Fluorous ponytail

Figure 6.7. Hydroformylation of an alkene using a rhodium complex bearing a fluorous ponytail.[l, 22]...

The concept makes use of the complimentary strengths and weaknesses of the two unconventional media. While ionic liquids are known to be excellent solvents for many transition metal catalysts, the solubility of most transition metal complexes in scC02 is poor (if not modified with e. g. phosphine ligands with fluorous "ponytails" [64]). However, product isolation from scC02 is always very simple, while from an ionic catalyst solution it may become more and more complicated depending on the solubility of the product in the ionic liquid and on the product s boiling point. 

Figure 8.7 Fluorous and control ligands screened to gauge the effect of the fluorous ponytails on catalysis using Wilkinson s catalyst analogues...

In order to perform fluorous biphasic catalysis the (organometallic) catalyst needs to be solubilized in the fluorous phase by deploying fluorophilic ligands, analogous to the hydrophilic ligands used in aqueous biphasic catalysis. This is accomplished by incorporating so-called fluorous ponytails . 

In Chapter 7 we have already discussed the use of fluorous biphasic systems to facilitate recovery of catalysts that have been derivatized with fluorous ponytails . The relatively high costs of perfluoroalkane solvents coupled with their persistent properties pose serious limitations for their industrial application. Consequently, second generation methods have been directed towards the elimination of the need for perfluoro solvents by exploiting the temperature-dependent solubilities of fluorous catalysts in common organic solvents [42]. Thus, appropriately designed fluorous catalysts are soluble at elevated temperatures and essentially insoluble at lower temperatures, allowing for catalyst recovery by simple filtration. 

Alternatively, an insoluble fluorous support, such as fluorous silica [43], can be used to adsorb the fluorous catalyst. Recently, an eminently simple and effective method has been reported in which common commercial Teflon tape is used for this purpose [44]. This procedure was demonstrated with a rhodium-catalyzed hydrosilylation of a ketone (Fig. 9.27). A strip of Teflon tape was introduced into the reaction vessel and when the temperature was raised the rhodium complex, containing fluorous ponytails, dissolved. When the reaction was complete the temperature was reduced and the catalyst precipitated onto the Teflon tape which could be removed and recycled to the next batch. 

Although most soluble homogeneous catalysts could be made fluorous-soluble by attaching fluorous ponytails to the catalyst core in appropriate size and number [9], transition metal complexes have mostly been converted to fluorous-soluble through ligand modification [10]. The most effective fluorocarbon moieties are linear or branched perfluoroalkyl chains with high carbon number that may con-... 

The fluorine-substituted tertiary phosphine P(C6F5)3 (132) (Scheme 9) was prepared by the reaction of C6F5MgBr and PC13 in diethyl ether.276 This procedure was also extended to the preparation of phosphorus(III) ligands with fluorous ponytails such as PPh3 ra(CH2CH2C6F13) (n= 1-3, (133)) (Equation (32)) and the ditertiary phosphine (134).277,278... 

A tin hydride attached to fluorous ponytails is quite versatile in respect of easy separation of the desired products from organotin compounds. When an iodoalkene was treated with NaCNBHs in the presence of a catalytic amount of fluor-oalkyltin hydride, the desired tricyclic ketones were produced (Scheme 12.133) [240]. Application of the crude mixture to a small plug of fluorous reversed-phase silica gel enabled facile separation of the cyclic ketone obtained (fluorous solid-phase extraction, FSPE). In contrast, when tris(trimethylsilyl)silane (TTMSH) was employed as a hydride source chromatographic purification of the nonpolar and somewhat volatile products was impossible because of contamination by nonpolar silicon-containing byproducts. 

Under the same fluorine loading, an increased number of fluorous ponytails usually results in higher P values. 

Molecules bearing longer fluorous ponytails show increased P values, together with decreased absolute solubilities in both phases. 

Fluorophihcity decreases in the presence of functional groups capable of intermolecular attractive interactions (through orientation forces, hydrogen bonds or induction forces). This effect can be counterbalanced by introducing more fluorous ponytails in the molecular structure. 

Branching and flexibility of the fluorous ponytails could have influence on the phase behavior of a fluorous molecule. 

Rule 2 The longer the fluorous ponytail, the higher the partition coefficient, and the lower the absolute solubility in both phases. On the other hand, an increase in the proportion of anti-fluorous (fluorophobic) [13, 14] domains in the molecule increases the absolute solubility in the organic phase. 

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