Pd(0)/PR3-catalyzed intermolecular arylation of sp3 C-H bonds

Versatile reactivity of Pd-catalysts: mechanistic features of the mono-N-protected amino acid ligand and cesium-halide base in Pd-catalyzed C–H bond functionalization

The C–H functionalization strategies, complexity in Pd-catalyzed chemical transformations, unprecedented Pd-clustering, base (Cs-halide) and weakly coordinated amino acid ligand effects.

Palladium(0)-catalyzed cyclopropanation of benzyl bromides via C(sp3)–H bond activation

A novel Pd(0)-catalyzed domino cyclopropanation reaction was established. The process involves a Heck-type coupling reaction and a C(sp³)–H activation.

Selective fluorination of alkyl C–H bonds via photocatalysis

We report the generation of cationic N-radicals from Selectfluor® via energy transfer with anthraquinone as a photocatalyst for the fluorination of unactivated C–H bonds.

Cross-coupling of remote meta-C-H bonds directed by a U-shaped template

meta-C-H arylation and methylation of 3-phenylpropanoic acid and phenolic derivatives were developed using an easily removable nitrile template. The combination of a weakly coordinating U-shaped template and mono-protected amino acid ligand was crucial for the cross-coupling of C-H bonds with organoborons.

Direct β-functionalization of cyclic ketones with aryl ketones via the merger of photoredox and organocatalysis

The direct β-coupling of cyclic ketones with aryl ketones has been achieved via the synergistic combination of photoredox catalysis and organocatalysis. Diaryl oxymethyl or aryl-alkyl oxymethyl radicals, transiently generated via single-electron reduction of ketone precursors, readily merge with β-enaminyl radical species, generated by photon-induced enamine oxidation, to produce γ-hydroxyketone adducts. Experimental evidence indicates that two discrete reaction pathways can be operable in this process depending upon the nature of the ketyl radical precursor and the photocatalyst.

An efficient palladium-catalyzed C-H alkoxylation of unactivated methylene and methyl groups with cyclic hypervalent iodine (i(3+) ) oxidants

All the hype: The title reaction has been developed for the facile synthesis of a variety of complex alkyl ethers. Cyclic hypervalent iodine (I(3+) ) reagents serve as oxidants for this unique CH alkoxylation reaction. The reaction demonstrates excellent reactivity, good functional-group tolerance, and high yields. Q=8-aminoquinoline-derived auxiliary.

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.

Developing ligands for palladium(II)-catalyzed C-H functionalization: intimate dialogue between ligand and substrate

Homogeneous transition-metal-catalyzed reactions are indispensable to all facets of modern chemical synthesis. It is thus difficult to imagine that for much of the early 20th century, the reactivity and selectivity of all known homogeneous metal catalysts paled in comparison to their heterogeneous and biological counterparts. In the intervening decades, advances in ligand design bridged this divide, such that today some of the most demanding bond-forming events are mediated by ligand-supported homogeneous metal species. While ligand design has propelled many areas of homogeneous catalysis, in the field of Pd(II)-catalyzed C-H functionalization, suitable ligand scaffolds are lacking, which has hampered the development of broadly practical transformations based on C-H functionalization logic. In this Perspective, we offer an account of our research employing three ligand scaffolds, mono-N-protected amino acids, 2,6-disubstituted pyridines, and 2,2'-bipyridines, to address challenges posed by several synthetically versatile substrate classes. Drawing on this work, we discuss principles of ligand design, such as the need to match a ligand to a particular substrate class, and how ligand traits such as tunability and modularity can be advantageous in reaction discovery.

Palladium-catalyzed through-space C(sp(3))-H and C(sp(2))-H bond activation by 1,4-palladium migration: efficient synthesis of [3,4]-fused oxindoles

Palladium two step: Linear anilides were converted into the title compounds in good to excellent yields through a palladium-catalyzed domino carbopalladation/1,4-palladium shift sequence. The C(sp(3) )-H activation involves a seven-membered palladacycle, and is chemoselective in the presence of competitive C(sp(2) )H bonds. DMA=N,N-dimethylacetamide, OPiv=pivalate.

Pd(II)-catalyzed C-H iodination using molecular I2 as the sole oxidant

Pd-catalyzed ortho-C-H iodination directed by a weakly coordinating amide auxiliary using I2 as the sole oxidant was developed. This reaction is compatible with a wide range of heterocycles including pyridines, imidazoles, oxazoles, thiazoles, isoxazoles, and pyrazoles.

Photoredox activation for the direct β-arylation of ketones and aldehydes

The direct β-activation of saturated aldehydes and ketones has long been an elusive transformation. We found that photoredox catalysis in combination with organocatalysis can lead to the transient generation of 5π-electron β-enaminyl radicals from ketones and aldehydes that rapidly couple with cyano-substituted aryl rings at the carbonyl β-position. This mode of activation is suitable for a broad range of carbonyl β-functionalization reactions and is amenable to enantioselective catalysis.

Palladium(0)-catalyzed alkynylation of C(sp3)-H bonds

The alkynylation of β-C(sp(3))-H bonds in aliphatic amides with alkynyl halides has been enabled using Pd(0)/N-heterocyclic carbene (NHC) and Pd(0)/phosphine (PR3) catalysts. This is the first example of utilizing [AlkynylPd(II)L(n)] complexes to activate C(sp(3))-H bonds.

Rhodium-catalyzed intramolecular silylation of unactivated C(sp3)-H bonds

The treatment of a variety of hydrosilanes, each incorporating a benzylic C(sp(3))-H bond, with a rhodium catalyst resulted in intramolecular dehydrogenative silylation. This silylation reaction was found to occur at typically unreactive C(sp(3))-H bonds located at terminal positions on alkyl chains. Interestingly, the rhodium catalyst also promoted regioselective silylation at a site internal to an alkyl chain.

Ligand-enabled methylene C(sp3)-H bond activation with a Pd(II) catalyst

Pd(II) insertion into β-methylene C(sp(3))-H bonds was enabled by a mutually repulsive and electron-rich quinoline ligand. Ligand tuning led to the development of a method that allows for installation of an aryl group on a range of acyclic and cyclic amides containing β-methylene C(sp(3))-H bonds.