Directed C–H activation has emerged as a major approach for developing synthetically useful reactions, owing to the proximity-induced reactivity and selectivity enabled by coordinating functional groups1–6. In contrast, development of palladium-catalyzed non-directed C–H activation has faced significant challenges associated with the lack of sufficiently active palladium catalysts7–8. Current palladium catalysts are only reactive with electron-rich arenes unless an excess of arene is used9–18, which limits synthetic applications. Herein, we disclose a 2-pyridone ligand that significantly enhances the reactivity of a palladium catalyst, allowing for Pd(II)-catalyzed non-directed C–H activation of a broad range of aromatic substrates using the various arenes as the limiting reagent. The significance of this finding is demonstrated by the direct functionalization of advanced synthetic intermediates, drug molecules, and natural products that cannot be utilized in excessive quantities. The potential of this methodology to be expanded to a variety of transformations is indicated by the development of both C–H olefination and C–H carboxylation protocols. Furthermore, the site selectivity in this transformation is governed by a combination of steric and electronic effects, with the pyridone ligand enhancing the influence of sterics on the selectivity, thus providing complementary selectivity to directed C–H functionalization.

Achieving selective C–H activation at a single and strategic site in the presence of multiple C–H bonds can provide a powerful and generally useful retrosynthetic disconnection. In this context, the directing group serves as a compass to guide the transition metal to C–H bonds using distance and geometry as powerful recognition parameters to distinguish between proximal and distal C–H bonds. However, the installation and removal of directing groups is a practical drawback. To improve the utility of this approach, one can seek solutions in three directions. First, simplifying the directing group; Second, use of common functional groups or protecting groups as directing groups; Third, attaching the directing group to substrates via a transient covalent bond to render directing groups catalytic. This review describes the rational development of an extremely simple and yet broadly applicable directing group for Pd(II), Rh(III) and Ru(II) catalysts, namely, the N-methoxy amide (CONHOMe). Through collective efforts in the community, a wide range of C–H activation transformations using this type of simple directing groups has been developed.

Here we report the development of a versatile 3-acetylamino-2-hydroxypyridine class of ligands that promote meta-C–H arylation of anilines, heterocyclic aromatic amines, phenols, and 2-benzyl heterocycles using norbornene as a transient mediator. More than 120 examples are presented, demonstrating this ligand scaffold enables a wide substrate and coupling partner scope. Meta-C–H arylation with heterocyclic aryl iodides as coupling partners is also realized for the first time using this ligand. The utility for this transformation for drug discovery is showcased by allowing the meta-C–H arylation of a lenalidomide derivative. The first steps towards a silver free protocol for this reaction are also demonstrated.

Meta-C–H amination and meta-C–H alkynylation of aniline and phenol substrates using a modified norbornene (methyl bicyclo[2.2.1]hept-2-ene-2-carboxylate) as a transient mediator has been developed for the first time. Both the identification of a mono-protected 3-amino-2-hydroxypyridine/pyridone type ligand and the use of a modified norbornene as a mediator are crucial for the realization of these two unprecedented meta-C–H transformations. A variety of substrates are compatible with both meta-C–H amination and meta-C–H alkynylation. Amination and alkynylation of heterocyclic substrates including indole, indoline, and indazole afford the desired products in moderate to high yields.

A range of Rh(III)-catalyzed ortho-C-H functionalizations have been developed; however, extension of this reactivity to remote C-H functionalizations through large-ring rhodacyclic intermediates has yet to be demonstrated. Herein we report the first example of the use of a U-shaped nitrile template to direct Rh(III)-catalyzed remote meta-C-H activation via a postulated 12-membered macrocyclic intermediate. Because the ligands used for Rh(III) catalysts are significantly different from those of Pd(II) catalysts, this offers new opportunities for future development of ligand-promoted meta-C-H activation reactions.

Pd-catalyzed C–H functionalization of mandelic acid and α-phenylglycine is reported. We have developed different protocols for the arylation, iodination, acetoxylation, and olefination of these substrates based on two different (Pd(II)/Pd(IV) and Pd(II)/Pd(0)) catalytic cycles. Four crucial features of these protocols are advantageous for practical applications. First, the α-hydroxyl and amino groups are protected with simple protecting groups such as acetates (Ac, Piv) and carbamates (Boc, Fmoc), respectively. Second, these protocols do not involve installation and removal of a directing group. Third, monoselectivity is accomplished. Fourth, no epimerization occurs at the vulnerable α-chiral centers.

Meta-C–H functionalization of benzylamines has been developed using a Pd(II)/transient mediator strategy. Using 2-pyridone ligands and 2-carbomethoxylnorbornene (NBE-CO2Me) as the mediator, arylation, amination, and chlorination of benzylamines are realized. This protocol features a broad substrate scope and is compatible with heterocylic coupling partners. Moreover, the loading of the Pd can be lowered to 2.5 mol% by using the optimal ligand.

Pd-catalyzed meta-C–H chlorination of anilines and phenols is developed using norbornene as the mediator. The presence of heterocycles, including indole, thiophene and indazole, are tolerated. The identification of a new pyridone-based ligand is crucial for the success of this meta-C–H chlorination reaction. Subsequent diverse transformations of the chlorinated products demonstrate the versatility of meta-C–H chlorination.