Methylene transfer from SnMe groups mediated by a rhodium(I) pincer complex: Sn-C, C-C, and C-H bond activation

The PCP-Rh(I) complex 1a based on the [1,3-phenylenebis(methylene)]bis(diisopropylphosphine) ligand reacts with [diazo(phenyl)methyl]trimethylstannane (2) at room temperature to give novel pincer-type phenyl(dimethylstannyl)methylene]hydrazinato complex 3a. The reaction sequence involves a unique combination of Sn-C bond cleavage, C-C bond formation, C-H activation and intramolecular deprotonation of a rhodium hydride intermediate, which results in methylene transfer from an SnMe group to the pincer system and PCP-chelate expansion. A methylene-transfer reaction was also demonstrated with tetramethyltin as the methylene source in the presence of KOC(CH(3))(3) at room temperature. The resulting unstable "chelate-expanded" Rh(I) complex [(C(10)H(5)(CH(2)PiPr(2))(2))(CH(2))Rh(L)] (L=N(2), THF; 4a) was isolated as its carbonyl derivative 5a. Heating 4a in benzene yielded an equimolar amount of toluene and 1a, which demonstrates the ability of the Rh(I) pincer complex to extract a methylene group from an unactivated alkyl tin substrate and transfer it, via C-C followed by C-H activation, to an arene. Use of fluorobenzene resulted in formation of fluorotoluene. Catalytic methylene-group transfer mediated by 1a was not possible, because of formation of o-xylylene complex 8 under the reaction conditions. Steric parameters play a decisive role in the reactivity with tin compounds; while iPrP derivative 1 a underwent facile reactions, tBuP complex 1b was inert.     

Palladium(II)-catalyzed one-pot syntheses of 9-(pyridin-2-yl)-9H-carbazoles through a tandem C-H activation/C-X (X=C or N) formation process

A new one-pot synthesis of 9-(pyridin-2-yl)-9H-carbazoles through the simultaneous C-H activation and palladium(II)-catalyzed cross-coupling of N-phenylpyridin-2-amines with potassium aryltrifluoroborates is presented. Silver acetate and 1,4-dioxane proved to be the best oxidant and solvent, respectively. The product yields fluctuated from modest to excellent and the reaction showed sufficient functional group tolerance. p-Benzoquinone served as an important ligand for the transmetalation and reductive elimination steps in the catalytic process. The kinetic isotope effects (k(H)/k(D)) for the first and second C-H activation/C-C or C-N formation steps were measured as 2.14 and 1.18, respectively. Finally, a rational catalytic mechanism is presented based on all experimental evidence.     

Extrusion of CO from aryl ketones: rhodium(I)-catalyzed C-C bond cleavage directed by a pyridine group

Snipping tool: the rhodium(I)-catalyzed extrusion of carbon monoxide from biaryl ketones and alkyl/alkenyl aryl ketones was developed to produce biaryls and alkyl/alkenyl arenes, respectively, in high yields. A wide range of functionalities are tolerated. Not only does this method provide an alternative pathway to construct useful scaffolds, but also offers a new strategy for C-C bond activation.     

Au(I) /Au(III) catalysis: an alternative approach for C-C oxidative coupling

When reacted in the presence of external oxidants, gold complexes are capable of catalyzing oxidative homo- and cross-coupling reactions involving the formation of new C-C bonds. Over the last few years, several cascade processes have been reported in which coupling is preceded by a gold-mediated aryl C-H functionalization or nucleophilic addition. These reactions combine the unique reactivity of gold with oxidative coupling, enabling the construction of C-C bonds between coupling partners that are not easily accessed using alternative catalysts. In this Concept paper, the development of gold-catalyzed oxidative coupling reactions is discussed focusing on C-C bond-forming reactions of broad synthetic appeal.