The Role of Palladium Pincer Complexes in Heck Reactions

Even though various C-C cross-coupling reactions have been successfully catalyzed with palladium pincer complexes, often with excellent performances [22], their primarily application nowadays lies in the Heck reaction, of which the role of pincer-type catalysts in this reaction still is unclear and therefore under debate. 

The Role of Palladium Pincer Complexes in Heck Reactions 

Apart from their high catalytic activity in the Heck reaction, palladium pincer-catalyzed C-C cross-coupling reactions are of particular interest from mechanistic points of view. While classical Pd°/Pd cycles and mechanisms in which the palladium center of pincer-type cross-coupling catalysts does not change its oxidation state and has in most cases been excluded to be operative [23], considerable 

High reaction temperatures were found to be crucial for their irreversible decomposition into nanoparticles, because recent investigations on xylene-derived 

Finally, it should be mentioned that two years later, an enantioselective oxidative (boron) Heck-type reaction was reported by Jung and coworkers [32] for a dinuclear NHC-derived CNO-based pincer complex for which for the first time a Pd /Pd mechanism was suggested to be operative with palladium pincer-type crosscoupling catalysts. Reaction mechanisms without a change of the palladium s oxidation state have never been proposed to be operative for Heck cross-couphng reactions, but have been shown to be operative for xylene-derived selenium-based pincer complexes and related systems in the cross-couphng of vinyl epoxides (and aziridines) with organoboronic acids [24d, 33]. 

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