A general method for copper-catalyzed arene cross-dimerization

A general method for a highly regioselective copper-catalyzed cross-coupling of two aromatic compounds using iodine as an oxidant has been developed. The reactions involve an initial iodination of one arene followed by arylation of the most acidic C-H bond of the other coupling component. Cross-coupling of electron-rich arenes, electron-poor arenes, and five- and six-membered heterocycles is possible in many combinations. Typically, a 1/1.5 to 1/3 ratio of coupling components is used, in contrast to existing methodology that often employs a large excess of one of the arenes. Common functionalities such as ester, ketone, aldehyde, ether, nitrile, nitro, and amine are well-tolerated.     

Ligand-free CuO nanospindle catalyzed arylation of heterocycle C-H bonds

CuO nanospindles have been developed to efficiently catalyze the direct arylation of heterocycle C-H bonds with moderate to excellent yields. This reaction can be applied to heterocycles such as benzoxazole, benzothiazole, and 1-methylbenzimidazole in the presence of a more environmentally friendly inorganic base like K(2)CO(3) under ligand-free catalytic conditions. In addition, the catalyst can be recycled and reused without any significant decrease in catalytic activity.     

Direct arylation of benzoxazole C-H bonds with iodobenzene diacetates

A Pd (OAc)₂-catalyzed direct arylation of benzoxazole C-H bonds has been achieved with iodobenzene diacetates as the arylation reagent in moderate to good yields. The procedure tolerates a series of functional groups, such as methoxy, nitro, cyano, chloro, and bromo groups.     

Mechanistic understanding of the unexpected meta selectivity in copper-catalyzed anilide C-H bond arylation

DFT calculations suggest that the unexpected meta product in the copper-catalyzed arylation of anilide is formed via a Heck-like four-membered-ring transition state involving a Cu(III)-Ph species. A competitive electrophilic substitution mechanism delivers the ortho product when a methoxy group is present at the meta position of pivanilide. A series of experiments including kinetic studies support the involvement of a Cu(I) catalyst.     

Direct arylation of aromatic C-H bonds catalyzed by Cp*Ir complexes

The C-H bond of benzene was directly arylated by reaction with aryl iodides in the presence of a catalytic amount of a pentamethylcyclopentadienyliridium complex and potassium tert-butoxide.     

Highly regioselective arylation of sp3 C-H bonds catalyzed by palladium acetate

A new palladium-catalyzed arylation process based on C-H activation has been developed. The utilization of pyridine-containing directing groups allows the beta-arylation of carboxylic acid derivatives and gamma-arylation of amine derivatives. Both primary and secondary sp3 C-H bonds, as well as sp2 C-H bonds, are reactive.     

Ligand-free copper-catalyzed arylation of amidines

Copper-catalyzed cross-coupling reactions of amidine salts were utilized to synthesize monoarylated amidines in moderate to high yields with ligand-free conditions. DMF was the superior solvent for the N-arylation of benzamidines, while MeCN was used in the formation of N-aryl amidines in moderate to high yield.     

Direct arylation of unactivated aromatic C-H bonds catalyzed by a stable organic radical

A stable zwitterionic radical can catalyze direct arylation of unactivated aromatic C-H bonds via a chain homolytic aromatic substitution mechanism in the presence of potassium tert-butoxide.     

Direct, copper-catalyzed oxidation of aromatic C-H bonds with hydrogen peroxide under acid-free conditions

The direct oxidation of benzene into phenol using hydrogen peroxide has been achieved in the absence of any acid with Tp(x)Cu(NCMe) complexes as the catalysts. In the case of anthracenes as the substrates, valuable anthraquinones have been quantitatively obtained in the same manner.     

General and efficient copper-catalyzed amidation of saturated C-H bonds using N-halosuccinimides as the oxidants

We have developed a general and efficient method for copper-catalyzed amidation of saturated C-H bonds under mild conditions, and the used substrates include benzylic reagents, the N, N-dimethylaniline derivatives, the free carboxamides, and sulfonamides. The protocol uses inexpensive and readily available CuBr/ N-halosuccinimide (NBS or NCS) as the catalyst/oxidant, so it provides practical applications for synthesis of various amides via C-H activation.     

A highly selective catalytic method for the oxidative functionalization of C-H bonds

This communication describes a new and highly practical Pd(II)-catalyzed method for the regio- and chemoselective oxidative functionalization of arenes and alkanes. Carbon-hydrogen bonds of substrates that contain a variety of directing groups (e.g., pyridine, azobenzene, pyrazole, and imine derivatives) are selectively transformed into esters, ethers, and aryl-halides under mild conditions. The scope of this reaction in terms of substrate, directing group, and oxidant is described, and a preliminary catalytic cycle is proposed.     

C-H arylation of pyridines: high regioselectivity as a consequence of the electronic character of C-H bonds and heteroarene ring

We report a new catalytic protocol for highly selective C-H arylation of pyridines containing common and synthetically versatile electron-withdrawing substituents (NO(2), CN, F and Cl). The new protocol expands the scope of catalytic azine functionalization as the excellent regioselectivity at the 3- and 4-positions well complements the existing methods for C-H arylation and Ir-catalyzed borylation, as well as classical functionalization of pyridines. Another important feature of the new method is its flexibility to adapt to challenging substrates by a simple modification of the carboxylic acid ligand or the use of silver salts. The regioselectivity can be rationalized on the basis of the key electronic effects (repulsion between the nitrogen lone pair and polarized C-Pd bond at C2-/C6-positions and acidity of the C-H bond) in combination with steric effects (sensitivity to bulky substituents).     

Mechanistic aspects of samarium-mediated sigma-bond activations of arene C-H and arylsilane Si-C bonds.

To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH3 to Ph2SiH2 and SiH4, the samarium phenyl complex [Cp*2SmPh]2 (1) was prepared by oxidation of Cp2*Sm (2) with HgPh2. Compound 1 thermally decomposes to yield benzene and the phenylene-bridged disamarium complex Cp*2Sm(mu-1,4-C6H4)SmCp*2 (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp*2SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate = k1[1] + k2[1]2. Complex 1 efficiently transfers its phenyl group to PhSiH3, with formation of Ph2SiH2 and [Cp*2Sm(mu-H)]2 (4). Quantitative Si-C bond cleavage of C6F5SiH3 is effected by the samarium hydride complex 4, yielding silane and [Cp*2Sm(mu-C6F5)]2 (5). In contrast, Si-H activation takes place upon reaction of 4 with o-MeOC6H4SiH3, affording the samarium silyl species [structure: see text] Cp*2SmSiH2(o-MeOC6H4) (7). Complex 7 rapidly decomposes to [Cp*2Sm(mu-o-MeOC6H4)]2 (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C6F5)2 and Hg(o-MeOC6H4)2, respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in sigma-bond metathesis reactions, are discussed.     

Nickel-catalyzed sp2 C-H bonds arylation of N-aromatic heterocycles with Grignard reagents at room temperature

A novel protocol for nickel-catalyzed direct sp(2) C-H bond arylation of purines has been developed. This new reaction proceeded efficiently at room temperature using Grignard reagent as the coupling partner within 5 hours in good to high yields. This approach provides a new access to a variety of C8-arylpurines which are potentially of great importance in medicinal chemistry.     

Evidence for the net addition of arene C-H bonds across a Ru(II)-hydroxide bond

TpRu(PMe3)2(OH) (1) reacts with C6D6 to initiate H/D exchange between the hydroxide ligand and the deuterated benzene. In addition, complex 1 catalyzes H/D exchange between H2O and C6D6. Mechanistic and computational studies suggest that a likely reaction pathway for the H/D exchange involves loss of PMe3 to produce {TpRu(PMe3)(OH)}, followed by the net addition of a benzene C-H(D) bond across the Ru-OH bond to form the putative complex TpRu(PMe3)(OH2)(Ph).     

Carboxylation of arene C-H bonds with CO2: a DFT-based approach to catalyst design

A prototypical catalytic cycle for the direct carboxylation of unactivated arene C-H bonds with CO(2) based on ruthenium(II) pincer complexes as catalysts is proposed and investigated by density functional theory (DFT) methods. The energetic span model is used to predict the turnover frequency (TOF) of various potential catalysts, evaluating their efficiency for this reaction. In addition to modifications of the catalyst structure, we also investigated the effect of the substrate, the solvent, and the influence of a base on the thermodynamics and kinetics of the reaction. Turnover frequencies in the range of 10(5)-10(7) h(-1) are predicted for the best systems. Alternative reaction pathways that might prevent the reaction are also investigated. In all cases, either the respective intermediates are found to be unstable or activation barriers are found to be very high, thereby indicating that these alternative pathways will not interfere with the proposed catalytic cycle. As a result, several ruthenium pincer complexes are suggested as very promising candidates for experimental investigation as catalysts for the carboxylation of arene C-H bonds with CO(2).