Pick your Pd partners: A number of catalytic systems have been developed for palladium-catalyzed CH activation/CC bond formation. Recent studies concerning the palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle are discussed (see scheme), and the versatility and practicality of this new mode of catalysis are presented. Unaddressed questions and the potential for development in the field are also addressed.

In the past decade, palladium-catalyzed CH activation/CC bond-forming reactions have emerged as promising new catalytic transformations; however, development in this field is still at an early stage compared to the state of the art in cross-coupling reactions using aryl and alkyl halides. This Review begins with a brief introduction of four extensively investigated modes of catalysis for forming CC bonds from CH bonds: PdII/Pd0, PdII/PdIV, Pd0/PdII/PdIV, and Pd0/PdII catalysis. A more detailed discussion is then directed towards the recent development of palladium(II)-catalyzed coupling of CH bonds with organometallic reagents through a PdII/Pd0 catalytic cycle. Despite the progress made to date, improving the versatility and practicality of this new reaction remains a tremendous challenge.

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Palladium(II)-catalyzed C-H activation/C-C cross-coupling reactions: versatility and practicality.

About Supramolecular Assemblies of π-Conjugated Systems

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

The selective transformation of ubiquitous but inert C–H bonds to other functional groups has far-reaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. The past twenty years have seen many examples of C–H bond activation at transition-metal centres, often under remarkably mild conditions and with high selectivity. Although profitable practical applications have not yet been developed, our understanding of how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion are, has progressed considerably. In fact, the recent development of promising catalytic systems highlights the potential of organometallic chemistry for useful C–H bond activation strategies that will ultimately allow us to exploit Earth's alkane resources more efficiently and cleanly.

Understanding and exploiting C–H bond activation

Beyond Oil and Gas: The Methanol Economy

Platinum catalysts are reported for the direct, low-temperature, oxidative conversion of methane to a methanol derivative at greater than 70 percent one-pass yield based on methane. The catalysts are platinum complexes derived from the bidiazine ligand family that are stable, active, and selective for the oxidation of a carbon-hydrogen bond of methane to produce methyl esters. Mechanistic studies show that platinum(II) is the most active oxidation state of platinum for reaction with methane, and are consistent with reaction proceeding through carbon-hydrogen bond activation of methane to generate a platinum-methyl intermediate that is oxidized to generate the methyl ester product.

https://science.sciencemag.org/content/sci/280/5363/560.full.pdf

Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative

Acetic acid is an important petrochemical that is currently produced from methane (or coal) in a three-step process based on carbonylation of methanol. We report a direct, selective, oxidative condensation of two methane molecules to acetic acid at 180°C in liquid sulfuric acid. Carbon-13 isotopic labeling studies show that both carbons of acetic acid originate from methane. The reaction is catalyzed by palladium, and the results are consistent with the reaction occurring by tandem catalysis, involving methane C-H activation to generate Pd-CH 3 species, followed by efficient oxidative carbonylation with methanol, generated in situ from methane, to produce acetic acid.

http://science.sciencemag.org/content/sci/301/5634/814.full.pdf

Catalytic, Oxidative Condensation of CH4 to CH3COOH in One Step via CH Activation

Some of the most efficient homogeneous catalysts for the lowtemperature, selective oxidation of methane to functionalized products employ a mechanism involving C H activation with an electrophilic substitution mechanism. Several such systems have been reported based on the cations Hg, Pd, and Pt. These catalyst systems typically operate by two general steps that involve: A) C H activation by coordination of the methane to the inner sphere of the catalyst (E) followed by cleavage of the C Hbond by overall electrophilic substitution to generate E CH3 intermediates, and B) oxidative functionalization involving redox reactions of E CH3 to generate the desired oxidized product CH3X. [4a]

Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold.

Acetic acid is an important petrochemical that is currently produced from methane (or coal) in a three-step process based on carbonylation of methanol. We report a direct, selective, oxidative condensation of two methane molecules to acetic acid at 180°C in concentrated sulfuric acid. 13C isotopic labeling studies show that both carbons of acetic acid originate from two methane molecules in a formal eight-electron redox reaction. The reaction is catalyzed by palladium and the results are consistent with the reaction occurring by tandem catalysis involving methane C–H activation to generate Pd-CH3 species, followed by efficient oxidative carbonylation with methanol, generated in situ from methane, to produce acetic acid. Control Experiments suggest that the carbonylation occurs via the in situ oxidation of CH3OH to low levels of CO.

Doug Taube

Papers

Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative

Catalytic, Oxidative Condensation of CH4 to CH3COOH in One Step via CH Activation

Selective oxidation of methane to methanol catalyzed, with C-H activation, by homogeneous, cationic gold.

Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative

Homogeneous, catalytic, oxidative coupling of methane to acetic acid in one step