Synthesis using aromatic homolytic substitution—recent advances

N-Heterocyclic Carbene Catalysis for Polycyclic Benzazepines Assembly: Regioselective Intramolecular Tandem Radical Cyclization

A variety of polycyclic benzazepines were rapidly constructed by NHC-catalyzed regioselective redox-neutral intramolecular tandem cyclization. Initial mechanistic studies revealed that a SET radical process was possibly involved.

Photochemical direct alkylation of heteroarenes with alkanes, alcohols, amides, and ethers

Direct functionalization of heteroarenes with simple alkanes utilizing anthracene as a photoredox catalyst has been established. This approach provides a sustainable alternative, avoiding costly reagents or peroxides. The method demonstrates a broad substrate scope, enabling regioselective alkylation of various heteroarenes, including azoles, pyridines, quinolines, isoquinolones, and quinoxalinones under mild conditions. A range of alkyl sources, such as alkanes, ethers, dioxane, trioxane, alcohol, and alkylamides were viable substrates. A plausible catalytic cycle was proposed based on the preliminary mechanistic evidence.

Defunctionalization Enabled by Intramolecular Radical Aromatic Ipso Substitution

A chemoselective and regioselective copper-promoted defunctionalization procedure has been developed, enabling the rapid construction of various N-polyheterocycles. Initial mechanistic studies reveal that a single-electron transfer radical process is potentially involved.

Construction of Functionalized Oxindoles by Quinone-Carbonate Synergistically Triggered Intermolecular Radical Coupling

We disclose a rapid and nontoxic procedure to construct various oxindoles. This method harnesses the power of a catalytic amount of quinone in synergy with Cs2CO3, showcasing remarkable compatibility with a wide range of functional groups. Mechanistic investigations reveal that it operates via a radical pathway, likely initiated by the single-electron transfer from quinone-Cs2CO3 complexes. This pivotal electron transfer event leads to the generation of a crucial alkyl radical intermediate, contributing to the overall success and efficacy of the transformation.

α-Diimine-mediated C-H functionalization of arenes for aryl-aryl cross-coupling reactions

Easily accessible methods for direct C-H arylation of arenes have been explored in the presence of transition metal catalysts to facilitate C-C bond formation; however, the absence of transition-metal impurities is a significant concern in the preparation of active pharmaceutical ingredients (APIs). Herein, we examine the use of bis(imino)acenaphthene (BIAN) as a potential single-electron transfer initiator in transition metal-free C-C bond-forming reactions. Using this approach, arenes are coupled to several aryl and heteroaryl halides. Based upon preliminary mechanistic evidence and crystallographic probation of an active initiator species, we tentatively propose a potassium-stabilized 'metal-free' radical pathway is in operation.

Diverse Reactions of Formazanate/Formazan with Tetrylenes: Reduction, C-H Bond Activation, Substitution and Addition

The reactivity of the formazanate potassium salt [LtBu K(thf)] (LtBu= PhNNC(4-t BuPh)NNPh) with the group 14 chlorotetrylenes [{PhC(t BuN)2 }ECl] (E=Si, Ge, Sn) was investigated. Three corresponding compounds with unique configurations were formed, demonstrating the diverse reactivity of the system. In addition to the anticipated salt metathesis reactions of the potassium salt with the chlorine function of tetrylenes, unexpected reduction/insertion steps into the N=N bond of the formazanate (Si, Ge) and subsequent C-H activation (Ge) were also observed. Furthermore, when the neutral formazan ligand [LtBu H] was exposed to silylenes [{PhC(t BuN)2 }SiCl] and [LPh SiNMePy], substitution and addition reactions occurred. These discoveries significantly enrich the diversity of formazanate/formazan redox chemistry, opening up new avenues for exploration in this field.

Convergent Syntheses of Isomeric Imidazolospiroketones as Templates for Acetyl-CoA Carboxylase (ACC) Inhibitors

The synthesis of imidazole fused spirocyclic ketones as templates for acetyl-CoA carboxylase (ACC) inhibitors is reported. By completing the spirocyclic ring closure via divergent pathways, the synthesis of these regioisomers from common intermediates was developed. Through an aldehyde homologation/transmetalation strategy, one isomer was formed selectively. The second desired isomer was obtained via an intramolecular aromatic homolytic substitution reaction. Preparation of these isomeric spiroketones provided templates which, upon elaboration, led to key structure-activity relationship (SAR) points for delivery of potent ACC inhibitors.

Direct C2-H alkylation of indoles driven by the photochemical activity of halogen-bonded complexes

A light-driven metal-free protocol for the synthesis of sulfone-containing indoles under mild conditions is reported. Specifically, the process is driven by the photochemical activity of halogen-bonded complexes formed upon complexation of a sacrificial donor, namely 1,4-diazabicyclo[2.2.2]octane (DABCO), with α-iodosulfones. The reaction provides a variety of densely functionalized products in good yields (up to 96% yield). Mechanistic investigations are reported. These studies provide convincing evidences for the photochemical formation of reactive open-shell species.

Metal-Free Generation of γ-Cyanoalkyl Radicals by N-Heterocyclic Carbene Catalysis: Assembly of 6-Cyanoalkyl Phenanthridines

A number of γ-cyanoalkyl radicals were generated by sustainable N-heterocyclic carbene catalysis in tin-, transition-meal-, and light-free conditions, followed by insertion into biaryl isonitriles, thus leading to the rapid assembly of a variety of diversely functionalized 6-cyanoalkyl phenanthridines. A preliminary mechanism study revealed that a single-electron transfer radical process was possibly involved.

Redox-Neutral Generation of Iminyl Radicals by N-Heterocyclic Carbene Catalysis: Rapid Access to Phenanthridines from Vinyl Azides

An N-heterocyclic carbene-catalyzed oxidant-, metal- and light-free iminyl radical generation pathway stemming from the reaction of vinyl azide and α-bromo ester is uncovered. This newly developed methodology is successfully applied to the redox-neutral construction of a number of diversified phenanthridine derivatives with nice functional group compatibility. Insights from the mechanism study reveal that this NHC-catalyzed transformation potentially proceeds through an alkyl radical addition-initiated HAS process, with the iminyl radical as an active intermediate.

Construction of N-Polyheterocycles by N-Heterocyclic Carbene Catalysis via a Regioselective Intramolecular Radical Addition/Cyclization Cascade

N-Polyheterocycles are rapidly accessed by N-heterocyclic carbene (NHC) catalysis through regioselective sequential radical addition/cyclization in the absence of any metals or oxidants. The transformation occurs under mild conditions and enjoys a wide substrate scope with excellent functional group compatibility. Furthermore, a gram-scale synthesis is also conducted. Preliminary mechanistic studies reveal the potential involvement of an NHC radical cation intermediate.

The Photochemical Activity of a Halogen-Bonded Complex Enables the Microfluidic Light-Driven Alkylation of Phenols

A mild light-driven protocol for the direct alkylation of phenols is reported. The process is driven by the photochemical activity of a halogen-bonded complex formed upon complexation of the in situ generated electron-rich phenolate anion with the α-iodosulfone. The reaction proceeds rapidly (10 min) under microfluidic conditions, delivering a wide variety of ortho-alkylated products (27 examples, up to 97% yield, >20:1 regioselectivity, on a gram scale), including densely functionalized bioactive phenol derivatives.

Examining the Role of Aryldiazonium Salts in Surface Electroinitiated Polymerization

Historically, the irreversible reduction of aryldiazonium salts has provided a reliable method to modify surfaces, demonstrating a catalogue of suitable diazonium salts for targeted applications. This work expands the knowledge of diazonium salt chemistry to participate in surface electroinitiated emulsion polymerization (SEEP). The influence of concentration, electronic effects, and steric hindrance/regiochemistry of the diazonium salt initiator on the production of polymeric films is examined. The objective of this work is to determine if a polymer film can be tailored, controlling the thickness, density, and surface homogeneity using specific diazonium chemistry. The data presented herein demonstrate a significant difference in polymer films that can be achieved when selecting a variety of diazonium salts and vinylic monomers. A clear trend aligns with the electron-rich diazonium salt substitution providing the thickest films (up to 70.9 ± 17.8 nm) with increasing diazonium concentration and electron-withdrawing substitution achieving optimal homogeneity for the surface of the film at a 5 mM diazonium concentration.

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.

Multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds: fluorine as a detachable chemical handle

We highlighted the recent advances in the field of multiple-fold C–F bond functionalization for the synthesis of (hetero)cyclic compounds.

The synthesis of seven- and eight-membered rings by radical strategies

Radical strategies for preparation of seven- or eight-membered rings.

Planar Blatter radicals through Bu3SnH- and TMS3SiH-assisted cyclization of aryl iodides: azaphilic radical addition

Radical chain cyclization of aryl iodides provides an efficient synthesis of planar Blatter radicals, and, for the first time, access to functionalized sulphur-containing analogues.

Transition-metal-free oxindole synthesis: quinone-K2CO3 catalyzed intramolecular radical cyclization

A novel and highly efficient transition-metal-free approach for the conversion of α-bromoanilides to 3,3-disubstituted oxindoles is described. This transformation is promoted by catalytic amount of 9,10-phenanthrenequinone (PQ) together with K2CO3,...

Transition-Metal- and Light-Free Generation of an Iminyl Radical: Facile Approach to Oxindoles and Isoquinolinediones with a Quaternary Carbon Center via Cyanoalkylarylation

We have developed an efficient and non-toxic method for the environmental-friendly generation of an iminyl radical from cyclobutanone oxime ester via direct thermolysis in the absence of light, transition metals, "tin", and other activators. This redox-neutral cyanoalkylarylation protocol enjoys a wide substrate scope and a good functional group tolerance, providing facile access to oxindoles and isoquinolinediones with a quaternary carbon center that are difficult to prepare by traditional methods.

Radical chain monoalkylation of pyridines

The monoalkylation of N-methoxypyridinium salts with alkyl radicals generated from alkenes (via hydroboration with catecholborane), alkyl iodides (via iodine atom transfer) and xanthates is reported. The reaction proceeds under neutral conditions since no acid is needed to activate the heterocycle and no external oxidant is required. A rate constant for the addition of a primary radical to N-methoxylepidinium >10⁷ M^-1 s^-1 was experimentally determined. This rate constant is more than one order of magnitude larger than the one measured for the addition of primary alkyl radicals to protonated lepidine demonstrating the remarkable reactivity of methoxypyridinium salts towards radicals. The reaction has been used for the preparation of unique pyridinylated terpenoids and was extended to a three-component carbopyridinylation of electron-rich alkenes including enol esters, enol ethers and enamides.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

Real Metal-Free C–H Arylation of (Hetero)arenes: The Radical Way

Synthetic methodologies involving the formation of carbon–carbon bonds from carbon–hydrogen bonds are of significant synthetic interest, both for efficiency in terms of atom economy and for their undeniable usefulness in late-stage functionalization approaches. Combining these aspects with being metal-free, the radical C–H intermolecular arylation procedures covered by this review represent both powerful and green methods for the synthesis of (hetero)biaryl systems.

1 Introduction

2 Arylation with Arenediazonium Salts and Related Derivatives

2.1 Ascorbic Acid as the Reductant

2.2 Hydrazines as Reductants

2.3 Gallic Acid as the Reductant

2.4. Polyanilines as Reductants

2.5 Chlorpromazine Hydrochloride as the Reductant

2.6 Phenalenyl-Based Radicals as Reductants

2.7 Electrolytic Reduction of Diazonium Salts

2.8 Visible-Light-Mediated Arylation

3 Arylation with Arylhydrazines: Generation of Aryl Radicals Using an Oxidant

4 Arylation with Diaryliodonium Salts

5 Arylation with Aryl Halides

6 Conclusions

Construction of Quaternary Carbon Center via NHC Catalysis Initiated by an Intermolecular Heck-Type Alkyl Radical Addition

A quaternary carbon center containing an oxindole motif is constructed via NHC-catalyzed transition-metal and aldehyde-free intermolecular Heck-type alkyl radical addition initiated annulation. This redox-neutral protocol also features a simple procedure, broad substrate scope, good functional group tolerance and could be smoothly amplified to a gram scale. The mechanism study shows that the reaction possibly undergoes two folds of SET processes with an NHC radical cation intermediate involved.

Visible-Light-Induced Synthesis of 1,2,3,4-Tetrahydroquinolines through Formal [4+2] Cycloaddition of Acyclic α,β-Unsaturated Amides and Imides with N,N-Dialkylanilines

1,2,3,4-Tetrahydroquinolines should be applicable to the development of new pharmaceutical agents. A facile synthesis of 1,2,3,4-tetrahydroquinolines that is achieved by a photoinduced formal [4+2] cycloaddition reaction of acyclic α,β-unsaturated amides and imides with N,N-dialkylanilines under visible-light irradiation, in which a new Ir^III complex photosensitizer, a thiourea, and an oxidant act cooperatively in promoting the reaction, is reported. The photoreaction enables the synthesis of a wide variety of 1,2,3,4-tetrahydroquinolines, while controlling the trans/cis diastereoselectivity (>99:1) and constructing contiguous stereogenic centers. A chemoselective cleavage of an acyclic imide auxiliary is demonstrated.

Intramolecular Oxidative Coupling between Unactivated Aliphatic C-H and Aryl C-H Bonds

Direct oxidative coupling of different inert C-H bonds is the most straightforward and environmentally benign method to construct C-C bonds. In this paper, we developed a Pd-catalyzed intramolecular oxidative coupling between unactivated aliphatic and aryl C-H bonds. This chemistry showed great potential to build up fused cyclic scaffolds from linear substrates through oxidative couplings. Privileged chromane and tetralin scaffolds were constructed from readily available linear starting materials in the absence of any organohalides and organometallic partners.

Transition-Metal-Catalyzed Addition/Cyclization Reactions of the C-N Multiple Bonds Containing Species

This article describes our research work for the past decade, which involves the transition-metal-catalyzed cyclization reactions of the C-N multiple bonds containing species and their synthetic applications to access various heterocyclic compounds. The concepts of reactions including four types of coupling with a subsequent cyclization are (1) the transition-metal performs as a Lewis acid to activate a nitrile and accelerate the nucleophilic addition, (2) the transition-metal-catalyzed 1,2-insertion reaction of nitrile, (3) the Cu-catalyzed C-N coupling reaction of imine, and (4) the Co-catalyzed addition/cyclization reaction of imine. These methods can be used to synthesize various N-containing aromatic heterocycles with higher efficiency, and can be applied to the synthesis of relevent natural alkaloids, their derivatives as well as biologically active compounds.

NHC-catalyzed oxindole synthesis via single electron transfer

A NHC-catalyzed metal-free oxindole synthesis method is developed.

Installing the “magic methyl” – C–H methylation in synthesis

Following notable cases of remarkable potency increases in methylated analogues of lead compounds, this review documents the state-of-the-art in C–H methylation technology.

Light-driven, heterogeneous organocatalysts for C-C bond formation toward valuable perfluoroalkylated intermediates

The favorable exploitation of carbon nitride (CN) materials in photocatalysis for organic synthesis requires the appropriate fine-tuning of the CN structure. Here, we present a deep investigation of the structure/activity relationship of CN in the photocatalytic perfluoroalkylation of organic compounds. Four types of CN bearing subtle structural differences were studied via conventional characterization techniques and innovative nuclear magnetic resonance (NMR) experiments, correlating the different structures with the fundamental mechanistic nexus and especially highlighting the importance of the halogen bond strength between the reagent and the catalyst surface. The optimum catalyst exhibited an excellent performance, with a very wide reaction scope, and could prominently trigger the model reaction using natural sunlight. The work lays a platform for establishing a new approach in the development of heterogeneous photocatalysts for organic synthesis related to medical, agricultural, and material chemistry.

Electrochemical oxidative decarboxylation and 1,2-aryl migration towards the synthesis of 1,2-diaryl ethers

Carboxylic acid compounds are important chemicals and are widely present in various natural products. They are not only nucleophiles, but also radical precursors. Classic transition-metal-catalyzed and photochemical decarboxylation have shown their excellent site selectivity in radical chemistry. However, electrochemical decarboxylation with a long history hasn't got enough attention in recent years. In this work, the electrochemical oxidative decarboxylation and 1,2-aryl migration of 3,3-diarylpropionic acids have been introduced to construct C–O bonds with alcohols. Remarkably, this transformation can proceed smoothly without metal catalysts and external oxidants.

Carboxylic acid compounds are important chemicals and are widely present in various natural products. The conversion of carboxylic acids into valuable compounds is a promising field.

NaH-mediated direct C-H arylation in the presence of 1,10-phenanthroline

Transition-metal-free coupling of haloarenes with unactivated arenes has been developed in the presence of NaH and 1,10-phenanthroline. Various haloarenes bearing methyl, methoxy, halogen (fluoride, chloride, and bromide), cyano, trifluoromethyl, ester, and amide groups can be cross-coupled with unactivated arenes, or heteroarenes in this reaction.

Site-Selective Thiolation of (Multi)halogenated Heteroarenes

A general and simple strategy for the site-selective thiolation of various pharmaceutically relevant electron-rich heteroarenes with thiols is reported. This mild and reliable photocatalytic protocol enables C-S coupling at the most electron-rich position of the (multi)halogenated substrates, complementing established methodologies. Experimental and computational studies suggest a radical chain mechanism with the key step being a homolytic aromatic substitution of the heteroaryl halide by an electrophilic thiyl radical, highlighting an underdeveloped reactivity mode.

K2S2O8 mediated C-3 arylation of quinoxalin-2(1H)-ones under metal-, photocatalyst- and light-free conditions

Two facile and effective C-3 arylation protocols of quinoxalin-2(1H)-ones with arylhydrazines and aryl boronic acids respectively via free radical cross-coupling reactions under metal-, photocatalyst- and light-free conditions have been unveiled. K2S2O8 has been used as an efficient oxidant to generate aryl radicals from arylhydrazines and aryl boronic acids under two different reaction conditions. The generated aryl radicals undergo a free radical coupling reaction at the C-3 position of quinoxalin-2(1H)-ones producing 3-arylquinoxalin-2(1H)-ones in good to excellent yields. The involvement of radicals in the course of the reaction has been demonstrated by radical trapping experiments with 2,2,6,6-tetramethylpiperidine-1-oxyl.

Copper-catalyzed C–H [3 + 2] annulation of N-substituted anilines with α-carbonyl alkyl bromides via C(sp3)–Br/C(sp2)–H functionalization

A copper-catalyzed C–H [3 + 2] annulation of N-substituted anilines with α-carbonyl alkyl bromides for the synthesis of 3,3′-disubstituted oxindoles is developed.

Use of Nitrogen-Doped Carbon Nanodots for the Photocatalytic Fluoroalkylation of Organic Compounds

The use of amine-rich N-doped carbon nanodots (NCNDs) for the photochemical radical perfluoroalkylation of organic compounds is reported. This operationally simple approach occurs under mild conditions producing valuable new C-C bonds. The chemistry is driven by the ability of NCNDs to directly reach an electronically excited state upon light absorption, thereby successively triggering the formation of reactive radical species from simple perfluoroalkyl iodides. Preliminary mechanistic studies are also reported.