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

Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts

Dendritic catalysis—Basic concepts and recent trends

Formation and Role of Palladium Chalcogenide and Other Species in Suzuki–Miyaura and Heck C–C Coupling Reactions Catalyzed with Palladium(II) Complexes of Organochalcogen Ligands: Realities and Speculations

Nature uses the principles of encapsulation and supramolecular chemistry to bind and orientate substrates within active catalytic sites. Over the years, synthetic chemistry has generated a number of small molecule active site mimics capable of catalysing reactions involving bound substrates. Another approach uses larger molecules that better represent an enzymes globular structure. These molecules mimic an enzymes structure by incorporating binding/catalytic sites within the globular structure of the polymer. As such, the electronic and steric properties around the binding/catalytic site(s) can be controlled and fine-tuned. One class of polymer that is particularly adept at mimicking the globular structure of enzymes are dendritic polymers. This review will concentrate on the use of hyperbranched polymers as synthetic enzyme mimics.

Catalytic hyperbranched polymers as enzyme mimics; exploiting the principles of encapsulation and supramolecular chemistry.

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

Soluble polysiloxanes of various architectures (linear, star-shaped and hyperbranched), having vinyl, 2-butylthioethyl and 2-diphenylphosphinoethyl side groups have been used as supports for palladium(II) catalysts. Catalytic activity of such immobilized palladium complexes was tested in model Mizoroki–Heck reactions. The activity of the complexes in terms of yield and turnover number was comparable to that of PdCl 2 (PhCN) 2 . Polysiloxane-supported catalysts show good stability and can be reused several times. Catalysts immobilized on linear polymers show generally better stability than those immobilized on branched structures. Mercury poisoning test indicated that the true catalytic species is the supported complex. According to XPS analysis, palladium in the complexes with polysiloxanes is present as Pd(II). XRF shows however a significant metal leaching after 5–10 reaction cycles.

Soluble polysiloxane-supported palladium catalysts for the Mizoroki-Heck reaction

Synthesis and characterization of novel polythiourethane hardeners for epoxy resins

Epoxy resins cured with ionic liquids as novel supports for metal complex catalysts

Abstract In this work, the application of the thiol-functionalized epoxy resin encapsulated on magnetic core as supports for palladium catalysts is reported. The study focuses on obtaining of heterogeneous catalysts which can be separated by magnetic field. Palladium complex catalyst [PdCl2(PhCN)2] has been heterogenized by anchoring to these supports via ligand exchange reaction. The characterization of polymeric supports and heterogenized palladium catalysts has involved research methods like time-of-flight secondary ion mass spectrometry (TOF-SIMS), scanning electron microscopy (SEM) and nitrogen BET surface area measurements. The activity and stability during long-term use of the investigated catalytic systems were tested in a Heck and hydrogenation reaction. The influence of the type of thiols used as epoxy hardeners and the morphology of the supports on the catalytic properties of epoxy-supported palladium catalysts was discussed.

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Magnetic recykling of complex catalysts immobilized on thiol-functionalized polymer supports

Abstract The selective hydrogenation of cinnamaldehyde has been studied using palladium catalysts based on epoxy resin cured with ionic liquids. Characterization of palladium catalysts has involved the following methods: IR spectroscopy, X-ray photoelectron spectroscopy (XPS), AAS spectroscopy and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The investigated catalysts showed high stability and good recycling efficiency what makes them useful for a prolonged use. The factors influencing the selectivity of the supported catalysts were discussed.

https://www.degruyter.com/downloadpdf/j/pjct.2013.15.issue-4/pjct-2013-0063/pjct-2013-0063.xml

Karolina Wąsikowska

Papers

Soluble polysiloxane-supported palladium catalysts for the Mizoroki-Heck reaction

Synthesis and characterization of novel polythiourethane hardeners for epoxy resins

Epoxy resins cured with ionic liquids as novel supports for metal complex catalysts

Magnetic recykling of complex catalysts immobilized on thiol-functionalized polymer supports

Hydrogenation of cinnamaldehyde over supported palladium catalysts