Visible-Light-Mediated Utilization of α-Aminoalkyl Radicals: Addition to Electron-Deficient Alkenes Using Photoredox Catalysts

Aminomethylations of electron-deficient compounds-bringing iron photoredox catalysis into play

The α-functionalisation of N-containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(iii)(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(iii)(phtmeimb)2]PF6 can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy)3]Cl2 under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value.

Embracing heterogeneous photocatalysis: evolution of photocatalysts in annulation of dimethylanilines and maleimides

Recent advances in visible-light-promoted construction of tetrahydroquinolines from dimethylanilines and maleimides are documented. Homogeneous and heterogeneous photocatalytic systems, as well as the reaction mechanism, are emphasized. The mechanism of this photocatalytic annulation reaction is quite clear, i.e., dimethylanilines and maleimides serve as the radical precursors and radical acceptors, respectively. This annulation reaction could serve as an excellent platform for evaluating novel oxidative heterogeneous photocatalytic systems, which could further inspire chemists in this field to develop more efficient photocatalytic systems. Significant opportunities are expected in the future for heterogeneous photocatalysis strategies.

Functionalized Polyamine Synthesis with Photoredox Catalysis

Polyamines, such as putrescine and spermidine, are pivotal in various biological processes across living organisms. Despite their significance, structurally modified polyamines offer a less-explored avenue for discovering bioactive compounds. The limitation is attributed to the synthetic difficulty of accessing functionalized polyamines. In this study, we accomplished photoredox-catalyzed functionalization of polyamines to diversify their structure. The rapid functionalization allows attaching fluorophores to the target polyamine, facilitating the development of molecular probes for advancing chemical biology studies.

Transition-Metal-Free Anti-Markovnikov Hydroarylation of Alkenes with Aryl Chlorides through Consecutive Photoinduced Electron Transfer

The hydroarylation of alkenes has emerged as a powerful strategy for arene functionalization. However, aryl chlorides remain a large challenge in this type of reaction due to the chemical inertness of the C(sp2)-Cl bond and high negative reduction potential. Herein, we report an anti-Markovnikov radical hydroarylation of alkenes with aryl chlorides via visible-light photoredox catalysis. The key reactive aryl radicals can be efficiently achieved from aryl chlorides by consecutive photoinduced electron transfer. This transition-metal-free protocol features mild conditions, a wide substrate scope, and functional group tolerance, producing a diverse range of linear alkylarenes in moderate to good yields. The reaction is proposed to proceed through a radical-polar crossover pathway.

Deconstructive Carboxylation of Activated Alkenes with Carbon Dioxide

Carboxylation with carbon dioxide (CO2 ) represents one notable methodology to produce carboxylic acids. In contrast to carbon-heteroatom bonds, carbon-carbon bond cleavage for carboxylation with CO2 is far more challenging due to their inherent and less favorable orbital directionality for interacting with transition metals. Here we report a photocatalytic protocol for the deconstructive carboxylation of alkenes with CO2 to generate carboxylic acids in the absence of transition metals. It is emphasized that our protocol provides carboxylic acids with obviously unchanged carbon numbers when terminal alkenes were used. To show the power of this strategy, a variety of pharmaceutically relevant applications including the modular synthesis of propionate nonsteroidal anti-inflammatory drugs and the late-stage carboxylation of bioactive molecule derivatives are demonstrated.

Nickel/photoredox dual catalyzed arylalkylation of nonactivated alkenes

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

Red-Light-Driven Atom Transfer Radical Polymerization for High-Throughput Polymer Synthesis in Open Air

Photoinduced reversible-deactivation radical polymerization (photo-RDRP) techniques offer exceptional control over polymerization, providing access to well-defined polymers and hybrid materials with complex architectures. However, most photo-RDRP methods rely on UV/visible light or photoredox catalysts (PCs), which require complex multistep synthesis. Herein, we present the first example of fully oxygen-tolerant red/NIR-light-mediated photoinduced atom transfer radical polymerization (photo-ATRP) in a high-throughput manner under biologically relevant conditions. The method uses commercially available methylene blue (MB+) as the PC and [X-CuII/TPMA]+ (TPMA = tris(2-pyridylmethyl)amine) complex as the deactivator. The mechanistic study revealed that MB+ undergoes a reductive quenching cycle in the presence of the TPMA ligand used in excess. The formed semireduced MB (MB•) sustains polymerization by regenerating the [CuI/TPMA]+ activator and together with [X-CuII/TPMA]+ provides control over the polymerization. This dual catalytic system exhibited excellent oxygen tolerance, enabling polymerizations with high monomer conversions (>90%) in less than 60 min at low volumes (50-250 μL) and high-throughput synthesis of a library of well-defined polymers and DNA-polymer bioconjugates with narrow molecular weight distributions (Đ < 1.30) in an open-air 96-well plate. In addition, the broad absorption spectrum of MB+ allowed ATRP to be triggered under UV to NIR irradiation (395-730 nm). This opens avenues for the integration of orthogonal photoinduced reactions. Finally, the MB+/Cu catalysis showed good biocompatibility during polymerization in the presence of cells, which expands the potential applications of this method.

Nickel/Photoredox Dual-Catalyzed Regiodivergent Aminoalkylation of Unactivated Alkyl Halides

A mild and regiodivergent aminoalkylation of unactivated alkyl halides is disclosed via a dual photoredox/nickel catalysis. Bipyridyl-type ligands without an ortho-substituent control the site-selective coupling at the original position, while ortho-disubstituted ligands tune the site-selectivity at a remote, unprefunctionalized position. Mechanistic studies combined with DFT calculations give insight into the mechanism and the origins of the ligand-controlled regioselectivity. Notably, this redox-neutral, regiodivergent alkyl-alkyl coupling features mild conditions, broad substrate scope for both alkyl coupling partners, and excellent site-selectivity and offers a straightforward way for α-alkylation of tertiary amines to synthesize structurally diverse alkylamines and value-added amino acid derivatives.

Visible-Light-Curable Acrylic Resins toward UV-Light-Blocking Adhesives for Foldable Displays

Current technological advances in the organic light-emitting diode panel design of foldable smartphones demand advanced adhesives with UV-blocking abilities, beyond their conventional roles of bonding objects and relieving deformation stress. However, optically clear adhesives (OCAs) with UV-blocking ability cannot be prepared using conventional UV-curing methods relying on a photoinitiator. Herein, a new acrylic resin that can be efficiently cured using visible light without oxygen removal is presented, which may be used to develop UV-blocking OCAs for use in current flexible displays. A novel photocatalyst and a specific combination of additives facilitate sufficiently rapid curing under visible light in the presence of UV-absorbers. Only a very small amount of the highly active photocatalyst is required to prepare UV-blocking OCA films with very high transparency in the visible region. Using this system, a UV-blocking OCA that nearly meets the specifications of an OCA used in commercialized foldable smartphones is realized. This technology can also be utilized in other applications that require highly efficient visible light curing, such as optically clear resins, dental resins, and 3D/4D-printable materials.

Recent Developments on Photochemical Synthesis of 1,n-Dicarbonyls

1,n-dicarbonyls are one of the most fascinating chemical feedstocks finding abundant usage in the field of pharmaceuticals. Besides, they are utilized in a plethora of synthesis in general synthetic organic chemistry. A number of 'conventional' methods are available for their synthesis, such as the Stetter reaction, Baker-Venkatraman rearrangement, oxidation of vicinal diols, and oxidation of deoxybenzoins, synonymous with unfriendly reagents and conditions. In the last 15 years or so, photocatalysis has taken the world of synthetic organic chemistry by a remarkable renaissance. It is fair to say now that everybody loves the light and photoredox chemistry has opened a new gateway to organic chemists towards milder, more simpler alternatives to the previously available methods, allowing access to many sensitive reactions and products. In this review, we present the readers with the photochemical synthesis of a variety of 1,n-dicarbonyls. Diverse photocatalytic pathways to these fascinating molecules have been discussed, placing special emphasis on the mechanisms, giving the reader an opportunity to find all these significant developments in one place.

Tantalum ureate complexes for photocatalytic hydroaminoalkylation

Using a tantalum ureate pre-catalyst, photocatalytic hydroaminoalkylation of unactivated alkenes with unprotected amines at room temperature is demonstrated. The combination of Ta(CH₂SiMe₃)₃Cl₂ and a ureate ligand with a saturated cyclic backbone resulted in this unique reactivity. Preliminary investigations of the reaction mechanism suggest that both the thermal and photocatalytic hydroaminoalkylation reactions begin with N-H bond activation and subsequent metallaaziridine formation. However, a select tantalum ureate complex, through ligand to metal charge transfer (LMCT), results in photocatalyzed homolytic metal-carbon bond cleavage and subsequent addition to unactivated alkene to afford the desired carbon-carbon bond formation. Origins of ligand effects on promoting homolytic metal-carbon bond cleavage are explored computationally to support enhanced ligand design efforts.

Visible Light Induced C-H/N-H and C-X Bonds Reactions

Herein, we report efficient visible light-induced photoredox reactions of C–H/N–H and C–X Bonds. These methods have provided access to varied portfolio of synthetically important γ-ketoesters, azaspirocyclic cyclohexadienones spirocyclohexadienones, multisubstituted benzimidazole derivatives, substituted N,2-diarylacetamide, 2-arylpyridines and 2-arylquinolines in good yields and under mild conditions. Moreover, we have successfully discussed the construction through visible light-induction by an intermolecular radical addition, dearomative cyclization, aryl migration and desulfonylation. Similarly, we also spotlight the visible light-catalyzed aerobic C–N bond activation from well-known building blocks through cyclization, elimination and aromatization. The potential use of a wide portfolio of simple ketones and available primary amines has made this transformation very attractive.

Regiospecific α-methylene functionalisation of tertiary amines with alkynes via Au-catalysed concerted one-proton/two-electron transfer to O2

Regioselective transformations of tertiary amines, which are ubiquitously present in natural products and drugs, are important for the development of novel medicines. In particular, the oxidative α-C-H functionalisation of tertiary amines with nucleophiles via iminium cations is a promising approach because, theoretically, there is almost no limit to the type of amine and functionalisation. However, most of the reports on oxidative α-C-H functionalisations are limited to α-methyl-selective or non-selective reactions, despite the frequent appearance of α-methylene-substituted amines in pharmaceutical fields. Herein, we develop an unusual oxidative regiospecific α-methylene functionalisation of structurally diverse tertiary amines with alkynes to synthesise various propargylic amines using a catalyst comprising Zn salts and hydroxyapatite-supported Au nanoparticles. Thorough experimental investigations suggest that the unusual α-methylene regiospecificity is probably due to a concerted one-proton/two-electron transfer from amines to O₂ on the Au nanoparticle catalyst, which paves the way to other α-methylene-specific functionalisations.

Remote and Proximal Hydroaminoalkylation of Alkenes Enabled by Photoredox/Nickel Dual Catalysis

A mild and site-selective hydroaminoalkylation of activated and unactivated alkenes via dual photoredox/Ni catalysis is developed. This dual catalytic strategy enables exclusive access to α-selective products, which is complementary to previously reported photocatalytic hydroaminoalkylation of activated alkenes that provides the β-selective products. The chain-walking of a Ni-H intermediate toward a carbonyl allows for the hydroaminoalkylation of unactivated alkenes at remote sp³ C-H sites. This method tolerates a broad substrate scope of both amines and alkenes as well as providing a streamlined synthesis of value-added β-amino acid derivatives from readily available starting materials.

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.

Visible-Light-Induced Dual C(sp3)-H Bond Functionalization of Tertiary Amine via Hydrogen Transfer to Carbene and Subsequent Cycloaddition

Herein, we describe the dual C(sp³)-H bond functionalization of a tertiary amine through hydride-transfer-induced dehydrogenation, followed by cycloaddition, using an easily preparable diazoester as a new type hydride-acceptor precursor under mild, redox-neutral conditions. With carbene as a hydrogen acceptor, this method was demonstrated by the preparation of a broad range of functionalized isoxazoldines in moderate to good yields.

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.

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

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

Regio-, Diastereo-, and Enantioselective Decarboxylative Hydroaminoalkylation of Dienol Ethers Enabled by Dual Palladium/Photoredox Catalysis

Intermolecular photocatalytic hydroaminoalkylation (HAA) of alkenes have emerged as a powerful method for the construction of alkyl amines. Although there are some studies aiming at stereoselective photocatalytic HAA reactions, the alkenes are limited to electrophilic alkenes. Herein, we report a highly regio-, diastereo-, and enantioselective HAA of electron-rich dienol ethers and α-amino radicals derived from α-amino acids using a unified photoredox and palladium catalytic system. This decarboxylative 1,2-Markovnikov addition enables the construction of vicinal amino tertiary ethers with high levels of regio- (up to >19 : 1 rr), diastereo- (up to >19 : 1 dr), and enantioselectivity control (up to >99 % ee). Mechanistic studies support a reversible hydropalladation as a key step.

Tropane and related alkaloid skeletons via a radical [3+3]-annulation process

Tropanes and related bicyclic alkaloids are highly attractive compounds possessing a broad biological activity. Here we report a mild and simple protocol for the synthesis of N-arylated 8-azabicyclo[3.2.1]octane and 9-azabicyclo[3.3.1]nonane derivatives. It provides these valuable bicyclic alkaloid skeletons in good yields and high levels of diastereoselectivity from simple and readily available starting materials using visible-light photoredox catalysis. These bicyclic aniline derivatives are hardly accessible via the classical Robinson tropane synthesis and represent a particularly attractive scaffold for medicinal chemistry. This unprecedented annulation process takes advantage of the unique reactivity of ethyl 2-(acetoxymethyl)acrylate as a 1,3-bis radical acceptor and of cyclic N,N-dialkylanilines as radical 1,3-bis radical donors. The success of this process relies on efficient electron transfer processes and highly selective deprotonation of aminium radical cations leading to the key α-amino radical intermediates.

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.

Direct Arylation of Distal and Proximal C(sp3)-H Bonds of t-Amines with Aryl Diazonium Tetrafluoroborates via Photoredox Catalysis

A visible light-mediated arylation protocol for t-amines has been reported through the coupling of γ- and α-amino alkyl radicals with different aryl diazonium salts using Ru(bpy)₃Cl₂·6H₂O as a photocatalyst. Structurally different 9-aryl-9,10-dihydroacridine, 1-aryl tetrahydroisoquinoline, hexahydropyrrolo[2,1-a]isoquinoline, and hexahydro-2H-pyrido[2,1-a]isoquinoline frameworks with different substitution patterns have been synthesized in good yield using this methodology.

Thiol-Mediated α-Amino Radical Formation via Visible-Light-Activated Ion-Pair Charge-Transfer Complexes

Visible-light-activated electron donor-acceptor complexes offer distinct reaction pathways for the synthesis of complex molecules under mild conditions. Herein, we report a method for the reductive generation of α-amino radicals via the reaction of a visible-light-activated ion-pair charge-transfer complex formed between an in situ-generated alkyl-iminium ion and a thiophenolate. This distinct activation mode is demonstrated through the development of a multicomponent coupling reaction to form substituted aminomethyl-cyclopentanes from secondary amines, cyclopropyl aldehydes, and alkenes. The operationally straightforward transformation displays broad scope and provides a means to generate cyclic amine-containing scaffolds from readily available feedstocks.

Site-Selective α-C-H Functionalization of Trialkylamines via Reversible Hydrogen Atom Transfer Catalysis

Trialkylamines are widely found in naturally occurring alkaloids, synthetic agrochemicals, biological probes, and especially pharmaceuticals agents and preclinical candidates. Despite the recent breakthrough of catalytic alkylation of dialkylamines, the selective α-C(sp³)-H bond functionalization of widely available trialkylamine scaffolds holds promise to streamline complex trialkylamine synthesis, accelerate drug discovery, and execute late-stage pharmaceutical modification with complementary reactivity. However, the canonical methods always result in functionalization at the less-crowded site. Herein, we describe a solution to switch the reaction site through fundamentally overcoming the steric control that dominates such processes. By rapidly establishing an equilibrium between α-amino C(sp³)-H bonds and a highly electrophilic thiol radical via reversible hydrogen atom transfer, we leverage a slower radical-trapping step with electron-deficient olefins to selectively forge a C(sp³)-C(sp³) bond with the more-crowded α-amino radical, with the overall selectivity guided by the Curtin-Hammett principle. This subtle reaction profile has unlocked a new strategic concept in direct C-H functionalization arena for forging C-C bonds from a diverse set of trialkylamines with high levels of site selectivity and preparative utility. Simple correlation of site selectivity and ¹³C NMR shift serves as a qualitative predictive guide. The broad consequences of this dynamic system, together with the ability to forge N-substituted quaternary carbon centers and implement late-stage functionalization techniques, hold potential to streamline complex trialkylamine synthesis and accelerate small-molecule drug discovery.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent-controlled stereodivergent carbocyclisation of aryl aldimine-derived, photocatalytically generated, α-amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α-amino radical species underwent a single stereoselective cyclisation to give trans-configured amino-indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis-fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good-to-excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

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.

Anti-Markovnikov hydro(amino)alkylation of vinylarenes via photoredox catalysis

Photoredox catalysis is a powerful means to generate odd-electron species under mild reaction conditions from a wide array of radical precursors. Herein, we present the application of this powerful catalytic manifold to address the hydroalkylation and hydroaminoalkylation of electronically diverse vinylarenes. This reaction allows for generalized alkene hydroalkylation leveraging common alkyl radical precursors, such as organotrifluoroborate salts and carboxylic acids. Furthermore, utilizing easily accessible α-silyl amine reagents or tertiary amines directly, secondary and tertiary amine moieties can be installed onto monoaryl and diaryl alkenes to access valuable products, including γ,γ-diarylamines pharmacophores. Thus, under a unified system, both hydroalkylation and hydroaminoalkylation of alkenes are achieved. The substrate scope is evaluated through 57 examples, the synthetic utility of the method is demonstrated, and preliminary mechanistic insights are presented.

Many useful chemical scaffolds include carbon or nitrogen substitutions at two or three atoms away from benzene. Here, the authors show a unified hydroalkylation and hydroaminoalkylation protocol to access these structures via a regioselective photocatalytic addition to simple styrenes.

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.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.

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.

Tandem Photoredox-Chiral Phosphoric Acid Catalyzed Radical-Radical Cross-Coupling for Enantioselective Synthesis of 3-Hydroxyoxindoles

A photochemical protocol that couples diarylamines and α-ketoesters to afford the chiral 3-hydroxyoxindoles through tandem photoredox and chiral phosphoric acid catalysis is developed. The reaction involves an enantioselective photochemical radical-radical cross-coupling process. The chiral phosphoric acid is discovered to play crucial roles by decreasing the reductive potentials of α-ketoesters and stereocontrolling the downstream asymmetric radical-radical cross-coupling via the formation of pentacoordinate complex.

Cooperative Photoredox- and Nickel-Catalyzed Alkylative Cyclization Reactions of Alkynes with 4-Alkyl-1,4-dihydropyridines

Cooperative photoredox- and nickel-catalyzed alkylative cyclization reactions of iodoalkynes with 4-alkyl-1,4-dihydropyridines as alkylation reagents under visible light irradiation have been achieved to afford the corresponding alkylated cyclopentylidenes in good to high yields. Introduction of substituents at the propargylic position of iodoalkynes has led to the stereoselective formation of E-isomers. The present reaction system provides a novel synthetic method for alkylative cyclization reactions of both terminal and internal alkynes with cooperative photoredox and nickel catalysis.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

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.

A Photocatalytic Regioselective Direct Hydroaminoalkylation of Aryl-Substituted Alkenes with Amines

A photocatalytic method for the α-selective hydroaminoalkylation of cinnamate esters has been developed. The reaction involves the regioselective addition of α-aminoalkyl radicals generated from aniline derivatives or aliphatic amines to the α-position of unsaturated esters. The scope of aromatic alkenes was extended to styrenes undergoing hydroaminoalkylation with anti-Markovnikov selectivity, which confirms the importance of the aromatic group at the β-position. Simple scale-up is demonstrated under continuous flow conditions, highlighting the practicality of the method.

Recent Advances in Photo-mediated Radiofluorination

Carbon-fluorine bond formations have received a lot of attention because organofluorine compounds are widely used in pharmaceutical, agricultural, and materials science applications. In particular, the incorporation of fluorine-18, which is a commonly used radioisotope for radiopharmaceuticals for positron emission tomography (PET), a molecular imaging tool for the visualization of biochemical events, human metabolism processes, and the measurement and diagnosis of diseases in humans, plays a crucial role in clinical and preclinical studies. Several synthetic methodologies for carbon-fluorine-18 bond formation have been developed. However, conventional fluorination methods have some remaining drawbacks such as the high temperature and basic environment. Photo-induced catalysis is an emerging technique that allow chemists to achieve the synthesis of target molecular architectures under mild conditions. Moreover, several radiofluorination strategies have been developed via photocatalysis. In this review, we focused on describing recent advances in the field of light-mediated radiofluorination.

Transition-Metal-Free α Csp3 -H Cyanation of Sulfonamides

This report describes the site-selective α-functionalization of sulfonylamide derivatives through the in-situ generation of imine intermediates. The N-F sulfonylamides, which could facilitate the elimination to generate imines, are coupled with TBACN to efficiently and mildly afford α-amino cyanides. Comparing with Strecker reaction, this transformation offers a complementary strategy to efficiently construct α-amino cyanides from direct α C-H functionalization of sulfonylamindes. The reaction is also characterized by broad substrate scope and flash chromatography column free workup. More importantly, the new two-electron pathway to generate imines through manipulation of the leaving group allows us to achieve excellent α site-selectivity.

Dendritic fibrous nanosilica-supported dendritic IL/Ru(ii) as photocatalysts for the dicarbofunctionalization of styrenes with carbon dioxide and amines

The effectual utilization of heterogeneous catalysts from nano sources through chemical moderation for the α-aminomethylcarboxylation of alkenes with carbon dioxide and amines is an attractive area to study. Dendritic fibrous nanosilica (DFNS) is a cost-effective, resistant, plenteous, and reproducible source with dandelion-like fibrous anatomy. The present paper is a report on an easy method to provide a family of new DFNS-supported dendritic imidazolium IL/Ru(ii) heterogeneous catalysts DFNS/IL/Ru (1-3) with high ionic density from DFNS. A positive dendritic effect was perceived in the chemical stabilization performance of CO2. DFNS/IL/Ru(ii) was appropriately identified by UV-vis spectroscopy, XPS, SEM, TEM, FT-IR spectroscopy, and TGA. It was discovered that DFNS/IL/Ru(ii) has high catalytic activity for the synthesis of quinoline-2-one through the annulation of ortho-heteroaryl anilines and CO2. DFNS/IL/Ru (3) could be reutilized ten continuous times with no notable reduction in the catalytic activity. Notably, the coveted quinoline-2-one was prepared on a multi-gram scale by deploying DFNS/IL/Ru (3) as a green heterogeneous catalyst. Owing to the attendance of the zwitterionic liquid functional groups on the exterior layer of the bio-based DFNS/IL/Ru (3) catalyst, DFNS/IL/Ru (3) expressed the highest catalytic activity. This approach provides highly functional γ-amino acids in proper yields with great selective power. This paper announces the first nanocatalyst for this transformation, comprising the DFNS-supported Ru N-heterocyclic carbine complex.

Visible-Light-Driven Organophotocatalyzed Mono-, Di-, and Tri-C(sp3)-H Alkylation of Phosphoramides

A photocatalytic metal-free, visible-light-driven, highly atom-economic, direct multiple α-C(sp³)-H alkylation of phosphoramides and thiophosphoramides is demonstrated under environmentally benign conditions. Economically viable and commercially available Eosin-Y is used as an HAT photocatalyst for mono-α-C(sp³)-H alkylation of phosphoramide derivatives. Remarkably, di- and tri-C(sp³)-H alkylation of phosphoramides and thiophosphoramides using an acridinium photocatalyst is reported with good yield and selectivity. Mechanistic studies reveal that monoalkylation of phosphoramides by Eosin-Y follows the HAT mechanism, whereas di- and tri-C(sp³)-H alkylation by the acridinium photocatalyst follows the SET mechanism.