Radical-Mediated Strategies for the Functionalization of Alkenes with Diazo Compounds

Photocatalytic Enantioselective Radical Cascade Multicomponent Minisci Reaction of β-Carbolines Using Diazo Compounds as Radical Precursors

Here, a photocatalytic asymmetric multicomponent cascade Minisci reaction of β-carbolines with enamides and diazo compounds is reported, enabling an effective enantioselective radical C─H functionalization of β-carbolines with high yields and enantioselectivity (up to 83% yield and 95% ee). This enantioselective multicomponent Minisci protocol exhibits step economy, high chemo-/enantio-selective control, and good functional group tolerance, allowing access to a variety of valuable chiral β-carbolines. Notably, diazo compounds are suitable radical precursors in enantioselective cascade radical reactions. Moreover, the efficiency and practicality of this approach are demonstrated by the asymmetric synthesis of bioactive compounds and natural products.

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.

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.

Nucleophilic Organocatalyst for Photochemical Carbon Radical Generation via SN2 Substitution

Photochemical generation of radicals is a powerful way to construct various molecules. But most of these methods rely on initiators or the redox properties of radical precursors. Herein, we report a photochemical organic catalyst that reacts with benzyl halide to generate carbon radical via an SN2 pathway. This nucleophilic catalyst can be easily prepared and is bench-stable. The SN2 process does not rely on the redox properties of halides, showing potential synthetic utility. Control experiments and UV-vis spectroscopic analysis indicate that the SN2 substitution adduct is the key intermediate.

Light-Driven Synthesis and Functionalization of Bicycloalkanes, Cubanes and Related Bioisosteres

Bicycloalkanes, cubanes and their structural analogues have emerged as bioisosteres of (hetero)arenes. To meet increasing demand, the chemical community has developed a plethora of novel synthetic methods. In this review, we assess the progress made in the field of light-driven construction and functionalization of such relevant molecules. We have focused on diverse structural targets, as well as on reaction processes giving access to: (i) [1.1.1]-bicyclopentanes (BCPs); (ii) [2.2.1]-bicyclohexanes (BCHs); (iii) [3.1.1]-bicycloheptanes (BCHeps); and (iv) cubanes; as well as other structurally related scaffolds. Finally, future perspectives dealing with the identification of novel reaction manifolds to access new functionalized bioisosteric units are discussed.

Palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines

Herein, we report a visible-light-mediated palladium-catalyzed three-component radical-polar crossover carboamination of 1,3-dienes or allenes with diazo esters and amines, affording unsaturated γ- and ε-amino acid derivatives with diverse structures. In this methodology, the diazo compound readily transforms into a hybrid α-ester alkylpalladium radical with the release of dinitrogen. The radical intermediate selectively adds to the double bond of a 1,3-diene or allene, followed by the allylpalladium radical-polar crossover path and selective allylic substitution with the amine substrate, thereby leading to a single unsaturated γ- or ε-amino acid derivative. This approach proceeds under mild and simple reaction conditions and shows high functional group tolerance, especially in the incorporation of various bioactive molecules. The studies on scale-up reactions and diverse derivatizations highlight the practical utility of this multicomponent reaction protocol.

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.

Photocatalytic Synthesis of Indanone, Pyrone, and Pyridinone Derivatives with Diazo Compounds as Radical Precursors

We disclose herein a photocatalytic radical cascade cyclization of diazoalkanes for the divergent synthesis of important carbocycles and heterocycles. Under the optimal reaction conditions, various indanone, pyrone, and pyridinone derivatives can be obtained in moderate to good yields. Mechanistic experiments support the formation of carbon-centered radicals from diazoalkanes through the proton-coupled electron transfer process. Scale-up reaction using continuous flow technology and useful downstream application of the formed heterocycles further render the strategy attractive and valuable.

Metal-free synthesis of 1,3-dichloro-1,5-diarylpentan-5-ones via cascade oxidative radical addition of styrenes with CHCl3

A novel and efficient metal-free cascade oxidative radical addition of styrenes is developed for the construction of 1,3-dichloro-1,5-diarylpentan-5-ones. This protocol presents a practical one-pot procedure that delivers highly functionalized 1,3-dichloro-1,5-diarylpentan-5-ones in moderate-to-good yields with a broad substrate scope under mild conditions.

Iodine-promoted transfer of dihydrogen from ketones to alkenes, triphenylmethyl, and diphenylmethyl derivatives

Herein, a novel class of transfer hydrogenation agent, cycloheptanone, was successfully employed in metal-free hydrogenation facilitated by iodine. A series of alkenes, triphenylmethyl derivatives, and diphenylmethyl derivatives were reduced to the desired compounds in moderate to excellent yields. The transfer hydrodeuteration of alkenes using α-deuterated cyclododecanone exhibited high regioselectivity. Preliminary mechanism studies confirmed the origins of the two hydrogen atoms involved in the reduction of alkenes. The current study paves the way for the use of ketones as unique transfer hydrogenation agents in chemical synthesis.

Visible-Light-Mediated Radical Hydroalkylative Cyclization of 1,6-Enynes

A radical hydroalkylative cyclization approach accessing various alkenyl heterocyclic compounds was developed using dimethyl malonate and 1,6-enynes in the presence of visible-light photoredox catalysis. The use of Ir(dtbbpy)(ppy)2PF6 as a photosensitizer enables carbon atom radical formation and initiates the cascade cyclization reaction under mild conditions.

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.

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.

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.

Aryl versus Alkyl Redox-Active Diazoacetates ─ Light-Induced C-H Insertion or 1,2-Rearrangement

Diazo compounds with redox-active leaving groups are versatile reagents for orthogonal functionalizations, previously utilized in the Rh-catalyzed synthesis of highly substituted cyclopropanes. Photochemical activation of aryl-substituted diazoacetates generates carbenes, whereas redox-active esters can furnish C-radicals via the photoexcitation of EDA complexes. However, the photochemical behavior of these two functionalities, while present in one molecule, remains to be defined. We demonstrate that under light irradiation, reactions occur only on the diazo moiety, leaving the NHPI functionality intact. Not only aryl- but also alkyl-substituted NHPI diazoacetates are activated by blue light; either C-H insertion or the hydrogen/carbon 1,2-rearrangement occurs depending on the aryl/alkyl group.

One-Pot (3 + 2) Cycloaddition-Isomerization-Oxidation of 2,2,2-Trifluorodiazoethane and Styryl Derivatives

A facile access to 5-aryl-3-trifluoromethylpyrazoles has been developed by a one-pot (3 + 2) cycloaddition-isomerization-oxidation sequence employing 2,2,2-trifluorodiazoethane and styryl derivatives. A broad variety of functional groups and good yields are achieved under mild conditions. Additionally, the functionalization of 3-trifluoromethylpyrazoles was studied. DFT calculations of the cycloaddition transition state energies are consistent with the experimentally observed reactivity.

A Guide to Tris(4-Substituted)-triphenylmethyl Radicals

Triphenylmethyl (trityl, Ph3C•) radicals have been considered the prototypical carbon-centered radical since their discovery in 1900. Tris(4-substituted)-trityls [(4-R-Ph)3C•] have since been used in many ways due to their stability, persistence, and spectroscopic activity. Despite their widespread use, existing synthetic routes toward tris(4-substituted)-trityl radicals are not reproducible and often lead to impure materials. We report here robust syntheses of six electronically varied (4-RPh)3C•, where R = NMe2, OCH3, tBu, Ph, Cl, and CF3. The characterization reported for the radicals and related compounds includes five X-ray crystal structures, electrochemical potentials, and optical spectra. Each radical is best accessed using a stepwise approach from the trityl halide, (RPh)3CCl or (RPh)3CBr, by controllably removing the halide with subsequent 1e- reduction of the trityl cation, (RPh)3C+. These syntheses afford consistently crystalline trityl radicals of high purity for further studies.

Photocatalytic Three-Component Synthesis of 3-Heteroarylbicyclo[1.1.1]pentane-1-acetates

Herein, we report a visible-light-induced three-component reaction involving [1.1.1]propellane, diazoates, and various heterocycles for the synthesis of 3-heteroarylbicyclo[1.1.1]pentane-1-acetates. Throughout this reaction, the radicals generated from diazoate species react with [1.1.1]propellane in an addition reaction to form bicyclo[1.1.1]pentane (BCP) radicals that subsequently react with heterocycles, leading to the formation of 1,3-disubstituted BCP acetates. Notably, this methodology exhibits excellent functional group compatibility, high atom economy, and mild reaction conditions, thus facilitating suitable synthetic access to 1,3-disubstituted BCP acetates.

Visible Light-Driven α-Diazoketones as Denitrogenated Synthons: Synthesis of Fluorinated N-Heterocycles via Multicomponent Cyclization Reactions

We disclose herein an efficient photochemical formal [3+2+1] annulation strategy for the transformation of diazocarbonyl compounds into various fluorinated nitrogen-containing heterocycles. This transformation is characterized by reacting fluoroalkyl radicals with α-diazoketones, which are used as infrequent denitrogenated synthons under visible light. Moreover, a wide range of N-heterocycles containing precious CF₃ and perfluoroalkylated groups are constructed in moderate to good yields. Notably, this photochemical strategy may provide a fruitful path for the synthesis of complex organofluorides via diazo/fluorine/radical chemistry.

Visible-Light-Induced Radical gem-Iodoallylation of 2,2,2-Trifluorodiazoethane

A visible-light-induced radical gem-iodoallylation of CF₃CHN₂ was developed under mild conditions, delivering a variety of α-CF₃-substituted homoallylic iodide compounds in moderate to excellent yields. The transformation features broad substrate scope, good functional group compatibility, and operational simplicity. The described protocol provides a convenient and attractive tool to apply CF₃CHN₂ as CF₃-introduction reagent in radical synthetic chemistry.

Iodine-Mediated Coupling of 2,2,2-Trifluorodiazoethane and Alkynes To Access Bistrifluoromethylated 1,3,5-Trienes

Multi-component reaction of higher degree ultilization of diazo molecules for polyene formation is highly intriuging but still underexplored. We present herein an unprecedented coupling of 2,2,2-trifluorodiazoethane and aryl alkynes mediated by iodine under visible light. The multi-component reaction involving two diazo units and two alkyne units provides a straightforward and step-economic access to bistrifluoromethylated 1,3,5-trienes in high stereoselectivity by creation of three C═C bonds in a single step under mild conditions.

A Photoenzymatic Strategy for Radical-Mediated Stereoselective Hydroalkylation with Diazo Compounds

Carbene insertion reactions initiated with diazo compounds have been widely used to develop unnatural enzymatic reactions. However, alternative functionalization of diazo compounds in enzymatic processes has been unexploited. Herein, we describe a photoenzymatic strategy for radical-mediated stereoselective hydroalkylation with diazo compounds. This method generates carbon-centered radicals through an ene reductase catalyzed photoinduced electron transfer process from diazo compounds, enabling the synthesis of γ-stereogenic carbonyl compounds in good yields and stereoselectivities. This study further expands the possible reaction patterns in photo-biocatalysis and offers a new approach to solving the selectivity challenges of radical-mediated reactions.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Photochemical synthesis of 1,2,4-triazoles via addition reaction of triplet intermediates to diazoalkanes and azomethine ylide intermediates

The reactivity of diazoalkanes most commonly proceeds through the formation of carbene intermediates or dipolar cycloaddition reactions. The reaction of diazoalkanes with intermediates with unpaired electrons, however, is much less elaborated. Herein, we report on the photochemical reaction of acceptor-only diazoalkanes with azodicarboxylates. Photoexcitation of the latter results in the formation of a triplet species, which undergoes an addition reaction with diazoalkanes and formation of an azomethine ylide followed by dipolar cycloaddition reaction with organic nitriles to give a 1,2,4-triazole. The application of this transformation was elaborated in a broad and general substrate scope (48 examples), including scale-up via flow chemistry and downstream transformations. Experimental and computational studies were performed to elucidate the reaction mechanism and to rationalize the reaction outcome.

Photocatalytic Regioselective Difunctionalization of Alkenes with Diazo Compounds and tert-Butyl Nitrite: Access to γ-Oximino Esters

A visible-light photocatalytic regioselective difunctionalization of alkenes with diazo compounds and tert-butyl nitrite has been developed. The protocol provides an efficient approach to γ-oximino esters under mild conditions. Significantly, this transformation not only shows the good compatibility of nucleophilic diazo compounds and electrophilic tert-butyl nitrite but also displays diazo compounds generating alkyl radicals that preferred addition to alkenes over nitroso radicals.

Copper(I)-Catalyzed Cross-Coupling of Arylsulfonyl Radicals with Diazo Compounds: Assembly of Arylsulfones

A novel copper-catalyzed cross-coupling of arylsulfonyl radicals with diazo compounds is described for the synthesis of various arylsulfones under mild conditions. In this reaction, the cheap, environmentally friendly, and readily available inorganic K₂S₂O₅ is employed as the sulfur dioxide source for providing arylsulfonyl radicals. In addition, a radical mechanism involving the insertion of sulfur dioxide with aryl radicals followed by the coupling of arylsulfonyl radicals with copper carbenes is proposed.

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.

Photochemical single-step synthesis of β-amino acid derivatives from alkenes and (hetero)arenes

β-Amino acids are frequently found as important components in numerous biologically active molecules, drugs and natural products. In particular, they are broadly utilized in the construction of bioactive peptides and peptidomimetics, thanks to their increased metabolic stability. Despite the number of methodologies established for the preparation of β-amino acid derivatives, the majority of these methods require metal-mediated multistep manipulations of prefunctionalized substrates. Here we disclose a metal-free, energy-transfer enabled highly regioselective intermolecular aminocarboxylation reaction for the single-step installation of both amine and ester functionalities into alkenes or (hetero)arenes. A bifunctional oxime oxalate ester was developed to simultaneously generate C-centred ester and N-centred iminyl radicals. This mild method features a remarkably broad substrate scope (up to 140 examples) and excellent tolerance of sensitive functional groups, and substrates that range from the simplest ethylene to complex (hetero)arenes can participate in the reaction, thus offering a general and practical access to β-amino acid derivatives.

Photoredox-Catalyzed Carbonyl Alkylative Amination with Diazo Compounds: A Three-Component Reaction for the Construction of γ-Amino Acid Derivatives

A photoredox-catalyzed reaction of secondary amines, aldehydes, diazo compounds, and Hantzsch ester is reported, affording biologically active γ-amino acid derivatives in high yields. This one-pot process tolerates a broad range of functional groups and various drug molecules and biologically active compounds. Remarkably, a gram-scale reaction and diverse transformations of γ-amino acid derivatives were successfully performed, and the utility of the products is demonstrated in the synthesis of therapeutic agent pregabalin.

C-H chlorination of (hetero)anilines via photo/organo co-catalysis

Chlorinated (hetero)anilines are a class of important structural motifs that are widely present in synthetic building blocks and pharmaceuticals. Despite recent advancements, direct aniline chlorination still suffers from ortho/para and mono/poly chlorination selectivity problems. Herein, we disclose a photo-redox and organo co-catalyzed chlorination method for anilines. This method has great substrate generality and excellent mono-chlorination selectivity. Another merit of this method is the late-stage modification of drug molecules, which would be useful in medicinal chemistry.

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.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Cobalt-Catalyzed Four-Component Carbonylation of Methylarenes with Ethylene and Alcohols

Direct conversion of raw materials to fine chemicals is greatly economically influential. We developed a non-expensive cobalt-catalyzed multicomponent carbonylative reaction for the synthesis of γ-aryl carboxylic acid esters from readily available methylarene, ethylene, and CO, which are widely found in multiple FDA-approved drugs.

Functionalization of DNA-Tagged Alkenes with Diazo Compounds via Photocatalysis

To explore potential chemical space using DNA-encoded library (DEL) technology, the development of various types of robust DNA-compatible reactions is urgently needed. Diazo compounds, which serve as valuable building blocks and important synthons in synthetic chemistry, have been rarely applied in DEL synthesis, probably because of their potential modifications of the bases and phosphate backbone of DNA. Herein we report two cases of DNA-compatible reactions with alkenes and diazo compounds, providing corresponding hydroalkylation and cyclopropanation products in moderate to excellent yields. Notably, these transformations not only provide new access to C(sp³)-C(sp³) bond formation in DELs with excellent functional group tolerance but also represent practical ligation methods to introduce functionalized molecules into DNA.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

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.

Water-promoted regio-selective trifluoromethylation of vinyl conjugated diazoacetates

An efficient trifluoromethylation reaction of vinyldiazoacetates under mild reaction conditions was depicted.

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...