Photoredox-Catalyzed Site-Selective α-C(sp3 )-H Alkylation of Primary Amine Derivatives

Sulfonamide-directed site-selective functionalization of unactivated C(sp3)-H enabled by photocatalytic sequential electron/proton transfer

The generation of alkyl radical from C(sp3)-H substrates via hydrogen atom abstraction represents a desirable yet underexplored strategy in alkylation reaction since involving common concerns remain adequately unaddressed, such as the harsh reaction conditions, limited substrate scope, and the employment of noble metal- or photo-catalysts and stoichiometric oxidants. Here, we utilize the synergistic strategy of photoredox and hydrogen atom transfer (HAT) catalysis to accomplish a general and practical functionalization of unactived C(sp3)-H centers with broad reaction scope, high functional group compatibility, and operational simplicity. A combination of validation experiments and density functional theory reveals that the N-centered radicals, generated from free N - H bond in a stepwise electron/proton transfer event, are the key intermediates that enable an intramolecular 1,5-HAT or intermolecular HAT process for nucleophilic carbon-centered radicals formation to achieve heteroarylation, alkylation, amination, cyanation, azidation, trifluoromethylthiolation, halogenation and deuteration. The practical value of this protocol is further demonstrated by the gram-scale synthesis and the late-stage functionalization of natural products and drug derivatives.

A Radical Approach for Asymmetric α-C-H Addition of N-Sulfonyl Benzylamines to Aldehydes

Efficient synthesis of enantioenriched amines is of great importance due to their significant synthetic and biological applications. Photoredox-mediated asymmetric α-amino C(sp3)-H functionalization offers an atom-economical and sustainable approach to access chiral amines. However, the development of analogous reactions is in its early stages, generally affording chiral amines with a single stereocenter. Herein, we present a novel synergistic triple-catalysis approach for the asymmetric α-C-H addition of readily available N-sulfonyl amines to aldehydes under mild conditions. This method allows for the efficient synthesis of a diverse array of valuable β-amino alcohols bearing vicinal stereocenters. Unlike previous reports, our protocol employs a radical approach using earth-abundant Cr catalysis. Quinuclidine plays a dual role by facilitating highly selective hydrogen-atom transfer to generate α-amino radicals and promoting the dissociation of the Cr-O bond, which is crucial for the overall catalytic cycle as evidenced by control, NMR, and DFT experiments. Preliminary mechanistic studies, including radical trapping, nonlinear effect, Stern-Volmer plot, kinetic isotope effect, and Hammett plot, offer valuable insights into the reaction pathway.

Total Synthesis of Incarnatapeptins A and B

The first total synthesis of incarnatapeptins A and B, two novel marine natural products, was accomplished from readily available (S)-1-benzyloxycarbonylhexahydropyridazine-3-carboxylic acid. This route, whose longest linear sequence was 12 steps, provided the incarnatapeptins A and B in yields of 26.5 % and 19.7 %, respectively, and enabled the structure and stereochemistry of both natural products to be unambiguously confirmed. Highlights of our synthesis include the photoredox-mediated decarboxylative 1,4-addition reaction and a novel and practical N-acylation paradigm promoted by silver carbonate. The unusual facile atropisomerism of some linear peptidic intermediates was also observed by TLC analysis in the course of this work.

Ligand-Controlled Regiodivergent Nickel-Catalyzed Hydroaminoalkylation of Unactivated Alkenes

Ligand modulation of transition-metal catalysts to achieve optimal reactivity and selectivity in alkene hydrofunctionalization is a fundamental challenge in synthetic organic chemistry. Hydroaminoalkylation, an atom-economical approach for alkylating amines using alkenes, is particularly significant for amine synthesis in the pharmaceutical, agrochemical, and fine chemical industries. However, the existing methods usually require specific substrate combinations to achieve precise regio- and stereoselectivity, which limits their practical utility. Protocols allowing for regiodivergent hydroaminoalkylation from the same starting materials, controlling both regiochemical and stereochemical outcomes, are currently absent. Herein, we report a ligand-controlled, regiodivergent nickel-catalyzed hydroaminoalkylation of unactivated alkenes with N-sulfonyl amines. The reaction initiates with amine dehydrogenation and involves aza-nickelacycle intermediates. Tritert-butylphosphine promotes branched regioselectivity and syn diastereoselectivity, whereas ethyldiphenylphosphine enables linear selectivity, yielding regioisomers with inverse orientation. Systematic evaluation of diverse monodentate phosphine ligands reveals distinct regioselectivity cliffs, and % Vbur (min), a ligand steric descriptor, was established as a predictive parameter correlating ligand structure to regioselectivity. Computational investigations supported experimental findings, offering mechanistic insights into the origins of regioselectivity. Our method provides an efficient and predictable route for amine synthesis, demonstrating broad substrate scope, excellent tolerance toward various functional groups, and practical advantages. These include the use of readily available starting materials and cost-effective nickel(II) salts as precatalysts.

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

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

α-Difluoroalkylation of Benzyl Amines with Trifluoromethylarenes

An α-difluoroalkylation of benzyl amines with trifluoromethylarenes is disclosed herein. This protocol is characterized by its operational simplicity, excellent chemoselectivity and broad scope-even with advanced synthetic intermediates-, thus offering a new entry point to medicinally-relevant α-difluoroalkylated amines from simple, yet readily accessible, precursors.

Oxidative Imidation of Benzylic and Cycloalkane C(sp3)-H Bond Donors Using N-Aroyloxyquinuclidinium Salts and Nitriles under Photoredox Catalysis

A series of N-aroyloxyquinuclidinium salts were prepared and used as reagents to perform efficient three-component Ritter-Mumm-type oxidative C-H imidation of donors of 1° and 2° benzylic C-H bonds used as limiting reagents with nitriles as a source of imide nitrogen under photocatalytic conditions; these reagents also exhibit somewhat lower reactivity toward cycloalkanes.

Photoredox-Nickel Dual-Catalyzed C-Alkylation of Secondary Nitroalkanes: Access to Sterically Hindered α-Tertiary Amines

The preparation of tertiary nitroalkanes via the nickel-catalyzed alkylation of secondary nitroalkanes using aliphatic iodides is reported. Previously, catalytic access to this important class of nitroalkanes via alkylation has not been possible due to the inability of catalysts to overcome the steric demands of the products. However, we have now found that the use of a nickel catalyst in combination with a photoredox catalyst and light leads to much more active alkylation catalysts. These can now access tertiary nitroalkanes. The conditions are scalable as well as air and moisture tolerant. Importantly, reduction of the tertiary nitroalkane products allows rapid access to α-tertiary amines.

Directed Photochemically Mediated Nickel-Catalyzed (Hetero)arylation of Aliphatic C-H Bonds

Site-selective functionalization of unactivated C(sp3)-H centers is challenging because of the ubiquity and strength of alkyl C-H bonds. Herein, we disclose a position-selective C(sp3)-C(sp2) cross-coupling reaction. This process engages C(sp3)-H bonds and aryl bromides, utilizing catalytic quantities of a photoredox-capable molecule and a nickel precatalyst. Using this technology, selective C-H functionalization arises owing to a 1,6-hydrogen atom transfer (HAT) process that is guided by a pendant alcohol-anchored sulfamate ester. These transformations proceed directly from N-H bonds, in contrast to previous directed, radical-mediated, C-H arylation processes, which have relied on prior oxidation of the reactive nitrogen center in reactions with nucleophilic arenes. Moreover, these conditions promote arylation at secondary centers in good yields with excellent selectivity.

A Chiral Amine Transfer Approach to the Photocatalytic Asymmetric Synthesis of α-Trialkyl-α-tertiary Amines

A long-standing challenge within radical chemistry is that of controlling the absolute stereochemistry of the products. Here, we report the stereocontrolled addition of α-amino radicals reductively generated from imines via visible-light-mediated photoredox-catalysis to alkenes, giving rise to enantioenriched α-trialkyl-α-tertiary amines. This process exploits a commercially available phenylglycinol derivative as a source of both nitrogen and chiral information. DFT studies support a stereochemical model whereby an intramolecular H-bond rigidifies the transition state of the enantiodetermining step.

The recent advances in cobalt-catalyzed C(sp3)-H functionalization reactions

Over the past decades, reactions involving C-H functionalization have become a hot theme in organic transformations because they have a lot of potential for the streamlined synthesis of complex molecules. C(sp³)-H bonds are present in most organic species. Since organic molecules have massive significance in various aspects of life, the exploitation and functionalization of C(sp³)-H bonds hold enormous importance. In recent years, the first-row transition metal-catalyzed direct and selective functionalization of C-H bonds has emerged as a simple and environmentally friendly synthetic method due to its low cost, unique reactivity profiles and easy availability. Therefore, research advancements are being made to conceive catalytic systems that foster direct C(sp³)-H functionalization under benign reaction conditions. Cobalt-based catalysts offer mild and convenient reaction conditions at a reasonable expense compared to conventional 2^nd and 3^rd-row transition metal catalysts. Consequently, the probing of Co-based catalysts for C(sp³)-H functionalization is one of the hot topics from the outlook of an organic chemist. This review primarily focuses on the literature from 2018 to 2022 and sheds light on the substrate scope, selectivity, benefits and limitations of cobalt catalysts for organic transformations.

Enantioselective Giese Additions of Prochiral α-Amino Radicals

Amines featuring an adjacent stereocenter are important building blocks, and recent years have seen remarkable growth in methods forming these via prochiral α-amino radical intermediates. However, very few can exert control over the newly formed stereocenter. We disclose a strategy to overcome this in the context of one of the most widely used radical carbon-carbon bond forming reactions, the Giese reaction. Incorporation of a removable basic heteroarene into the substrate enables a network of attractive noncovalent interactions between a phosphoric acid catalyst, the subsequently formed α-amino radical, and the Giese acceptor, allowing the catalyst to exert control during the C-C bond forming step. Deprotection of the products leads to analogues of γ-aminobutyric acid. We anticipate that this strategy will be applicable to other asymmetric radical transformations in which catalyst control is presently challenging.

Visible-light-induced dual catalysis for N-α C(sp3)-H amination and alkenylation of N-alkyl benzamides

The amination and alkenylation of the C(sp3)-H bond at the N-α position of secondary benzamides were both realized in this work by using N-hydroxyphthalimide (NHPI) imidate esters as substrates under a dual catalysis involving a photoredox catalyst and hydrogen atom transfer (HAT) catalyst. The developed methods significantly extended the scope of applications of the N-α position C(sp3)-H bond functionalization with regard to secondary N-alkylamides. More importantly, new reaction models in photoredox catalysis have been established. Based on corresponding experiments and density functional theory (DFT) calculations on the critical reaction steps combined with information reported previously, we proposed a synergistic photo- and organocatalytic reaction process for the C(sp3)-H bond functionalization and also clarified the occurrence of a chain process in the reaction pathway.

Oxidative Trifluoroacetoxylation of 1°, 2°, and 3° Benzylic C(sp3)-H Bond Donors Using N-Trifluoroacetoxyquinuclidinium Salts under Photoredox Catalysis

N-Trifluoroacetoxyquinuclidinium trifluoroacetate was prepared in situ from quinuclidine N-oxide and (CF₃CO)₂O. Except for some electron-poor substrates, this reagent allows for the high-yielding oxidative trifluoroacetoxylation of 1°, 2°, and 3° benzylic C-H bonds under photocatalytic conditions. The trifluoroacetoxylation of an ibuprofen methyl ester allowed the selective functionalization of a 2° benzylic C-H bond. For alkylbenzenes, hydrogen-atom transfer from a benzylic C-H bond to a quinuclidine cation radical was proposed to be the reaction-product-determining step.

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C-H Bonds

Electrostatic attraction between two groups of opposite charge, typically known as ion-pairing, offers unique opportunities for the design of systems to enable selectivity control in chemical reactions. Catalysis using noncovalent interactions is an established and vibrant research area, but it is noticeable that hydrogen bonding interactions are still the main interaction of choice in system design. Opposite charges experience the powerful force of Coulombic attraction and have the ability to exert fundamental influence on the outcome of reactions that involve charged reagents, intermediates or catalysts. In this Perspective, we will examine how ion-pairing interactions have been used to control selectivity in C-H bond functionalization processes. This broad class of reactions provides an interesting and thought-provoking lens through which to examine the application of ion-pairing design strategies because it is one that encompasses great mechanistic diversity, poses significant selectivity challenges, and perhaps most importantly is of immense interest to synthetic chemists in both industry and academia. We survey reactions that proceed via radical and ionic mechanisms alongside those that involve transition metal catalysis and will deal with control of site-selectivity and enantioselectivity. We anticipate that as this emerging area develops, it will become an ever-more important design strategy for selectivity control.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

Transition-metal-catalyzed C-H bond alkylation using olefins: recent advances and mechanistic aspects

Transition metal catalysis has contributed immensely to C-C bond formation reactions over the last few decades, and alkylation is no exception. The superiority of such methodologies over traditional alkylation is evident from minimal reaction steps, shorter reaction times, and atom economy while also allowing control over regio- and stereo-selectivity. In particular, hydrocarbonation of alkenes has grabbed increased attention due its fundamental ability to effectively and selectively synthesise a wide range of industrially and pharmaceutically relevant moieties. This review attempts to provide a scientific viewpoint and a systematic analysis of the recent developments in transition-metal-catalyzed alkylation of various C-H bonds using simple and activated olefins. The key features and mechanistic studies involved in these transformations are described briefly.

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

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

Triflamides and Triflimides: Synthesis and Applications

Among the variety of sulfonamides, triflamides (CF₃SO₂NHR, TfNHR) occupy a special position in organic chemistry. Triflamides are widely used as reagents, efficient catalysts or additives in numerous reactions. The reasons for the widespread use of these compounds are their high NH-acidity, lipophilicity, catalytic activity and specific chemical properties. Their strong electron-withdrawing properties and low nucleophilicity, combined with their high NH-acidity, makes it possible to use triflamides in a vast variety of organic reactions. This review is devoted to the synthesis and use of N-trifluoromethanesulfonyl derivatives in organic chemistry, medicine, biochemistry, catalysis and agriculture. Part of the work is a review of areas and examples of the use of bis(trifluoromethanesulfonyl)imide (triflimide, (CF₃SO₂)₂NH, Tf₂NH). Being one of the strongest NH-acids, triflimide, and especially its salts, are widely used as catalysts in cycloaddition reactions, Friedel–Crafts reactions, condensation reactions, heterocyclization and many others. Triflamides act as a source of nitrogen in C-amination (sulfonamidation) reactions, the products of which are useful building blocks in organic synthesis, catalysts and ligands in metal complex catalysis, and have found applications in medicine. The addition reactions of triflamide in the presence of oxidizing agents to alkenes and dienes are considered separately.

Hydroaminoalkylation for the Catalytic Addition of Amines to Alkenes or Alkynes: Diverse Mechanisms Enable Diverse Substrate Scope

Hydroaminoalkylation is a powerful, atom-economic catalytic reaction for the reaction of amines with alkenes and alkynes. This C-H functionalization reaction allows for the atom-economic alkylation of amines using simple alkenes or alkynes as the alkylating agents. This transformation has significant potential for transformative approaches in the pharmaceutical, agrochemical, and fine chemical industries in the preparation of selectively substituted amines and N-heterocycles and shows promise in materials science for the synthesis of functional and responsive aminated materials. Different early transition-metal, late transition-metal, and photoredox catalysts mediate hydroaminoalkylation by distinct mechanistic pathways. These mechanistic insights have resulted in the development of new catalysts and reaction conditions to realize hydroaminoalkylation with a broad range of substrates: activated and unactivated, terminal and internal, C-C double and triple bonds with aryl or alkyl primary, secondary, or tertiary amines, including N-heterocyclic amines. By deploying select catalysts with specific substrate combinations, control over regioselectivity, diastereoselectivity, and enantioselectivity has been realized. Key barriers to widespread adoption of this reaction include air and moisture sensitivity for early transition-metal catalysts as well as a heavy dependence on amine protecting or directing groups for late transition-metal or photocatalytic routes. Advances in improved catalyst robustness, substrate scope, and regio-/stereoselective reactions with early- and late transition-metal catalysts, as well as photoredox catalysis, are highlighted, and opportunities for further catalyst and reaction development are included. This perspective shows that hydroaminoalkylation has the potential to be a disruptive and transformative strategy for the synthesis of selectively substituted amines and N-heterocycles from simple amines and alkenes.

Electrocatalytic Isomerization of Allylic Alcohols: Straightforward Preparation of β-Aryl-Ketones

Electrochemical synthesis has been rapidly developing over the past few years. Here, we report a practical and eco-friendly electrocatalytic isomerization of allylic alcohols to their corresponding carbonyl compounds. This reaction can be carried out in undivided cells without the addition of external chemical oxidants and metal catalysts. Moreover, this reaction features a broad substrate scope including challenging allylic alcohols bearing tri- and tetra-substituted olefins and affords straightforward access to diverse β-aryl-ketones. Mechanistic investigations suggest that the reactions proceed through a radical process. This study represents a unique example in which electrochemistry enables hydrogen atom transfer in organic allylic alcohol substrates using a simple organocatalyst.

Efficient Aliphatic Hydrogen-Isotope Exchange with Tritium Gas through the Merger of Photoredox and Hydrogenation Catalysts

Employment of a combination of an organophotoredox catalyst with Wilkinson's catalyst (Rh(PPh₃)₃Cl) has given rise to an unprecedented method for hydrogen-isotope exchange (HIE) of aliphatic C(sp³)-H bonds of complex pharmaceuticals using T₂ gas directly. Wilkinson's catalyst, commonly used for catalytic hydrogenations, was exploited as a precatalyst for activation of D₂ or T₂ and hydrogen atom transfer. In this combined methodology and mechanistic study, we demonstrate that by coupling photocatalysis with Rh catalysis, carbon-centered radicals generated via photoredox catalysis can be intercepted by Rh-hydride intermediates to deliver an effective hydrogen atom donor for hydrogen-isotope labeling of complex molecules in one step. By optimizing the ratio of the photocatalyst and Wilkinson's catalyst to balance the rate of the dual catalytic cycles, we can achieve efficient HIE and high recovery yield. This protocol was readily applied to direct HIE of C(sp³)-H bonds in 10 complex drug molecules, showing high isotope incorporation efficiency and exceptionally good functional group tolerance and demonstrating this approach as a practical and attractive labeling method for deuteration and tritiation.

Cu(II)-Catalyzed N-Directed Distal C(sp3)-H Heteroarylation of Aliphatic N-Fluorosulfonamides

A copper-catalyzed δ-regioselective C(sp³)-H heteroarylation of N-fluorosulfonamides has been developed. A broad range of heteroarenes were well tolerated and reacted with various N-fluorosulfonamides to give the corresponding heteroarylated amides in good yields. Notably, all types (1°, 2°, and 3°) of δ-C(sp³)-H bonds in the N-fluorosulfonamides could be regioselectively activated through the 1,5-HAT process. This protocol provides a practical strategy for the functionalization of heteroarenes and amides via forging a C(sp³)-C(sp²) bond.

Investigations into mechanism and origin of regioselectivity in the metallaphotoredox-catalyzed α-arylation of N-alkylbenzamides

A mechanistic study on the α-arylation of N-alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides is reported herein. This study elucidates the origins of site-selectivity of the transformation, which is controlled by the generation of a hydrogen atom transfer (HAT) agent by a photocatalyst and bromide ions in solution. Tetrabutylammonium bromide was identified as a crucial additive and source of a potent HAT agent, which led to increases in yields and a lowering of the stoichiometries of the aryl bromide coupling partner. NMR titration experiments and Stern–Volmer quenching studies provide evidence for complexation to and oxidation of bromide by the photocatalyst, while elementary steps involving deprotonation of the N-alkylbenzamide or 1,5-HAT were ruled out through mechanistic probes and kinetic isotope effect analysis. This study serves as a valuable tool to better understand the α-arylation of N-alkylbenzamides, and has broader implications in halide-mediated C–H functionalization reactions.

A mechanistic study of the α-arylation of N-alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides elucidates the origins of site-selectivity of the transformation and identifies the hydrogen atom transfer agent.

The Morita–Baylis–Hillman reaction for non-electron-deficient olefins enabled by photoredox catalysis

A strategy for overcoming the limitation of the Morita–Baylis–Hillman (MBH) reaction, which is only applicable to electron-deficient olefins, has been achieved via visible-light induced photoredox catalysis in this report.

Iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides for synthesis of α-tertiary amines

A protocol for the synthesis of α-tertiary amines was developed by iterative addition of carbon nucleophiles to N,N-dialkyl carboxamides. Nucleophilic 1,2-addition of organolithium reagents to carboxamides forms anionic tetrahedral carbinolamine (hemiaminal) intermediates, which are subsequently treated with bromotrimethylsilane (Me₃SiBr) followed by organomagnesium (Grignard) reagents, organolithium reagents or tetrabutylammonium cyanide, affording α-tertiary amines. Employment of (trimethylsilyl)methylmagnesium bromide as the 2^nd nucleophile allowed for aza-Peterson olefination of the resulting α-tertiary (trimethylsilyl)methylamines with acidic work-up, resulting in the formation of 1,1-diarylethylenes.

Metal-Free Aminomethylation of Aromatic Sulfones Promoted by Eosin Y

A metal-free α-aminomethylation of heteroaryls promoted by eosin Y under green light irradiation is reported. A large variety of α-trimethylsilylamines as precursor of α-aminomethyl radical species were engaged to functionalize sulfonyl-heteroaryls following a Homolytic Aromatic Substitution (HAS) pathway. This method has provided a range of α-aminoheteroaryl compounds including a functionalized natural product. The mechanism of this late-stage functionalization of aryls was investigated and suggests the formation of a sulfonyl radical intermediate over a reductive quenching cycle.

Late-Stage N-Me Selective Arylation of Trialkylamines Enabled by Ni/Photoredox Dual Catalysis

The diversity and wide availability of trialkylamines render them ideal sources for rapid construction of complex amine architectures. Herein, we report that a nickel/photoredox dual catalysis strategy affects site-selective α-arylation of various trialkylamines. Our catalytic system shows exclusive N-Me selectivity with a wide range of trialkylamines under mild conditions, even in the context of late-stage arylation of pharmaceutical compounds bearing this common structural motif. Mechanistic studies indicate the unconventional behavior of Ni catalyst upon intercepting the α-amino radicals, in which only the primary α-amino radical undergoes a successful cross-coupling process.

Alkoxy Radicals See the Light: New Paradigms of Photochemical Synthesis

Alkoxy radicals are highly reactive species that have long been recognized as versatile intermediates in organic synthesis. However, their development has long been impeded due to a lack of convenient methods for their generation. Thanks to advances in photoredox catalysis, enabling facile access to alkoxy radicals from bench-stable precursors and free alcohols under mild conditions, research interest in this field has been renewed. This review comprehensively summarizes the recent progress in alkoxy radical-mediated transformations under visible light irradiation. Elementary steps for alkoxy radical generation from either radical precursors or free alcohols are central to reaction development; thus, each section is categorized and discussed accordingly. Throughout this review, we have focused on the different mechanisms of alkoxy radical generation as well as their impact on synthetic utilizations. Notably, the catalytic generation of alkoxy radicals from abundant alcohols is still in the early stage, providing intriguing opportunities to exploit alkoxy radicals for diverse synthetic paradigms.

Photocatalytic Hydroaminoalkylation of Styrenes with Unprotected Primary Alkylamines

Catalytic, intermolecular hydroaminoalkylation (HAA) of styrenes provides a powerful disconnection for pharmacologically relevant γ-arylamines, but current methods cannot utilize unprotected primary alkylamines as feedstocks. Metal-catalyzed HAA protocols are also highly sensitive to α-substitution on the amine partner, and no catalytic solutions exist for α-tertiary γ-arylamine synthesis via this approach. We report a solution to these problems using organophotoredox catalysis, enabling a direct, modular, and sustainable preparation of α-(di)substituted γ-arylamines, including challenging electron-neutral and moderately electron-rich aryl groups. A broad range of functionalities are tolerated, and the reactions can be run on multigram scale in continuous flow. The method is applied to a concise, protecting-group-free synthesis of the blockbuster drug Fingolimod, as well as a phosphonate mimic of its in vivo active form (by iterative α-C-H functionalization of ethanolamine). The reaction can also be sequenced with an intramolecular N-arylation to provide a general and modular access to valuable (spirocyclic) 1,2,3,4-tetrahydroquinolines and 1,2,3,4-tetrahydronaphthyridines. Mechanistic and kinetic studies support an irreversible hydrogen atom transfer activation of the alkylamine by the azidyl radical and some contribution from a radical chain. The reaction is photon-limited and exhibits a zero-order dependence on amine, azide, and photocatalyst, with a first-order dependence on styrene.

Modular Photocatalytic Synthesis of α-Trialkyl-α-Tertiary Amines

Molecules displaying an α-trialkyl-α-tertiary amine motif provide access to an important and versatile area of biologically relevant chemical space but are challenging to access through existing synthetic methods. Here, we report an operationally straightforward, multicomponent protocol for the synthesis of a range of functionally and structurally diverse α-trialkyl-α-tertiary amines, which makes use of three readily available components: dialkyl ketones, benzylamines, and alkenes. The strategy relies on the of use visible-light-mediated photocatalysis with readily available Ir(III) complexes to bring about single-electron reduction of an all-alkyl ketimine species to an α-amino radical intermediate; the α-amino radical undergoes Giese-type addition with a variety of alkenes to forge the α-trialkyl-α-tertiary amine center. The mechanism of this process is believed to proceed through an overall redox neutral pathway that involves photocatalytic redox-relay of the imine, generated from the starting amine-ketone condensation, through to an imine-derived product. This is possible because the presence of a benzylic amine component in the intermediate scaffold drives a 1,5-hydrogen atom transfer step after the Giese addition to form a stable benzylic α-amino radical, which is able to close the photocatalytic cycle. These studies detail the evolution of the reaction platform, an extensive investigation of the substrate scope, and preliminary investigation of some of the mechanistic features of this distinct photocatalytic process. We believe this transformation will provide convenient access to previously unexplored α-trialkyl-α-tertiary amine scaffolds that should be of considerable interest to practitioners of synthetic and medicinal chemistry in academic and industrial institutions.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp² or sp³ C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Recent Advances in Photoredox-Mediated Radical Conjugate Addition Reactions: An Expanding Toolkit for the Giese Reaction

Photomediated Giese reactions are at the forefront of radical chemistry, much like the classical tin-mediated Giese reactions were nearly forty years ago. With the global recognition of organometallic photocatalysts for the mild and tunable generation of carbon-centered radicals, chemists have developed a torrent of strategies to form previously inaccessible radical intermediates that are capable of engaging in intermolecular conjugate addition reactions. This Review summarizes advances in photoredox-mediated Giese reactions since 2013, with a focus on the breadth of methods that provide access to crucial carbon-centered radical intermediates that can engage in radical conjugate addition processes.

Direct, Site-Selective and Redox-Neutral α-C-H Bond Functionalization of Tetrahydrofurans via Quantum Dots Photocatalysis

As one of the most ubiquitous bulk reagents available, the intrinsic chemical inertness of tetrahydrofuran (THF) makes direct and site-selective C(sp³ )-H bond activation difficult, especially under redox neutral condition. Here, we demonstrate that semiconductor quantum dots (QDs) can activate α-C-H bond of THF via forming QDs/THF conjugates. Under visible light irradiation, the resultant alkoxyalkyl radical directly engages in radical cross-coupling with α-amino radical from amino C-H bonds or radical addition with alkene or phenylacetylene, respectively. In contrast to stoichiometric oxidant or hydrogen atom transfer reagents required in previous studies, the scalable benchtop approach can execute α-C-H bond activation of THF only by a QD photocatalyst under redox-neutral condition, thus providing a broad of value added chemicals starting from bulk THFs reagent.

Recent progress in the radical α-C(sp3)-H functionalization of ketones

The direct use structurally simple ketones as α-ketone radical sources for α-C(sp³)-H functionalization is a sustainable and powerful approach for constructing complex and multifunctional chemical scaffolds with diverse applications. The reactions of α-ketone radicals with alkenes, alkynes, enynes, imides, and imidazo[1,2-a]pyridines have broadened the structural diversity and complexity of ketones. Through chosen illustrative examples, we outline the recent progress in the development of methods that enable the radical α-C(sp³)-H functionalization of ketones, with an emphasis on radical initiation systems and possible mechanisms of the transformations. The application of these strategies is illustrated by the synthesis of several biologically active molecules and drug molecules. Further subdivision is based on substrate type and reaction type.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

Electrooxidative dearomatization of biaryls: synthesis of tri- and difluoromethylated spiro[5.5]trienones

Radical spirocyclization via dearomatization has emerged as an attractive strategy for the rapid synthesis of structurally diverse spiro molecules. We report the use of electrochemistry to perform an oxidative dearomatization of biaryls leading to tri- and difluoromethylated spiro[5.5]trienones in a user friendly undivided cell set-up and a constant current mode. The catalyst- and chemical oxidant-free dearomatization procedure features ample scope, and employs electricity as the green and sole oxidant.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

Photocatalytic carbocarboxylation of styrenes with CO2 for the synthesis of γ-aminobutyric esters

Metal-free photoredox-catalyzed carbocarboxylation of various styrenes with carbon dioxide (CO2) and amines to obtain γ-aminobutyric ester derivatives has been developed (up to 91% yield, 36 examples). The radical anion of (2,3,4,6)-3-benzyl-2,4,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBnBN) possessing a high reduction potential (-1.72 V vs. saturated calomel electrode (SCE)) easily reduces both electron-donating and electron-withdrawing group-substituted styrenes.

Triarylamine-based porous coordination polymers performing both hydrogen atom transfer and photoredox catalysis for regioselective α-amino C(sp3)-H arylation

Direct functionalization of C(sp³)–H bonds in a predictable, selective and recyclable manner has become a central challenge in modern organic chemistry. Through incorporating different triarylamine-containing ligands into one coordination polymer, we present herein a heterogeneous approach to the combination of hydrogen atom transfer (HAT) and photoredox catalysis for regioselective C–H arylation of benzylamines. The different molecular sizes and coordination modes of the ligands, tricarboxytriphenylamine (H₃TCA) and tris(4-(pyridinyl)phenyl)amine (NPy3), in one coordination polymer consolidate the triarylamine (Ar₃N) moiety into a special structural intermediate, which enhances the chemical and thermal stability of the polymers and diminishes structural relaxation during the catalytic process. The inherent redox potentials of Ar₃N moieties prohibit the in situ formed Ar₃N˙⁺ to earn an electron from C(sp³)–H nucleophiles, but allow the abstraction of a hydrogen atom from C(sp³)–H nucleophiles, enabling the formation of the C(sp³)˙ radical and the cross-coupling reaction to proceed at the most electron-rich sites with excellent regioselectivity. The new heterogeneous photoredox HAT approach skips several interactions between transient species during the typical synergistic SET/HAT cycles, demonstrating a promising redox-economical and reagent-economical heterogeneous platform that has not been reported for α-amino C–H arylation to form benzylamine derivatives. Control experiments based on monoligand coordination polymers suggested that the mixed-ligand approach improved the photochemical and photophysical properties, providing important insight into rational design and optimization of recyclable photocatalysts for rapid access to complex bioactive molecules and late-stage functionalized pharmaceuticals.

The efficiency of photosensitization and hydrogen atom transfer (HAT) catalysis is balanced in a recyclable heterogeneous manner by the modification of the N-central conformation in Cd-MIX.

α-C-H/N-H Annulation of Alicyclic Amines via Transient Imines: Preparation of Polycyclic Lactams

Polycyclic lactams are prepared in a single operation from o-toluamides and cyclic amines in a process that involves transient cyclic imines, species that are conveniently obtained in situ from the corresponding lithium amides and simple ketone oxidants. Imines thus generated, such as 1-pyrroline and 1-piperideine, engage lithiated o-toluamides in a facile annulation process. Undesired side reactions such as imine deprotonation and o-toluamide dimerization are suppressed through the judicious choice of reaction conditions.