Photochemical Carbene Transfer Reactions of Aryl/Aryl Diazoalkanes-Experiment and Theory*

Purple light-induced Ritter-type reaction of diazophosphonates: access to α-amido-β-keto phosphonates

A Ritter-type reaction of diazophosphonates to synthesize α-amido-β-keto phosphonates has been reported in this study under purple light in the absence of a photocatalyst. This protocol shows that the synthesis of the amide functionality involves in situ generation of a carbene, followed by C-N bond formation with a nitrile. The purple LED irradiation alone is sufficient for the efficient transformation to afford synthetic routes to various amide moieties. A rationalization of the reaction mechanism was well supported by control experiments. A library of α-amido-β-keto phosphonates has been well documented for the synthetic community.

Photosensitizer-Free Photoinduced Ground-State Triplet Carbene-Assisted Persistent Aryloxy Radical Generation via Hydrogen Atom Transfer

The traditional intermolecular O-H insertion strategy is typically associated with the reactivity exhibited by the singlet spin state, or it can alter the spin state from triplet to singlet by hydrogen bonding. Herein, we report diazoarylidene succinimide that generates a persistent ground-state triplet carbene under visible light (Blue LED, 456 nm) without a photosensitizer. This triplet carbene undergoes an intramolecular O-H insertion via hydrogen atom transfer, forming a persistent aryloxy radical without altering its spin state and leading to biologically relevant 2H-chromenes.

Visible-Light-Promoted N-H Insertion/Controllable Transformation of Diazoalkanes and 3-Aminomethylated Maleimides

The utilization of photogenerated carbene species to perform N-H insertion reactions has attracted considerable attention in the past few years. In this Article, we disclose a visible-light-promoted N-H insertion of 3-aminomethylated maleimides with aryl diazoacetates under sole blue LED irradiation. Continuous flow reactor technology was exploited to improve the reaction efficiency. By simply varying the reaction conditions, the formed N-H insertion products could be selectively transferred to bioimportant octahydropyrrolo[3,4-c]pyrroles and E-selective trisubstituted olefins.

Spin States Matter─from Fundamentals toward Synthetic Methodology Development and Drug Discovery

ConspectusThe potent reactivity of carbenes and nitrenes has been traditionally harnessed by the employment of a transition-metal catalyst in which the reactivity of the metal carbene/nitrene intermediates can be controlled via the judicious tuning of the metal catalyst. In recent years, progress made in this research area has unveiled novel strategies to directly access free carbenes or nitrenes under visible-light-mediated conditions without the necessity of a metal catalyst for stabilization of the carbene/nitrene intermediate. Such photochemical approaches present new opportunities to leverage orthogonal reactions with classic metal-catalyzed transformations.In this Account, we describe the major contributions from our group over the past years pushing the boundaries of light-mediated carbene and nitrene transfer reactions. In the first section, the development from purely singlet carbene chemistry toward methods that allow access to triplet carbene intermediates will be dissected. We describe how the triplet spin state of reagents provides a rich array of novel synthetic methods that build on the fundamentals of spin conservation. We lay out the different strategies in accessing the triplet spin state of carbenes (i.e., via electronic stabilization, via triplet sensitization with suitable photocatalysts, or via exploitation of geometric features of these intermediates), followed by an analysis of how the triplet spin state can be employed to leverage reactions distinct to the classic singlet carbene chemistry.The second part focuses on free nitrene intermediates, whereby both photochemical and photocatalytic strategies are analyzed and compared. We initiate with a discussion of the reactivity of iminoiodinanes as nitrene precursors in the presence of a photocatalyst or under photochemical conditions and how these two approaches result in fundamentally distinct nitrogen-based intermediates. While a nitrene radical anion is formed under photocatalytic conditions, triplet nitrene is generated under photochemical conditions. We commence with an outline of the basic reactivity of nitrene transfer reactions under both conditions, with a focus on the reaction with substrates containing double bonds. Finally, the latest developments in advanced cycloaddition chemistry beyond classic aziridination reactions are examined, with a special emphasis on the relay of the triplet nitrene reactivity to enable a Pauson-Khand-like (2 + 2 + 1) cycloaddition reaction that offers convenient access to high value bioisosteres in drug discovery.The work from our group on spin-dependent reactivities offers insight into important fundamentals in synthesis, where the spin state of the reactive intermediate will dictate the reaction outcome. We hope this may inspire others to widen the scope of applications of light-mediated carbene and/or nitrene transfer reactions, and furthermore, we anticipate that these understandings may also enable the development of advanced catalytic systems featuring triplet metal carbene/nitrene intermediates.

Visible Light Induced B-H Bond Insertion Reaction with Diazo Compounds

A protocol induced by visible light for the direct insertion of α-carbonyl carbenes into the B-H bond of amine-borane adducts has been developed under conditions that are free of metal and photocatalyst. This approach provides a straightforward route to various organoboron compounds from diazo compounds and amine-borane adducts with moderate to good yields. Mechanistic investigations reveal that this photoinduced reaction proceeds through concerted carbene insertion into the B-H bond, and the photoinduced generation of free carbene from α-diazo esters may be the rate-determining step.

The efficient synthesis of three-membered rings via photo- and electrochemical strategies

Three-membered rings, such as epoxides, aziridines, oxaziridines, cyclopropenes, vinyloxaziridines, and azirines, are recognized as crucial pharmacophores and building blocks in organic chemistry and drug discovery. Despite the significant advances in the synthesis of these rings through photo/electrochemical methods over the past decade, there has currently been no focused discussion and updated overviews on this topic. Therefore, we presented this review article on the efficient synthesis of three-membered rings using photo- and electrochemical strategies, covering the literature since 2015. In this study, a conceptual overview and detailed discussions were provided to illustrate the advancement of this field. Moreover, a brief discussion outlines the current challenges and opportunities in synthesizing the three-membered rings using these strategies.

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.

Stereoselective Synthesis of Silyl Enol Ethers with Acylsilanes and α,β-Unsaturated Ketones

Acylsilanes are emerging bench-stable reagents for the generation of electron-rich oxycarbenes that are difficult to access with unstable diazo compounds. Herein, we report a siloxycarbene-mediated stereoselective synthesis of silyl enol ethers through visible-light-induced intermolecular reactions between acylsilanes and α,β-unsaturated ketones. Both the solvent and low temperature are important for the success of the reaction. This approach features atomic economics, exclusive stereocontrol, and broad substrate scope. The synthetic potential of this methodology is demonstrated by gram-scale reaction and various downstream transformations including that requiring configuration purity of the silyl enol ethers.

Visible Light-Induced Reactions of Diazo Compounds and Their Precursors

In recent years, visible light-induced reactions of diazo compounds have attracted increasing attention in organic synthesis, leading to improvement of existing reactions, as well as to the discovery of unprecedented transformations. Thus, photochemical or photocatalytic generation of both carbenes and radicals provide milder tools toward these key intermediates for many valuable transformations. However, the vast majority of the transformations represent new reactivity modes of diazo compounds, which are achieved by the photochemical decomposition of diazo compounds and photoredox catalysis. In particular, the use of a redox-active photocatalysts opens the avenue to a plethora of radical reactions. The application of these methods to diazo compounds led to discovery of transformations inaccessible by the classical reactivity associated with carbenes and metal carbenes. In most cases, diazo compounds act as radical sources but can also serve as radical acceptors. Importantly, the described processes operate under mild, practical conditions. This Review describes this subfield of diazo compound chemistry, particularly focusing on recent advancements.

Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

Cyclopropanation of Alkenes with Halodiazirines as Halocarbene Precursors in Continuous Flow

The cyclopropanation reaction of alkenes with photolytically-generated chlorocarbenes from chlorodiazirines is reported as an effective way to prepare substituted 3-chloro-3-aryl-cyclopropanes. This practical and efficient approach allows the synthesis of various 3-chloro-3-aryl-cyclopropanes (32 examples) in continuous flow in 5-minute residence time under light-emitting diode (LED) irradiation. The conditions using 380 nm LED irradiation were successfully extended to the synthesis of substituted 3-bromo-3-aryl-cyclopropanes (3 examples).

Visible Light-Promoted Regioselective Benzannulation of Vinyl Sulfoxonium Ylides with Ynoates

Herein, we report a highly regioselective [4 + 2]-annulation of vinyl sulfoxonium ylides with ynoates under light-mediated conditions. The reaction proceeds through the new dienyl sulfoxonium ylide, which undergoes photolysis under blue light irradiation to give highly substituted naphthalene scaffolds. The method presented here operates at room temperature and does not require the addition of an external photosensitizer. The in situ-generated dienyl sulfoxonium ylide absorbs light and acts as a photosensitizer for the formation of arenes. The synthetic potential of these benzannulations was further illustrated by various synthetic transformations and a scale-up reaction. Moreover, control experiments and quantum chemical calculations reveal the mechanistic details of the developed reaction.

Silver-Catalyzed Dearomative Skeletal Editing of Indazoles by Donor Carbene Insertion

Given the prevalence of heterocyclic scaffolds in drug-related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene-mediated dearomative N-N bond cleavage leading to skeletal hopping between indazole and 1,2-dihydroquinazoline via a highly selective single-carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene-substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late-stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C-N bond.

Unlocking the reactivity of diazo compounds in red light with the use of photochemical tools

Structurally diversified diazoalkanes can be activated under red light irradiation relying on direct photolysis, photosensitization or photoredox catalysis.

Heavy-Atom Tunneling in Bicyclo[4.1.0]hepta-2,4,6-trienes

Heavy-atom tunneling limits the lifetime and observability of bicyclo[4.1.0]hepta-2,4,6-triene, a key intermediate in the rearrangement of phenylcarbene. Bicyclo[4.1.0]hepta-2,4,6-triene had been proposed as the primary intermediate of the rearrangement of phenylcarbene, but despite many efforts evaded its characterization even in cryogenic matrices. By introducing fluorine substituents into the ortho-positions of the phenyl ring of phenylcarbene, the highly strained cyclopropene 1,5-difluorobicyclo[4.1.0]hepta-2,4,6-triene becomes stable enough to be characterized in argon matrices. However, even at 3 K this cyclopropene is only metastable and rearranges via heavy-atom tunneling to the corresponding cycloheptatetraene. Calculations suggest that fluorination is necessary to slow down the tunneling rearrangement of the bicycloheptatriene. The parent bicycloheptatriene rapidly rearranges via heavy-atom tunneling and therefore cannot be detected under matrix isolation conditions.

Visible-Light-Induced Synthesis of 3-Alkyl Chromones under Catalyst- and Additive-Free Conditions

Herein, we reported an efficient and facile visible-light-induced 3-alkyl chromone synthesis from easily accessible o-hydroxyaryl enaminones and α-diazo esters. In this protocol, excellent yields were obtained with a broad substrate scope at room temperature, tolerating various functional groups. Of note is that this eco-friendly methodology features catalyst- and additive-free, mild reaction conditions, simple operation procedure, and easy scale-up, which affords a convenient pathway for the preparation of 3-alkyl chromones. Experimental results and density functional theory (DFT) computation analyses confirm the participation of carbene species and active cyclopropane intermediate.

Carbene Formation or Reduction of the Diazo Functional Group? An Unexpected Solvent-Dependent Reactivity of Cyclic Diazo Imides

The control of the reactivity of diazo compounds is commonly achieved by the choice of a suitable catalyst, e.g. via stabilization of singlet carbenes or radical intermediates. Herein, we report on the light-promoted reactivity of cyclic diazo imides with thiols, where the choice of solvent results in two fundamentally different reaction pathways. In dichloromethane (DCM), a carbene is formed initially and engages in a cascade C-H functionalization/thiolation reaction to deliver indane-fused pyrrolidines in good to excellent yields. When switching to acetonitrile solvent, the carbene pathway is shut down and an unusual reduction of the diazo compound occurs under otherwise identical reaction conditions, where the aryl thiol acts as reductant. A combined set of experimental and computational studies was carried out to obtain mechanistic understanding and to support that indane formation proceeds via the insertion of a triplet carbene, while the reduction of diazo imides proceeds via an electron transfer process.

Photochemical reactions of a diamidocarbene: cyclopropanation of bromonaphthalene, addition to pyridine, and activation of sp3 C-H bonds

We report unprecedented photochemistry for the diamidocarbene 1. Described within are the double cyclopropanation of 1-bromonaphthalene, the double addition to pyridine, and remarkably, the insertion into the unactivated sp3 C-H bonds of cyclohexane, tetramethylsilane, and n-pentane to give compounds 2-6, respectively. All compounds have been fully characterized, and the solid state structure of 4 was obtained using single crystal electron diffraction.

Visible-Light-Induced Trifluoromethylsulfonylation Reaction of Diazo Compounds Enabled by Manganese Catalysis

A visible-light-induced trifluoromethylsulfonylation reaction of diazo compounds is herein reported. This developed synthetic method captures the relatively rare trifluoromethyl sulfone radicals via coordination to the Mn(acac)₃ catalyst, delivering the corresponding α-trifluoromethyl sulfone esters in good to moderate yields (up to 82%). This protocol exhibits broad substrate scope and is easily carried out under mild reaction conditions. Furthermore, a plausible mechanism of the reaction was investigated through DFT calculations.

Electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles

With the fast development of synthetic chemistry, the introduction of functional group into organic molecules has attracted increasing attention. In these reactions, the difunctionalization of unsaturated bonds, traditionally with one nucleophile and one electrophile, is a powerful strategy for the chemical synthesis. In this work, we develop a different path of electrochemical oxidative difunctionalization of diazo compounds with two different nucleophiles. Under metal-free and external oxidant-free conditions, a series of structurally diverse heteroatom-containing compounds hardly synthesized by traditional methods (such as high-value alkoxy-substituted phenylthioacetates, α-thio, α-amino acid derivatives as well as α-amino, β-amino acid derivatives) are obtained in synthetically useful yields. In addition, the procedure exhibits mild reaction conditions, excellent functional-group tolerance and good efficiency on large-scale synthesis. Importantly, the protocol is also amenable to the key intermediate of bioactive molecules in a simple and practical process.

Visible-light-induced organocatalytic enantioselective N-H insertion of α-diazoesters enabled by indirect free carbene capture

While asymmetric insertion of metal carbenes into H-X (X = C, N, O, etc.) bonds has been well-established, asymmetric control over free carbenes is challenging due to the presence of strong background reactions and lack of any anchor for a catalyst interaction. Here we have achieved the first photo-induced metal-free asymmetric H-X bond insertion of this type. With visible light used as a promoter and a chiral phosphoric acid used as a catalyst, α-diazoesters and aryl amines underwent smooth N-H bond insertion to form enantioenriched α-aminoesters with high efficiency and good enantioselectivity under mild conditions. Key to the success was the use of DMSO as an additive, which served to rapidly capture the highly reactive free carbene intermediate to form a domesticated sulfoxonium ylide.

Visible light-induced carbene reactivity of acceptor diazoalkanes: deconstructive difunctionalizations of cyclic ethers with nucleophiles

A visible light-induced carbene reactivity of acceptor diazoalkanes has been developed for the synthesis of difunctionalized ethers from cyclic ethers and various N/O/S nucleophiles.

Cobalt-Catalyzed Nitrogen Atom Insertion in Arylcycloalkenes

Developing strategies enabling the modification of underlying molecular frameworks facilitates access to underexplored chemical spaces. Skeletal editing is an emerging technology for late-stage diversification of bioactive molecules. However, the current state of this knowledge remains undeveloped. This work describes a simple protocol that "inserts" a nitrogen atom into arylcycloalkenes to form the corresponding N-heterocycles. The use of an inexpensive cobalt catalyst under aqueous and open-air conditions makes this protocol very practical. Examples of late-stage modification of compounds of pharmaceutical interest and complex fused ring compounds further demonstrated the potentially broad applicability of this methodology.

Photo‐Induced Homologation of Carbonyl Compounds for Iterative Syntheses

We describe a photo-induced reaction for the in situ generation of highly reactive alkyl diazo species from carbonyl precursors via photo-excitation of N-tosylhydrazone anions. The diazo intermediates undergo efficient C-H insertion of aldehydes, leading to the productive synthesis of aldehydes and ketones. The method is applicable to the iterative synthesis of densely functionalized carbonyl compounds through sequential trapping of the diazo species with various aldehydes. The reaction proceeds without the need of any catalyst by light irradiation and features high functional group tolerance. More than 70 examples, some performed on a gram-scale, demonstrate the broad applicability of this reaction sequence in synthesis.

Visible-Light-Promoted Carbene Insertion and Decarbonylation for the Synthesis of α-Substituted γ-Ketoesters

Herein, we report a blue visible-light-promoted approach for preparing a variety of α-substituted γ-ketoester derivatives through carbene insertion and the decarbonylation of enaminones and diazoesters. These reactions use readily available starting materials and transition-metal-free, eco-friendly procedures that are amenable to gram-scale synthesis and wide functional group tolerance. This methodology may be useful for constructing polysubstituted heterocycles with potential biological activity.

Visible-Light-Induced Imide Synthesis through a Nitrile Ylide Formation/Trapping Cascade

A visible-light-promoted three component reaction of diazo compounds, nitriles, and carboxylic acids is reported. The reaction utilizes acceptor-only diazo compounds as carbene precursors and nitriles as carbene-trapping reagents to form the key nitrile ylides. Under the optimal reaction conditions, a wide range of imide products were obtained in good to excellent yields. The gram-scale synthesis and synthetic application of the imide products to form isoquinoline-1,3(2H,4H)-dione derivatives further proved the value of this method.

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.

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.

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.

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.

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.

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.

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...

Dioxiranes: A Half-Century Journey

Dioxiranes are multi-tasking reagents inheriting mild and selective oxygen transfer attributes. These oxidants are accessed from the reaction of ketones with an oxidant and are employed stoichiometrically or catalytically (in...

Visible-Light-Mediated Strategies for the Preparation of Oxime Ethers Derived from O-H Insertions of Oximes into Aryldiazoacetates

Two visible-light-mediated O-H insertion protocols involving oximes and aryldiazoacetates leading to different products depending on the solvent employed are reported. In DCM, direct O-H insertion takes place. In THF, there is the additional incorporation of the ring-opened form of this solvent into the structure of the product. These metal-free protocols are mild and tolerant to air and moisture. The preparation of an acaricide has been developed as an example of synthetic application.

Visible-Light-Mediated Strategies to Assemble Alkyl 2-Carboxylate-2,3,3-Trisubstituted β-Lactams and 5-Alkoxy-2,2,4-Trisubstituted Furan-3(2H)-ones Using Aryldiazoacetates and Aryldiazoketones

Two new visible-light-mediated strategies are described starting from aryldiazoacetates. The first approach describes their reaction with azides to afford the corresponding imines, and then reaction with aryldiazoketones produces alkyl 2-carboxylate-2,3,3-trisubstituted β-lactams. The second approach describes the reaction with sulfoxides to afford the corresponding sulfoxonium ylides, followed by reaction with aryldiazoketones to produce 5-alkoxy-2,2,4-trisubstituted furan-3(2H)-ones. These protocols take advantage of the photolysis of aryldiazoacetates and the photochemically promoted Wolff rearrangement of aryldiazoketones.

Photocatalytic gem-Difluoroolefination Reactions by a Formal C-C Coupling/Defluorination Reaction with Diazoacetates

The photolysis of diazoalkanes to conduct singlet carbene transfer reactions of colored diazoalkanes has recently attracted significant interest in organic synthesis. Herein, we describe a photocatalytic approach that allows the access of triplet carbene intermediates via energy transfer to conduct highly efficient gem‐difluoroolefination reactions with α‐trifluoromethyl styrenes. The use of a tertiary amines proved pivotal to unlock this unusual reaction pathway and to prevent undesired cyclopropanation pathways. The amine further facilitates the ultimate abstraction of fluoride to yield gem‐difluoroolefins (43 examples, up to 88 % yield), which is supported by experimental and theoretical mechanistic studies. We explored this synthesis method with a broad substrate scope, ranging from simple olefins and heterocyclic olefins towards the decoration of pharmaceutically relevant building blocks.

A DFT Study on the Molecular Mechanism of Additions of Electrophilic and Nucleophilic Carbenes to Non-Enolizable Cycloaliphatic Thioketones

The molecular mechanisms of addition of dihalocarbenes and dimethoxycarbene to thioketones derived from 2,2,4,4-tetrmethylcyclobutane-1,3-dione were examined on the basis of the DFT wb97xd/6-311g(d,p)(PCM) calculations. Obtained results demonstrated that the examined processes exhibit polar nature and in the case of electrophilic dichloro-, and dibromocarbenes are initiated by the attack of carbene species onto the sulfur atom of the C=S group. Remarkably, reactions involving more electrophilic carbenes (dichloro-, and dibromocarbene) proceeds via stepwise mechanism involving thiocarbonyl ylide as a transient intermediate. In contrast, analogous reactions with nucleophilic dimethoxycarbene occur via a single step reaction, which can be considered as the [2 + 1] cycloaddition reaction initiated by the attack onto the C=S bond. A computational study showed that difluorocarbene tends to react as a nucleophilic species and resembles rather dimethoxycarbene and not typical dihalocarbene species. Significantly higher reactivity of the thioketone unit in comparison to the ketone group, both present in 3-thioxo-2,2,4,4-tetramthylcyclobutanone molecule, was rationalized in the light of DFT computational study.