysts

Transition metal-catalyzed enantioselective C-H bond functionalizations have become efficient methods for the synthesis of complex optically active molecules. Heterogeneous catalysts for this chemistry remain largely unexplored despite the advantages they offer in terms of ease of separation and reuse of catalysts. Herein, we report the development of heterogeneous chiral Rh catalysts for continuous-flow enantioselective hydroacylations. Heterogeneous catalysts could be prepared simply by mixing supports and Rh complexes. The prepared catalysts exhibited excellent activity and enantioselectivity affording optically active ketones in quantitative yields with 99 % ee's. Under the optimized reaction conditions, a turnover number >300 was achieved without the leaching of Rh species. The catalysts exhibited a wide substrate scope and in sequential-flow reactions with other heterogeneous catalysts, the syntheses of biologically active molecules and functional materials were demonstrated.

Stereoselective [2 + 2] photodimerization: a viable strategy for the synthesis of enantiopure cyclobutane derivatives

The [2 + 2] photodimerization of cinnamic acid derivatives to afford enantiopure cyclobutanes has been investigated. The use of a chiral auxiliary represents a convenient and straightforward method to exert enantiocontrol on the reaction. By exploiting Evans oxazolidinones, the stereoselective light-driven cyclisation affords a functionalised cyclobutane ring with up to 99% enantiocontrol after removing the chiral auxiliary. In-flow experiments allowed us to improve further the efficiency of the methodology, leading to high conversion and excellent enantioselectivity.

A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)Ir(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir(cod)py][SZO] (1) and Me₃SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir(cod)(OSi(O^tBu)₃]₂, which forms [Ir(cod)P(^tBu)₂Ph][SZO] (2), followed by reaction with pyridine to form 1. FTIR and ¹⁵N{¹H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.

Enantioselective Organophotocatalytic Telescoped Synthesis of a Chiral Privileged Active Pharmaceutical Ingredient

The continuous flow, enantioselective, organophotoredox catalytic asymmetric alkylation of aldehydes was studied, by using a homemade, custom‐designed photoreactor for reactions under cryogenic conditions. Going from microfluidic conditions up to a 10 mL mesofluidic reactor, an increase of productivity by almost 18000 % compared to the batch reaction was demonstrated. Finally, for the first time, a stereoselective photoredox organocatalytic continuous flow reaction in a fully telescoped process for an active pharmaceutical ingredient (API)synthesis was successfully achieved. The final process consists of four units of operation: visible light‐driven asymmetric catalytic benzylation under continuous flow, inline continuous work‐up, neutralisation and a final oxidative amidation step afforded the pharmaceutically active molecule in 95 % e.e.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Rxn Rover: automation of chemical reactions with user-friendly, modular software

Automation of chemical reactions through tools such as Rxn Rover in research and development is an enabling technology to reduce cost and waste management in technology transformations towards renewable feedstocks and energy in the chemical industry.

An enantio- and diastereoselective approach to indoloquinolizidines in continuous flow

A solvent-free enantioselective Michael addition mediated by a polymer-supported Jørgensen–Hayashi catalyst and a domino Pictet–Spengler plus lactamisation sequence has been reported in continuous flow.

Chiral Heterogeneous Scandium Lewis Acid Catalysts for Continuous-Flow Enantioselective Friedel-Crafts Carbon-Carbon Bond-Forming Reactions

While continuous-flow reactions with chiral heterogeneous catalysts provide a highly efficient method to synthesize optically active compounds, chiral heterogeneous Lewis acid catalysis has been less extensively explored. We have developed the first example of chiral heterogeneous Sc catalysts, which demonstrated excellent activity and selectivity for continuous-flow enantioselective Friedel-Crafts reactions of isatins with indoles. Noncovalent interactions between chiral Sc complexes and heteropoly acid-anchored amine-functionalized SiO₂ as support were utilized for the synthesis. The heteropoly acid was found to be crucial for the preparation, activity, and selectivity of the catalysts. The chiral ligand could be easily tuned without chemical modification and the continuous-flow synthesis of a biologically active compound was achieved.

Immobilisation and flow chemistry: tools for implementing biocatalysis

The merger of enzyme immobilisation and flow chemistry has attracted the attention of the scientific community during recent years. Immobilisation enhances enzyme stability and enables recycling, flow chemistry allows process intensification. Their combination is desirable for the development of more efficient and environmentally friendly biocatalytic processes. In this feature article, we aim to point out important metrics for successful enzyme immobilisation and for reporting flow biocatalytic processes. Relevant examples of immobilised enzymes used in flow systems in organic, biphasic and aqueous systems are discussed. Finally, we describe recent developments to address the cofactor recycling hurdle.

Continuous Flow Synthesis of α-Trifluoromethylthiolated Esters and Amides from Carboxylic Acids: a Telescoped Approach

A continuous flow approach to access α-trifluoromethylthiolated esters and amides using commercially available arylacetic acids and N-(trifluoromethylthio)phthalimide as the electrophilic reagent is described. The experimental protocol involves the in-flow conversion of the carboxylic acid into N-acylpyrazole followed by the α-trifluoromethylthiolation in a PTFE coil reactor and final reaction with primary or secondary amines, or alcohols, to afford in a telescoped process α-substituted SCF₃ amides and esters, respectively, in good overall yield and short reaction times.

Continuous flow asymmetric synthesis of chiral active pharmaceutical ingredients and their advanced intermediates

Catalytic enantioselective transformations provide well-established and direct access to stereogenic synthons that are broadly distributed among active pharmaceutical ingredients (APIs). These reactions have been demonstrated to benefit considerably from the merits of continuous processing and microreactor technology. Over the past few years, continuous flow enantioselective catalysis has grown into a mature field and has found diverse applications in asymmetric synthesis of pharmaceutically active substances. The present review therefore surveys flow chemistry-based approaches for the synthesis of chiral APIs and their advanced stereogenic intermediates, covering the utilization of biocatalysis, organometallic catalysis and metal-free organocatalysis to introduce asymmetry in continuously operated systems. Single-step processes, interrupted multistep flow syntheses, combined batch/flow processes and uninterrupted one-flow syntheses are discussed herein.

Editorial Catalysts: Supported Metal Catalysts and Their Applications in Fine Chemicals

Heterogeneous catalysis is an essential tool for the development of both emerging and established chemical processes, as well as for their economic and environmental sustainability [...]

Nitrogen-Doped Carbon Enables Heterogeneous Asymmetric Insertion of Carbenoids into Amines Catalyzed by Rhodium Nanoparticles

Development of stable heterogeneous catalyst systems is a crucial subject to achieve sustainable society. Though metal nanoparticles are robust species, the study of asymmetric catalysis by them has been restricted because methods to activate metal nanoparticles without causing metal leaching were limited. We developed Rh nanoparticle catalysts (NCI-Rh) supported on nitrogen-doped carbon as a solid ligand to interact with metals for asymmetric insertion of carbenoids into N-H bonds cocatalyzed by chiral phosphoric acid. Nitrogen dopants played a crucial role in both catalytic activity and enantioselectivity while almost no catalysis was observed with Rh nanoparticles immobilized on supports without nitrogen dopants. Various types of chiral α-amino acid derivatives were synthesized in high yields with high enantioselectivities and NCI-Rh could be reused in seven runs. Furthermore, we demonstrated the corresponding continuous-flow reaction using a column packed with NCI-Rh. The desired product was obtained efficiently for over 90 h through the reactivation of NCI-Rh and the chiral source could be recovered.

Horeau amplification in the sequential acylative kinetic resolution of (±)-1,2-diols and (±)-1,3-diols in flow

The sequential acylative kinetic resolution (KR) of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols using a packed bed microreactor loaded with the polystyrene-supported isothiourea, HyperBTM, is demonstrated in flow. The sequential KRs of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols exploits Horeau amplification, with each composed of two successive KR processes, with each substrate class significantly differing in the relative rate constants for each KR process. Optimisation of the continuous flow set-up for both C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diol substrate classes allowed isolation of reaction products in both high enantiopurity and yield. In addition to the successful KR of C2-symmetric (±)-1,2-syn and (±)-1,3-anti-diols, the application of this process to the more conceptually-complex scenario involving the sequential KR of C1-symmetric (±)-1,3-anti-diols was demonstrated, which involves eight independent rate constants.

Recent Advances in Continuous-Flow Reactions Using Metal-Free Homogeneous Catalysts

Developments that result in high-yielding, low-cost, safe, scalable, and less-wasteful processes are the most important goals in synthetic organic chemistry. Continuous-flow reactions have garnered much attention due to many advantages over conventional batch reactions that include precise control of short reaction times and temperatures, low risk in handling dangerous compounds, and ease in scaling up synthesis. Combinations of continuous-flow reactions with homogeneous, metal-free catalysts further enhances advantages that include low-cost and ready availability, low toxicity, higher stability in air and water, and increased synthetic efficiency due to the avoidance of the time-consuming removal of toxic metal traces. This review summarizes recently reported continuous-flow reactions using metal-free homogeneous catalysts and classifies them either as acidic catalysts, basic catalysts, or miscellaneous catalysts. In addition, we compare the results between continuous-flow conditions and conventional batch conditions to reveal the advantages of using flow reactions with metal-free homogeneous catalysts.

Telescoped Continuous Flow Synthesis of Optically Active γ-Nitrobutyric Acids as Key Intermediates of Baclofen, Phenibut, and Fluorophenibut

The two-step flow asymmetric synthesis of chiral γ-nitrobutyric acids as key intermediates of the GABA analogues baclofen, phenibut, and fluorophenibut is reported on a multigram scale. The telescoped process comprises an enantioselective Michael-type addition facilitated by a polystyrene-supported heterogeneous organocatalyst under neat conditions followed by in situ-generated performic acid-mediated aldehyde oxidation. Simple access to valuable optically active substances is provided with key advances in terms of productivity and sustainability compared to those of previous batch approaches.

Isothiourea-Catalyzed Acylative Kinetic Resolution of Tertiary α-Hydroxy Esters

A highly enantioselective isothiourea-catalyzed acylative kinetic resolution (KR) of acyclic tertiary alcohols has been developed. Selectivity factors of up to 200 were achieved for the KR of tertiary alcohols bearing an adjacent ester substituent, with both reaction conversion and enantioselectivity found to be sensitive to the steric and electronic environment at the stereogenic tertiary carbinol centre. For more sterically congested alcohols, the use of a recently-developed isoselenourea catalyst was optimal, with equivalent enantioselectivity but higher conversion achieved in comparison to the isothiourea HyperBTM. Diastereomeric acylation transition state models are proposed to rationalize the origins of enantiodiscrimination in this process. This KR procedure was also translated to a continuous-flow process using a polymer-supported variant of the catalyst.

Recent Advances in Asymmetric Iron Catalysis

Asymmetric transition-metal catalysis represents a fascinating challenge in the field of organic chemistry research. Since seminal advances in the late 60s, which were finally recognized by the Nobel Prize to Noyori, Sharpless and Knowles in 2001, the scientific community explored several approaches to emulate nature in producing chiral organic molecules. In a scenario that has been for a long time dominated by the use of late-transition metals (TM) catalysts, the use of 3d-TMs and particularly iron has found, recently, a widespread application. Indeed, the low toxicity and the earth-abundancy of iron, along with its chemical versatility, allowed for the development of unprecedented and more sustainable catalytic transformations. While several competent reviews tried to provide a complete picture of the astounding advances achieved in this area, within this review we aimed to survey the latest achievements and new concepts brought in the field of enantioselective iron-catalyzed transformations.

Immobilization of Arabidopsis thaliana Hydroxynitrile Lyase (AtHNL) on EziG Opal

Arabidopsis thaliana hydroxynitrile lyase (AtHNL) catalyzes the selective synthesis of (R)-cyanohydrins. This enzyme is unstable under acidic conditions, therefore its immobilization is necessary for the synthesis of enantiopure cyanohydrins. EziG Opal is a controlled porosity glass material for the immobilization of His-tagged enzymes. The immobilization of His6-tagged AtHNL on EziG Opal was optimized for higher enzyme stability and tested for the synthesis of (R)-mandelonitrile in batch and continuous flow systems. AtHNL-EziG Opal achieved 95% of conversion after 30 min of reaction time in batch and it was recycled up to eight times with a final conversion of 80% and excellent enantioselectivity. The EziG Opal carrier catalyzed the racemic background reaction; however, the high enantioselectivity observed in the recycling study demonstrated that this was efficiently suppressed by using citrate/phosphate buffer saturated methyl-tert-butylether (MTBE) pH 5 as reaction medium. The continuous flow system achieved 96% of conversion and excellent enantioselectivity at 0.1 mL min−1. Lower conversion and enantioselectivity were observed at higher flow rates. The specific rate of AtHNL-EziG Opal in flow was 0.26 mol h−1 genzyme−1 at 0.1 mL min−1 and 96% of conversion whereas in batch, the immobilized enzyme displayed a specific rate of 0.51 mol h−1 genzyme−1 after 30 min of reaction time at a similar level of conversion. However, in terms of productivity the continuous flow system proved to be almost four times more productive than the batch approach, displaying a space-time-yield (STY) of 690 molproduct h−1 L−1 genzyme−1 compared to 187 molproduct h−1 L−1 genzyme−1 achieved with the batch system.

Asymmetric copper-catalyzed fluorination of cyclic β-keto esters in a continuous-flow microreactor

A highly enantioselective homogeneous fluorination of cyclic β-keto esters catalyzed by diphenylamine linked bis(oxazoline)-Cu(OTf)2 complexes has been established in a continuous flow microreactor. The microreactor allowed an efficient transformation with reaction times ranging from 0.5 to 20 min, and the desired products were afforded in high yields (up to 99%) with excellent enantioselectivities (up to 99% ee) at a low catalyst loading of 1 mol%.

Recent advances in continuous-flow organocatalysis for process intensification

The progresses on continuous-flow organocatalysis from 2016 to early 2020 are reviewed with focus on transition from batch to flow.

The rise of continuous flow biocatalysis – fundamentals, very recent developments and future perspectives

Very recent developments in the field of biocatalysis in continuously operated systems. Special attention on the future perspectives in this key emerging technological area ranging from process analytical technologies to digitalization.

A metal-free synthesis of 1,1-diphenylvinylsulfides with thiols via thioetherification under continuous-flow conditions

A continuous-flow chemistry facilitated protocol that allows efficient access to a novel aggregation-induced emission (AIE) luminogen 1,1-diphenylvinylsulfides utilizing thiols under metal-free and mild conditions.