Catalytic Carbene Insertion into C−H Bonds

Conformational Locking Induced Enantioselective Diarylcarbene Insertion into B-H and O-H Bonds Using a Cationic Rh(I)/Diene Catalyst

Transition-metal-catalyzed enantioselective transformations of aryl/aryl carbene are inherently challenging due to the difficulty in distinguishing between two arene rings in the reaction process thus remain largely less explored. The few successful examples reported so far, without exception, have all been catalyzed by Rh(II)-complexes. Herein, we describe our successful development of a novel cationic Rh(I)/chiral diene catalytic system capable of efficient enantioselective B-H and O-H insertions with diaryl diazomethanes, allowing the access to a broad range of gem-diarylmethine boranes and gem-diarylmethine ethers in good yields with high enantioselectivities. Notably, previously unattainable asymmetric diarylcarbene insertion into the O-H bond was achieved for the first time. A remarkable feature of this newly designed Rh(I)/diene catalyst bearing two ortho-amidophenyl substitutents is that it can distinguish between two arene rings of the diaryl carbene through a stereochemically selective control of π-π stacking interactions. DFT calculations indicate that the rotation-restricted conformation of Rh(I)/diene complex played an important role in the highly enantioselective carbene transformations. This work provides an interesting and unprecedented stereocontrol mode in diaryl metal carbene transformations.

Recent advances in catalytic enantioselective carbometallation of cyclopropenes and cyclobutenes

Enantioenriched small carbocycles are key structures in numerous natural products and pharmaceutically important molecules as well as vital intermediates in organic synthesis. Although various catalytic approaches for the construction of such molecules from acyclic precursors have been developed, direct enantioselective functionalization of preformed three-membered and four-membered rings represents the most straightforward and modular strategy, enabling rapid and diversified synthesis of enantioenriched cyclopropanes and cyclobutanes from a single set of starting materials without the need for the incorporation of specific functional groups. In this Feature Article, we have summarized the recent advances in catalytic enantioselective functionalization of cyclopropenes and cyclobutenes through carbometallation. The plausible mechanisms of such reactions and future of this field are also discussed.

A Lead(II) Substituted Triplet Carbene

Reaction of the pincer-type ligand L3 supported complex [L3PbBr][BArF24] (1) with Li[(C(═N2)TMS)] furnishes [L3Pb(C(═N2)TMS)][BArF24] (2). Diazo-compound 2 eliminates dinitrogen upon irradiation affording formal plumba-alkyne 3, which persists in cold fluoroarene solutions. Variable temperature UV/Vis and NMR spectroscopies in combination with quantum-chemical calculations identify 3 as a metal-substituted triplet carbene. In-crystallo irradiation of [L3Pb(C(═N2)TMS)(tol)][BArF24] (2·tol) provides a snapshot of intermolecular C-H bond insertion with toluene (4).

C-H Insertion from Isolable Copper Benzylidenes

Despite the utility of copper catalysts for the insertion of carbene moieties into C-H bonds, the copper carbene intermediate often invoked in these transformations has not been isolated. Herein, we describe the synthesis and structural characterization of a series of copper benzylidenes utilizing the sterically encumbered dipyrrin ligand (EmL)H. These isolated copper carbenes demonstrate intramolecular insertion into the primary C(sp3)-H bond of the ligand (EmL)H and intermolecular insertion into ethereal and allylic C-H bonds. The copper carbenes isolated are best described as Cu(I) carbene adducts akin to canonical Fischer carbenes, given their diamagnetic ground state and electrophilic carbene reactivity. Furthermore, the insertion chemistry can be rendered catalytic utilizing a more sterically exposed dipyrrin ligand (ArFL)H. The ability to isolate and observe stoichiometric C-H insertion and olefin cyclopropanation from well-characterized copper benzylidenes illuminates their viability as catalytic intermediates and their participation in potential catalyst deactivation pathways.

Reactivity of Diruthenium Bisborylene Complexes: Formation of B-C and B-H Bonds via Borylene Ligand Coupling

Thermal or photoinduced isomerization of diruthenium bridging bisborylene complexes [{Cp*(H)2Ru}2(μ-BAr)2] (1a, Ar = Ph; 1b, Ar = 3,4,5-F3C6H2) led to nido-ruthenacarboranes 2a and 2b with newly formed B-C and B-H bonds. The reaction mechanism was analyzed by deuterium-labeling experiments and density functional theory calculations. Additionally, 2-fold B-H coupling between borylene and two hydrido ligands of 1a can be achieved, assisted by Lewis base IPr2Me2 to generate a dinuclear bridging borylene complex [(Cp*Ru)2(μ-H)2(μ-BPh)] (3). Our results provide new reactivity patterns for borylene-based functionalizations.

Recent advances in metal carbene-induced semipinacol rearrangements

As has been well-recognized, the semipinacol rearrangements (SPRs) function as a powerful and versatile tool for the construction of all-carbon and heteroatom-containing quaternary stereocenters, which are present in various natural products and bioactive molecules. In recent years, considerable attention has been paid to exploring the metal carbene-induced semipinacol rearrangements, providing an attractive and powerful strategy for obtaining various important carbonyl compounds. However, to date, no review has been published that summarizes the significant advances in the preparation of functionalized carbonyl compounds using these migration rearrangement reactions. In this review article, we have summarised the recent advances in the field of metal carbene-induced SPR reactions according to different metal classifications. Mechanistic insights, synthetic applications, and their limitations are discussed. The challenges and opportunities in this field are also outlined.

Chiral Bis(binaphthyl) Cyclopentadienyl Ligands for Rhodium-Catalyzed Desymmetrization of Diarylmethanes via Selective Arene Coordination

Owing to substantial advances in the past several decades, transition-metal-catalyzed asymmetric reactions have garnered considerable attention as pivotal methods for constructing chiral molecules from abundant, readily available achiral counterparts. These advances are largely attributed to the development of chiral ligands that control stereochemistry through steric repulsion and other noncovalent interactions between the ligands and functional groups or prochiral centers on the substrates. However, stereocontrol weakens dramatically with increasing distance between the reaction site and the functional group or prochiral center. Herein, we report a symphonic strategy for remote stereocontrol of Rh(III)-catalyzed asymmetric benzylic C-H bond addition reactions of diarylmethanes in which the two aryl motifs differ at the meta and/or para position. Specifically, catalysts bearing a new type of chiral cyclopentadienyl (Cp) ligand differentiate between the two aromatic rings of the diarylmethane by arene-selective η6 coordination, setting up an opportunity for ligand-controlled stereoselective benzylic deprotonation and subsequent stereoselective addition to the 1,1-bis(arylsulfonyl)ethylene.

In Situ Observation of Elusive Dirhodium Carbenes and Studies on the Innate Role of Carboxamidate Ligands in Dirhodium Paddlewheel Complexes: A Combined Experimental and Computational Approach

Carboxamidates as equatorial ligands in dirhodium paddlewheel catalysts are widely believed to increase selectivity at the expense of reactivity. The results of the combined experimental and computational approach described in this paper show that one has to beware of such generalizations. First, 103Rh NMR revealed how strongly primary carboxamidates impact the electronic nature of the rhodium center they are bound to; at the same time, such ligands stabilize donor/acceptor carbenes by engaging their ester carbonyl group into peripheral interligand hydrogen bonding. This array benefits selectivity as well as reactivity if maintained along the entire reaction coordinate of a catalytic cyclopropanation. In settings where the hydrogen bond needs to be distorted for the reaction to proceed, however, it constitutes a significant enthalpic handicap. Representative examples for each scenario were analyzed by DFT; in both cases, the cyclopropanation step rather than carbene formation was found to be turnover-limiting. While this conclusion somehow contradicts the literature, it implied that the direct observation of highly reactive dirhodium carbenes in truly catalytic settings might be possible, even though the intermediates carry olefinic sites amenable to intramolecular cyclopropanation. Such in situ monitoring by NMR is without precedent, yet it was successful with the homoleptic catalyst [Rh2(OPiv)4] as well as with its heteroleptic sibling [Rh2(OPiv)3(acam)] comprising an acetamidate (acam); in the latter case, the carbene bound to the rhodium atom at the [O3N]-face was observed, which concurs with the computational data that this species is stabilized by the forecited interligand hydrogen bonding.

Main-group compounds selectively activate natural gas alkanes under room temperature and atmospheric pressure

Most C-H bond activations of natural gas alkanes rely on transition metal complexes. Activations by using main-group systems have been reported but required heating or photo-irradiation under high atmospheric pressure with rather low regioselectivity. Here we report that Lewis acid-carbene adducts facilely undergo oxidative additions to C-H bonds of ethane, propane and n-butane with high selectivity under room temperature and atmospheric pressure. The Lewis acids can be moved by the addition of a base and the carbene-derived products can be easily converted into aldehydes. This work offers a route for main-group element compounds to selectively functionalise C-H bonds of natural gas alkanes and other small molecules.

Recent Advances in Synthetic Methods by Photocatalytic Single-Electron Transfer Chemistry of Pyridine N-Oxides

By adoption of the enabling technology of modern photoredox catalysis and photochemistry, the generation of reactive and versatile pyridine N-oxy radicals can be facilely achieved from single-electron oxidation of pyridine N-oxides. This Synopsis highlights recent methodologies mediated by pyridine N-oxy radicals in developing (1) pyridine N-oxide-based hydrogen atom transfer catalysts for C(sp3)-H functionalizations and (2) β-oxyvinyl radical-mediated cascade reactions. In addition, recent research revealed that direct photoexcitation of pyridine N-oxides allowed for the generation of alkyl carbon radicals from alkylboronic acids.

Crosslinking Vinyl-Addition Polynorbornenes via Difunctional Diazirines to Generate Low Dielectric-Constant and Low Dielectric-Loss Thermosets

Thermosets having low dielectric constant (Dk < 3) and low dielectric dissipation factor (Df < 0.003), high glass transition temperature (Tg > 150 °C), and good adhesion to copper are desirable for the low loss layers of the copper clad laminates (CCL) in next generation printed circuit boards. Three different difunctional diazirines are evaluated for both thermal and photochemical crosslinking of a high Tg vinyl-addition polynorbornene resin: poly(5-hexyl-1-norbornene) (poly(HNB)). The substrate polymer, crosslinked by the carbenes generated from the activated diazirines, forms thermosets with Dk < 2.3 and Df < 0.001 at 10 GHz depending on the identity of the diazirine and the loading. The Dk and Df values for one composition are stable for 1600 h at 125 °C in air and for 1400 h at 85 °C and 85% relative humidity, suggesting good long-term reliability of this thermoset. Adhesion of poly(HNB) to copper can be enhanced by priming the copper surface with a diazirine prior to high temperature lamination; peel strength values of greater than 7.5 N cm-1 are achieved. Negative-tone photopatterning of poly(HNB) with diazirines upon exposure to 365 nm light is demonstrated.

Tunable electrochemical diverse sulfurization of sulfoxonium ylides with disulfides

An electrochemical protocol for the synthesis of sulfursulfoxonium ylides and 1,3-dithioketals by reacting sulfoxonium ylides with disulfides has been developed under simple and mild conditions. By changing the solubility of the raw materials and the dielectric parameters of the electrolyte, sulfurization enabled a selective dehydrogenation of C-S and the construction of 1,3-dithioketals. The transformation is an ideal approach to prepare organosulfur reagents with a broad functional group tolerance as well as high selectivity, which leads to vicinal difunctionalized organosulfur compounds.

Unique Reactivity of Triazolyl Diazoacetates under Photochemical Conditions

Under light irradiation, aryldiazo acetates can generate either singlet or triplet carbenes depending on the reaction conditions, but heteroaryl diazo compounds have remained underexplored in this context. Herein, we found that triazolyl diazoacetates exhibit higher reactivity than their aryl counterparts. They even react with dichloromethane (DCM), a common, inert solvent, for photoreactions involving diazo reagents, giving halogenated products. Theoretical studies show that all reactions involve carbenes but progress via different pathways depending on the solvent used.

Modular Synthesis of Tetrasubstituted Vinyl Sulfides via One-Pot Sequential Carbene Transfer Reaction from Thiols with α-Diazo Carbonyl Compounds

We present a one-pot reaction that offers an efficient approach to synthesizing tetrasubstituted vinyl sulfides with high stereoselectivity. This method involves the sequential Wolff rearrangement, ylide formation, and [1,4]-aryl transfer by utilizing aryl and alkyl thiols and α-diazo carbonyl compounds as substrates. Notably, this reaction features commercially available materials, straightforward operation, atom economy, and broad substrate scope. Moreover, the primary photophysical properties (aggregation-induced emission effect) of the products were also investigated, which might be useful in functional materials via structural modification.

Transition-Metal-Catalyzed Directed C-H Bond Functionalization with Iodonium Ylides: A Review of the Last 5 Years

Transition-metal-catalyzed directed C-H functionalization with various carbene precursors has been widely employed for constructing a wide range of complex and diverse active molecules through metal carbene migratory insertion processes. Among various carbene precursors, iodonium ylides serve as a novel and emerging carbene precursor with features including easy accessibility, thermal stability and high activity, which have attracted great attention from organic chemists and have achieved tremendous success in organic transformation. In this review, recent progress on the application of iodonium ylides with multifunctional coupling characteristics in C-H bond activation reactions is summarized, and the potential of iodonium ylides is discussed.

Asymmetric Carbene Transfer: Enhancing Chemical Diversity for Drug Discovery

The quest to explore chemical space is vital for identifying novel disease targets, impacting both the effectiveness and safety profile of therapeutic agents. The tangible chemical space, currently estimated at a conservative 108 synthesized compounds, pales in comparison to the theoretically conceivable diversity of 1060 molecules. To bridge this vast gap, organic chemists are spearheading innovative methodologies that promise to broaden this limited chemical diversity. A beacon of this progressive wave is Asymmetric Carbene Transfer (ACT), a burgeoning strategy that significantly boosts molecular diversity with efficient bond-formation and precise chiral control. This review focuses on the capabilities of ACT in creating pharmaceutically significant molecules, encompassing an array of natural products and bioactive compounds. Through the lens of ACT, we discern its substantial influence on drug discovery, paving the way for novel therapeutic avenues by expanding the boundaries of molecular diversity. This review will shed light on prospective methodological developments of ACT and articulate their conceivable contributions to the medicinal chemistry arena.

Asymmetric Synthesis and Biological Evaluation of Platensilin, Platensimycin, Platencin, and Their Analogs via a Bioinspired Skeletal Reconstruction Approach

Platensilin, platensimycin, and platencin are potent inhibitors of β-ketoacyl-acyl carrier protein synthase (FabF) in the bacterial and mammalian fatty acid synthesis system, presenting promising drug leads for both antibacterial and antidiabetic therapies. Herein, a bioinspired skeleton reconstruction approach is reported, which enables the unified synthesis of these three natural FabF inhibitors and their skeletally diverse analogs, all stemming from a common ent-pimarane core. The synthesis features a diastereoselective biocatalytic reduction and an intermolecular Diels-Alder reaction to prepare the common ent-pimarane core. From this intermediate, stereoselective Mn-catalyzed hydrogen atom-transfer hydrogenation and subsequent Cu-catalyzed carbenoid C-H insertion afford platensilin. Furthermore, the intramolecular Diels-Alder reaction succeeded by regioselective ring opening of the newly formed cyclopropane enables the construction of the bicyclo[3.2.1]-octane and bicyclo[2.2.2]-octane ring systems of platensimycin and platencin, respectively. This skeletal reconstruction approach of the ent-pimarane core facilitates the preparation of analogs bearing different polycyclic scaffolds. Among these analogs, the previously unexplored cyclopropyl analog 47 exhibits improved antibacterial activity (MIC80 = 0.0625 μg/mL) against S. aureus compared to platensimycin.

Unlocking catalytic potential: a rhodium(II)-based coordination polymer for efficient carbene transfer reactions with donor/acceptor diazoalkanes

Herein, we report the use of a molecular-defined rhodium(II) coordination polymer (Rh-CP) as a heterogeneous, recyclable catalyst in carbene transfer reactions. We showcase the application of this heterogeneous catalyst in a range of carbene transfer reactions and conclude with the functionalization of natural products and drug molecules.

α-Carbonyl Rh-Carbenoid Initiated Cascade Assembly of Diazobarbiturates with Alkylidene Pyrazolones for Synthesis of Spirofuropyrimidines

Catalyzed by Rh2(esp)2 (10 mol%) and (±)-BINAP (20 mol%) in DCE at 80 °C, the cascade assembly between diazobarbiturates and alkylidene pyrazolones proceeded readily and produced spiro-furopyrimidines in 38-96% chemical yields. The chemical structure of the prepared spirofuro-pyrimidines was firmly confirmed by X-ray diffraction analysis.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Photoinduced [3+2] Cycloaddition of Alkyl-Acceptor Diazoalkanes: Diversity-Oriented Synthesis of Pyrazolines Containing a Quaternary Center

We present a new [3+2] cycloaddition reaction between alkyl-acceptor diazoalkanes under visible light irradiation. By employing easily accessible alkyl-acceptor-type diazoalkanes or their precursor hydrazones as both 1,3-dipoles and dipolarophiles, a diverse range of pyrazoline derivatives featuring a quaternary center have been efficiently synthesized in a predictable manner, with excellent functional group tolerance and good yields. Furthermore, scale-up experiments and downstream transformations of the product were also detailed.

Rh(III)-catalysed C-H annulation of cis-stilbene acids with 2-diazo-1,3-diketones: facile access to 6,7-dihydrobenzofuran-4(5H)-one and α-pyrone scaffolds

An efficient Rh(III)-catalysed C-H functionalization, tandem annulation of cis-stilbene acids using 2-diazo-1,3-diketones was devised. This protocol solely afforded 6,7-dihydrobenzofuran-4(5H)-ones using alicyclic diazocarbonyls via decarbonylation and α-pyrones with aliphatic diazo compounds. The chameleonic nature of cis-stilbene acid was observed with various diazo compounds by altering the additives. This synthetic method furnished good atom-economy and wide functional group tolerance, and also explained the use of carboxylic acids as a directing group. In addition, a mechanistic investigation of the catalysed reaction using ESI-MS, and the fluorescence properties of α-pyrones were well explored.

Alkoxy Diazomethylation of Alkenes by Photoredox-Catalyzed Oxidative Radical-Polar Crossover

Herein, we present the discovery and development of the first photoredox-catalyzed alkoxy diazomethylation of alkenes with hypervalent iodine reagents and alcohols. This multicomponent process represents a new disconnection approach to diazo compounds and is featured by a broad scope, mild reaction conditions, and excellent selectivity. Key to the process was the generation of diazomethyl radicals, which engaged alkenes and alcohols in an inter- and intramolecular fashion by a photoredox-catalyzed oxidative radical-polar crossover leading to unexplored β-alkoxydiazo compounds. The synthetic utility of such diazo compounds was demonstrated with a series of transformations involving C-H, N-H, and O-H insertions as well as in the construction of complex sp3-rich heterocycles.

Catalytic asymmetric carbenoid α-C-H insertion of ether

Significant advancements have been made in catalytic asymmetric α-C-H bond functionalization of ethers via carbenoid insertion over the past decade. Effective asymmetric catalytic systems, featuring a range of chiral metal catalysts, have been established for the enantioselective synthesis of diverse ether substrates. This has led to the generation of various enantioenriched, highly functionalized oxygen-containing structural motifs, facilitating their application in the asymmetric synthesis of bioactive natural products.

Origin of the Conformational Stability of Dirhodium(II) Tetrakis[N-phthaloyl-(S)-tert-leucinate] and Its Halogenated Derivatives

In order to elucidate the origins of the stable structures of dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate] and the four derivatives with halogenated aromatic rings, the conformational stability and intramolecular interactions were investigated by DFT calculations. In all of these complexes, the conformation in which all ligands face in the same direction is the most stable. When adjacent ligands are in the same orientation, destabilization due to exchange repulsion is larger than that when they are in opposite orientations. However, this destabilizing effect is reversed by the sum of the stabilizing effects of the electronic and charge transfer interactions. The imide carbonyl group plays an important role in these stabilizing interactions. The negatively charged site and bond orbitals in the imide carbonyl group interact with the positively charged sites and bond orbitals in the aromatic ring, the carboxylate group, and the α-position of the carboxylate group in the adjacent ligands. In addition, the lone-pair orbitals of the halogen atoms contribute to conformational stabilization by interacting with the vacant orbitals in the adjacent ligands. However, the combinations of these charged sites or bond orbitals, which effectively contribute to the stabilization, are different for each complex.

Dioxane promoted photochemical O-alkylation of 1,3-dicarbonyl compounds beyond carbene insertion into C-H and C-C bonds

A photochemical synthesis of enol ethers and furan-3(2H)-ones from 1,3-dicarbonyl compounds and aryl diazoacetates has been developed. Significantly, 1,4-dioxane promoted O-alkylation of various 1,3-dicarbonyl compounds beyond previous carbene insertion into C-H and C-C bonds has been disclosed.

Palladium-Catalyzed Regioselective Insertion of Carbenes into γ-C(sp3)-H Bonds of Aliphatic Amines

A migratory insertion of carbenes into distal γ-C(sp3)-H bonds of aliphatic amines has been successfully developed. The synergistic interplay among a palladium catalyst, picolinamide directing group, a carefully selected base additive, and an essential ligand proved crucial in achieving high yields. These findings hold significant value for advancing the exploration of regioselective carbene insertions into nonactivated C(sp3)-H bonds.

Trifluoroethylation and Pentafluoropropylation of C(sp3)-H Bonds

Polyfluorinated substituents often enhance effectiveness, improve the stability within metabolic processes, and boost the lipophilicity of biologically active compounds. However, methods for their introduction into aliphatic carbon chains remain very limited. A potentially general route to integrate the fluorinated scaffolds into organic molecules involves insertion of fluorine-containing carbenes into C(sp3)-H bonds. The electron-withdrawing characteristics of perfluoroalkyl groups enhances the reactivity of these carbenes which should enable the functionalization of unactivated C(sp3)-H bonds. Curiously, it appears that use of perfluoroalkyl-containing carbenes in alkane C-H functionalization is exceedingly rare. This concept describes photolysis, enzymatic catalysis, and transition metal catalysis as three primary approaches to C(sp3)-H functionalization by trifluoromethylcarbene and its homologues.

Photodriven Radical-Polar Crossover Cyclization Strategy: Synthesis of Pyrazolo[1,5-a]pyridines from Diazo Compounds

This work demonstrates the synthesis of a variety of perfluoroalkyl heterocycles via a visible-light-driven radical-polar crossover cyclization strategy. In this process, single-electron reduction/SNV-type/cyclization sequences follow the radical addition reaction of a diazoester, which differs from the current role of diazoesters as radical precursors/acceptors. This transformation demonstrates excellent functional group compatibility and allows for the modification of many bioactive molecules with diazoesters. Such a reaction could represent a novel approach to the photochemical transformation of diazo compounds.

Silica nanoparticles and kaolin clay decorated with VO2+ in aerobic oxidative destruction of BTEX contaminants

The importance of controlled hydrocarbon oxidation has sparked interest in methods that catalyze this process. In this vein, controlled oxidative degradation of BTEX compounds (benzene, toluene, ethylbenzene and xylenes) which are hazardous air and industrial waste water contaminants is very considerable. Accordingly, the reactive VO2+ species was anchored onto silica nanoparticles (VO-SNP) to catalyze the conversion of BTEX into useful compounds. The synthesized heterogeneous VO-SNP catalyst was characterized using different techniques such as FTIR, FETEM, FESEM, XRD, EDX, ICP and XPS. Interestingly, the catalyst performed the activation of the relatively inert C-H bonds of BTEX to produce oxygenated compounds under quite mild and eco-friendly conditions at room temperature with no extra additives. Furthermore, we introduced VO2+ species onto mineral kaolin sheets (VO-kaolin) as a vanadyl decorated natural solid support and the results showed less efficiency compared to VO-SNP.

Reagent Engineering for Group Transfer Biocatalysis

Biocatalysis has become a major driver in the innovation of preparative chemistry. Enzyme discovery, engineering and computational design have matured to reliable strategies in the development of biocatalytic processes. By comparison, substrate engineering has received much less attention. In this Minireview, we highlight the idea that the design of synthetic reagents may be an equally fruitful and complementary approach to develop novel enzyme-catalysed group transfer chemistry. This Minireview discusses key examples from the literature that illustrate how synthetic substrates can be devised to improve the efficiency, scalability and sustainability, as well as the scope of such reactions. We also provide an opinion as to how this concept might be further developed in the future, aspiring to replicate the evolutionary success story of natural group transfer reagents, such as adenosine triphosphate (ATP) and S-adenosyl methionine (SAM).

Polycyclic Pyrazoles from Alkynyl Cyclohexadienones and Nonstabilized Diazoalkanes via [3 + 2]-Cycloaddition/[1,5]-Sigmatropic Rearrangement/Aza-Michael Reaction Cascade

An efficient method for the stereoselective synthesis of "all center substituted" polycyclic pyrazoles from alkynyl cyclohexa-2,5-dienones and nonstabilized diazoalkanes via sequential [3 + 2]-cycloaddition/[1,5]-sigmatropic rearrangement and aza-Michael reactions is reported. The developed process is highly regioselective and stereoselective. It employs a wide substrate scope to furnish structurally diverse linear and bridged [4.4.n.0] ring-fused pyrazoles in moderate to good yields. One-pot and gram-scale syntheses and synthetic transformations have also been showcased.

Dirhodium-Catalyzed Enantioselective Synthesis of Difluoromethylated Cyclopropanes via Enyne Cycloisomerization

(Difluoromethylated cyclopropane represents an important motif, which is widely found in bioactive and functional molecules. Despite significant progress in modern chemistry, the atom-economic and enantioselective synthesis of difluoromethylated cyclopropanes is still challenging. Herein, an Rh2 (II)-catalyzed asymmetric enyne cycloisomerization is described to construct chiral difluoromethylated cyclopropane derivatives with up to 99% yield and 99% ee in low catalyst loading (0.2 mol%), which can be easily transformed into highly functionalized difluoromethylated cyclopropanes with vicinal all-carbon quaternary stereocenters by ozonolysis. Mechanistic studies and the crystal structures of alkyne-dirhodium complexes reveal that the cooperative weak hydrogen bondings between the substrates and the dirhodium catalyst may play key roles in this reaction.).

Photoproximity Labeling from Single Catalyst Sites Allows Calibration and Increased Resolution for Carbene Labeling of Protein Partners In Vitro and on Cells

The cell surface proteome (surfaceome) plays a pivotal role in virtually all extracellular biology, and yet we are only beginning to understand the protein complexes formed in this crowded environment. Recently, a high-resolution approach (μMap) was described that utilizes multiple iridium-photocatalysts attached to a secondary antibody, directed to a primary antibody of a protein of interest, to identify proximal neighbors by light-activated conversion of a biotin-diazirine to a highly reactive carbene followed by LC/MS (Geri, J. B.; Oakley, J. V.; Reyes-Robles, T.; Wang, T.; McCarver, S. J.; White, C. H.; Rodriguez-Rivera, F. P.; Parker, D. L.; Hett, E. C.; Fadeyi, O. O.; Oslund, R. C.; MacMillan, D. W. C. Science2020, 367, 1091-1097). Here we calibrated the spatial resolution for carbene labeling using site-specific conjugation of a single photocatalyst to a primary antibody drug, trastuzumab (Traz), in complex with its structurally well-characterized oncogene target, HER2. We observed relatively uniform carbene labeling across all amino acids, and a maximum distance of ∼110 Å from the fixed photocatalyst. When targeting HER2 overexpression cells, we identified 20 highly enriched HER2 neighbors, compared to a nonspecific membrane tethered catalyst. These studies identify new HER2 interactors and calibrate the radius of carbene photoprobe labeling for the surfaceome.

Enzymatic Assembly of Diverse Lactone Structures: An Intramolecular C-H Functionalization Strategy

Lactones are cyclic esters with extensive applications in materials science, medicinal chemistry, and the food and perfume industries. Nature's strategy for the synthesis of many lactones found in natural products always relies on a single type of retrosynthetic strategy, a C-O bond disconnection. Here, we describe a set of laboratory-engineered enzymes that use a new-to-nature C-C bond-forming strategy to assemble diverse lactone structures. These engineered "carbene transferases" catalyze intramolecular carbene insertions into benzylic or allylic C-H bonds, which allow for the synthesis of lactones with different ring sizes and ring scaffolds from simple starting materials. Starting from a serine-ligated cytochrome P450 variant previously engineered for other carbene-transfer activities, directed evolution generated a variant P411-LAS-5247, which exhibits a high activity for constructing a five-membered ε-lactone, lactam, and cyclic ketone products (up to 5600 total turnovers (TTN) and >99% enantiomeric excess (ee)). Further engineering led to variants P411-LAS-5249 and P411-LAS-5264, which deliver six-membered δ-lactones and seven-membered ε-lactones, respectively, overcoming the thermodynamically unfavorable ring strain associated with these products compared to the γ-lactones. This new carbene-transfer activity was further extended to the synthesis of complex lactone scaffolds based on fused, bridged, and spiro rings. The enzymatic platform developed here complements natural biosynthetic strategies for lactone assembly and expands the structural diversity of lactones accessible through C-H functionalization.

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronicc i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronict r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.

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.

Systematic Studies of Functional Group Tolerance and Chemoselectivity in Carbene-Mediated Intramolecular Cyclopropanation and Intermolecular C-H Functionalization

Generating reliable data on functional group compatibility and chemoselectivity is essential for evaluating the practicality of chemical reactions and predicting retrosynthetic routes. In this context, we performed systematic studies using a functional group evaluation kit including 26 kinds of additives to assess the functional group tolerance of carbene-mediated reactions. Our findings revealed that some intermolecular heteroatom-hydrogen insertion reactions proceed faster than intramolecular cyclopropanation reactions. Lewis basic functionalities inhibited rhodium-catalyzed C-H functionalization of indoles. While performing these studies, we observed an unexpected C-H functionalization of a 1-naphthol variant used as an additive.

Influence of Metal Identity and Complex Nuclearity in Kumada Cross-Coupling Polymerizations with a Pyridine Diimine-Based Ligand Scaffold

Cross-coupling polymerizations have fundamentally changed the field of conjugated polymers (CPs) by expanding the scope of accessible materials. Despite the prevalence of cross-coupling in CP synthesis, almost all polymerizations rely on mononuclear Ni or Pd catalysts. Here, we report a systematic exploration of mono- and dinuclear Fe and Ni precatalysts with a pyridine diimine ligand scaffold for Kumada cross-coupling polymerization of a donor thiophene and an acceptor benzotriazole monomers. We observe that variation of the metal identity from Ni to Fe produces contrasting polymerization mechanisms, while complex nuclearity has a minimal impact on reactivity. Specifically, Fe complexes appear to catalyze step-growth Kumada polymerizations and can readily access both Csp2-Csp3 and Csp2-Csp2 cross-couplings, while Ni complexes catalyze chain-growth polymerizations and predominantly Csp2-Csp2 cross-couplings. Thus, our work sheds light on important design parameters for transition metal complexes used in cross-coupling polymerizations, demonstrates the viability of iron catalysis in Kumada polymerization, and opens the door to novel polymer compositions.

Experimental and Computational Studies on Uranium Diazomethanediide Complexes

Uranium diazomethanediide complexes can be prepared and their synthesis, structure and reactivity were explored. Reaction of the uranium imido compound [η5 -1,2,4-(Me3 Si)3 C5 H2 ]2 U=N(p-tolyl)(dmap) (1) or [η5 -1,3-(Me3 C)2 C5 H3 ]2 U=N(p-tolyl)(dmap) (4) with Me3 SiCHN2 cleanly yields the first isocyanoimido metal complexes [η5 -1,2,4-(Me3 Si)3 C5 H2 ]2 U(=NNC)(μ-CNN=)U(dmap)[η5 -1,2,4-(Me3 Si)3 C5 H2 ]2 (2) and {[η5 -1,3-(Me3 C)2 C5 H3 ]2 U[μ-(=NNC)]}6 (5), respectively. Both compounds exhibit remarkable thermal stability and were fully characterized. According to density functional theory (DFT) studies the bonding between the Cp2 U2+ and [NNC]2- moieties is strongly polarized with a significant 5 f orbital contribution, which is also reflected in the reactivity of these complexes. For example, complex 5 acts as a nucleophile toward alkylsilyl halides and engages in a [2+2] cycloaddition with CS2 , but no reaction occurs in the presence of internal alkynes.

Enantioselective Multifunctionalization with Rh Carbynoids

Multifunctionalization from the interception of active intermediates is an attractive synthetic strategy for the efficient construction of complex molecular scaffolds in an atom and step economic fashion. However, the design of reactions involving metal carbynoids that exhibit carbene/carbocation behavior is currently limited, and developing catalyst-controlled highly enantioselective versions poses significant challenges. In this study, we present the first asymmetric trifunctionalization reactions with rhodium carbynoids. This reaction unveils the distinctive reactivity of the carbynoid precursor, enabling it to react with simultaneously two nucleophiles and one electrophile. This process involves the formation of two distinct carbene ylides with the alcohol/carbamate and the trapping of one ylide with the imine, resulting in the formation of three new bonds. Furthermore, this strategy allows for the divergent synthesis of a wide array of β-amino esters in high yields and exceptional enantioselectivity.

Introducing N-Sulfinylamines into Visible-Light-Induced Carbene Chemistry for the Synthesis of Diverse Amides and α-Iminoesters

A rare example of visible-light-mediated diverse reactivity of N-sulfinylamines with different types of carbene precursors has been disclosed. Acylsilanes and aryldiazoacetates have been utilized as nucleophilic and electrophilic carbene precursors into the N═S═O linchpin, to achieve valuable amides and α-iminoesters, respectively. Interestingly, diazocarbonyls can also participate in the amidation reaction with N-sulfinylamines via in situ generated ketenes. This operationally simple modular method offers a mild, transition-metal-free, and coupling-reagent-free protocol to fabricate structurally diverse amides and a promptly accessible technique to achieve α-iminoesters, where visible light remains as a key promoter.

Copper-catalyzed atroposelective formal [4+1] annulation of 1,2-diketones with vinyl cations

The enantioselective transformation of easily accessible 1,2-diketones represents a quick pathway towards enantioenriched molecules. Herein, we disclose a copper-catalyzed atroposelective formal [4+1] annulation of 1,2-diketones with vinyl cations, enabling the efficient and atom-economical construction of axially chiral arylpyrroles bearing 1,3-dioxole moieties with good to excellent enantioselectivities under mild reaction conditions. Importantly, this methodology constitutes the first enantioselective formal [4+1] annulation of 1,2-diketones.

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

Ruthenium-Catalyzed Chemo-Selective Carbene Insertion into C-H Bond of Styrene over Cyclopropanation: C-C Bond Formation

The C-C bond formation reactions are important in organic synthesis. Heck reaction is known to arylate the terminal carbon of olefins; however, direct alkylation of the terminal carbon of olefin is limited. Herein, we report a novel ruthenium-catalyzed selective cross-coupling reaction of styrene and α-diazoesters to form a new C-C bond over cyclopropanation via the C-H insertion process for the first time. Using this novel methodology, a wide variety of substrates have been utilized and a variety of α-vinylated benzylic esters and densely functionalized olefins have been synthesized with good stereoselectivity under mild reaction conditions. The overall reaction process proceeds through the carbene insertion into styrene to form the desired products in good to excellent yields with proper stereoselectivity. The selective C-H inserted product, wide substrate scope, and excellent functional group tolerance are the best features of this work.

Catalytic, asymmetric carbon-nitrogen bond formation using metal nitrenoids: from metal-ligand complexes via metalloporphyrins to enzymes

The introduction of nitrogen atoms into small molecules is of fundamental importance and it is vital that ever more efficient and selective methods for achieving this are developed. With this aim, the potential of nitrene chemistry has long been appreciated but its application has been constrained by the extreme reactivity of these labile species. This liability however can be attenuated by complexation with a transition metal and the resulting metal nitrenoids have unique and highly versatile reactivity which includes the amination of certain types of aliphatic C-H bonds as well as reactions with alkenes to afford aziridines. At least one new chiral centre is typically formed in these processes and the development of catalysts to exert control over enantioselectivity in nitrenoid-mediated amination has become a growing area of research, particularly over the past two decades. Compared with some synthetic methods, metal nitrenoid chemistry is notable in that chemists can draw from a diverse array of metals and catalysts , ranging from metal-ligand complexes, bearing a variety of ligand types, via bio-inspired metalloporphyrins, all the way through to, very recently, engineered enzymes themselves. In the latter category in particular, rapid progress is being made, the rate of which suggests that this approach may be instrumental in addressing some of the outstanding challenges in the field. This review covers key developments and strategies that have shaped the field, in addition to the latest advances, up until September 2023.

Bifunctionality of dirhodium tetracarboxylates in metallaphotocatalysis

Metallaphotocatalysis has been recognized as a pivotal catalysis enabling new reactivities. Traditional metallaphotocatalysis often requires two or more separate catalysts and exhibits flaw in cost and substrate-tolerance, thus representing an await-to-solve issue in catalysis. We herein realize metallaphotocatalysis with a bifunctional dirhodium tetracarboxylate ([Rh2]) alone. The [Rh2] shows an photocatalytic activity of promoting singlet oxygen (1O2) oxidation. By harnessing its photocatalytic activity, the [Rh2] catalyzes a photochemical cascade reaction (PCR) via combination of carbenoid chemistry and 1O2 chemistry. The PCR is characterized by high atom-efficiency, excellent stereoselectivities, mild conditions, scalable synthesis, and pharmaceutically interesting products. DFT calculations-aided mechanistic study rationalizes the reaction pathway and interprets the origin of stereoselectivities of the PCR. The products show inhibitory activity against PTP1B, being promising in the treatment of type II diabetes and cancers. Overall, here we show the bifunctional [Rh2] merges Rh-carbenoid chemistry and 1O2 chemistry.

Diazo-Transfer Reaction of Nonactivated Ketones with 2-Azido-1,3-bis(2,6-diisopropylphenyl)imidazolium Hexafluorophosphate (IPrAP)

The diazo-transfer reaction of nonactivated ketone under mild reaction conditions was developed. Various nonactivated ketones such as aryl methyl ketones, sec-alkyl methyl ketones, and cyclic ketones were transformed into their corresponding α-diazoketones in one step by treating 2-azido-1,3-bis(2,6-diisopropylphenyl)imidazolium hexafluorophosphate (IPrAP) in the presence of iPr2NH in ethylene glycol. In the reaction of IPrAP with prim-alkyl methyl ketone and prim-alkyl aryl ketones, migratory amidation proceeded under the reaction conditions to afford the corresponding amides.

Development of Catalytic Site-Selective C-H Oxidation

Direct C-H bond oxygenation is a strong and useful tool for the construction of oxygen functional groups. After Chen and White's pioneering works, various non-heme-type iron and manganese complexes were introduced, leading to strong development in this area. However, for this method to become a truly useful tool for synthetic organic chemistry, it is necessary to make further efforts to improve site-selectivity, and catalyst durability. Recently, we found that non-heme-type ruthenium complex cis-1 presents efficient catalysis in C(sp3 )-H oxygenation under acidic conditions. cis-1-catalysed C-H oxygenation can oxidize various substrates including highly complex natural compounds using hypervalent iodine reagents as a terminal oxidant. Moreover, the catalyst system can use almost stoichiometric water molecules as the oxygen source through reversible hydrolysis of PhI(OCOR)2 . It is a strong tool for producing isotopic-oxygen-labelled compounds. Moreover, the environmentally friendly hydrogen peroxide can be used as a terminal oxidant under acidic conditions.

Recent Advances in Monofluorinated Carbenes, Carbenoids, Ylides, and Related Species

The synthesis of monofluorinated compounds is of great interest because of the vast applications of organofluorine compounds. Recently, the introduction of monofluorocarbene synthons has emerged as an important strategy for the synthesis of fluorine-containing products. In contrast to direct fluorination, in which C-F bonds are formed, the use of monofluorinated carbenes and related reactive species involves C-C or C-X bond formation while delivering valuable fluorine atoms into the target structure. Owing to increased knowledge on carbon-carbon and carbon-heteroatom bond formations, monofluorinated carbenes have enormous potential for the synthesis of organofluorine compounds, which, in our opinion, has not yet been fully exploited. This review summarizes the recent advances in the synthetic applications of monofluorinated carbenes, carbenoids, ylides, and related species.