Visible‐Light‐Promoted Iminyl‐Radical Formation from Acyl Oximes: A Unified Approach to Pyridines,Quinolines, and Phenanthridines

A unified strategy involving visible‐light‐induced iminyl‐radical formation has been established for the construction of pyridines, quinolines, and phenanthridines from acyl oximes. With fac‐[Ir(ppy)₃] as a photoredox catalyst, the acyl oximes were converted by 1 e^? reduction into iminyl radical intermediates, which then underwent intramolecular homolytic aromatic substitution (HAS) to give the N‐containing arenes. These reactions proceeded with a broad range of substrates at room temperature in high yield. This strategy of visible‐light‐induced iminyl‐radical formation was successfully applied to a five‐step concise synthesis of benzo[c]phenanthridine alkaloids.     

Visible‐Light‐Promoted Iminyl‐Radical Formation from Acyl Oximes: A Unified Approach to Pyridines,Quinolines, and Phenanthridines

A unified strategy involving visible‐light‐induced iminyl‐radical formation has been established for the construction of pyridines, quinolines, and phenanthridines from acyl oximes. With fac‐[Ir(ppy)₃] as a photoredox catalyst, the acyl oximes were converted by 1 e⁻ reduction into iminyl radical intermediates, which then underwent intramolecular homolytic aromatic substitution (HAS) to give the N‐containing arenes. These reactions proceeded with a broad range of substrates at room temperature in high yield. This strategy of visible‐light‐induced iminyl‐radical formation was successfully applied to a five‐step concise synthesis of benzo[c]phenanthridine alkaloids.     

Visible‐Light Photocatalytic Radical Alkenylation of α‐Carbonyl Alkyl Bromides and Benzyl Bromides

Through the use of [Ru(bpy)₃Cl₂] (bpy=2,2′‐bipyridine) and [Ir(ppy)₃] (ppy=phenylpyridine) as photocatalysts, we have achieved the first example of visible‐light photocatalytic radical alkenylation of various α‐carbonyl alkyl bromides and benzyl bromides to furnish α‐vinyl carbonyls and allylbenzene derivatives, prominent structural elements of many bioactive molecules. Specifically, this transformation is regiospecific and can tolerate primary, secondary, and even tertiary alkyl halides that bear β‐hydrides, which can be challenging with traditional palladium‐catalyzed approaches. The key initiation step of this transformation is visible‐light‐induced single‐electron reduction of C? Br bonds to generate alkyl radical species promoted by photocatalysts. The following carbon? carbon bond‐forming step involves a radical addition step rather than a metal‐mediated process, thereby avoiding the undesired β‐hydride elimination side reaction. Moreover, we propose that the Ru and Ir photocatalysts play a dual role in the catalytic system: they absorb energy from the visible light to facilitate the reaction process and act as a medium of electron transfer to activate the alkyl halides more effectively. Overall, this photoredox catalysis method opens new synthetic opportunities for the efficient alkenylation of alkyl halides that contain β‐hydrides under mild conditions.     

Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O‐aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible‐light‐mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et₃N was proposed as a key step of this process.     

Visible‐Light‐Promoted Dearomative Fluoroalkylation of β‐Naphthols through Intermolecular Charge Transfer

The first visible‐light‐promoted dearomative fluoroalkylation of β‐naphthols was realized without the assistance of any transition‐metal catalysts or external photosensitizers. Inexpensive fluoroalkyl iodides were directly used as efficient fluoroalkylation reagents under very mild reaction conditions. The scope of this process was found to be general and broad, and both trifluoromethyl and perfluoroalkyl groups (‐C₄F₉, ‐C₆F₁₃, and ‐C₈F₁₇) were installed in excellent yields. Preliminary mechanistic studies suggest that visible‐light‐promoted intermolecular charge transfer within the naphtholate–fluoroalkyl iodide electron donor–acceptor (EDA) complex induces a single electron transfer in the absence of photocatalysts.     

Visible‐Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes

The visible‐light‐promoted diastereodivergent intramolecular oxyamination of alkenes is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon of highly diastereoselective ring‐opening of aziridines controlled by electron sacrifices was observed. Highly diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti‐aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pK_a values of the electron sacrifices.     

Radical Addition Enables 1,2‐Aryl Migration from a Vinyl‐Substituted All‐Carbon Quaternary Center

An efficient method for photocatalytic perfluoroalkylation of vinyl‐substituted all‐carbon quaternary centers involving 1,2‐aryl migration has been developed. The rearrangement reactions use fac‐Ir(ppy)₃, visible light and commercially available fluoroalkyl halides and can generate valuable multisubstituted perfluoroalkylated compounds in a single step that would be challenging to prepare by other methods. Mechanistically, the photoinduced alkyl radical addition to an alkene leads to the migration of a vicinal aryl substituent from its adjacent all‐carbon quaternary center with the concomitant generation of a C‐radical bearing two electron‐withdrawing groups that is further reduced by a hydrogen donor to complete the domino sequence.     

Fullerol–Titania Charge‐Transfer‐Mediated Photocatalysis Working under Visible Light

The development of visible‐light‐active photocatalysts is being investigated through various approaches. In this study, C₆₀‐based sensitized photocatalysis that works through the charge transfer (CT) mechanism is proposed and tested as a new approach. By employing the water‐soluble fullerol (C₆₀(OH)_x) instead of C₆₀, we demonstrate that the adsorbed fullerol activates TiO₂ under visible‐light irradiation through the “surface–complex CT” mechanism, which is largely absent in the C₆₀/TiO₂ system. Although fullerene and its derivatives have often been utilized in TiO₂‐based photochemical conversion systems as an electron transfer relay, their successful photocatalytic application as a visible‐light sensitizer of TiO₂ is not well established. Fullerol/TiO₂ exhibits marked visible photocatalytic activity not only for the redox conversion of 4‐chlorophenol, I^?, and Cr^VI, but also for H₂ production. The photoelectrode of fullerol/TiO₂ also generates an enhanced anodic photocurrent under visible light as compared with the electrodes of bare TiO₂ and C₆₀/TiO₂, which confirms that the visible‐light‐induced electron transfer from fullerol to TiO₂ is particularly enhanced. The surface complexation of fullerol/TiO₂ induced a visible absorption band around 400–500 nm, which was extinguished when the adsorption of fullerol was inhibited by fluorination of the surface of TiO₂. The transient absorption spectroscopic measurement gave an absorption spectrum ascribed to fullerol radical cations (fullerol^.+) the generation of which should be accompanied by the proposed CT. The theoretical calculation regarding the absorption spectra for the (TiO₂ cluster+fullerol) model also confirmed the proposed CT, which involves excitation from HOMO (fullerol) to LUMO (TiO₂ cluster) as the origin of the visible‐light absorption.     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

Visible‐Light‐Promoted Trifluoromethylthiolation of Styrenes by Dual Photoredox/Halide Catalysis

Herein, we report a new visible‐light‐promoted strategy to access radical trifluoromethylthiolation reactions by combining halide and photoredox catalysis. This approach allows for the synthesis of vinyl–SCF₃ compounds of relevance in pharmaceutical chemistry directly from alkenes under mild conditions with irradiation from household light sources. Furthermore, alkyl–SCF₃‐containing cyclic ketone and oxindole derivatives can be accessed by radical‐polar crossover semi‐pinacol and cyclization processes. Inexpensive halide salts play a crucial role in activating the trifluoromethylthiolating reagent towards photoredox catalysis and aid the formation of the SCF₃ radical.     

Silver Quantum Cluster (Ag9)‐Grafted Graphitic Carbon Nitride Nanosheets for Photocatalytic Hydrogen Generation and Dye Degradation

We report the visible‐light photocatalytic properties of a composite system consisting of silver quantum clusters [Ag₉(H₂MSA)₇] (H₂MSA=mercaptosuccinic acid) embedded on graphitic carbon nitride nanosheets (AgQCs‐GCN). The composites were prepared through a simple chemical route; their structural, chemical, morphological, and optical properties were characterized by using X‐ray diffraction (XRD), energy dispersive X‐ray spectroscopy, transmission electron microscopy, UV/Vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy. Embedment of [Ag₉(H₂MSA)₇] on graphitic carbon nitride nanosheets (GCN) resulted in extended visible‐light absorption through multiple single‐electron transitions in Ag quantum clusters and an effective electronic structure for hydroxyl radical generation, which enabled increased activity in the photocatalytic degradation of methylene blue and methyl orange dye molecules compared with pristine GCN and silver nanoparticle‐grafted GCN (AgNPs‐GCN). Similarly, the amount of hydrogen generated by using AgQCs‐GCN was 1.7 times higher than pristine GCN. However, the rate of hydrogen generated using AgQCs‐GCN was slightly less than that of AgNPs‐GCN because of surface hydroxyl radical formation. The plausible photocatalytic processes are discussed in detail.     

Visible‐light photocatalytic activity of Ag@MIL‐125(Ti) microspheres

Ag nanoparticle (NP)‐decorated MIL‐125(Ti) microspheres (Ag@MIL‐125(Ti)) were firstly fabricated via a facile hydrothermal and following photo‐reduction method. The photocatalysts were characterized using X‐ray diffraction, scanning and transmission electron microscopies, X‐ray photoelectron spectroscopy and UV–visible diffuse reflectance spectroscopy. The characterization results indicated that Ag NPs were dispersed on the surface of MIL‐125(Ti) microspheres, and the Ag NPs had a uniform diameter of about 40 nm. The composites exhibited excellent visible‐light absorption, due to the modification with the Ag NPs. The photocatalytic activity for the visible‐light‐promoted degradation of Rhodamine B was improved through the optimization of the amount of Ag loaded as a co‐catalyst, this amount being determined as 3 wt%. Additionally, studies performed using radical scavengers indicated that O₂^? and e^? served as the main reactive species. The catalyst can be reused at least five times without significant loss of its catalytic activity. Furthermore, a photocatalytic mechanism for degradation of organics over Ag@MIL‐125(Ti) is also proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Visible‐Light‐Promoted C(sp3)−H Alkylation by Intermolecular Charge Transfer: Preparation of Unnatural α‐Amino Acids and Late‐Stage Modification of Peptides

Disclosed herein is the visible‐light‐promoted deaminative C(sp³)?H alkylation of glycine and peptides using Katritzky salts as electrophiles. Simple reaction conditions and excellent functional‐group tolerance provide a general strategy for the efficient preparation of unnatural α‐amino acids and precise modification of peptides with unnatural α‐amino‐acid residues. Mechanistic studies suggest that visible‐light‐promoted intermolecular charge transfer within a glycine–Katritzky salt electron donor‐acceptor (EDA) complex induces a single‐electron transfer process without the assistance of photocatalyst.     

A Visible‐Light‐Induced α‐H Chlorination of Alkylarenes with Inorganic Chloride under NanoAg@AgCl

An efficient, photocatalytic chlorination of alkylarene α‐H groups using NaCl/HCl as a chlorine source has been developed, which involves a radical mechanism under visible‐light (including sunlight) conditions. A chlorine radical is proposed to be formed by an electron transfer from chloride ion to O₂ in air through the bandgap hole of the semiconductor AgCl. The chlorination protocol is characterized by its use of natural sunlight or other visible light, mild conditions, cheap source of chlorine, green solvent, and high selectivity. The yield of benzylchloride is 95 % with a toluene conversion as high as 40 %, which rivals traditional chlorination methods.     

Radical‐Based C−C Bond‐Forming Processes Enabled by the Photoexcitation of 4‐Alkyl‐1,4‐dihydropyridines

We report herein that 4‐alkyl‐1,4‐dihydropyridines (alkyl‐DHPs) can directly reach an electronically excited state upon light absorption and trigger the generation of C(sp³)‐centered radicals without the need for an external photocatalyst. Selective excitation with a violet‐light‐emitting diode turns alkyl‐DHPs into strong reducing agents that can activate reagents through single‐electron transfer manifolds while undergoing homolytic cleavage to generate radicals. We used this photochemical dual‐reactivity profile to trigger radical‐based carbon–carbon bond‐forming processes, including nickel‐catalyzed cross‐coupling reactions.     

Solvent‐Free Photooxidation of Alkanes by Dioxygen with 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone via Photoinduced Electron Transfer

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) which acts as a super photooxidant. Solvent‐free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser‐induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.     

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

The synthetic utility of tertiary amines to oxidatively generate α‐amino radicals is well established, however, primary amines remain challenging because of competitive side reactions. This report describes the site‐selective α‐functionalization of primary amine derivatives through the generation of α‐amino radical intermediates. Employing visible‐light photoredox catalysis, primary sulfonamides are coupled with electron‐deficient alkenes to efficiently and mildly construct C?C bonds. Interestingly, a divergence between intermolecular hydrogen‐atom transfer (HAT) catalysis and intramolecular [1,5] HAT was observed through precise manipulation of the protecting group. This dichotomy was leveraged to achieve excellent α/δ site‐selectivity.     

Redox‐Neutral α‐Allylation of Amines by Combining Palladium Catalysis and Visible‐Light Photoredox Catalysis

An unprecedented α‐allylation of amines was achieved by combining palladium catalysis and visible‐light photoredox catalysis. In this dual catalysis process, the catalytic generation of allyl radical from the corresponding π‐allylpalladium intermediate was achieved without additional metal reducing reagents (redox‐neutral). Various allylation products of amines were obtained in high yields through radical cross‐coupling under mild reaction conditions. Moreover, the transformation was applied to the formal synthesis of 8‐oxoprotoberberine derivatives which show potential anticancer properties.     

Visible‐Light‐Induced Trifluoromethylation of Isonitrile‐Substituted Methylenecyclopropanes: Facile Access to 6‐(Trifluoromethyl)‐7,8‐Dihydrobenzo[k]phenanthridine Derivatives

A new visible‐light‐induced trifluoromethylation of isonitrile‐substituted methylenecyclopropanes is developed. A range of substituted 6‐(trifluoromethyl)‐7,8‐dihydrobenzo[k]phenanthridine derivatives are readily furnished by this newly developed tandem reaction with moderate to good yields. This reaction allows the direct formation of two six‐membered rings and three new C?C bonds, including the C?CF₃ bond, under visible light irradiation.     

α‐Aminoxy‐Acid‐Auxiliary‐Enabled Intermolecular Radical γ‐C(sp3)−H Functionalization of Ketones

A method for site‐specific intermolecular γ‐C(sp³)?H functionalization of ketones has been developed using an α‐aminoxy acid auxiliary applying photoredox catalysis. Regioselective activation of an inert C?H bond is achieved by 1,5‐hydrogen atom abstraction by an oxidatively generated iminyl radical. Tertiary and secondary C‐radicals thus formed at the γ‐position of the imine functionality undergo radical conjugate addition to various Michael acceptors to provide, after reduction and imine hydrolysis, the corresponding γ‐functionalized ketones.