Merging Halogen-Atom Transfer (XAT) and Cobalt Catalysis to Override E2-Selectivity in the Elimination of Alkyl Halides: A Mild Route toward contra-Thermodynamic Olefins

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.

Synthesis of α-Bromo Arylethyl Sulfonyl Fluorides and β-Arylethenesulfonyl Fluorides via Copper-Catalyzed Meerwein Arylation

A practical copper-catalyzed process for the synthesis of the β-arylethenesulfonyl fluorides is described. A series of α-bromo arylethyl sulfonyl fluorides was prepared via Meerwein reaction from arenediazonium tetrafluoroborates and ethenesulfonyl fluoride (ESF) under mild conditions. The following β-arylethenesulfonyl fluorides were further obtained through a β-elimination reaction. This protocol features excellent regio- and stereoselectivity and broad substrate scope.

Biomimetic Photoexcited Cobaloxime Catalysis in Organic Synthesis

Drawing inspiration from nature has long been a cornerstone of chemical innovation, with natural systems offering a wealth of untapped potential for discovery. In this minireview, we delve into the burgeoning field of cobaloxime catalysis in organic synthesis, which mimics the catalytic activity of the natural organometallic alkylcobalamine enzymes. Our focus lies on elucidating the latest advancements in this area, as well as delineating the primary mechanistic pathways at play. By describing, and comparing these mechanisms, we provide a comprehensive overview of the current state-of-the-art, while also shedding light on the key unresolved challenges that await further exploration.

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

Photoinduced Ligand-to-Metal Charge Transfer in Base-Metal Catalysis

The absorption of light by photosensitizers has been shown to offer novel reactive pathways through electronic excited state intermediates, complementing ground state mechanisms. Such strategies have been applied in both photocatalysis and photoredox catalysis, driven by generating reactive intermediates from their long-lived excited states. One developing area is photoinduced ligand-to-metal charge transfer (LMCT) catalysis, in which coordination of a ligand to a metal center and subsequent excitation with light results in the formation of a reactive radical and a reduced metal center. This mini review concerns the foundations and recent developments in ligand-to-metal charge transfer in transition metal catalysis focusing on the organic transformations made possible through this mechanism.

Hydroalkylation of styrenes enabled by boryl radical mediated halogen atom transfer

In this study, we present an inexpensive, stable, and easily available boryl radical source (BPh4Na) employed in a Halogen Atom Transfer (XAT) methodology. This mild and convenient strategy unlocks the use of not only alkyl iodides as radical precursors but also of the more challenging alkyl and aryl bromides to generate C-centered radicals. The generated radicals were further engaged in the anti-Markovnikov hydroalkylation of electronically diverse styrenes, therefore achieving the formation of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds. A series of experimental and computational studies revealed the prominent role of BPh4Na in the halogen abstraction step.

Iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer and hydrogen atom transfer

Site selective functionalization of inert remote C(sp3)-H bonds to increase molecular complexity offers vital potential for chemical synthesis and new drug development, thus it has been attracting ongoing research interest. In particular, typical β-C(sp3)-H arylation methods using chelation-assisted metal catalysis or metal-catalyzed oxidative/photochemical in situ generated allyl C(sp3)-H bond processes have been well developed. However, radical-mediated direct β-C(sp3)-H arylation of carbonyls remains elusive. Herein, we describe an iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer (XAT) and hydrogen atom transfer (HAT). The method involves diethylaminoethyl radical-mediated generation of an aryl radical intermediate via XAT, then directed 1,5-HAT to form the remote alkyl radical intermediate and radical-radical coupling with cyanoarenes, and is applicable to a broad scope of unactivated remote C(sp3)-H bonds like β-C(sp3)-H bonds of o-iodoaryl-substituted alkanones and α-C(sp3)-H bonds of o-iodoarylamides. Experimental findings are supported by computational studies (DFT calculations), revealing that this method operates via a radical-relay stepwise mechanism involving multiple SET, XAT, 1,5-HAT and radical-radical coupling processes.

Electrochemical Reduction of Perfluorooctanoic Acid (PFOA): An Experimental and Theoretical Approach

Perfluorooctanoic acid (PFOA) is an artificial chemical of global concern due to its high environmental persistence and potential human health risk. Electrochemical methods are promising technologies for water treatment because they are efficient, cheap, and scalable. The electrochemical reduction of PFOA is one of the current methodologies. This process leads to defluorination of the carbon chain to hydrogenated products. Here, we describe a mechanistic study of the electrochemical reduction of PFOA in gold electrodes. By using linear sweep voltammetry (LSV), an E0' of -1.80 V vs Ag/AgCl was estimated. Using a scan rate diagnosis, we determined an electron-transfer coefficient (αexp) of 0.37, corresponding to a concerted mechanism. The strong adsorption of PFOA into the gold surface is confirmed by the Langmuir-like isotherm in the absence (KA = 1.89 × 1012 cm3 mol-1) and presence of a negative potential (KA = 3.94 × 107 cm3 mol-1, at -1.40 V vs Ag/AgCl). Based on Marcus-Hush's theory, calculations show a solvent reorganization energy (λ0) of 0.9 eV, suggesting a large electrostatic repulsion between the perfluorinated chain and water. The estimated free energy of the transition state of the electron transfer (ΔG‡ = 2.42 eV) suggests that it is thermodynamically the reaction-limiting step. 19F - 1H NMR, UV-vis, and mass spectrometry studies confirm the displacement of fluorine atoms by hydrogen. Density functional theory (DFT) calculations also support the concerted mechanism for the reductive defluorination of PFOA, in agreement with the experimental values.

Hydrogen Atom Transfer (HAT)-Mediated Remote Desaturation Enabled by Fe/Cr-H Cooperative Catalysis

An iron/chromium system (Fe(OAc)2, CpCr(CO)3H) catalyzes the preparation of β,γ- or γ,δ-unsaturated amides from 1,4,2-dioxazol-5-ones. An acyl nitrenoid iron complex seems likely to be responsible for C-H activation. A cascade of three H• transfer steps appears to be involved: (i) the abstraction of H• from a remote C-H bond by the nitrenoid N, (ii) the transfer of H• from Cr to N, and (iii) the abstraction of H• from a radical substituent by the Cr•. The observed kinetic isotope effects are consistent with the proposed mechanism if nitrenoid formation is the rate-determining step. The Fe/Cr catalysts can also desaturate substituted 1,4,2-dioxazol-5-ones to 3,5-dienamides.

Visible-Light-Induced Three-Component 1,2-Alkylpyridylation of Alkenes via a Halogen-Atom Transfer Process

Visible-light-induced three-component 1,2-alkylpyridylation of alkenes with unactivated alkyl iodides and aryl cyanides is reported via a photocatalytic halogen-atom transfer (XAT) strategy. This metal-free protocol utilizes readily available tertiary alkylamine as the terminal reductant to smoothly convert alkyl iodides into the corresponding carbon radical species. The reaction features a broad substrate scope, excellent functional group tolerance, high efficiency, and mild reaction conditions. The practicability of this methodology is further demonstrated in the late-stage difunctionalization of bioactive molecules.

Photoinduced reductive Reformatsky reaction of α-haloesters and aldehydes or ketones by cooperative dual-metal catalysis

A photoinduced reductive Reformatsky reaction by cooperative dual-metal catalysis is described. This methodology enables the implementation of this venerable reaction in environmentally friendly conditions, obviating the need for a stoichiometric amount of metals. A broad range of synthetically useful β-hydroxy esters can be efficiently prepared in moderate to high yields using this protocol.

Photocatalytic Enantioselective Hydrosulfonylation of α,β-Unsaturated Carbonyls with Sulfonyl Chlorides

This research explores the enantioselective hydrosulfonylation of various α,β-unsaturated carbonyl compounds via the use of visible light and redox-active chiral Ni-catalysis, facilitating the synthesis of enantioenriched α-chiral sulfones with remarkable enantioselectivity (exceeding 99 % ee). A significant challenge entails enhancing the reactivity between chiral metal-coordinated carbonyl compounds and moderate electrophilic sulfonyl radicals, aiming to minimize the background reactions. The success of our approach stems from two distinctive attributes: 1) the Cl-atom abstraction employed for sulfonyl radical generation from sulfonyl chlorides, and 2) the single-electron reduction to produce a key enolate radical Ni-complex. The latter process appears to enhance the feasibility of the sulfonyl radical's addition to the electron-rich enolate radical. An in-depth investigation into the reaction mechanism, supported by both experimental observations and theoretical analysis, offers insight into the intricate reaction process. Moreover, the versatility of our methodology is highlighted through its successful application in the late-stage functionalization of complex bioactive molecules, demonstrating its practicality as a strategy for producing α-chiral sulfones.

Photoinduced Palladium-Catalyzed Regio- and Chemoselective Elimination of Primary Alkyl Bromides: A Mild Route to Synthesize Unactivated Terminal Olefins

Presented is a highly efficient method for visible-light-induced regio- and chemoselective elimination of alkyl halides yielding unactivated terminal olefins vital in organic synthesis. Achieved through ligand control, the reaction exhibits remarkable regioselectivity and suppresses undesired side reactions, particularly 1,5-hydrogen atom transfer (HAT). The process favors primary alkyl halides while preserving secondary and tertiary alkyl bromides, thereby enabling the incorporation of terminal olefins in complex molecules for late-stage functionalization.

Photocatalytic Umpolung Strategy for the Synthesis of α-Amino Phosphine Oxides and Deuterated Derivatives

Direct, economical, and green synthesis of deuterated α-amino phosphine oxides remains an elusive challenge in synthetic chemistry. Herein, we report a visible-light-driven umpolung strategy for synthesizing deuterated α-amino phosphine oxides from isocyanide using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene as the photocatalyst and D2O as the deuterium source. Moreover, the streamlined and sustainable methodology can be applied in the modification of amino acids, natural products, and drugs. The strong antiproliferative activity of the desired products indicates that the method could provide a novel privileged scaffold for antitumor drug development.

Electrochemical halogen-atom transfer alkylation via α-aminoalkyl radical activation of alkyl iodides

Alkyl halides, widely recognized as important building blocks and reagents in organic synthesis, can serve as versatile alkyl radical precursors in radical-based transformations. However, generating alkyl radicals directly from unactivated alkyl halides under mild conditions remains a challenge due to their extremely low reduction potentials. To address this issue, α-aminoalkyl radicals were employed as efficient halogen-atom transfer (XAT) reagents in the photoredox activation of unactivated alkyl halides. Here, we report an effective electrooxidation strategy for generating alkyl radicals from unactivated alkyl iodides via an electrochemical halogen-atom transfer (e-XAT) process under mild conditions. The α-aminoalkyl radicals generated by anodic oxidation are demonstrated to be efficient XAT reagents in these transformations. This facile electricity-driven strategy obviates the need for sacrificial anodes and external chemical oxidants. The method successfully applies to a wide variety of alkyl iodides, including primary, secondary, and tertiary, as well as structurally diverse olefins, exhibiting excellent functional group tolerance. Moreover, we further demonstrate the utility of this strategy by rapidly functionalizing complex molecules and biomolecules.

Strain-Inducing Positional Alkene Isomerization

Small, strained ring systems are important pharmacophores in medicinal chemistry and versatile intermediates in organic synthesis. However, the kinetic and thermodynamic instability of many strained organic molecules renders them challenging to prepare. Here, we report a strain-inducing positional alkene isomerization reaction that provides mild and selective access to cyclobutene building blocks from readily obtained cyclobutylidene precursors. This endergonic isomerization relies on the sequential and synergistic action of a decatungstate polyanion photocatalyst and cobaloxime co-catalyst to store potential energy in the form of ring strain. The versatility of the cyclobutene products is demonstrated through diverse subsequent strain-releasing transformations. Mechanistic studies reveal a steric basis for strain-selective product formation.

Ketone Synthesis via Irradiation-Induced Generation of a Persistent Ketyl Radical from Acyl Azolium Salts

A novel three-component α-acylated difunctionalization of alkenes strategy has been developed on the basis of a direct hydrogen atom transfer (HAT) process of photoinduced acyl azolium salts. With simple irradiation without the catalyst, a variety of olefins can be directly converted into ketone derivatives, including 1,4-dione, β-silyl ketone, 1,5-dione, etc. Mechanistic investigations indicated that the unique reactivity of the acyl azonium triplet excited state is crucial to the strategy's success.

Pd/Xu-Phos-catalyzed asymmetric elimination of fully substituted enol triflates into axially chiral trisubstituted allenes

The β-H elimination, as one of the most important elementary reactions in transition metal chemistry, is a key step in quenching the carbon-palladium bond for the Heck reaction. However, the β-H elimination of the alkenyl palladium species leading to allene is an energetically unfavored process, and therefore, it has been a long-standing challenge in control of this process via enantioselective manner. We developed a concise and efficient methodology to construct trisubstituted chiral allenes from stereodefined fully substituted enol triflates by the enantioselective β-H elimination of the alkenyl palladium species under mild conditions. The identified Xu-Phos play a crucial role in the chemoselectivity and enantioselectivity. Multiple linear regression analysis shows the important steric effect on enantioselectivity. DFT computation results allow us to propose an intramolecular base (^-OAc)-assisted deprotonation mechanism for this progress. Distortion-interaction and energy decomposition analysis indicate that the difference in electrostatic energy (E_elec) of the two intramolecular base-assisted deprotonation transition states dominates the stereoselectivity.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

Molecular Engineering of Metal-Organic Layers for Sustainable Tandem and Synergistic Photocatalysis

Metal-organic layers (MOLs), a monolayered version of metal-organic frameworks (MOFs), have recently emerged as a novel two-dimensional molecular material platform to design multifunctional catalysts. MOLs inherit the intrinsic molecular tunability of MOFs and yet have more accessible and modifiable building blocks. Here we report molecular engineering of six MOLs via modulated solvothermal synthesis between HfCl4 and three photosensitizing ligands followed by postsynthetic modification with two carboxylate-containing cobaloximes for tandem and synergistic photocatalysis. Morphological and structural characterization by transmission electron microscopy and atomic force microscopy and compositional analysis by inductively coupled plasma-mass spectrometry and nuclear magnetic resonance spectroscopy establish the MOLs as flat nanoplates with a periodic lattice structure of hexagonal symmetry. The MOLs efficiently catalyze tandem dehydrogenative coupling reactions and synergistic Heck-type coupling reactions. The most active MOL catalyst was used for the gram-scale synthesis of vesnarinone, a cardiotonic agent, in 80% yield with a turnover number of 400 and in eight consecutive reaction cycles without significant loss of activities.

Effective Formation of New C(sp2 )-S Bonds via Photoactivation of Alkylamine-based Electron Donor-Acceptor Complexes

A novel visible light promoted formation of C_Aryl- S bonds through electron donor-acceptor (EDA) complexes of alkylamines with 5- and 6-membered (hetero)arene halides is presented. This represents the first EDA-based thiolation method not relying on π-π or a thiolate-anion-π interactions and provides a facile access to heteroarene radicals, which can be suitably trapped by disulfide derivatives to form the corresponding versatile arylsulfides. Mechanistic investigations on the aspects of the whole process were conducted by spectroscopic measurements, demonstrating the hypothesized EDA complex formation. Moreover, the strength of this method has been proven by a gram-scale synthesis of thiolated products and the late-stage derivatization of an anticoagulant drug.

Photocatalytic Deuterium Atom Transfer Deuteration of Electron-Deficient Alkenes with High Functional Group Tolerance

Due to its mild reaction conditions and unique chemoselectivity, hydrogen atom transfer (HAT) hydrogenation represents an indispensable method for the synthesis of complex molecules. Its analog using deuterium, deuterium atom transfer (DAT) deuteration, is expected to enable access to complex deuterium-labeled compounds. However, DAT deuteration has been scarcely studied for synthetic purposes, and a method that possesses the favorable characteristics of HAT hydrogenations has remained elusive. Herein, we report a protocol for the photocatalytic DAT deuteration of electron-deficient alkenes. In contrast to the previous DAT deuteration, this method tolerates a variety of synthetically useful functional groups including haloarenes. The late-stage deuteration also allows access to deuterated amino acids as well as donepezil-d₂ . Thus, this work demonstrates the potential of DAT chemistry to become the alternative method of choice for preparing deuterium-containing molecules.

Photoinduced Halogen-Atom Transfer by N-Heterocyclic Carbene-Ligated Boryl Radicals for C(sp3)-C(sp3) Bond Formation

Herein, we present a comprehensive study on the use of N-heterocyclic carbene (NHC)-ligated boryl radicals to enable C(sp³)-C(sp³) bond formation under visible-light irradiation via Halogen-Atom Transfer (XAT). The methodology relies on the use of an acridinium dye to generate the boron-centered radicals from the corresponding NHC-ligated boranes via single-electron transfer (SET) and deprotonation. These boryl radicals subsequently engage with alkyl halides in an XAT step, delivering the desired nucleophilic alkyl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally relevant compounds and biologically active molecules. The key role of NHC-ligated boryl radicals in the operative reaction mechanism has been elucidated through a combination of experimental, spectroscopic, and computational studies. This methodology stands as a significant advancement in the chemistry of NHC-ligated boryl radicals, which had long been restricted to radical reductions, enabling C-C bond formation under visible-light photoredox conditions.

Transition-metal free C-N bond formation from alkyl iodides and diazonium salts via halogen-atom transfer

Construction of C-N bond continues to be one part of the most significant goals in organic chemistry because of the universal applications of amines in pharmaceuticals, materials and agrochemicals. However, E2 elimination through classic S_N2 substitution of alkyl halides lead to generation of alkenes as major side-products. Thus, formation of a challenging C(sp³)-N bond especially on tertiary carbon center remains highly desirable. Herein, we present a practical alternative to prepare primary, secondary and tertiary alkyl amines with high efficiency between alkyl iodides and easily accessible diazonium salts. This robust transformation only employs Cs₂CO₃ promoting halogen-atom transfer (XAT) process under transition-metal-free reaction conditions, thus providing a rapid method to assemble diverse C(sp³)-N bonds. Moreover, diazonium salts served as alkyl radical initiator and amination reagent in the reaction. Mechanism studies suggest this reaction undergo through halogen-atom transfer process to generate active alkyl radical which couples with diazonium cations to furnish final products.

ICl-Mediated Functional Group Interconversion from Methyl Homopropargyl Ether to α-Iodo-γ-chloroketone

An ICl-mediated highly chemo- and regioselective functional group interconversion from methyl homopropargyl ether to α-iodo-γ-chloro-ketone is reported. Density functional theory (DFT)-calculated reaction coordinate and potential energy surface support the high chemo-selectivity observed for the formation of α-iodo-γ-chloroketone over furan. The five-membered oxonium ring formation-ring opening mechanism is a potential template for the preparation of polyfunctionalized carbonyl compounds.

Halogen-atom and group transfer reactivity enabled by hydrogen tunneling

The generation of carbon radicals by halogen-atom and group transfer reactions is generally achieved using tin and silicon reagents that maximize the interplay of enthalpic (thermodynamic) and polar (kinetic) effects. In this work, we demonstrate a distinct reactivity mode enabled by quantum mechanical tunneling that uses the cyclohexadiene derivative γ-terpinene as the abstractor under mild photochemical conditions. This protocol activates alkyl and aryl halides as well as several alcohol and thiol derivatives. Experimental and computational studies unveiled a noncanonical pathway whereby a cyclohexadienyl radical undergoes concerted aromatization and halogen-atom or group abstraction through the reactivity of an effective H atom. This activation mechanism is seemingly thermodynamically and kinetically unfavorable but is rendered feasible through quantum tunneling.

The Merger of Benzophenone HAT Photocatalysis and Silyl Radical-Induced XAT Enables Both Nickel-Catalyzed Cross-Electrophile Coupling and 1,2-Dicarbofunctionalization of Olefins

A strategy for both cross-electrophile coupling and 1,2-dicarbofunctionalization of olefins has been developed. Carbon-centered radicals are generated from alkyl bromides by merging benzophenone hydrogen atom transfer (HAT) photocatalysis and silyl radical-induced halogen atom transfer (XAT) and are subsequently intercepted by a nickel catalyst to forge the targeted C(sp³)–C(sp²) and C(sp³)–C(sp³) bonds. The mild protocol is fast and scalable using flow technology, displays broad functional group tolerance, and is amenable to a wide variety of medicinally relevant moieties. Mechanistic investigations reveal that the ketone catalyst, upon photoexcitation, is responsible for the direct activation of the silicon-based XAT reagent (HAT-mediated XAT) that furnishes the targeted alkyl radical and is ultimately involved in the turnover of the nickel catalytic cycle.

α-Amino Radical Halogen Atom Transfer Agents for Metallaphotoredox-Catalyzed Cross-Electrophile Couplings of Distinct Organic Halides

α-Amino radicals from simple tertiary amines were employed as halogen atom transfer (XAT) agents in metallaphotoredox catalysis for cross-electrophile couplings of organic bromides with organic iodides. This XAT strategy proved to be efficient for the generation of carbon radicals from a range of partners (alkyl, aryl, alkenyl, and alkynyl iodides). The reactivities of these radical intermediates were captured by nickel catalysis with organobromides including aryl, heteroaryl, alkenyl, and alkyl bromides, enabling six diverse C-C bond formations. Classic named reactions including Negishi, Suzuki, Heck, and Sonogashira reactions were readily achieved in a net-reductive fashion under mild conditions. More importantly, the cross coupling was viable with either organic bromide or iodide as limiting reactant based on the availability of substrates, which is beneficial to the late-stage functionalization of complex molecules. The scalability of this method in batch and flow was investigated, further demonstrating its applicability.

Visible-Light-Driven α-Aminoalkyl Radical-Mediated C(sp3)-C(sp) Cross-Coupling of Iodoalkanes and Alkynyl Bromides

We herein report a simple protocol for metal-free cross-coupling between unactivated alkyl iodides and terminal alkynyl bromides promoted by visible light. The salient features of this transformation are the utilization of an organic photocatalyst and commercially available tri-n-butylamine as a reductant. This protocol couples a variety of unactivated iodoalkanes containing different functional groups and with a variety of terminal alkynyl bromides under mild reaction conditions to afford the substituted alkynes in good yields.

Cobalt-catalyzed chemoselective dehydrogenation through radical translocation under visible light

The transformations that allow the direct removal of hydrogen from their corresponding saturated counterparts by the dehydrogenative strategy are a dream reaction that has remained largely underexplored. In this report, a straightforward and robust cobaloxime-catalyzed photochemical dehydrogenation strategy via intramolecular HAT is described for the first time. The reaction proceeds through an intramolecular radical translocation followed by the cobalt assisted dehydrogenation without needing any other external photosensitizers, noble-metals or oxidants. With this approach, a series of valuable unsaturated compounds such as α,β-unsaturated amides, enamides and allylic and homoallylic sulfonamides were obtained in moderate to excellent yields with good chemo- and regioselectivities, and the synthetic versatility was demonstrated by a range of transformations. And mechanistic studies of the method are discussed.

The dehydrogenative reactions proceeded through selective 1,n-hydrogen atom transfer (n = 5–7) for remote C–H activation by cobaloxime catalysis.

Catalytic defluorinative ketyl-olefin coupling by halogen-atom transfer

Ketyl–olefin coupling reactions stand as one of the fundamental chemical transformations in synthetic chemistry and have been widely employed in the generation of complex molecular architectures and natural product synthesis. However, catalytic ketyl–olefin coupling, until the recent development of photoredox chemistry and electrosynthesis through single-electron transfer mechanisms, has remained largely undeveloped. Herein, we describe a new approach to achieve catalytic ketyl–olefin coupling reactions by a halogen-atom transfer mechanism, which provides innovative and efficient access to various gem-difluorohomoallylic alcohols under mild conditions with broad substrate scope. Preliminary mechanistic experimental and computational studies demonstrate that this radical-to-polar crossover transformation could be achieved by sequentially orchestrated Lewis acid activation, halogen-atom transfer, radical addition, single-electron reduction and β-fluoro elimination.

A catalytic ketyl–olefin coupling reaction including sequentially orchestrated Lewis acid activation, halogen-atom transfer, radical addition, single-electron reduction and β-fluoro elimination has been developed.

Asymmetric C-H Dehydrogenative Allylic Alkylation by Ternary Photoredox-Cobalt-Chiral Primary Amine Catalysis under Visible Light

We report herein an asymmetric C-H dehydrogenative allylic alkylation by a synergistic catalytic system involving a chiral primary amine, a photoredox catalyst, and a cobaloxime cocatalyst. The ternary catalytic system enables the coupling of β-ketocarbonyls and olefins with good yields and high enantioselectivities. Mechanism studies disclosed a cooperative radical addition process with a chiral α-imino radical and Co(II)-metalloradical wherein the chiral primary aminocatalyst and the cobaloxime catalyst work in concert to control the stereoinduction.

Tetrachlorophthalimides as Organocatalytic Acceptors for Electron Donor-Acceptor Complex Photoactivation

Excitation of photoactive electron donor-acceptor (EDA) complexes is an effective way to generate radicals. Applications in a catalytic regime typically use catalytic donors. Herein, we report that readily available electron-poor tetrachlorophthalimides can act as effective organocatalytic acceptors to trigger the formation of EDA complexes with a variety of radical precursors not amenable to previous catalytic methods. Excitation with visible light generates carbon radicals under mild conditions. The versatility of this EDA complex catalytic platform allowed us to develop mechanistically distinct radical reactions, including in combination with a cobalt-based catalytic system. Quantum yield measurements established that a closed catalytic cycle is operational, which hints at the ability of tetrachlorophthalimides to readily turn over and govern each catalytic cycle.

Selective, Transition Metal-free 1,2-Diboration of Alkyl Halides, Tosylates, and Alcohols

Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal‐free, simple and efficient strategy to access alkyl 1,2‐bis(boronate esters) via regio‐ and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N‐dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late‐stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.

Radical Aza-Heck Cyclization of Imidates via Energy Transfer, Electron Transfer, and Cobalt Catalysis

A radical aza-Heck cyclization has been developed to afford functionally rich products with four contiguous C-heteroatom bonds. This multi-catalytic strategy provides rapid syntheses of dense, medicinally relevant motifs by enabling the conversion of alcohol-derived imidates to heteroatom-rich fragments containing vinyl oxazolines/oxazoles, allyl amines, β-amino alcohols/halides, and combinations thereof. Mechanistic insights of this process show how three distinct photocatalytic cycles cooperate to enable: (1) imidate radical generation by energy transfer, (2) dehydrogenation by Co catalysis, and (3) catalyst turnover by electron transfer.

Visible Light-Driven, Gold(I)-Catalyzed Preparation of Symmetrical (Hetero)biaryls by Homocoupling of Arylazo Sulfones

The preparation of symmetrical (hetero)biaryls via arylazo sulfones has been successfully carried out upon visible light irradiation in the presence of PPh3AuCl as the catalyst. The present protocol led to the efficient synthesis of a wide range of target compounds in an organic-aqueous solvent under photocatalyst-free conditions.

Visible-Light Enabled C(sp3)-C(sp2) Cross-Electrophile Coupling via Synergistic Halogen-Atom Transfer (XAT) and Nickel Catalysis

We herein report the first visible-light-mediated cross-coupling of unactivated alkyl iodides with aryl bromides through synergistic halogen atom transfer (XAT) and nickel catalysis. This simple protocol operates under mild reaction conditions and tolerates a variety of functional groups affording C(sp³)-C(sp²) cross-coupling products in good to moderate yields.

Electrocatalytic Allylic C-H Alkylation Enabled by a Dual-Function Cobalt Catalyst

The direct functionalization of allylic C-H bonds with nucleophiles minimizes pre-functionalization and converts inexpensive, abundantly available materials to value-added alkenyl-substituted products but remains challenging. Here we report an electrocatalytic allylic C-H alkylation reaction with carbon nucleophiles employing an easily available cobalt-salen complex as the molecular catalyst. These C(sp³ )-H/C(sp³ )-H cross-coupling reactions proceed through H₂ evolution and require no external chemical oxidants. Importantly, the mild conditions and unique electrocatalytic radical process ensure excellent functional group tolerance and substrate compatibility with both linear and branched terminal alkenes. The synthetic utility of the electrochemical method is highlighted by its scalability (up to 200 mmol scale) under low loading of electrolyte (down to 0.05 equiv) and its successful application in the late-stage functionalization of complex structures.

Visible-light-mediated intramolecular radical cyclization of α-brominated amide-tethered alkylidenecyclopropanes

A ring-opening/cyclization cascade reaction of α-brominated amide-tethered alkylidenecyclopropanes in the presence of photocatalyst 4CzIPN under visible-light irradiation was developed to afford polycyclic benzazepine derivatives in good yields with broad substrate scope and good functional tolerance. A plausible mechanism involving a halogen atom transfer (XAT) process and a radical chain process is proposed for this reaction. This study provides a concise and practical strategy for the synthesis of benzazepine derivatives.

Regioselective Deaminative Allylation of Aliphatic Amines via Dual Cobalt and Organophotoredox Catalysis

Despite the rapid progress in C-C bond-forming reactions using Katritzky salts, their deaminative allylation remains a challenge. Inspired by the metallaphotoredox-catalyzed allylic substitution regime, here, we report the deaminative allylation of Katritzky salts via cobalt/organophotoredox dual catalysis. This cross-electrophile coupling enables regioselective allylation using a variety of allylic esters, overcoming the substrate limitations of reported protocols. Mechanistic studies indicate the involvement of a π-allyl cobalt complex as a radicalophile that mediates C-C bond formation.