Perfluorocarbon solvents

On the heels of work by Zhu and Horvath and Rabai, perfluorocarbon solvents and fluorous reagents have been used increasingly in organic syntheses. Ruorous compounds often partition preferentially into a fluorous phase in organic/fluorous liquid-liquid extraction, thus providing easy separation of the compounds. Tris[(2-perfluorohexyl)ethyl]tin hydride combines the favorable radical reaction chemistry of trialkyltin hydrides with the favorable separation features of fluorous compounds. 

In the Liquid-Phase Photofluorination [39,44] process the reactant is injected at a very slow constant rate into an inert fluorocarbon solvent which is saturated by fluorine and under U.V. irradiation. Conditions are chosen to ensure that the concentration of fluorine and fluorine radicals is always much higher than the concentration of the substrate. This method is only suitable for the perfluorination of substrates, such as partially fluorinated ethers (see Section 2.5) and amines (see Section 2.7), that are both soluble in perfluorocarbon solvents and can withstand such vigorous reaction conditions. 

Fig. 1. The growth in the number of publications using perfluorocarbon solvents.

The first report of a perfluorocarbon as a solvent for synthesis pre-dates Horvath and Rabai s fluorous biphase paper by 14 years. Kelly [70] investigated the flash photolysis of chromium hexacarbonyl in perfluoroheptane where the chromium pentacarbonyl (generated during photolysis) was shown to be more reactive in perfluorocarbon solvents than in hydrocarbon solvents. This effect, presumably, arises from the weak solvation of the unsaturated metal centre by the perfluorocarbon. 

Multi-phase purification using perfluorocarbon solvents... 

This reaction uses a fluorinated tetraalkyldistan-noxane catalyst in perfluorohexanes. Here, the catalyst and solvent form one layer, while the ester and alcohol reactants form a separate organic layer. On heating, the two layers are sufficiently miscible for the reaction to occur on cooling, the organic and fluorinated layers separate. The pure ester is recovered by evaporation of the ethanol co-product. The catalyst can be recovered from the perfluorocarbon solvent. 

Fluorous biphasic catalysis has emerged since the late 1990s as an attractive alternative to traditional catalysis methods [1], Fluorous techniques take advantage of the temperature-dependent miscibility of organic and perfluorocarbon solvents to provide easier isolation of products and recovery of a fluorinated catalyst. The large-scale use of fluorous solvents, however, has drawbacks cost, and concern over environmental persistence. 

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