Supported palladium catalysts for fine chemicals synthesis

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Abstract

The contribution reviews the application of heterogeneous Pd catalysts for the manufacture of fine chemicals with special emphasis of their organic synthetic potential. In a first part, some background is given on the scope and limitations of homogeneous and heterogeneous catalysis and on significant parameters and the various types of supported Pd catalysts. Then, the application of supported Pd catalysts for important classes of transformations are reviewed organized according to reaction type. The general statements are illustrated with relevant examples from the literature and from our own laboratories. Hydrogenation, hydrogenolysis, and dehydrogenation reactions are considered to be mature technologies with an extremely broad scope both for small-scale laboratory applications and large to very large-scale manufacturing processes. Oxidation reactions and CC coupling reactions usually have a rather narrow scope and only relatively few have been developed to the technical stage. Especially for CC coupling reactions, the nature of the active Pd species is under debate because it is not clear whether the reaction takes place on the metallic surface or whether leached soluble Pd complexes are the active catalysts.

Introduction

Without any doubt, there is an increasing tendency to apply catalytic methods for the synthesis and production of multifunctional, complex fine, and specialty chemicals such as agrochemicals and pharmaceuticals. On the one hand, the catalysts used are homogeneous metal complexes with organic ligands ([1] emphasis on industrial applications; [2] up-to-date monograph) and on the other hand heterogeneous catalysts, either in the metallic state or as oxides [3]. From an industrial point of view, catalysts not soluble in the same phase as the organic reactant have the inherent advantage of easy separation and very often also of better handling properties, but homogeneous catalysts are better defined and understood (for a short comparison see Table 1).

In the field of fine chemicals, palladium is arguably the most versatile and the most widely applied catalytic metal. However, with the exception of hydrogenation/dehydrogenation where many commercial processes with Pd catalysts are in operation, most reactions described in the open as well the patent literature are carried out with homogeneous Pd catalysts, either with or without organic ligands. This is best illustrated by the fact that several books have been devoted exclusively to the application of homogeneous Pd catalysts to organic synthesis, the most recent one by Tsuji in 1996 [4].1 Homogeneous Pd catalysts have several important advantages: (i) many different metal precursors are known and available; (ii) Pd forms complexes with a wide variety of organic ligands with P, N, and O atoms; (iii) many of these complexes are relatively easy to prepare and to handle; (iv) many Pd-catalyzed reactions give reliable results and are easy to run in ordinary equipment; (v) the functional group tolerance is often very good. The list of synthetically useful transformations is now truly impressive and explains why Pd catalysis has found its place into the repertoire of so many organic chemists.

The literature for the application of heterogeneous Pd catalysts in fine chemical synthesis is much less broad, and until a decade ago was restricted to hydrogenation and dehydrogenation reactions [5], [6], [7]. This situation is changing slowly and in the last few years the application of supported metallic palladium for CC bond-forming transformations has increased significantly. However, it must be pointed out that the heterogeneous examples listed in Table 2 in many cases have only been carried out with model substrates and under non-optimized conditions.

Section snippets

Catalyst types

As a rule, heterogeneous catalysts are still chosen on an empirical basis by trial and error, and it is rarely understood why a given catalyst is superior to another one. Many factors influence the catalytic properties of such catalysts, and it is important to realize that even today it is not possible to adequately characterize a heterogeneous catalyst on a molecular level (also see the contributions by M.V. Twigg and B. Coq).

Of the many parameters affecting the catalytic performance of a

Some general comments

Palladium on carbon (Pd/C) are the most widely used hydrogenation catalysts both in research laboratories of academia and the chemical industry. There are probably hundreds of Pd-catalyzed processes in operation for the production of fine chemicals and of biologically active ingredients. The catalytic profile of Pd catalysts differs significantly from hydrogenation catalysts based on Pt, Rh, Ru, Ni, or Cu in the following properties:

  • Pd is the most active metal for hydrogenolysis reactions, i.e.

Dehydrogenation reactions

Due to its high affinity to hydrogen atoms and low activity for the saturation of aromatic rings, palladium is the metal of choice for aromatization of unsaturated or partially unsaturated rings via dehydrogenation [58], [59]. Examples of partially saturated starting materials which lead to aromatic products are shown in Fig. 15 [60], [61], [62]. If a hydrogen acceptor like nitrobenzene or α-methylstyrene is used, dehydrogenation usually occurs at temperatures of 200°C and below, as can be seen

Oxidation reactions

Relatively few reports exist on the use of heterogeneous Pd catalysts for oxidation reactions using either oxygen or similar oxidants. An important exception is the selective oxidation of alcohols to aldehydes or ketones and of aldehydes to carboxylic acids in presence of metallic Pd and other platinum group metals on carbon supports [64], [65]. Especially useful is the highly selective oxidation of carbohydrates with oxygen or H2O2-catalyzed by Pd-Bi/C [66]. An example is shown in Fig. 17,

CC coupling and carbonylation reactions

For the organic chemist, palladium-catalyzed CC-forming reactions have become very important synthetic tools and many of these reactions were used both in academia and in industry so extensively that they became name reactions [4], [68]. In general, soluble palladium complexes with ligands like phosphines, amines, or carbenes are used as the catalysts where the choice of the ligand allows tuning of the catalytic activity and selectivity. Despite the potential benefits of using heterogeneous

Conclusions

The most important conclusions of this contribution can be summarized as follows.

  • Palladium is the most versatile and the most widely applied metal for catalytic organic synthesis and the manufacture of fine chemicals.

  • Two types of catalysts are used: heterogeneous catalysts where metallic Pd is supported on a carrier and homogeneous catalysts consisting of mononuclear Pd (ligand) complexes.

  • The most important applications for heterogeneous catalysts both in preparative chemistry and on a

Acknowledgements

We would like to thank Bernd Siebenhaar for fruitful discussions and valuable contributions.

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