Palladium Nanoparticles as Efficient Green Homogeneous and Heterogeneous Carbon−Carbon Coupling Precatalysts: A Unifying View

Abstract

Pd catalysis of C−C bond formations is briefly reviewed from the angle of nanoparticles (NPs) whether they are homogeneous or heterogeneous precatalysts and whether they are intentionally preformed or generated from a Pd derivative such as Pd(OAc)₂. The most studied reaction is the Heck coupling of halogenoarenes with olefins that usually proceeds at high temperature (120−160 °C). Under such conditions, the Pd^II precursor is reduced to Pd⁰, forming PdNPs from which Pd atom leaching, subsequent to oxidative addition of the aryl halide onto the PdNP surface, is the source of very active molecular catalysts. Other C−C coupling reactions (Suzuki, Sonogashira, Stille, Negishi, Hiyama, Corriu−Kumada, Ullmann, and Tsuji−Trost) can also be catalyzed by species produced from preformed PdNPs. For catalysis of these reactions, leaching of active Pd atoms from the PdNPs may also provide a viable molecular mechanistic scheme. Thus, the term “PdNP catalysis of C−C coupling” used in this review refers to this function of PdNPs as precursors of catalytically active Pd species (i.e., the PdNPs are precatalysts of C−C coupling reactions).