meta C-H Arylation of Electron-Rich Arenes: Reversing the Conventional Site Selectivity

meta-C-H functionalization of phenylethyl and benzylic alcohol derivatives via Pd/NBE relay catalysis

The transition metal-catalyzed meta-C-H functionalization of alcohols and their hydroxylamine derivatives remains underdeveloped. Herein, we report an efficient meta-C-H arylation of both phenylethyl and benzylic alcohols and their hydroxylamine derivatives using a readily removable oxime ether directing group. Using electronically activated 2-carbomethoxynorbornene as the transient mediator and 3-trifluoromethyl-2-pyridone as the enabling ligand, this reaction features a broad substrate scope and good functional group tolerance. More importantly, with this oxime-directed meta-C-H functionalization, this method provides a dual approach for efficient access to both meta-substituted alcohols and hydroxylamines using two sets of simple deprotection conditions. This protocol leads to the efficient synthesis of bioactive compounds possessing promising reactivities for the treatment of pulmonary fibrosis.

Control over Anion Coordination on Pd(II), Cu(I), and Ag(I) with Regioisomeric Phosphine-Carboxylate Ligands

The coordination of anionic donors is involved at various stages of catalytic cycles in transition-metal catalysis, but control over the spatial positioning of anions around a metal center is a challenge in coordination chemistry. Here we show that regioisomeric phosphine-carboxylate ligands provide spatial anion control on palladium(II) centers by favoring either κ2, cis-κ1, or trans-κ1 coordination of the carboxylate donor. Additionally, the palladium(II) carboxylates, which contain a methyl donor, upon protonation, deliver metal-alkyl complexes that feature a coordinated carboxylic acid. Such complexes can be considered as models for the minima that follow the concerted metalation-deprotonation transition state for C-H activation. The predictability of the coordination modes is further demonstrated on silver(I) and copper(I) centers, for which less common structures of mononuclear and dinuclear complexes can be obtained by using spatial anion control. Our results demonstrate the potential for spatial control over carboxylate anions in coordination chemistry.

Cr-Catalyzed Intramolecular Arylative Cross-Coupling of Unactivated C-H Bonds with C-Halide Bonds

We report here a chromium-catalyzed intramolecular arylation of unactivated C-H bonds with C-halide bonds under mild conditions. This reaction was enabled by using a low-cost CrCl2 salt as the precatalyst in combination with allylmagnesium bromide and E/Z-mixed 1-halo-2-styrylarenes as substates, providing a strategy for the construction of functionalized phenanthrene compounds without using external ligands.

Native Amino Group Directed Meta-Selective C-H Arylation of Primary Amines Via Pd/Norbornene Catalysis

The selective functionalization of remote C-H bonds in free primary amines holds significant promise for the late-stage diversification of pharmaceuticals. However, to date, the direct functionalization of the meta position of amine substrates lacking additional directing groups remains underexplored. In this Letter, we present a successful meta-C-H arylation of free primary amine derivatives using aryl iodides, resulting in synthetically valuable yields. This meta-selective C-H functionalization is achieved through a sequence involving native amino-directed Pd-catalyzed seven-membered cyclometalation, followed by the utilization of a norbornene-type transient mediator.

Zwitterionic Aqua Palladacycles with Noncovalent Interactions for meta-Selective Suzuki Coupling of 3,4-Dichlorophenol and 3,4-Dichlorobenzyl Alcohol in Water

The site-selective reaction of substrates with multiple reactive sites has been a focus of the current synthetic chemistry. The use of attractive noncovalent interactions between the catalyst and substrate is emerging as a versatile approach to address site-selectivity challenges. Herein, we designed and synthesized a series of palladacycles, to control meta-selective Suzuki coupling of 3,4-dichlorophenol and 3,4-dichlorobenzyl alcohol. Noncovalent interactions directed zwitterionic aqua palladacycles catalyzed meta-selective Suzuki couplings of 3,4-dichloroarenes bearing hydroxyl in water have been developed. Experiments and density functional theory (DFT) calculations demonstrated that the electrostatic interactions play a critical role in meta-selective coupling of 3,4-dichlorophenol, while meta-selective coupling of 3,4-dichlorobenzyl alcohol arises due to the hydrogen-bonding interactions.

Late-Stage C-H Activation of Drug (Derivative) Molecules with Pd(ll) Catalysis

This review comprehensively analyses representative examples of Pd(II)-catalyzed late-stage C-H activation reactions and demonstrates their efficacy in converting C-H bonds at multiple positions within drug (derivative) molecules into diverse functional groups. These transformative reactions hold immense potential in medicinal chemistry, enabling the efficient and selective functionalization of specific sites within drug molecules, thereby enhancing their pharmacological activity and expanding the scope of potential drug candidates. Although notable articles have focused on late-stage C-H functionalization reactions of drug-like molecules using transition-metal catalysts, reviews specifically focusing on late-stage C-H functionalization reactions of drug (derivative) molecules using Pd(II) catalysts are required owing to their prominence as the most widely utilized metal catalysts for C-H activation and their ability to introduce a myriad of functional groups at specific C-H bonds. The utilization of Pd-catalyzed C-H activation methodologies demonstrates impressive success in introducing various functional groups, such as cyano (CN), fluorine (F), chlorine (Cl), aromatic rings, olefin, alkyl, alkyne, and hydroxyl groups, to drug (derivative) molecules with high regioselectivity and functional-group tolerance. These breakthroughs in late-stage C-H activation reactions serve as invaluable tools for drug discovery and development, thereby offering strategic options to optimize drug candidates and drive the exploration of innovative therapeutic solutions.

meta-C-H Arylation of Aniline Derivatives via Palladium/ S,O-Ligand/Norbornene Cooperative Catalysis

Aromatic amines are ubiquitous moieties in organic molecules and their direct functionalization is of great interest in many research areas due to their prevalence in pharmaceuticals and organic electronics. While several synthetic tools exist for the ortho- and para-functionalization of anilines, the functionalization of the less reactive meta-position is not easy to achieve with current methods. To date, the meta-C-H arylation of aniline derivatives has been restricted to either the use of directing groups & templates, or their transformation into anilides & quaternary anilinium salts. Herein, we report the first general and efficient meta-C-H-arylation of non-directed aniline derivatives via cooperative catalysis with a palladium-S,O-ligand-norbornene system. The reaction proceeds under mild conditions with a wide range of aniline derivatives and aryl iodides, while being operationally simple and scalable. Our preliminary mechanistic investigation-including the isolation of several palladium complexes and deuterium experiments-reveal useful insights into the substituent-effects of both the aniline-substrate and the norbornene-mediator during the meta-C-H activation step.

Synthesis of meta-arylphenol derivatives via acid-promoted rearrangement of cyclohexadienones

A highly effective strategy for the synthesis of meta-arylphenol derivatives through the selective rearrangement of 4-alkyl-4-aryl-2,5-cyclohexadienones under metal-free conditions was developed, in which acid-promoted [1,2]-migration of the aryl group at C-4 occurred exclusively when the alkyl group at C-4 was a methyl group. Treatment of 4-methyl-4-aryl-2,5-cyclohexadienones with 37% HCl in Ac2O at room temperature provided polysubstituted meta-arylphenyl acetates in 75-94% yields. The application of this protocol in the synthesis of polycyclic aromatic compounds was also described.

Remote C-H Olefination of Heterocyclic Biaryls Enabled by Reversibly Bound Templates

Remote C-H functionalization of heterocyclic biaryls will be of great importance in synthesis and medicinal chemistry. Through adjusting the geometric relationship of the directing atom and target C-H bonds, two new catalytic templates have been developed to enable the functionalization of the more hindered ortho-C-H bonds of heterobiaryls bearing directing heteroatom at the meta- or para-positions, affording unprecedented site-selectivity. The use of template chaperone also overcomes product inhibition and renders the directing templates catalytic. The utility of this protocol was demonstrated by olefination of heterocyclic biaryls with various substituents, overriding conventional steric and electronic effects. These ortho-C-H olefinated heterobiaryls are sterically hindered and can often be challenging to prepare through aryl-aryl coupling reactions.

Site-Selective C-H Functionalization of Carbazoles

Carbazole alkaloids hold great potential in pharmaceutical and material sciences. However, the current approaches for C1 functionalization of carbazoles rely on the use of a pre-installed directing group, severely limiting their applicability and hindering their overall efficiency. Herein, we report for the first time the development of direct Pd-catalyzed C-H alkylation and acylation of carbazoles assisted by norbornene (NBE) as a transient directing mediator. Notably, the involvement of a six-membered palladacycle intermediate was suggested in this case, representing the first example of such intermediacy within the extensively studied Pd/norbornene reactions realm.

meta-Selective C-H arylation of phenols via regiodiversion of electrophilic aromatic substitution

Electrophilic aromatic substitution is among the most widely used mechanistic manifolds in organic chemistry. Access to certain substitution patterns is, however, precluded by intrinsic and immutable substituent effects that ultimately restrict the diversity of the benzenoid chemical space. Here we demonstrate that the established regioselectivity of electrophilic aromatic substitution can be overcome simply by diverting the key σ-complex intermediate towards otherwise inaccessible substitution products. This 'regiodiversion' strategy is realized through the development of a general and concise method for the meta-selective C-H arylation of sterically congested phenols. Consisting of a Bi(V)-mediated electrophilic arylation and a subsequent aryl migration/rearomatization, our process is orthogonal to conventional C-H activation and cross-coupling approaches, and does not require prefunctionalization of the substrate. Mechanistically informed applications in synthesis showcase its utility as a versatile and enabling route to highly functionalized, contiguously substituted aromatic building blocks that defy synthesis via existing methods.

Non-directed C-H arylation of electron-deficient arenes by synergistic silver and Pd3 cluster catalysis

Transition-metal clusters have attracted great attention in catalysis due to their unique reactivity and electronic properties, especially for novel substrate binding and activation modes at the bridging coordination sites of metal clusters. Although palladium complexes have demonstrated outstanding catalytic performance in various transformations, the catalytic behaviors of polynuclear palladium clusters in many important synthetic methodologies remain much less explored so far. Herein, we disclose the use of an atomically defined tri-nuclear palladium (Pd₃Cl) species as a catalyst precursor in Ag(I)-assisted direct C-H arylation with aryl iodides under mild conditions. This catalyst system leads to the formation of synthetically important biaryls in good yields with high site selectivities without the assistance of directing groups.

Dual ligand approach increases functional group tolerance in the Pd-catalysed C-H arylation of N-heterocyclic pharmaceuticals

The excellent functional group tolerance of the Suzuki-Miyaura cross-coupling reactions has been decisive for their success in the pharmaceutical industry. Highly diversified (hetero)aromatic scaffolds can be effectively coupled in the final step(s) of a convergent synthetic route. In contrast, electrophilic Pd catalysts for non-directed C-H activation are particularly sensitive to inhibition by coordinating groups in pharmaceutical precursors. While C-H arylation enables the direct conversion of (hetero)aromatics without preinstalled functional or directing groups, its functional group tolerance should be increased to be viable in late-stage cross-couplings. In this work, we report on a dual ligand approach that combines a strongly coordinating phosphine ligand with a chelating 2-hydroxypyridine for the highly robust C-H coupling of bicyclic N-heteroaromatics with aryl bromide scaffolds. The catalyst speciation was studied via in situ XAS measurements, confirming the coordination of both ligands under the reaction conditions. The C-H activation catalyst was shown to be tolerant to a wide range of pharmaceutically relevant scaffolds, including examples of late-stage functionalization of known drug molecules.

Silver-Free C-H Activation: Strategic Approaches towards Realizing the Full Potential of C-H Activation in Sustainable Organic Synthesis

The activation of carbon-hydrogen bonds is considered as one of the most attractive techniques in synthetic organic chemistry because it bears the potential to shorten synthetic routes as well as to produce complementary product scopes compared to traditional synthetic strategies. However, many current methods employ silver salts as additives, leading to stoichiometric metal waste and thereby preventing the full potential of C-H activation to be exploited. Therefore, the development of silver-free protocols has recently received increasing attention. Mechanistically, silver can serve various roles in C-H activation and thus, avoiding the use of silver requires different approaches based on the role it serves in a given process. In this Review, we present the comparison of silver-based and silver-free methods. Focusing on the strategic approaches to develop silver-free C-H activation, we provide the reader with the means to develop sustainable methods for C-H activation.

Site-Selective C-H Arylation of Diverse Arenes Ortho to Small Alkyl Groups

Catalytic systems for direct C-H activation of arenes commonly show preference for electronically activated and sterically exposed C-H sites. Here we show that a range of functionally rich and pharmaceutically relevant arene classes can undergo site-selective C-H arylation ortho to small alkyl substituents, preferably endocyclic methylene groups. The C-H activation is experimentally supported as being the selectivity-determining step, while computational studies of the transition state models indicate the relevance of non-covalent interactions between the catalyst and the methylene group of the substrate. Our results suggest that preference for C(sp2 )-H activation next to alkyl groups could be a general selectivity mode, distinct from common steric and electronic factors.

Ligand-Enabled Sequential C(sp3)-H and C(sp2)-H Diolefination Reaction via Palladium Catalyst

Palladium-catalyzed sequential C(sp³)-H and C(sp²)-H bond diolefination reaction of o-toluidine has been realized for the first time using acetyl-protected aminoethyl phenyl thioether ligands. This novel reaction allows for preparation of the conjugated diene structure via an immediate second olefination on the basis of the first C(sp³)-H olefination in one pot. Various triflyl-protected anilines and acrylates were used as coupling partners elegantly. Furthermore, the unpurified diolefination products can be easily converted to tetrahydroquinoline derivatives.

S,O-Ligand Promoted meta-C-H Arylation of Anisole Derivatives via Palladium/Norbornene Catalysis

Reversing the conventional site-selectivity of C-H activation processes provides new retrosynthetic disconnections to otherwise unreactive bonds. Here, we report a new catalytic system based on palladium/norbornene and an S,O-ligand for the meta-C-H arylation of aryl ethers that significantly outperforms previously reported systems. We demonstrate the unique ability of this system to employ alkoxyarene substrates bearing electron donating and withdrawing substituents. Additionally, ortho-substituted aryl ethers are well tolerated, overcoming the "ortho constraint", which is the necessity to have a meta-substituent on the alkoxyarene to achieve high reaction efficiency, by enlisting novel norbornene mediators. Remarkably, for the first time the monoarylation of alkoxyarenes is achieved efficiently enabling the subsequent introduction of a second, different aryl coupling partner to rapidly furnish unsymmetrical terphenyls. Further insight into the reaction mechanism was achieved by isolation and characterization of some Pd-complexes-before and after meta C-H activation-prior to evaluation of their respective catalytic activities.

Study of Pd-catalyzed Selective Mono- and Di-C(sp3)-H Bond Activation: A Bi-ligand Model

Controlling the number of C-H bond activation is a long-standing challenge in organic synthesis. Recently, Yu's group demonstrated that in Pd-catalyzed alanine's arylation, pyridine-type ligands favor a mono-C-H bond activation, while quinoline-type ligands favor a di-C-H bond activation. To disclose the underlying principles, a theoretical study (density functional theory (DFT)) has been carried out. Our study indicates that a mono-ligand model, which is generally adopted in the community, does not reproduce the experimentally observed mono-/di-selectivity, while a bi-ligand model can rationalize the experimental observations well, including the observed diastereoselectivity in diarylation. The electron-rich pyridine-type ligands with less steric congestion can promote the C-H bond activation reaction of alanine derivatives. The quinoline-type ligands have a better π back-donation interaction with the metal, which makes a more active C-H bond activation than the pyridine-type ligands for this reaction. This bi-ligand model, which is a necessity, allows the understanding and future design of a dual ligand effect in C-H bond activation.

meta-Allylation of Arenes via Ruthenium-Catalyzed Cross-Dehydrogenative Coupling

A successful example of oxidative meta-dehydrogenative allylation of arenes with alkenes has been developed using Ru(PPh₃)₃Cl₂ as a catalyst and DTBP as an oxidant. In the allylation process, pyrimidines, pyrazoles, and purines, found widely in nucleosides, were effective auxiliary groups. Gram-scale experiments took place smoothly under optimized conditions. Mechanistic studies indicated that ruthenium-catalyzed meta-dehydrogenative allylation was a free-radical process. The allylation process developed herein provides an efficient and practical strategy to prepare versatile meta-allylated arenes.

Introduction to Spatial Anion Control for Direct C–H Arylation

C–H activation of functionally rich molecules without the need for directing groups promises shorter organic syntheses and late-stage diversification of molecules for drug discovery. We highlight recent examples of palladium-catalyzed nondirected functionalization of C–H bonds in arenes as limiting substrates with a focus on the development of the concept of spatial anion control for direct C–H arylation.

1 C–H Activation and the CMD Mechanism

2 Nondirected C–H Functionalizations of Arenes as Limiting Substrates

3 Nondirected C–H Arylation

4 Spatial Anion Control for Direct C–H Arylation

5 Coordination Chemistry with Spatial Anion Control

6 Conclusion

Three-Component Ruthenium-Catalyzed meta-C-H Alkylation of Phenol Derivatives

Herein, we realized the multicomponent reactions of phenol derivatives via a six-membered cycloruthenated intermediate for the first time. This strategy exhibited good substrate suitability and functional group tolerance with various phenol derivatives and provided a potential synthetic drug approach. Mechanistic studies showed that a radical might be involved in this process. In addition, the meta alkylated phenol was obtained by further removal of the directing group.

Insight into the mechanism of the arylation of arenes via norbornene relay palladation through meta- to para-selectivity

A theoretical insight was shown into the origin of site-selectivity in the arylation of arenes by a norbornene relay palladation through meta- to para-selectivity.

Mechanistic insight into construction of axially chiral biaryls via palladium/chiral norbornene cooperative catalysis: a DFT-based computational study

DFT calculations rationalize the enantioselectivity and clarify the reaction sequence of two aryl halides as well as the substantial role of the ortho-ester group in the aryl bromide.

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.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Highly regioselective and stereoselective synthesis of C-Aryl glycosides via nickel-catalyzed ortho-C-H glycosylation of 8-aminoquinoline benzamides

C-Aryl glycosides are of high value as drug candidates. Here a novel and cost-effective nickel catalyzed ortho-CAr-H glycosylation reaction with high regioselectivity and excellent α-selectivity is described. This method shows great functional group compatibility with various glycosides, showing its synthetic potential. Mechanistic studies indicate that C-H activation could be the rate-determining step.

Dienolate Annulation Approach for Assembly of Densely Substituted Aromatic Architectures

The efficient assembly of complex aromatic structures from simple acyclic building blocks is reported. An anion-cascade union of an enoate and a conjugated imine affords cyclohexenone products, which are readily aromatized to phenols. By engaging the intermediate cyclohexenones with Grignard reagents, a facile addition/elimination proceeds yielding chiral cyclohexadienes, which are then aromatized. In a complementary approach, the cyclohexenone products are converted into enol triflates, which provides a gateway to diverse aromatic architectures following cross-couplings and aromatization steps.

Advancing the Logic of Chemical Synthesis: C-H Activation as Strategic and Tactical Disconnections for C-C Bond Construction

The design of synthetic routes by retrosynthetic logic is decisively influenced by the transformations available. Transition-metal-catalyzed C-H activation has emerged as a powerful strategy for C-C bond formation, with myriad methods developed for diverse substrates and coupling partners. However, its uptake in total synthesis has been tepid, partially due to their apparent synthetic intractability, as well as a lack of comprehensive guidelines for implementation. This Review addresses these issues and offers a guide to identify retrosynthetic opportunities to generate C-C bonds by C-H activation processes. By comparing total syntheses accomplished using traditional approaches and recent C-H activation methods, this Review demonstrates how C-H activation enabled C-C bond construction has led to more efficient retrosynthetic strategies, as well as the execution of previously unattainable tactical maneuvers. Finally, shortcomings of existing processes are highlighted; this Review illustrates how some highlighted total syntheses can be further economized by adopting next-generation ligand-enabled approaches.

Facile Access to Hetero-poly-functional Arenes and meta-Substituted Arenes via Two-Step Dimetalation and Mg/Halogen-Exchange Protocol

The Grignard reagent, iPrMgCl and its lithium chloride-enhanced 'turbo' derivative iPrMgCl⋅LiCl have been employed to investigate the single iodo/magnesium exchange reactions of the trisubstituted arenes, 2,5-diiodo-N,N-diisopropylbenzamide 1, 1,4-diiodo-2-methoxybenzene 2, and 1,4-diiodo-2-(trifluoromethyl)benzene 3. These three arenes themselves were initially prepared by a double ortho-, meta'-deprotonation of N,N-diisopropylbenzamide, anisole and (trifluoromethyl)benzene, respectively, using the sodium magnesiate reagent [Na₄ Mg₂ (TMP)₆ (nBu)₂ ] (where TMP is 2,2,6,6-tetramethylpiperidide), and subsequent electrophilic quenching with iodine/THF solution. Thus, by following a combined deprotonation and magnesium/halogen exchange strategy, the simple monosubstituted arenes can be converted to trisubstituted diiodoarenes, which can ultimately be transformed into the corresponding mono-magnesiated arenes, in THF at -40 °C, within seconds in good yields. The other functional group (OMe, NiPr₂ or CF₃ respectively) present on the di-iodoarenes helps direct the exchange reaction to the ortho position, whereas subsequent addition of different electrophiles permits the preparation of hetero-poly-functional-arenes, with three different substituents in their structure. Intriguingly, if water is used as the electrophile, a new and facile route to prepare meta-substituted arenes, which cannot be easily obtained by conventional processes, is forthcoming. In contrast to directed ortho-metalation (DoM) chemistry, this reaction sequence can be thought of as InDirect meta-Metalation (IDmM). The scope of the chemistry has been tested further by exposing the initial unreacted iodo-functionality at the meta-position to a second Mg/I-exchange reaction and subsequent functionalization.

Direct Arylation of Simple Arenes with Aryl Bromides by Synergistic Silver and Palladium Catalysis

The direct, catalytic arylation of simple arenes in small excess with aryl bromides is disclosed. The developed method does not require the assistance of directing groups and relies on a synergistic catalytic cycle in which phosphine-ligated silver complexes cleave the aryl C-H bond, while palladium catalysts enable the formation of the biaryl products. Mechanistic experiments, including kinetic isotope effects, competition experiments, and hydrogen-deuterium exchange, support a catalytic cycle in which cleavage of the C-H bond by silver is the rate-determining step.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.

Meta-Dehydrogenative Alkylation of Arenes with Ethers, Ketones, and Esters Catalyzed by Ruthenium

A meta-dehydrogenative alkylation of arenes with cyclic ethers, ketones, and esters catalyzed by ruthenium is achieved in the presence of a di-tert-butyl peroxide (DTBP) oxidant. Interestingly, when quinoline and isoquinoline are employed as the directing group, or a chain ether as alkylation reagent, the system produces Minisci reaction products. Mechanistic study indicates that meta-dehydrogenative alkylation is a radical process initiated by DTBP with the assistance of a C_Ar-Ru bond ortho/para-directing effect.

Spatial Anion Control on Palladium for Mild C-H Arylation of Arenes

C-H arylation of arenes without the use of directing groups is a challenge, even for simple molecules, such as benzene. We describe spatial anion control as a concept for the design of catalytic sites for C-H bond activation, thereby enabling nondirected C-H arylation of arenes at ambient temperature. The mild conditions enable late-stage structural diversification of biologically relevant small molecules, and site-selectivity complementary to that obtained with other methods of arene functionalization can be achieved. These results reveal the potential of spatial anion control in transition-metal catalysis for the functionalization of C-H bonds under mild conditions.

The site-selectivity and mechanism of Pd-catalyzed C(sp2)-H arylation of simple arenes

Control over site-selectivity is a critical challenge for practical application of catalytic C–H functionalization reactions in organic synthesis. Despite the seminal breakthrough of the Pd-catalyzed C(sp²)–H arylation of simple arenes via a concerted metalation–deprotonation (CMD) pathway in 2006, understanding the site-selectivity of the reaction still remains elusive. Here, we have comprehensively investigated the scope, site-selectivity, and mechanism of the Pd-catalyzed direct C–H arylation reaction of simple arenes. Counterintuitively, electron-rich arenes preferably undergo meta-arylation without the need for a specifically designed directing group, whereas electron-deficient arenes bearing fluoro or cyano groups exhibit high ortho-selectivity and electron-deficient arenes bearing bulky electron-withdrawing groups favor the meta-product. Comprehensive mechanistic investigations through a combination of kinetic measurements and stoichiometric experiments using arylpalladium complexes have revealed that the Pd-based catalytic system works via a cooperative bimetallic mechanism, not the originally proposed monometallic CMD mechanism, regardless of the presence of a strongly coordinating L-type ligand. Notably, the transmetalation step, which is influenced by a potassium cation, is suggested as the selectivity-determining step.

The transmetalation step, not the C–H activation step, is suggested as the selectivity-determining step in Pd-catalyzed C–H arylation of simple arenes.

Structurally Modified Norbornenes: A Key Factor to Modulate Reaction Selectivity in the Palladium/Norbornene Cooperative Catalysis

Palladium/norbornene (Pd/NBE) cooperative catalysis has received enormous attention and found numerous synthetic applications in the past two decades. Considering the critical roles that NBE plays in the catalytic cycle, the use of structurally modified NBEs (smNBEs), starting from 2015, has become an important approach to address limitations and modulate reaction selectivity in Pd/NBE catalysis. This Perspective highlights the development of three types of smNBEs: C1-substituted, C2-substituted, and C5-substituted or C5,C6-disubstituted NBEs, as well as their synthetic applications toward site-selective C-H functionalization. A focus is on the structure-activity relationship of smNBEs in these reactions, and rationales for using smNBEs in many cases have also been provided.

Palladium-Catalyzed ortho-C-H Glycosylation/ipso-Alkenylation of Aryl Iodides

This report describes the first example of palladium-catalyzed ortho-C-H glycosylation/ipso-alkenylation of aryl iodides, and the easily accessible glycosyl chlorides are used as a glycosylation reagent. The reaction is compatible with the functional groups of the substrates, and a series of C-aryl glycosides have been synthesized in good to excellent yield and with excellent diastereoselectivity. It is found that a cheap 5-norbornene-2-carbonitrile as a transient mediator can effectively promote this reaction. In addition, ipso-arylation and cyanation were also realized by the strategy.

meta-Selective C-H Arylation of Fluoroarenes and Simple Arenes

Fluorine is known to promote ortho-C-H metalation. Based upon this reactivity, we employed an activated norbornene that traps the ortho-palladation intermediate and is then relayed to the meta position, leading to meta-selective C-H arylation of fluoroarenes. Deuterium experiment suggests that this meta-arylation is initiated by ortho C-H activation and the catalytic cycle is terminated by C-2 protonation. A dual-ligand system is crucial for the observed high reactivity and site selectivity. Applying this approach to simple benzene or other arenes also affords arylation products with good yield and site selectivity.

Catalytic Behavior of Mono-N-Protected Amino-Acid Ligands in Ligand-Accelerated C-H Activation by Palladium(II)

Mono-N-protected amino acids (MPAAs) are increasingly common ligands in Pd-catalyzed C-H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C-H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand-accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand-to-metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C-H activation and catalytic C-H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50-100-fold increase in rate when only 0.05 equivalents of MPAA are present relative to PdII . These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to PdII enables a single MPAA to support C-H activation at multiple PdII centers.