Cross-Dehydrogenative C–H Amination of Indoles under Aerobic Photo-oxidative Conditions

Photoredox/Nickel Dual Catalysis-Enabled Cross-Dehydrogenative C-H Amination of Indoles with Unactivated Amine

Herein, a direct cross-dehydrogenative C-H amination of indoles has been successfully achieved, enabled by the merger of photocatalysis with nickel catalysis. This developed process does not require stoichiometric oxidants and prefunctionalization of amine partners, providing a concise platform for C-N bond formation. Moreover, the synthetic practicality of this transformation was well revealed by its high step- and atom-economy, high reaction efficiency, and broad functional group tolerance.

Photocatalysed C-H amidation of indoles enabled by tert-butyl alkyl((perfluoropyridin-4-yl)oxy)carbamate

Development of a new catalytic and straightforward strategy to construct C-N bonds is playing a pivotal role in synthetic chemistry. Here, we report a photocatalysed protocol to access direct C-H amidation of indoles, enabled by a rationally designed tert-butyl alkyl((perfluoropyridin-4-yl)oxy)carbamate. A series of biologically important aminoindoles were prepared under mild conditions with excellent regioselectivity and broad substrate scope.

Visible-Light-Driven C-H Imidation of Arenes and Heteroarenes by a Phosphonium Ylide Organophotoredox Catalyst: Application to C-H Functionalization of Alkenes

Phosphonium ylide catalysis through an oxidative quenching cycle has been developed for visible-light-driven C-H imidation of arenes and heteroarenes. The present protocol could be applied not only to trihalomethylative lactonization reactions involving trifluoromethyl, trichloromethyl, and tribromomethyl radicals but also to the first example of an organophotoredox-catalyzed imidative lactonization reaction involving a nitrogen-centered electrophilic radical species.

A Two-Step Synthesis of Unprotected 3-Aminoindoles via Post Functionalization with Nitrostyrene

A novel, low-cost method for the preparation of not easily accessible free 3-aminoindoles has been developed. This approach is based on a well-established reaction between indoles and nitrostyrene in the presence of phosphorous acid, which results in the formation of 4'-phenyl-4'H-spiro[indole-3,5'-isoxazoles]. The latter could be transformed to corresponding aminated indoles by reaction with hydrazine hydrate in good or excellent yields upon microwave-assisted heating.

[Development of Molecular Transformation Reactions Using Visible Light and Main Group Elements]

Owing to their ability to induce excitation of specific molecular orbitals or initiate chemical reactions, photochemical reactions have the potential to be more effective at selectively activating target molecules than thermal reactions. The thermal reactions transfer thermal energy to activate molecules, which often leads to the activation of multiple molecular species, including undesired ones, resulting in non-selectivity. This nonselectivity may result in undesirable side reactions or decrease reaction efficiency. Additionally, photochemical reactions can induce selective activation by absorbing specific wavelengths of light. However, visible light-driven photocatalytic reactions typically require expensive transition metal catalysts or organic dyes, leaving plenty of room for improvement. To address the aforementioned issues, the photochemical properties of the main group elements, such as halogens, were optimized and methodologies for visible light-induced reactions were developed. Activation of molecular halogen, halogen-carbon bonds, and halogen bonding interactions were independently investigated and various methodologies were reported. These developed reactions are excellent methodologies that use inexpensive raw materials and are thus predicted to contribute significantly toward sustainability.

Visible-Light-Induced Oxyalkylation of 1,2,4-Triazine-3,5(2H, 4H)-diones with Ethers via Oxidative Cross-Dehydrogenative Coupling

An efficient and convenient method to synthesize 6-oxyalkylated 1,2,4-triazine-3,5(2H, 4H)-diones has been developed via visible-light-induced cross-dehydrogenative coupling reaction between 1,2,4-triazine-3,5(2H, 4H)-diones and ethers with a wide range of functional group tolerance. The present transformation employs the cheap and low-toxic 2-tert-butylanthraquinone as a metal-free photocatalyst and air as a green oxidant at room temperature. Moreover, this reaction can also be driven by sunlight as a clean energy resource. The synthetic utility of this method is further demonstrated by gram-scale reaction and application in the preparation of key intermediates of bioactive molecules.

N-Acylimino-λ3-iodanes from the Metathesis of Iodosoarenes and Nitriles for the Photoinduced C-H Perfluoroacylamination of (Hetero)Arenes

An efficient and scalable synthesis of N-perfluoroacylimino-λ³-iodanes was achieved via an unprecedented metathesis between iodosoarenes and perfluoroalkanenitriles. The perfluoroacylamino groups of the iodanes could be introduced to aromatic and heteroaromatic rings using photoirradiation.

Recent Developments in the Photochemical Synthesis of Functionalized Imidazopyridines

Imidazopyridines constitute one of the most important scaffolds in medicinal chemistry, as their skeleton could be found in a myriad of biologically active molecules. Although numerous strategies were elaborated for imidazopyridine preparation in the 2010s, novel eco-compatible synthetic approaches have emerged, conscious of climate change concerns. In this framework, photochemical methods have been promoted to conceive this heterocyclic motif over the last decade. This review covers the recently published works on synthesizing highly functionalized imidazopyridines by light induction.

Copper-Catalyzed Regiospecific Amination of Heteroarenes with Quinoneimides

In this work, an unprecedented and widely applicable strategy for the regioselective C-3 amination of indoles and C-2 amination of heteroarenes (pyrrole and benzofuran) is presented in a simple, high-efficiency way. This protocol is also one of the few methods for the efficient construction of C-N bonds of quinoneimides by the 1,6-addition reaction.

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.

Recent Progress in Organophotoredox Reaction

In the past decade, visible light photoredox catalysis has been established as a gentle and powerful strategy for the activation of organic molecules. As an important part of it, organic...

Iron–iodine co-catalysis towards tandem C–N/C–C bond formation: one-pot regioselective synthesis of 2-amino-3-alkylindoles

An efficient intermolecular C2,3-H aminoalkylation of indoles with 9H-xanthenes and azoles via iron–iodine co-catalyzed tandem C–N/C–C bond formation has been developed.

Three-component synthesis of α-indole-β-sulfonyl tetrahydrofurans under metal-free conditions

An efficient multi-component reaction has been developed for the synthesis of 2,3-disubstituted tetrahydrofurans in a “one pot” manner, starting from readily available 2-arylindoles, arylsulfonyl azides, and tetrahydrofuran under simple and easily operated reaction conditions.

Copper-Catalyzed Aminoarylation of Alkenes via Aminyl Radical Addition and Aryl Migration

We describe a new strategy for aminoarylation of alkenes by copper-catalyzed smiles rearrangement using O-benzoylhydroxylamines as the amine reagent. This method affords various β-amino amide derivatives possessing a quaternary carbon center with wide functional group tolerance and high regioselectivity. The mechanistic studies indicate that the transformation can involve aminyl radical intermediates under acid-free condition.

Electro-Oxidative C-N Bond Formation through Azolation of Indole Derivatives: An Access to 3-Substituent-2-(Azol-1-yl)indoles

A practical protocol to synthesize 3-substituent-2-(azol-1-yl)indole derivatives has been developed via an electrochemical oxidative cross coupling process under mild conditions. This electro-oxidative C-N bond formation strategy tolerates a range of functional groups and is amenable to gram scale synthesis. Moreover, this method was applied to the late-stage functionalization of bioactive molecules.

Cross-dehydrogenative Coupling of N-Aryl Tetrahydroisoquinolines Catalyzed by an Anthraquinone-containing Polymeric Photosensitizer

This work reports the photocatalytic application of an anthraquinone-containing polymeric photosensitizer (AQ-PHEMA) in the visible light-induced cross-dehydrogenative-coupling of N-aryl tetrahydroisoquinolines with several nucleophiles, including nitromethane, 1-methyl-2-alkyl ketone and dialkyl (aryl) phosphine oxide. The results revealed that the reaction could be catalyzed by AQ-PHEMA efficiently to afford a series of 1-substituted-2-aryl-1,2,3,4-tetrahydroisoquinolines in good to excellent yields with nice substrate tolerance under aerobic conditions at room temperature. The practical application potential was also showcased by a gram-scale synthesis. More importantly, the utilization of AQ-PHEMA as a heterogeneous photosensitizer also showed nice recyclability and reusability of the catalyst, whereas AQ-PHEMA can be easily separated and reused for at least 8 times without significant loss of photocatalytic activity.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Direct amidation of metallaaromatics: access to N-functionalized osmapentalynes via a 1,5-bromoamidated intermediate

The direct C–H amidation or imidation of metallaaromatics with N-bromoamides or imides has been achieved under mild conditions and leads to the formation of a family of N-functionalized metallapentalyne derivatives. A unique 1,5-bromoamidated species has been identified, and can be viewed as a σ^H-adduct intermediate in a nucleophilic aromatic substitution. The 1,5-addition of both electrophilic and nucleophilic moieties into the metallaaromatic framework demonstrates a novel pathway in contrast to the typical radical process of arene C–H amidation involving N-haloamide reagents.

The direct C–H amidation of metallapentalyne has been achieved under mild conditions in which key 1,5-bromoamidated intermediates was determined.

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.

Iron-catalyzed cross-dehydrogenative C–H amidation of benzofurans and benzothiophenes with anilines

An efficient iron-catalyzed radical cross-dehydrogenative aromatic C–H amidation provides a straightforward access to structurally diverse diarylamine derivatives incorporating benzofuran/benzothiophene motifs.

Visible-Light-Induced Metal-/Photocatalyst-Free C-H Bond Imidation of Arenes

In this study, a visible-light-induced intermolecular C-H bond imidation of arenes was achieved at ambient condition. By using simple phthalimide with (diacetoxyiodo)benzene and molecular iodine, direct metal-/photocatalyst-free C-N bond formation was achieved. The imidation protocol was designed by using time-dependent density functional theory calculations and experimentally demonstrated for 28 substrates with as high as 96% yield. Mechanistic studies indicated that radical-mediated aromatic substitution occurred via photolysis of N-iodophthalimide under visible-light irradiation.

Chemistry With N-Centered Radicals Generated by Single-Electron Transfer-Oxidation Using Photoredox Catalysis

This review covers the recent literature on oxidative generation of N-centered radicals using photoredox catalysis. The concept of proton-coupled electron transfer is briefly discussed. Applications of such reactive N-centered radicals in cascade processes comprising arene amidation, alkene amidation, C—C bond cleavage reactions, and remote C—H functionalization are addressed. In addition, novel reagents allowing for clean oxidative N-radical generation are discussed.

Visible-Light Catalytic Photooxygenation of Monoterpene Indole Alkaloids: Access to Spirooxindole-1,3-oxazines

Few natural oxindole alkaloids possess an exceptional spiro-[(1,3)oxazinan-3,6'-oxindole] core structure, which results from an unusual oxidative indole rearrangement. The Rauvolfia alkaloid reserpine can be converted into the spirooxindole-1,3-oxazines dioxyreserpine and trioxyreserpine through efficient visible-light catalytic photooxygenation with anthraquinone photocatalysts. A mechanistic investigation sheds new light on the photooxidative rearrangement of reserpine and related monoterpene indole alkaloids, and the spirooxindole-1,3-oxazine products can be valorized by reductive ring opening, to obtain cis-decahydroisoquinolines as new enantiopure synthetic building blocks, as demonstrated for dioxyreserpine.

Amidyl Radicals by Oxidation of α-Amido-oxy Acids: Transition-Metal-Free Amidofluorination of Unactivated Alkenes

A three-component transition-metal-free amidofluorination of unactivated alkenes and styrenes is presented. α-Amido-oxy acids are introduced as efficient and easily accessible amidyl radical precursors that are oxidized by a photoexcited organic sensitizer (Mes-Acr-Me) to the corresponding carboxyl radical. Sequential CO₂ and aldehyde/ketone fragmentation leads to an N-centered radical that adds to an alkene. Commercial Selectfluor is used to trap the adduct radical through fluorine-atom transfer. The transformation features by high functional-group tolerance, broad substrate scope, and practical mild conditions. Mechanistic studies support the radical nature of the cascade.

Organic dye-catalyzed radical ring expansion reaction

Herein, we reported an attractive method for synthesizing medium-sized rings that are catalyzed by erythrosine B under fluorescent light irradiation. This synthetic approach featured mild conditions, a facile procedure, a broad substrate scope, and moderate-to-good yields.

Herein, we reported an attractive method for synthesizing medium-sized rings that are catalyzed by erythrosine B under fluorescent light irradiation.

Visible light catalyzed synthesis of quinolines from (aza)-Morita–Baylis–Hillman adducts

A mild and efficient protocol for the synthesis of quinoline scaffolds from (aza)-MBH adducts under visible light catalysis has been established.

Recent advances in radical-based C–N bond formation via photo-/electrochemistry

This review highlights the recent advances in cross-dehydrogenative amination for C–N bond construction from C–H/N–H cross-coupling partners through photocatalytic and/or electronic techniques.

Tunable Cu(i)-catalyzed site-selective dehydrogenative amination of β,γ-unsaturated hydrazones for divergent synthesis of pyrazol-4-ones and 1,6-dihydropyradazines

A new tunable Cu(i)-catalyzed site-selective dehydrogenative amination of β,γ-unsaturated hydrazones has been established using readily accessible dioxygen as a green oxidant.

Synthesis of Arylamines via Aminium Radicals

Arylamines constitute the core structure of many therapeutic agents, agrochemicals, and organic materials. The development of methods for the efficient and selective construction of these structural motifs from simple building blocks is desirable but still challenging. We demonstrate that protonated electron-poor O-aryl hydroxylamines give aminium radicals in the presence of Ru(bpy)3 Cl2 . These highly electrophilic species undergo polarized radical addition to aromatic compounds in high yield and selectivity. We successfully applied this method to the late-stage modification of chiral catalyst templates, therapeutic agents, and natural products.