Epoxides using peracids

Oxidation of primary N-aminobenzimidazole 32 with PhI(OAc)2 4 in the presence of olefins gives aziridines 34 [54]. Similar oxidations are effected by lead tetraacetate. The reaction was initially proposed to involve the intermediacy of AT-nitrene as a reactive species, thought to be produced through reductive a-elimination of amino-A3-iodane 33. Recent mechanistic studies on lead tetraacetate oxidation, however, suggests that the acetoxyamine 35 instead of AT-nitrene is the aziridinating species, and the reaction proceeds through a transition state 36 similar to that of epoxidation using peracids [55]. 

Cycloaddition reactions of ketenes to imines derived from glycosylamine and aldehydes provide access to P-lactams, although the level of selectivity observed with these N-linked auxiliaries is modest. The alkenyl moiety of glycoside 212 undergoes diastereoselective epoxidation using peracid in 80 %d.e. Simmons-Smith cyclopropanation of 212 was reported earlier (Vol. 25, p.335). 

FIGURE 11.48 Mechanism of alkene epoxidation using peracid. 

What if we wanted to add two OH groups to an alkene to give a 1,2-diol We have already seen a method for the synthesis of frara-diols. First, we convert the alkene to an epoxide using peracid then we do an 5 2 opening of the epoxide with hydroxyl ion (Figure 11.67). 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

On a laboratory scale, one particular method of producing in situ peracids for epoxidation of a wide range of substrates under mild conditions is via the use of urea hydrogen peroxide (UHP) in the presence of organic anhydrides.27 The anhydride must be added slowly to the UHP, solvent and substrate to generate the peracid. If the UHP is added to the substrate, solvent and anhydride, the unstable and potentially explosive diacyl peroxides can be formed. Table 3.2 illustrates a number of substrates which have been epoxidized using this reagent system. 

The approaches to anhydrous or essentially anhydrous solutions include reaction of the acid anhydride with hydrogen peroxide in the presence of a solvent,44 oxidation of the analogous aldehyde45 and azeotropic removal of water during peracid formation.46 By far the easiest and safest method is to simply extract the equilibrium peracid into an appropriate solvent.47 The solvents usually employed are ethyl acetate or isopropyl acetate. A range of substrates has been epoxidized using such extracted peracids (Figure 3.12). 

The —N=C— bond in imines, especially Schiff bases formed from aromatic aldehydes and amines, can be epoxidized by peracids to form oxaziridines [N(0)C].326 Unlike epoxides, oxaziridines will oxygen-transfer.327 Chiral oxaziridines have been used to carry out enantioselective epoxidations,328 although these compounds are often prepared by non-peroxygen routes.329 Oxaziridines can also be rearranged to oximes or nitrones (Figure 3.85).330... 

Barton oxidation was the key to form the 1,2-diketone 341 in surprisingly high yield, in order to close the five-membered ring (Scheme 38). The conditions chosen for the deprotection of the aldehyde, mercuric oxide and boron trifluoride etherate, at room temperature, immediately led to aldol 342. After protection of the newly formed secondary alcohol as a benzoate, the diketone was fragmented quantitatively with excess sodium hypochlorite. Cyclization of the generated diacid 343 to the desired dilactone 344 proved very difficult. After a variety of methods failed, the use of lead tetraacetate (203), precedented by work performed within the stmcmre determination of picrotoxinin (1), was spectacularly successful (204). In 99% yield, the simultaneous formation of both lactones was achieved. EIcb reaction with an excess of tertiary amine removed the benzoate of 344 and the double bond formed was epoxidized with peracid affording p-oxirane 104 stereoselectively. Treatment of... 

In a manner exactly analogous to the a-hydroxylation of ketone silyl enol ethers (Sections 2.3.2.1.3.i and 2.3.2.2.3.i) the corresponding ester silyl ketene acetals may be epoxidized by peracid and subsequently cleaved with fluoride to reveal the a-hydroxy ester. Yields are good if hexanes are employed as solvent, while competing hydrolysis hampers the process in other media. The equivalent lactone hydroxylations are, however, not possible since hydrolysis is the dominant process even in hexane. This solvent limitation may prove restrictive to the widespread use of this technique. 

Less regularly used reagents are rerf-butyl hydroperoxide/terr-butyliithium,600,601 ozone.602 dioxiranes,603,604 fluorine/ water/acetonitrile,605 or A, A -diethylhydroxylaminc.606 Alkenes carrying a donor substituent can also be epoxidized with peracids.607-609 Fluorinated allylic alcohols give, under Sharpless conditions, epoxides in good yield and enantiosclectivity.610... 

Recent studies have attempted to improve the efficiency of epoxidation under milder conditions that minimize the formation of byproducts. Chemo-enzymatic epoxidation uses the immobilized lipase from Candida antartica (Novozym 435) (56) to catalyze conversion of fatty acids to peracids with 60% hydrogen peroxide. The fatty acid is then self-epoxidized in an intermolecular reaction. The lipase is remarkably stable under the reaction conditions and can be recovered and reused 15 times without loss of activity. Competitive lipolysis of triacylglycerols is inhibited by small amounts of fatty acid, allowing the reaction to be carried out on intact oils (57). Rapeseed oil with 5% of rapeseed fatty acids was converted to epoxidized rapeseed oil in 91% yield with no hydroxy byproducts. Linseed oil was epoxidized in 80% yield. Methyl esters are also epoxidized without hydrolysis under these conditions. 

The most widely accepted method for epoxidation of alkenes remains oxidation with organic peracids. The early work (up to 1970) in this field shows that a large number of dienes and polyenes were oxidized in this manner . The most commonly used peracids are peracetic, monoperphthalic and perbenzoic acids which are most dominant in industrial applications. On the other hand, in laboratory procedures m-chloroperbenzoic acid, MCPBA, is often used, with trifluoroperacetic acid cited in more difficult transformations. Recently, the transportation of m-chloroperbenzoic acid has been restricted and the use of other peroxygen agents has been gaining acceptance as a general alternative. Among the substrate types epoxidized it would be especially worthy to point out polyunsaturated... 

Hydrocarboxylation of the Ce-Cs a-olefins with cobaltcarbonyl/pyridine catalysts at 200 °C and 20 MPa gives predominantly the linear carboxylic acids. The acids and their esters are used as additives for lubricants. The Ce-Cio a-olefins are hydroformylated to odd-numbered linear primary alcohols, which are converted to polyvinylchloride (PVC) plasticizers with phthalic anhydride. Oligomerization of (preferably) 1 -decene, applying BF3 catalysts, gives oligomers used as synthetic lubricants known as poly-a-olefins (PAO) or synthetic hydrocarbons (SHC) [11, 12]. The C10-C12 a-olefins can be epoxidized by peracids this opens up a route to bifunctional derivatives or ethoxylates as nonionic surfactants [13]. 

Recent developments indicate that new processes and raw materials will soon provide new types of epoxy resins. The most significant development is the epoxidation method using peracids as means of epoxidation. Per-acids are produced commercially either by the interaction of acids with hydrogen peroxide or by the oxidation of aldehydes. Peracids will epox-idize double bonds under controlled conditions, preferably in absence of water and free acids. It is anticipated that polyepoxides based on these processes will soon be available. 

Epoxidation. The peracid is useful for epoxidation of olefinic double bonds. This epoxidation differs from epoxidation with /-butyl hydroperoxide-transition metals in that a double bond furthest removed from the hydroxyl group is more readily attacked than a double bond in the allylic position. 

Epoxidation is carried out by four different procedures (i) epoxidation with peracids such as peracetic acid or perbenzoic acid in the presence of an acid catalyst (ii) epoxidation with organic and inorganic peroxides, including transition metal catalysts (iii) epoxidation with halohydrins using hypoha-lous acids and their salts and (iv) epoxidation with molecular oxygen. 

Olefins, including medium-ring cyclic olefins, have been epoxidized using a polymer supported peracid, thus cis-cyclo-octene was epoxidized in 95% yield. Agerol (293) has been converted into ageratriol (294) by two routes, epoxidation and base promoted isomerization, and photosensitized oxidation followed by sodium boro-hydride reduction. ... 

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