Non‐Conventional Media as Strategy to Overcome the Solvent Dilemma in Chemoenzymatic Tandem Catalysis

Multifunctional Catalysts for Cascade Reactions in Biomass Processing

Multifunctional catalysts have received considerable attention in the cascade reactions of biomass processing. A cascade (or tandem) reaction is realized when multiple reaction steps that require different catalysts are performed in a one-step process. These reactions require bi- or multifunctional catalysts or catalyst mixtures to serve successfully at each reaction step. In this review article, we discuss the major factors of the catalyst design influencing the structure-property relationships, which could differ depending on the catalyst type. The major factors include the amounts and strengths of acidic and basic sites, interactions between those and metal sites, synergetic effects, nanoparticle sizes and morphology, nanostructures, porosity, etc. The catalysts described in this review are based on zeolites, mesoporous solids, MOFs, and enzymes. The importance of continuous cascade processes is also examined.

Redox Biocatalysis in Lidocaine-Based Hydrophobic Deep Eutectic Solvents: Non-Conventional Media Outperform Aqueous Conditions

Redox biocatalysis is an essential pillar of the chemical industry. Yet, the enzymes' nature restricts most reactions to aqueous conditions, where the limited substrate solubility leads to unsustainable diluted biotranformations. Non-aqueous media represent a strategic solution to conduct intensified biocatalytic routes. Deep eutectic solvents (DESs) are designable solvents that can be customized to meet specific application needs. Within the large design space of combining DES components (and ratios), hydrophobic DESs hold the potential to be both enzyme-compatible - keeping the enzymes' hydration -, and solubilizers for hydrophobic reactants. We explored two hydrophobic DESs, lidocaine/oleic acid, and lidocaine/decanoic acid, as reaction media for carbonyl reduction catalyzed by horse liver alcohol dehydrogenase, focusing on the effect of water contents and on maximizing substrate loadings. Enzymes remained highly active and stable in the DESs with 20 wt % buffer, whereas the reaction performance in DESs outperformed the pure buffer system with hydrophobic substrates (e. g., cinnamaldehyde to form the industrially relevant cinnamyl alcohol), with a 3-fold specific activity. Notably, the cinnamaldehyde reduction was for the first time performed at 800 mM (~100 g L-1) with full conversion, which opens up new avenues to industrial applications of hydrophobic DESs for enzyme catalysis.

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.

Corner Engineering: Tailoring Enzymes for Enhanced Resistance and Thermostability in Deep Eutectic Solvents

Deep eutectic solvents (DESs), heralded for their synthesis simplicity, economic viability, and reduced volatility and flammability, have found increasing application in biocatalysis. However, challenges persist due to a frequent diminution in enzyme activity and stability. Herein, we developed a general protein engineering strategy, termed corner engineering, to acquire DES-resistant and thermostable enzymes via precise tailoring of the transition region in enzyme structure. Employing Bacillus subtilis lipase A (BSLA) as a model, we delineated the engineering process, yielding five multi-DESs resistant variants with highly improved thermostability, such as K88E/N89 K exhibited up to a 10.0-fold catalytic efficiency (kcat /KM ) increase in 30 % (v/v) choline chloride (ChCl): acetamide and 4.1-fold in 95 % (v/v) ChCl: ethylene glycol accompanying 6.7-fold thermal resistance improvement than wild type at ≈50 °C. The generality of the optimized approach was validated by two extra industrial enzymes, endo-β-1,4-glucanase PvCel5A (used for biofuel production) and esterase Bs2Est (used for plastics degradation). The molecular investigations revealed that increased water molecules at substrate binding cleft and finetuned helix formation at the corner region are two dominant determinants governing elevated resistance and thermostability. This study, coupling corner engineering with obtained molecular insights, illuminates enzyme-DES interaction patterns and fosters the rational design of more DES-resistant and thermostable enzymes in biocatalysis and biotransformation.

Bioelectrocatalytic Synthesis: Concepts and Applications

Bioelectrocatalytic synthesis is the conversion of electrical energy into value-added products using biocatalysts. These methods merge the specificity and selectivity of biocatalysis and energy-related electrocatalysis to address challenges in the sustainable synthesis of pharmaceuticals, commodity chemicals, fuels, feedstocks and fertilizers. However, the specialized experimental setups and domain knowledge for bioelectrocatalysis pose a significant barrier to adoption. This review introduces key concepts of bioelectrosynthetic systems. We provide a tutorial on the methods of biocatalyst utilization, the setup of bioelectrosynthetic cells, and the analytical methods for assessing bioelectrocatalysts. Key applications of bioelectrosynthesis in ammonia production and small-molecule synthesis are outlined for both enzymatic and microbial systems. This review serves as a necessary introduction and resource for the non-specialist interested in bioelectrosynthetic research.

Biocatalysis in Water or in Non-Conventional Media? Adding the CO2 Production for the Debate

Biocatalysis can be applied in aqueous media and in different non-aqueous solutions (non-conventional media). Water is a safe solvent, yet many synthesis-wise interesting substrates cannot be dissolved in aqueous solutions, and thus low concentrations are often applied. Conversely, non-conventional media may enable higher substrate loadings but at the cost of using (fossil-based) organic solvents. This paper determines the CO2 production-expressed as kg CO2·kg product-1-of generic biotransformations in water and non-conventional media, assessing both the upstream and the downstream. The key to reaching a diminished environmental footprint is the type of wastewater treatment to be implemented. If the used chemicals enable a conventional (mild) wastewater treatment, the production of CO2 is limited. If other (pre)treatments for the wastewater are needed to eliminate hazardous chemicals and solvents, higher environmental impacts can be expected (based on CO2 production). Water media for biocatalysis are more sustainable during the upstream unit-the biocatalytic step-than non-conventional systems. However, processes with aqueous media often need to incorporate extractive solvents during the downstream processing. Both strategies result in comparable CO2 production if extractive solvents are recycled at least 1-2 times. Under these conditions, a generic industrial biotransformation at 100 g L-1 loading would produce 15-25 kg CO2·kg product-1 regardless of the applied media.

Hydrophobic Deep Eutectic Solvents as Greener Substitutes for Conventional Extraction Media: Examples and Techniques

Deep eutectic solvents (DESs) are multicomponent designer solvents that exist as stable liquids over a wide range of temperatures. Over the last two decades, research has been dedicated to developing noncytotoxic, biodegradable, and biocompatible DESs to replace commercially available toxic organic solvents. However, most of the DESs formulated until now are hydrophilic and disintegrate via dissolution on coming in contact with the aqueous phase. To expand the repertoire of DESs as green solvents, hydrophobic DESs (HDESs) were prepared as an alternative. The hydrophobicity is a consequence of the constituents and can be modified according to the nature of the application. Due to their immiscibility, HDESs induce phase segregation in an aqueous solution and thus can be utilized as an extracting medium for a multitude of compounds. Here, we review literature reporting the usage of HDESs for the extraction of various organic compounds and metal ions from aqueous solutions and absorption of gases like CO₂. We also discuss the techniques currently employed in the extraction processes. We have delineated the limitations that might reduce the applicability of these solvents and also discussed examples of how DESs behave as reaction media. Our review presents the possibility of HDESs being used as substitutes for conventional organic solvents.

Expanding the Synthetic Toolbox through Metal-Enzyme Cascade Reactions

The combination of metal-, photo-, enzyme-, and/or organocatalysis provides multiple synthetic solutions, especially when the creation of chiral centers is involved. Historically, enzymes and transition metal species have been exploited simultaneously through dynamic kinetic resolutions of racemates. However, more recently, linear cascades have appeared as elegant solutions for the preparation of valuable organic molecules combining multiple bioprocesses and metal-catalyzed transformations. Many advantages are derived from this symbiosis, although there are still bottlenecks to be addressed including the successful coexistence of both catalyst types, the need for compatible reaction media and mild conditions, or the minimization of cross-reactivities. Therefore, solutions are here also provided by means of catalyst coimmobilization, compartmentalization strategies, flow chemistry, etc. A comprehensive review is presented focusing on the period 2015 to early 2022, which has been divided into two main sections that comprise first the use of metals and enzymes as independent catalysts but working in an orchestral or sequential manner, and later their application as bionanohybrid materials through their coimmobilization in adequate supports. Each part has been classified into different subheadings, the first part based on the reaction catalyzed by the metal catalyst, while the development of nonasymmetric or stereoselective processes was considered for the bionanohybrid section.

Where Chemocatalysis Meets Biocatalysis: In Water

Chemoenzymatic catalysis, by definition, involves the merging of sequential reactions using both chemocatalysis and biocatalysis, typically in a single reaction vessel. A major challenge, the solution to which, however, is associated with numerous advantages, is to run such one-pot processes in water: the majority of enzyme-catalyzed processes take place in water as Nature's reaction medium, thus enabling a broad synthetic diversity when using water due to the option to use virtually all types of enzymes. Furthermore, water is cheap, abundantly available, and environmentally friendly, thus making it, in principle, an ideal reaction medium. On the other hand, most chemocatalysis is routinely performed today in organic solvents (which might deactivate enzymes), thus appearing to make it difficult to combine such reactions with biocatalysis toward one-pot cascades in water. Several creative approaches and solutions that enable such combinations of chemo- and biocatalysis in water to be realized and applied to synthetic problems are presented herein, reflecting the state-of-the-art in this blossoming field. Coverage has been sectioned into three parts, after introductory remarks: (1) Chapter 2 focuses on historical developments that initiated this area of research; (2) Chapter 3 describes key developments post-initial discoveries that have advanced this field; and (3) Chapter 4 highlights the latest achievements that provide attractive solutions to the main question of compatibility between biocatalysis (used predominantly in aqueous media) and chemocatalysis (that remains predominantly performed in organic solvents), both Chapters covering mainly literature from ca. 2018 to the present. Chapters 5 and 6 provide a brief overview as to where the field stands, the challenges that lie ahead, and ultimately, the prognosis looking toward the future of chemoenzymatic catalysis in organic synthesis.

A Deep Eutectic Solvent Thermomorphic Multiphasic System for Biocatalytic Applications

The applicability of a thermomorphic multiphasic system (TMS) composed of a hydrophobic deep eutectic solvent (DES) and an aqueous potassium phosphate buffer with a lower critical solution temperature (LCST) phase change for homogeneous biocatalysis was investigated. A lidocaine‐based DES with the fatty acid oleic acid as a hydrogen‐bond donor was studied. Phase diagrams were determined and presented within this study. We tested different additional components to the solvent system and observed a decrease in the cloud point of approximately 0.026 °C per concentration unit. Distribution studies revealed a clear distribution of the protein in the aqueous buffer phase (>95 %), whereas the hydrophobic substrate and educt accumulated (>95 %) in the DES‐enriched layer. Finally, a reduction catalyzed by horse liver alcohol dehydrogenase was performed in a larger‐scale experiment, and the biocatalyst could be recycled by simply removing the DES phase for three recycling runs.

Enzymatic Cascade for the Synthesis of 2,5-Furandicarboxylic Acid in Biphasic and Microaqueous Conditions: 'Media-Agnostic' Biocatalysts for Biorefineries

5-hydroxymethylfurfural (HMF) is produced upon dehydration of C6 sugars in biorefineries. As the product, it remains either in aqueous solutions, or is in situ extracted to an organic medium (biphasic system). For the subsequent oxidation of HMF to 2,5-furandicarboxylic acid (FDCA), 'media-agnostic' catalysts that can be efficiently used in different conditions, from aqueous to biphasic, and to organic (microaqueous) media, are of interest. Here, the concept of a one-pot biocatalytic cascade for production of FDCA from HMF was reported, using galactose oxidase (GalOx) for the formation of 2,5-diformylfuran (DFF), followed by the lipase-mediated peracid oxidation of DFF to FDCA. GalOx maintained its catalytic activity upon exposure to a range of organic solvents with only 1 % (v/v) of water. The oxidation of HMF to 2,5-diformylfuran (DFF) was successfully established in ethyl acetate-based biphasic or microaqueous systems. To validate the concept, the reaction was conducted at 5 % (v/v) water, and integrated in a cascade where DFF was subsequently oxidized to FDCA in a reaction catalyzed by Candida antarctica lipase B.

Enzyme immobilization in hydrogels: A perfect liaison for efficient and sustainable biocatalysis

Biocatalysis is an established chemical synthesis technology that has by no means been restricted to research laboratories. The use of enzymes for organic synthesis has evolved greatly from early development to proof-of-concept - from small batch production to industrial scale. Different enzyme immobilization strategies contributed to this success story. Recently, the use of hydrogel materials for the immobilization of enzymes has been attracting great interest. Within this review, we pay special attention to recent developments in this key emerging field of research. Firstly, we will briefly introduce the concepts of both biocatalysis and hydrogel worlds. Then, we list recent interesting publications that link both concepts. Finally, we provide an outlook and comment on future perspectives of further exploration of enzyme immobilization strategies in hydrogels.

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.

Parameters Influencing Lipase-Catalyzed Glycolipid Synthesis by (Trans-)Esterification Reaction

Glycolipids are biodegradable, non-toxic surfactants with a wide range of applications. Enzymatic esterification or transesterification facilitated in reaction media of low water activity is a reaction strategy for the production of tailor-made glycolipids as a high structural diversity can be achieved. Organic solvents, ionic liquids, and deep eutectic solvents have been applied as reaction media. However, several challenges need to be addressed for efficient (trans-)esterification reactions, especially for the lipophilization of polar substrates. Therefore, crucial parameters in (trans-)esterification reactions in conventional and non-conventional media are discussed and compared in this review with a special focus on glycolipid synthesis.

Multicatalytic Hybrid Materials for Biocatalytic and Chemoenzymatic Cascades—Strategies for Multicatalyst (Enzyme) Co-Immobilization

During recent decades, the use of enzymes or chemoenzymatic cascades for organic chemistry has gained much importance in fundamental and industrial research. Moreover, several enzymatic and chemoenzymatic reactions have also served in green and sustainable manufacturing processes especially in fine chemicals, pharmaceutical, and flavor/fragrance industries. Unfortunately, only a few processes have been applied at industrial scale because of the low stabilities of enzymes along with the problematic processes of their recovery and reuse. Immobilization and co-immobilization offer an ideal solution to these problems. This review gives an overview of all the pathways for enzyme immobilization and their use in integrated enzymatic and chemoenzymatic processes in cascade or in a one-pot concomitant execution. We place emphasis on the factors that must be considered to understand the process of immobilization. A better understanding of this fundamental process is an essential tool not only in the choice of the best route of immobilization but also in the understanding of their catalytic activity.

Applied biocatalysis beyond just buffers - from aqueous to unconventional media. Options and guidelines

In nature, enzymes conventionally operate under aqueous conditions. Because of this, aqueous buffers are often the choice for reaction media when enzymes are applied in chemical synthesis. However, to meet the demands of an industrial application, due to the poor water solubility of many industrially relevant compounds, an aqueous reaction system will often not be able to provide sufficient substrate loadings. A switch to a non-aqueous solvent system can provide a solution, which is already common for lipases, but more challenging for biocatalysts from other enzyme classes. The choices in solvent types and systems, however, can be overwhelming. Furthermore, some engineering of the protein structure of biocatalyst formulation is required. In this review, a guide for those working with biocatalysts, who look for a way to increase their reaction productivity, is presented. Examples reported clearly show that bulk water is not necessarily required for biocatalytic reactions and that clever solvent systems design can support increased product concentrations thereby decreasing waste formation. Additionally, under these conditions, enzymes can also be combined in cascades with other, water-sensitive, chemical catalysts. Finally, we show that the application of non-aqueous solvents in biocatalysis can actually lead to more sustainable processes. At the hand of flowcharts, following simple questions, one can quickly find what solvent systems are viable.

Choline Chloride/Glycerol Promoted Synthesis of 3,3-Disubstituted Indol-2-ones

Objective:

3,3-Disubstituted indol-2-one derivatives have wider applications in pharmaceuticals and they are key intermediates for the synthesis of many kinds of drug candidates. The development of an efficient and practical method to prepare this class of compound is highly desirable from both environmental and economical points of views.

Methods:

In order to establish an effective synthetic method for preparing 3,3-disubstituted indol-2-one derivatives, the bis-condensation reaction of isatin and 1H-indene-1,3(2H)-dione was selected as a model reaction. A variety of natural deep eutectic solvent (NADES) were prepared and used for this reaction. The generality and limitation of the established method were also investigated.

Results:

It was found that model reaction can be carried out in natural deep eutectic solvent (NADES) based on choline chloride (ChCl) at 80 oC under microwave irradiation. This protocol with a broad substrate applicability afforded various 2,2'-(2-oxoindoline-3,3-diyl)bis(1H-indene-1,3(2H)-dione) derivatives in high yields.

Conclusion:

simple and efficient procedure has been developed for synthesis of 2,2'-(2-oxoindoline-3,3-diyl)bis(1H-indene-1,3(2H)-dione), spiro[indoline-3,7'-pyrano[5,6-c:5,6-c']dichromene]-2,6',8'-trione, and spiro[indoline-3,9'-xan-thene] trione via bis-condensation between isatin with 1,3-indandione, 4-hydroxycoumarin or 1,3-cyclohexanedione in nat-ural deep eutectic solvent (NADES) based on choline chloride (ChCl) and glycerol (Gl) under microwave irradiation. The salient features of this protocol are avoidance of any additive/catalyst and toxic organic solvent, clean reaction profiles, non-chromatographic purification procedure, and high to excellent yield. Furthermore, the use of NADES as green reaction medium reduces burden on environment and makes the present method environmentally sustainable.

Advances in deep eutectic solvents and water: applications in metal- and biocatalyzed processes, in the synthesis of APIs, and other biologically active compounds

This review highlights recent advances in metal- and biocatalyzed transformations, in the synthesis of APIs and other biologically active compounds, when employing deep eutectic solvents and water as environmentally responsible solvents.

Deep eutectic solvent-catalyzed Meyer-Schuster rearrangement of propargylic alcohols under mild and bench reaction conditions

The Meyer-Schuster rearrangement of propargylic alcohols into α,β-unsaturated carbonyl compounds has been revisited by setting up an atom-economic process catalyzed by a deep eutectic solvent FeCl3·6H2O/glycerol. Isomerizations take place smoothly, at room temperature, under air and with short reaction times. The unique solubilizing properties of the eutectic mixture enabled the use of a substrate concentration up to 1.0 M with the medium being recycled up to ten runs without any loss of catalytic activity.