Palladium-catalyzed meta-selective C-H bond activation with a nitrile-containing template: computational study on mechanism and origins of selectivity

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.

Generating High-Precision Force Fields for Molecular Dynamics Simulations to Study Chemical Reaction Mechanisms Using Molecular Configuration Transformer

Theoretical studies on chemical reaction mechanisms have been crucial in organic chemistry. Traditionally, calculating the manually constructed molecular conformations of transition states for chemical reactions using quantum chemical calculations is the most commonly used method. However, this way is heavily dependent on individual experience and chemical intuition. In our previous study, we proposed a research paradigm that used enhanced sampling in molecular dynamics simulations to study chemical reactions. This approach can directly simulate the entire process of a chemical reaction. However, the computational speed limited the use of high-precision potential energy functions for simulations. To address this issue, we presented a scheme for training high-precision force fields for molecular modeling using a previously developed graph-neural-network-based molecular model, molecular configuration transformer. This potential energy function allowed for highly accurate simulations at a low computational cost, leading to more precise calculations of the mechanism of chemical reactions. We applied this approach to study a Claisen rearrangement reaction and a carbonyl insertion reaction catalyzed by manganese.

Pd(II)/LA-catalyzed acetanilide olefination with dioxygen

Transition-metal-catalyzed aromatic olefination through direct C-H activation represents an atom and step-economic route for versatile pharmaceutical syntheses, and in many cases, different stoichiometric oxidants are frequently employed for achieving a reasonable catalytic efficiency of the transition metal ions. Herein, we report a Lewis acid promoted Pd(II)-catalyzed acetanilide olefination reaction with atmospheric dioxygen as the oxidant source. The linkage of the Lewis acid to the Pd(II) species through a diacetate bridge significantly improved its catalytic efficiency, and independent kinetic studies on the olefination step revealed that adding the Lewis acid significantly accelerated the olefination rate as well as the C-H activation step. A strong basicity of the internal base in the Pd(II) salt also benefited the olefination reaction plausibly through base-assisted β-hydride elimination.

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.

The Essence in Selectivity of Copper-Mediated Intermolecular Nucleophilic Substitution of a meta C-H Bond in 2-Methyl-N-methoxyaniline: A Theoretical Study

The detailed mechanism for NHC-Cu(I)-catalyzed intermolecular nucleophilic substitution of the C-H bonds at aniline (2-methyl-N-methoxyaniline) was studied via DFT methods to reveal the essence of the selectivity. Calculations revealed that the meta C-H functionalization proceeds via two nucleophilic attacks on the aromatic ring rather than a one-step meta C-H substitution to give the experimentally observed major product. The reaction is initiated by activation of the substrate via oxidative addition with an NHC-Cu(I) catalyst, through which an umpolung occurs at the ring. From the activated intermediate, methoxyl group transfer to benzyl forms a resting state, while a nucleophile can attack the ortho position of benzyl to form a more stable intermediate. The nucleophile group can then transfer to the meta position by a 1,2-Wagner-Meerwein rearrangement to form the final product through a proton shuttle. In contrast, other transfer processes affording ortho- or para-substituted products encounter higher activation barriers. This work investigates the relationship of product selectivity with the umpolung of the aromatic ring, as well as the priority of a nucleophilic attack at the ortho position of the aromatic, 1,2-Wagner-Meerwein rearrangement from the ortho-substituted intermediate, and proton shuttle from the meta-substituted intermediate.

Plausible PEPPSI catalysts for direct C-H functionalization of five-membered heterocyclic bioactive motifs: synthesis, spectral, X-ray crystallographic characterizations and catalytic activity

In this study, a series of benzimidazolium salts were synthesized as asymmetric N-heterocyclic carbene (NHC) precursors. Nine novel palladium complexes with the general formula [PdX2(NHC)(pyridine)] were synthesized using benzimidazolium salts in the PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) theme. All synthesized Pd(ii) complexes are stable. The synthesized compounds were thoroughly characterized by respective spectroscopic techniques, such as 1HNMR, 13C NMR, FTIR spectroscopy, X-ray crystallography and elemental analysis. The geometric structure of the palladium N-heterocyclic carbene has been optimized in the framework of density functional theory (DFT) using the B3LYP-D3 dispersion functional with LANL2DZ as a basis set. The on/off mechanism of pyridine assisted Pd-NHC complexes made them the best C-H functionalized catalysts for regioselective C-5 arylated products. Five membered heterocyclic compounds such as 2-acetyl furan, furfuryl acetate 2-acetylthiophene and N-methylpyrrole-2-carboxaldehyde were treated with numerous aryl bromides and arylchlorides under optimal catalytic reaction conditions. Interestingly, all the prepared catalysts possessed essential structural features that facilitated the formation of desired coupling products in quantitative yield with excellent selectivity. The arylation reaction of bromoacetophenone was highly catalytically active with only 1 mol% catalyst loading at 150 °C for 2 hours. To check the efficiency of the synthesized complexes, three different five member heterocyclic substrates (2-acetylfuran, 2-acetylthiophen, 2-propylthaizole) were tested with a number of aryl bromides bearing both electron-donating and electron-withdrawing groups on para position. The data in Tables 2-4. Indicated that electron-donating groups on the para position of aryl halide decreased the catalytic conversion while electron-withdrawing groups increased the catalytic conversion this was due to the high nucleophilicity of the electron-donating substituents.

The crucial role of silver(I)-salts as additives in C-H activation reactions: overall analysis of their versatility and applicability

Transition-metal catalyzed C-H activation reactions have been proven to be useful methodologies for the assembly of synthetically meaningful molecules. This approach bears intrinsic peculiarities that are important to be studied and comprehended in order to achieve its best performance. One example is the use of additives for the in situ generation of catalytically active species. This strategy varies according to the type of additive and the nature of the pre-catalyst that is being used. Thus, silver(I)-salts have proven to play an important role, due to the resulting high reactivity derived from the pre-catalysts of the main transition metals used so far. While being powerful and versatile, the use of silver-based additives can raise concerns, since superstoichiometric amounts of silver(I)-salts are typically required. Therefore, it is crucial to first understand the role of silver(I) salts as additives, in order to wisely overcome this barrier and shift towards silver-free systems.

Macrocyclization via remote meta-selective C-H olefination using a practical indolyl template

The synthesis of macrocyclic compounds with different sizes and linkages remains a great challenge via transition metal-catalysed intramolecular C-H activation. Herein, we disclose an efficient macrocyclization strategy via Pd-catalysed remote meta-C-H olefination using a practical indolyl template. This approach was successfully employed to access macrolides and coumarins. In addition, the intermolecular meta-C-H olefination also worked well and was exemplified by the synthesis of antitumor drug belinostat from inexpensive and readily available benzenesulfonyl chloride. Notably, catalytic copper acetate and molecular oxygen were used in place of silver salts as oxidants. Furthermore, for the first time, the formation of a macrocyclophane cyclopalladated intermediate was detected through in situ Fourier-transform infrared monitoring experiments and ESI-MS.

Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc)₂ and [(η²-C₂H₄)₂Rh(μ-OAc)]₂ with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu₂(η²-C₂H₄)₃(μ-OPiv)₆ and [(η²-C₂H₄)₂Rh(μ-OPiv)₂]₂(μ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)-containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of mono-substituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C-H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of mono-substituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C-H activation step for Pd catalysis involving significant η¹-arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rh-mediated arene C-H activation.

Distal meta-C-H functionalization of α-substituted cinnamates

Development of a novel strategy for the palladium-catalyzed selective meta-C-H activation of α-substituted cinnamates and their heterocyclic analogues with various alkenes using nitrile as a directing group (DG) has been described. Importantly, we introduced naphthoquinone, benzoquinones, maleimides and sulfolene as coupling partners in the meta-C-H activation reaction for the first time. Notably, allylation, acetoxylation and cyanation were also achieved through distal meta-C-H functionalization. This novel protocol also includes the coupling of various olefin-tethered bioactive molecules with high selectivity.

A Cheap and Efficient Oxidant (n-Bu)4NNO3-Enabled C(sp2)- and C(sp3)-H Olefination at Room Temperature

To further promote the widely practical application of C-H activation, developing green and mild reaction conditions has invariably been the objective of researchers, especially when it comes to remote C-H activation reactions. Herein, we report a new cheap and powerful (n-Bu)₄NNO₃ oxidant. This oxidant is efficient and universal for Pd(II)-catalyzed sp² and sp³ C-H olefination and allows the reaction to be carried out at room temperature. Because of this, we attempted to make C-H functionalization more economical and environmentally benign.

Functionalized Cycloolefin Ligand as a Solution to Ortho-Constraint in the Catellani-Type Reaction

The Catellani reaction, i.e., the Pd/norbornene (NBE) catalysis, has been evolved into a versatile approach to multisubstituted arenes via the ortho-functionalization/ipso-termination process of a haloarene. Despite significant advances over the past 25 years, this reaction still suffered from an intrinsic limitation in the substitution pattern of haloarene, referred to as "ortho-constraint". When an ortho substituent is absent, the substrate often fails to undergo an effective mono ortho-functionalization process, and either ortho-difunctionalization products or NBE-embedded byproducts predominate. To tackle this challenge, structurally modified NBEs (smNBEs) have been developed, which were proved effective for the mono ortho-aminative, -acylative, and -arylative Catellani reactions of ortho-unsubstituted haloarenes. However, this strategy is incompetent for solving the ortho-constraint in Catellani reactions with ortho-alkylation, and to date there lacks a general solution to this challenging but synthetically useful transformation. Recently, our group developed the Pd/olefin catalysis, in which an unstrained cycloolefin ligand served as a covalent catalytic module to enable the ortho-alkylative Catellani reaction without NBE. In this work, we show that this chemistry could afford a new solution to ortho-constraint in the Catellani reaction. A functionalized cycloolefin ligand bearing an amide group as the internal base was designed, which allowed for mono ortho-alkylative Catellani reaction of iodoarenes suffering from ortho-constraint before. Mechanistic study revealed that this ligand is capable of both accelerating the C-H activation and inhibiting side reactions, which accounts for its superior performance. The present work showcased the uniqueness of the Pd/olefin catalysis as well as the power of rational ligand design in metal catalysis.

Multifaceted role of silver salts as ligand scavengers and different behavior of nickel and palladium complexes: beyond halide abstraction

The reaction of [NiArBr(PPh₃)₂] with AgBF₄ brings about the abstraction of both the halide and phosphine from the nickel center by silver. When the reaction is carried out in CH₂Cl₂/toluene a mixture of the cationic aquo derivatives [NiAr(H₂O)(PPh₃)₂]BF₄ (2) and [NiAr(H₂O)₂(PPh₃)]BF₄ (3) is formed, along with AgBr and [Ag(PPh₃)_n]BF₄. When the same reaction is carried out in acetone as the solvent, it leads to the completely different complex [NiAr(κ²-O, O-MeC(O)CH₂C(OH)Me₂)(PPh₃)] (5), bearing a chelating ligand formed by the aldol self-condensation of acetone. Phosphine abstraction by silver is less favorable for the analogous palladium(II) complexes and only occurs if a large excess of AgBF₄ is used. Thus, silver salts can be safely used as halide scavengers for palladium derivatives. However, the generation of cationic Ni complexes from neutral precursors by halide extraction with a silver salt may produce naked species, different than those expected, and highly reactive in certain media.

Overlooked potential of N,N-bidentate directing-groups in Ni-catalyzed C-H functionalization of benzamides

The Ni-catalyzed reactions of benzamides with bicyclic alkenes were explored using DFT calculations. An unprecedented "N-H deprotonation circumvented" catalytic mechanism was proposed, over the more common N-H/C-H activation mechanism, in which (i) the circumvention of N-H deprotonation ensures the presence of N-H⋯O hydrogen bond interaction, thereby stabilizing the critical ortho-C-H functionalization TS; and (ii) the N-H moiety retention results in a weak N⋯Ni σ-coordination, which is flexible to the configurational conversion during the key alkene insertion. These overlooked aspects of the functionalized N,N-bidentate directing groups will aid the design of new related catalytic reactions.

Pd(II)/Lewis Acid Catalyzed Intramolecular Annulation of Indolecarboxamides with Dioxygen through Dual C-H Activation

Transition-metal ion catalyzed intramolecular dual C-H activation to construct polycyclic heteroarene skeletons is merited for its step and atom-economic advantages in organic synthesis. However, in most cases, stoichiometric oxidants, elevated temperature, and other harsh conditions were commonly faced for this reaction, which apparently block the synthetic applications. Herein, we report a Pd(II)/LA (LA: Lewis acid) catalyzed intramolecular dual C-H activation to construct indoloquinolinone derivatives under mild conditions with dioxygen as the sole oxidant. It was found that adding LA such as Sc³⁺ to Pd(OAc)₂ sharply improved its catalytic efficiency, whereas Pd(OAc)₂ alone was very sluggish. The activity improvement was attributed to the linkage of the Sc³⁺ cation to the Pd(II) species through a diacetate bridge that significantly enhanced the electrophilic properties of Pd(II) for dual C-H activation.

The Renaissance of Organo Nitriles in Organic Synthesis

In the arena of functional group-oriented organic synthesis, the nitrile or cyano functionality is of immense importance. The presence of nucleophilic N -atom, π-coordinating ability of the triple bond, and electrophilic C-center imparts unique and interesting reactivity. Owing to the ability of the nitrile to transform into various other functional groups or intermediates, the chemistry is very rich and diverse. In particular, the involvement of nitrile in numerous organic reactions such as inter- or intramolecular alkyne insertion, [2 + 2 + 2] cycloaddition with alkynes, [3 + 2] cycloaddition with azides, [4 + 2] cycloaddition with diene allow the synthesis of many important carbocycles and heterocycles. Furthermore, the nitrile serves as a directing group in many C-H bond functionalization reactions to introduce diverse functionality and participate as a radical acceptor in radical cascade strategies to obtain a large variety of functional molecules. This review mainly focuses on the reactivity and diverse synthetic application of the nitrile including C-H bond functionalization, alkyne insertion, cycloaddition, and thermal or photochemical cascade strategy. The objective of the current review aims at bringing out the striking collection of various nitrile-triggered organic transformations.

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.

Ligand-Promoted Fluorinated Olefination of Isatins at the C5 Position via a Palladium Catalyst

A palladium-catalyzed nondirected fluorinated olefination was developed. The oxalyl amide ligand greatly improved the yield of the reaction. A wide variety of isatin derivatives were well tolerated and yielded the corresponding products in moderate to good yields. Various fluorinated olefins were also compatible. The application and synthesis of bioactive compounds such as a Metisazone derivative highlight the synthetic value of this approach.

Meta-Selective C-H Functionalization of Arylsilanes Using a Silicon Tethered Directing Group

We describe meta-selective C-H functionalization of arylsilanes using a Si-tethered directing group. The current method enables a selective alkenylation of arenes bearing a variety of functional groups, and several electron-deficient olefins are also applicable as coupling partners. Further functional group transformations of the silicon-tethered directing group provide multisubstituted arenes efficiently.

Palladium-Catalyzed Regioselective B(9)-Amination of o-Carboranes and m-Carboranes in HFIP with Broad Nitrogen Sources

Amination of carboranes has a good application prospect in organic and pharmaceutical synthesis. However, the current methods used for this transformation suffer from limitations. Herein, we report a practical method for a highly regioselective formation of a B-N bond by Pd(II)-catalyzed B(9)-H amination of o- and m-carboranes in hexafluoroisopropanol (HFIP) with different nitrogen sources under air atmosphere. The silver salt and HFIP solvent play critical roles in the present protocol. The mechanistic study reveals that the silver salt acts as a Lewis acid to promote the electrophilic palladation step by forming a heterobimetallic active catalyst PdAg(OAc)₃; the strong hydrogen-bond-donating ability and low nucleophilicity of HFIP enhance the electrophilic ability of Pd(II). It is believed that these N-containing carboranes are potentially of great importance in the synthesis of new pharmaceuticals.

Deprotonation from an OH on myo-Inositol Promoted by μ2-Bridges with Possible Regioselectivity/Chiral Selectivity

Single-crystal structures of myo-inositol complexes with erbium ([Er₂(C₆H₁₁O₆)₂(H₂O)₅Cl₂]Cl₂(H₂O)₄, denoted ErI hereafter) and strontium (Sr(C₆H₁₂O₆)₂(H₂O)₂Cl₂, denoted SrI hereafter) are described. In ErI, deprotonation occurs on an OH of myo-inositol, although the complex is synthesized in an acidic solution, and the pK_a values of all of the OHs in myo-inositol are larger than 12. The deprotonated OH is involved in a μ₂-bridge. The polarization from two Er³⁺ ions activates the chemically relatively inert OH and promotes deprotonation. In the stable conformation of myo-inositol, there are five equatorial OHs and one axial OH. The deprotonation occurs on the only axial OH, suggesting that the deprotonation possesses characteristics of regioselectivity/chiral selectivity. Two Er³⁺ ions in the μ₂-bridge are stabilized by five-membered rings formed by chelating Er³⁺ with an O-C-C-O moiety. As revealed by the X-ray crystallography study, the absolute values of the O-C-C-O torsion angles decrease from ∼60 to ∼45° upon chelating. Since the O-C-C-O moiety is within a six-membered ring, the variation of the torsion angle may exert distortion of the chair conformation. Quantum chemistry calculation results indicate that an axial OH flanked by two equatorial OHs (double ax-eq motif) is favorable for the formation of a μ₂-bridge, accounting for the selectivity. The double ax-eq motif may be used in a rational design of high-performance catalysts where deprotonation with high regioselectivity/chiral selectivity is carried out.

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.

Recyclable Aliphatic Nitrile-Template Enabled Remote meta-C-H Functionalization at Room Temperature

This article describes the development of a new aliphatic nitrile-template-directed remote meta-selective C-H olefin functionalization reaction of arenes. Remarkably, unlike the previous reports, this process is feasible at room temperature and enabled the formation of products with excellent regioselectivity. The present protocol encompasses a broad spectrum of substituted dihydrocinnamic acids and olefins, producing meta-C-H olefinated products (up to 96% yield). In addition, the efficacy of the present method has been showcased by the synthesis of various drug analogues (e.g., cholesterol, estrone, ibuprofen, and naproxen). Significantly, the robustness of meta-olefination was also demonstrated by gram-scale synthesis. The new nitrile-based meta-directing template, in particular, could be easily synthesized in two steps and recycled under mild conditions.

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

Unconventional mechanism and selectivity of the Pd-catalyzed C-H bond lactonization in aromatic carboxylic acid

The search for more effective and highly selective C-H bond oxidation of accessible hydrocarbons and biomolecules is a greatly attractive research mission. The elucidating of mechanism and controlling factors will, undoubtedly, help to broaden scope of these synthetic protocols, and enable discovery of more efficient, environmentally benign, and highly practical new C-H oxidation reactions. Here, we reveal the stepwise intramolecular S_N2 nucleophilic substitution mechanism with the rate-limiting C-O bond formation step for the Pd(II)-catalyzed C(sp³)-H lactonization in aromatic 2,6-dimethylbenzoic acid. We show that for this reaction, the direct C-O reductive elimination from both Pd(II) and Pd(IV) (oxidized by O₂ oxidant) intermediates is unfavorable. Critical factors controlling the outcome of this reaction are the presence of the η³-(π-benzylic)-Pd and K⁺-O(carboxylic) interactions. The controlling factors of the benzylic vs ortho site-selectivity of this reaction are the: (a) difference in the strains of the generated lactone rings; (b) difference in the strengths of the η³-(π-benzylic)-Pd and η²-(π-phenyl)-Pd interactions, and (c) more pronounced electrostatic interaction between the nucleophilic oxygen and K⁺ cation in the ortho-C-H activation transition state. The presented data indicate the utmost importance of base, substrate, and ligand in the selective C(sp³)-H bond lactonization in the presence of C(sp²)-H.

Pd(ii)-catalyzed meta-C–H bromination and chlorination of aniline and benzoic acid derivatives

The classic electrophilic bromination leads to ortho- and para-bromination of anilines due to their electron-rich properties. Herein we report the development of an unprecedented Pd-catalyzed meta-C–H bromination of aniline derivatives using commercially available N-bromophthalimide (NBP), which overcomes the competing ortho/para-selectivity of electrophilic bromination of anilines. The addition of acid additives is crucial for the success of this reaction. A broad range of substrates with various substitution patterns can be tolerated in this reaction. Moreover, benzoic acid derivatives bearing complex substitution patterns are also viable with this mild bromination reaction, and meta-C–H chlorination is also feasible under similar reaction conditions. The ease of the directing group removal and subsequent diverse transformations of the brominated products demonstrate the application potential of this method and promise new opportunities for drug discovery.

An unprecedented Pd-catalyzed meta-C–H bromination and chlorination of highly substituted aniline and benzoic acid derivatives using N-bromophthalimide is reported.

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.

Metal-free three-component cyanoalkylation of quinoxalin-2(1H)-ones with vinylarenes and azobis(alkylcarbonitrile)s

A K2S2O8-mediated C3 cyanoalkylation of quinoxalin-2(1H)-ones via a three-component radical cascade reaction with vinylarenes and azobis(alkylcarbonitrile)s has been achieved.

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.

Rhodium-Catalyzed meta-Selective C-H Carboxylation Reaction of 1,1-Diarylethylenes via Hydrorhodation-Rhodium Migration

A meta-selective C-H carboxylation reaction of 1,1-diarylethylene derivatives with CO₂ by using a rhodium catalyst with NaOⁱ Pr as a stoichiometric reductant has been achieved. Together with hydrogenation of the ethylene moiety, a carboxyl group was introduced to the meta-position of the aryl ring with high selectivity over the ortho-positions. Experimental and computational mechanistic studies indicate that this carboxylation reaction proceeds via hydrorhodation on the ethylene moiety, followed by 1,4-rhodium migration and successive 1,2-rhodium migration on the aryl ring. The use of a bulky phosphine ligand seems to be the key to this unusual aryl-to-aryl 1,2-rhodium shift.

Mechanistic Investigation of Palladium-Catalyzed meta-C-H Bond Activation of Arenes with a Carboxyl Directing Group

The mechanism of Pd(II)-catalyzed meta-C-H bond olefination of arenes with a carboxyl directing group (DG)-containing template has been investigated with density functional theory. The reaction includes three major steps: C-H bond activation, alkene insertion, and β-hydride elimination. The C-H activation step, which proceeds via a concerted metalation-deprotonation pathway, is found to be the rate- and regioselectivity-determining step. We proposed a mono-N-protected amino acid (MPAA)/DG-assisted C-H activation model, in which the carboxyl DG coordinates with the Pd center and delivers it to the meta-position of arene, and the bidentate dianionic MPAA acts as a base for deprotonation. There is a hydrogen bonding interaction between the carboxyl DG and the carboxylate group of MPAA. An alternative Pd(OAc)₂-catalyzed mechanism without involvement of MPAA is also operative. The template is conformationally flexible, and multiple low-energy transition-state conformations contribute to the regioselectivity.

Sequential C-H activation enabled expedient delivery of polyfunctional arenes

Modular construction of polyfunctional arenes from abundant feedstocks stands as an unremitting pursue in synthetic chemistry, accelerating the discovery of drugs and materials. Herein, using the multiple C-H activation strategy with versatile imidate esters, the expedient delivery of molecular libraries of densely functionalized sulfur-containing arenes was achieved, which enabled the concise construction of biologically active molecules, such as Bipenamol.

Mechanism of Pd-catalysed C(sp3)-H arylation of thioethers with Ag(I) additives

Mechanistic studies reveal that Pd-catalyzed C(sp3)-H arylation of thioethers with silver(i) additives takes place via C(sp3)-H activation, oxidative addition and reductive elimination, wherein all steps proceed via the heterodimeric Pd-Ag pathway. Besides, the active heterodimeric Pd-Ag species are detected by mass spectrometry via control experiments.