Palladium nanoparticles as efficient green homogeneous and heterogeneous carbon-carbon coupling precatalysts: A unifying view

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 degrees C). Under such conditions, the Pd-II precursor is reduced to Pd-0, 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).