Olefin epoxidation applications

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

Most of the methods for the epoxidation of electon-poor olefins are applications or variants of the Weitz-Scheffer reaction using alkaline H202. The requirement of a nucleophilic peroxy oxygen and the efficiency of the purely organic systems appear to be the main reasons why metal peroxides have been moderately used for this reaction. 

Considerable improvements have been made in the field of organorhenium and organomolybdenum oxide catalyst heterogenisation over the last decade. These include, in particular, the application of zeolites and mesoporous materials as supporting systems. Such heterogeneous systems have proven to be, in most cases, efficient and selective catalysts in various reactions, especially olefin epoxidation and (to a lesser extent) silane oxidation, other oxidation... 

Hydrogen peroxide is a mild oxidant and its use in olefin epoxidations requires the application of appropriate catalysts. The oxidation with aqueous hydrogen peroxide in the presence of tungstic acid yields the corresponding epoxides (equation 7)9. 

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. 

Sakthivel, A., Zhao, J., Hanzlik, M. and Kuhn, F. E. Heterogenization of CpMo(CO)(3)Cl on mesoporous materials and its application as olefin epoxidation catalyst, J. Chem. Soc., Dalton Trans. 2004, 3338-3341. 

F. E. Kiihn, A. M. Santos, W. A. Herrmann, Organorhenium(VII) and organomolybdenum(VI) oxides Syntheses and application in olefin epoxidation, Dalton Trans. (2005) 2483. 

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. 

Together with hydrogenation and isomerization, epoxidation completes the trio of commercially significant applications of enantioselective homogeneously catalyzed reactions. Stereospecific olefin epoxidation is distinctive in that it creates two chiral centers simultaneously. The enantioselective epoxidation method developed by Sharpless and co-workers is an important asymmetric transformation known today. This method involves the epoxidation of allylic alcohols with tert.-butyl hydroperoxide and titanimn isopropoxide in the presence of optically active pure tartrate esters (Eq. 3-14). 

A major application of the HOF CH3CN complex is olefin epoxidation. Practically any type of olefin, including electron-deficient ones, can be epoxidized by this reagent. In addition, the reagent oxidizes tertiary C-H bonds to C-OH, sulfides (including unreactive ones) to sulfones, and primary amines to nitfo compounds. Some representative applications are depicted below ... 

Sakthivel, A., Zhao, J., Hanzfik, M., et al. (2004). Heterogenisation of CpMo(CO)3Cl on Meso-porous Materials and its Application as Olefin Epoxidation Catalyst, Dalton Trans., 20, pp. 3338-3341. 

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... 

As electrophilic substitutes for peracids, the use of borate ester induced decomposition of alkyl hydroperoxides and molybdenum VI peroxy-complexes have been reported in the recent literature. Although these reagents have led to the epoxidation of olefins in greater than 90% yield there are no reports yet of their application to steroid olefins. 

The first application of the Jacobsen-Katsuki epoxidation reaction to kinetic resolution of prochiral olefins was nicely displayed in the total synthesis of (+)-teretifolione B by Jacobsen in 1995. 

CDP840 is a selective inhibitor of the PDE-IV isoenzyme and interest in the compound arises from its potential application as an antiasthmatic agent. Chemists at Merck Co. used the asymmetric epoxidation reaction to set the stereochemistry of the carbon framework and subsequently removed the newly established C-O bonds." Epoxidation of the trisubstituted olefin 51 provided the desired epoxide in 89% ee and in 58% yield. Reduction of both C-O bonds was then accomplished to provide CDP840. 

Epoxidation of olefins with meta-chloroperbenzoic acid, (MCPBA) remains to this day among the most widely used methods for research-scale applications [16], Discovered by Nikolai Prilezahev in 1909 [17], it became popular only decades later, mostly through the works of Daniel Swern in the 1940s [18]. Despite its simplicity, and not unlike most epoxidation methods in use today, it suffers from undesired epoxide opening caused by the slight acidity of the reaction milieu. Although acid-catalyzed side reactions can sometimes be minimized by use of buffered systems... 

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

New interesting applications have been in the epoxidation of difficult olefin compounds (including hexafluoropropene) with NaOCl, side-chain chlorination of substituted toluenes, diazotization of pentafluoroaniline, polymerization with free radicals, etc. 

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