Designing fluorous catalysts

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. 

Richter B, de Wolf E, van Koten G, Deelman BJ (2000) J Org Chem 65 3885 Gladysz JA, Correa da Costa R (2004) Strategies for the Recovery of Fluorous Catalysts and Reagents Design and Evaluation. In Gladysz JA, Curran DP, Horvdth IT (eds) Handbook of Fluorous Chemistry. WUey, Weinheim, p 24 Palo D, Erkey C (1998) J Chem Eng Data 43 47... 

Finally, it should be noted that Lewis acids and bases can also be used in other non-conventional media, as described in Chapter 7, e.g. fluorous solvents, supercritical carbon dioxide and ionic liquids by designing the catalyst, e.g. for solubility in a fluorous solvent or an ionic liquid, to facilitate its recovery and reuse. For example, the use of the ionic liquid butylmethylimidazolium hydroxide, [bmim][OH], as both a catalyst and reaction medium for Michael additions (Fig. 2.45) has been recently reported [151]. 

In 2001, it has been independently reported [2, 3] that the fluorous solvent can be skipped by designing fluorinated catalysts that themselves have a temperature-dependent phase miscibility - that is, solubility - in ordinary organic solvents. 

In order to extract non-fluorous products from reactions involving fluorous solvents in a rational way, partition coefficients must be known. The design and optimization of fluorous catalysts and reagents require analogous data. In 1999, only a few partition coefficients involving fluorous and organic phases had been... 

Several years ago, Professor Hoveyda designed the chelated Ru complex 12a as a versatile and stable metathesis catalyst. Dennis P. Curran of the University of Pittsburgh has now introduced (J. Org. Chem. 2005, 70, 1636) the fluorous-tagged Ru catalyst 12b. The fluorous tag allows the facile recovery of most of the active catalyst. The advantages of this are two-fold the valuable catalyst can be re-used, and there will potentially be less Ru contamination in the cyclized product. 

A chapter written in 1996 covers hydroformylation catalyzed by organometallic complexes in detail,219 whereas a review written 5 years later gives a summary of the advances on hydroformylation with respect to synthetic applications.220 A selection of papers in a special journal issue has been devoted to carbonylation reactions.221 A major area of the research has been the development of fluorous biphasic catalysis and the design of new catalysts for aqueous/organic biphasic catalysis to achieve high activity and regioselectivity of linear or branched aldehyde formation. 

The allylation of carbonyl compounds with allyltin reagents is still an active area of organotin chemistry from the methodological point of view, and also for synthetic applications. For completeness we should add several alternative techniques, such as the development of trifluoromethanesulphonic acid as a Bronsted acid catalyst for the allylation of aldehydes in water123, or the design of fluorous allyltin reagents for the platinum-catalysed allylation of aldehydes124. 

Solubility switching behaviour is one of the main benefits of fluorous biphasic catalysis (discussed in Chapter 7) however, other specially designed catalysts... 

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