Advancing Organic Photoredox Catalysis: Mechanistic Insight through Time-Resolved Spectroscopy

The rapid development of light-activated organic photoredox catalysts has led to the proliferation of powerful synthetic chemical strategies with industrial and pharmaceutical applications. Despite the advancement in synthetic approaches, a detailed understanding of the mechanisms governing these reactions has lagged. Time-resolved optical spectroscopy provides a method to track organic photoredox catalysis processes and reveal the energy pathways that drive reaction mechanisms. These measurements are sensitive to key processes in organic photoredox catalysis such as charge or energy transfer, lifetimes of singlet or triplet states, and solvation dynamics. The sensitivity and specificity of ultrafast spectroscopic measurements can provide a new perspective on the mechanisms of these reactions, including electron-transfer events, the role of solvent, and the short lifetimes of radical intermediates.

Photocatalytic systems: reactions, mechanism, and applications

The photocatalytic field revolves around the utilization of photon energy to initiate various chemical reactions using non-adsorbing substrates, through processes such as single electron transfer, energy transfer, or atom transfer. The efficiency of this field depends on the capacity of a light-absorbing metal complex, organic molecule, or substance (commonly referred to as photocatalysts or PCs) to execute these processes. Photoredox techniques utilize photocatalysts, which possess the essential characteristic of functioning as both an oxidizing and a reducing agent upon activation. In addition, it is commonly observed that photocatalysts exhibit optimal performance when irradiated with low-energy light sources, while still retaining their catalytic activity under ambient temperatures. The implementation of photoredox catalysis has resuscitated an array of synthesis realms, including but not limited to radical chemistry and photochemistry, ultimately affording prospects for the development of the reactions. Also, photoredox catalysis is utilized to resolve numerous challenges encountered in medicinal chemistry, as well as natural product synthesis. Moreover, its applications extend across diverse domains encompassing organic chemistry and catalysis. The significance of photoredox catalysts is rooted in their utilization across various fields, including biomedicine, environmental pollution management, and water purification. Of course, recently, research has evaluated photocatalysts in terms of cost, recyclability, and pollution of some photocatalysts and dyes from an environmental point of view. According to these new studies, there is a need for critical studies and reviews on photocatalysts and photocatalytic processes to provide a solution to reduce these limitations. As a future perspective for research on photocatalysts, it is necessary to put the goals of researchers on studies to overcome the limitations of the application and efficiency of photocatalysts to promote their use on a large scale for the development of industrial activities. Given the significant implications of the subject matter, this review seeks to delve into the fundamental tenets of the photocatalyst domain and its associated practical use cases. This review endeavors to demonstrate the prospective of a powerful tool known as photochemical catalysis and elucidate its underlying tenets. Additionally, another goal of this review is to expound upon the various applications of photocatalysts.

Visible light-promoted [3+2] cyclization reaction of vinyl azides with perfluoroalkyl-substituted-imidoyl sulfoxonium ylides

Visible-light-induced [3+2] cyclization of vinyl azides with perfluoroalkyl-substituted imidoyl sulfoxonium ylides has been developed for the first time. In this transformation, perfluoroalkyl-substituted imidoyl sulfoxonium ylides are firstly employed as a carbon radical precursor under visible light irradiation, providing a new and efficient method for the construction of perfluoroalkyl-substituted 1-pyrrolines.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

Total Synthesis of the Diterpenes (+)-Randainin D and (+)-Barekoxide via Photoredox-Catalyzed Deoxygenative Allylation

We report the first enantioselective total synthesis of diterpenoid randainin D, which possesses a hydroazulenone core with a β-substituted butenolide moiety on the cycloheptane ring. The trans-5/7 ring system was formed via a highly challenging ring-closing metathesis delivering the tetrasubstituted cycloheptenone. The butenolide moiety was installed via a novel deoxygenative allylation under Ir-photoredox catalysis, employing methyl oxalate as a red/ox tag. Moreover, the developed allylation was successfully utilized in the 7-step total synthesis of (+)-barekoxide. This study suggests that this deoxygenative allylation method is a promising strategy for the formation of Cq-C(sp3) bonds (Cq = quaternary center) in the context of natural product synthesis.

Visible Light Mediated Organophotoredox Catalyzed Synthesis of Tetraketones Using Tertiary Amines as Alkyl Synthons

Eosin Y catalyzed syntheses of bioactive tetraketones using cyclic-1,3-diketones and tertiary amines as alkyl synthons under 18 W blue LED have been accomplished. The condition is very mild that uses air as a green oxidant and avoids previously used harsh conditions like high temperature. Diverse arrays of tertiary amines first undergo reductive quenching of excited photocatalysts to form iminium ions that upon subsequent attack by cyclic-1,3-diketones give rise to tetraketones. This method is demonstrated with 31 examples with yields up to 76%. The feasibility of this reaction in the presence of eosin Y catalyst under a solar condition is also demonstrated with significant examples.

Tunable C-H functionalization and dearomatization enabled by an organic photocatalyst

C-H functionalization and dearomatization constitute fundamental transformations of aromatic compounds, which find wide applications in various research areas. However, achieving both transformations from the same substrates with a single catalyst by operating a distinct mechanism remains challenging. Here, we report a photocatalytic strategy to modulate the reaction pathways that can be directed toward either C-H functionalization or dearomatization under redox-neutral or net-reductive conditions, respectively. Two sets of indoles and indolines bearing tertiary alcohols are divergently furnished with good yields and high selectivity. The key to success is the introduction of isoazatruxene ITN-2 as a novel photocatalyst (PC), which outperforms the commonly used PCs. The ready synthesis and high modulability of isoazatruxene type PCs indicate their great application potential.

Harnessing the Potential of Electron Donor-Acceptor Complexes and N-Centered Radicals: Expanding the Frontiers of Isoquinoline Derivative Synthesis

Research on synthesizing nitrogen-containing heterocyclic scaffolds is important because these structures are commonly found in Nature, such as in the alkaloids' family. In our study, we propose a new method to synthesize the isoquinoline core using an electron donor-acceptor (EDA) complex strategy. Our mechanistic investigations have confirmed that our synthesis process operates through an EDA mechanism, which is not extensively discussed in the literature, particularly regarding its applications on alkynyl substrates. This EDA strategy has proven to be a simple and straightforward way to produce isoquinoline scaffolds and their derivatives without the need for metal catalysts.

Photochemical Povarov-type Reactions: Electron Donor-Acceptor Photoactivation by Visible Light

This report investigates the mechanism of photochemical Povarov-type reactions of N,N-dialkylanilines and maleimides in polar solvents (DMF or dioxane) in the presence of light. Fundamental aspects of the electron donor-acceptor (EDA) photoactivation pathway proposed to underpin this chemistry are examined through integrated experimental and computational studies. This approach provided evidence supporting the involvement of an EDA complex in facilitating this chemistry via a reaction mechanism that does not involve a triplet manifold. Most notably, our findings indicate that relying solely on UV-vis absorption spectroscopic data to either account for or predict reactivity in synthetic experiments may not always provide the complete picture. More specifically, this relates to considering UV-vis absorption spectroscopic data, calculated values for association constants (KEDA) and molar extinction coefficients (ε), with the reactivity observed in associated synthetic reactions in practice.

Switchable Decarboxylation by Energy- or Electron-Transfer Photocatalysis

Kolbe dimerization and Hofer-Moest reactions are well-investigated carboxylic acid transformations, wherein new carbon-carbon and carbon-heteroatom bonds are constructed via electrochemical decarboxylation. These transformations can be switched by choosing an electrode that allows control of the reactive intermediate, such as carbon radical or carbocation. However, the requirement of a high current density diminishes the functional group compatibility with these electrochemical reactions. Here, we demonstrate the photocatalytic decarboxylative transformation of activated carboxylic acids in a switchable and functional group-compatible manner. We discovered that switching between Kolbe-type or Hofer-Moest-type reactions can be accomplished with suitable photocatalysts by controlling the reaction pathways: energy transfer (EnT) and single-electron transfer (SET). The EnT pathway promoted by an organo-photocatalyst yielded 1,2-diarylethane from arylacetic acids, whereas the ruthenium photoredox catalyst allows the construction of an ester scaffold with two arylmethyl moieties via the SET pathway. The resulting radical intermediates were coupled to olefins to realize multicomponent reactions. Consequently, four different products were selectively obtained from a simple carboxylic acid. This discovery offers new opportunities for selectively synthesizing multiple products via switchable reactions using identical substrates with minimal cost and effort.

A Recent Update on the Visible Light-promoted Organic Transformations - A Mini-review

Visible light-induced reactions are a rapidly developing and powerful technique to promote organic transformations. They provide green and sustainable chemistry and have recently received increasing attention from chemists due to their wide application in organic synthesis. Light energy is eco-friendly, cheap, green, and inexhaustible with potential industrial and pharmaceutical applications. In this review, the most recent advances in visible light-induced reactions (2021-till date) have been highlighted.

Photocatalytic (3 + 2) dipolar cycloadditions of aziridines driven by visible-light

Herein, we document the design and development of a novel (3 + 2) cycloaddition reaction aided by the activity of an organic photocatalyst and visible light. The process is extremely fast, taking place in a few minutes, with virtually complete atom economy. A large variety of structurally diverse aziridines were used as masked ylides in the presence of different types of dipolarophiles (28 examples with up to 94% yield and >95 : 5 dr). Mechanistic insights obtained from photophysical, electrochemical and experimental studies highlight that the chemistry is driven by the in situ generation of the reactive ylide through two consecutive electron-transfer processes. We also report an aerobic cascade process, where an additional oxidation step grants access to a vast array of pyrrole derivatives (19 examples with up to 95% yield). Interestingly, the extended aromatic core exhibits a distinctive absorption and emission profile, which can be easily used to tag the effectiveness of this covalent linkage.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Iron Photoredox Catalysis-Past, Present, and Future

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.

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

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

Expanding the 'aplysinospin cascade' through DNA-templated [2+2] photocycloaddition

Inspired by the unique ability of nucleic acids to template chemical transformations that are otherwise impossible in solution, we embarked on the generalisation of our DNA-templated [2+2] photo-induced homo- and heterodimerization of aplysinopsins. Our process ensures a straightforward access to cyclobutane containing natural products and analogues thereof. Most importantly, this conceptual biomimetic achievement presents interesting arguments to build a biosynthetic scenario.

Polarity Transduction Enables the Formal Electronically Mismatched Radical Addition to Alkenes

The formation of carbon-carbon bonds via the intermolecular addition of alkyl radicals to alkenes is a cornerstone of organic chemistry and plays a central role in synthesis. However, unless specific electrophilic radicals are involved, polarity matching requirements restrict the alkene component to be electron deficient. This limits the scope of a fundamentally important carbon-carbon bond forming process that could otherwise be more universally applied. Herein, we introduce a polarity transduction strategy that formally overcomes this electronic limitation. Vinyl sulfonium ions are demonstrated to react with carbon-centered radicals, giving adducts that undergo in situ or sequential nucleophilic displacement to provide products that would be inaccessible via traditional methods. The broad generality of this strategy is demonstrated through the derivatization of unmodified complex bioactive molecules.

Light-accelerated "on-water" hydroacylation of dialkyl azodicarboxylates

The hydroacylation of dialkyl azodicarboxylates has received a lot of attention lately due to the great importance of acyl hydrazides in organic chemistry. Herein, we report an inexpensive and green photochemical approach, where light irradiation (390 nm) significantly accelerates the reaction between dialkyl azodicarboxylates and aldehydes, while water is employed as the solvent. A variety of aromatic and aliphatic aldehydes were converted into their corresponding acyl hydrazides in good to excellent yields in really short reaction times (15-210 min) and the reaction mechanism was also studied. Applications of this reaction in the syntheses of Vorinostat and Moclobemide were demonstrated.

Radical-Based Route to Functionalized Tetralin: Formal Total Synthesis of (±)-Hamigeran B

A formal synthetic route to hamigeran B, an antiviral marine natural product with a unique tricyclic molecular architecture, has been developed. The key chemical transformations in the present route include a novel zinc(II)porphyrin-catalyzed photoredox radical cascade cyclization to access a functionalized tetralin, a catalyst-free benzylic radical bromination with NBS by visible-light irradiation, and a samarium(II)-induced cyclization of brominated tetralone possibly via an orthoquinodimethane-like intermediate.

[2 + 2]-Cycloaddition-derived cyclobutane natural products: structural diversity, sources, bioactivities, and biomimetic syntheses

This review summarizes the structural diversity, bioactivities, and biomimetic synthesis of [2 + 2]-type cyclobutane natural products, along with discussion of their biosynthesis, stereochemical analysis, racemic occurrence, and biomimetic synthesis.

Strategies for the synthesis of Stemona alkaloids: an update

Covering: 2009 to 2022The Stemona alkaloids, which are found in plant species from the family Stemonaceae, represent a tremendously large and structurally-diverse family of natural products. This review presents and discusses a selection of case studies, grouped by alkaloid class, that showcase the key strategies and overall progress that has been made in the synthesis of Stemona alkaloids and related compounds since 2009. Structural reassignments that have been reported over this period are also identified where necessary.

NickelII-catalyzed asymmetric photoenolization/Mannich reaction of (2-alkylphenyl) ketones

A diastereo- and enantioselective photoenolization/Mannich (PEM) reaction of ortho-alkyl aromatic ketones with benzosulfonimides was established by utilizing a chiral N,N'-dioxide/Ni(OTf)2 complex as the Lewis acid catalyst. It afforded a series of benzosulfonamides and the corresponding ring-closure products, and a reversal of diastereoselectivity was observed through epimerization of the benzosulfonamide products under continuous irradiation. On the basis of the control experiments, the role of the additive LiNTf2 in achieving high stereoselectivity was elucidated. This PEM reaction was proposed to undergo a direct nucleophilic addition mechanism rather than a hetero-Diels-Alder/ring-opening sequence. A possible transition state model with a photoenolization process was proposed to explain the origin of the high level of stereoinduction.

Metal-free deoxygenative coupling of alcohol-derived benzoates and pyridines for small molecules and DNA-encoded libraries synthesis

Alcohols are among the most widely occurring functional groups found in naturally abundant, biologically relevant organic compounds, which in many cases are considered feedstock chemicals. Herein, we report a metal-free method for the deoxygenative coupling of alcohol-derived benzoates and pyridines promoted by visible light. Given the practical, mild and water-compatible conditions, small molecules and DNA headpieces can be successfully functionalized with a range of primary, secondary and tertiary alcohols. This protocol is distinguished by its wide substrate scope and broad applicability, even in the context of late-stage functionalization and DNA-drug coupling reactions.

Photochemical Synthesis of Indolocarbazoles through Tandem Indolization/Dimerization/Mannich Cyclization from Allenes

Indolocarbazole alkaloids and their derivatives were discovered to have potent protein kinase and topoisomerase I inhibitory activities. Disclosed herein is the photochemical synthesis of the indolocarbazole ring system from N-allenyl-2-iodoanilines. The tandem protocol included visible-light-mediated 5-exo-trig radical cyclization and subsequent radical dimerization, followed by acid-promoted deprotection and intramolecular Mannich cyclization. This strategy showed exceptional functional group tolerance and was successfully applied in the concise synthesis of natural products tjipanazoles B and D.

Visible light-promoted photocatalyst-free activation of persulfates: a promising strategy for C-H functionalization reactions

The employment of renewable energy resources is highly desirable according to the twelve principles of green chemistry. In this context, visible light promoted organic transformations have gained much attention from synthetic chemists due to the employment of renewable energy. However, the inability of the majority of organic molecules to absorb visible light encouraged the use of photocatalysts in visible light-mediated organic transformations. As a result, different types of photocatalysts like transition-metal containing photoredox catalysts, organophotoredox catalysts, heterogeneous photocatalysts, etc. have emerged over the years. On the other hand, persulphates (K₂S₂O₈, Na₂S₂O₈, and (NH₄)₂S₂O₈) have been widely used as oxidants in various oxidative organic transformations under thermal and photochemical conditions. The initial formation of an active persulfate radical anion from a persulfate anion is the crucial step for these oxidative transformations and the conversions under visible light are generally carried out employing different photocatalysts. Although numerous methodologies have been successfully developed employing these photocatalysts, the development of new processes under photocatalyst-free conditions are more preferable from the viewpoint of sustainable development. Persulphates could be very useful for various organic transformations through C-H functionalizations under photocatalyst-free visible light irradiation. In this review, we will exemplify the efficiency of persulphates in various oxidative organic transformations under visible light irradiation without the employment of any photocatalysts. The utilities and mechanistic pathways of the methodologies will also be highlighted.

Radical coupling decreases synthetic burden

Cross coupling of light-promoted β-keto radicals enables natural product syntheses.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Modeling and Simulation of Reaction Environment in Photoredox Catalysis: A Critical Review

From the pharmaceutical industry’s point of view, photoredox catalysis has emerged as a powerful tool in the field of the synthesis of added-value compounds. With this method, it is possible to excite the catalyst by the action of light, allowing electron transfer processes to occur and, consequently, oxidation and reduction reactions. Thus, in association with photoredox catalysis, microreactor technology and continuous flow chemistry also play an important role in the development of organic synthesis processes, as this technology offers high yields, high selectivity and reduced side reactions. However, there is a lack of a more detailed understanding of the photoredox catalysis process, and computational tools based on computational fluid dynamics (CFD) can be used to deal with this and boost to reach higher levels of accuracy to continue innovating in this area. In this review, a comprehensive overview of the fundamentals of photoredox catalysis is provided, including the application of this technology for the synthesis of added-value chemicals in microreactors. Moreover, the advantages of the continuous flow system in comparison with batch systems are pointed out. It was also demonstrated how modeling and simulation using computational fluid dynamics (CFD) can be critical for the design and optimization of microreactors applied to photoredox catalysis, so as to better understand the reagent interactions and the influence of light in the reaction medium. Finally, a discussion about the future prospects of photoredox reactions considering the complexity of the process is presented.

Photoactive Metal Carbonyl Complexes Bearing N-Heterocyclic Carbene Ligands: Synthesis, Characterization, and Viability as Photoredox Catalysts

This report details the synthesis and characterization of a small family of previously unreported, structurally related chromium, molybdenum, tungsten, manganese, and iron complexes bearing N-heterocyclic carbene and carbonyl supporting ligands. These complexes have the general form [ML(CO)₃X] or [ML(CO)₃], where X = CO or Br and L = 1-phenyl-3-(2-pyridyl)imidazolin-2-ylidene. Where possible, the solid-state, spectroscopic, electrochemical, and photophysical properties of these molecules were studied using a combination of experiment and theory. Photophysical studies reveal that decarbonylation occurs when these complexes are exposed to ultraviolet light, with the CO ligand being replaced with a labile acetonitrile solvent molecule. To obtain insights into the potential utility, scope, and applications of these complexes in visible-light-mediated photoredox catalysis, their capacity to facilitate a range of photoinduced reactions via the reductive or oxidative functionalization of organic molecules was investigated. These chromium, molybdenum, and manganese catalysts efficiently facilitated atom-transfer radical addition processes. In light of their photolability, these types of catalysts may potentially allow for the development of photoinduced reactions involving less conventional inner-sphere electron-transfer pathways.

A combined experimental and theoretical study on the reactivity of nitrenes and nitrene radical anions

Nitrene transfer reactions represent one of the key reactions to rapidly construct new carbon-nitrogen bonds and typically require transition metal catalysts to control the reactivity of the pivotal nitrene intermediate. Herein, we report on the application of iminoiodinanes in amination reactions under visible light photochemical conditions. While a triplet nitrene can be accessed under catalyst-free conditions, the use of a suitable photosensitizer allows the access of a nitrene radical anion. Computational and mechanistic studies rationalize the access and reactivity of triplet nitrene and nitrene radical anion and allow the direct comparison of both amination reagents. We conclude with applications of both reagents in organic synthesis and showcase their reactivity in the reaction with olefins, which underline their markedly distinct reactivity. Both reagents can be accessed under mild reaction conditions at room temperature without the necessity to exclude moisture or air, which renders these metal-free, photochemical amination reactions highly practical.

Gaining in-depth understanding of photochemical processes is key for developing more sustainable and efficient chemical transformations. Here the authors show that under visible light photochemical conditions, iminoiodinanes undergo formation of triplet nitrenes or nitrene radical anions, depending on the use of a photosensitizer; These reagents are studied in amination reactions with olefins.

UV light-driven asymmetric vinylogous aldol reaction of isatins with 2-alkylbenzophenones and enantioselective synthesis of 3-hydroxyoxindoles

A UV light-driven enantioselective synthesis of 3-hydroxyoxindoles is reported for the first time.

Photoinduced Decarboxylative Radical Coupling Reaction of Multiply Oxygenated Structures by Catalysis of Pt-Doped TiO2

A new reaction system was devised for decarboxylative radical coupling reactions by heterogeneous semiconductor photoredox catalysis. When an α-alkoxy carboxylic acid and Pt-doped TiO₂ in EtOAc were irradiated with a violet light-emitting diode at room temperature, the photogenerated electron hole of TiO₂ oxidatively induced the ejection of CO₂ via the formation of a carboxyl radical to produce the corresponding α-alkoxy radical. C(sp³)-C(sp³) bond formation between the radicals led to dimers with reductive conversion of protons to H₂ by the photogenerated electron. Alternatively, in the presence of an electron-deficient olefin, an intermolecular radical addition reaction occurred, resulting in the formation of a 1,4-adduct via single-electron reduction and subsequent protonation. These operationally simple and mild transformations are amenable to the one-step assembly of densely oxygenated linear and branched carbon chains.

Synthesis of Pyrrolidine- and γ-Lactam-Containing Natural Products and Related Compounds from Pyrrole Scaffolds

Polycyclic alkaloid natural products featuring pyrrolidine and pyrrolidinone motifs remain enduring targets of total synthesis endeavors. Pyrrole and its derivatives have been exploited to access many such frameworks, including alkaloids belonging to the Aspidosperma, Stemona, and batzelladine families. In this article, a selection of exemplars that highlight the utility of pyrrole-based approaches to facilitate total syntheses of pyrrolidine- and pyrrolidinone-containing alkaloids and related molecules are showcased.

Photoredox-Catalyzed Cascade Reactions Involving Aryl Radical: Total Synthesis of (±)-Norascyronone A and (±)-Eudesmol

Herein, we have developed two types of photoredox-catalyzed cascade reactions using diaryliodonium salts for the concise synthesis of norascyronone A and β-eudesmol. A rationally designed photoredox-catalyzed arylation/cyclization/Friedel-Crafts cascade reaction of enone was exploited to generate the norascyronone polycyclic skeleton. A visible-light-induced radical cyclization/acyloxy-migration reaction was explored to forge the decalin skeleton of eudesmol, and mechanistic studies indicated the reaction was initiated by one-electron oxidation of the enol ester.

Visible-Light Induced C(sp2 )-H Amidation with an Aryl-Alkyl σ-Bond Relocation via Redox-Neutral Radical-Polar Crossover

We report an approach for the intramolecular C(sp² )-H amidation of N-acyloxyamides under photoredox conditions to produce δ-benzolactams with an aryl-alkyl σ-bond relocation. Computational studies on the designed reductive single electron transfer strategy led us to identify N-[3,5-bis(trifluoromethyl)benzoyl] group as the most effective amidyl radical precursor. Upon the formation of an azaspirocyclic radical intermediate by the selective ipso-addition with outcompeting an ortho-attack, radical-polar crossover was then rationalized to lead to the rearomative ring-expansion with preferential C-C bond migration.

Visible light-driven conjunctive olefination

Carboxylic acids and aldehydes are ubiquitous in chemistry and are native functionalities in many bioactive molecules and natural products. As such, a general cross-coupling process that involves these partners would open new avenues to achieve molecular diversity. Here we report a visible-light-mediated and transition metal-free conjunctive olefination that uses an alkene 'linchpin' with a defined geometry to cross-couple complex molecular scaffolds that contain carboxylic acids and aldehydes. The chemistry merges two cornerstones of organic synthesis-namely, the Wittig reaction and photoredox catalysis-in a catalytic cycle that couples a radical addition process with the redox generation of a phosphonium ylide. The methodology allows the rapid structural diversification of bioactive molecules and natural products in a native form, with a high functional group tolerance, and also forges a new alkene functional group with a programmable E-Z stereochemistry.

Photocatalytic gem-Difluoroolefination Reactions by a Formal C-C Coupling/Defluorination Reaction with Diazoacetates

The photolysis of diazoalkanes to conduct singlet carbene transfer reactions of colored diazoalkanes has recently attracted significant interest in organic synthesis. Herein, we describe a photocatalytic approach that allows the access of triplet carbene intermediates via energy transfer to conduct highly efficient gem‐difluoroolefination reactions with α‐trifluoromethyl styrenes. The use of a tertiary amines proved pivotal to unlock this unusual reaction pathway and to prevent undesired cyclopropanation pathways. The amine further facilitates the ultimate abstraction of fluoride to yield gem‐difluoroolefins (43 examples, up to 88 % yield), which is supported by experimental and theoretical mechanistic studies. We explored this synthesis method with a broad substrate scope, ranging from simple olefins and heterocyclic olefins towards the decoration of pharmaceutically relevant building blocks.

Solvent-Minimized Synthesis of 4CzIPN and Related Organic Fluorophores via Ball Milling

The mechanochemical synthesis of 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile and related organic fluorophores/photocatalysts via a solvent-minimized four-fold S_NAr pathway is herein described. Employing sodium tert-butoxide as base, and negating the need for any air/moisture-sensitive reaction set-ups, a selection of organic dyes was synthesized in just 1 h using this ball-milling technique. Furthermore, the transformation was then showcased on a multigram scale.

Evolution towards green radical generation in total synthesis

The use of radicals as intermediates in total synthesis has evolved since their initial use in the latter half of the twentieth century. Radical generation from metal hydride methodologies has shifted to "greener" techniques including catalytic metal-mediated systems, electrochemical and photoredox-mediated processes. This review will focus on these classical and contemporary methods for radical generation and their applications in recent total syntheses.

Fluorescent Hybrid Porous Polymers as Sustainable Heterogeneous Photocatalysts for Cross-Dehydrogenative Coupling Reactions

A series of hybrid porous polymers (HPPs) based on polyhedral oligomeric silsesquioxane (POSS) were synthesized, characterized, and successfully used as metal-free heterogeneous photocatalysts for cross-dehydrogenative coupling reactions (CDC), for which the aza-Henry coupling of tetrahydroisoquinolines and nitroalkanes was studied as the model reaction. The reactions run smoothly at room temperature under visible (blue) light irradiation using gaseous oxygen as an oxidant under atmospheric pressure. These novel metal-free heterogeneous photocatalysts can be readily recovered and reused without a significant loss of reactivity. Mechanistic investigations revealed the intermediacy of ¹O₂, obtained from ³O₂ sensitization (energy transfer) by the photoexcited catalyst.