Epoxides synthesis from alkenes

Scheme 8.13. 1,2-Diol synthesis from alkenes via direct osmylation or epoxidation followed by hydrolysis.

The most important oxirane syntheses are by addition of an oxygen atom to a carbon-carbon double bond, i.e. by the epoxidation of alkenes, and these are considered in Section 5.05.4.2.2. The closing, by nucleophilic attack of oxygen on carbon, of an OCCX moiety is dealt with in Section 5.05.4.2.1 (this approach often uses alkenes as starting materials). Finally, oxirane synthesis from heterocycles is considered in Section 5.05.4.3 one of these methods, thermal rearrangement of 1,4-peroxides (Section 5.05.4.3.2), has assumed some importance in recent years. The synthesis of oxiranes is reviewed in (B-73MI50500) and (64HC(19-1U). 

Scheme 12.11. Synthesis of Epoxides from Alkenes Using Peroxy Acids...

Figure 4.20 Continuous removal of product by stripping synthesis of epoxides from alkenes applying a batch process with resting cells or a fermenter connected to absorption, extraction and distillation...

Synthesis of cyclic carbonates from alkenes epoxidation-cum-cycloaddition... 

Historically, the asymmetric synthesis of epoxides derived from electron-poor alkenes, for example a, (3-unsaturated ketones, has not received as much attention as the equivalent reaction for electron-rich alkenes (vide supra). However, a recent flurry of research activity in this area has uncovered several... 

Since the epoxidation of alkenes with peracids was discovered by Prilezajew in 1909 [29], epoxides have played a major role in organic chemistry and industry, providing important intermediates for the synthesis of more complex molecules. Metal-catalyzed epoxidation reactions have received much attention in recent decades since the discovery of the Sharpless epoxidation [30, 31], but most epoxides were prepared from alkenes primarily by their interaction with peracids. 

In recent years, dioxiranes have become workhorses for a variety of selective transformations in organic synthesis, from epoxidation of alkenes to the conversion of alcohols into fee corresponding ketones <99CJC308>. Dioxirane-mediated epoxidation continues to be the method of choice for complex substrates wife acid-sensitive functionality. Thus, fee dimethyl-dioxirane (DMD)-mediated epoxidation of the silylated stilbene lactam 159 has been reported as a key step in fee synthesis of protoberberines <99JOC877>. 

By reasoning backward, recalling that epoxides are made from alkenes by reaction with peroxy acids, we develop a proper synthesis. 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

The epoxide 6 is naturally electrophilic, but where does the epoxide come from By far the most important method of epoxide synthesis is the treatment of alkenes 19 with peroxy acids RCO3H 21. Alkenes are naturally nucleophilic 2 they react with bromine to give dibromides 20 and with electrophilic peroxyacids 21 to give epoxides. Again, these reactions convert nucleophilic alkenes into electrophilic derivatives. A very popular reagent for epoxidation is mCPBA (meta-chloro-perbenzoic acid) 21 R = 3-chlorophenyl but many other compounds are used. 

Radical Epoxide Reduction Borane-free Two-step Synthesis of anti-Markownikow Alcohols from Alkenes... 

Both chemical and enzymatic synthetic methods for the asymmetric oxidation of the carbon-carbon double bond have been developed [46], but the area of carbon-carbon double bond oxidations has been shaped by the breakthrough discovery of asymmetric epoxidation of allylic alcohols with the Katsuki-Sharpless method [47]. Catalytic asymmetric synthesis of epoxides from alkenes by Jacobsen... 

Of the numerous other aziridine syntheses, there are several multistep procedures from alkenes. Though not strictly within the scope of this review, the practising chemist will wish to consider their merits alongside the rect syntheses, and the main possibilities are summarized in Scheme 8. There are several good recent reviews, - and two older compilations remain very useful. - Syntheses of those intermediates of Scheme 8 accessible from alkenes are described in later sections of tiie present review, and syntheses of epoxides (Volume 6, Chapter 1.1 and Volume 7, Chapters 3.1 and 3.2) and triazolines (Volume 5, Ch ter 3.1) are described elsewhere in Comprehensive Organic Synthesis . It is important to note that by careful choice of route one can either commence with alkene (14) and retain the cisitrans stereochemistry in the resulting aziridine (16), or start with alkene (13) and change the cisitrans relatitm-ships of the substituents. 

Most of the examples of seleniranes and telluriranes shown as the unstable intermediates in the organic synthesis are derived from oxiranes. As discussed previously in Section 1.07.6.2, seleno-cyanate anions react with epoxides at room temperature to deposit selenium via the selenirane intermediate and form the corresponding alkenes. On the other hand, triphenylphosphine selenide and trifluoroacetic acid constitute an effective and mild combination of reagents for carrying out the deoxygenation of epoxides (67) to alkenes via cyclic intermediate (68) (Scheme 12) <73CC253>. 

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