Site-Selective Acceptorless Dehydrogenation of Aliphatics Enabled by Organophotoredox/Cobalt Dual Catalysis

Tandem Azolation/Aromatization of Tetrahydronaphthalenes with Hydrogen Evolution via Organophotoredox/Cobalt Dual Catalysis

Reported herein is a photoredox/cobaloxime dual-catalytic approach to execute tandem dehydrogenative azolation and aromatization of tetrahydronaphthalene for rapid construction of N-(β-naphthyl)azole architectures. This protocol highlights noble metal-free and external oxidants-free conditions, step- and atom-economy, and site-selectivity. A preliminary mechanistic study has uncovered that the transformation undergoes a N-centered radical mediated C-H/N-H cross-coupling followed by dehydrogenative aromatization of saturated naphthyl surrogates under visible light irradiation, and DFT calculations elucidate the site-selectivity.

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.

Light-activated hypervalent iodine agents enable diverse aliphatic C-H functionalization

The functionalization of aliphatic C-H bonds is a crucial step in the synthesis and transformation of complex molecules relevant to medicinal, agricultural and materials chemistry. As such, there is substantial interest in the development of general synthetic platforms that enable the efficient diversification of aliphatic C-H bonds. Here we report a hypervalent iodine reagent that releases a potent hydrogen atom abstractor for C-H activation under mild photochemical conditions. Using this reagent, we demonstrate selective (N-phenyltetrazole)thiolation of aliphatic C-H bonds for a broad scope of substrates. The synthetic utility of the thiolated products is showcased through various derivatizations. Simply by altering the radical trapping agent, our method can directly transform C-H bonds into diverse functionalities, including C-S, C-Cl, C-Br, C-I, C-O, C-N, C-C and C=C bonds.

Photocatalytic acceptorless dehydrogenation of flavanones by cationic Eosin Y as a bifunctional catalyst

We report the first example of photocatalytic acceptorless dehydrogenation using cationic Eosin Y as a bifunctional photocatalyst, without metal catalysts or HAT reagents. Under Bayesian optimized conditions, a wide range of flavones were synthesized in moderate to excellent yields, many of which were reported with biological activities. Mechanistic studies suggest that flavones likely form through two HAT processes, with hydrogen release occurring via photoredox.

Ni-catalysed acceptorless dehydrogenative aromatisation of cyclohexanones enabled by concerted catalysis specific to supported nanoparticles

The dehydrogenative aromatisation of cyclohexanone derivatives has had a transformative influence on the synthesis of aromatic compounds because functional groups can be easily introduced at desired positions via classic organic reactions without being limited by ortho-, meta- or para-orientations. However, research is still limited on acceptorless dehydrogenative aromatisation, especially with regard to nonprecious-metal catalysts. Ni is a promising candidate catalyst as a congener of Pd, but thermally Ni-catalysed dehydrogenative aromatisation has not been reported even in an oxidative manner because of the difficulty of β-hydride elimination and the fast re-insertion of Ni-H species. Here, we report a CeO2-supported Ni(0) nanoparticle catalyst for acceptorless dehydrogenative aromatisation of cyclohexanone derivatives. This catalyst is widely applicable to various compounds such as cyclohexanols, cyclohexylamines, N-heterocycles, enamines and β-heteroatom-substituted ketones. Through various experiments, we demonstrate that the present reaction was achieved by the concerted catalysis utilizing metal ensembles unique to supported metal nanoparticle catalysts.

Photoinduced Cobaloxime-Catalyzed Regio- and Diastereoselective Hydrogen-Evolution C(sp3)-H Phosphorylation of Bicyclo[1.1.0]butanes

Radical-initiated functionalization of bicyclo[1.1.0]butanes (BCBs) is a straightforward approach to accessing diverse cyclobutane derivatives. However, selective C(sp3)-H functionalization at the C2 position of BCBs remains scarce. Herein, a mild protocol for the hydrogen-evolution of C2 C(sp3)-H phosphorylation with BCBs enabled by photoinduced cobaloxime catalysis was realized in a regio- and diastereoselective manner. This oxidant- and additional photocatalyst-free method enabled C(sp3)-H phosphorylation with a wide range of BCBs and diarylphosphine oxides. The mechanism was studied via control experiments and DFT calculation. Moreover, the efficiency of this approach was highlighted in the synthesis of high-value, structurally complex molecules.

Biomimetic Catalytic Remote Desaturation of Aliphatic Alcohols

Herein we present photoinduced cobaloxime-catalyzed selective remote desaturation of aliphatic alcohols. This transformation, which proceeds efficiently at room temperature, facilitates the synthesis of valuable cyclic and acyclic allylic and homoallylic alcohols from readily available saturated aliphatic alcohols. Remarkably, this method obviates the need for external oxidants, noble metal catalysts, and phosphine ligands.

Catalytic acceptorless complete dehydrogenation of cycloalkanes

The advancement of an effective hydrogen liberation technology from liquid organic hydrogen carriers, particularly cycloalkanes such as cyclohexane and methylcyclohexane, holds significance in realizing a hydrogen-centric society. However, the attainment of homogeneous catalytic acceptorless dehydrogenation characterized by elevated selectivity for thorough aromatization under mild conditions remains unrealized. In this study, a catalyst system, facilitated by a double hydrogen atom transfer processes, has been devised for the catalytic acceptorless dehydrogenation of inert cycloalkanes at ambient temperature under visible light irradiation. Through the synergistic utilization of tetrabutylammonium chloride and thiophosphoric acid hydrogen atom transfer catalysts, successful catalytic acceptorless dehydrogenation with comprehensive aromatization has been accomplished with potential liquid organic hydrogen carrier candidates and showcased high functional group tolerance.

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.

Visible-Light-Enabled Catalytic Intramolecular Double Oxidation of Olefins to ortho-Hydroxylactones

We have effectively utilized cost-effective 2-bromoanthraquinone as a photocatalyst to develop an efficient and environmentally friendly method for producing o-hydroxy lactones under mild visible light irradiation. Importantly, this protocol only relies on oxygen as an oxidant, completely eliminating the need for additional chemical reagents and showcasing a sustainable approach to chemical transformation. Operating at room temperature, we utilized a mixed solvent system of DMF and CHCl3, which greatly facilitated the selective conversion of various 2-vinylbenzoic acids and carboxylic acids to functional o-hydroxyl lactones. The process also exhibited excellent diastereoselectivity. Moreover, this versatile strategy is compatible with a wide range of biologically active and complex molecules, offering new opportunities for late-stage structural modifications of these compounds.

Chemoselective Dehydrogenation and Hetero-Arylation of Amides via Radical Translocation Enabled by Photoexcited Triplet Ketone Catalysis

We herein report the chemoselective dehydrogenation and heteroarylation of amides through photoexcited triplet ketone catalysis. Under mild reaction conditions, the generated aryl radical through the halogen atom transfer (XAT) process further undergoes an intramolecular 1,5-HAT event to generate an α-amido alkyl radical, which then intercepted with either cobalt or a reduced cyano arene radical leads to dehydrogenated and heteroarylated products, respectively, in good yields.

Alkenylation of unactivated alkanes: synthesis of Z-alkenes via dual Co-TBADT catalysis

Hydroalkylation of terminal alkynes via C-H activation is the most atom-economical and straightforward method for synthesizing alkenes. They remain confined to using C(sp2)-H or activated C(sp3)-H bonds. A chelating group enabled the alkenylation of C(sp3)-H bonds, resulting in E alkenes. Protocols by which alkenylation of unactivated C(sp3)-H bonds occurs without a chelating group via metal-hydride or radical pathways remain unknown. Our cobalt-HAT catalysis achieves the desired Z alkene with excellent regio- and diastereoselectivity via C-H activation.

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.

Radical-triggered translocation of C-C double bond and functional group

Multi-site functionalization of molecules provides a potent approach to accessing intricate compounds. However, simultaneous functionalization of the reactive site and the inert remote C(sp3)-H poses a formidable challenge, as chemical reactions conventionally occur at the most active site. In addition, achieving precise control over site selectivity for remote C(sp3)-H activation presents an additional hurdle. Here we report an alternative modular method for alkene difunctionalization, encompassing radical-triggered translocation of functional groups and remote C(sp3)-H desaturation via photo/cobalt dual catalysis. By systematically combining radical addition, functional group migration and cobalt-promoted hydrogen atom transfer, we successfully effectuate the translocation of the carbon-carbon double bond and another functional group with precise site selectivity and remarkable E/Z selectivity. This redox-neutral approach shows good compatibility with diverse fluoroalkyl and sulfonyl radical precursors, enabling the migration of benzoyloxy, acetoxy, formyl, cyano and heteroaryl groups. This protocol offers a resolution for the simultaneous transformation of manifold sites.

Synthesis of non-canonical amino acids through dehydrogenative tailoring

Amino acids are essential building blocks in biology and chemistry. Whereas nature relies on a small number of amino acid structures, chemists desire access to a vast range of structurally diverse analogues1-3. The selective modification of amino acid side-chain residues represents an efficient strategy to access non-canonical derivatives of value in chemistry and biology. While semisynthetic methods leveraging the functional groups found in polar and aromatic amino acids have been extensively explored, highly selective and general approaches to transform unactivated C-H bonds in aliphatic amino acids remain less developed4,5. Here we disclose a stepwise dehydrogenative method to convert aliphatic amino acids into structurally diverse analogues. The key to the success of this approach lies in the development of a selective catalytic acceptorless dehydrogenation method driven by photochemical irradiation, which provides access to terminal alkene intermediates for downstream functionalization. Overall, this strategy enables the rapid synthesis of new amino acid building blocks and suggests possibilities for the late-stage modification of more complex oligopeptides.

1,3-Difunctionalization of Alkenes by Cobaloxime Photocatalysis

Represented herein is the first 1,3-difunctionalization of alkenes via photocatalysis. A single cobaloxime is used to carry out two catalytic cycles in which cobaloxime is used not only as a photocatalyst to initiate the reaction but also as a metal catalyst for the β-H elimination process. Electron-deficient alkenes, electron-rich alkenes, and unactivated alkenes could be directly converted to 1,3-bisphosphorylated products, even unsymmetric 1,3-bisphosphorylated products, with only H2 as a byproduct under extremely mild reaction conditions.

Photoredox/Cu-Catalyzed Decarboxylative C(sp3)-C(sp3) Coupling to Access C(sp3)-Rich gem-Diborylalkanes

gem-Diborylalkanes are highly valuable building blocks in organic synthesis and pharmaceutical chemistry due to their ability to participate in multi-step cross-coupling transformations, allowing for the rapid generation of molecular complexity. While progress has been made in their synthetic metholodology, the construction of β-tertiary and C(sp3)-rich gem-diborylalkanes remains a synthetic challenge due to substrate limitations and steric hindrance issues. An approach is presented that utilizes synergistic photoredox and copper catalysis to achieve efficient C(sp3)-C(sp3) cross-coupling of alkyl N-hydroxyphthalimide esters, which can easily be obtained from alkyl carboxylic acids, with diborylmethyl species, providing a series of C(sp3)-rich gem-diborylalkanes with 1°, 2°, and even 3° β positions. Furthermore, this approach can also be applied to complex medicinal compounds and natural products, offering rapid access to molecular complexity and late-stage functionalization of C(sp3)-rich drug candidates. Mechanistic experiments revealed that diborylmethyl Cu(I) species participated in both the photoredox process and the key C(sp3)-C(sp3) bond-forming step.

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.

Stereoselective benzylic C(sp3)-H alkenylation enabled by metallaphotoredox catalysis

Selective activation of the benzylic C(sp3)-H bond is pivotal for the construction of complex organic frameworks. Achieving precise selectivity among C-H bonds with comparable energetic and steric profiles remains a profound synthetic challenge. Herein, we unveil a site- and stereoselective benzylic C(sp3)-H alkenylation utilizing metallaphotoredox catalysis. Various linear and cyclic (Z)-all-carbon tri- and tetrasubstituted olefins can be smoothly obtained. This strategy can be applied to complex substrates with multiple benzylic sites, previously deemed unsuitable due to the uncontrollable site-selectivity. In addition, sensitive functional groups such as terminal alkenyl and TMS groups are compatible under the mild conditions. The exceptional site-selectivity and broad substrate compatibility are attributed to the visible-light catalyzed relay electron transfer-proton transfer process. More importantly, we have extended this methodology to achieve enantioselective benzylic C(sp3)-H alkenylation, producing highly enantioenriched products. The applicability and scalability of our protocol are further validated through late-stage functionalization of complex structures and gram-scale operations, underscoring its practicality and robustness.

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.

Tandem dehydrogenation-olefination-decarboxylation of cycloalkyl carboxylic acids via multifold C-H activation

Dehydrogenation chemistry has long been established as a fundamental aspect of organic synthesis, commonly encountered in carbonyl compounds. Transition metal catalysis revolutionized it, with strategies like transfer-dehydrogenation, single electron transfer and C-H activation. These approaches, extended to multiple dehydrogenations, can lead to aromatization. Dehydrogenative transformations of aliphatic carboxylic acids pose challenges, yet engineered ligands and metal catalysis can initiate dehydrogenation via C-H activation, though outcomes vary based on substrate structures. Herein, we have developed a catalytic system enabling cyclohexane carboxylic acids to undergo multifold C-H activation to furnish olefinated arenes, bypassing lactone formation. This showcases unique reactivity in aliphatic carboxylic acids, involving tandem dehydrogenation-olefination-decarboxylation-aromatization sequences, validated by control experiments and key intermediate isolation. For cyclopentane carboxylic acids, reluctant to aromatization, the catalytic system facilitates controlled dehydrogenation, providing difunctionalized cyclopentenes through tandem dehydrogenation-olefination-decarboxylation-allylic acyloxylation sequences. This transformation expands carboxylic acids into diverse molecular entities with wide applications, underscoring its importance.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

B(C6F5)3-Catalyzed Dehydrogenation of Pyrrolidines to Form Pyrroles

Pyrroles are important N-heterocycles found in medicines and materials. The formation of pyrroles from widely accessible pyrrolidines is a potentially attractive strategy but is an underdeveloped approach due to the sensitivity of pyrroles to the oxidative conditions required to achieve such a transformation. Herein, we report a catalytic approach that employs commercially available B(C6F5)3 in an operationally simple procedure that allows pyrrolidines to serve as direct synthons for pyrroles. Mechanistic studies have revealed insights into borane-catalyzed dehydrogenative processes.

Phenalenyl-Based Photocatalyst for Bioinspired Oxidative Dehydrogenation of N-Heterocycles and Benzyl Alcohols

The environmental benefits of molecular oxygen as the oxidizing agent in oxidation reactions that synthesize fine chemicals cannot be overstated. Increased interest in developing robust photocatalysts is stimulated by the fact that the current photocatalytic transformation boom has made previously inaccessible synthetic approaches possible. Motivated by enzymatic catalysis, employing a reusable phenalenyl-based photocatalyst, we have successfully developed oxidative dehydrogenation utilizing molecular oxygen as a greener oxidant. Under photoinduced oxidative dehydrogenation conditions, different types of saturated N-heterocycles and alcohols were successfully dehydrogenated. The versatility of this bioinspired protocol is demonstrated by the fact that a wide variety of N-heteroaromatics, such as quinoline, carbazole, quinoxaline, acridine, and indole derivatives, as well as aldehydes and ketones, were successfully synthesized. Detailed mechanistic studies validate the proposed mechanism. Fluorescence lifetime and CV experiments revealed the crucial role of water on the efficiency of the reaction. The present protocol also provides chemoselectivity and scalability, leading to superior results and allowing for the functionalization of bioactive molecules at a late stage in a sustainable manner.

Electrochemical Lactonization Enabled by Unusual Shono-Type Oxidation from Functionalized Benzoic Acids

A novel method for electrochemical lactonization via C(sp3)-H functionalization was developed. This metal- and oxidant-free strategy enabled the efficient synthesis of various lactones. Gram-scale reaction and derivatization of the lactone product demonstrated the synthetic utility of this methodology. Mechanistic studies using control experiments and CV curves elucidated the proposed intramolecular HAT and the oxidative cyclization pathway. An unusual Shono-type oxidation was realized through this electrochemical approach, proceeding without a traditional nucleophilic addition process.

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.

Cooperative Fe/Co-Catalyzed Remote Desaturation for the Synthesis of Unsaturated Amide Derivatives

Unsaturated amides represent common functional groups found in natural products and bioactive molecules and serve as versatile synthetic building blocks. Here, we report an iron(II)/cobalt(II) dual catalytic system for the syntheses of distally unsaturated amide derivatives. The transformation proceeds through an iron nitrenoid-mediated 1,5-hydrogen atom transfer (1,5-HAT) mechanism. Subsequently, the radical intermediate undergoes hydrogen atom abstraction from vicinal methylene by a cobaloxime catalyst, efficiently yielding β,γ- or γ,δ-unsaturated amide derivatives under mild conditions. The efficiency of Co-mediated HAT can be tuned by varying different auxiliaries, highlighting the generality of this protocol. Remarkably, this desaturation protocol is also amenable to practical scalability, enabling the synthesis of unsaturated carbamates and ureas, which can be readily converted into various valuable molecules.

Visible-Light-Initiated Air-Oxygenation of Alkylarenes to Carbonyls Mediated by Carbon Tetrabromide in Water

Synthesizing benzyl skeleton derivatives via direct oxidation of functionalized benzylic C-H bonds has received extensive research attention. Herein, a method was developed to prepare carbonyl compounds via photoinduced aerobic oxidation of ubiquitous benzylic C-H bonds mediated by bromine radicals and tribromomethane radicals. This method employed commercially available CBr4 as a hydrogen atom transfer reagent precursor, air as an oxidant, water as a reaction solvent, and tetrabutylammonium perchlorate (TBAPC) as an additive under mild conditions. A series of substrates bearing different functional groups was converted to aromatic carbonyls in moderate to good yields. Moreover, a low environmental factor (E-factor value=0.45) showed that the proposed method is ecofriendly and environmentally sustainable.

Substituent-Determined Intramolecular Hydrogen Transfer for Photopromoted Intermolecular Cycloaddition of Anthraquinones with Aryl Olefins

The formation of intramolecular hydrogen bonds in anthraquinones makes them inert to photoinduced reactions; therefore, it is a great challenge to phototransform these compounds. Herein, we reported a formal visible-light-induced [4 + 2] cycloaddition of both 1-hydroxyanthraquinones and 1-aminoanthraquinones with olefins under external photocatalyst-free conditions with high regioselectivity. More than 60 substrates are disclosed, demonstrating the reliability of this protocol to construct diverse functionalized anthraquinone derivatives.

Oxidative Dehydrogenation of Alkanes through Homogeneous Base Metal Catalysis

Preparing valuable olefins from cheap and abundant alkane resources has long been a challenging task in organic synthesis, which mainly suffers from harsh reaction conditions and narrow scopes. Homogeneous transition metals catalyzed dehydrogenation of alkanes has attracted much attention for its excellent catalytic activities under relatively milder conditions. Among them, base metal catalyzed oxidative alkane dehydrogenation has emerged as a viable strategy for olefin synthesis for its usage of cheap catalysts, compatibility with various functional groups, and low reaction temperature. In this review, we discuss recent development of base metal catalyzed alkane dehydrogenation under oxidative conditions and their application in constructing complex molecules.

Aromatization of cyclic hydrocarbons via thioether elimination reaction

Herein, the diversity-oriented aromatization of cyclic hydrocarbons via potassium ethyl xanthogenate (EtOCS2K)/NH4I-mediated methylthiyl radical addition and thioether elimination was investigated under transition-metal-free conditions. The methylthiyl radical species were generated in situ via the NH4I-mediated decomposition of DMSO following which EtOCS2K promoted the breaking of carbon-sulfur bonds of thioether.

Stereodivergent Zweifel Olefination and its Mechanistic Dichotomy

Stereoselective Zweifel olefination using boronate complexes carrying two different reactive π-systems was achieved to synthesize vinyl heteroarenes and conjugated 1,3-dienes in good yield and up to 100 % stereoselectivity, which remains unexplored until now. Most importantly, we report the unprecedented formation of E vs. Z-vinyl heteroarenes for different heteroarenes under identical conditions. Density functional theory (DFT) investigations unveil the mechanistic dichotomy between olefin and heteroarene activation followed by 1,2-migration, leading to E or Z-vinyl heteroarenes respectively. We also report a previously unknown reversal of stereoselectivity by using 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an electrophile. The Zweifel olefination using a boronate complex that carries two different olefins was previously unexplored due to significant challenges associated with the site-selective activation of olefins. We have solved this problem and reported the site-selective activation of olefins for the stereoselective synthesis of 1,3-dienes.

Azido-alkynylation of alkenes through radical-polar crossover

We report an azido-alkynylation of alkenes allowing a straightforward access to homopropargylic azides by combining hypervalent iodine reagents and alkynyl-trifluoroborate salts. The design of a photocatalytic redox-neutral radical polar crossover process was key to develop this transformation. A variety of homopropargylic azides possessing electron-rich and -poor aryls, heterocycles or ether substituents could be accessed in 34-84% yield. The products are synthetically useful building blocks that could be easily transformed into pyrroles or bioactive amines.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Direct synthesis of alkylated 4-hydroxycoumarin derivatives via a cascade Cu-catalyzed dehydrogenation/conjugate addition sequence

An efficient approach for the direct synthesis of alkylated 4-hydroxycoumarin derivatives via a Cu-catalyzed cascade dehydrogenation/conjugate addition sequence starting from simple saturated ketones and 4-hydroxycoumarins has been developed. This protocol features excellent functional-group tolerance, easy scale-up, and a broad substrate scope including bioactive molecules. More importantly, a series of marketed drugs, such as warfarin, acenocoumarol, coumachlor, and coumafuryl, can be obtained by this method.

Sequential Modifications of Metal-Organic Layer Nodes for Highly Efficient Photocatalyzed Hydrogen Atom Transfer

Herein, we report the synthesis of a bifunctional photocatalyst, Zr-OTf-EY, through sequential modifications of metal cluster nodes in a metal-organic layer (MOL). With eosin Y and strong Lewis acids on the nodes, Zr-OTf-EY catalyzes cross-coupling reactions between various C-H compounds and electron-deficient alkenes or azodicarboxylate to afford C-C and C-N coupling products, with turnover numbers of up to 1980. In Zr-OTf-EY-catalyzed reactions, Lewis acid sites bind the alkenes or azodicarboxylate to increase their local concentrations and electron deficiency for enhanced radical additions, while EY is stabilized by site isolation on the MOL to afford a long-lived catalyst for hydrogen atom transfer. The proximity between photostable EY sites and Lewis acids on the nodes of Zr-OTf-EY enhances the catalytic efficiency by approximately 400 times over the homogeneous counterpart in the cross-coupling reactions.

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

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.

Multiple remote C(sp3)-H functionalizations of aliphatic ketones via bimetallic Cu-Pd catalyzed successive dehydrogenation

The dehydrogenation-triggered multiple C(sp3)-H functionalizations at remote positions γ, δ or ε, ζ to carbonyl groups of aliphatic ketones with aryl/alkenyl carboxylic acids as coupling partners have been achieved using a bimetallic Cu-Pd catalyst system. This reaction allows access to alkenylated isocoumarins and their derivatives in generally good yields with high functional group tolerance. The identification of bimetallic Cu-Pd synergistic catalysis for efficient successive dehydrogenation of aliphatic ketones, which overcomes the long-standing challenge posed by the successive dehydrogenation desaturation of terminally unsubstituted alkyl chains in aliphatic ketones, is essential to achieving this bimetallic Cu-Pd catalyzed dehydrogenation coupling reaction.

Acceptorless Dehydrogenation of Aliphatics, Amines, and Alcohols with Homogeneous Catalytic Systems

The dehydrogenation of saturated substrates is fundamentally essential for producing value-added unsaturated organic molecules both in academia and industry. In recent years, homogeneously catalyzed acceptorless C–C, C–N, and C–O bond desaturations have attracted increasing attention due to high atom economy, environmentally benign nature, and wide availability of the starting materials. This short review discusses the acceptorless dehydrogenation of aliphatics, alcohols, and amines by homogeneous catalytic systems based on two categories of reaction mechanisms: thermal transition-metal-catalyzed two-electron pathway and photoredox catalyzed or electrochemically driven one-electron pathway.

1 Introduction

2 Catalytic Acceptorless Dehydrogenation of Aliphatics

3 Catalytic Acceptorless Dehydrogenation of Amines

4 Catalytic Acceptorless Dehydrogenation of Alcohols

5 Conclusion

Selective dehydrogenation of small and large molecules by a chloroiridium catalyst

The dehydrogenation of abundant alkane feedstocks to olefins is one of the mostly intensively investigated reactions in organic catalysis. A long-standing, pervasive challenge in this transformation is the direct dehydrogenation of unactivated 1,1-disubstituted ethane, an aliphatic motif commonly found in organic molecules. Here, we report the design of a diphosphine chloroiridium catalyst for undirected dehydrogenation of this aliphatic class to form valuable 1,1-disubstituted ethylene. Featuring high site selectivity and excellent functional group compatibility, this catalytic system is applicable to late-stage dehydrogenation of complex bioactive molecules. Moreover, the system enables unprecedented dehydrogenation of polypropene with controllable degree of desaturation, dehydrogenating more than 10 in 100 propene units. Further derivatizations of the resulting double bonds afford functionalized polypropenes.

Energy- and atom-efficient chemical synthesis with endergonic photocatalysis

Endergonic photocatalysis is the use of light to perform catalytic reactions that are thermodynamically unfavourable. While photocatalysis has become a powerful tool in facilitating chemical transformations, the light-energy efficiency of these processes has not gathered much attention. Exergonic photocatalysis does not take full advantage of the light energy input, producing low-energy products and heat, whereas endergonic photocatalysis incorporates a portion of the photon energy into the reaction, yielding products that are higher in free energy than the reactants. Such processes can enable catalytic, atom-economic syntheses of reactive compounds from bench-stable materials. With respect to environmental friendliness and carbon neutrality, endergonic photocatalysis is also of interest to large-scale industrial manufacturing, where better energy efficiency, less waste and value addition are highly sought. We therefore assess here the thermochemistry of several classes of reported photocatalytic transformations to showcase current advances in endergonic photocatalysis and point to their industrial potential.