Oxidations on Immobilized Molecular Catalysts

Abstract

IntroductionThe immobilization of homogeneous catalysts has been the focal point of extensive research effort over the past few decades. Heterogenization facilitates separation of the catalyst from the reagents and products, simplifies the recovery of the often expensive or toxic catalysts, and also allows the reuse or continuous operation of the immobilized catalyst. The immobilization of a homogeneous oxidation catalyst often greatly enhances its stability, due to the suppression of bimolecular deactivation pathways. For example, soluble metal complexes tend to react with each other, leading to oxidative degradation of the organic ligands. Another non-productive side reaction is the formation of inactive μ-oxo-bridged dimers. Such self-destruction and dimerization processes are almost impossible with a properly immobilized complex. Many approaches allow the careful control of metal complex loading of the support, and often truly site-isolated metal centers are obtained. Moreover, immobilization sometimes leads to an improved activity of the catalyst. For example, the microenvironment of the catalytic center may provide the correct polarity and acidobasicity for the desired catalytic activity. * Corresponding author.In this chapter, existing immobilization techniques are reviewed with special focus on their application in liquid-phase catalytic oxidations. General aspects such as the type of metal and oxidant, the choice of support, and the issue of leaching are discussed. The discussion is limited to the immobilization of achiral oxidation catalysts; chiral catalysts and metal-free organic catalysts are not included. The focus is on reactions employing clean, inexpensive oxidants such as molecular oxygen, hydrogen peroxide, or organic peroxides. Finally, the concepts are illustrated with examples selected among early seminal work and more recent publications. Only synthetically useful oxidative transformations of organic compounds are discussed; bleaching or deep oxidation of organics and oxidation of inorganic compounds are not included. Solid catalytic materials are only discussed if there exist homogeneous analogues for these materials. The immobilization of nanoclusters (e.g., Au, Pd) is not included, however. Emphasis is placed on examples where the heterogeneous nature of the catalysis has clearly been proven, preferably by employing stringent filtration tests. Immobilization MethodsIn designing an immobilization method for homogeneous oxidation catalysts, it is preferable to first have a clear idea of all chemical states of the catalyst that must be withheld by the support. This implies knowledge of the catalytic cycle, and of all metal species in the system. Only then can a mechanism for catalyst immobilization be proposed which is effective for all chemical states of the metal involved, particularly for the most oxidized form of the redox metal. Although this is a more demanding approach, it may often be the only rewarding method [1].Several different methodologies and concepts may be followed to achie...