Reactive Palladium Carbenes: Migratory Insertion and Other Carbene-Hydrocarbyl Coupling Reactions on Well-Defined Systems

Abstract

Palladium complexes with carbene ligands are among the best known and more extensively used catalysts. Those carbenes are usually NHC or N,N-disubstituted derivatives and their use relies on the robust nature of the carbene ligands and their role as auxiliary ligands that modify the metal's elec- tronic and steric features. In contrast, there are other types of carbenes that, when coordinated to palladium, lead to transfor- mations in which the carbene fragment is involved. In the last decade, palladium-catalyzed reactions have been extended to the use of carbene precursors that can be functionalized with the formation of several C–C bonds in one reaction step. Plausi- ble mechanisms have been proposed where a carbene–hydro- carbyl coupling occurs in the palladium coordination sphere. This microreview aims to collect and discuss the examples of organometallic reactions in well-characterized palladium com- plexes where the carbene is one of the fragments involved in C–C bond formation. These examples are key in understanding and supporting the mechanisms proposed for the Pd-catalyzed of C–C coupling reactions of carbenes.