Catalytic N-radical cascade reaction of hydrazones by oxidative deprotonation electron transfer and TEMPO mediation

Electrocatalytic dehydrogenative and defluorinative coupling between aldehyde-derived N,N-dialkylhydrazones and fluoromalonates: synthesis of 2-pyrazolines

An electrocatalytic synthesis of 2-pyrazolines via dehydrogenative and defluorinative cross-coupling reactions between (hetero)arylaldehyde-derived N,N-dialkylhydrazones and fluoromalonates is disclosed. Salient features of this work include (i) readily available starting materials, (ii) practical reaction conditions, and (ii) a formal oxidative (4 + 1)-cycloaddition via triple C-H bond functionalization. Cyclic voltammetry analyses support the electrocatalytic formation of an α-fluoromalonyl radical.

Photocatalytic Bicyclization of Indole-Tethered 1,6-Enynes for Diastereoselective Synthesis of Pyrrolo[3,2,1-jk]carbazoles

A visible-light-driven photocatalytic protocol is established for the diastereoselective synthesis of pyrrolo[3,2,1-jk]carbazoles via a radical-triggered multicomponent bicyclization reaction starting from readily available indole-tethered 1,6-enynes and α-benzyl-α-bromomalonates under mild conditions. This photocatalytic approach exhibits a wide substrate compatibility and excellent tolerability toward various functional groups and boasts the benefit of efficient ring formation and chemical bond creation.

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.

Generation of Aromatic N-Heterocyclic Radicals for Functionalization of Unactivated Alkenes

Nitrogen-centered radicals (NCRs) have been widely recognized as versatile synthetic intermediates for the construction of nitrogen containing molecules of high value. As such, there has been a long-standing interest in the field of organic synthesis to develop novel nitrogen-based radicals and explore their inherent reactivity. In this study, we present the generation of aromatic N-heterocyclic radicals and their application in a novel and diverse functionalization of unactivated alkenes. Bench-stable aromatic N-heterocyclic pyridinium salts were employed as crucial NCR precursors, which enabled the efficient conversion of various unactivated alkenes into medicinally relevant alkylated N-heterocyclic amines. This approach offers an unexplored retrosynthetic disconnection for the synthesis of related molecules that commonly possess therapeutic value. Furthermore, this platform can be extended to the synthesis of densely functionalized heterocyclic amines by utilizing disulfides and diethyl bromomalonate as radical quenchers. Mechanistic investigations indicate an energy transfer (EnT) pathway involving the formation of a transient aromatic N-heterocyclic radical, radical addition to unactivated alkenes, and subsequent generation of the amination product through either hydrogen atom transfer (HAT) or radical addition processes.

Transition metal-free [3 + 3] annulation of cyclopropanols with β-enamine esters to assemble nicotinate derivatives

A metal-free and efficient approach for the synthesis of structurally important nicotinates through 4-HO-TEMPO-mediated [3 + 3] annulation of cyclopropanols with β-enamine esters is presented. This protocol features high atom efficiency, green waste, simple operation and broad substrate scope. Moreover, the experiments of gram-scale synthesis and recovery of oxidants make this strategy more sustainable and practical.

Photocatalytic Intramolecular Alkene Hydroamination of N-Alkoxy Ureas: An Approach to Imidazolinones

Imidazolinones were obtained in good yields by intramolecular hydroamination of N-alkoxy ureas in the presence of an organic photocatalyst and an inorganic base. In this reaction, the N-alkoxy urea anion generated by deprotonation undergoes photocatalyzed single-electron-transfer oxidation to generate the corresponding radical, which cyclizes to afford the imidazolinone ring. This new protocol grants access to an array of complex molecules containing a privileged imidazolinone core.

Oxygen-Promoted 6-endo-trig Cyclization of β,γ-Unsaturated Hydrazones/Ketoximes with Diazonium Tetrafluoroborates for Pyridazin-4(1H)-ones/Oxazin-4(1H)-ones

An oxidative 6-endo-trig cyclization and [2 + 2] cycloaddition of β,γ-unsaturated hydrazones/ketoximes and diazonium tetrafluoroborates for a synthetic strategy to pyridazin-4(1H)-ones/oxazin-4(1H)-ones under metal-free conditions is presented in a one-pot procedure. This protocol features excellent functional group tolerance and remarkable regioselectivity. A mechanistic study has been verified via 18O labeling and the H218O labeling method, in which O2 acts as both a reaction component and an oxidant.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Iodine-Mediated Oxidative Annulation of β,γ-Unsaturated Hydrazones in Dimethyl Sulfoxide: A Strategy to Build 1,6-Dihydropyridazines and Pyrroles

Simple, commercially available iodine was successfully employed as a highly efficient and chemoselective catalyst for the oxidative annulation of β,γ-unsaturated hydrazones to produce 1,6-dihydropyridazines under mild conditions for the first time. Interestingly, when active β,γ-unsaturated hydrazone compounds containing electron-donating groups, such as furyl, thienyl, and cycloalkyl, were used, pyrroles were obtained. A gram-scale preparation experiment and further derivatization of pyridazines demonstrated the potential applicability of our synthesis method. Experimental studies and density functional theory calculations unveiled the origin of the chemoselectivity determining the formation of different products.

Energy-transfer-induced [3+2] cycloadditions of N-N pyridinium ylides

Photocycloaddition is a powerful reaction to enable the conversion of alkenes into high-value synthetic materials that are normally difficult to obtain under thermal conditions. Lactams and pyridines, both prominent in pharmaceutical applications, currently lack effective synthetic strategies to combine them within a single molecular structure. Here we describe an efficient approach to diastereoselective pyridyl lactamization via a photoinduced [3+2] cycloaddition, based on the unique triplet-state reactivity of N-N pyridinium ylides in the presence of a photosensitizer. The corresponding triplet diradical intermediates allow the stepwise radical [3+2] cycloaddition with a broad range of activated and unactivated alkenes under mild conditions. This method exhibits excellent efficiency, diastereoselectivity and functional group tolerance, providing a useful synthon for ortho-pyridyl γ- and δ-lactam scaffolds with syn-configuration in a single step. Combined experimental and computational studies reveal that the energy transfer process leads to a triplet-state diradical of N-N pyridinium ylides, which promotes the stepwise cycloaddition.

Visible Light-Induced Decomposition of Acyl Peroxides Using Isocyanides: Synthesis of Heteroarenes by Radical Cascade Cyclization

Visible light-mediated facile synthesis of heteroarenes, namely, isoquinolines, benzothiazoles, and quinazolines, is demonstrated by employing isocyanides and inexpensive acyl peroxides. It is shown for the first time that singlet-excited isocyanides decompose acyl peroxides into aryl/alkyl radicals. The latter attack isocyanides, yielding imidoyl radicals that subsequently cyclize to afford heteroarene products. The protocol involving radical cascade reactions obviates the requirement of any external photocatalyst, oxidant, additive, and base.

Modular synthesis of unsaturated aza-heterocycles via copper catalyzed multicomponent cascade reaction

The unsaturated aza-heterocycles such as tetrahydropyridines pose significant applications in both drug discovery and development. However, the methods to construct polyfunctionalized tetrahydropyridines are still limited. Herein, we report a modular synthesis of tetrahydropyridines via copper catalyzed multicomponent radical cascade reaction. The reaction features mild conditions and broad substrate scope. In addition, the reaction could scale up to gram scale with similar yield. A variety of 1,2,5,6-tetrahydropyridines with C3 and C5 substituents could be assembled from simple starting materials. More importantly, the products could serve as versatile intermediate to access various functionalized aza-heterocycles which further demonstrates its utility.

Metal-Catalyzed, Photo-Assisted Selective Transformation of Tertiary Alkylbenzenes and Polystyrenes into Carbonyl Compounds

Every year, thousands of tons of polystyrene are produced and discarded, filling landfills and polluting the marine environment. Although several degradation alternatives have been proposed, the need for an effective procedure for the chemical recycling of polystyrene still remains. Here, a vanadium-catalyzed reaction, assisted by visible light, promoted the direct, selective conversion of tertiary alkylbenzenes into acetophenone and other ketone derivatives. Likewise, standard polystyrene samples as well as polystyrenes from insulation and packaging waste could be chemically recycled into acetophenone in a scalable way regardless of their molecular weight, polydispersity, or form. Preliminary mechanistic investigations revealed the participation of singlet oxygen, superoxide, and hydroxyl radical species in this homogenously catalyzed process. Acetophenone could be used as an additive to accelerate the reaction and to increase the yields in some cases.

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.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Deoxygenative gem-difluorovinylation of aliphatic alcohols

An unprecedented deoxygenative gem-difluorovinylation of aliphatic alcohols using α-trifluoromethyl alkenes is achieved under photocatalytic conditions. Inexpensive Ph₃P acts as an efficient O-atom transfer reagent to facilitate the deoxygenation of alcohols for the generation of reactive alkyl radical species. Remarkable features of this reaction include mild conditions, simple operation and broad scope. The synthetic utility of this reaction was validated by the success of two-step one-pot reactions, scale-up synthesis and chemoselective monodeoxygenation of diols.

Highly Selective Synthesis of Seven-Membered Azaspiro Compounds by a Rh(I)-Catalyzed Cycloisomerization/Diels-Alder Cascade of 1,5-Bisallenes

The synthesis of spiro compounds featuring seven- and six-membered rings in the spirobicyclic motif is successfully achieved through a cascade process encompassing a rhodium(I)-catalyzed cycloisomerization followed by a highly selective Diels–Alder homodimerization. The scope of the reaction is analyzed based on a series of synthetic substrates, and control experiments and DFT calculations led us to justify the exquisite degree of selectivity observed.

Visible light-induced N-radical 5-exo/6-endo cyclization of alkenyl amides: facile access to isoindolinones/isoquinolinones

An efficient N-centered radical intramolecular cyclization reaction of alkenyl amides induced by visible light was described. In this process, an alkenyl amide underwent 5-exo/6-endo cyclization to selectively yield two critical alkaloid structures, namely isoindolinones and isoquinolinones.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Regioselective Synthesis of Pyrazolo[1,5-a]pyridine via TEMPO-Mediated [3 + 2] Annulation-Aromatization of N-Aminopyridines and α,β-Unsaturated Compounds

A TEMPO-mediated [3 + 2] annulation-aromatization protocol for the preparation of pyrazolo[1,5-a]pyridines from N-aminopyridines and α,β-unsaturated compounds was developed. The procedure offered multisubstituted pyrazolo[1,5-a]pyridines in good to excellent yield with high and predictable regioselectivity. The modification of marketed drugs including Loratadine, Abiraterone, and Metochalcone, and a one-pot three-step gram scale synthesis of key intermediate for the preparation of Selpercatinib were demonstrated. Mechanism studies show that TEMPO serves both as a Lewis acid and as an oxidant.

A Photocatalytic System Composed of Benzimidazolium Aryloxide and Tetramethylpiperidine 1-Oxyl to Promote Desulfonylative α-Oxyamination Reactions of α-Sulfonylketones

A new photocatalytic system was developed for carrying out desulfonylative α-oxyamination reactions of α-sulfonylketones in which α-ketoalkyl radicals are generated. The catalytic system is composed of benzimidazolium aryloxide betaines (BI⁺-ArO^-), serving as visible light-absorbing electron donor photocatalysts, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), playing dual roles as an electron donor for catalyst recycling and a reagent to capture the generated radical intermediates. Information about the detailed nature of BI⁺-ArO^- and the photocatalytic processes with TEMPO was gained using absorption spectroscopy, electrochemical measurements, and density functional theory calculations.

An umpolung strategy for rapid access to thermally activated delayed fluorescence (TADF) materials based on phenazine

Herein, Ag(I)-promoted regioselective intramolecular radical nucleophilic addition/rearrangement of 2-aryl diazaboroles has been accomplished for the first time to construct phenazine structures. This protocol is an umpolung strategy based on the classical electrophilic mechanism, and therefore, a reversed regioselectivity was observed, which provides an opportunity to prepare sterically hindered phenazines. The resulting thermally activated delayed fluorescence (TADF) materials based on phenazine exhibit emission bands from green to red with high quantum yields and moderate fluorescence lifetimes as solid films.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

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 indole synthesis via intramolecular C–N bond formation: desulfonylative C(sp2)–H functionalization

LED visible-light-induced redox neutral desulfonylative C(sp2)–H functionalization for the synthesis of N-substituted indoles in the absence of any additional additive has been established on the basis of KIE, Hammett plotting and DFT calculations.

Radical 1,4/5-Amino Shift Enables Access to Fluoroalkyl-Containing Primary β(γ)-Aminoketones under Metal-Free Conditions

A novel radical 1,4/5-amino shift from the oxygen center of alkene-tethered diphenyl ketoxime ethers to the carbon center to achieve high value-added fluoroalkyl-containing primary β(γ)-amino-ketones is reported. Mechanism studies reveal that the migration is triggered by the alkene addition of fluoroalkyl radical derived from the electron donor-acceptor (EDA) complex of Togni's reagent II or fluoroalkyl iodides and quinuclidine, and involves a unique 5(6)-exo-trig cyclization of the carbon-centered radical onto the N-atom of ketoxime ethers followed by a cascade sequence of N-O bond cleavage and dehydrogenation. Notably, besides Togni's reagent II and fluoroalkyl iodides, this protocol is also compatible with other radical precursors to provide various functionalized primary aminoketones.

KOtBu-Promoted Halogen-Bond-Assisted Intramolecular C-S Cross-Coupling of o-Iodothioanilides for the Synthesis of 2-Substituted Benzothiazoles

An efficacious and mild KO^tBu-promoted intramolecular C-S cross-coupling of ortho-iodothioanilides in conjunction with a catalytic quantity of phenanthroline as an additive has been described for the convenient synthesis of 2-substituted benzothiazoles. The methodology is suitable for attaining a wide variety of 2-alkyl- and 2-aryl-substituted benzothiazoles. Single-crystal XRD, DFT calculations, NMR, and UV studies suggest that halogen bonds between the units of ortho-iodothioanilides may assist in the electron transfer process.

Alkene Synthesis by Photo-Wolff-Kischner Reaction of Sulfur Ylides and N-Tosylhydrazones

A visible-light-driven and room temperature photo-Wolff-Kischner reaction of sulfur ylides and N-tosylhydrazones has been developed for the first time to provide modular access to alkene synthesis. The high functional group tolerance and broad substrate scope were demonstrated by more than 60 examples. Both E- and Z-olefinic stereochemistry in the products could be controlled with excellent stereoselectivity. A series of mechanistic studies support that the reaction should proceed through a radical-carbanion crossover pathway, specifically involving addition of photo-generated sulfur ylide radical cations to N-tosylhydrazones to form carbanions and subsequent Wolff-Kischner process.

Photoredox Generation of N-Centered Hydrazonyl Radicals Enables the Construction of Dihydropyrazole-Fused gem-Difluoroalkenes

An efficient visible-light-promoted N-radical-mediated tandem radical cyclization/defluorinated alkylation of β,γ-unsaturated hydrazones, and α-trifluoromethyl alkenes is described. This protocol provides a general and effective route to synthesize various dihydropyrazole-fused gem-difluoroalkenes at moderate to excellent yields under redox-neutral, metal-free, and mild conditions.

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.

Metal-Free Tandem Oxidative Cyclization for the Synthesis of 1,2-Dihydropyridazines and Pyrazoles

Mediated by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), a novel oxidative coupling of hydrazones and 1,3-diarylpropenes has been disclosed to generate appealing β,γ-unsaturated hydrazones, which further undergo 5-exo-trig or 6-endo-trig cascade cyclization to give the respective 1,2-dihydropyridazines or pyrazoles selectively under metal-free conditions. The mechanisms of the coupling and subsequent cyclization are proposed.

O-Perhalopyridin-4-yl Hydroxylamines: Amidyl-Radical Generation Scaffolds in Photoinduced Direct Amination of Heterocycles

Reported herein is the design and synthesis of new O-perhalopyridin-4-yl hydroxylamines as shelf-stable and versatile amidyl-radical precursors. The novel amination reagents can be easily prepared via a single synthetic step from inexpensive commercially available starting materials using monoprotected HONH₂ as amino source. The synthetic potency of the developed reagents was well demonstrated by direct amination of a series of quinoxalin-2(1H)-ones and their analogues under photocatalytic conditions, even without any additive and photocatalysts.

Ru-catalysed C(sp2)-H vinylation/annulation of benzoic acids and alkynes: rapid access to medium-sized lactones

An unprecedented ruthenium catalysed [4+4] annulation of readily available benzoic acids and alkynes is reported for the first time. The carboxylate group acts as both a directing group and an internal nucleophilic reagent to facilitate a C(sp²)-H vinylation/annulation cascade. This reaction avoids the classically oxidative [4+2] annulation, allowing the efficient synthesis of a wide array of eight-membered lactones under oxidant-free conditions. Moreover, this catalytic system can be successfully extended to [4+3] and [4+5] annulations for the assembly of seven- and nine-membered lactones.

Ruthenium-catalysed C–H/C–N bond activation: facile access to isoindolinones

A facile ruthenium-catalysed C–H/C–N bond activation and the subsequent annulation of readily available benzoic acids with in situ generated formaldimines are developed for the efficient synthesis of a wide range of biologically important isoindolinones.

Transition-metal-free decarboxylative ipso amination of aryl carboxylic acids

An unprecedented DMAP-catalysed decarboxylative amination of carboxylic acids has been achieved under metal free conditions, enabling the convenient synthesis of structurally diverse aryl and alkyl amines.

Photoredox-neutral alkene aminoarylation for the synthesis of 1,4,5,6-tetrahydropyridazines

A mild and redox-neutral protocol is developed for the synthesis of 1,4,5,6-tetrahydropyridazines via photoredox catalysis.

Electrochemical oxidative synthesis of 2-benzoylquinazolin-4(3H)-one via C(sp3)–H amination under metal-free conditions

An electrochemically induced C(sp3)–H amination of 2-aminobenzamides with ketones using TBAI as a catalyst was developed, and provided 2-benzoylquinazolin-4(3H)-ones under metal-free conditions.