Versatile and robust C–C activation by chelation-assisted manganese catalysis

Mn(I)-Catalyzed Site-Selective C-H Activation: Unlocking Access to 3-Arylated Succinimides from 2-Arylpyridines and Maleimides

An efficient and cost-effective Mn(I)-catalyzed site-selective C-H activation of 2-arylpyridines with maleimides has been described. This approach facilitates the synthesis of 3-arylated succinimide derivatives with high site selectivity, chemoselectivity, catalytic efficiency, and outstanding tolerance to numerous functional groups. The practicality of this approach is further evidenced by its successful application in large-scale reactions and the conversion of the synthesized succinimide derivatives into other valuable compounds.

Construction of quaternary alkyl motifs through palladium-catalyzed oxidative coupling of 1,3-dicarbonyl compounds with alkenes followed by C-C bond cleavage

A palladium-catalyzed coupling reaction has been developed for the generation of tertiary alkylation products by reacting olefins with diversely functionalized 1,3-dicarbonyls. The reaction involves the tertiary C-H alkylation of 1,3-dicarbonyls with olefins to produce a tertiary alcohol, followed by C-C bond cleavage.

Reversible C-C bond formation in group 4 metal complexes: nitrile extrusion via β-aryl elimination

Pyridylamides of zirconium and hafnium with [C,N,N]-ligands reversibly insert nitriles into M-CAr bonds leading to an observable equilibrium between the starting [C,N,N]-complexes and newly formed [N,N,N]-complexes with a ketimide moiety in a 7-membered metallacycle. The discovered reversible insertion of nitriles into M-CAr bonds represents an unprecedented example of β-aryl elimination from a ketimide ligand in early transition metal complexes. Experimental and computational studies suggest thermodynamic and electronic reasons for this reactivity. Weak orbital overlap between the ketimide nitrogen and the metal, and an unfavorable 7-membered metallacycle destabilize the product of insertion into the M-CAr bond, while the pyridylamide moiety acts as a directing group making the reverse process viable. The influence of non-chelate spectator ligands on the metal center and substituents in nitrile on the thermodynamic stability of the [N,N,N]-complexes was also studied. Exploiting β-carbon elimination in complexes of early transition metals may extend the range of catalysts that are accessible for C-C activation processes in the future.

Mechanistic Investigation of the Rhodium-Catalyzed Transfer Hydroarylation Reaction Involving Reversible C-C Bond Activation

Carbon-carbon (C-C) bonds are ubiquitous but are among the least reactive bonds in organic chemistry. Recently, catalytic approaches to activate C-C bonds by transition metals have demonstrated the synthetic potential of directly reorganizing the skeleton of small molecules. However, these approaches are usually restricted to strained molecules or rely on directing groups, limiting their broader impact. We report a detailed mechanistic study of a rare example of catalytic C-C bond cleavage of unstrained alcohols that enables reversible ketone transfer hydroarylation under Rh-catalysis. Combined insight from kinetic analysis, in situ nuclear magnetic resonance (NMR) monitoring, and density functional theory (DFT) calculations supports a symmetric catalytic cycle, including a key reversible β-carbon elimination event. In addition, we provide evidence regarding the turnover-limiting step, the catalyst resting state, and the role of the sterically encumbered NHC ligand. The study further led to an improved catalytic system with the discovery of two air-stable precatalysts that showed higher activity for the transformation in comparison to the original conditions.

Transition-Metal-Catalyzed C-C Bond Formation from C-C Activation

ConspectusC-C single bonds are ubiquitous in organic compounds. The activation and subsequent functionalization of C-C single bonds provide a unique opportunity to synthesize conventionally inaccessible molecules through the rearrangement of carbon skeletons, often with a favorable atom and step economy. However, the C-C bonds are thermodynamically and kinetically inert. Consequently, the activation of C-C bonds is particularly attractive yet challenging in the field of organic chemistry. In the past decade, we sought to develop efficient strategies to carry out transition-metal-catalyzed diverse C-C cleavage/C-C forming reactions and to obtain some insights into the intrinsic reactivities of different C-C bonds. With our efforts, readily available alcohols, carboxylic acids, and ketones served as suitable substrates for the catalytic C-C coupling reactions, which are reviewed in this Account. In 2009, we observed a Ni-catalyzed cross coupling of aryl nitriles with arylboronic esters through C-CN cleavage. Encouraged by these results, we are interested in transition-metal-catalyzed C-C bond activation. Due to their broad availability, we then turned our attention to C-C cleavage of carboxylic acids. Rhodium-catalyzed decarbonylative coupling of carboxylic acids with (hetero)arenes was then achieved through oxidative addition of in situ formed, more reactive mixed anhydrides to Rh(I) without the need for oxidants that are commonly required for the decarboxylative coupling of carboxylic acids. Subsequently, the decarbonylation of more challenging unstrained aryl ketones was realized under Rh catalysis assisted by N-containing directing groups. Following this work, a group exchange of aryl ketones with carboxylic acids was achieved through 2-fold C-C bond cleavage. By employing the chelation strategy, Rh-catalyzed C-C bond activation of secondary benzyl alcohols was also accomplished through β-carbon elimination of the rhodium alcoholate intermediates. The competing oxidation of secondary alcohols to ketones via β-hydrogen elimination of the same intermediates was suppressed as thermodynamically favorable five-membered rhodacycles are formed after β-carbon elimination. Different types of transformations of alcohols, including the Heck-type reaction with alkenes, cross coupling with arylsilanes, and Grignard-type addition with aldehydes or imines, have been achieved, showing the great potential of secondary alcohols in the formation of C-C bonds. These C-C bond-forming reactions are complementary to traditional cross couplings of aryl halides with organometallic reagents. However, these transformations produce small molecules as byproducts. To improve the atom economy, we then investigated C-C bond transformations of strained-ring cyclic compounds. Ni-catalyzed intermolecular cyclization of benzocyclobutenones with alkynes was recently achieved via the uncommon cleavage of the C1-C8 bond by employing a removable blocking strategy. Rh-catalyzed intramolecular annulation of benzocyclobutenols with alkynes was also achieved. In summary, our developments demonstrate the great potential of transition-metal-catalyzed C-C bond activation for the formation of new C-C bonds. To further expand the synthetic utility of C-C bond activation, more efforts are required to expand the substrate scope and to achieve earth-abundant metal-catalyzed transformations.

Ruthenium-Catalyzed Activation of Nonpolar C-C Bonds via π-Coordination-Enabled Aromatization

Activation of C-C bonds allows editing of molecular skeletons, but methods for selective activation of nonpolar C-C bonds in the absence of a chelation effect or a driving force derived from opening of a strained ring are scarce. Herein, we report a method for ruthenium-catalyzed activation of nonpolar C-C bonds of pro-aromatic compounds by means of π-coordination-enabled aromatization. This method was effective for cleavage of C-C(alkyl) and C-C(aryl) bonds and for ring-opening of spirocyclic compounds, providing an array of benzene-ring-containing products. The isolation of a methyl ruthenium complex intermediate supports a mechanism involving ruthenium-mediated C-C bond cleavage.

Sliver-Catalyzed 1,3-Aza-Benzyl Migration of Allyl Alcohol

Carbon migration of alkenyl alcohols has been recognized as an increasingly viable methodology in organic synthesis. Herein, we disclose a silver-catalyzed 1,3-aza-benzyl migration of allyl alcohols by utilizing chelation-assisted selective cleavage of an unstrained C(sp³)-C(sp³) bond. This approach provides an available, efficient, high atom-economic, and environmentally benign procedure, leading to alkylation products with broad substrate scopes and excellent yields. The migration proceeds via a one-pot, two-step process involving a free-state alkyl metal species.

C-H bond activation and sequential addition to two different coupling partners: a versatile approach to molecular complexity

Sequential multicomponent C-H bond addition is a powerful approach for the rapid, modular generation of molecular complexity in a single reaction. In this approach, C-H bonds are typically added across π-bonds or π-bond isosteres, followed by subsequent coupling to another type of functionality, thereby forming two σ-bonds in a single reaction sequence. Many sequential C-H bond addition reactions have been developed to date, including additions across both conjugated and isolated π-systems followed by coupling with reactants such as carbonyl compounds, cyanating reagents, aminating reagents, halogenating reagents, oxygenating reagents, and alkylating reagents. These atom-economical reactions transform ubiquitous C-H bonds under mild conditions to more complex structures with a high level of regiochemical and stereochemical control. Surprising connectivities and diverse mechanisms have been elucidated in the development of these reactions. Given the large number of possible combinations of coupling partners, there are enormous opportunities for the discovery of new sequential C-H bond addition reactions.

Palladium-Catalyzed Stagewise Strain-Release-Driven C-C Activation of Bicyclo[1.1.1]pentanyl Alcohols

A palladium-catalyzed chemoselective coupling of readily available bicyclo[1.1.1]pentanyl alcohols (BCP-OH) with various halides is reported, which offers expedient approaches to a wide range of cyclobutanone and β,γ-enone skeletons via single or double C-C activation. The chemistry shows a broad substrate scope in terms of both the range of BCP-OH and halides including a series of natural product derivatives. Moreover, dependency of reaction chemodivergence on base additive has been investigated through experimental and density functional theory (DFT) studies, which is expected to significantly enrich the reaction modes and increase the synthetic potential of BCP-OH in preparing more complex molecules.

From Amides to Urea Derivatives or Carbamates with Chemospecific C-C Bond Cleavage at Room Temperature

The ureas and carbamates are common motifs in pharmaceuticals, agrochemicals, biologically active compounds and organocatalysis applications. Herein, we report a significant advance in this area and present the general method...

Regio- and stereo-selective construction of cis-indeno[1,2-c]isoxazoles via a C–H allylation/1,3-dipolar cycloaddition cascade

Synthesis of fused cis-indole[1,2-c]isoxazoles with excellent regio- and stereo-selectivity via a rhodium-catalyzed C–H allylation/intramolecular 1,3-dipolar cycloaddition cascade has been realized.

N-Atom Deletion in Nitrogen Heterocycles

Excising the nitrogen in secondary amines, and coupling the two residual fragments is a skeletal editing strategy that can be used to construct molecules with new skeletons, but which has been largely unexplored. Here we report a versatile method of N-atom excision from N-heterocycles. The process uses readily available N-heterocycles as substrates, and proceeds by N-sulfonylazidonation followed by the rearrangement of sulfamoyl azide intermediates, providing various cyclic products. Examples are provided of deletion of nitrogen from natural products, synthesis of chiral O-heterocycles from commercially available chiral β-amino alcohols, formal inert C-H functionalization through a sequence of N-directed C-H functionalization and N-atom deletion reactions in which the N-atom can serve as a traceless directing group.

Orthogonal cross-coupling through intermolecular metathesis of unstrained C(aryl)-C(aryl) single bonds

While metathesis reactions involving carbon-carbon double bonds, namely olefin metathesis, have been well established with broad utility in organic synthesis and materials science, direct metathesis of kinetically less accessible C-C single bonds is extremely rare. Here we report a ruthenium-catalysed reversible C-C single-bond metathesis reaction that allows redox- and pH-neutral biaryl synthesis. Assisted by directing groups, unstrained homo-biaryl compounds undergo aryl exchanges to generate cross-biaryl products, catalysed by a well-defined air-stable ruthenium(II) complex. Functional groups reactive under typical cross-coupling reactions, such as halogen, silyl and boronate moieties, are compatible under the metathesis conditions. Mechanistic studies disclose an intriguing 'olefin-metathesis-like' pathway that involves an unexpected heptacoordinated, 18-electron closed-shell intermediate. The distinct reaction mode discovered here is expected to inspire the development of more general C-C single-bond metathesis and orthogonal cross-coupling reactions.

Reusable Manganese Catalyst for Site-Selective Pyridine C-H Arylations and Alkylations

Herein, we disclose a recyclable, hybrid manganese catalyst for site‐selective azine C−H activation by weak amide assistance. The novel, reusable catalyst enabled C3–H arylation and C3–H alkylation with ample scope, and was characterized by detailed transmission electron microscopy analysis.

Metal-catalysed C-Het (F, O, S, N) and C-C bond arylation

The formation of C-aryl bonds has been the focus of intensive research over the last decades for the construction of complex molecules from simple, readily available feedstocks. Traditionally, these strategies involve the coupling of organohalides (I, Br, Cl) with organometallic reagents (Mg, Zn, B, Si, Sn,…) such as Kumada-Corriu, Negishi, Suzuki-Miyaura, Hiyama and Sonogashira cross-couplings. More recently, alternative methods have provided access to these products by reactions with less reactive C-Het (F, O, S, N) and C-C bonds. Compared to traditional methods, the direct cleavage and arylation of these chemical bonds, the essential link in accessible feedstocks, has become increasingly important from the viewpoint of step-economy and functional-group compatibility. This comprehensive review aims to outline the development and advances of this topic, which was organized into (1) C-F bond arylation, (2) C-O bond arylation, (3) C-S bond arylation, (4) C-N bond arylation, and (5) C-C bond arylation. Substantial attention has been paid to the strategies and mechanistic investigations. We hope that this review can trigger chemists to discover more efficient methodologies to access arylation products by cleavage of these C-Het and C-C bonds.

Rhodium-Catalyzed Dealkenylative Arylation of Alkenes with Arylboronic Compounds

The C-C bond formation reaction represents a fundamental and important transformation in synthetic chemistry, and exploring new types of C-C bond formation reactions is recognized as appealing, yet challenging. Herein, we disclose the first example of rhodium-catalyzed dealkenylative arylation of alkenes with arylboronic compounds, thereby providing an unconventional access to bi(hetero)aryls with excellent chemoselectivity. In this method, C(aryl)-C(alkenyl) and C(alkenyl)-C(alkenyl) bonds in various alkenes and 1,3-dienes can be cleaved via a hydrometalation and followed by β-carbon elimination pathway for Suzuki-Miyaura reactions. Furthermore, a series of novel organic fluorescent molecules with excellent photophysical properties has been efficiently constructed with this protocol.

Rhodaelectro-catalyzed chemo-divergent C-H activations with alkylidenecyclopropanes for selective cyclopropylations

Herein, we report on selectivity control in C-H activations with alkylidenecyclopropanes (ACPs) for the chemo-selective assembly of cyclopropanes or dienes. Thus, unprecedented rhodaelectro-catalyzed C-H activations were realized with diversely decorated ACPs with a wide substrate scope and electricity as the sole oxidant.

Synthesis of Nitrile Bearing Acyclic Quaternary Centers through Co(III)-Catalyzed Sequential C-H Bond Addition to Dienes and N-Cyanosuccinimide

Herein we disclose a three-component strategy to access quaternary centers bearing nitriles by cobalt-catalyzed C-H bond activation and sequential addition to internally substituted 1,3-dienes and an electrophilic cyanating reagent with high regio and stereocontrol. 2-Aryl and alkyl monosubstituted dienes provide α-aryl and α-alkyl α-methyl-substituted nitriles, respectively. An even wider variety of functionality can be installed at the quaternary carbon by using 1,2-disubstituted dienes. The synthetic utility of the nitrile products was successfully demonstrated by various transformations, including conversions to γ-lactones and tetrazoles. The observed connectivity in the products along with studies with deuterium labeled reactants provide insight into the mechanism. Formation of a 7-membered cobaltacycle by C-H activation and migratory insertion of the diene is followed by β-hydride elimination and hydride reinsertion to give a 6-membered cobaltacycle that then reacts with the cyanating agent.

Palladium-catalyzed asymmetric allylic amination of a vinylethylene carbonate with N-heteroaromatics

Highly enantioselective allylic amination of a vinylethylene carbonate with N-heteroaromatics is enabled by asymmetric palladium catalysis for the synthesis of chiral acyclic nucleosides and isonucleosides.

Green strategies for transition metal-catalyzed C–H activation in molecular syntheses

Sustainable strategies for the activation of inert C–H bonds towards improved resource-economy.

Temporary or removable directing groups enable activation of unstrained C-C bonds

Carbon-carbon (C-C) bonds constitute basic skeletons in most organic molecules. One can imagine that selective manipulation of C-C bonds would provide a direct approach to edit or alter molecular scaffolds but has been an ongoing challenge. Due to the kinetic inertness of C-C bonds, the common strategies of activating these bonds by transition metals rely on either the use of highly strained substrates or the assistance of a permanent directing group (DG), in which strain relief or formation of stable metallocycles becomes the driving force. To allow more common and less strained compounds utilized as substrates for C-C activation, the use of temporary and removable DGs has emerged as an attractive strategy in the past two decades. A variety of C-C bonds in unstrained or less strained organic molecules now can be converted to more reactive metal-carbon bonds, and further downstream transformations have led to diverse synthetic methods. This review highlights the development of catalytic approaches that can activate unstrained C-C bonds enabled by temporary or removable DGs. The content is mainly divided based on the nature of the DGs: temporary and removable. Applications of these methods in syntheses of natural products or bioactive molecules are also discussed.

Regiodivergent C-H and Decarboxylative C-C Alkylation by Ruthenium Catalysis: ortho versus meta Position-Selectivity

Ruthenium(II)biscarboxylate complexes enabled the selective alkylation of C-H and C-C bonds at the ortho- or meta-position. ortho-C-H Alkylations were achieved with 4-, 5- as well as 6-membered halocycloalkanes. Furthermore, the judicious choice of the directing group allowed for a full control of ortho-/meta-selectivities. Detailed mechanistic studies by experiment and computation were performed and provided strong support for an oxidative addition/reductive elimination process for ortho-alkylations, while a homolytic C-X cleavage was operative for the meta-selective transformations.

Dehydroxyalkylative halogenation of C(aryl)-C bonds of aryl alcohols

We herein report Cu mediated side-directed dehydroxyalkylative halogenation of aryl alcohols. C(aryl)-C bonds of aryl alcohols were effectively cleaved, affording the corresponding aryl chlorides, bromides and iodides in excellent yields. Aryl alcohols could serve as both aromatic electrophilic and radical synthetic equivalents during the reaction.

A Highly Efficient Dimeric Manganese-Catalyzed Selective Hydroarylation of Internal Alkynes

We have developed a general and site-predictable manganese-catalyzed hydroarylation of internal alkynes in the presence of water, under an air atmosphere without the involvement of ligand. The unique catalytic feature of this reaction is highlighted by comparison with other widely used transition metal catalysts including palladium, rhodium, nickel, or copper. The simple operation, high efficiency and excellent functional group compatibility make this protocol practical for more than 90 structurally diverse internal alkynes, overcoming the influence of both electronic and steric effect of alkynes. Its exclusive regio- and chemoselectivity originates from the unique reactivity of the manganese-based catalyst towards an inherent double controlled strategy of sterically hindered propargyl alcohols without the installing of external directing groups. Its synthetic robustness and practicality have been illustrated by the concise synthesis of bervastatin, a hypolipidemic drug, and late-stage modification of complex alkynes with precise regioselectivity.

Synthesis of thioethers, arenes and arylated benzoxazoles by transformation of the C(aryl)-C bond of aryl alcohols

Transformation of aryl alcohols into high-value functionalized aromatic compounds by selective cleavage and functionalization of the C(aryl)–C(OH) bond is of crucial importance, but very challenging by far. Herein, for the first time, we report a novel and versatile strategy for activation and functionalization of C(aryl)–C(OH) bonds by the cooperation of oxygenation and decarboxylative functionalization. A diverse range of aryl alcohol substrates were employed as arylation reagents via the cleavage of C(aryl)–C(OH) bonds and effectively converted into corresponding thioether, arene, and arylated benzoxazole products in excellent yields, in a Cu based catalytic system using O₂ as the oxidant. This study offers a new way for aryl alcohol conversion and potentially offers a new opportunity to produce high-value functionalized aromatics from renewable feedstocks such as lignin which features abundant C(aryl)–C(OH) bonds in its linkages.

Aryl alcohols served as aryl agents by effective activation of C(aryl)–C(OH) bonds, and were transformed into corresponding thioether, arene, and arylated benzoxazole products in excellent yields.

Solvent-free and room temperature microwave-assisted direct C7 allylation of indolines via sequential C–H and C–C activation

A Ru or Rh-catalyzed efficient and atom-economic C7 allylation of indolines with vinylcyclopropanes was developed via sequential C–H and C–C activation. A wide range of substrates were well tolerated to afford the corresponding allylated indolines in high yields and E/Z selectivities under microwave irradiation. The obtained allylated indolines could further undergo transformations to afford various value-added chemicals. Importantly, this reaction proceeded at room temperature under solvent-free conditions.

A Ru or Rh-catalyzed direct C7 allylation of indolines with vinylcyclopropanes via sequential C–H/C–C activation under microwave irradiation has been disclosed.

Weak Coordination-Guided Regioselective Direct Redox-Neutral C4 Allylation of Indoles with Morita-Baylis-Hillman Adducts

A weak carbonyl coordination-guided regioselective C4 allylation of indoles is demonstrated using the versatile Morita-Baylis-Hillman adduct in the presence of Rh catalysts in a redox-neutral fashion. The substrate scope, functional group diversity, oxidant free character, mechanistic aspects, and synthetic utilities are important practical features.

Late-stage peptide C-H alkylation for bioorthogonal C-H activation featuring solid phase peptide synthesis

Methods for the late-stage diversification of structurally complex peptides hold enormous potential for advances in drug discovery, agrochemistry and pharmaceutical industries. While C-H arylations emerged for peptide modifications, they are largely limited to highly reactive, expensive and/or toxic reagents, such as silver(I) salts, in superstoichiometric quantities. In sharp contrast, we herein establish the ruthenium(II)-catalyzed C-H alkylation on structurally complex peptides. The additive-free ruthenium(II)carboxylate C-H activation manifold is characterized by ample substrate scope, racemization-free conditions and the chemo-selective tolerance of otherwise reactive functional groups, such as electrophilic ketone, bromo, ester, amide and nitro substituents. Mechanistic studies by experiment and computation feature an acid-enabled C-H ruthenation, along with a notable protodemetalation step. The transformative peptide C-H activation regime sets the stage for peptide ligation in solution and proves viable in a bioorthogonal fashion for C-H alkylations on user-friendly supports by means of solid phase peptide syntheses.

Visible-Light-Enabled Ruthenium-Catalyzed meta-C-H Alkylation at Room Temperature

Visible-light-induced ruthenium catalysis has enabled remote C-H alkylations with excellent levels of position control under exceedingly mild conditions at room temperature. The metallaphotocatalysis occurred under exogenous-photosensitizer-free conditions and features an ample substrate scope. The robust nature of the photo-induced mild meta-C-H functionalization is reflected by the broad functional group tolerance, and the reaction can be carried out in an operationally simple manner, setting the stage for challenging secondary and tertiary meta-C-H alkylations by ruthenaphotoredox catalysis.