Water metal-complex catalysis

The application of aqueous / supercritical biphasic media is not restricted to metal complex catalysis but has proven effective also for enzymatic and whole-cell biocatalysis [36]. In general, water plays an important role in coimection with biocatalysis. If water is completely absent, enzymes are often not catalytically active under supercritical conditions [37]. In the literature many examples of biocatalysis with supercritical fluids containing various amounts of water are known and a detailed account of this field is outside the scope of the present discussion. One example to highlight the use of a true biphasic system is the carboxylation of pyrrole... 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

This review primarily focuses on the numerous reactions catalysed by water soluble transition metal complexes with emphasis on their implications for development of new environmentally benign processes in aqueous media based on the easy and quantitative separation of products from the catalyst as well as the avoidance of organic solvents. Moreover it contains, to our knowledge, the first comprehensive overview of water soluble ligands which play a key role for the development of efficient organometallic catalysis in aqueous media. 

A few years ago, a new class of ligands namely the sulfonated phosphites (for examples see Table 7, 132, 133) was described.283 287 They show remarkable stabilities in water compared to conventional phosphites such as P(OPh)3 and rhodium catalysts modified with 132 exhibited much higher catalytic activities in the hydroformylation of 1-tetradecene than conventional Rh/P(OPh)3 or Ph/PPh3 catalysts even at lower reaction temperatures.285,286 Sulfonated phosphite ligands may play a role in the emerging field of biphasic catalysis in ionic liquids15 22 or in combination with membrane separation of the metal complexes of these bulky ligands. 

The term Counter Phase Transfer Catalysis (CPTC) was coined by Okano214,215 to describe biphasic reactions catalysed by water soluble transition metal complexes which involve transport of an organic-soluble reactant into the aqueous phase where the catalytic reaction takes place. Similarly, Mathias and Vaidya564,565 gave the name Inverse Phase Transfer Catalysis to describe this kind of biphasic catalysis which contrasts with classical Phase Transfer Catalysis where the reaction occurs in the organic phase and does not involve formation of micelles.389,564... 

The general principle of two-phase catalysis in polar solvents, for example, in water, is shown in the simplified diagram of Fig. 1. The metal complex catalyst, which can be solubilized by hydrophilic ligands, converts the reactants A + B into the product C. The product is more soluble in the second than in the first phase and can be separated from the catalyst medium by simple phase separation. Excellent mixing and contacting of the two phases are necessary for efficient catalytic reaction, and thus the reactor is normally well stirred. 

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