Arene-Limited Nondirected C-H Activation of Arenes

Site-Selective Ortho/Ipso C-H Difunctionalizations of Arenes using Thianthrene as a Leaving Group

Site-selective ortho/ipso C-H difunctionalizations of aromatic compounds were designed to afford polyfunctionalized arenes including challenging 1,2,3,4-tetrasubstituted ones (62 examples, up to 97 % yields). To ensure the excellent regioselectivity of the process while keeping high efficiency, an original strategy based on a "C-H thianthenation/Catellani-type reaction" sequence was developed starting from simple arenes. Non-prefunctionalized arenes were first regioselectively converted into the corresponding thianthrenium salts. Then, a palladium-catalyzed, norbornene (NBE)-mediated process allowed the synthesis of ipso-olefinated/ortho-alkylated polyfunctionalized arenes using a thianthrene as a leaving group (revisited Catellani reaction). Pleasingly, using a commercially available norbornene (NBE) and a unique catalytic system, synthetic challenges known for the Catellani reaction with aryl iodides were smoothly and successfully tackled with the "thianthrenium" approach. The protocol was robust (gram-scale reaction) and was widely applied to the two-fold functionalization of various arenes including bio-active compounds. Moreover, a panel of olefins and alkyl halides as coupling partners was suitable. Pleasingly, the "thianthrenium" strategy was successfully further applied to the incorporation of other groups at the ipso (CN/alkyl/H, aryl) and ortho (alkyl, aryl, amine, thiol) positions, showcasing the generality of the process.

Ligand-Enabled ortho-Selective C-H Olefination of Tertiary Aniline Derivatives

A highly ortho-selective CAr-H olefination of tertiary anilines without a directing group was developed. This reaction tolerated various substituted arenes and olefin coupling partners, affording ortho-olefination products in moderate to good yields. Preliminary mechanistic studies showed that N-Ac-d-Ala, Ag2CO3, and BQ were the key factors for tuning the regioselectivity from para to ortho. Density functional theory was used to achieve a theoretical understanding of the ortho selectivity.

Non-Directed C-H Arylation of Anisole Derivatives via Pd/S,O-Ligand Catalysis

Non-directed C-H arylation is one of the most efficient methods to synthesize biaryl compounds without the need of the prefuctionalization of starting materials, or the installment and removal of directing groups on the substrate. A direct C-H arylation of simple arenes as limiting reactants remains a challenge. Here we disclose a non-directed C-H arylation of anisole derivatives as limiting reagents with aryl iodides under mild reaction conditions. The arylated products are obtained in synthetically useful yields and the arylation of bioactive molecules is also demonstrated. Key to the success of this methodology is the use of a one-step synthesized S,O-ligand.

Allylsilane as a versatile handle in photoredox catalysis

Organosilanes have secured a special place in the synthetic world for several decades. However, among them, allylsilanes are a choice reagent for organic chemists to develop novel organic transformations. In recent years researchers have proved that visible-light photoredox catalysis has emerged as one of the most mild, sustainable, straightforward, and efficient strategies to construct simple to complex molecules with or without enantioselectivity. This review provides an in-depth analysis of recent advances and strategies employed in visible-light photoredox catalysis for allylsilane and its analogues for the development of various organic transformations. The review is divided into sections, each focused on a specific reactivity of allylsilane under light irradiation with C(sp2) center arene or alkene, C(sp2) center carbonyl, and C(sp3) center carbon functionality. In this review, we present optimization data, reaction scope, and mechanistic aspects to bring forward specific reactivity and selectivity trends of allylsilane in photoredox conditions.

B(C6 F5 )3 -Catalyzed [2+3]-Cyclative o,m-diC-H Functionalization of Phenols

Metal-free catalytic C-H functionalization is highly desired for the construction of C-C bonds. We herein report a highly chemoselective consecutive C-H [2+3]-cyclative functionalization for the simultaneous formation of two C-C bonds with construction of polycyclic phenols catalyzed by commercially available and low-cost B(C6 F5 )3 . This catalytic system tolerates a wide range of substrate scope, providing a series of 2,6,7,8-tetrahydroacenaphthylen-3-ol-type polycyclic compounds efficiently. Several derivatizations of the catalytic products have also been conducted to show the potential application of this method in synthesis of polycyclic compounds.

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.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Dinuclear Gold-Catalyzed para-Selective C-H Arylation of Undirected Arenes by Noncovalent Interactions

The regioselectivity of C-H functionalization is commonly achieved by directing groups, electronic factors, or steric hindrance, which facilitate the identification of reaction sites. However, such strategies are less effective for reactants such as simple monofluoroarenes due to their relatively low reactivity and the modest steric demands of the fluorine atom. Herein, we present an undirected gold-catalyzed para-C-H arylation of a wide array of monofluoroarenes using air-stable aryl silanes and germanes at room temperature. A high para-regioselectivity (up to 98 : 2) can be realized by utilizing a dinuclear dppm(AuOTs)2 (dppm=bis(diphenylphosphino)methane) as the catalyst and hexafluorobenzene as the solvent. This provides a general and practical protocol for the concise construction of structurally diverse para-arylated monofluoroarenes through C-H activation manner. It features excellent functional group tolerance and a broad substrate scope (>80 examples). Besides, this strategy is also robust for other simple monosubstituted arenes and heteroarenes. Our mechanistic studies and theoretical calculations suggest that para-C-H selectivity arises from highly electrophilic and structurally flexible dinuclear Ar-Au(III)-Au(I) species, coupled with noncovalent interaction induced by hexafluorobenzene.

Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C-H Activation of Arenes

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiNDipp}AlI] ({SiNDipp} = {CH2SiMe2NDipp}2) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiNDipp}AlM]2. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiNDipp}Na2] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C-H oxidative addition of arene C-H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C-H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M+ cation, which becomes increasingly favorable as group 1 is descended.

Controlling Reactivity and Selectivity in the Nondirected C-H Activation of Arenes with Palladium

ConspectusAromatic structures are widespread motifs throughout organic chemistry, and C-H activation has been recognized as a major tool for enabling their sustainable and efficient functionalization. Through C-H activation, arenes can be modified without the need for prefunctionalization, leading to inherent atom- and step-economic advantages over traditional methods. However, for the development of synthetically useful methods, several hurdles have to be overcome. The strength of C-H bonds necessitates the development of sufficiently reactive catalysts, while the presence of multiple C-H bonds within a substrate poses challenges in terms of site-selectivity. Traditionally these challenges have been addressed by substrate control. By attaching different directing groups (DGs), the reactivity of the respective arene was significantly enhanced and the DG guided the metal in close proximity to specific C-H bonds, resulting in high site-selectivity. However, the introduction and removal of the DG add additional steps to the synthetic sequence, and the scope of the reaction is limited to a specific substrate class. The development of complementary nondirected methods that can be applied to a broad range of arenes without the necessity to carry a specific functional group that coordinates to Pd (referred to as simple arenes) is therefore highly desirable. However, the intrinsically lower reactivity of such substrates and the absence of a selectivity-determining DG pose significant challenges that can be solved only by the development of highly efficient catalysts. Consequently, the field of nondirected C-H activation, especially with respect to Pd-catalyzed methods, remained comparatively underdeveloped when we initiated our research program in 2017. At that time, state-of-the-art methods required the arene to be used in large excess, precluding its use in late-stage functionalization. Since organopalladium species are among the most versatile synthetic intermediates, we realized that developing a system, which can effectively and selectively activate C-H bonds in simple arenes with the arene as the limiting reagent, would be a powerful tool in synthetic organic chemistry. This account summarizes our groups' research toward the development and application of catalytic systems offering this desired reactivity and focuses explicitly on Pd-catalyzed nondirected C-H functionalization reactions of arenes, where the arene is employed as a limiting reagent. After an introduction that summarizes the state of Pd-catalyzed C-H activation of arenes before 2017 and the associated challenges, experimental and mechanistic details about the development of the first arene-limited, nondirected C-H functionalization of simple arenes with palladium will be discussed. This reactivity was enabled by the identification and combination of two complementary ligands, an N-heterocycle and an amino acid-derived ligand. Afterward we will discuss the expansion of this dual-ligand approach to further arene-limited transformations. Finally, we describe two methodologies that originated from the observations we made during our studies, namely, the late-stage deuteration of simple arenes and a highly selective olefination method that uses noncovalent interactions to induce meta selectivity.

Diversification of Complex Diaryl Ethers via Diaryliodonium Intramolecular Aryl Rearrangement

In this study, we present an efficient site-selective O-arylation method applicable to a broad range of complex arenes involving intramolecular aryl rearrangement. The reaction was facilitated by diaryliodonium salts bearing vicinal trifluoromethanesulfonate (OTf) groups. The procedure was initiated with selective C-H bond activation of arenes, which were then converted into diaryl ethers through nucleophilic aromatic substitution (SNAr). This synthetic method successfully affords complex diaryl ether derivatives, showcasing its practicality for the diversification of functionalized arenes and pharmaceutical agents.

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.

FeCl3-catalyzed oxidative amidation of benzylic C-H bonds enabled by a photogenerated chlorine-radical

Herein, we report the development of iron-catalyzed benzylic C-H oxidative amidation reactions via photoinduced ligand-to-metal charge transfer (LMCT). These reactions exhibit a broad substrate scope (60 examples) and offer operationally simple, scalable procedures for accessing valuable products from methylarenes in a single step. Mechanistic studies and control experiments confirm the participation of a photogenerated chlorine radical in facilitating the hydrogen atom transfer (HAT) from the benzylic C-H bond to initiate the reaction.

Electrocatalyzed direct arene alkenylations without directing groups for selective late-stage drug diversification

Electrooxidation has emerged as an increasingly viable platform in molecular syntheses that can avoid stoichiometric chemical redox agents. Despite major progress in electrochemical C-H activations, these arene functionalizations generally require directing groups to enable the C-H activation. The installation and removal of these directing groups call for additional synthesis steps, which jeopardizes the inherent efficacy of the electrochemical C-H activation approach, leading to undesired waste with reduced step and atom economy. In sharp contrast, herein we present palladium-electrochemical C-H olefinations of simple arenes devoid of exogenous directing groups. The robust electrocatalysis protocol proved amenable to a wide range of both electron-rich and electron-deficient arenes under exceedingly mild reaction conditions, avoiding chemical oxidants. This study points to an interesting approach of two electrochemical transformations for the success of outstanding levels of position-selectivities in direct olefinations of electron-rich anisoles. A physical organic parameter-based machine learning model was developed to predict position-selectivity in electrochemical C-H olefinations. Furthermore, late-stage functionalizations set the stage for the direct C-H olefinations of structurally complex pharmaceutically relevant compounds, thereby avoiding protection and directing group manipulations.

Extended Single-Electron Transfer Model and Dynamically Associated Energy Transfer Event in a Dual-Functional Catalyst System

Organic photocatalysis has been developed flourishingly to rely on bimolecular energy transfer (EnT) or oxidative/reductive electron transfer (ET), promoting a variety of synthetic transformations. However, there are rare examples to merge EnT and ET processes rationally within one chemical system, of which the mechanistic investigation still remains in its infancy. Herein, the first mechanistic illustration and kinetic assessments of the dynamically associated EnT and ET paths were conducted for realizing the C-H functionalization in a cascade photochemical transformation of isomerization and cyclization by using the dual-functional organic photocatalyst of riboflavin. An extended single-electron transfer model of transition-state-coupled dual-nonadiabatic crossings was explored to analyze the dynamic behaviors in the proton transfer-coupled cyclization. This can also be used to clarify the dynamic correlation with the EnT-driven E → Z photoisomerization that has been kinetically evaluated by using Fermi's golden rule with the Dexter model. The present computational results of electron structures and kinetic data contribute to a fundamental basis for understanding the photocatalytic mechanism of the combined operation of EnT and ET strategies, which will guide the design and manipulation for the implementation of multiple activation modes based on a single photosensitizer.

Dual-Ligand Catalyst for the Nondirected C-H Olefination of Heteroarenes

Pd(II)-catalyzed nondirected C-H functionalization of heteroarenes is a significant challenge for the following reasons: poor reactivity of electron-deficient heterocycles and the unproductive coordination of Lewis basic nitrogen atoms. Existing methodologies using palladium catalysis often employ a large excess of heterocycle substrates to overcome these hurdles. Despite recent advances in nondirected functionalization of arenes that allow them to be used as limiting reagents, the reaction conditions are incompatible with electron-deficient heteroarenes. Herein we report a dual-ligand catalyst that enables Pd(II)-catalyzed nondirected C-H olefination of heteroarenes without using a large excess of substrate. In general, the use of 1-2 equiv of substrates was sufficient to obtain synthetically useful yields. The reactivity was rationalized by the synergy between two types of ligands: a bidentate pyridine-pyridone ligand promotes C-H cleavage; the monodentate heterocycle substrate acts as a second ligand to form a cationic Pd(II) complex that has high affinity for arenes. The proposed dual-ligand cooperation is supported by a combination of X-ray, kinetics, and control experiments.

Palladium-Catalyzed Chemoselective Oxygenation of C(sp2)-H and C(sp3)-H Bonds in Isatins

The palladium-catalyzed chemoselective C(sp²)-H and C(sp³)-H bond oxygenation of substituted isatin derivatives is reported. This mild protocol exhibits the C5 C(sp²)-H oxygenation of isatins through electrophilic intermolecular C-H palladation in concentrated solutions using PhI(OAc)₂ or Selectfluor as an oxidant, whereas it exhibits-N-CH₃ C(sp³)-H oxygenation in dilute solutions via carbonyl-assisted intramolecular palladation in the presence of K₂S₂O₈. This oxygenation reaction provides a direct and unified approach for synthesizing diverse oxygenated isatins with sensitive functionalities, including biorelevant compounds.

Mechanistic studies of the palladium-catalyzed S,O-ligand promoted C-H olefination of aromatic compounds

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has been shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions.

Rhodium(III)-catalyzed C-H alkylation of arylhydrophthalazinediones with α-Cl ketones as sp3-carbon alkylated agents

A Rh(III)-catalyzed C-H bond direct alkylation between 2-arylphthalazine-1,4-diones and α-Cl ketones, which are sp³-carbon synthons, under mild conditions has been disclosed. The corresponding phthalazine derivatives are readily obtained in moderate to excellent yields with a wide range of substrates and high functional group tolerance. The practicality and utility of this method are demonstrated by the derivatization of the product.

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.

Charge-controlled Pd catalysis enables the meta-C–H activation and olefination of arenes

The regioselective C-H activation of arenes remains one of the most promising techniques for accessing highly important functionalized motifs. Such functionalizations can generally be achieved through directed and non-directed processes. The directed approach requires a covalently attached directing group (DG) on the substrate to induce reactivity and selectivity and therefore intrinsically leaves a functional group at the point of attachment within the molecule, even after the tailored DG has been removed. Conversely, non-directed methods typically suffer from regioselectivity issues, especially for unbiased substrates. Herein, we report a unique approach that employs weak charge-charge and charge-dipole interactions to enable the meta-selective activation and olefination of arenes to address these challenges in Pd catalysis. The charged moiety can easily be converted to uncharged simple arenes by hydrogenation or cross-coupling. In-depth mechanistic studies prove that the charge is responsible for the observed selectivity. We expect our studies to be generalizable and thereby enable further regioselective transformations.

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.

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.

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.

Dual Ligand Enabled Nondirected C-H Chalcogenation of Arenes and Heteroarenes

Chalcogenide motifs are present as principal moieties in a vast array of natural products and complex molecules. Till date, the construction of these chalcogen motifs has been restricted to either the use of directing groups or the employment of a large excess of electronically activated arenes, typically employed as a cosolvent. Despite being highly effective, these methods have their own limitations in the step economy and the deployment of an excess amount of arenes. Herein, we report the evolution of a catalytic system employing arene-limited, nondirected thioarylation of arenes and heteroarenes using a complimentary dual-ligand approach. The reaction is controlled by a combination of steric and electronic factors, and the utilization of a suitable ligand enables the generation of products on a complimentary spectrum to that generated by classical methods. The combination of ligands remains imperative in the reaction protocol with theoretical calculations pointing towards a monoprotected amino acid ligand being crucial in the concerted metalation deprotonation (CMD) mechanism by a characteristic [5,6]-palladacyclic transition state, while the pyridine moiety assists in the active catalyst species formation and product release. Combined experimental and computational mechanistic investigations point toward the C-H activation step being both regio- and rate-determining. Interestingly, oxidative addition of the diphenyl disulfide substrate is found to be unlikely, and an alternative transmetalation-like mechanism involving the Pd-Ag heterometallic complex is proposed to be operative.

Selective ortho-C-H Activation in Arenes without Functional Groups

Aromatic C-H activation in alkylarenes is a key step for the synthesis of functionalized organic molecules from simple hydrocarbon precursors. Known examples of such C-H activations often yield mixtures of products resulting from activation of the least hindered C-H bonds. Here we report highly selective ortho-C-H activation in alkylarenes by simple iridium complexes. We demonstrate that the capacity of the alkyl substituent to override the typical preference of metal-mediated C-H activation for the least hindered aromatic C-H bonds results from transient insertion of iridium into the benzylic C-H bond. This enables fast iridium insertion into the ortho-C-H bond, followed by regeneration of the benzylic C-H bond by reductive elimination. Bulkier alkyl substituents increase the ortho selectivity. The described chemistry represents a conceptually new alternative to existing approaches for aromatic C-H bond activation.

Metal-catalysed C-H bond activation and borylation

Transition metal-catalysed direct borylation of hydrocarbons via C-H bond activation has received a remarkable level of attention as a popular reaction in the synthesis of organoboron compounds owing to their synthetic versatility. While controlling the site-selectivity was one of the most challenging issues in these C-H borylation reactions, enormous efforts of several research groups proved instrumental in dealing with selectivity issues that presently reached an impressive level for both proximal and distal C-H bond borylation reactions. For example, in the case of ortho C-H bond borylation reactions, innovative methodologies have been developed either by the modification of the directing groups attached with the substrates or by creating new catalytic systems via the design of new ligand frameworks. Whereas meta and para selective C-H borylations remained a formidable challenge, numerous innovative concepts have been developed within a very short period of time by the development of new catalytic systems with the employment of various noncovalent interactions. Moreover, significant advancements have occurred for aliphatic C(sp³)-H borylations as well as enantioselective borylations. In this review article, we aim to discuss and summarize the different approaches and findings related to the development of directed proximal ortho, distal meta/para, aliphatic (racemic and enantioselective) borylation reactions since 2014. Additionally, considering the C-H borylation reaction as one of the most important mainstream reactions, various applications of this C-H borylation reaction toward the synthesis of natural products, therapeutics, and applications in materials chemistry will be summarized in the last part of this review article.

Formal C-H/C-I Metathesis: Site-Selective C-H Iodination of Anilines Using Aryl Iodides

Functional group metathesis has the potential to render mild reaction conditions for C-H functionalization. Protocols for the meta- and ortho-C-H iodination of aniline derivatives via formal C(sp²)-H/C(sp²)-I metathesis using 2-nitrophenyl iodides as mild iodinating reagents are reported herein. These protocols led to the production of a range of valuable iodinated aniline derivatives. These results demonstrate the potential of developing novel site-selective C-H activation reactions with electron-rich compounds, since mild reagents can often been utilized in functional group metathesis reactions.

Divergent Regioselective Csp2-H Difluoromethylation of Aromatic Amines Enabled by Nickel Catalysis

Herein, the first catalytic protocol for nickel-catalyzed ortho or para position difluoromethylation of various aromatic amines has been developed with the assistance of a bidentate phosphine ligand, offering an invaluable synthesis means to construct extensive p-difluoromethylated products and difluorooxindole derivatives with significant functional fragments. Furthermore, the gram-scale reaction, broad substrate scope, excellent functional-group compatibility, late-stage difluoromethylation of pesticides, and even formal synthesis of HDAC6 inhibitors further demonstrate the usefulness of this method.

Remote steric control for undirected meta-selective C-H activation of arenes

Regioselective functionalization of arenes remains a challenging problem in organic synthesis. Steric interactions are often used to block sites adjacent to a given substituent, but they do not distinguish the remaining remote sites. We report a strategy based on remote steric control, whereby a roof-like ligand protects the distant para site in addition to the ortho sites, and thereby enables selective activation of meta carbon-hydrogen (C-H) bonds in the absence of ortho or para substituents. We demonstrate this concept for iridium-catalyzed meta-selective borylation of various monosubstituted arenes, including complex drug molecules. This strategy has the potential to expand the toolbox of C-H bond functionalization to previously nondifferentiable reaction sites.

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

Ruthenium–NHC complex-catalyzed P(iii)-directed C–H borylation of arylphosphines

Bidentate NHC-based ruthenium catalyst for P(III)-directed ortho C–H borylation of arylphosphines.

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.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Construction of α-Amino Azines via Thianthrenation-Enabled Photocatalyzed Hydroarylation of Azine-Substituted Enamides with Arenes

α-Amino azines are widely found in pharmaceuticals and ligands. Herein, we report a practical method for accessing this class of compounds via photocatalyzed hydroarylation of azine-substituted enamides with the in situ-generated aryl thianthrenium salts as the radical precursor. This reaction features a broad substrate scope, good functional group tolerance, and mild conditions and is suitable for the late-stage installation of α-amino azines in complex structures.

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes

Indenyl (Ind) rhodium N-heterocyclic carbene (NHC) complexes [Rh(η⁵ -Ind)(NHC)(L)] were synthesised for 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with L=C₂ H₄ (1), CO (2 a) and cyclooctene (COE; 3), for 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C₂ H₄ )₂ ] did not give the desired SIPr complex, thus demonstrating the "indenyl effect" in the synthesis of 1. Oxidative addition of HSi(OEt)₃ to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)₃ }(H)(SIPr)] (6) with loss of COE. Tethered-fluorenyl NHC rhodium complexes [Rh{(η⁵ -C₁₃ H₈ )C₂ H₄ N(C)C₂ H_x NR}(L)] (x=4, R=Dipp, L=C₂ H₄ : 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5-31 %) in comparison to good yields for the monodentate complexes (49-79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B₂ pin₂ (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl-tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B₂ pin₂ at 120 and 140 °C, respectively, was monitored by ¹¹ B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4-7 days, thus demonstrating catalysed sp³ C-H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C₂ H₄ and COE ligands, whereas CO complexes were inert. Evidence for C-H bond activation in the alkyl groups of the NHC ligands was obtained.

Palladium-Catalyzed Nondirected Late-Stage C-H Deuteration of Arenes

We describe a palladium-catalyzed nondirected late-stage deuteration of arenes. Key aspects include the use of D2O as a convenient and easily available deuterium source and the discovery of highly active N,N-bidentate ligands containing an N-acylsulfonamide group. The reported protocol enables high degrees of deuterium incorporation via a reversible C-H activation step and features extraordinary functional group tolerance, allowing for the deuteration of complex substrates. This is exemplified by the late-stage isotopic labeling of various pharmaceutically relevant motifs and related scaffolds. We expect that this method, among other applications, will prove useful as a tool in drug development processes and for mechanistic studies.

Selective, radical-free activation of benzylic C-H bonds in methylarenes

We report rare examples of exclusive benzylic C-H oxidative addition in industrially important methylarenes using simple η4-arene iridium complexes. Mechanistic studies showed that coordinatively unsaturated η2-arene intermediates are responsible for the selective activation of benzylic, not aromatic C-H bonds and formation of stable benzyl complexes after trapping with a phosphine ligand.

C-H Bond Functionalization of (Hetero)aryl Bromide Enabled Synthesis of Brominated Biaryl Compounds

Aryl bromide is one of the most important compounds in organic chemistry, because it is widely used as synthetic building blocks enabling quick access to a wide array of bioactive molecules, organic materials, and polymers via the versatile cutting-edge transformations of C-Br bond. Direct C-H bond functionalization of aryl bromide is considered to be an efficient way to prepare functionalized aryl bromides; however, it is rarely explored possibly due to the relatively low reactivity of aryl bromide toward C-H bond activation. We herein report a palladium-catalyzed coupling reaction between aryl iodide and aryl bromide for preparing brominated biaryl compounds via a silver-mediated C-H bond activation pathway.

Mechanism of the Arene-Limited Nondirected C-H Activation of Arenes with Palladium*

Recently palladium catalysts have been discovered that enable the directing-group-free C-H activation of arenes without requiring an excess of the arene substrate, thereby enabling methods for the late-stage modification of complex organic molecules. The key to success has been the use of two complementary ligands, an N-acyl amino acid and an N-heterocycle. Detailed experimental and computational mechanistic studies on the dual-ligand-enabled C-H activation of arenes have led us to identify the catalytically active species and a transition state model that explains the exceptional activity and selectivity of these catalysts. These findings are expected to be highly useful for further method development using this powerful class of catalysts.