Metallic palladium, PdO, and palladium supported on metal oxides for the Suzuki-Miyaura cross-coupling reaction: A unified view of the process of formation of the catalytically active species in solution

The Pd-catalysed Suzuki–Miyaura reaction is a powerful and widely used method for the synthesis of asymmetric biaryls, which has found increasing industrial application for the production of pharmaceuticals, fine chemicals and materials. Despite its widespread popularity, there is still a need to better understand some features of the whole process. Although the catalytic cycle is well established in all its key steps (aryl halide oxidative addition to Pd(0), transmetalation, C–C bond formation/reductive elimination of the coupling product), the effective nature of catalysis when a Pd-containing solid material is employed as the catalyst is still debated. More specifically, the question is whether the oxidative addition of Ar–X occurs on the surface of the catalyst (heterogeneous catalysis) or on leached metal atoms (homogeneous catalysis). This critical review is an attempt to answer this question, drawing upon findings from recent research based on the application of different forms of metallic palladium, PdO, and metal oxide-supported Pd catalysts to the Suzuki–Miyaura coupling. On the basis of the results of studies conducted so far, there is convergence towards a unique scenario, namely, the solid (pre)catalyst acts as a “reservoir” of soluble catalytically active palladium species. Furthermore, there is strong evidence, at least for catalysts of the type Pd/MxOy, that the noble metal is released in the form of Pd2+ ions, generally from amorphous PdO present on the surface.