Cation Radical Accelerated Nucleophilic Aromatic Substitution via Organic Photoredox Catalysis

Arene and Heteroarene Functionalization Enabled by Organic Photoredox Catalysis

ConspectusAromatic functionalization reactions are some of the most fundamental transformations in organic chemistry and have been a mainstay of chemical synthesis for over a century. Reactions such as electrophilic and nucleophilic aromatic substitution (EAS and SNAr, respectively) represent the two most fundamental reaction classes for arene elaboration and still today typify the most utilized methods for aromatic functionalization. Despite the reliable reactivity accessed by these venerable transformations, the chemical space that can be accessed by EAS and SNAr reactions is inherently limited due to the electronic requirements of the substrate. In the case of EAS, highly active electrophiles are paired with electron-neutral to electron-rich (hetero)arenes. For SNAr, highly electron-deficient (hetero)arenes that possess appropriate nucleofuges (halides, -NO2, etc.) are required for reactivity. The inherent limitations on (hetero)arene reactivity presented an opportunity to develop alternative reactivity to access increased chemical space to expand the arsenal of reactions available to synthetic chemists.For the past decade, our research has concentrated on developing novel methods for arene functionalization, with a particular focus on electron-neutral and electron-rich arenes and applying these methods to late-stage functionalization. Specifically, electron-rich arenes undergo single electron oxidation by a photoredox catalyst under irradiation, forming arene cation radicals. These cation radicals act as key intermediates in various transformations. While electron-rich arenes are typically unreactive toward nucleophiles, arene cation radicals are highly reactive and capable of engaging with common nucleophiles.This Account details the dichotomy of reactivity accessed via arene cation radicals: C-H functionalization by nucleophiles under aerobic conditions or cation radical accelerated nucleophilic aromatic substitution (CRA-SNAr) in anaerobic settings. Based on experimental and computational studies, we propose that reversible nucleophilic addition to arene cation radicals can occur at the ipso-, para-, or ortho-positions relative to the most electron-releasing group. Under aerobic conditions, intermediates formed by para- or ortho-addition typically undergo an additional irreversible oxidation step, resulting in C-H functionalization as the major outcome. Conversely, in the absence of an external oxidant, C-H functionalization is not observed, and ipso-addition predominates, releasing an alcohol or HF nucleofuge, leading to SNAr products. Building on the success of these arene functionalization transformations, we also explored their applications to positron emission tomography (PET) radiotracer development. Both C-H functionalization and SNAr with 18F- and 11CN- have been applied to radiofluorination and radiocyanation of arenes, respectively. Applications of the radiotracers synthesized by these methods have been demonstrated in preclinical and clinical models.

Photoredox-Catalyzed Nucleophilic Aromatic Substitution of Halophenols with Azoles via Oligomeric Phenylene Oxide Radicals

Nucleophilic aromatic substitution (SNAr) reactions are widely employed in organic synthesis yet typically require the use of electron-deficient arenes for efficient reactivity. Herein, we report a photocatalytic protocol for formal SNAr of electron-rich 4-halophenols with azole nucleophiles under mild, redox-neutral conditions. The transformation proceeds via a two-stage mechanism consisting of initial halophenol oligomerization to produce a key oligo(phenylene oxide) intermediate and its subsequent breakdown through SNAr with the azole enabled by photoredox-catalyzed arene umpolung. Reaction monitoring, stoichiometric control experiments, and luminescence quenching data implicate phenoxyl radicals and Brønsted acid-activated oligo(phenylene oxide) radicals as the reactive species in the oligomerization and the SNAr stages, respectively. The synthetic utility of this method is demonstrated across 17 (pseudo)halophenols bearing a variety of leaving groups (F, Cl, Br, OMs, and OTs) and 22 azole examples.

Engineered enzymes for enantioselective nucleophilic aromatic substitutions

Nucleophilic aromatic substitutions (SNAr) are among the most widely used processes in the pharmaceutical and agrochemical industries1-4, allowing convergent assembly of complex molecules through C-C and C-X (X = O, N, S) bond formation. SNAr reactions are typically carried out using forcing conditions, involving polar aprotic solvents, stoichiometric bases and elevated temperatures, which do not allow for control over reaction selectivity. Despite the importance of SNAr chemistry, there are only a handful of selective catalytic methods reported that rely on small organic hydrogen-bonding or phase-transfer catalysts5-11. Here we establish a biocatalytic approach to stereoselective SNAr chemistry by uncovering promiscuous SNAr activity in a designed enzyme featuring an activated arginine12. This activity was optimized over successive rounds of directed evolution to afford an engineered biocatalyst, SNAr1.3, that is 160-fold more efficient than the parent and promotes the coupling of electron-deficient arenes with carbon nucleophiles with near-perfect stereocontrol (>99% enantiomeric excess (e.e.)). SNAr1.3 can operate at a rate of 0.15 s-1, perform more than 4,000 turnovers and can accept a broad range of electrophilic and nucleophilic coupling partners, including those that allow construction of challenging 1,1-diaryl quaternary stereocentres. Biochemical, structural and computational studies provide insights into the catalytic mechanism of SNAr1.3, including the emergence of a halide binding pocket shaped by key catalytic residues Arg124 and Asp125. This study brings a landmark synthetic reaction into the realm of biocatalysis to provide an efficient and versatile platform for catalytic SNAr chemistry.

Mechanistic insights into the regiodivergent insertion of bicyclo[1.1.0]butanes towards carbocycle-tethered N-heteroarenes

Ring scaffolds constitute important sub-structures in nature and across the chemical industries. However, their straight-forward introduction into a target molecule or cross-linkage between cyclic motifs of choice comprise major challenges for methodology development. Herein, the interconnection of two prominent representatives of the 2D and 3D cyclic chemical space-namely N-heteroarenes and unsaturated carbocycles-in the form of hybrid cyclobutane-tethered N-heteroarenes is targeted. The diastereoselective introduction of decorated cyclobutanes is promoted by the insertion of strained bicyclo[1.1.0]butanes (BCBs) into the C-S bond of C2-thioether aza-arenes. In-depth density functional theory (DFT) studies provide insights on the key factors governing the unexpected regiodivergent insertion outcomes. A broad scope of mono- and bicyclic aza-arenes along with mono- and disubstituted BCBs are shown to be competent. Detailed mechanistic studies support an oxidative activation of the N-heteroarenes.

Electro-oxidative Deoxyfluorination of Arenes with NEt3·3HF

This report describes the design, development, and optimization of an electrochemical deoxyfluorination of arenes using a tetrafluoropyridine-derived leaving group. NEt3·3HF serves as the fluoride source, and the reactions are conducted using either constant potential or constant current electrolysis in an undivided electrochemical cell. Mechanistic studies support a net oxidative pathway, in which initial single-electron oxidation generates a radical cation intermediate that is trapped by fluoride. The resulting radical undergoes a second oxidation reaction, followed by the loss of the leaving group to yield the fluoroarene product.

Fundamentals and Challenges of Ligand Modification in Heterogeneous Electrocatalysis

The development of efficient catalytic materials in the energy field could promote the structural transformation from traditional fossil fuels to sustainable energy. In heterogeneous catalytic reactions, ligand modification is an effective way to regulate both electronic and steric structures of catalytic sites, thus paving a prospective avenue to design the interfacial structures of heterogeneous catalysts for energy conversion. Although great achievements have been obtained for the study and applications of heterogeneous ligand-modified catalysts, the systematical refinements of ligand modification strategies are still lacking. Here, we reviewed the ligand modification strategy from both the mechanistic and applicable scenarios by focusing on heterogeneous electrocatalysis. We elucidated the ligand-modified catalysts in detail from the perspectives of basic concepts, preparation, regulation of physicochemical properties of catalytic sites, and applications in different electrocatalysis. Notably, we bridged the electrocatalytic performance with the electronic/steric effects induced by ligand modification to gain intrinsic structure-performance relations. We also discussed the challenges and future perspectives of ligand modification strategies in heterogeneous catalysis.

Recent Advances in C-O Bond Cleavage of Aryl, Vinyl, and Benzylic Ethers

Transition metal-catalyzed cross-coupling with aryl halides has revolutionized the way of diversifying aromatic compounds. Aryl ethers are attractive alternatives to aromatic halides as coupling partners considering the accessibility and potential environmental benefits. The last two decades have witnessed a striking success in the field of C-O bond activation of aryl ethers, including the construction of C-C bond and C-X bond, as well as reductive deoxygenation. Here, we present a comprehensive review of C-O bond activation in the context of aryl, vinyl, and benzylic ethers. This review elaborates on the current state-of-the-art methods, categorized by different catalytic systems, including transition metal catalysis, photoredox catalysis, and other innovative approaches. The newly developed methods allow C-O bond activation under mild conditions with exceptional functional group tolerance, potentially enabling the late-stage functionalization of pharmaceuticals. The limitations and future perspectives of the methods are also presented.

Mechanism-Based Regiocontrol in SNAr: A Case Study of Ortho-Selective Etherification with Chloromagnesium Alkoxides

Although nucleophilic aromatic substitution (SNAr) is routinely employed as a practical alternative to transition-metal-catalyzed cross-coupling, the mechanistic basis for reactivity and regioselectivity remains underexplored and is an active area of research. This article reports a SNAr-based etherification of 2,4-difluoroarylcarboxamides as a model system and shows that the ortho/para regioselectivity spans >500:1 to 1:∼20 simply by varying the reaction conditions via high-throughput experimentation (HTE). An in-depth characterization of the ortho-selective lead conditions is presented, and these insights are used to build a reactivity model that self-consistently accounts for the regioselectivity and reaction scope. This article discusses synthetic implications of condition-dependent magnesium coordination and Schlenk equilibria and demonstrates that consideration of molecular-level stoichiometry and isomerism is an essential prerequisite for rationalizing reactivity and regioselectivity in SNAr.

One-Step Synthesis of [18F]Aromatic Electrophile Prosthetic Groups via Organic Photoredox Catalysis

To avoid the harsh conditions that are oftentimes adopted in direct radiofluorination reactions, conjugation of bioactive ligands with 18F-labeled prosthetic groups has become an important strategy to construct novel PET agents under mild conditions when the ligands are structurally sensitive. Prosthetic groups with [18F]fluoroarene motifs are especially appealing because of their stability in physiological environments. However, their preparation can be intricate, often requiring multistep radiosynthesis with functional group conversions to prevent the decomposition of unprotected reactive prosthetic groups during the harsh radiofluorination. Here, we report a general and simple method to generate a variety of highly reactive 18F-labeled electrophiles via one-step organophotoredox-mediated radiofluorination. The method benefits from high step-economy, reaction efficiency, functional group tolerance, and easily accessible precursors. The obtained prosthetic groups have been successfully applied in PET agent construction and subsequent imaging studies, thereby demonstrating the feasibility of this synthetic method in promoting imaging and biomedical research.

Direct, Selective α-Aryloxyalkyl Radical Cyanation and Allylation of Aryl Alkyl Ethers

We report the site-selective α-aryloxyalkyl C-H cyanation and allylation of aryl alkyl ethers using an acridinium photocatalyst with phosphate base under LED irradiation (456 nm). Oxidation of the aryl alkyl ether to its corresponding radical cation by the excited stated photocatalyst allowed facile deprotonation of the ArOC(sp3)-H bond to afford an α-aryloxyalkyl radical, which was trapped with sulfone substrates, resulting in expulsion of a sulfonyl radical and formation of allylated or cyanated products.

Controllable Pyridine N-Oxidation-Nucleophilic Dechlorination Process for Enhanced Dechlorination of Chloropyridines: The Cooperation of HCO4- and HO2. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2024;58:4438-4449. [PMID: 38330552 DOI: 10.1021/acs.est.3c09878] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Dechlorination of chloropyridines can eliminate their detrimental environmental effects. However, traditional dechlorination technology cannot efficiently break the C-Cl bond of chloropyridines, which is restricted by the uncontrollable nonselective species. Hence, we propose the carbonate species-activated hydrogen peroxide (carbonate species/H2O2) process wherein the selective oxidant (peroxymonocarbonate ion, HCO4-) and selective reductant (hydroperoxide anion, HO2-) controllably coexist by manipulation of reaction pH. Taking 2-chloropyridine (Cl-Py) as an example, HCO4- first induces Cl-Py into pyridine N-oxidation intermediates, which then suffer from the nucleophilic dechlorination by HO2-. The obtained dechlorination efficiencies in the carbonate species/H2O2 process (32.5-84.5%) based on the cooperation of HCO4- and HO2- are significantly higher than those in the HO2--mediated sodium hydroxide/hydrogen peroxide process (0-43.8%). Theoretical calculations confirm that pyridine N-oxidation of Cl-Py can effectively lower the energy barrier of the dechlorination process. Moreover, the carbonate species/H2O2 process exhibits superior anti-interference performance and low electric energy consumption. Furthermore, Cl-Py is completely detoxified via the carbonate species/H2O2 process. More importantly, the carbonate species/H2O2 process is applicable for efficient dehalogenation of halogenated pyridines and pyrazines. This work offers a simple and useful strategy to enhance the dehalogenation efficiency of halogenated organics and sheds new insights into the application of the carbonate species/H2O2 process in practical environmental remediation.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Electrochemical Arene Radical Cation Promoted Spirocyclization of Biaryl Ynones: Access to Alkoxylated Spiro[5,5]trienones

Herein, a novel electrochemical arene radical cation promoted dearomative spirocyclization of biaryl ynones with alcohols is described, providing a conceptually novel transformation mode for producing diverse alkoxylated spiro[5,5]trienones. The catalyst- and chemical-oxidant-free spirocyclization protocol features broad substrate scope and high functional group tolerance. Mechanistic studies reveal that the generation of arene radical cation via anodic single-electron oxidation is crucial, with sequential 6-endo-dig cyclization, dissociation of hemiketal, anodic oxidation, and nucleophilic attack of alcohols.

Exo-cage catalysis and initiation derived from photo-activating host-guest encapsulation

Coordination cage catalysis has commonly relied on the endogenous binding of substrates, exploiting the cavity microenvironment and spatial constraints to engender increased reactivity or interesting selectivity. Nonetheless, there are issues with this approach, such as the frequent occurrence of product inhibition or the limited applicability to a wide range of substrates and reactions. Here we describe a strategy in which the cage acts as an exogenous catalyst, wherein reactants, intermediates and products remain unbound throughout the course of the catalytic cycle. Instead, the cage is used to alter the properties of a cofactor guest, which then transfers reactivity to the bulk-phase. We have exemplified this approach using photocatalysis, showing that a photoactivated host-guest complex can mediate [4 + 2] cycloadditions and the aza-Henry reaction. Detailed in situ photolysis experiments show that the cage can both act as a photo-initiator and as an on-cycle catalyst where the quantum yield is less than unity.

Divergent Photosensitizer Controlled Reactions of 4-Hydroxycoumarins and Unactivated Olefins: Hydroarylation and Subsequent [2+2] Cycloaddition

Herein, we report a photoinduced approach for hydroarylation of unactivated olefins using 4-hydroxycoumarins as the arylating reagent. Key to the success of this reaction is the conversion of nucleophilic 4-hydroxycoumarins into electrophilic carbon radicals via photocatalytic arene oxidation, which not only circumvents the polarity-mismatch issue encountered under ionic conditions but also accommodates a broad substrate scope and inhibits side reactions that were previously observed. Moreover, divergent reactivity was achieved by changing the photocatalyst, enabling a subsequent [2+2] cycloaddition to deliver cyclobutane-fused pentacyclic products that are otherwise challenging to access in high yields and with high diastereoselectivity. Mechanistic studies have elucidated the mechanism of the reactions and the origin of the divergent reactivity.

Photoinduced Borylation of the Inert C(sp3)-O Bond of Alkyl Heteroaryl Ethers

A photoinduced reductive Calkyl-O borylation of alkyl heteroaryl ethers with very negative reduction potential in the presence of 4-dimethylaminopyridine (DMAP) and bis(catecholato)diborane(B2cat2) was developed. Despite the high reducing power, various substrates with liable functional groups were well-tolerated as well as ethers derived from natural products and medicinal-relevant compounds. Mechanistic investigation implied that an intra-single electron transfer process in an electron donor-acceptor complex formed from ethers with the adduct of B2cat2 and DMAP should be involved.

Organic photoredox-catalyzed oxidative azolation of unactivated fluoroarenes

Inexpensive and commercially available 2,4,6-triphenylpyrrolium tetrafluoroborate (TPT) is used as an organic photocatalyst for the nucleophilic aromatic substitution of unactivated fluoroarenes with pyrazole derivatives (SNAr) to form azole arenes. The use of organic photoredox catalysis enables the easy operation of this method under mild conditions. Various fluorinated aromatic compounds are suitable electrophiles for this transformation.

Contemporary photoelectrochemical strategies and reactions in organic synthesis

In recent years, with the development of organic synthetic chemistry, a variety of organic synthetic methods have been discovered and applied in practical production. Photochemistry and electrochemistry have been widely used in organic synthesis recently due to their advantages such as mild conditions and green and environmental protection and have now been developed into two of the most massive synthetic strategies in the field of organic synthesis. In order to further enhance the potential of photochemistry and electrochemistry and to overcome the limitations of each, organic synthetic chemists have worked to combine the two synthetic strategies together to develop photoelectrochemistry as a new synthetic method. Photoelectrochemistry achieves the complementary advantages and disadvantages of photochemistry and electrochemistry, avoids the problem of using stoichiometric oxidants or reductants in photochemistry and easy dimerization in electrochemistry, generates highly reactive reaction intermediates under mild conditions, and achieves reactions that are difficult to accomplish by single photochemistry or electrochemistry. This review summarizes the research progress in the field of photoelectrochemistry from the perspective of photoelectro-chemical catalysts in recent years, analyzes the catalytic mechanism of various catalysts in detail, and gives a brief outlook on the research direction and development prospects in this field.

Chemoselective Decarboxylative Protonation Enabled by Cooperative Earth-Abundant Element Catalysis

Decarboxylative protonation is a general deletion tactic to replace polar carboxylic acid groups with hydrogen or its isotope. Current methods rely on the pre-activation of acids, non-sustainable hydrogen sources, and/or expensive/highly oxidizing photocatalysts, presenting challenges to their wide adoption. Here we show that a cooperative iron/thiol catalyst system can readily achieve this transformation, hydrodecarboxylating a wide range of activated and unactivated carboxylic acids and overcoming scope limitations in previous direct methods. The reaction is readily scaled in batch configuration and can be directly performed in deuterated solvent to afford high yields of d-incorporated products with excellent isotope incorporation efficiency; characteristics not attainable in previous photocatalyzed approaches. Preliminary mechanistic studies indicate a radical mechanism and kinetic results of unactivated acids (KIE=1) are consistent with a light-limited reaction.

Photoredox-Catalyzed Synthesis of β-Amino Alcohols: Hydroxymethylation of Imines with α-Silyl Ether as Hydroxymethyl Radical Precursor

Carbon-carbon bond formation is an efficient approach for the synthesis of amino alcohols using two simple starting materials. Herein, we present a novel method for a divergent synthesis of β-amino ethers and β-amino alcohols in a sequential one-pot protocol under high-efficiency, mild, and metal- or metal-free conditions. Especially, TMSCH₂OPMP was developed as a synthetic equivalent of α-hydroxymethyl radical in an in situ photocatalyzed oxidative PMP group deprotection strategy under air. A preliminary mechanistic investigation provides evidence for reaction mechanism involving a photoinduced α-alkoxy methyl radical and superoxide.

Chiral Anthranyl Trifluoromethyl Alcohols: Structures, Oxidative Dearomatization and Chiroptical Properties

Chiral trifluoromethyl alcohol groups were introduced at the hindered ortho positions of 9,10-diphenylanthracenes to investigate their effects on the physical properties and reactivity towards oxidative dearomatization. In such compact structures, the position in different quadrants and the preferred orientation of the -CH(OH)CF₃ groups were determined by the relative and absolute configurations of each stereoisomer, respectively. As a consequence, the stereochemistry governs the organization of the H-bonded molecules in single crystals (homochiral dimers vs ribbon), whereas in chlorinated solvents, they all behave as discrete compounds. Concerning their reactivity, the stereospecific dearomative oxidation of these molecules leads to 9,10-bis-spiro-isobenzofuran-anthracenes, when using organic single-electron transfer oxidants. The chiroptical properties of the alcohols and the corresponding dearomatized products were compared and showed an important modulation of the intensity.

Direct C-H Radiocyanation of Arenes via Organic Photoredox Catalysis

Innovative labeling methods to incorporate the short-lived positron emitter carbon-11(¹¹C) into bioactive molecules are attractive for positron emission tomography (PET) tracer discovery. Herein, we report a direct C-H radiocyanation method that incorporates [¹¹C]cyanide (¹¹CN^-) to a series of functional electron-rich arenes via photoredox catalysis. This photoredox-mediated radiocyanation can proceed in an aerobic environment and is not moisture sensitive, which allows for ease of reaction setup and for scalable synthesis of ¹¹C-aryl nitriles from readily available precursors.

Radicals as Exceptional Electron-Withdrawing Groups: Nucleophilic Aromatic Substitution of Halophenols Via Homolysis-Enabled Electronic Activation

While heteroatom-centered radicals are understood to be highly electrophilic, their ability to serve as transient electron-withdrawing groups and facilitate polar reactions at distal sites has not been extensively developed. Here, we report a new strategy for the electronic activation of halophenols, wherein generation of a phenoxyl radical via formal homolysis of the aryl O-H bond enables direct nucleophilic aromatic substitution of the halide with carboxylate nucleophiles under mild conditions. Pulse radiolysis and transient absorption studies reveal that the neutral oxygen radical (O•) is indeed an extraordinarily strong electron-withdrawing group [σp-(O•) = 2.79 vs σp-(NO2) = 1.27]. Additional mechanistic and computational studies indicate that the key phenoxyl intermediate serves as an open-shell electron-withdrawing group in these reactions, lowering the barrier for nucleophilic substitution by more than 20 kcal/mol relative to the closed-shell phenol form of the substrate. By using radicals as transient activating groups, this homolysis-enabled electronic activation strategy provides a powerful platform to expand the scope of nucleophile-electrophile couplings and enable previously challenging transformations.

Photochemical C(sp2 )-H Pyridination via Arene-Pyridinium Electron Donor-Acceptor Complexes

This report describes the development of a photochemical method for C(sp2 )-H pyridination that leverages the photoexcitation of electron donor-acceptor (EDA) complexes. Experimental and DFT studies show that black light (λmax ≈350 nm) irradiation of solutions of protonated pyridines (acceptors) and aromatic C-H substrates (donors) results in single electron transfer to form aryl radical cation intermediates that can be trapped with pyridine nucleophiles under aerobic conditions. With some modification of the reaction conditions, this EDA activation mode is also effective for promoting the oxidatively triggered SN Ar pyridination of aryl halides. Overall, this report represents an inexpensive and atom-economical approach to photochemical pyridination reactions that eliminates the requirement of an exogenous photocatalyst.

Highly selective α-aryloxyalkyl C-H functionalisation of aryl alkyl ethers

We report highly selective photocatalytic functionalisations of alkyl groups in aryl alkyl ethers with a range of electron-poor alkenes using an acridinium catalyst with a phosphate base and irradiation with visible light (456 nm or 390 nm). Experiments indicate that the reaction operates via direct single-electron oxidation of the arene substrate ArOCHRR' to its radical cation by the excited state organic photocatalyst; this is followed by deprotonation of the ArOC-H in the radical cation to yield the radical ArOC˙RR'. This radical then attacks the electrophile to form an intermediate alkyl radical that is reduced to complete the photocatalytic cycle. The oxidation step is selective for activated arenes (ArOR) over their non-activated counterparts and the subsequent deprotonation of the methoxy group affords the α-aryloxyalkyl radical that leads to a wide range of functionalised products in good to excellent yield.

Successive Cleavage and Reconstruction of Lignin β-O-4 Models and Polymer to Access Quinoxalines

The construction of N-heterocyclic compounds from lignin remains a great challenge due to the complex lignin structure and the involvement of multiple steps, including the cleavage of lignin C-O linkages and the formation of heterocyclic aromatic rings. Herein, the first example of KOH mediated sustainable synthesis of quinoxaline derivatives from lignin β-O-4 model compounds in a one-pot fashion under transition-metal-free conditions has been achieved. Mechanistic studies suggested that this transformation includes highly coupled cascade steps of cleavage of C-O bonds, dehydrative condensation, sp³ C-H bond oxidative activation, and intramolecular dehydrative coupling reaction. With this protocol, a wide range of functionalized quinoxalines, including an important drug compound AG1295, were synthesized from lignin β-O-4 model compounds and β-O-4 polymer, showcasing the application potential of lignin in pharmaceutical synthesis.

Mechanistic Investigations into Amination of Unactivated Arenes via Cation Radical Accelerated Nucleophilic Aromatic Substitution

A mechanistic investigation into the amination of electron-neutral and electron-rich arenes using organic photoredox catalysis is presented. Kinetic and computational data support rate-limiting nucleophilic addition into an arene cation radical using both azole and primary amine nucleophiles. This finding is consistent with both fluoride and alkoxide nucleofuges, supporting a unified mechanistic picture using cation radical accelerated nucleophilic aromatic substitution (CRA-SNAr). Electrochemistry and time-resolved fluorescence spectroscopy confirm the key role solvents play in enabling selective arene oxidation in the presence of amines. The synthetic limitations of xanthylium salts are elucidated via photophysical studies. An alternative catalyst scaffold with improved turnover numbers is presented.

Electrophotocatalysis: Combining Light and Electricity to Catalyze Reactions

Visible-light photocatalysis and electrocatalysis are two powerful strategies for the promotion of chemical reactions that have received tremendous attention in recent years. In contrast, processes that combine these two modalities, an area termed electrophotocatalysis, have until recently remained quite rare. However, over the past several years a number of reports in this area have shown the potential of combining the power of light and electrical energy to realize new catalytic transformations. Electrophotocatalysis offers the ability to perform photoredox reactions without the need for large quantities of stoichiometric or superstoichiometric chemical oxidants or reductants by making use of an electrochemical potential as the electron source or sink. In addition, electrophotocatalysis is readily amenable to the generation of open-shell photocatalysts, which tend to have exceptionally strong redox potentials. In this way, potent yet selective redox reactions have been realized under relatively mild conditions. This Perspective highlights recent advances in the area of electrophotocatalysis and provides some possible avenues for future work in this growing area.

Transition-metal-free synthesis of pyrimidines from lignin β-O-4 segments via a one-pot multi-component reaction

Heteroatom-participated lignin depolymerization for heterocyclic aromatic compounds production is of great importance to expanding the product portfolio and meeting value-added biorefinery demand, but it is also particularly challenging. In this work, the synthesis of pyrimidines from lignin β-O-4 model compounds, the most abundant segment in lignin, mediated by NaOH through a one-pot multi-component cascade reaction is reported. Mechanism study suggests that the transformation starts by NaOH-induced deprotonation of Cα-H bond in β-O-4 model compounds, and involves highly coupled sequential cleavage of C-O bonds, alcohol dehydrogenation, aldol condensation, and dehydrogenative aromatization. This strategy features transition-metal free catalysis, a sustainable universal approach, no need of external oxidant/reductant, and an efficient one-pot process, thus providing an unprecedented opportunity for N-containing aromatic heterocyclic compounds synthesis from biorenewable feedstock. With this protocol, an important marine alkaloid meridianin derivative can be synthesized, emphasizing the application feasibility in pharmaceutical synthesis.

Organophotocatalytic Regioselective C-H Alkylation of Electron-Rich Arenes Using Activated and Unactivated Alkenes

Direct alkylation of the C-H bond arenes in a selective manner is a long-standing challenge. Herein, a metal-free photocatalytic regioselective C-H alkylation method for electron-rich arenes with both activated and unactivated alkenes was developed. The reaction tolerates a wide range of aromatic rings with diverse substitution patterns, as well as terminal and internal alkenes, providing a general and straightforward metal-free method for C-C bond formation from inert C-H bonds. Moreover, alkynes are also compatible to give the C-H vinylation of electron-rich arenes.

Aroyl Fluorides as Bifunctional Reagents for Dearomatizing Fluoroaroylation of Benzofurans

The 2,3-dihydrobenzofuran scaffold is widely found in natural products and biologically active compounds. Herein, dearomatizing 2,3-fluoroaroylation of benzofurans with aroyl fluorides as bifunctional reagents to access 2,3-difunctionalized dihydrobenzofurans is reported. The reaction that occurs by cooperative NHC/photoredox catalysis provides 3-aroyl-2-fluoro-2,3-dihydrobenzofurans with moderate to good yield and high diastereoselectivity. Cascades proceed via radical/radical cross-coupling of a benzofuran radical cation generated in the photoredox catalysis cycle with a neutral ketyl radical formed through the NHC catalysis cycle. The redox-neutral transformation exhibits broad substrate scope and high functional group compatibility. With anhydrides as bifunctional reagents, dearomatizing aroyloxyacylation of benzofurans is achieved and the strategy can also be applied to N-acylated indoles to afford 3-aroyl-2-fluoro-dihydroindoles.

Cobalt-Catalyzed Regioselective para-Amination of Azobenzenes via Nucleophilic Aromatic Substitution of Hydrogen

The metal-catalyzed nucleophilic aromatic substitution of hydrogen (S_NArH) via coordination of the substituent on the aromatic ring to the metal catalyst, in terms of reactivity, substrate type, and reaction selectivity, complements the transition metal-catalyzed C-H functionalization that proceeds via C-H metalation but remains an elusive target. Described herein is the development of an unprecedented cobalt-catalyzed para-selective amination of azobenzenes, which is essentially a metal-promoted S_NArH process as revealed by Hammett analysis, thus illustrating the concept that coordination of the substituent on the arene ring to the metal catalyst may result in electrophilic activation of the arene ring toward S_NArH. This cobalt-catalyzed protocol allows the use of a variety of both aliphatic amines and anilines as aminating reagents, tolerates electronically diverse substituents of azobenzene, and furnishes the corresponding products in good yields with a regiospecific selectivity for para-amination.

Photoinduced Arylation of Acridinium Salts: Tunable Photoredox Catalysts for C-O Bond Cleavage

A photoinduced arylation of N-substituted acridinium salts has been developed and has exhibited a high functional group tolerance (e.g., halogen, nitrile, ketone, ester, and nitro). A broad range of well-decorated C9-arylated acridinium-based catalysts with fine-tuned photophysical and photochemical properties, namely, excited-state lifetimes and redox potentials have been synthetized in a one-step procedure. These functionalized acridinium salts were later evaluated in the photoredox-catalyzed fragmentation of 1,2-diol derivatives (lignin models). Among them, 2-bromophenyl substituted N-methyl acridinium has outperformed all photoredox catalysts, including commercial Fukuzumi's catalyst, for the selective C^βO-Ar bond cleavage of diol monoarylethers to afford 1,2-diols in good yields.

Radical generation and fate control for photocatalytic biomass conversion

Photocatalysis is an emerging approach for sustainable chemical production from renewable biomass under mild conditions. Active radicals are always generated as key intermediates, in which their high reactivity renders them versatile for various upgrading processes. However, controlling their reaction is a challenge, especially in highly functionalized biomass frameworks. In this Review, we summarize recent advanced photocatalytic systems for selective biomass valorization, with an emphasis on their distinct radical-mediated reaction patterns. The strategies for generating a specific radical intermediate and controlling its subsequent conversion towards desired chemicals are also highlighted, aiming to provide guidance for future studies. We believe that taking full advantage of the unique reactivity of radical intermediates would provide great opportunities to develop more efficient photocatalytic systems for biomass valorization.

Arene radiofluorination enabled by photoredox-mediated halide interconversion

Positron emission tomography (PET) is a powerful imaging technology that can visualize and measure metabolic processes in vivo and/or obtain unique information about drug candidates. The identification of new and improved molecular probes plays a critical role in PET, but its progress is somewhat limited due to the lack of efficient and simple labelling methods to modify biologically active small molecules and/or drugs. Current methods to radiofluorinate unactivated arenes are still relatively limited, especially in a simple and site-selective way. Here we disclose a method for constructing C-18F bonds through direct halide/18F conversion in electron-rich halo(hetero)arenes. [18F]F- is introduced into a broad spectrum of readily available aryl halide precursors in a site-selective manner under mild photoredox conditions. Notably, our direct 19F/18F exchange method enables rapid PET probe diversification through the preparation and evaluation of an [18F]-labelled O-methyl tyrosine library. This strategy also results in the high-yielding synthesis of the widely used PET agent L-[18F]FDOPA from a readily available L-FDOPA analogue.

The advent and development of organophotoredox catalysis

In the last decade, photoredox catalysis has unlocked unprecedented reactivities in synthetic organic chemistry. Seminal advancements in the field have involved the use of well-studied metal complexes as photoredox catalysts (PCs). More recently, the synthetic community, looking for more sustainable approaches, has been moving towards the use of purely organic molecules. Organic PCs are generally cheaper and less toxic, while allowing their rational modification to an increased generality. Furthermore, organic PCs have allowed reactivities that are inaccessible by using common metal complexes. Likewise, in synthetic catalysis, the field of photocatalysis is now experiencing a green evolution moving from metal catalysis to organocatalysis. In this feature article, we discuss and critically comment on the scientific reasons for this ongoing evolution in the field of photoredox catalysis, showing how and when organic PCs can efficiently replace their metal counterparts.

Aryl Transfer Strategies Mediated by Photoinduced Electron Transfer

Radical aryl migrations are powerful techniques to forge new bonds in aromatic compounds. The growing popularity of photoredox catalysis has led to an influx of novel strategies to initiate and control aryl migration starting from widely available radical precursors. This review encapsulates progress in radical aryl migration enabled by photochemical methods─particularly photoredox catalysis─since 2015. Special attention is paid to descriptions of scope, mechanism, and synthetic applications of each method.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Metal-Free, Rapid, and Highly Chemoselective Reduction of Aromatic Nitro Compounds at Room Temperature

In this study, we developed a metal-free and highly chemoselective method for the reduction of aromatic nitro compounds. This reduction was performed using tetrahydroxydiboron [B₂(OH)₄] as the reductant and 4,4'-bipyridine as the organocatalyst and could be completed within 5 min at room temperature. Under optimal conditions, nitroarenes with sensitive functional groups, such as vinyl, ethynyl, carbonyl, and halogen, were converted into the corresponding anilines with excellent selectivity while avoiding the undesirable reduction of the sensitive functional groups.

Recent advances in photochemical construction of aromatic C–P bonds via C–hetero bond cleavage

Photochemical C–P bond cross-coupling in aromatics via C–X (X = F, Cl, Br, I), C–N bond and C–O bond cleavages with/without photosensitizer were summarized in this review.

Cleavage via Selective Catalytic Oxidation of Lignin or Lignin Model Compounds into Functional Chemicals

Lignin, a complex aromatic polymer with different types of methoxylated phenylpropanoid connections, enables the sustainable supply of value-added chemicals and biofuels through its use as a feedstock. Despite the development of numerous methodologies that upgrade lignin to high-value chemicals such as drugs and organic synthesis intermediates, the variety of valuable products obtained from lignin is still very limited, mainly delivering hydrocarbons and oxygenates. Using selective oxidation and activation cleavage of lignin, we can obtain value-added aromatics, including phenols, aldehydes, ketones, and carboxylic acid. However, biorefineries will demand a broad spectrum of fine chemicals in the future, not just simple chemicals like aldehydes and ketones containing simple C = O groups. In particular, most n-containing aromatics, which have found important applications in materials science, agro-chemistry, and medicinal chemistry, such as amide, aniline, and nitrogen heterocyclic compounds, are obtained through n-containing reagents mediating the oxidation cleavage in lignin. This tutorial review provides updates on recent advances in different classes of chemicals from the catalytic oxidation system in lignin depolymerization, which also introduces those functionalized products through a conventional synthesis method. A comparison with traditional synthetic strategies reveals the feasibility of the lignin model and real lignin utilization. Promising applications of functionalized compounds in synthetic transformation, drugs, dyes, and textiles are also discussed.

Synthesis of phenanthridines via a novel photochemically-mediated cyclization and application to the synthesis of triphaeridine

Readily synthesized biphenyl-2-carbaldehyde O-acetyl oximes were exposed to UV radiation affording phenanthridines. The scope and limitations of this novel reaction were explored. For example, exposure of 2',3'-dimethoxy-[1,1'-biphenyl]-2-carbaldehyde O-acetyl oxime to UV radiation afforded 4-methoxyphenanthridine in 54% yield. This methodology was applied to the synthesis of trisphaeridine to afford the product in four linear steps in an overall yield of 6.5% from 1-bromo-2,4,5-trimethoxybenzene.