Epoxidation of Alkenes in Fluorinated Alcohol Solvents

The first reports on the use of fluorinated alcohols, and in particular of HFIP in oxidations with hydrogen peroxide, can be found in the patent hterature of the late 1970s and early 1980s [19,20]. Typically, 60% aqueous hydrogen peroxide was used in the presence of metal catalysts. A number of reports on alkene epoxidations in fluorinated alcohols, both in the absence and in the presence of additional catalysts, have followed. 

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In 2000, Neimann and Neumann reported on alkene epoxidation by H2O2 in fluorinated alcohol solvents without the addition of further catalysts [21]. Shortly thereafter, in 2001, Sheldon et al. reported about their results, also on alkene epoxidation in fluorinated alcohol solvents [22]. In the latter study, it became clear that buffering the reaction mixtures, preferably by addition of Na2H PO4 improves the overall efficiency of the process, presumably by suppressing acidotalyzed degradation of the product epoxides. Scheme 4.3 summarizes the results obtained using TFE as solvent, whereas the results for HFIP are summarized in Scheme 4.4. 

The epoxidation of alkenes is accelerated in fluorinated alcohol solvents the factors responsible for this rate acceleration have been examined <2006JA8421, 2006JA13412>. Chiral dioxiranes can now be used in a catalytic sense for the synthesis of vinyl m-epoxides <2006AGE4475>. 

The catalysts applied to alkene epoxidation in fluorinated alcohol solvents can be subdivided into those which are metal/chalconide-based and those which are purely organic in nature (Scheme 4.5). The former comprise arsanes/arsane oxides [27,28], arsonic acids [29, 30], seleninic acids/diselenides ]31-35], and rhenium compounds such as Re207 and MTO (methylrhenium trioxide) ]36,37]. As shown in Scheme 4.5, their catalytic activity is ascribed to the intermediate formation of, for example, perseleninic/perarsonic adds or bisperoxorhenium complexes. In other words, their catalytic effect is due to the equilibrium transformation of hydrogen peroxide to kmetically more active peroxidic spedes. 

It was noted in Sections 4.3.2.2 and 4.3.2.3 that arsonic acids and seleninic adds are efficient catalysts for the epoxidation of alkenes. For both types of catalyst, significant enhancement of catalyst activity and selectivity was observed in fluorinated alcohol solvents compared to, for example, 1,4-dioxane. 

Fluorous solvents proved to be highly effective in epoxidation of alkenes. H202 can be used in combination with trifluoroacetone,27 perfluoroacetone,28 or a mixture of perfluoroacetone and hexafluoro-2-propanol.29 In fluorinated alcohols as solvents uncatalyzed epoxidations with aqueous H202 are performed.30,31... 

The high solubility of the MTO catalyst in almost any solvent opens up a broad spectrum of reaction media from vhich to choose when performing epoxidations. The most commonly used solvent, however, is still dichloromethane. From an environmental point of view this is certainly not the most appropriate solvent in large scale epoxidations. Interesting solvent effects for the MTO-catalyzed epoxida-tion were reported by Sheldon and coworkers, who performed the reaction in trifluoroethanol [86]. The change from dichoromethane to the fluorinated alcohol allowed for a further reduction of the catalyst loading down to 0.1 mol%, even for terminal alkene substrates. It should be pointed out that this protocol does require 60% aqueous hydrogen peroxide for efficient epoxidations. 

As a consequence of the above summarized properties of fluorinated alcohols, they are ideal solvents for the generation of cationic or radical-cationic species or reaction intermediates. This eflect has been exploited numerous times, for example, in the investigation of organic radical cations [11]. At the same time, it lies at the heart of the catalysis of alkene epoxidation with hydrogen peroxide and of Baeyer-Villiger-type oxidations of carbonyl compounds. The mechanisms of the latter two reaction types are discussed in more detail in Sections 4.3.1.1 (epoxidation) and Section 4.5.1 (Baeyer-VUliger). 

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