A Diverse Library of Chiral Cyclopropane Scaffolds via Chemoenzymatic Assembly and Diversification of Cyclopropyl Ketones

Electron donor-acceptor complex-driven photocatalyst-free synthesis of nitrocyclopropanes

Herein, a visible light-promoted metal-free protocol for the synthesis of nitrocyclopropanes under mild conditions is reported. Specifically, the process is driven by the photochemical activity of ternary EDA complexes formed upon complexation of α-bromonitrostyrenes and DIPEA in the presence of benzaldehyde. This reaction provides a variety of densely functionalized cyclopropanes with good selectivity under mild reaction conditions. Mechanistic investigations on the aspects of the process also demonstrate formation of the hypothesized EDA complex.

Stereospecific Synthesis of 1,2,3-Trisubstituted Cyclopropanes from Pseudoglycals

Strategic design for constructing optically pure 1,2,3-trisubstituted cyclopropanes with three contiguous stereocenters represents a formidable challenge in synthetic organic chemistry. Herein, we report a simple and highly enantiospecific transformation of pseudoglycals into chiral 1,2,3-trisubstituted cyclopropanes, featuring three consecutive stereocenters, involving a stereospecific [1,4]-Wittig rearrangement. The effects of functional group orientation and conformational preferences were studied. Successful gram-scale preparations and subsequent derivatization reactions yielded various cyclopropane scaffolds with multiple stereocenters.

Automated Flow Synthesis of Artificial Heme Enzymes for Enantiodivergent Biocatalysis

The remarkable efficiency with which enzymes catalyze small-molecule reactions has driven their widespread application in organic chemistry. Here, we employ automated fast-flow solid-phase synthesis to access catalytically active full-length enzymes without restrictions on the number and structure of noncanonical amino acids incorporated. We demonstrate the total syntheses of iron-dependent Bacillus subtilis myoglobin (BsMb) and sperm whale myoglobin (SwMb). The synthetic enzymes displayed excellent enantioselectivity and yield in carbene transfer reactions. Absolute control over enantioselectivity in styrene cyclopropanation was achieved using synthetic L- and D-BsMb mutants, which delivered each enantiomer of cyclopropane product in identical and opposite enantiomeric enrichment. BsMb mutants outfitted with noncanonical amino acids were used to facilitate detailed structure-activity relationship studies, revealing a previously unrecognized hydrogen-bonding interaction as the primary driver of enantioselectivity in styrene cyclopropanation. We anticipate that our approach will advance biocatalysis by providing reliable and rapid access to fully synthetic enzymes possessing noncanonical amino acids.

Well Defined Phosphine Free Ni-Catalyzed Dehydrogenation of Secondary Alcohols for the Synthesis of Ketones and Ketazines

In this work, we unveil a novel synthesis of bench stable Ni (II) complexes supported by tetradentate Schiff-base ligands and the complexes were devoid of any phosphine or phosphine-based ligand. These Ni-complexes were successfully applied for the dehydrogenation of secondary alcohols for ketone and ketazine syntheses. Secondary alcohols with different functional groups were well tolerated during catalytic cycle. Moreover, we successfully extended this protocol for the synthesis of biologically significant ketones and ketazines. On the basis of various control experiments, probable reaction pathway was proposed, and an acceptorless alcohol dehydrogenation mechanism was suggested.

Mechanistic investigation of sustainable heme-inspired biocatalytic synthesis of cyclopropanes for challenging substrates

Engineered heme proteins exhibit excellent sustainable catalytic carbene transfer reactivities toward olefins for value-added cyclopropanes. However, unactivated and electron-deficient olefins remain challenging in such reactions. To help design efficient heme-inspired biocatalysts for these difficult situations, a systematic quantum chemical mechanistic study was performed to investigate effects of olefin substituents, non-native amino acid axial ligands, and natural and non-natural macrocycles with the widely used ethyl diazoacetate. Results show that electron-deficient substrate ethyl acrylate has a much higher barrier than the electron-rich styrene. For styrene, the predicted barrier trend is consistent with experimentally used heme analogue cofactors, which can significantly reduce barriers. For ethyl acrylate, while the best non-native axial ligand only marginally improves the reactivity versus the native histidine model, a couple of computationally studied macrocycles can dramatically reduce barriers to the level comparable to styrene. These results will facilitate the development of better biocatalysts in this area.

Engineering Neuroglobin for Synthesis of Chiral Organoborons via Carbene B-H Insertion

Organoborons have recently received much attention, while a biocatalytic platform for the synthesis of chiral organoborons is limited only to Rma cytochrome c. In this study, we exploited the other heme protein, neuroglobin (Ngb), and engineered a quadruple mutant, A15C/H64G/V68F/F28M Ngb, by redesigning the heme active site using the structural information on A15C Ngb and molecular docking studies. The enzyme was shown to be efficient in catalyzing carbene transfer B-H insertion reactions between pyridine/quinoline boranes and benzyl 2-diazopropanoates and their derivatives (29 examples). The designed cavity in the heme distal site favors the binding of large volume substrates such as those containing a quinoline, naphthyl, or biphenyl group. As further determined by the X-ray crystallography of 6c, the chiral products are in the R-configuration, with up to 98:2 e.r. Furthermore, both the whole cell and cell lysate containing the enzyme are reactive toward the B-H insertion reactions. This study presents a convenient biocatalytic platform that may be generally applicable for the synthesis of functional chiral organoborons.

Redox Engineering of Myoglobin by Cofactor Substitution to Enhance Cyclopropanation Reactivity

Design of metal cofactor ligands is essential for controlling the reactivity of metalloenzymes. We investigated a carbene transfer reaction catalyzed by myoglobins containing iron porphyrin cofactors with one and two trifluoromethyl groups at peripheral sites (FePorCF3 and FePor(CF3)2, respectively), native heme and iron porphycene (FePc). These four myoglobins show a wide range of Fe(II)/Fe(III) redox potentials in the protein of +147 mV, +87 mV, +42 mV and -198 mV vs. NHE, respectively. Myoglobin reconstituted with FePor(CF3)2 has a more positive potential, which enhances the reactivity of a carbene intermediate with alkenes, and demonstrates superior cyclopropanation of inert alkenes, such as aliphatic and internal alkenes. In contrast, engineered myoglobin reconstituted with FePc has a more negative redox potential, which accelerates the formation of the intermediate, but has low reactivity for inert alkenes. Mechanistic studies indicate that myoglobin with FePor(CF3)2 generates an undetectable active intermediate with a radical character. In contrast, this reaction catalyzed by myoglobin with FePc includes a detectable iron-carbene species with electrophilic character. This finding highlights the importance of redox-focused design of the iron porphyrinoid cofactor in hemoproteins to tune the reactivity of the carbene transfer reaction.

Computational design of myoglobin-based carbene transferases for monoterpene derivatization

Carbene transfer reactions have emerged as pivotal methodologies for the synthesis of complex molecular architectures. Heme protein-catalyzed carbene transfer reactions have shown promising results on model compounds. However, their limited substrate scope has hindered their application in natural product functionalization. Building upon the foundation of previously published work on a carbene transferase-myoglobin variant, this study employs computer-aided protein engineering to design myoglobin variants, using either docking or the deep learning-based LigandMPNN method. These variants were utilized as catalysts in carbene transfer reactions with a selection of monoterpene substrates featuring C-C double bonds, leading to seven target products. This cost-effective methodology broadens the substrate scope for heme protein-catalyzed reactions, thereby opening novel pathways for research in heme protein functionalities and offering fresh perspectives in the synthesis of bioactive molecules.

Biocatalytic Strategy for the Highly Stereoselective Synthesis of Fluorinated Cyclopropanes

Fluorinated cyclopropanes are highly desired pharmacophores in drug discovery owing to the rigid nature of the cyclopropane ring and the beneficial effects of C-F bonds on the pharmacokinetic properties, cell permeability, and metabolic stability of drug molecules. Herein a biocatalytic strategy for the stereoselective synthesis of mono-fluorinated and gem-difluoro cyclopropanes is reported though the use of engineered myoglobin-based catalysts. In particular, this system allows for a broad range of gem-difluoro alkenes to be cyclopropanated in the presence of diazoacetonitrile with excellent diastereo and enantiocontrol (up to 99 : 1 d.r. and 99 % e.e.), thereby enabling a transformation not currently accessible with chemocatalytic methods. The synthetic utility of the present approach is further exemplified through the gram-scale synthesis of a key gem-difluorinated cyclopropane intermediate useful for the preparation of fluorinated bioactive molecules.

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.

Biocatalytic strategy for the construction of sp3-rich polycyclic compounds from directed evolution and computational modelling

Catalysis with engineered enzymes has provided more efficient routes for the production of active pharmaceutical agents. However, the potential of biocatalysis to assist in early-stage drug discovery campaigns remains largely untapped. In this study, we have developed a biocatalytic strategy for the construction of sp3-rich polycyclic compounds via the intramolecular cyclopropanation of benzothiophenes and related heterocycles. Two carbene transferases with complementary regioisomer selectivity were evolved to catalyse the stereoselective cyclization of benzothiophene substrates bearing diazo ester groups at the C2 or C3 position of the heterocycle. The detailed mechanisms of these reactions were elucidated by a combination of crystallographic and computational analyses. Leveraging these insights, the substrate scope of one of the biocatalysts could be expanded to include previously unreactive substrates, highlighting the value of integrating evolutionary and rational strategies to develop enzymes for new-to-nature transformations. The molecular scaffolds accessed here feature a combination of three-dimensional and stereochemical complexity with 'rule-of-three' properties, which should make them highly valuable for fragment-based drug discovery campaigns.

Chemoenzymatic synthesis of optically active α-cyclopropyl-pyruvates and cyclobutenoates via enzyme-catalyzed carbene transfer with diazopyruvate

Cyclopropanes are recurrent structural motifs in natural products and bioactive molecules. Recently, biocatalytic cyclopropanations have emerged as a powerful approach to access enantioenriched cyclopropanes, complementing chemocatalytic approaches developed over the last several decades. Here, we report the development of a first biocatalytic strategy for cyclopropanation using ethyl α-diazopyruvate as a novel enzyme-compatible carbene precursor. Using myoglobin variant Mb(H64V,V68G) as the biocatalyst, this method afforded the efficient synthesis of α-cyclopropylpyruvates in high diastereomeric ratios and enantiomeric excess (up to 99% ee). The ketoester moiety in the cyclopropane products can be used to synthesize diverse optically pure cyclopropane derivatives. Furthermore, the enzymatically obtained α-cyclopropylpyruvate products could be converted into enantiopure cyclobutenoates via a metal-free photochemical ring expansion without loss of optical activity.

Amberlyst-A26-Mediated Corey-Chaykovsky Cyclopropanation of 9-Alkylidene-9H-fluorene under Continuous Process

Herein, we have developed a continuous-process for the direct cyclopropanation of various alkenes nonconjugated with carbonyl using trimethylsulfoxonium iodide as a methylene source via the Corey-Chaykovsky cyclopropanation reaction in the presence of Amberlyst-A26 as a heterogeneous base. Several 9-alkylidene-9H-fluorene derivatives successfully undergo Corey-Chaykovsky cyclopropanation to afford spiro[cyclopropane-1,9'-fluorene] in excellent yields under the continuous-process module. Furthermore, continuous process for the cyclopropanation of 3-benzylideneindolin-2-one derivatives using Amberlyst-A26 as a heterogeneous base has been described, which afford spiro[cyclopropane-1,3'-indolin]-2'-one derivatives.

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.

Multifaceted personality and roles of heme enzymes in industrial biotechnology

Heme enzymes are the most prominent category of iron-containing metalloenzymes with the capability of catalyzing an astonishingly wide range of reactions like epoxidation, hydroxylation, demethylation, desaturation, reduction, sulfoxidation, and decarboxylation. Various enzymes in this category are P450s, heme peroxidases, catalases, myoglobin, cytochrome C, and others. Besides this, the natural promiscuity and amenability of these enzymes to protein engineering and evolution have also added several non-native reactions such as C-H, N-H, S-H insertions, cyclopropanation, and other industrially important reactions to their capabilities. Surprisingly, all of these reactions and their wide substrate scopes are attributed to changes in the active site scaffold of different heme enzymes as the center of all enzymes is constituted by a porphyrin ring containing iron. Multiple prominent research groups across the world, including 2018, Nobel Laureate Frances Arnold's group, have shown keen interest in engineering and evolving these enzymes for utilizing their industrial potential. Besides engineering the active site, researchers have also explored the possibility of these enzymes catalyzing non-native reactions by replacing the center porphyrin ring with other cofactors or by changing the iron in the porphyrin ring with other metal ions along with engineering the active site and thereby creating novel artificial metalloenzymes. Thus, in this mini-review from our group, for the first time, we are trying to catalog various activities catalyzed by heme enzymes and their engineered variants and their active usage in various industries along with shedding light on their potential for use in various applications in the future.

Exploiting and Engineering Neuroglobin for Catalyzing Carbene N-H Insertions and the Formation of Quinoxalinones

It is desired to design and construct more efficient enzymes with better performance to catalyze carbene N-H insertions for the synthesis of bioactive molecules. To this end, we exploited and designed a series of human neuroglobin (Ngb) mutants. As shown in this study, a double mutant, A15C/H64G Ngb, with an additional disulfide bond and a modified heme active site, exhibited yields up to >99% and total turnover numbers up to 33000 in catalyzing the carbene N-H insertions for aromatic amine derivatives, including those with a large size such as 1-aminopyrene. Moreover, for o-phenylenediamine derivatives, they underwent two cycles of N-H insertions, followed by cyclization to form quinoxalinones, as confirmed by the X-ray crystal structures. This study suggests that Ngb can be designed into a functional carbene transferase for efficiently catalyzing carbene N-H insertion reactions with a range of substrates. It also represents the first example of the formation of quinoxalinones catalyzed by an engineered heme enzyme.

Semipinacol Rearrangement of Cyclopropenylcarbinols for the Synthesis of Highly Substituted Cyclopropanes

An electrophile-induced semipinacol rearrangement of cyclopropenylcarbinols is reported. This transformation gives access to various polyfunctionalized cyclopropanes under mild metal-free conditions. The scope of the reaction includes iodine, sulfur and selenium electrophiles, aryl and strained ring migrating groups, and diverse substitution patterns on the cyclopropene. The reaction is particularly efficient for the synthesis of small ring-containing spirocycles, which are important rigid three-dimensional building blocks for medicinal chemistry.

Ligand-Metal Cooperation Enables Net Ring-Opening C-C Activation / Difunctionalization of Cyclopropyl Ketones

Reactions that cleave C-C bonds and enable functionalization at both carbon sites are powerful strategic tools in synthetic chemistry. Stereodefined cyclopropyl ketones have become readily available and would be an ideal source of 3-carbon fragments, but general approaches to net C-C activation / difunctionalization are unknown. Herein we demonstrate the cross-coupling of cyclopropyl ketones with organozinc reagents and chlorotrimethylsilane to form 1,3-difunctionalized, ring-opened products. A combination of experimental and theoretical studies rule out more established mechanisms and shed light on how cooperation between the redox-active terpyridine (tpy) ligand and the nickel atom enables the C-C bond activation step. The reduced (tpy•-)NiI species activates the C-C bond via a concerted asynchronous ring-opening transition state. The resulting alkylnickel(II) intermediate can then be engaged by aryl-, alkenyl-, and alkylzinc reagents to furnish cross-coupled products. This allows quick access to products that are difficult to make by conjugate addition methods, such as β-allylated and β -benzylated enol ethers. The utility of this approach is demonstrated in the synthesis of a key (±)-taiwaniaquinol B intermediate and the total synthesis of prostaglandin D1.

Photosensitized O2 enables intermolecular alkene cyclopropanation by active methylene compounds

Cyclopropanes are key features in many preclinical, clinical, and commercial drugs, as well as natural products. The most prolific technique for their synthesis is the metal-catalyzed reaction of an alkene with a diazoalkane, a highly energetic reagent requiring stringent safety precautions. Discovery of alternative innocuous reagents remains an ongoing challenge. Herein, we report a simple photoredox-catalyzed intermolecular cyclopropanation of unactivated alkenes with active methylene compounds. The reaction proceeds in neutral solvent under air or dioxygen (O2) with a photoredox catalyst excited by blue light-emitting diode light and an iodine co-catalyst that is either added as molecular iodine or generated in situ from alkyl iodides. Mechanistic investigations indicate that photosensitized O2 plays a vital role in the generation of carbon-centered radicals for both the addition of active methylene compounds to alkenes and the ring closure.

Engineering an Oxygen-Binding Protein for Photocatalytic CO2 Reductions in Water

While native CO2 -reducing enzymes display remarkable catalytic efficiency and product selectivity, few artificial biocatalysts have been engineered to allow understanding how the native enzymes work. To address this issue, we report cobalt porphyrin substituted myoglobin (CoMb) as a homogeneous catalyst for photo-driven CO2 to CO conversion in water. The activity and product selectivity were optimized by varying pH and concentrations of the enzyme and the photosensitizer. Up to 2000 TON(CO) was attained at low enzyme concentrations with low product selectivity (15 %), while a product selectivity of 74 % was reached by increasing the enzyme loading but with a compromised TON(CO). The efficiency of CO generation and overall TON(CO) were further improved by introducing positively charged residues (Lys or Arg) near the active stie of CoMb, which demonstrates the value of tuning the enzyme secondary coordination sphere to enhance the CO2 -reducing performance of a protein-based photocatalytic system.

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.

Engineered Myoglobin Catalysts for Asymmetric Intermolecular Cyclopropanation Reactions

Biocatalysis has covered an increasingly important role in the synthesis and manufacturing of pharmaceuticals and other high value compounds. In the interest of expanding the range of synthetically useful reactions accessible via biocatalysts, our group has explored the potential and application of engineered myoglobins for 'abiological' carbene transfer catalysis. These transformations provide a direct route for the construction of new carbon-carbon and carbon-heteroatom bonds, including the synthesis of cyclopropane rings, which are key motifs and pharmacophores in many drugs and bioactive natural products. In this award article, we survey the progress made by our group toward the development of myoglobin-based catalysts for asymmetric intermolecular cyclopropanation reactions. The high stereoselectivity exhibited by these biocatalysts in these reactions, combined with their broad substrate scope, scalability, and robustness to high substrate loading and organic co-solvents, contribute to make these systems particularly useful for chemical synthesis and biocatalysis at the preparative scale. Extension of the scope of biocatalytic carbene transfer reactions to include different classes of carbene donor reagents has created new opportunities for the asymmetric synthesis of functionalized cyclopropanes. Furthermore, the integration of myoglobin-catalyzed stereoselective cyclopropanations with chemical diversification of the enzymatic products has furnished attractive chemoenzymatic strategies to access a diverse range of optically active cyclopropane scaffolds of high value for drug discovery, medicinal chemistry, and the synthesis of natural products.

Silver(I)-Catalyzed Oxidative Cyclopropanation of 1,6-Enynes: Synthesis of 3-Aza-bicyclo[3.1.0]hexane Derivatives

A simple Ag(I)-catalyzed oxidative cyclopropanation of heteroatom-tethered 1,6-enynes for the establishment of valuable functionalized 3-aza-bicyclo[3.1.0]hexane is presented, which allows the formation of multiple chemical bonds in one step under 20 mol % silver(I) catalysts and air conditions. This approach is highly atom economical, easy to perform, and free of external oxidants and features good to excellent yields and gram-scale synthesis. The preliminary study showed that an uncommon silver carbenoid intermediate might be involved in this process.

Enantioselective Single and Dual α-C-H Bond Functionalization of Cyclic Amines via Enzymatic Carbene Transfer

Cyclic amines are ubiquitous structural motifs found in pharmaceuticals and biologically active natural products, making methods for their elaboration via direct C-H functionalization of considerable synthetic value. Herein, we report the development of an iron-based biocatalytic strategy for enantioselective α-C-H functionalization of pyrrolidines and other saturated N-heterocycles via a carbene transfer reaction with diazoacetone. Currently unreported for organometallic catalysts, this transformation can be accomplished in high yields, high catalytic activity, and high stereoselectivity (up to 99:1 e.r. and 20,350 TON) using engineered variants of cytochrome P450 CYP119 from Sulfolobus solfataricus. This methodology was further extended to enable enantioselective α-C-H functionalization in the presence of ethyl diazoacetate as carbene donor (up to 96:4 e.r. and 18,270 TON), and the two strategies were combined to achieve a one-pot as well as a tandem dual C-H functionalization of a cyclic amine substrate with enzyme-controlled diastereo- and enantiodivergent selectivity. This biocatalytic approach is amenable to gram-scale synthesis and can be applied to drug scaffolds for late-stage C-H functionalization. This work provides an efficient and tunable method for direct asymmetric α-C-H functionalization of saturated N-heterocycles, which should offer new opportunities for the synthesis, discovery, and optimization of bioactive molecules.

Dehaloperoxidase Catalyzed Stereoselective Synthesis of Cyclopropanol Esters

Chiral cyclopropanols are highly desirable building blocks for medicinal chemistry, but the stereoselective synthesis of these molecules remains challenging. Here, a novel strategy is reported for the diastereo- and enantioselective synthesis of cyclopropanol derivatives via the biocatalytic asymmetric cyclopropanation of vinyl esters with ethyl diazoacetate (EDA). A dehaloperoxidase enzyme from Amphitrite ornata was repurposed to catalyze this challenging cyclopropanation reaction, and its activity and stereoselectivity were optimized via protein engineering. Using this system, a broad range of electron-deficient vinyl esters were efficiently converted to the desired cyclopropanation products with up to 99.5:0.5 diastereomeric and enantiomeric ratios. In addition, the engineered dehaloperoxidase-based biocatalyst is able to catalyze a variety of other abiological carbene transfer reactions, including N-H/S-H carbene insertion with EDA as well as cyclopropanation with diazoacetonitrile, thus adding to the multifunctionality of this enzyme and defining it as a valuable new scaffold for the development of novel carbene transferases.

Preparation of Chiral Enantioenriched Densely Substituted Cyclopropyl Azoles, Amines, and Ethers via Formal SN2′ Substitution of Bromocylopropanes

Enantiomerically enriched cyclopropyl ethers, amines, and cyclopropylazole derivatives possessing three stereogenic carbon atoms in a small cycle are obtained via the diastereoselective, formal nucleophilic substitution of chiral, non-racemic bromocyclopropanes. The key feature of this methodology is the utilization of the chiral center of the cyclopropene intermediate, which governs the configuration of the two adjacent stereocenters that are successively installed via 1,4-addition/epimerization sequence.

Peroxidase Activity of Myoglobin Variants Reconstituted with Artificial Cofactors

Myoglobin (Mb) can react with hydrogen peroxide (H₂ O₂ ) to form a highly active intermediate compound and catalyse oxidation reactions. To enhance this activity, known as pseudo-peroxidase activity, previous studies have focused on the modification of key amino acid residues of Mb or the heme cofactor. In this work, the Mb scaffold (apo-Mb) was systematically reconstituted with a set of cofactors based on six metal ions and two ligands. These Mb variants were fully characterised by UV-Vis spectroscopy, circular dichroism (CD) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS) and native mass spectrometry (nMS). The steady-state kinetics of guaiacol oxidation and 2,4,6-trichlorophenol (TCP) dehalogenation catalysed by Mb variants were determined. Mb variants with iron chlorin e6 (Fe-Ce6) and manganese chlorin e6 (Mn-Ce6) cofactors were found to have improved catalytic efficiency for both guaiacol and TCP substrates in comparison with wild-type Mb, i. e. Fe-protoporphyrin IX-Mb. Furthermore, the selected cofactors were incorporated into the scaffold of a Mb mutant, swMb H64D. Enhanced peroxidase activity for both substrates were found via the reconstitution of Fe-Ce6 into the mutant scaffold.

Designing Artificial Metalloenzymes by Tuning of the Environment beyond the Primary Coordination Sphere

Metalloenzymes catalyze a variety of reactions using a limited number of natural amino acids and metallocofactors. Therefore, the environment beyond the primary coordination sphere must play an important role in both conferring and tuning their phenomenal catalytic properties, enabling active sites with otherwise similar primary coordination environments to perform a diverse array of biological functions. However, since the interactions beyond the primary coordination sphere are numerous and weak, it has been difficult to pinpoint structural features responsible for the tuning of activities of native enzymes. Designing artificial metalloenzymes (ArMs) offers an excellent basis to elucidate the roles of these interactions and to further develop practical biological catalysts. In this review, we highlight how the secondary coordination spheres of ArMs influence metal binding and catalysis, with particular focus on the use of native protein scaffolds as templates for the design of ArMs by either rational design aided by computational modeling, directed evolution, or a combination of both approaches. In describing successes in designing heme, nonheme Fe, and Cu metalloenzymes, heteronuclear metalloenzymes containing heme, and those ArMs containing other metal centers (including those with non-native metal ions and metallocofactors), we have summarized insights gained on how careful controls of the interactions in the secondary coordination sphere, including hydrophobic and hydrogen bonding interactions, allow the generation and tuning of these respective systems to approach, rival, and, in a few cases, exceed those of native enzymes. We have also provided an outlook on the remaining challenges in the field and future directions that will allow for a deeper understanding of the secondary coordination sphere a deeper understanding of the secondary coordintion sphere to be gained, and in turn to guide the design of a broader and more efficient variety of ArMs.

Diastereoselective Transfer of Tri(di)fluoroacetylsilanes-Derived Carbenes to Alkenes

Stereoselective cyclopropanation reaction of alkenes is usually achieved by metal complexes via singlet-metal-carbene intermediates. However, previous transition-metal-catalyzed cyclopropanation of alkenes with acylsilanes afforded low diastereoselectivity. Herein, we report the first visible-light-induced transition-metal-free cyclopropanation reaction of terminal alkenes with trifluoroacetylsilanes and difluoroacetylsilanes. Both aromatic and aliphatic alkenes as well as electron-deficient alkenes are suitable substrates for the highly cis-selective [2+1] cyclization reaction. A combination of experimental and computational studies identified triplet carbenes as being key intermediates in this transformation. The gram scale reaction and late-stage functionalization demonstrated the synthetic potential of this strategy.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

A Green Chemistry Approach toward the Stereospecific Synthesis of Densely Functionalized Cyclopropanes via the Solid-State Photodenitrogenation of Crystalline 1-Pyrazolines

The cyclopropane ring features prominently in active pharmaceuticals, and this has spurred the development of synthetic methodologies that effectively incorporate this highly strained motif into such molecules. As such, elegant solutions to prepare densely functionalized cyclopropanes, particularly ones embedded within the core of complex structures, have become increasingly sought-after. Here we report the stereospecific synthesis of a set of cyclopropanes with vicinal quaternary stereocenters via the solvent-free solid-state photodenitrogenation of crystalline 1-pyrazolines. Density functional theory calculations at the M062X/6-31+G(d,p) level of theory were used to determine the origin of regioselectivity for the synthesis of the 1-pyrazolines; favorable in-phase frontier molecular orbital interactions are responsible for the observation of a single pyrazoline regioisomer. It was also shown that the loss of N₂ may take place via a highly selective solid-state thermal reaction. Scalability of the solid-state photoreaction is enabled through aqueous nanocrystalline suspensions, making this method a "greener" alternative to effectively facilitate the construction of cyclopropane-containing molecular scaffolds.

Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors

Reactive carbenes generated from diazo compounds are key intermediates for a range of organic reactions to afford synthetically useful organic compounds. The majority of these reactions have been carried out using transition metal catalysts. However, the formation of carbene intermediates using main group elements has not been widely investigated for synthetic purposes. Recent studies have demonstrated that triarylboranes can be used for the in situ generation of reactive carbene intermediates in both stoichiometric and catalytic reactions. These new reactivities of triarylboranes have gained significant attention in synthetic chemistry particularly in catalytic studies. The range of organic compounds that have been synthesized through these reactions are important as pharmaceuticals or agrochemicals. In this perspective, we highlight the recent progress and ongoing challenges of carbene transfer reactions generated from their corresponding diazo precursors using triarylboranes as catalysts. We also highlight the stoichiometric use of triarylboranes in which the boranes not only activate the diazo functionality to afford a carbene intermediate but also actively participate in the reactions as a reagent. The different mechanisms for activation and carbene transfer are described along with the mechanistic and computational studies that have aided the elucidation of these reaction pathways. Potential opportunities for the use of boranes as a catalyst toward different carbene transfer reactions and their future prospects are discussed.

Highly stereoselective and enantiodivergent synthesis of cyclopropylphosphonates with engineered carbene transferases

Organophosphonate compounds have represented a rich source of biologically active compounds, including enzyme inhibitors, antibiotics, and antimalarial agents. Here, we report the development of a highly stereoselective strategy for olefin cyclopropanation in the presence of a phosphonyl diazo reagent as carbene precursor. In combination with a ‘substrate walking’ protein engineering strategy, two sets of efficient and enantiodivergent myoglobin-based biocatalysts were developed for the synthesis of both (1R,2S) and (1S,2R) enantiomeric forms of the desired cyclopropylphosphonate ester products. This methodology enables the efficient transformation of a broad range of vinylarene substrates at a preparative scale (i.e. gram scale) with up to 99% de and ee. Mechanistic studies provide insights into factors that contribute to make this reaction inherently more challenging than hemoprotein-catalyzed olefin cyclopropanation with ethyl diazoacetate investigated previously. This work expands the range of synthetically useful, enzyme-catalyzed transformations and paves the way to the development of metalloprotein catalysts for abiological carbene transfer reactions involving non-canonical carbene donor reagents.

Two enantiocomplementary myoglobin-based carbene transfer biocatalysts were developed for the synthesis of cyclopropylphosphonate esters with high diastereo- and enantioselectivity and in high yields.

Asymmetric Radical Process for General Synthesis of Chiral Heteroaryl Cyclopropanes

A highly efficient catalytic method has been developed for asymmetric radical cyclopropanation of alkenes with in situ-generated α-heteroaryldiazomethanes via Co(II)-based metalloradical catalysis (MRC). Through fine-tuning the cavity-like environments of newly-synthesized D2-symmetric chiral amidoporphyrins as the supporting ligand, the optimized Co(II)-based metalloradical system is broadly applicable to α-pyridyl and other α-heteroaryldiazomethanes for asymmetric cyclopropanation of wide-ranging alkenes, including several types of challenging substrates. This new catalytic methodology provides a general access to valuable chiral heteroaryl cyclopropanes in high yields with excellent both diastereoselectivities and enantioselectivities. Combined computational and experimental studies further support the underlying stepwise radical mechanism of the Co(II)-based olefin cyclopropanation involving α- and γ-metalloalkyl radicals as the key intermediates.

Copper(II) Acetate-Induced Oxidation of Hydrazones to Diazo Compounds under Flow Conditions Followed by Dirhodium-Catalyzed Enantioselective Cyclopropanation Reactions

A tandem system comprising in-line diazo compound synthesis and downstream consumption in a rhodium-catalyzed cyclopropanation reaction has been developed. Passing hydrazone through a silica column absorbed with Cu(OAc)₂-H₂O/N,N-dimethylaminopyridine oxidized the hydrazone to generate an aryldiazoacetate in flow. The crude aryldiazoacetate elutes from this column directly into a downstream cyclopropanation reaction, catalyzed by the chiral dirhodium tetracarboxylates, Rh₂(R-p-Ph-TPCP)₄ and Rh₂(R-PTAD)₄. This convenient flow to batch method was applied to the synthesis of a range of 1,2-diarylcyclopropane-1-carboxylates in high yields and with high levels of enantioselectivity.

Repurposed and artificial heme enzymes for cyclopropanation reactions

Heme enzymes are some of the most versatile catalysts in nature. In recent years it has been found that they can also catalyze reactions for which there are no equivalents in nature. This development has been driven by the abiological catalytic reactivity reported for bio-inspired and biomimetic iron porphyrin complexes. This review focuss es on heme enzymes for catalysis of cyclopropanation reactions. The two most important approaches used to create enzymes for cyclopropanation are repurposing of heme enzymes and the various strategies used to improve these enzymes such as mutagenesis and heme replacement, and artificial heme enzymes. These strategies are introduced and compared. Moreover, lessons learned with regard to mechanism and design principles are discussed.