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.

Facile Synthesis of Functionalized Phenoxy Quinolines: Antibacterial Activities against ESBL Producing Escherichia coli and MRSA, Docking Studies, and Structural Features Determination through Computational Approach

The synthesis of new 6-Bromoquinolin-4-ol derivatives (3a–3h) by Chan–Lam coupling utilizing different types of solvents (protic, aprotic, and mixed solvents) and bases was studied in the present manuscript. Furthermore, their potential against ESBL producing Escherichia coli (ESBL E. coli) and methicillin-resistant Staphylococcusaureus (MRSA) were investigated. Commercially available 6-bromoquinolin-4-ol (3a) was reacted with different types of aryl boronic acids along with Cu(OAc)₂ via Chan–Lam coupling methodology utilizing the protic and aprotic and mixed solvents. The molecules (3a–3h) exhibited very good yields with methanol, moderate yields with DMF, and low yields with ethanol solvents, while the mixed solvent CH₃OH/H₂O (8:1) gave more excellent results as compared to the other solvents. The in vitro antiseptic values against ESBL E. coli and MRSA were calculated at five different deliberations (10, 20, 30, 40, 50 mg/well) by agar well diffusion method. The molecule 3e depicted highest antibacterial activity while compounds 3b and 3d showed low antibacterial activity. Additionally, MIC and MBC standards were calculated against the established bacteria by broth dilution method. Furthermore, a molecular docking investigation of the derivatives (3a–3h) were performed. Compound (3e) was highly active and depicted the least binding energy of −5.4. Moreover, to investigate the essential structural and physical properties, the density functional theory (DFT) findings of the synthesized molecules were accomplished by using the basic set PBE0-D3BJ/def2-TZVP/SMD water level of the theory. The synthesized compounds showed an energy gap from 4.93 to 5.07 eV.

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.

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.

Rational Development of Remote C-H Functionalization of Biphenyl: Experimental and Computational Studies

A simple and efficient nitrile-directed meta-C-H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C-H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C-H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd-Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C-H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C-H bond in the concerted metalation-deprotonation (CMD) process.

Ligand-Promoted Non-Directed C-H Cyanation of Arenes

This article reports the first example of a 2-pyridone accelerated non-directed C-H cyanation with an arene as the limiting reagent. This protocol is compatible with a broad scope of arenes, including advanced intermediates, drug molecules, and natural products. A kinetic isotope experiment (k_H /k_D =4.40) indicates that the C-H bond cleavage is the rate-limiting step. Also, the reaction is readily scalable, further showcasing the synthetic utility of this method.

Complementary Strategies for Directed C(sp3 )-H Functionalization: A Comparison of Transition-Metal-Catalyzed Activation, Hydrogen Atom Transfer, and Carbene/Nitrene Transfer

The functionalization of C(sp3 )-H bonds streamlines chemical synthesis by allowing the use of simple molecules and providing novel synthetic disconnections. Intensive recent efforts in the development of new reactions based on C-H functionalization have led to its wider adoption across a range of research areas. This Review discusses the strengths and weaknesses of three main approaches: transition-metal-catalyzed C-H activation, 1,n-hydrogen atom transfer, and transition-metal-catalyzed carbene/nitrene transfer, for the directed functionalization of unactivated C(sp3 )-H bonds. For each strategy, the scope, the reactivity of different C-H bonds, the position of the reacting C-H bonds relative to the directing group, and stereochemical outcomes are illustrated with examples in the literature. The aim of this Review is to provide guidance for the use of C-H functionalization reactions and inspire future research in this area.

Palladium-Catalyzed Pyrazole-Directed sp3 C-H Bond Arylation for the Synthesis of β-Phenethylamines

We have developed a method for palladium-catalyzed, pyrazole-directed sp³ C-H bond arylation by aryl iodides. The reaction employs a Pd(OAc)₂ catalyst at 5-10 mol % loading and silver(I) oxide as a halide-removal agent, and it proceeds in acetic acid or acetic acid/hexafluoroisopropanol solvent. Ozonolysis of the pyrazole moiety affords pharmaceutically important β-phenethylamines.

Ligand-Enabled β-C-H Arylation of α-Amino Acids Without Installing Exogenous Directing Groups

Herein we report acid-directed β-C(sp³ )-H arylation of α-amino acids enabled by pyridine-type ligands. This reaction does not require the installation of an exogenous directing group, is scalable, and enables the preparation of Fmoc-protected unnatural amino acids in three steps. The pyridine-type ligands are crucial for the development of this new C(sp³ )-H arylation.