Site-Selective Alkenylation of δ-C(sp3)–H Bonds with Alkynes via a Six-Membered Palladacycle

Computational Insights into Palladium-Catalyzed Site-Selective Anilide and Benzamide-Type [3+2] Annulation via Double C-H Bond Activation

The mechanism of palladium-catalyzed annulation reactions of benzamide- and anilide-type aromatic systems with maleimides is investigated using density functional theory. Double C-H bond activation is key to forming the desired annulation product. The first C-H bond activation for anilide- and amide-type ligands can occur at the ortho and benzylic C-H bonds, while the second C-H activation occurs at the meta carbon of the aromatic rings. For the anilide-type system, ortho and benzylic C-H bond activations occur via four- and five-membered palladacycles, respectively. In contrast, for the benzamide-type system, ortho and benzylic C-H bond activations occur via five- and six-membered palladacycles, respectively. The energy span model suggests that the initial C-H bond activation step at the benzylic position determines the turnover frequency for both anilide- and benzamide-type systems. Energy decomposition analysis and distortion-interaction/activation-strain analyses are employed to understand the electronic and steric factors controlling the turnover frequency-determining transition state.

Palladium-Catalyzed Selective Benzylic C-H Alkylation of Aromatic Sulfonamides with Maleimides

An efficient method for the selective benzylic C-H alkylation of sulfonamides using maleimides has been developed. The reaction proceeds via the benzylic C(sp3)-H bond activation of sulfonamide in the presence of a Pd(II) catalyst without requiring any oxidant, additive, or external ligand. This methodology is highly compatible with a wide variety of substituted maleimides. A plausible reaction mechanism is also proposed to account for this alkylation reaction.

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

Palladium-Catalyzed Regioselective Insertion of Carbenes into γ-C(sp3)-H Bonds of Aliphatic Amines

A migratory insertion of carbenes into distal γ-C(sp3)-H bonds of aliphatic amines has been successfully developed. The synergistic interplay among a palladium catalyst, picolinamide directing group, a carefully selected base additive, and an essential ligand proved crucial in achieving high yields. These findings hold significant value for advancing the exploration of regioselective carbene insertions into nonactivated C(sp3)-H bonds.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Ligand-Enabled Palladium(II)-Catalyzed γ-C(sp3)-H Arylation of Primary Aliphatic Amines

The Pd(II)/sulfoxide-2-hydroxypyridine catalytic system shows promising activity in C-H activation chemistry. In this study, we showcase how this catalytic system solves the problem of native primary amine-directed γ-C(sp3)-H arylation. Primary amines with different complexities are compatible with the established methodology, and the range of applicable substrates can be expanded to include pyridine, oxime ether, and pyridine N-oxide.

Pd-Catalyzed Picolinamide-Directed Late-Stage Chalcogenation of Tryptophan-Containing Peptides

This report describes the Pd-catalyzed late-stage chalcogenation of tryptophan-containing peptides with disulfides/diselenides in moderate to good yields. It comprises broad substrate scope, functional group diversity, late-stage modification of drug molecules, and various valuable synthetic transformations, including room temperature easy removal of the picolinamide auxiliary, which could be applicable to tune the structure and function of peptides.

Late-Stage Diversification of Peptides via Pd-Catalyzed Site-Selective δ-C(sp2)-H Fluorination and Amination

Site-selective C-H fluorination is an attractive strategy for directly transforming inert C-H bonds into C-F bonds, yet it remains a significant challenge. Herein, we have developed an efficient and versatile strategy for site-selective fluorination and amination of phenylalanine-containing peptides via late-stage Pd-catalyzed δ-C(sp2)-H activation, providing a valuable tool for the in situ synthesis of fluorinated indoline scaffolds within peptides.

Simplifying the Synthesis of Nonproteinogenic Amino Acids via Palladium-Catalyzed δ-Methyl C-H Olefination of Aliphatic Amines and Amino Acids

Transition metal-catalyzed directing group assisted C-H functionalizations provide a straightforward access to a wide variety of nonproteinogenic amino acids. While altering the side chain of an existing natural amino acids is one way, introducing a functional group to an aliphatic amine to synthesize versatile unnatural amino acids is another exciting avenue. In this work, we explore both the possibilities by the palladium-catalyzed δ-C(sp³)-H olefination of aliphatic amines and amino acids. A diverse substrate scope including sequential difunctionalizations followed by post synthetic transformations were achieved to understand the applicability of the current protocol. An in-depth mechanistic study was carried out to learn the mode of the reaction pathway.

Site-Selective Modification of Peptides via Late-Stage Pd-Catalyzed Tandem Reaction of Phenylalanine with Benzoquinone

An efficient and straightforward approach for site-selective functionalization of phenylalanine and phenylalanine-containing peptide via a Pd-catalyzed tandem reaction has been developed. The robust method underwent dual C-H activation, including C-C coupling with benzoquinone and intramolecular C-N cyclization, providing a feasible and rapid synthetic route to incorporate 4-benzoquinone-indoline fragments into peptides.

Palladium-catalyzed intramolecular asymmetric hydrocyclopropanylation of alkynes: synthesis of cyclopropane-fused γ-lactams

A palladium-catalyzed intramolecular asymmetric hydrocyclopropanylation of alkynes via C(sp³)-H activation has been developed for the synthesis of cyclopropane-fused γ-lactams. The presented strategy proceeds in a selective and 100% atom-economical manner. A range of cyclopropane-fused γ-lactams were prepared from readily available substrates in good yields and enantioselectivities with a chiral phosphoramidite ligand.

Late-stage C-H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C-H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.

Pd(ii)-catalyzed β- and γ-C-(sp3)-H dienylation with allenyl acetates

Recent years have seen the emergence of transition metal catalyzed C-H activation as a powerful synthetic tool in organic chemistry. Allenes have fascinated synthetic chemists due to their unique reactivity. While directing group assisted functionalization of C(sp²)-H bonds with allenes is well documented in the literature, their coupling with more challenging aliphatic C(sp³)-H bonds remains elusive. In this regard, we hereby report a Pd(ii) catalyzed 8-aminoquinoline directed aliphatic C(sp³)-H dienylation protocol using allenyl acetates. A variety of carboxylic acids including fatty acids and amino acids were efficiently functionalized at β and γ-positions to afford diversely functionalized 1,3-dienes. Preliminary mechanistic studies revealed the crucial role of the base in the success of the transformation. The reaction proceeds via regioselective 2,3-migratory insertion of the allene with the alkylpalladium(ii) species followed by β-acetoxy elimination.

Selective editing of a peptide skeleton via C-N bond formation at N-terminal aliphatic side chains

The applications of peptides and peptidomimetics have been demonstrated in the fields of therapeutics, diagnostics, and chemical biology. Strategies for the direct late-stage modification of peptides and peptidomimetics are highly desirable in modern drug discovery. Transition-metal-catalyzed C-H functionalization is emerging as a powerful strategy for late-stage peptide modification that is able to construct functional groups or increase skeletal diversity. However, the installation of directing groups is necessary to control the site selectivity. In this work, we describe a transition metal-free strategy for late-stage peptide modification. In this strategy, a linear aliphatic side chain at the peptide N-terminus is cyclized to deliver a proline skeleton via site-selective δ-C(sp3)-H functionalization under visible light. Natural and unnatural amino acids are demonstrated as suitable substrates with the transformations proceeding with excellent regio- and stereo-selectivity.

Multiple remote C(sp3)-H functionalizations of aliphatic ketones via bimetallic Cu-Pd catalyzed successive dehydrogenation

The dehydrogenation-triggered multiple C(sp³)-H functionalizations at remote positions γ, δ or ε, ζ to carbonyl groups of aliphatic ketones with aryl/alkenyl carboxylic acids as coupling partners have been achieved using a bimetallic Cu-Pd catalyst system. This reaction allows access to alkenylated isocoumarins and their derivatives in generally good yields with high functional group tolerance. The identification of bimetallic Cu-Pd synergistic catalysis for efficient successive dehydrogenation of aliphatic ketones, which overcomes the long-standing challenge posed by the successive dehydrogenation desaturation of terminally unsubstituted alkyl chains in aliphatic ketones, is essential to achieving this bimetallic Cu-Pd catalyzed dehydrogenation coupling reaction.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Photo-Induced Ruthenium-Catalyzed Double Remote C(sp2 )-H / C(sp3 )-H Functionalizations by Radical Relay

Distal C(sp2 )-H and C(sp3 )-H functionalizations have recently emerged as step-economical tools for molecular synthesis. However, while the C(sp2 )-C(sp3 ) construction is of fundamental importance, its formation through double remote C(sp2 )-H/C(sp3 )-H activation has proven elusive. By merging the ruthenium-catalyzed meta-C(sp2 )-H functionalization with an aliphatic hydrogen atom transfer (HAT) process, we, herein, describe the catalyzed twofold remote C(sp2 )-H/C(sp3 )-H functionalizations via photo-induced ruthenium-mediated radical relay. Thus, meta-C(sp2 )-H arene bonds and remote C(sp3 )-H alkane bonds were activated by a single catalyst in a single operation. This process was accomplished at room temperature by visible light-notably without exogenous photocatalysts. Experimental and computational theory studies uncovered a manifold comprising ortho-C-H activation, single-electron-transfer (SET), 1,n-HAT (n=5-7) and σ-activation by means of a single ruthenium(II) catalyst.

Remote Stereocenter through Amide-Directed, Rhodium-Catalyzed Enantioselective Hydroboration of Unactivated Internal Alkenes

Despite the frequent occurrence of γ-branched amines in bioactive molecules, the direct catalytic asymmetric synthesis of this structural motif containing a remote stereocenter remains an important synthetic challenge. Here, we report an amide-directed, rhodium-catalyzed highly diastereo- and enantioselective hydroboration of unactivated internal alkenes. This method provided facile access to enantioenriched amines containing β,γ-vicinal stereocenters. The application of this strategy to the synthesis of bioactive molecules was demonstrated. Computational studies indicated that migratory insertion of the alkene into rhodium hydride controls the enantioselectivity.

Synthesis of allylanilines via scandium-catalysed benzylic C(sp3)-H alkenylation with alkynes

The ortho-selective benzylic C(sp³)-H alkenylation of 2-methyl tertiary anilines with internal alkynes has been achieved for the first time by using a half-sandwich scandium catalyst. This protocol provides a straightforward route for the synthesis of a new family of 2-allylaniline derivatives, featuring broad substrate scope, 100% atom-efficiency, high yields, and high chemo-, regio-, and stereoselectivity.

δ-C-H Halogenation Reactions Enabled by a Nitrogen-Centered Radical Precursor

Nitrogen-centered radicals are versatile synthetic intermediates with the ability to undergo diverse reactions such as hydrogen atom transfer (HAT), β-scission, and addition across unsaturated systems. A long-standing impediment to the wider adoption of these intermediates in synthesis has been the difficulty of their generation. Herein we disclose a new hydrazonyl carboxylic acid precursor to nitrogen-centered radicals and its application toward remote C-H fluorination and chlorination reactions of sulfonyl-protected alkyl amines via 1,5-HAT.

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.

Palladacycle-Catalyzed Regioselective Heck Reaction Using Diaryliodonium Triflates and Aryl Iodides

We describe a P-containing palladacycle-catalyzed regioselective Heck reaction of 2,3-dihydrofuran with diaryliodonium salts and aryl iodides to afford 2-aryl-2,5-dihydrofurans and 2-aryl-2,3-dihydrofurans, respectively, in good yields. Mechanistic studies revealed that the oxidative addition of diaryliodonium salts to palladacycles to form Pd(IV) species showed high chemoselectivity and that electron-rich aryl moieties were preferentially transferred to the Heck product. DFT calculations indicated that the regioselectivity-determining step is the reductive elimination reaction rather than the isomerization and reinsertion of Pd(IV)-hydride intermediates into the double bond.

Theoretical Investigate of the α-Substitution Effect on γ-C(sp3)−H Arylation of Amines: Structure-Reactivity Relationship (SRR) Studies

Structure-reactivity relationship (SRR) studies to understand the α-substitution effect toward Pd-catalyzed γ-C(sp3)−H arylation enabled by catalytic transient directing groups have carefully been performed by DFT calculations. Selected transient directing groups...

Site-Selective Pd-Catalyzed C(sp3 )-H Arylation of Heteroaromatic Ketones

A ligand‐controlled site‐selective C(sp³)−H arylation of heteroaromatic ketones has been developed using Pd catalysis. The reaction occurred selectively at the α‐ or β‐position of the ketone side‐chain. The switch from α‐ to β‐arylation was realized by addition of a pyridone ligand. The α‐arylation process showed broad scope and high site‐ and chemoselectivity, whereas the β‐arylation was more limited. Mechanistic investigations suggested that α‐arylation occurs through C−H activation/oxidative addition/reductive elimination whereas β‐arylation involves desaturation and aryl insertion.

Pd-catalyzed bidentate auxiliary assisted remote C(sp3)-H functionalization

Pd-catalyzed C-H functionalisation affords effective synthetic tools to construct C-C and C-X bonds. Despite the challenges, the distal functionalization of C(sp³)-H bonds has witnessed significant developments and the use of bidentate auxiliaries has garnished this area by providing an opportunity to control reactivity as well as selectivity beyond proximal sites. This article covers the recent developments on the Pd-catalyzed bidentate auxiliary-assisted distal C(sp³)-H functionalization and is categorized based on the nature of functionalizations.

Toolbox for Distal C-H Bond Functionalizations in Organic Molecules

Transition metal catalyzed C-H activation has developed a contemporary approach to the omnipresent area of retrosynthetic disconnection. Scientific researchers have been tempted to take the help of this methodology to plan their synthetic discourses. This paradigm shift has helped in the development of industrial units as well, making the synthesis of natural products and pharmaceutical drugs step-economical. In the vast zone of C-H bond activation, the functionalization of proximal C-H bonds has gained utmost popularity. Unlike the activation of proximal C-H bonds, the distal C-H functionalization is more strenuous and requires distinctly specialized techniques. In this review, we have compiled various methods adopted to functionalize distal C-H bonds, mechanistic insights within each of these procedures, and the scope of the methodology. With this review, we give a complete overview of the expeditious progress the distal C-H activation has made in the field of synthetic organic chemistry while also highlighting its pitfalls, thus leaving the field open for further synthetic modifications.

Stereoselective Synthesis of C-Vinyl Glycosides via Palladium-Catalyzed C-H Glycosylation of Alkenes

C-vinyl glycosides are an important class of carbohydrates and pose a unique synthetic challenge. A new strategy has been developed for stereoselective synthesis of C-vinyl glycosides via Pd-catalyzed directed C-H glycosylation of alkenes with glycosyl chloride donors using an easily removable bidentate auxiliary. Both the γ C-H bond of allylamines and the δ C-H bond of homoallyl amine substrates can be glycosylated in high efficiency and with excellent regio- and stereoselectivity. The resulting C-vinyl glycosides can be further converted to a variety of C-alkyl glycosides with high stereospecificity. These reactions offer a broadly applicable method to streamline the synthesis of complex C-vinyl glycosides from easily accessible starting materials.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

Ligand-Enabled δ-C(sp3 )-H Borylation of Aliphatic Amines

Directed C-H functionalization has been realized as a complimentary technique to achieve borylation at a distal position of aliphatic amines. Here, we demonstrated the oxidative borylation at the distal δ-position of aliphatic amines using various borylating agents, a palladium catalyst, and a rightly tuned ligand in the presence of a cheap oxidant. Moreover, an organopalladium δ-C(sp³ )-H-activated intermediate has been isolated and crystallographically characterized to get mechanistic insight.

Assembly of Tetrahydroquinolines and 2-Benzazepines by Pd-Catalyzed Cycloadditions Involving the Activation of C(sp3)-H Bonds

Cycloaddition reactions are among the most practical strategies to assemble cyclic products; however, they usually require the presence of reactive functional groups in the reactants. Here, we report a palladium-catalyzed formal (4 + 2) cycloaddition that involves the activation of C(sp³)–H bonds and provides a direct, unconventional entry to tetrahydroquinoline skeletons. The reaction utilizes amidotolyl precursors and allenes as annulation partners, and is catalyzed by Pd(II) precursors in combination with specific N-acetylated amino acid ligands. The reactivity can be extended to ortho-methyl benzylamides, which provide for the assembly of appealing tetrahydro-2-benzazepines in a formal (5 + 2) annulation process.

Palladium-catalyzed functionalizations of acidic and non-acidic C(sp3)-H bonds - recent advances

A tremendous upsurge has been seen in the recent decade for the proximal and remote functionalization of activated and unactivated substrates via palladium redox pathways. This feature article discusses some of the recent reports on direct as well as indirect C(sp3)-H functionalization via cross-coupling reactions under palladium catalysis. Activated substrates (possessing acidic C(sp3)-H) including enones, ketones, aldehydes, silylenol ethers, esters, silyl ketene acetals, amides, cyano, α-amino esters, and O-carbamates, capable of undergoing cross-coupling reactions at the α-, β-, γ-, δ- and ε-positions, will be discussed. To overcome the challenging task of achieving regioselectivity, a variety of innovative modifications have been reported. The reports of C-H activations based on directing group, and as native functionality have been illustrated at the β-, γ- and δ-positions. Substrates such as α-amino esters, carbonyls, carboxylic acids and their derivatives, afford site-selective C(sp3)-H functionalization via varied-sized reactive metallacycles and are a unique class of substrates whose C(sp3)-H functionalizations were earlier considered as very difficult.

Late-stage stitching enabled by manganese-catalyzed C─H activation: Peptide ligation and access to cyclopeptides

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging. Despite major accomplishments, these strategies heavily rely on noble-metal catalysis. Herein, we report on a manganese(I)-catalyzed peptide C─H hydroarylation that enabled the stitching of peptidic and sugar fragments, under exceedingly mild and racemization-free conditions. This convergent approach represents an atom-economical alternative to traditional iterative peptide synthesis. The robustness of the manganese(I) catalysis regime is reflected by the full tolerance of a plethora of sensitive functional groups. Our strategy enabled an expedient access to challenging cyclic peptides by a modular late-stage macrocyclization of structurally complex peptides.