Ponytails fluorous ligands

If nonvolatile liquids are to be used to avoid the problems associated with volatile organic solvents, then it is very desirable that there is some convenient way of recovering the reaction products from the liquid. This approach is used in the biphasic systems described in Chapters 2-5. In the fluorous biphase (Chapter 3), reagents and catalysts are fine-tuned by adding perfluoroalkyl chains, known as ponytails , to ensure that only those chemicals will mix with the fluorous layer. Purification is simply a matter of separating the two phases. Transition metal catalysts with fluorous ligands will remain in the fluorous phase, and the whole catalyst-solvent mixture may be reused for another batch of reactions, as shown schematically in Figure 1.20b. 

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 1.6 Like dissolves like perfluoroalkyl ponytails make phosphines more soluble in a fluorous solvent. These phosphines are suitable ligands for metal catalysts, and will therefore aid the solubility of these catalysts in fluorous solvents...

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

From our cooperation partners. Profs. Gladysz and Dinjus, we received ligands with perfluorinated chains ( ponytails ), which show a thermomorphic solubility in organic solvents (P(et-Rf8)2(m-me-bz)) or maybe extracted with fluorous solvents (P(et-Rfs)3). P(et-Rf6)(z-pr)2 with only one perfluorinated... 

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 . 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

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... 

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