Visible light-promoted reactions with diazo compounds: a mild and practical strategy towards free carbene intermediates

Stereoselective Synthesis of Tetrasubstituted Olefins via Visible-Light-Promoted Iodine-Mediated Homo-Coupling of Diazo

A visible-light-promoted iodine-mediated homo-coupling of diazo was first described. A series of tetrasubstituted olefins were synthesized in high yields and with low to high Z-selectivities from phenyldiazoacetates. For 3-diazooxindoles, isoindigo derivatives were provided in moderate to high yields and with excellent E-selectivities. Experimental results showed that the reaction proceeded through a diiodo intermediate. The synthetic usefulness of this reaction was illustrated by the synthesis of maleimide derivatives and dispiro epoxy.

Photocatalytic para-Selective C-H Functionalization of Anilines with Diazomalonates

Visible-light-induced para-selective C-H functionalization of anilines over N-H insertion was developed using diazomalonates with the help of an Ir(III) photocatalyst. The para-selective radical-radical cross coupling proceeded via C-centered radical intermediates generated from both anilines and diazomalonates. The photochemistry of anilines could be extended to other N-heterocycles, such as indole and carbazole. The reaction pathway for the selective C-C coupling was validated by electrochemical and photophysical experiments as well as computational studies.

Photochemical Synthesis of Succinic Ester-Containing Phenanthridines from Diazo Compounds as 1,4-Dicarbonyl Precursors

We disclosed herein a straightforward photochemical method for the construction of phenanthridines containing a synthetically useful succinate unit. The reaction occurred under visible-light irradiation with cheap eosin Y Na as photoredox catalyst and a diazo compound as the succinate precursor. Under the optimal reaction conditions, a wide range of phenanthridines were obtained in moderate to good yields. Note that the succinate units in the final heterocycles could be easily transformed into many valuable structures, such as γ-butyrolactone, dihydrofuran-2(3H)-one, and tetrahydrofuran. Mechanistic experiments were performed to support the proposed mechanism.

Visible-Light-Mediated (sp 3)Cα-H Functionalization of Ethers Enabled by Electron Donor-Acceptor Complex

A synthetically beneficial visible-light-mediated protocol has been disclosed to achieve C-H amination of readily available feedstocks cyclic and acyclic ethers. A rarely identified N-bromosuccinamide-tetrahydrofuran electron donor-acceptor complex served as an initiator to functionalize both α-diazoketones and dialkyl azodicarboxylates. This developed methodology gives an alternative and milder way to construct the C-N bond and can be explored for the formation of C-C bond to perform arylation and allylation reactions.

Visible Light-Promoted Diazoacetates and Nitriles Generating Nitrilium Ions Trapped by Benzotriazoles and Carboxylic Acids

A visible light-promoted generation of nitrilium ions from diazoacetates and nitriles has been developed. The reaction utilized visible light transformation of diazoacetates to the free carbene that could be trapped by nitriles to generate nitrilium ions, followed by nucleophilic attack on the benzotriazoles and carboxylic acids. This protocol provides an efficient and practical approach to N-imidoylbenzotriazoles and diacylglycine esters in good to excellent yields.

TBAI-Catalyzed S-H and N-H Insertion Reactions of α-Diazoesters with Thiophenols and Amines under Metal-Free Conditions

Mild, convenient, and effective TBAI-catalyzed S-H and N-H insertion reactions of α-diazoesters with thiophenols and aromatic amines under metal-free conditions have been described, furnishing a straightforward and general platform for the synthesis of various thioethers and 2-amino-2-oxoacetates in moderate to excellent yields. Moreover, this strategy features simple operation, mild conditions, broad substrate scope, and easy scale-up.

Visible-Light-Induced, Single-Metal-Catalyzed, Directed C-H Functionalization: Metal-Substrate-Bound Complexes as Light-Harvesting Agents

C-H functionalization represents one of the most rapidly advancing areas in organic synthesis and is regarded as one of the key concepts to minimize the ecological and economic footprint of organic synthesis. The ubiquity and low reactivity of C-H bonds in organic molecules, however, poses several challenges, and often necessitates harsh reaction conditions to achieve this goal, although it is highly desirable to achieve C-H functionalization reactions under mild conditions. Recently, several reports uncovered a conceptually new approach towards C-H functionalization, where a single transition-metal complex can be used as both the photosensitizer and catalyst to promote C-H bond functionalization in the absence of an exogeneous photosensitizer. In this Minireview, we will provide an overview on recent achievements in C-H functionalization reactions, with an emphasis on the photochemical modulation of the reaction mechanism using such catalysts.

Fe-Catalyzed Selective Formal Insertion of Diazo Compounds into C(sp)-C(sp3) Bonds of Propargyl Alcohols: Access to Alkyne-Substituted All-Carbon Quaternary Centers

The construction of all-carbon quaternary centers, especially those containing an alkyne-substituted framework, represents an important challenge in organic synthesis. Here we present a novel Fe-catalyzed selective formal insertion of diazo compounds into C(sp)-C(sp³) bonds of propargyl alcohols under mild conditions that enables the streamlined construction of alkyne-substituted all-carbon quaternary centers. This unique strategy starts with in situ generation of an ester group in the presence of carboxylic acids, followed by insertion of metal-carbene into C(sp)-C(sp³) bonds, which may open up a new reaction mode for exploring metal-carbene insertion into acyclic C-C bonds.

Functionalized Cyclopentenes via the Formal [4+1] Cycloaddition of Photogenerated Siloxycarbenes from Acyl Silanes

This work describes the first formal cycloaddition reaction of photogenerated nucleophilic carbenes derived from acylsilanes with electrophilic dienes. The resulting transient donor-acceptor cyclopropane rearranges to its stable and highly functionalized cyclopentene isomer in an unprecedented metal-free process. The cyclopropanation-vinyl cyclopropane rearrangement sequence was corroborated by computational calculations. The cyclopropane formation corresponds to a higher energetic barrier, and the vinylcyclopropane-cyclopentene rearrangement proceeds through different mechanisms, although of comparable energies, depending on the stereochemistry of the cyclopropane.

Continuous Flow Technology as an Enabler for Innovative Transformations Exploiting Carbenes, Nitrenes, and Benzynes

Miniaturization offered by microreactors provides for superb reaction control as well as excellent heat and mass transfer. By performing chemical reactions in microreactors or tubular systems under continuous flow conditions, increased safety can be harnessed which allows exploitation of these technologies for the generation and immediate consumption of high-energy intermediates. This Synopsis demonstrates the use of flow technology to effectively exploit benzynes, carbenes, and nitrenes in synthetic chemistry programs.

Photocatalytic 1,2-oxo-alkylation reaction of styrenes with diazoacetates

We report on the photocatalytic 1,2-difunctionalization reaction of styrenes with acceptor-only diazoalkanes. In the presence of DABCO and tBuOOH, the carbene reactivity of diazoalkanes can be suppressed and a 1,2 oxo-alkylation reaction can be achieved (32 examples, up to 94% yield) without the formation of cyclopropane by-products via the formation of radical intermediates from ethyl diazoacetate.

Organophotocatalytic Regioselective C-H Alkylation of Electron-Rich Arenes Using Activated and Unactivated Alkenes

Direct alkylation of the C-H bond arenes in a selective manner is a long-standing challenge. Herein, a metal-free photocatalytic regioselective C-H alkylation method for electron-rich arenes with both activated and unactivated alkenes was developed. The reaction tolerates a wide range of aromatic rings with diverse substitution patterns, as well as terminal and internal alkenes, providing a general and straightforward metal-free method for C-C bond formation from inert C-H bonds. Moreover, alkynes are also compatible to give the C-H vinylation of electron-rich arenes.

Direct Observation of the Dynamics of Ylide Solvation by Hydrogen-bond Donors Using Time-Resolved Infrared Spectroscopy

The photoexcitation of α-diazocarbonyl compounds produces singlet carbene intermediates that react with nucleophilic solvent molecules to form ylides. The zwitterionic nature of these newly formed ylides induces rapid changes in their interactions with the surrounding solvent. Here, ultrafast time-resolved infrared absorption spectroscopy is used to study the ylide-forming reactions of singlet carbene intermediates from the 270 nm photoexcitation of ethyl diazoacetate in various solvents and the changes in the subsequent ylide-solvent interactions. The results provide direct spectroscopic observation of the competition between ylide formation and C-H insertion in reactions of the singlet carbene with nucleophilic solvent molecules. We further report the specific solvation dynamics of the tetrahydrofuran (THF)-derived ylide (with a characteristic IR absorption band at 1636 cm-1) by various hydrogen-bond donors and the coordination by lithium cations. Hydrogen-bonded ylide bands shift to a lower wavenumber by -19 cm-1 for interactions with ethanol, -14 cm-1 for chloroform, -10 cm-1 for dichloromethane, -9 cm-1 for acetonitrile or cyclohexane, and -16 cm-1 for Li+ coordination, allowing the time evolution of the ylide-solvent interactions to be tracked. The hydrogen-bonded ylide bands grow with rate coefficients that are close to the diffusional limit. We further characterize the specific interactions of ethanol with the THF-derived ylide using quantum chemical (MP2) calculations and DFT-based atom-centered density matrix propagation trajectories, which show preferential coordination to the α-carbonyl group. This coordination alters the hybridization character of the ylidic carbon atom, with the greatest change toward sp2 character found for lithium-ion coordination.

A Computational Study on the Photochemical O-H Functionalization of Alcohols with Diazoacetates

In this computational study, we provide a detailed analysis of the underlying reaction mechanism and show that a singlet carbene is initially formed. Depending on the pK_A of the alcohol, this singlet carbene can engage in direct protonation or enol formation to yield the O-H functionalization product. On the contrary, propargylic alcohols take up a dual role and form a complex with the carbene intermediate that leads to facile cyclopropenation reactions.

Visible-Light-Induced Organocatalyzed [2+1] Cyclization of Alkynes and Trifluoroacetylsilanes

The synthesis of common cyclopropenes has been widely studied, but the synthesis of cyclopropenols is a significant challenge. Herein, we highlight our recent work on the synthesis of trifluoromethylated cyclopropenols through [2+1] cycloaddition reaction between alkynes and trifluoroacetylsilanes under visible-light-induced organocatalysis. The novel amphiphilic donor-acceptor carbenes derived from trifluoroacetylsilanes can react effectively with both activated and unactivated alkynes. Broad substrate scope and good functional group tolerance have been achieved. Besides, the synthetic potential of this reaction was highlighted by a gram-scale reaction and the one-pot diastereoselective synthesis of trifluoromethylated cyclopropanols.

H-F bond insertions into α-diazo carbonyl compounds

A reaction for H-F bond insertion into α-diazo carbonyl compounds is reported. The protocol describes a simple reaction setup employing commercially available HF·pyr (Olah reagent) as the fluorine source. The method is rapid and practical, and allows access to a broad range of α-fluorinated carbonyl compounds in generally good yields.

Recent advances in transition-metal-catalyzed carbene insertion to C-H bonds

C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp³)-H, aryl (aromatic) C(sp²)-H, heteroaryl (heteroaromatic) C(sp²)-H bonds, alkenyl C(sp²)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Blue Light-Promoted Cross-Coupling of α-Diazo Esters with Isocyanides: Synthesis of Ester-Functionalized Ketenimines

A metal-free scalable synthesis of functionalized ketenimines from alkyl α-(aryl/heteroaryl)-α-diazoacetates and alkyl isocyanides induced by blue light irradiation has been developed. The reaction proceeds at room temperature without any photocatalyst and provides ketenimines in moderate to good yields. Density functional theory (DFT) calculations and the experimental study showed that aryl(alkoxycarbonyl)carbenes in both singlet and triplet states can react with isocyanides but only the reaction of the former leads to the smooth formation of ketenimines. The obtained ketenimines were used for the synthesis of functionalized amidines under mild metal-free conditions.

Enabling Cyclopropanation Reactions of Imidazole Heterocycles via Chemoselective Photochemical Carbene Transfer Reactions of NHC-Boranes

Herein we report a site-selective cyclopropanation of N-heterocyclic carbene (NHC)-borane complexes via photochemical carbene transfer reactions. By subtle changes to the reaction conditions, this approach can be further extended toward the difunctionalization of NHC-boranes via cyclopropanation and the B-H insertion reaction. Further investigations in photochemical continuous-flow applications and synthetic transformations proved the utility of the method. Theoretical calculations and control experiments were performed to explain the observed selectivity.

Proton or Carbene Transfer? On the Dark and Light Reaction of Diazoalkanes with Alcohols

The formal alkylation reaction of OH groups with diazoalkanes under catalyst-free reaction conditions finds broad application in organic synthesis. However, even today, this reaction is mainly limited to the use of diazomethane as reaction partner. In this combined experimental and theoretical study, we aim at a fundamental understanding of the reaction of diazoalkanes with alcohols to make this transformation amenable to a generalized approach towards formal alkylation reactions of alcohols with diazoalkanes. Experimental and theoretical studies suggest a direct proton transfer only in exceptional cases. In a more general setting, such O-H functionalization proceed both under dark and photochemical conditions via a key hydrogen-bonded singlet carbene intermediate that undergoes a protonation-addition mechanism. We conclude with applications of this approach in O-H functionalization reactions of alcohols, including simple fluorinated, halogenated and aliphatic alcohols and showcase functional-group tolerance of this method in the reaction of biologically active and pharmaceutically relevant alcohols.

Synthesis of Cyclopropenols Enabled by Visible-Light-Induced Organocatalyzed [2+1] Cyclization

Although the synthesis of common cyclopropenes has been well studied, the access to cyclopropenols is rather limited. Herein, we report the first synthesis of α-trifluoromethylated cyclopropenols via 2+1 cycloaddition reactions between alkynes and trifluoroacylsilanes, enabled by visible-light-induced organocatalysis. The novel ambiphilic donor-acceptor carbenes derived from trifluoroacetylsilanes reacted efficiently with both activated and non-activated alkynes. The reaction features simple operation, mild conditions, broad substrate scope and good functional group tolerance. The synthetic potential of the reaction is highlighted by the gram-scale reactions and first synthesis of α-trifluoromethylated cyclopropanols through the combination of the 2+1 cyclization and high diastereoselective hydrogenation reaction in one pot.

Generation of (amino)(boryloxy)carbenes from carbamoylboranes and their coupling reaction with aldehydes

Carbamoylboranes were found to react with various aldehydes under heating conditions to give α-hydroxycarboxamides in good yields. A DFT study supports the mechanism, which involves thermally generated (amino)(boryloxy)carbene intermediates. To our knowledge, this is the first report on the generation of (amino)(boryloxy)carbene intermediates from carbamoylboranes and its application to carbon-carbon bond-forming reactions.

Asymmetric Catalytic Rearrangements with α-Diazocarbonyl Compounds

α-Diazocarbonyl compounds serve as nucleophiles, dipoles, carbene precursors, and rare electrophiles, enabling a vast array of organic transformations under the influence of metal catalysts. Among them, rearrangement processes are attractive and provide straightforward and efficient accesses to one-carbon extension adducts or heteroatom-containing molecules. The reactions occur upon the release of dinitrogen after nucleophilic addition or before ylide formation. Although significant progress has been made for these two types of rearrangement reactions, the issue of enantiocontrol is challenging because the final optically enriched products are generated via multistep transformations and the inherent spacial arrangement of the intermediates has more or less influence on the regio- and enantioselectivity.In this Account, we collected several rearrangements of α-diazocarbonyl compounds, showcasing the efficient catalysts and tailored strategies for tackling enantioselective varieties of these two types of rearrangement reactions. Our research group initiated the catalytic asymmetric reactions of α-diazocarbonyl compounds during the development of chiral Feng N,N'-dioxide-metal complex catalysts and others. As a kind of useful chiral Lewis acid catalyst chiral N,N'-dioxide-metal complexes are favorable for the activation of various carbonyl compounds, accelerating the diastereo- and enantioselective nucleophilic addition of α-diazoesters and the sequential rearrangements in either an intermolecular or intramolecular manner. Aldehydes, acyclic and cyclic ketone derivatives, and α,β-unsaturated ketones could participate in efficient asymmetric homologation reactions, and an obvious ligand-acceleration effect is observed in these processes. For example, the Roskamp-Feng reaction of aldehydes gives optically active β-ketoesters through a H-shift, overwhelming the aryl group shift or oxygen attack. The shift preference and enantiocontrol in the homologation of acyclic and cyclic ketone derivatives could be under excellent control of the chiral catalysts. An unusual electrophilic α-amination of aryl/alkyl ketones and even a complicated homologation/dyotropic rearrangement/interconversion/[3 + 2] cycloaddition cascade used to construct dimeric polycyclic compounds were discovered as a result of the selection of chiral ligands and additives. On the basis of the understanding of the interaction of the functional group with N,N'-dioxide-metal complexes in catalysis and the key enantio-determining issues in ylide-based rearrangements, we designed new α-diazocarbonyl compounds by introducing a pyrazole-1-carboxyl group as the acceptor unit, which could benefit the formation of both carbenoid species and the chiral catalyst-bound ylides to deliver stereoselectivity. Taking advantage of Ni(II) or Co(II) complexes of Feng N,N'-dioxide ligands, we realized several kinds of enantioselective [2,3]-sigmatropic rearrangements, such as the Doyle-Kirmse reaction with allylic sulfides or selenides, [2,3]-Stevens rearrangements of vinyl-substituted α-diazo pyrazoleamides with thioacetates, Sommelet-Hauser rearrangements of aryl-substituted α-diazo pyrazoleamides with thioamides, and thio-Claisen rearrangements of 2-thio-indoles as well. Moreover, this strategy was shown to be applicable to highly γ-selective and enantioselective insertion into N-H bonds of secondary amines with vinyl-substituted α-diazo pyrazoleamides.

A combined experimental and theoretical study on the reactivity of nitrenes and nitrene radical anions

Nitrene transfer reactions represent one of the key reactions to rapidly construct new carbon-nitrogen bonds and typically require transition metal catalysts to control the reactivity of the pivotal nitrene intermediate. Herein, we report on the application of iminoiodinanes in amination reactions under visible light photochemical conditions. While a triplet nitrene can be accessed under catalyst-free conditions, the use of a suitable photosensitizer allows the access of a nitrene radical anion. Computational and mechanistic studies rationalize the access and reactivity of triplet nitrene and nitrene radical anion and allow the direct comparison of both amination reagents. We conclude with applications of both reagents in organic synthesis and showcase their reactivity in the reaction with olefins, which underline their markedly distinct reactivity. Both reagents can be accessed under mild reaction conditions at room temperature without the necessity to exclude moisture or air, which renders these metal-free, photochemical amination reactions highly practical.

Gaining in-depth understanding of photochemical processes is key for developing more sustainable and efficient chemical transformations. Here the authors show that under visible light photochemical conditions, iminoiodinanes undergo formation of triplet nitrenes or nitrene radical anions, depending on the use of a photosensitizer; These reagents are studied in amination reactions with olefins.

Emerging Applications of Aryl Trifluoromethyl Diazoalkanes and Diazirines in Synthetic Transformations

Aryl trifluoromethyl diazoalkanes and diazirines have become unique as reactants in synthetic methodology. As privileged compounds containing CF₃ groups and ease of synthetic access, aryl trifluoromethyl diazoalkanes and diazirines have been highlighted for their versatility in applications toward a wide range of synthetic transformations. This Perspective highlights the synthetic applications of these reactants as precursors of stabilized metal carbenes, i.e., donor-acceptor-substituted ones.

Visible Light Induced Brønsted Acid Assisted Pd-Catalyzed Alkyl Heck Reaction of Diazo Compounds and N-Tosylhydrazones

A mild visible light-induced palladium-catalyzed alkyl Heck reaction of diazo compounds and N-tosylhydrazones is reported. A broad range of vinyl arenes and heteroarenes with high functional group tolerance, as well as a range of different diazo compounds, can efficiently undergo this transformation. This method features Brønsted acid-assisted generation of hybrid palladium C(sp3 )-centered radical intermediate, which allowed for new selective C-H functionalization protocol.

Metal-free visible-light-induced multi-component reactions of α-diazoesters leading to S-alkyl dithiocarbamates

A metal-free and visible-light-promoted strategy has been developed for the synthesis of S-alkyl dithiocarbamates through multicomponent reactions of α-diazoesters, amines and CS2.

A Visible Light-Mediated Three-Component Strategy Based on the Ring-Opening of Cyclic Ethers with Aryldiazoacetates and Nucleophiles

A blue light-promoted reaction between aryldiazoacetates and different nucleophiles has been developed in the presence of THF (and other cyclic ethers) as solvent, allowing the incorporation of these three elements...

Unlocking novel reaction pathways of diazoalkanes with visible light

Photochemistry has recently attracted the interest of synthetic chemists to conduct photolysis reactions of diazoalkanes. In this feature article, we provide a concise overview on this field, starting with discoveries...

Elemental sulfur as the “S” source: visible-light-mediated four-component reactions leading to thiocyanates

An eco-friendly and photocatalyst-free visible-light-promoted four-component reaction of α-diazoesters, elemental sulfur, cyclic ethers and TMSCN leading to thiocyanates is described.

Indium(iii)-catalyzed efficient synthesis of 3-arylhydrazonoindolin-2-ones and their fluorescent metal sensing studies

In(iii)-catalyzed strategy for the preparation of 3-arylhydrazonoindolin-2-ones from 3-diazoindoles is developed and used for Fe3+ and Hg2+ ion sensing.

Intramolecular Photochemical [2+1]-Cycloadditions of Nucleophilic Siloxy Carbenes

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds. This cyclopropanation process requires only visible light irradiation to proceed, circumventing the use of exogenous (photo)catalysts, sensitisers or additives and showcases a vastly underexplored mode of reactivity for nucleophilic carbenes in chemical synthesis. The discovery of additional transformations including a cyclopropanation/retro-Michael/Michael cascade process to afford chromanones and a photochemical C–H insertion reaction are also described.

Visible light induced singlet nucleophilic carbenes undergo rapid [2 + 1]-cycloaddition with tethered olefins to afford unique bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane scaffolds.