Palladium-Catalyzed Enantioselective β-C(sp3)-H Activation Reactions of Aliphatic Acids: A Retrosynthetic Surrogate for Enolate Alkylation and Conjugate Addition

Versatile Copper-Catalyzed γ-C(sp3)-H Lactonization of Aliphatic Acids

Site-selective C(sp3)-H oxidation is of great importance in organic synthesis and drug discovery. γ-C(sp3)-H lactonization of free carboxylic acids provides the most straightforward means to prepare biologically important lactone scaffolds from abundant and inexpensive carboxylic acids; however, a versatile catalyst for this transformation with a broad substrate scope remains elusive. Herein, we report a simple yet broadly applicable and scalable γ-lactonization reaction of free aliphatic acids enabled by a copper catalyst in combination with inexpensive Selectfluor as the oxidant. This lactonization reaction exhibits compatibility with tertiary, benzylic, allylic, methylene, and primary γ-C-H bonds, affording access to a wide range of structurally diverse lactones such as spiro, fused, and bridged lactones. Notably, exclusive γ-methylene C-H lactonization of cycloalkane carboxylic acids and cycloalkane acetic acids was observed, giving either fused or bridged γ-lactones that are difficult to access by other methods. δ-C-H lactonization was only favored in the presence of tertiary δ-C-H bonds. The synthetic utility of this methodology was demonstrated by the late-stage functionalization of amino acids, drug molecules, and natural products, as well as a two-step total synthesis of (iso)mintlactones (the shortest synthesis reported to date).

Enantioselective remote methylene C-H (hetero)arylation of cycloalkane carboxylic acids

Stereoselective construction of γ- and δ-stereocenters in carbonyl compounds is a pivotal objective in asymmetric synthesis. Here, we report chiral bifunctional oxazoline-pyridone ligands that enable enantioselective palladium-catalyzed remote γ-C-H (hetero)arylations of free cycloalkane carboxylic acids, which are essential carbocyclic building blocks in organic synthesis. The reaction establishes γ-tertiary and α-quaternary stereocenters simultaneously in up to >99% enantiomeric excess, providing access to a wide range of cyclic chiral synthons and bioactive molecules. The sequential enantioselective editing of two methylene C-H bonds can be achieved by using chiral ligands with opposite configuration to construct carbocycles containing three chiral centers. Enantioselective remote δ-C-H (hetero)arylation is also realized to establish δ-stereocenters that are particularly challenging to access using classical methodologies.

Construction of Si-Stereogenic Silanols by Palladium-Catalyzed Enantioselective C-H Alkenylation

The construction of silicon-stereogenic silanols via Pd-catalyzed intermolecular C-H alkenylation with the assistance of a commercially available L-pyroglutamic acid has been realized for the first time. Employing oxime ether as the directing group, silicon-stereogenic silanol derivatives could be readily prepared with excellent enantioselectivities, featuring a broad substrate scope and good functional group tolerance. Moreover, parallel kinetic resolution with unsymmetric substrates further highlighted the generality of this protocol. Mechanistic studies indicate that L-pyroglutamic acid could stabilize the Pd catalyst and provide excellent chiral induction. Preliminary computational studies unveil the origin of the enantioselectivity in the C-H bond activation step.

Pd(0)-Catalyzed Intermolecular Methylene C(sp3)-H Silylation by Using N-Heterocyclic Carbene Ligands

The direct functionalization of methylene C(sp3)-H bonds is one of the greatest challenges in transition metal-catalyzed C-H activation. Although Pd(0)-catalyzed intramolecular cyclization reactions of methylene C(sp3)-H bonds have been reported, intermolecular functionalization remains to be discovered. Herein, we report the first example of a Pd(0)-catalyzed intermolecular methylene C(sp3)-H functionalization reaction. By use of a N-heterocyclic carbene ligand, the methylene C(sp3)-H bonds of 1-(benzyloxy)-2-iodobenzenes are activated and disilylated with hexamethyldisilane, affording disilylated products.

Palladium-Catalyzed C-H Olefination of Imidazo[1,2a] pyridine Carboxamide in Aqueous Ethanol under Oxygen

The advancement of sustainable chemistry and changes in the economy are strongly intertwined. Reaction time, cost savings, moderate temperatures, and generation of the fewest byproducts are frequently achieved by using catalytic processes. Herein, we report the C-H olefination of imidazo[1,2a] pyridine carboxamides with various acrylates in the presence of Pd (OAc)2 with O2 as the oxidant in aqueous ethanol rather than using non-ecofriendly solvents. The C-H activation features most user-friendly reaction conditions, excellent yield as well as plenty substrate scope and applicable for C-H deuteriation of the corresponding heteroarenes with D2O. Experimental mechanistic studies indicate that C-H activation step succeeded after formation of tetra coordinated square planer Pd-substrate adduct.

Synthesis of R-GABA Derivatives via Pd(II) Catalyzed Enantioselective C(sp3)-H Arylation and Virtual Validation with GABAB1 Receptor for Potential leads

GABA (γ-amino butyric acid) analogues like baclofen, tolibut, phenibut, etc., are well-known GABAB1 inhibitors and pharmaceutically important drugs. However, there is a huge demand for more chiral GABA aryl analogues with promising pharmacological actions. Here, we demonstrate the chiral ligand acetyl-protected amino quinoline (APAQ) mediated enantioselective synthesis of GABAB1 inhibitor drug scaffolds from easily accessible GABA via Pd-catalyzed C(sp3)-H activation. The synthetic methodology shows moderate to good yields, up to 74% of ee. We have successfully demonstrated the deprotection and removal of the directing group to synthesize R-tolibut in 86% yield. Further, we employed computation to probe the binding of R-GABA analogues to the extracellular domain of the human GABAB1 receptor. Our Rosetta-based molecular docking calculations show better binding for four R-enantiomers of GABA analogues than R-baclofen and R-phenibut. In addition, we employed GROMACS MD simulations and MMPB(GB)SA calculations to identify per-residue contribution to binding free energy. Our computational results suggest analogues (3R)-4-amino-3-(3,4-dimethylphenyl) butanoic acid, (3R)-4-amino-3-(3-fluorophenyl) butanoic acid, (3R)-3-(4-acetylphenyl)-4-aminobutanoic acid, (3R)-4-amino-3-(4-methoxyphenyl) butanoic acid, and (3R)-4-amino-3-phenylbutanoic acid are potential leads which could be synthesized from our methodology reported here.

Enantioselective functionalization of unactivated C(sp3)-H bonds through copper-catalyzed diyne cyclization by kinetic resolution

Site- and stereoselective C-H functionalization is highly challenging in the synthetic chemistry community. Although the chemistry of vinyl cations has been vigorously studied in C(sp3)-H functionalization reactions, the catalytic enantioselective C(sp3)-H functionalization based on vinyl cations, especially for an unactivated C(sp3)-H bond, has scarcely explored. Here, we report an asymmetric copper-catalyzed tandem diyne cyclization/unactivated C(sp3)-H insertion reaction via a kinetic resolution, affording both chiral polycyclic pyrroles and diynes with generally excellent enantioselectivities and excellent selectivity factors (up to 750). Importantly, this reaction demonstrates a metal-catalyzed enantioselective unactivated C(sp3)-H functionalization via vinyl cation and constitutes a kinetic resolution reaction based on diyne cyclization. Theoretical calculations further support the mechanism of vinyl cation-involved C(sp3)-H insertion reaction and elucidate the origin of enantioselectivity.

Metal-catalyzed asymmetric reactions enabled by organic peroxides

As a class of multifunctional reagents, organic peroxides play vital roles in the chemical industry, pharmaceutical synthesis and polymerization reactions. Metal-catalyzed asymmetric catalysis has emerged as one of the most straightforward and efficient strategies to construct enantioenriched molecules, and an increasing number of metal-catalyzed asymmetric reactions enabled by organic peroxides have been disclosed by researchers in recent years. Despite remarkable progress, the types of asymmetric reactions facilitated by organic peroxides remain limited and the catalysis systems need to be further broadened. To the best of our knowledge, there is still no review devoted to summarizing the reactions from this perspective. In this review, we will endeavor to highlight the advances in metal-catalyzed asymmetric reactions enabled by organic peroxides. We hope that this survey will summarize the functions of organic peroxides in catalytic reactions, improve the understanding of these compounds and inspire future developments in this area.

Reversal of Regioselectivity in Asymmetric C-H Bond Annulation with Bromoalkynes under Cobalt Catalysis

Metal-catalyzed asymmetric C-H bond annulation strategy offers a versatile platform, allowing the construction of complex P-chiral molecules through atom- and step-economical fashion. However, regioselective insertion of π-coupling partner between M-C bond with high enantio-induction remain elusive. Using commercially available Co(II) salt and chiral-Salox ligands, we demonstrate an unusual protocol for the regio-reversal, enantioselective C-H bond annulation of phosphinamide with bromoalkyne through desymmetrization. The reaction proceeds through ligand-assisted enantiodetermining cyclocobaltation followed by regioselective insertion of bromoalkyne between Co-C, subsequent reductive elimination, and halogen exchange with carboxylate resulted in P-stereogenic compounds in excellent ee (up to >99 %). The isolation of cobaltacycle involved in the catalytic cycle and the outcome of control experiments provide support for a plausible mechanism.

Photocatalyzed Enantioselective Functionalization of C(sp3)-H Bonds

Owing to its diverse activation processes including single-electron transfer (SET) and hydrogen-atom transfer (HAT), visible-light photocatalysis has emerged as a sustainable and efficient platform for organic synthesis. These processes provide a powerful avenue for the direct functionalization of C(sp3)-H bonds under mild conditions. Over the past decade, there have been remarkable advances in the enantioselective functionalization of the C(sp3)-H bond via photocatalysis combined with conventional asymmetric catalysis. Herein, we summarize the advances in asymmetric C(sp3)-H functionalization involving visible-light photocatalysis and discuss two main pathways in this emerging field: (a) SET-driven carbocation intermediates are followed by stereospecific nucleophile attacks; and (b) photodriven alkyl radical intermediates are further enantioselectively captured by (i) chiral π-SOMOphile reagents, (ii) stereoselective transition-metal complexes, and (iii) another distinct stereoscopic radical species. We aim to summarize key advances in reaction design, catalyst development, and mechanistic understanding, to provide new insights into this rapidly evolving area of research.

Palladium-Catalyzed Enantioselective Directed C(sp3)-H Functionalization Using C5-Substituted 8-Aminoquinoline Auxiliaries

8-Aminoquinoline (AQ) has proven to be a highly effective bidentate directing group for palladium-catalyzed C-H functionalization reactions. However, enantiocontrol of AQ-directed C(sp3)-H functionalization reactions has been challenging. Herein, a new protocol is presented for the Pd-catalyzed enantioselective arylation of unactivated β C(sp3)-H bonds of alkyl carboxamides with aryl iodides using a C5-iodinated 8-aminoquinolines (IQ) auxiliary in conjugation with a BINOL ligand. Additionally, a C5-aryl substituted 8-aminoquinoline auxiliary can facilitate enantioselective alkenylation and alkynylation of benzylic C(sp3)-H bonds of 3-arylpropanamides with the corresponding bromide reagents under similar conditions.

Enantioselective Intermolecular C-H Functionalization of Primary Benzylic C-H Bonds Using ((Aryl)(diazo)methyl)phosphonates

Catalyst-controlled C-H functionalization using donor/acceptor carbenes has been shown to be an efficient process capable of high levels of site control and stereocontrol. This study demonstrated that the scope of the donor/acceptor carbene C-H functionalization can be extended to systems where the acceptor group is a phosphonate. When using the optimized dirhodium catalyst, Rh2(S-di-(4-Br)TPPTTL)4, ((aryl)(diazo)methyl)phosphonates undergo highly enantioselective (84-99% ee) and site-selective (>30:1 r.r.) benzylic C-H functionalization. The phosphonate group is much more sterically demanding than the previously studied carboxylate ester group, leading to much higher selectivity for a primary site versus more sterically crowded positions. The effectiveness of this methodology has been demonstrated by the late-stage primary C-H functionalization of estrone, adapalene, (S)-naproxen, clofibrate, and gemfibrozil derivatives.

Rhodium-Catalyzed Two-Fold, Regioselective and Enantioselective C-H Activation: an Efficient Strategy to Chiral Single-Benzene-Based Fluorophores

A Rh-catalyzed two-fold, regioselective and enantioselective C-H activation via chiral transient directing group strategy has been demonstrated in moderate to good yields with commendable enantioselectivities. The newly synthesized chiral fluorophores exhibit favorable photophysical properties, including large Stokes shifts, good fluorescence quantum yields, aggregation-induced emission in aqueous solution, and intense emission and circularly polarized luminescence in the solid state, indicating great potential applications as chiral fluorescent probes or optoelectronic materials.

Rhodium(III)-Catalyzed C-H Cascade Annulation of Arylhydrazines with 2-Diazo-1,3-indandiones for the Synthesis of Tetracyclic Indeno[1,2-b]indoles

An efficient approach for the preparation of tetracyclic indeno[1,2-b]indoles via Rh(III)-catalyzed C-H cascade annulation between arylhydrazines and diazo indan-1,3-diones has been established. In addition, a series of indeno[1,2-b]indoles were obtained in up to 96% yield with a wide range of substrates and high functional group tolerance. Finally, the diverse transformations of the desired products demonstrate the synthetic utility and utilization of this protocol.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Carboxylate-Directed Pd-Catalyzed β-C(sp3)-H Arylation of N-Methyl Alanine Derivatives for Diversification of Bioactive Peptides

This study presents a Pd(II)-catalyzed method for the β-C(sp3)-H arylation of N-Cbz- or N-Fmoc-protected N-methyl alanines, providing ready access to building blocks for N-methylated peptide synthesis. For this transformation, the native carboxylate was exploited as the directing group, attributing its success to the use of a monoprotected amino-pyridine ligand. Its synthetic utility was demonstrated by facile generation of nine analogues of the naturally occurring N-methylated cyclic peptide cycloaspeptide A.

Ligand-Enabled Double γ-C(sp3 )-H Functionalization of Aliphatic Acids: One-Step Synthesis of γ-Arylated γ-Lactones

γ-methylene C(sp3 )-H functionalization of linear free carboxylic acids remains a significant challenge. Here in we report a Pd(II)-catalyzed tandem γ-arylation and γ-lactonization of aliphatic acids enabled by a L,X-type CarboxPyridone ligand. A wide range of γ-arylated γ-lactones are synthesized in a single step from aliphatic acids in moderate to good yield. Arylated lactones can readily be converted into disubstituted tetrahydrofurans, a prominent scaffold amongst bioactive molecules.

Cu-mediated enantioselective C-H alkynylation of ferrocenes with chiral BINOL ligands

A wide range of Cu(II)-catalyzed C-H activation reactions have been realized since 2006, however, whether a C-H metalation mechanism similar to Pd(II)-catalyzed C-H activation reaction is operating remains an open question. To address this question and ultimately develop ligand accelerated Cu(II)-catalyzed C-H activation reactions, realizing the enantioselective version and investigating the mechanism is critically important. With a modified chiral BINOL ligand, we report the first example of Cu-mediated enantioselective C-H activation reaction for the construction of planar chiral ferrocenes with high yields and stereoinduction. The key to the success of this reaction is the discovery of a ligand acceleration effect with the BINOL-based diol ligand in the directed Cu-catalyzed C-H alkynylation of ferrocene derivatives bearing an oxazoline-aniline directing group. This transformation is compatible with terminal aryl and alkyl alkynes, which are incompatible with Pd-catalyzed C-H activation reactions. This finding provides an invaluable mechanistic information in determining whether Cu(II) cleaves C-H bonds via CMD pathway in analogous manner to Pd(II) catalysts.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

Nickel-Catalyzed Functionalization Reactions Involving C-H Bond Activation via an Amidate-Promoted Strategy and Its Extension to the Activation of C-F, C-O, C-S, and C-CN Bonds

ConspectusThe development of functionalization reactions involving the activation of C-H bonds has evolved extensively due to the atom and step economy associated with such reactions. Among these reactions, chelation assistance has been shown to provide a powerful solution to the serious issues of reactivity and regioselectivity faced in the activation of C-H bonds. The vast majority of C-H functionalization reactions reported thus far has involved the use of precious metals. Kleiman and Dubeck reported the cyclonickelation of azobenzene and NiCp2 in which an azo group directs a Ni center to activate the ortho C-H bond in close proximity. Although this stoichiometric reaction was discovered earlier than that for other transition-metal complexes, its development as a catalytic reaction was delayed. No general catalytic systems were available for Ni-catalyzed C-H functionalization reactions for a long time. This Account details our group's development of Ni(0)- and Ni(II)-catalyzed chelation-assisted C-H functionalization reactions. It also highlights how the new strategy can be extended to the activation of other unreactive bonds.In the early 2010s, we found that the Ni(0)-catalyzed reaction of aromatic amides that contain a 2-pyridinylmethylamine moiety as a directing group with alkynes results in C-H/N-H oxidative annulation to give isoquinolinones. In addition, the combination of a Ni(II) catalyst and an 8-aminoquinoline directing group was found to be a superior combination for developing a wide variety of C-H functionalization reactions with various electrophiles. The reactions were proposed to include the formation of unstable Ni(IV) and/or Ni(III) species; the generation of such high-valence Ni species was rare at that time, but since then, many papers dealing with DFT and organometallic studies have appeared in the literature in attempts to understand the mechanism. Based on our in-depth considerations of the mechanism with respect to why an N,N-bidentate directing group is required, we realized that the formation of a N-Ni bond by the oxidative addition of a N-H bond to a Ni(0) species or a ligand exchange between a N-H bond and Ni(II) species is the key step. We concluded that the precoordination of the N(sp2) atom in the directing group positions the Ni species to be in close proximity to the N-H bond which permits the formation of a N-Ni bond. Based on this working hypothesis, we carried out the reaction using KOtBu as a base and found that the Ni(0)-catalyzed reaction of aromatic amides that do not contain such a specific directing group with alkynes results in the formation of the desired isoquinolinone, in which an amidate anion acts as the actual directing group. Remarkably, this strategy was found to be applicable to the activation of various other unreactive bonds such as C-F, C-O, C-S, and C-CN.

Asymmetric (4+1) Annulations by Cascade Allylation and Transient σ-Alkyl-Pd(II) Initiated Allylic Csp3 -H Activation

A unique Pd-catalyzed approach for asymmetric (4+1) annulations via cascade allylation and transient σ-alkyl-Pd(II) initiated methylene Csp3 -H activation is reported. The enolate fragment derived from the decarboxylation of vinyl methylene carbonate is crucial to stabilize the key intermediate. These reactions enable the synthesis of various useful dihydrobenzofurans with excellent enantioselectivity, typically >95 : 5 er, and exclusive (Z)-stereoselectivity. Compared with the well-established annulations via Heck-type C-H activations, this protocol showcases a conceptually new way to generate σ-alkyl-Pd(II) species that could initiate challenging asymmetric Csp3 -H activations.

Hydrogen-bond-acceptor ligands enable distal C(sp3)-H arylation of free alcohols

The functionalization of C-H bonds in organic molecules is one of the most direct approaches for chemical synthesis. Recent advances in catalysis have allowed native chemical groups such as carboxylic acids, ketones and amines to control and direct C(sp3)-H activation1-4. However, alcohols, among the most common functionalities in organic chemistry5, have remained intractable because of their low affinity for late transition-metal catalysts6,7. Here we describe ligands that enable alcohol-directed arylation of δ-C(sp3)-H bonds. We use charge balance and a secondary-coordination-sphere hydrogen-bonding interaction-evidenced by structure-activity relationship studies, computational modelling and crystallographic data-to stabilize L-type hydroxyl coordination to palladium, thereby facilitating the assembly of the key C-H cleavage transition state. In contrast to previous studies in C-H activation, in which secondary interactions were used to control selectivity in the context of established reactivity8-13, this report demonstrates the feasibility of using secondary interactions to enable challenging, previously unknown reactivity by enhancing substrate-catalyst affinity.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes

1,3-Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small-molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C-H/C-C functionalization strategy for the stereospecific synthesis of cis-γ-functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan-2-ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish-Yang procedure. This intermediate then undergoes a ligand-enabled, palladium-catalyzed C-C cleavage/functionalization to produce valuable cis-γ-(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters.

Palladium-Mediated C(sp3)-H Bond Activation of N-Methyl-N-(pyridin-2-yl)benzamide: Direct Arylation/Alkylation and Mechanistic Investigation

Herein, we present a facile synthetic methodology to produce a range of N-(CH₂-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides via palladium-mediated C(sp³)-H bond activation. The N-methyl-N-(pyridin-2-yl)benzamide precursor was first reacted with palladium(II) acetate in a stoichiometric manner to obtain the key dinuclear palladacycle intermediates, whose structures were elucidated by mass spectrometric, NMR spectroscopic, and X-ray crystallographic studies in detail. The subsequent C(sp³)-H bond functionalizations on the N-methyl group of the starting substrate show facile productions of the corresponding N-(CH₂-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides with good functional group tolerance. A plausible mechanism was proposed based on density functional theory calculations in conjunction with kinetic isotope effect experiments. Finally, the synthetic transformation from the prepared N-(CH₂-aryl)-N-(pyridin-2-yl)benzamides through debenzoylation to N-(CH₂-aryl)-2-aminopyridine was successfully demonstrated.

Unlocking the Nucleophilicity of Strong Alkyl C-H Bonds via Cu/Cr Catalysis

Direct functionalization of inert C-H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C-H bonds are transformed into nucleophilic alkyl-Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C-H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.

β- and γ-C(sp3 )-H Heteroarylation of Free Carboxylic Acids: A Modular Synthetic Platform for Diverse Quaternary Carbon Centers

PdII -catalyzed C(sp3 )-H activation of free carboxylic acids represents a significant advance from conventional cyclopalladation initiated reactions. However, developing a modular synthetic platform for diverse quaternary and tertiary carbon centers based on this reactivity, two challenges remain to be addressed: mono-selectivity in each consecutive C-H functionalization step; compatibility with heteroatoms. While the exclusive mono-selectivity was achieved by β-lactonization/nucleophilic attack, the latter limitation remains to be overcome. Herein, we report the PdII -catalyzed β- and γ-C(sp3 )-H heteroarylation of free carboxylic acids using pyridine-pyridone ligands capable of overcoming these limitations. A sequence of three consecutive C(sp3 )-H activation reactions of pivalic acid provides an unique platform for constructing diverse quaternary carbon centers containing heteroaryls which could serve as an enabling tool for escaping the flat land in medicinal chemistry.

C-H Bond Activation Relay (CHAR) of Proline Ester Derivatives Promoted by In Situ Triarylamine Radical Cation: Selective Synthesis of 4-Bromopyrrole Derivatives

Using the in situ generated triarylamine radical cation as an initiator, the sp³ C-H bond of proline esters was smoothly oxidized and brominated through C-H activation relay (CHAR), giving a series of 4-bromopyrroles in good yields with high regioselectivity. The mechanistic study revealed that the oxidation of the active C-H bond initiated the followed 1,5-HAT and bromination, which provides a new method to realize the functionalization of the remote C-H bond.

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.

MPAI-Ligand Accelerated Pd-Catalyzed C(sp3)-H Arylation of Free Aliphatic Acids

Here, we report the development of a class of bifunctional monoprotected amino-imidazoline (MPAI) ligands and their applications in Pd-catalyzed C(sp³)-H arylation of free aliphatic acids. The newly developed MPAI ligand allows the use of 1.0 equiv of aliphatic acids containing an alpha hydrogen for the first time.

Carboxylic Acid Directed γ-Lactonization of Unactivated Primary C-H Bonds Catalyzed by Mn Complexes: Application to Stereoselective Natural Product Diversification

Reactions that enable selective functionalization of strong aliphatic C-H bonds open new synthetic paths to rapidly increase molecular complexity and expand chemical space. Particularly valuable are reactions where site-selectivity can be directed toward a specific C-H bond by catalyst control. Herein we describe the catalytic site- and stereoselective γ-lactonization of unactivated primary C-H bonds in carboxylic acid substrates. The system relies on a chiral Mn catalyst that activates aqueous hydrogen peroxide to promote intramolecular lactonization under mild conditions, via carboxylate binding to the metal center. The system exhibits high site-selectivity and enables the oxidation of unactivated primary γ-C-H bonds even in the presence of intrinsically weaker and a priori more reactive secondary and tertiary ones at α- and β-carbons. With substrates bearing nonequivalent γ-C-H bonds, the factors governing site-selectivity have been uncovered. Most remarkably, by manipulating the absolute chirality of the catalyst, γ-lactonization at methyl groups in gem-dimethyl structural units of rigid cyclic and bicyclic carboxylic acids can be achieved with unprecedented levels of diastereoselectivity. Such control has been successfully exploited in the late-stage lactonization of natural products such as camphoric, camphanic, ketopinic, and isoketopinic acids. DFT analysis points toward a rebound type mechanism initiated by intramolecular 1,7-HAT from a primary γ-C-H bond of the bound substrate to a highly reactive Mn^IV-oxyl intermediate, to deliver a carbon radical that rapidly lactonizes through carboxylate transfer. Intramolecular kinetic deuterium isotope effect and ¹⁸O labeling experiments provide strong support to this mechanistic picture.

Hydride Relay Exchange Mechanism for the Heterocyclic C-H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C-H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)₂ complexes were investigated using the density functional theory (DFT) method. The Pd(0)L₂(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (S_EAr) mechanism for the C-H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C-H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C-H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C-H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C-H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C-H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.

One-Step Synthesis of β-Alkylidene-γ-lactones via Ligand-Enabled β,γ-Dehydrogenation of Aliphatic Acids

Ligand-enabled Pd-catalyzed regioselective α,β-dehydrogenation of carbonyl compounds via β-methylene C-H activation has recently emerged as a promising transformation. Herein, we report the realization of β,γ-dehydrogenation and subsequent vinyl C-H olefination reactions of free carboxylic acids, thus providing a unique method for the structural diversification of aliphatic acids containing α-quaternary centers through sequential functionalizations of two β-C-H bonds and one γ-C-H bond. This tandem dehydrogenation-olefination-lactonization reaction offers a one-step preparation of β-alkylidene-γ-lactones, which are often difficult to prepare through conventional methods, from inexpensive and abundant free aliphatic acids. A variety of free aliphatic acids, such as isosteviol and grandiflorolic acid natural products, and olefins are compatible with the reported protocol. The newly designed bidentate oxime ether-pyridone and morpholine-pyridone ligands are crucial for this tandem reaction to proceed. Notably, these ligands also enable preferential methylene C-H activation over the previously reported, competing process of methyl C-H bond olefination.

Pd(II)-Catalyzed Synthesis of Bicyclo[3.2.1] Lactones via Tandem Intramolecular β-C(sp3)-H Olefination and Lactonization of Free Carboxylic Acids

Bicyclo[3.2.1] lactones are chemical scaffolds found in numerous bioactive natural products. Herein, we detail the development of a novel palladium(II)-catalyzed tandem intramolecular β-C(sp³)-H olefination and lactonization reaction that rapidly transforms linear carboxylic acid possessing a tethered olefin into the bicyclo[3.2.1] lactone motif. This transformation features a broad substrate scope, shows excellent functional group compatibility, and can be extended to the preparation of the related seven-membered bicyclo[4.2.1] lactones. Additionally, we demonstrate the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of the meroterpenoid cochlactone A. We anticipate that this compelling reaction may provide a novel synthetic disconnection that can be broadly applied toward the preparation of a variety of bioactive natural products.

Application of a transient directing strategy in cyclization reactions via C–H activation

This review introduces seven types of cyclization reactions via C–H activation using a transient directing strategy.