Catalytic Arene meta-C–H Functionalization Exploiting a Quinoline-Based Template

Palladium(II)-catalyzed annulation of N-methoxy amides and arynes: computational mechanistic insights and substituents effects

CONTEXT

The combined use of transition metal-catalyzed C-H activation with aryne annulation reactions has emerged as an important strategy in organic synthesis. In this study, the mechanisms of the palladium(II)-catalyzed annulation reaction of N-methoxy amides and arynes were computationally investigated by density functional theory. The role of methoxy amide as a directing group was elucidated through the calculation of three different pathways for the C-H activation step, showing that the pathway where amide nitrogen acts as a directing group is preferable. At the reductive elimination transition state, an unstable seven-membered ring is formed preventing the lactam formation. A substituent effect study based on an NBO analysis, Hammet, and using a More O'Ferall-Jenks plot indicates that the C-H activation step proceeds via an electrophilic concerted metalation-deprotonation (eCMD) mechanism. The results show that electron-withdrawing groups increase the activation barrier and contribute to an early Pd-C bond formation and a late C-H bond breaking when compared with electron-donating substituents. Our computational results are in agreement with the experimental data provided in the literature.

METHODS

All calculations were performed using Gaussian 16 software. Geometry optimizations, frequency analyses at 393.15 K, and IRC calculations were conducted at the M06L/Def2-SVP level of theory. Corrected electronic energies, NBO charges, and Wiberg bond indexes were computed at the M06L/Def2-TZVP//M06L/Def2-SVP level of theory. Implicit solvent effects were considered in all calculations using the SMD model, with acetonitrile employed as the solvent.

Rh(III)-catalyzed selective C2 C-H acyloxylation of indoles

Herein, we present the first highly regio- and chemoselective C2 C-H acyloxylation of indole under rhodium catalysis and an N-quinolinyl auxiliary. This strategy accommodates a wide range of indoles and structurally diverse carboxylic acids with good reaction efficiencies to yield functionalized indoles. The utility of this logic was demonstrated by the concise synthesis of the functionalized 2-oxindole derivatives. Preliminary mechanistic studies indicate that catalyst turnover of RhIII-RhIV/V-RhII/III-RhIII might be involved in this catalytic C-H transformation.

Pd-Catalyzed Tandem Pathway for Stereoselective Synthesis of (E)-1,3-Enyne from β-Nitroalkenes by Using a Sacrificial Directing Group

The involvement of nitroalkenes instead of minimal one alkyne motif for (E)-1,3-enynes synthesis through a palladium catalyzed stereoselective bond forming pathway at room temperature is presented. Implication of nitro group as a sacrificial directing group, formation of magical alkyne on a newly developed Csp 3 -Csp 3 bond with initial palladium-MBH adduct make this methodology distinctive. This protocol features an unprecedented sequential acetate addition, carbon-carbon bond formation, isomerization of double bond and nitromethane degradation in a tandem catalytic walk via dancing hybridization. Mechanistic understanding through identification of intermediates and computational calculations furnishes complete insight into the tandem catalytic pathway. Broad substrates scope and functional groups tolerance make this synthetic methodology magnificent and dynamic. This represents the first example of stereoselective 1,3-enyne synthesis exclusively from alkene substrates by introducing the concept of sacrificial directing group.

Access to unsaturated bicyclic lactones by overriding conventional C(sp3)-H site selectivity

Transition metal catalysis plays a pivotal role in transforming unreactive C-H bonds. However, regioselective activation of distal aliphatic C-H bonds poses a tremendous challenge, particularly in the absence of directing templates. Activation of a methylene C-H bond in the presence of methyl C-H is underexplored. Here we show activation of a methylene C-H bond in the presence of methyl C-H bonds to form unsaturated bicyclic lactones. The protocol allows the reversal of the general selectivity in aliphatic C-H bond activation. Computational studies suggest that reversible C-H activation is followed by β-hydride elimination to generate the Pd-coordinated cycloalkene that undergoes stereoselective C-O cyclization, and subsequent β-hydride elimination to provide bicyclic unsaturated lactones. The broad generality of this reaction has been highlighted via dehydrogenative lactonization of mid to macro ring containing acids along with the C-H olefination reaction with olefin and allyl alcohol. The method substantially simplifies the synthesis of important bicyclic lactones that are important features of natural products as well as pharmacoactive molecules.

Synthesis of Ferrocene 1,3-Derivatives by Distal C-H Activation

The third position of cyclopentadienyl ring of a monosubstituted ferrocene has remained as an inaccessible chemical space for direct functionalization. Until recently, functionalizing the C(3)-position while bypassing the predominantly active C(2)-position is the most challenging task. Herein, we report a distal C-H functionalization of monosubstituted ferrocenes using an easily removable directing group with precise site-selectivity, under a PdII / mono-N-protected amino-acid ligand catalytic system. The robust synthetic protocol leads to the synthesis of ferrocene 1,3-derivatives with broad scope in olefins while functionalizing ferrocenyl methylamine in moderate to good yields via a highly strained ferrocene appended 12-membered palladacycle intermediate.

Photoinduced meta-Selective C-H Oxygenation of Arenes

The merger of photocatalysis and transition-metal catalysis has recently emerged as an adaptable platform for the development of innovative and environmentally benign synthetic methodologies. In contrast to classical transformation by Pd complexes, photoredox Pd catalysis operates through a radical pathway in the absence of a radical initiator. Using the synergistic merger of photoredox and Pd catalysis, we have developed a highly efficient, regioselective, and general meta-oxygenation protocol for diverse arenes under mild reaction conditions. The protocol showcases the meta-oxygenation of phenylacetic acids and biphenyl carboxylic acids/alcohols and is also amenable for a series of sulfonyls and phosphonyl-tethered arenes, irrespective of the nature and position of the substituents. Unlike thermal C-H acetoxylation which operates through the Pd^II/Pd^IV catalytic cycle, this metallaphotocatalytic C-H activation involves Pd^II/Pd^III/Pd^IV intermediacy. The radical nature of the protocol is established through radical quenching experiments and EPR analysis of the reaction mixture. Furthermore, the catalytic path of this photoinduced transformation is established through control reactions, absorption spectroscopy, luminescence quenching, and kinetic studies.

3-Arylsulfonylquinolines from N-Propargylamines via Cascaded Oxidative Sulfonylation Using DABSO

We report a cascaded oxidative sulfonylation of N-propargylamine via a three-component coupling reaction using DABCO·(SO₂)₂ (DABSO). 3-Arylsulfonylquinolines were obtained by mixing diazonium tetrafluoroborate, N-propargylamine, and DABSO under argon atmosphere in dichloroethane (DCE) for 1 h. In a radical pathway, DABSO was utilized as the sulfone source and an oxidant in this radical-mediated cascaded reaction.

Native carboxyl group-assisted C-H acetoxylation of hydrocinnamic and phenylacetic acids

The use of a native directing group to promote C-H activation is highly desirable. Herein, we report a method of native carboxyl-assisted, Pd(II)-catalyzed ortho-C-H acetoxylation of both hydrocinnamic and phenylacetic acids that can be found in many biologically active molecules as the key moieties. Based on the broad scope and the application potential showcased with drug molecules, it is anticipated that this C-H acetoxylation reaction will find attractive applicability in future synthetic endeavors.

Empirical Guidelines for the Development of Remote Directing Templates through Quantitative and Experimental Analyses

The ability to differentiate and selectively activate remote C-H bonds represents a perennial challenge in the field of C-H activation. Since its first report in 2012, a now-established "directing template" (DT) approach remains demonstrably effective for the functionalization of remote C-H bonds. As selectivity is hypothesized to be principally determined by the optimal positioning of the reactive catalyst to a target C-H bond, a DT's spatial factors are particularly important toward achieving high selectivity, though a systematic study on its requisite factors remain unelucidated. Through an in-depth analysis of 119 structurally unique published remote DTs, this report summarizes the key factors that are central toward achieving high selectivity at defined aryl positions, which are experimentally corroborated through the development of new aliphatic meta and para-selective DTs for electronically unbiased arenes. These empirical rules, which summarize key distance and geometric factors, are expected to be useful tools for the future development of site-selective arene C-H activation as well as other reactions that rely on covalent/noncovalent DT-mediated remote regioselection.

Emergence of Pyrimidine-Based meta-Directing Group: Journey from Weak to Strong Coordination in Diversifying meta-C-H Functionalization

C-H activation has emerged as a powerful transformative synthetic tool to construct complex molecular frameworks, which are ubiquitous in natural products, medicines, dyes, polymers, and many more. However, reactivity and selectivity, arising from the inertness of C-H bonds and their overabundance in organic molecules, are the two major fundamental challenges in developing various carbon-carbon (C-C) and carbon-heteroatom (C-X) bond formation reactions via C-H activation technique. Functional groups with coordinating capacity to the transition metal catalysts, profoundly known as directing groups (DGs), have shown great promise in exerting selective C-H activation, often called site-selective or regioselective transformation of a target molecule. Advent of directing group (DG)-assisted strategies not only has resolved the selectivity issues but also offers a unique solution to the rapid synthesis of complex molecules in a convenient and predictable manner. Our laboratory, in this regard, is fascinated by the prospect of DG-assisted distal C-H functionalization of arenes, in which the target C-H bond is remotely located from the existing directing group. Notably, in opposition to proximal ortho-C-H activation, which proceeded via an energetically favorable five- to seven-membered metallacycle, distal C-H activation remained a formidable challenge as it required formation of a large macrocyclic metallacycle. Therefore, designing a suitable directing template that would maintain the required distance and geometric relationship between the target C-H bond and the appended directing auxiliary in order to ensure the prolific delivery of the metal catalyst to the closest proximity of targeted distal C-H bond was the key to success. In this regard, the Yu group devised an elegant "U-shaped" template for the first time to execute distal meta-C-H activation recruiting a cyano-based directing group. Our initial effort to diversify the scope of meta-C-H functionalization using a cyano-based template led us to realize that the "cyano-based DGs" are intrinsically limited with weak coordinating ability, competitive binding mode (end-on vs side-on), and incompatibility with acidic and basic reaction conditions. In search of a robust directing auxiliary, we were intrigued by the possibility of using the strongly coordinating ability of pyrimidine and quinoline-based DGs.In this Account, we describe our journey from the weakly coordinating cyano-based DG to the strongly coordinating pyrimidine-based DG to achieve diverse meta-C-H functionalization of electronically and sterically unbiased arenes. While some of the functionalizations were achieved by finding suitable reaction conditions, others were led by mechanistic understanding. Notably, initial development in this realm was constrained with short linkers, in which the DG was attached to the arene of interest through 2-4 atoms. In later studies, we demonstrated that the selective meta-C-H activation can be attained even though the DG is 10-atoms away from the targeted arene. More importantly, a transient DG was successfully utilized to deliver meta-C-H olefination of arenes via in situ imine formation, which provided a step-economic route to meta-C-H activation.We hope that this Account will stimulate further template design and will provide a guiding platform for the future development of distal meta-C-H functionalization.

RhodiumIII-Catalyzed Remote Difunctionalization of Arenes Assisted by a Relay Directing Group

Rhodium-catalyzed diverse tandem twofold C-H bond activation reactions of para olefin-tethered arenes have been realized, with unsaturated reagents such as internal alkynes, dioxazolones, and isocyanates being the coupling partner as...

Directing Group Assisted Rhodium Catalyzed meta-CH Alkynylation of Arenes

Site-selective C-H alkynylation of arenes to produce aryl alkynes is a highly desirable transformation due to its prevalence in various natural products, drug molecules and in material sciences. To ensure...

Insight into the Mechanism and Regioselectivity of Pd(OAc)2-Catalyzed C-O Bond Activation via a β-O Elimination Approach: A Computational Study

The density functional theory investigations were carried out to elucidate the mechanism and the origin of regioselectivity for the Pd(OAc)₂-catalyzed carbon-oxygen bond activation in the reaction between 4-phenoxy-N-(quinolin-8-yl) butanamide and N-methylindole. The reaction proceeded through four main stages in succession: C-H activation, β-O elimination, nucleo-palladation of the new C-C bond formation, and proto-depalladation steps. A total of six pathways were considered since there were two possible forms of C-O bond breaking in the β-O elimination step and six reaction channels of nucleophilic attack in the crucial nucleo-palladation step. The computational results indicate that the common first step (C-H bond activation step) occurs via a concerted metalation deprotonation (CMD) mechanism. The nucleo-palladation was the rate-determining step for all six reaction pathways. The results also show that the most favorable pathway for the whole reaction is the one (denoted as path b1) in which phenol was removed in the second stage and the hydrogen atom of N-methylindole attacked the oxygen atom of acetate group of the intermediate in the third stage. According to the analyses of noncovalent interaction (NCI) and the reduced density gradient (RDG), the most favored pathway benefits from the strong attractive interaction and weak repulsive interaction in its key transition state. Furthermore, structural, natural bond orbital charge, and energy analyses of the transition states reveal the origin of the regioselectivity. This is a good explanation of the experimental phenomenon and benefits future design of a new strategy for a similar reaction.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

C-H activation reactions of nitroarenes: current status and outlook

Ring substitution reactions of nitroarenes remain an under-developed area of organic synthesis, confined to the narrow domains of S_NAr and S_NArH reactions. While searching for alternative methodologies, we took stock of the C-H activation reactions of nitroarenes which unearthed a variety of examples of nitro directed regioselective C-H functionalization reactions such as ortho-arylation, -benzylation/alkylation, and -allylation, oxidative Heck and C-H arylation reactions on (hetero)aromatic rings. A collective account of these reactions is presented in this review to showcase the existing landscape of C-H activation reactions of nitroarenes, to create interest in this field for further development and propagate this strategy as a superior alternative for ring substitution reactions of nitroarenes. The prospect of merging the C-H activation of nitroarenes with C-NO₂ activation, thereby harnessing NO₂ as a transformable multitasking directing group, is also illustrated.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

Direct C4-Acetoxylation of Tryptophan and Tryptophan-Containing Peptides via Palladium(II)-Catalyzed C-H Activation

An efficient regioselective palladium(II)-catalyzed C(sp2)-H 4-acetoxylation of tryptophan and tryptophan-containing peptides is described. This transformation achieves the direct construction of C-O bonds at the tryptophan C4-position and features good functional group tolerance. The 4-hydroxyl compound was obtained by removing acetyl after C4-acetoxylation of tryptophan derivatives and tryptophan-containing dipeptides. This method provides a novel strategy for the synthesis of 4-substituted tryptophan derivatives and modification of tryptophan-containing peptides.

Recent development in transition metal-catalysed C-H olefination

Transition metal-catalysed functionalizations of inert C-H bonds to construct C-C bonds represent an ideal route in the synthesis of valuable organic molecules. Fine tuning of directing groups, catalysts and ligands has played a crucial role in selective C-H bond (sp2 or sp3) activation. Recent developments in these areas have assured a high level of regioselectivity in C-H olefination reactions. In this review, we have summarized the recent progress in the oxidative olefination of sp2 and sp3 C-H bonds with special emphasis on distal, atroposelective, non-directed sp2 and directed sp3 C-H olefination. The scope, limitation, and mechanism of various transition metal-catalysed olefination reactions have been described briefly.

Removal and modification of directing groups used in metal-catalyzed C–H functionalization: the magical step of conversion into ‘conventional’ functional groups

This feature review is focused on recent approaches for removing versatile directing groups.

Carboxyl-Assisted meta-Selective C-H Functionalizations of Benzylsulfonamides

A protocol of carboxyl-group-assisted, Pd(II)-catalyzed remote meta-C(sp²)-H olefination and arylation of benzylsulfonamides has been developed. It was supposed to proceed through a κ² coordination of the carboxyl group to the Pd center. These findings demonstrated the versatility of carboxyl-assisted remote meta-C-H activation strategy and might stimulate the exploration of novel reactivity and selectivity of other traditional chelating groups in different contexts.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

para-Selective Arylation of Arenes: A Direct Route to Biaryls by Norbornene Relay Palladation

Biaryl compounds are extremely important structural motifs in natural products, biologically active components and pharmaceuticals. Selective synthesis of biaryls by distinguishing the subtle reactivity difference of distal arene C-H bonds are significantly challenging. Herein, we describe para-selective C-H arylation, which is acheived by a unique combination of a meta-directing group and norbornene as a transient mediator. Upon direct meta-C-H palladation, one-bond relay palladation occurs in presence of norbornene and subsequently para-C-H arylation is achieved for sulfonates, phosphonates and phenols bearing 2,6-disubstitution patterns. The protocol is amenable to electron-deficient aryl iodides. Multisubstituted arenes and phenols are obtained by postsynthetic modification of the products. The protocol allows the synthesis of hexa-substituted benzene by sequential selective distal C-H functionalization.

Achieving Site-Selectivity for C-H Activation Processes Based on Distance and Geometry: A Carpenter's Approach

The ability to differentiate between highly similar C-H bonds in a given molecule remains a fundamental challenge in organic chemistry. In particular, the lack of sufficient steric and electronic differences between C-H bonds located distal to functional groups has prevented the development of site-selective catalysts with broad scope. An emerging approach to circumvent this obstacle is to utilize the distance between a target C-H bond and a coordinating functional group, along with the geometry of the cyclic transition state in directed C-H activation, as core molecular recognition parameters to differentiate between multiple C-H bonds. In this Perspective, we discuss the advent and recent advances of this concept. We cover a wide range of transition-metal-catalyzed, template-directed remote C-H activation reactions of alcohols, carboxylic acids, sulfonates, phosphonates, and amines. Additionally, we review eminent examples which take advantage of non-covalent interactions to achieve regiocontrol. Continued advancement of this distance- and geometry-based differentiation approach for regioselective remote C-H functionalization reactions may lead to the ultimate realization of molecular editing: the freedom to modify organic molecules at any site, in any order.

Alkyne Linchpin Strategy for Drug:Pharmacophore Conjugation: Experimental and Computational Realization of a Meta-Selective Inverse Sonogashira Coupling

The late-stage functionalization (LSF) of pharmaceutical and agrochemical compounds by the site-selective activation of C-H bonds provides access to diverse structural analogs and expands synthetically-accessible chemical space. We report a C-H functionalization LSF strategy that hinges on the use of an alkyne linchpin to assemble conjugates of sp²-rich marketed pharmaceuticals and agrochemicals with sp³-rich 3D fragments and natural products. This is accomplished through a template-assisted inverse Sonogashira reaction that displays high levels of selectivity for the meta position. This protocol is also amenable to distal structural modifications of α-amino acids. The transformation of alkyne functionality to other functional groups further highlights the applicative potential. Computational and experimental mechanistic studies shed light on the detailed mechanism. Turnover-limiting 1,2-migratory insertion of the bromoalkyne coupling partner occurs after relatively fast C-H activation. While this insertion occurs unselectively, regioconvergence results from one of the adducts undergoing a 1,2-trialkylsilyl migration to form the alkynylated product. A heterobimetallic Pd-Ag transition structure is essential for product formation in the β-bromide elimination step.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Coligand driven diverse organometallation in benzothiazolyl-hydrazone derivatized pyrene: ortho vs. peri C–H activation

Divergent aromatic C–H activation at both ortho and peri positions in polyaromatic hydrocarbons has been successfully accomplished by suitable variation of the coligand.

Palladium-catalysed Heck-type alkenylation reactions in the synthesis of quinolines. Mechanistic insights and recent applications

Recent developments in Pd(0)- and Pd(ii)-catalysed alkenylation reactions for the synthesis of quinolines focusing on mechanistic understanding.

A directing group-assisted ruthenium-catalyzed approach to access meta-nitrated phenols

meta-Selective C–H nitration of phenol derivatives was developed using a Ru-catalyzed σ-activation strategy. Cu(NO₃)₂·3H₂O was employed as the nitrating source, whereas Ru₃(CO)₁₂ was found to be the most suitable metal catalyst for the protocol.

Overriding ortho selectivity by template assisted meta-C–H activation of benzophenones

A regioselective meta-C–H activation strategy for benzophenone was successfully developed by overriding the inherent ketone-directed ortho-selectivity.

Intramolecular Remote C-H Activation via Sequential 1,4-Palladium Migration To Access Fused Polycycles

An unprecedented intramolecular remote C-H activation via sequential 1,4-palladium migration with an aromatic ring as a conveyor has been described. This reaction provides an efficient route to construct diverse polycyclic frameworks in moderate to good yield via palladium-catalyzed remote C-H activation/alkene insertion, arylation, alkenylation, and the Heck reaction. The preliminary mechanistic studies revealed that the 1,4-palladium migration process was reversible.

Rhodium catalyzed template-assisted distal para-C-H olefination

Rhodium catalysis has been extensively used for ortho-C-H functionalization reactions, and successfully extended to meta-C-H functionalization. Its application to para-C-H activation remains an unmet challenge. Herein we disclose the first example of such a reaction, with the Rh-catalyzed para-C-H olefination of arenes. The use of a Si-linked cyanobiphenyl unit as a traceless directing group leads to highly para-selective arene-olefin couplings.