Catalytic dehydrogenative cross-coupling: forming carbon-carbon bonds by oxidizing two carbon-hydrogen bonds

Copper-Catalyzed Borrowing Hydrogen Reaction for α-Alkylation of Amides with Alcohols

We report the first example of copper-catalyzed α-alkylation of acetamides with alcohols via a borrowing hydrogen strategy. Catalyzed by the in situ-generated copper particles, acetamides and various substituted benzyl or alkyl alcohols were transformed into functionalized amides in good yields with excellent selectivity. Compared with previous work, this process is simple using commercially available Cu(OAc)2 as a precatalyst, without an additional ligand or a metal complex, and easier. Mechanistic studies revealed that aldehyde and α,β-unsaturated amides were the intermediates of this reaction and also disclosed the role of copper in alcohol dehydrogenation and C═C bond hydrogenation.

C-H Bond Sulfonylation from Thianthrenium Salts and DABCO·(SO2)2: Synthesis of 2-Sulfonylindoles

A three-component reaction of 1-(1H-indol-1-yl)isoquinolines or 1-(pyridin-2-yl)-1H-indoles, DABCO·(SO2)2, and thianthrenium salts under synergistic photoredox and palladium catalysis is accomplished. This direct C-H bond sulfonylation of indoles with the insertion of sulfur dioxide under mild conditions works efficiently, giving rise to a wide range of 2-sulfonated indoles in moderate to good yields under mild conditions. In this protocol, the generality of aryl/alkyl thianthrenium salts is demonstrated as well. A photoredox radical process combined with palladium catalysis is proposed.

Enantioselective nickel-catalyzed anodic oxidative dienylation and allylation reactions

Precision control of stereochemistry in radical reactions remains a formidable challenge due to the prevalence of incidental racemic background reactions resulting from undirected substrate oxidation in the absence of chiral induction. In this study, we devised an thoughtful approach-electricity-driven asymmetric Lewis acid catalysis-to circumvent this impediment. This methodology facilitates both asymmetric dienylation and allylation reactions, resulting in the formation of all-carbon quaternary stereocenters and demonstrating significant potential in the modular synthesis of functional and chiral benzoxazole-oxazoline (Boox) ligands. Notably, the involvement of chiral Lewis acids in both the electrochemical activation and stereoselectivity-defining radical stages offers innovative departures for designing single electron transfer-based reactions, significantly underscoring the relevance of this approach as a multifaceted and universally applicable strategy for various fields of study, including electrosynthesis, organic chemistry, and drug discovery.

Palladium-Catalyzed Arylation of C(sp2)-H Bonds and C(sp3)-H Bonds with 4-Amino-benzotriazole as the Bidentate Directing Group

The arylation of C(sp2)-H and C(sp3)-H bonds in carboxylic acids catalyzed by Pd(II) with 4-aminobentriazole as the directing group was investigated. In addition to activation of the C(sp2)-H bond, selective arylation of alkyl carboxylic acids and amino acids in the β position can also be achieved. This strategy involved a 5,5-bicyclic Pd intermediate complex whose structure was determined by X-ray single crystal diffraction analysis. Importantly, the DG (directing group) can be easily removed under mild conditions.

Generation of New Synthons for Synthesis Through Activation of Nitromethane

Nitromethane is used as a common solvent, stabilizer, and fuel additive. Nitromethane has also been used as a sustainable building block and convenient reagent in chemical synthesis. In this Minireview, we summarize the recent advances in activation of nitromethane, using nitromethane as the source of cyano group, nitrogen, methylamine, formyl group, C1, nitroso, and oxime.

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.

Recent advances in cross-coupling of alcohols via borrowing hydrogen catalysis

Application of the borrowing hydrogen strategy facilitates utilization of abundantly available alcohols for linear or branched long-chain alcohols. Selective synthesis of such alcohols is highly challenging and involves the utilization of transition metal catalysts towards the desired cross-coupled product. Herein, we have highlighted recent advances (from 2015 to 2023) towards the synthesis of higher alcohols. Major focus has been given to the development of ligands, including transition metal catalysts. Judicious catalyst design plays a key role in the alkylation process and is summarised in this review.

Tandem Reactions of Electrophilic Indoles toward Indolizines and Their Subsequent Transformations through Pd(II)-Mediated C-H Functionalization to Access Polyring-Fused N-Heterocycles

A simple and efficient synthetic approach for generating a library of structurally novel indolizines has been developed via sequential 1,3-dipolar cycloaddition-ring opening processes. Using this methodology, a series of indolizines bearing different substituents were made in moderate to good yields. The presence of two functionalizable C-H bonds in these indolizine motifs makes them attractive for accessing fused indolizine scaffolds. In this line, we have introduced palladium-mediated site-selective C-H functionalizations, where the N-center and the two C-H centers of the indolizine moiety can be readily functionalized to generate fused N-heterocycles. Utilizing a Pd-mediated dual C-H activation of 5-benzoyl-substituted indolizine afforded 6H-indeno-indolizine, and a tetracene, viz., indolizino[2,1-b]indoles, was produced in the same substrate by the Pd-catalyzed selective C-H amination in the presence of oxygen.

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

Controlling the transformation of versatile and reactive allenes is a considerable challenge. Herein, we report an efficient silylboronate-mediated cross-coupling reaction of organic fluorides with allenes to construct a series of sterically demanding α-ethynyl-containing all-carbon quaternary centers (ACQCs), using catalyst-free silyl-radical-relay reactions to selectively functionalize highly inert C-F bonds in organic fluorides. The key to the success of this transformation lies in the radical rearrangement of an in situ-generated allenyl radical to form a bulky tertiary propargyl radical; however, the transformation does not show efficiency when using the propargyl isomer directly. This unique reaction enables the cross-coupling of a tertiary carbon radical center with a C(sp2)-F bond or a benzylic C(sp3)-F bond. α-Ethynyl-containing ACQCs with (hetero)aromatic substituents and benzyl were efficiently synthesized in a single step using electronically and sterically diverse organic fluorides and allenes. The practical utility of this protocol is showcased by the late-stage functionalization of bioactive molecules and the modification of a liquid crystalline material.

Metal free cross-dehydrogenative N-N coupling of primary amides with Lewis basic amines

Hydrazides, N-N containing structural motifs, are important due to their presence in a wide variety of biologically significant compounds. While the homo N-N coupling of two NH moieties to form the hydrazide N-N bond is well developed, the cross-dehydrogenative hetero N-N coupling remains very unevolved. Here we present an efficient intermolecular N-N cross-coupling of a series of primary benzamides with broad range of Lewis basic primary and secondary amines using PhI(OAc)2 as both a terminal oxidant and a cross-coupling mediator, without the need for metal catalysts, high temperatures, and inert atmospheres, and with substantial potential for use in the late-stage functionalization of drugs.

Electroreductive Remote Benzylic C(sp3)-H Arylation of Aliphatic Ethers Using Cyanoarenes for the Synthesis of α-(Hetero)aryl Ethers

An iodoarene-driven electroreductive remote C(sp3)-H arylation of unsymmetrical 1-(o-iodoaryl)alkyl ethers with cyanoarenes for the site selective synthesis of α-(hetero)aryl ethers is developed. With the introduction of cyanoarenes as both aryl sources and electron transfer mediators, this method includes an iodoarene-driven strategy to enable the regiocontrollable formation of two new bonds, one C(sp2)-H bond, and one C(sp2)-C(sp3) bond, in a single reaction step through the sequence of halogen atom transfer (XAT), hydrogen atom transfer (HAT), radical-radical coupling, and decyanation.

A Practical and Regioselective Strategy for Aromatic C-H Difunctionalization via Site-Selective C-H Thianthrenation

Herein, we present a novel Catellani-type reaction that employed aryl-thianthrenium salts as aryl substrates to trigger the subsequent palladium/norbornene cooperatively catalyzed progress. This strategy can achieve site-selective C-H difunctionalization of aryl compounds without directing groups or a known initiating reagent. A series of functionalized syntheses of bioactive molecules further demonstrated the potential of this strategy.

Photoinduced Radical Desulfurative C(sp3)-C(sp2) Coupling via Electron Donor-Acceptor Complexes

Herein, we disclose a radical desulfurative C-C coupling protocol for the synthesis of 4-alkylpyridines. A variety of substituents on both benzyl thiols and 4-cyanopyridines are tolerated. The reaction is carried out under mild and photocatalyst- and transition-metal-free conditions. Preliminary mechanistic studies show that an electron donor-acceptor complex is formed between benzyl thiols and 4-cyanopyridines under alkaline conditions. Then, a variety of 1°, 2°, and 3° C(sp3)-centered radicals was formed by cleavage of the C-S bond, and the 4-alkylpyridines were achieved through a radical-radical coupling with the pyridyl radical anion.

Recent advances in oxidative phenol coupling for the total synthesis of natural products

Covering: 2008 to 2023This review will describe oxidative phenol coupling as applied in the total synthesis of natural products. This review covers catalytic and electrochemical methods with a brief comparison to stoichiometric and enzymatic systems assessing their practicality, atom economy, and other measures. Natural products forged by C-C and C-O oxidative phenol couplings as well as from alkenyl phenol couplings will be addressed. Additionally, exploration into catalytic oxidative coupling of phenols and other related species (carbazoles, indoles, aryl ethers, etc.) will be surveyed. Future directions of this particular area of research will also be assessed.

Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3  ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

Hole-Transporting Materials based on Oligo(hetero)aryls with a Naphthodithiophene Core - Succinct Synthesis by Twofold Direct C-H Olefination

This work demonstrated the first synthetic application of direct C-H olefinations in the step-saving preparation of various hole-transporting materials (HTM) for efficient perovskite solar cells (PSC). Cross-dehydrogenative couplings of naphthodithiophene (NDT) with vinyl arenes under palladium-catalysis facilely generated various new oligo(hetero)aryls with internal alkenes. Reaction conditions were optimized, which gave the product isolated yields of up to 71 % with high (E)-stereoselectivity. These readily accessible NDT core-based small molecules involving olefin as π-spacers displayed immediate power conversion efficiencies of up to 17.2 % without a device oxidation process that is required for the commercially available spiro-OMeTAD and most other existing HTMs while fabricated in corresponding PSC devices.

A Catalytic Method to Activate Nitromethane by the Cooperation of Homo- and Heterogeneous Catalysis

The achievement of directly activating and utilizing bulk small molecules has remained a longstanding objective in the field of chemical synthesis. The present work reports a catalytic activation method for bulk chemical nitromethane (MeNO2 ). This method combines homogeneous Lewis acid with recyclable heterogeneous Brønsted acid catalysis, featuring practicality, sustainability, and low cost, thus solving the inherent drawbacks of previous Nef processes where stoichiometric reductants or activators were required. By combining the advantages of both homo- and heterogeneous catalysts, this chemistry may not only offer new opportunities for the further development of MeNO2 as a nitrogen source for organic synthesis, but also promote the catalysis design in synthetic chemistry.

Half-sandwich ruthenium complexes with acylhydrazone ligands: synthesis and catalytic activity in the N-alkylation of hydrazides

Novel half-sandwich ruthenium complexes termed [(p-cymene)RuClL] were synthesized by chelating arylhydrazone ligands with [(p-cymene)RuCl2]2 and were then fully characterized using different spectroscopic and analytical techniques. The crystal structure of complex 4 indicated that the hydrazone ligands bonded to the ruthenium ion in a bidentate manner through the imine nitrogen and imidazolate oxygen, exhibiting a pseudo-octahedral geometry centered by the ruthenium atom. The as-fabricated air and moisture stable half-sandwich ruthenium complexes demonstrated excellent catalytic activity towards the N-alkylation of hydrazides under mild conditions. Under the catalysis of ruthenium complexes, acyl hydrazides were reacted with different types of alcohols in a one-pot reaction, resulting in N-alkylation hydrazides with different substituents. This catalyst exhibited characteristics such as high catalytic efficiency, broad substrate scope, and mild reaction conditions, indicating that it has great potential for industrial applications.

NNN manganese complex-catalyzed α-alkylation of methyl ketones using alcohols: an experimental and computational study

We present here a phosphine-free, quinoline-based pincer Mn catalyst for α-alkylation of methyl ketones using primary alcohols as alkyl surrogates. The C-C bond formation reaction proceeds via a hydrogen auto-transfer methodology. The sole by-product formed is water, rendering the protocol atom efficient. Electronic structure theory studies corroborated the proposed mechanism.

Discovery of Organic Optoelectronic Materials Powered by Oxidative Ar-H/Ar-H Coupling

Efficient and streamlined synthetic methods that facilitate the rapid build-up of structurally diverse π-conjugated systems are of paramount importance in the quest for organic optoelectronic materials. Among these methods, transition-metal-catalyzed oxidative Ar-H/Ar-H coupling reactions between two (hetero)arenes have emerged as a concise and effective approach for generating a wide array of bi(hetero)aryl and fused heteroaryl structures. This innovative approach bypasses challenges associated with substrate pre-activation processes, thereby allowing for the creation of frameworks that were previously beyond reach using conventional Ar-X/Ar-M coupling reactions. These inherent advantages have ushered in new design patterns for organic optoelectronic molecules that deviate from traditional methods. This ground-breaking approach enables the transcendence of the limitations of repetitive material structures, ultimately leading to the discovery of novel high-performance materials. In this Perspective, we provide an overview of recent advances in the development of organic optoelectronic materials through the utilization of transition-metal-catalyzed oxidative Ar-H/Ar-H coupling reactions. We introduce several notable synthetic strategies in this domain, covering both directed and non-directed oxidative Ar-H/Ar-H coupling strategies, dual chelation-assisted strategy and directed ortho-C-H arylation/cyclization strategy. Additionally, we shed light on the role of oxidative Ar-H/Ar-H coupling reactions in the advancement of high-performance organic optoelectronic materials. Finally, we discuss the current limitations of existing protocols and offer insights into the future prospects for this field.

Selenated NHC-Pd(II) Pincer Complex Catalyzed, Temperature-Dependent Selective Hydroamination and Oxidative Amination of Olefins: Formation of Enamine Esters and β-Amino Esters under Solvent-Free and Aerobic Conditions

NHC-Pd(II) pincer catalyzed oxidative amination and hydroamination of olefins is developed under solvent-free aerobic conditions. Reaction offered a temperature-controlled synthesis of (Z)-enamine and β-amino esters to provide easy access and remarkable functional group tolerance for a variety of enamines. The developed approach renders an opportunity of scalability and flexibility, and besides this, the produced enamines can be transformed into many N-containing heterocycles, underscoring its potential usage in synthetic and pharmaceutical chemistry. Moreover, it is the first report for coupling of aniline with styrene.

Pyrimidine-morpholine hybrids as potent druggable therapeutics for Alzheimer's disease: Synthesis, biochemical and in silico analyses

The identification of effective and druggable cholinesterase inhibitors to treat progressive neurodegenerative Alzheimer's disorder remains a continuous drug discovery hunt. In this perspective, the present study investigates the design and discovery of pyrimidine-morpholine hybrids (5a-l) as potent cholinesterase inhibitors. Palladium-catalyzed Suzuki-Miyaura cross-coupling reaction was employed to introduce the structural diversity on the pyrimidine heterocyclic core. A range of commercially available boronic acids was successfully coupled showing a high functional group tolerance. In vitro cholinesterase inhibitory potential using Ellman's method revealed significantly strong potency. Compound 5h bearing a meta-tolyl substituent at 2-position of pyrimidine ring emerged as a lead candidate against AChE with an inhibitory potency of 0.43 ± 0.42 µM, ∼38-fold stronger value than neostigmine (IC50 = 16.3 ± 1.12 µM). Compound 5h also showed the lead inhibition against BuChE with an IC50 value of 2.5 ± 0.04 µM. The kinetics analysis of 5h revealed the non-competitive mode of inhibition against AChE whereas computational modelling results of potent leads depicted diverse contacts with the binding site amino acid residues. Molecular dynamics simulations revealed the stability of biomolecular system, while, ADME analysis demonstrated druglikeness behaviour of potent compounds. Overall, the investigated pyrimidine-morpholine scaffold presented a remarkable potential to be developed as efficacious anti-Alzheimer's drugs.

K2S2O8 promoted metal-free direct C-alkylation of acetophenones with alcohols

Herein, we report a metal-free synthetic methodology for the C-alkylation of acetophenones following a hydrogen borrowing-like pathway using the commercially available inorganic oxidant K2S2O8 in conjunction with KOtBu. This study articulates the potential of K2S2O8 in fast initiation of the oxidation of benzyl alcohols to develop an atom-economical, easy, and more efficient methodology for the C-alkylation of various acetophenones and synthesis of a variety of substituted quinolines. Experimental data from control experiments, literature and characterization of intermediates through spectroscopic techniques support the proposed plausible mechanism.

Unusual Kinetics Induced by Ligands in Rhodium(III)-Catalyzed Dehydrogenative Olefination Reactions

In this study, we investigate the effects of ligands on C-H activation during rhodium(III)-catalyzed C-H bond olefination reactions using well-defined [CpXRhIII] catalytic systems with three representative CpX (Cp (η5-C5H5), CpCF3 (η5-C5Me4CF3), and Cp* (η5-C5Me5)) ligands. Our results demonstrate that C-H activation as the rate-limiting step is significantly influenced by the steric properties of the CpX ligands. Moreover, we observe a dramatic acceleration of the simple [CpRhIII]-catalyzed C-H olefination reaction with acid coproducts such as HOAc, implying an autocatalytic C-H activation process.

Study on the Mechanism of Ru-Catalyzed Cyclization of Aromatic Amides with Allylphosphine Oxides

The mechanism of Ru-catalyzed cyclization of aromatic amides with allylphosphine oxides is studied by density functional theory calculation (DFT). The results show that, first, a 5-membered Ru ring intermediate is formed by N-H and C-H diprotons via the concerted metalation-deprotonation mechanism (CMD) and then the allylphosphine oxide is inserted through the ring-extending reaction to form a 7-membered ring intermediate. Next, reduction elimination is followed via intramolecular hydrogen transfer isomerization. At last, with the assistance of acetic acid, Ru (II) → Ru (IV) → Ru (II) complexes occur from the 7-membered Ru ring intermediate, and the final product is formed by reduction elimination and protonation reaction, while the catalyst is released to participate in the next cycle.

Photoredox/Nickel Dual Catalysis-Enabled Cross-Dehydrogenative C-H Amination of Indoles with Unactivated Amine

Herein, a direct cross-dehydrogenative C-H amination of indoles has been successfully achieved, enabled by the merger of photocatalysis with nickel catalysis. This developed process does not require stoichiometric oxidants and prefunctionalization of amine partners, providing a concise platform for C-N bond formation. Moreover, the synthetic practicality of this transformation was well revealed by its high step- and atom-economy, high reaction efficiency, and broad functional group tolerance.

Organic photoredox catalyzed C(sp3)-H functionalization of saturated aza-heterocycles via a cross-dehydrogenative coupling reaction

Herein we present a novel protocol to access α-functionalized saturated aza-heterocycles, and a variety of nucleophilic groups, such as indole, naphthol, phenol, pyrrole, furyl, nitromethyl, and cyano, could be easily installed into saturated aza-heterocycles. Furthermore, a range of biologically valuable 3,3'-diindolylmethane derivatives could also be readily accessed under mild photocatalytic conditions.

Ir-Catalyzed B(3)-Amination of o-Carboranes with Amines via Acceptorless Dehydrogenative BH/NH Cross-Coupling

An efficient and convenient strategy for Ir-catalyzed selective B(3)-amination of o-carboranes with amines via acceptorless BH/NH dehydrocoupling was developed, affording a series of B(3)-aminated-o-carboranes in moderate to high isolated yields with H2 gas as a sole by-product. Such an oxidant-free system endues the protocol sustainability, atom-economy and environmental friendliness. A reaction mechanism via an Ir(I)-Ir(III)-Ir(I) catalytic cycle involving oxidative addition, dehydrogenation and reductive elimination was proposed.

Visible light-triggered selective C(sp2)-H/C(sp3)-H coupling of benzenes with aliphatic hydrocarbons

The direct and selective coupling of benzenes with aliphatic hydrocarbons is a promising strategy for C(sp2)-C(sp3) bond formation using readily available starting materials, yet it remains a significant challenge. In this study, we have developed a simplified photochemical system that incorporates catalytic amounts of iron(III) halides as multifunctional reagents and air as a green oxidant to address this synthetic problem. Under mild conditions, the reaction between a strong C(sp2)-H bond and a robust C(sp3)-H bond has been achieved, affording a broad range of cross-coupling products with high yields and commendable chemo-, site-selectivity. The iron halide acts as a multifunctional reagent that responds to visible light, initiates C-centered radicals, induces single-electron oxidation to carbocations, and participates in a subsequent Friedel-Crafts-type process. The gradual release of radical species and carbocation intermediates appears to be critical for achieving desirable reactivity and selectivity. This eco-friendly, cost-efficient approach offers access to various building blocks from abundant hydrocarbon feedstocks, and demonstrates the potential of iron halides in sustainable synthesis.

Palladium-Catalyzed Direct Selanylation of Chalcogenophenes and Arenes Assisted by 2-(Methylthio)amide

The direct and selective conversion of a C-H bond into a C-Se bond remains a significant challenge, which is even more intricate with substrates having an innate regioselectivity under several reaction conditions, such as chalcogenophenes. We overrode their selectivity toward selanylation using palladium, copper, and the 2-(methylthio)amide directing group. This chelation-assisted direct selanylation was also suitable for mono and double ortho functionalization of arenes. The mechanistic studies indicate high-valent Pd(IV) species in the catalytic cycle, a reversible C-H activation step, and Cu(II) as a sequestering agent for organoselenide byproducts.

Metal-free photoinduced C(sp3)-H/C(sp3)-H cross-coupling to access α‑tertiary amino acid derivatives

The cross-dehydrogenative coupling (CDC) reaction is the most direct and efficient method for constructing α-tertiary amino acids (ATAAs), which avoids the pre-activation of C(sp3)-H substrates. However, the use of transition metals and harsh reaction conditions are still significant challenges for these reactions that urgently require solutions. This paper presents a mild, metal-free CDC reaction for the construction of ATAAs, which is compatible with various benzyl C-H substrates, functionalized C-H substrates, and alkyl substrates, with good regioselectivity. Notably, our method exhibits excellent functional group tolerance and late-stage applicability. According to mechanistic studies, the one-step synthesized and bench-stable N-alkoxyphtalimide generates a highly electrophilic trifluoro ethoxy radical that serves as a key intermediate in the reaction process and acts as a hydrogen atom transfer reagent. Therefore, our metal-free and additive-free method offers a promising strategy for the synthesis of ATAAs under mild conditions.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

Visible-light promoted photoredox catalysis in flow: addition of biologically important α‑amino radicals to michael acceptors

Visible light promoted photoredox catalyzed formation of α-amino radicals from cyclic tertiary amine compounds and their subsequent addition to Michael acceptors performed in flow conditions allowed access to a wide range of functionalized N-aryl-substituted tetrahydroisoquinolines (THIQs) and N-aryl-substituted tetrahydro-β-carbolines (THBCs). Visible light in conjunction with Ru(bpy)3Cl2 photocatalyst allowed the formation and high reactivities of α-amino radicals in flow conditions at room temperature. These reactions gave valuable products with high efficiencies; some previously unavailable reaction pathways photo or thermal reaction conditions; i.e. direct synthesis of 1-substituted (THBCs) via α-amino radical path were successfully realized in flow. The use of custom-made FEP tube microreactor proved to be the key to succesfull α-amino-radical formation and overall reaction performance in flow. Three types of light transparent custom-made microfluidic devices were tested, among them glass/silicon and FEP type reactor showed very good results in the conversion of tested compounds. Plausible reaction mechanism is proposed in accordance with known principles of photo activation of tertiary amines. Visible light promoted C(sp3)-H functionalization of N-aryl-protected tetrahydroisoquinolines and N-aryl-protected tetrahydro-β-carbolines in microflow conditions via a-amino radical pathway with various coupling partners in excellent yields and efficiencies.

Recent advances in oxidative catalytic applications of polyoxovanadate-based inorganic-organic hybrids

Polyoxovanadates (POVs) have received widespread attention in catalytic applications due to their various structures and remarkable redox properties. By introducing a second transition metal, POV-based inorganic-organic hybrid (POVH) catalysts show increasing stability and more catalytic active sites compared with pure POVs. In this perspective article, POVH materials as oxidative catalysts have been classified into two main categories according to the interactions between transition metal-complex units and POV clusters: (i) hybrids with metal-organic units act as isolated cations and (ii) hybrids with an organic ligand coordinate to the second transition metal, which is further linked to a POV cluster via oxygen bridges directly or indirectly to give zero-, one-, two- or three-dimensional supramolecular structures. The oxidative conversion of organic compounds, including thiophene derivatives, thioethers, alkanes, alcohols, and alkenes, and oxidative detoxification of a sulfur mustard simulant or degradation of lignin, along with the oxidative photo/electrocatalytic transformation of organic compounds catalyzed by POVH materials, are discussed in detail. Furthermore, the challenges and prospects toward the development of POVH catalysts are explored briefly from our perspectives.

Regioselective Formal β-Allylation of Carbonyl Compounds Enabled by Cooperative Nickel and Photoredox Catalysis

The Tsuji-Trost reaction between carbonyl compounds and allylic precursors has been widely used in the synthesis of natural products and pharmaceutical compounds. As the α-C-H bond is far more acidic than the β-C-H bond, carbonyl compounds undergo highly regioselective allylation at the α-position and their β-allylation is therefore highly challenging. This innate α-reactivity conversely hampers diversity, especially if the corresponding β-allylation product is targeted. Herein, we present a formal intermolecular β-C-C bond formation reaction of a broad range of aldehydes and ketones with different allyl electrophiles through cooperative nickel and photoredox catalysis. β-Selectivity is achieved via initial transformation of the aldehydes and ketones to their corresponding silyl enol ethers. The overall transformation features mild conditions, excellent regioselectivity, wide functional group tolerance and high reaction efficiency. The introduced facile and regioselective β-allylation of carbonyl compounds proceeding through cooperative catalysis allows the preparation of valuable building blocks that are difficult to access from aldehydes and ketones using existing methodology.

Non-noble metal-catalyzed cross-dehydrogenation coupling (CDC) involving ether α-C(sp3)-H to construct C-C bonds

Ether derivatives are widespread as essential building blocks in various drugs, natural products, agrochemicals, and materials. Modern economy requires developing green strategies with improved efficiency and reduction of waste. Due to its atom and step-economy, the cross-dehydrogenative coupling (CDC) reaction has become a major strategy for ether functionalization. This review covers C-H/C-H cross-coupling reactions of ether derivatives with various C-H bond substrates via non-noble metal catalysts (Fe, Cu, Co, Mn, Ni, Zn, Y, Sc, In, Ag). We discuss advances achieved in these CDC reactions and hope to attract interest in developing novel methodologies in this field of organic chemistry.

Dehydrogenative Cross-Coupling of α,β-Unsaturated Compounds with Unactivated Olefins via Co(III) Catalysis

An oxidative cross-coupling of α,β-unsaturated compounds with unactivated alkenes via cobalt-catalyzed vinylic C-H activation has been developed. The present catalytic reaction was examined with various differently functionalized unsaturated compounds and unactivated olefins. In these reactions, highly valuable amide functionalized butadienes and indenones were prepared in good to excellent yields. A possible reaction mechanism is proposed involving directed olefinic C-H activation through a base-assisted deprotonation pathway.

Phosphine-Catalyzed Allylic Alkylation of (Hetero)Aryl Alkynes with Pronucleophiles: Concise Total Synthesis of (±)-Esermethole

Allylic alkylations are valuable in the construction of versatile carbon-carbon bonds, which are mostly catalyzed by noble transition metals with additional waste byproduct generation. Here, we present the first organophosphine-catalyzed allylic alkylation of (hetero)aryl alkynes with various carbo-nucleophiles. The methodology is highly atom economical and compatible with a wide substrate scope (more than 38 examples). Moreover, the reaction could be easily scaled up, and deuterium labeling experiments have been conducted to elucidate the plausible mechanism. Finally, the protocol has been utilized to achieve the concise total synthesis of natural product (±)-esermethole.

Asymmetric Hydrative Aldol Reaction (HAR) via Vinyl-Gold Promoted Intermolecular Ynamide Addition to Aldehydes

Herein, we reported an intermolecular asymmetric hydrative aldol reaction through vinyl-gold intermediate under ambient conditions. This tandem alkyne hydration and sequential nucleophilic addition afforded a "base-free" approach to β-hydroxy amides with high efficiency (up to 95 % yields, >50 examples). Vinyl gold intermediate was applied as reactive nucleophile and Fe(acac)3 was used as the critical co-catalyst to prevent undesired protodeauration, allowing this transformation to proceed under mild conditions with good functional group tolerance and excellent stereoselectivity (>20 : 1 d.r. and up to 99 % ee).

Selective Cross-Dehydrogenative Coupling of Various Acyclic Enamides with Heteroarenes via Rh(III)-Catalyzed C-H Activation

The developed methodology describes an efficient Rh(III)-catalyzed oxidative C-H/C-H cross-coupling between acyclic enamides and heteroarenes. This cross dehydrogenative coupling (CDC) reaction offers advantages, including excellent regioselectivity and stereoselectivity, good functional group compatibility, and a broad substrate scope. Mechanistically, Rh(III)-catalyzed β-C(sp2)-H activation of acyclic enamides is proposed to be the critical step.

Pd(II), Ni(II), and Cu(II) complexes of α,α'-ditolylmethanone dipyrroethene

Dipyrromethenes containing two pyrrole rings connected by one meso-carbon are versatile monoanionic bidentate ligands and form coordination complexes with many metals/nonmetals/metalloids. Dipyrroethenes containing one additional meso-carbon compared to dipyrromethenes have more space between coordinating pyrrole nitrogens and provide a good coordination environment but have not been explored as ligands in coordination chemistry. Dipyrroethenes are dianionic bidentate ligands and by suitable modifications, the coordination environment of dipyrroethenes can be changed further. Herein, we successfully synthesized α,α'-ditolylmethanone dipyrroethene which is a bipyrrolic tetradentate ligand with an ONNO ligand core and used it for the synthesis of novel Pd(II), Ni(II), and Cu(II) metal complexes by treating it with respective metal salts in CH₂Cl₂/CH₃OH at room temperature. The X-ray crystallographic structure of the metal complexes showed that the M(II) ion was coordinated to the ONNO atoms of the ligand in a perfect square planar geometry. The NMR studies of Pd(II) and Ni(II) complexes also supported the highly symmetric nature of the metal complexes. The absorption spectra of the metal complexes showed strong bands in the region of 300-550 nm. The electrochemical studies of metal complexes revealed that only ligand-based oxidation and reduction were observed. The DFT and TD-DFT studies were in agreement with the experimental observations. Our preliminary studies indicated that the Pd(II) complex can be used as a catalyst for the Fujiwara-Moritani olefination reaction.

Redox-neutral C-H annulation strategies for the synthesis of heterocycles via high-valent Cp*Co(III) catalysis

A variety of biologically active molecules, pharmaceuticals, and natural products consist of a nitrogen-containing heterocyclic backbone. The majority of them are isoquinolones, indoles, isoquinolines, etc.; thereby the synthesis and derivatization of such heterocycles are synthetically very relevant. Also, certain naphthol derivatives have high synthetic utility as agrochemicals and in dye industries. Previous approaches have utilized ruthenium, rhodium, or iridium which may not be desirable due to the high toxicity, low abundance, and high cost of such 4d and 5d metals. Moreover, the need for an external oxidant during the reaction also adds by-products to the system. A high-valent cobalt-catalyzed redox-neutral C-H functionalization strategy has emerged to be a far better alternative in this regard. The use of the non-noble metal cobalt allows for selectivity and specificity in product formation. Also, the redox-neutral concept avoids the use of an external oxidant either due to the presence of a metal in a non-variable oxidation state throughout the catalytic cycle or due to the presence of an oxidizing directing group or an oxidizing coupling partner. Such an oxidizing directing group not only directs the catalyst to a specific reaction site by chelation but also regenerates the catalyst at the end of the cycle. Certain bonds such as N-O, N-N, N-Cl, N-S, and C-S are the main game-players behind the oxidizing property of such directing groups. In the other case, the directing group only chelates the catalyst to a reaction center, whereas the oxidation is carried out by the upcoming group/coupling partner. Overall, merging the redox-neutral concept with the high-valent cobalt catalysis is paving the way forward toward a sustainable and environmentally friendly approach. This review critically describes the mechanistic understanding, scope, limitations, and synthesis of various biologically relevant heterocycles via the redox-neutral concept in the high-valent Cp*Co(III)-catalyzed C-H functionalization chemistry domain.

C(sp3)-H Aerobic Alkenylation of Tetrahydroisoquinolines via Organic Electrosynthesis

A method for the C(sp³)-H alkenylation of N-aryl-tetrahydroisoquinoline (THIQ) has been developed by the combination of electrooxidation and a copper catalyst. The corresponding products were obtained with good to excellent yields under mild conditions. Besides, the addition of TEMPO as an electron mediator is crucial to this transformation, since the oxidative reaction could proceed under a low electrode potential. In addition, the catalytic asymmetric variant has also been demonstrated with good enantioselectivity.

Iridium catalysed C2 site-selective methylation of indoles using a pivaloyl directing group through weak chelation-assistance

Here we present an iridium catalysed C2-selective methylation of indoles using methyltrifluoroborate as a source of methyl group. The iridium catalyst selectively discriminates the indole C2 and C4 C-H bonds by coordination with a pivaloyl directing group.

Electrochemical Cross-Dehydrogenative Coupling of Isochroman and Unactivated Ketones

An unprecedented electrochemical cross-dehydrogenative coupling reaction between isochroman and unactivated ketones to directly synthesize α-substituted isochromans has been developed. This strategy provides a facile and efficient procedure to the direct activation of C(sp³)-H bond adjacent to the O atom of isochroman. The method features high atom economy, chemical oxidant-free, and mild conditions, in which methanesulfonic acid (MsOH) acts as both electrolyte and catalyst, making the process more convenient and environmentally friendly. Gram-scale experiment and synthesis of antitumor active compounds demonstrate the great potential of this protocol for practical applications.

Palladium-Catalyzed C2-Selective Oxidative Olefination of Benzo[b]thiophene 1,1-Dioxides with Styrenes and Acrylates

Here, we disclose a novel Pd(II)-catalyzed oxidative Heck reaction of benzo[b]thiophene 1,1-dioxides with styrenes and acrylates. This transformation features broad functional group tolerance and high C2 selectivity. Furthermore, the photoluminescence properties of C-2 alkenylated products have been characterized, which illustrates the potential usefulness of our protocol in constructing π-conjugated fluorescent molecules.

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Ru(II)-Catalyzed Regioselective Redox-Neutral [4 + 2] Annulation of N-Chlorobenzamides with 1,3-Diynes at Room Temperature for the Synthesis of Isoquinolones

Herein, we report Ru(II)-catalyzed C-H/N-H bond functionalization of N-chlorobenzamides with 1,3-diynes via regioselective (4 + 2) annulation for the synthesis of isoquinolones under redox-neutral conditions at room temperature. This represents the first example of C-H functionalization of N-chlorobenzamides using an inexpensive and commercially available [Ru(p-cymene)Cl₂]₂ catalyst. The reaction is operationally simple, works in the absence of any silver additives, and is also applicable to a broad range of substrates with good functional group tolerance. The synthetic utility of the isoquinolone is demonstrated for the synthesis of bis-heterocycles consisting of isoquinolone-pyrrole and isoquinolone-isocoumarin scaffolds.

C-Alkylation by Phenalenyl-Based Molecule via a Borrowing Hydrogen Pathway

The present study demonstrates the first transition-metal-free catalytic C-alkylation via a borrowing hydrogen pathway for the α-alkylation of ketone, synthesis of substituted quinoline, and 9-monoalkylation of fluorene. With applications on diversification of biologically active molecules and gram-scale synthesis, a preliminary investigation of the reaction mechanism has been carried out, suggesting a radical-mediated borrowing hydrogen pathway.