Photocatalytic Activation of Less Reactive Bonds and Their Functionalization via Hydrogen-Evolution Cross-Couplings

Engineering Atomic Sites and Proton Transfer Microenvironments for Bioinspired Photocatalytic Alcohol-Amine Coupling

Achieving a precise understanding and accurate design of heterogeneous catalysts based on bioinspired principles is challenging yet crucial to digging out optimal materials for artificial catalysis. Here, an ADH-mimicking dual-site photocatalyst (YCuCdS) is developed, and demonstrates the powerful effects of atomic site configuration and proton transfer environments on alcohol-amine coupling. Mechanism studies reveal that the alcohol substrate is effectively dehydrogenated at the Y sites, forming the carbonyl intermediates that rapidly experience condensation with the amine. Meanwhile, the released hydrogen species (Hads) migrate from adjacent Cu sites to active S atoms, promoting H2 production and hindering the over-hydrogenation of imine. As a result, a high imine yield of 92% is achieved, along with an H2 production rate of 7400 µmol g-1 h-1. This work showcases an effective strategy for the design of heterogeneous catalysts with bioinspiration.

Photocatalytic synthesis of ethylene glycol and hydrogen from methyl tert-butyl ether

In this work, we have developed a green and sustainable strategy for the synthesis of ethylene glycol, which is a highly valuable compound in chemical industry. In contrast to the currently applied energy-intensive process based on petroleum resources, this work demonstrates the photocatalytic pathway of methanol dehydrogenative coupling to produce ethylene glycol, utilizing methyl tert-butyl ether as the substrate to protect the hydroxyl group against oxidation. Photocatalytic tests reveal efficient C-C coupling of methyl tert-butyl ether with Pt/C-TiO(B)-650 catalyst under light irradiation, with the target product 1,2-di-tert-butoxyethane at a selectivity of 67% and a Pt-based turnover frequency of 2754 h-1. Scale up test demonstrates high stability of the system, reaching an accumulated turnover number of 120 000 as well as isolation of 13 g of the coupling product after 130 h irradiation. The target ethylene glycol is obtained by the hydrolysis of the dimer using the regenerable acidic resin catalyst.

Tandem Azolation/Aromatization of Tetrahydronaphthalenes with Hydrogen Evolution via Organophotoredox/Cobalt Dual Catalysis

Reported herein is a photoredox/cobaloxime dual-catalytic approach to execute tandem dehydrogenative azolation and aromatization of tetrahydronaphthalene for rapid construction of N-(β-naphthyl)azole architectures. This protocol highlights noble metal-free and external oxidants-free conditions, step- and atom-economy, and site-selectivity. A preliminary mechanistic study has uncovered that the transformation undergoes a N-centered radical mediated C-H/N-H cross-coupling followed by dehydrogenative aromatization of saturated naphthyl surrogates under visible light irradiation, and DFT calculations elucidate the site-selectivity.

Visible-Light-Induced Radical Hydroaroylation of Vinyl Boronic Ester with Aroyl Chlorides to Access β-Boryl Ketones

A visible-light-induced radical hydroaroylation of vinyl boronic ester with aroyl chlorides has been developed, yielding a variety of β-boryl ketones in moderate to good yields with broad functional group tolerance. This metal-free transformation offers a convenient and practical route to access valuable β-boryl ketones from readily available aroyl chlorides and vinyl boronic ester.

Palladium/Norbornene Cooperative Catalysis 2-Fold C-H/C-X Coupling: Construction of Polycyclic Aromatic Hydrocarbons from Brominated Benzimidazoles

A palladium/norbornene (NBE)-catalyzed 2-fold C-H/C-X coupling reaction of aryl iodides is reported. Bromine-substituted benzimidazoles were chosen as electrophilic and termination reagents, and the versatile polycyclic aromatic hydrocarbon products were successfully obtained. The strategy overcomes the challenge of catalyst poisoning by heterocyclic substrates. In addition, the imidazole moiety in the product is endowed with a localization role, thus enabling the compounds to be applied in a wider synthetic scenario, and the fluorescence persisted. Furthermore, the bioactivity evaluation has identified three promising leading compounds 3b, 4e, and 4i.

Photoinduced Autopromoted Ni-Catalyzed Three-Component Arylsulfonation Inspired by Density Functional Theory/Time-Dependent Density Functional Theory-Simulated Photoactive Nickel Species

The structure of the novel photoactive nickel species was simulated by density functional theory (DFT)/time-dependent density functional theory (TD-DFT) calculations. The application of the simplified photoactive nickel catalyst was demonstrated in a photoinduced nickel-catalyzed three-component arylsulfonation of 1,6-enynes. This reaction was autopromoted and proceeded in the absence of an additional photocatalyst. This methodology exhibited mild conditions, a broad substrate scope, and high efficiency.

Photocatalytic applications of covalent organic frameworks: synthesis, characterization, and utility

Photocatalysis has emerged as an energy efficient and safe method to perform organic transformations, and many semiconductors have been studied for use as photocatalysts. Covalent organic frameworks (COFs) are an established class of crystalline, porous materials constructed from organic units that are easily tunable. COFs importantly display semiconductor properties and respectable photoelectric behaviour, making them a strong prospect as photocatalysts. In this review, we summarize the design, synthetic methods, and characterization techniques for COFs. Strategies to boost photocatalytic performance are also discussed. Then the applications of COFs as photocatalysts in a variety of reactions are detailed. Finally, a summary, challenges, and future opportunities for the development of COFs as efficient photocatalysts are entailed.

Photoinduced Decarboxylative Thioacylation of N-Hydroxyphthalimide Esters with Tetraalkylthiuram Disulfides

Dithiocarbamate is a key structural sequence in pharmaceuticals and agrochemicals, and its synthesis is crucial in organic chemistry. Although significant progress has been made in related synthesis research, developing a practical and universal synthesis method remains fascinating. Herein, we report a new visible-light-induced decarboxylation coupling reaction between N-hydroxyphthalimide esters and tetraalkylthiuram disulfides, which uses Ir(ppy)3 as a photocatalyst to promote the generation of corresponding decarboxylation thioacylation product-dithiocarbamates in high yields. This redox-neutral protocol uses inexpensive and readily available starting material under mild reaction conditions, exhibiting broad substrate scope and wide functional group compatibility. This method can be further used for post modification of complex natural products and bioactive drugs.

Customized Photoelectrochemical C-N and C-P Bond Formation Enabled by Tailored Deposition on Photoanodes

Photoelectrochemistry (PEC) is burgeoning as an innovative solution to organic synthesis. However, the current PEC systems suffer from limited reaction types and unsatisfactory performances. Herein, we employ efficient BiVO4 photoanodes with tailored deposition layers for customizing two PEC approaches toward C-N and C-P bond formation. Our process proceeds under mild reaction conditions, deploying easily available substrates and ultra-low potentials. Beyond photocatalysis and electrocatalysis, customized PEC offers high efficiency, good functional group tolerance, and substantial applicability for decorating drug molecules, highlighting its promising potential to enrich the synthetic toolbox for broader organic chemistry of practical applications.

C-H Labeling with [18F]Fluoride: An Emerging Methodology in Radiochemistry

Fluorine-18 is the most routinely employed radioisotope for positron emission tomography, a dynamic nuclear imaging modality. The radiolabeling of C-H bonds is an attractive method for installing fluorine-18 into organic molecules since it can preclude the cumbersome prefunctionalization of requisite precursors. Although electrophilic "F+" reagents (e.g., [18F]F2) are effective for C-H radiolabeling, state-of-the-art methodologies predominantly leverage high molar activity nucleophilic [18F]fluoride sources (e.g., [18F]KF) with substantial (pre)clinical advantages. Reflecting this, multiple nucleophilic C-H radiolabeling techniques of high utility have been disclosed over the past decade. However, the adoption of (pre)clinical C-H radiolabeling has been slow, and PET imaging agents are still routinely prepared via methods that, despite a high level of practicality, are limited in scope (e.g., SNAr, SN2 radiofluorinations). By addressing the drawbacks inherent to these strategies, C-H radiofluorination and radiofluoroalkylation carry the potential to complement and supersede state-of-the-art labeling methods, facilitating the expedited production of PET agents used in disease staging and drug development. In this Outlook, we showcase recent C-H labeling developments with fluorine-18 and discuss the merits, potential, and barriers to adoption in (pre)clinical settings. In addition, we highlight trends, challenges, and directions in this emerging field of study.

Oxidative Organic Transformations Photocatalyzed by NDI in Visible Light

In this work, we report the synthesis and photocatalytic properties of N,N-bis(n-hexyl)-2-bromo-6-(n-hexylamino)-1,4,5,8-naphthalenetetracarboxylic diimide photocatalyst, NDI-PC, in visible light. In the presence of air or oxidant, NDI-PC efficiently enables multiple photooxygenations of isoquinolines, thiocyanation of phenylimidazopyridines, functionalization of quinolinones by allowing regioselective installation of an SCN, SeCN, SPh, SePh, Cl, Br, or I group at the C-3 position, and isomerization of alkenes. Mechanistic investigations suggest an oxidative photoredox process for oxygenation and C-H functionalization, while isomerization is believed to proceed through a photosensitization pathway.

A Photoredox Thiol-yne Reaction for the Synthesis of Vinyl Sulfide-Based Coumarins and its Effect on Fluorescence Properties

Coumarins still remain one of the most widely explored fluorescent dyes, with a broad spectrum of applications spanning various fields, such as molecular imaging, bioorganic chemistry, materials chemistry, or medical sciences. Their fluorescence is strongly based on a push-pull mechanism involving an electron-donating group (EDG), mainly located at the C7 or C8 positions of the dye core. Unfortunately, up to now, these positions have been very limited to hydroxyl or amino groups. In this study, we present in detail the synthesis of the first series of coumarins bearing a vinyl sulfide as the EDG at the C7 position. These derivatives were prepared by thiol-yne reaction, promoted by ruthenium- or porphyrin-based photoredox catalysis, enabling rapid late-stage diversification. We also functionalized coumarins with short peptides, and BSA protein as a proof-of-concept study, in a single-step process. This strategy, capable of proceeding under aqueous conditions, overcomes the protection/deprotection steps usually required by traditional methods, which also use strong bases and organic solvents. Moreover, the photophysical properties such as absorption and emission of obtained coumarins (for 3-CF3, 3-benzothiazole, 6-8-difluoro derivatives), predominantly exhibited large Stokes shifts (up to 204 nm) and maintained intramolecular charge transfer (ICT) characteristics.

A supramolecular photosensitizer based on triphenylamine and pyrazine with aggregation-induced emission properties for high-efficiency photooxidation reactions

Recently, there has been a great interest in the study of photocatalysts (PCs) and photosensitizers (PSs) in the field of organic photocatalysis. In the present study, a pure organic thermally activated delayed fluorescence (TADF) molecule 4,4'-(12-(pyridin-4-yl)dibenzo[f,h]pyrido[2,3-b]quinoxaline-3,6-diyl)bis(N,N-diphenylaniline) (DPQ-TPA) was designed and synthesized, which not only have excellent TADF property and small energy splitting (ΔEST), but also can self-assembly in water to form cross-linked nanoparticles with exceptional aggregation-induced emission (AIE) characteristics. DPQ-TPA exhibits excellent remarkable selectivity and notably enhances the production capacity of reactive oxygen species (ROS), particularly 1O2, which was employed as a highly effective photocatalyst in the photooxidation reaction of phosphine and hydroazobenzenes under blue light irradiation with high yields up to 94% and 91%, respectively. This work expands the potential application of (donor-acceptor) D-A type AIE-TADF molecules in photocatalytic organic transformations through supramolecular self-assembly.

Sulfonamide-directed site-selective functionalization of unactivated C(sp3)-H enabled by photocatalytic sequential electron/proton transfer

The generation of alkyl radical from C(sp3)-H substrates via hydrogen atom abstraction represents a desirable yet underexplored strategy in alkylation reaction since involving common concerns remain adequately unaddressed, such as the harsh reaction conditions, limited substrate scope, and the employment of noble metal- or photo-catalysts and stoichiometric oxidants. Here, we utilize the synergistic strategy of photoredox and hydrogen atom transfer (HAT) catalysis to accomplish a general and practical functionalization of unactived C(sp3)-H centers with broad reaction scope, high functional group compatibility, and operational simplicity. A combination of validation experiments and density functional theory reveals that the N-centered radicals, generated from free N - H bond in a stepwise electron/proton transfer event, are the key intermediates that enable an intramolecular 1,5-HAT or intermolecular HAT process for nucleophilic carbon-centered radicals formation to achieve heteroarylation, alkylation, amination, cyanation, azidation, trifluoromethylthiolation, halogenation and deuteration. The practical value of this protocol is further demonstrated by the gram-scale synthesis and the late-stage functionalization of natural products and drug derivatives.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Photocatalytic Generation of Carbocation from Thiols and Application to Cross-Nucleophile Coupling

Represented herein is a simple thiol identified as an effective precursor to photochemically form a carbocation. Thanks to the thiyl radical rapid transformation to disulfide, which serves not only to stabilize the generated thiyl radical but also to allow the second electron transfer to form a carbocation. The resulting carbocations, including primary benzylic, secondary, and tertiary carbocations, can smoothly couple with nitrogen, oxygen, and carbon nucleophilic coupling partners as well as complex drug molecules, accompanied by elemental sulfur formation in air.

Redox Relay-Induced C-S Radical Cross-Coupling Strategy: Application in Nontraditional Site-Selective Thiocyanation of Quinoxalinones

Oxidative cross-coupling is a powerful strategy to form C-heteroatom bonds. However, oxidative cross-coupling for constructing C-S bond is still a challenge due to sulfur overoxidation and poisoning transition-metal catalysts. Now, electrochemical redox relay using sulfur radicals formed in situ from inorganic sulfur source offers a solution to this problem. Herein, electrochemical redox relay-induced C-S radical cross-coupling of quinoxalinones and ammonium thiocyanate with bromine anion as mediator is presented. The electrochemical redox relay comprised initially the formation of sulfur radical via indirect electrochemical oxidation, simultaneous electrochemical reduction of the imine bond, electro-oxidation-triggered radical coupling involving dearomatization-rearomatization, and the reformation of the imine bond through anodic oxidation. Applying this strategy, various quinoxalinones bearing multifarious electron-deficient/-rich substituents at different positions were well compatible with moderate to excellent yields and good steric hindrance compatibility under constant current conditions in an undivided cell without transition-metal catalysts and additional redox reagents. Synthetic applications of this methodology were demonstrated through gram-scale preparation and follow-up transformation. Notably, such a unique strategy may offer new opportunities for the development of new quinoxalinone-core leads.

Tandem Vinylogous Aldol and Intramolecular [2 + 2] Cycloaddition toward Benzocyclobutenes by UV Light Photocatalysis

A facile and efficient radical tandem vinylogous aldol and intramolecular [2 + 2] cycloaddition reaction for direct synthesis of cyclobutane-containing benzocyclobutenes (BCBs) under extremely mild conditions without using any photocatalysts is reported. This approach exhibited definite compatibility with functional groups and afforded new BCBs with excellent regioselectivity and high yields. Moreover, detailed mechanism studies were carried out both experimentally and theoretically. The readily accessible, low-cost, and ecofriendly nature of the developed strategy will endow it with attractive applications in organic and medicinal chemistry.

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

Metal-Free, Hindered, Regioselective Access to Multifunctional Groups Diarylamines via SN Ar Substitution of P-Nitroso Aromatic Methyl Ether by Arylamines

Multifunctional groups diarylamines, an innovative product, efficiently produced from arylamines and p-nitrosoanisole derivatives by intermolecular SN Ar under weak acid conditions. This SN Ar proceeds under mild reaction conditions, and more significantly, the substrates involved do not necessarily require strong electron-withdrawing groups. Moreover, this SN Ar is characterized by resistance to space crowding, tolerance to halogen and nitroso functional groups, and high regioselectivity. Mechanistic observations suggest that the SN Ar is the result of the transfer of the positive charge center of the protonated nitroso group to the p-methoxy group.

Anodic Cyclizations, Densely Functionalized Synthetic Building Blocks, and the Importance of Recent Mechanistic Observations

Anodic cyclization reactions can provide a versatile method for converting newly obtained chiral lactols to densely functionalized cyclic building blocks. The method works by first converting the lactol into an electron-rich olefin and then oxidatively generating a radical cation that is trapped by a nucleophile. Historically, such reactions have benefited from the use of less polar radical cations when the trapping nucleophile is a heteroatom and more polar radical cations when the reaction forms C-C bonds. This forced one to optimize underperforming reactions by resynthesizing the substrate. Here, we show that by taking advantage of methods that serve to drive a reversible initial cyclization reaction toward the product, this dichotomy and need to manipulate the substrate can be avoided. Two such methods were utilized: a faster second oxidation step and a mediated electrolysis. Both led to successful cyclizations using a polar radical cation and heteroatom nucleophiles.

Cross-Coupling of Benzylic and Aldehydic C-H Bonds via Photocatalytic Tandem Radical-Radical Coupling and Acceptorless Alcohol Dehydrogenation

The construction of a C-C bond by cross-coupling of two different C-H bonds with the release of hydrogen gas represents an ideal yet challenging bond formation strategy. Herein, we report a photocatalytic metal-free cross-coupling of benzylic and aldehydic C-H bonds by synergistic catalysis of organophotocatalyst 4CzIPN and a thiol, which affords the corresponding α-aryl ketones in acceptable yields along with hydrogen evolution. The mechanistic investigation indicates a radical-radical coupling to give an intermediary alcohol, followed by an acceptorless alcohol dehydrogenation.

Metal-free visible light mediated direct C-H amination of benzoxazole with secondary amines

An efficient visible light mediated, eosin Y catalyzed direct C-H oxidative amination of benzoxazoles with secondary amines has been developed, which providing a straightforward, green, and environmentally benign access to a wide variety of substituted benzoxazole-2-amines under mild reaction conditions. The biological studies such as drug-likeness and molecular docking are also carried out on the molecule.

Al2 O3 -coated BiVO4 Photoanodes for Photoelectrocatalytic Regioselective C-H Activation of Aromatic Amines

Photoelectrochemistry is becoming an innovative approach to organic synthesis. Generally, the current photoelectrocatalytic organic transformations suffer from limited reaction type, low conversion efficiency and poor stability. Herein, we develop efficient and stable photoelectrode materials using metal oxide protective layer, with a focus on achieving regioselective activation of amine compounds. Notably, our photoelectrochemistry process is implemented under mild reaction conditions and does not involve any directing groups, transition metals or oxidants. The results demonstrate that beyond photocatalysis and electrocatalysis, photoelectrocatalysis exhibits high efficiency, remarkable repeatability and good functional group tolerance, highlighting its great potential for applications.

Host-Guest Photosensitizer of a Cationic BODIPY Derivative and Cucurbit[7]uril for High-Efficiency Visible Light-Induced Photooxidation Reactions

In recent years, there has been a notable surge of interest in the fields of organic and pharmaceutical research about photocatalysts (PCs) and photosensitizers (PSs). In this study, a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) molecule adorned with quaternary ammonium (TMB) functionality was meticulously designed and synthesized. This compound has remarkable characteristics such as exceptional water solubility, great optical qualities, and commendable photostability. It can form a 1:1 complex (TMB-CB[7]) with cucurbit[7]uril (CB[7]) through host-guest interactions in the aqueous solution and shows obvious fluorescence enhancement. The reactive oxygen species (ROS) including superoxide anion radical (O2·-) and singlet oxygen (1O2) generation ability of TMB-CB[7] were promoted compared with that of TMB in the aqueous solution. More interestingly, the ROS generated from TMB-CB[7] can be used as PCs for aerobic cross dehydrogenation coupling reactions and photooxidation reactions in water with high yields of 89 and 95%, respectively. Therefore, the utilization of a host-guest PS presents a novel and environmentally friendly approach for conducting photocatalyzed organic processes under ambient conditions using visible light.

Photochemical reductive deamination of alpha-amino aryl alkyl ketones

Photochemical reductive deamination of alpha-amino aryl alkyl ketones under photosensitizer-free conditions is presented. This protocol features high efficiency and selectivity. A plausible reaction pathway is proposed based on ultraviolet-visible absorption investigation, control experiments and deuterium-labelling studies. Mechanistic study reveals that the alpha-hydrogen atom of the ketone product originated from water.

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Manganese/Cobalt Bimetallic Relay Catalysis for Divergent Dehydrogenative Difluoroalkylation of Alkenes

The involvement of manganese radical for halogen atom transfer (XAT) reactions has been esteemed as one reliable method but encountered with limited catalytic models. In this paper, a novel bimetallic relay catalysis of Mn2 (CO)10 and cobaloxime has been developed for divergent dehydrogenative difluoroalkylation of alkenes using commercially available difluoroalkyl bromides. A wide range of structurally diverse terminal, cyclic and internal alkenes as well as tetrasubstituted alkenes are found to be good coupling partners to deliver difluoroalkylated allylic products and difluoromethylated cyclic products, accompanied with the production of H2 as the by-product. This bimetallic relay strategy features broad substrate scope, mild reaction conditions and excellent functional group compatibility. Its success represents an important step-forward to expedite the construction of a rich library of difluoroalkylated products.

Photochemical reactions of a diamidocarbene: cyclopropanation of bromonaphthalene, addition to pyridine, and activation of sp3 C-H bonds

We report unprecedented photochemistry for the diamidocarbene 1. Described within are the double cyclopropanation of 1-bromonaphthalene, the double addition to pyridine, and remarkably, the insertion into the unactivated sp3 C-H bonds of cyclohexane, tetramethylsilane, and n-pentane to give compounds 2-6, respectively. All compounds have been fully characterized, and the solid state structure of 4 was obtained using single crystal electron diffraction.

Photochemical Hydrogen Atom Transfer Catalysis for Dehydrogenation of Alcohols To Form Carbonyls

Controllable oxidation of alcohols to carbonyls is one of the fundamental transformations in organic chemistry. Herein, we report an unprecedented visible-light-mediated metal-free oxidation of alcohols to carbonyls with hydrogen evolution. By synergistic combination of organophotocatalyst 4CzIPN and a thiol hydrogen atom transfer catalyst, a broad range of alcohols, including primary and secondary benzylic alcohols as well as aliphatic alcohols, were readily oxidized to carbonyls in moderate to excellent yields. A site-selective oxidation has also been achieved by this protocol. Mechanistic investigation indicates that the oxidation proceeds through an oxidative radical-polar crossover process to obtain an α-oxy carbon cation.

Photoredox-Catalyzed Synthesis of 3-Sulfonylated Pyrrolin-2-ones via a Regioselective Tandem Sulfonylation Cyclization of 1,5-Dienes

A mild, visible-light-induced, regioselective cascade sulfonylation-cyclization of 1,5-dienes with sulfonyl chlorides through the intermolecular radical addition/cyclization of alkenes C(sp²)-H was developed. This procedure proceeds well and affords a mild and efficient route to a range of monosulfonylated pyrrolin-2-ones at room temperatures.

Metal-free visible-induced C(sp2)-C(sp2) coupling of quinoxalin-2(H)-ones via oxidative cleavage of the C-N bond

A C(sp²)-C(sp²) reaction between aromatic hydrazines and quinoxalines has been developed through a photocatalytic system. The protocol is established for C(sp²)-N bond cleavage and direct C(sp²)-H functionalization for the coupling of C(sp²)-C(sp²) via photocatalysis under mild and ideal air conditions without the presence of a strong base and metal. The mechanistic studies reveal that the generation of a benzene radical via the oxidative cleavage of aromatic hydrazines for the cross-coupling of C(sp²)-C(sp²) with the assistance of a photocatalyst is essential. The process exhibits excellent compatibility with functional groups and provides convenient access to various 3-arylquinoxalin-2(1H)-ones in good to excellent yields.

Ruthenium-Encapsulated Porphyrinic Organic Polymer as a Photoresponsive Oxidoreductase Mimetic Nanozyme for Colorimetric Sensing

The advantages of porosity and stable unpaired electrons of porphyrinic organic polymers (POPs) with free radicals are exclusive and potentially practical functionalities and combining the semiconductor-like characteristics of these materials and metal ions has been an effective way to assemble an efficient photocatalytic system. Herein, a new ruthenium (Ru) ion-encapsulated porphyrinic organic polymer (POP/Ru) is facilely synthesized as a proper photoresponsive nanozyme with unique photo-oxidase properties. Surprisingly, the proposed POP/Ru revealed outstanding photoresponsive oxidase-mimicking activity due to the synergetic effect of the integration of Ru and π-electrons of POP, which boosts charge separation and transport. POP/Ru was applied to the oxidation of o-phenylenediamine (o-PDA) as a chromogenic probe for producing a colorimetric signal. The kinetic study reveals that these photo-oxidase mimics have a significant affinity for the o-PDA chromogenic agent owing to a lower K_m and superior V_max. Further findings demonstrate that the presence of the l-arginine (l-Arg) target causes an inhibition effect on the photo-nanozymatic colorimetry of POP/Ru. This research develops the applications of the comprehensive colorimetric strategy for ultrasensitive l-Arg monitoring with a limit of detection (LOD) of 15.2 nM in the dynamic range of 4.0 nM-340 μM and illuminates that the proposed photo-oxidase nanozyme as a visual strategy is feasible in l-Arg environmentally friendly colorimetric detection in juice samples.

N-Allylation of Azoles with Hydrogen Evolution Enabled by Visible-Light Photocatalysis

Direct N-allylation of azoles with hydrogen evolution has been achieved through the synergistic combination of organic photocatalysis and cobalt catalysis. The protocol bypasses stoichiometric oxidants and prefunctionalization of alkenes and produces hydrogen (H₂) as the byproduct. This transformation highlights high step- and atom-economy, high efficiency, and broad functional group tolerance for further derivatization, which opens a door for C-N bond formation that is valuable in heterocyclic chemistry.

Selective oxidation of benzylic alcohols via synergistic bisphosphonium and cobalt catalysis

A synergistic photocatalytic system using a bisphosphonium catalyst and a cobalt catalyst has been developed, enabling the selective oxidation of benzylic alcohols under oxidant-free and environmentally benign conditions. High efficiencies have been obtained for a variety of alcohol substrates, and exclusive selectivity for aldehyde products has been achieved across the board. Furthermore, this photocatalytic system proved to be efficient when performed under continuous-flow conditions, even using a simple and easily assembled continuous-flow setup.

Mn-catalyzed electrooxidative radical phosphorylation of 2-isocyanobiaryls

As an efficient and green synthesis method, the electrocatalysis hydrogen evolution coupling reaction has been widely used by chemists to realize the combining of two nucleophiles. In this work, an alternative method to synthesize 6-phosphorylated phenanthridines has been developed by synergistically utilizing electrocatalysis and Mn catalysis, with moderate to relatively good yields achieved. Mild and oxidant-free conditions make this synthetic method applicable in various settings.

Building a cobaloxime-based metal-organic framework for photocatalytic aerobic oxidation of arylboronic acids to phenols

Herein, the design and synthesis of an unprecedented cobaloxime-based zirconium metal-organic framework (Zr-TCPCo) with an she net is reported. This heterogeneous material as a photocatalyst exhibits excellent catalytic activity for aerobic oxidation of arylboronic acids to phenols. Recycling experiments demonstrate the stability and reusability of Zr-TCPCo as a robust catalyst.

Direct Excitation of Aldehyde to Activate the C(sp2 )-H Bond by Cobaloxime Catalysis toward Fluorenones Synthesis with Hydrogen Evolution

A new way to form fluorenones via the direct excitation of substrates instead of photocatalyst to activate the C(sp² )-H bond under redox-neutral condition is reported. Our design relies on the photoexcited aromatic aldehyde intermediates that can be intercepted by cobaloxime catalyst through single electron transfer for following β-H elimination. The generation of acyl radical and successful interception by a metal catalyst cobaloxime avoid the use of a photocatalyst and stoichiometric external oxidants, affording a series of highly substituted fluorenones, including six-membered ketones, such as xanthone and thioxanthone derivatives in good to excellent yields, and with hydrogen as the only byproduct. This catalytic system features a readily available metal catalyst, mild reaction conditions and broad substrate scope, in which sunlight reaction and scale-up experiments by continuous-flow approach make the new methodology sustainable and amenable for potentially operational procedures.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Selective Reductive Coupling of Vinyl Azaarenes and Alkynes via Photoredox Cobalt Dual Catalysis

A visible light-induced Co-catalyzed highly regio- and stereoselective reductive coupling of vinyl azaarenes and alkynes has been developed. Notably, Hünig's base together with simple ethanol has been successfully applied as the hydrogen sources instead of commonly used Hantzsch esters in this catalytic photoredox reaction. This approach has considerable advantages for the straightforward synthesis of stereodefined multiple substituted alkenes bearing an azaarene motif, such as excellent regioselectivity (>20 : 1 for >30 examples) and stereoselectivity (>20 : 1 E/Z), broad substrate scope and good functional group compatibility under mild reaction conditions, which has been utilized in the concise synthesis of natural product monomorine I. A reasonable catalytic reaction pathway involving protolysis of the cobaltacyclopentene intermediate has been proposed based on the mechanistic studies.

α-Acylation of Alkenes by a Single Photocatalyst

A direct strategy for the difunctionalization of alkenes, with acylation occurring at the more substituted alkene position, would be attractive for complex ketone synthesis. We report herein a reaction driven by a single photocatalyst that enables α-acylation in this way with the introduction of a fluoromethyl, alkyl, sulfonyl or thioether group at the β-position of the alkene with high chemo- and regioselectivity under extremely mild conditions. Crucial to the success of this method are rate differences in the kinetics of radical generation through single-electron transfer (SET) between different radical precursors and the excited photocatalyst (PC*). Thus, the β-position of the alkene is first occupied by the group derived from the radical precursor that can be generated most readily, and α-keto acids could be used as an electrophilic reagent for the α-acylation of alkenes.

Synthesis of 10-Phenanthrenols via Photosensitized Triplet Energy Transfer, Photoinduced Electron Transfer, and Cobalt Catalysis

Due to the inert redox activity and high triplet energy, radical chemistry of 1,3-dicarbonyl compounds usually requires prefunctionalization substrates, external oxidant, and high-energy UV light. Here, we report a visible-light-driven photocatalyst/cobaloxime system composed of a photosensitized energy transfer reaction (PEnT) and photoinduced electron transfer reaction (PET) and with an interrupted 6π-photocyclization/dehydrogenative aromatization in one pot to synthesize 10-phenanthrenols. Preliminary mechanistic studies revealed that fac-Ir(ppy)₃ plays the dual roles of energy transfer catalysis for photocycloaddition via 1,2-biradical intermediates of 1,3-dicarbonyl compounds and photoredox/cobaloxime catalysis dehydrogenative aromatization of 1,4-biradical rather than the intermediates via 6π photocyclization in the tandem reaction. In contrast to previous well-established radical chemistry of 1,3-dicarbonyl compounds, we provide a new strategy for the activation of 1,3-dicarbonyl compounds under visible light catalysis, affording a novel cyclization strategy with extremely high atom economy for the synthesis of 10-phenanthrenols.

Ni(II)-Mediated Photochemical Oxidative Esterification of Aldehydes with Phenols

The photopromoted, Ni-catalyzed acceptorless dehydrogenation esterification of phenols and aromatic aldehydes has been achieved in an oxidant- and external photosensitizer-free manner. This reliable and atom-economical transformation was tolerant to a wide range of functional groups and proceeded efficiently to give various aryl benzoates in moderate to high yields. Additionally, this photocatalytic system displayed high activity for the hydrogen-evolution cross coupling of aliphatic aldehydes and phenols employing dual nickel and aromatic aldehyde catalysis.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Photochemistry Journey to Multielectron and Multiproton Chemical Transformation

The odyssey of photochemistry is accompanied by the journey to manipulate "electrons" and "protons" in time, in space, and in energy. Over the past decades, single-electron (1e^-) photochemical transformations have brought marvelous achievements. However, as each photon absorption typically generates only one exciton pair, it is exponentially challenging to accomplish multielectron and proton photochemical transformations. The multistep differences in thermodynamics and kinetics urgently require us to optimize light harvesting, expedite consecutive electron transfer, manipulate the interaction of catalysts with substrates, and coordinate proton transfer kinetics to furnish selective bond formations. Tandem catalysis enables orchestrating different photochemical events and catalytic transformations from subpicoseconds to seconds, which facilitates multielectron redox chemistries and brings consecutive, value-added reactivities. Joint efforts in molecular and material design, mechanistic understanding, and theoretical modeling will bring multielectron and proton synthetic opportunities for fuels, fertilizers, and chemicals with enhanced versatility, efficiency, selectivity, and scalability, thus taking better advantage of photons (i.e., sunlight) for our sustainable society.