Enzyme-Catalyzed Carbonyl Olefination by theE. coliProtein YfeX in the Absence of Phosphines

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.

Catalytic Synthesis of 1H-2-Benzoxocins: Cobalt(III)-Carbene Radical Approach to 8-Membered Heterocyclic Enol Ethers

The metallo-radical activation of ortho-allylcarbonyl-aryl N-arylsulfonylhydrazones with the paramagnetic cobalt(II) porphyrin catalyst [CoII(TPP)] (TPP = tetraphenylporphyrin) provides an efficient and powerful method for the synthesis of novel 8-membered heterocyclic enol ethers. The synthetic protocol is versatile and practical and enables the synthesis of a wide range of unique 1H-2-benzoxocins in high yields. The catalytic cyclization reactions proceed with excellent chemoselectivities, have a high functional group tolerance, and provide several opportunities for the synthesis of new bioactive compounds. The reactions are shown to proceed via cobalt(III)-carbene radical intermediates, which are involved in intramolecular hydrogen transfer (HAT) from the allylic position to the carbene radical, followed by a near-barrierless radical rebound step in the coordination sphere of cobalt. The proposed mechanism is supported by experimental observations, density functional theory (DFT) calculations, and spin trapping experiments.

An Enzymatic Platform for the Highly Enantioselective and Stereodivergent Construction of Cyclopropyl-δ-lactones

Abiological enzymes offers new opportunities for sustainable chemistry. Herein, we report the development of biological catalysts derived from sperm whale myoglobin that exploit a carbene transfer mechanism for the asymmetric synthesis of cyclopropane-fused-δ-lactones, which are key structural motifs found in many biologically active natural products. While hemin, wild-type myoglobin, and other hemoproteins are unable to catalyze this reaction, the myoglobin scaffold could be remodeled by protein engineering to permit the intramolecular cyclopropanation of a broad spectrum of homoallylic diazoacetate substrates in high yields and with up to 99 % enantiomeric excess. Via an alternate evolutionary trajectory, a stereodivergent biocatalyst was also obtained for affording mirror-image forms of the desired bicyclic products. In combination with whole-cell transformations, the myoglobin-based biocatalyst was used for the asymmetric construction of a cyclopropyl-δ-lactone scaffold at a gram scale, which could be further elaborated to furnish a variety of enantiopure trisubstituted cyclopropanes.

Organic solvent stability and long-term storage of myoglobin-based carbene transfer biocatalysts

Recent years have witnessed a rapid increase in the application of enzymes for chemical synthesis and manufacturing, including the industrial-scale synthesis of pharmaceuticals using multienzyme processes. From an operational standpoint, these bioprocesses often require robust biocatalysts capable of tolerating high concentrations of organic solvents and possessing long shelflife stability. In this work, we investigated the activity and stability of myoglobin (Mb)-based carbene transfer biocatalysts in the presence of organic solvents and after lyophilization. Our studies demonstrate that Mb-based cyclopropanases possess remarkable organic solvent stability, maintaining high levels of activity and stereoselectivity in the presence of up to 30%-50% (v/v) concentrations of various organic solvents, including ethanol, methanol, N,N-dimethylformamide, acetonitrile, and dimethyl sulfoxide. Furthermore, they tolerate long-term storage in lyophilized form, both as purified protein and as whole cells, without significant loss in activity and stereoselectivity. These stability properties are shared by Mb-based carbene transferases optimized for other type of asymmetric carbene transfer reactions. Finally, we report on simple protocols for catalyst recycling as whole-cell system and for obviating the need for strictly anaerobic conditions to perform these transformations. These findings demonstrate the robustness of Mb-based carbene transferases under operationally relevant conditions and should help guide the application of these biocatalysts for synthetic applications.

Multireference Ground and Excited State Electronic Structures of Free- versus Iron Porphyrin-Carbenes

Iron porphyrin carbenes (IPCs) are important reaction intermediates in engineered carbene transferase enzymes and homogeneous catalysis. However, discrepancies between theory and experiment complicate the understanding of IPC electronic structure. In the literature, this has been framed as whether the ground state is an open- vs closed-shell singlet (OSS vs CSS). Here we investigate the structurally dependent ground and excited spin-state energetics of a free carbene and its IPC analogs with variable trans axial ligands. In particular, for IPCs, multireference ab initio wave function methods are more consistent with experiment and predict a mixed singlet ground state that is dominated by the CSS (Fe(II) ← {:C(X)Y}⁰) configuration (i.e., electrophilic carbene) but that also has a small, non-negligible contribution from an Fe(III)-{C(X)Y}^-• configuration (hole in d(xz), i.e., radical carbene). In the multireference approach, the "OSS-like" excited states are metal-to-ligand charge transfer (MLCT) in nature and are energetically well above the CSS-dominated ground state. The first, lowest energy of these "OSS-like" excited states is predicted to be heavily weighted toward the Fe(III)-{C(X)Y}^-• (hole in d(yz)) configuration. As expected from exchange considerations, this state falls energetically above a triplet of the same configuration. Furthermore, potential energy surfaces (PESs) along the IPC Fe-C(carbene) bond elongation exhibit increasingly strong mixings between CSS/OSS characters, with the Fe(III)-{C(X)Y}^-• configuration (hole in d(xz)) growing in weight in the ground state during bond elongation. The relative degree of electrophilic/radical carbene character along this structurally relevant PES can potentially play a role in reactivity and selectivity patterns in catalysis. Future studies on IPC reaction coordinates should evaluate contributions from ground and excited state multireference character.

Broadening the scope of biocatalytic C-C bond formation

The impeccable control over chemo-, site-, and stereoselectivity possible in enzymatic reactions has led to a surge in the development of new biocatalytic methods. Despite carbon-carbon (C-C) bonds providing the central framework for organic molecules, development of biocatalytic methods for their formation has been largely confined to the use of a select few lyases over the last several decades, limiting the types of C-C bond-forming transformations possible through biocatalytic methods. This Review provides an update on the suite of enzymes available for highly selective biocatalytic C-C bond formation. Examples will be discussed in reference to the (1) native activity of enzymes, (2) alteration of activity through protein or substrate engineering for broader applicability, and (3) utility of the biocatalyst for abiotic synthesis.

C-H Amination via Nitrene Transfer Catalyzed by Mononuclear Non-Heme Iron-Dependent Enzymes

Expanding the reaction scope of natural metalloenzymes can provide new opportunities for biocatalysis. Mononuclear non-heme iron-dependent enzymes represent a large class of biological catalysts involved in the biosynthesis of natural products and catabolism of xenobiotics, among other processes. Here, we report that several members of this enzyme family, including Rieske dioxygenases as well as α-ketoglutarate-dependent dioxygenases and halogenases, are able to catalyze the intramolecular C-H amination of a sulfonyl azide substrate, thereby exhibiting a promiscuous nitrene transfer reactivity. One of these enzymes, naphthalene dioxygenase (NDO), was further engineered resulting in several active site variants that function as C-H aminases. Furthermore, this enzyme could be applied to execute this non-native transformation on a gram scale in a bioreactor, thus demonstrating its potential for synthetic applications. These studies highlight the functional versatility of non-heme iron-dependent enzymes and pave the way to their further investigation and development as promising biocatalysts for non-native metal-catalyzed transformations.

A de novo peroxidase is also a promiscuous yet stereoselective carbene transferase

While the bottom-up design of enzymes appears to be an intractably complex problem, a minimal approach that combines elementary, de novo-designed proteins with intrinsically reactive cofactors offers a simple means to rapidly access sophisticated catalytic mechanisms. Not only is this method proven in the reproduction of powerful oxidative chemistry of the natural peroxidase enzymes, but we show here that it extends to the efficient, abiological—and often asymmetric—formation of strained cyclopropane rings, nitrogen–carbon and carbon–carbon bonds, and the ring expansion of a simple cyclic molecule to form a precursor for NAD+, a fundamentally important biological cofactor. That the enzyme also functions in vivo paves the way for its incorporation into engineered biosynthetic pathways within living organisms.

By constructing an in vivo-assembled, catalytically proficient peroxidase, C45, we have recently demonstrated the catalytic potential of simple, de novo-designed heme proteins. Here, we show that C45’s enzymatic activity extends to the efficient and stereoselective intermolecular transfer of carbenes to olefins, heterocycles, aldehydes, and amines. Not only is this a report of carbene transferase activity in a completely de novo protein, but also of enzyme-catalyzed ring expansion of aromatic heterocycles via carbene transfer by any enzyme.

Spin modification of iron(ii) complexes via covalent (dative) and dispersion guided non-covalent bonding with N-heterocyclic carbenes: DFT, DLPNO-CCSD(T) and MCSCF studies

Spin crossover from high spin Fe(ii)-phthalocyanine to low or intermediate spin via either dative covalent or non-covalent interaction by just varying the substituent using the same core ligand.

Golden Mutagenesis: An efficient multi-site-saturation mutagenesis approach by Golden Gate cloning with automated primer design

Site-directed methods for the generation of genetic diversity are essential tools in the field of directed enzyme evolution. The Golden Gate cloning technique has been proven to be an efficient tool for a variety of cloning setups. The utilization of restriction enzymes which cut outside of their recognition domain allows the assembly of multiple gene fragments obtained by PCR amplification without altering the open reading frame of the reconstituted gene. We have developed a protocol, termed Golden Mutagenesis that allows the rapid, straightforward, reliable and inexpensive construction of mutagenesis libraries. One to five amino acid positions within a coding sequence could be altered simultaneously using a protocol which can be performed within one day. To facilitate the implementation of this technique, a software library and web application for automated primer design and for the graphical evaluation of the randomization success based on the sequencing results was developed. This allows facile primer design and application of Golden Mutagenesis also for laboratories, which are not specialized in molecular biology.

Broadening the Scope of Biocatalysis in Sustainable Organic Synthesis

This Review is aimed at synthetic organic chemists who may be familiar with organometallic catalysis but have no experience with biocatalysis, and seeks to provide an answer to the perennial question: if it is so attractive, why wasn't it extensively used in the past? The development of biocatalysis in industrial organic synthesis is traced from the middle of the last century. Advances in molecular biology in the last two decades, in particular genome sequencing, gene synthesis and directed evolution of proteins, have enabled remarkable improvements in scope and substantially reduced biocatalyst development times and cost contributions. Additionally, improvements in biocatalyst recovery and reuse have been facilitated by developments in enzyme immobilization technologies. Biocatalysis has become eminently competitive with chemocatalysis and the biocatalytic production of important pharmaceutical intermediates, such as enantiopure alcohols and amines, has become mainstream organic synthesis. The synthetic space of biocatalysis has significantly expanded and is currently being extended even further to include new-to-nature biocatalytic reactions.

Biocatalytic Strategy for Highly Diastereo- and Enantioselective Synthesis of 2,3-Dihydrobenzofuran-Based Tricyclic Scaffolds

2,3-Dihydrobenzofurans are key pharmacophores in many natural and synthetic bioactive molecules. A biocatalytic strategy is reported here for the highly diastereo- and enantioselective construction of stereochemically rich 2,3-dihydrobenzofurans in high enantiopurity (>99.9% de and ee), high yields, and on a preparative scale via benzofuran cyclopropanation with engineered myoglobins. Computational and structure-reactivity studies provide insights into the mechanism of this reaction, enabling the elaboration of a stereochemical model that can rationalize the high stereoselectivity of the biocatalyst. This information was leveraged to implement a highly stereoselective route to a drug molecule and a tricyclic scaffold featuring five stereogenic centers via a single-enzyme transformation. This work expands the biocatalytic toolbox for asymmetric C-C bond transformations and should prove useful for further development of metalloprotein catalysts for abiotic carbene transfer reactions.

Stereodivergent Intramolecular Cyclopropanation Enabled by Engineered Carbene Transferases

We report the development of engineered myoglobin biocatalysts for executing asymmetric intramolecular cyclopropanations resulting in cyclopropane-fused γ-lactones, which are key motifs found in many bioactive molecules. Using this strategy, a broad range of allyl diazoacetate substrates were efficiently cyclized in high yields with up to 99% enantiomeric excess. Upon remodeling of the active site via protein engineering, myoglobin variants with stereodivergent selectivity were also obtained. In combination with whole-cell transformations, these biocatalysts enabled the gram-scale assembly of a key intermediate useful for the synthesis of the insecticide permethrin and other natural products. The enzymatically produced cyclopropyl-γ-lactones can be further elaborated to furnish a variety of enantiopure trisubstituted cyclopropanes. This work introduces a first example of biocatalytic intramolecular cyclopropanation and provides an attractive strategy for the stereodivergent preparation of fused cyclopropyl-γ-lactones of high value for medicinal chemistry and the synthesis of natural products.

Expression and Characterization of a Dye-Decolorizing Peroxidase from Pseudomonas Fluorescens Pf0-1

The consumption of dyes is increasing worldwide in line with the increase of population and demand for clothes and other colored products. However, the efficiency of dyeing processes is still poor and results in large amounts of colored effluents. It is desired to develop a portfolio of enzymes which can be used for the treatment of colored wastewaters. Herein, we used genome sequence information to discover a dye-decolorizing peroxidase (DyP) from Pseudomonas fluorescens Pf-01. Two genes putatively encoding for DyPs were identified in the respective genome and cloned for expression in Escherichia coli, of which one (PfDyP B2) could be overexpressed as a soluble protein. PfDyP B2 shows some typical features known for DyPs which includes the ability to convert dyes at the expense of hydrogen peroxide. Interestingly, t-butyl hydroperoxide could be used as an alternative substrate to hydrogen peroxide. Immobilization of PfDyP B2 in calcium-alginate beads resulted in a significant increase in stability: PfDyP B2 retains 80% of its initial activity after 2 h incubation at 50 °C, while the soluble enzyme is inactivated within minutes. PfDyP B2 was also tested with aniline and ethyl diazoacetate as substrates. Based on GC-MS analyses, 30% conversion of the starting material was achieved after 65 h at 30 °C. Importantly, this is the first report of a DyP-catalyzed insertion of a carbene into an N-H bond.

Effect of proximal ligand substitutions on the carbene and nitrene transferase activity of myoglobin

Engineered myoglobins were recently shown to be effective catalysts for abiological carbene and nitrene transfer reactions. Here, we investigated the impact of substituting the conserved heme-coordinating histidine residue with both proteinogenic (Cys, Ser, Tyr, Asp) and non-proteinogenic Lewis basic amino acids (3-(3'-pyridyl)-alanine, p-aminophenylalanine, and β-(3-thienyl)-alanine), on the reactivity of this metalloprotein toward these abiotic transformations. These studies showed that mutation of the proximal histidine residue with both natural and non-natural amino acids result in stable myoglobin variants that can function as both carbene and nitrene transferases. In addition, substitution of the proximal histidine with an aspartate residue led to a myoglobin-based catalyst capable of promoting stereoselective olefin cyclopropanation under nonreducing conditions. Overall, these studies demonstrate that proximal ligand substitution provides a promising strategy to tune the reactivity of myoglobin-based carbene and nitrene transfer catalysts and provide a first, proof-of-principle demonstration of the viability of pyridine-, thiophene-, and aniline-based unnatural amino acids for metalloprotein engineering.

Tryptamine Synthesis by Iron Porphyrin Catalyzed C-H Functionalization of Indoles with Diazoacetonitrile

The functionalization of C-H bonds with non-precious metal catalysts is an important research area for the development of efficient and sustainable processes. Herein, we describe the development of iron porphyrin catalyzed reactions of diazoacetonitrile with N-heterocycles yielding important precursors of tryptamines, along with experimental mechanistic studies and proof-of-concept studies of an enzymatic process with YfeX enzyme. By using readily available FeTPPCl, we achieved the highly efficient C-H functionalization of indole and indazole heterocycles. These transformations feature mild reaction conditions, excellent yields with broad functional group tolerance, can be conducted on gram scale, and thus provide a unique streamlined access to tryptamines.

Origin of high stereocontrol in olefin cyclopropanation catalyzed by an engineered carbene transferase

Recent advances in metalloprotein engineering have led to the development of a myoglobin-based catalyst, Mb(H64V,V68A), capable of promoting the cyclopropanation of vinylarenes with high efficiency and high diastereo- and enantioselectivity. Whereas many enzymes evolved in nature often exhibit catalytic proficiency and exquisite stereoselectivity, how these features are achieved for a non-natural reaction has remained unclear. In this work, the structural determinants responsible for chiral induction and high stereocontrol in Mb(H64V,V68A)-catalyzed cyclopropanation were investigated via a combination of crystallographic, computational (DFT), and structure-activity analyses. Our results show the importance of steric complementarity and non-covalent interactions involving first-sphere active site residues, heme-carbene, and the olefin substrate, in dictating the stereochemical outcome of the cyclopropanation reaction. High stereocontrol is achieved through two major mechanisms. First, by enforcing a specific conformation of the heme-bound carbene within the active site. Second, by controlling the geometry of attack of the olefin on the carbene via steric occlusion, attractive van der Waals forces and protein-mediated π-π interactions with the olefin substrate. These insights could be leveraged to expand the substrate scope of the myoglobin-based cyclopropanation catalyst toward non-activated olefins and to increase its cyclopropanation activity in the presence of a bulky α-diazo-ester. This work sheds first light into the origin of enzyme-catalyzed enantioselective cyclopropanation, furnishing a mechanistic framework for both understanding the reactivity of current systems and guiding the future development of biological catalysts for this class of synthetically important, abiotic transformations.

Myoglobin-Catalyzed C-H Functionalization of Unprotected Indoles

Functionalized indoles are recurrent motifs in bioactive natural products and pharmaceuticals. While transition metal-catalyzed carbene transfer has provided an attractive route to afford C3-functionalized indoles, these protocols are viable only in the presence of N-protected indoles, owing to competition from the more facile N-H insertion reaction. Herein, a biocatalytic strategy for enabling the direct C-H functionalization of unprotected indoles is reported. Engineered variants of myoglobin provide efficient biocatalysts for this reaction, which has no precedents in the biological world, enabling the transformation of a broad range of indoles in the presence of ethyl α-diazoacetate to give the corresponding C3-functionalized derivatives in high conversion yields and excellent chemoselectivity. This strategy could be exploited to develop a concise chemoenzymatic route to afford the nonsteroidal anti-inflammatory drug indomethacin.

Catalytic iron-carbene intermediate revealed in a cytochrome c carbene transferase

Recently, heme proteins have been discovered and engineered by directed evolution to catalyze chemical transformations that are biochemically unprecedented. Many of these nonnatural enzyme-catalyzed reactions are assumed to proceed through a catalytic iron porphyrin carbene (IPC) intermediate, although this intermediate has never been observed in a protein. Using crystallographic, spectroscopic, and computational methods, we have captured and studied a catalytic IPC intermediate in the active site of an enzyme derived from thermostable Rhodothermus marinus (Rma) cytochrome c High-resolution crystal structures and computational methods reveal how directed evolution created an active site for carbene transfer in an electron transfer protein and how the laboratory-evolved enzyme achieves perfect carbene transfer stereoselectivity by holding the catalytic IPC in a single orientation. We also discovered that the IPC in Rma cytochrome c has a singlet ground electronic state and that the protein environment uses geometrical constraints and noncovalent interactions to influence different IPC electronic states. This information helps us to understand the impressive reactivity and selectivity of carbene transfer enzymes and offers insights that will guide and inspire future engineering efforts.

An Artificial Heme Enzyme for Cyclopropanation Reactions

An artificial heme enzyme was created through self-assembly from hemin and the lactococcal multidrug resistance regulator (LmrR). The crystal structure shows the heme bound inside the hydrophobic pore of the protein, where it appears inaccessible for substrates. However, good catalytic activity and moderate enantioselectivity was observed in an abiological cyclopropanation reaction. We propose that the dynamic nature of the structure of the LmrR protein is key to the observed activity. This was supported by molecular dynamics simulations, which showed transient formation of opened conformations that allow the binding of substrates and the formation of pre-catalytic structures.

Carbene Transfer Reactions Catalysed by Dyes of the Metalloporphyrin Group

Carbene transfer reactions are very important transformations in organic synthesis, allowing the generation of structurally challenging products by catalysed cyclopropanation, cyclopropenation, carbene C-H, N-H, O-H, S-H, and Si-H insertion, and olefination of carbonyl compounds. In particular, chiral and achiral metalloporphyrins have been successfully explored as biomimetic catalysts for these carbene transfer reactions under both homogeneous and heterogeneous conditions. In this work the use of synthetic metalloporphyrins (MPorph, M = Fe, Ru, Os, Co, Rh, Ir, Sn) as homogeneous or heterogeneous catalysts for carbene transfer reactions in the last years is reviewed, almost exclusively focused on the literature since the year 2010, except when reference to older publications was deemed to be crucial.

Exploiting and engineering hemoproteins for abiological carbene and nitrene transfer reactions

The surge in reports of heme-dependent proteins as catalysts for abiotic, synthetically valuable carbene and nitrene transfer reactions dramatically illustrates the evolvability of the protein world and our nascent ability to exploit that for new enzyme chemistry. We highlight the latest additions to the hemoprotein-catalyzed reaction repertoire (including carbene Si-H and C-H insertions, Doyle-Kirmse reactions, aldehyde olefinations, azide-to-aldehyde conversions, and intermolecular nitrene C-H insertion) and show how different hemoprotein scaffolds offer varied reactivity and selectivity. Preparative-scale syntheses of pharmaceutically relevant compounds accomplished with these new catalysts are beginning to demonstrate their biotechnological relevance. Insights into the determinants of enzyme lifetime and product yield are providing generalizable cues for engineering heme-dependent proteins to further broaden the scope and utility of these non-natural activities.

Highly Diastereo- and Enantioselective Synthesis of Trifluoromethyl-Substituted Cyclopropanes via Myoglobin-Catalyzed Transfer of Trifluoromethylcarbene

We report an efficient strategy for the asymmetric synthesis of trifluoromethyl-substituted cyclopropanes by means of myoglobin-catalyzed olefin cyclopropanation reactions in the presence of 2-diazo-1,1,1-trifluoroethane (CF3CHN2) as the carbene donor. These transformations were realized using a two-compartment setup in which ex situ generated gaseous CF3CHN2 is processed by engineered myoglobin catalysts expressed in bacterial cells. This approach was successfully applied to afford a variety of trans-1-trifluoromethyl-2-arylcyclopropanes in high yields (61-99%) and excellent diastereo- and enantioselectivity (97-99.9% de and ee). Furthermore, mirror-image forms of these products could be obtained using myoglobin variants featuring stereodivergent selectivity. These reactions provide a convenient and effective biocatalytic route to the stereoselective synthesis of key fluorinated building blocks of high value for medicinal chemistry and drug discovery. This work expands the range of carbene-mediated transformations accessible via metalloprotein catalysts and introduces a potentially general strategy for exploiting gaseous and/or hard-to-handle carbene donor reagents in biocatalytic carbene transfer reactions.

Highly Stereoselective Biocatalytic Synthesis of Key Cyclopropane Intermediate to Ticagrelor

Extending the scope of biocatalysis to important non-natural reactions such as olefin cyclopropanation will open new opportunities for replacing multi-step chemical syntheses of pharmaceutical intermediates with efficient, clean, and highly selective enzyme-catalyzed processes. In this work, we engineered the truncated globin of Bacillus subtilis for the synthesis of a cyclopropane precursor to the antithrombotic agent ticagrelor. The engineered enzyme catalyzes the cyclopropanation of 3,4-difluorostyrene with ethyl diazoacetate on a preparative scale to give ethyl-(1R, 2R)-2-(3,4-difluorophenyl)-cyclopropanecarboxylate in 79% yield, with very high diastereoselectivity (>99% dr) and enantioselectivity (98% ee), enabling a single-step biocatalytic route to this pharmaceutical intermediate.

Non-natural carbenoid and nitrenoid insertion reactions catalyzed by heme proteins

Despite increasing interest in using enzymes as tools for synthesis, many reactions discovered through the creativity of synthetic chemists remain beyond the scope of biocatalysis. This vacancy in the field has compelled researchers to develop strategies to adapt protein scaffolds for new reactivity. Heme proteins have recently been shown to activate synthetic precursors to generate reactive metallocarbenoid and metallonitrenoid species that enable the biosynthetic construction of novel C-C, C-N, and other bonds using mechanisms not previously explored by Nature. By interrogating heme proteins with synthetic, non-natural reagents, scientists are merging the reaction space traditionally dominated by organocatalysis and transition metal catalysis with the mild reaction conditions, selectivity, and adaptability imparted by native protein scaffolds.

Computation Sheds Insight into Iron Porphyrin Carbenes' Electronic Structure, Formation, and N-H Insertion Reactivity

Iron porphyrin carbenes constitute a new frontier of species with considerable synthetic potential. Exquisitely engineered myoglobin and cytochrome P450 enzymes can generate these complexes and facilitate the transformations they mediate. The current work harnesses density functional theoretical methods to provide insight into the electronic structure, formation, and N-H insertion reactivity of an iron porphyrin carbene, [Fe(Por)(SCH3)(CHCO2Et)](-), a model of a complex believed to exist in an experimentally studied artificial metalloenzyme. The ground state electronic structure of the terminal form of this complex is an open-shell singlet, with two antiferromagnetically coupled electrons residing on the iron center and carbene ligand. As we shall reveal, the bonding properties of [Fe(Por)(SCH3)(CHCO2Et)](-) are remarkably analogous to those of ferric heme superoxide complexes. The carbene forms by dinitrogen loss from ethyl diazoacetate. This reaction occurs preferentially through an open-shell singlet transition state: iron donates electron density to weaken the C-N bond undergoing cleavage. Once formed, the iron porphyrin carbene accomplishes N-H insertion via nucleophilic attack. The resulting ylide then rearranges, using an internal carbonyl base, to form an enol that leads to the product. The findings rationalize experimentally observed reactivity trends reported in artificial metalloenzymes employing iron porphyrin carbenes. Furthermore, these results suggest a possible expansion of enzymatic substrate scope, to include aliphatic amines. Thus, this work, among the first several computational explorations of these species, contributes insights and predictions to the surging interest in iron porphyrin carbenes and their synthetic potential.