Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Photochemical direct alkylation of heteroarenes with alkanes, alcohols, amides, and ethers

Direct functionalization of heteroarenes with simple alkanes utilizing anthracene as a photoredox catalyst has been established. This approach provides a sustainable alternative, avoiding costly reagents or peroxides. The method demonstrates a broad substrate scope, enabling regioselective alkylation of various heteroarenes, including azoles, pyridines, quinolines, isoquinolones, and quinoxalinones under mild conditions. A range of alkyl sources, such as alkanes, ethers, dioxane, trioxane, alcohol, and alkylamides were viable substrates. A plausible catalytic cycle was proposed based on the preliminary mechanistic evidence.

Anomalous Reaction Pathways to Methane Production in Photocatalytic Ethanol Oxidation

Photocatalytic reduction reactions occasionally utilize sacrificial agents to scavenge photogenerated holes, thus enhancing the kinetics and efficiency of electron harvesting. However, exploring alternative hole-mediated oxidation reactions and their potential impact on photoredox processes is limited. This study investigates the products resulting from the oxidation of ethanol, a commonly used hole scavenger, and the underlying mechanisms involved. We examine a homogeneous eosin Y photoreaction scheme containing a Cu complex coordinated with an N-heterocyclic carbene, a combination often employed in CO2 conversion. Under visible-light excitation, this photosystem yields methane as an unusual product, alongside acetaldehyde and carbon monoxide. Mechanistic analysis reveals that ethanol undergoes a catalytic cascade involving oxidative processes, C-C bond cleavage, and intermolecular hydrogen atom transfer. Notably, the Lewis-acidic metal center of the Cu complex activates a novel pathway for ethanol oxidation. This work presents the influence of catalyst selection and reaction condition optimization on the emergence of new or unexpected catalytic processes.

Building Catalytic Reactions One Electron at a Time

ConspectusClassical education in organic chemistry and catalysis, not the least my own, has centered on two-electron transformations, from nucleophilic attack to oxidative addition. The focus on two-electron chemistry is well-founded, as this brand of chemistry has enabled incredible feats of synthesis, from the development of life-saving pharmaceuticals to the production of ubiquitous commodity chemicals. With that said, this approach is in many ways complementary to the approach of nature, where enzymes frequently make use of single-electron "radical" steps to achieve challenging reactions with exceptional selectivity, including light detection and C-H hydroxylation. While the power of radical elementary steps is undeniable, the fundamental understanding of─and ability to apply─these in catalysis remains underdeveloped, constraining the palette with which chemists can make new reactions.Motivation to remedy this traditional underemphasis on radical catalysis has been intensified by the runaway success of outer-sphere photoredox catalysis, not only confirming the versatility of radicals in anthropogenic catalysis but also teaching the value of robust and well-understood catalytic cycles for reaction design. Indeed, I would argue the success of outer-sphere photoredox catalysis has been fueled by strong fundamental understanding of its underlying radical elementary steps, with consideration of single-electron transfer (SET) energetics allowing new reactions to be designed de novo with enviable confidence. However, outer-sphere photoredox catalysis is an outlier in this regard, with other mechanistic approaches remaining underexplored.Our research group is part of a growing movement to expand the vocabulary of synthetic radical catalysis beyond the traditional outer-sphere photoredox SET manifold, assembling new cycles comprised of hydrogen atom transfer (HAT), light-induced homolysis (LIH), and radical ligand transfer (RLT) steps in new combinations to achieve challenging transformations. These efforts have been made possible by the ever-growing understanding of these radical elementary steps and discovery of catalyst systems with significant mechanistic flexibility, most recently iron/thiol (Fe/S) cocatalysis.In this Account, I will focus on our efforts applying HAT and LIH steps in Fe/S cocatalysis, sharing broad guidelines we have found helpful for using these steps and demonstrating how they can be combined to make new reactions using three case studies: radical hydrogenation (HAT + HAT), decarboxylative protonation (LIH + HAT), and alkene hydrofluoroalkylation (LIH + HAT, with an intervening radical alkene addition). These efforts have highlighted the importance of several key parameters, including bond dissociation energy (BDE) and radical polarity, and I hope our findings similarly provide a valuable framework to others designing new radical catalytic reactions.

Ligand-to-Metal Charge Transfer (LMCT) Catalysis: Harnessing Simple Cerium Catalysts for Selective Functionalization of Inert C-H and C-C Bonds

ConspectusChemists have long pursued harnessing light energy and photoexcitation processes for synthetic transformations. Ligand-to-metal charge transfer (LMCT) in high-valent metal complexes often triggers bond homolysis, generating oxidized ligand-centered radicals and reduced metal centers. While photoinduced oxidative activations can be enabled, this process, typically seen as photochemical decomposition, remains underexplored in catalytic applications. To mitigate decomposition during LMCT excitation, we developed a catalytic cycle integrating in situ coordination, LMCT, and ligand homolysis to activate ligated alcohols transiently into alkoxy radicals. This catalytic approach leverages Ce(IV) LMCT excitation and highly reactive alkoxy radical intermediates for selective functionalizations of C(sp3)-H and C(sp3)-C(sp3) bonds under mild conditions. In this Account, we discuss these advancements, highlighting the practical utility of cost-effective cerium salts as catalysts and their potential to develop innovative transformations, addressing long-standing synthetic challenges.Selective functionalization of chemically inert C(sp3)-H bonds has long posed a significant challenge. We first detail our research using LMCT-enabled alkoxy radical-mediated hydrogen atom transfer (HAT) processes for selective C(sp3)-H functionalizations. Using readily available CeCl3, we established a general protocol for employing free alcohols in the Barton reaction. By integrating LMCT and HAT catalysis, we introduced a selective photocatalytic strategy for functionalizing feedstock alkanes, converting gaseous hydrocarbons into valuable products. Employing simple cerium salts like Ce(OTf)3 and CeCl3, we achieved selective C-H amination of methane and ethane at ambient temperature, achieving turnover numbers of 2900 and 9700, respectively. This catalytic manifold has been further exploited to address the site-selectivity challenge in the C-H functionalization of linear alkanes. The use of methanol as a cocatalyst enabled preferential functionalization of the most electron-rich sites, achieving a high intrinsic selectivity over 12:1 of secondary vs primary sites in pentane and hexane.Next, we discuss the catalytic utilization of alkoxy-radical-mediated β-scission, a frequently encountered side reaction in HAT transformations, for selective cleavage and functionalization of C-C bonds. The versatility of the LMCT catalytic platform facilitates the generation of alkoxy radicals from various free alcohols. In our initial demonstration of LMCT-enabled C(sp3)-C(sp3) bond activation, we developed a cerium-catalyzed ring-opening and amination of cycloalkanols, providing an effective protocol for cleaving unstrained C-C bonds. This strategy has been successfully applied to various radical cross-coupling processes, leading to innovative transformations such as ring expansions of cycloalkanols, dehydroxymethylative alkylation, amination, alkenylation, and ring expansions of cyclic ketones. These results highlight the synthetic potential of employing LMCT-mediated β-scission and ubiquitous C-C bonds as unconventional functional handles for generating molecular complexity.Lastly, we delve into our mechanistic investigations. Beyond the catalytic application of Ce(IV) LMCT in various transformations, we have undertaken comprehensive mechanistic studies. These investigations encompass characterization of Ce(IV) alkoxide complexes to elucidate their structures, evaluation of their photoactivity and selectivity in radical generation, and elucidation of kinetic pathways associated with transient LMCT excited states. Our research has revealed ultrafast bond homolysis, back electron transfer, and the selectivity of heteroleptic complexes in homolysis, providing crucial insights for advancing LMCT catalysis.

Photoexcited Copper-Catalyzed Enantioselective Allylic C(sp3)-H Acyloxylation of Acyclic Internal Alkenes

The functionalization of C-H bonds streamlines the synthesis of complex molecules by eliminating the need for substrate preactivation. Traditionally, the Kharasch-Sosnovsky reaction, which directly oxidizes allylic C-H bonds into allylic esters under copper catalysis, has been hampered by long reaction times, limited substrate scope, and low enantioselectivity with acyclic olefins. Herein, we present a novel, visible light-driven, copper-catalyzed asymmetric Kharasch-Sosnovsky reaction that overcomes these challenges. This method expands the substrate scope to include acyclic internal alkenes and improves reaction conditions using eco-friendly visible light catalysis. It enhances radical reactivity and achieves superior enantioselectivity and regioselectivity in producing allylic C-H acyloxylation products. This breakthrough significantly advances direct C-H functionalization techniques, offering a more efficient and sustainable approach to synthesizing chiral molecules.

Stereoselective and site-divergent synthesis of C-glycosides

Carbohydrates play important roles in medicinal chemistry and biochemistry. However, their synthesis relies on specially designed glycosyl donors, which are often unstable and require multi-step synthesis. Furthermore, the catalytic and stereoselective installation of arylated quaternary stereocentres on sugar rings remains a formidable challenge. Here we report a facile and versatile method for the synthesis of diverse C-R (where R is an aryl, heteroaryl, alkenyl, alkynyl or alkyl) glycosides from readily available and bench-stable 1-deoxyglycosides. The reaction proceeds under mild conditions and exhibits high stereoselectivity across a broad range of glycosyl units. This protocol can be used to synthesize challenging 2-deoxyglycosides, unprotected glycosides, non-classical glycosides and deuterated glycosides. We further developed the catalyst-controlled site-divergent functionalization of carbohydrates for the synthesis of various unexplored carbohydrates containing arylated quaternary stereocentres that are inaccessible by existing methods. The synthetic utility of this strategy is further demonstrated in the synthesis of pharmaceutically relevant molecules and carbohydrates.

Direct C(sp3)-H functionalization with aryl and alkyl radicals as intermolecular hydrogen atom transfer (HAT) agents

Recent years have witnessed the emergence of direct intermolecular C(sp3)-H bond functionalization using in situ generated aryl/alkyl radicals as a unique class of hydrogen atom transfer (HAT) agents. A variety of precursors have been exploited to produce these radical HAT agents under photocatalytic, electrochemical or thermal conditions. To date, viable aryl radical precursors have included aryl diazonium salts or aryl azosulfones, diaryliodonium salts, O-benzoyl oximes, aryl sulfonium salts, aryl thioesters, and aryl halides; and applicable alkyl radical sources have included tetrahalogenated methanes (e.g., CCl3Br, CBr4 and CF3I), N-hydroxyphthalimide esters, alkyl bromides, and acetic acid. This review summarizes the current advances in direct intermolecular C(sp3)-H functionalization through key HAT events with in situ generated aryl/alkyl radicals and categorizes the procedures by the specific radical precursors applied. With an emphasis on the reaction conditions, mechanisms and representative substrate scopes of these protocols, this review aims to demonstrate the current trends and future challenges of this emerging field.

CdS Quantum Dot Gels as a Direct Hydrogen Atom Transfer Photocatalyst for C-H Activation

Here, we report CdS quantum dot (QD) gels, a three-dimensional network of interconnected CdS QDs, as a new type of direct hydrogen atom transfer (d-HAT) photocatalyst for C-H activation. We discovered that the photoexcited CdS QD gel could generate various neutral radicals, including α-amido, heterocyclic, acyl, and benzylic radicals, from their corresponding stable molecular substrates, including amides, thio/ethers, aldehydes, and benzylic compounds. Its C-H activation ability imparts a broad substrate and reaction scope. The mechanistic study reveals that this reactivity is intrinsic to CdS materials, and the neutral radical generation did not proceed via the conventional sequential electron transfer and proton transfer pathway. Instead, the C-H bonds are activated by the photoexcited CdS QD gel via a d-HAT mechanism. This d-HAT mechanism is supported by the linear correlation between the logarithm of the C-H bond activation rate constant and the C-H bond dissociation energy (BDE) with a Brønsted slope α=0.5. Our findings expand the currently limited direct hydrogen atom transfer photocatalysis toolbox and provide new possibilities for photocatalytic C-H activation.

Photocatalytic Asymmetric Acyl Radical Truce-Smiles Rearrangement for the Synthesis of Enantioenriched α-Aryl Amides

The radical Truce-Smiles rearrangement is a straightforward strategy for incorporating aryl groups into organic molecules for which asymmetric processes remains rare. By employing a readily available and non-expensive chiral auxiliary, we developed a highly efficient asymmetric photocatalytic acyl and alkyl radical Truce-Smiles rearrangement of α-substituted acrylamides using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom-transfer photocatalyst, along with aldehydes or C-H containing precursors. The rearranged products exhibited excellent diastereoselectivities (7 : 1 to >98 : 2 d.r.) and chiral auxiliary was easily removed. Mechanistic studies allowed understanding the transformation in which density functional theory (DFT) calculations provided insights into the stereochemistry-determining step.

Photocatalyzed Direct C(sp3)-H Alkenylation of Unactivated Alkanes via Tandem C-C Activation of Cyclopropenes

A highly adaptable method has been developed for the alkenylation of a broad spectrum of inert alkanes, employing milder reaction conditions. Tetrabutylammonium decatungstate (TBADT) serves as a photocatalyst for hydrogen atom transfer (HAT), instigating the formation of transient alkyl radicals through C(sp3)-H functionalization. These radicals exhibit regioselective addition to cyclopropenes, followed by the subsequent activation of C-C bonds, forming the corresponding vinylated derivatives. This methodology accommodates diverse unreactive C(sp3)-H bond motifs and multisubstituted cyclopropenes, enabling the efficient synthesis of highly functionalized olefins with high diastereoselectivity.

Electrophotocatalytic Hydroxymethylation of Azaarenes with Methanol

The merging of electrochemistry and photocatalysis allowed the required selectivity for the hydroxymethylation of functionalized azaarenes with methanol, including bioactive substrates. The two electrophotocatalytic protocols developed in this work address this transformation, using nontoxic and readily available reagents under mild reaction conditions with electricity as the only "sacrificial oxidant".

Recent Advances in Synthetic Methods by Photocatalytic Single-Electron Transfer Chemistry of Pyridine N-Oxides

By adoption of the enabling technology of modern photoredox catalysis and photochemistry, the generation of reactive and versatile pyridine N-oxy radicals can be facilely achieved from single-electron oxidation of pyridine N-oxides. This Synopsis highlights recent methodologies mediated by pyridine N-oxy radicals in developing (1) pyridine N-oxide-based hydrogen atom transfer catalysts for C(sp3)-H functionalizations and (2) β-oxyvinyl radical-mediated cascade reactions. In addition, recent research revealed that direct photoexcitation of pyridine N-oxides allowed for the generation of alkyl carbon radicals from alkylboronic acids.

Photoredox cross-dehydrogenative C(sp2)-C(sp3) coupling of heteroarenes with secondary amines through 1,5-HAT

The functionalization of α-C(sp3)-H bonds in amines has become a focal point of contemporary research. Here, we report a new approach utilizing photocatalysis α-C(sp3)-H bond functionalization in alicyclic and aliphatic secondary amines facilitated by intramolecular 1,5-hydrogen atom transfer (HAT). This finding unlocks a sustainable method for rapidly constructing complex heterocyclics via cross-dehydrogenative C-C coupling of protected amines and nitrogen-containing heterocycles. This protocol boasts broad applicability to various substrates, exhibits tolerance to numerous functional groups, and supports the late-stage modification of drug molecules.

Visible-light-induced redox-neutral difunctionalization of alkenes and alkynes

The twelve principles of green chemistry illuminate the pathway in the direction of sustainable and eco-friendly synthesis, marking a fundamental shift in synthetic organic chemistry paradigms. In this realm, harnessing the power of visible light for the difunctionalization of various skeletons without employing any external oxidant or reductant, specifically termed as redox-neutral difunctionalization, has attracted tremendous interest from synthetic organic chemists due to its low cost, easy availability and environmentally friendly nature in contrast to traditional metal-catalyzed difunctionalizations. This review presents an overview of recent updates on visible-light-induced redox-neutral difunctionalization reactions with literature coverage up to May 2024.

Titanium in photocatalytic organic transformations: current applications and future developments

Titanium, as an important transition metal, has garnered extensive attention in both industry and academia due to its excellent mechanical properties, corrosion resistance, and unique reactivity in organic synthesis. In the field of organic photocatalysis, titanium-based compounds such as titanium dioxide (TiO2), titanocenes (Cp2TiCl2, CpTiCl3), titanium tetrachloride (TiCl4), tetrakis(isopropoxy)titanium (Ti(OiPr)4), and chiral titanium complexes have demonstrated distinct reactivity and selectivity. This review focuses on the roles of these titanium compounds in photocatalytic organic reactions, and highlights the reaction pathways such as photo-induced single-electron transfer (SET) and ligand-to-metal charge transfer (LMCT). By systematically surveying the latest advancements in titanium-involved organic photocatalysis, this review aims to provide references for further research and technological innovation within this fast-developing field.

N-Heterocyclic Carbene Enabled Functionalization of Inert C(Sp3)-H Bonds via Hydrogen Atom Transfer (HAT) Processes

Developing methods to directly transform C(sp3) -H bonds is crucial in synthetic chemistry due to their prevalence in various organic compounds. While conventional protocols have largely relied on transition metal catalysis, recent advancements in organocatalysis, particularly with radical NHC catalysis have sparked interest in the direct functionalization of "inert" C(sp3) -H bonds for cross C-C coupling with carbonyl moieties. This strategy involves selective cleavage of C(sp3) -H bonds to generate key carbon radicals, often achieved via hydrogen atom transfer (HAT) processes. By leveraging the bond dissociation energy (BDE) and polarity effects, HAT enables the rapid functionalization of diverse C(sp3)-H substrates, such as ethers, amines, and alkanes. This mini-review summarizes the progress in carbene organocatalytic functionalization of inert C(sp3)-H bonds enabled by HAT processes, categorizing them into two sections: 1) C-H functionalization involving acyl azolium intermediates; and 2) functionalization of C-H bonds via reductive Breslow intermediates.

Single-Atom Fe-Catalyzed Acceptorless Dehydrogenative Coupling to Quinolines

A single-atom iron catalyst was found to exhibit exceptional reactivity in acceptorless dehydrogenative coupling for quinoline synthesis, outperforming known homogeneous and nanocatalyst systems. Detailed characterizations, including aberration-corrected HAADF-STEM, XANES, and EXAFS, jointly confirmed the presence of atomically dispersed iron centers. Various functionalized quinolines were efficiently synthesized from different amino alcohols and a range of ketones or alcohols. The iron single-atom catalyst achieved a turnover number (TON) of up to 105, far exceeding the results of current homogeneous and nanocatalyst systems. Detailed mechanistic studies verified the significance of single-atom Fe sites in the dehydrogenation process.

Recent advances in dual photoredox/nickel catalyzed alkene carbofunctionalised reactions

Alkene carbofunctionalization reactions have great potential for synthesizing complex molecules and constructing complex structures in natural products and medicinal chemistry. Recently, dual photoredox/nickel catalysis has emerged as a novel strategy for alkene carbofunctionalization. Nickel offers numerous advantages over other transition metals, such as cost-effectiveness, abundance, and low toxicity, and moreover, it has many oxidation states. Nickel catalysts exhibit excellent catalytic activity in dual photoredox/transition metal catalysis, facilitating the formation of carbon-carbon or carbon-heteroatom bonds in organic transformations. This review highlights the latest advancements in dual photoredox/nickel-catalyzed alkene carbofunctionalizations and includes the literature published from 2020 to 2024.

Shining light on the nitro group: distinct reactivity and selectivity

The nitro moiety is an indispensable functional group in organic synthesis due to its facile introduction and reduction to the corresponding amines for a plethora of organic transformations. Owing to its distinct electronegative and conventional properties, it has been used for activated aromatic nucleophilic substitution (SNAr) reactions, Smiles reactions, Henry reactions, acyl anion equivalents, etc. Recently, the excellent photochemical properties of nitroarenes have been rediscovered by several groups, and their untapped potential in organic synthesis under UV or visible light irradiation has been exploited. Photoexcited nitroarenes can undergo facile reduction to amines, azo-coupling, metal-free reductive C-N coupling with boronic acids via a 1,2-boronate shift, hydrogen atom transfer (HAT), oxygen atom transfer for anaerobic oxidation of organic molecules, molecular editing via nitrene intermediates, denitrative coupling of β-nitrostyrene, radical α-alkylation of nitroalkanes, etc. They have also been used as a photolabile protecting group in medicinal chemistry and chemical biology applications. Here, we summarise the recent findings on visible-light-mediated transformations involving nitro-containing organic molecules.

Organofunctionalized borotungstate polyoxometalates as tunable photocatalysts for oxidative dimerization of amines

Organofunctionalized borotungstate Keggin polyoxometalates, ( n Bu4N)3H[HBW11O39(P(O)Ph)2] (PBW11), ( n Bu4N)3H[HBW11O39(As(O)Ph)2] (AsBW11), and ( n Bu4N)4[HBW11O39(PhSiOSiPh)] (SiBW11), were synthesized and structurally characterized. Cyclic voltammetry showed that the electronic properties of the clusters are dependent on the nature of the appended main group atoms (P, As, or Si). The first reduction potentials were found to shift positively with respect to that of the unmodified parent species ( n Bu4N)5[BW12O40], with PBW11 showing the largest shift at +100 mV. All clusters were evaluated as photocatalysts for the oxidative dimerization of amines where the organophosphonate hybrid PBW11 was found to be the most active. This study demonstrates how organofunctionalization of polyoxometalates may be used to tune and improve their performance as photocatalysts for organic reactions.

Site-selective α-C(sp3)-H arylation of dialkylamines via hydrogen atom transfer catalysis-enabled radical aryl migration

Site-selective C(sp3)-H arylation is an appealing strategy to synthesize complex arene structures but remains a challenge facing synthetic chemists. Here we report the use of photoredox-mediated hydrogen atom transfer (HAT) catalysis to accomplish the site-selective α-C(sp3)-H arylation of dialkylamine-derived ureas through 1,4-radical aryl migration, by which a wide array of benzylamine motifs can be incorporated to the medicinally relevant systems in the late-stage installation steps. In contrast to previous efforts, this C-H arylation protocol exhibits specific site-selectivity, proforming predominantly on sterically more-hindered secondary and tertiary α-amino carbon centers, while the C-H functionalization of sterically less-hindered N-methyl group can be effectively circumvented in most cases. Moreover, a diverse range of multi-substituted piperidine derivatives can be obtained with excellent diastereoselectivity. Mechanistic and computational studies demonstrate that the rate-determining step for methylene C-H arylation is the initial H atom abstraction, whereas the radical ipso cyclization step bears the highest energy barrier for N-methyl functionalization. The relatively lower activation free energies for secondary and tertiary α-amino C-H arylation compared with the functionalization of methylic C-H bond lead to the exceptional site-selectivity.

Direct ketone synthesis from primary alcohols and alkenes enabled by a dual photo/cobalt catalysis

Catalytic methods to couple alcohol and alkene feedstocks are highly valuable in synthetic chemistry. The direct oxidative coupling of primary alcohols and alkenes offers a streamlined approach to ketone synthesis. Currently, available methods are based on transition metal-catalyzed alkene hydroacylation, which involves the generation of an electrophilic aldehyde intermediate from primary alcohol dehydrogenation. These methods generally require high reaction temperatures and a high loading of precious metal catalysts and are predominantly effective for branch-selective reactions with electron-rich alkenes. Herein, we designed a dual photo/cobalt-catalytic method to manipulate the reactivity of nucleophilic ketyl radicals for the synthesis of ketones from primary alcohols and alkenes in complementary reactivity and selectivity. This protocol exhibits exceptional scope across both primary alcohols and alkenes with high chemo- and regio-selectivity under mild reaction conditions. Mechanism investigations reveal the essential role of cobalt catalysis in enabling efficient catalysis and broad substrate scope.

Revealing the nature of covalently tethered distonic radical anions in the generation of heteroatom-centered radicals: evidence for the polarity-matching PCET pathway

Recognition of the intermediacy and regulation of reactivity patterns of radical intermediates in radical chemistry have profound impacts on harnessing and developing the full potential of open-shell species in synthetic settings. In this work, the possibility of in situ formation of O/N-X intermediates from Brønsted base covalently tethered carbonyl hypohalites (BCTCs) for the generation of heteroatom-centered radicals has certainly been excluded by NMR experiments and density functional theory calculations. Instead, the spectroscopic analyses reveal that the BCTCs serve as precursors of tether-tunable distonic radical anions (TDRAs) which have been unequivocally substantiated to be involved in the direct cleavage of O/N-H bonds to generate the corresponding heteroatom-centered radicals. Meanwhile, a deep insight into the properties and reactivities of the resulting TDRAs indicates that the introduction of a tethered Brønsted base on the parent open-shell species reinforces their stabilities and leads to a reversal of electrophilicity. Moreover, the dual descriptor values and electrophilicity indices are calculated based on eleven reported radical reactions involving various electrophilic/nucleophilic radical species, further confirming their validity in the prediction of the polar effect and the polarity-matching consistency between nucleophilic TDRAs and protic O/N-H bonds. The additional halogen-free experiments mediated by the combination of phthaloyl peroxide and TEMPO also prove the feasibility of the TDRA-assisted philicity-regulation approach. Lastly, detailed intrinsic bond orbital (IBO) and Hirschfeld spin population analyses are employed to elucidate that the H-atom abstraction processes are the polarity-matching proton-coupled electron transfer (PCET) pathways, with a degree of oxidative asynchronicity.

Stereoselective benzylic C(sp3)-H alkenylation enabled by metallaphotoredox catalysis

Selective activation of the benzylic C(sp3)-H bond is pivotal for the construction of complex organic frameworks. Achieving precise selectivity among C-H bonds with comparable energetic and steric profiles remains a profound synthetic challenge. Herein, we unveil a site- and stereoselective benzylic C(sp3)-H alkenylation utilizing metallaphotoredox catalysis. Various linear and cyclic (Z)-all-carbon tri- and tetrasubstituted olefins can be smoothly obtained. This strategy can be applied to complex substrates with multiple benzylic sites, previously deemed unsuitable due to the uncontrollable site-selectivity. In addition, sensitive functional groups such as terminal alkenyl and TMS groups are compatible under the mild conditions. The exceptional site-selectivity and broad substrate compatibility are attributed to the visible-light catalyzed relay electron transfer-proton transfer process. More importantly, we have extended this methodology to achieve enantioselective benzylic C(sp3)-H alkenylation, producing highly enantioenriched products. The applicability and scalability of our protocol are further validated through late-stage functionalization of complex structures and gram-scale operations, underscoring its practicality and robustness.

Lewis-Acid-Promoted Visible-Light-Mediated C(sp3)-H Bond Functionalization of Arylvinylpyridines via Diradical Hydrogen Atom Transfer

A visible-light-induced intramolecular diradical-mediated hydrogen atom transfer (DHAT) of primary, secondary, and tertiary C(sp3)-H bonds and subsequent cyclization is described. This transformation is enabled by triplet energy transfer upon Lewis acid coordination to alkyl-substituted arylvinylpyridines and gives access to a variety of benzocyclobutenes (>40 examples, 32-96% yield). Notably, tri- and tetrasubstituted olefins with tertiary C(sp3)-H bonds effectively delivered sterically hindered products with adjacent all-carbon quaternary centers. Mechanistic evidence and density functional theory (DFT) calculations suggest that Lewis acid coordination was crucial for the success by modulating the reactivity of the diradical intermediates to unlock a challenging carbon-to-carbon DHAT and subsequent cyclization with a rather low barrier, which allows the functionalization of benzylic C(sp3)-H bonds to construct otherwise inaccessible benzocyclobutenes.

Photochemical Aerobic Upcycling of Polystyrene Plastics via Synergistic Indirect HAT Catalysis

Plastic pollution constitutes an evergrowing urgent environmental problem, since overaccumulation of plastic waste, arising from the immense increase of the production of disposable plastic products, overcame planet's capacity to properly handle them. Chemical upcycling of polystyrene constitutes a convenient method for the conversion of plastic waste into high-added value chemicals, suggesting an attractive perspective in dealing with the environmental crisis. We demonstrate herein a novel, easy-to-perform organocatalytic photoinduced aerobic protocol, which proceeds via synergistic indirect hydrogen atom transfer (HAT) catalysis under LED 390 nm Kessil lamps as the irradiation source. The developed method employs a BrCH2CN-thioxanthone photocatalytic system and was successfully applied to a variety of everyday-life plastic products, leading to the isolation of benzoic acid after simple base-acid work up in yields varying from 23-49 %, while a large-scale experiment was successfully performed, suggesting that the photocatalytic step is susceptible to industrial application.

Substituent Effects in the Photophysical and Electrochemical Properties of Meso-Tetraphenylporphyrin Derivatives

Porphyrins were identified some years ago as a promising, easily accessible, and tunable class of organic photoredox catalysts, but a systematic study on the effect of the electronic nature and of the position of the substituents on both the ground-state and the excited-state redox potentials of these compounds is still lacking. We prepared a set of known functionalized porphyrin derivatives containing different substituents either in one of the meso positions or at a β-pyrrole carbon, and we determined their ground- and (singlet) excited-state redox potentials. We found that while the estimated singlet excited-state energies are essentially unaffected by the introduction of substituents, the redox potentials (both in the ground- and in the singlet excited-state) depend on the electron-withdrawing or electron-donating nature of the substituents. Thus, the presence of groups with electron-withdrawing resonance effects results in an enhancement of the reduction facility of the photocatalyst, both in the ground and in the excited state. We next prepared a second set of four previously unknown meso-substituted porphyrins, having a benzoyl group at different positions. The reduction facility of the porphyrin increases with the proximity of the substituent to the porphine core, reaching a maximum when the benzoyl substituent is introduced at a meso position.

Iron-Catalyzed Multicomponent C-H Alkylation of in Situ Generated Imines via Photoinduced Ligand-to-Metal Charge Transfer

Herein, we describe a novel photoinduced iron-catalyzed strategy for multicomponent C-H alkylation of in situ generated imines. By utilizing the alkyl radicals generated through iron-mediated photocatalytic C-H activation, the imines formed in situ are further subjected to addition reactions, resulting in the synthesis of various secondary and tertiary amine products. This method is simple to operate and does not require additional oxidants. It is applicable to inert alkane substrates such as cyclic alkanes, cyclic ethers, toluene, and ketones. The reaction is also compatible with various aromatic amines, alkyl amines, halogenated aromatic amines, as well as aromatic aldehydes, alkyl aldehydes, and cinnamaldehyde, among other different types of aldehydes.

Bis(pinacolato)diboron-Enabled Ni-Catalyzed Reductive Arylation/Vinylation of Alkyl Electrophiles

Herein, the use of economically and environmentally friendly bis(pinacolato)diboron (B2Pin2) is described as a non-metallic reductant in mediating Ni-catalyzed C(sp3)-C(sp2) reductive cross-coupling of alkyl electrophiles with aryl/vinyl halides. This method exhibits excellent suitability for heteroaryl halides and alkyl halides/Katritzky salts. The present study is compatible with an in situ halogenation of alcohol method, allowing for selective mono-functionalization of diols and bio-relevant alcohols (e.g., carbohydrates). The use of B2Pin2 shows potential for easy scalability without introducing additional metal impurities into the products. It is observed for the first time in the realm of cross-electrophile coupling chemistry that B2Pin2 can sever as a reductant to reduce NiII to Ni0. This mechanistic insight may inspire the development of new reductive bond-forming methodologies that can otherwise be difficult to achieve with a metal reductant.

A Ru-porphyrin metal-organic framework with Mn2+ paddlewheel nodes for the selective oxidation of C(sp3)-H bonds

The activation and selective functionalization of inert C(sp3)-H bonds is fundamental for industrial applications and occupies a very important place in industry, but it remains a great challenge in current synthetic chemistry. In this paper, we report an approach for activating reactive tert-butyl peroxyl radicals by modifying Ru-porphyrin into metal-organic frameworks (MOFs) for the activation of inert C(sp3)-H bonds. Under mild conditions, the Ru-porphyrinyl MOF can activate the peroxyl radical, extracting a hydrogen atom from the inert C(sp3)-H bond. Mn2+ paddlewheels with unsaturated coordination sites were introduced into the MOF, and direct oxidative conversion using environmentally friendly oxygen provides a new pathway to activate the inert C(sp3)-H bond.

The Subtle Mechanism of Nickel-Photocatalyzed C(sp3)-H Cross-Coupling

This computational study revises and reformulates the mechanism for the cross-coupling reaction between chlorobenzene and tetrahydrofuran catalyzed by a Ni complex with the assistance of an Ir photocatalyst. This is a representative process of transition-metal photocatalysis, and variations of it have been reported by different experimental authors. It has been also the subject of previous computational studies, which we revise and extend. Density functional theory (DFT) calculations and microkinetic modeling indicate that the most efficient mechanism takes place through an energy-transfer step and involves a NiIII complex.

p-Diarylboryl Halothiophenols as Multifunctional Catalysts via Photoactive Intramolecular Frustrated Lewis Pairs

p-Diarylboryl halothiophenols are developed and unequivocally characterized. Their photophysical properties and catalytic performance are unveiled by experimental and theoretical investigations. This novel class of triarylboranes behaves as a Brønsted acid to generate the corresponding borylthiophenolate that can absorb visible light to undergo intramolecular charge transfer to form a radical pair consisting of a boron radical anion and thiyl radical, which acts as a single-electron reductant while engaging in hydrogen atom transfer to regenerate the parent borylthiophenol. The synthetic relevance of this mode of action is demonstrated by the establishment of unique catalysis that integrates three different yet tunable functions in a single catalytic cycle, thereby allowing borylthiophenols to solely promote the assembly of sterically congested 1,2-diols and 1,2-aminoalcohol derivatives via radical-radical cross-coupling.

Synthesis of alcohols: streamlined C1 to Cn hydroxyalkylation through photoredox catalysis

Naturally occurring and readily available α-hydroxy carboxylic acids (AHAs) are utilized as platforms for visible light-mediated oxidative CO2-extrusion furnishing α-hydroxy radicals proved to be versatile C1 to Cn hydroxyalkylating agents. The direct decarboxylative Giese reaction (DDGR) is operationally simple, not requiring activator or sacrificial oxidants, and enables the synthesis of a diverse range of hydroxylated products, introducing connectivity typically precluded from conventional polar domains. Notably, the methodology has been extended to widely used glycolic acid resulting in a highly efficient and unprecedented C1 hydroxyhomologation tactic. The use of flow technology further facilitates scalability and adds green credentials to this synthetic methodology.

Hydrogen Atom Abstraction and Reduction Study of 21-Thiaporphyrin and 21,23-Dithiaporphyrin

The metal-free porphyrins protonation has gained interest over five decades because its structure modification and hardly monoacid intermediate isolation. Here, upon the hydrogen atom abstraction processes, one step diproptonated H3STTP(BF4)2 (STTP = 5,10,15,20-tetraphenyl-21-thiaporphyrin) (3) and stepwise protonated HS2TTPSbCl6 (5) and diprotonated H2S2TTP(BF4)2 (6) (S2TTP = 5,10,15,20-tetraphenyl-21,23-thiaporphyrin) compounds were obtained using HSTTP and S2TTP with oxidants. The closed-shell protonated compounds were fully characterized using XRD, UV-vis, IR and NMR spectra. In addition, the reduced 19π compounds [K(2,2,2)]HSTTP (2) and [K(2,2,2)]S2TTP (7) were synthesized by the ligands with reductant KC8 in THF solution. These two open-shell compounds were characterized with UV-vis, IR and EPR spectroscopies. The semiempirical ZINDO/S method was employed to analyze the HOMO/LUMO gap lever and identify the electronic transitions of the UV-vis spectra of the closed- and open-shell porphyrin compounds.

C(sp3)-H (N-Phenyltetrazole)thiolation as an Enabling Tool for Molecular Diversification

Methods enabling the broad diversification of C(sp3)-H bonds from a common intermediate are especially valuable in chemical synthesis. Herein, we report a site-selective (N-phenyltetrazole)thiolation of aliphatic and (hetero)benzylic C(sp3)-H bonds using a commercially available disulfide to access N-phenyltetrazole thioethers. The thioether products are readily elaborated in diverse fragment couplings for C-C, C-O, or C-N construction. The C-H functionalization proceeds via a radical-chain pathway involving hydrogen atom transfer by the electron-poor N-phenyltetrazolethiyl radical. Hexafluoroisopropanol was found to be essential to reactions involving aliphatic C(sp3)-H thiolation, with computational analysis consistent with dual hydrogen bonding of the N-phenyltetrazolethiyl radical imparting increased radical electrophilicity to facilitate the hydrogen atom transfer. Substrate is limiting reagent in all cases, and the reaction displays an exceptional functional group tolerance well suited to applications in late-stage diversification.

Recent advances in oxidative degradation of plastics

Oxidative degradation is a powerful method to degrade plastics into oligomers and small oxidized products. While thermal energy has been conventionally employed as an external stimulus, recent advances in photochemistry have enabled photocatalytic oxidative degradation of polymers under mild conditions. This tutorial review presents an overview of oxidative degradation, from its earliest examples to emerging strategies. This review briefly discusses the motivation and the development of thermal oxidative degradation of polymers with a focus on underlying mechanisms. Then, we will examine modern studies primarily relevant to catalytic thermal oxidative degradation and photocatalytic oxidative degradation. Lastly, we highlight some unique studies using unconventional approaches for oxidative polymer degradation, such as electrochemistry.

Two- and three-photon processes during photopolymerization in 3D laser printing

The performance of a photoinitiator is key to control efficiency and resolution in 3D laser nanoprinting. Upon light absorption, a cascade of competing photophysical processes leads to photochemical reactions toward radical formation that initiates free radical polymerization (FRP). Here, we investigate 7-diethylamino-3-thenoylcoumarin (DETC), belonging to an efficient and frequently used class of photoinitiators in 3D laser printing, and explain the molecular bases of FRP initiation upon DETC photoactivation. Depending on the presence of a co-initiator, DETC causes radical generation either upon two-photon- or three-photon excitation, but the mechanism for these processes is not well understood so far. Here, we show that the unique three-photon based radical formation by DETC, in the absence of a co-initiator, results from its excitation to highly excited triplet states. They allow a hydrogen-atom transfer reaction from the pentaerythritol triacrylate (PETA) monomer to DETC, enabling the formation of the reactive PETA alkyl radical, which initiates FRP. The formation of active DETC radicals is demonstrated to be less spontaneous. In contrast, photoinitiation in the presence of an onium salt co-initiator proceeds via intermolecular electron transfer after the photosensitization of the photoinitiator to the lowest triplet excited state. Our quantum mechanical calculations demonstrate photophysical processes upon the multiphoton activation of DETC and explain different reactions for the radical formation upon DETC photoactivation. This investigation for the first time describes possible pathways of FRP initiation in 3D laser nanoprinting and permits further rational design of efficient photoinitiators to increase the speed and sensitivity of 3D laser nanoprinting.

A General Approach for Photo-Oxidative Degradation of Various Polymers

The escalating demand for plastics has resulted in a surge of plastic waste worldwide, posing a monumental environmental challenge. To address this issue, a versatile photo-oxidative degradation method applicable to seven distinct polymer families is proposed, comprising poly(isobutyl vinyl ether) (PIBVE), poly(2,3-dihydrofuran) (PDHF), poly(vinyl acetate) (PVAc), poly(n-butyl acrylate) (PBA), poly(methyl acrylate) (PMA), poly(vinyl chloride) (PVC), poly(dimethyl acrylamide) (PDMA), poly(ethylene oxide) (PEO), poly(oligo(ethylene glycol) methyl ether acrylate) (PEGMEA), and even poly(methyl methacrylate) (PMMA). This method employs photo-mediated hydrogen atom transfer (HAT) followed by oxidation to promote polymer degradation. This reaction is carried out under aerobic condition in the presence of iron trichloride (FeCl3) as a photocatalyst in combination with low-intensity purple light irradiation. The process can degrade up to 97% of the polymer in less than 3 h. This degradation process can be easily controlled by switching the light off, which allows for precise modulation of the degradation rate, enhancing the effectiveness of the method. Overall, this method provides a sustainable method for degrading various polymer types with low energy input.

Visible-Light-Photocatalyzed Self-Cyclopropanation Reactions of Dibenzoylmethanes for the Synthesis of Cyclopropanes

A new self-cyclopropanation of 1,3-diphenylpropane-1,3-dione, leading to tetrasubstituted cyclopropane containing three contiguous stereogenic centers with high stereoselectivity, has been achieved through violet-light-emitting diode-irradiated photocatalysis, featuring both cycloaddition and a distinctive rearrangement. Diverging from conventional cyclopropanation pathways, this reaction yields a tetrasubstituted cyclopropane through unprecedented rearrangement and cascade reactions.

Benzylic C(sp3)-H fluorination

The selective fluorination of C(sp3)-H bonds is an attractive target, particularly for pharmaceutical and agrochemical applications. Consequently, over recent years much attention has been focused on C(sp3)-H fluorination, and several methods that are selective for benzylic C-H bonds have been reported. These protocols operate via several distinct mechanistic pathways and involve a variety of fluorine sources with distinct reactivity profiles. This review aims to give context to these transformations and strategies, highlighting the different tactics to achieve fluorination of benzylic C-H bonds.

Visible-Light-Promoted Thiolation of Benzyl Chlorides with Thiosulfonates via a Photoactive Electron Donor-Acceptor Complex

Visible-light-promoted thiolation of benzyl chlorides with thiosulfonates is disclosed via an electron donor-acceptor complex strategy. In addition to efficiently delivering a series of arylbenzylsulfide compounds, versatile thioglycosides were also successfully constructed by applying the metal- and photocatalyst-free protocol. Preliminary mechanistic studies suggest that a radical-radical coupling process was involved in this transformation.

Photoinduced Decatungstate Anion-Catalyzed 1,4-Difunctionalization of 1,3-Butadienes via C-H Activation

This paper outlines an innovative three-component coupling strategy for the 1,4-difunctionalization of 1,3-butadiene, utilizing sodium decatungstate (NaDT) as a hydrogen atom transfer (HAT) photocatalyst. The photoinduced process efficiently generates homoallylic amino acid esters with 100% atom economy, employing readily available components under mild reaction conditions. This light-induced protocol eliminates the need for an additional transition metal catalysts, additives, or equivalent reducing agents. The study explored various C(sp3)-H bearing partners, butadienes, and α-iminoesters, demonstrating the versatility and synthetic utility of this method.

Dehydrogenative Coupling of Alkylamines with Primary Alcohols Forming α-Amino Ketones

Acceptorless dehydrogenative coupling reactions between C-H bonds offer straightforward and atom-economical methods connecting readily available materials while liberating gaseous hydrogen as the sole byproduct. Despite the growing interest in such transformations, their realization still poses a significant challenge. Here we report a photoinduced dehydrogenative coupling reaction of alkylamines with primary alcohols. C-H bonds adjacent to nitrogen and oxygen are site-selectively cleaved, and a C-C bond is created between the carbon atoms in a cross-selective manner to produce α-amino ketones. Diverse polar functionalities such as esters, amides, and carboxylic acids survived, demonstrating the broad applicability of the present method.

Direct photocatalytic C-H functionalization mediated by a molybdenum dioxo complex

Direct photocatalytic C-H activation mediated by MoO2Cl2(bpy-tBu), a unique photoactive metal OXO, is presented. The limiting step, reoxidation to the Mo dioxo, is evaluated and proposed to occur via a key Cl- loss event. Photocatalyst degradation occurs upon substitution of bpy-tBu with H2O generated during catalysis.

Leuco Ethyl Violet as Self-Activating Prodrug Photocatalyst for In Vivo Amyloid-Selective Oxygenation

Aberrant aggregates of amyloid-β (Aβ) and tau protein (tau), called amyloid, are related to the etiology of Alzheimer disease (AD). Reducing amyloid levels in AD patients is a potentially effective approach to the treatment of AD. The selective degradation of amyloids via small molecule-catalyzed photooxygenation in vivo is a leading approach; however, moderate catalyst activity and the side effects of scalp injury are problematic in prior studies using AD model mice. Here, leuco ethyl violet (LEV) is identified as a highly active, amyloid-selective, and blood-brain barrier (BBB)-permeable photooxygenation catalyst that circumvents all of these problems. LEV is a redox-sensitive, self-activating prodrug catalyst; self-oxidation of LEV through a hydrogen atom transfer process under photoirradiation produces catalytically active ethyl violet (EV) in the presence of amyloid. LEV effectively oxygenates human Aβ and tau, suggesting the feasibility for applications in humans. Furthermore, a concept of using a hydrogen atom as a caging group of a reactive catalyst functional in vivo is postulated. The minimal size of the hydrogen caging group is especially useful for catalyst delivery to the brain through BBB.

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

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

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

Tandem dehydrogenation-olefination-decarboxylation of cycloalkyl carboxylic acids via multifold C-H activation

Dehydrogenation chemistry has long been established as a fundamental aspect of organic synthesis, commonly encountered in carbonyl compounds. Transition metal catalysis revolutionized it, with strategies like transfer-dehydrogenation, single electron transfer and C-H activation. These approaches, extended to multiple dehydrogenations, can lead to aromatization. Dehydrogenative transformations of aliphatic carboxylic acids pose challenges, yet engineered ligands and metal catalysis can initiate dehydrogenation via C-H activation, though outcomes vary based on substrate structures. Herein, we have developed a catalytic system enabling cyclohexane carboxylic acids to undergo multifold C-H activation to furnish olefinated arenes, bypassing lactone formation. This showcases unique reactivity in aliphatic carboxylic acids, involving tandem dehydrogenation-olefination-decarboxylation-aromatization sequences, validated by control experiments and key intermediate isolation. For cyclopentane carboxylic acids, reluctant to aromatization, the catalytic system facilitates controlled dehydrogenation, providing difunctionalized cyclopentenes through tandem dehydrogenation-olefination-decarboxylation-allylic acyloxylation sequences. This transformation expands carboxylic acids into diverse molecular entities with wide applications, underscoring its importance.

Zwitterionic Acridinium Amidate: A Nitrogen-Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer

The development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C-H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited-state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C-H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen-bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Photogeneration of Chlorine Radical from a Self-Assembled Fluorous 4CzIPN•Chloride Complex: Application in C-H Bond Functionalization

The chlorine radical is a strong HAT (Hydrogen Atom Transfer) agent that is very useful for the functionalization of C(sp3)-H bonds. Albeit highly attractive, its generation from the poorly oxidizable chloride ion mediated by an excited photoredox catalyst is a difficult task. We now report that 8Rf8-4CzIPN, an electron-deficient fluorous derivative of the benchmark 4CzIPN photoredox catalyst belonging to the donor-acceptor carbazole-cyanoarene family, is not only a better photooxidant than 4CzIPN, but also becomes an excellent host for the chloride ion. Combining these two properties ultimately makes the self-assembled 8Rf8-4CzIPN•Cl- dual catalyst highly reactive in redox-neutral Giese-type C(sp3)-H bond alkylation reactions promoted by the chlorine radical. Additionally, because of its fluorous character, the efficient separation/recovery of 8Rf8-4CzIPN could be envisioned.