Expanding the Synthetic Toolbox through Metal-Enzyme Cascade Reactions

Lipase-Palladium Co-Catalyzed Dynamic Kinetic Resolution of Racemic Allylic Esters

Dynamic kinetic resolution (DKR) by combining lipase-catalyzed esterification of racemic sec-alcohols and in situ racemization has been widely studied; however, reports on DKR involving lipase-catalyzed hydrolysis of racemic esters are scarce. This problem is probably due to the lack of more effective and general racemization methods. Herein, we report the enhanced hydrolytic DKR of racemic allylic esters. The discovery of the monodentate ligand P[C6H3-2,6-(OMe)2]3, which in situ generates Pd complex(es) highly reactive to racemization and the NaOAc-mediated acceleration of racemization, are notable breakthroughs. Consequently, the DKR of racemic allylic esters can be completed in a few hours at 40 °C in most cases, yielding optically active allylic alcohols (93 % ee to >99 % ee) in 58-91 % isolated yields with minimal side reactions.

Tandem Switch-Triggered On-Demand Synthesis of Aromatic Amines in High Yields

Tandem catalysis stands as a beacon of chemical sustainability. Although bifunctional catalysts have achieved wide success in two-step tandem reactions, achieving multi-step catalysis with three or more distinct and potentially incompatible catalytic sites and components remains an ambitious challenge. Here, we present a "tandem switch" strategy that transforms these incompatibilities into functional advantages, enabling on-demand production of primary, secondary, and tertiary aromatic amines, all with yields exceeding 96 %. A kinetic switch, enabled by phosphotungstic acid functionalization of the Ni-Ni(Al)O heterojunction catalyst, modulates the Ni-Ni and Ni-O boundary microenvironment to simultaneously accelerate the rate-determining steps in nitrobenzene hydrogenation, N-alkylation, and aza-Michael addition. Meanwhile, a thermodynamic switch, controlled by the competitive adsorption of ethanol, hydrogen, and acrylonitrile, stepwise minimizes Gibbs free energy to ensure a seamless reaction cascade. Hence, by toggling these tandem switches on or off, we achieve selective regulation of nitrobenzene conversion pathways into the production of targeted aromatic amine. Techno-economic analysis shows the developed process significantly reduces material and energy consumption for sustainable amine production.

Topology of Gemini-shaped Hexagonal Heterojunction for Efficient Stereoconvergent Transformation via Dynamic Kinetic Resolution

Despite recent advances in the combination of kinetic resolution and racemization for efficient stereoconvergent transformation, the poor stability and limited reaction activities of the products restrict their wide application in industrial production. To overcome these problems, Gemini-shaped hexagons with para-heterojunctions for one-dimensional and two-dimensional supramolecular polymers were designed via hydrogen-bonding adhesion by racemization catalyst 1 and kinetic resolution 2 in this work. The polymers from the assembly of Gemini-shaped hexagons exhibit rapid catalytic behaviour with efficient selectivity for the desired configuration in the synthesis of tertiary alcohols with contiguous stereocenters through dynamic kinetic resolution for the nanoscale heterojunctions of dissimilar catalysts. Among them, the developed 2D polymers gave outstanding enantioselectivities and diastereoselectivities (>99 % ee, 20 : 1 dr) through the cooperation of adjacent dissimilar catalysts. The heterojunctions varying dimensions and distances of dissimilar catalysts provide new insight for increasing the enantioselectivity of chiral organocatalysts.

One-pot hydroaminomethylation of an alkene under formation of primary amines by combining hydroformylation at elevated syngas pressure and biocatalytic transamination in water

We report a novel one-pot chemoenzymatic synthesis of primary amines in water, combining rhodium-catalysed hydroformylation of styrene with a biocatalytic transamination. This process is starting from styrene at 50 mM substrate loading on a 10 mL preparative scale. Combined towards a one-pot process with both steps running concurrently, this chemoenzymatic synthesis involves a 6-DPPon/rhodium-catalysed hydroformylation of styrene at 20 bar of syngas, forming the iso- and n-aldehydes and an enzymatic transamination of the in situ-formed aldehydes to the corresponding primary amines catalysed by the amine transaminase from Chromobacterium violaceum, yielding the desired primary amines with 99% conversion.

Chemoenzymatic C,C-Bond Forming Cascades by Cryptic Vanadium Haloperoxidase Catalyzed Bromination

Inspired by natural cryptic halogenation in C,C-bond formation, this study developed a synthetic approach combining biocatalytic bromination with transition-metal-catalyzed cross-coupling. Using the cyanobacterial AmVHPO, a robust and sustainable bromination-arylation cascade was created. Genetic modifications allowed enzyme immobilization, enhancing the compatibility between biocatalysis and chemocatalysis. This mild, efficient method for synthesizing biaryl compounds provides a foundation for future biochemo cascade reactions harnessing halogenation as a traceless directing tool.

Hybrid Metal Catalysts as Valuable Tools in Organic Synthesis: An Overview of the Recent Advances in Asymmetric C─C Bond Formation Reactions

Carbon-carbon bond formation represents a key reaction in organic synthesis, resulting in paramount importance for constructing the carbon backbone of organic molecules. However, traditional metal-based catalysis, despite its advantages, often struggles with issues related to efficiency, selectivity, and sustainability. On the other hand, while biocatalysis offers superior selectivity due to an extraordinary recognition process of the substrate, the scope of its applicable reactions remains somewhat limited. In this context, Artificial Metalloenzymes (ArMs) and Metallo Peptides (MPs) offer a promising and not fully explored solution, merging the two fields of transition metal catalysis and biotransformations, by inserting a catalytically active metal cofactor into a customizable protein scaffold or coordinating the metal ion directly to a short and tunable amino acid (Aa) sequence, respectively. As a result, these hybrid catalysts have gained attention as valuable tools for challenging catalytic transformations, providing systems with new-to-nature properties in organic synthesis. This review offers an overview of recent advances in the development of ArMs and MPs, focusing on their application in the asymmetric carbon-carbon bond-forming reactions, such as carbene insertion, Michael additions, Friedel-Crafts and cross-coupling reactions, and cyclopropanation, underscoring the versatility of these systems in synthesizing biologically relevant compounds.

BioLindlar Catalyst: Ene-Reductase-Promoted Selective Bioreduction of Cyanoalkynes to Give (Z)-Cyanoalkenes

The direct synthesis of alkenes from alkynes usually requires the use of transition-metal catalysts. Unfortunately, efficient biocatalytic alternatives for this transformation have yet to be discovered. Herein, the selective bioreduction of electron-deficient alkynes to alkenes catalysed by ene-reductases (EREDs) is described. Alkynes bearing ketone, aldehyde, ester, and nitrile moieties have been effectively reduced with excellent conversions and stereoselectivities, observing clear trends for the E/Z ratios depending on the nature of the electron-withdrawing group. In the case of cyanoalkynes, (Z)-alkenes were obtained as the major product, and the reaction scope was expanded to a wide variety of aromatic substrates (up to >99 % conversion, and Z/E stereoselectivities of up to >99/1). Other alkynes containing aldehyde, ketone, or ester functionalities also proved to be excellent substrates, and interestingly gave the corresponding (E)-alkenes. Preparative biotransformations were performed on a 0.4 mmol scale, producing the desired (Z)-cyanoalkenes with good to excellent isolated yields (63-97 %). This novel reactivity has been rationalised through molecular docking by predicting the binding poses of key molecules in the ERED-pu-0006 active site.

Chemical CO2 fixation using a cyanido bridged heterometallic Zn(II)-Mn(II) 2D coordination polymer

A new cyanido bridged Zn(II)-Mn(II) mixed-metal coordination polymer, {[Zn(μ-L)(μ-CN)2Mn0.5]·(CH3OH)}n (1), has been synthesized by the reaction of Zn(CN)2, Mn(II) salts and a hydrazone ligand (HL = (E)-N'-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide) in methanol. Compound 1 was characterized using various analytical methods (including elemental analysis, photoluminescence, FT-IR, XRD, SEM, and EDX analyses, and TGA), and its structure was determined by X-ray analysis. These analyses confirmed the formation of a mixed metal Zn(II)-Mn(II) coordination polymer containing both cyanide and hydrazone bridging ligands. This mixed metal coordination polymer exhibits interesting emission spectra by having several emissions via excitation at 230, 270, 375 and 385 nm. The catalytic activity of compound 1 in chemical CO2 fixation was investigated in the presence of epoxides, and the effects of various parameters on its catalytic performance were evaluated. The results of catalytic studies show that compound 1 can efficiently catalyze the chemical CO2 fixation reaction under mild conditions. The amount of co-catalyst, temperature of the reaction, nature of the solvent and also the substituent connected to the epoxide ring are some of the important parameters that have considerable effects on the catalytic activity of 1.

Isothiourea-Catalysed Acylative Dynamic Kinetic Resolution of Tetra-substituted Morpholinone and Benzoxazinone Lactols

The development of methods to allow the selective acylative dynamic kinetic resolution (DKR) of tetra-substituted lactols is a recognised synthetic challenge. In this manuscript, a highly enantioselective isothiourea-catalysed acylative DKR of tetra-substituted morpholinone and benzoxazinone-derived lactols is reported. The scope and limitations of this methodology have been developed, with high enantioselectivity and good to excellent yields (up to 89 %, 99 : 1 er) observed across a broad range of substrate derivatives incorporating substitution at N(4) and C(2), di- and spirocyclic substitution at C(5) and C(6), as well as benzannulation (>35 examples in total). The DKR process is amenable to scale-up on a 1 g laboratory scale. The factors leading to high selectivity in this DKR process have been probed through computation, with an N-C=O⋅⋅⋅isothiouronium interaction identified as key to producing ester products in highly enantioenriched form.

One-pot chemo- and photo-enzymatic linear cascade processes

The combination of chemo- and photocatalyses with biocatalysis, which couples the flexible reactivity of the photo- and chemocatalysts with the highly selective and environmentally friendly nature of enzymes in one-pot linear cascades, represents a powerful tool in organic synthesis. However, the combination of photo-, chemo- and biocatalysts in one-pot is challenging because the optimal operating conditions of the involved catalyst types may be rather different, and the different stabilities of catalysts and their mutual deactivation are additional problems often encountered in one-pot cascade processes. This review explores a large number of transformations and approaches adopted for combining enzymes and chemo- and photocatalytic processes in a successful way to achieve valuable chemicals and valorisation of biomass. Moreover, the strategies for solving incompatibility issues in chemo-enzymatic reactions are analysed, introducing recent examples of the application of non-conventional solvents, enzyme-metal hybrid catalysts, and spatial compartmentalization strategies to implement chemo-enzymatic cascade processes.

Merging High-Pressure Syngas Metal Catalysis and Biocatalysis in Tandem One-Pot Processes for the Synthesis of Nonchiral and Chiral Alcohols from Alkenes in Water

While simultaneously proceeding reactions are among the most fascinating features of biosynthesis, this concept of tandem processes also offers high potential in the chemical industry in terms of less waste production and improved process efficiency and sustainability. Although examples of one-pot chemoenzymatic syntheses exist, the combination of completely different reaction types is rare. Herein, we demonstrate that extreme "antipodes" of the "worlds of catalysis", such as syngas-based high-pressure hydroformylation and biocatalyzed reduction, can be combined within a tandem-type one-pot process in water. No significant deactivation was found for either the biocatalyst or the chemocatalyst. A proof-of-concept for the one-pot process starting from 1-octene was established with >99 % conversion and 80 % isolated yield of the desired alcohol isomers. All necessary components for hydroformylation and biocatalysis were added to the reactor from the beginning. This concept has been extended to the enantioselective synthesis of chiral products by conducting the hydroformylation of styrene and an enzymatic dynamic kinetic resolution in a tandem mode, leading to an excellent conversion of >99 % and an enantiomeric ratio of 91 : 9 for (S)-2-phenylpropanol. The overall process runs in water under mild and energy-saving conditions, without any need for intermediate isolation.

Molecular Engineering and Morphology Control of Covalent Organic Frameworks for Enhancing Activity of Metal-Enzyme Cascade Catalysis

Metal-enzyme integrated catalysts (MEICs) that combine metal and enzyme offer great potential for sustainable chemoenzymatic cascade catalysis. However, rational design and construction of optimal microenvironments and accessible active sites for metal and enzyme in individual nanostructures are necessary but still challenging. Herein, Pd nanoparticles (NPs) and Candida antarctica lipase B (CALB) are co-immobilized into the pores and surfaces of covalent organic frameworks (COFs) with tunable functional groups, affording Pd/COF-X/CALB (X = ONa, OH, OMe) MEICs. This strategy can regulate the microenvironment around Pd NPs and CALB, and their interactions with substrates. As a result, the activity of the COF-based MEICs in catalyzing dynamic kinetic resolution of primary amines is enhanced and followed COF-OMe > COF-OH > COF-ONa. The experimental and simulation results demonstrated that functional groups of COFs modulated the conformation of CALB, the electronic states of Pd NPs, and the affinity of the integrated catalysts to the substrate, which contributed to the improvement of the catalytic activity of MEICs. Further, the MEICs are prepared using COF with hollow structure as support material, which increased accessible active sites and mass transfer efficiency, thus improving catalytic performance. This work provides a blueprint for rational design and preparation of highly active MEICs.

Strategies to Design Chemocatalytic Racemization of Tertiary Alcohols: State of the Art & Utilization for Dynamic Kinetic Resolution

The synthesis of enantiomerically pure tertiary alcohols is an important issue in organic synthesis of a range of pharmaceuticals including molecules such as the anti-HIV drug Efavirenz. A conceptually elegant approach to such enantiomers is the dynamic kinetic resolution of racemic tertiary alcohols, which, however, requires efficient racemization strategies. The racemization of tertiary alcohols is particularly challenging due to various side reactions that can occur because of their high tendency for elimination reactions. In the last few years, several complementary catalytic concepts for racemization of tertiary alcohols have been developed, characterized by efficient racemization and suppression of unwanted side-reactions. Besides resins bearing sulfonic acid moieties and a combination of boronic acid and oxalic acid as heterogeneous and homogeneous Brønsted-acids, respectively, immobilized oxovanadium and piperidine turned out to be useful catalysts. The latter two catalysts, which have already been applied to different types of substrates, also have proven good compatibility with lipase, thus leading to the first two examples of chemoenzymatic dynamic kinetic resolution of tertiary alcohols. In this review, the difficulties in racemizing tertiary alcohols are specifically described, and the recently developed complementary concepts to overcome these hurdles are summarized.

Enantioselective Synthesis of Secondary Amines by Combining Oxidative Rearrangement and Biocatalysis in a One-Pot Process

This contribution describes the development of chemoenzymatic one-pot processes, which combine an oxidative rearrangement and a biotransformation catalyzed by an imine reductase (IRED), for the synthesis of highly enantiomerically enriched secondary amines, such as an aryl-substituted pyrrolidine and a benzazepine. The benefits of this chemoenzymatic one-pot approach include high overall conversions (up to >99%), high enantiomeric excesses (up to >99% ee), and a straightforward synthetic approach toward secondary amines without the need to isolate the formed intermediate. For the initial chemical reaction, namely, the oxidative rearrangement, PhI(OAc)2 in methanol is used as a non-natural reagent, whereas the enzymatic step requires only stoichiometric amounts of d-glucose along with catalytic amounts of IRED, glucose dehydrogenase (GDH), and the cofactor NADPH. This methodology, demonstrating the compatibility of a "classic" organic synthesis using a non-natural, highly reactive reagent and a subsequent biocatalytic step, can be applied for different amines as substrates, thus making this concept a versatile tool in synthetic organic chemistry in general and for enantioselective synthesis of heterocyclic secondary amines in particular.

Manganese/Enzyme Sequential Catalytic Pathway for the Production of Optically Active γ-Functionalized Alcohols

A brief, practical catalytic process for the production of optically active γ-functionalized alcohols from relevant alkenes has been developed by using a robust Mn(III)/air/(Me2SiH)2O catalytic system combined with lipase-catalyzed kinetic resolution. This approach demonstrates exceptional tolerance toward proximal functional groups present on alkenes, enabling the achievement of high yields and exclusive enantioselectivity. Under this sequential catalytic system, the chiral alkene precursors can also be converted into γ-functionalized alcohols and related acetates as separable single enantiomers.

Design of 3D-Printed Heterogeneous Reactor Systems To Overcome Incompatibility Hurdles when Combining Metal and Enzyme Catalysis in a One-Pot Process

Combining chemo- and biocatalysis enables the design of novel economic and sustainable one-pot processes for the preparation of industrial chemicals, preferably proceeding in water. While a range of proofs-of-concept for the compatibility of such catalysts from these two different "worlds of catalysis" have recently been demonstrated, merging noncompatible chemo- and biocatalysts for joint applications within one reactor remained a challenge. A conceptual solution is compartmentalization of the catalytic moieties by heterogenization of critical catalyst components, thus "shielding" them from the complementary noncompatible catalyst, substrate or reagent. Exemplified for a one-pot process consisting of a metal-catalyzed Wacker oxidation and enzymatic reduction as noncompatible individual reactions steps, we demonstrate that making use of 3D printing of heterogeneous materials containing Cu as a critical metal component can overcome such incompatibility hurdles. The application of a 3D-printed Cu-ceramic device as metal catalyst component allows an efficient combination with the enzyme and the desired two-step transformation of styrene into the chiral alcohol product with high overall conversion and excellent enantioselectivity. This compartmentalization concept based on 3D printing of heterogenized metal catalysts represents a scalable methodology and opens up numerous perspectives to be used as a general tool also for other related chemoenzymatic research challenges.

A review of nanotechnology in enzyme cascade to address challenges in pre-treating biomass

Nanotechnology has become a revolutionary technique for improving the preliminary treatment of lignocellulosic biomass in the production of biofuels. Traditional methods of pre-treatment have encountered difficulties in effectively degrading the intricate lignocellulosic composition, thereby impeding the conversion of biomass into fermentable sugars. Nanotechnology has enabled the development of enzyme cascade processes that present a potential solution for addressing the limitations. The focus of this review article is to delve into the utilization of nanotechnology in the pretreatment of lignocellulosic biomass through enzyme cascade processes. The review commences with an analysis of the composition and structure of lignocellulosic biomass, followed by a discussion on the drawbacks associated with conventional pre-treatment techniques. The subsequent analysis explores the importance of efficient pre-treatment methods in the context of biofuel production. We thoroughly investigate the utilization of nanotechnology in the pre-treatment of enzyme cascades across three distinct sections. Nanomaterials for enzyme immobilization, enhanced enzyme stability and activity through nanotechnology, and nanocarriers for controlled enzyme delivery. Moreover, the techniques used to analyse nanomaterials and the interactions between enzymes and nanomaterials are introduced. This review emphasizes the significance of comprehending the mechanisms underlying the synergy between nanotechnology and enzymes establishing sustainable and environmentally friendly nanotechnology applications.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Stereodivergent Protein Engineering of Fatty Acid Photodecarboxylase for Light-Driven Kinetic Resolution of Sec-Alcohol Oxalates

Stereodivergent engineering of one enzyme to create stereocomplementary variants for synthesizing optically pure molecules with tailor-made (R) or (S) configurations on an optional basis is highly desirable and challenging. This study aimed to engineer fatty acid photodecarboxylase from Chlorella variabilis (CvFAP) using the focused rational iterative site-specific mutagenesis (FRISM) strategy to obtain two highly stereocomplementary variants with excellent selectivity (both giving products with up to 99 % e.e.). These variants were used for the CvFAP-catalyzed light-driven kinetic resolution of oxalates or oxamic acids prepared from the corresponding sec-alcohols or amines, providing a new biotransformation process for preparing chiral sec-alcohols and amines. Molecular dynamics simulation, kinetic data and transient spectra revealed the source of selectivity. This study represents the first example of the kinetic resolution of sec-alcohols or amines catalyzed by a pair of stereocomplementary CvFAPs.

Cell-free chemoenzymatic cascades with bio-based molecules

For the valorization of various bio-based feedstocks, the combination of different catalytic systems with biocatalysis in chemoenzymatic cascades has been shown to have high potential. However, the development of such integrated catalytic systems is often limited by catalyst incompatibility. Therefore, incorporating novel catalytic concepts into the chemoenzymatic valorization of bio-based feedstocks is currently of great interest. This article provides an overview of the methods/approaches used to advance the development of chemoenzymatic cascades for the catalytic upgrading of bio-based feedstocks. It specifically focuses on recent developments in the combination of enzymes with organo- and chemocatalysis. Furthermore, current applications and future perspectives of integrating novel catalytic systems such as photo- and electrocatalysis toward new synthetic routes for the utilization of the often highly functionalized bio-based compounds are reviewed.

Carbonyl Olefin Metathesis and Dehydrogenative Cyclization of Aromatic Ketones and gem-Difluoroalkenes

The beauty of one-pot cascade reaction lies in the efficient disconnection and construction of several bonds in a single reaction flask, without the isolation of any intermediates. Herein, we report the first photoinduced thermally promoted cascade reactions of readily available aromatic ketones and aromatic gem-difluoroalkenes for the synthesis of phenanthrenes which possess potential utility in drug design and materials science. The reaction combines carbonyl-olefin metathesis (cascade photoinduced [2+2] cyclization and thermally controlled retro [2+2] cyclization) and dehydrogenative cyclization (cascade photoinduced conrotatory 6π electrocyclization and collidine-promoted dehydrogenative aromatization) together in one pot. The oxidant-free, acid-free and metal-free reaction shows broad substrate scope and wide functional group tolerance.

From nature to industry: Harnessing enzymes for biocatalysis

Biocatalysis harnesses enzymes to make valuable products. This green technology is used in countless applications from bench scale to industrial production and allows practitioners to access complex organic molecules, often with fewer synthetic steps and reduced waste. The last decade has seen an explosion in the development of experimental and computational tools to tailor enzymatic properties, equipping enzyme engineers with the ability to create biocatalysts that perform reactions not present in nature. By using (chemo)-enzymatic synthesis routes or orchestrating intricate enzyme cascades, scientists can synthesize elaborate targets ranging from DNA and complex pharmaceuticals to starch made in vitro from CO2-derived methanol. In addition, new chemistries have emerged through the combination of biocatalysis with transition metal catalysis, photocatalysis, and electrocatalysis. This review highlights recent key developments, identifies current limitations, and provides a future prospect for this rapidly developing technology.