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

Visible Light-Promoted and Catalyst-Controlled α-Aminoalkylation and α-Diaminoalkylation of Morita-Baylis-Hillman Acetates with N-Methyl Arylamines: Divergent Synthesis of N-Containing Alkyl Acrylates and γ,γ-Diaminobutyl Esters

Visible light-promoted α-aminoalkyl radical-triggered α-aminoalkylation and α-diaminoalkylation of Morita-Baylis-Hillman acetates with N-methyl arylamines to synthesize N-containing alkyl acrylates and γ,γ-diaminobutyl esters was reported. Photoinduced α-aminoalkylation is carried out with Na2-eosin Y as an organophotocatalyst at room temperature under metal- and oxidant-free conditions. In particular, the α-diaminoalkylation is performed via α-aminoalkyl radical addition/elimination followed by a catalyst-controlled selective α-aminoalkyl radical addition strategy in one step under the [Ir(dtbbpy)(bpy)2)]PF6/KF catalytic system. The reaction is highly atom- and step-economic, with high selectivity, furnishing N-containing alkyl acrylates and butyl esters in moderate to good yields with wide substrate scope.

Paired electrocatalysis enabled oxidative coupling of styrenes with alkyl radicals

A paired electrocatalysis strategy for intermolecular oxidative cross-dehydrocoupling between styrenes and ethers or p-methylphenol derivatives using ketone as a mild oxidant is described. This approach enables the generation of Csp3 carbon-centered radicals through anodic oxidation, followed by reductive coupling of ketones at the cathode, ultimately yielding valuable oxidative alkylation products.

Two-Fold Oxidative Coupling of Furan with Indole Provides Modular Access to a New Class of Tetra-(Hetero)Arylated Furans with Up to Four Different Substituents

Highly arylated propeller-shaped heteroarenes constitute an intriguing class of molecular scaffolds for material science applications. Among these, tetraarylated furans demonstrate differentiated properties as compared to other similar heterocyclic cores. The synthetic complexity to access tetraarylated furans increases significantly with increasing number of different peripheral aryl groups. There are only a very limited number of methodologies available to access furans with four different (hetero)aryl substituents. Notably, none of these involve direct oxidative coupling on the furan core as the method of choice. Herein, we report the first methodology based on a sequential two-fold oxidative C-C coupling of furans with indoles to access bis(indolyl)furans (BIFs) - a new class of 'extremely congested' tetra-(hetero)arylated furans with up to four different substituents. The reaction is mediated by inexpensive, earth-abundant FeCl3⋅6H2O and displays high efficiency, wide substrate scope, modularity and aqueous compatibility. Moreover, we also present the first validation of the distinct aggregation-caused quenching (ACQ) property of the tetraarylated furans beyond only phenyls as peripheral groups and disclose new mechanistic underpinnings for the same.

Nondirected Ortho C-H Arylation for One-Pot Synthesis of Biaryl Scaffolds via In Situ Generated Mixed Acetal

Herein, we introduce a mild and efficient method for synthesizing aniline biaryls and unsymmetrical phenol biaryls through iodine-catalyzed coupling of quinone imine ketals (QIKs)/quinonemonoacetals (QMAs) and β-naphthols. This approach allows for the unusual formation of ortho-substituted anilines and phenols, valuable in pharmaceuticals and advanced materials but typically difficult to produce. Our method achieves high ortho-selectivity without needing transition metals or external/internal templates. The process involves a [3,3] sigmatropic rearrangement of the in situ generated mixed acetal, which is the key intermediate. Notable features include scalability, broad functional group tolerance, and late-stage derivatization of natural products using a cost-effective, air-tolerant catalytic system, eliminating the need for hazardous catalysts.

Ruthenium-catalyzed Heck coupling of 3-arylidene-oxindoles with alkenes: a facile synthesis of 3-allylidene-2(3H)-oxindoles

A simple and efficient Ru(II)-catalyzed olefination of 3-(arylbenzylidene)indolin-2-ones with alkenes is described. This is an atom and step-economical strategy with a wide substrate scope, good functional group tolerance, and suitability for gram scale synthesis. A plausible mechanism is also proposed for this synthetic transformation involving the formation of a 5-membered ruthenacycle and insertion of the alkene followed by β-hydride elimination to deliver the desired product.

Chromium-catalyzed sustainable C-C and C-N bond formation: C-alkylation and Friedländer quinoline synthesis using alcohols

The synthesis of a novel phosphine-based pincer chromium(II) complex CrCl2(PONNH) (Cr-1) is reported in this study. The complex exhibited promising catalytic performance in C-C and C-N bond formation using the borrowing hydrogen methodology. Cr-1 catalyzed the α-alkylation of ketones using primary alcohols as alkyl surrogates in the presence of catalytic amount of a base. Cr-1 was also found to catalyze the β-alkylation of secondary alcohols using primary alcohols. In addition, the dehydrogenative annulation of 2-aminobenzyl alcohols with ketones to form quinolines was achieved using Cr-1 as the catalyst. Based on the mechanistic investigation, a plausible mechanism based on metal-ligand cooperation is proposed. The reactions are redox-neutral, atom-efficient, and produce water as the only by-product, thus contributing to green chemistry.

Rhodium-Catalyzed Meta-C-H Arylation of Arenes with Varied Linker Lengths: Bridging Catalytic Selectivity with Structural Diversity

The directing group (DG)-assisted approach has so far been the major route to achieve selective C-H activation at both proximal and distal positions. While rhodium catalysts are highly effective in DG-assisted ortho-C-H arylation, meta-C-H arylation with rhodium has not yet been reported. In this study, we present the first example of Rh-catalyzed meta-C-H arylation of arenes. We found that the 2-cyanophenyl-based directing group, in conjunction with arylboronic acids, selectively promotes meta-arylation with complete mono-selectivity. Despite significant advancements in meta-C-H activation for substrates with shorter linkers, such as hydrocinnamic acids, benzyl alcohols/amines, etc., meta-C-H activation of substrates with longer alkyl chains remains challenging with limited literature examples. We demonstrated that arenes with varying chain lengths, including conformationally flexible and less rigid ones such as 4-phenylbutanoic acid, 5-phenylvaleric acid, 6-phenylcaproic acid, 3-phenylpropanol, and 4-phenylbutanol underwent meta-arylation with high levels of regiocontrol. From a synthetic perspective, this approach could be valuable as it allows to produce biaryl derivatives of flexible arenes with native functional groups at the meta-position. The synthetic utility of this strategy is demonstrated through the total synthesis of CNBCA, a bioactive compound possessing promising potency against the SHP2 enzyme activity in vitro.

L-Proline catalyzed multi-component synthesis of N-pyridyl-tetrahydroisoquinolines and their α-C(sp3)-H oxygenation

Herein, we report an effective multi-component synthesis that starts with readily available starting materials and accesses poly-substituted pyridine derivatives by using L-proline as a benign catalyst. This process uses cyclic amines, aldehydes, and malononitrile in a condensation reaction to produce a variety of pyridine derivatives under mild conditions. Furthermore, depending on the catalysts used, the selective synthesis of an amide and/or an aldehyde functionality is achieved through α-C(sp3)-H oxygenation of the tertiary amine moiety in the resultant pyridine derivatives. The pyridine ring's nitrogen atom plays a crucial role in accelerating C-H oxygenation at the α-position of the tertiary amine, highlighting the synthetic versatility and usefulness of this method.

Photoredox-Catalyzed Alkynylation of C(sp3)-H Bonds Adjacent to a Nitrogen Atom of Tertiary Amines with Alkynyl Bromides

A novel and robust alkynylation of C(sp3)-H bonds adjacent to a nitrogen atom of tertiary amines with alkynyl bromides as radical alkynylating reagents has been realized under visible-light irradiation. A range variety of tertiary amines including N-arylamines and N-alkylamine have been coupled with both aromatic and aliphatic alkynyl bromides to furnish 51 examples of propargylamines in moderate to excellent yields (31-80% yields). The possible mechanism was a radical addition-elimination process based on preliminary mechanistic studies.

Diastereoselective Homocoupling of Benzylic C(sp3)-H Bonds Enabled by Halogen Transfer

A transition-metal- and harsh-oxidant-free strategy for diastereoselective homocoupling of benzylic α-boryl carbanions has been developed. Central to this methodology is the ability of the halogen transfer reagent to seamlessly integrate halogenation and substitution within a compatible process. Additionally, this methodology is also applicable to the homocoupling of diarylmethanes and alkylheteroarenes. Substrates bearing oxidatively sensitive functional groups were well-tolerated. Preliminary studies suggest that the hydrogen bond between two boryl groups contributes to the high diastereoselectivities.

Pd-Catalyzed Dual C-H Activation/Cyclization: Convergent and Divergent Synthesis of 1-Azahelicenes

Herein, we report a convergent synthesis of 1-azahelicenes using easily available quinoline derivatives and cyclic diaryliodonium salts as starting materials. This reaction undergoes a palladium-catalyzed dual C-H activation/cyclization process to give facile access to a wide range of 1-aza[5]helicenes and 1-aza[6]helicenes with abundant functional groups (including F, Cl, Br, I, CF3, SeR, SR, and heteroaryl) in moderate to excellent yields, thereby providing new opportunities to fine-tune the properties of the helicene backbone. In addition, the obtained products could be further transformed into helicene-based Lewis base catalysts and redox switch materials easily. Notably, one selected 1-azahelicene shined bright yellow light by aggregation. These features enlarge the chemical space of 1-azahelicenes and inspire further utilization in other areas of research.

Photoelectrocatalytic [4+2] Annulation for S-Heterocycle Assembly Enabled by Proton-Coupled Electron Transfer (PCET)

Cross-dehydrogenative couplings (CDC) present an efficient strategy for the assembly of biorelevant heterocycles, but are thus far largely limited to toxic transition metals and rather harsh reaction conditions. In sharp contrast, we, herein report on a mild photoelectrocatalyzed CDC-[4+2] annulation enabling the synthesis of functionalized isothiochromenes enabled by a proton-coupled electron transfer (PCET) strategy. The transformative photoelectrocatalysis obviated toxic transition-metal, high reaction temperatures, and stoichiometric chemical redox reagents. This approach was characterized by exceedingly mild conditions, ample substrate scope, and a commercially available catalyst. Gram-scale reactions and a telescoped synthesis route reflected the unique potential in the green synthesis of important S-heterocycles.

Cobalt-Catalyzed Divergence in C(sp3)-H Functionalization of 9H-Fluorene: A Streamlined Approach Utilizing Alcohols

Sustainable chemical production demands the creation of innovative catalysts and catalytic technologies. While the development of coherent and robust catalytic systems using earth-abundant transition metals is essential, it remains a significant challenge. Herein, an expedient divergence strategy for tandem dehydrogenative C(sp3)-H alkylation and cyclization reactions of 9H-fluorene using a newly developed N,N-bidentate cobalt catalytic system is developed. This method capitalizes on the use of abundant and readily accessible alcohol. Demonstrating wide-ranging applicability, the catalytic protocol successfully integrated a diverse array of fluorenes and alcohols. This includes benzylic, heteroaromatic, and aliphatic primary and secondary alcohols, amassing a total of 75 distinct examples. When applied to sterically bulky alcohols, the reaction preferentially undergoes alkenylation, yielding a tetrasubstituted olefin as the main product. In the case of diols, the expected outcome is Dual-fluorescence at both terminal positions. This process leads to difluorination, followed by a cyclization step, culminating in the formation of a relatively unprecedented spiro compound. The scalability of this method has been validated through gram-scale synthesis. Control experiments have shed light on the reaction mechanism, indicating that it progresses through an unsaturated intermediate and adheres to the borrowing hydrogen pathway.

Customized Photoelectrochemical C-N and C-P Bond Formation Enabled by Tailored Deposition on Photoanodes

Photoelectrochemistry (PEC) is burgeoning as an innovative solution to organic synthesis. However, the current PEC systems suffer from limited reaction types and unsatisfactory performances. Herein, we employ efficient BiVO4 photoanodes with tailored deposition layers for customizing two PEC approaches toward C-N and C-P bond formation. Our process proceeds under mild reaction conditions, deploying easily available substrates and ultra-low potentials. Beyond photocatalysis and electrocatalysis, customized PEC offers high efficiency, good functional group tolerance, and substantial applicability for decorating drug molecules, highlighting its promising potential to enrich the synthetic toolbox for broader organic chemistry of practical applications.

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.

RuII-Catalyzed C-H Activated Diverse Cyclization with Transformation of Substrate-DG to Functional Groups: Synthesis of Functionalized Indoles and Indenones

We present an elegant and efficient method for Ru(II)-catalyzed C-H activation, followed by a diverse range of intermolecular cross-dehydrogenative coupling reactions. This process is facilitated by an intrinsic directing group (DG) and includes the in situ transformation of the DG into common and useful functional groups. Notably, this method avoids the installation and deinstallation of the directing group. Our approach enables the selective functionalization of benzimidate, coupled with the cyclization of o-alkynyl-aniline, resulting in the high-yield synthesis of diverse compounds such as indoles, and indenones. The sequential formation of C-N, C-C, and C-O bonds, followed by hydrolysis, underscores the versatile in situ transformation of the directing group. This work not only broadens the synthetic toolbox for constructing complex heterocyclic structures but also highlights the potential for sustainable and selective synthesis of valuable compounds.

Rhodium-Catalyzed Arene Alkenylation: Selectivity and Reaction Mechanism as a Function of In Situ Oxidant Identity

Rhodium catalyzed arene alkenylation reactions with arenes and olefins using dioxygen as the direct oxidant (e.g., ACS Catal. 2020, 10, 11519), Cu(II) carboxylates (e.g., Science 2015, 348, 421; J. Am. Chem. Soc. 2017, 139, 5474) or Fe(III) carboxylate clusters (e.g., ACS Catal. 2024, 14, 10295), in the presence or absence of dioxygen, have been reported. These processes involve heating catalyst precursor [(η2-C2H4)2Rh(μ-OAc)]2, olefin, arene, and oxidant at temperatures between 120 and 200 °C. Herein, we report comparative studies of Rh-catalyzed arene alkenylation as a function of oxidant identity. This work includes comparisons of catalysis using Cu(II) carboxylates in the presence and absence of dioxygen, catalysis with only dioxygen as the oxidant, and Fe(III) carboxylates in the presence and absence of dioxygen. We report studies of catalysis with each oxidant including reagent concentration dependencies and kinetic isotope effect experiments using C6H6 or C6D6 and protio- or deutero carboxylic acid. Additionally, we probe ortho/meta/para regioselectivity for reactions of ethylene with monosubstituted arenes and Markovnikov/anti-Markovnikov selectivity with monosubstituted olefins. These studies indicate that the variation of oxidant identity impacts catalyst speciation, the reaction mechanism, and the reaction rate. Consequently, distinct Markovnikov/anti-Markovnikov and ortho/meta/para selectivities are observed for catalysis with each oxidant.

Factors Affecting the Formation and Transformation of the Intermediates in Pd(II)-Catalyzed Aromatic C-H Activation: A Comprehensive Study with the Pd(II)/LA Platform

How the factors affecting the formation and transformation of the intermediates in Pd(II)-catalyzed aromatic C-H activation: A comprehensive study with the Pd(II)/LA platform. Using the Pd(II)/Lewis acid (LA)-catalyzed C-H activation of electron-rich acetanilides as a platform, the C-H activation intermediates, including the precomplex η2-intermediate, the agostic hydrogen intermediate, and the palladacycle compound have been well-characterized. This work presents how the catalyst source, substrate, and solvent affect the formation of the η2-intermediate and its equilibrium with the agostic hydrogen intermediate, and the transformation of the agostic hydrogen intermediate to the palladacycle compound through C-H activation. The findings disclosed above are provided as a guideline for the catalyst design of the oxidative olefination of acetanilide with dioxygen, and the catalytic efficiency matched well with the mechanistic findings.

Paired Electrolysis Enables Reductive Heck Coupling of Unactivated (Hetero)Aryl Halides and Alkenes

The formation of carbon-carbon (C-C) bonds is a cornerstone of organic synthesis. Among various methods to construct Csp2-Csp3 bonds, the reductive Heck reaction between (hetero)aryl halides and alkenes stands out due to its potential efficiency and broad substrate availability. However, traditional reductive Heck reactions are limited by the use of precious metal catalysts and/or limited aryl halide and alkene compatibility. Here, we present an electrochemically mediated, metal- and catalyst-free reductive Heck reaction that tolerates both unactivated (hetero)aryl halides and diverse alkenes such as vinyl boronates and silanes. Detailed electrochemical and deuterium-labeling studies support that this transformation likely proceeds through a paired electrolysis pathway, in which acid generated by the oxidation of N,N-diisopropylethylamine (DIPEA) at the anode intercepts an alkyl carbanion formed after radical-polar crossover at the cathode. As such, this approach offers a sustainable method for the construction of Csp2-Csp3 bonds from (hetero)aryl halides and alkenes, paving the way for the development of other electrochemically mediated olefin difunctionalization reactions.

Catalytic dehydrogenative synthesis of α,β-unsaturated secondary amides without external oxidants

Direct dehydrogenative synthesis of α,β-unsaturated secondary amides still represents an elusive transformation. Herein we describe a palladium-catalyzed redox-neutral desaturation to prepare α,β-conjugated secondary amides. Without external oxidants, this approach relies on the N-O bond cleavage as the driving force to achieve formal dehydrogenation. Complementary to known protocols, this transformation is enabled by the unique reactivity of hydroxamate, thereby representing a novel strategy to accomplish carbonyl desaturation. Desired conjugated secondary amides can be efficiently synthesized in the presence of more reactive esters and even ketones, thus providing a solution to the long-standing issue of α,β-unsaturated secondary amides via C-C desaturation.

Cobalt-Catalyzed Allylic Alkylation at sp3-Carbon Centers

The rising demand and financial costs of noble transition metal catalysts have emphasized the need for sustainable catalytic approaches. Over the past few years, base-metal catalysts have emerged as ideal candidates to replace their noble-metal counterparts because of their abundance and easiness of handling. Despite the significant advancements achieved with precious transition metals, earth-abundant cobalt catalysts have emerged as efficient alternatives for allylic substitution reactions. In this review, allylic alkylations at sp3-carbon centers mediated by cobalt will be discussed, with a special focus on the mechanistic features, scope, and limitations.

CuBr2-mediated dehydrogenative [4+2] annulation of 1-naphthyl-1,3-indandiones and alkenes

Intermolecular annulation reactions of 1-naphthyl-1,3-indandiones with alkenes proceed efficiently in the presence of a copper catalyst to generate spirocarbocycle compounds. Various spirocyclic molecules bearing an all-carbon quaternary center could be obtained by this novel method with good yields, excellent regioselectivity, and good functional group tolerance. A radical mechanism is proposed based on the HRMS analysis results of control experiments.

Recent advances in enzymatic carbon-carbon bond formation

Enzymatic carbon-carbon (C-C) bond formation reactions have become an effective and invaluable tool for designing new biological and medicinal molecules, often with asymmetric features. This review provides a systematic overview of key C-C bond formation reactions and enzymes, with the focus of reaction mechanisms and recent advances. These reactions include the aldol reaction, Henry reaction, Knoevenagel condensation, Michael addition, Friedel-Crafts alkylation and acylation, Mannich reaction, Morita-Baylis-Hillman (MBH) reaction, Diels-Alder reaction, acyloin condensations via Thiamine Diphosphate (ThDP)-dependent enzymes, oxidative and reductive C-C bond formation, C-C bond formation through C1 resource utilization, radical enzymes for C-C bond formation, and other C-C bond formation reactions.

Double C-H Amination of Naphthylamine Derivatives by the Cross-Dehydrogenation Coupling Reaction

A high-efficiency tandem process has been developed for the formation of two C-N bonds through a cross-dehydrogenative coupling (CDC) amination of spiro[acridine-9,9'-fluorene]s (SAFs) with amines. This method offers a strategically innovative and atom-economical approach to obtaining diamine-substituted SAFs. Notably, the approach eliminates the need for metal catalysts and other additives, relying solely on O2 as the oxidant. A self-activation mechanism has been proposed to elucidate the effective double amination in the CDC process.

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.

Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene

Fe(II) carboxylates react with dioxygen and carboxylic acid to form Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 (X = acetate or pivalate), which is an active oxidant for Rh-catalyzed arene alkenylation. Heating (150-200 °C) the catalyst precursor [(η2-C2H4)2Rh(μ-OAc)]2 with ethylene, benzene, Fe(II) carboxylate, and dioxygen yields styrene >30-fold faster than the reaction with dioxygen in the absence of the Fe(II) carboxylate additive. It is also demonstrated that Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 is an active oxidant under anaerobic conditions, and the reduced material can be reoxidized to Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 by dioxygen. At optimized conditions, a turnover frequency of ∼0.2 s-1 is achieved. Unlike analogous reactions with Cu(II) carboxylate oxidants, which undergo stoichiometric Cu(II)-mediated production of phenyl esters (e.g., phenyl acetate) as side products at temperatures ≥150 °C, no phenyl ester side product is observed when Fe carboxylate additives are used. Kinetic isotope effect experiments using C6H6 and C6D6 give k H/k D = 3.5(3), while the use of protio or monodeutero pivalic acid reveals a small KIE with k H/k D = 1.19(2). First-order dependencies on Fe(II) carboxylate and dioxygen concentration are observed in addition to complicated kinetic dependencies on the concentration of carboxylic acid and ethylene, both of which inhibit the reaction rate at a high concentration. Mechanistic studies are consistent with irreversible benzene C-H activation, ethylene insertion into the formed Rh-Ph bond, β-hydride elimination, and reaction of Rh-H with Fe6(μ-OH)2(μ3-O)2(μ-X)12(HX)2 to regenerate a Rh-carboxylate complex.

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.

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.