Fluorous biphasic concept

Figure 6.1 The fluorous biphase concept illustrated for the hydro formylation of an alkene (substrate) to an...

The fluorous biphasic concept was introduced by the Hungarian chemists Istvan Horvath and Jozsef Rabai in 1994 [148], A fluorous biphasic system consists of a fluorous phase (a perfluoroalkane-, perfluorodialkyl ether-, or perfluorotrialkylamine-rich phase) containing a fluorous-soluble reagent or catalyst, and a second phase,... 

Interestingly, the first experiments demonstrating the fluorous biphasic concept were actually made in 1991 by M. Vogt, a German PhD student at the Rheinisch-Westfalischen Technischen Hochschule in Aachen. Unfortunately, Vogt (and his supervisor ) never published these results except as a PhD thesis hidden at the technical school library in Aachen, so their work remained secret. It came to light only some years after the field of fluorous biphasic chemistry was established internationally [151]. 

The successful demonstration of the fluorous biphasic concept for performing organometallic catalysis sparked extensive interest in the methodology and it has subsequently been applied to a wide variety of catalytic reactions, including hydrogenation [59], Heck and Suzuki couplings [60, 61] and polymerizations [62]. The publication of a special Symposium in print devoted to the subject [63] attests to the broad interest in this area. 

It is generally considered that the major obstacle to the commercialization of reactions employing the fluorous biphasic concept is the cost of the fluorous solvent and the cost of the ligand, which must contain very large amounts of fluorine to retain the catalyst within the fluorous phase. Cost may be reduced by a proper selection... 

The fluorous biphase concept was successfully used for Pd catalyzed Sonoga-shira couplings of arylbromides and alkynes by Bannwarth and his co-workers [47]. The reactions are carried out at 100°C in a mixture of DMF and l,2,2,3,4,4,5,5,6,6-decafluoro-l,3-bis(trifluoromethy])cyclohexane with Cul (5%) as promotor and a catalyst concentration of 2%. Electron-deficient and electron-rich bromoarenes as well as three different types of alkynes bearing a silyl, an aromatic or an aliphatic substituent were investigated. The catalysts were prepared from... 

Fig. 1 The fluorous biphase concept for the catalytic conversion of substrates A and B to product P. The attachment of appropriate fluorous ponytails to the ligands of the homogeneous catalyst ensures that the fluorous catalyst remains in the fluorous phase.

It is noteworthy that this research dates back to the early nineties, that is, before the fluorous biphase concept became generally known because of the work of Horvath. However, the results were not made widely available and became published in the open literature only in 1999. 

Similar to the fluorous biphasic concept, a system was developed that formed a homogenous phase at elevated temperature, but phase-separated at room temperature [49c]. It was found that a mixture of functionalized PNIPAM polymer, ethanol, heptane and water exhibited these properties. The hydrogenation of 1-octadecene and 1-dodecene using a phosphine functionalized PNIPAM with a rhodium precursor were taken as test reactions and the high activity was foimd was similar to that of RhCl(PPh3)3. At room temperature the mixture phase separated and the catalysis stopped since the catalyst is completely insoluble in heptane. The substrate is dissolved in the heptane allowing a facile catalyst/product separation without the loss of activity. The concept is obviously limited to substrates that show... 

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