Organocatalysis in aqueous media

Unravelling structural features of small molecules for photochemical transformation of environmental contaminants

Small molecules, including natural metabolites, organic matter decomposition products, and engineered oxidation byproducts, are widespread in aquatic environment. However, the limited understanding of the photochemical interactions of these small molecules with water pollutants hampers the development of effective environmental protection strategies. This study explores the structural features governing the photochemical transformation of toxic oxyanions by α- and β-dicarbonyl compounds. By integrating experimental observations with quantum chemical calculations, a robust correlation network was constructed. The correlation network reveals that the reactivity of small organic molecules with oxyanions could be quantitively predicted by their intrinsic properties, such as electronic transition energy, bond dissociation energy, molecular softness, molecular orbital gap, atomic charge, and molecular surface local ionization energy. This network maps the relationship between the molecular architecture of chemicals and their photochemical behaviors. This perspective offers fresh insights into the photochemical behaviors of small molecules in diverse environmental and chemical contexts and are helpful for developing advanced water treatment strategies toward a sustainable future.

Design and Evolution of an Artificial Friedel-Crafts Alkylation Enzyme Featuring an Organoboronic Acid Residue

Creating artificial enzymes by the genetic incorporation of noncanonical amino acids with catalytic side chains would expand the enzyme chemistries that have not been discovered in nature. Here, we report the design of an artificial enzyme that uses p-boronophenylalanine as the catalytic residue. The artificial enzyme catalyzes Michael-type Friedel-Crafts alkylation through covalent activation. The designer enzyme was further engineered to afford high yields with excellent enantioselectivities. We next developed a practical method for preparative-scale reactions by whole-cell catalysis. This enzymatic C-C bond formation reaction was combined with palladium-catalyzed dearomative arylation to achieve the efficient synthesis of spiroindolenine compounds.

Chiral Diselenophosphoric Acids for Ion Pair Catalysis: A Novel Approach to Enhance Both Proton Donating and Proton Accepting Properties

The activation of poorly reactive substrates via strong chiral acids is a central topic in asymmetric ion pair catalysis these days. Despite highly successful scaffolds such as N-triflylphosphoramides, these catalysts either lack C2-symmetry or provide multiple H-bond acceptor sites, leading to lower ee values for certain reactions. We present BINOL-based diselenophosphoric acids (DSA) as an extremely promising alternative. Using an intertwined approach of synthesis and NMR studies, we developed a synthetic approach to DSA with up to 98 % NMR yield. The obtained acids provide both very high proton donor and proton acceptor properties, a bifunctionality, which is key to catalytic applications. Indeed, first reactivity test proved the much higher acidity of DSA and its ability to initiate Mukaiyama-Mannich reaction and protodesilylation of silyl ethers. Together with their C2-symmetry, the single donor and single acceptor situation, the decreased tendency of self-association, and the straightforward synthesis with potential 3,3'-substitution, the DSA provide all features ideal for the further development of ion pair catalysis.

Design, Preparation, and Implementation of Axially Chiral Benzotetramisoles as Lewis Base Catalysts for Asymmetric Cycloadditions

Developing novel catalysts with potent activity is of great importance in organocatalysis. In this study, we designed and prepared a new class of benzotetramisole Lewis base catalysts (AxBTM) that have both central and axial chirality. This unique feature of these catalysts results in a three-dimensional microenvironment with multi-layers of chirality. The performance of the developed catalysts was tested in a series of cycloaddition reactions. These included the AxBTM-catalyzed (2+2) cycloaddition between α-fluoro-α-aryl anhydride with imines or oxindoles, and the sequential gold/AxBTM-catalyzed (4+2) cycloaddition of enynamides with pentafluorophenyl esters. The interplay between axial and central chirality had a collaborative effect in regulating the stereochemistry in these cycloadditions, leading to high levels of stereoselectivity that would otherwise be challenging to achieve using conventional BTM catalysts. However, the (2+2) and (4+2) cycloadditions have different predilections for axial and central chirality combinations.

Chemoenzymatic Synthesis of 2-Aryl Thiazolines from 4-Hydroxybenzaldehydes Using Vanillyl Alcohol Oxidases

Nitrogen heterocycles are commonly found in bioactive natural products and drugs. However, the biocatalytic tools for nitrogen heterocycle synthesis are limited. Herein, we report the discovery of vanillyl alcohol oxidases (VAOs) as efficient biocatalysts for the one-pot synthesis of 2-aryl thiazolines from various 4-hydroxybenzaldehydes and aminothiols. The wild-type biocatalyst features a broad scope of 4-hydroxybenzaldehydes. Though the scope of aminothiols is limited, it could be improved via semi-rational protein engineering, generating a variant to produce previously inaccessible cysteine-derived bioactive 2-aryl thiazolines using the wild-type VAO. Benefiting from the derivatizable functional groups in the enzymatic products, we further chemically modified these products to expand the chemical space, offering a new chemoenzymatic strategy for the green and efficient synthesis of structurally diverse 2-aryl-thiazoline derivatives to prompt their use in drug discovery and catalysis.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

Advances in organocatalyzed synthesis of organic compounds

The recent advancements in utilizing organocatalysts for the synthesis of organic compounds have been described in this review by focusing on their simplicity, effectiveness, reproducibility, and high selectivity which lead to excellent product yields. The organocatalytic methods for various derivatives, such as indoles, pyrazolones, anthrone-functionalized benzylic amines, maleimide, polyester, phthalimides, dihydropyrimidin, heteroaryls, N-aryl benzimidazoles, stilbenoids, quinazolines, quinolines, and oxazolidinones have been specifically focused. The review provides more understanding by delving into potential reaction mechanisms. We anticipate that this collection of data and findings on successful synthesis of diverse compound derivatives will serve as valuable resources and stimulating current and future research efforts in organocatalysis and industrial chemistry.

The Michael donor-acceptor reactivity of curcumins in the synthesis of diverse multi-functional scaffolds

Curcumin is a key constituent of turmeric with a variety of biological activities. From a chemical point of view, curcumin contains different functional groups that can undergo multiple transformations such as Michael addition, cycloaddition, click reaction, polymerisation, etc. Among these, Michael-type reactions under benign conditions constitute a captivating domain of curcumin's reactivity. To the best of our knowledge, no review focusing on the Michael donor-acceptor reactivity of curcumins has been published to date. Herein, we have compiled the chemistry of curcumins with respect to their chemical synthesis, biosynthesis, and involvement in chemical transformations, especially in Michael additions with advances in mechanistic aspects and understanding.

Cross aldol OPAL bioconjugation outcompetes intramolecular hemiaminal cyclisation of proline adjacent N-terminal α-oxo aldehydes at acidic pH

Novel methods to construct small molecule-protein bioconjugates are integral to the development of new biomedicines for a variety of diseases. C-C linked bioconjugates are increasingly desirable in this application due to their in vivo stability and can be accessed through cross aldol bioconjugation of reactive α-oxo aldehyde handles easily introduced at the N-terminus of proteins by periodate oxidation. We previously developed an organocatalyst-mediated protein aldol ligation (OPAL) for chemical modification of these reactive aldehydes, but the efficiency of this method was limited when a proline residue was directly adjacent to the N-terminus due to intramolecular hemiaminal formation. Herein we explore the competition between this cyclisation and the OPAL modification and demonstrate bioconjugation can be favoured through use of acidic pH for both oxidation and OPAL, and optimisation of reaction conditions and organocatalyst. We then showcase the utility of this acidic-OPAL in modification of the cholera toxin B-subunit (CTB), a homo-pentameric protein of biomedical promise.

Modifying the Catalytic Activity of Lipopeptide Assemblies with Nucleobases

Biohybrid catalysts that operate in aqueous media are intriguing for systems chemistry. In this paper, we investigate whether control over the self-assembly of biohybrid catalysts can tune their properties. As a model, we use the catalytic activity of functional hybrid molecules consisting of a catalytic H-dPro-Pro-Glu tripeptide, derivatized with fatty acid and nucleobase moieties. This combination of simple biological components merged the catalytic properties of the peptide with the self-assembly of the lipid, and the structural ordering of the nucleobases. The biomolecule hybrids self-assemble in aqueous media into fibrillar assemblies and catalyze the reaction between butanal and nitrostyrene. The interactions between the nucleobases enhanced the order of the supramolecular structures and affected their catalytic activity and stereoselectivity. The results point to the significant control and ordering that nucleobases can provide in the self-assembly of biologically inspired supramolecular catalysts.

Development and Applications of Water-Compatible Reactions: A Journey to Be Continued

ConspectusThe pursuit of novel and eco-friendly methods in organic synthesis is gaining prominence, with a strong emphasis on green transformations using renewable and sustainable resources. Among these environmentally conscious approaches, water-compatible reactions stand out for their many advantages. Water, as a solvent, offers unmatched abundance, cost-efficiency, and environmental compatibility compared to organic solvents. Its use eliminates the need for complex protection and deprotection steps for reactive functional groups in multistep synthesis and enables the use of water-soluble substrates like proteins and carbohydrates. Water-compatible reactions also provide opportunities to combine with enzymes, resulting in chemoenzymatic transformations that can increase efficiency. Additionally, these reactions facilitate site-specific modification and the bioconjugation of biomolecules, leading to bioconjugate therapeutics.Over nearly three decades, our research group has been dedicated to developing innovative water-compatible methodologies and concepts. This Account provides a comprehensive overview of our contributions since 1994. Our central strategy revolves around integrating green chemistry principles into our methods, focusing on (i) developing reactions that can operate under mild conditions, including room temperature, atmospheric pressure, and physiological pH; (ii) designing atom-economical reactions that minimize waste production; (iii) replacing toxic and flammable organic solvents with eco-friendly alternatives like water and ethanol; and (iv) reducing reliance on metals or halogenated compounds in specific reactions.In this Account, we detail our achievements in developing efficient methodologies in aqueous media, highlighting their scope, limitations, asymmetric control, and applications for synthesizing complex molecules and functionalizing peptides and proteins. Mechanistic investigations underlying these developments are also discussed when applicable. Furthermore, we offer insights into the reasoning behind our work and address future opportunities and challenges in this area of research. We hope that this Account will inspire continued interest and foster new breakthroughs. By exploring innovative and broadly applicable strategies that expand the water-compatible synthetic toolbox, we aim to pave the way for the truly green and sustainable synthesis of complex molecules and pharmaceuticals.

Organocatalytic Asymmetric Vinylogous Michael Addition of Electron-Deficient Aryl Alkane Nucleophiles to Enals

We report herein a protocol for an organocatalyzed asymmetric vinylogous Michael addition of aryl alkane nucleophiles with enals under base- and additive-free conditions. A series of allylic building blocks were obtained in 60%-93% yield and 88-99% ee with 20 mol % diphenylprolinol silyl ether as catalyst. This protocol has advantages such as excellent chemoselectivity and regioselectivity, good tolerance of functionalities, and simple reaction conditions.

Biocompatible α-Methylenation of Metabolic Butyraldehyde in Living Bacteria

Small molecule organocatalysts are abundant in all living organisms. However, their use as organocatalysts in cells has been underexplored. Herein, we report that organocatalytic aldol chemistry can be interfaced with living Escherichia coli to enable the α-methylenation of cellular aldehydes using biogenic amines such as L-Pro or phosphate. The biocompatible reaction is mild and can be interfaced with butyraldehyde generated from D-glucose via engineered metabolism to enable the production of 2-methylenebutanal (2-MB) and 2-methylbutanal (2-MBA) by anaerobic fermentation, and 2-methylbutanol (2-MBO) by whole-cell catalysis. Overall, this study demonstrates the combination of non-enzymatic organocatalytic and metabolic reactions in vivo for the sustainable synthesis of valuable non-natural chemicals that cannot be accessed using enzymatic chemistry alone.

Reversible 2'-OH acylation enhances RNA stability

The presence of a hydroxyl group at the 2'-position in its ribose makes RNA susceptible to hydrolysis. Stabilization of RNAs for storage, transport and biological application thus remains a serious challenge, particularly for larger RNAs that are not accessible by chemical synthesis. Here we present reversible 2'-OH acylation as a general strategy to preserve RNA of any length or origin. High-yield polyacylation of 2'-hydroxyls ('cloaking') by readily accessible acylimidazole reagents effectively shields RNAs from both thermal and enzymatic degradation. Subsequent treatment with water-soluble nucleophilic reagents removes acylation adducts quantitatively ('uncloaking') and recovers a remarkably broad range of RNA functions, including reverse transcription, translation and gene editing. Furthermore, we show that certain α-dimethylamino- and α-alkoxy- acyl adducts are spontaneously removed in human cells, restoring messenger RNA translation with extended functional half-lives. These findings support the potential of reversible 2'-acylation as a simple and general molecular solution for enhancing RNA stability and provide mechanistic insights for stabilizing RNA regardless of length or origin.

Enolate-Azide [3 + 2]-Cycloaddition Reaction Suitable for DNA-Encoded Library Synthesis

The DNA-encoded chemical library (DEL) is a powerful hit selection technique in either basic science or innovative drug discovery. With the aim to circumvent the issue concerning DNA barcode damage in a conventional on-DNA copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC), we have successfully developed the first DNA-compatible enolate-azide [3 + 2] cycloaddition reaction. The merits of this DEL chemistry include metal-free reaction and high DNA fidelity, high conversions and easy operation, broad substrate scope, and ready access to the highly substituted 1,4,5-trisubstituted triazoles. Thus, it will not only further enrich the DEL chemistry toolbox but also will have great potential in practical DEL synthesis.

Going Full Circle with Organocatalysis and Biocatalysis: The Latent Potential of Cofactor Mimics in Asymmetric Synthesis

Many enzymes work in tandem with small molecule cofactors, which have inspired organocatalyst designs. Chemical modification of cofactor scaffolds has increased organocatalytic reactivity and reaction scope. This synopsis presents a selection of recent advances in the use of cofactors (native and mimics) in organocatalysis and biocatalysis. We aim to highlight the benefits of combining fundamental knowledge gained in both bio- and organo-catalysis for asymmetric biocatalysis.

Utilization of vanillin to prepare sulfated Calix[4]resorcinarene as efficient organocatalyst for biodiesel production based on methylation of palmitic acid and oleic acid

Recently, biodiesel production from palm oils has been thoroughly investigated to substitute crude oil due to its scarcity. However, the biodiesel production process is time-consuming due to its slow kinetics; thus, concentrated sulfuric acid has been used to fasten the reaction process in some industries. Unfortunately, sulfuric acid is a toxic, corrosive, and non-environmentally friendly catalyst. In this study, we prepared sulfated Calix[4]resorcinarene derived from vanillin as an efficient organocatalyst to replace sulfuric acid. The catalytic activity of sulfated Calix[4]resorcinarenes was evaluated through the methylation of palmitic acid and oleic acid as model compounds due to their abundant amounts in palm oil. The Calix[4]resorcinarene and sulfated Calix[4]resorcinarenes have been obtained through a one-pot reaction in 71.8-98.3% yield. Their chemical structures were confirmed by using FTIR, NMR and HRMS spectrometry analyses. The results showed that the sulfated Calix[4]resorcinarene exhibited high catalytic activity for methyl palmitate and methyl oleate productions in 94.8 ± 1.8 and 97.3 ± 2.1% yield, respectively, which was comparable to sulfuric acid (96.3 ± 1.8 and 95.9 ± 2.5%). The optimum condition was achieved by using 0.020 wt equivalent of organocatalyst for 6 h reaction process at 338 K. The methylation of palmitic acid and oleic acid fits well with the first-order kinetic model (R² = 0.9940-0.9999) with a reaction rate constant of 0.6055 and 1.1403 h^-1, respectively. Further investigation reveals that the hydroxyl group of vanillin plays a pivotal role in the organocatalytic activity of sulfated Calix[4]resorcinarene.

Tailoring the Chemical Structure of Nitrogen-Doped Carbon Dots for Nano-Aminocatalysis in Aqueous Media

Amine-rich carbon dots (NCDs) have become promising nano-aminocatalytic platforms in organic synthesis. These nanomaterials can be effectively produced through straightforward bottom-up approaches using inexpensive nitrogen-containing molecular precursors as a starting material. However, to date, there is still a limited understanding of how the molecular features of these precursors affect the catalytic activity of the resulting nanoparticles. This study concerns the production of a new family of NCDs, which use l-arginine and different alkyl diamines as starting materials. The surface amines of all these NCDs were comprehensively characterized, thus allowing us to provide a correlation between the structural features of the nanoparticles and their catalytic performance with a selected amino-catalyzed organic transformation. Importantly, the most active nano-aminocatalysts, namely, NCDs-3, were then used as a basis for the formation of a wide variety of functionalized organic compounds in water under mild reaction conditions.

On-Demand Release of Secondary Amine Bases for the Activation of Catalysts and Crosslinkers

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

Recent Advances in Greener Asymmetric Organocatalysis Using Bio-Based Solvents

Efficient synthetic methods that avoid the extensive use of hazardous reagents and solvents, as well as harsh reaction conditions, have become paramount in the field of organic synthesis. Organocatalysis is notably one of the best tools in building chemical bonds between carbons and carbon-heteroatoms; however, most examples still employ toxic volatile organic solvents. Although a portfolio of greener solvents is now commercially available, only ethyl alcohol, ethyl acetate, 2-methyltetrahydrofuran, supercritical carbon dioxide, ethyl lactate, and diethyl carbonate have been explored with chiral organocatalysts. In this review, the application of these bio-based solvents in asymmetric organocatalytic methods reported in the last decade is discussed, highlighting the proposed mechanism pathway for the transformations.

Reactivity and mechanism in chemical and synthetic biology

Physical organic chemistry and mechanistic thinking provide a strong intellectual framework for understanding the chemical logic of evolvable informational macromolecules and metabolic transformations in living organisms. These concepts have also led to numerous successes in designing and applying tools to delineate biological function in health and disease, chemical ecology and possible alternative chemistries employed by extraterrestrial life. A symposium at the 2020 Pacifichem meeting was scheduled in December 2020 to discuss designing and exploiting expanded genetic alphabets, methods to understand the biosynthesis of natural products and re-engineering primary metabolism in bacteria. The COVID-19 pandemic led to postponement of in-person discussions, with the symposium eventually being held on 20-21 December 2021 as an online event. This issue is a written record of work presented on biosynthetic pathways and enzyme catalysis, engineering microorganisms with new metabolic capabilities, and the synthesis of non-canonical, nucleobases for medical applications and for studies of alternate chemistries for living organisms. The variety of opinion pieces, reviews and original research articles provide a starting point for innovations that clarify how complex biological systems emerge from the rules of chemical reactivity and mechanism. This article is part of the themed issue 'Reactivity and mechanism in chemical and synthetic biology'.

A recyclable stereoauxiliary aminocatalyzed strategy for one-pot synthesis of indolizine-2-carbaldehydes

Indolizine-carbaldehydes with the easily modifiable carbaldehyde group are important synthetic targets as versatile precursors for distinct indolizines. However, the efficient one-pot construction of trisubstituted indolizine-2-carbaldehydes represents a long-standing challenge. Herein, we report an unprecedented recyclable stereoauxiliary aminocatalytic approach via aminosugars derived from biomass, which enable the efficient one-pot synthesis of desired trisubstituted indolizine-2-carbaldehydes via [3+2] annulations of acyl pyridines and α,β-unsaturated aldehyde. Compared to the steric shielding effect from α-anomer, a stereoauxiliary effect favored by β-anomer of D-glucosamine is supported by control experiments. Furthermore, polymeric chitosan containing predominantly β-D-anhydroglucosamine units also shows excellent catalytic performance in aqueous solutions for the conversion of various substrates, large-scale synthesis and catalytic cycling experiments. Thus, our approach advances the existing methodologies by providing a rich library of indolizine-2-aldehydes. In addition, it delivers an efficient protocol for a set of late-stage diversification and targeted modifications of bioactive molecules or drugs, as showcased with 1,2,3-trisubstituted indolizine-2-carbaldehydes.

Enantioselective Synthesis of Spiro Heterocyclic Compounds Using a Combination of Organocatalysis and Transition-Metal Catalysis

Over the last ten years, the combination of organocatalysis with transition metal (TM) catalysis has become one of the most important toolboxes used for synthesizing optically pure compounds containing chiral quaternary centers, including spiro heterocyclic molecules. The dominant method in the enantioselective synthesis of spiro heterocyclic compounds based on synergistic catalysis includes chiral aminocatalysis and NHC catalysis, as already established covalent organocatalytic strategies. Another area of organocatalysis widely combined with TM catalysis producing enantiomerically enriched spiro heterocyclic compounds is non-covalent catalysis, dominated by chiral phosphoric acids, thiourea, and squaramide derivatives. This review article aims to summarize enantioselective methods used for constructing spirocyclic heterocycles based on a combination of organocatalysis and transition metal catalysis.

Asymmetric Organocatalysis: A Survival Guide to Medicinal Chemists

Majority of drugs act by interacting with chiral counterparts, e.g., proteins, and we are, unfortunately, well-aware of how chirality can negatively impact the outcome of a therapeutic regime. The number of chiral, non-racemic drugs on the market is increasing, and it is becoming ever more important to prepare these compounds in a safe, economic, and environmentally sustainable fashion. Asymmetric organocatalysis has a long history, but it began its renaissance era only during the first years of the millennium. Since then, this field has reached an extraordinary level, as confirmed by the awarding of the 2021 Chemistry Nobel Prize. In the present review, we wish to highlight the application of organocatalysis in the synthesis of enantio-enriched molecules that may be of interest to the pharmaceutical industry and the medicinal chemistry community. We aim to discuss the different activation modes observed for organocatalysts, examining, for each of them, the generally accepted mechanisms and the most important and developed reactions, that may be useful to medicinal chemists. For each of these types of organocatalytic activations, select examples from academic and industrial applications will be disclosed during the synthesis of drugs and natural products.

Enantio- and Diastereoselective Michael Addition of Cyclic Ketones/Aldehydes to Nitroolefins in Water as Catalyzed by Proline-Derived Bifunctional Organocatalysts

New l-proline-derived bifunctional secondary amine organocatalysts were synthesized for enantioselective Michael reactions in water as a solvent. Application of these catalysts in Michael additions provided high yield (up to 97%) with high stereoselectivity (dr up to 99:1 and ee up to 99%). The effect of phenyl group at (R)-C₆ in the catalyst was investigated and played a key role in successful catalysis by density functional theory computational calculations. The synthetic utility of this reaction was demonstrated by the formal synthesis of Sch 50971, which is a novel histamine H₃ receptor agonist.

Prebiotic triose glycolysis promoted by co-catalytic proline and phosphate in neutral water

Proline and phosphate promote a near-quantitative aldol reaction between glycolaldehyde phosphate and formaldehyde at neutral pH in water. Our results demonstrate the important role of general acid-base catalysis in water and underscore the essential role that amino acid catalysis may have played in early evolution of life's core metabolic pathways.

Mechanistic Insights into the Selective Synthesis of 4H-Pyran Derivatives On-Water Using Naturally Occurring Alginate from Sargassum muticum: Experimental and DFT Study

The naturally occurring sodium alginate (SA) biopolymer from the Sargassum muticum (Yendo) Fensholt was employed as a green organocatalyst for the synthesis of 4H-pyran derivatives. The naturally extracted macromolecule was fully characterized using different analyses, including nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Energy Dispersive X-ray Analysis (EDX). The catalytic activity of SA was investigated in the one-pot reaction between aldehydes, malononitrile, and 1,3-dicarbonyl compounds in water at room temperature, and the corresponding 2-amino-3-cyano-4H-pyran derivatives were obtained with good to excellent yields. This organocatalyst was easily separated from the reaction mixture and reused for at least two consecutive cycles without a significant loss of its catalytic activity or selectivity. From the mechanistic point of view, density functional theory (DFT) and NCI analyses were performed for the first time to explain the regioselectivity outcomes for the synthesis of 2-amino-3-cyano-4H-pyran derivatives using SA as a green organocatalyst.

Switchable aqueous catalytic systems for organic transformations

In living organisms, enzyme catalysis takes place in aqueous media with extraordinary spatiotemporal control and precision. The mechanistic knowledge of enzyme catalysis and related approaches of creating a suitable microenvironment for efficient chemical transformations have been an important source of inspiration for the design of biomimetic artificial catalysts. However, in "nature-like" environments, it has proven difficult for artificial catalysts to promote effective chemical transformations. Besides, control over reaction rate and selectivity are important for smart application purposes. These can be achieved via incorporation of stimuli-responsive features into the structure of smart catalytic systems. Here, we summarize such catalytic systems whose activity can be switched 'on' or 'off' by the application of stimuli in aqueous environments. We describe the switchable catalytic systems capable of performing organic transformations with classification in accordance to the stimulating agent. Switchable catalytic activity in aqueous environments provides new possibilities for the development of smart materials for biomedicine and chemical biology. Moreover, engineering of aqueous catalytic systems can be expected to grow in the coming years with a further broadening of its application to diverse fields.

l-Asparagine-EDTA-amide silica-coated MNPs: a highly efficient and nano-ordered multifunctional core-shell organocatalyst for green synthesis of 3,4-dihydropyrimidin-2(1H)-one compounds

In this study, new l-asparagine grafted on 3-aminopropyl-modified Fe3O4@SiO2 core-shell magnetic nanoparticles using the EDTA linker (Fe3O4@SiO2-APTS-EDTA-asparagine) was prepared and its structures properly confirmed using different spectroscopic, microscopic and magnetic methods or techniques including FT-IR, EDX, XRD, FESEM, TEM, TGA and VSM. The Fe3O4@SiO2-APTS-EDTA-asparagine core-shell nanomaterial was found, as a highly efficient multifunctional and recoverable organocatalyst, to promote the efficient synthesis of a wide range of biologically-active 3,4-dihydropyrimidin-2(1H)-one derivatives under solvent-free conditions. It was proved that Fe3O4@SiO2-APTS-EDTA-asparagine MNPs, as a catalyst having excellent thermal and magnetic stability, specific morphology and acidic sites with appropriate geometry, can activate the Biginelli reaction components. Moreover, the environmental-friendliness and nontoxic nature of the catalyst, cost effectiveness, low catalyst loading, easy separation of the catalyst from the reaction mixture and short reaction time are some of the remarkable advantages of this green protocol.

Derivatives of Natural Organocatalytic Cofactors and Artificial Organocatalytic Cofactors as Catalysts in Enzymes

Catalytically active non-metal cofactors in enzymes carry out a variety of different reactions. The efforts to develop derivatives of naturally occurring cofactors such as flavins or pyridoxal phosphate and the advances to design new, non-natural cofactors are reviewed here. We report the status quo for enzymes harboring organocatalysts as derivatives of natural cofactors or as artificial ones and their application in the asymmetric synthesis of various compounds.

Prebiotic Catalytic Peptide Ligation Yields Proteinogenic Peptides by Intramolecular Amide Catalyzed Hydrolysis Facilitating Regioselective Lysine Ligation in Neutral Water

The prebiotic origin of catalyst-controlled peptide synthesis is fundamental to understanding the emergence of life. Building on our recent discovery that thiols catalyze the ligation of amino acids, amides, and peptides with amidonitriles in neutral water, we demonstrate the outcome of ligation depends on pH and that high pK_a primary thiols are the ideal catalysts. While the most rapid thiol catalyzed peptide ligation occurs at pH 8.5–9, the most selective peptide ligation, that tolerates all proteinogenic side chains, occurs at pH 7. We have also identified the highly selective mechanism by which the intermediate peptidyl amidines undergo hydrolysis to α-peptides while demonstrating that the hydrolysis of amidines with nonproteinogenic structures, such as β- and γ-peptides, displays poor selectivity. Notably, this discovery enables the highly α-selective protecting-group-free ligation of lysine peptides at neutral pH while leaving the functional ε-amine side chain intact.

Two-Dimensional Metal-Organic Framework Nanosheets: Synthesis and Applications in Electrocatalysis and Photocatalysis

Two-dimensional metal-organic nanosheets (2D MONs) are an emerging class of ultrathin, porous, and crystalline materials. The organic/inorganic hybrid nature offers MONs distinct advantages over other inorganic nanosheets in terms of diversity of organic ligands and metal notes. Compared to bulk three-dimensional metal-organic frameworks, 2D MONs possess merits of high density and readily accessible catalytic sites, reduced diffusion pathways for reactants/products, and fast electron transport. These features endow MONs with enhanced physical/chemical properties and are ideal for heterogeneous catalysis. In this Review, state-of-the-art synthetic methods for the fabrication of 2D MONs were summarized. The advances of 2D MONs-based materials for electrocatalysis and photocatalysis, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂ RR), and electro-/photocatalytic organic transformations were systematically discussed. Finally, the challenges and perspectives regarding future design and synthesis of 2D MONs for high-performance electrocatalysis and photocatalysis were provided.

Cyclo(His-Pro) Exerts Protective Carbonyl Quenching Effects through Its Open Histidine Containing Dipeptides

Cyclo(His-Pro) (CHP) is a cyclic dipeptide which is endowed with favorable pharmacokinetic properties combined with a variety of biological activities. CHP is found in a number of protein-rich foods and dietary supplements. While being stable at physiological pH, CHP can open yielding two symmetric dipeptides (His-Pro, Pro-His), the formation of which might be particularly relevant from dietary CHP due to the gastric acidic environment. The antioxidant and protective CHP properties were repeatedly reported although the non-enzymatic mechanisms were scantly investigated. The CHP detoxifying activity towards α,β unsaturated carbonyls was never investigated in detail, although its open dipeptides might be effective as already observed for histidine containing dipeptides. Hence, this study investigated the scavenging properties of TRH, CHP and its open derivatives towards 4-hydroxy-2-nonenal. The obtained results revealed that Pro-His possesses a marked activity and is more reactive than l-carnosine. As investigated by DFT calculations, the enhanced reactivity can be ascribed to the greater electrophilicity of the involved iminium intermediate. These findings emphasize that the primary amine (as seen in l-carnosine) can be replaced by secondary amines with beneficial effects on the quenching mechanisms. Serum stability of the tested peptides was also evaluated, showing that Pro-His is characterized by a greater stability than l-carnosine. Docking simulations suggested that its hydrolysis can be catalyzed by serum carnosinase. Altogether, the reported results evidence that the antioxidant CHP properties can be also due to the detoxifying activity of its open dipeptides, which might be thus responsible for the beneficial effects induced by CHP containing food.

Tandem Friedel-Crafts-Alkylation-Enantioselective-Protonation by Artificial Enzyme Iminium Catalysis

The incorporation of organocatalysts into protein scaffolds holds the promise of overcoming some of the limitations of this powerful catalytic approach. Previously, we showed that incorporation of the non-canonical amino acid para-aminophenylalanine into the non-enzymatic protein scaffold LmrR forms a proficient and enantioselective artificial enzyme (LmrR_pAF) for the Friedel-Crafts alkylation of indoles with enals. The unnatural aniline side-chain is directly involved in catalysis, operating via a well-known organocatalytic iminium-based mechanism. In this study, we show that LmrR_pAF can enantioselectively form tertiary carbon centres not only during C-C bond formation, but also by enantioselective protonation, delivering a proton to one face of a prochiral enamine intermediate. The importance of various side-chains in the pocket of LmrR is distinct from the Friedel-Crafts reaction without enantioselective protonation, and two particularly important residues were probed by exhaustive mutagenesis.

Metal-Mediated Organocatalysis in Water: Serendipitous Discovery of Aldol Reaction Catalyzed by the [Ru(bpy)2(nornicotine)2]2+ Complex

The [Ru(bpy)2(Nor)2]2+ complex (Nor = nornicotine) is an efficient catalyst for the aldol reaction of acetone with activated benzaldehydes in a buffered aqueous solution. The metal plays the role of an activator for the nornicotine organocatalyst ligands. The resulting catalytic activity is potentiated by a factor of about 4.5 as compared to free nornicotine. Similar rate enhancements can be achieved by using Zn(II) cations as the activator. The observations are rationalized with the reduced basicity of the pyrrolidine N in nornicotine due to the enhanced electron withdrawal of the metal-complexed pyridyl moiety.

Remote C(sp3 )-H Acylation of Amides and Cascade Cyclization via N-Heterocyclic Carbene Organocatalysis

The direct functionalization of inert C(sp³ )-H bonds under environmentally benign catalytic conditions remains a challenging task in synthetic chemistry. Here, we report an organocatalytic remote C(sp³ )-H acylation of amides and cascade cyclization through a radical-mediated 1,5-hydrogen atom transfer mechanism using N-heterocyclic carbene as the catalyst. Notably, a diversity of nitrogen-containing substrates, including simple linear aliphatic carbamates and ortho-alkyl benzamides, can be successfully applied to this organocatalytic system. With the established protocol, over 120 examples of functionalized δ-amino ketones and isoquinolinones with diverse substituents were easily synthesized in up to 99 % yield under mild conditions. The robustness and generality of the organocatalytic strategy were further highlighted by the successful acylation of unactivated C(sp³ )-H bonds and late-stage modification of pharmaceutical molecules. Then, the asymmetric control of the radical reaction was attempted and proven feasible by using a newly designed chiral thiazolium catalyst, and moderate enantioselectivity was obtained at the current stage. Preliminary mechanistic investigations including several control reactions, KIE experiments, and computational studies shed light on the organocatalytic radical reaction mechanism.

C-H deuteration of organic compounds and potential drug candidates

C-H deuteration has been intricately developed to satisfy the urgent need for site-selectively deuterated organic frameworks. Deuteration has been primarily used to study kinetic isotope effects of reactions but recently its significance in pharmaceutical chemistry has been discovered. Deuterium labelled compounds have stolen the limelight since the inception of the first FDA-approved deuterated drug, for the treatment of chorea-associated Huntington's disease, and their pharmacological importance was realised by chemists, although surprisingly very late. Various approaches were developed to carry out site-selective deuteration. However, the most common and efficient method is hydrogen isotope exchange (HIE). This review summarises deuteration methods of various organic motifs containing C(sp²)-H and C(sp³)-H bonds utilizing C-H bond functionalisation as a key step along with a variety of catalysts, and exemplifies their biological relevance.

The Role of the Fused Ring in Bicyclic Triazolium Organocatalysts: Kinetic, X-ray, and DFT Insights

Bicyclic triazolium scaffolds are widely employed in N-heterocyclic carbene (NHC) organocatalysis. While the incorporation of a fused ring was initially for synthetic utility in accessing chiral, modular triazolyl scaffolds, recent results highlight the potential for impact upon reaction outcome with the underpinning origins unclear. The common first step to all triazolium-catalyzed transformations is C(3)-H deprotonation to form the triazolylidene NHC. Herein, we report an analysis of the impact of size of the fused (5-, 6-, and 7-membered, n = 1, 2, and 3, respectively) ring on the C(3) proton transfer reactions of a series of bicyclic triazolium salts. Rate constants for the deuteroxide-catalyzed C(3)-H/D-exchange of triazolium salts, kDO, were significantly influenced by the size of the adjacent fused ring, with the kinetic acidity trend, or protofugalities, following the order kDO (n = 1) > kDO (n = 2) ≈ kDO (n = 3). Detailed analyses of X-ray diffraction (XRD) data for 20 triazolium salts (including 16 new structures) and of computational data for the corresponding triazolylidene NHCs provide insight on structural effects of alteration of fused ring size. In particular, changes in internal triazolyl NCN angle and positioning of the most proximal CH2 with variation in fused ring size are proposed to influence the experimental protofugality order.

Pedal to the Metal: The Homogeneous Catalysis of the Native Chemical Ligation Reaction

The native chemical ligation reaction of peptide thioesters with cysteinyl peptides is a pivotal chemical process in the production of native or modified peptides and proteins, and well beyond in the preparation of various biomolecule analogs and materials. To benefit from this reaction at its fullest and to access all the possible applications, the experimentalist needs to know the factors affecting its rate and how to control it. This concept article presents the fundamental principles underlying the rate of the native chemical ligation and its homogeneous catalysis by nucleophiles. It has been prepared to serve as a quick guide in the search for an appropriate catalyst.

An atomistic view on the uptake of aromatic compounds by cyclodextrin immobilized on mesoporous silica

The effect of immobilized $$\upbeta$$ β -cyclodextrin (bCD) molecules inside a mesoporous silica support on the uptake of benzene and p-nitrophenol from aqueous solution was investigated using all-atom molecular dynamics (MD) simulations. The calculated adsorption isotherms are discussed with respect to the free energies of binding for a 1:1 complex of bCD and the aromatic guest molecule. The adsorption capacity of the bCD-containing material significantly exceeds the amount corresponding to a 1:1 binding scenario, in agreement with experimental observations. Beside the formation of 1:2 and, to a lesser extent, 1:3 host:guest complexes, also host–host interactions on the surface as well as more unspecific host–guest interactions govern the adsorption process. The demonstrated feasibility of classical all-atom MD simulations to calculate liquid phase adsorption isotherms paves the way to a molecular interpretation of experimental data that are too complex to be described by empirical models.