Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?

Much effort has gone into the immobilization of homogeneous catalysts based on the idea that in this way the catalysts could be not only separated more easily from the product but also reused several times, thus reducing the cost of the catalyst use. So far none of these immobilized catalysts have been used by industry. In this article we critically review the use of immobilized homogeneous catalysts from the point of view of process development for the pharmaceutical and fine chemical industry. The first and foremost question that needs to be answered is: will immobilizing a homogeneous catalyst really lead to lower costs? The answer is thus far always no. This is caused mostly by the fact that homogeneous catalysts are not stable and thus there is little point in immobilizing them. The second reason is the extra added cost that is incurred in immobilizing the catalysts. Other problems are lower rates, sometimes lower selectivities and metal leaching. Three different areas are discussed. The research on immobilized metathesis catalysts is analyzed in detail; in general the immobilized catalysts do not achieve sufficient turnovers to be interesting for industrial use. Very many publications have appeared on immobilized palladium catalysts that were used for C-C bond-forming reactions, such as Suzuki, Heck or Sonoga-shira reactions. These catalysts are invariably converted to nanoparticles after the first run. Although these catalysts can be reused, there is no reason to use an expensive support based on immobilized ligands. This also does not protect the product from palladium contamination. Even more effort has gone into the immobilization of homogeneous hydrogenation catalysts. Most of these catalysts suffer from the same problems as the other immobilized catalysts: catalyst deactivation, low turnover numbers, and leaching of the metal. In addition, the heterogenization adds complexity to the system, increasing risk and prolonging process development. 1 Introduction 2 Immobilized Catalysts in Metathesis Reactions 2.1 Immobilization via the "Fixed" Ligand 2.2 Immobilization via a Pseudo-Halide Substituent 2.3 "Boomerang" Catalysts 2.4. Soluble Polymers 2.5 Miscellaneous Strategies 3 Immobilized Palladium Complexes for Heck, Suzuki, Sonogashira and Other C-C Bond-Forming Reactions 3.1 Catalysts Based on Immobilized Nitrogen Ligands 3.2 Catalysts Based on Immobilized Thiol Ligands 3.3 Catalysts Based on Immobilized Phosphine Ligands 3.4 Catalysts Based on Immobilized NHC Ligands 3.5 Catalysts Based on Immobilized Palladacycles 3.6 Immobilized Palladium Catalysts in Flow 4 Hydrogenation with Immobilized Catalysts 4.1 Case Studies of Covalently Bound Complexes 4.2 Case Studies of Ionically Bound Complexes 4.3 Case Studies of Physisorbed Complexes 5 Conclusion and Outlook