Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.

The Cross-Dehydrogenative Coupling of C sp 3H Bonds: A Versatile Strategy for CC Bond Formations

Cyclometalated phosphine-based pincer complexes: Mechanistic insight in catalysis, coordination, and bond activation

Since the first reports in the late 1970s on transition metal complexes contain- ing pincer-type ligands—named after the particular coordination mode of these ligands—these systems have at- tracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typical- ly, such a ligand comprises an anionic aryl ring which is ortho,ortho-disubsti- tuted with heteroatom substituents, for example, CH2NR2 ,C H 2PR2 or CH2SR, which generally coordinate to the met- al center, and therefore support the MC s bond. This commonly results in a terdentate and meridional coordina- tion mode consisting of two metalla- cycles which share the MC bond. Detailed studies of the formation and the properties of a large variety of pincers containing platinum group metal complexes have provided direct access to both a fundamental under- standing of a variety of reactions in organometallic chemistry and to a range of new applications of these complexes. The discovery of alkane dehydrogenation catalysts, the mecha- nistic elucidation of fundamental transformations (for example, CC bond activation), the construction of the first metallodendrimers for sustain- able homogeneous catalysis, and the engineering of crystalline switches for materials processing represent only a few of the many highlights which have emanated from these numerous inves- tigations. This review discusses the synthetic methodologies that are cur- rently available for the preparation of platinum group metal complexes con- taining pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineer- ing of sensors, switches, and catalysts.

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts

The potential of palladacycles: more than just precatalysts

Selective hydrogenolysis of the aromatic carbon-oxygen (C-O) bonds in aryl ethers is an unsolved synthetic problem important for the generation of fuels and chemical feedstocks from biomass and for the liquefaction of coal. Currently, the hydrogenolysis of aromatic C-O bonds requires heterogeneous catalysts that operate at high temperature and pressure and lead to a mixture of products from competing hydrogenolysis of aliphatic C-O bonds and hydrogenation of the arene. Here, we report hydrogenolyses of aromatic C-O bonds in alkyl aryl and diaryl ethers that form exclusively arenes and alcohols. This process is catalyzed by a soluble nickel carbene complex under just 1 bar of hydrogen at temperatures of 80 to 120°C; the relative reactivity of ether substrates scale as Ar-OAr>>Ar-OMe>ArCH2-OMe (Ar, Aryl; Me, Methyl). Hydrogenolysis of lignin model compounds highlights the potential of this approach for the conversion of refractory aryl ether biopolymers to hydrocarbons.

https://science.sciencemag.org/content/sci/332/6028/439.full.pdf

Selective, Nickel-Catalyzed Hydrogenolysis of Aryl Ethers

Reaction of [RhCl(C8H14)2]2 (C8H14 = cyclooctene) with 2 equiv of the aryl methyl ether phosphine 1 in C6D6 results in an unprecedented metal insertion into the strong sp2−sp3 aryl−O bond. This remarkable reaction proceeds even at room temperature and occurs directly, with no intermediacy of C−H activation or insertion into the adjacent weaker ArO−CH3 bond. Two new phenoxy complexes (8 and 9), which are analogous to the product of insertion into the ArO−CH3 bond (had it taken place) were prepared and shown not to be intermediates in the Ar−OCH3 bond cleavage process. Thus, aryl−O bond activation by the nucleophilic Rh(I) is kinetically preferred over activation of the alkyl−O bond. The phenoxy Rh(I)−η1-N2 complex (8) is in equilibrium with the crystallographically characterized Rh(I)−μ-N2−Rh(I) dimer (12). Reaction of [RhCl(C8H14)2]2 with 2 equiv of the aryl methyl ether phosphine 2, PPh3, and excess HSiR3 (R = OCH2CH3, CH2CH3) results also in selective metal insertion into the aryl−O bond and formation o...

Alkyl− and Aryl−Oxygen Bond Activation in Solution by Rhodium(I), Palladium(II), and Nickel(II). Transition-Metal-Based Selectivity

SELECTIVE chemical transformations involving hydrocarbons are a major goal in synthetic chemistry. Transition-metal complexes are capable of promoting a range of selective transformations of organic molecules under mild conditions1,2, and much effort has been devoted to studying their reactivity towards hydrocarbons3–7. Here we report a reaction promoted by a rhodium complex in which a methylene (CH2) group can be abstracted from a methyl (CH3) group and inserted into a variety of bonds, such as Si-H, Si-Si and C-H. Our approach presently requires a certain coordination geometry in the metal complex, involving chelation of the metal centre, but we believe that it can be extended to other substrates capable of metal coordination and may eventually lead to a general strategy for CH2 transfer from hydrocarbons to other molecules under mild conditions.

Transfer of methylene groups promoted by metal complexation

Reaction of [RhClL2]2 (L = cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-(CH2PtBu2)2C6H3} (1) in toluene results in a selective metal insertion into the strong Ar−Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp3−sp3 ArCH2−CH3 bond. The identity of complex Rh(Et){2,6-(CH2PtBu2)2C6H3}Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η1-N2){2,6-(CH2PtBu2)2C6H3} (7) with EtI. It is possible to direct the bond activation process toward the benzylic C−H bonds of the aryl−alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms (tBu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH2−CH3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar−CH2CH3 bond activation process. Thus, aryl−C activation by Rh(I) is kinetically preferred over activation of the alkyl−C bond in this system. Moreover...

Shyh-Yeon Liou

Papers

Alkyl− and Aryl−Oxygen Bond Activation in Solution by Rhodium(I), Palladium(II), and Nickel(II). Transition-Metal-Based Selectivity

Transfer of methylene groups promoted by metal complexation

Carbon-carbon bond activation by rhodium(i) in solution. comparison of sp2-sp3 vs sp3-sp3 c-c, c-h vs c-c, and ar-ch3 vs ar-ch2ch3 activation

Selective activation of alkyl- and aryl-oxygen single bonds in solution with transition metal complexes

Consecutive cyclometalation by platinum(II)