Transition-metal-free α-arylation of oxindoles via visible-light-promoted electron transfer

Catalyst- and additive-free cascade radical addition/cyclization of N-arylacrylamides with trifluoropyruvates

Herein, we introduce a photocatalyst- and additive-free method for the preparation of valuable 3,3-disubstituted oxindoles bearing trifluoromethyl alcohol moieties from readily available acrylamides and cheap trifluoropyruvates. The excited trifluoropyruvates under ultraviolet-light irradiation react efficiently with acrylamides delivering a variety of trifluoromethyl oxindoles with broad functional group tolerance and moderate to good yields. This protocol features mild reaction conditions, simple operation and ready scalability.

Photo-induced imino functionalizations of alkenes via intermolecular charge transfer

A catalyst-free photosensitized strategy has been developed for regioselective imino functionalizations of alkenes via the formation of an EDA complex. This photo-induced protocol facilitates the construction of structurally diverse β-imino sulfones and vinyl sulfones in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with an intermolecular charge transfer between oximes and sulfinates, followed by fragmentation to generate a persistent iminyl radical and transient sulfonyl radical. This catalyst-free protocol also features excellent regioselectivity, broad functional group tolerance and mild reaction conditions. The late stage functionalization of natural product derived compounds and total synthesis of some bioactive molecules have been demonstrated to highlight the utility of this protocol. Meanwhile, the compatibility of different donors has proved the generality of this strategy.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

Shining light on halogen-bonding complexes: a catalyst-free activation mode of carbon-halogen bonds for the generation of carbon-centered radicals

The discovery of new activation modes for the creation of carbon-centered radicals is a task of great interest in organic chemistry. Classical activation modes for the generation of highly reactive radical carbon-centered intermediates typically relied on thermal activation of radical initiators or irradiation with unsafe energetic UV light of adequate reaction precursors. In recent years, photoredox chemistry has emerged as a leading strategy towards the catalytic generation of C-centered radicals, which enabled their participation in novel synthetic organic transformations which is otherwise very challenging or even impossible to take place. As an alternative to these activation modes for the generation of C-centered radicals, the pursuit of greener, visible-light initiated reactions that do not necessitate a photoredox/metal catalyst has recently caught the attention of chemists. In this review, we covered recent transformations, which rely on photoactivation with low-energy light of a class of EDA complexes, known as halogen-bonding adducts, for the creation of C-centered radicals.

Enforced Electronic-Donor-Acceptor Complex Formation in Water for Photochemical Cross-Coupling

The amino alcohol meglumine solubilizes organic compounds in water and enforces the formation of electron donor acceptor (EDA) complexes of haloarenes with indoles, anilines, anisoles or thiols, which are not observed in organic solvents. UV-A photoinduced electron transfer within the EDA complexes induces the mesolytic cleavage of the halide ion and radical recombination of the arenes leading, after rearomatization and proton loss to C-C or C-S coupling products. Depending on the substitution pattern selective and unique cross-couplings are observed. UV and NMR measurements reveal the importance of the assembly for the photoinduced reaction. Enforced EDA aggregate formation in water allows new activation modes for organic photochemical synthesis.

1,3,2‐Diazaphospholene‐Catalyzed Reductive Cyclizations of Organohalides**

1,3,2‐diazaphospholenes hydrides (DAP‐Hs) are highly nucleophilic organic hydrides serving as main‐group catalysts for a range of attractive transformations. DAP hydrides can act as stoichiometric hydrogen atom transfer agents in radical reactions. Herein, we report a DAP‐catalyzed reductive radical cyclization of a broad range of aryl and alkyl halides under mild conditions. The pivotal DAP catalyst turnover was achieved by a DBU‐assisted σ‐bond metathesis between the formed DAP halide and HBpin, which rapidly regenerates DAP‐H. The transformation is significantly accelerated by irradiation with visible light. Mechanistic investigations indicate that visible light irradiation leads to the formation of DAP dimers, which are in equilibrium with DAP radicals and accelerate the cyclization. The direct use of (DAP)₂ enabled a catalytic protocol in the absence of light.

Transition metal-free functionalization of 2-oxindoles via sequential aldol and reductive aldol reactions using rongalite as a C1 reagent

A sequential one-pot classical aldol, transition-metal and hydride-free reductive aldol reaction is reported here for C(sp3)- H functionalization of 2-oxindoles using the multifaceted reagent rongalite. Here, rongalite functions as a hydride-free reducing agent and double C1 unit donor. This protocol enables the synthesis of a wide range of 3-methylindoline-2-ones and 3-(hydroxymethyl)-3-methylindolin-2-ones from 2-oxindoles (65-95% yields), which are the synthetic precursors for many natural products. Some of the important aspects of this synthetic approach include one-pot methylation and hydroxymethylation, low-cost rongalite (ca. $0.03 per 1 g), mild reaction conditions and applicability to gram-scale synthesis.

Oxindole synthesis via polar-radical crossover of ketene-derived amide enolates in a formal [3 + 2] cycloaddition

Herein we introduce a simple, efficient and transition-metal free method for the preparation of valuable and sterically hindered 3,3-disubstituted oxindoles via polar-radical crossover of ketene derived amide enolates. Various easily accessible N-alkyl and N-arylanilines are added to disubstituted ketenes and the resulting amide enolates undergo upon single electron transfer oxidation a homolytic aromatic substitution (HAS) to provide 3,3-disubstituted oxindoles in good to excellent yields. A variety of substituted anilines and a 3-amino pyridine engage in this oxidative formal [3 + 2] cycloaddition and cyclic ketenes provide spirooxindoles. Both substrates and reagents are readily available and tolerance to functional groups is broad.

Redox-neutral access to 3,3′-disubstituted oxindoles via radical coupling reactions

An efficient radical protocol was developed for the assembly of valuable functional oxindoles in the absence of metals, photocatalysts, and bases.

Electrochemical Umpolung C-H Functionalization of Oxindoles

Herein, we present a general electrochemical method to access unsymmetrical 3,3-disubstituted oxindoles by direct C–H functionalization where the oxindole fragment behaves as an electrophile. This Umpolung approach does not rely on stoichiometric oxidants and proceeds under mild, environmentally benign conditions. Importantly, it enables the functionalization of these scaffolds through C–O, and by extension to C–C or even C–N bond formation.

Iron-Catalyzed Oxidative Cross-Coupling of Phenols and Tyrosine Derivatives with 3-Alkyloxindoles

In this study, a novel iron-catalyzed oxidative cross-coupling reaction between phenols and 3-alkyloxindole derivatives is reported. The efficient method, which is based on the FeCl₃ catalyst and the t-BuOOt-Bu oxidant in 1,2-dichloroethane at 70 °C, affords 3-alkyl-3-(hydroxyaryl)oxindole compounds with a high degree of selectivity. The generality of the conditions was proven by reacting various substituted phenols, naphthols, and tyrosine derivatives with 3-alkyloxindoles. To apply the chemistry for the conjugation of tyrosine-containing short peptides with oxindolylalanine (Oia) derivatives, the reaction conditions were modified [Fe(O₂CCF₃)₃ catalyst, t-BuOOt-Bu, HFIP, 70 °C], and amino acids with acid-stable N-protecting groups were used.

Recent Advances in the Transition-Metal-Free Arylation of Hetero­arenes

Transition-metal-free arylation reactions have attracted considerable attention for economic and environmental reasons over the past 40 years. In recent years, much effort has been made to develop efficient transition-metal-free approaches for the arylation of heteroarenes. Covering the literature from 2015 to early 2021, this review aims to provide a thorough overview of the synthetic and mechanistic aspects of these atom-economical and environmentally benign reactions.

1 Introduction

2 Arylation of Pre-functionalized Heteroarenes

2.1 Arylation of Heteroaryl Halides

2.2 Decarboxylative Arylation of Heteroarenes

3 Direct C–H Arylation of Heteroarenes

3.1 C(sp2)–H Arylation

3.2 C(sp3)–H Arylation

4 N-Arylation of Heteroarenes

5 Summary and Outlook

Construction of sterically congested oxindole derivatives via visible-light-induced radical-coupling

The oxindole scaffold represents an important structural feature in many natural products and pharmaceutically relevant molecules. Herein, we report a visible-light-induced modular methodology for the synthesis of complex 3,3'-disubstituted oxindole derivatives. A library of valuable fluoroalkyl-containing highly sterically congested oxindole derivatives can be synthesized by a catalytic three-component radical coupling reaction under mild conditions (metal & photocatalyst free, >80 examples). This strategy shows high functional group tolerance and broad substrate compatibility (including a wide variety of terminal or non-terminal alkenes, conjugated dienes and enynes, and a broad array of polyfluoroalkyl iodide and oxindoles), which enables modular modification of complex drug-like compounds in one chemical step. The success of solar-driven transformation, large-scale synthesis, and the late-stage functionalization of bioactive molecules, as well as promising tumor-suppressing biological activities, highlights the potential for practical applications of this strategy. Mechanistic investigations, including a series of control experiments, UV-vis spectroscopy and DFT calculations, suggest that the reaction underwent a sequential two-step radical-coupling process and the photosensitive perfluoroalkyl benzyl iodides are key intermediates in the transformation.

An Efficient Approach for 3,3-Disubstituted Oxindoles Synthesis: Aryl Iodine Catalyzed Intramolecular C-N Bond Oxidative Cross-Coupling

Herein, we report the first intramolecular C-N bond formation of phenylpropanamide derivatives via organocatalytic oxidative reactions, affording 3,3-disubstituted oxindole derivatives with up to 99% yield. The high efficiency of this reaction is exemplified by the transition metal-free mild conditions and the ability to perform the reaction on a gram scale. Meanwhile, the DFT calculation of the catalytic oxidative transformation pathway has also been studied.

Enhancing the Potential of Miniature-Scale DNA-Compatible Radical Reactions via an Electron Donor-Acceptor Complex and a Reversible Adsorption to Solid Support Strategy

DNA-encoded library (DEL) technology is a powerful tool in the discovery of bioactive probe molecules and drug leads. Mostly, the success in DEL technology stems from the molecular diversity of the chemical libraries. However, the construction of DELs has been restricted by the idiosyncratic needs and the required low concentration (∼1 mM or less) of the library intermediate. Here, we report visible-light-promoted on-DNA radical coupling reactions via an electron donor-acceptor (EDA) complex and a reversible adsorption to solid support (RASS) strategy. This protocol provides a unique solution to the challenges of increasing the reactivity of highly diluted DNA substrates and reducing the residues of heavy metals from photocatalysts. A series of on-DNA indole sulfone and selenide derivatives were obtained with good to quantitative conversions. It is anticipated that these mild-condition on-DNA radical reactions will significantly improve the chemical diversity of DELs and find widespread utility to DEL construction.

B(C6F5)3-Catalyzed Electron Donor-Acceptor Complex-Mediated Aerobic Sulfenylation of Indoles under Visible-Light Conditions

An efficient B(C₆F₅)₃-catalyzed aerobic oxidative C-S cross-coupling reaction of thiophenol with indoles was developed, affording a wide range of diaryl sulfides in good yields. An electron donor-acceptor complex between B(C₆F₅)₃ and indoles was formed, facilitating the photoinduced single-electron transfer (SET) from indole substrates to the B(C₆F₅)₃ catalyst. This protocol demonstrates a new reaction model using B(C₆F₅)₃ as a single-electron oxidant.

α-C(sp3)-H Arylation of Cyclic Carbonyl Compounds

α-C(sp3)-H arylation is an important type of C-H functionalization. Various biologically significant natural products, chemical intermediates, and drugs have been effectively prepared via C-H functionalization. Cyclic carbonyl compounds comprise of cyclic ketones, enones, lactones, and lactams. The α-C(sp3)-H arylation of these compounds have been exhibited high efficiency in forming C(sp3)-C(sp2) bonds, played a crucial role in organic synthesis, and attracted majority of interests from organic and medicinal communities. This review focused on the most significant advances including methods, mechanism, and applications in total synthesis of natural products in the field of α-C(sp3)-H arylations of cyclic carbonyl compounds in recent years.

Visible-Light-Driven C-S Bond Formation Based on Electron Donor-Acceptor Excitation and Hydrogen Atom Transfer Combined System

Developed herein is a visible-light-driven synthesis of sulfides by an electron donor-acceptor/single electron transfer and hydrogen atom transfer combined system without transition metals and strong oxidants. This reaction proceeds through the excitation of an electron donor-acceptor complex between a thiolate and an aryl halide, followed by the hydrogen atom transfer from an alkane to the generated aryl radical.

Synthetic reactions driven by electron-donor-acceptor (EDA) complexes

The reversible, weak ground-state aggregate formed by dipole-dipole interactions between an electron donor and an electron acceptor is referred to as an electron-donor-acceptor (EDA) complex. Generally, upon light irradiation, the EDA complex turns into the excited state, causing an electron transfer to give radicals and to initiate subsequent reactions. Besides light as an external energy source, reactions involving the participation of EDA complexes are mild, obviating transition metal catalysts or photosensitizers in the majority of cases and are in line with the theme of green chemistry. This review discusses the synthetic reactions concerned with EDA complexes as well as the mechanisms that have been shown over the past five years.

Photocatalyst- and Transition-Metal-Free Visible-Light-Promoted Intramolecular C(sp2)-S Formation

A photocatalyst- and transition-metal-free visible-light-induced cyclization of ortho-halothiobenzanilides has been developed. Upon irradiation with visible light, substrates undergo dehalogenative cyclization to 2-aryl benzothiazoles with high efficiency and selectivity. This photocyclization exhibits a high tolerance to various functional groups, is applicable for the synthesis of 2-alkyl benzothiazoles, and is easy to set up for gram-scale reaction.

Contemporary methods for generation of aryl radicals

The synthetic utility of aryl radicals has been established in the last century, however, their broad applications were hampered by ineffective generation methods. It was in the last decade, that a rapid development of various redox systems took place, thus triggering a renaissance of aryl radical chemistry. This tutorial review focuses on the start-of-the-art methods for generation of aryl radicals. Primarily, various light-induced systems, including photoredox catalysis, visible light transition metal catalysis, and chemistry of electron donor-acceptor complexes, are reviewed. The main current precursors of aryl radicals are evaluated together with the selected examples of their modern applications.

A transition-metal-free, base-promoted annulation/ring-cleavage/ring-reconstruction cascade reaction: a facile access to N-protection free indole-indenones

An unprecedented base-promoted reaction of 2-halogenated arylglyoxals with 2-oxindoles is accomplished under metal-free conditions, furnishing a wide range of biologically important (NH)-indeno[2,1-b]indol-6(5H)-ones in useful to good yields.

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Hantzsch Ester-Mediated Synthesis of Phenanthridines under Visible-Light Irradiation

An efficient photocatalytic synthesis of phenanthridines mediated by an organo-photoredox initiator Hantzsch ester has been developed via denitrogenative intramolecular annulation of benzotriazolyl chalcones. The highly reducing photoactivated Hantzsch ester facilitates the transformation of benzotriazolyl chalcones into phenanthridinyl chalcones through photoinduced electron transfer (PET) and hydrogen atom transfer (HAT) processes. The mild reaction conditions utilizing inexpensive Hantzsch ester as photosensitizer, wide reaction scope and excellent functional group tolerance are notable attributes of the methodology.

A light- and heat-driven glycal diazidation approach to nitrogenous carbohydrate derivatives with antiviral activity

The aminated mimetics of 2-keto-3-deoxy-sugar acids such as the anti-influenza clinical drugs oseltamivir (Tamiflu) and zanamivir (Relenza) are important bioactive molecules. Development of synthetic methodologies for accessing such compound collections is highly desirable. Herein, we describe a simple, catalyst-free glycal diazidation protocol enabled by visible light-driven conditions. This new method requires neither acid promoters nor transition-metal catalysts and takes place at ambient temperature within 1-2 hours. Notably, the desired transformations could be promoted by thermal conditions as well, albeit with lower efficacy compared to the light-induced conditions. Different sugar acid-derived glycal templates have been converted into a range of 2,3-diazido carbohydrate analogs by harnessing this mild and scalable approach, leading to the discovery of new antiviral agents.

Synthetic Methods Driven by the Photoactivity of Electron Donor-Acceptor Complexes

The association of an electron-rich substrate with an electron-accepting molecule can generate a new molecular aggregate in the ground state, called an electron donor-acceptor (EDA) complex. Even when the two precursors do not absorb visible light, the resulting EDA complex often does. In 1952, Mulliken proposed a quantum-mechanical theory to rationalize the formation of such colored EDA complexes. However, and besides a few pioneering studies in the 20th century, it is only in the past few years that the EDA complex photochemistry has been recognized as a powerful strategy for expanding the potential of visible-light-driven radical synthetic chemistry. Here, we explain why this photochemical synthetic approach was overlooked for so long. We critically discuss the historical context, scientific reasons, serendipitous observations, and landmark discoveries that were essential for progress in the field. We also outline future directions and identify the key advances that are needed to fully exploit the potential of the EDA complex photochemistry.

Redox-neutral photochemical Heck-type arylation of vinylphenols activated by visible light

Disclosed herein is a photochemical Heck-type arylation of vinylphenols with non-activated aryl and heteroaryl halides under visible light irradiation. Preliminary mechanistic studies suggested that the colored vinylphenolate anions acted as a strong reducing photoactivator to directly activate (hetero)aryl halides without the need for any sacrificial reductants. The photochemically generated aryl radicals coupled with another molecule of vinylphenol to afford the Heck-type arylation product in a regiospecific and stereoselective manner. The developed photochemical arylation protocol showed exceptional functional group tolerance and was successfully applied in the challenging late-stage modification of natural products without any protection-deprotection procedures.

Photoinduced Deoxygenative Borylations of Aliphatic Alcohols

A photochemical method for converting aliphatic alcohols into boronic esters is described. Preactivation of the alcohol as a 2-iodophenyl-thionocarbonate enables a novel Barton-McCombie-type radical deoxygenation that proceeds efficiently with visible light irradiation and without the requirement for a photocatalyst, a radical initiator, or tin or silicon hydrides. The resultant alkyl radical is intercepted by bis(catecholato)diboron, furnishing boronic esters from a diverse range of structurally complex alcohols.

Visible-light-induced intramolecular charge transfer in the radical spirocyclisation of indole-tethered ynones

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols. This initiates a novel radical chain sequence, based on dearomatising spirocyclisation with concomitant C–S bond formation. Sulfur-containing spirocycles are formed in high yields using this simple and mild synthetic protocol, in which neither transition metal catalysts nor photocatalysts are required. The proposed mechanism is supported by various mechanistic studies, and the unusual radical initiation mode represents only the second report of the use of an intramolecular electron donor–acceptor complex in synthesis.

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols.