Epoxidation

Epoxidation


The epoxide functional group consists of a three-member ring with two carbon atoms and one oxygen atom. These moieties are found in many naturally occurring organic molecules, as well as in industrial starting materials. Epoxides are most commonly synthesized from reacting alkenes with peracids [12]. One of the most widely used epoxidizing reagents is meta-chloroperoxybenzoic acid, or m-CPBA, because of its mild conditions and high yield. However, it also produces chlorinated waste and is potentially explosive. H2O2 is a better choice in terms of both atom economy and innocuous products. Both H2O2 and m-CPBA donate one oxygen atom to the alkene substrate. However, this is ~50% by weight of the H2O2 whereas only ~10% of the m-CPBA. Studies with various POMs show that they might offer safer, selective, and high yielding routes to epoxide synthesis.

[SiW10O36]-8

Perhaps the most promising POM catalyst for the H2O2-based epoxidation of alkenes is the Mizuno group’s [SiW10O36]-8 anion in the dilacunary Keggin structure prepared as the tetrabutylammonium salt [12]. This method offers >99% selectivity of products and >99% efficiency of H2O2 at room temperature (Figure 2). The efficiency of H2O2 describes whether or not H2O2 is decomposed in a side reaction. Since this wastes one of the reagents, it is an undesirable process. Fortunately, Mizuno’s method does not suffer from this [13]. The process offers greater than 90% conversion of substrate to product in less than 12 hours for most substrates studied. The H2O2 efficiency has made other POMs unattractive for industrial applications.

[SiW10(Fe3(OH2))2O38]-6

Another system investigated by the Mizuno group is the related structure g-[SiW10{Fe3(OH2)}2O38]6- [14]. This compound is the same as the one above except that the two lacunary positions have now been filled with iron atoms. This catalytic system is a homogeneous catalyst for the O2 epoxidation of alkenes. This process offers the benefits that water sensitive alkenes might be oxidized since no aqueous H2O2 needs be used and that O2 is a very cheap and abundant reagent. However, this method does suffer some drawbacks. By using O2 gas, the reaction has the potential for explosion. The conversions are generally low except for rare cases, and the selectivity is also lower. A very large drawback for use in laboratory preparations is that the reaction is slow (experimental results were recorded after two and a half weeks of reaction) [14]. Additionally, the use of a gaseous reagent makes this an unappealing method for general use. This method may be useful for industrial large scale preparations when a continuous flow of products is maintained. For conversion and selectivity see Figure 2.


Figure 2: This graph compares the activity and selectivity of the two catalysts studied by the Mizuno group. The solid bars represent the % conversion for each substrate, and the checkered bars indicate the selectivity for the epoxide. The reaction time for SiW10 (red) is between 4 and 8 hours, whereas the reaction time for SiFe2W10 (blue) was 385 hours.

View Graph as an Enlargement

 

[ZnW(VO)2(ZnW9O34)2]-12

A third catalytic system has been developed for the catalytic epoxidation of allylic alcohols, consisting of [ZnW(VO)2(ZnW9O34)2]12- [15]. This process is not only regioselective, giving the appropriate functionality at the appropriate site, it is also stereoselective, yielding the threo isomer preferentially to the erythro isomer with ratios as high as 95:5. The key difference in this reaction as compared to the Mizuno compounds is that the source of oxygen is the sterically hindered chiral {(4R, 5R)-5-[(hydroperoxy-diphenyl)methyl]-2,2-dimethyl-1,3-dioxolan-4-yl}diphenylmethanol, or TADOOH [16]. This compound must be used in a stoichiometric amount, making this a less attractive catalyst from a cost perspective. Fortunately, a,a,a’,a’,-tetraaryl-1,3-dioxolane-4,5-dimethanol, or TADDOL, can be recovered from the reaction, and in a reaction with H2O2 the TADOOH can be regenerated [16]. The advantage to this method is should the reaction require the threo stereochemistry, this method is capable of yielding it.

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