Photocatalytic Generation of 2-Azolyl Radicals: Intermediates for the Azolylation of Arenes and Heteroarenes via C–H Functionalization

Generation and Application of Aryl Radicals Under Photoinduced Conditions

Photoinduced aryl radical generation is a powerful strategy in organic synthesis that facilitates the formation of diverse carbon-carbon and carbon-heteroatom bonds. The synthetic applications of photoinduced aryl radical formation in the synthesis of complex organic compounds, including natural products, physiologically significant molecules, and functional materials, have received immense attention. An overview of current developments in photoinduced aryl radical production methods and their uses in organic synthesis is given in this article. A generalized idea of how to choose the reagents and approach for the generation of aryl radicals is described, along with photoinduced techniques and associated mechanistic insights. Overall, this article offers a critical assessment of the mechanistic results as well as the selection of reaction parameters for specific reagents in the context of radical cascades, cross-coupling reactions, aryl radical functionalization, and selective C-H functionalization of aryl substrates.

N-Iodosuccinimide-Mediated Synthesis of Benzo-Fused Bisimidazoles Enabled by a One-Pot Tandem Reaction of Fluorinated Propargyl Amidines

A N-iodosuccinimide (NIS)-mediated divergent and efficient tandem reaction between fluorinated propargyl amidines and aromatic o-diamines without any metal catalyst and additive under mild reaction conditions was developed for the synthesis of benzo-fused bisimidazoles in moderate to excellent yields. Preliminary mechanistic studies suggested that this reaction proceeded by an intermediate of secondary amine derived from 5-iodomethyl imidazole, and NIS played another role of oxidation reagent to promote the formation of a benzimidazole motif.

Construction of bi(hetero)aryls via dicyanopyrazine-mediated photochemical cross-coupling

A photochemical cross-coupling protocol towards bi(hetero)aryls has been developed. The coupling reactions were mediated by dicyanopyrazine photoredox catalyst, while a photoinduced disproportionation process has been identified as an accompanying mechanism, especially for pyrrole derivatives. The developed method allows the cross-coupling of five-membered rings such as pyrrole, imidazole, thiazole and oxazole as well as various diazines (pyridine and pyrimidine) and benzene derivatives. A plausible mechanism of the reaction has also been disclosed. The practical application and relevance of the developed method were demonstrated by constructing an atorvastatin core or by the gradual functionalization of benzo[c][1,2,5]thiadiazole. In total, twenty-one bi(hetero)aryls were prepared in yields ranging from 19 to 95%.

Photoactive Copper Complexes: Properties and Applications

Photocatalyzed and photosensitized chemical processes have seen growing interest recently and have become among the most active areas of chemical research, notably due to their applications in fields such as medicine, chemical synthesis, material science or environmental chemistry. Among all homogeneous catalytic systems reported to date, photoactive copper(I) complexes have been shown to be especially attractive, not only as alternative to noble metal complexes, and have been extensively studied and utilized recently. They are at the core of this review article which is divided into two main sections. The first one focuses on an exhaustive and comprehensive overview of the structural, photophysical and electrochemical properties of mononuclear copper(I) complexes, typical examples highlighting the most critical structural parameters and their impact on the properties being presented to enlighten future design of photoactive copper(I) complexes. The second section is devoted to their main areas of application (photoredox catalysis of organic reactions and polymerization, hydrogen production, photoreduction of carbon dioxide and dye-sensitized solar cells), illustrating their progression from early systems to the current state-of-the-art and showcasing how some limitations of photoactive copper(I) complexes can be overcome with their high versatility.

Palladium Metallaphotoredox-Catalyzed 2-Arylation of Indole Derivatives

Given that biaryl motifs are found in many useful molecules, including pesticides, pharmaceuticals, functional materials, and polymers, the development of methods for their construction is important. Herein, we report a two-step method for C(sp²)-H/C(sp²)-H cross-coupling reactions to synthesize 2-arylindole derivatives by combining palladium catalysis and photocatalysis. This mild, dual-catalysis method showed good functional group tolerance and a wide substrate scope and could be used for late-stage functionalization of oligopeptides, drugs, and natural products.

Ionic Reactivity of 2-Isocyanoaryl Thioethers: Access to 2-Halo and 2-Aminobenzothia/Selenazoles

An ionic cascade insertion/cyclization reaction of thia-/selena-functionalized arylisocyanides has been successfully developed for the efficient and practical synthesis of 2-halobenzothiazole/benzoselenazole derivatives. This synthetic protocol, incorporating a halogen atom when forming the five-membered ring of benzothia/selenazoles, is different from the existing ones, where halogenation of the preformed benzothia/selenazole precursors happens. Additionally, a facile access to 2-aminobenzothiazoles is also achieved by the one-pot cascade reaction of 2-isocyanoaryl thioethers, iodine, and amines.

Photochemical C-H arylation of heteroarenes for DNA-encoded library synthesis

DNA-encoded library (DEL) technology has emerged as a time- and cost-efficient technique for the identification of therapeutic candidates in the pharmaceutical industry. Although several reaction classes have been successfully validated in DEL environments, there remains a paucity of DNA-compatible reactions that harness building blocks (BBs) from readily available substructures bearing multifunctional handles for further library diversification under mild, dilute, and aqueous conditions. In this study, the direct C-H carbofunctionalization of medicinally-relevant heteroarenes can be accomplished via the photoreduction of DNA-conjugated (hetero)aryl halides to deliver reactive aryl radical intermediates in a regulated fashion within minutes of blue light illumination. A broad array of electron-rich and electron-poor heteroarene scaffolds undergo transformation in the presence of sensitive functional groups.

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.

Selective C3–H nitration of 2-sulfanilamidopyridines with tert-butyl nitrite

A selective C3–H nitration of bioactive 2-sulfanilamidopyridine derivatives, including corticosteroid 11-β-dehydrogenase isozyme, secretory phospholipase A2 inhibitor and human neutrophil elastase inhibitor, has been reported.

Product selectivity in the photoreaction of aryl sulfonates and mesylate of estrone derivatives in sustainable and micellar media: a steady-state investigation

The photochemical reaction of sulfonate steroids in a sustainable environment was carried out successfully under steady-state conditions. Significant selectivity in photoproduct formation was also observed.

Photoredox-mediated hydroalkylation and hydroarylation of functionalized olefins for DNA-encoded library synthesis

DNA-encoded library (DEL) technology features a time- and cost-effective interrogation format for the discovery of therapeutic candidates in the pharmaceutical industry. To develop DEL platforms, the implementation of water-compatible transformations that facilitate the incorporation of multifunctional building blocks (BBs) with high C(sp3) carbon counts is integral for success. In this report, a decarboxylative-based hydroalkylation of DNA-conjugated trifluoromethyl-substituted alkenes enabled by single-electron transfer (SET) and subsequent hydrogen atom termination through electron donor-acceptor (EDA) complex activation is detailed. In a further photoredox-catalyzed hydroarylation protocol, the coupling of functionalized, electronically unbiased olefins is achieved under air and within minutes of blue light irradiation through the intermediacy of reactive (hetero)aryl radical species with full retention of the DNA tag integrity. Notably, these processes operate under mild reaction conditions, furnishing complex structural scaffolds with a high density of pendant functional groups.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Photoinduced Hydrocarboxylation via Thiol-Catalyzed Delivery of Formate Across Activated Alkenes

Herein we disclose a new photochemical process to prepare carboxylic acids from formate salts and alkenes. This redox-neutral hydrocarboxylation proceeds in high yields across diverse functionalized alkene substrates with excellent regioselectivity. This operationally simple procedure can be readily scaled in batch at low photocatalyst loading (0.01% photocatalyst). Furthermore, this new reaction can leverage commercially available formate carbon isotologues to enable the direct synthesis of isotopically labeled carboxylic acids. Mechanistic studies support the working model involving a thiol-catalyzed radical chain process wherein the atoms from formate are delivered across the alkene substrate via CO₂^•- as a key reactive intermediate.

Selective photoredox direct arylations of aryl bromides in water in a microfluidic reactor

Carrying out photoredox direct arylation couplings between aryl halides and aryls in aqueous solutions of surfactants enables unprecedented selectivity with respect to the competing dehalogenation process, thanks to the partition coefficient of the selected sacrificial base. The use of a microfluidic reactor dramatically improves the reaction time, without eroding the yields and selectivity. The design of a metal free sensitizer, which also acts as the surfactant, sizeably improves the overall sustainability of arylation reactions and obviates the need for troublesome purification from traces of metal catalysts. The generality of the method is investigated over a range of halides carrying a selection of electron withdrawing and electron donating substituents.

Coupling Photocatalysis and Substitution Chemistry to Expand and Normalize Redox-Active Halides

Photocatalysis can generate radicals in a controlled fashion and has become an important synthetic strategy. However, limitations due to the reducibility of alkyl halides prevent their broader implementation. Herein we explore the use of nucleophiles that can substitute the halide and serve as an electron capture motif that normalize the variable redox potentials across substrates. When used with photocatalysis, bench-stable, commercially available collidinium salts prove to be excellent radical precursors with a broad scope.

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.

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Catalyst- and Additive-Free Method for the Synthesis of 2-Substituted Benzothiazoles from Aromatic Amines, Aliphatic Amines, and Elemental Sulfur

Under catalyst- and additive-free conditions, a novel, convenient, environmentally friendly method was developed for the synthesis of 2-substituted benzothiazoles via the three-component one pot reaction from aromatic amines, aliphatic amines, and elemental sulfur. The reaction achieves double C-S and one C-N bond formations via cleavage of two C-N bonds and multiple C-H bonds. Furthermore, the mechanism research shows that DMSO acts as an oxidant in the cyclization reaction.

Photochemical Behavior of Some Estrone Aryl and Methyl Sulfonates in Solution: Preparative and Mechanistic Studies

Direct irradiation of estrone aryl and methyl sulfonates in different organic solvents under nitrogen atmosphere was investigated under steady-state conditions. The estrone derivatives reacted efficiently through the photo-Fries rearrangement reaction involving [1;3]-sulfonyl migration providing the ortho-sulfonyl estrone derivatives and estrone as the photoproducts. In addition, estrone and 2-arylsulfonyl estrone derivatives were epimerized involving a Norrish Type-I reaction. Chemical quenching and photosensitization experiments on the photoreaction have been also carried out to establish the photoreactive excited state. Likewise, the solvent effect and the nature of the sulfonyl group on the photoreactions have been also studied.

Intermolecular Reactions of Pyridyl Radicals with Olefins via Photoredox Catalysis

Pyridines are valuable motifs in a number of bioactive and functional molecules. The chemoselective functionalization of these structures from stable and widely available starting materials is a meaningful goal. We have demonstrated selective formation of pyridyl radicals at any position (2-, 3-, 4-pyridyl), through the action of a reducing photoredox catalyst. These radicals readily engage alkenes to deliver high-value products. Alteration of the reaction medium has enabled the use of a diverse range of alkene subtypes in a highly divergent and chemoselective manner.

Utilising excited state organic anions for photoredox catalysis: activation of (hetero)aryl chlorides by visible light-absorbing 9-anthrolate anions

A novel photocatalytic concept based on photoexcitation of an organic anionic ground state catalyst for direct C–H (het)arylations using (het)aryl chlorides.

Radical Hydroarylation of Functionalized Olefins and Mechanistic Investigation of Photocatalytic Pyridyl Radical Reactions

We report the photoredox alkylation of halopyridines using functionalized alkene and alkyne building blocks. Selective single-electron reduction of the halogenated pyridines provides the corresponding heteroaryl radicals, which undergo anti-Markovnikov addition to the alkene substrates. The system is shown to be mild and tolerant of a variety of alkene and alkyne subtypes. A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.

Photoredox Catalysis for Building C-C Bonds from C(sp2)-H Bonds

Transition metal-catalyzed C-H bond functionalizations have been the focus of intensive research over the last decades for the formation of C-C bonds from unfunctionalized arenes, heteroarenes, alkenes. These direct transformations provide new approaches in synthesis with high atom- and step-economy compared to the traditional catalytic cross-coupling reactions. However, such methods still suffer from several limitations including functional group tolerance and the lack of regioselectivity. In addition, they often require harsh reaction conditions and some of them need the use of strong oxidant, in a stoichiometric amount, avoiding these processes to be truly eco-friendly. The use of photoredox catalysis has contributed to a significant expansion of the scope of C(sp²)-H bond functionalizations which include the direct arylations, (perfluoro)alkylations, acylations, and even cyanations. Most of these transformations involve the photochemical induced generation of a radical followed by its regioselective addition to arenes, heteroarenes, or alkenes, leading to the building of a new C(sp²)-C bond. The use of photoredox catalysis plays crucial roles in these reactions promoting electron transfer, enabling the generation of radical species and single electron either oxidation or reduction. Such reactions operating at room temperature allow the building of C-C bonds with high chemo-, regio-, or stereoselectivity. This review surveys the formation of C(sp²)-C bonds initiated by photoredox catalysis which involves a C(sp²)-H bond functionalization step, describes the advantages compared to traditional C(sp²)-H bond functionalizations, and presents mechanistic insights into the role played by the photoredox catalysts.

A General Copper Catalyst for Photoredox Transformations of Organic Halides

A broadly applicable copper catalyst for photoredox transformations of organic halides is reported. Upon visible light irradiation in the presence of catalytic amounts of [(DPEphos)(bcp)Cu]PF₆ and an amine, a range of unactivated aryl and alkyl halides were shown to be smoothly activated through a rare Cu(I)/Cu(I)*/Cu(0) catalytic cycle. This complex efficiently catalyzes a series of radical processes, including reductions, cyclizations, and direct arylation of arenes.

Synthesis of Arylated Nucleobases by Visible Light Photoredox Catalysis

Arylated nucleobases were synthesized by visible light photocatalysis using rhodamine 6G as photoredox catalyst and N,N-diisopropylethylamine as sacrificial electron donor. The high redox potential of this catalyst system is achieved by a consecutive photoinduced electron transfer process (conPET) and allows the room temperature conversion of brominated and chlorinated nucleobases or nucleobase precursors as starting materials. In contrast to many transition-metal-based syntheses, a direct C-H arylation of nitrogen-containing halogenated heterocycles is possible without protection of the N-H groups. The method provides a simple, metal-free alternative for the synthesis of biologically interesting arylated heterocycles under mild conditions.

A mild catalytic system for radical conjugate addition of nitrogen heterocycles

A catalytic redox system for the direct conjugate addition of pyridines and diazines to Michael acceptors has been developed.

The direct addition of pyridine and diazine units to electron-poor alkenes has been achieved via a redox radical mechanism that is enabled by limiting the effective concentration of the hydrogen-atom source. The described method is tolerant of acidic functional groups and is generally applicable to the union of a wide range of Michael acceptors and 6-membered heterocyclic halides.

Visible Light Photocatalysis for the Generation and Use of Reactive Azolyl and Polyfluoroaryl Intermediates

Photocatalysis offers several mechanistically unique pathways that are not rivaled by mainstream catalysis. Primarily, the ability to convert photochemical energy into single electron oxidation and reduction events provides a new dimension for chemists to consider when choosing how to activate a molecule or approach a complex synthesis. Since most organic molecules do not absorb light in the visible region, they are impervious to direct visible light photochemistry, which provides an opportunity for photocatalysis in which a visible light absorbing compound can serve as a mediator. In this Account, we discuss the consequences of catalyst mediated, photoinduced electron transfer to several classes of reducible arenes. While the bulk of the work discussed within this Account utilizes iridium-based photocatalysts, in principle the chemistry is not limited to this class of photocatalyst, and the principles should be more general. Instead, this Account focuses largely on the consequences of single electron transfer to poly- and perfluorinated arenes and 2-halo azoles. Electron transfer converts these stable molecules into reactive intermediates whose behavior often depends entirely on the identity of the halogen that undergoes substitution. The result is both diverse chemistry and an alternative way of thinking about the chemical reactivity of these motifs. Specifically, we discuss our efforts and those of others to develop strategies for the generation of radicals or radical anions from perfluoroarenes and azoles and the behavior of these intermediates as implied by reactions in which they participate. The divergent pathway is illustrated by 2-bromoazoles, which yield azolyl radicals and can be utilized for addition to π-bonds, while use of the 2-chloroazole substrate leads to an entirely different reaction profile. Under the appropriate reaction conditions, the reactive and transient intermediates are useful coupling partners and often provide unrivaled access to new chemical space. The odd electron species can form challenging bonds with minimal prefunctionalization of the coupling partner. For instance, some of the intermediates can be utilized for C-H functionalizations to selectively make crowded amines or to synthesize biarenes substituted at every ortho position. While photocatalysis is not the only manner of accomplishing electron transfer, the catalytic generation of the reactive species in which the concentration of the transient odd electron species is kept low, provides a synthetic handle that can be used to improve reaction outcomes. This is elegantly demonstrated in a number of examples in which redox sensitive groups located on substrates survive the reaction. In addition, the underlying basic concepts associated with radical anion fragmentation are reviewed and provide the backdrop for discussion throughout the Account.