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

Minisci C-H Alkylation of Heterocycles with Unactivated Alkyl Iodides Enabled by Visible Light Photocatalysis

In this work, we developed a general catalytic strategy that allows Minisci C-H alkylation of a variety of heterocycles using unactivated alkyl halide as an alkyl radical source under visible light photocatalysis. Mild reaction conditions, employing 4CzIPN as an organophotocatalyst and aerial oxygen as a green terminal oxidant, a broad scope, good functional group tolerance, and late-stage C-H alkylation of bioactive and pharmaceutically relevant molecules are advantages of the protocol. Preliminary mechanistic studies indicate the involvement of the α-amino alkyl radical and the alkyl radical and further involvement of aerial oxygen under our reaction conditions.

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Oxygen Atom Transfer Reactions of Colloidal Metal Oxide Nanoparticles

Redox transformations at metal oxide (MOx)/solution interfaces are broadly important, and oxygen atom transfer (OAT) is one of the simplest and most fundamental examples of such reactivity. OAT is a two-electron transfer process, well-known in gas/solid reactions and catalysis. However, OAT is rarely directly observed at oxide/water interfaces, whose redox reactions are typically proposed to occur in one-electron steps. Reported here are stoichiometric OAT reactions of organic molecules with aqueous colloidal titanium dioxide and iridium oxide nanoparticles (TiO2 and IrOx NPs). Me2SO (DMSO) oxidizes reduced TiO2 NPs with the formation of Me2S, and IrOx NPs transfer O atoms to a water-soluble phosphine and a thioether. The reaction stoichiometries were established and the chemical mechanisms were probed using typical solution spectroscopic techniques, exploiting the high surface areas and transparency of the colloids. These OAT reactions, including a catalytic example, utilize the ability of the individual NPs to accumulate many electrons and/or holes. Observing OAT reactions of two different materials, in opposite directions, is a step toward harnessing oxide nanoparticles for valuable multi-electron and multi-hole transformations.

Highly Diastereoselective Synthesis of 5/6-Fused Bicyclic Ring Systems via Radical Cyano Group Migration

Here we report a highly diastereoselective cyano group transfer radical cyclization reaction to construct 5/6-fused bicyclic ring systems that bear three contiguous and congested stereogenic elements, with 100% atom economy under catalyst-free and near-ultraviolet light irradiation conditions. Mechanistic investigations and density functional theory calculations suggest that the diastereoselectivity is governed by the conformational distribution of the triplet diradical intermediate and the rate of reverse intersystem crossing (RISC) before radical coupling.

Hydroalkylation of Vinylarenes by Transition-Metal-Free In Situ Generation of Benzylic Nucleophiles Using Tetramethyldisiloxane and Potassium tert-Butoxide

Hydrosilanes and Lewis bases are known to promote various reductive defunctionalizations, rearrangements, and silylation reactions, facilitated by enigmatic silicon/Lewis base-derived reactive intermediates. Despite the wide variety of transformations enabled by this reagent combination, no examples of intermolecular C(sp3)-C(sp3) forming reactions have been reported. In this work, we've identified 1,1,3,3-tetramethyldisiloxane (TMDSO) and KOtBu as a unique reagent combination capable of generating benzylic nucleophiles in situ from styrene derivatives, which can subsequently react with alkyl halides to give a new C(sp3)-C(sp3) linkage via formal hydroalkylation. Mechanistic experiments suggest that the reaction proceeds through a key hydrogen atom transfer (HAT) step from a hydrosilane reducing agent to styrene, affording a benzylic radical that undergoes reductive radical polar crossover (RRPC) and subsequent SN2 alkylation.

Heterocoupling Two Similar Benzyl Radicals by Dual Photoredox/Cobalt Catalysis

Transition-metal-regulated radical cross coupling enables the selective bonding of two distinct transient radicals, whereas the catalytic method for sorting two almost identical transient radicals, especially similar benzyl radicals, is still rare. Herein, we show that leveraging dual photoredox/cobalt catalysis can selectively couple two similar benzyl radicals. Using easily accessible methylarenes and phenylacetates (benzyl N-hydroxyphthalimide (NHPI) esters) as benzyl radical sources, a range of unsymmetrical 1,2-diarylethane classes via the 1°-1°, 1°-2°, 1°-3°, 2°-2°, 2°-3° and 3°-3° couplings were obtained with broad functional group tolerance. Besides the photochemical continuous flow synthesis, the one-pot procedure that directly uses phenylacetic acids and NHPI as the starting materials to avoid the pre-preparation of benzyl NHPI esters for the gram-scale synthesis is also feasible and affords good yields, showcasing the synthetic utility of our protocol.

Isolated Neutral Organic Radical Unveiled Solvent-Radical Interaction in Highly Reducing Photocatalysis

Diffusion-limited kinetics is a key mechanistic debate when consecutive photoelectron transfer (conPET) is discussed in photoredox catalysis. In situ generated organic photoactive radicals can access catalytic systems as reducing as alkaline metals that can activate remarkably stable bonds. However, in many cases, the extremely short-lived transient nature of these doublet state open-shell species has led to debatable mechanistic studies, hindering adoption and development. Herein, we document the use of an isolated and stable neutral organic nPrDMQA radical as a highly photoreducing species. The isolated radical offers a unique platform to investigate the mechanism behind the photocatalytic activity of organic photocatalyst radicals. The involvement of reduced solvent is observed, formed by single electron transfer (SET) between the short-lived excited state nPrDMQA radical and the solvent. In our detailed mechanistic studies, spectroscopic and chemical affirmation of solvent reduction is strongly evident. Reduction of aryl halides, including difluoroarenes is presented as a model study of the conPET method. Further, the activation of N2O, a greenhouse gas that is yet to be activated by photoredox catalysis, is showcased in the absence of a transition metal.

Metallaphotoredox-catalyzed alkynylcarboxylation of alkenes with CO2 and alkynes for expedient access to β-alkynyl acids

Carboxylation with CO2 offers an attractive and sustainable access to valuable carboxylic acids. Among these methods, direct C-H carboxylation of terminal alkynes with CO2 has attracted much attention for one-carbon homologation of alkynes, enabling rapid synthesis of propiolic acids. In contrast, the multi-carbons homologation of alkynes with CO2 to construct important non-conjugated alkynyl-containing acids has not been reported. Herein, we present alkynylcarboxylation of alkenes with CO2 via photoredox and copper dual catalysis. This protocol provides a direct and practical method to form valuable non-conjugated alkynyl acids from readily available alkynes, alkenes and CO2. Additionally, this approach also features mild (room temperature, 1 atm of CO2) and redox-neutral conditions, high atom and step economy, good functional group tolerance, and high selectivities. Moreover, diverse transformations of the β-alkynyl acid products and the rapid synthesis of bioactive molecule (GPR40/FFA1 agonist) further illustrate the synthetic utility of this methodology.

Modular assembly of amines and diborons with photocatalysis enabled halogen atom transfer of organohalides for C(sp3)-C(sp3) bond formation

In the past few years, the direct activation of organohalides by ligated boryl radicals has emerged as a potential synthetic tool for cross-coupling reactions. In most existing methods, ligated boryl radicals are accessed from NHC-boranes or amine-boranes. In this work, we report a new photocatalytic platform by modular assembly of readily available amines and diboron esters to access a library of ligated boryl radicals for reaction screening, thus enabling the cross-coupling of organohalides and alkenes including both activated and unactivated ones for C(sp3)-C(sp3) bond formation by using the assembly of DABCO A1 and B2Nep2B1. The strategy features operational simplicity, mild conditions and good functional group tolerance. A range of organohalides including activated alkyl chlorides, alkyl bromides (1°, 2° and 3° C-Br) as well as aromatic bromides are applicable in the strategy. Experimental and computational studies rationalize the proposed mechanism.

Merging halogen atom transfer, ring-expansion and oxidation by electron-rich arenediazonium salts: modular assembly of cyclohexenone derivatives

As fundamental structural scaffolds in numerous natural products and pharmaceutical molecules, the construction of cyclohexenone architectures has remained a pivotal focus in organic chemistry. However, established strategies to synthesize cyclohexenone derivatives via Dowd-Beckwith ring-expansion reaction invariably involve the use of transition metals and photoirradiation. Herein, we present a novel transition-metal- and photoirradiation-free pathway to access such structures from α-iodomethyl β-keto esters with electron-rich arenediazonium salts as inexpensive radical initiators and oxidants under mild reaction conditions. The unique aspect of this reactivity is the integration of halogen atom transfer, ring-expansion, and oxidation in one-pot. Further investigation reveals that this method is applicable for modifying complex biologically active molecules, such as epiandrosterone derivatives.

Co-Catalytic Coupling of Alkyl Halides and Alkenes: the Curious Role of Lutidine

Continuous pressure to shorten synthetic sequences along with the concomitant expansion of scope makes the use of alkyl bromides, chlorides, and oxygen based leaving groups- which are abundant and readily available feedstocks, highly attractive for C-C bond synthesis. However, selective activation of these bonds to generate radical intermediates remains challenging and is generally unfeasible using traditional activation strategies. Herein, we report a dual catalytic activation strategy to access primary, secondary, and tertiary alkyl radicals from respective alkyl chlorides and bromides, as well as primary tosylates and trifluoroacetates. While the method relies on visible light and a photocatalyst to facilitate electron transfer, based on reduction potentials, the substrates are not expected to be reduceable, and yet they are reduced in the presence of lutidine. Ultimately, our investigation revealed that lutidine was a precatalyst and ultimately led to the use of lutidinium iodide salt which served as a critical cocatalyst that resulted in improved reaction profiles. Our studies revealed two critical roles that lutidinium iodide salts play which made it possible to engage otherwise unreactive substrates: nucleophilic exchange and halogen atom transfer by the lutidinium radical. In short, this work converts unactivated alkyl chlorides, bromides, tosylates, and trifluoroacetates to radicals that can be used for C-C bond formation without the need for preactivation─effectively expediting synthesis.

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.

Metallaphotoredox Enabled Single Carbon Atom Insertion into Alkenes for Allene Synthesis

Efficient methods for synthesizing allenes from readily available starting materials pose a persistent challenge in organic chemistry. In this work, we present a novel two-stage protocol for allene synthesis involving the single-atom insertion into alkenes, facilitated by synergistic photoredox and cobalt catalysis. Diverging from conventional methods such as the Doering-LaFlamme reaction, this photochemical rearrangement approach operates efficiently under mild conditions in a radical-based manner. The protocol exhibits a broad substrate scope and demonstrates applicability in the late-stage diversification of alkene-containing natural products and bioactive molecules. Preliminary mechanistic studies and density functional theory (DFT) calculations offer insights into the reaction pathway, indicating a radical mechanism involving fleeting cyclopropyl carbene intermediates followed by rapid ring opening to form allenes.

Catalytic Reductive Homocoupling of Benzyl Chlorides Enabled by Zirconocene and Photoredox Catalysis

The bibenzyl skeleton is prevalent in numerous natural products and other biologically active compounds. Radical homocoupling provides a straightforward approach for synthesizing bibenzyls in a single step with the reductive homocoupling of benzyl halides undergoing extensive development. Unlike benzyl bromides and other tailored precursors used in visible-light-mediated homocoupling, benzyl chlorides offer greater abundance and chemical stability. Nevertheless, achieving chemoselective cleavage of the C-Cl bond poses significant challenges, with only a limited number of studies reported to date. Herein, we demonstrate a catalytic reductive homocoupling of benzyl chlorides facilitated by zirconocene and photoredox catalysis. This cooperative catalytic system promotes C-Cl bond cleavage in benzyl chlorides under mild conditions and supports the homocoupling of a wide range of benzyl chlorides, including those derived from pharmaceutical agents. Our preliminary mechanistic investigations highlight the pivotal role of hydrosilane in the catalytic cycle.

Photoredox-Catalyzed Site-Selective Intermolecular C(sp3)-H Alkylation of Tetrahydrofurfuryl Alcohol Derivatives

4'-Selective alkylation of nucleosides has been recognized as one of the ideal and straightforward approaches to chemically modified nucleosides, but such a transformation has been scarce and less explored. In this Letter, we combine a visible-light-mediated photoredox catalysis and hydrogen atom transfer (HAT) auxiliary to achieve β-C(sp3)-H alkylation of alcohol on tetrahydrofurfuryl alcohol scaffolds and exploit it for 4'-selective alkylation of nucleosides. The reaction involves an intramolecular 1,5-HAT process and stereocontrolled Giese addition of the resultant radicals.

Alternative and Sustainable Route to Explore a New Class of Amidines by Photochemical Synergistic Effect of Copper/Nitroxyl Radical Catalysis via Halogen-Atom Transfer

Amidines are a vital class of bioactive compounds and often necessitate multiple components for their synthesis. Therefore, exploring efficient and sustainable methodologies for their synthesis is indispensable. Herein, we disclose an alternative and greener method for synthesizing an unexplored new class of amidines through the photochemical synergistic effect of copper/nitroxyl radical catalysis. This approach facilitates site-selective radical amination of unactivated imine C(sp2)-H bond in C,N,N-cyclic imines over favored selectivity via halogen-atom transfer (XAT). This greener method ticks 11 out of 12 green chemistry metrics (GCM), effectively sidestepping the need for oxidants, bases, ligands, multistep processes, and harsh conditions, distinguishing it from conventional methods described in previous studies. Kinetic, spectroscopic, and computational tools have been employed to elucidate the synergistic effect of Cu/nitroxyl radical, the role of light, XAT, the influence of substituents, and the order of the reaction in the catalytic cycle.

Unveiling Heavier Dihydropyridine Chalcogenol Esters in Metallaphotoredox Catalyst-Enabled Regioselective Hydrothio(seleno)carbonylation

Herein, aromaticity-driven thio(seleno)ester group transfer from novel 1,4-dihydropyridine thio(seleno)esters to alkene feedstocks is disclosed by merging palladium and photoredox catalysis. In this process, photoactivation of dihydropyridine thio(seleno)esters is integrated with regioselective hydrometalation of alkenes, avoiding photoinduced Pd-C bond homolysis of organopalladium intermediates. Additionally, a regioselective hydroselenocarbonylation of an alkene is accomplished for the first time using a bench-stable selenoester reagent. The activation mode of novel dihydropyridine thioesters has been illustrated by detailed mechanistic studies, spectroscopic analysis, intermediate trapping, and isotope labeling experiments.

Deoxygenative photochemical alkylation of secondary amides enables a streamlined synthesis of substituted amines

Secondary amines are vital functional groups in pharmaceuticals, agrochemicals, and natural products, necessitating efficient synthetic methods. Traditional approaches, including N-monoalkylation and reductive amination, suffer from limitations such as poor chemoselectivity and complexity. Herein, we present a streamlined deoxygenative photochemical alkylation of secondary amides, enabling the efficient synthesis of α-branched secondary amines. Our method leverages triflic anhydride-mediated semi-reduction of amides to imines, followed by a photochemical radical alkylation step. This approach broadens the synthetic utility of amides, facilitating late-stage modifications of drug-like molecules and the synthesis of saturated N-substituted heterocycles. The pivotal role of flow technology in developing a scalable and robust process underscores the practicality of this method, significantly expanding the organic chemist's toolbox for complex amine synthesis.

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.

Cross-Coupling of Carbonyl Derivatives and N-Arylamines Enabled by Visible Light for Easy Access to 1,2-Amino Alcohols

We disclosed a new strategy for the synthesis of 1,2-amino alcohols enabled by visible light without the requirement of a photocatalyst and metal. Under light irradiation at 400 nm, the reaction of carbonyl derivatives and N-arylamines proceeds via an electron-donor-acceptor (EDA) intermediate, obtaining diverse vicinal amino alcohols decorated with a two-electron-rich/-deficient aryl group.

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.

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.

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.

Better Together: Photoredox/Copper Dual Catalysis in Atom Transfer Radical Polymerization

Photomediated Atom Transfer Radical Polymerization (photoATRP) is an activator regeneration method, which allows for the controlled synthesis of well-defined polymers via light irradiation. Traditional photoATRP is often limited by the need for high-energy ultraviolet or violet light. These could negatively affect the control and selectivity of the polymerization, promote side reactions, and may not be applicable to biologically relevant systems. This drawback can be circumvented by an introduction of the catalytic amount of photocatalysts, which absorb visible and/or NIR light and, therefore, controlled, regenerative ATRP can be performed with the dual-catalytic cycle. Herein, a critical summary of recent developments in the field of dual-catalysis concerning Cu-catalyzed ATRP is provided. Contributions of involved species are examined mechanistically, followed by challenges and future directions towards the next generation of advanced functional macromolecular materials.

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.

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.

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.

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.

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.

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.

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.

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

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.