Ligand‐Enabled Pd II ‐Catalyzed Iterative γ‐C(sp3)−H Arylation of Free Aliphatic Acid

Chiral Transient Directing Group Strategies in Asymmetric Synthesis

The development of novel methodologies for catalytic enantioselective functionalization reactions enabled by chiral transient directing groups is accompanying in a paradigm shift in the field of asymmetric synthesis. In particular, these highly atom- and step-economic enantioinduction processes commonly proceed either via enantioselective C-H functionalization, or via enantioselective hydroarylation of the pro-chiral substrates generating point, axial or planar chirality. The use of the transient directing group strategy in C-H functionalizations precludes the stoichiometric installations and removal of directing groups and enables efficient, more compatible and economical chemical routes. This minireview highlights asymmetric transition-metal-catalyzed methodologies involving chiral transient directing groups together with the scope, utility and future perspective of the field.

Harnessing the Efficacy of 2-Pyridone Ligands for Pd-Catalyzed (β/γ)-C(sp3)-H Activations

Mechanisms of palladium-aminooxyacetic acid and 2-pyridone-enabled cooperative catalysis for the β- and γ-C(sp³)-H functionalizations of ketones are investigated with density functional theory. 2-Pyridone-assisted dissociation of the trimeric palladium acetate [Pd₃(OAc)₆] is found to be crucial for these catalytic pathways. The evolution of the [6,6]-membered palladacycles (Int-4) are elucidated and are active complexes in Pd(II/IV) catalytic cycles. Nevertheless, 2-pyridone acts as an external ligand, which accelerates β-C(sp³)-H activation. Computational investigations suggest that the C(sp³)-H bond activation is the rate-limiting step for both the catalytic processes. To overcome the kinetic inertness, an unsubstituted aminooxyacetic acid auxiliary is used for the β-C(sp³)-H activation pathway to favor the formation of the [5,6]-membered palladacycle intermediate, Int-IV. Among the several modeled ligands, 3-nitro-5-((trifluoromethyl)sulfonyl)pyridine-2(1H)-one (L8) is found to be highly valuable for both the (β/γ)-C(sp³)-H functionalization catalytic cycles. A favorable free energy pathway of late-stage functionalization of (R)-muscone paves the path to design other bioactive molecules.

Thioether Ligand-Enabled Cationic Palladium(II)-Catalyzed Electrophilic C-H Arylation of α,β-Unsaturated Oxime Ethers

The use of the cationic palladium(II) catalyst realized electrophilic C-H arylation of α,β-unsaturated O-SEM oximes with arylboronic acids. This Pd-catalyzed electrophilic C-H arylation is facilitated by employing alkyl aryl thioether ligands, and optimization of the ligand structure greatly improves the yield. The resulting α,β-unsaturated oximes would provide access to multisubstituted heterocyclic compounds.

Diverse strategies for transition metal catalyzed distal C(sp3)-H functionalizations

Transition metal catalyzed C(sp3)-H functionalization is a rapidly growing field. Despite severe challenges, distal C-H functionalizations of aliphatic molecules by overriding proximal positions have witnessed tremendous progress. While usage of stoichiometric directing groups played a crucial role, reactions with catalytic transient directing groups or methods without any directing groups are gaining more attention due to their practicality. Various innovative strategies, slowly but steadily, circumvented issues related to remote functionalizations of aliphatic molecules. A systematic compilation has been presented here to provide insights into the recent developments and future challenges in the field. The Present perspective is expected to open up a new dimension and provide an avenue for deep insights into the distal C(sp3)-H functionalizations that could be applied routinely in various pharmaceutical and agrochemical industries.

Temperature-modulated selective C(sp3)-H or C(sp2)-H arylation through palladium catalysis

Transition metal-catalysed C–H bond functionalisations have been extensively developed in organic and medicinal chemistry. Among these catalytic approaches, the selective activation of C(sp³)–H and C(sp²)–H bonds is particularly appealing for its remarkable synthetic versatility, yet it remains highly challenging. Herein, we demonstrate the first example of temperature-dependent selective C–H functionalisation of unactivated C(sp³)–H or C(sp²)–H bonds at remote positions through palladium catalysis using 7-pyridyl-pyrazolo[1,5-a]pyrimidine as a new directing group. At 120 °C, C(sp³)–H arylation was triggered by the chelation of a rare [6,5]-fused palladacycle, whereas at 140 °C, C(sp²)–H arylation proceeded instead through the formation of a 16-membered tetramer containing four 7-pyridyl-pyrazolo[1,5-a]pyrimidine–palladium chelation units. The subsequent mechanistic study revealed that both C–H activations shared a common 6-membered palladacycle intermediate, which was then directly transformed to either the [6,5]-fused palladacycle for C(sp³)–H activation at 120 °C or the tetramer for C(sp²)–H arylation at 140 °C with catalytic amounts of Pd(OAc)₂ and AcOH. Raising the temperature from 120 °C to 140 °C can also convert the [6,5]-fused palladacycle to the tetramer with the above-mentioned catalysts, hence completing the C(sp²)–H arylation ultimately.

Unprecedented 16-membered tetramer or [6,5]-fused palladacycle, mutually shadowboxing-like transformed from the shared common intermediate, accomplishes the Pd-catalysed temperature-dependent selective arylation of C(sp²)–H or C(sp³)–H.

RETRACTED: Asymmetric remote C-H borylation of aliphatic amides and esters with a modular iridium catalyst

Site selectivity and stereocontrol remain major challenges in C-H bond functionalization chemistry, especially in linear aliphatic saturated hydrocarbon scaffolds. We report the highly enantioselective and site-selective catalytic borylation of remote C(sp³)-H bonds γ to the carbonyl group in aliphatic secondary and tertiary amides and esters. A chiral C-H activation catalyst was modularly assembled from an iridium center, a chiral monophosphite ligand, an achiral urea-pyridine receptor ligand, and pinacolatoboryl groups. Quantum chemical calculations support an enzyme-like structural cavity formed by the catalyst components, which bind the substrate through multiple noncovalent interactions. Versatile synthetic utility of the enantioenriched γ-borylcarboxylic acid derivatives was demonstrated.

A direct route to six and seven membered lactones via γ-C(sp3)-H activation: a simple protocol to build molecular complexity

Lactones comprise a class of valuable compounds having biological as well as industrial importance. Development of a methodology to synthesize such molecules directly from readily available materials such as aliphatic carboxylic acid is highly desirable. Herein, we have reported synthesis of δ-lactones and ε-lactones via selective γ-C(sp³)–H activation. The γ-C–H bond containing aliphatic carboxylic acids provide six or seven membered lactones depending on the olefin partner in the presence of a palladium catalyst. A mechanistic investigation suggests that C–H activation is the rate-determining step. Further transformations of the lactones have been carried out to showcase the applicability of the present strategy.

Six and seven membered lactones have been synthesized directly from readily available aliphatic acids.

Peptide late-stage C(sp3)-H arylation by native asparagine assistance without exogenous directing groups

There is a strong demand for novel native peptide motifs for post-synthetic modifications of peptides without pre-installation and subsequent removal of directing groups. Herein, we report an efficient method for peptide late-stage C(sp3)-H arylations assisted by the unmodified side chain of asparagine (Asn) without any exogenous directing group. Thereby, site-selective arylations of C(sp3)-H bonds at the N-terminus of di-, tri-, and tetrapeptides have been achieved. Likewise, we have constructed a key building block for accessing agouti-related protein (AGRP) active loop analogues in a concise manner.

Distal γ-C(sp3 )-H Olefination of Ketone Derivatives and Free Carboxylic Acids

Reported herein is the distal γ-C(sp3 )-H olefination of ketone derivatives and free carboxylic acids. Fine tuning of a previously reported imino-acid directing group and using the ligand combination of a mono-N-protected amino acid (MPAA) and an electron-deficient 2-pyridone were critical for the γ-C(sp3 )-H olefination of ketone substrates. In addition, MPAAs enabled the γ-C(sp3 )-H olefination of free carboxylic acids to form diverse six-membered lactones. Besides alkyl carboxylic acids, benzylic C(sp3 )-H bonds also could be functionalized to form 3,4-dihydroisocoumarin structures in a single step from 2-methyl benzoic acid derivatives. The utility of these protocols was demonstrated in large scale reactions and diversification of the γ-C(sp3 )-H olefinated products.

Ligand-Enabled γ-C(sp3 )-H Olefination of Free Carboxylic Acids

We report the ligand-enabled C-H activation/olefination of free carboxylic acids in the γ-position. Through an intramolecular Michael addition, δ-lactones are obtained as products. Two distinct ligand classes are identified that enable the challenging palladium-catalyzed activation of free carboxylic acids in the γ-position. The developed protocol features a wide range of acid substrates and olefin reaction partners and is shown to be applicable on a preparatively useful scale. Insights into the underlying reaction mechanism obtained through kinetic studies are reported.

Catalytic Behavior of Mono-N-Protected Amino-Acid Ligands in Ligand-Accelerated C-H Activation by Palladium(II)

Mono-N-protected amino acids (MPAAs) are increasingly common ligands in Pd-catalyzed C-H functionalization reactions. Previous studies have shown how these ligands accelerate catalytic turnover by facilitating the C-H activation step. Here, it is shown that MPAA ligands exhibit a second property commonly associated with ligand-accelerated catalysis: the ability to support catalytic turnover at substoichiometric ligand-to-metal ratios. This catalytic role of the MPAA ligand is characterized in stoichiometric C-H activation and catalytic C-H functionalization reactions. Palladacycle formation with substrates bearing carboxylate and pyridine directing groups exhibit a 50-100-fold increase in rate when only 0.05 equivalents of MPAA are present relative to PdII . These and other mechanistic data indicate that facile exchange between MPAAs and anionic ligands coordinated to PdII enables a single MPAA to support C-H activation at multiple PdII centers.

Pd(II)-Catalyzed Direct γ-C(sp3)-H Arylation between Free β2-Amino Esters and β3-Amino Esters and Aryl Iodides Using a Catalytic Transient Directing Group

Pd(II)-catalyzed direct γ-C(sp³)-H arylation coupling with free β²-amino esters and β³-amino esters using a commercially available catalytic transient directing group has been developed. This approach features high efficiency, broad substrate tolerance, easily accessible starting materials, and mild reaction conditions.

Transient Directing Group Enabled Pd-catalyzed γ-C(sp3)-H Oxygenation of Alkyl Amines

We report a general protocol for γ-C(sp³)-H acyloxylation and alkoxylation of free amines using 2-hydroxynicotinaldehyde as the transient directing group. In the presence of an electrophilic fluorinating bystanding oxidant and acetic acid, a wide range of aliphatic amines could be oxygenated selectively at the γ-methyl positions. A vast variety of aryl, heteroaryl, and aliphatic acids could also be successfully coupled under this C-O bond formation reaction to afford amine containing esters. Switching the nucleophile from acids to alcohols enables alkoxylation of free amines. Importantly, natural products and drug molecules such as ibuprofen, isozepac, fenbufen, and lithocholic acid are all compatible coupling partners. Notably, synthesis of these mono-protected amino alcohols from free amino alcohols using conventional selective protection are not always feasible.

Systematic Investigation of the Scope of Transannular C-H Heteroarylation of Cyclic Secondary Amines for Synthetic Application in Medicinal Chemistry

Transannular C-H heteroarylation of amines provides rapid access to complex scaffolds that are otherwise difficult to synthesize. Wide adaptation of this emerging reaction for medicinal chemistry requires a broad understanding of substrate scope and more robust experimental conditions. In this article, we report a new ligand to promote the transannular reaction of a range of fused- and bridged-bicyclic secondary amines with a broad set of heteroarenes. The method was also successfully applied to the arylation of one spiro-bicyclic amine, a class of substrates that has not been studied in the context of transannular C-H activation reactions. The broad application of this transannular C-H heteroarylation methodology is currently hampered by the difficulty of removing the directing group. The development of a new directing group that is easier to remove will expand the utility of this reaction.

Ligand-Controlled Direct γ-C-H Arylation of Aldehydes

The first example of Pd^II -catalyzed γ-C(sp³ )-H functionalization of aliphatic and benzoheteroaryl aldehydes has been developed using a transient ligand and an external ligand, concurrently. A wide array of γ-arylated aldehydes were readily accessed without preinstalling internal directing groups. The catalytic mechanism was studied by performing deuterium-labelling experiments, which indicated that the γ-C(sp³ )-H bond cleavage is the rate-limiting step during the reaction process. This reaction could be performed on a gram scale, and also demonstrated its potential application in the synthesis of new mechanofluorochromic materials with blue-shifted mechanochromic properties.

Synthesis of amino acids and peptides with bulky side chains via ligand-enabled carboxylate-directed γ-C(sp3)-H arylation

Amino acids and peptides with bulky side chains are of significant importance in organic synthesis and modern medicinal chemistry. The efficient synthesis of these molecules with full enantiocontrol and high diversity remains challenging. Herein we report a Pd-catalyzed ligand-enabled γ-C(sp3)-H arylation of tert-leucine and its derived peptides without using an external directing group (DG) via a less favored six-membered palladacycle. Structurally diverse bulky side chain amino acids and peptides were accessed in a step-economic fashion and the reaction could be conducted on a gram scale with retention of chirality. The resulting amino acids can be used as chiral ligands in Co(iii)-catalyzed enantioselective C(sp3)-H amidation. It is worth noting that the weakly coordinating carboxylate DG outcompetes the strongly coordinating bidentate DG of the peptide backbone, providing the products of γ-C(sp3)-H arylation of Tle residue exclusively. This protocol represents the first example of late stage C(sp3)-H functionalization of peptides using a weakly coordinating directing group.

Late-stage functionalization of peptides via a palladium-catalyzed C(sp3)–H activation strategy

Recent advances in the late-stage modification of peptides via palladium-catalyzed C(sp³)–H functionalization are summarized.

Palladium-Catalyzed C8-Arylation of Naphthalenes through C-H Activation: A Combined Experimental and Computational Study

Herein, a direct C8-arylation reaction of 1-amidonaphthalenes is described. By using diaryliodonium salts as arylating agents, the palladium-catalyzed C-H activation reaction showed perfect C8 regioselectivity and a wide functional group tolerance. In most cases, the desired polyaromatic compounds were isolated in good to excellent yields. To explain the observed regioselectivity, DFT calculations were performed and highlighted the crucial role of the amide directing group. Finally, the utility of this method is showcased by the synthesis of benzanthrone derivatives.