Organometallic catalysis, aqueous biphasic systems

The best solvent is no solvent and if a solvent (diluent) is needed then water is preferred [100]. Water is non-toxic, non-inflammable, abundantly available and inexpensive. Moreover, owing to its highly polar character one can expect novel reactivities and selectivities for organometallic catalysis in water. Furthermore, this provides an opportunity to overcome a serious shortcoming of homogeneous catalysts, namely the cumbersome recovery and recycling of the catalyst. Thus, performing the reaction in an aqueous biphasic system, whereby the... 

Many other C-C bond forming reactions involving organometallic catalysis have been successfully performed in an aqueous biphasic system. Examples are shown in Fig. 7.13 and include Heck [47, 48] and Suzuki couplings [48] and the Rhone-Poulenc process for the synthesis of geranylacetone, a key intermediate in the manufacture of vitamin E, in which the key step is Rh/tppts catalyzed ad-... 

To determine the mode of operation (see below) it is useful to group these homogeneous catalysts into aqueous biphasic systems (see Section 3.1.1.1) and nonaqueous biphasic systems (see Section 3.1.1.2). Gas-liquid-liquid reactions are also involved in organometallic phase transfer catalysis, e. g., in biphasic carbonylation of benzyl chloride to phenylacetic acid by the catalyst system NaCo(CO)4/Bu4NBr/NaOH [21]. Here, the biphasic system consists of an organic solvent and aqueous alkali. 

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). 

As exemplified in the previous sections, organometallic catalysis in water and bi-phasic water-containing systems has not only been intensively examined during recent years but its scope has also considerably broadened. Between 1945 and 1997, the publishing date of the first edition of this book, about 1650 articles dealing with catalysis in aqueous media appeared in scientific journals. Approximately the same number of articles has been published between 1998 and the end of 2002. In the first half of 2003 nearly 200 new articles appeared [1], Most of the research dedicated to reactions in aqueous-phase and aqueous biphasic or multiphase systems refer to the reactions described in the previous sections. In this chapter some other new results, not described above in more detail, on reactions in aqueous media will be presented. 

Fluorous biphase organometallic catalysis is now a well-established area and provides a complementary approach to aqueous and ionic biphase organometallic catalysis [50]. Since each catalytic chemical reaction could have its own perfectly designed catalyst (the chemzyme), the possibility to select from biphase systems ranging from fluorous to aqueous systems provides a powerful portfolio for catalyst designers. 

Not surprisingly the ideal form of the process is aqueous biphase catalysis, in which the organometallic two-phase catalyst resides in a stationary aqueous solution in the reaction system. This is not only the most convenient arrangement on both the laboratory and industrial scale, but also the optimal modification wich respect to cost and environmental considerations. Use of water as the second phase has its limitations however, especially when the water solubility of the starting materials proves too low, preventing adequate transfer of organic substrate into the aqueous phase or at the phase boundary, and consequently reducing the reaction rate to such an extent that it becomes unacceptable. Cases... 

In the 15 years since its invention, fluorous (biphasic) catalysis has become a well-established area and provides a complementary approach to other variants of biphasic catalysis. Fluorous solvents are nontoxic and environmentally benign. Owing to their low surface tension, they do not form emulsions and due to their high density separate readily from other components in solvent mixtures. All these characteristics could make fluorous biphase catalysis superior to the analogous aqueous systems. Conversely, the same features that make fluorous solvents immiscible with many organic liquids make them bad solvents of organometallic catalysts, too, and chemical modifications of the catalysts (ligands) are required to attain sufficient solubility. Moreover, to retain the catalyst in the fluorous phase... 

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