Braca

Benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical catalyzes cascade cyclization reactions to construct polycyclic compounds both intra- and intermolecularly. This catalytic cascade radical reaction allows rapid construction of complex molecular architectures through the formation of two or three bonds in a single operation, respectively.

Abstract

Cascade radical cyclization constitutes an atom- and step-economic route for rapid assembly of polycyclic molecular skeletons. Although an array of redox-active metal catalysts has recently shown robust applications in enabling various catalytic cascade radical processes, the use of free organic radical as the catalyst, which is capable of triggering strategically distinct cascades, has rarely been developed. Here, we disclosed that the benzimidazolium-based N-heterocyclic carbene (NHC)-boryl radical is capable of catalyzing cascade cyclization reactions in both intra- and intermolecular pathways, assembling [5,5] fused bicyclic and [6,6,6] fused tricyclic molecules, respectively. The catalytic reactions start with the chemo- and regioselective addition of the boryl radical catalyst to a tethered alkene or alkyne moiety, followed by either an intramolecular formal [3+2] or an intermolecular [2+2+2] cycloaddition process to construct bicyclo[3.3.0]octane or tetrahydrophenanthridine skeletons, respectively. Eventually, a β-elimination occurs to release the boryl radical catalyst, completing a catalytic cycle. High to excellent diastereoselectivity is achieved in both catalytic reactions under substrate control.

Nature Catalysis, Published online: 01 April 2024; doi:10.1038/s41929-024-01140-5

Polar and steric effects usually dictate the regioselectivity in homolytic aromatic substitution. Now a method for direct ortho-selective C–H amination of aromatics with diverse side chains as directing groups is disclosed, by which the iron catalyst coordinates both the substrate and the aminyl radical.

Unnatural amino acids (UAAs) have shown great potential to enhance the pharmacological properties of peptides and represent a key motif in a wide range of biologically relevant natural products. Herein, we disclose a protocol that facilitates the asymmetric synthesis of UAAs from widely abundant amines through photoredox-mediated α-amino C–H bond activation. The platform utilizes either the strongly oxidizing organic acridinium or decatungstate as a photocatalyst to generate α-amino radicals from a variety of amines, which are then effectively coupled with a chiral glyoxylate-derived N-sulfinyl imine. The disclosed platform provides a general entry to various decorated unnatural α-amino acid derivatives, accommodating a diverse array of functional groups.

Radical-type carbene transfer catalysis is an efficient method for the direct functionalization of C–H and C=C bonds. However, carbene radical complexes are currently formed via high-energy carbene precursors, such as diazo compounds or iodonium ylides. Many of these carbene precursors require additional synthetic steps, have an explosive nature or generate halogenated waste. Con-sequently, the utilization of carbene radical catalysis is limited by specific carbene precursors to access the carbene radical inter-mediate. In this study, we generate a cobalt(III) carbene radical complex from dimethyl malonate, which is commercially available and bench-stable. EPR and NMR spectroscopy were used to identify the intermediates and showed that the cobalt(III) carbene radical complex is formed upon light irradiation. In presence of styrene, carbene transfer occurred, forming cyclopropane as the product. With this photochemical method, we demonstrate that dimethyl malonate can be used as an alternative carbene precursor in the formation of a cobalt(III) carbene radical complex.

Nature, Published online: 28 March 2024; doi:10.1038/s41586-024-07341-z

Copper-catalyzed dehydrogenation or lactonization of C(sp³)−H bonds

The importance of protein templates in artificial enzyme design is illustrated through genetic code expansion. Incorporation of a secondary amine into the nucleotide-binding DHFR and multidrug-binding LmrR resulted in catalytic entities, with the former favoring the use of NADPH as the hydride source for reactions, whereas the latter required biomimetic 1-benzyl-1,4-dihydronicotinamide (BNAH).

Abstract

Secondary amines, due to their reactivity, can transform protein templates into catalytically active entities, accelerating the development of artificial enzymes. However, existing methods, predominantly reliant on modified ligands or N-terminal prolines, impose significant limitations on template selection. In this study, genetic code expansion was used to break this boundary, enabling secondary amines to be incorporated into alternative proteins and positions of choice. Pyrrolysine analogues carrying different secondary amines could be incorporated into superfolder green fluorescent protein (sfGFP), multidrug-binding LmrR and nucleotide-binding dihydrofolate reductase (DHFR). Notably, the analogue containing a D-proline moiety demonstrated both proteolytic stability and catalytic activity, conferring LmrR and DHFR with the desired transfer hydrogenation activity. While the LmrR variants were confined to the biomimetic 1-benzyl-1,4-dihydronicotinamide (BNAH) as the hydride source, the optimal DHFR variant favorably used the pro-R hydride from NADPH for stereoselective reactions (e.r. up to 92 : 8), highlighting that a switch of protein template could broaden the nucleophile option for catalysis. Owing to the cofactor compatibility, the DHFR-based secondary amine catalysis could be integrated into an enzymatic recycling scheme. This established method shows substantial potential in enzyme design, applicable from studies on enzyme evolution to the development of new biocatalysts.

Due to the scarcity of C–F bond forming enzymatic activities in nature and the contrasting ubiquity of organofluorine moieties in bioactive compounds, developing new biocatalytic fluorination reactions represents a preeminent challenge in enzymology, biocatalysis, and synthetic biology. Additionally, catalytic asymmetric C(sp3)–H fluorination remains a challenging problem facing synthetic chemists. Although many nonheme Fe halogenases have been discovered to promote C(sp3)–H halogenation reactions, to date, efforts to convert these Fe halogenases to fluorinases have remained unsuccessful. We repurposed a plant-derived natural nonheme enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) to catalyze unnatural enantioselective C(sp3)–H fluorination via a radical rebound mechanism. Directed evolution afforded C–H fluorinating enzyme ACCOCHF displaying 200-fold higher activity, substantially improved chemoselectivity and excellent enantioselectivity, converting a range of substrates into enantioenriched organofluorine products. Notably, almost all the beneficial mutations were found to be distal to the Fe centre, underscoring the importance of substrate tunnel engineering in nonheme Fe biocatalysis. Computational studies revealed that the radical rebound step with the Fe(III)–F intermediate has an exceedingly low activation barrier of 3.4 kcal/mol, highlighting a new avenue to expand the catalytic repertoire of enzymes to encompass asymmetric C–F bond formation.

Nature Synthesis, Published online: 28 March 2024; doi:10.1038/s44160-024-00507-7

Methods for enzymatic C–F bond formation are rare. Now an enzymatic method for enantioselective C(sp3)–F bond formation is reported, through reprogramming non-haem iron enzyme (S)-2-hydroxypropylphosphonate epoxidase. Mechanistic studies reveal that the process proceeds through an iron-mediated radical fluorine transfer process.

Proteins and enzymes can be repurposed by the introduction of artificial cofactors or non-canonical amino acids (ncAAs). These artificial biocatalytic constructs turned into valuable tools to perform new-to-nature reactions with biocatalysts increasing their scope. This perspective focuses on the limitations and future application for in vivo biosynthetic pathways.

Abstract

Astonishing progress has been achieved in unlocking new-to-nature biocatalysis in the past decades. The progress in protein engineering enabled research to efficiently incorporate artificial structural elements into enzyme design. Recent trends include cofactor mimetics, artificial metalloenzymes and non-canonical amino acids. In this perspective article, we present the state-of-the-art, discuss recent examples and our view on what we call artificial biocatalysis. Although these artificial systems undoubtedly increase the scope of biocatalysis, their applicability remains challenging. Fundamental questions regarding the impact of this research field are addressed in this perspective.

Photogenerated radicals have good redox properties, thus promoting the widespread application of photocatalysis for environmental treatment and organic synthesis. The generation and application of photogenerated radicals are related to carriers and reactive molecules, such as a variety of alkane radicals in photocatalytic dissolution. In this conceptual article, we discuss some basic properties of photogenerated radicals and recent related research work.

Abstract

Free radicals are increasingly recognized as active intermediate reactive species that can participate in various redox processes, significantly influencing the mechanistic pathways of reactions. Numerous researchers have investigated the generation of one or more distinct photogenerated radicals, proposing various hypotheses to explain the reaction mechanisms. Notably, recent research has demonstrated the emergence of photogenerated radicals in innovative processes, including organic chemical reactions and the photocatalytic dissolution of precious metals. To harness the potential of these free radicals more effectively, it is imperative to consolidate and analyze the processes and action modes of these photogenerated radicals. This conceptual paper delves into the latest advancements in understanding the mechanics of photogenerated radicals.

A new polyHIPE (high internal phase emulsion) resin served as a solid support system for both a halogenase enzyme and a benzothiadiazole (BTZ) photosensitizer. It was then used in a continuous flow system for the electrophilic bromination of aromatic substrates. This support offers excellent single pass and recirculation yields. It is stable and reusable and allows for straightforward product separation owing to its heterogeneous nature.

Abstract

The use of a dual resin for photobiocatalysis, encompassing both a photocatalyst and an immobilized enzyme, brings several challenges, including effective immobilization, maintaining photocatalyst and enzyme activity and ensuring sufficient light penetration. However, the benefits, such as integrated processes, reusability, easier product separation, and potential for scalability, can outweigh these challenges, making dual resin systems promising for efficient and sustainable photobiocatalytic applications. In this study, we employed a photosensitizer-containing porous emulsion-templated polymer as a functional support that is used to covalently anchor a chloroperoxidase from Curvularia inaequalis (CiVCPO). We demonstrate the versatility of this heterogeneous photobiocatalytic material, which enables the bromination of four aromatic substrates, including rutin—a natural occurring flavonol—under blue light (456 nm) irradiation and continuous flow conditions.

Organohalogen compounds are extensively used as building blocks, intermediates, pharmaceuticals, and agrochemicals due to their unique chemical and biological properties. Installing halogen substituents, however, frequently requires functionalized starting materials and multistep functional group interconversion. Several classes of halogenases evolved in nature to enable halogenation of a different classes of substrates; for example, site-selective halogenation of electron rich aromatic compounds is catalyzed by flavin-dependent halogenases (FDHs). Mechanistic studies have shown that these enzymes use FADH2 supplied by a flavin reductase (FRed) to reduce O2 to water with concomitant oxidation of X- to HOX (X = Cl, Br, I). This species travels through a tunnel within the enzyme to access the FDH active site. Here, it is believed to H-bond to an active site lysine proximal to bound substrate, enabling electrophilic halogenation with selectivity imparted via molecular recognition, rather than directing groups or strong electronic activation. The unique selectivity of FDHs led to several early biocatalysis efforts, preparative halogenation was rare, and the hallmark catalyst-controlled selectivity of FDHs did not translate to non-native substrates. FDH engineering was limited to site-directed mutagenesis, which resulted in modest changes in site-selectivity or substrate preference. To address these limitations, we optimized expression conditions for the FDH RebH and its cognate FRed, RebF. We then showed that RebH could be used for preparative halogenation of non-native substrates with catalyst-controlled selectivity. We reported the first examples in which the stability, substrate scope, and site selectivity of an FDH were improved to synthetically useful levels via directed evolution. X-ray crystal structures of evolved FDHs and reversion mutations showed that random mutations throughout the RebH structure were critical to achieving high levels of activity and selectivity on diverse aromatic substrates, and these data were used in combination with molecular dynamics simulations to develop predictive model for FDH selectivity. Finally, we used family-wide genome mining to identify a diverse set of FDHs with novel substrate scope and complementary regioselectivity on large, three-dimensionally complex compounds. The diversity of our evolved and mined FDHs allowed us to pursue synthetic applications beyond simple aromatic halogenation. For example, we established that FDHs catalyze enantioselective reactions involving desymmetrization, atroposelective halogenation, and halocyclization. These results highlight the ability of FDH active sites to tolerate different substrate topologies. This utility was further expanded by our recent studies on the single component FDH/FRed, AetF. While we were initially drawn to AetF because it does not require a separate FRed, we found that it halogenates substrates that are not halogenated efficiently or at all by other FDHs and provides high enantioselectivity for reactions that could only be achieved using RebH variants after extensive mutagenesis. Perhaps most notably, AetF catalyzes site-selective aromatic iodination and enantioselective iodoetherification. Together, these studies highlight the origins of FDH engineering, the utility and limitations of the enzymes developed to date, and the promise of FDHs for an ever-expanding range of biocatalytic halogenation reactions.

Photoenzymatic catalysis is an emerging platform for asymmetric synthesis. In most of these reactions, the protein templates a charge transfer complex between the cofactor and substrate, which absorbs in the blue region of the electromagnetic spectrum. Here, we report the engineering of a photoenzymatic ‘ene’-reductase to utilize red light (620 nm) for a radical cyclization reaction. Mechanistic studies indicate that red light ac-tivity is achieved by introducing a broadly absorbing shoulder off the previously identified cyan absorption feature. Molecular dynamics simulations, docking, and excited-state calculations suggest that red light absorption is a 𝜋→ 𝜋* transition from flavin to the substrate, while the cyan feature is the red-shift of the flavin 𝜋→ 𝜋* transition, which occurs upon substrate binding. Differences in the excitation event help to disfavor alkylation of the flavin cofactor, a pathway for catalyst decomposition observed with cyan light but not red.

Metalloradical catalysis (MRC) represents a versatile approach for controlling reactivity and selectivity in radical chemistry. By harnessing metalloradicals as one-electron catalysts, MRC facilitates the homolytic activation of substrates, leading to the generation of metal-entangled organic radicals as pivotal intermediates. The distinctive stepwise radical mechanism in MRC paves the way for innovative advancements in the field of organic synthesis.

Abstract

Since Friedrich Wöhler's groundbreaking synthesis of urea in 1828, organic synthesis over the past two centuries has predominantly relied on the exploration and utilization of chemical reactions rooted in two-electron heterolytic ionic chemistry. While one-electron homolytic radical chemistry is both rich in fundamental reactivities and attractive with practical advantages, the synthetic application of radical reactions has been long hampered by the formidable challenges associated with the control over reactivity and selectivity of high-energy radical intermediates. To fully harness the untapped potential of radical chemistry for organic synthesis, there is a pressing need to formulate radically different concepts and broadly applicable strategies to address these outstanding issues. In pursuit of this objective, researchers have been actively developing metalloradical catalysis (MRC) as a comprehensive framework to guide the design of general approaches for controlling over reactivity and stereoselectivity of homolytic radical reactions. Essentially, MRC exploits the metal-centered radicals present in open-shell metal complexes as one-electron catalysts for homolytic activation of substrates to generate metal-entangled organic radicals as the key intermediates to govern the reaction pathway and stereochemical course of subsequent catalytic radical processes. Different from the conventional two-electron catalysis by transition metal complexes, MRC operates through one-electron chemistry utilizing stepwise radical mechanisms.

Nature Chemical Biology, Published online: 11 March 2024; doi:10.1038/s41589-024-01579-4

Reprogramming of the genetic code allows the synthesis of proteins using new building blocks, thus opening the door to the development of a wider variety of medicines and biocatalysts; however, it is currently limited to α-amino acids. A new study has now reported the incorporation of β-linked and α,α-disubstituted monomers into a ribosome-synthesized protein.

Nature, Published online: 07 March 2024; doi:10.1038/d41586-024-00690-9

A Dune-loving worm palaeontologist makes the case that worms have been just as important on Earth as they are in the blockbuster film.