Photo-induced Decarboxylative Heck-Type Coupling of Unactivated Aliphatic Acids and Terminal Alkenes in the Absence of Sacrificial Hydrogen Acceptors

Photoinduced Cobalt-Catalyzed Carbonyl Umpolung Alkenylation

Herein we introduce a cobalt-catalyzed umpolung strategy for radical carbonyl alkenylation. This method provides an alternative pathway for producing Grignard-type products from (hetero)aryl vinyl bromides and aliphatic aldehydes without pre-functionalization. The method is distinguished by its broad substrate scope, mild reaction conditions, and simple catalytic system. Upon specific activation of the carbonyl group with AcI, a cobaloxime complex catalyst facilitates the selective reduction of the resulting α-oxy iodide through an atom transfer process.

Iron-Cobalt Dual Catalysis for the Synthesis of Alkenyl Amino Acids and Modification of Peptides

Herein, we report an Fe/Co dual-catalyzed strategy for synthesizing alkenyl unnatural amino acids and peptide modifications. This approach utilizes aspartic acid and glutamic acid derivatives as alkyl radical precursors. It avoids the use of expensive photoredox catalysts and substrate preactivation while preserving the chirality of the amino acids. Furthermore, this strategy enables both modification of peptides and the synthesis of amino-acid-based drug candidates for boron-neutron capture therapy (BNCT).

Photocatalytic Decarboxylative Heck-Type Postpolymerization Modification of Polyacrylates

Postpolymerization modification of commodity polymers has emerged as a powerful strategy for synthesizing functional polymers that are challenging to obtain via direct polymerization. Moreover, it offers opportunities to upcycle existing polymers and enhance their material properties. In this work, we introduce a photocatalytic decarboxylative Heck-type approach to upcycle polyacrylates. By employing an acridine-based photocatalyst in combination with a cobaloxime catalyst, hydrolyzed polyacrylates with varying carboxylic acid contents were efficiently converted into polymers bearing olefin functionalities. The degree of olefin incorporation notably influenced the thermal properties of the resulting materials. Importantly, this protocol is applicable to a broad range of vinyl comonomers, yielding 1,3-diene copolymers with exclusive 1,2-insertion selectivity. Furthermore, a variety of terminal alkenes can be utilized to functionalize the acrylate copolymers, and subsequent thiol-ene click reactions further expand the versatility of the accessible polymers.

Photocatalytic Strategy for Decyanative Transformations Enabled by Amine-Ligated Boryl Radical

Decyanation after α-functionalization by exploiting the inherent properties of cyano groups enables the strategic assembly of a carbon scaffold. Herein, we demonstrate an amine-ligated boryl radical-mediated cyano group transfer (CGT) strategy of malononitriles under photocatalytic conditions. This strategy allows for the cleavage of C(sp3)-CN and the formation of C(sp3)-D and C(sp3) to realize decyanative deuteration and cyclization via radical-polar crossover. Computational studies successfully demonstrated the reactivity of CGT promoters can be accurately assessed.

Photocatalysis Enables Chemodivergent Radical Polar Crossover: Ritter-Type Amidation vs Heck-Type Olefin Carbofunctionalizations

Three-component alkene difunctionalization reactions constitute an ideal platform to rapidly build molecular complexity, enabling the simultaneous introduction of two distinct, orthogonal functional groups into the C═C bond in a single step. Herein, a photoredox catalyzed Ritter-type carboamidation of electronically diverse styrenes harnessing non-stabilized, nucleophilic primary radicals generated from readily-accessible carboxylic acid-derived redox active esters is reported. Furthermore, it is found that Heck-type products are chemoselectively obtained by simply switching aryl olefin acceptors with 1,1-diarylolefins. In the context of photocatalytic chemodivergent radical polar crossover, the synthesis of various trisubstituted alkenes was achieved, simultaneously revealing a divergence in the activation of redox-active esters toward reduction. In-depth mechanistic studies demonstrated both transformation pathways, while DFT calculations indicated the origin of product switchability. Both Ritter-type and Heck-type olefin carbofunctionalizations are scalable up to 4 mmol scale in batch and continuous flow, proving the synthetic utility of the methodology.

Cobalt-based Photocatalysis: From Fundamental Principles to Applications in the Generation of C-X (X=C, O, N, H, Si) Bond

Over the past few decades, the merger of photocatalysis and transition metal-based catalysis or self-photoexcitation of transition metals has emerged as a useful tool in organic transformations. In this context, cobalt-based systems have attracted significant attention as sustainable alternatives to the widely explored platinum group heavy metals (iridium, rhodium, ruthenium) for photocatalytic chemical transformations. This review encompasses the basic types of cobalt-based homogeneous photocatalytic systems, their working principles, and the recent developments (2018-2024) in C-X (X=C, N, O, H, Si) bond formations. Noteworthy to mention that cobalt-based heterogeneous photocatalysis is beyond the scope of the present review. An elaborate presentation on the mechanistic intricacies of cobalt-based photocatalysis, without any external photocatalyst, and cobalt-based dual organophotoredox catalysis have been provided in this comprehensive review, excluding the dual-metal photoredox catalysis. To the best of our knowledge, this is the only contemporary review encompassing the aforementioned two major types of cobalt-based photocatalysis, in general synthetic chemistry, covering all types of C-X bond formations spanning a range of the last six years.

Photoinduced Late-Stage Radical Decarboxylative and Deoxygenative Coupling of Complex Carboxylic Acids and Their Derivatives

The simple and efficient conversion of carboxylic acids into structurally diverse organic molecules is highly desirable in chemical synthesis. This review covers recent developments in photocatalytic methodology for late-stage transformations of complex carboxylic acids and their derivatives enabled by radical decarboxylation and deoxygenation, highlighting some representative and significant contributions in this field. These advancements are categorized based on the reactivity patterns exhibited by the carboxylic acids. Several activation modes to generate alkyl or aryl radical intermediates during decarboxylation of carboxylic acids are presented, namely, single-electron transfer (SET) oxidation, ligand-to-metal charge transfer (LMCT), SET reduction, and energy transfer (EnT) processes. On the other hand, direct activation of C-O bonds in carboxylic acids mediated by phosphoranyl radicals has been discussed and illustrates their potential synthetic application for the synthesis of complex aldehydes, ketones and amides.

Photocatalytic Alkene-Migrative Chain Elongation of 2-Phosphinostyrenes with Aldehydes

The photocatalytic alkene-migrative chain elongation reaction of 2-phosphinostyrenes with aldehydes under mild conditions in response to blue light was demonstrated. A broad range of aldehydes, both aliphatic and aromatic, participated in this reaction to afford alkene-phosphine oxides in a Z-selective manner. Mechanistic experiments suggested the formation of benzophospholene-based ylide intermediates via photocatalytic cyclization of phosphinostyrenes followed by solvent-mediated proton transfer under base-free reaction conditions.

B12-Catalyzed Carbonylation of Carbon Tetrahalides: Using a Broad Range of Visible Light to Access Diverse Carbonyl Compounds

Visible-light-driven organic synthesis is a green and sustainable method for producing fine chemicals and is highly desirable at both laboratory and industrial scales. In this study, we developed a broad-range (including the red region) visible-light-driven carbonylation of CCl4, CBr4, and CBr3F with nucleophiles, such as amines and alcohols, using a B12-Mg2+/TiO2 hybrid catalyst. Carbonyl molecules such as ureas, carbamates, carbonate esters, and carbamoyl fluorides were synthesized with high selectivity and efficiency under mild conditions. Diffuse reflectance UV-vis spectroscopy, femtosecond time-resolved diffuse reflectance spectroscopy, and density functional theory calculations revealed the reaction mechanism is a combination of SN2 and single-electron transfer. This is a rare example of a low-energy, red-light-driven photocatalysis, which has been a highly desired organic reaction in recent years. We believe that this study provides a general platform to access diverse carbonyl molecules and could promote photocatalytic carbonylation reactions on a pilot scale.

Biomimetic Dehydrogenative Intermolecular Formal Allylic Amidation of Branched α-Olefins

Allylic amide moieties are commonly encountered in natural products and are privileged structures in pharmaceuticals and agrochemicals. Moreover, because allylic amide can be to converted into an array of high-value motifs, they have been widely employed in organic synthesis. However, the development of catalytic systems for intermolecular allylic amidation of olefins, particularly branched α-olefins, has proven to be challenging. Here, a biomimetic, synergistic catalytic method is reported that combines photoredox, cobalt, and Brønsted base catalysis for the synthesis of substituted allylic amides from branched α-olefins and simple imides without using oxidants. This low-cost, operationally simple method features a broad substrate scope and excellent functional group compatibility. Moreover, it is successfully used for the functionalization of several structurally complex molecules demonstrating the method's potential utility for medicinal chemistry applications. Mechanistic studies revealed that C(sp3)─N bond formation is mediated by a nitrogen-centered radical intermediate, which is generated via a sequence involving deprotonation and single-electron oxidation.

Modular alkene synthesis from carboxylic acids, alcohols and alkanes via integrated photocatalysis

Alkenes serve as versatile building blocks in diverse organic transformations. Despite notable advancements in olefination methods, a general strategy for the direct conversion of carboxylic acids, alcohols and alkanes into alkenes remains a formidable challenge owing to their inherent reactivity disparities. Here we demonstrate an integrated photochemical strategy that facilitates a one-pot conversion of these fundamental building blocks into alkenes through a sequential C(sp3)-C(sp3) bond formation-fragmentation process, utilizing an easily accessible and recyclable phenyl vinyl ketone as the 'olefination reagent'. This practical method not only offers an unparalleled paradigm for accessing value-added alkenes from abundant and inexpensive starting materials but also showcases its versatility through various complex scenarios, including late-stage on-demand olefination of multifunctional molecules, chain homologation of acids and concise syntheses of bioactive molecules. Moreover, initiating from carboxylic acids, alcohols and alkanes, this protocol presents a complementary approach to traditional olefination methods, making it a highly valuable addition to the research toolkit for alkene synthesis.

Enzyme-Like Catalysis of Vinyl Copolymer Carrying Boron Directly Connected to Backbone: Catalytic Esterification through Cooperation of Boron with Neighboring Carboxylic Anhydride

Alternating-rich copolymer of vinylboronic acid pinacol ester (VBpin) and maleic anhydride (MAH) was found to catalyze direct dehydrative esterification of carboxylic acid and alcohol. The key to the catalytic function is the activation of the MAH unit by the neighboring Lewis acidic boron directly connected to the backbone through the formation of five-membered ring. The effects of the side-chain cooperation were clarified through comparisons with the polymers having similar structures and a conventional titanium catalyst as well as the analyses of reactions with carboxylic acid or alcohol. The catalytic activity was enhanced as the molecular weight was higher, which is owing to the structural feature that boron is directly attached to the backbone. The cooperative catalysis is of interest because of its conceptual similarity with enzyme.

Photocatalytic Cross-Coupling of Aldehydes and Alkenes for Aryl Vinyl Ketones by a Single Catalyst

Herein is the first example of photocatalytic cross-coupling of alkenes with aldehydes by a single catalyst without an external photosensitizer and any additives. Irradiation of the aromatic aldehyde and cobaloxime catalyst results in the formation of an acyl radical, which undergoes radical addition with alkene or indole and subsequently β-H elimination to afford alkenyl ketone. The reaction features cheap and readily available raw materials, a broad substrate scope, and mild conditions, even for late-stage derivatization of bioactive compounds.

Visible light induced cooperative carbonylation and (hetero)aryl migration: synthesis of multi-carbonyl compounds

Carbonylative transformation represents one of the most straightforward procedures for the synthesis of carbonyl-containing compounds. However, the carbonylative procedure toward 1,4-diketones is still limited which are key moieties with potent applications in various areas. Herein, we report a new strategy for the synthesis of multi-carbonyl compounds containing a 1,4-diketone skeleton through remote heteroaryl migration of traditionally restricted 1,3-migratory substrates utilizing carbon monoxide (CO) as the C1 synthon and diazonium compounds as the starting material.

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.

Catalytic tert-alkylation of enamides via C-C bond cleavage under photoredox conditions

Efficient C-C bond cleavage is recognized as a persistent challenge in the field of synthetic methodology. In this study, we found that tertiary alkyl radicals are smoothly formed from tertiary alkylated dienones (BHT adducts) via SET, using PDI as a photocatalyst. Resulting tert-alkyl radicals could be applied to the tert-alkylation of enamides. The driving force of this C-C bond cleavage reaction is the mesolytic cleavage of the BHT adducts. The mechanistic study revealed that PDI anion radical is the key active species during the catalytic cycle.

Cooperative Photoredox and Cobalt-Catalyzed Acceptorless Dehydrogenative Functionalization of Cyclopropylamides towards Allylic N,O-Acyl-acetal Derivatives

We disclose herein a novel photoredox and cobalt co-catalyzed ring-opening/acceptorless dehydrogenative functionalization of mono-donor cyclopropanes. This sustainable and atom-economic approach allows the rapid assembly of a wide range of allylic N,O-acyl-acetal derivatives. The starting materials are readily available and the reaction features mild conditions, broad substrate scope, and excellent functional group compatibility. The optimized conditions accommodate assorted cycloalkylamides and primary, secondary, and tertiary alcohols, with applications in late-stage functionalization of pharmaceutically relevant compounds, stimulating further utility in medicinal chemistry. Moreover, selective nucleophilic substitutions with various carbon nucleophiles were achieved in a one-pot fashion, offering a reliable avenue to access some cyclic and acyclic derivatives.

Photoinduced Cobalt Catalyzed Radical Heck-Type Reaction of Alkyl Electrophiles

We report here a general alkylation reaction of terminal alkenes with nucleophilic cobaloxime complex catalysis under visible light irradiation. A broad range of vinyl arenes and heteroarenes, as well as a diverse set of alkyl(pseudo)halides or strained rings as alkyl electrophiles, can efficiently undergo the transformation up to the gram scale. Mechanistic study reveals the radical nature of the reaction and corroborates our design involving alkylcobaloxime as the key intermediate.

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Manganese/Cobalt Bimetallic Relay Catalysis for Divergent Dehydrogenative Difluoroalkylation of Alkenes

The involvement of manganese radical for halogen atom transfer (XAT) reactions has been esteemed as one reliable method but encountered with limited catalytic models. In this paper, a novel bimetallic relay catalysis of Mn2 (CO)10 and cobaloxime has been developed for divergent dehydrogenative difluoroalkylation of alkenes using commercially available difluoroalkyl bromides. A wide range of structurally diverse terminal, cyclic and internal alkenes as well as tetrasubstituted alkenes are found to be good coupling partners to deliver difluoroalkylated allylic products and difluoromethylated cyclic products, accompanied with the production of H2 as the by-product. This bimetallic relay strategy features broad substrate scope, mild reaction conditions and excellent functional group compatibility. Its success represents an important step-forward to expedite the construction of a rich library of difluoroalkylated products.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Metal-free visible-induced C(sp2)-C(sp2) coupling of quinoxalin-2(H)-ones via oxidative cleavage of the C-N bond

A C(sp²)-C(sp²) reaction between aromatic hydrazines and quinoxalines has been developed through a photocatalytic system. The protocol is established for C(sp²)-N bond cleavage and direct C(sp²)-H functionalization for the coupling of C(sp²)-C(sp²) via photocatalysis under mild and ideal air conditions without the presence of a strong base and metal. The mechanistic studies reveal that the generation of a benzene radical via the oxidative cleavage of aromatic hydrazines for the cross-coupling of C(sp²)-C(sp²) with the assistance of a photocatalyst is essential. The process exhibits excellent compatibility with functional groups and provides convenient access to various 3-arylquinoxalin-2(1H)-ones in good to excellent yields.

N-Allylation of Azoles with Hydrogen Evolution Enabled by Visible-Light Photocatalysis

Direct N-allylation of azoles with hydrogen evolution has been achieved through the synergistic combination of organic photocatalysis and cobalt catalysis. The protocol bypasses stoichiometric oxidants and prefunctionalization of alkenes and produces hydrogen (H₂) as the byproduct. This transformation highlights high step- and atom-economy, high efficiency, and broad functional group tolerance for further derivatization, which opens a door for C-N bond formation that is valuable in heterocyclic chemistry.

Selective oxidation of benzylic alcohols via synergistic bisphosphonium and cobalt catalysis

A synergistic photocatalytic system using a bisphosphonium catalyst and a cobalt catalyst has been developed, enabling the selective oxidation of benzylic alcohols under oxidant-free and environmentally benign conditions. High efficiencies have been obtained for a variety of alcohol substrates, and exclusive selectivity for aldehyde products has been achieved across the board. Furthermore, this photocatalytic system proved to be efficient when performed under continuous-flow conditions, even using a simple and easily assembled continuous-flow setup.

Photoexcited cobalt catalysed endo-selective alkyl Heck reaction

Herein, we report an intramolecular endo-selective Heck reaction of iodomethylsilyl ethers of phenols and alkenols. The reaction leads to the formation of seven- and eight-membered siloxycycles in excellent yields, which could be further converted into the corresponding allylic alcohols upon oxidation. Thus, this method could be used for the selective (Z)-hydroxymethylation of o-hydroxystyrenes and alkenols. Rapid scan EPR experiments and DFT calculations suggest a concerted β-hydrogen elimination event to take place in the triplet state.

Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.

Palladium-catalysed deaminative/decarboxylative cross-coupling of organoammonium salts with carboxylic acids

A palladium-catalysed carbon-carbon bond-forming reaction via deaminative/decarboxylative cross-coupling of organoammonium salts with carboxylic acids was developed. Under the reaction conditions, polyfluoroaromatic carboxylic acids, propiolic acids and α-cyano benzyl carboxylic acid reacted smoothly with benzyl ammonium salts to produce the corresponding carbon-carbon coupling products in good-to-excellent yields.

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.

Direct Excitation of Aldehyde to Activate the C(sp2 )-H Bond by Cobaloxime Catalysis toward Fluorenones Synthesis with Hydrogen Evolution

A new way to form fluorenones via the direct excitation of substrates instead of photocatalyst to activate the C(sp² )-H bond under redox-neutral condition is reported. Our design relies on the photoexcited aromatic aldehyde intermediates that can be intercepted by cobaloxime catalyst through single electron transfer for following β-H elimination. The generation of acyl radical and successful interception by a metal catalyst cobaloxime avoid the use of a photocatalyst and stoichiometric external oxidants, affording a series of highly substituted fluorenones, including six-membered ketones, such as xanthone and thioxanthone derivatives in good to excellent yields, and with hydrogen as the only byproduct. This catalytic system features a readily available metal catalyst, mild reaction conditions and broad substrate scope, in which sunlight reaction and scale-up experiments by continuous-flow approach make the new methodology sustainable and amenable for potentially operational procedures.

PCET-Mediated Ring-Opening Alkenylation of Cycloalkanols via Dual Photoredox and Cobalt Catalysis

The construction of molecular skeletons and modification of molecules using widely available and easily prepared alcohols as radical precursors for coupling reactions are significant and challenging subjects. We herein report a straightforward strategy for the dehydrogenative ring-opening alkenylation of cycloalkanols with alkenes by combining a proton-coupled electron transfer strategy and a dual photoredox and cobalt catalysis system. With this approach, a series of distally unsaturated ketones were obtained in 17-83% yields with high E selectivity.

Direct Aroylation of Olefins through a Cobalt/Photoredox-Catalyzed Decarboxylative and Dehydrogenative Coupling with α-Oxo Acids

A photoredox/cobalt dual catalytic procedure has been developed that allows benzoylation of olefins. Here the photoredox catalyst effects the decarboxylation of α-ketoacids to form benzoyl radicals. After addition of this radical to styrenes, the cobalt catalyst abstracts a H-atom. Hydrogen evolution from the putative cobalt hydride intermediate allows a Heck-like aroylation without the need for a stoichiometric oxidant. Mechanistic studies reveal that electronically different styrenes lead to a curved Hammett plot, thus suggesting a change in product-determining step in the catalytic mechanism.

Transition metal-free approach for late-stage benzylic C(sp3)-H etherifications and esterifications

We report a transition metal-free approach for the regioselective functionalization of benzylic C(sp³)-H bonds using alcohols and carboxylic acids as the nucleophiles. This straightforward and general route has provided various benzylic ethers and esters, including twelve pharmaceutically relevant compounds.

Site-Selective Functionalization of Carboranes at the Electron-Rich Boron Vertex: Photocatalytic B-C Coupling via a Carboranyl Cage Radical

Functionalization of carboranes in a vertex-specific manner is a perennial challenge. Here, we report a photocatalytic B-C coupling for the selective functionalization of carboranes at the boron site which is most distal to carbon. This reaction was achieved by the photo-induced decarboxylation of carborane carboxylic acids to generate boron vertex-centered carboranyl radicals. Theoretical calculations also demonstrate that the reaction more easily occurs at the boron site bearing higher electron density owing to the lower energy barrier for a single-electron transfer to generate a carboranyl radical. By using this strategy, a number of functionalized carboranes could be accessed through alkylation, alkenylation, and heteroarylation under mild conditions. Moreover, both a highly efficient blue emitter with a solid-state luminous efficiency of 42 % and a drug candidate for boron neutron capture therapy (BNCT) containing targeting and fluorine units were obtained.

Alkyl/Glycosyl Sulfoxides as Radical Precursors and Their Use in the Synthesis of Pyridine Derivatives

We report here the use of simple and readily available alkyl sulfoxides as precursors to radicals and their application in the preparation of pyridine derivatives. We show that alkyl sulfoxides, N-methoxy pyridinium salts and fluoride anions form electron donor-acceptor (EDA) complexes in solution, which, upon visible light irradiation, undergo a radical chain process to afford various pyridine derivatives smoothly. This reaction displays broad scope with respect to both sulfoxides and N-methoxy pyridiniums. The synthetic versatility of sulfoxides as a handle in chemistry adds to their power as radical precursors. Glycosyl sulfoxides are converted to the corresponding pyridyl C-glycosides with high stereoselectivities. Computational and experimental studies provide insights into the reaction mechanism.

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.

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.

Synthesis of Pleuromutilin

Synthesis of a potent inhibitor of bacterial protein synthesis, pleuromutilin, is described. Assembly of the critical cyclooctane fragment relies on an oxidative ring-expansion, and complete stereochemical relay in the synthetic sequence is enabled by the judicious choice of tactics. The requisite connectivity pattern of the perhydroindanone motif is rapidly established in a sequence of cycloaddition and radical cyclization events. Application of this strategy allows for preparation of the target natural product in 16 steps from commercially available material.

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.

A Triple Photoredox/Cobalt/Brønsted Acid Catalysis Enabling Markovnikov Hydroalkoxylation of Unactivated Alkenes

We demonstrate Markovnikov hydroalkoxylation of unactivated alkenes using alcohols through a triple catalysis consisting of photoredox, cobalt, and Brønsted acid catalysts under visible light irradiation. The triple catalysis realizes three key elementary steps in a single catalytic cycle: (1) Co(III) hydride generation by photochemical reduction of Co(II) followed by protonation, (2) metal hydride hydrogen atom transfer (MHAT) of alkenes by Co(III) hydride, and (3) oxidation of the alkyl Co(III) complex to alkyl Co(IV). The precise control of protons and electrons by the three catalysts allows the elimination of strong acids and external reductants/oxidants that are required in the conventional methods.

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.