Enantioselective Synthesis of α-Trifluoromethyl Amines via Biocatalytic N-H Bond Insertion with Acceptor-Acceptor Carbene Donors

Asymmetric photoenzymatic incorporation of fluorinated motifs into olefins

Enzymes capable of assimilating fluorinated feedstocks are scarce. This situation poses a challenge for the biosynthesis of fluorinated compounds used in pharmaceuticals, agrochemicals, and materials. We developed a photoenzymatic hydrofluoroalkylation that integrates fluorinated motifs into olefins. The photoinduced promiscuity of flavin-dependent ene-reductases enables the generation of carbon-centered radicals from iodinated fluoroalkanes, which are directed by the photoenzyme to engage enantioselectively with olefins. This approach facilitates stereocontrol through interaction between a singular fluorinated unit and the enzyme, securing high enantioselectivity at β, γ, or δ positions of fluorinated groups through enzymatic hydrogen atom transfer-a process that is notably challenging with conventional chemocatalysis. This work advances enzymatic strategies for integrating fluorinated chemical feedstocks and opens avenues for asymmetric synthesis of fluorinated compounds.

Stereodivergent Synthesis of Pyridyl Cyclopropanes via Enzymatic Activation of Pyridotriazoles

Hemoproteins have recently emerged as powerful biocatalysts for new-to-nature carbene transfer reactions. Despite this progress, these strategies have remained largely limited to diazo-based carbene precursor reagents. Here, we report the development of a biocatalytic strategy for the stereoselective construction of pyridine-functionalized cyclopropanes via the hemoprotein-mediated activation of pyridotriazoles (PyTz) as stable and readily accessible carbene sources. This method enables the asymmetric cyclopropanation of a variety of olefins, including electron-rich and electrodeficient ones, with high activity, high stereoselectivity, and enantiodivergent selectivity, providing access to mono- and diarylcyclopropanes that incorporate a pyridine moiety and thus two structural motifs of high value in medicinal chemistry. Mechanistic studies reveal a multifaceted role of 7-halogen substitution in the pyridotriazole reagent toward favoring multiple catalytic steps in the transformation. This work provides the first example of asymmetric olefin cyclopropanation with pyridotriazoles, paving the way to the exploitation of these attractive and versatile reagents for enzyme-catalyzed carbene-mediated reactions.

Expanding chemistry through in vitro and in vivo biocatalysis

Living systems contain a vast network of metabolic reactions, providing a wealth of enzymes and cells as potential biocatalysts for chemical processes. The properties of protein and cell biocatalysts-high selectivity, the ability to control reaction sequence and operation in environmentally benign conditions-offer approaches to produce molecules at high efficiency while lowering the cost and environmental impact of industrial chemistry. Furthermore, biocatalysis offers the opportunity to generate chemical structures and functions that may be inaccessible to chemical synthesis. Here we consider developments in enzymes, biosynthetic pathways and cellular engineering that enable their use in catalysis for new chemistry and beyond.

Iron(II) Phthalocyanine-Catalyzed Homodimerization and Tandem Diamination of Diazo Compounds with Primary Amines: Access to Construct Substituted 2,3-Diaminosuccinonitriles in One-Pot

We herein first report the homodimerization and tandem diamination of diazo compounds with primary amines catalyzed by the iron(II) phthalocyanine (PcFe(II)), which can construct one C-C bond and two C-N bonds within 20 min in one-pot. Compared to the traditional metal-catalyzed N-H insertion reaction between amines with diazo reagents, the developed reaction almost does not generate the N-H insertion product, but the homodimerization/tandem diamination product. The proposed mechanism studies indicate that primary amines play a crucial role in the homocoupling of diazo compounds via dimerization of iron(III)-acetonitrile radical generated from the reaction between diazoacetonitrile with PcFe(II) coordinated by bis(amines); the β-hydride elimination is involved, and then, the attack of primary amines toward the carbon atoms on the formed C-C bond is followed. Moreover, this novel reaction can be used to effectively prepare substituted 2,3-diaminosuccinonitriles with high yields and even up to >99:1 d.r., encouragingly these products contain both 1,2-diamines and succinonitrile motifs, which are two classes of important organic compounds with significant applications in many yields. This reaction is also suitable for the gram-scale preparation of 2,3-bis(phenylamino)succinonitrile (2a) with a yield of 84%. Therefore, the developed reaction represents a new type of transformation.

Rh-Catalyzed Chemodivergent [3+3] Annulations of Diazoenals and α-Aminoketones: Direct Synthesis of Functionalized 1,2-Dihydropyridines and Fused 1,4-Oxazines

Novel rhodium-catalyzed [3+3] annulations of diazoenals and α-amino ketones has been disclosed here. The reactivity of diazoenals has been switched from carbenoid to vinylogous NH-insertion by altering acyclic to cyclic α-amino ketones. In this direction, we report an efficient strategy to synthesize 1,2-dihydropyridines (DHPs) and fused 1,4-oxazines. Mechanistic investigation revealed that the formyl group is necessary for carbenoid [3+3] annulation and the cyclohexyl group is the dictating factor for vinylogous NH- insertion. The synthetic utility of 1,2-dihydropyridines was demonstrated by synthesizing piperidine, pyrido[1,2-a]indole, and 2-pyridone scaffolds. Further, structural diversification of fused 1,4-oxazines resulted in the short synthesis of hexahydroquinolin-2(1H)-ones, hexahydro quinolines and tetrahydroquinolinones via ring opening rearrangement and a new oxidative deformylation, respectively.

Recent advances in biocatalytic C-N bond-forming reactions

Molecules containing C-N bonds are of paramount importance in a diverse array of organic-based materials, natural products, pharmaceutical compounds, and agricultural chemicals. Biocatalytic C-N bond-forming reactions represent powerful strategies for producing these valuable targets, and their significance in the field of synthetic chemistry has steadily increased over the past decade. In this review, we provide a concise overview of recent advancements in the development of C-N bond-forming enzymes, with a particular emphasis on the inherent chemistry involved in these enzymatic processes. Overall, these enzymatic systems have proven their potential in addressing long-standing challenges in traditional small-molecule catalysis.

Aminoquinoline-Based Tridentate (NNN)-Copper Catalyst for C-N Bond-Forming Reactions from Aniline and Diazo Compounds

A new tridentate Cu2+ complex based on (E)-1-(pyridin-2-yl)-N-(quinolin-8-yl)methanimine (PQM) was generated and characterized to support the activation of diazo compounds for the formation of new C-N bonds. This neutral Schiff base ligand was structurally characterized to coordinate with copper(II) in an equatorial fashion, yielding a distorted octahedral complex. Upon characterization, this copper(II) complex was used to catalyze an efficient and cost-effective protocol for C-N bond formation between N-nucleophiles and copper carbene complexes arising from the activation of diazo carbonyl compounds. A substrate scope of approximately 15 different amine-based substrates was screened, yielding 2° or 3° amine products with acceptable to good yields under mild reaction conditions. Reactivity towards phenol and thiophenol were also screened, showing relatively weak C-O or C-S bond formation under optimized conditions.

Silver(I)/Dirhodium(II) Catalytic Platform for Asymmetric N-H Insertion Reaction of Heteroaromatics

Transition-metal-catalyzed enantioselective N-H insertion reactions of carbene species offer a powerful and straightforward strategy to produce chiral nitrogen-containing compounds. Developing highly selective insertion reactions using indole variants can meet synthetic demand. Herein we present an asymmetric insertion reaction into N-H bonds of the aromatic heterocycles using donor/acceptor-substituted diazo compounds based on a heteronuclear catalytic platform. Although a previously developed catalysis comprising chiral silver catalyst or dirhodium(II,II) paddlewheel complexes with and without chiral phosphoric acid showed modest performance, a unique combination of widely available Rh2(OAc)4 and silver(I) phosphate dimer [(S)-TRIP-Ag]2 enabled asymmetric carbene insertion reactions (up to 98% ee). Moreover, the Ag/Rh catalytic system facilitated regioselective and enantioselective C-H functionalization of protic indoles. Mechanistic investigation based on density functional theory indicated that an in situ-generated Ag-Rh trimetallic enolate is protonated in a chiral environment.

Mechanistic manifold in a hemoprotein-catalyzed cyclopropanation reaction with diazoketone

Hemoproteins have recently emerged as promising biocatalysts for new-to-nature carbene transfer reactions. However, mechanistic understanding of the interplay between productive and unproductive pathways in these processes is limited. Using spectroscopic, structural, and computational methods, we investigate the mechanism of a myoglobin-catalyzed cyclopropanation reaction with diazoketones. These studies shed light on the nature and kinetics of key catalytic steps in this reaction, including the formation of an early heme-bound diazo complex intermediate, the rate-determining nature of carbene formation, and the cyclopropanation mechanism. Our analyses further reveal the existence of a complex mechanistic manifold for this reaction that includes a competing pathway resulting in the formation of an N-bound carbene adduct of the heme cofactor, which was isolated and characterized by X-ray crystallography, UV-Vis, and Mössbauer spectroscopy. This species can regenerate the active biocatalyst, constituting a non-productive, yet non-destructive detour from the main catalytic cycle. These findings offer a valuable framework for both mechanistic analysis and design of hemoprotein-catalyzed carbene transfer reactions.

Visible-Light-Driven C,N-Selective Heteroarylation of N-Fluoroalkyl Hydroxylamine Reagents with Quinoxalin-2(1H)-ones

Herein, we disclose a direct and powerful strategy for the synthesis of highly valuable α-trifluoromethylamine and N-trifluoroethylamine derivatives from a visible-light-promoted C,N-selective heteroarylation of N-trifluoroethyl hydroxylamine reagents with quinoxalin-2(1H)-ones under ambient conditions. The chemoselectivity of the process (trifluoroalkylation or N-trifluoroethylamination) can easily be dictated and modulated by a selection of N-trifluoroethyl hydroxylamine substrates. The key to success is the protecting group on the N atom of hydroxylamine reagents, which can control the process of 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Remarkable features of this method include mild conditions, easy operation, high selectivity, and excellent functional group tolerability. More importantly, the trifluoroalkylated products can be readily derivatized into other interesting imidazo-fused heterocycles that would be of great potential for the exploitation of pharmaceutically relevant molecules.

Engineering of Dual-Function Vitreoscilla Hemoglobin: A One-Pot Strategy for the Synthesis of Unnatural α-Amino Acids

Despite a well-developed and growing body of work on the directed evolution of hemoproteins, the potential of hemoproteins to catalyze non-natural reactions remains underexplored. This paper reports a new biocatalytic strategy for the one-pot synthesis of unnatural α-amino acids. Engineered variants of dual-function Vitreoscilla hemoglobin were found to efficiently catalyze N-H insertion and C-H sp3 alkylation, providing moderate to excellent yields (57%-95%) of unnatural α-amino acid derivatives and turnover numbers (1425-2375).

Reversing the Enantioselectivity of Enzymatic Carbene N-H Insertion Through Mechanism-Guided Protein Engineering

We report a computationally driven approach to access enantiodivergent enzymatic carbene N-H insertions catalyzed by P411 enzymes. Computational modeling was employed to rationally guide engineering efforts to control the accessible conformations of a key lactone-carbene (LAC) intermediate in the enzyme active site by installing a new H-bond anchoring point. This H-bonding interaction controls the relative orientation of the reactive carbene intermediate, orienting it for an enantioselective N-nucleophilic attack by the amine substrate. By combining MD simulations and site-saturation mutagenesis and screening targeted to only two key residues, we were able to reverse the stereoselectivity of previously engineered S-selective P411 enzymes. The resulting variant, L5_FL-B3, accepts a broad scope of amine substrates for N-H insertion with excellent yields (up to >99 %), high efficiency (up to 12 300 TTN), and good enantiocontrol (up to 7 : 93 er).

Biocatalytic Fluoroalkylation Using Fluorinated S-Adenosyl-l-methionine Cofactors

Modification of organic molecules with fluorine functionalities offers a critical approach to develop new pharmaceuticals. Here, we report a multienzyme strategy for biocatalytic fluoroalkylation using S-adenosyl-l-methionine (SAM)-dependent methyltransferases (MTs) and fluorinated SAM cofactors prepared from ATP and fluorinated l-methionine analogues by an engineered human methionine adenosyltransferase hMAT2AI322A. This work introduces the first example of biocatalytic 3,3-difluoroallylation. Importantly, this strategy can be applied to late-stage site-selective fluoroalkylation of complex molecule vancomycin with conversions up to 99%.

A Photoenzymatic Strategy for Radical-Mediated Stereoselective Hydroalkylation with Diazo Compounds

Carbene insertion reactions initiated with diazo compounds have been widely used to develop unnatural enzymatic reactions. However, alternative functionalization of diazo compounds in enzymatic processes has been unexploited. Herein, we describe a photoenzymatic strategy for radical-mediated stereoselective hydroalkylation with diazo compounds. This method generates carbon-centered radicals through an ene reductase catalyzed photoinduced electron transfer process from diazo compounds, enabling the synthesis of γ-stereogenic carbonyl compounds in good yields and stereoselectivities. This study further expands the possible reaction patterns in photo-biocatalysis and offers a new approach to solving the selectivity challenges of radical-mediated reactions.

Ligand-enabled silver-catalyzed carbene insertion into the N–H bond of aliphatic and electron-rich aromatic amines

The stable effect of tris(pyrazolyl)borate ligand and 1,2-diaminocyclohexane ligand on the silver metal center was used respectively, to realize the efficient N–H bond insertion reaction of carbene to aliphatic amines and aromatic amines.

New Additions to the Arsenal of Biocatalysts for Noncanonical Amino Acid Synthesis

Noncanonical amino acids (ncAAs) merge the conformational behavior and native interactions of proteinogenic amino acids with nonnative chemical motifs and have proven invaluable in developing modern therapeutics. This blending of native and nonnative characteristics has resulted in essential drugs like nirmatrelvir, which comprises three ncAAs and is used to treat COVID-19. Enzymes are appearing prominently in recent syntheses of ncAAs, where they demonstrate impressive control over the stereocenters and functional groups found therein. Here we review recent efforts to expand the biocatalyst arsenal for synthesizing ncAAs with natural enzymes. We also discuss how new-to-nature enzymes can contribute to this effort by catalyzing reactions inspired by the vast repertoire of chemical catalysis and acting on substrates that would otherwise not be used in synthesizing ncAAs. Abiotic enzyme-catalyzed reactions exploit the selectivity afforded by a macromolecular catalyst to access molecules not available to natural enzymes and perhaps not even chemical catalysis.

YfeX - A New Platform for Carbene Transferase Development with High Intrinsic Reactivity

Carbene transfer biocatalysis has evolved from basic science to an area with vast potential for the development of new industrial processes. In this study, we show that YfeX, naturally a peroxidase, has great potential for the development of new carbene transferases, due to its high intrinsic reactivity, especially for the N-H insertion reaction of aromatic and aliphatic primary and secondary amines. YfeX shows high stability against organic solvents (methanol and DMSO), greatly improving turnover of hydrophobic substrates. Interestingly, in styrene cyclopropanation, WT YfeX naturally shows high enantioselectivity, generating the trans product with 87 % selectivity for the (R,R) enantiomer. WT YfeX also catalyzes the Si-H insertion efficiently. Steric effects in the active site were further explored using the R232A variant. Quantum Mechanics/Molecular Mechanics (QM/MM) calculations reveal details on the mechanism of Si-H insertion. YfeX, and potentially other peroxidases, are exciting new targets for the development of improved carbene transferases.

Co-Crystal Structure-Guided Optimization of Dual-Functional Small Molecules for Improving the Peroxygenase Activity of Cytochrome P450BM3

We recently developed an artificial P450–H₂O₂ system assisted by dual-functional small molecules (DFSMs) to modify the P450BM3 monooxygenase into its peroxygenase mode, which could be widely used for the oxidation of non-native substrates. Aiming to further improve the DFSM-facilitated P450–H₂O₂ system, a series of novel DFSMs having various unnatural amino acid groups was designed and synthesized, based on the co-crystal structure of P450BM3 and a typical DFSM, N-(ω-imidazolyl)-hexanoyl-L-phenylalanine, in this study. The size and hydrophobicity of the amino acid residue in the DFSM drastically affected the catalytic activity (up to 5-fold), stereoselectivity, and regioselectivity of the epoxidation and hydroxylation reactions. Docking simulations illustrated that the differential catalytic ability among the DFSMs is closely related to the binding affinity and the distance between the catalytic group and heme iron. This study not only enriches the DFSM toolbox to provide more options for utilizing the peroxide-shunt pathway of cytochrome P450BM3, but also sheds light on the great potential of the DFSM-driven P450 peroxygenase system in catalytic applications based on DFSM tunability.

Gold(I) α‐Trifluoromethyl Carbenes: Synthesis, Characterization and Reactivity Studies

Aryl trifluoromethyl diazomethanes 2‐R (R=Ph, OMe, CF₃) are readily decomposed by the (o‐carboranyl)diphosphine gold(I) complex 1. The resulting α‐CF₃ substituted carbene complexes 3‐R have been characterized by multi‐nuclear NMR spectroscopy as well as X‐ray crystallography (for 3‐Ph). The bonding situation was thoroughly assessed by computational means, showing stabilization of the electrophilic carbene center by π‐donation from the aryl substituent and backdonation from Au, as enhanced by the chelating P^P ligand. Reactivity studies under stoichiometric and catalytic conditions substantiate typical carbene‐type behavior for 3‐Ph.