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

Photolytic Access to Oxaspirodecanes and Chromenes from Vinyldiazo Ester Cycloaddition with p-Quinones: A Vinylcarbene Is Not Involved

Synthetic methods that provide access to skeletally diverse heterocyclic motifs are poised to accelerate drug discovery and streamline the synthesis of advanced intermediates and materials. However, the development of such synthetic methods necessitates leveraging previously unexplored mechanistic pathways. We report herein an efficient blue light-emitting diode light-induced reaction of vinyldiazoacetates and quinones that produces spirocyclic dihydrofurans, featuring the synthetically challenging oxaspiro[4,5]decane core of numerous medicinal agents, agrochemicals, and natural products. In a departure from the well-established photochemical reactivity of diazo compounds, these reactions do not involve vinylcarbene intermediates formed by photolytic dinitrogen extrusion. Instead, they result from photoexcitation of the quinone to its triplet state with subsequent triplet energy transfer to the vinyldiazo ester. The subsequent addition of the vinylogous carbon of the triplet vinyldiazoacetate to the quinone oxygen affords the triplet diradical that collapses to the spirocyclic dihydrofuran upon the loss of dinitrogen. A strain release-driven and Bro̷nsted acid-catalyzed rearrangement of the spirocyclic products unravels the fused bicyclic ring system of equally synthetically and medicinally valuable chromenes, enabling facile skeletal diversification of the important heterocyclic motifs.

Photocatalytic Synthesis of Isoquinolinediones via the Cascade Reaction of N-Alkyl-N-methylacrylamides with Diazo Compounds

Carbon radicals generated from acceptor diazo compounds prefer to add to electron-rich olefins and heteroarenes due to their containing electron-withdrawing groups. Herein, a cascade reaction of acceptor diazo compounds with electron-deficient olefins is developed. In the presence of visible light and Ru catalyst, diazo compounds generate carbon radicals via proton-coupled electron transfer process, followed by addition with various N-alkyl-N-methylacrylamides and subsequent cyclization to give a series of acyl-substituted isoquinolinediones.

Photoinduced Diazo Carbene-Promoted C(sp3)-H Oxidative Cross-Coupling Reaction for α-Triazolation of Isochromans

A photocatalytic diazo triplet carbene-promoted C(sp3)-H oxidative coupling of isochromans and triazoles was developed in up to 83% yield at room temperature in air. The radical-like triplet carbenes were used as efficient HAT acceptors, and the possible synergistic HAT enabled the unprecedented process with a high regioselectivity.

Photoinduced synthesis of functionalized spiro[2.3]hexane via an additive-free approach

A general green protocol for the synthesis of spiro[2.3]hexane avoiding the use of harmful and toxic reagents is described. Spirocyclic scaffolds can be constructed by using alkenes of low reactivity under visible-light irradiation only. Several synthetic advantages of this method are exhibited, including mild conditions, good functional-group tolerance, operational simplicity, and scalability. Mechanistic studies indicate that C-C bond formation occurs almost simultaneously with a light-sustained initiation process.

Illuminating Palladium Catalysis

ConspectusThe past decade has witnessed significant advancements of visible-light-induced photocatalysis, establishing it as a powerful and versatile tool in organic synthesis. The major focus of this field has centered on the development of methodologies that either rely solely on photocatalysts or combine photocatalysis with other catalytic methods, such as transition metal catalysis, to address a broader and more diverse array of transformations. Within this rapidly evolving area, a subfield that we refer to as transition metal photocatalysis has garnered significant attention due to its growing impact and mechanistic uniqueness. A distinguishing feature of this subfield is the dual functionality of a single transition metal complex, which not only acts as a photocatalyst to initiate photochemical processes but also functions as a traditional catalyst, facilitating key bond-breaking and bond-forming events. As such, an exogenous photocatalyst is not required in transition metal photocatalysis. However, the implications of harnessing both the excited- and ground-state reactivities of the transition metal complex can extend beyond this simplification. One of the most compelling aspects of this area is that photoexcited transition metal complexes can exhibit unique reactivities inaccessible through conventional thermal or dual photocatalytic approaches. These distinct reactivities can be leveraged to accomplish novel transformations either by engaging an entirely different substrate pool or by unlocking new reactivities of known substrates.In 2016, our group pioneered the use of phosphine-ligated palladium catalysts that can be photoexcited upon visible-light irradiation to engage diverse substrates in radical reactions. In our initial discovery, we showed that photoexcitation can redirect the well-established oxidative addition of a Pd(0) complex into aryl iodides toward an unprecedented radical process, generating hybrid aryl Pd(I) radical species. We subsequently extended this novel strategy to the formation of alkyl radicals from alkyl halides. These reactive radical intermediates have been harnessed in a wide variety of transformations, including desaturation, alkyl Heck reactions, and alkene difunctionalization cascades, among others.Seeking to further expand this new avenue, we achieved the first example of asymmetric palladium photocatalysis in the context of allylic C-H amination, where the palladium catalyst now plays triple duty by additionally controlling the stereochemical outcome of the reaction. In parallel to reaction discovery, we have also established that diazo compounds, strained molecules, and electron-deficient alkenes can serve as alkyl radical precursors beyond organic halides and redox-active esters. Notably, the engagement of electron-deficient alkenes has been made possible by the photoinduced hydricity enhancement of Pd-H species, representing a new mode of photoexcited reactivity.This Account presents our discovery and development of visible-light-induced palladium catalysis, organized by the type of transformations explored. Given the rapid progress in the field, we anticipate that this Account will provide readers with guiding principles and inspiration for designing and developing more efficient and novel transformations.

Nickel-Catalyzed Branched Hydroalkylation of Alkenes with Diazo Compounds

A nickel-catalyzed, branched-selective hydroalkylation of alkenes using diazo compounds has been developed. This protocol enables the functionalization of both activated and unactivated alkenes, in both directed and nondirected manners. Mono-, di-, and trisubstituted alkenes can be effectively transformed. Highly diastereoselective hydroalkylations have also been demonstrated. The method provides a novel approach for introducing an α-carbonyl moiety to alkenes, which is currently inaccessible by existing methods. Preliminary mechanistic investigations suggest a carbene-type mechanism, which is unusual for nickel catalysis.

The Cascade Reaction Chemistry of Diazo Compounds with Intentionally Designed Alkene to Access Esterified Heterocycles

Exploration of the cascade reactivity of diazo compounds with alkenes is a challenging and largely unmet goal. Herein, we disclose a light-mediated de novo synthesis of esterified heterocycles under mild conditions. The reaction displays a broad functional group tolerance, including a wide variety of alkenes, diazo compounds, and some bioactive molecules. Importantly, the synthetic appeal was demonstrated for synthesizing indoleamine 2,3-dioxygenase inhibitor analogue, the deethylated derivative of natural product leucomidine C, and the anti-inflammatory agent AN669.

Synergistic photocatalysis enables aerobic oxo-hydrazination of α-diazoacetates with azobenzenes

A photocatalytic oxo-hydrazination of α-diazoacetates with azobenzenes has been developed. With air as an oxygen source, the reaction proceeded smoothly and afforded previously unknown N,N'-diarylhydrazino-containing oxoacetates. Mechanistically, the reaction is enabled by cooperation of photoredox catalysis, energy transfer photocatalysis and direct photoexcitation.

Visible-Light-Induced Copper-Catalyzed Radical Reactions of Diazo Arylidene Succinimides to Access the Pyromellitic Diimide (PMDI) Core

The synthesis of pyromellitic diimides (PMDIs) through visible-light-promoted copper-catalyzed reaction of diazo arylidene succinimides has been accomplished without the use of external oxidants. This transformation involves a carbon radical from diazo arylidene succinimides with a copper catalyst or photocatalyst via the proton-coupled electron transfer (PCET) process. This approach successfully challenges a long-standing paradigm in the synthesis of PMDIs. Notably, copper complex (CuNCS) formed in situ proved to be playing a pivotal role to drive the reaction via photoinitiation. Additionally, we synthesized a PMDI molecule known for its prominent aggregation-induced emission (AIE) property. For the very first time, we have synthesized unsymmetrical PMDIs by employing the developed protocol.

Visible light induced cooperative carbonylation and (hetero)aryl migration: synthesis of multi-carbonyl compounds

Carbonylative transformation represents one of the most straightforward procedures for the synthesis of carbonyl-containing compounds. However, the carbonylative procedure toward 1,4-diketones is still limited which are key moieties with potent applications in various areas. Herein, we report a new strategy for the synthesis of multi-carbonyl compounds containing a 1,4-diketone skeleton through remote heteroaryl migration of traditionally restricted 1,3-migratory substrates utilizing carbon monoxide (CO) as the C1 synthon and diazonium compounds as the starting material.

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.

Expanding Reaction Profile of Allyl Carboxylates via 1,2-Radical Migration (RaM): Visible-Light-Induced Phosphine-Catalyzed 1,3-Carbobromination of Allyl Carboxylates

Allyl carboxylates are useful synthetic intermediates in a variety of organic transformations, including catalytic nucleophilic/electrophilic allylic substitution reactions and 1,2-difunctionalization reactions. However, the catalytic 1,3-difunctionalization of allyl carboxylates remains elusive. Herein, we report the first photoinduced, phosphine-catalyzed 1,3-carbobromination of allyl carboxylates, affording a range of valuable substituted isopropyl carboxylates (sIPC). The transformation has broad functional group tolerance, is amenable to the late-stage modification of complex molecules and gram-scale synthesis, and expands the reaction profiles of allyl carboxylates and phosphine catalysis. Preliminary experimental and computational studies suggest a non-chain-radical mechanism involving the formation of an electron donor-acceptor complex, 1,2-radical migration (RaM), and Br-atom transfer processes. We anticipate that the 1,2-RaM reactivity of allyl carboxylates and the phosphine-catalyzed radical reaction will both serve as a platform for the development of new transformations in organic synthesis.

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.