Peroxycarboxylic acids epoxidation

Solvent effects in the peroxycarboxylic acid epoxidations are significant. The epoxidation rates in ether or ethyl acetate are approximately one-tenth of those in benzene or chloroform. The much slower epoxidation with peroxycarboxylic acids with intermolecular hydrogen bonding is indicative of the development of a cyclic transition state. 

The stereoselectivity of epoxidation with peroxycarboxylic acids has been well studied. Addition of oxygen occurs preferentially from the less hindered side of the molecule. Norbornene, for example, gives a 96 4 exo endo ratio.76 In molecules where two potential modes of approach are not very different, a mixture of products is formed. 

In studies of the asymmetric epoxidation of olefins, chiral peroxycarboxylic acid induced epoxidation seldom gives enantiomeric excess over 20%.1 Presumably, this is due to the fact that the controlling stereocenters in peroxycarboxylic acids are too remote from the reaction site. An enantiomeric excess of over 90% has been reported for the poly-(Y)-alanine-catalyzcd epoxidation of chalcone.2 The most successful nonmetallic reagents for asymmetric epoxidation have been the chiral TV-sulfonyloxaziridincs3 until asymmetric epoxidation reactions mediated by chiral ketones were reported. Today, the... 

Scheme 12.3 Chemoenzymatic generation of the peroxycarboxylic acid and subsequent epoxidation of the fatty material.

The stereoselectivity of epoxidation with peroxycarboxylic acids has been well studied. Addition of oxygen occurs preferentially from the less hindered side of the... 

Although none of these reagent combinations have been as generally usefirl as the peroxycarboxylic acids, they serve to illustrate that epoxidizing activity is not unique to the peroxy acids. 

Direct Oxidation with Stoichiometric Oxidants. Discovered by Prilezhaev in 1909,211 the typical epoxidation reaction of alkenes is their oxidation with organic peracids. Of the large number of different peroxycarboxylic acids used in... 

The transformation of carboxylic acids and their functional derivatives to the corresponding peroxycarboxylic acids or diacyl peroxides are generally known reactions.200-201 Among the hydroperoxy derivatives, trifluoroperacetic acid is a frequently used epoxidizing reagent in organic chemistry and is usually prepared in situ.200-201... 

Alkenes can be oxidized with peroxycarboxylic acids, RC03H, to give oxacy-clopropanes (oxiranes, epoxides), which are three-membered cyclic ethers ... 

The most important method of preparation involves oxidation, or epoxidation, of an alkene with a peroxycarboxylic acid, RC03H. This reaction achieves suprafacial addition of oxygen across the double bond, and is a type of electrophilic addition to alkenes (see Exercise 15-53) ... 

The epoxidation of olefins by peroxycarboxylic acids is quite important and has been reviewed repeatedly2,6,9. The two characteristic features of epoxidations with peroxycarboxylic acids are such that the epoxidations are accelerated by both increasing electron density of the carbon-carbon double bond and electron-attracting groups on the peroxycarboxylic acid. 

The reaction of alkenes with peroxycarboxylic acids to produce epoxides was discovered by Prilezhaev over 80 years ago.14 It is still the most widely used method for epoxidation, and considerable work has been carried out to elucidate the mechanism. The commonly accepted explanation for oxirane formation involves a cyclic polar process where the proton is transferred intramolecularly to the carbonyl oxygen, with simultaneous attack by the alkene rc-bond. This concerted process was suggested by Bartlett,15 and because of the unique planar transition structure it is referred to as the butterfly mechanism (Figure 3.2). 

Figure 3.2 Bartlett butterfly mechanism for the epoxidation of alkenes with peroxycarboxylic acids.

Peroxycarboxylic acids can attack either the top side or the bottom side of a double bond. The epoxide resulting from top-side attack on ris-4-octene has two chirality centers, but because it has a plane of symmetry, it is a meso compound. The two epoxides are identical. 

The epoxidation of alkenes with peroxycarboxylic acids gives the corresponding oxirane by an electrophilic 1,1-addition mechanism as outlined in Eq. (5-31) [77, 86, 86a, 87, 516]. 

Anti hydroxylation of an alkene is readily achieved with peroxycarboxylic acids. - Acid-catalyzed ting opening of the initial product, an oxirane (epoxide), forms the monoester of a 1,2-diol, hydrolysis of which affords the parent diol. Alternative reagents which are often used for anti hydroxylation of alkenes are hydrogen peroxide with oxides of timgsten - or selenium, - and iodine-silver benzoate (Prdvost reaction). ... 

The most widely used and, presumably, the most chemoselective reagents for the epoxidation of nucleophilic C—C double bonds are the peroxycarboxylic acids (see Houben-Weyl, Vol. IV/ 1 a, p 184, Vol. Vl/3, p 385, Vol. E13/2, p 1258). Using chloroform as solvent, epoxidation rates are particularly high79. Reactive or acid/base sensitive epoxides can often be obtained with dimethyldioxirane (see Houben-Weyl, Vol. R13/2, p 1256 and references 15, 16, 87-90), peracid imides (see Houben-Weyl, Vol. IV/1 a, p 205, Vol. VI/3, p 401, Vol. E13/2, p 1276) (prepared in situ from nitriles and hydrogen peroxide), hydroperoxy acetals (see Houben-Weyl, Vol. El3/2, p 1253) or peroxycarbonic acid derivatives (see Houben-Weyl, Vol. IV/la, p 209 and references 17-19) as oxidants. For less reactive alkenes, potassium hydrogen persulfate is a readily available reagent for direct epoxidation20. 

It is noteworthy that selenium, arsenic and boron compounds are also effective catalysts for the selective epoxidation of alkenes by H2O2 (equations 34-36). It is generally thought that peroxyselenic and peroxyarsonic acids act as reactive intermediates in a way similar to that of peroxycarboxylic acids. Metaboric acid, HBO2, acts as both an epoxidation catalyst and a dehydrating agent. The resulting orthoboric acid can be dehydrated back to metaboric acid. ... 

Alkenes undergo reaction with peroxycarboxylic acids (RCO3H) to give three-membered-ring cyclic ethers called epoxides. For example, 4-octene reacts with a peroxyacid to yield 4,5-epoxyoctane ... 

Prilezhaev reaction Oxidation of alkenes to epoxides using peroxycarboxylic acids. 362... 

Epoxides are three-membered cyclic ethers. The simplest, ethylene oxide is prepared from ethylene and oxygen. Epoxides are prepared more generally from alkenes using a peroxycarboxylic acid. 

Selectivity in epoxidation reactions frequently depends on the reagent chosen and also on reaction conditions. The epoxidation of cholesterol using a range of peroxycarboxylic acids, including MMPP, gives a mixture in which the a-epoxide... 

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