On the nature of the active species in palladium catalyzed Mizoroki-Heck and Suzuki-Miyaura couplings - Homogeneous or heterogeneous catalysis, a critical review

A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true "catalysts" in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems. 1 Introduction 2 Mechanism 3 Homogeneous Precatalysts 3.1 Homogeneous Precatalysts with N, O, P or S Donor Ligands 3.2 "Naked" Homogeneous Precatalysts 3.3 Palladacycle-Based Homogeneous Precatalysts 3.4 N-Heterocyclic Carbene-Containing Precatalysts 4 Heterogeneous Precatalysts 4.1 Pd(0) Particles on Oxide or Carbon Supports 4.2 Pd(II) Exchanged Supports 4.3 Supported Metal-Ligand Complexes 4.4 Soluble or Polymer Nanosphere-Supported Pd Nanoparticle Precatalysts 4.5 Polymer or Oxide "Encapsulated" Precatalysts 5 Distinguishing Between Soluble vs. Insoluble, Molecular vs. Nanoparticle Catalysts 5.1 Aryl Iodides vs. Bromides vs. Chlorides 5.2 Filtration or Split Tests 5.3 Kinetic Studies vs. Catalyst Recycle with Consistent Yield 5.4 Reaction Selectivity 5.5 Correlation of Reaction Rate with Concentration of Palladium in Solution 5.6 "Quantitative" Catalyst Recovery 5.7 Mercury Test 5.8 Selective Poisons for Solid Catalysts 5.9 Selective Poisons for Soluble Catalysts 5.10 3-Phase Test 6 Summary and Conclusions.