Visible‐Light‐Induced Photoredox Catalysis of Dye‐Sensitized Titanium Dioxide: Selective Aerobic Oxidation of Organic Sulfides

TiO₂ photoredox catalysis has recently attracted much interest for use in performing challenging organic transformations under mild reaction conditions. However, the reaction scheme is hampered by the fact that TiO₂ can only be excited by UV light of wavelengths λ shorter than 385 nm. One promising strategy to overcome this issue is to anchor an organic, preferably metal‐free dye onto the surface of TiO₂. Importantly, we observed that the introduction of a catalytic amount of the redox mediator TEMPO [(2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl] ensured the stability of the anchored dye, alizarin red S, thereby resulting in the selective oxidation of organic sulfides with O₂. This result affirms the essential role of the redox mediator in enabling the organic transformations by visible‐light photoredox catalysis.     

Late‐Stage Functionalization of Biologically Active Heterocycles Through Photoredox Catalysis

The direct C? H functionalization of heterocycles has become an increasingly valuable tool in modern drug discovery. However, the introduction of small alkyl groups, such as methyl, by this method has not been realized in the context of complex molecule synthesis since existing methods rely on the use of strong oxidants and elevated temperatures to generate the requisite radical species. Herein, we report the use of stable organic peroxides activated by visible‐light photoredox catalysis to achieve the direct methyl‐, ethyl‐, and cyclopropylation of a variety of biologically active heterocycles. The simple protocol, mild reaction conditions, and unique tolerability of this method make it an important tool for drug discovery.     

Synthesis of N‐Containing Heterocyclic Compounds Using Visible‐light Photoredox Catalysis

Nitrogen heterocycles represent a highly important class of compounds which are widely used in materials science, agrochemistry, and medicinal chemistry. Therefore, there is continuing interest in the development of convenient, efficient, and environmentally benign synthetic methods for the construction of nitrogen containing heterocycles. Due to its natural abundance, ease of use, and promising application in industry, the use of visible light as a driving force for chemical reactions has received considerable attention in the past few years. This account summarizes the synthesis of N‐heterocycles using visible‐light photoredox catalysis published in the last two years, according to the size and type of the formed N‐heterocyclic rings. In the context of seminal works of others in this area, a concise summary of the contributions of the authors is also offered.     

Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O‐aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible‐light‐mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et₃N was proposed as a key step of this process.     

可见光促进的氧化还原催化反应在天然产物全合成中的应用

可见光促进的光氧化还原催化在近年来取得了重要进展, 许多可见光光氧化还原的反应被用于复杂分子的构建. 本文概述了这一领域的最新成果, 重点介绍该策略作为关键步骤在天然产物及类天然产物化合物的全合成中的应用.     

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.     

Tandem Photoredox Catalysis: Enabling Carbonylative Amidation of Aryl and Alkylhalides

We report a new visible-light-mediated carbonylative amidation of aryl, heteroaryl, and alkyl halides. A tandem catalytic cycle of [Ir(ppy)₂(dtb-bpy)]⁺ generates a potent iridium photoreductant through a second catalytic cycle in the presence of DIPEA, which productively engages aryl bromides, iodides, and even chlorides as well as primary, secondary, and tertiary alkyl iodides. The versatile in situ generated catalyst is compatible with aliphatic and aromatic amines, shows high functional-group tolerance, and enables the late-stage amidation of complex natural products.     

Visible‐Light‐Induced Photoredox Catalysis with a Tetracerium‐Containing Silicotungstate

The development of visible‐light‐induced photocatalysts for chemoselective functional group transformations has received considerable attention. Polyoxometalates (POMs) are potential materials for efficient photocatalysts because their properties can be precisely tuned by changing their constituent elements and structures and by the introduction of additional metal cations. Furthermore, they are thermally and oxidatively more stable than the frequently utilized organometallic complexes. The visible‐light‐responsive tetranuclear cerium(III)‐containing silicotungstate TBA₆[{Ce(H₂O)}₂{Ce(CH₃CN)}₂(μ₄‐O)(γ‐SiW₁₀O₃₆)₂] (CePOM; TBA=tetra‐n‐butylammonium) has now been synthesized; when CePOM was irradiated with visible light (λ>400 nm), a unique intramolecular Ce^III‐to‐POM(W^VI) charge transfer was observed. With CePOM, the photocatalytic oxidative dehydrogenation of primary and secondary amines as well as the α‐cyanation of tertiary amines smoothly proceeded in the presence of O₂ (1 atm) as the sole oxidant.     

Radical Fluoroalkylation of Isocyanides with Fluorinated Sulfones by Visible‐Light Photoredox Catalysis

The radical fluoroalkylation of isocyanides with fluorinated sulfones is enabled by visible‐light photoredox catalysis. A wide range of readily available mono‐, di‐, and trifluoromethyl heteroaryl sulfones can thus be used as efficient radical fluoroalkylation reagents under mild conditions. This method not only describes a new synthetic application of fluorinated sulfones, but also provides a new route to fluoroalkyl radicals.     

Functionalization of C‐H Bonds by Photoredox Catalysis

Visible‐light photoredox catalysis has been successfully used in the functionalization of inert C?H bonds including C(sp²)‐H bonds of arenes and C(sp³)‐H bonds of aliphatic compounds over the past decade. These transformations are typically promoted by the process of single‐electron‐transfer (SET) between substrates and photo‐excited photocatalyst upon visible light irradiation (household bulbs or LEDs). Compared with other synthetic strategies, such as the transition‐metal catalysis and traditional radical reactions, visible‐light photoredox approach has distinct advantages in terms of operational simplicity and practicability. Versatile direct functionalization of inert C(sp²)‐H and C(sp³)‐H bonds including alkylation, trifluoromethylation, arylation and amidation, has been achieved using this practical strategy.     

Dual Catalysis Sees the Light: Combining Photoredox with Organo‐, Acid,and Transition‐Metal Catalysis

The photoredox activation of organic substrates with visible light is a powerful methodology that generates reactive radical species under very mild conditions. When combined with another catalytic process in a dual catalytic system, novel, visible‐light‐promoted transformations have been realized that do not proceed using either catalyst in isolation. In this minireview, the state of the art in organic reactions mediated by dual catalytic systems merging photoredox activation with organo‐, acid or metal catalysis is discussed.     

Metal‐Free,Room‐Temperature,Radical Alkoxycarbonylation of Aryldiazonium Salts through Visible‐Light Photoredox Catalysis

The first radical alkoxycarboxylation of aryldiazonium salts using CO gas through visible‐light‐induced photoredox catalysis (16 W blue LEDs) has been developed. This reaction is entirely metal‐free, is carried out at room temperature with a low loading of an organic dye as a photocatalyst (0.5 mol %), and provides a wide range of arylcarboxylic acid esters in high yields. Importantly, this photocatalytic system can be successfully extended to other carboxylation reactions.     

Enantioselective α‐Alkylation of Aldehydes by Photoredox Organocatalysis: Rapid Access to Pharmacophore Fragments from β‐Cyanoaldehydes

The combination of photoredox catalysis and enamine catalysis has enabled the development of an enantioselective α‐cyanoalkylation of aldehydes. This synergistic catalysis protocol allows for the coupling of two highly versatile yet orthogonal functionalities, allowing rapid diversification of the oxonitrile products to a wide array of medicinally relevant derivatives and heterocycles. This methodology has also been applied to the total synthesis of the lignan natural product (?)‐bursehernin.     

Perylene-Grafted Silicas: Mechanistic Study and Applications in Heterogeneous Photoredox Catalysis

A mechanistic study is herein presented for the use of heterogeneous photocatalysts based on perylene moieties. First, the successful immobilization of perylene diimides (PDI) on silica matrices is demonstrated, including their full characterization by means of electronic microscopy, surface area measurements, powder XRD, thermogravimetric analysis, and FTIR, ²⁹Si and ¹³C solid-state NMR, fluorescence, and diffuse reflectance spectroscopies. Then, the photoredox activity of the material was tested by using two model reactions, alkene oxidation and 4-nitrobenzylbromide reduction, and mechanistic studies were performed. The mechanistic insights into their photoredox activity show they have promising dual photocatalytic activity for both organic oxidations and reductions.     

CH Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis: In Situ Generation of the Oxidant

A combination of ruthenium and photoredox catalysis allowed the ortho olefination of phenols. Using visible light, the direct C? H functionalization of o‐(2‐pyridyl)phenols occurred, and diverse phenol ethers were obtained in good yields. The regeneration of the ruthenium catalyst was accomplished by a photoredox‐catalyzed oxidative process.     

Photoredox catalyst-mediated direct regioselective phosphonylation of indoles

An efficient hetero-cross-dehydrogenative-coupling (hetero-CDC) reaction between N-protected indoles and phosphites to 2-indolyphosphites is described. The regioselective methodology took place in the presence of photo redox catalyst Ru(bpy)₃(PF₆)₂ combined with oxygen as an clean oxidant when exposed to visible light, furnishing the 2-indolyphosphites as the exclusive products in moderate to good yields with good functional group tolerance. Moreover, the phosphonylation protocol was proved by the control reaction to proceed through the radical pathway.     

Surface-Plasmonic-Field-Induced Photoredox Catalysis and Mediated Electron Transfer for Washing-Free DNA Detection

Distance-dependent electromagnetic radiation and electron transfer have been commonly employed in washing-free fluorescence and electrochemical bioassays, respectively. In this study, we combined the two distance-dependent phenomena for sensitive washing-free DNA detection. A distance-dependent surface plasmonic field induces rapid photoredox catalysis of surface-bound catalytic labels, and distance-dependent mediated electron transfer allows for rapid electron transfer from the surface-bound labels to the electrode. An optimal system consists of a chemically reversible acceptor (Ru(NH₃)₆³⁺), a chemically reversible photoredox catalyst (eosin Y), and a chemically irreversible donor (triethanolamine). Side reactions with O₂ do not significantly decrease the efficiency of photoredox catalysis. Energy transfer quenching between the electrode and the label can be lowered by increasing the distance between them. Washing-free DNA detection had a detection limit of approximately 0.3 nm in buffer and 0.4 nm in serum without a washing step.     

Anti-Markovnikov Hydroazidation of Alkenes by Visible-Light Photoredox Catalysis

The anti-Markovnikov hydroazidation of alkenes has been accomplished under visible-light irradiation by using [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ as the photocatalyst and trimethylsilyl azide as the azidating agent. The reactions were greatly facilitated by water, the beneficial effect of which can be attributed to its participation in the reaction as the hydrogen donor, as indicated by deuterium isotope experiments. The reactions proceed under solvent free conditions in the presence of water. 4-Dimethylaminopyridine also exhibited a beneficial effect on the reactions. The present method enabled hydroazidation of several types of unactivated alkenes with good yields and high regioselectivity.     

Visible‐Light‐Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible‐light‐promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl–SCF₃ compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl–SCF₃‐containing cyclic ketone and oxindole derivatives can be accessed by radical‐polar crossover semi‐pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF₃ radical.