Recent advances in visible-light photoredox-catalyzed nitrogen radical cyclization

Visible Light-Promoted Radical Cascade Bicyclization to Access Partially Saturated Pyrrolo[2,3-b]Pyridines

A visible light-promoted cascade [2+2+1] bicyclization reaction of N-cyanamide alkenes and activated alkyl bromides has been successfully established. This reaction exhibits mild conditions, metal-free characteristics, and excellent atom- and step-economy, along with environmental friendliness. It demonstrates broad substrate tolerance and scalability to gram-scale synthesis, underscoring its potential for practical applications. Preliminary mechanistic studies suggest that the reaction proceeds through a photoinduced single-electron transfer (SET) process and a 1,5-hydrogen atom transfer (HAT) process mediated by an iminyl radical.

Silver-catalyzed generation of acyl radicals from α-imino-oxy acids: access to acylated 3-CF3-2-oxindoles

A unified strategy to generate acyl radicals from α-imino-oxy acids via selective C-C bond activation is reported. In the presence of a silver catalyst, α-imino-oxy acids could undergo decarboxylation and N-O and C-C bond homolysis to deliver the acyl radicals, which were subsequently captured by N-alkyl-N-aryl-2-(trifluoromethyl)acrylamides to access acylated 3-CF3-2-oxindoles.

Photocatalytic Strecker-Type Reaction for the Synthesis of Primary α-Aminonitriles

A practical photoinduced direct N-O cleavage of oxime ethers via a single electron transfer (SET) process was developed, enabling controlled generation of N-H imines via iminyl radical intermediates. By employing this strategy, an efficient Strecker-type reaction was established to construct a variety of primary α-aminonitriles using TMSCN as a cyanide source. This protocol showed exceptional tolerance to various functional groups, delivering the corresponding products in good yields. Mechanistic investigations indicate the involvement of iminyl radicals and a radical/polar crossover sequence.

Near-Infrared-Light-Induced Iron(I) Dimer Enabled Abstraction of Ester Group from Cycloketone Oxime Esters

Photoinduced dimeric metal complexes have been extensively utilized in halogen atom transfer (XAT) reactions. In this study, we successfully achieved the abstraction of ester group from cyclobutanone oxime esters via iron(I)-dimer catalysis under near-infrared (NIR) light (730 nm) excitation, enabling the efficient synthesis of cyanoalkylated alkenes, quinazolinones, and 3,3-disubstituted oxindoles. Mechanistic investigations confirmed the NIR-induced functional group abstraction process.

Pyrrolidine synthesis via ring contraction of pyridines

A ring contraction of easily available cyclic compounds to smaller cycles that are valuable but difficult to synthetically access is one of important skeletal editing strategies. Pyrrolidine synthesis via a ring contraction of pyridines, which are abundant, cheap, and readily available bulk chemicals in chemical industry, is highly promising to accelerate drug discovery and development research due to the great demand of pyrrolidine skeletons in medicinal molecules. Herein we report a photo-promoted ring contraction of pyridines with silylborane to afford pyrrolidine derivatives bearing a 2-azabicyclo[3.1.0]hex-3-ene skeleton. The reaction demonstrates broad substrate scope with high functional group compatibility, realizing facile access to 6-silyl-2-azabicyclo[3.1.0]hex-3-ene derivatives that work as powerful synthons for the synthesis of functionalized pyrrolidines and nitrogen-containing compounds. The reaction mechanism is clarified to proceed via 2-silyl-1,2-dihydropyridine and vinylazomethine ylide as intermediates, which are connected via photochemical or thermal silyl migration.

Solvent-controlled silver catalyzed radical transformation of α-imino-oxy acids with cyclic aldimines

A silver-catalyzed cross coupling of cyclic aldimines and α-imino-oxy acids has been developed. The solvent-dependent reaction could selectively deliver either cyclic imine moiety retained nitriles or ring-opened oxonitriles in moderate yields. The mechanistic studies show that the reaction undergoes a radical pathway.

Radical Alkylcyanation of 1,6-Enynes with Isonitriles as Bifunctional Reagents

We report a radical cyano-cyclization of 1,6-enynes with isonitriles via photochemically driven nickel catalysis, forging alkenyl nitrile-tethered γ-lactams under mild conditions. This reaction leverages the photoexcitation of in situ generated nickel (isonitrile) species to facilitate isonitriles serving as alkyl radical precursors and cyanide sources. The reaction accommodates a wide range of substrates, exhibiting excellent regioselectivity and Z/E stereoselectivity.

Conversion of Acids to S-Alkyl Dithiocarbamates by Decarboxylative Sulfuration Using Visible-Light Photocatalysis

S-Alkyl dithiocarbamates, as an important class of sulfur-containing compounds, play pivotal roles in diverse fields, yet methods for the synthesis that start from simple, readily available feedstocks and exhibit mild conditions and structurally diverse products are scarce. In this work, we developed an efficient approach for the synthesis of various S-alkyl dithiocarbamates via visible-light photocatalysis with readily available and structurally diverse alkyl carboxylic acids (primary, secondary, and tertiary acids, amino acids, etc.) and disulfide tetraalkylthiuram as the starting materials. This protocol features high efficiency, mild reaction conditions, a broad substrate scope, and good functional group tolerance. Potential applications are further demonstrated by a sunlight experiment, H2O as a solvent, gram-scale synthesis, and facile synthesis of bioactive molecules.

Photoredox-catalyzed intramolecular nucleophilic amidation of alkenes with β-lactams

The direct nucleophilic addition of amides to unfunctionalized alkenes via photoredox catalysis represents a facile approach towards functionalized alkylamides. Unfortunately, the scarce nucleophilicity of amides and competitive side reactions limit the utility of this approach. Herein, we report an intramolecular photoredox cyclization of alkenes with β-lactams in the presence of an acridinium photocatalyst. The approach uses an intramolecular nucleophilic addition of the β-lactam nitrogen atom to the radical cation photogenerated in the linked alkene moiety, followed by hydrogen transfer from the hydrogen atom transfer (HAT) catalyst. This process was used to successfully prepare 2-alkylated clavam derivatives.

Visible-Light-Induced Photocatalytic Deoxygenative Benzylation of Quinoxalin-2-(1H)-ones with Carboxylic Acid Anhydrides

A visible-light-induced photocatalytic deoxygenative benzylation of quinoxalin-2-(1H)-ones is herein described. This novel approach provides a mild, simple, and practical route to 3-benzylquinoxalin-2(1H)-ones from ubiquitous and safe carboxylic acid anhydrides. A wide range of substrates with different substituents were well-tolerated and efficiently transformed to various functionalized 3-benzylquinoxalin-2(1H)-ones with great potential for valuable applications in drug discovery. Mechanistic investigations suggest H2O as a proton source, while hydroxyl-containing quinoxalin-2(1H)-ones may be key intermediates of the photocatalytic deoxygenative process.

Generation of alkyl and acyl radicals by visible-light photoredox catalysis: direct activation of C-O bonds in organic transformations

Alkyl and acyl radicals play a critical role in the advancement of chemical synthesis. The generation of acyl and alkyl radicals by activation of C-O bonds using visible-light photoredox catalysis offers a mild and environmentally benign approach to useful chemical transformations. Alcohols, carboxylic acids, anhydrides, xanthates, oxalates, N-phthalimides, and thiocarbonates are some examples of alkyl and acyl precursors that can produce reactive radicals by homolysis of the C-O bond. These radicals can then go through a variety of transformations that are beneficial for the construction of synthetic materials that are otherwise difficult to access. This study summarizes current developments in the use of organic photocatalysts, transition-metal photoredox catalysts, and metallaphotocatalysts to produce acyl and alkyl radicals driven by visible light.

Nitrogen-Centered Radicals Derived from Azidonucleosides

Azido-modified nucleosides have been extensively explored as substrates for click chemistry and the metabolic labeling of DNA and RNA. These compounds are also of interest as precursors for further synthetic elaboration and as therapeutic agents. This review discusses the chemistry of azidonucleosides related to the generation of nitrogen-centered radicals (NCRs) from the azido groups that are selectively inserted into the nucleoside frame along with the subsequent chemistry and biological implications of NCRs. For instance, the critical role of the sulfinylimine radical generated during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxy pyrimidine nucleotides as well as the NCRs generated from azidonucleosides by radiation-produced (prehydrated and aqueous) electrons are discussed. Regio and stereoselectivity of incorporation of an azido group ("radical arm") into the frame of nucleoside and selective generation of NCRs under reductive conditions, which often produce the same radical species that are observed upon ionization events due to radiation and/or other oxidative conditions that are emphasized. NCRs generated from nucleoside-modified precursors other than azidonucleosides are also discussed but only with the direct relation to the same/similar NCRs derived from azidonucleosides.

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.

Photoredox Synthesis of Silicon-Containing Isoindolin-1-ones and Deuterated Analogues Through Hydrosilylation and Deuterium-silylation

An efficient, practical, and metal-free protocol for the synthesis of silicon-containing isoindolin-1-ones and deuterated analogues via the synergistic combination of an organic photoredox and hydrogen atom transfer process is described. This strategy features mild reaction conditions, high atom economy, and excellent functional group compatibility, delivering a myriad of structurally diverse and valuable products with good to excellent yields.

Integrating Olefin Carboamination and Hofmann-Löffler-Freytag Reaction by Radical Deconstruction of Hydrazonyl N-N Bond

As a type of elementary organic compounds containing N-N single bond, hydrazone involved chemical conversions are extremely extensive, but they are mainly limited to N2-retention and N2-removal modes. We report herein an unprecedented protocol for the realization of division utilization of the N2-moiety of hydrazone by a radical facilitated N-N bond deconstruction strategy. This new conversion mode enables the successful combination of alkene carboamination and Hofmann-Löffler-Freytag reaction by the reaction of N-homoallyl mesitylenesulfonyl hydrazones with ethyl difluoroiodoacetate under photocatalytic redox neutral conditions. Mechanism studies reveal that the reaction undergoes a radical relay involving addition, crucial remote imino-N migration and H-atom transfer. Consequently, a series of structurally significant ϵ-N-sulphonamide-α,α-difluoro-γ-amino acid esters are efficiently produced via continuous C-C bond and dual C-N bonds forging.

Visible-Light-Promoted and EDA Complex-Driven [4 + 2] Annulation for the Construction of Naphtho[1',2':4,5]imidazo[1,2-a]pyridines

A green visible-light-promoted and electron donor-acceptor (EDA) complex-driven synthetic strategy for the construction of value-added naphtho[1',2':4,5]imidazo[1,2-a]pyridines from 2-arylimidazo[1,2-a]pyridines with Z-α-bromocinnamaldehydes has been accomplished under photocatalyst- and transition-metal-free conditions. This efficient annulation approach provides a new and straightforward pathway for the annulative π-extension of imidazo[1,2-a]pyridine-based aromatics. Moreover, the sustainable methodology exhibits simple operation, a wide range of substrates, benign conditions, and good functional group compatibility.

Phototriggered Fluoroalkylation/Cyclization of Unactivated 1-Acryloyl-2-cyanoindoles: Synthesis of RCOCF2-Substituted Pyrrolo[1,2-a]indolediones

A photochemical approach toward RCOCF2-substituted pyrrolo[1,2-a]indolediones was developed by the radical cascade difluoroalkylation/cyclization reaction of unactivated 1-acryloyl-2-cyanoindoles with ethyl iododifluoroacetate or iododifluoramides under visible-light irradiation. This transition-metal- and photosensitizer-free protocol afforded diverse difluoroalkylated pyrrolo[1,2-a]indolediones in moderate to good yields under mild reaction conditions. Most appealingly, the reaction can proceed smoothly under sunlight irradiation, which opens a new avenue toward difluoroalkylated pyrrolo[1,2-a]indolediones.

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.

Advances in heterocycle synthesis through photochemical carbene transfer reactions

Heterocyclic skeletons are commonly found in various bioactive molecules and pharmaceutical compounds, making them crucial in areas such as medicinal chemistry, materials science, and the realm of natural product synthesis. In recent years, the rapid advancements of visible light methodologies in organic synthesis have shown promising potential for the development of light-induced carbene transfer reactions. This is particularly significant as most organic molecules do not absorb visible light. Free carbene, known for its high activity, is frequently utilized for insertion reactions or cyclopropanation reactions. This review focuses on the photochemical strategy for the construction of heterocyclic skeletons, specifically highlighting the methods that employ visible light-promoted carbene transfer reactions.

Visible-Light-Promoted α-C(sp3)-H Amination of Ethers with Azoles and Amides

A visible-light-induced highly efficient C(sp3)-H amination of ethers with amides and azoles has been presented under mild conditions via a nitrogen- and carbon-centered radical coupling process. This protocol successfully utilizes 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and tert-butyl nitrite (TBN) as cocatalysts to deliver the aminated products of ethers under aerobic conditions. Notably, the developed reaction features the corresponding products in good yields (up to 93%) with a wide substrate scope. The mechanistic study indicates that C-N bond formation proceeds via a direct radical cross-coupling process. Preliminary biological activity analysis indicates that the resulting products have good and selective inhibitory activity on osteosarcoma (OS) cell lines and are promising for use as hits for drug discovery.

Reductive Radical-Polar Crossover Enabled Carboxylative Alkylation of Aryl Thianthrenium Salts with CO2 and Styrenes

Carboxylic functionalities are among the pivotal groups in bioactive molecules and in the synthesis of new lead compounds because of their unique character in the formation of hydrogen bonds and the possibility of constructing molecular complexes via amide couplings. We adopt the reductive radical-polar crossover strategy to introduce carboxyalkyl groups into arenes with styrenes and CO2 via thianthrenium salts. This protocol exhibits excellent potential as a straightforward and modular platform for site-selective carboxylative derivation of bioactive molecules.

Enantioselective Cyanofunctionalization of Aromatic Alkenes via Radical Anions

Alkene radical ions constitute an integral and unique class of reactive intermediates for the synthesis of valuable compounds because they have both unpaired spins and charge. However, relatively few synthetic applications of alkene radical anions have emerged due to a dearth of generally applicable and mild radical anion generation approaches. Precise control over the chemo- and stereoselectivity in alkene radical anion-mediated processes represents another long-standing challenge due to their high reactivity. To overcome these issues, here, we develop a new redox-neutral strategy that seamlessly merges photoredox and copper catalysis to enable the controlled generation of alkene radical anions and their orthogonal enantioselective cyanofunctionalization via distonic-like species. This new strategy enables highly regio-, chemo-, and enantioselective hydrocyanation, deuterocyanation, and cyanocarboxylation of alkenes without stoichiometric reductants or oxidants under visible light irradiation. This protocol provides a new blueprint for the exploration of the transformation potential of alkene radical anions.

Photocatalytic Carbosulfonylation/Cyclization of N-Homoallyl and N-Allyl Aldehyde Hydrazones toward Sulfonylated Tetrahydropyridazines and Dihydropyrazoles

N'-Benzylidene-N-homoallylacetohydrazides were designed and synthesized as novel skeletons for the construction of functionalized tetrahydropyridazines. A series of aryl- and alkylsulfonylated tetrahydropyridazines were obtained in yields of up to 94% employing sulfonyl chlorides as the sulfonyl radical sources under visible-light irradiation. Besides, sulfonylated dihydropyrazoles were also produced from N-allyl-N'-benzylideneacetohydrazides. Mechanistic investigations indicated that both energy transfer and single electron transfer processes were involved in accomplishing the radical 6/5-endo-trig cyclization to the C═N bond.

New Mode of Asymmetric Induction for Enantioselective Radical N-Heterobicyclization via Kinetically Stable Chiral Radical Center

Enantioselective radical N-heterobicyclization of N-allylsulfamoyl azides have been developed via metalloradical catalysis (MRC). The Co(II)-based catalytic system can homolytically activate the organic azides with varied electronic and steric properties for asymmetric radical N-heterobicyclization under mild conditions without the need of oxidants, allowing for stereoselective construction of chiral [3.1.0]-bicyclic sulfamoyl aziridines in excellent yields with high diastereoselectivities and enantioselectivities. The key to achieving the enantioselective radical process relies on catalyst development through ligand design. We demonstrate that the use of new-generation D2-symmetric chiral bridged amidoporphyrin ligand HuPhyrin with judicious variation of the alkyl bridge length can dictate both reactivity and selectivity of Co(II)-based MRC. We present both experimental and computational studies that shed light on the working details of the unprecedented mode of asymmetric induction consisting of enantioface-selective radical addition and stereospecific radical substitution. We showcase the synthetic applications of the resulting enantioenriched bicyclic aziridines through a number of stereospecific transformations.

Photocatalytic Boryl Radicals Triggered Sequential B─N/C─N Bond Formation to Assemble Boron-Handled Pyrazoles

Vinyldiazo compounds are one of the most important synthons in the construction of a cyclic ring. Most photochemical transformations of vinyldiazo compounds are mainly focusing on utilization of their C═C bond site, while reactions taking place at terminal nitrogen atom are largely unexplored. Herein, a photocatalytic cascade radical cyclization of LBRs with vinyldiazo reagents through sequential B─N/C─N bond formation is described. The reaction starts with the addition of LBRs (Lewis base-boryl radicals) at diazo site, followed by intramolecular radical cyclization to access a wide range of important boron-handled pyrazoles in good to excellent yields. Control experiments, together with detailed mechanism studies well explain the observed reactivity. Further studies demonstrate the utility of this approach for applications in pharmaceutical and agrochemical research.

A three-component reaction of cyclobutanone oxime esters, sulfur dioxide and N-alkyl-N-methacryloyl benzamides

A three-component reaction of cyclobutanone oxime esters, DABCO·(SO2)2 and N-alkyl-N-methacryloyl benzamides is described. This reaction proceeds without the addition of any oxidant or transition metal, affording sulfonyl-containing isoquinoline-1,3-(2H,4H)-diones in moderate to good yields. Various functional groups are tolerated well in this transformation. Mechanistic studies suggest that a radical pathway is involved, including β-scission, sulfur dioxide insertion, and intramolecular cyclization processes.

Visible-Light-Driven C,N-Selective Heteroarylation of N-Fluoroalkyl Hydroxylamine Reagents with Quinoxalin-2(1H)-ones

Herein, we disclose a direct and powerful strategy for the synthesis of highly valuable α-trifluoromethylamine and N-trifluoroethylamine derivatives from a visible-light-promoted C,N-selective heteroarylation of N-trifluoroethyl hydroxylamine reagents with quinoxalin-2(1H)-ones under ambient conditions. The chemoselectivity of the process (trifluoroalkylation or N-trifluoroethylamination) can easily be dictated and modulated by a selection of N-trifluoroethyl hydroxylamine substrates. The key to success is the protecting group on the N atom of hydroxylamine reagents, which can control the process of 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Remarkable features of this method include mild conditions, easy operation, high selectivity, and excellent functional group tolerability. More importantly, the trifluoroalkylated products can be readily derivatized into other interesting imidazo-fused heterocycles that would be of great potential for the exploitation of pharmaceutically relevant molecules.

Recent advances in metal directed C-H amidation/amination using sulfonyl azides and phosphoryl azides

Transition metal-catalyzed C-N bond formation reactions have gained popularity as a method for selectively transforming common C-H bonds into N-functionalized molecules. This approach is particularly useful for synthesizing aminated molecules, which require aminating reagents and amidated building blocks. Over the past two decades, significant advancements have been achieved in transition-metal-catalyzed C-H functionalization, with organic azides emerging as promising amino sources and internal oxidants. This review focuses on recent developments in utilizing sulfonyl and phosphoryl azides as building blocks for directed intra- and intermolecular C-H functionalization reactions. Specifically, it discusses methods for synthesizing sulfonamidates and phosphoramidates using sulfonyl and phosphoryl azides, respectively. The article highlights the potential of C-H functionalization reactions with organic azides for efficiently and sustainably synthesizing N-functionalized molecules, providing valuable insights into the latest advancements in this field.

Silver-catalyzed cyclization of α-imino-oxy acids to fused tetralone derivatives

A silver-catalyzed intramolecular radical relay cyclization of α-imino-oxy acids under mild conditions has been described. This reaction offers facile access to a diverse range of fused tetralone derivatives with exquisite stereoselectivity in moderate to good yields (40-98%). Experimental studies show that the reaction undergoes a decarboxylation and acetone fragmentation/1,5-hydrogen atom transfer (HAT)/cyclization process.

Visible-light promoted intramolecular carboamination of alkynes for the synthesis of oxazolidinone-fused isoquinolinones

An efficient method for the synthesis of isoquinolinone derivatives via photopromoted carboamination of alkynes is developed. Starting from the readily available propargyl alcohol derivatives, the polycyclic isoquinolinone derivatives could be obtained with good aryl and heterocycle tolerance. Both terminal and alkyl substituted alkynes could be employed. This protocol is operationally easy, and easily conducted on a gram-scale. A possible mechanism involving radical addition and cyclization following aromatization was proposed.

Visible-light-initiated external photocatalyst-free synthesis of α,α-difluoro-β-ketoamides from 4-aminocoumarins

A concise and efficient ring-opening difluorination strategy was developed for the synthesis of highly functionalized hydroxy-containing α,α-difluoro-β-ketoamides from the one-pot multicomponent reaction of 4-aminocoumarins, NFSI, and water in dimethyl carbonate (DMC) as a green solvent. The reactions were smoothly achieved under visible light irradiation in air at room temperature without the addition of any other external photocatalysts. With this protocol, various α,α-difluoro-β-ketoamides were successfully synthesized under mild conditions (25 examples, 73-91% yields). This transition-metal-free synthetic procedure shows good functional group compatibility and attractive practical potential for large-scale synthesis.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Copper-Catalyzed Difluoroalkylation Reaction

This review describes recent advances in copper-catalyzed difluoroalkylation reactions. The RCF₂ radical is generally proposed in the mechanism of these reactions. At present, various types of copper-catalyzed difluoroalkylation reactions have been realized. According to their characteristics, we classify these difluoroalkylation reactions into three types.

Site-Selective, Photocatalytic Vinylogous Amidation of Enones

Despite the broad interest in organic compounds possessing a γ-aminocarbonyl motif, limited strategies for their synthesis have been reported. Herein, we describe a mild and efficient method for the site-selective amidation of unsaturated enones with electrophilic N-centered radicals as a key intermediate. The photocatalytic vinylogous reaction of dienolates with N-amino pyridinium salts affords γ-amido carbonyl compounds. This process is high-yielding, scalable, and tolerates a broad range of unsaturated α,β-unsaturated carbonyls, including biologically relevant compounds, as starting materials.

Chemodivergent photocatalytic access to 1-pyrrolines and 1-tetralones involving switchable C(sp3)–H functionalization

A chemodivergent photocatalytic approach to 1-pyrrolines and 1-tetralones from alkyl bromides and vinyl azides has been developed through chemoselectively controllable intermolecular [3 + 2] and [4 + 2] cyclization. This photoredox-neutral two-component protocol involves intermolecular radical addition and switchable distal C(sp³)–H functionalization enabled by iminyl radical-mediated 1,5-hydrogen atom transfer. Meanwhile, chemoselectivity between C(sp³)–N bond formation and C(sp³)–C(sp²) bond formation is precisely switched by photocatalysts (Ru(bpy)₃(PF₆)₂ vs. fac-Ir(ppy)₃) and additives (base vs. acid).

Visible-Light-Induced Photoredox-Catalyzed Selective 1,4-Difluoroalkylesterification of 1-Aryl-1,3-dienes

A selective three-component 1,4-difluoroalkylesterification of 1-aryl-1,3-dienes enabled by dual photoredox and copper catalysis is described. This protocol uses commercially available CF₂-reagents as radical precursors and carboxylic acids as oxygen-based nucleophiles, providing access to difluoroalkylated allylic esters. This protocol could be extended to intramolecular two-component 1,4-difluoroalkylesterification to access 3-substituted benzobutyrolactones. Preliminary mechanistic studies support a radical process.

N-O Bond Activation by Energy Transfer Photocatalysis

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp²) and N(sp³) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of ¹O₂, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp³)-N radical-radical coupling or intermolecular C(sp³)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

Synthesis of Pyrrole via Formal Cycloaddition of Allyl Ketone and Amine under Metal-Free Conditions

A new metal-free synthesis of pyrrole from allyl ketone and amine has been established. The reaction proceeds via an thiolative activation of the C-C double bond with dimethyl(methylthio)sulfonium trifluoromethanesulfonate, followed by a nucleophilic ring-opening addition of primary amine to the generated episulfonium intermediate, and then an internal condensation and aromatization. This mild procedure provides a novel strategy to the construction of substituted pyrroles through a formal [4 + 1] cycloaddition reaction.

Recent advances in 3-aminoindazoles as versatile synthons for the synthesis of nitrogen heterocycles

Nitrogen-based heterocycles are an important class of structural scaffolds distributed in biologically active natural products, medicinal chemistry, and agrochemicals. Hence, there is increasing interest in the development of novel synthetic strategies for the construction of these privileged structural motifs. Recently, 3-aminoindazoles have emerged as versatile synthons participating in a variety of condensation annulation, denitrogenative transannulation and rearrangement ring expansion reactions, which provide efficient synthetic routes for the formation of nitrogen heterocycles. This review systematically highlights for the first time the most recent advances in 3-aminoindazoles to provide a deep understanding of using 3-aminoindazoles as versatile synthons in organic transformations for synthetic and medicinal chemists.

Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction

The use of metal-free carbon nitride and light to drive catalytic transformations constitutes a sustainable strategy for organic synthesis. At the moment, enhancing the intrinsic activity of CN catalysts by tuning the interfacial coupling between catalyst and substrate remains challenging. Herein, we demonstrate that urea-derived carbon nitride catalysts with the abundant −NH₂ groups and the relative positive charged surface could effectively complex with the deprotonated anionic intermediate to improve the adsorption of organic reactants on the catalyst surface. The decreased oxidation potential and upshift in its highest occupied molecular orbital position make the electron abstraction kinetics by the catalyst more energetically favorable. The prepared catalyst is thus utilized for the photocatalytic cyclization of nitrogen-centered radicals for the synthesis of diverse pharmaceutical-related compounds (33 examples) with high activity and reusability, which shows competent performance to the homogeneous catalysts.

Carbon nitride catalysts with positively charged surfaces and abundant −NH2 are found to be effective photocatalysts for dihydropyrazole synthesis. A surface-mediated mechanism where deprotonated intermediates interact with the surface is proposed.