Photoredox Catalysis in Organic Chemistry

Research on the metabolites and key metabolic enzymes of allocryptopine in chicken liver microsomes via stable isotope tracing technology

Allocryptopine (ALL), a principal active component of the novel veterinary medicine Bopu Powder®, has gained widespread application in the poultry farming sector for the effective management of Escherichia coli (E. coli) diarrhea. In order to explore the metabolites and the pivotal enzymes associated with ALL, this study was conducted employing an in vitro chicken liver microsomal incubation. The metabolites of ALL were analyzed and identified by combining isotope tracing technology with the application of mass spectrometry fragmentation patterns. The key metabolic enzymes involved in the biotransformation of ALL were explored using the CYP450 recombinant enzyme method, which facilitated the identification of the enzymes contributing to ALL's metabolic pathway. The liver microsomal metabolism investigation revealed a total of five metabolites, with the predominant being M2 (harmol or 3-hydroxy-4-methoxy-6-methyl-5,7,8,15-tetrahydro-[1,3]dioxolo[4',5':4,5]benzo[1,2-g]benzo[c]azecin-14(6 H)-one). The recombinant enzyme analysis conclusively identified CYP2D6 as the pivotal CYP450 isoenzyme that plays a central role in the metabolic pathway of the principal ALL metabolite, M2. This research not only expands our comprehension of the biotransformation process of ALL but also provides significant scientific evidence for the clinical safety of ALL, which was of great importance for guiding the application of ALL in the field of veterinary medicine.

Visible-Light-Induced Decarboxylative Annulation of α,β-Unsaturated Acids with Amines and α-Keto Acids for 2,4-Diarylquinoline Synthesis

An efficient and sustainable approach for the synthesis of 2,4-diarylquinolines has been developed via a visible-light-promoted metal-free three-component decarboxylative annulation pathway. This one-pot protocol combines readily available feed-stock α,β-unsaturated acids, aromatic amines, and α-keto acids in a cascade manner to access substituted quinolines under eco-benign conditions. Moreover, mechanistic insights suggest initial C-C cross coupling followed by decarboxylative 6π electrocyclic annulation to afford the desired products. The broad substrates scope and excellent functional group tolerance make this protocol more attractive and synthetically applicable toward the construction of complex N-heterocycles.

N-Protonated Acridinium Catalyst Enables Anti-Markovnikov Hydration of Unconjugated Tri- and Disubstituted Olefins

The preparation of alcohols with anti-Markovnikov selectivity directly from olefins and water is a sought-after reaction due to its atom-economy and potential cost-effectiveness. Herein, we present the first general method for direct, catalytic anti-Markovnikov hydration of unconjugated tri- and disubstituted olefins. The key advancement is made possible by an oxidative (E*red = 2.15 V) N-H acridinium catalyst, which allowed for the functionalization of alkenes that were previously unreactive in such transformations due to their high oxidation potential. The developed protocol is not limited to hydration but also enables other hydrofunctionalizations, such as hydroetherifications, following the same mechanistic pathway.

Disentangling Driving Force Effects, Polar Effects, e-/H+ Imbalance, and Other Influences on H-Atom Transfer Reactions

Hydrogen atom transfer (HAT) reactions and their kinetic barriers ΔGHAT‡ are important in organic and inorganic chemistry. This study examines factors that influence ΔGHAT‡, reporting the kinetics and thermodynamics of HAT from various ruthenium bis(acetylacetonate) pyridine-imidazole complexes to nitroxyl radicals. Across these 36 reactions, the ΔGPT° and ΔGHAT° can be independently varied, with different sets of Ru complexes primarily tuning either their pKas or their E°s. The ΔΔGHAT‡ are analyzed using multiple linear free energy relationships (LFERs), the first largely experimental study of its kind. The barriers vary most strongly with the overall driving force, ΔΔGHAT‡ = 0.28 × ΔΔGHAT°, but are also affected by HAT intrinsic barriers (λ), sterics, and the thermochemical e-/H+ imbalance of the reactions, |ΔGPT° - ΔGET°|. The latter is a small but significant effect, revealed only by comparing LFERs. The imbalance analysis is closely related to traditional explanations of polar effects, but it is quantitative: ΔGHAT‡ shifts by ∼4% with changes in |ΔGPT° - ΔGET°|. This is the same dependence as was observed for purely organic HAT from toluenes─a remarkable result because traditional explanations of organic polar effects, e.g., using X-H bond polarities, do not apply to the Ru complexes in which the e- and H+ are spatially separated. This work demonstrates the strong similarities between different kinds of HAT reactions when viewed through the lens of H+/e- (PCET) free energies. This lens also shows that ΔGHAT‡ are ∼10-fold more sensitive to changes in ΔGHAT° and λ than to the e-/H+ free-energy imbalance.

Catalytic asymmetric photocycloaddition reactions mediated by enantioselective radical approaches

The use of olefins in the construction of cyclic compounds represents a powerful strategy for advancing the pharmaceutical industry. Photocycloaddition has attracted significant interest from chemists due to its ability to exploit simple and readily available olefins along with their reaction patterns under mild conditions. Moreover, the sustainable and versatile pathways for generating highly reactive intermediates can greatly enrich both substrate diversity and reaction patterns. As a result, numerous photocycloaddition reactions have been successfully developed, particularly asymmetric [2+2], [3+2], and [4+2] photocycloadditions mediated by enantioselective radical approaches, achieving remarkable enantioselectivities. This review offers a comprehensive overview of this rapidly evolving field, organizing the discussion into three distinct reaction types that facilitate the construction of enantioenriched derivatives of cyclobutanes, cyclopentanes, and cyclohexanes. Emphasis is placed on analyzing and summarizing established strategies aimed at circumventing the challenges posed by racemic background transformations. Additionally, the exploration of asymmetric [3+2] and [4+2] photocycloaddition reactions will be interwoven with a detailed discussion of the various substrate types involved. This systematic framework seeks to enhance understanding of the strategies employed to manage the high reactivity of radicals while achieving high enantioselectivity. Importantly, it aims to guide readers in identifying uncharted radical-based cycloaddition pathways, which possess significant potential to broaden the diversity of complex cyclic molecules.

Red-light mediated formylation of indoles using a helical carbenium ion as a photoredox catalyst

Low-energy photoredox catalysis has gained significant attention in developing organic transformations due to its ability to achieve high penetration depth and minimum health risks. Herein, we disclose a red-light (λ = 640 nm)-mediated C-3 formylation of indoles utilizing a helical carbenium ion as a photocatalyst and 2,2-dimethoxy-N,N-dimethylethanamine as a formylating source. These protocols exhibit a broad substrate scope under mild conditions with efficient scalability for the synthesis of C-3 formylated indoles.

Asymmetric Photoredox Catalytic Minisci-Type Reactions of α-Bromide Amides

An asymmetric photoredox catalytic Minisci-type reaction between α-bromide amides and imine-containing azaarenes has been successfully developed. This catalyst system employs a chiral phosphoric acid alongside 3DPAFIPN as a photosensitizer. The reaction produces a diverse array of valuable amides, featuring azaarene-substituted tertiary carbon stereocenters at the β-position, in high yields (up to 85%) and good to excellent enantioselectivities (up to >99% enantiomeric excess (ee)). Importantly, this work marks the first example of asymmetric radical addition to simple azaarenes utilizing radicals functionalized with electron-withdrawing carbonyl groups, which are conventionally considered unfavorable for such transformations, especially in an enantioselective manner.

Cobalt-based Photocatalysis: From Fundamental Principles to Applications in the Generation of C-X (X=C, O, N, H, Si) Bond

Over the past few decades, the merger of photocatalysis and transition metal-based catalysis or self-photoexcitation of transition metals has emerged as a useful tool in organic transformations. In this context, cobalt-based systems have attracted significant attention as sustainable alternatives to the widely explored platinum group heavy metals (iridium, rhodium, ruthenium) for photocatalytic chemical transformations. This review encompasses the basic types of cobalt-based homogeneous photocatalytic systems, their working principles, and the recent developments (2018-2024) in C-X (X=C, N, O, H, Si) bond formations. Noteworthy to mention that cobalt-based heterogeneous photocatalysis is beyond the scope of the present review. An elaborate presentation on the mechanistic intricacies of cobalt-based photocatalysis, without any external photocatalyst, and cobalt-based dual organophotoredox catalysis have been provided in this comprehensive review, excluding the dual-metal photoredox catalysis. To the best of our knowledge, this is the only contemporary review encompassing the aforementioned two major types of cobalt-based photocatalysis, in general synthetic chemistry, covering all types of C-X bond formations spanning a range of the last six years.

Direct Stereoselective Synthesis of 2-Deoxyglycosides via Visible-Light-Induced Photoacid-Catalyzed Activation of Glycosyl o-[1-(p-MeO-Phenyl)vinyl]benzoates (PMPVBs) as Donors

The photoacid-catalyzed synthesis of 2-deoxy glycosides is presented using stable glycosyl o-[1-(p-MeO-Phenyl)vinyl]benzoate (PMPVB) donors and employing the eosin Y and diphenyl disulfide (PhSSPh) catalytic system in the presence of blue LED lights. The remote activation of the alkene functionality under the photoacid catalysis followed by a 5-exo-trig cyclization led to the generation of oxocarbenium ions that were trapped to provide the glycosylated products in excellent yields and decent selectivities under mild conditions. This method is also useful for the photoacid-catalyzed synthesis of p-methoxybenzyl-alkyl ethers.

Visible-Light-Induced [4 + 3]-Annulation of Carbonyl Ylides with Alkenyl Pyrazolinone for Constructing [4.2.1]-Oxo-Bridged Oxocine Skeleton

Herein, we present a visible-light-induced protocol for the synthesis of highly functionalized oxo-bridged oxocine skeletons. This method generates carbenes via visible-light-induced ortho-acyl diazo compounds, which are rapidly intercepted by the oxygen atom of an intermolecular acyl group to form a cyclic 1,3-dipole. The in situ generated highly reactive 1,3-dipole undergoes a facile formal [4 + 3] cycloaddition with alkenyl pyrazolinone, yielding [4.2.1]-oxo-bridged oxocine compounds.

E→Z contra-Thermodynamic isomerization of alkenes with SEGPHOS

Herein we disclose a photosensitized E→Z contra-thermodynamic isomerization of vinyl sulfones, employing SEGPHOS as a promoter. The reaction was performed successfully on 23 vinyl sulfones and was extended to three other olefins. Different mechanistic experiments were performed as well as photophysical studies.

Photocatalytic Strategy for Decyanative Transformations Enabled by Amine-Ligated Boryl Radical

Decyanation after α-functionalization by exploiting the inherent properties of cyano groups enables the strategic assembly of a carbon scaffold. Herein, we demonstrate an amine-ligated boryl radical-mediated cyano group transfer (CGT) strategy of malononitriles under photocatalytic conditions. This strategy allows for the cleavage of C(sp3)-CN and the formation of C(sp3)-D and C(sp3) to realize decyanative deuteration and cyclization via radical-polar crossover. Computational studies successfully demonstrated the reactivity of CGT promoters can be accurately assessed.

Photocatalytic oxidative hydroacylation with alcohols

Herein, we report a photocatalytic method for oxidative hydroacylation with alcohols. Under photoirradiation and a catalytic amount of TBADT, different electrophiles (azodicarboxylates, N-phenylmaleimide, benzylidenemalononitrile and phenyl vinyl sulfone) underwent hydroacylation with alcohols in good yields. The method was also applied to achieve a convenient synthesis of the anti-depressant drug moclobemide.

Bioorthogonal Activation of Deep Red Photoredox Catalysis Inducing Pyroptosis

The revolutionary impact of photoredox catalytic processes has ignited novel avenues for exploration, empowering us to delve into nature in unprecedented ways and to pioneer innovative biotechnologies for therapy and diagnosis. However, integrating artificial photoredox catalysis into living systems presents significant challenges, primarily due to concerns over low targetability, low compatibility with complex biological environments, and the safety risks associated with photocatalyst toxicity. To address these challenges, herein, we present a novel bioorthogonally activatable photoredox catalysis approach. In this approach, potent photocatalyst selection via atom replacement of the rhodamine core yielded the bioorthogonally activatable photocatalyst (PC-Tz). The introduction of 1,2,4,5-tetrazine quenched its photocatalytic properties, which were restored upon an intracellular inverse electron-demand Diels-Alder (iEDDA) reaction with trans-cyclooctene (TCO) localized in mitochondria. This reaction led to remarkable photocatalytic oxidation of nicotinamide adenine dinucleotide (NADH), effectively manipulating the mitochondrial electron transport chain (ETC) under hypoxic conditions in cancer cells. Additionally, photocatalytic pyroptotic cell death was observed through a caspase-3/gasdermin E (GSDME) pathway, achieving notable antitumor efficacy and adenosine triphosphate (ATP) reduction in tumor cells. To the best of our knowledge, this represents the first example of bioorthogonally activatable photoredox catalysis, opening new avenues for chemists to spatiotemporally control activity in specific cell organelles without disrupting other native biological processes.

Visible-light-induced radical-cascade alkylation/cyclization of acrylamides: access to 3,3-dialkylated oxindoles

A visible-light-induced deoxygenative alkylation/cyclization of acrylamides with alcohols activated by CS2 has been developed by using xanthate salts as alkyl radical precursors in the presence of tricyclohexylphosphine. It proceeds through a tandem radical addition/cyclization process, and this protocol provides a reliable and practical approach to building the skeleton of 3,3-disubstituted oxindoles in moderate to good yields. Notable features of this reaction include readily available starting reagents, broad substrate scope and mild reaction conditions.

Photoredox/Sulfide Dual Catalysis for Modular Synthesis of Multi-substituted Furan Rings via Catalytic Indirect Reductive Quenching

The catalytic indirect reductive quenching method is facilitated by a combination of Ir(III) photoredox and sulfide dual-catalysis system. This study demonstrated a method for synthesizing multi-substituted furans by using a photoredox/sulfide dual-catalysis system. This method enables the synthesis of various furan derivatives, including spirofurans and phthalans. The utility of this system was demonstrated through gram-scale synthesis of the pharmaceutical molecule talopram. Mechanistic studies and density functional theory calculations suggested the formation of sulfonium species via sulfide radical cations, followed by intramolecular cyclization to produce the desired furan derivatives.

Organophotocatalyst Enabled Deoxycyclopropanation of Alcohols

Cyclopropane fragments, which widely exist in marketed drugs and natural products, can confer special pharmacological properties to small-molecule drugs. Therefore, developing methods to construct cyclopropanes is of great significance. Nevertheless, the introduction of cyclopropane primarily relies on already-formed cyclopropyl groups, which significantly restricts the diversity of cyclopropane skeletons. Late-stage direct cyclopropanation is still a challenging task. Herein, a photo-induced intermolecular deoxycyclopropanation reaction that employs alcohols as substrates, and 1 mol.% of 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) as organophotocatalyst is reported. This method proceeds with high transformation efficiency (up to 98% yield) and exhibits broad functional group tolerance, such as primary, secondary, and tertiary alcohols as well as various activated β-halogenated alkenes. This process is mild, easy to operate, and has low equipment requirements. The power of this technology is demonstrated by the late-stage functionalization of five marketed drugs and five natural products.

Visible-Light-Induced Intramolecular Radical Cyclization for the Synthesis of Benzimidazo-isoquinolineones and Pyrrolidones

A visible-light-induced radical cyclization of α-brominated amides has been developed to construct benzimidazole[2,1-a]isoquinolin-6(5H)-ones and pyrrolidonesin in yields up to 97%. The methodology exhibits a certain functional-group compatibility with 43 examples and operational simplicity, offering a rapid and concise approach to obtain five- and six-membered rings under the irradiation of 18 W blue LED. Additionally, large-scale reaction of this method for synthesizing benzimidazo-isoquinolineones and the further transformations to useful pyrrolidones were also conducted and afforded positive results.

Visible Light Triggerable CO Releasing Micelles

Carbon monoxide (CO), along with nitric oxide and hydrogen sulfide, is one of a trinity of known gasotransmitters, or endogenously produced gaseous molecules that signal and regulate a panoply of physiological functions. CO releasing molecules (CORMs) are chemical tools that enable the study and application of this ephemeral gas, that, ideally, release CO on-demand when externally stimulated. Surveying the available triggers, photolysis is potentially advantageous: It is contactless and grants practitioners unparalleled spatial and temporal control. However, current phototriggered CORMs are capricious and do not meet current needs. Presented here is a highly efficient platform for the visible light triggered release of CO gas. This platform is built on a unique CO containing functionality, the cyclopropenone, which undergoes facile decarbonylation through visible light (470 nm) mediated photoredox catalysis. Due to the exothermic strain-release that occurs upon formation of CO, this photoreaction is rapid, quantitative, and has tunable release rates. To render this photo-CORM water-soluble, deliverable, and to keep reactants in proximity, necessary components were polymerized into block copolymers that self-assemble into CO releasing micelles (CORMIs). This platform was compared directly to other state-of-the-art CORMs, showing significantly improved CO production efficiency, lower toxicity, tunable release rates, and consistent efficacy in ex vivo and in vitro settings.

Demonstrating Synergistic Activity of Magnetic Iron Oxide Nano Photocatalyst for C-H Activation in Heterogeneous Phase

This report describes a dual catalytic approach for the versatile C-H arylation of arenes under photo-excitation at room temperature. The cooperative catalysis utilizes iron oxide magnetic nanoparticles (which mostly contain Fe3O4 along with some γ-Fe2O3) as the potential photocatalyst, which merges with the Pd-catalyzed C-H activation cycle for the reductive generation of aryl radical from aryl diazonium salt, revealing its photocatalytic activities. The method is applicable to a wide range of aryl coupling partners and different directing groups, demonstrating excellent productivity, nice co-operativity and recyclability. Adequate control experiments and mechanistic studies assisted in establishing the radical-based reaction mechanism for the C-H arylation occurring in the heterogeneous phase.

Photoinduced B-H Arylation of N-Heterocyclic Carbene Boranes with Sulfonyl(hetero)arenes

Arylborane complexes ligated by N-heterocyclic carbenes (NHCs) can be synthesized by photoirradiation of a mixture of NHC-boranes and sulfonyl(hetero)arenes. The reaction occurs under mild and convenient conditions without any photocatalyst, which are realized by a radical chain mechanism involving NHC-boryl radicals and sulfonyl radicals. This reaction offered the opportunity to reveal the photophysical property of a 2-borylnaphtho[1,2-d]thiazole derivative.

Copper-Catalyzed Asymmetric C(sp3)-H Benzylation: Stereoselective Synthesis of Unnatural Aromatic Amino Acids

The general synthesis of chiral unnatural aromatic amino acids has rarely been reported. We herein describe a visible light-promoted copper-catalyzed enantioselective C(sp3)-H benzylation of glycine derivatives. The method demonstrated compatibility in coupling various N-hydroxyphthalimide (NHP) esters derived from aromatic acids with glycine derivatives, providing a general protocol for synthesizing analogues of phenylalanine, tryptophan, and tyrosine. This protocol features mild conditions, high enantioselectivity, excellent functional group tolerance, and important impacts on the development of peptide drugs.

Iron-Catalyzed SO2-Retaining Smiles Rearrangement through Decarboxylation

Radical Smiles rearrangements have emerged as powerful methodologies for constructing carbon-carbon bonds through intramolecular radical addition and fragmentation under milder conditions, with SO2 released as a byproduct. However, SO2-retaining Smiles rearrangements, which can yield valuable alkyl sulfone derivatives, have been scarcely explored. In this study, we present an unprecedented iron-catalyzed SO2-retaining Smiles rearrangement initiated by the decarboxylation of aliphatic carboxylic acids. This approach provides a mild, cost-effective, and versatile pathway to sulfone-containing compounds, demonstrating broad substrate scope and functional group tolerance. It offers a promising strategy for synthesizing γ- and δ-aryl substituted alkyl sulfones, which are traditionally challenging to produce.

Iterative One-Carbon Homologation of Unmodified Carboxylic Acids

The one-carbon homologation of carboxylic acids is a valuable route to construct families of homologues, which play fundamental roles in chemistry and biology. However, known procedures are based on multistep sequences, use harsh conditions or are limited in scope. Thus, almost a century after the discovery of the original Arndt-Eistert homologation sequence, a general method to directly convert carboxylic acids into their corresponding homologues remains elusive. Exploiting the photoredox reactivity of nitroethylene, we disclose a practical visible-light-induced homologation of unmodified carboxylic acids. Iterations of the procedure reveal an exceptionally tunable strategy for the construction of inert carbon spacers, opening new opportunities in synthesis.

Giese-type alkylation of dehydroalanine derivatives via silane-mediated alkyl bromide activation

The rising popularity of bioconjugate therapeutics has led to growing interest in late-stage functionalization (LSF) of peptide scaffolds. α,β-Unsaturated amino acids like dehydroalanine (Dha) derivatives have emerged as particularly useful structures, as the electron-deficient olefin moiety can engage in late-stage functionalization reactions, like a Giese-type reaction. Cheap and widely available building blocks like organohalides can be converted into alkyl radicals by means of photoinduced silane-mediated halogen-atom transfer (XAT) to offer a mild and straightforward methodology of alkylation. In this research, we present a metal-free strategy for the photochemical alkylation of dehydroalanine derivatives. Upon abstraction of a hydride from tris(trimethylsilyl)silane (TTMS) by an excited benzophenone derivative, the formed silane radical can undergo a XAT with an alkyl bromide to generate an alkyl radical. Consequently, the alkyl radical undergoes a Giese-type reaction with the Dha derivative, forming a new C(sp3)-C(sp3) bond. The reaction can be performed in a phosphate-buffered saline (PBS) solution and shows post-functionalization prospects through pathways involving classical peptide chemistry.

Recent developments in the photoredox catalyzed Minisci-type reactions under continuous flow

The Minisci reaction constitutes a straightforward and convenient strategy to achieve the direct C-H functionalization of heterocyclic molecules. This radical-based reaction platform has received increasing attention due to the predictability of its outcome according to the nature of the radicals and heterocycles involved. Considering the importance of these heterocyclic scaffolds in the development and production of drug molecules, it is inevitable that scaling up this reaction manifold is of utmost importance. This review will present recent strategies to achieve the goal, which mostly involve implementing the reaction conditions under continuous flow.

Recent Applications of Sulfonium Salts in Synthesis and Catalysis

The use of sulfonium salts in organic synthesis has experienced a dramatic increase during the last years that can arguably be attributed to three main factors; the development of more direct and efficient synthetic methods that make easily available sulfonium reagents of a wide structural variety, their intrinsic thermal stability, which facilitates their structural modification, handling and purification even on large scale, and the recognition that their reactivity resembles that of hypervalent iodine compounds and therefore, they can be used as replacement of such reagents for most of their uses. This renewed interest has led to the improvement of already existing reactions, as well as to the discovery of unprecedented transformations; in particular, by the implementation of photocatalytic protocols. This review aims to summarize the most recent advancements on the area focusing on the work published during and after 2020. The scope of the methods developed will be highlighted and their limitations critically evaluated.

100 μs Luminescence Lifetime Boosts the Excited State Reactivity of a Ruthenium(II)-Anthracene Complex in Photon Upconversion and Photocatalytic Polymerizations with Red Light

The triplet excited state lifetime of a photosensitizer is an essential parameter for diffusion-controlled energy- and electron-transfer, which occurs usually in a competitive manner to the intrinsic decay of a triplet excited state. Here we show the decisive role of luminescence lifetime in the triplet excited state reactivity toward energy- and electron transfer. Anchoring two phenyl anthracene chromophores to a ruthenium(II) polypyridyl complex (RuII ref) leads to a RuII triad with a luminescence lifetime above 100 μs, which is more than 40 times longer than that of the prototypical complex. The obtained RuII triad sensitizes energy transfer to anthracene-based annihilators more efficiently than RuII ref and enables red-to-blue photon upconversion with a pseudo anti-Stokes shift of 0.94 eV and a moderate upconversion efficiency near 1 % in aerated solution. Particularly, RuII triad allows rapid photoredox catalytic polymerizations of acrylate and acrylamide monomers under aerobic condition with red light, which are kinetically hindered for RuII ref. Our work shows that excited state lifetime of a photosensitizer governs the dynamics of the excited state reactions, which seems an overlooked but important aspect for photochemistry.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Novel electron donor-acceptor (EDA) complex promoted arylation of 2-oxo-2H-chromene-3-carbonitriles under visible light irradiation

An efficient and operationally simple photochemical method has been demonstrated under transition metal-free, photocatalyst-free, and oxidant-free conditions. In recent times, diaryliodonium salts have become one of the most popular arylating sources under photoinduced conditions. Herein, we developed a visible light induced arylation of heterocycles using an EDA complex that is formed in situ from 2,6-lutidine and diaryliodonium triflate. Under light irradiation, the EDA complex generates the aryl radical that undergoes addition with 2-oxo-2H-chromene-3-carbonitriles via an SET process. This method serves as an effective tool to access biologically active and pharmaceutically relevant coumarin scaffolds.

Visible-Light-Induced Copper-Catalyzed Radical Reactions of Diazo Arylidene Succinimides to Access the Pyromellitic Diimide (PMDI) Core

The synthesis of pyromellitic diimides (PMDIs) through visible-light-promoted copper-catalyzed reaction of diazo arylidene succinimides has been accomplished without the use of external oxidants. This transformation involves a carbon radical from diazo arylidene succinimides with a copper catalyst or photocatalyst via the proton-coupled electron transfer (PCET) process. This approach successfully challenges a long-standing paradigm in the synthesis of PMDIs. Notably, copper complex (CuNCS) formed in situ proved to be playing a pivotal role to drive the reaction via photoinitiation. Additionally, we synthesized a PMDI molecule known for its prominent aggregation-induced emission (AIE) property. For the very first time, we have synthesized unsymmetrical PMDIs by employing the developed protocol.

Chemoselective Dehydrogenation and Hetero-Arylation of Amides via Radical Translocation Enabled by Photoexcited Triplet Ketone Catalysis

We herein report the chemoselective dehydrogenation and heteroarylation of amides through photoexcited triplet ketone catalysis. Under mild reaction conditions, the generated aryl radical through the halogen atom transfer (XAT) process further undergoes an intramolecular 1,5-HAT event to generate an α-amido alkyl radical, which then intercepted with either cobalt or a reduced cyano arene radical leads to dehydrogenated and heteroarylated products, respectively, in good yields.

Visible-Light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photocycloaddition Reactions

We have studied 2-(2-aminophenyl)benzothiazole and related derivatives for their photophysical properties in view of employing them as new and readily tunable organic photocatalysts. Their triplet energies were estimated by DFT calculations to be in the range of 52-57 kcal mol-1, suggesting their suitability for the [2+2] photocycloaddition of unsaturated acyl imidazoles with styrene derivatives. Experimental studies have shown that 2-(2-aminophenyl)benzothiazoles comprising alkylamino groups (NHMe, NHiPr) or the native amino group provide the best photocatalytic results in these visible-light mediated [2+2] reactions without the need of any additives, yielding a range of cyclobutane derivatives. A combined experimental and theoretical approach has provided insights into the underlying triplet-triplet energy transfer process.

Molybdenum Disulfide-Based Catalysts in Organic Synthesis: State of the Art, Open Issues, and Future Perspectives

In the field of heterogeneous organic catalysis, molybdenum disulfide (MoS2) is gaining increasing attention as a catalytically active material due to its low toxicity, earth abundance, and affordability. Interestingly, the catalytic properties of this metal-based material can be improved by several strategies. In this Perspective, through the analysis of some explicative examples, the main approaches used to prepare highly efficient MoS2-based catalysts in relevant organic reactions are summarized and critically discussed, namely: i) increment of the specific surface area, ii) generation of the metallic 1T phase, iii) introduction of vacancies, iv) preparation of nanostructured hybrids/composites, v) doping with transition metal ions, and vi) partial oxidation of MoS2. Finally, emerging trends in MoS2-based materials catalysis leading to a richer organic synthesis are presented.

A Mild and Chemoselective Photoredox-Catalyzed Reduction of Aromatic Ketones

A mild, chemoselective reduction of aromatic ketones was discovered and investigated. The combination of photoredox and Lewis acid catalysis with an organic hydrogen source reduced aromatic ketones in good to high yield. Optimization found 2-phenylbenzothiazoline to be a sufficiently strong source of hydrogen in combination with an iridium photosensitizer and lanthanum triflate. Effective photomediated reduction of some substrates was also observed in the absence of photocatalyst and Lewis acid or with photocatalyst only. While yields were typically higher under catalytic conditions, some acid-sensitive substrates were more effectively reduced in the absence of Lewis acid. The reaction was generally high yielding, and chemoselecte, while tolerant of complex and functionally rich molecules.

Polymer Connectivity Governs Electrophotocatalytic Activity in the Solid State

The reductive functionalization of inert substrates like chloroarenes is a critical yet challenging transformation relevant to both environmental remediation and organic synthesis. Combining electricity and light is an emerging approach to access the deeply reducing potentials required for single electron transfer to chloroarenes, yet this approach is held back by the poor stability and mechanistic ambiguity of current homogeneous systems. Incorporating redox-active moieties into insoluble organic materials represents a promising strategy to unlock new heterogeneous catalytic activity while improving catalyst stability. Herein, we demonstrate the first example of heterogeneous electrophotocatalysis using redox-active rylene diimide polymers for the reduction of chloroarenes. In particular, we find that the electrophotocatalytic activity varies significantly not just as a function of the rylene diimide but also of the redox-inactive polymer backbone. In particular, PTCDA-en, a flexible, non-conjugated perylenediimide polymer, outperforms all other tested materials as an electrophotocatalyst. Using transient absorption spectroscopy, we reveal that precomplexation between the closed-shell PTCDA-en 2- and the haloarene substrate is key to productive catalysis. Overall, our work represents the first example of heterogeneous electrophotocatalysis using an insoluble redox-active organic material and provides critical insights into how polymer structure dictates electrophotocatalytic activity in the solid state, guiding the development of next-generation heterogeneous (electro)photocatalysts for sustainable synthesis.

Visible Light Photoredox Catalysis in the Synthesis of Phosphonate-Substituted 1,10-Phenanthrolines

Photoredox-catalyzed phosphonylation of bromo-substituted 1,10-phenanthrolines under visible light irradiation was studied. The reaction was shown to proceed under mild conditions with Eosin Y as a photocatalyst in DMSO under blue light irradiation. It is transition-metal-free and affords the target phosphonate-substituted 1,10-phenanthrolines in moderate yields (26-51%) in 22 to 40 h. The rate and selectivity of the reaction depend largely on the position of the bromine atom, as well as on the nature and position of other substituents in the 1,10-phenanthroline core.

Photoinduced Intermolecular Hydroamination and Hydroetherification of Electron-Rich Alkenes With Low Catalyst Loadings

A photochemical method based on visible-light irradiation (blue LEDs/sunlight) has been developed for the intermolecular hydroamination and hydroetherification of electron-rich alkenes. This photochemical method is compatible with a wide range of azoles and electron-rich alkenes, such as vinyl ethers, vinyl sulfides and enamides, and is performed with low concentrations of photocatalyst (1000 ppm). Comprehensive mechanistic studies indicate that this process is initiated by the formation of an active radical cation intermediate through single electron oxidation of azole, which is mediated by excited Acr-Mes+ BF4 -.

Direct alkene functionalization via photocatalytic hydrogen atom transfer from C(sp3)-H compounds: a route to pharmaceutically important molecules

Direct functionalization of alkenes with C(sp3)-H substrates offers unique opportunities for the rapid construction of pharmaceuticals and natural products. Although significant progress has been made over the past decades, the development of green, high step-economy methods to achieve these transformations under mild conditions without the need for pre-functionalization of C(sp3)-H bonds remains a substantial challenge. Therefore, the pursuit of such methodologies is highly desirable. Recently, the direct activation of C(sp3)-H bonds via photocatalytic hydrogen atom transfer (HAT), especially from unactivated alkanes, has shown great promise. Given the potential of this approach to generate a wide range of pharmaceutically relevant compounds, this review highlights the recent advancements in the direct functionalization of alkenes through photocatalytic HAT from C(sp3)-H compounds, as well as their applications in the synthesis and diversification of drugs, natural products, and bioactive molecules, aiming to provide medicinal chemists with a practical set of tools.

Photoredox-catalyzed arylative and aryl sulfonylative radical cascades involving diaryliodonium reagents: synthesis of functionalized pyrazolones

We disclose a photoredox-catalyzed arylative radical cascade between N'-arylidene-N-acryloylhydrazides and diaryliodonium reagents to obtain the corresponding benzylated pyrazolones in good yields. The protocol was extended to three-component coupling involving the 1,4-diazabicyclo[2.2.2]octane bis(sulfur dioxide) (DABSO) adduct as a sulfur dioxide surrogate for the synthesis of arylsulfonylated pyrazolones. Both reactions exhibit broad scope, scalability, and high functional group tolerance.

Green-Light Hydrosilylation Photocatalysis with Platinum(II)Metalla-N-Heterocyclic Carbene Complexes

Platinum(II) metalla-N-heterocyclic carbene complexes featuring pyridyl heterocyclic moiety demonstrate remarkable catalytic efficiency in alkyne hydrosilylation under green light irradiation. The photocatalytic properties of complexes are rationalised by the photo-induced charge transfer occurring in extended condensed system identified with the help of various experimental (UV/vis and emission spectroscopy, cyclic voltammetry) and theoretical methods (DFT/TD-DFT, IFCT analysis).

Visible-Light-Mediated Vicinal Dihalogenation of Unsaturated C-C Bonds Using Dual-Functional Group Transfer Reagents

The growing demand for chemical production continues to drive the development of sustainable and efficient methods for introducing molecular complexity. In this context, the exploration of unconventional functional group transfer reagents (FGTRs) has led to significant advancements in practical and atom-efficient synthetic protocols. Aiming to advance the field of valuable organic synthesis, herein we report the successful development of carbon-based, bench-stable, modular, and inexpensive reagents implemented in dual halogen transfer to unsaturated hydrocarbons via photocatalytic activation of reagents based on a radical-polar crossover mechanism. This method beneficially enables vicinal dichlorination, dibromination, and bromo-chlorination reactions of olefins, offering practical alternatives to the use of toxic binary halogens. Detailed mechanistic studies, combining experimental, spectroscopic, and theoretical investigations, revealed a distinctive photocatalytic single-electron transfer reduction of FGTR. This process triggers mesolytic carbon-halogen bond cleavage, followed by a radical 1,2-halide rearrangement, leading to the continuous generation of dihalogen species in the reaction medium. The wide applicability of the developed protocol is demonstrated through an extensive scope of unsaturated molecules, including additional operations on strain-release dihalogenation.

Bioorthogonal Synthetic Chemistry Enabled by Visible-Light Photocatalysis

The field of bioorthogonal chemistry has revolutionized our ability to interrogate and manipulate biological systems at the molecular level. However, the range of chemical reactions that can operate efficiently in biological environments without interfering with the native cellular machinery, remains limited. In this context, the rapidly growing area of photocatalysis offers a promising avenue for developing new type of bioorthogonal tools. The inherent mildness, tunability, chemoselectivity, and external controllability of photocatalytic transformations make them particularly well-suited for applications in biological and living systems. This minireview summarizes recent advances in bioorthogonal photocatalytic technologies, with a particular focus on their potential to enable the selective generation of designed products within biologically relevant or living settings.

Photocatalytic Decarboxylative Cross-Coupling of α,β-Unsaturated Acids with Amines for α-Ketoamides via C-N Bond Formation

An unprecedented oxidative decarboxylative chemical domain of α,β-unsaturated acids and amines for C-N cross-coupled α-ketoamidation is disclosed. Molecular oxygen as a source oxygen in amide and water oxygen in the ketone segment furnished a green and sustainable synthesis of α-ketoamide from feedstock acids and amines. Mechanistically, photocatalyst travels with reductive quenching cycle, whereas pallado-cycle proceeded through oxidative C-N bond formation. Broad substrate scope, functional group tolerance, and CO2 and H2O as traceless byproducts make the present methodology more efficient and attractive.

Leveraging the Redox Promiscuity of Nickel To Catalyze C-N Coupling Reactions

This perspective details advances made in the field of Ni-catalyzed C-N bond formation. The use of this Earth abundant metal to decorate amines, amides, lactams, and heterocycles enables direct access to a variety of biologically active and industrially relevant compounds in a sustainable manner. Herein, different strategies that leverage the propensity of Ni to facilitate both one- and two-electron processes will be surveyed. The first part of this Perspective focuses on strategies that facilitate C-N couplings at room temperature by accessing oxidized Ni(III) intermediates. In this context, advances in photochemical, electrochemical, and chemically mediated processes will be analyzed. A special emphasis has been put on providing a comprehensive explanation of the different mechanistic avenues that have been proposed to facilitate these chemistries; either Ni(I/III) self-sustained cycles or Ni(0/II/III) photochemically mediated pathways. The second part of this Perspective details the ligand designs that also enable access to this reactivity via a two-electron Ni(0/II) mechanism. Finally, we discuss our thoughts on possible future directions of the field.

Electrophotocatalysis for Organic Synthesis

Electrocatalysis and photocatalysis have been the focus of extensive research efforts in organic synthesis in recent decades, and these powerful strategies have provided a wealth of new methods to construct complex molecules. Despite these intense efforts, only recently has there been a significant focus on the combined use of these two modalities. Nevertheless, the past five years have witnessed rapidly growing interest in the area of electrophotocatalysis. This hybrid strategy capitalizes on the enormous benefits of using photons as reagents while also employing an electric potential as a convenient and tunable source or sink of electrons. Research on this topic has led to a number of methods for C-H functionalization, reductive cross-coupling, and olefin addition among others. This field has also seen the use of a broad range of catalyst types, including both metal and organocatalysts. Of particular note has been work with open-shell photocatalysts, which tend to have comparatively large redox potentials. Electrochemistry provides a convenient means to generate such species, making electrophotocatalysis particularly amenable to this intriguing class of redox catalyst. This review surveys methods in the area of electrophotocatalysis as applied to organic synthesis, organized broadly into oxidative, reductive, and redox neutral transformations.

Exploring Photoredox Catalytic Reactions as an Entry to Glycosyl-α-amino Acids

The synthesis of glycosyl-α-amino acids presents a significant challenge due to the need for precise glycosidic linkages connecting carbohydrate moieties to amino acids while maintaining stereo- and regiochemical fidelity. Classical methods relying on ionic intermediates (2e-) often involve intricate synthetic procedures, particularly when dealing with 2-N-acetamido-2-deoxyglycosides linked to α-amino acids-a crucial class of glycoconjugates that play important biological roles. Considering the growing prominence of photocatalysis, this study explores various photoredox catalytic approaches to achieving glycosylation reactions. Our focus lies on the notoriously difficult case of 2-N-acetamido-2-deoxyglycosyl-α-amino acids, which could be obtained efficiently by two methodologies that involved, on the one hand, photoredox Giese reactions using a chiral dehydroalanine (Dha) as an electron density-deficient alkene in these radical 1,4-additions and, on the other hand, photoredox glycosylations using selenoglycosides as glycosyl donors and hydroxyl groups of protected amino acids as acceptors.

Overriding Cage Effect in Electron Donor-Acceptor Photoactivation of Diaryliodonium Reagents: Synthesis of Chalcogenides

In recent times, diaryliodonium reagents (DAIRs) have witnessed a resurgence as arylating reagents, especially under photoinduced conditions. However, reactions proceeding through electron donor-acceptor (EDA) complex formation with DAIRs are restricted to electron-rich reacting partners serving as donors due to the well-known cage effect. We discovered a practical and high-yielding visible-light-induced EDA platform to generate aryl radicals from the corresponding DAIRs and use them to synthesize key chalcogenides. In this process, an array of DAIRs and dichalcogenides react in the presence of 1,4 diazabicyclo[2.2.2]octane (DABCO) as a cheap and readily available donor, furnishing a variety of di(hetero)aryl and aryl/alkyl chalcogenides in good yields. The method is scalable, features a broad scope with good yields, and operates under open-to-air conditions. The photoinduced chalcogenation technology is suitable for late-stage functionalizations and disulfide bioconjugations and facilitates access to biologically relevant thioesters, dithiocarbamates, sulfoximines, and sulfones. Moreover, the method applies to synthesizing diverse pharmaceuticals, such as vortioxetine, promazine, mequitazine, and dapsone, under amenable conditions.

Iodine(III)-Mediated Photochemical C-H Azolation

A systematic radical polarity analysis framework is formulated herein for the projection of radical reactivity patterns. An iodine(III)-mediated photochemical C-H azolation reaction has been envisaged and developed based on the set of empirical guidelines. The synthesis features an environmentally benign reagent, mild reaction conditions, an operationally simple protocol, and a broad substrate scope. The inclusive demonstration of reactivity for ether, thioether, amide, benzylic, and allylic C-H bonds promises wide-ranging synthetic utility.

Photocatalytic Diheteroarylation of [1.1.1]Propellane for the Construction of 1,3-Diheteroaryl Bicyclo[1.1.1]pentanes

Herein, we report a visible light-induced diheteroarylation reaction of [1.1.1]propellane to synthesize 1,3-diheteroaryl bicyclo[1.1.1]pentanes (BCPs). In this approach, heteroaryl radicals are generated from heteroaryl halides via photocatalysis and subsequently added to [1.1.1]propellane. The in situ generated BCP radicals are then trapped by various heterocycles to furnish 1,3-diheteroaryl BCPs. Notably, this strategy features metal-free, mild conditions and utilizes inexpensive catalyst. For the first time, the diheteroarylation of [1.1.1]propellane could be achieved via a radical strategy, allowing for the efficient synthesis of 1,3-diheteroaryl BCPs with various applications in organic and medicinal chemistry.