Iron-Catalyzed Photoredox Alcohol α-C-H Alkylation and Tandem Intramolecular Cyclization: Facile Access to Multisubstituted 2,3-Dihydrofurans and γ-Butyrolactones

Multisubstituted furans occupy a pivotal position within the realms of synthetic chemistry and pharmacological science due to their distinctive chemical configurations and inherent properties. We herein introduce a tandem difunctionalization protocol of alcohols for the efficient synthesis of multisubstituted 2,3-dihydrofurans and γ-butyrolactones through the combination of photocatalysis and iron catalysis under mild conditions. Photoredox alcohol α-C(sp3)-H activation and Pinner-type intramolecular cyclization are two key processes. This method features significant convenience, economic benefits, and environmental friendliness.

Organophotoredox-Catalyzed C-H Functionalizations of Benzophospholes

An organophotoredox-catalyzed oxidative C-H functionalization of benzophospholes has been developed. The C-H alkoxycarbonylation with methyl carbazate occurs in the presence of Rose bengal, whereas Eosin Y enables the dehydrogenative coupling with secondary phosphine oxides and ethers, delivering the C-H phosphinylated and alkylated products. The scope of coupling partners is complementary to that of conventional metal-promoted C-H activation, thus successfully expanding the chemical space of substituted phospholes accessed by C-H functionalization protocols.

Thiobenzoic Acid-Catalyzed Cα-H Cross Coupling of Benzyl Alcohols with α-Ketoacid Derivatives

The C-H alkylation of benzyl alcohols with α-ketoacid derivatives was achieved in the presence of thiobenzoic acid with or without Ru or Ir photoredox catalysts. The thiobenzoic acid serves as a photoexcited single-electron reducing reagent and a hydrogen atom transfer catalyst, while addition of the metal photoredox catalyst assists the electron transfer and improves the reaction efficiency. Various functional groups were tolerant of the reaction conditions, and sterically hindered diols were produced in good to high yield.

Metal Free Functionalization of Saturated Heterocycles with Vinylarenes and Pyridine Enabled by Photocatalytic Hydrogen Atom Transfer

Saturated heterocycles are important class of structural scaffolds in small-molecule drugs, natural products, and synthetic intermediates. Here, we disclosed a metal free, mild, and scalable functionalization of saturated heterocycles using vinylarenes as a linchpin approach. Key to success of this transformation is the employing of simple and cheap benzophenone as a hydrogen atom transfer (HAT) catalyst. This operationally robust process was used for the making of diverse functionalized saturated heterocycles. Furthermore, aldehydes, alkane, and alcohol have been functionalized under the optimized conditions. The potential pharmaceutical utility of the procedure has also been demonstrated by late-stage functionalization of bioactive natural compounds and pharmaceutical molecules. Initial mechanism studies and control experiments were performed to elucidate the mechanism of the reactions.

Hydrosulfonylation of Alkynes for Stereodivergent Synthesis of Vinyl Sulfones: Synthetic Strategy and Mechanistic Insights

Direct synthesis of thermodynamically less favorable (Z)-vinyl sulfones presents a notable challenge in organic synthesis. In addition, the development of a stereodivergent synthesis for (E)- and (Z)-vinyl sulfones is crucial but remains elusive. In this study, we present a hydrosulfonylation of aryl-substituted alkynes, achieving a stereodivergent synthesis of (E)- and (Z)-vinyl sulfones by leveraging both thermodynamic and kinetic controls. Notably, the synthesis of challenging (Z)-vinyl sulfones was achieved through a kinetically controlled process without the need for a catalyst. To synthesize (E)-vinyl sulfones, unconventional visible light-mediated isomerization was employed as a means of facilitating the transition to the thermodynamically favored form. The present study encompasses a comprehensive experimental and computational investigation, which provides valuable insights into the reaction mechanism. This investigation reveals two plausible isomerization pathways: a novel double spin-flip mechanism and a hydrogen atom transfer process in the presence of eosin Y. This study not only advances our understanding of isomerization mechanisms beyond conventional energy-transfer routes but also offers a robust and switchable strategy for synthesizing (E)- and (Z)-vinyl sulfones, thereby providing a versatile avenue for the creation of valuable compounds in the fields of organic synthesis and medicinal chemistry.

Multi-stimuli-responsive polymer degradation by polyoxometalate photocatalysis and chloride ions

Photocatalytic polymer degradation based on harnessing the abundant light energy present in the environment is one of the promising approaches to address the issue of plastic waste. In this study, we developed a multi-stimuli-responsive photocatalytic polymer degradation system facilitated by the photocatalysis of a polyoxometalate [γ-PV2W10O40]5- in conjunction with chloride ions (Cl-) as harmless and abundant stimuli. The degradation of various polymers was significantly accelerated in the presence of Cl-, which was attributed to the oxidation of Cl- by the polyoxometalate photocatalysis into a highly reactive chlorine radical that can efficiently generate a carbon-centered radical for subsequent polymer degradation. Although organic and organometallic photocatalysts decomposed under the conditions for photocatalytic polymer degradation in the presence of Cl-, [γ-PV2W10O40]5- retained its structure even under these highly oxidative conditions.

TBADT-Mediated Photocatalytic Stereoselective Radical Alkylation of Chiral N-Sulfinyl Imines: Towards Efficient Synthesis of Diverse Chiral Amines

Herein we describe a sustainable and efficient photocatalytic method for the stereoselective radical alkylation of chiral sulfinyl imines. By employing readily available non-prefunctionalized radical precursors and the cost-effective TBADT as a direct HAT photocatalyst, we successfully obtain diverse chiral amines with high yields and excellent diastereoselectivity under mild conditions. This method provides an efficient approach for accessing a diverse array of medicinally relevant compounds, including both natural and synthetic α-amino acids, aryl ethyl amines, and other structural motifs commonly found in approved pharmaceuticals and natural product.

Guest-Regulated Generation of Reactive Oxygen Species from Porphyrin-Based Multicomponent Metallacages for Selective Photocatalysis

The development of novel materials for highly efficient and selective photocatalysis is crucial for their practical applications. Herein, we employ the host-guest chemistry of porphyrin-based metallacages to regulate the generation of reactive oxygen species and further use them for the selective photocatalytic oxidation of benzyl alcohols. Upon irradiation, the sole metallacage (6) can generate singlet oxygen (1O2) effectively via excited energy transfer, while its complex with C70 (6⊃C70) opens a pathway for electron transfer to promote the formation of superoxide anion (O2⋅-), producing both 1O2 and O2⋅-. The addition of 4,4'-bipyridine (BPY) to complex 6⊃C70 forms a more stable complex (6⊃BPY) via the coordination of the Zn-porphyrin faces of 6 and BPY, which drives fullerenes out of the cavities and restores the ability of 1O2 generation. Therefore, benzyl alcohols are oxidized into benzyl aldehydes upon irradiation in the presence of 6 or 6⊃BPY, while they are oxidized into benzoic acids when 6⊃C70 is employed as the photosensitizing agent. This study demonstrates a highly efficient strategy that utilizes the host-guest chemistry of metallacages to regulate the generation of reactive oxygen species for selective photooxidation reactions, which could promote the utilization of metallacages and their related host-guest complexes for photocatalytic applications.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

Flavin-Mediated Photocatalysis Provides a General Platform for Sulfide C-H Functionalization

Functionalized sulfides are important in many areas of science, ranging from chemical biology through drug discovery to organic materials chemistry. Sulfides bearing pendant reactive groups in the α-position are particularly useful; however, methods for the selective valorization of simple sulfides or the late-stage functionalization of complex sulfides by the convenient addition of valuable functionality are underexplored. Here we exemplify a general reaction platform for sulfide functionalization by showcasing three modes of α-sulfur C-H functionalization; cyanation, alkenylation, and alkynylation. Using inexpensive and commercially available riboflavin tetraacetate and visible light, decoration of both feedstock and complex sulfides proceeds in a good yield and with high selectivity. Methionine-containing peptides can also be selectively functionalized and a tolerance screen using amino-acid dopants suggests that the platform is compatible with most amino-acid side chains and thus is a potential tool for bioconjugation.

Photoredox-Catalyzed α-C-H Monoalkylation of Symmetric Polyols in the Presence of CO2

Achieving the selective modification of symmetric poly-hydroxylated compounds presents a significant challenge due to the presence of identical active sites. Herein, we address this challenge through the design of a ternary catalytic system that includes a photoredox catalyst, a hydrogen atom transfer promotor and a carbonation catalyst. This catalytic system enables the reversible carbonation of acyclic polyols under CO2 atmosphere, which modulates the reactivity of its distinct C-H bonds toward hydrogen atom transfers. An exquisite selectivity for the monoalkylation is achieved in a variety of unprotected light polyols, yielding valuable building blocks in short reaction times. Mechanistic and computational studies demonstrate that the formation of an intramolecular hydrogen bond between the transient carbonate and the free alcohol is pivotal for the kinetic and thermodynamic activation of a specific alcohol.

Terminal C(sp3)-H borylation through intermolecular radical sampling

Hydrogen atom transfer (HAT) processes can overcome the strong bond dissociation energies (BDEs) of inert C(sp3)-H bonds and thereby convert feedstock alkanes into value-added fine chemicals. Nevertheless, the high reactivity of HAT reagents, coupled with the small differences among various C(sp3)-H bond strengths, renders site-selective transformations of straight-chain alkanes a great challenge. Here, we present a photocatalytic intermolecular radical sampling process for the iron-catalyzed borylation of terminal C(sp3)-H bonds in substrates with small steric hindrance, including unbranched alkanes. Mechanistic investigations have revealed that the reaction proceeds through a reversible HAT process, followed by a selective borylation of carbon radicals. A boron-sulfoxide complex may contribute to the high terminal regioselectivity observed.

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.

Modular synthesis of congested β2,2-amino acids via the merger of photocatalysis and oxidative functionalisations

A two-step protocol for the modular synthesis of β2- and α-quaternary β2,2-amino acid derivatives is reported. The key steps are a photocatalytic hydroalkylation reaction, followed by an oxidative functionalisation to access N-protected β-amino acids, esters, and amides. This strategy can be effectively scaled up via continuous-flow technology.

Automated self-optimization, intensification, and scale-up of photocatalysis in flow

The optimization, intensification, and scale-up of photochemical processes constitute a particular challenge in a manufacturing environment geared primarily toward thermal chemistry. In this work, we present a versatile flow-based robotic platform to address these challenges through the integration of readily available hardware and custom software. Our open-source platform combines a liquid handler, syringe pumps, a tunable continuous-flow photoreactor, inexpensive Internet of Things devices, and an in-line benchtop nuclear magnetic resonance spectrometer to enable automated, data-rich optimization with a closed-loop Bayesian optimization strategy. A user-friendly graphical interface allows chemists without programming or machine learning expertise to easily monitor, analyze, and improve photocatalytic reactions with respect to both continuous and discrete variables. The system's effectiveness was demonstrated by increasing overall reaction yields and improving space-time yields compared with those of previously reported processes.

C-H Alkylation of Cubanes via Catalytic Generation of Cubyl Radicals

A catalytic method for the C-H alkylation of cubanes is described. Some hydrogen atom transfer catalysts enable the direct abstraction of a hydrogen atom from the C-H bond of cubanes, followed by conjugate addition of the generated cubyl radicals to electron-deficient alkenes. Synthetic applications of the functionalization method developed are also described.

In Operando Visualization of Elementary Turnovers in Photocatalytic Organic Synthesis

We report the in operando visualization of the photocatalytic turnovers on single eosin Y (EY) through a redox-induced photoblinking phenomenon. The photocatalytic cyclization of thiobenzamide (TB) catalyzed by EY was investigated. The analysis of the intensity-versus-time trajectories of single EYs revealed the kinetics and dynamics of the elementary photocatalytic turnovers and the heterogeneity of the activity of individual EYs. The quenching turnover time showed a fast population and a slow population, which could be attributed to the singlet and triplet states of photoexcited EY. The slow quenching turnovers were more dominant at higher TB concentrations. The activity heterogeneity of EYs was studied over a series of reactant concentrations. Excess quenching reagent was found to decrease the percentage of active EYs. The method can be broadly applied to studying the elementary processes of photocatalytic organic reactions in operando.

Organo-Photoredox Catalyzed C(sp3 )-H Bond Arylation of Aliphatic Amides

A C(sp3 )-H bond arylation of aliphatic amides has been achieved via organophotoredox catalysis. The reaction could be realized at room temperature with visible light source and metal-free catalyst. Quinuclidine is employed as an efficient HAT reagent and a range of aliphatic amides is employed as both substrate and solvent in the reaction. This photocatalyzed transformation provides a convenient protocol to afford a board range of N-benzyl amides.

Photoredox-Catalyzed Radical-Radical Cross-Coupling of α-Ketoesters with Ethers: Access to Sterically Hindered α-Hydroxy Esters

We describe a photoredox catalysis method for synthesizing sterically hindered α-hydroxy esters from α-ketoesters and ethers through a radical-radical cross-coupling reaction. This approach utilizes commercially available Ir[dF(CF3)ppy]2(dtbbpy)PF6 as a photocatalyst and inexpensive and readily available nBu4NBr as a hydrogen atom transfer catalyst. Unactivated tetrahydrofuran and other ethers effectively react with various α-ketoesters to yield the desired products. The efficiency of this reaction is highlighted by its broad substrate scope, good functional group tolerance, and mild reaction conditions.

Sun Light Responsive 2D Covalent-Organic Frameworks Platform as a Catalysts Boost C-H Bond Arylation and Dopamine Regeneration

Photocatalysis is one of the most promising methods for producing organic compounds with a renewable source of energy. Two-dimensional covalent organic frameworks (2D COFs) are a type of polymer that has developed as a potential light-harvesting catalyst for artificial photosynthesis with a design-controllable platform that might be developed into a new type of cost-effective and metal-free photocatalyst. Here, we present a two-dimensional covalent organic framework synthesis technique as a low-cost and highly efficient visible light active flexible photocatalyst for C-H bond activation and dopamine regeneration. 2D COF were synthesized from tetramino-benzoquinone (TABQ) and terapthaloyl chloride monomer through condensation polymerization reaction and the resultant photocatalyst have remarkable performance due to its visible light-harvesting capacity, appropriate band gap, and highly organized π-electron channels. The synthesized photocatalyst is capable to convert dopamine into leucodopaminechrome with a higher yield (77.08%) and also capable to activate the C-H bond between 4-nitrobenzenediazonium tetrafluoroborate and pyrrole.

Photochemical Synthesis of S,N,O-Containing Polyheterocycles via an α-C(sp3)-H Functionalization/Radical Cyclization Cascade

A highly effective approach based on an organophotocatalytic α-C(sp3)-H functionalization/radical cyclization cascade has been developed. This method enables the synthesis of various tricyclic heterocycles containing S, O, and N atoms with excellent site selectivity and diastereoselectivity. Mechanistic investigations have confirmed that the reaction involves photoredox-triggered C(sp3)-H cleavage followed by a radical cyclization and aromatization process. These findings are expected to pave the way for developing cost-effective tandem radical reactions and synthesizing heterocyclic drugs.

Reductive Aldol Approach to Natural Products: Bioinspired Synthesis of abeo-11(12 → 13)-Oleanane Triterpenoids

A synthesis of alstoscholarinoid B (1) and 3β-acetoxy-11α-hydroxy-11(12 → 13)abeooleanan-12-al (2) has been accomplished in 7-9 steps and 10%-16% overall yield from oleanolic acid. This synthesis featured a bioinspired SmI2-mediated reductive aldol reaction to establish the abeo-11(12 → 13)-oleanane framework of both 1 and 2 and a retro-aldol/aldol/lactonization cascade to fully construct the skeleton of 1. Moreover, the investigation of the bioinspired aldol reaction also sheds light on the potential biogenesis of natural products.

2-Isopropylthioxanthone-Catalyzed Divergent Functionalization of Bicyclo[1.1.0]butanes under Visible-Light Irradiation

1,3-Functionalized cyclobutane structural motifs are ubiquitous in natural products and pharmaceuticals. Photoinduced alkylation of bicyclo[1.1.0]butanes (BCBs) offers a step-economical strategy for accessing 1,3-functionalized cyclobutane motifs. Herein, we disclose a general and mild photocatalytic protocol of bromoallylation and alkylation of BCBs in a metal, additive-free manner by using the same photocatalyst, 2-isopropylthioxanthone, in different catalytic roles. Furthermore, the synthetic utility of these products was illustrated in the synthesis of various valuable and complex cyclobutane derivatives.

Transition metal-free visible light photoredox-catalyzed remote C(sp3)-H borylation enabled by 1,5-hydrogen atom transfer

The borylation of unreactive carbon-hydrogen bonds is a valuable method for transforming feedstock chemicals into versatile building blocks. Here, we describe a transition metal-free method for the photoredox-catalyzed borylation of unactivated C(sp³)-H bond, initiated by 1,5-hydrogen atom transfer (HAT). The remote borylation was directed by 1,5-HAT of the amidyl radical, which was generated by photocatalytic reduction of hydroxamic acid derivatives. The method accommodates substrates with primary, secondary and tertiary C(sp³)-H bonds, yielding moderate to good product yields (up to 92%) with tolerance for various functional groups. Mechanistic studies, including radical clock experiments and DFT calculations, provided detailed insight into the 1,5-HAT borylation process.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Photoredox-HAT Catalysis for Primary Amine α-C-H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

The synergistic use of (organo)photoredox catalysts with hydrogen-atom transfer (HAT) cocatalysts has emerged as a powerful strategy for innate C(sp³)-H bond functionalization, particularly for C-H bonds α- to nitrogen. Azide ion (N₃^-) was recently identified as an effective HAT catalyst for the challenging α-C-H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N₃^- to photoexcited 4CzIPN reveals the participation of the S₁ excited electronic state of the organic photocatalyst as an electron acceptor, but the N₃^• radical product of this reaction is not observed. Instead, both time-resolved infrared and UV-visible spectroscopic measurements implicate rapid association of N₃^• with N₃^- (a favorable process in acetonitrile) to form the N₆^•- radical anion. Electronic structure calculations indicate that N₃^• is the active participant in the HAT reaction, suggesting a role for N₆^•- as a reservoir that regulates the concentration of N₃^•.

Modular Synthesis of Polar Spirocyclic Scaffolds Enabled by Radical Chemistry

Herein, we report a highly modular strategy to access spirocyclic scaffolds from abundant starting materials, i.e., cyclic ketones and α-amino or oxamic acids. The sequence proceeds through a straightforward Knoevenagel condensation, followed by a domino Giese-type reaction/base-mediated cyclization process, to deliver a broad scope of polar spirocyclic scaffolds in good to excellent yields. The products can be readily diversified, thus increasing the versatility of our method to gain rapid access to libraries of potential druglike molecules.

Sequential Modifications of Metal-Organic Layer Nodes for Highly Efficient Photocatalyzed Hydrogen Atom Transfer

Herein, we report the synthesis of a bifunctional photocatalyst, Zr-OTf-EY, through sequential modifications of metal cluster nodes in a metal-organic layer (MOL). With eosin Y and strong Lewis acids on the nodes, Zr-OTf-EY catalyzes cross-coupling reactions between various C-H compounds and electron-deficient alkenes or azodicarboxylate to afford C-C and C-N coupling products, with turnover numbers of up to 1980. In Zr-OTf-EY-catalyzed reactions, Lewis acid sites bind the alkenes or azodicarboxylate to increase their local concentrations and electron deficiency for enhanced radical additions, while EY is stabilized by site isolation on the MOL to afford a long-lived catalyst for hydrogen atom transfer. The proximity between photostable EY sites and Lewis acids on the nodes of Zr-OTf-EY enhances the catalytic efficiency by approximately 400 times over the homogeneous counterpart in the cross-coupling reactions.

Allylic hydroxylation of enones useful for the functionalization of relevant drugs and natural products

Enones are privileged structural motifs in bioactive natural products and pharmaceuticals, but the γ-hydroxylation of enones is challenging. Here we show a mild and efficient method for the direct C(sp³)-H hydroxylation of enones via visible-light-induced hydrogen-atom transfer (HAT), which facilitates γ-hydroxylation of primary, secondary, and tertiary C-H bonds of different enones without involving metal and peroxide. The mechanism study shows that Na₂-eosin Y serves as both the photocatalyst and the source of catalytic bromine radical species in the HAT-based catalytic cycle, and finally sacrifices itself completely by oxidative degradation to produce bromine radical and a major product phthalic anhydride in an environmentally friendly way. This scalable method was demonstrated by plenty of substrates (41 examples) including 10 clinical drugs and 15 natural products to be useful for the late-stage functionalization of enone-containing compounds, and, in particular, has potential application in industry for large-scale production.

Visible light-induced direct alkylation of the purine C8-H bond with ethers

The one-step visible light-induced direct alkylation of the C₈-H bond for purine derivatives by ethers was developed using Eosin Y as the photocatalyst and t-BuOOH as the oxidant at room temperature. This method describes the coupling of the α-C of the ether to the C₈ of purine. Of particular interest is that substrates include purines with various functional groups and even unprotected 9H-purines. The protocol provides an effective method for the synthesis of 8-alkylpurine derivatives with high atom economy and high regioselectivity.

Ultrathin 2D Cerium-Based Metal-Organic Framework Nanosheet That Boosts Selective Oxidation of Inert C(sp3 )H Bond through Multiphoton Excitation

The development of methodologies for inducing and tailoring activities of catalysts is an important issue in various catalysis. The ultrathin 2D monolayer metal-organic framework (MOF) nanosheets with more accessible active sites and faster diffusion obtained by exfoliating 3D layered MOFs are of great potential as heterogeneous catalysts, but the rational design and preparation of 3D layered MOFs remains a grand challenge. Herein, a novel weak electrostatic interaction strategy to construct a 3D layered cerium-bearing MOF by coordinating chlorine-capped cerium nodes and linear photoactive methyl viologen (MV⁺ ) organic linkers is used. Under multiphoton excitation, the MV⁺ ligands and CeCl chromophores are triggered consecutively to form the high activity chlorine radical (Cl^• ) for activation of inert C(sp³ )H bond through a hydrogen atom transfer. Benefiting from framework confinement effects, synergistic effects of two active sites and/or flexibility of the ultrathin framework nanosheets with high surface utilization, the observed activities increase in the order CeCl₃ /MV⁺  < bulk 3D MOF crystals < 2D MOF nanosheets in photocatalysis. This work not only contributes a new strategy to construct 3D layered MOFs and their ultrathin nanosheets but also paves the way to use nanostructured MOFs to handle synergy of multiple molecular catalysts.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

Direct Synthesis of Thioesters from Feedstock Chemicals and Elemental Sulfur

The development of a mild, atom- and step-economical catalytic strategy that effectively generates value-added molecules directly from readily available commodity chemicals is a central goal of organic synthesis. In this context, the thiol-ene click chemistry for carbon-sulfur (C-S) bond construction has found widespread applications in the synthesis of pharmaceuticals and functional materials. In contrast, the selective carbonyl thiyl radical addition to carbon-carbon multiple bonds remains underdeveloped. Herein, we report a carbonyl thiyl radical-based thioester synthesis through three-component coupling from feedstock aldehydes, alkenes, or alkynes and elemental sulfur by direct photocatalyzed hydrogen atom transfer. This method represents an orthogonal strategy to the conventional thiol-based nucleophilic substitution and exhibits a remarkably broad substrate scope ranging from simple commodity chemicals such as ethylene and acetylene to complex pharmaceutical molecules. This protocol can be easily extended to the synthesis of thiolactones, oligomer/polymers, and thioacids. Its synthetic utility has been demonstrated by a two-step synthesis of the drug esonarimod. Mechanistic studies indicate that the use of elemental sulfur to trap acyl radicals is both thermodynamically and kinetically favored, illustrating its great potential for the synthesis of sulfur-containing molecules.

Stepwise on-demand functionalization of multihydrosilanes enabled by a hydrogen-atom-transfer photocatalyst based on eosin Y

Organosilanes are of vital importance for modern human society, having found widespread applications in functional materials, organic synthesis, drug discovery and life sciences. However, their preparation remains far from trivial, and on-demand synthesis of heteroleptic substituted silicon reagents is a formidable challenge. The generation of silyl radicals from hydrosilanes via direct hydrogen-atom-transfer (HAT) photocatalysis represents the most atom-, step-, redox- and catalyst-economic pathway for the activation of hydrosilanes. Here, in view of the green characteristics of neutral eosin Y (such as its abundance, low cost, metal-free nature, absorption of visible light and excellent selectivity), we show that using it as a direct HAT photocatalyst enables the stepwise custom functionalization of multihydrosilanes, giving access to fully substituted silicon compounds. By exploiting this strategy, we realize preferable hydrogen abstraction of Si-H bonds in the presence of active C-H bonds, diverse functionalization of hydrosilanes (for example, alkylation, vinylation, allylation, arylation, deuteration, oxidation and halogenation), and remarkably selective monofunctionalization of di- and trihydrosilanes.

Photochemical synthesis of pyrano[2,3-d]pyrimidine scaffolds using photoexcited organic dye, Na2 eosin Y as direct hydrogen atom transfer (HAT) photocatalyst via visible light-mediated under air atmosphere

The Knoevenagel-Michael cyclocondensation of barbituric acid/1,3-dimethylbarbituric acid, malononitrile, and arylaldehyde derivatives was used to construct a multicomponent green tandem method for the metal-free synthesis of pyrano[2,3-d]pyrimidine scaffolds. At room temperature in aqueous ethanol, photo-excited state functions generated from Na₂ eosin Y were employed as direct hydrogen atom transfer (HAT) catalysts by visible light mediated in the air atmosphere. This research looks towards expanding the use of a non-metallic organic dye that is both affordable and readily available. Because of its good yields, energy-effectiveness, high atom economy, time-saving qualities of the reaction, and operational simplicity, Na₂ eosin Y is photochemically produced with the least amount of a catalyst. As a result, various ecological and sustainable chemical properties are met. Surprisingly, such cyclization may be carried out on a gram scale, indicating the reaction's potential industrial application.

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.

Visible-Light-Active Coumarin- and Quinolinone-Based Photocatalysts and Their Applications in Chemical Transformations

Organic dyes have been actively studied as useful photocatalysts because they allow access to versatile structural flexibility and green synthetic applications. The identification of a new class of robust organic chromophores is, therefore, in high demand to increase structural diversity and variability. Although coumarins and quinolinones have long been acknowledged as organic chromophores, their ability to participate in photoinduced transformations is somewhat less familiar. Fascinated by their chromophoric features and adaptable platform, our group is interested in the identification of fluorescent bioactive molecules and in the development of new photoinduced synthetic methods using coumarins and quinolinones as photocatalysts. This account provides an overview of our recent progress in the discovery and application of light-absorbing coumarin and quinolinone derivatives in photochemistry and medicinal chemistry.

A Vitamin B2 -Photocatalysed Approach to Methionine Analogues

Access to new non-canonical amino acid residues is crucial for medicinal chemistry and chemical biology. Analogues of the amino acid methionine have been far less explored-despite their use in biochemistry, pharmacology and peptide bioconjugation. This is largely due to limited synthetic access. Herein, we exploit a new disconnection to access non-natural methionines through the development of a photochemical method for the radical α-C-H functionalization of sulfides with alkenes, in water, using inexpensive and commercially-available riboflavin (vitamin B₂ ) as a photocatalyst. Our photochemical conditions allow the two-step synthesis of novel methionine analogues-by radical addition to unsaturated amino acid derivatives-and the chemoselective modification of peptide side-chains to yield non-natural methionine residues within small peptides. The mechanism of the bio-inspired flavin photocatalysis has been probed by experimental, DFT and TDDFT studies.

A Vitamin B2-Photocatalysed Approach to Methionine Analogues

Access to new non-canonical amino acid residues is crucial for medicinal chemistry and chemical biology. Analogues of the amino acid methionine have been far less explored-despite their use in biochemistry, pharmacology and peptide bioconjugation. This is largely due to limited synthetic access. Herein, we exploit a new disconnection to access non-natural methionines through the development of a photochemical method for the radical α-C-H functionalization of sulfides with alkenes, in water, using inexpensive and commercially-available riboflavin (vitamin B2) as a photocatalyst. Our photochemical conditions allow the two-step synthesis of novel methionine analogues-by radical addition to unsaturated amino acid derivatives-and the chemoselective modification of peptide side-chains to yield non-natural methionine residues within small peptides. The mechanism of the bio-inspired flavin photocatalysis has been probed by experimental, DFT and TDDFT studies.

Copper- and Visible-Light-Catalyzed Cascade Radical Cyclization of N-Propargylindoles with Cyclic Ethers

An efficient visible-light-assisted, copper-catalyzed tandem radical cyclization of N-propargylindoles with cyclic ethers is established. A series of 2-oxoalkyl-9H-pyrrolo[1,2-a]indol-9-ones with potential biological activities were synthesized in moderate yields by using a dual catalytic system with copper acetate as a transition metal catalyst and eosin Y as a visible light catalyst. The investigation of reaction mechanism shows that it goes through a cascade oxoalkyl radical addition, cyclization, and oxidation process.

Heteroarylation of unactivated C-H bonds suitable for late-stage functionalization

The late-stage introduction of diverse heterocycles onto complex small molecules enables efficient access to new medicinally relevant compounds. An attractive approach to such a transformation would utilize the ubiquitous aliphatic C-H bonds of a complex substrate. Herein, we report a system that enables direct C-H heteroarylation using a stable, commercially available O-alkenylhydroxamate with heterocyclic sulfone partners. The C-H heteroarylation proceeds efficiently with a range of aliphatic substrates and common heterocycles, and is a rare example of heteroarylation of strong C-H bonds. Importantly, the present approach is amenable to late-stage functionalization as the substrate is the limiting reagent in all cases.

Photocatalyst-free hydroacylations of electron-poor alkenes and enones under visible-light irradiation

Herein we present a photocatalyst- and additive-free radical hydroacylation of electron-poor double bonds under mild reaction conditions. Using 4-acyl-Hantzsch ester radical reservoirs, various Michael acceptors, enones and para-quinone methide substrates could be used. The protocol enabled further derivatizations and it could also be extended to a few unactivated alkenes. Moreover, the nature of the radical process was also investigated.

Aliphatic C-H Functionalization Using Pyridine N-Oxides as H-Atom Abstraction Agents

The alkylation and heteroarylation of unactivated tertiary, secondary, and primary C(sp3)-H bonds was achieved by employing an acridinium photoredox catalyst along with readily available pyridine Noxides as hydrogen atom transfer (HAT) precursors under visible light. Oxygen-centered radicals, generated by single-electron oxidation of the Noxides, are the proposed key intermediates whose reactivity can be easily modified by structural adjustments. A broad range of aliphatic C-H substrates with electron-donating or -withdrawing groups as well as various olefinic radical acceptors and heteroarenes were well tolerated.