Ponytails

The interest in catalyst recyclability has led to the development of biphasic catalysts for hydro-boration.22 Derivitization of Wilkinson s catalyst with fluorocarbon ponytails affords [Rh(P (CH2)2(CF2)5CF3 3)3Cl] which catalyzes FIBcat addition to norbornene in a mixture of C6FnCF3 and tetrahydrofuran (TF1F) or toluene (alternatively a nonsolvent system can be used with just the fluorocarbon and norbornene) to give exo-norborneol in 76% yield with a turnover number up to 8,500 (Scheme 4). Mono-, di- and trisubstituted alkenes can all be reacted under these conditions. The catalyst can be readily recycled over three runs with no loss of activity.23... 

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 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...

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. 

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

For systems where the catalyst is required in the CO2 phase a modification of the hgand periphery to increase the solubihty in the supercritical medium is usually necessary. This has to be mostly done via introduction of perfluorinated tags ( ponytails ) which causes expensive and/or sometimes difficult synthetic operations. 

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... 

Following the publication of the first example of fluorous biphase catalysis by Horvath and Rabai in 1994 [1], the immediate focus was to develop catalysts that would exhibit very biased partition coefficients with respect to fluorous and organic solvents. Such liquids are normally immiscible at room temperature. This was done by attaching ponytails of the formula (CH2)m(CF2) -iCF3 (abbreviated (CH2)mRf )> including arrays emanating from silicon atoms [2]. Catalysis was then effected at elevated temperatures, where fluorous and organic solvents are commonly miscible, with prod-uct/catalysis separation at the low-temperature two-phase limit. 

As detailed elsewhere, the fluorous palladacycle acetates and hahdes 7 and 8 were synthesized [38,39]. These feature three Rfg ponytails, and were poorly soluble in common organic solvents at room temperature, and insoluble in DMF. However, they were very soluble in DMF at higher temperatures. All were effective catalyst precursors for Heck reactions (100-140 °C), and precipitated (as the halides) upon cooling. However, a number of control experiments established that 7 and 8 served as steady-state sources of colloidal palladium nanoparticles, formed anew with each cycle imtil the palladacycles were exhausted. These, or low-valent Pd(0) species derived therefrom, were the true catalysts. 

In crystalhne 14-Rf8, one ponytail on each phosphorus atom extends above the palladiiun square plane, and the other below. Both run parallel to the ponytails on the trans phosphorus atom, which are nearly in van der Waals contact (Fig. 6, middle). In accord with the tendency for fluorous phases to segregate, the lattice is divided into fluorous and non-fluorous domains (Fig. 6, bottom), with the ponytails of neighboring molecules in comparable van der Waals contact. 

To our knowledge, there have been no previous attempts to develop a broad class of molecular catalysts that have temperature-dependent solubilities. When molecular catalysts are covalently bound to polymeric supports, they generally assume the solubihty properties of the host polymer. In the above fluorous catalysts, we Hke to think that a short segment of polymer is being grafted onto a molecular catalyst, hi other words, the ponytails can be viewed as pieces of Teflon , which impart more and more of the solubility characteristics of the polymer as they are lengthened. 

In conclusion, the new recycHng procedures described above offer virtually unhmited possibihties for optimization and catalyst engineering . The lengths and other structural features of the ponytails are easily varied. There are innumerable types of possible fluoropolymer supports, as well many additional classes of fluorous supports. Accordingly, a variety of further refinements and developments can be expected in the near future. 

Gladysz JA (2004) Overview of structural classes of ponytails. In Gladysz JA, Curran DP, Horvath IT (eds) Handbook of fluorous chemistry. WUey, Weinheim, Chap 5... 

Fig. 22. Schematic illustration of the approach used to carry out fluorous biphasic catalysis using dendrimer-encapsulated metal nanoparticles modified on their exterior with perfluoroether ponytails. Note that the ponytails can be attached by either electrostatic or covalent means. Reprinted with permission from Ref. 103 Copyright 2000 American Chemical Society...

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