Beyond Friedel and Crafts: Directed Alkylation of C−H Bonds in Arenes

Electroreductive alkylations of (hetero)arenes with carboxylic acids

Carboxylic acids are widely available and generally inexpensive from abundant biomass feedstocks, and they are suitable and generic coupling partners in synthetic chemistry. Reported herein is an electroreductive coupling of stable and versatile carboxylic acids with (hetero)arenes using protons as the hydrogen source. The application of an earth-abundant titanium catalyst has significantly improved the deoxygenative reduction process. Preliminary mechanistic studies provide insights into the deoxygenative reduction of in-situ generated ketone pathway, and the intermediacy generation of ketyl radical and alkylidene titanocene. Without the necessity of pressurized hydrogen or stoichiometric hydride as reductants, this protocol enables highly selective and straightforward synthesis of various functionalized and structurally diverse alkylbenzenes under mild conditions. The utility of this reaction is further demonstrated through practical and valuable isotope incorporation from readily available deuterium source.

A two carbon homologation of Friedel-Crafts alkylation enabled by photochemical alkene stitching: modular assembly of cyclolignans

Herein we report an efficient two-carbon homologated variant of Friedel-Crafts alkylation via photochemical radical alkene stitching. Readily available feedstock alkenes are used as bridges between photogenerated alkyl radicals and arenes, opening a route to γ-aryl-carbonyls for chemo-divergent access to aryltetralone and γ-lactones, a gateway to 2,7'-cyclolignans.

Friedel-Crafts reactions for biomolecular chemistry

Chemical tools and principles have become central to biological and medical research/applications by leveraging a range of classical organic chemistry reactions. Friedel-Crafts alkylation and acylation are arguably some of the most well-known and used synthetic methods for the preparation of small molecules but their use in biological and medical fields is relatively less frequent than the other reactions, possibly owing to the notion of their plausible incompatibility with biological systems. This review demonstrates advances in Friedel-Crafts alkylation and acylation reactions in a variety of biomolecular chemistry fields. With the discoveries and applications of numerous biomolecule-catalyzed or -assisted processes, these reactions have garnered considerable interest in biochemistry, enzymology, and biocatalysis. Despite the challenges of reactivity and selectivity of biomolecular reactions, the alkylation and acylation reactions demonstrated their utility for the construction and functionalization of all the four major biomolecules (i.e., nucleosides, carbohydrates/saccharides, lipids/fatty acids, and amino acids/peptides/proteins), and their diverse applications in biological, medical, and material fields are discussed. As the alkylation and acylation reactions are often fundamental educational components of organic chemistry courses, this review is intended for both experts and nonexperts by discussing their basic reaction patterns (with the depiction of each reaction mechanism in the ESI) and relevant real-world impacts in order to enrich chemical research and education. The significant growth of biomolecular Friedel-Crafts reactions described here is a testament to their broad importance and utility, and further development and investigations of the reactions will surely be the focus in the organic biomolecular chemistry fields.

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.

Electrochemically Driven para-Selective C(sp2)-H Alkylation Enabled by Activation of Alkyl Halides without Sacrificial Anodes

With alkyl halides (I, Br, Cl) as a coupling partner, an electrochemically driven strategy for para-selective C(sp2)-H alkylation of electron-deficient arenes (aryl esters, aldehydes, nitriles, and ketones) has been achieved to access diverse alkylated arenes in one step. The reaction enables the activation of alkyl halides in the absence of sacrificial anodes, achieving the formation of C(sp2)-C(sp3) bonds under mild electrolytic conditions. The utility of this protocol is reflected in high site selectivity, broad substrate scope, and scalable.

H2 O ⋅ B(C6 F5 )3 -Catalyzed para-Alkylation of Anilines with Alkenes Applied to Late-Stage Functionalization of Non-Steroidal Anti-Inflammatory Drugs

Anilines are core motifs in a variety of important molecules including medicines, materials and agrochemicals. We report a straightforward procedure that allows access to new chemical space of anilines via their para-C-H alkylation. The method utilizes commercially available catalytic H2 O ⋅ B(C6 F5 )3 and is highly selective for para-C-alkylation (over N-alkylation and ortho-C-alkylation) of anilines, with a wide scope in both the aniline substrates and alkene coupling partners. Readily available alkenes are used, and include new classes of alkene for the first time. The mild reaction conditions have allowed the procedure to be applied to the late-stage-functionalization of non-steroidal anti-inflammatory drugs (NSAIDs), including fenamic acids and diclofenac. The formed novel NSAID derivatives display improved anti-inflammatory properties over the parent NSAID structure.

Unlocking regioselective meta-alkylation with epoxides and oxetanes via dynamic kinetic catalyst control

Regioselective arene C-H bond alkylation is a powerful tool in synthetic chemistry, yet subject to many challenges. Herein, we report the meta-C-H bond alkylation of aromatics bearing N-directing groups using (hetero)aromatic epoxides as alkylating agents. This method results in complete regioselectivity on both the arene as well as the epoxide coupling partners, cleaving exclusively the benzylic C-O bond. Oxetanes, which are normally unreactive, also participate as alkylating reagents under the reaction conditions. Our mechanistic studies reveal an unexpected reversible epoxide ring opening process undergoing catalyst-controlled regioselection, as key for the observed high regioselectivities.

π-Electron Fluctuation-Induced P+ /C- Ambiphilic Interaction for Intramolecular CAr -H Bond Activation

Herein, we describe how computational mechanistic understanding has led directly to the discovery of new 2H-phosphindole for C-CAr bond activation and dearomatization reaction. We uncover an unexpected intramolecular C-H bond activation with a 2H-phosphindole derivative. This new intriguing experimental observation and further theoretical studies led to an extension of the reaction mechanism with 2H-phosphindole. Through DFT calculations, we confirm that within a five-membered ring, the polarizable PC3 unit orchestrates the formation of an electrophilic phosphorus atom (P+ ) and a nucleophilic carbon atom (C- ). This kinetically accessible ambiphilic phosphorus/carbon couple is spatially separated by geometric constraints, and their reactivity is modulated through structural resonance.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Asymmetric Catalytic Friedel-Crafts Reactions of Unactivated Arenes

Since its discovery more than a century ago, the Friedel-Crafts reaction has manifested itself as a powerful method for the introduction of carbon substituents to arenes. Despite its potential generality, the scope of the reaction is intrinsically limited by the arene's nucleophilicity, which has previously restrained the applicability of asymmetric variants to activated substrates. To overcome this fundamental limitation, we report herein an asymmetric Friedel-Crafts reaction of unactivated, purely hydrocarbon arenes, alkoxybenzenes, and heteroarenes with N,O-acetals to give enantioenriched arylglycine esters. Highly regio- and stereoselective C-C bond formation was achieved using strong and confined Brønsted acid organocatalysts, enabling the first asymmetric catalytic Friedel-Crafts reaction of simple alkylbenzenes.

Catalyst-controlled regiodivergent C-H bond alkenylation of 2-pyridylthiophenes

A novel and effective Rh^III- and Pd^II-controlled switchable C-H alkenylation of 2-pyridylthiophenes with alkenes is realized. The alkenylation reactions proceeded smoothly in a highly regio- and stereo-selective manner to afford a broad range of C3- and C5-alkenylated products. Depending on the catalyst employed, the reactions involve two typical approaches: C3-alkenylation via chelation-assisted rhodation and C5-alkenylation via electrophilic palladation. This regiodivergent synthetic protocol was successfully applied for the straightforward building of π-conjugated difunctionalized 2-pyridylthiophenes, which may show great potential in organic electronic materials.

Pyridine C(sp2)-H bond functionalization under transition-metal and rare earth metal catalysis

Pyridine is a crucial heterocyclic scaffold that is widely found in organic chemistry, medicines, natural products, and functional materials. In spite of the discovery of several methods for the synthesis of functionalized pyridines or their integration into an organic molecule, new methodologies for the direct functionalization of pyridine scaffolds have been developed during the past two decades. In addition, transition-metal-catalyzed C-H functionalization and rare earth metal-catalyzed reactions have flourished over the past two decades in the development of functionalized organic molecules of concern. In this review, we discuss recent achievements in the transition-metal and rare earth metal-catalyzed C-H bond functionalization of pyridine and look into the mechanisms involved.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

para-Selective, Direct C(sp2)-H Alkylation of Electron-Deficient Arenes by the Electroreduction Process

Direct para-selective C(sp²)-H alkylation of electron-deficient arenes based on the electroreduction-enabled radical addition of alkyl bromides has been developed under mild conditions. In the absence of any metals and redox agents, the simple electrolysis system tolerates a variety of primary, secondary, and tertiary alkyl bromides and behaves as an important complement to the directed alkylation of the C(sp²)-H bond and the classic Friedel-Crafts alkylation. This electroreduction process provides a more straightforward, environmentally benign, and effective alkylation method for electron-deficient arenes.

Directing Group Repositioning Strategy Enabled Site- and Enantioselective Addition of Heteroaromatic C-H Bonds to Acyclic Internal Alkenes

Despite the notable advances achieved in the Murai-type hydroarylations, highly enantioselective catalytic addition of native (hetero)arenes to internal alkenes remains a prominent challenge. Herein, we report a directing group repositioning strategy, which enables the iridium-catalyzed enantioselective addition of heteroarenes including furan, benzofuran, and thiophene to internal enamides. The C-H bond at the C2 position of the heteroarene is site-selectively cleaved and added regioselectively to the β-position of an enamide, affording a valuable β-heteroaryl amide with high enantioselectivity. Mechanistic studies indicate that the rate and the enantioselectivity are determined by separate elementary steps.

Transition Metal-Catalyzed C-H Functionalization Through Electrocatalysis

Electrochemically promoted transition metal-catalyzed C-H functionalization has emerged as a promising area of research over the last few decades. However, development in this field is still at an early stage compared to traditional functionalization reactions using chemical-based oxidizing agents. Recent reports have shown increased attention on electrochemically promoted metal-catalyzed C-H functionalization. From the standpoint of sustainability, environmental friendliness, and cost effectiveness, electrochemically promoted oxidation of a metal catalyst offers a mild, efficient, and atom-economical alternative to traditional chemical oxidants. This Review discusses advances in the field of transition metal-electrocatalyzed C-H functionalization over the past decade and describes how the unique features of electricity enable metal-catalyzed C-H functionalization in an economic and sustainable way.

Triazole-enabled, iron-catalysed linear/branched selective C-H alkylations with alkenes

Iron-catalysed C-H alkylations with alkenes were achieved on benzamides by N-triazole assistance. A notable switch of the regioselectivity from linear to branched was observed depending on the nature of the olefin employed. The approach allowed for the synthesis of a family of decorated benzamides with ample scope and high levels of chemo-, regio- and site-selectivity.

C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BAr^F₄] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ-C:C-(CH₂C(Me)₂)C₆H₄]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr₄^F = B[(3,5-(CF₃)₂)C₆H₃]₄). Each of these compounds promoted facile directed C(sp²)-H activation with exclusive selectivity for ortho-alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.

Palladium-catalysed construction of butafulvenes

Butafulvene is a constitutional isomer of benzene, comprising a cyclobutene skeleton bearing two exocyclic conjugated methylene units. As a result of the intrinsic high strain energy and anti-aromaticity, the preparation of butafulvene compounds has been a fundamental issue for the development of butafulvene chemistry. Here an efficient palladium-catalysed coupling protocol involving propargylic compounds has been developed, providing a solid and versatile strategy for the rapid assembly of symmetric butafulvene derivatives. Based on mechanistic studies, two complementary mechanisms, both involving palladium catalysis, have been confirmed. With the mechanism unveiled, the synthesis of non-symmetric butafulvenes has also been achieved. Advantages of this strategy include tolerance to a wide range of propargylic molecules, mild reaction conditions, simple catalytic systems and easy scalability. The synthetic potential of the products as platform molecules for cyclobutene derivatives has also been demonstrated.

Recent advances in transition-metal-catalyzed directed C-H alkenylation with maleimides

Transition-metal-catalyzed directed C-H alkenylation with maleimides has attracted much attention in recent years, as maleimide core moieties are present in various natural products and pharmaceuticals. In addition, these derivatives can be readily modified into biologically important compounds including succinimides, pyrrolidines and γ-lactams. The efficient chelation-assisted inert C-H bond activation strategy provides straightforward access to a wide array of structurally diverse molecules containing maleimide units. This review describes the major progress and mechanistic investigations on Heck-type reaction/cyclization of maleimides with organic molecules until early 2022.

Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C-H Functionalization

A cobalt-catalyzed intermolecular three-component coupling of arenes, ethylene, and alkynes was developed using the well-defined air-stable cationic bis(phosphine) cobalt(I) complex, [(dcype)Co(η⁶-C₇H₈)][BAr^F₄] (dcype = 1,2-bis(dicyclohexylphosphino)ethane; BAr^F₄ = B[(3,5-(CF₃)₂)C₆H₃]₄), as the precatalyst. All three components were required for turnover and formation of ortho-homoallylated arene products. A range of directing groups including amide, ketone, and 2-pyridyl substituents on the arene promoted the reaction. The cobalt-catalyzed method exhibited broad functional group tolerance allowing for the late-stage functionalization of two drug molecules, fenofibrate and haloperidol. A series of control reactions, deuterium labeling studies, resting state analysis, as well as synthesis of substrate- and product-bound η⁶-arene complexes supported a pathway involving C(sp²)–H activation from a cobalt(III) metallacycle.

Metal-catalyzed asymmetric heteroarylation of alkenes: diverse activation mechanisms

This review summarizes the state-of-the-art in transition metal-catalyzed asymmetric alkylation of heteroarenes using alkenes (covering literature from 2000 to late 2021). Based on elementary reactions on metals for substrate activation, these reactions are broadly classified in several categories: (A) concerted oxidative addition of heteroaryl C-H bonds on rhodium(I) and iridium(I), (B) ligand-to-ligand hydrogen transfer (LLHT) on low-valent 3d metal complexes of nickel and cobalt, (C) different ways for deprotonation of heteroaryl C-H bonds by late transition metal complexes, especially palladium, including electrophilic aromatic substitution and a related mechanism, base-assisted intramolecular electrophilic substitution, concerted and nonconcerted metalation deprotonation, (D) σ-bond metathesis by d⁰ early transition metal complexes, (E) electrophilic activation of olefins by Pd(II), Pt(II) and Au(I), and (F) metal hydride insertion of aryl olefins and dienes. The demand to achieve enantiocontrol in the heteroarylation reactions has also driven innovation in chiral ancillary ligands, exemplified by extremely bulky, chiral N-heterocyclic carbenes for nickel catalysts, bulky monodentate oxazolines for Wacker-type reactions and chiral cyclopentadienyl ligands for half-sandwich complexes of scandium.

Alkylation of Nitropyridines via Vicarious Nucleophilic Substitution

Electrophilic nitropyridines react with sulfonyl-stabilized carbanions to give products of C-H alkylation via vicarious nucleophilic substitution. The process consists of formation of the Meisenheimer-type adduct followed by base-induced β-elimination of the sulfinic acid (e.g., PhSO₂H). Mechanistic studies reveal that in the latter step alkyl substituent and adjacent nitro group tend to planarize for effective stabilization of benzyl anion, and thus, adduct of hindered isopropyl carbanion remains stable toward elimination for steric reasons.

Overcoming peri- and ortho-selectivity in C–H methylation of 1-naphthaldehydes by a tunable transient ligand strategy

This work provides a highly regioselective peri- and ortho-C–H methylation of 1-naphthaldehyde by using a transient ligand strategy, enabling practical synthesis of multi-substituted naphthalene-based bioactive molecules and natural products.

Atom and step economical synthesis of acyclic quaternary centers via iridium-catalyzed hydroarylative cross-coupling of 1,1-disubstituted alkenes

Quaternary benzylic centers are accessed with high atom and step economy by intermolecular Ir-catalyzed hydroarylation of non-polarized 1,1-disubstituted alkenes.

Copper-Catalyzed Direct Perfluoroalkylation of Heteroarenes

An efficient and broadly applicable process is reported for the copper-catalyzed direct perfluoroalkylation of C-H bonds in heteroarenes with commercially available perfluoroalkyl iodides. This reaction is based on a simple combination of copper(I) iodide and 1,10-phenanthroline enabling the easy reduction of perfluoroalkyl iodides to the corresponding radical species that add to a wide range of heteroarenes including benzofurans, benzothiophenes, (aza)indoles, furans and pyrroles. High levels of regioselectivity were obtained in all cases and the efficiency and robustness of this process was highlighted by the direct perfluoroalkylation of furan-containing peptides.

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.

Iron-catalyzed alkylation of carbazole derivatives via hydroarylation of styrenes

The first direct and selective 3,6-di-alkylation of carbazoles via iron-catalyzed hydroarylation of styrenes is demonstrated. This simple, general and efficient method could deliver a wide range of di-benzyl-carbazoles with high chemo- and regio-selectivity at room temperature in up to 96% yield with no need for a noble-metal catalyst, directing group or additives.

B(C6F5)3-Catalyzed Sequential Additions of Terminal Alkynes to para-Substituted Phenols: Selective Construction of Congested Phenol-Substituted Quaternary Carbons

We have developed a borane-catalyzed sequential addition of terminal alkynes to para-substituted phenols, which affords a wide range of ortho-propargylic alkylated phenols bearing congested quaternary carbons. Control experiments combined with DFT calculations suggest that the reaction undergoes a sequential phenol alkenylation/hydroalkynylation process. Further extension of this strategy to the construction of triaryl-substituted quaternary carbons demonstrates the broad utility of this method.

Ruthenium(II)-Catalyzed Redox-Neutral C-H Alkylation of Arylamides with Unactivated Olefins

A Ru(II)-catalyzed weak chelating group-aided ortho-C-H alkylation of arylamides with unactivated olefins in a redox-neutral fashion has been demonstrated. The present alkylation reaction was well-suited for various substituted arylamides and unactivated aliphatic alkenes. In this alkylation reaction, pivalic acid plays dual role in which it delivers the proton source in a protonation step and the corresponding acetate moiety deprotonates the ortho-C-H bond of the arylamides.

Iridium-Catalyzed Hydroarylation via C-H Bond Activation

Hydroarylation reactions via C-H activation, which compensate for shortcomings of classical methods based on the Friedel-Crafts reaction, is one of the most attractive methods to synthesize substituted arenes. This Personal Account reviews our recent studies on iridium-catalyzed intermolecular hydroarylation of vinyl ethers, alkynes, bicycloalkenes, and 1,3-dienes, and intramolecular hydroarylation of m-allyloxyphenyl ketones, where asymmetric addition reactions are included. A cationic iridium catalyst, which is generated from chloroiridium [IrCl] and NaBAr^F ₄ [Ar^F =3,5-(CF₃ )₂ C₆ H₃ ], or a hydroxoiridium [Ir(OH)] complex is effective in catalyzing the hydroarylation depending on the substrates. 1,5-Cyclooctadiene (cod), chiral dienes, and conventional bisphosphines function as ligands controlling the high reactivity and selectivity of the catalysts in the hydroarylation. H/D exchange reaction of alkenes by use of a key intermediate of the hydroarylation reaction is also described.

Rh-Catalyzed General Method for Directed C-H Functionalization via Decarbonylation of in-Situ-Generated Acid Fluorides from Carboxylic Acids

A Rh-catalyzed decarbonylative C-H coupling of in-situ-generated acid fluorides with amide substrates bearing ortho-Csp²-H bonds has been developed. This method enables alkyl, aryl, and alkenyl carboxylic acids to undergo decarbonylative coupling with C-H bonds of (hetero)aromatic or alkenyl amides in generally good yields via the in situ conversion of carboxylic acids into acid fluorides and also allows for the functionalization of a series of structurally complex carboxyl-containing natural products and pharmaceuticals as well as pharmaceutical amide derivatives.

Site-Selective C-H alkylation of Complex Arenes by a Two-Step Aryl Thianthrenation-Reductive Alkylation Sequence

Herein, we present an undirected para-selective two-step C-H alkylation of complex arenes useful for late-stage functionalization. The combination of a site-selective C-H thianthrenation with palladium-catalyzed reductive electrophile cross-coupling grants access to a diverse range of synthetically useful alkylated arenes which cannot be accessed otherwise with comparable selectivity, diversity, and practicality. The robustness of this transformation is further demonstrated by thianthrenium-based reductive coupling of two complex fragments.

Enantioselective Intermolecular Murai-Type Alkene Hydroarylation Reactions

Strategies that enable the efficient assembly of complex building blocks from feedstock chemicals are of paramount importance to synthetic chemistry. Building upon the pioneering work of Murai and co-workers in 1993, C–H-activation-based enantioselective hydroarylations of alkenes offer a particularly promising framework for the step- and atom-economical installation of benzylic stereocenters. This short review presents recent intermolecular enantioselective Murai-type alkene hydroarylation methodologies and the mechanisms by which they proceed.

1 Introduction

2 Enantioselective Hydroarylation Reactions of Strained Bicyclic Alkenes

3 Enantioselective Hydroarylation Reactions of Electron-Rich Acyclic Alkenes

4 Enantioselective Hydroarylation Reactions of Electron-Poor Acyclic Alkenes

5 Enantioselective Hydroarylation Reactions of Minimally Polarized Acyclic Alkenes

6 Conclusion and Outlook

Iridium-catalyzed C-H methylation and d 3-methylation of benzoic acids with application to late-stage functionalizations

Late-stage functionalization (LSF) has over the past years emerged as a powerful approach in the drug discovery process. At its best, it allows for rapid access to new analogues from a single drug-like molecule, bypassing the need for de novo synthesis. To be successful, methods able to tolerate the diverse functional groups present in drug-like molecules that perform under mild conditions are required. C-H methylation is of particular interest due to the magic methyl effect in medicinal chemistry. Herein we report an iridium-catalyzed carboxylate-directed ortho C-H methylation and d ₃-methylation of benzoic acids. The method uses commercially available reagents and precatalyst and requires no inert atmosphere or exclusion of moisture. Substrates bearing electron-rich and electron-poor groups were successfully methylated, including compounds with competing directing/coordinating groups. The method was also applied to the LSF of several marketed drugs, forming analogues with increased metabolic stability compared with the parent drug.

Pd-catalyzed cross-electrophile Coupling/C-H alkylation reaction enabled by a mediator generated via C(sp3)-H activation

Transition-metal-catalyzed cross-electrophile C(sp²)–(sp³) coupling and C–H alkylation reactions represent two efficient methods for the incorporation of an alkyl group into aromatic rings. Herein, we report a Pd-catalyzed cascade cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides. Methoxy and benzyloxy groups, which are ubiquitous functional groups and common protecting groups, were utilized as crucial mediators via primary or secondary C(sp³)–H activation. The reaction provides an innovative and convenient access for the synthesis of alkylated phenol derivatives, which are widely found in bioactive compounds and organic functional materials.

A cascade Pd-catalyzed cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides has been developed by using an ortho-methoxy or benzyloxy group as a mediator via C(sp³)–H activation.

Silver-Catalysed Hydroarylation of Highly Substituted Styrenes

Hydroarylation is an effective strategy to rapidly increase the complexity of organic structures by transforming flat alkene moieties into three-dimensional frameworks. Many strategies have already been developed to achieve the hydroarylation of styrenes, however most of these reports examine the hydroarylation of unpolar, β-mono- or β-unsubstituted styrenes, while exploring mainly electron-rich benzene nucleophiles. Herein, we report a mild and general catalytic system for the selective hydroheteroarylation of multiply substituted styrenes and heteroaromatic styrenes. Mechanistic analysis of the reaction led to the discovery of commercially available 2,2':5',2''-terthiophene as a key reagent.

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.