Pd(II)-Catalyzed Intermolecular Arylation of Unactivated C(sp3)–H Bonds with Aryl Bromides Enabled by 8-Aminoquinoline Auxiliary

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.

Ligand-Enabled Pd(II)-Catalyzed β-Methylene C(sp3)-H Arylation of Free Aliphatic Acids

Ligand development has enabled rapid advances in Pd(II)-catalyzed β-methyl C(sp3)-H activation of free carboxylic acids. However, there are only a handful of reports of free-acid-directed β-methylene C(sp3)-H activation, all of which are limited to intramolecular reactions. Herein, we report the first Pd(II)-catalyzed intermolecular β-methylene C(sp3)-H arylation of free aliphatic acids, which is enabled by bidentate pyridine-pyridone ligands. The bite angle of this ligand has been discovered to play a key role in promoting β-methylene C-H activation of free carboxylic acid. This new transformation provides a disconnection for alkylation of arenes with simple aliphatic acids. A variety of free aliphatic acids, including the antiasthmatic drug seratrodast, were compatible with the reported protocol.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Access to Hetero-Benzyl Scaffolds via Transient-Ligand-Enabled Direct γ-C(sp3)-H Arylation of 3-Methylheteroarene-2-Carbaldehydes

An efficient and straightforward method has been developed for the synthesis of β-benzyl-substituted 5-membered heterocyclic carbaldehydes via transient directing-group-enabled direct γ-C(sp³)-H arylation of 3-methylheteroarene-2-carbaldehydes. A wide range of 3-methylheteroarene carbaldehydes undergo coupling with a variety of aryl iodides, including less reactive iodo pyridine derivatives to provide a library of highly selective functionalized products in good to excellent yields. Some of these products have been successfully utilized in synthesizing useful synthetic intermediates.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C–H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp³)–H halogenation products rather than C–C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C–X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp³)–H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C–H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.

Visible light irradiation of 8-aminoquinoline Pd(ii) complexes initiates photoinduced electron transfer with alkyl halides, affording C–H halogenation over C–C bond adducts. A method for inert C(sp³)–H bond halogenation (Br, Cl and I) is reported.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Computational Study of the Ni-Catalyzed C-H Oxidative Cycloaddition of Aromatic Amides with Alkynes

The mechanism of Ni-catalyzed ortho C(sp²)-H oxidative cycloaddition of aromatic amides with internal alkynes containing 2-pyridinylmethylamine directing group was investigated using density functional theory (DFT) calculations. The C-H cleavage step proceeds via σ-complex-assisted metathesis (σ-CAM) with an alkenyl-Ni(II) complex. This is in contrast to the more common carboxylate/carbonate-assisted concerted metalation-deprotonation mechanism in related Ni-catalyzed C-H bond functionalization reactions with N,N-bidentate directing groups. In this reaction, the alkyne not only serves as the coupling partner, but also facilitates the σ-CAM C-H metalation both kinetically and thermodynamically. The subsequent functionalization of the five-membered nickelacycle proceeds via alkyne insertion into the Ni-C bond to form a seven-membered nickelacycle. This process proceeds with high levels of regioselectivity to form a C-C bond with sterically more encumbered alkyne terminus. This unusual regioselectivity is due to steric repulsions with the directing group that is coplanar with the alkyne in the migratory insertion transition state. The C-N bond reductive elimination to form the isoquinolone cycloadduct is promoted by PPh₃ complexation to the Ni center and the use of flexible 2-pyridinylmethylamine directing group. The origin of the cis-trans isomerism of alkene byproduct was also explained by computations.

Pd/Cu-Catalyzed Cascade C(sp3)-H Arylation and Intramolecular C-N Coupling: A One-Pot Synthesis of 3,4-2 H-Quinolinone Skeletons

In this letter, we successfully explored a cascade Pd/Cu-catalyzed intermolecular C(sp³)-H arylation of amides and intramolecular C-N coupling reaction. This method provides a one-pot strategy to synthesize 3,4-2 H-quinolinone with good regioselectivity of C-H arylation and C-N coupling from C-I and C-X bonds from readily available starting materials.

Overcoming the Limitations of γ- and δ-C-H Arylation of Amines through Ligand Development

L,X-Type transient directing groups (TDGs) based on a reversible imine linkage have emerged as broadly useful tools for C-H activation of ketones and free amines. However, competitive binding interactions among multiple reaction components (TDG itself, substrate, and substrate-TDG adduct) with the palladium catalyst often lead to the formation of multiple unreactive complexes, rendering ligand development extremely challenging. Herein, we report the finding of versatile 2-pyridone ligands that addresses these problems and significantly improves the γ-methylene arylation of alkyl amines, extending the coupling partners to a wide range of medicinally important heteroaryl iodides and even previously unreactive heteroaryl bromides. The combination of an appropriate transient directing group and pyridone ligand has also enabled the δ-arylation of alkyl amines. Notably, our transient directing group design reveals the importance of matching the size of the Pd-chelation with different transient directing groups and the size of palladacycles generated from γ- and δ-C-H bonds: TDGs that coordinate with Pd(II) to form a six-membered chelate are selective toward γ-C-H bonds, whereas TDGs that coordinate with Pd(II) via a five-membered chelate tend to activate δ-C-H bonds. These findings provide an avenue for developing protecting group free and selective C-H functionalization using the transient directing group strategy.

Direct Palladium-Catalyzed β-Arylation of Lactams

A direct and catalytic method is reported here for β-arylation of N-protected lactams with simple aryl iodides. The transformation is enabled by merging soft enolization of lactams, palladium-catalyzed desaturation, Ar-X bond activation, and aryl conjugate addition. The reaction is operated under mild reaction conditions, is scalable, and is chemoselective. Application of this method to concise syntheses of pharmaceutically relevant compounds is demonstrated.

C-H functionalisation of cycloalkanes

Among organic compounds hydrocarbons are inexpensive and possibly the most abundant among natural resources. Developing strategies for selective functionalisation of inert hydrocarbon C-H bonds is one of the most ideal synthetic paths that a synthetic chemist could think of. This critical review focuses on the recent development of various directed and non-directed cycloalkylations leading to the formation of carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. Apart from various transition metal catalysed cycloalkylations, this review also covers various metal-free cycloalkylation processes.

Remote alkylation of N-(quinolin-8-yl)benzamides with alkyl bromides via ruthenium(ii)-catalyzed C–H bond activation

A ruthenium(ii) catalyzed remote C-5 alkylation of the quinoline ring of N-(quinolin-8-yl)benzamides with alkyl bromides via C–H bond activation is described.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Pd-catalysed ligand-enabled carboxylate-directed highly regioselective arylation of aliphatic acids

α-amino acids bearing aromatic side chains are important synthetic units in the synthesis of peptides and natural products. Although various β-C-H arylation methodologies for amino acid derivatives involving the assistance of directing groups have been extensively developed, syntheses that directly employ N-protected amino acids as starting materials remain rare. Herein, we report an N-acetylglycine-enabled Pd-catalysed carboxylate-directed β-C(sp3)-H arylation of aliphatic acids. In this way, various non-natural amino acids can be directly prepared from phthaloylalanine in one step in good to excellent yields. Furthermore, a series of aliphatic acids have been shown to be amenable to this transformation, affording β-arylated propionic acid derivatives in moderate to good yields. More importantly, this ligand-enabled direct β-C(sp3)-H arylation could be easily scaled-up to 10 g under reflux conditions, highlighting the potential utility of this synthetic method.

Nickel-catalyzed selective C-5 fluorination of 8-aminoquinolines with NFSI

The first nickel-catalyzed selective C-5 fluorination of 8-aminoquinoline derivatives was achieved using NFSI as the “F” source.

Iron(iii)-catalyzed chelation assisted remote C–H bond oxygenation of 8-amidoquinolines

We have developed a simple, efficient oxygenation of C–H bonds that are geometrically inaccessible in 8-amidoquinolines with inexpensive and nontoxic iron(iii) catalyst. The chelation assisted remote benzoxylation protocol tolerates a wide substrate scope and is operationally simple.

Palladium-Catalyzed Transformations of Alkyl C-H Bonds

This Review summarizes the advancements in Pd-catalyzed C(sp³)-H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C-H bonds, many C(sp³)-H activation/C-C and C-heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp³)-H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C-H functionalization reactions.

Synthesis of β-arylated alkylamides via Pd-catalyzed one-pot installation of a directing group and C(sp(3))-H arylation

The synthesis of β-arylated alkylamides via alkyl C-H bond arylation has been realized by means of direct one-pot reactions of acyl chlorides, aryl iodides and 8-aminoquinoline. Depending on the structure of the starting materials, both single and double β-arylated alkylamides could be accessed.

Palladium-catalyzed β-C(sp3)–H arylation of phthaloyl alanine with hindered aryl iodides: synthesis of complex β-aryl α-amino acids

An efficient protocol for palladium-catalyzed PE auxiliary-directed C(sp³)–H arylation reaction with sterically hindered aryl iodides was developed.

DFT studies on the mechanism of palladium catalyzed arylthiolation of unactive arene to diaryl sulfide

The cleavage of S–N bond prefers to take place via concerted σ-bond metathesis rather than oxidative addition proposed in experiment.

Aminoquinoline-assisted vinylic C–H arylation of unsubstituted acrylamide for the selective synthesis of Z olefins

A method for Pd-catalyzed, aminoquinoline-directed arylation of vinylic C–H bonds with aryl iodides has been developed. This reaction represents a rare example of Pd-catalyzed vinylic C–H functionalization of unsubstituted acrylamide, allowing for preparation of Z-olefins.

Palladium-catalyzed arylation of β-methylene C(sp3)–H bonds at room temperature: desymmetrization of simple cycloalkyl carboxylic acids

A new protocol for Pd-catalyzed β methylene C–H arylation of N-quinolyl cycloalkylcarboxamides with aryl iodides at room temperature is reported.

Palladium-Catalyzed Directed C(sp3)-H Arylation of Saturated Heterocycles at C-3 Using a Concise Optimization Approach

Saturated heterocycles, such as THFs, pyrrolidines, piperidines and THPs, are essential components of many biologically active compounds. Examples of C-H functionalization on these important ring systems remain scarce, especially at unactivated positions. Here we report the development of conditions for the palladium-catalyzed stereoselective C(sp3)-H arylation at unactivated 3-positions of 5- and 6-membered N- and O-heterocycles with aminoquinoline directing groups. Subtle differences in substrate structures altered their reactivity significantly; and different conditions were required to achieve high yields in each case. Successful conditions were developed using a short empirical optimization approach to cover reaction space with a limited set of variables. Excellent cis-selectivity was achieved in all cases, except for the THP substrate where minor trans-products were formed through a different palladacyclic intermediate. Here, differences in reactivity and selectivity with other directing groups were examined.