Biphasic systems organometallic catalysis

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. 

In conjunction with the work being done on fluorous biphasic systems, the development of fluorinated ligands has also made an impact in the area of supercritical carbon dioxide (SCCO2) catalysis. Supercritical CO2 has been heavily studied as a reaction medium for organometallic chemistry due to its unique properties. Its... 

Multi-phase organometallic catalysis, in particular, liquid-liquid biphasic catalysis involving two immiscible phases, may offer the possibility of circumventing the problems associated with the homogeneous process such as product separation, catalyst recycling, and the use of organic solvents. The concept of this system implies that the catalyst is soluble in only one phase whereas the substrates/products remain in the other phase. The reaction can take place in one (or both) of the phases or at the interface. In most cases, the catalyst phase can be reused and the products/ substrates are simply removed from the reaction mixture by decantation. 

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

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