Intermolecular Dehydrogenative Heck Reactions

Synthesis of 1H-Isoindoles via Ruthenium(II)-Catalyzed Cyclization of Benzimidates with Alkenes

A Ru(II)-catalyzed efficient synthesis of 1H-isoindoles via the cyclization of benzimidates with alkenes has been demonstrated. This methodology exhibits high compatibility with various functionalized activated and unactivated olefins containing different sensitive functional groups. This protocol provides an effective method for synthesizing various 1H-isoindole derivatives in decent to excellent yields. Notably, the ortho-alkenylation of benzimidates with unactivated alkenes was achieved. A potential reaction mechanism has been suggested that involves C-H activation, 1,2 insertion, and β-hydride elimination subsequent to aza-Michael addition.

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.

Expedient access to polysubstituted acrylamides via strain-release-driven dual phosphine and palladium catalysis

Polysubstituted acrylamides are ubiquitous in bioactive molecules and natural products. However, synthetic methods for the assembly of these important motifs remain underdeveloped. Herein, we report the expedient synthesis of structurally diverse and synthetically challenging polysubstituted acrylamides from readily available aromatic amines, cyclopropenones (CpOs), and aryl halides via the synergistic merging of nucleophilic phosphine-mediated amidation and palladium-catalyzed C-H arylation. The reaction is scalable, and some obtained acrylamides proved to be solid state luminogens with obvious aggregation-induced emission (AIE) properties, demonstrating the synthetic potential in drug discovery and material development.

Asymmetric C-H Dehydrogenative Alkenylation via a Photo-induced Chiral α‑Imino Radical Intermediate

The direct alkenylation with simple alkenes stands out as the most ideal yet challenging strategy for obtaining high-valued desaturated alkanes. Here we present a direct asymmetric dehydrogenative α-C(sp3)-H alkenylation of carbonyls based on synergistic photoredox-cobalt-chiral primary amine catalysis under visible light. The ternary catalytic system enables the direct coupling of β-keto-carbonyls and alkenes through a cooperative radical addition-dehydrogenation process involving a chiral α-imino radical and Co(II)-metalloradical intermediate. A catalytic H-transfer process involving nitrobenzene is engaged to quench in situ generated cobalt hydride species, ensuring a chemoselective alkenylation in good yields and high enantioselectivities.

Unveiling the promising anticancer activity of palladium(II)-aryl complexes bearing diphosphine ligands: a structure-activity relationship analysis

In continuation of our previous works on the cytotoxic properties of organopalladium compounds, in this contribution we describe the first systematic study of the anticancer activity of Pd(II)-aryl complexes. To this end, we have prepared and thoroughly characterized a wide range of palladium derivatives bearing different diphosphine, aryl and halide ligands, developing, when necessary, specific synthetic protocols. Most of the synthesized compounds showed remarkable cytotoxicity towards ovarian and breast cancer cell lines, with IC50 values often comparable to or lower than that of cisplatin. The most promising complexes ([PdI(Ph)(dppe)] and [PdI(p-CH3-Ph)(dppe)]), characterized by a diphosphine ligand with a low bite angle, exhibited, in addition to excellent cytotoxicity towards cancer cells, low activity on normal cells (MRC5 human lung fibroblasts). Specific immunofluorescence tests (cytochrome c and H2AX assays), performed to clarify the possible mechanism of action of this class of organopalladium derivatives, seemed to indicate DNA as the primary cellular target, whereas caspase 3/7 assays proved that the complex [PdI(Ph)(dppe)] was able to promote intrinsic apoptotic cell death. A detailed molecular docking analysis confirmed the importance of a diphosphine ligand with a reduced bite angle to ensure a strong DNA-complex interaction. Finally, one of the most promising complexes was tested towards patient-derived organoids, showing promising ex vivo cytotoxicity.

Mono and di ortho-C-H acetoxylation of 2-aryloxyquinoline-3-carbaldehydes

2-Aryloxyquinolines are well known for various biological activities. In this report, we have developed a novel protocol for introducing an acetoxy functional group on the aryl sp2 carbon of 2-aryloxyquinoline-3-carbaldehyde using a palladium catalyst for the first time. Interestingly, this C-H acetoxylation reaction is highly chemo- and site-selective. By modifying the reaction conditions, mono or di ortho-C-H acetoxylation products have been synthesized selectively with good yields and with good functional group tolerance.

Ligand-free Pd-catalyzed highly selective arylation of activated and unactivated alkenes via oxidative and reductive heck coupling

In this work, an eco-friendly, green, efficient approach for oxidative and reductive Heck-Mizoroki (HM) reactions was developed, which offered acceptable yields from first-pass experiments. Mono-arylation was achieved without the use of ligands, directing groups, or prefunctionalized alkenes. Considering mild reaction conditions, good functional group compatibility, and great regioselectivity, the method can find broad applications in novel medicine and material development and discovery processes.

Challenges and recent advancements in the synthesis of α,α-disubstituted α-amino acids

α,α-Disubstituted α-amino acids (α-AAs) have improved properties compared to other types of amino acids. They serve as modifiers of peptide conformation and as precursors of bioactive compounds. Therefore, it has been a long-standing goal to construct this highly valuable scaffold efficiently in organic synthesis and drug discovery. However, access to α,α-disubstituted α-AAs is highly challenging and largely unexplored due to their steric constraints. To overcome these, remarkable advances have been made in the last decades. Emerging strategies such as synergistic enantioselective catalysis, visible-light-mediated photocatalysis, metal-free methodologies and CO2 fixation offer new avenues to access the challenging synthesis of α,α-disubstituted α-AAs and continuously bring additional contributions to this field. This review article aims to provide an overview of the recent advancements since 2015 and discuss existing challenges for the synthesis of α,α-disubstituted α-AAs and their derivatives.

Synthesis of Spiroisoindolinones via Ru(II)-Catalyzed Spiroannulation of N-Acyl Ketimines with Aryl Isocyanates/Isothiocyanates through Aromatic C-H Bond Activation

Herein, we disclose the first report on Ru(II)-catalyzed amidation/thioamidation of 3-hydroxy-3-arylisoindolinones with isocyanates/isothiocyanates, respectively. The reaction furnishes spiroisoindolinones via sequential C-H functionalization of ortho C-H bond followed by intramolecular cyclization in moderate to high yields (up to 94%). Moreover, the developed strategy is highly atom-economical and site-selective and shows diverse substrate generality. Also, synthesized spiroisoindolinones undergo several chemical transformations.

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.

One-Step Protocol for the Synthesis of Cyanoacrylates Promoted by Elemental Sulfur from p-Quinone Methides and Cyanoacetates under Basic Conditions

Cyanoacrylates have a wide range of biological activities and are extensively applied in production and daily life. Classic synthetic routes to cyanoacrylates have many limitations. Herein, we demonstrate an elemental sulfur-promoted method for the synthesis of β,β-diaryl cyanoacrylates by a tandem 1,6-Michael addition/oxidation/elimination process from p-QMs and cyanoacetates under optimal conditions. The effective protocol has good substrate scopes and yields, and the ratio of inseparable E/Z isomers of cyanoacrylates is also determined by 1HNMR.

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

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

DMSO-promoted direct δ-selective arylation of p-quinone methenylpiperidine bearinides to generate fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources

Fuchsones have wide applications in modern society. Present methods for generating fuchsones have many disadvantages and there are significant limitations for further exploration of fuchsone applications. Herein, we describe a DMSO-promoted direct δ-selective arylation of p-QMs to synthesize symmetrical and unsymmetrical fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources. As unprecedented methods, these novel strategies present a great advantage and significance for further exploration of fuchsones and the development of new applications.

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.

Diverse Synthetic Transformations Using 4-Bromo-3,3,4,4-tetrafluorobut-1-ene and Its Applications in the Preparation of CF2 CF2 -Containing Sugars, Liquid Crystals, and Light-Emitting Materials

Organic molecules with fluoroalkylene scaffolds, especially a tetrafluoroethylene (CF2 CF2 ) moiety, in their molecular structures exhibit unique biological activities, or can be applied to functional materials such as liquid crystals and light-emitting materials. Although several methods for the syntheses of CF2 CF2 -containing organic molecules have been reported to date, they have been limited to methods using explosives and fluorinating agents. Therefore, there is an urgent need to develop simple and efficient approaches to synthesize CF2 CF2 -containing organic molecules from readily available fluorinated substrates using carbon-carbon bond formation reactions. This personal account summarizes the simple and efficient transformation of functional groups at both ends of 4-bromo-3,3,4,4-tetrafluorobut-1-ene and discusses its synthetic applications to biologically active fluorinated sugars and functional materials, such as liquid crystals and light-emitting molecules.

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.

Radical-mediated photoredox hydroarylation with thiosulfonate

Herein, we report a novel visible light-induced photocatalytic system that enables intramolecular hydroarylation of unactivated alkenes. Thiosulfonate compounds were found to be the key radical precursor that mediates the Minisci-type intramolecular cyclization reaction. Under the optimal reaction conditions, a wide range of pyridyquinazolinone and pyrroloquinazolinone products were obtained in moderate to good yields.

Rhodium(III)-catalyzed oxidative cross-coupling of benzoxazinones with styrenes via C-H activation

Vinylarenes represent an important class of core skeleton embedded in natural products, organic materials, and pharmaceutical molecules. Therefore, numerous efforts have been devoted to developing efficient methods for their preparation. Among them, transition-metal-catalyzed oxidative coupling of arenes and alkenes has proved to be a powerful method due to its high atom and step economy. Although a wide range of oxidative alkenylations of arenes have been developed, the alkenes employed in most cases are still limited to electron-deficient alkenes. Reported herein is a Rh(III)-catalyzed C-H cross-coupling of benzoxazinones and simple unactivated styrenes to furnish a variety of vinylarene scaffolds. This established protocol is characterized by wide functional group compatibility, high yields, and excellent regio- and chemo-selectivity. Mechanistic studies and gram-scale experiments on this high-value conversion are disclosed. Moreover, the potential utility of this method was highlighted by a series of further transformations.

Ru(II)-Catalyzed C-H Alkenylation of Benzimidates with Unactivated Olefins: A Route to ortho-Alkenylated Benzonitriles

A Ru(II)-catalyzed C-H alkenylation of benzimidates with unactivated alkenes providing ortho-alkenylated benzonitriles in good to excellent yields in a highly regio- and stereoselective manner is described. In the reaction, an imidate group converted into a nitrile under the reaction conditions. The alkenylation reaction was compatible with various substituted benzimidates as well as functionalized unactivated olefins, including ibuprofen-, neproxen-, coumarin-, and cholesterol-substituted alkenes. A feasible reaction mechanism was proposed to account for the present alkenylation reaction.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

Progress in copper-catalysed/mediated intramolecular dehydrogenative coupling

Transition metal-catalysed C-H functionalization reactions are one of the most efficient synthetic methodologies to construct carbon-carbon and carbon-heteroatom bonds. The initial developments in the field were largely dominated by expensive transition metal catalysts. However, in the past decade, the focus of the catalyst shifted to first-row transition metals and copper catalysis contributed significantly. Abundant, cost-effective, and less toxic copper catalysts are an ideal green alternative to palladium and similar metals. The intramolecular dehydrogenative coupling itself developed as a prominent area of focus as the strategy straightaway affords complex polycyclic scaffolds in one pot. Regioselective activation of inert C-H bonds were made possible with copper catalysts and interestingly, oxygen served as the terminal oxidant in most of the cases. In the present review the focus is on the intramolecular dehydrogenative coupling reactions between carbon-hydrogen and heteroatom-hydrogen bonds to afford carbon-carbon and carbon-hetero atom bonds, catalysed/mediated by copper salts. Though the intermolecular dehydrogenative coupling reactions of copper have already been reviewed more than once, to the best of our knowledge this is the first comprehensive account of copper-based intramolecular dehydrogenative coupling.

Denitrative Mizoroki–Heck reaction of unactivated alkenes

A general palladium-catalyzed denitrative Mizoroki–Heck reaction of unactivated alkenes has been developed with high E/Z selectivity.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Rh(III)-Catalyzed Enone Carbonyl/Ketone-Directed Aerobic C-H Olefination of Aromatics with Unactivated Olefins

A Rh(III)-catalyzed weak enone carbonyl/ketone-assisted aerobic oxidative C-H olefination of aromatics with unactivated alkenes has been developed. This protocol involves cross-dehydrogenative Heck-type olefination reaction of various substituted biologically relevant chalcones and aromatic ketones such as acetophenones and chromones with various functionalized unactivated olefins in moderate to good yields. Further, ortho-alkylation of chalcones with norbornene is also demonstrated. A possible reaction mechanism involving weak chelation-assisted C-H activation/insertion/β-hydride elimination was proposed and supported by the deuterium labeling studies.

Palladium-Catalyzed Sequential Three-Component Cross-Coupling to 1,3-Dienes: Employing Alkenes as Hydride and Alkenyl Donors

This report discloses a novel Pd-catalyzed sequential three-component multiple reaction of alkenes, bromoalkynes, and boronic acids using alkenes as hydride and alkenyl donors, leading to highly stereoselective assembly of (Z,E)-1,3-diene derivatives. Mechanistic studies demonstrate that the generation and reutilization of palladium hydride species are critical to the success of this transformation. In addition, the good functional group compatibility, late-stage modification, and investigation of photophysical properties of 1,3-diene products illustrate the synthetic value of this strategy.

Recent advances in theoretical studies on transition-metal-catalyzed regioselective C-H functionalization of indoles

Indole compounds are widely found in natural products and drug candidates. The transition-metal-catalyzed regioselective C-H bond functionalization of indoles as the most efficient method for the synthesis of various functionalized indoles has been extensively studied in the past two decades due to its advantages of step economy and atom economy. In general, the catalysts included the transition-metals (Pd, Rh, Ru, Cu, Co, Fe, Zn, and Ga) and these reactions were accomplished with a remarkably wide range of coupling reagents for construction of various C-C and C-X (X = N, O, S) bonds. However, the general and important rules of the regioselectivity are not clear to date. Therefore, a comprehensive analysis through previous reported theoretical studies on transition-metal-catalyzed regioselective C-H bond functionalization of indoles was crucial and significant. In this review, we found that when the C-H bond activation process was the rate-determining step, the regioselectivity ordinarily occurred at the C7 or C4 positions (on benzene ring), and otherwise, the regioselectivity often occurred at C2 position (on pyrrole ring). For indoline substrates, the C-H bond functionalization occurred at the benzene ring. General rules of the regioselectivities for transition-metal-catalyzed C-H bond functionalization of indoles. This review collects major advances in the transition-metal-catalyzed C-H bond functionalization of indoles and indolines.

A regio- and stereoselective Heck-Matsuda process for construction of γ-aryl allylsulfonyl fluorides

A highly efficient regio- and stereoselective Heck–Matsuda method was developed employing aryl diazoniums and allylsulfonyl fluorides for the construction of a class of novel γ-aryl allylsulfonyl fluorides in the presence of Pd(OAc)₂ and PPh₃. The method features excellent regio- and stereoselectivity (up to 100% E-selectivity), broad substrate scope and mild reaction conditions. Further application of γ-aryl allylsulfonyl fluoride in SuFEx reactions was achieved to provide their corresponding sulfonates and sulfonamides in excellent yields.

A Heck–Matsuda reaction of aryl diazoniums with allylsulfonyl fluorides for the construction of γ-aryl allylsulfonyl fluorides was developed.

Electrochemically driven regioselective C-H phosphorylation of group 8 metallocenes

Metallocenes are privileged backbones for synthesis and catalysis. However, the direct dehydrogenative C−H functionalization of unsymmetric metallocenes suffers from reactivity and selectivity issues. Herein, we report an electrochemically driven regioselective C−H phosphorylation of group 8 metallocenes. Mechanistic investigations indicate this dehydrogenative cross coupling occurs through an electrophilic radical substitution of the metallocene with a phosphoryl radical, facilitated by the metallocene itself. This work not only offers an efficient and divergent synthesis of phosphorylated metallocenes, but also provides a guide to interpret the reactivity and regioselectivity for the C−H functionalization of unsymmetric metallocenes.

Metallocene-based phosphines are compounds with potential use in catalysis. Here, the authors report the electrochemical regioselective functionalization of group 8 metallocenes with phosphine oxides; over 60 examples of phosphorylated (benzo)ferrocenes and ruthenocenes can be accessed via this method without the need for a preinstalled directing group.

Scandium-Catalyzed Benzylic C(sp3)-H Alkenylation of Tertiary Anilines with Alkynes

This work describes the chemo- and stereoselective benzylic C(sp³)-H alkenylation of tertiary ortho-methyl anilines with internal alkynes using a simple β-diketiminato scandium catalyst. This protocol offers an efficient method for the synthesis of a new family of tertiary ortho-allylanilines in high yields. The resultant alkenylation products facilely underwent further chemical transformation to other valuable anilines. A cationic scandium benzyl species was isolated from a stoichiometric reaction and confirmed to be the catalytic intermediate.

Base-promoted direct E-selective olefination of organoammonium salts with sulfones toward stilbenes and conjugated 1,3-dienes

A base-promoted direct deaminative olefination of organoammonium salts was developed. Only mediated by KO^tBu, a series of benzyl and cinnamyl ammonium salts reacted smoothly with sulfones, producing the valuable stilbenes and related 1,3-diene derivatives in good to high yields with good functional group tolerance and excellent E-selectivity. With this developed method, biologically active resveratrol and DMU-212 were also successfully prepared, which further demonstrates the practicality of this reaction.

Cu(I)-Catalyzed C-H Alkenylation of Tertiary C(sp3)-H Bonds of 3-Aryl Benzofuran-2(3H)-ones to Give Z- and E-Styrene Containing Quaternary Carbon Centers with 99/1 Regioselectivity

The synthesis of isomerically pure olefins containing all-carbon quaternary centers is a challenging issue. Herein, we developed an efficient protocol for the synthesis of (Z)-olefins (27 examples, yield up to 97%, Z/E up to 99/1) and (E)-olefins (16 examples, yield up to 94%, E/Z up to 99/1) containing all-carbon quaternary centers. This protocol is adopted for the copper-catalyzed regioselective C-H alkenylation of the tertiary C(sp³)-H bond of 3-aryl benzofuran-2(3H)-ones with alkyne and alkenes. A diverse range of functional groups in the substrates is well-tolerated, such as F, Cl, Br, Me, OMe, ester, CF₃, etc. A gram scale experiment was performed in good yield, and the radical mechanisms are also proposed based on the control experiments.

Redox-Neutral Ru(0)-Catalyzed Alkenylation of 2-Carboxaldimine-heterocyclopentadienes

A new Ru₃(CO)₁₂-catalyzed directed alkenylation of 2-carboxaldimine-heterocyclopentadienes has been accomplished. This process allows coupling of furan, pyrrole, indole, and thiophene 2-carboxaldimines with electron-poor alkenes such as acrylates, vinylsulfones, and styrenes. This regio- and chemoselective oxidative C-H coupling does not require the presence of an additional sacrificial oxidant. Density functional theory calculations allowed us to propose a mechanism and unveiled the nature of the H₂ acceptor.

DFT Case Study on the Comparison of Ruthenium-Catalyzed C-H Allylation, C-H Alkenylation, and Hydroarylation

Density functional calculations at the B3LYP-D3+IDSCRF/TZP-DKH(-dfg) level of theory have been performed to understand the mechanism of ruthenium-catalyzed C-H allylation reported in the literature in depth. The plausible pathway consisted of four sequential processes, including C-H activation, migratory insertion, amide extrusion, and recovery of the catalyst, in which C-H activation was identified as the rate-determining step. The amide extrusion step could be promoted kinetically by trifluoroacetic acid since its mediation lowered the free-energy barrier from 32.1 to 12.2 kcal/mol. Additional calculations have been performed to explore other common pathways between arenes and alkenes, such as C-H alkenylation and hydroarylation. A comparison of the amide extrusion and β-H elimination steps established the following reactivity sequence of the leaving groups: protonated amide group > β-H group > unprotonated amide group. The suppression of hydroarylation was attributed to the sluggishness of the Ru-C protonation step as compared to the amide extrusion step. This study can unveil factors favoring the C-H allylation reaction.

Remote ortho-C-H functionalization via medium-sized cyclopalladation

Compared to the tremendous progress made in directed ortho-C-H functionalization via five- or six-membered cyclopalladation, protocols with the ability to selectively activate more remote C-H bonds through the intermediacy of larger, less favorable, seven- or eight-membered metalacycles are particularly challenging and remain rare. However, such a strategy would provide new retrosynthetic opportunities for generating structural diversity and complexity. Intense recent research based on the use of either mono-anionic bidentate or monodentate directing groups is characterizing this approach as an increasingly viable tool for selective C-C and C-X bond-forming reactions. This short review provides an overview of these strategies with an emphasis on mechanistic details, synthetic applicability, limitations, and key challenges.