High-turnover palladium catalysts in cross-coupling and Heck chemistry: A critical overview

This review discusses the problems associated with developing high-turnover catalysts for the cross-coupling and Heck reactions. New developments in the area, principally constituted by pallada-cycles and coordinatively unsaturated Pd catalysts featuring bulky phosphanes of high donicity, are reviewed from a mechanistic and synthetic standpoint, and compared with more traditional catalysts obtained from conventional mono- and polydentate N-and P-based ligands, as well as Pd catalysts without strong ligands, such as Pd colloids or heterogeneous catalysts. Carbene ligands are also briefly presented. Whereas a single, most promising approach to high-turnover Pd catalysis cannot presently be defined, it is clear that the new "PdL1" catalysts (where L-1 is a monodentate bulky P ligand of high donicity) represent the latest, most important development in Pd research, certainly from the standpoint of scope and probably also from the standpoint of efficiency. High turnovers with these catalysts have been described and their use will certainly increase in the next few years. The review ends with a brief discussion containing practical considerations on how to choose a high TON catalyst for a given Heck or cross-coupling reaction of interest. 1 Introduction 2 Different Paradigms in High TON Research 3 Palladacycle Pre-Catalysts 4 Highly Coordinatively Unsaturated Palladium Catalysts 5 Carbene Ligands 6 Bi- or Multidentate P- or N-Based Ligands 7 "Ligandless" Catalysts 8 Stable Colloids and Nanoparticles 9 Traditional Catalytic Systems 10 Polymer-Supported Catalysts 11 Heterogeneous Catalysts 12 Practical Considerations and Conclusion.