Organometallic catalyst recycling

Fig. 3.5-1 Organometallic catalyst recycling and product recovery using an scCOj-IL biphasic system.

It is reasonable to suppose that a simple organic compound will be less affected by the connection to a support than a more structurally complex and somehow more delicate enzyme. Metal-free catalysts are also more readily amenable than organometallic catalysts to anchoring on a support in order to easily separate the product from the catalyst, and to recover and recycle the latter. Indeed, it has repeatedly been shown that the use of a metal-based catalyst immobilized on a support is often problematic because of possible, extensive metal leaching and requires catalyst regeneration by metal replenishment before recycling. 

During the last decade there have been many new applications of water-soluble organometallic catalysts in the field of organic synthesis. Although the first aim of this new concept was the easy separation of the catalyst fi-om the reaction products for its eventual recycling, new selectivities and sometimes higher activities were found using water as the reaction medium. 

Since the first edition of this book a great number of articles have been published in which the different techniques to separate the catalyst from the products via two liquid phases were applied. Some general review articles in books and journals about multiphase homogeneous catalysis, catalyst recycling and fluid-fluid systems have been published [96-102]. Other review articles concentrate on aqueous organometallic chemistry and catalysis [103-108], on biphasic systems with ionic liquids [109, 110], or on fluorous solvents [111, 112] (cf. Sections 7.2, 7.3). 

This systematic classification of recycling methods can be related directly to the solubility properties of the organometallic catalysts. The majority of these are poorly soluble in CO2, which therefore acts as an anti-solvent for solutions containing such species. However, either substrates or products may act as entrainers which serve to enhance the CO2 solubility, so in some cases additional catalyst modification may be necessary to render them sufficiently C02-phobic for efficient separation. In the other two approaches, the catalysts need to be C02-philic to ensure sufficient solubility in the C02-based media under the reaction conditions. This behavior is exhibited by certain volatile and nonpolar complexes such as transition metal carbonyl complexes, and also by metal complexes containing suitably modified ligands (e.g., containing perfluoroalkyl groups). 

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