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

Light-promoted aromatic denitrative chlorination

Nitroarenes are readily accessible bulk chemicals and can serve as versatile starting materials for a series of synthetic reactions. However, due to the inertness of the CAr-NO2 bond, the direct denitrative substitution reaction with unactivated nitroarenes remains challenging. Chemists rely on sequential reduction and diazotization followed by the Sandmeyer reaction or the nucleophilic aromatic substitution of activated nitroarenes to realize nitro group transformations. Here we develop a general denitrative chlorination reaction under visible-light irradiation, in which the chlorine radical replaces the nitro moiety through the cleavage of the CAr-NO2 bond. This practical method works with a wide range of unactivated nitro(hetero)arenes and nitroalkenes, is not sensitive to air or moisture and can proceed smoothly on a decagram scale. This transformation differs fundamentally from previous nucleophilic aromatic substitution reactions under thermal conditions in both synthesis and mechanism. Density functional theory calculations reveal the possible pathway for the substitution reaction.

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.

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.

Photomediated Hexafluoroisopropoxylation of Unactivated Aryl Halides

The use of fluoroalkoxy groups, such as trifluoromethoxy and 2,2,2-trifluoroethoxy groups, in pharmaceutical and agrochemical development has increased dramatically in recent years. However, hexafluoroisopropoxy groups have remained significantly under-represented, presumably due to limited synthetic methods for accessing this substituent in good yields. Herein, we report a mild, photochemical nucleophilic aromatic substitution (SNAr) approach for the synthesis of hexafluoroisopropyl aryl ethers from unactivated and abundant aryl halides. Notably, aryl chloride and bromide functionality are efficiently engaged by this methodology in addition to the traditional aryl fluoride nucleofuge. This method provided access to a diverse array of hexafluoroisopropyl aryl ethers, including multiple examples of late-stage functionalization of active pharmaceutical ingredients. A simple flow system was adapted for 10x scale-up, maintaining good yield for the reaction. Initial mechanistic studies indicate single electron oxidation of the arene as a key step in product formation.

Directed nucleophilic aromatic substitution reaction

In this study, we discovered a directed nucleophilic aromatic substitution reaction, "directed SNAr (dSNAr)," in the reaction of ortho-iodobezamides and amine in the presence of pyridine. The reaction proceeded ortho-specifically and did not require a strong electron-withdrawing group on the arene substrate. Most reactions proceeded at room temperature in the presence of Py, and a wide range of amine nucleophiles can be applied. Furthermore, the reactions with benzamide substituted with multiple halogens were found to be 100% ortho-selective.

Catalytic Concerted SNAr Reactions of Fluoroarenes by an Organic Superbase

We herein propose that the catalytic concerted SNAr reaction is a powerful method to prepare functionalized aromatic scaffolds. Classic stepwise SNAr reactions involving addition/elimination processes require the use of electron-deficient aromatic halides to stabilize Meisenheimer intermediates, despite their widespread use in medicinal chemistry research. Recent efforts have been made to develop concerted SNAr reactions involving a single transition state, allowing the use of electron-rich substrates based on the use of stoichiometric amounts of strong bases or reactive nucleophiles. This study demonstrates that, without the use of such reagents, the organic superbase t-Bu-P4 efficiently catalyzes the concerted SNAr reactions of aryl fluorides regardless of their electronic nature. The key to establishing this system is the dual activation of aryl fluoride and anionic nucleophiles by the t-Bu-P4 catalyst. Furthermore, this catalysis allows excellent functional group tolerance, utilization of diverse nucleophiles, and late-stage functionalization of bioactive compound derivatives. These findings make possible diverse applications in chemical synthesis and pharmaceutical development.

Acridine/Lewis Acid Complexes as Powerful Photocatalysts: A Combined Experimental and Mechanistic Study

A class of in-situ generated Lewis acid (LA) activated acridine complexes is reported, which act as potent photochemical catalysts for the oxidation of a variety of protected secondary amines. Acridine/LA complexes exhibit tunable excited state reduction potentials ranging from +2.07 to 2.38 V vs. SCE. The ytterbium triflate complex of 3,6-di-t-butyl-9-mesitylacridine catalyzes a photochemical Giese-type reaction of Boc-protected secondary amines with challenging conjugate acceptors such as acrylates, that are inaccessible to the analogous acridinium (t-Bu-Mes-Acr⊕) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular 13C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by t-Bu-Mes-Acr⊕, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced t-Bu-Mes-Acr• to the α-keto radical intermediate as the first irreversible step in the catalytic cycle. Intriguingly, we find that the reduced acridine/LA complexes are better ground state reductants (-0.72 to -0.74 V vs SCE) relative to t-Bu-Mes-Acr• (-0.59 V vs SCE) and predict that the increased substrate reactivity stems from a lower energy barrier for this key SET event.

Photocatalytic furan-to-pyrrole conversion

The identity of a heteroatom within an aromatic ring influences the chemical properties of that heterocyclic compound. Systematically evaluating the effect of a single atom, however, poses synthetic challenges, primarily as a result of thermodynamic mismatches in atomic exchange processes. We present a photocatalytic strategy that swaps an oxygen atom of furan with a nitrogen group, directly converting the furan into a pyrrole analog in a single intermolecular reaction. High compatibility was observed with various furan derivatives and nitrogen nucleophiles commonly used in drug discovery, and the late-stage functionalization furnished otherwise difficult-to-access pyrroles from naturally occurring furans of high molecular complexity. Mechanistic analysis suggested that polarity inversion through single electron transfer initiates the redox-neutral atom exchange processes at room temperature.

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.

Unlocking flavin photoacid catalysis through electrophotochemistry

Molecular flavins are one of the most versatile photocatalysts. They can coordinate single and multiple electron transfer processes, gift hydrogen atoms, form reversible covalent linkages that support group transfer mechanisms, and impart photonic energy to ground state molecules, priming them for downstream reactions. But one mechanism that has not featured extensively is the ability of flavins to act as photoacids. Herein, we disclose our proof-of-concept studies showing that electrophotochemistry can transform fully oxidized flavin quinones to super-oxidized flavinium photoacids that successfully guide proton-transfer and deliver acid-catalyzed products. We also show that these species can adopt a second mechanism wherein they react with water to release hydroxyl radicals that facilitate hydrogen-atom abstraction and sp3C-H functionalization protocols. Together, this unprecedented bimodal reactivity enables electro-generated flavinium salts to affect synthetic chemistries previously unknown to flavins, greatly expanding their versatility as catalysts.

Carbon Free Radical (R⋅) Inactivates NF-κB for Radical Capping Therapy

Inactivating hyperactivated transcription factors can overcome tumor therapy resistance, but their undruggable features limit the development of conventional inhibitors. Here, we report that carbon-centered free radicals (R⋅) can inactivate NF-κB transcription by capping the active sites in both NF-κB and DNA. We construct a type of thermosensitive R⋅ initiator loaded amphiphilic nano-micelles to facilitate intracellular delivery of R⋅. At a temperature of 43 °C, the generated R⋅ engage in electrophilic radical addition towards double bonds in nucleotide bases, and simultaneously cap the sulfhydryl residues in NF-κB through radical chain reaction. As a result, both NF-κB nuclear translocation and NF-κB-DNA binding are suppressed, leading to a remarkable NF-κB inhibition of up to 94.1 %. We have further applied R⋅ micelles in a clinical radiofrequency ablation tumor therapy model, showing remarkable NF-κB inactivation and consequently tumor metastasis inhibition. Radical capping strategy not only provides a method to solve the heat-sink effect in clinic tumor hyperthermia, but also suggests a new perspective for controllable modification of biomacromolecules in cancer therapy.

Alkoxy Radical Generation Mediated by Sulfoxide Cation Radicals for Alcohol-Directed Aliphatic C-H Functionalization

The C-H functionalization of remote, unactivated C-H bonds offers a unique method of garnering structural complexity in a synthesis. The use of directing groups has provided a means of enacting C-H functionalization on these difficult-to-access bonds; however, the installation and removal of directing groups on a substrate require additional synthetic manipulations, detracting from both the efficiency and economic feasibility of a transformation. The use of alkoxy radicals as transient directing groups for the functionalization of remote C-H bonds allows access to the synthesis of complex molecules without the need for additional functionality. Herein, we report a method for alkoxy radical formation from unactivated alcohols and reactivity mediated by photoredox-generated sulfoxide cation radicals. This protocol leverages the unique reactivity of alkoxy radicals to implement different reaction manifolds: 1,5-hydrogen atom transfer (HAT), cyclization, and β-scission. Furthermore, it was discovered that this methodology could be utilized to impose radical group transfer reactions via the β-scission pathway. Stern-Volmer analysis supports the formation of an alkoxy radical via the intermediacy of a sulfurane radical rather than a proton-coupled electron transfer (PCET) mechanism.

Oxidative two-state photoreactivity of a manganese(IV) complex using near-infrared light

Highly reducing or oxidizing photocatalysts are a fundamental challenge in photochemistry. Only a few transition metal complexes with Earth-abundant metal ions have so far advanced to excited state oxidants. All these photocatalysts require high-energy light for excitation, and their oxidizing power has not been fully exploited due to energy dissipation before reaching the photoactive state. Here we demonstrate that the complex [Mn(dgpy)2]4+, based on Earth-abundant manganese and the tridentate 2,6-diguanidylpyridine ligand (dgpy), evolves to a luminescent doublet ligand-to-metal charge transfer (2LMCT) excited state (1,435 nm, 0.86 eV) with a lifetime of 1.6 ns after excitation with low-energy near-infrared light. This 2LMCT state oxidizes naphthalene to its radical cation. Substrates with extremely high oxidation potentials up to 2.4 V enable the [Mn(dgpy)2]4+ photoreduction via a high-energy quartet 4LMCT excited state with a lifetime of 0.78 ps, proceeding via static quenching by the solvent. This process minimizes free energy losses and harnesses the full photooxidizing power, and thus allows oxidation of nitriles and benzene using Earth-abundant elements and low-energy light.

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.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C-N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.

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.

Macromolecular Photoediting Using Single-Electron Logic

Accessing the chemistry of reactive intermediates under mild conditions has significantly expanded the available chemical space for molecular transformations. Nowhere is this more apparent than in the context of photoredox catalysis. Despite abundant literature precedents for using this powerful methodology to build complex targets, there are comparatively few reports that leverage photoredox catalysis for macromolecular editing. Here, we report a mild photoredox approach that enables both the functionalization and degradation of polyalkenamers to valuable feedstocks. Irradiation with visible light (including natural sunlight) in the presence of a pyrillium photoredox catalyst promoted facile chain scission in a variety of substrates. This metal-free approach transformed high molar mass materials (>300 kDa) to low molar mass species (<15 kDa) within 10 min. Moreover, we could completely degrade macromolecules into a range of useful targets (C16-C29 species) within 96 h. Mechanistic and kinetic experiments were carried out to understand this reactivity, which could be coupled with hydrofunctionalizations to create tailored products.

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.

Triplet quenching pathway control with molecular dyads enables the identification of a highly oxidizing annihilator class

Metal complex - arene dyads typically act as more potent triplet energy donors compared to their parent metal complexes, which is frequently exploited for increasing the efficiencies of energy transfer applications. Using unexplored dicationic phosphonium-bridged ladder stilbenes (P-X2+) as quenchers, we exclusively observed photoinduced electron transfer photochemistry with commercial organic photosensitizers and photoactive metal complexes. In contrast, the corresponding pyrene dyads of the tested ruthenium complexes with the very same metal complex units efficiently sensitize the P-X2+ triplets. The long-lived and comparatively redox-inert pyrene donor triplet in the dyads thus provides an efficient access to acceptor triplet states that are otherwise very tricky to obtain. This dyad-enabled control over the quenching pathway allowed us to explore the P-X2+ photochemistry in detail using laser flash photolysis. The P-X2+ triplet undergoes annihilation producing the corresponding excited singlet, which is an extremely strong oxidant (+2.3 V vs. NHE) as demonstrated by halide quenching experiments. This behavior was observed for three P2+ derivatives allowing us to add a novel basic structure to the very limited number of annihilators for sensitized triplet-triplet annihilation in neat water.

Redox-neutral carbon-heteroatom bond formation under photoredox catalysis

Recently, visible-light-mediated photoredox catalysis has been emerging as one of the fastest growing fields in organic chemistry because of its low cost, easy availability and environmental benignness. In the past five years, a new yet challenging trend, visible-light-induced redox-neutral carbon-heteroatom bond formation reaction involving presumed radical intermediates, has been flourishing rapidly. Although mostly transition metal-based photoredox catalysts were reported, a few organophotoredox catalysts have also shown efficacy towards carbon-heteroatom bond formation reactions. This review intends to summarize the recent research progress in redox-neutral carbon-heteroatom bond formations based on active intermediate(s) involved under photoredox catalysis.

Visible-light acridinium-based organophotoredox catalysis in late-stage synthetic applications

The field of photoredox catalysis has been transformed by the use of organic photocatalysts, which give access to re-activities that were previously only possible with transition-metal photocatalysts. Recent advancements in the use of an acridinium photocatalyst in organic synthesis are covered in this review. Both the late-stage functionalization of biorelevant molecules and the activation of inert chemical bonds are explored, with an emphasis on their mechanistic features.