Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

Halide Perovskite Induces Halogen/Hydrogen Atom Transfer (XAT/HAT) for Allylic C-H Amination

Allylic C-H amination has emerged as a powerful tool to construct allylamines, common motifs in molecular therapeutics. Such reaction implies an oxidative path for C-H activation but furnishes reductive amines, inferring mild oxidants' inactivity for C-H oxidation but strong oxidants' detriment to products. Herein we report a heterogeneous catalytic approach that manipulates halogen-vacancies of perovskite photocatalyst and exploits halogenated-solvents (i.e. CH2Cl2, CH2Br2) as mild oxidants for selective C-H allyl amination with 19,376 turnovers. CsPbBr3 nanocrystals induce cooperative hydrogen-atom-transfer (HAT, C-H oxidation, and halogen-vacancy CsPbBr3-x formation) and halogen-atom-transfer (XAT, CsPbBr3-x-induced solvent reduction) under a radical chain mechanism. Terminal/internal olefins are amenable to forge aromatic/aliphatic, cyclic/acyclic, secondary/tertiary allylamines (70 examples), including drugs or their derivatives.

Photocatalytic Hydrodichloromethylation of Unactivated Alkenes with Chloroform

A visible-light-induced method for the hydrodichloromethylation of unactivated alkenes using chloroform (CHCl3) was developed, employing pyridine·BH3 as the halogen atom transfer (XAT) reagent. The strategy showed a broad functional group tolerance, and 29 examples of unactivated alkenes, including complex natural products or drug derivatives, have been established with good yields. Mechanistic studies indicated that CHCl3 serves as both the source of a dichloromethyl radical and a hydrogen atom transfer (HAT) reagent, and the borane short-chain reaction process was involved in this system. This method represents a novel approach for hydrodichloromethylation of unactivated alkenes without using an additional HAT reagent.

Photoinduced Autopromoted Ni-Catalyzed Three-Component Arylsulfonation Inspired by Density Functional Theory/Time-Dependent Density Functional Theory-Simulated Photoactive Nickel Species

The structure of the novel photoactive nickel species was simulated by density functional theory (DFT)/time-dependent density functional theory (TD-DFT) calculations. The application of the simplified photoactive nickel catalyst was demonstrated in a photoinduced nickel-catalyzed three-component arylsulfonation of 1,6-enynes. This reaction was autopromoted and proceeded in the absence of an additional photocatalyst. This methodology exhibited mild conditions, a broad substrate scope, and high efficiency.

Visible-Light-Induced Radical Carbon Oximation of Styrenes Using N-Nitrosoamine and Organic Halides

An efficient visible-light-induced radical carbon oximation of styrenes with 1-nitrosopyrrolidine and organic halides is developed. The reaction proceeds smoothly in the absence of a transition metal and a photocatalyst under mild conditions, producing a wide range of functionalized oximes in moderate to good yields. Mechanistic studies reveal that the reaction involves the generation of nucleophilic α-amino alkyl radicals and subsequent halogen atom transfer (XAT) with organic halides.

Boryl Radical Mediated Hydro(gem-diboryl)alkylation of Alkenes with Sterically Hindered NHC Boranes

NHC boryl radical mediated halogen atom transfer (XAT) is useful in organic synthesis. Yet, most of the reaction ends only with reducing the halogen to hydrogen, that is, the C-X to C-H. This is especially dominant for electron-deficient alkyl halides, where the formed electrophilic radical reacts rapidly with NHC boranes. Herein, by employing a sterically hindered NHC borane as the boryl radical precursor (IPr·BH3), we were able to use the electrophilic-deficient alkyl halide (α-Iodide gem-di(B(pin))methane) in the C-C bond formation reaction. Mono-, disubstituted styrene, aliphatic alkenes, and heteroatom-substituted alkenes were used as reaction partners. Forty hydro(gem-diboryl)methylation products were obtained at room temperature in moderate to good yields. Detailed mechanistic studies revealed that the reaction mainly involved the radical process.

A Dual Catalytic Approach for the Halogen-Bonding-Mediated Reductive Cleavage of α-Bromodifluoroesters and Amides

While charge-transfer complexes involving halogen-bonding interactions have emerged as an alternative strategy for the photogeneration of carbon radicals, examples using (fluoro)alkyl bromides are limited. This report describes a dual catalytic approach for radical generation from α-bromodifluoroesters and amides under visible-light irradiation. Mechanistic studies suggest that the reaction proceeds through in situ bromide displacement using a catalytic iodide salt, generating a C-I bond that can be engaged by our halogen-bonding photocatalysis platform.

Radical pathways for 2,4-chromandione synthesis via photoexcitation of 4-hydroxycoumarins

4-Hydroxycoumarins are well-known for their ground-state nucleophilic behavior, which has been widely exploited for their functionalization. Herein, we reveal a previously unexplored photochemical reactivity: upon deprotonation and excitation with purple light, 3-substituted 4-hydroxycoumarins reach an excited state and act as single-electron transfer (SET) reductants, generating radicals from stable substrates. This newfound reactivity enables the direct synthesis of 3,3-disubstituted 2,4-chromandiones via a radical dearomatization process. By enabling the incorporation of alkyl and perfluoroalkyl fragments, this protocol offers a straightforward and mild route to access synthetically valuable chromanone scaffolds featuring a quaternary stereocenter. Comprehensive photophysical studies confirmed that deprotonated 4-hydroxycoumarins are potent SET reductants in their excited state, making them suitable for initiating radical-based transformations.

Photo-Induced Three-Component Reaction for the Construction Of α-Tertiary Amino Acid Derivatives

The synthesis of α-tertiary amino acids (ATAAs), which are pivotal components in natural metabolism and pharmaceutical innovation, continues to attract significant research interest. Despite substantial advancements, the pursuit of a facile, versatile, and resource-efficient methodology remains an area of active development. In this work, we introduce a visible light-triggered three-component reaction involving readily available nitrosoarenes, N-acyl pyrazoles, and allyl or (bromomethyl)benzenes under mild conditions. This approach enables the straightforward assembly of a wide array of ATAA derivatives (42 examples) in commendably high yields (up to 89 %). Mechanistic investigations elucidate that the reaction proceeds through a dehydration condensation between nitrosoarenes and N-acyl pyrazoles to generate ketimine intermediates. This is followed by a light-driven halogen atom transfer (XAT) process and a radical addition, culminating in the formation of the desired products. The approach showcases excellent functional group compatibility and late-stage derivatization potential, offering new insights and avenues for the synthesis of ATAA analogs.

Iron-Catalyzed Perfluoroalkylarylation of Styrenes with Arenes and Alkyl Iodides Enabled by Halogen Atom Transfer

A new iron-catalyzed three-component perfluoroalkylarylation of styrenes with alkyl halides and arenes has been established. Alkyl halides undergo halogen atom transfer with methyl radicals to form alkyl radicals in reactions initiated by a combination of tert-butyl peroxybenzoate and an iron catalyst, thus adducting to the olefins, which results in alkylarylation products. The protocol is compatible with a wide range of perfluoroalkyl and non-perfluoroalkyl halides, features excellent functional group tolerance, and enables the synthesis of structurally diverse 1,1-diaryl fluoro-substituted alkanes.

Divergent Synthesis of (E)- and (Z)-Alkenones via Photoredox C(sp3)-H Alkenylation-Dehydrogenation of o-Iodoarylalkanols with Alkynes

A photoredox C(sp3)-H alkenylation-dehydrogenation of o-iodoarylalkanols with terminal alkynes for the synthesis of (E)- and (Z)-quaternary carbon center-containing pent-4-en-1-ones is described. The stereoselectivity depends on the utilization of alkynes and photocatalysts. While using an organic photocatalyst like 4-DPAIPN manipulates the C(sp3)-H alkenylation-dehydrogenation of o-iodoarylalkanols with arylalkynes to assemble (E)-pent-4-en-1-ones, in the case of an Ir potocatalyst such as Ir(ppy)2(dtbbpy)PF6 the reaction with arylalkynes delivers (Z)-pent-4-en-1-ones. For alkylalkynes, the reaction furnishes (E)-pent-4-en-1-ones exclusively in the presence of 4-DPAIPN or Ir(ppy)2(dtbbpy)PF6.

Catalytic Aldehyde-Alkyne Couplings Triggered by Ketyl Radicals

A general and flexible platform for catalytic aldehyde-alkyne couplings triggered by ketyl radicals is described. This open-shell strategy necessitates only a catalytic quantity of a photoredox catalyst, along with Hünig's base (DIPEA) as a halogen atom transfer reagent. The reaction proceeds through sequential steps involving activation, halogen atom transfer, and radical addition. This carbonyl-alkyne coupling exhibits a wide substrate scope and functional group compatibility and has been successfully applied to the late-stage modification of complex architectures.

Iron-Catalyzed Cross-Electrophile Coupling for the Formation of All-Carbon Quaternary Centers

Quaternary carbon centers are desirable targets for drug discovery and complex molecule synthesis, yet the synthesis of these motifs within traditional cross-coupling paradigms remains a significant challenge due to competing β-hydride elimination pathways. In contrast, the bimolecular homolytic substitution (SH2) mechanism offers a unique and attractive alternative pathway. Metal porphyrin complexes have emerged as privileged catalysts owing to their ability to selectively form primary metal-alkyl complexes, thereby eliminating the challenges associated with tertiary alkyl complexation with a metal center. Herein, we report an iron-catalyzed cross-electrophile coupling of tertiary bromides and primary alkyl electrophiles for the formation of all-carbon quaternary centers through a biomimetic SH2 mechanism.

Overcoming Copper Reduction Limitation in Asymmetric Substitution: Aryl-Radical-Enabled Enantioconvergent Cyanation of Alkyl Iodides

Cu-catalyzed enantioconvergent cross-coupling of alkyl halides has emerged as a powerful strategy for synthesizing enantioenriched molecules. However, this approach is intrinsically limited by the weak reducing power of copper(I) species, which restricts the scope of compatible nucleophiles and necessitates extensive ligand optimization or the use of complex chiral scaffolds. To overcome these challenges, we introduce an aryl-radical-enabled strategy that decouples the alkyl halide activation step from the chiral Cu center. We demonstrate that merging aryl-radical-enabled iodine abstraction with Cu-catalyzed asymmetric radical functionalization enables the conversion of racemic α-iodoamides to enantioenriched alkyl nitrile products with good yield and enantioselectivity. The rational design of chiral ligands identified a new class of carboxamide-containing BOX ligands. Mechanistic studies support an aryl-radical-enabled pathway and the unique hydrogen-bonding ability in the newly designed BOX ligands. This aryl-radical-enabled asymmetric substitution reaction has the potential to significantly expand the scope of Cu-catalyzed enantioconvergent cross-coupling reactions.

Light-assisted functionalization of aryl radicals towards metal-free cross-coupling

The many synthetic possibilities that arise when using radical intermediates, in place of their polar counterparts, make contemporary radical chemistry research an exhilarating field. The introduction of photocatalysis has helped tame aryl radicals, leading to a resurgence of interest in their chemistry, and an expansion of viable coupling partners and attainable transformations. These methods are more selective and safer than classical approaches, and they utilize new radical precursors. Given the importance of sustainability in current organic synthesis and our interest in light-assisted metal-free transformations, this Review focuses on recent advances in the use of aryl radicals in photoinduced cross-couplings that do not rely on metals for the crucial bond-forming event, and it is structured according to the key step that the aryl radicals engage in.

Transition-Metal-Free Radical Relay Cascade Annulation of Amides: Access to Antitumor Active Benzo[b]azepine and Oxindole Derivatives

Efficient transition-metal-free synthesis of benzo[b]azepines and oxindoles is achieved via a radical relay cascade strategy employing halogen atom transfer (XAT) for aryl radical generation followed by intramolecular hydrogen atom transfer (HAT). Optimization yielded moderate to substantial yields under visible light irradiation. Preliminary biological assessments revealed promising anti-tumor activity for select compounds. This study underscores the potential of XAT-mediated radical relay cascades in medicinal chemistry and anticancer drug discovery.

Quenching Rate Constants of Lewis Base-Boryl Radical by Substrates: a Laser Flash Photolysis Study

The advanced strategy using Lewis base-boryl radicals (LBRs) has recently been proposed for the addition of alkyl substituents to the full-carbon quaternary center of an organic molecule. However, as the rate-determining step in the whole route, reaction rate constants of LBRs with substrates are extremely lacking. In this paper, 4-dimethylaminopyridine (DMAP)-BH2⋅ was selected as a representative of LBRs, and its reactions with six monochloro-substituted substrates, including three methyl chlorobenzoates and three chlorinated acetanilides were studied in experiments and theoretical calculations. The bimolecular reaction rate constants, kq, were determined using laser flash photolysis approach. By comparing activation energies along the two addition pathways, we have clarified the rate-determining step as the attacking to carbonyl oxygen instead of chlorine atom. Furthermore, noncovalent interaction (NCI) analyses on these substrates indicate that weak interactions, such as hydrogen-bonding and van der Waals interactions, have significant influence on the reactivity of these substrates. Our study provides concrete clues to extend this synthetic strategy.

Nickel-Catalyzed Photoredox Allenylation of Alkyl Halides

We report a dual Ni/photoredox-catalyzed cross-coupling method for propargyl carbonates and nonactivated alkyl bromides, facilitating the synthesis of a variety of substituted allenes under mild and practical conditions. Mechanistically, the reaction integrates Ni-catalyzed activation of the propargyl electrophile via SN2' oxidative addition at Ni(I) with silyl radical-induced activation of the alkyl halide through halogen-atom transfer. This methodology provides a gentle approach for introducing allenyl groups into complex halogenated aliphatic molecules, offering further opportunities for derivatization.

Energy-transfer-enabled photocatalytic transformations of aryl thianthrenium salts

Aryl thianthrenium salts are valuable in photocatalysis but traditionally require external electron donors for activation. This study introduces an energy transfer (EnT) strategy for the activation of aryl thianthrenium salts using 2,3,4,5,6-penta(carbazol-9-yl)benzonitrile (5CzBN) as a metal-free photocatalyst, eliminating the need for external donors. Utilizing this EnT approach, we achieve C-H deuteration of arenes under visible light with CDCl3 as a deuterium source to synthesize various deuterated aromatic compounds, including important natural products and pharmaceuticals. Additionally, this strategy enables diverse functionalizations including borylation, arylation, cyanation, and selenylation, enhancing the applicability of aryl sulfonium salts in environmentally friendly photocatalysis.

Dearomative hydroamination of heteroarenes catalyzed by the phenolate photocatalyst

Dearomative functionalization of heteroarenes offers an attractive and sustainable approach for the rapid construction of complex 3D heterocyclic scaffolds from planar structures. Despite progress in this field, dearomative amination of heteroarenes via a radical anion intermediate remains a challenge. Here, we report a photoredox-catalyzed dearomative hydroamination of heteroarenes with hydrazodiformates under mild and transition-metal-free reaction conditions. Various benzofurans and benzothiophenes can efficiently participate in this transformation. A series of mechanistic experiments revealed that heteroaryl radical anions are the crucial intermediates, generated through photo-induced electron transfer between the excited phenolate photocatalyst and heteroarenes.

Iodine(III)-Mediated Photochemical C-H Azolation

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

Advances in the Light-Promoted Transformations of N-Heterocyclic Carbene Ligated Boryl Radicals

Organoboron compounds are integral to modern life, with extensive applications in synthesis, materials science, medicine, and various other domains closely linked to human endeavors. NHC-BH3, noted for its stability, ease of synthesis, and high reactivity as a boryl radical precursor, has emerged as a key focus in boryl radical chemistry. Recently, the visible-light-induced single electron transfer (SET) and hydrogen atom transfer (HAT) processes have garnered considerable interest, presenting innovative strategies for generating boryl radicals from NHC-BH3. In the context of this review, our focus is on the synthesis of C–B and X–B bonds under visible light irradiation, facilitated by NHC-BH3. Furthermore, we explored the role of NHC-BH3 as a hydrogen donor or halogen atom transfer reagent in the construction of C–C bonds.

1 Introduction

2 Hydroboration

3 Borylation

4 Construction of X–B Bonds (X = N, O, S)

5 Halogen Atom Transfer Reagent and Hydrogen Donor

6 Conclusion

C(sp3)-F Bond Activation by Lewis Base-Boryl Radicals via Concerted Electron-Fluoride Transfer

Selective C-F bond activation through a radical pathway in the presence of multiple C-H bonds remains a formidable challenge, owing to the extraordinarily strong bond strength of the C-F bond. By the aid of density functional theory calculations, we disclose an innovative concerted electron-fluoride transfer mechanism, harnessing the unique reactivity of Lewis base-boryl radicals to selectively activate the resilient C-F bonds in fluoroalkanes. This enables the direct abstraction of a fluorine atom and subsequent generation of an alkyl radical, thus expanding the boundaries of halogen atom transfer reactions.

C-H Alkylation of Heterocycles via Light-Mediated Palladium Catalysis

Methods enabling direct C-H alkylation of heterocycles are of fundamental importance in the late-stage modification of natural products, bioactive molecules, and medicinally relevant compounds. However, there is a scarcity of a general strategy for the direct C-H alkylation of a variety of heterocycles using commercially available alkyl surrogates. We report an operationally simple palladium-catalyzed direct C-H alkylation of heterocycles using alkyl halides under the visible light irradiation with good scalability and functional group tolerance. Our studies suggest that the photoinduced alkylation proceeds through a cascade of events comprising, site-selective alkyl radical addition, base-assisted deprotonation, and oxidation. A combination of experiments and computations was employed for the generalization of this strategy, which was successfully translated towards the modification of natural products and pharmaceuticals.

Boryl radical-mediated halogen-atom transfer (XAT) enables the Sonogashira-like alkynylation of alkyl halides

Alkynes are a crucial class of materials with application across the wide range of chemical disciplines. The alkynylation of alkyl halides presents an ideal strategy for assembling these materials. Current methods rely on the intrinsic electrophilic nature of alkyl halides to couple with nucleophilic acetylenic systems, but these methods faces limitations in terms of applicability and generality. Herein, we introduce a different approach to alkynylation of alkyl halides that proceeds via radical intermediates and uses alkynyl sulfones as coupling partners. This strategy exploits the ability of amine-ligated boryl radicals to activate alkyl iodides and bromides through halogen-atom transfer (XAT). The resulting radicals then undergo a cascade of α-addition and β-fragmentation with the sulfone reagent, leading to the construction of C(sp3)-C(sp) bonds. The generality of the methodology has been demonstrated by its successful application in the alkynylation of complex and high-value molecules.

Modular Synthesis of Heterobenzylic Amines via Carbonyl Azinylative Amination

Transformations enabling the synthesis of α-alkyl, α'-2-azinyl amines by addition of 2-heteroaryl-based nucleophiles to in situ-generated and non-activated alkyl-substituted iminium ions are extremely rare. Approaches involving classical 2-azinyl organometallics, such as the corresponding Grignard reagents, often fail to produce the desired products. Here, we report an operationally straightforward solution to this problem through the development of a multicomponent coupling process wherein a soft 2-azinyl indium nucleophile, generated in situ from the corresponding 2-iodo heteroarene and indium powder, adds to an iminium ion that is also formed directly in the reaction. This modular carbonyl azinylative amination (CAzA) displays a broad scope and only a metal reductant is needed to generate a reactive 2-azinyl nucleophile. Beyond the addition to iminium ions, the 2-azinyl addition to polyfluoromethyl ketones forms the corresponding tertiary alcohols. Together, the products of these reactions possess a high degree of functionality, are typically challenging to synthesize by other methods, and contain motifs recognized as privileged in the context of pharmaceuticals and agrochemicals.

Photocatalytic Multisite Functionalization of Unactivated Terminal Alkenes by Merging Polar Cycloaddition and Radical Ring-Opening Process

Although highly appealing for rapid access of molecular complexity, multi-functionalization of alkenes that allows incorporation of more than two functional groups remains a prominent challenge. Herein, we report a novel strategy that merges dipolar cycloaddition with photoredox promoted radical ring-opening remote C(sp3)-H functionalization, thus enabling a smooth 1,2,5-trifunctionalization of unactivated alkenes. A highly regioselective [3+2] cycloaddition anchors a reaction trigger onto alkene substrates. The subsequent halogen atom transfer (XAT) selectively initiates ring-opening process, which is followed by a series of 1,5-hydrogen atom transfer (1,5-HAT) and intermolecular fluorine atom transfer (FAT) events. With this method, site-selective introduction of three different functional groups is accomplished and a broad spectrum of valuable β-hydroxyl-ϵ-fluoro-nitrile products are synthesized from readily available terminal alkenes.

Maximizing Photon-to-Electron Conversion for Atom Efficient Photoredox Catalysis

Photoredox catalysis is a powerful tool to access challenging and diverse syntheses. Absorption of visible light forms the excited state catalyst (*PC) but photons may be wasted if one of several unproductive pathways occur. Facile dissociation of the charge-separated encounter complex [PC•-:D•+], also known as (solvent) cage escape, is required for productive chemistry and directly governs availability of the critical PC•- intermediate. Competitive charge recombination, either inside or outside the solvent cage, may limit the overall efficiency of a photochemical reaction or internal quantum yield (defined as the moles of product formed per mole of photons absorbed by PC). Measuring the cage escape efficiency (ϕCE) typically requires time-resolved spectroscopy; however, we demonstrate how to estimate ϕCE using steady-state techniques that measure the efficiency of PC•- formation (ϕPC). Our results show that choice of electron donor critically impacts ϕPC, which directly correlates to improved synthetic and internal quantum yields. Furthermore, we demonstrate how modest structural differences between photocatalysts may afford a sizable effect on reactivity due to changes in ϕPC, and by extension ϕCE. Optimizing experimental conditions for cage escape provides photochemical reactions with improved atom economy and energy input, paving the way for sustainable design of photocatalytic systems.

Photoinduced Single Electron Reduction of the 4-O-5 Linkage in Lignin Models for C-P Coupling Catalyzed by Bifunctional N-Heterocyclic Carbenes

Catalytic activation of Caryl-O bonds is considered as a powerful strategy for the production of aromatics from lignin. However, due to the high reduction potentials of diaryl ether 4-O-5 linkage models, their single electron reduction remains a daunting challenge. This study presents the blue light-induced bifunctional N-heterocyclic carbene (NHC)-catalyzed one-electron reduction of diaryl ether 4-O-5 linkage models for the synthesis of trivalent phosphines. The H-bond between the newly devised bifunctional NHC and diaryl ethers is responsible for the success of the single electron transfer. Furthermore, this approach demonstrates selective one-electron reduction of unsymmetric diaryl ethers, oligomeric phenylene oxide, and lignin model.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

Dispersion-induced cooperative hydrogen atom transfer for radical iodoalkylation

Described herein is a novel visible-light-promoted three-component radical iodo-alkylative cyclization of alkynes using iodoform as a bifunctional iodine atom source. Visible-light irradiation of a polar-polar interaction complex of iodoform with malonate enables the cooperative hydrogen atom transfer process to generate alkyl radical and trigger a cascade reaction sequence.

Synthesis of Diarylalkanes by Photoreductive 1,2-Diarylation of Alkenes with Aryl Halides and Cyanoaromatics

We report a visible light-induced photoreductive strategy for three-component diarylation of alkenes with aryl halides and cyanoaromatics. Upon photoredox catalysis and with tertiary alkyl amines as the electron transfer agent, aryl halides selectively undergo halogen atom transfer to generate the aryl radicals and two C(sp2)-C(sp3) bonds between the cabron atoms are created in a radical addition and radical-radical coupling fashion to rapidly assemble diverse functionalized polyarylalkanes with high regio- and chemoselectivity. This method can be applied to broad feedstocks, including terminal alkenes, internal alkenes, aryl iodides, aryl bromides, aryl chlorides, electron-deficient benzonitriles, and isonicotinonitriles.

Defluorinative Haloalkylation of Unactivated Alkenes Enabled by Dual Photoredox and Copper Catalysis

A three-component defluorinative haloalkylation of alkenes with trifluoromethyl compounds and TBAX (X = Cl, Br) via dual photoredox/copper catalysis is reported. The mild conditions are compatible with a wide array of activated trifluoromethyl aromatics bearing diverse substituents, and various nonactivated terminal and internal alkenes, enabling straightforward access to synthetically valuable γ-gem-difluoroalkyl halides with high efficiency. Mechanistic studies indicate that the [Cu] complexes not only serve as XAT catalysts but also facilitate the SET reduction of trifluoromethyl groups by photocatalysts. Additionally, the resulting alkyl halide products can serve as versatile conversion intermediates for the synthesis of a diverse range of γ-gem-difluoroalkyl compounds.

Near-Infrared-Light-Induced Iron(I) Dimer-Enabled Radical Cascade Reactions of Fluoroalkyl Bromides for the Synthesis of Ring-Fused Quinazolinones

The use of an earth-abundant and inexpensive iron complex as a catalyst, coupled with near-infrared (NIR) light as the energy source, for radical reactions with alkyl halides has been far less developed. In this study, we report NIR light-mediated iron(I) dimer-catalyzed radical cascade reactions of fluoroalkyl bromides for the synthesis of ring-fused quinazolinones bearing a difluoromethyl group. In this process, the 3-bromo-1,10-phenanthroline ligand facilitates the reactivity of [CpFe(CO)2]2, thereby improving the efficiency of the reaction.

Overestimated Halogen Atom Transfer Reactivity of α-Aminoalkyl Radicals

Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on α-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between α-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of α-aminoalkyl radicals in XAT reactions is 103 to 105 times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that α-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of α-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for α-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.

Radical-Chain Hydrosilylation of Alkenes Enabled by Triplet Energy Transfer

Development of mild, robust and metal-free catalytic approach for the hydrosilylation of alkenes is critical to the advancement of modern organosilicon chemistry given their powerful capacity in the construction of various C-Si bonds. Herein, we wish to disclose a visible light-triggered organophotocatalytic strategy, which proceeds via a triplet energy transfer (EnT)-enabled radical chain pathway. Notably, this redox-neutral protocol is capable of accommodating a broad spectrum of electron-deficient and -rich alkenes with excellent functional group compatibility. Electron-deficient alkenes are more reactive and the reaction could be finished within a couple of minutes even in PBS solution with extremely low concentration, which suggests its click-like potential in organic synthesis. The preparative power of the transformations has been further highlighted in a number of complex settings, including the late-stage functionalization and scale-up experiments. Furthermore, although only highly reactive (TMS)3SiH is suitable hydrosilane substrate, our studies revealed the great reactivity and versatility of (TMS)3Si- group in diverse C-Si and Si-Si bond cleavage-based transformations, enabling the rapid introduction of diverse functional groups and the facile construction of valuable quaternary silicon architectures.

Photoreductive 1,4-Dicarbofunctionalization of 1,3-Enynes with Organoiodides and Cyanoarenes via Halogen-Atom Transfer

A photoreductive halogen-atom transfer (XAT) strategy for 1,4-dicarbofunctionalization of 1,3-enynes with organoiodides and cyanoarenes is disclosed, enabling access to functionalized allenes in a highly regio-, chemo-, and stereoselective manner. Upon the photoredox catalysis and the activation of Et3N XAT agents, the mild conditions and high functional group tolerance of this protocol enable the formation of two C-C bonds, including a C(sp3)-C(sp3) bond and a C(sp2)-C(sp2) bond, in a single reaction step, and provides a general avenue to polysubstituted allenes and late-stage modification of bioactive compounds.

Amine-Borane-Mediated, Nickel/Photoredox-Catalyzed Cross-Electrophile Coupling between Alkyl and Aryl Bromides

Nickel/photoredox catalysis has emerged as a powerful platform for exploring nontraditional and challenging cross-couplings. Herein, a metallaphotoredox catalytic protocol has been developed on the basis of a tertiary amine-ligated boryl radical-induced halogen atom transfer process under blue-light irradiation. A wide variety of aryl and heteroaryl bromides featuring different functional groups and pharmaceutical moieties were facilely coupled to rapidly install C(sp3)-enriched aromatic scaffolds. The compatibility of Lewis base-ligated borane with nickel catalysis was well exemplified to extend the chemical space for Ni-catalyzed cross-electrophile coupling.

Sodium Hypophosphite as a Halogen Atom Transfer (XAT) Agent under Photocatalytic Conditions

The ability of sodium hypophosphite to generate the phosphorus-centered radical, which can activate the carbon-halogen bond via the halogen atom transfer (XAT) is described. The hydroalkylation of nonactivated alkenes with methyl bromoacetate was performed using sodium hypophosphite as reducing agent under photocatalytic conditions. The key phosphorus centered radical is formed from the hypophosphite anion by hydrogen atom abstraction.

Tailoring the Degradation of Cyanoarene-Based Photocatalysts for Enhanced Visible-Light-Driven Halogen Atom Transfer

We present the strategic design of donor-acceptor cyanoarene-based photocatalysts (PCs) aiming to augment beneficial PC degradation for halogen atom transfer (XAT)-induced dehalogenation reactions. Our investigation reveals a competitive nature between the catalytic cycle and the degradation pathway, with the degradation becoming dominant, particularly for less activated alkyl halides. The degradation behavior of PCs significantly impacts the efficiency of the XAT process, leading to exploration into manipulating the degradation behavior in a desirable direction. Recognizing the variation in the nature and rate of PC degradation, as well as its influence on the reaction across the range of PC structures, we carefully engineered the PCs to develop a pre-catalyst, named 3DP-DCDP-IPN. This pre-catalyst undergoes rapid degradation into an active form, 3DP-DCDP-Me-BN, exhibited an enhanced reducing ability in its radical anion form to induce better PC regeneration and consequently effectively catalyzes the XAT reaction, even with a challenging substrate.

Photochemical pincer-catalyzed reductive cyclisation towards indolines and oxindoles

An organophotocatalytic method has been described towards the synthesis of indolines and oxindoles starting from unusual α-chloro amide and N-(2-chlorophenyl)-N-alkyl methacrylamide substrates. This marks a notable improvement since the earlier syntheses utilized iridium and gold catalysts, and involved C-I or C-Br bond cleavage as the initial step. Our photocatalyst is a pincer ligand that can be easily deprotonated to make a very strong reducing agent. The reductive cleavage of the carbon-chloride bond, and subsequent 5-exo-trig ring cyclization, followed by hydrogen atom abstraction, prepare the desired heterocycles under very mild reaction conditions. An atom economic use of KOtBu has been shown to demonstrate the unusual trifunctional role of the latter.

Challenges and Future Perspectives in Photocatalysis: Conclusions from an Interdisciplinary Workshop

Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.

XAT-Catalysis for Intramolecular Biaryl Synthesis

Radical ipso-substitution offers an alternative to organometallic approaches for biaryl synthesis, but usually requires stoichiometric reagents such as tributyltin hydride. Here, we demonstrate that visible light photoredox catalysis can be used for ipso-biaryl synthesis, via a halogen-atom transfer (XAT) regime. Using amide substrates that promote ipso- over unwanted ortho-addition, we demonstrate smooth biaryl formation with no constraint on the electronic character of the migrating arene ring. The photoreaction can be combined in one operation to achieve a formal arylation of the inert aniline C-N bond.

A toolbox approach to revealing a series of naphthocarbazoles to showcase photocatalytic reductive syntheses

The development of highly reducing photocatalysts to functionalize arenes via the generation of reactive aryl radicals under mild and environmentally benign reaction conditions has emerged as a noteworthy approach in the realm of organic synthesis. Herein, we report a readily synthesized series of novel naphthocarbazole derivatives (NCs) as organo-photocatalysts, which, upon irradiation under 390 nm light, acquire high reducing power to catalyze several reductive transformations. The promising properties revealed by in depth photophysical and electrochemical studies ( = -1.9 V to -2.07 V vs. SCE, τ = 5.59 to 7.12 ns) demonstrate NCs to be versatile catalysts, and notably, rational variation of the substituents (NC1-NC6) modulates their success as efficient photoreductants. Detailed DFT calculations of the frontier MO diagrams and energy levels revealed them to be non-donor-acceptor type molecular scaffolds. The applicability of the NCs as catalysts was demonstrated in reductive dehalogenative borylation, phosphorylation, and dehydrohalide intramolecular C-C coupling reactions, as well as the dimerization of carbonyls and imines. Visible-light-irradiated selective reductive desulfonylation from heteroaromatics and peptides further enhances their synthetic utility.

Halogen Atom Transfer-Induced Homolysis of C-F Bonds by the Excited-State Boryl Radical

A novel reactivity toward C-F bond functionalization has been developed, which could be designated as fluorine atom transfer (FAT). A photoexcited state of an N-heterocyclic carbene-ligated boryl radical exhibits a transcendent reactivity, capable of activating chemically inert carbon-fluorine bonds through homolysis. Combined experimental and computational studies suggest that the ligated boryl radical species directly abstracts a fluorine atom from the organofluoride substrates to provide valuable carbon-centered radicals.

Practical and regioselective halonitrooxylation of olefins to access β-halonitrates

Organic nitrates, as effective donors of the signaling molecule nitric oxide, are widely applied in the pharmaceutical industry. However, practical and efficient methods for accessing organic nitrates are still scarce, and achieving high regiocontrol in unactivated alkene difunctionalization remains challenging. Here we present a simple and practical method for highly regioselective halonitrooxylation of unactivated alkenes. The approach utilizes TMSX (X: Cl, Br, or I) and oxybis(aryl-λ3-iodanediyl) dinitrates (OAIDN) as sources of halogen and nitrooxy groups, with 0.5 mol % FeCl3 as the catalyst. Remarkably, high regioselectivity in the halonitrooxylation of aromatic alkenes can be achieved even without any catalyst. This protocol features easy scalability and excellent functional group compatibility, providing a range of β-halonitrates (127 examples, up to 99% yield, up to >20:1 rr). Notably, 2-iodoethyl nitrate, a potent synthon derived from ethylene, reacts smoothly with a variety of functional units to incorporate the nitrooxy group into the desired molecules.

A Radical Strategy for the Alkylation of Amides with Alkyl Halides by Merging Boryl Radical-Mediated Halogen-Atom Transfer and Copper Catalysis

Amide alkylation is a fundamental process in organic chemistry. However, the low nucleophilicity of amides means that divergent coupling with alkyl electrophiles is often not achievable. To circumvent this reactivity challenge, individual amine synthesis followed by amidation with standard coupling agents is generally required. Herein, we demonstrate a radical solution to this challenge by using an amine-borane complex and copper catalysis under oxidative conditions. While borohydride reagents are generally used as reducing agents in ionic chemistry, their conversion into amine-ligated boryl radicals diverts their reactivity toward halogen-atom transfer. This enables the conversion of alkyl halides into the corresponding alkyl radicals for amide functionalization via copper catalysis. The process is applicable to the N-alkylation of primary amides employing unactivated alkyl iodides and bromides, and it was also showcased in the late-state functionalization of both complex amide- and halide-containing drugs.

Visible light-induced halogen-atom transfer by N-heterocyclic carbene-ligated boryl radicals for diastereoselective C(sp3)-C(sp2) bond formation

Photoinduced halogen-atom transfer (XAT) has rapidly emerged as a programmable approach to generate carbon-centered radical intermediates, mainly relying on silyl and α-aminoalkyl radicals as halogen abstractors. More recently, ligated boryl radicals have also been proposed as effective halogen abstractors under visible-light irradiation. In this study, we describe the use of this approach to enable C(sp3)-C(sp2) bond formation via radical addition of carbon-centered radicals generated via XAT onto chloroalkynes. Our mechanistic investigation reveals a complex interplay of highly reactive radical intermediates which, under optimized conditions, delivered the targeted vinyl chlorides in excellent yields and Z : E ratios. Finally, we demonstrated the synthetic value of these products in transition metal-based cross-coupling reactions.

Site-Selective Distal C(sp3)-H Bromination of Aliphatic Amines as a Gateway for Forging Nitrogen-Containing sp3 Architectures

Herein, we disclose a new strategy that rapidly and reliably incorporates bromine atoms at distal, secondary C(sp3)-H sites in aliphatic amines with an excellent and predictable site-selectivity pattern. The resulting halogenated building blocks serve as versatile linchpins to enable a series of carbon-carbon and carbon-heteroatom bond-formations at remote C(sp3) sites, thus offering a new modular and unified platform that expediates the access to advanced sp3 architectures possessing valuable nitrogen-containing saturated heterocycles of interest in medicinal chemistry settings.

Organocatalyzed Carbonylation of Alkyl Halides Driven by Visible Light

Herein, we describe a new strategy for the carbonylation of alkyl halides with different nucleophiles to generate valuable carbonyl derivatives under visible light irradiation. This method is mild, robust, highly selective, and proceeds under metal-free conditions to prepare a range of structurally diverse esters and amides in good to excellent yields. In addition, we highlight the application of this activation strategy for 13C isotopic incorporation. We propose that the reaction proceeds by a photoinduced reduction to afford carbon-centered radicals from alkyl halides, which undergo subsequent single electron-oxidation to form a carbocationic intermediate. Carbon monoxide is trapped by the carbocation to generate an acylium cation, which can be attacked by a series of nucleophiles to give a range of carbonyl products.

Utilizing weakly donor-acceptor ternary π-conjugated architecture to achieve single-component white luminescence and stimulus-responsive room-temperature phosphorescence

Purely organic room-temperature phosphorescence (RTP) has garnered substantial attention for its delayed emission, environmental sensitivity, and potential diverse applications. However, the quest for high-performance RTP materials has always been a challenge. In this study, we introduce novel weakly donor-acceptor (D-A) ternary π-conjugated architecture to construct an efficient RTP system. The strategy utilizes synergistic effects of the analogous El-Sayed rule, halogen-free heavy-atom effect, reduction of the singlet-triplet energy gap, and manipulation of flexible molecular conformation. A remarkable enhancement in the phosphorescence-to-fluorescence ratio was achieved, elevating from 0.4 in carbazole to 35.2 in DBTDBTCZ. Furthermore, the RTP system demonstrates single-component white luminescence, yielding warm and cool white colors. Intriguingly, we unveil the novel position-dependent heavy-atom effects, discerningly promoting intersystem crossing or phosphorescence decay. Benefiting from efficient RTP, multifunctional applications of real-time humidity monitoring, oxygen sensing, anti-counterfeiting labeling, and white lighting are demonstrated.