Strategic Use of Visible-Light Photoredox Catalysis in Natural Product Synthesis

Overestimated Halogen Atom Transfer Reactivity of α-Aminoalkyl Radicals

Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on α-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between α-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of α-aminoalkyl radicals in XAT reactions is 103 to 105 times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that α-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of α-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for α-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.

Excited State Bond Homolysis of Vanadium(V) Photocatalysts for Alkoxy Radical Generation

Advancements in photocatalysis have transformed synthetic organic chemistry, using light as a powerful tool to drive selective chemical transformations. Recent approaches have focused on metal-halide ligand-to-metal charge transfer (LMCT) photoactivated bond homolysis reactions leveraged by earth-abundant elements to generate valuable synthons for radical-mediated cross-coupling reactions. Of recent utility, oxovanadium(V) LMCT photocatalysts exhibit selective alkoxy radical generation from aliphatic alcohols upon blue light (UVA) irradiation under mild conditions. The selective photochemical liberation of alkoxy radicals is valuable for applying late-stage fragmentation approaches in organic synthesis and depolymerization strategies for nonbiodegradable polymers. Steady-state and time-resolved spectroscopy were used to assign the electronic structure of three well-defined V(V) photocatalysts in their ground and excited states. We assign the excited state for this transformation at earth-abundant vanadium(V), interrogating the electronic structure using static UV-visible absorption, ultrafast transient absorption, and electron paramagnetic resonance spectroscopy coupled to computational approaches. These findings afford assignments of the short-lived excited state intermediates that dictate selective homolytic bond cleavage in metal alkoxides, illustrating the valuable insight gleaned from fundamental investigations of the molecular photochemistry responsible for light-escalated chemical transformations.

Alkylation of Glycine Derivatives through a Synergistic Single-Electron Transfer and Halogen-Atom Transfer Process

Here, we present a versatile method for forming C(sp3)-C(sp3) bonds, enabling the synthesis of a range of natural and non-natural amino acids. This approach utilizes readily available glycine derivatives and alkyl iodides, combining single-electron transfer and halogen-atom transfer processes. The utility of this step-economic and redox-economic C(sp3)-C(sp3) bond formation is further highlighted in the late-stage site-selective modifications of the glycine residue in short peptides.

Visible-Light-Induced Dearomative Annulation of Indoles toward Stereoselective Formation of Fused- and Spiro Indolines

Dearomatization approaches are attractive for their abilities to transform simple, planar arenes into complex, three-dimensional architectures. In particular, visible-light driven dearomatization strategies are significant because of their mild, green, and sustainable nature, enabling the fabrication of new chemical bonds via an electron transfer or energy transfer process. Indole compounds, being potentially bioactive and readily accessible, can be employed efficiently as building blocks for constructing diverse annulated frameworks under photocatalysis. Highly stereoselective radical cascade reactions of appropriate indole systems can provide complex cyclic scaffolds bearing multiple stereocenters. In fact, the past few years have witnessed the renaissance of dearomative cycloadditions of indoles via visible-light-induced photocatalysis. The present review highlights recent advances (2019-mid 2024) in visible-light-driven dearomative annulation of indoles leading to formation of polycyclic indolines, including angularly fused and spiro indolines. Most of the reactions described in this review are simple, providing quick access to the desired products. Additionally, characteristic reaction mechanisms are offered to provide an understand of how indole scaffolds show distinctive reactivity under photocatalytic conditions.

Dual role tertiary amines in photocatalytic cyclizations: towards sustainable syntheses of 1,3-dinitrogen heterocycles

A novel photocatalytic cascade has been engineered for the transformation of diketimines into 1,3-dinitrogen heterocycles, ingeniously repurposing methylamine a typically utilized sacrificial additive as a dual-function reagent, serving simultaneously as an electron donor and a methyl source. This innovation champions atom economy and underscores a commitment to sustainable chemical synthesis.

Challenges and Future Perspectives in Photocatalysis: Conclusions from an Interdisciplinary Workshop

Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.

Solar-assisted selective separation and recovery of precious group metals from deactivated air purification catalysts

The mitigation of environmental and energy crises could be advanced by reclaiming platinum group precious metals (PGMs) from decommissioned air purification catalysts. However, the complexity of catalyst composition and the high chemical inertness of PGMs significantly impede this process. Consequently, recovering PGMs from used industrial catalysts is crucial and challenging. This study delves into an environmentally friendly approach to selectively recover PGMs from commercial air purifiers using photocatalytic redox technology. Our investigation focuses on devising a comprehensive strategy for treating three-way catalysts employed in automotive exhaust treatment. By meticulously pretreating and modifying reaction conditions, we achieved noteworthy results, completely dissolving and separating rhodium (Rh), palladium (Pd), and platinum (Pt) within a 12-h time frame. Importantly, the solubility selectivity persists despite the remarkably similar physicochemical properties of Rh, Pd, and Pt. To bolster the environmental sustainability of our method, we harness sunlight as the energy source to activate the photocatalysts, facilitating the complete dissolution of precious metals under natural light irradiation. This eco-friendly recovery approach demonstrated on commercial air purifiers, exhibits promise for broader application to a diverse range of deactivated air purification catalysts, potentially enabling implementation on a large scale.

Construction of axially chiral molecules enabled by photoinduced enantioselective reactions

Axially chiral molecular scaffolds are widely found in pharmaceutical molecules, functionalized materials, and chiral ligands. The synthesis of these compounds has garnered considerable interest from both academia and industry. The construction of such molecules, enabled by transition metal catalysis and organocatalysis under thermodynamic conditions, has been extensively studied and well-reviewed. In recent years, photoinduced enantioselective reactions have emerged as powerful methods for the catalytic construction of axial chirality. In this review, we provide an overview of various synthetic strategies for the photoinduced construction of axial chirality, with a specific focus on reaction design and catalytic mechanisms. Additionally, we discuss the limitations of current methods and highlight future directions in this field.

Visible-Light-Mediated [2+2] Photocycloadditions of Alkynes

Visible-light-mediated [2+2] photocycloaddition reaction can be considered an ideal solution due to its green and sustainable properties, and is one of the most efficient methods to synthesize four-membered ring motifs. Although research on the [2+2] photocycloaddition of alkynes is challenging because of the diminished reactivity of alkynes, and the more significant ring strain of the products, remarkable achievements have been made in this field. In this article, we highlight the recent advances in visible-light-mediated [2+2] photocycloaddition reactions of alkynes, with focus on the reaction mechanism and the late-stage synthetic applications. Advances in obtaining cyclobutenes, azetines, and oxetene active intermediates continue to be breakthroughs in this fascinating field of research.

Charge-recombinative triplet sensitization of alkenes for DeMayo-type [2 + 2] cycloaddition

Synthetic photochemistry has undergone significant development, largely owing to the development of visible-light-absorbing photocatalysts (PCs). PCs have significantly improved the efficiency and precision of cycloaddition reactions, primarily through energy or electron transfer pathways. Recent research has identified photocatalysis that does not follow energy- or electron-transfer formalisms, indicating the existence of other, undiscovered photoactivation pathways. This study unveils an alternative route: a charge-neutral photocatalytic process called charge-recombinative triplet sensitization (CRTS), a mechanism with limited precedents in synthetic chemistry. Our investigations revealed CRTS occurrence in DeMayo-type [2 + 2] cycloaddition reactions catalyzed by indole-fused organoPCs. Our mechanistic investigations, including steady-state and transient spectroscopic analyses, electrochemical investigations, and quantum chemical calculations, suggest a mechanism involving substrate activation through photoinduced electron transfer, followed by charge recombination, leading to substrate triplet state formation. Our findings provide valuable insights into the underlying photocatalytic reaction mechanisms and pave the way for the systematic design and realization of innovative photochemical processes.

Photochemical radical decarboxylative disulfuration of α-keto acids and oxamic acids

A visible-light-induced directed decarboxylative disulfuration of α-keto acids and oxamic acids was developed. As a result, a series of versatile mono acyl disulfide derivatives was synthesized under mild and sustainable reaction conditions. This protocol has a broad substrate scope, good functional-group tolerance, and excellent synthetic applications.

Visible-Light-Driven Coupling of 1,3,4-Oxadiazoles and Hydroxamic Acid Derivatives

A visible-light-induced radical-radical cross-coupling reaction between 1,3,4-oxadiazoles and hydroxamic acid derivatives has been realized under base- and metal-free conditions. The protocol was characterized by broad substrate scope, excellent functional group tolerance, and simple operation procedures. By using this protocol, a variety of biologically important 5-aryl-1,3,4-oxadiazole-2-methylamines were obtained in good yields with excellent chemoselectivity.

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.

Unusual Regioselective C-H Difluoroalkylation of Heteroarenes under Photoredox Catalysis

We disclose a new general strategy for the site-selective difluoroalkylation of nonprefunctionalized heteroarenes, such as quinoxaline at the C-8 position, and benzothiadiazole, benzoxadiazole, and benzothiazole at the C-4 position via consecutive organophotoredox-catalyzed radical-radical cross-coupling and base-assisted hydrogen abstraction reactions. The current methodology represents a site-selective direct difluoroalkylative strategy to allow broad functional group tolerance and a wide substrate scope in good to excellent yields. Careful experimental investigations and detailed DFT calculations revealed the exact site-selectivity of the heteroarenes and a possible mechanistic pathway.

A Mechanistic Investigation of the N-Hydroxyphthalimide Catalyzed Benzylic Oxidation Mediated by Sodium Chlorite

A detailed investigation into the mechanistic course of N-hydroxyphthalimide catalyzed oxidation of benzylic centers using sodium chlorite as the stoichiometric oxidant is reported. Through a combination of experimental, spectroscopic, and computational techniques, the transformation is interrogated, providing improved reaction conditions and an enhanced understanding of the mechanism. Performing the transformation in the presence of acetic acid or a pH 4.5 buffer leads to extended reaction times but improves the catalyst lifetime, leading to the complete consumption of the starting material. Chlorine dioxide is identified as the active oxidant that is able to oxidize the N-hydroxyphthalimide anion to the phthalimide-N-oxyl radical, the proposed catalytically active species, which is able to abstract a hydrogen atom from the substrate. A second molecule of chlorine dioxide reacts with the resultant radical and, after loss of hypochlorous acid, leads to the observed product. Through a broad variety of techniques including UV/vis, EPR and Raman spectroscopy, isotopic labeling, and the use of radical traps, evidence for the mechanism is presented that is supported through electronic structural calculations.

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.

Allylsilane as a versatile handle in photoredox catalysis

Organosilanes have secured a special place in the synthetic world for several decades. However, among them, allylsilanes are a choice reagent for organic chemists to develop novel organic transformations. In recent years researchers have proved that visible-light photoredox catalysis has emerged as one of the most mild, sustainable, straightforward, and efficient strategies to construct simple to complex molecules with or without enantioselectivity. This review provides an in-depth analysis of recent advances and strategies employed in visible-light photoredox catalysis for allylsilane and its analogues for the development of various organic transformations. The review is divided into sections, each focused on a specific reactivity of allylsilane under light irradiation with C(sp2) center arene or alkene, C(sp2) center carbonyl, and C(sp3) center carbon functionality. In this review, we present optimization data, reaction scope, and mechanistic aspects to bring forward specific reactivity and selectivity trends of allylsilane in photoredox conditions.

Development of a Strategy for the Total Synthesis of Aspidosperma Alkaloids via the Cyclobutenone-Based PET-Initiated Cationic Radical-Driven [2+2]/Retro-Mannich Reaction

A novel strategy for the synthesis of Aspidosperma alkaloids has been achieved via a photoredox-initiated [2+2]/retro-Mannich reaction of tryptamine-substituted enaminones as a key step. The developed chemistry has been applied to the construction of the core tetracycle of Aspidosperma alkaloids (±)-aspidospermidine and (±)-limaspermidine.

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.

Blue-Light Induced Iron-Catalyzed Synthesis of γ,δ-Unsaturated Ketones

A visible-light-induced iron-catalyzed α-alkylation of ketones with allylic and propargylic alcohols as pro-electrophiles is reported. The diaminocyclopentadienone iron tricarbonyl complex plays a dual role by harvesting light and facilitating dehydrogenation and reduction steps without the help of any exogenous photosensitizer. γ,δ-Unsaturated ketones can now be accessed through this borrowing hydrogen methodology at room temperature. Mechanistic investigations revealed that the steric hindrance on the δ-position of either the dienone or ene-ynone intermediate is the key feature to prevent or decrease the competitive 1,6-reduction (and consequently the formation of the saturated ketone) and to favor the synthesis of a set of non-conjugated enones and ynones.

Synthesis of cyclobutane-fused chromanones via gold-mediated photocatalysis

Energy transfer (EnT) photocatalysis has emerged as a valuable tool for constructing complex organic scaffolds via [2 + 2]-cycloaddition reactions. Herein, we present the use of [Au(SIPr)(Cbz)] as a sensitizer for the [2 + 2]-cycloaddition of coumarins and unactivated alkenes. Widely used in EnT catalysis, iridium and organic sensitizers proved less efficient under the examined catalytic conditions. The developed protocol permits the synthesis of cyclobutane-fused chromanones from readily available starting materials. A wide range of alkenes and substituted coumarins, including naturally occurring compounds, were reacted under mild conditions leading to structurally complex products with good functional group tolerance. Mechanistic studies reveal a previously overlooked reaction pathway for energy transfer catalysis involving coumarins.

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.

Photochemistry in Medicinal Chemistry and Chemical Biology

Photochemistry has emerged as a transformative force in organic chemistry, significantly expanding the chemical space accessible for medicinal chemistry. Light-induced reactions enable the efficient synthesis of intricate organic structures and have found applications throughout the different stages of the drug discovery and development processes. Moreover, photochemical techniques provide innovative solutions in chemical biology, allowing precise spatiotemporal drug activation and targeted delivery. In this Perspective, we highlight the already numerous remarkable applications and the even more promising future of photochemistry in medicinal chemistry and chemical biology.

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.

Electrophotocatalytic hydrogenation of imines and reductive functionalization of aryl halides

The open-shell catalytically active species, like radical cations or radical anions, generated by one-electron transfer of precatalysts are widely used in energy-consuming redox reactions, but their excited-state lifetimes are usually short. Here, a closed-shell thioxanthone-hydrogen anion species (3), which can be photochemically converted to a potent and long-lived reductant, is generated under electrochemical conditions, enabling the electrophotocatalytic hydrogenation. Notably, TfOH can regulate the redox potential of the active species in this system. In the presence of TfOH, precatalyst (1) reduction can occur at low potential, so that competitive H2 evolution can be inhibited, thus effectively promoting the hydrogenation of imines. In the absence of TfOH, the reducing ability of the system can reach a potency even comparable to that of Na0 or Li0, thereby allowing the hydrogenation, borylation, stannylation and (hetero)arylation of aryl halides to construct C-H, C-B, C-Sn, and C-C bonds.

Photocatalyzed Enantioselective Functionalization of C(sp3)-H Bonds

Owing to its diverse activation processes including single-electron transfer (SET) and hydrogen-atom transfer (HAT), visible-light photocatalysis has emerged as a sustainable and efficient platform for organic synthesis. These processes provide a powerful avenue for the direct functionalization of C(sp3)-H bonds under mild conditions. Over the past decade, there have been remarkable advances in the enantioselective functionalization of the C(sp3)-H bond via photocatalysis combined with conventional asymmetric catalysis. Herein, we summarize the advances in asymmetric C(sp3)-H functionalization involving visible-light photocatalysis and discuss two main pathways in this emerging field: (a) SET-driven carbocation intermediates are followed by stereospecific nucleophile attacks; and (b) photodriven alkyl radical intermediates are further enantioselectively captured by (i) chiral π-SOMOphile reagents, (ii) stereoselective transition-metal complexes, and (iii) another distinct stereoscopic radical species. We aim to summarize key advances in reaction design, catalyst development, and mechanistic understanding, to provide new insights into this rapidly evolving area of research.

Facile fabrication of ternary NiTiFe-LDH ultrathin nanosheets for efficient conversion of amines into imines under visible light

Ternary NiTiFe-LDH with an ultrathin nanosheet morphology was successfully fabricated via a facile co-precipitation method, followed by refluxing, and was used as a catalyst for oxidative coupling of amines to produce imines under visible light. The obvious superior activity observed in NiTiFe-LDH ultrathin nanosheets compared with binary NiTi-LDH and bulk NiTiFe-LDH can be ascribed to an enhanced light absorption capability caused by the introduction of Fe3+ ions as well as the ultrathin nanosheets which can minimize the recombination of the photogenerated charge carriers and provide more catalytically active sites for the reaction. As a result, more catalytically active O2˙- radicals are generated over NiTiFe-LDH ultrathin nanosheets, which leads to their superior activity. This study not only shows the possibility of using LDHs in photocatalytic organic transformations but also demonstrates an effective strategy to promote the activity of LDH-based photocatalysts via simultaneous composition and morphology modulation of LDHs.

Photoredox Enabled Defluorinative Benzylation of Trifluoromethyl Alkenes with Alkylarenes

Herein, we report a photoredox enabled defluorinative benzylation of trifluoromethyl alkenes with readily available alkylarenes, which provides convenient access to a series of structurally valuable benzylated gem-difluoroalkenes under mild reaction conditions. The synthetic value of this protocol has been demonstrated by the transformations of several substrates bearing drug moieties, gram-scale reactions, and various further derivatizations of the gem-difluoroalkene products. The preliminary mechanistic investigations suggest a reaction pathway with rate-determining benzyl C-H bond cleavage of toluene followed by benzylic radical formation.

Hydroxytrifluoroethylation and Trifluoroacetylation Reactions via SET Processes

Hydroxytrifluoroethyl and trifluoroacetyl groups are of utmost importance in biologically active compounds, but methods to tether these motifs to organic architectures have been limited. Typically, the preparation of these compounds relied on the use of strong bases or multistep routes. The renaissance of radical chemistry in photocatalytic, transition metal mediated, and hydrogen atom transfer (HAT) processes have allowed the installation of these medicinally relevant fluorinated motifs. This review provides an overview of the methods available for the direct synthesis of hydroxytrifluoroethyl- and trifluoroacetyl-derived compounds governed by single-electron transfer processes.

Rational design of super reductive EDA photocatalyst for challenging reactions: a theoretical and experimental study

We reported a novel electron-donor-acceptor (EDA) photocatalyst formed in situ from isoquinoline, a diboron reagent, and a weak base. To further optimize the efficiency of this photocatalyst, Density Functional Theory (DFT) calculations were conducted to investigate the substituent effects on the properties of vertical excitation energy and redox potential. Subsequently, we experimentally validated these effects using a broader range of substituents and varying substitution positions. Notably, the 4-NH2 EDA complex derived from 4-NH2-isoquinoline exhibits the highest photocatalytic efficiency, enabling feasible metal free borylation of aromatic C-H bond and detosylaion of Ts-anilines under green and super mild conditions. These experimental results demonstrate the effectiveness of our strategy for photocatalyst optimization.

Generation and Coupling of Radical Species from α-Alkoxy Bridgehead Carboxylic Acid, Selenide, Telluride, Acyl Selenide, and Acyl Telluride

α-Alkoxy bridgehead radicals enable intermolecular construction of sterically congested C-C bonds due to their sterically accessible nature. We implemented these radical species into total syntheses of various densely oxygenated natural products and demonstrated their exceptional versatility. Herein, we employed different precursors to generate the same α-alkoxy bridgehead radical and compared the efficacy of the precursors for coupling reactions. Specifically, the bridgehead radical of the trioxaadamantane structure was formed from α-alkoxy carboxylic acid, selenide/telluride, and acyl selenide/acyl telluride, and reacted with 4-((tert-butyldimethylsilyl)oxy)cyclopent-2-en-1-one and 5-oxo-1-cyclopentene-1-carbonitrile. The efficiency of the bridgehead radical formation and subsequent coupling reaction significantly depended on the structures of the precursors and acceptors as well as the reaction conditions. Our findings provide new insights for selecting the appropriate substrates of key coupling reactions in the total synthesis of complex natural products.

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.

Photoinduced Reduction of Nitroarenes and Tandem C-N Cross-Coupling with Haloarenes

An efficient photocatalytic C-N cross-coupling of nitroarenes with haloarenes has been developed using simple and cheap Ni(acac)2 as a cocatalyst. The reaction is confirmed as a stepwise process: (1) metal free photoinduced reduction of nitroarenes into aniline derivatives and (2) photo- and Ni-catalyzed C-N cross-coupling of anilines with haloarenes. The reaction conditions are simple and mild, giving high-value diarylamines with good to excellent yields and good functional group tolerance.

Nickel/photoredox dual catalyzed arylalkylation of nonactivated alkenes

Alkene dicarbofunctionalization is an efficient strategy and operation-economic fashion for introducing complexity in molecules. A nickel/photoredox dual catalyzed arylalkylation of nonactivated alkenes for the simultaneous construction of one C(sp3)-C(sp3) bond and one C(sp3)-C(sp2) bond has been developed. The mild catalytic method provided valuable indanethylamine derivatives with wide substrate scope and good functional group compatibility. An enantioselective dicarbofunctionalization was also achieved with pyridine-oxazoline as a ligand. The efficiency of metallaphotoredox dicarbofunctionalization was demonstrated for the concise synthesis of pharmaceutically active compounds.

Photoelectrochemical oxidative C(sp3)-H borylation of unactivated hydrocarbons

Organoboron compounds are of high significance in organic synthesis due to the unique versatility of boryl substituents to access further modifications. The high demand for the incorporation of boryl moieties into molecular structures has witnessed significant progress, particularly in the C(sp3)-H borylation of hydrocarbons. Taking advantage of special characteristics of photo/electrochemistry, we herein describe the development of an oxidative C(sp3)-H borylation reaction under metal- and oxidant-free conditions, enabled by photoelectrochemical strategy. The reaction exhibits broad substrate scope (>57 examples), and includes the use of simple alkanes, halides, silanes, ketones, esters and nitriles as viable substrates. Notably, unconventional regioselectivity of C(sp3)-H borylation is achieved, with the coupling site of C(sp3)-H borylation selectively located in the distal methyl group. Our method is operationally simple and easily scalable, and offers a feasible approach for the one-step synthesis of high-value organoboron building blocks from simple hydrocarbons, which would provide ample opportunities for drug discovery.

Dioxygen concentration-dependent selective hydroxysulfonylation of olefins by rose bengal-sensitized photocatalysis

The synthesis of β-hydroxysulfones selectively in preference to β-ketosulfones by dye-sensitized photocatalysis is demonstrated by employing inexpensive and readily available olefins and sulfonyl hydrazides/N-hydroxy sulfonamides in the presence of oxygen. The facile hydroxysulfonylation reaction, which involves the use of rose bengal (RB) as a photocatalyst and dioxygen as an oxidant, permits access selectively to secondary and tertiary β-hydroxysulfones in good to excellent isolated yields at rt and is compatible with aryl, heteroaryl and alkyl sulfonyl hydrazides. Mechanistically, sulfonyl radicals are generated by a cascade of redox reactions, set off by the photocatalyst RB, between sulfonyl hydrazide and dioxygen. Attack of the sulfonyl radicals on olefins followed by oxygenation of the radical intermediates leads to hydroxysulfonylation products selectively without the requirement of a metal catalyst/additive/reductant/base, conforming to the tenets of sustainable chemistry. It is shown that the formations of β-hydroxysulfones and β-ketosulfones proceed concurrently through two different routes and that the outcome is crucially dependent on oxygen concentration with the former being formed predominantly under an oxygen atmosphere, a feature that is heretofore unprecedented to the best of our knowledge.

Spectral and Electrochemical Properties of Common Photocatalysts in Water: A Compendium for Aqueous Photoredox Catalysis

Electrochemical potentials of photocatalysts are solvent dependent. One of the largest discrepancies is observed when water is used in place of organic solvents as the reaction media. Unfortunately, the redox potentials for many photocatalysts in water have not been determined, at least under one unifying set of conditions, and this greatly hinders the rational design of sustainable and biocompatible photoredox reactions. Herein, we measure the spectral and electrochemical properties of the most common photoredox catalysts in water and catalog their absorption and fluorescence maxima and ground- and excited-state potentials.

Ketyl Radical Coupling Enabled by Polycyclic Aromatic Hydrocarbon Electrophotocatalysts

Herein, we report a new class of electrophotocatalysts, polycyclic aromatic hydrocarbons, that promote the reduction of unactivated carbonyl compounds to generate versatile ketyl radical intermediates. This catalytic platform enables previously challenging intermolecular ketyl radical coupling reactions, including those that classic reductants (e.g., SmI2/HMPA) have failed to promote. More broadly, this study outlines an approach to fundamentally expand the array of reactive radical intermediates that can be generated via electrophotocatalysis by obviating the need for rapid mesolytic cleavage following substrate reduction.

Photoredox and Copper Dual-Catalyzed Cyclization of Alkyne-tethered α-Bromocarbonyls

The synergistic systems of photoredox and copper catalyst have already appeared as a novel formation of green synthetic chemistry, which open new avenues for chemical synthesis applications. We describe a novel strategy for the cyclization of alkyne-tethered α-bromocarbonyls initiated by the cleavage of C(sp3 )-Br bond via the collaboration of photoredox and copper catalyst. The present protocol exhibits mildness using economical copper catalyst and visible-light at room temperature. The gram-scale and sunlight irradiation experiments proceeded smoothly to show the practicality of the methodology. It is notable that the newly generated oxygen in the product originates from H2 O.

Reductive photoredox transformations of carbonyl derivatives enabled by strongly reducing photosensitizers

Visible-light photoredox catalysis is well-established as a powerful and versatile organic synthesis strategy. However, some substrate classes, despite being attractive precursors, are recalcitrant to single-electron redox chemistry and thus not very amenable to photoredox approaches. Among these are carbonyl derivatives, e.g. ketones, aldehydes, and imines, which in most cases require Lewis or Brønsted acidic additives to activate via photoinduced electron transfer. In this work, we unveil a range of photoredox transformations on ketones and imines, enabled by strongly reducing photosensitizers and operating under simple, general conditions with a single sacrificial reductant and no additives. Specific reactions described here are umpolung C-C bond forming reactions between aromatic ketones or imines and electron-poor alkenes, imino-pinacol homocoupling reactions of challenging alkyl-aryl imine substrates, and γ-lactonization reactions of aromatic ketones with methyl acrylate. The reactions are all initiated by photoinduced electron transfer to form a ketyl or iminyl that is subsequently trapped.

Visible-Light-Mediated Generation of Acyl Radicals from Triazine Esters

Acyl radicals are significant synthetic active species in organic synthesis. However, their generation via green and compatible methods remains challenging. Herein, we report an unprecedented visible-light-mediated approach for generating aryl acyl radicals from readily available triazine esters. This protocol with mild and redox-neutral conditions affords a diverse array of oxindoles attached to alcohol groups in a single operation. The recycling of leaving groups and a range of visible-light-mediated reactions using triazine ester as an acyl radical precursor demonstrate the synthetic potential of this methodology.

Emerging Trends in Copper-Promoted Radical-Involved C-O Bond Formations

The C-O bond is ubiquitous in biologically active molecules, pharmaceutical agents, and functional materials, thereby making it an important functional group. Consequently, the development of C-O bond-forming reactions using catalytic strategies has become an increasingly important research topic in organic synthesis because more conventional methods involving strong base and acid have many limitations. In contrast to the ionic-pathway-based methods, copper-promoted radical-mediated C-O bond formation is experiencing a surge in research interest owing to a renaissance in free-radical chemistry and photoredox catalysis. This Perspective highlights and appraises state-of-the-art techniques in this burgeoning research field. The contents are organized according to the different reaction types and working models.

Confinement of Organic Dyes in UiO-66-Type Metal-Organic Frameworks for the Enhanced Synthesis of [1,2,5]Thiadiazole[3,4-g]benzoimidazoles

Organic dyes as non-noble metal photosensitizers have attracted increasing attention due to their environmental friendliness and sustainability but suffer from fast deactivation and low stability. Here, we reported a fruitful strategy by the confinement and stabilization of visible light-active signal unit organic dyes within the metal-organic frameworks (MOFs) and developed a series of heterogeneous photocatalysts dye@UiO-66s [dye = fluorescein (FL)/rhodamine B (RhB)/eosin Y (EY), UiO-66s = UiO-66, and Bim-UiO-66]. It has been demonstrated that the encapsulated dyes can effectively sensitize MOF hosts and dominate the band structures and photocatalytic activities of dye@UiO-66s regardless of the ligand functionalization of MOFs. Photocatalytic experiments showed that these dye@UiO-66s exhibit enhanced activities relative to free dyes and among them, FL@Bim-UiO-66 displays excellent efficiencies toward the green synthesis of new carbon-bridged annulations, [1,2,5]thiadiazole[3,4-g]benzoimidazoles in the yield of up to 98% at room temperature with outstanding stability and reusability. Furthermore, the intramolecular cyclization intermediate was captured and characterized by the single-crystal X-ray diffraction analysis.

The mechanism of visible light-induced C-C cross-coupling by Csp3-H bond activation

Csp3-C cross-coupling by activating Csp3-H bonds is a dream reaction for the chemical community, and visible light-induced transition metal-catalysis under mild reaction conditions is considered a powerful tool to achieve it. Advancement of this research area is still in its infancy because of the chemical and technical complexity of this catalysis. Mechanistic studies illuminating the operative reaction pathways can rationalize the increasing amount of experimental catalysis data and provide the knowledge allowing faster and rational advances in the field. This goal requires complementary experimental and theoretical mechanistic studies, as each of them is unfit to clarify the operative mechanisms alone. In this tutorial review we summarize representative experimental and computational mechanistic studies, highlighting weaknesses, strengths, and synergies between the two approaches.

Oxetane Synthesis via Alcohol C-H Functionalization

Oxetanes are strained heterocycles with unique properties that have triggered significant advances in medicinal chemistry. However, their synthesis still presents significant challenges that limit the use of this class of compounds in practical applications. In this Letter, we present a methodology that introduces a new synthetic disconnection to access oxetanes from native alcohol substrates. The generality of the approach is demonstrated by the application in late-stage functionalization chemistry, which is further exploited to develop a single-step synthesis of a known bioactive synthetic steroid derivative that previously required at least four synthetic steps from available precursors.

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.

Boosted Tetracycline and Cr(VI) Simultaneous Cleanup over Z-Scheme WO3/CoO p-n Heterojunction with 0D/3D Structure under Visible Light

In this study, a Z-Scheme WO₃/CoO p-n heterojunction with a 0D/3D structure was designed and prepared via a simple solvothermal approach to remove the combined pollution of tetracycline and heavy metal Cr(VI) in water. The 0D WO₃ nanoparticles adhered to the surface of the 3D octahedral CoO to facilitate the construction of Z-scheme p-n heterojunctions, which could avoid the deactivation of the monomeric material due to agglomeration, extend the optical response range, and separate the photogenerated electronhole pairs. The degradation efficiency of mixed pollutants after a 70 min reaction was significantly higher than that of monomeric TC and Cr(VI). Among them, a 70% WO₃/CoO heterojunction had the best photocatalytic degradation effect on the mixture of TC and Cr(VI) pollutants, and the removing rate was 95.35% and 70.2%, respectively. Meanwhile, after five cycles, the removal rate of the mixed pollutants by the 70% WO₃/CoO remained almost unchanged, indicating that the Z-scheme WO₃/CoO p-n heterojunction has good stability. In addition, for an active component capture experiment, ESR and LC-MS were employed to reveal the possible Z-scheme pathway under the built-in electric field of the p-n heterojunction and photocatalytic removing mechanism of TC and Cr(VI). These results offer a promising idea for the treatment of the combined pollution of antibiotics and heavy metals by a Z-scheme WO₃/CoO p-n heterojunction photocatalyst, and have broad application prospects: boosted tetracycline and Cr(VI) simultaneous cleanup over a Z-scheme WO₃/CoO p-n heterojunction with a 0D/3D structure under visible light.

Organophotoredox 1,6-Addition of 3,4-Dihydroquinoxalin-2-ones to para-Quinone Methides Using Visible Light

An organophotoredox 1,6-radical addition of 3,4-dihidroquinoxalin-2-ones to para-quinone methides catalyzed by Fukuzumi's photocatalyst is described under the irradiation of a HP Single LED (455 nm). The corresponding 1,1-diaryl compounds bearing a dihydroquinoxalin-2-one moiety (20 examples) are obtained with good to excellent yields under mild reaction conditions. Several experiments have been carried out in order to propose a reaction mechanism.

Enantioselective total syntheses of six natural and two proposed meroterpenoids from Psoralea corylifolia

The first enantioselective total syntheses of six natural and two proposed meroterpenoids isolated from Psoralea corylifolia have been achieved in 7-9 steps from 2-methylcyclohexanone. The current synthetic approaches feature a high level of synthetic flexibility, stereodivergent fashion and short synthetic route, thereby providing a potential platform for the preparation of numerous this-type meroterpenoids and their pseudo-natural products.

Photoredox-Catalyzed Multicomponent Synthesis of Functionalized γ-Amino Butyric Acids via Reductive Radical Polar Crossover

Multicomponent radical polar crossover (RPC) reactions are useful for leveraging both radical and polar bond-forming steps to rapidly build molecular complexity in a single transformation. However, multicomponent RPC reactions that utilize carbonyl π-bond electrophiles are underrepresented in the literature. Herein, we describe a mild, photoredox-catalyzed decarboxylative multicomponent RPC reaction that couples carboxylic acids, Michael acceptors, and carbonyl electrophiles for the formation of diversely functionalized γ-amino butyric acid derivatives. This transformation also facilitates the synthesis of complex and biologically relevant γ-lactam compounds.