L,X-Type transient directing groups (TDGs) based on a reversible imine linkage have emerged as broadly useful tools for C–H activation of ketones and free amines. However, competitive binding interactions among multiple reaction components (TDG itself, substrate, and substrate–TDG adduct) with the palladium catalyst often lead to the formation of multiple unreactive complexes, rendering ligand development extremely challenging. Herein, we report the finding of versatile 2-pyridone ligands that addresses these problems and significantly improves the γ-methylene arylation of alkyl amines, extending the coupling partners to a wide range of medicinally important heteroaryl iodides and even previously unreactive heteroaryl bromides. The combination of an appropriate transient directing group and pyridone ligand has also enabled the δ-arylation of alkyl amines. Notably, our transient directing group design reveals the importance of matching the size of the Pd-chelation with different transient directing groups and the size of palladacycles generated from γ- and δ-C–H bonds: TDGs that coordinate with Pd­(II) to form a six-membered chelate are selective toward γ-C–H bonds, whereas TDGs that coordinate with Pd­(II) via a five-membered chelate tend to activate δ-C–H bonds. These findings provide an avenue for developing protecting group free and selective C–H functionalization using the transient directing group strategy.

Hydroxylation of aryl carbon–hydrogen bonds with transition metal catalysts has proven challenging when oxygen is used as the oxidant. Here, we report a palladium complex bearing a bidentate pyridine/pyridone ligand that efficiently catalyzes this reaction at ring positions adjacent to carboxylic acids. Infrared, x-ray, and computational analysis support a possible role of ligand tautomerization from mono-anionic (L,X) to neutral (L,L) coordination in the catalytic cycle of aerobic carbon–hydrogen hydroxylation reaction. The conventional site selectivity dictated by heterocycles is overturned by this catalyst, thus allowing late-stage modification of compounds of pharmaceutical interest at previously inaccessible sites.

Dehydrogenative transformations of alkyl chains to alkenes through methylene carbon-hydrogen (C-H) activation remain a substantial challenge. We report two classes of pyridine-pyridone ligands that enable divergent dehydrogenation reactions through palladium-catalyzed b-methylene C-H activation of carboxylic acids, leading to the direct syntheses of a,b-unsaturated carboxylic acids or g-alkylidene butenolides. The directed nature of this pair of reactions allows chemoselective dehydrogenation of carboxylic acids in the presence of other enolizable functionalities such as ketones, providing chemoselectivity that is not possible by means of existing carbonyl desaturation protocols. Product inhibition is overcome through ligand-promoted preferential activation of C(sp 3 )-H bonds rather than C(sp 2 )-H bonds or a sequence of dehydrogenation and vinyl C-H alkynylation. The dehydrogenation reaction is compatible with molecular oxygen as the terminal oxidant.

The first example of free amine γ-C­(sp3)–H fluorination is realized using 2-hydroxynicotinaldehyde as the transient directing group. A wide range of cyclohexyl and linear aliphatic amines could be fluorinated selectively at the γ-methyl and methylene positions. Electron withdrawing 3,5-disubstituted pyridone ligands were identified to facilitate this reaction. Computational studies suggest that the turnover determining step is likely the oxidative addition step for methylene fluorination, while it is likely the C–H activation step for methyl fluorination. The explicit participation of Ag results in a lower energetic span for methylene fluorination and a higher energetic span for methyl fluorination, which is consistent with the experimental observation that the addition of silver salt is desirable for methylene but not for methyl fluorination. Kinetic studies on methyl fluorination suggest that the substrate and PdL are involved in the rate-determining step, indicating that the C–H activation step may be partially rate-determining. Importantly, an energetically preferred pathway has identified an interesting pyridone-assisted bimetallic transition state for the oxidative addition step in methylene fluorination, thus uncovering a potential new role of the pyridone ligand.

An Ir(III)-catalyzed direct C-H amidation/cyclization of benzamides using 2,2,2-trichloroethoxycarbonyl azide (TrocN3) as the aminocarbonyl source is reported. With the aid of cesium carboxylate, the reactions proceed efficiently and with high regioselectivity, producing various functionalized quinazoline-2,4(1H,3H)-diones, which are important building blocks and key synthetic intermediates for biologically and medicinally important compounds. During the reactions, two new C-N bonds were formed by breaking C-H and N-H bonds sequence.