Guangfa Shi

Palladium-Catalyzed Benzylation of Carboxylic Acids with Toluene via Benzylic C–H Activation

Direct benzylation of carboxylic acids with toluene has been developed via palladium-catalyzed C-H acyloxylation under 1 atm of oxygen. This reaction demonstrates good functional group tolerance and high yields, providing a facile, atom-economic, and efficient method for the synthesis of benzyl esters.

Silver-catalyzed C-H trifluoromethylation of arenes using trifluoroacetic acid as the trifluoromethylating reagent.

Direct trifluoromethylation of arenes using TFA as the trifluoromethylating reagent was achieved with Ag as the catalyst. This reaction not only provides a new protocol for aryl C-H trifluoromethylation, but the generation of CF3· from TFA may prove useful in other contexts and could potentially be extended to other trifluoromethylation reactions.

Palladium-Catalyzed C–H Ethoxycarbonyldifluoromethylation of Electron-Rich Heteroarenes

The first Pd-catalyzed C-H ethoxycarbonyldifluoromethylation with BrCF2CO2Et has been developed. The use of a bidentate phosphine ligand (Xantphos) is critical for the reaction to occur. A variety of electron-rich heteroarenes, including indoles, furans, thiophenes, and pyrroles, can be ethoxycarbonyldifluoromethylated in moderate to excellent yields. The reactions take place at the C-H bonds adjacent to the heteroatoms with high regioselectivity. This method provides a new protocol for the introduction of difuoroalkyl groups into electron-rich heteroarenes.

Synthesis of Fluorenes Starting from 2-Iodobiphenyls and CH2Br2 through Palladium-Catalyzed Dual C–C Bond Formation

A facile and efficient approach is developed for the synthesis of fluorene and its derivatives starting from 2-iodobiphenyls and CH2Br2. A range of fluorene derivatives can be synthesized under relatively mild conditions. The reaction proceeds via a tandem palladium-catalyzed dual C-C bond formation sequence through the key dibenzopalladacyclopentadiene intermediates, which are obtained from 2-iodobiphenyls through palladium-catalyzed C-H activation.

Sequential Difunctionalization of 2-Iodobiphenyls by Exploiting the Reactivities of a Palladacycle and an Acyclic Arylpalladium Species

A novel sequential difunctionalization reaction of 2-iodobiphenyl has been developed by exploiting the distinct reactivities of a palladacycle and an acyclic arylpalladium species. In this tandem reaction, an in situ formed dibenzopalladacyclopentadiene reacts selectively with an alkyl halide, after which the thus formed acyclic arylpalladium species selectively undergoes a Heck reaction with an alkene. This work demonstrates the strong relationship between the coordination mode of a transition metal complex and its reactivity, which could shed light on the mechanisms of other transition-metal-catalyzed reactions and offer the opportunity to develop other synthetically enabling organic transformations.

3.23 Carbon–Carbon σ -Bond Formation via C H Bond Functionalization

Transition-metal-catalyzed C H functionalization has gained considerable interest and made noticeable progress during the past decades, because of its attractive advantages over traditional organic reactions relying on the transformation and conversion of functional groups. In this context, carbon–carbon σ-bond formation via C H activation has been a major focus of studies. In the presence of transition metals as catalysts, C H bonds can be transformed into C C bonds by reacting with a variety of reaction partners employed in traditional transition-metal-catalyzed cross-coupling reactions, including (pseudo)halides, organometallics, C C multiple bonds, and polar multiple bonds. Furthermore, dual C H activation has also gained much attention because it produces no chemical wastes other than hydrogen. A wide array of transition metals have been shown to be effective for C C bond formation via C H activation, which not only include palladium, rhodium, and ruthenium catalysts but also less expensive and toxic copper, iron, and nickel complexes.