Pd(II)-Catalyzed Cross-Coupling of sp3 C−H Bonds with sp2 and sp3 Boronic Acids Using Air as the Oxidant

Quinoxaline: a new directing group for ortho C–H alkenylation / intramolecular ortho C–H cycloamination under open air leading to bioactive polynuclear N-heteroarenes

A two-step strategy involving quinoxaline directed the ortho C–H alkenylation/intramolecular ortho C–H cycloamination afforded new PDE4 inhibitors.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Synthesis of [POCOP]-pincer iron and cobalt complexes via Csp3–H activation and catalytic application of iron hydride in hydrosilylation reactions

C_sp3–H bond activation in pincer ligand (Ph₂PO(o-C₆H₂-(4,6-^tBu₂)))₂CH₂ (1) (POCH₂OP) was achieved by Fe(PMe₃)₄ and CoMe(PMe₃)₄ to afford (POCHOP)Fe(H) (PMe₃)₂ (2) and (POCHOP)Co(PMe₃)₂ (4).

Overcoming the limitations of directed C-H functionalizations of heterocycles

In directed C–H activation reactions, nitrogen and sulfur atoms present in heterocyclic substrates coordinate strongly with metal catalysts. This coordination, which can lead to catalyst poisoning or C–H functionalization at an undesired position, limits the application of C–H activation reactions in heterocycle-based drug discovery.^1–5 Herein, we report a robust and synthetically useful reaction that overcomes the complications associated with performing C–H functionalization reactions on heterocycles. Our approach employs a simple N-methoxy amide group, which serves as both a directing group and an anionic ligand to promote the in situ generation of the reactive PdX₂ (X = ArCONOMe) species from a Pd(0) source using air as the sole oxidant. In this way, the PdX₂ species is inherently anchored in close proximity with the target C–H bond adjacent to CONHOMe group, thus avoiding the interference from various heterocycles. Remarkably, this reaction overrides the conventional positional selectivity patterns observed with substrates containing strongly coordinating heteroatoms, including nitrogen, sulfur, and phosphorus. Thus, this operationally simple aerobic reaction demonstrates the feasibility of bypassing a fundamental limitation that has long plagued applications of directed C–H activation in medicinal chemistry.

Cu(OAc)2-catalyzed coupling of aromatic C-H bonds with arylboron reagents

Cu-catalyzed coupling of aryl C-H bonds with arylboron reagents was accomplished using a readily removable directing group, which provides a useful method for the synthesis of biaryl compounds. The distinct transmetalation step in this Cu-catalyzed C-H coupling with aryl borons provides unique evidence for the formation of an aryl cupperate intermediate.

Ligand-promoted alkylation of C(sp3)-H and C(sp2)-H bonds

9-Methylacridine was identified as a generally effective ligand to promote a Pd(II)-catalyzed C(sp³)–H and C(sp²)–H alkylation of simple amides with various alkyl iodides. This alkylation reaction was applied to the preparation of unnatural amino acids and geometrically controlled tri- and tetrasubstituted acrylic acids.

Palladium-catalysed C-H activation of aliphatic amines to give strained nitrogen heterocycles

The development of new chemical transformations based on catalytic functionalization of unactivated C-H bonds has the potential to simplify the synthesis of complex molecules dramatically. Transition metal catalysis has emerged as a powerful tool with which to convert these unreactive bonds into carbon-carbon and carbon-heteroatom bonds, but the selective transformation of aliphatic C-H bonds is still a challenge. The most successful approaches involve a 'directing group', which positions the metal catalyst near a particular C-H bond, so that the C-H functionalization step occurs via cyclometallation. Most directed aliphatic C-H activation processes proceed through a five-membered-ring cyclometallated intermediate. Considering the number of new reactions that have arisen from such intermediates, it seems likely that identification of distinct cyclometallation pathways would lead to the development of other useful chemical transformations. Here we report a palladium-catalysed C-H bond activation mode that proceeds through a four-membered-ring cyclopalladation pathway. The chemistry described here leads to the selective transformation of a methyl group that is adjacent to an unprotected secondary amine into a synthetically versatile nitrogen heterocycle. The scope of this previously unknown bond disconnection is highlighted through the development of C-H amination and carbonylation processes, leading to the synthesis of aziridines and β-lactams (respectively), and is suggestive of a generic C-H functionalization platform that could simplify the synthesis of aliphatic secondary amines, a class of small molecules that are particularly important features of many pharmaceutical agents.

Unlocking nature's C-H bonds

In an idealistic setting, it can be imagined that if every CH bond on an organic molecule could be selectively functionalized, the fields of chemical synthesis and drug discovery would be forever revolutionized. With the purpose of investigating the practicality of this idealistic scenario, our group has endeavored to unlock the potential of nature's CH bonds by developing palladium-catalyzed, site selective CH insertions that can be incorporated into both known and new catalytic cycles. To this end, we have developed a number of catalytic transformations that not only provide rapid diversification of simple starting materials and natural products through CH functionalization, but streamline the synthesis of a variety of natural products with biological activity and expand upon methods to access highly valuable enantiopure materials.

Palladium(II)-catalyzed enantioselective C(sp³)-H activation using a chiral hydroxamic acid ligand

An enantioselective method for Pd(II)-catalyzed cross-coupling of methylene β-C(sp(3))-H bonds in cyclobutanecarboxylic acid derivatives with arylboron reagents is described. High yields and enantioselectivities were achieved through the development of chiral mono-N-protected α-amino-O-methylhydroxamic acid (MPAHA) ligands, which form a chiral complex with the Pd(II) center. This reaction provides an alternative approach to the enantioselective synthesis of cyclobutanecarboxylates containing α-chiral quaternary stereocenters. This new class of chiral catalysts also show promises for enantioselective β-C(sp(3))-H activation of acyclic amides.

Late-Stage C-H Coupling Enables Rapid Identification of HDAC Inhibitors: Synthesis and Evaluation of NCH-31 Analogues

We previously reported the discovery of NCH-31, a potent histone deacetylase (HDAC) inhibitor. By utilizing our C-H coupling reaction, we rapidly synthesized 16 analogues (IYS-1 through IYS-15 and IYS-Me) of NCH-31 with different aryl groups at the C4-position of 2-aminothiazole core of NCH-31. Subsequent biological testing of these derivatives revealed that 3-fluorophenyl (IYS-10) and 4-fluorophenyl (IYS-15) derivatives act as potent pan-HDAC inhibitor. Additionally, 4-methylphenyl (IYS-1) and 3-fluoro-4-methylphenyl (IYS-14) derivatives acted as HDAC6-insensitive inhibitors. The present work clearly shows the power of the late-stage C-H coupling approach to rapidly identify novel and highly active/selective biofunctional molecules.

Ligand-enabled γ-C-H olefination and carbonylation: construction of β-quaternary carbon centers

Monoselective γ-C–H olefination and carbonylation of aliphatic acids has been accomplished by using a combination of a quinoline-based ligand and a weakly coordinating amide directing group. The reaction provides a new route for constructing richly functionalized all-carbon quaternary carbon centers at the β-position of aliphatic acids.

Ligand-controlled C(sp³)-H arylation and olefination in synthesis of unnatural chiral α-amino acids

The use of ligands to tune the reactivity and selectivity of transition metal catalysts for C(sp(3))-H bond functionalization is a central challenge in synthetic organic chemistry. Herein, we report a rare example of catalyst-controlled C(sp(3))-H arylation using pyridine and quinoline derivatives: The former promotes exclusive monoarylation, whereas the latter activates the catalyst further to achieve diarylation. Successive application of these ligands enables the sequential diarylation of a methyl group in an alanine derivative with two different aryl iodides, affording a wide range of β-Ar-β-Ar'-α-amino acids with excellent levels of diastereoselectivity (diastereomeric ratio > 20:1). Both configurations of the β-chiral center can be accessed by choosing the order in which the aryl groups are installed. The use of a quinoline derivative as a ligand also enables C(sp(3))-H olefination of a protected alanine.

Applications of C-H functionalization logic to cyclobutane synthesis

The application of C-H functionalization logic to target-oriented synthesis provides an exciting new venue for the development of new and useful strategies in organic chemistry. In this article, C-H functionalization reactions are explored as an alternative approach to access pseudodimeric cyclobutane natural products, such as the dictazole and the piperarborenine families. The use of these strategies in a variety of complex settings highlights the subtle geometric, steric, and electronic effects at play in the auxiliary guided C-H functionalization of cyclobutanes.

Ligand-enabled cross-coupling of C(sp3)-H bonds with arylboron reagents via Pd(II)/Pd(0) catalysis

There have been numerous developments in C–H activation reactions in the past decade. Attracted by the ability to directly functionalize molecules at ostensibly unreactive C–H bonds, chemists have discovered reaction conditions that enable reaction of C(sp²)–H and C(sp³)–H bonds with a variety of coupling partners. Despite these advances, the development of suitable ligands that enable catalytic C(sp³)–H bond functionalization remains a significant challenge. Here, we report the discovery of a mono-N-protected amino acid (MPAA) ligand that enables Pd(II)-catalyzed coupling of γ-C(sp³)–H bonds in triflyl-protected amines (R–NHTf) with arylboron reagents. Remarkably, no background reaction was observed in the absence of ligand. A variety of amine substrates and arylboron reagents were cross-coupled using this method. Arylation of optically active amino acid-derived substrates also provides a potential route for preparing non-protenogenic amino acids.

A highly efficient Pd-catalyzed decarboxylative ortho-arylation of amides with aryl acylperoxides

A Pd-catalyzed decarboxylative ortho-arylation of amides with aryl acylperoxides was developed. Anilides afforded ortho-arylation products, and N-methoxyarylamides gave phenanthridinones.

Ligand-accelerated ortho-C-H alkylation of arylcarboxylic acids using alkyl boron reagents

A protocol for the Pd(II)-catalyzed ortho-C-H alkylation of phenylacetic and benzoic acids using alkylboron reagents is disclosed. Monoprotected amino acid ligands (MPAA) were found to significantly promote reactivity. Both potassium alkyltrifluoroborates and alkylboronic acids were compatible coupling partners. The possibility of a radical alkyl transfer to Pd(II) was also investigated.

Scope and limitations of auxiliary-assisted, palladium-catalyzed arylation and alkylation of sp2 and sp3 C-H bonds

The scope of palladium-catalyzed, auxiliary-assisted direct arylation and alkylation of sp(2) and sp(3) C-H bonds of amine and carboxylic acid derivatives has been investigated. The method employs a palladium acetate catalyst, substrate, aryl, alkyl, benzyl, or allyl halide, and inorganic base in tert-amyl alcohol or water solvent at 100-140 °C. Aryl and alkyl iodides as well as benzyl and allyl bromides are competent reagents in this transformation. The picolinic acid auxiliary is used for amine γ-functionalization, and the 8-aminoquinoline auxiliary is used for carboxylic acid β-functionalization. Some optimization of base, additives, and solvent is required for achieving best results.