Distal γ‐C(sp 3 )−H Olefination of Ketone Derivatives and Free Carboxylic Acids

Palladium-Catalyzed Ligand-Enabled Cyclization of Substituted Aliphatic Carboxylic Acids with Allylic Electrophiles

A palladium-catalyzed cyclization of the β-C(sp3)-H bond of aliphatic carboxylic acids with allylic electrophiles providing five-membered γ-lactones in good to excellent yields is demonstrated. An acetyl-protected aminoethyl phenyl thioether ligand is used to promote the C-H activation reaction. A diverse range of allylic electrophiles such as allyl alcohols, allyl acetates, allyl sulfones, allyl phosphonate, allyl amine, and allyl ester have been utilized for this reaction. A feasible reaction mechanism has been proposed to account for the present cyclization reaction.

Tandem dehydrogenation-olefination-decarboxylation of cycloalkyl carboxylic acids via multifold C-H activation

Dehydrogenation chemistry has long been established as a fundamental aspect of organic synthesis, commonly encountered in carbonyl compounds. Transition metal catalysis revolutionized it, with strategies like transfer-dehydrogenation, single electron transfer and C-H activation. These approaches, extended to multiple dehydrogenations, can lead to aromatization. Dehydrogenative transformations of aliphatic carboxylic acids pose challenges, yet engineered ligands and metal catalysis can initiate dehydrogenation via C-H activation, though outcomes vary based on substrate structures. Herein, we have developed a catalytic system enabling cyclohexane carboxylic acids to undergo multifold C-H activation to furnish olefinated arenes, bypassing lactone formation. This showcases unique reactivity in aliphatic carboxylic acids, involving tandem dehydrogenation-olefination-decarboxylation-aromatization sequences, validated by control experiments and key intermediate isolation. For cyclopentane carboxylic acids, reluctant to aromatization, the catalytic system facilitates controlled dehydrogenation, providing difunctionalized cyclopentenes through tandem dehydrogenation-olefination-decarboxylation-allylic acyloxylation sequences. This transformation expands carboxylic acids into diverse molecular entities with wide applications, underscoring its importance.

Access to unsaturated bicyclic lactones by overriding conventional C(sp3)-H site selectivity

Transition metal catalysis plays a pivotal role in transforming unreactive C-H bonds. However, regioselective activation of distal aliphatic C-H bonds poses a tremendous challenge, particularly in the absence of directing templates. Activation of a methylene C-H bond in the presence of methyl C-H is underexplored. Here we show activation of a methylene C-H bond in the presence of methyl C-H bonds to form unsaturated bicyclic lactones. The protocol allows the reversal of the general selectivity in aliphatic C-H bond activation. Computational studies suggest that reversible C-H activation is followed by β-hydride elimination to generate the Pd-coordinated cycloalkene that undergoes stereoselective C-O cyclization, and subsequent β-hydride elimination to provide bicyclic unsaturated lactones. The broad generality of this reaction has been highlighted via dehydrogenative lactonization of mid to macro ring containing acids along with the C-H olefination reaction with olefin and allyl alcohol. The method substantially simplifies the synthesis of important bicyclic lactones that are important features of natural products as well as pharmacoactive molecules.

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.

Efficient Aerobic Oxidative Coupling of Methyl Heteroarenes with Indoles

Direct oxidative coupling of inert C(sp³ )-H bond has been a great challenge. Herein, an environmentally friendly aerobic oxidative coupling of α-methyl substituted N-heteroarenes with indoles is reported. A variety of diheteroaryl ketones were prepared in good yields (up to 72 %). This protocol features simple operation and broad substrates scope (26 examples). Significantly, a plausible mechanism about catalytic cycle was proposed, and two key intermediates were confirmed by high resolution mass spectrometry.

Transition-Metal-Catalyzed Annulations Involving the Activation of C(sp3 )-H Bonds

The selective functionalization of C(sp3 )-H bonds using transition-metal catalysis is among the more attractive transformations of modern synthetic chemistry. In addition to its inherent atom economy, such reactions open unconventional retrosynthetic pathways that can streamline synthetic processes. However, the activation of intrinsically inert C(sp3 )-H bonds, and the selection among very similar C-H bonds, represent highly challenging goals. In recent years there has been notable progress tackling these issues, especially with regard to the development of intermolecular reactions entailing the formation of C-C and C-heteroatom bonds. Conversely, the assembly of cyclic products from simple acyclic precursors using metal-catalyzed C(sp3 )-H bond activations has been less explored. Only recently has the number of reports on such annulations started to grow. Herein we give an overview of some of the more relevant advances in this exciting topic.

Direct β- and γ-C(sp3 )-H Alkynylation of Free Carboxylic Acids*

In this study we report the identification of a novel class of ligands for palladium-catalyzed C(sp3 )-H activation that enables the direct alkynylation of free carboxylic acid substrates. In contrast to previous synthetic methods, no introduction/removal of an exogenous directing group is required. A broad scope of acids including both α-quaternary and challenging α-non-quaternary can be used as substrates. Additionally, the alkynylation in the distal γ-position is reported. Finally, this study encompasses preliminary findings on an enantioselective variant of the title transformation as well as synthetic applications of the products obtained.