Industrial applications olefin epoxidation

The following examples illustrate the application of high-throughput screening tools together with heuristic search algorithms in the development of new enhanced catalyst for two fields of industrial interest, olefin epoxidation and the isomerization of light paraffins. 

Peroxidases have been used very frequently during the last ten years as biocatalysts in asymmetric synthesis. The transformation of a broad spectrum of substrates by these enzymes leads to valuable compounds for the asymmetric synthesis of natural products and biologically active molecules. Peroxidases catalyze regioselective hydroxylation of phenols and halogenation of olefins. Furthermore, they catalyze the epoxidation of olefins and the sulfoxidation of alkyl aryl sulfides in high enantioselectivities, as well as the asymmetric reduction of racemic hydroperoxides. The less selective oxidative coupHng of various phenols and aromatic amines by peroxidases provides a convenient access to dimeric, oligomeric and polymeric products for industrial applications. 

Over the past 25 years, biomimetic model systems have been extensively studied and a wide variety of interesting oxidation processes such as the epoxidation of olefins, the hydroxylation of aromatics and alkanes, the oxidation of alcohols to ketones, etc., have been accomplished some of these are also known in enantioselective versions with spectacular ee s. The vast majority of these transformations were obtained using monooxygen donors such as those mentioned above as primary oxidants. The complexity of the catalysts and the practical impossibility to use dioxygen as the terminal oxidant have so far prevented the use of such systems for large industrial applications, but some small applications in the synthesis of chiral intermediates for pharmaceuticals and agrochemicals, are finding their way to market. 

The simple organorhenium(VII) compound methyltrioxorhenium (Structure 1 in Scheme 1) - called MTO - has developed a plethora of applications in catalytic processes [1], This rapid development occurred in the decade of 1990-2000. The epoxidation of olefins (cf. Section 2.4.3) became attractive to industrial applications. There is sound evidence that MTO represents the most efficient catalyst for this process, being active even for highly dilute solutions of hydrogen peroxide. The latter oxidant is not decomposed by MTO, as opposed to many other metal complexes (cf. Section 3.3.13.1). 

This species, completely characterized also by X-ray diffraction studies as its tetrahexylammonium salt (Figure 1)5 was responsible for the epoxidation of a series of structurally diversified olefins with typical selectivities of ca. 95% and chemical yields in the range 85-95%. These catalysts have found industrial applications in the epoxidation of alkenes and in the oxidative cleavage of alkenes to carboxylic acids. The favourable characteristics of these catalysts are thermal stability, ease of synthesis, stability to oxidation, solubility- in both water and organic solvents, effectiveness as phase transfer catalysts. 

In addition to the above described procedures implying either direct oxidation of an olefinic double bond or stereoselective reduction of a ketone precursor, which, as discussed above, do not really provide very efficient ways for the large scale synthesis of enantiopure epoxides, some indirect strategies have also been explored. These are essentially based on the resolution of epoxide-ring bearing substrates as exemplified below. As will be seen, these approaches imply the use of cofactor-independent enzymes, which are in practice much easier to work with, and lead to very interesting results. As a matter fact, some of these processes are already used on an industrial scale, and it can be predicted that future industrial applications will continue to be essentially based on the use of these very promising easy-to-use biocatalysts. 

Olefin epoxidation is not only important in the manufacture of bulk chemicals, e. g. ethylene and propylene oxides, but is also a widely used transformation in the fine-chemicals industry [1], Ethylene oxide is manufactured by vapor-phase oxidation of ethylene, with air or oxygen, over a supported silver catalyst [2], This method is not generally applicable as olefins containing allylic or other reactive C-H bonds give complex mixtures of products with low epoxide selectivity. The method has recently been extended to some other olefins that do not contain reactive allylic C-H bonds, e. g. butadiene, styrene, norbornene, and tert-butyl ethylene [3]. Some of these products, e. g. butadiene monoepoxide and styrene oxide, have potential applications as fine chemicals/intermediates. 

There is not very often accordance among the different models for the acidity prediction of a given oxide structure. Moreover, it is also hard to justify the predicted acid properties with the catalytic activity of the oxide composition this is the case of titania-silica system [67, 104]. Ti02 -Si02 mixed oxide is a very important industrial material and catalyst in both the amorphous and crystalline phases which found several industrial applications (e.g., isomerization of olefins, epoxidation of olefins... 

In this multi-authored book selected authors in the field of oxidation provide the reader with an up to date of a number of important fields of modern oxidation methodology. Chapter 1 summarizes recent advances on the use of green oxidants such as H2O2 and O2 in the osmium-catalyzed dihydroxylation of olefins. Immobilization of osmium is also discussed and with these recent achievements industrial applications seem to be near. Another important transformation of olefins is epoxidation. In Chapter 2 transition metal-catalyzed epoxidations are reviewed and in Chapter 3 recent advances in organocatalytic ketone-catalyzed epoxidations are covered. Catalytic oxidations of alcohols with the use of environmentally benign oxidants have developed tremendously during the last decade and in Chapter 4 this area is reviewed. Aerobic oxidations catalyzed by N-hydroxyphtahmides (NHPI) are reviewed in Chapter 5. In particular oxidation of hydrocarbons via C-H activation are treated but also oxidations of aUcenes and alcohols are covered. 

Catalysis of oxidation reactions will continue to be of enormous importance in the future. Areas that continue to be of active interest are the development of efficient methods for the direct epoxidation of olefins, hydroxylation and substitution of aromatics as well as the selective oxidation of alkanes. The application of methods developed for industrial chemicals to the synthesis of more complex molecules is worthy of more attention. A few examples have been discussed in the text. On the whole, however, synthetic chemists have not exploited these methods. 

For all the olefins investigated under continuous-flow conditions, we obtained high conversions and yields ( 80%) of the desired epoxides. Moreover, the process was substantially accelerated, shortening the residence time from 24 to 48 h (batch process) to only 1 - 4 h, with a standard production of about 80 g/day, which makes this protocol appealing for applications in pharmaceutical industry. 

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