Olefin epoxidation with peracids

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

Bartlett [206] in 1960 suggested a mechanism of olefin epoxidation with peracids, based on nonionized molecules and containing a three-atom cycle in the transition complex ... 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

The direct epoxidation of olefins can be carried out with oxygen, per compounds, or alkaline hydrogen peroxide. In contrast to the above-mentioned processes, epoxidation with peracids occurs on the less sterically hindered side without Walden inversion. Direct epoxidation with oxygen... 

Olefin inversion via epoxides is reported by two groups of workers. One sequence involves epoxidation with peracid, which occurs with retention of stereochemistry, followed by deoxygenation with hexamethyldisilane and potassium methoxide in HMPT at 65 °C. The alternative procedure utilizes reaction of the episode with triphenylphosphine dihalides to give vicinal dihalides. Zinc reduction of the dihalides is specifically trans, and thus the sequence epoxidation-bromination-reduction gives overall inversion of olefin configuration. ... 

Epoxidations with peracids can exhibit high degree of chemoselectivity and these generally display preferences for reaction with more nucleophilic alkenes. This phenomenon is illustrated in Vandewalle s work directed towards the total synthesis of the sesquiterpene estafiatin (6, Equation 3) [52]. The final step included selective epoxidation of the trisubstituted olefin from its more accessible convex face (dr =97 3). 

The reactions of olefins with peracids to form epoxides allows for the selective oxidation of carbon-carbon double bonds in the presence of other functional groups which may be subject to oxidation (for example, hydroxyl groups). The epoxides that result are easily cleaved by strong acids to diols or half-esters of diols and are therefore useful intermediates in the synthesis of polyfunctional compounds. 

The reaction of allenes with peracids and other oxygen transfer reagents such as dimethyldioxirane (DM DO) or hydrogen peroxide proceeds via allene oxide intermediates (Scheme 17.17). The allene oxide moiety is a versatile functionality. It encompasses the structural features of an epoxide, an olefin and an enol ether. These reactive intermediates may then isomerize to cyclopropanones, react with nucleophiles to give functionalized ketones or participate in a second epoxidation reaction to give spirodioxides, which can react further with a nucleophile to give hydroxy ketones. 

It is of some interest to speculate briefly oonocming the nature of the peracid-imine reaction. It xb quite possible that this reaction is analogous to the epoxidation of olefins with peracids and involves a similar oydks transition state (X). An equally attractive if less obvious possibility is that the imine reaction proceeds through addition of the peracid to the azomethine followed by internal nucleophilic displace ment of the basic nitrogen atom on the peroxide bond. This reaction... 

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]. 

The rates of metal-catalyzed epoxidations are also influenced by the structure of the olefin and the structure of the hydroperoxide. The relative rates of epoxidation of a series of olefins using a mixture of r-Bu02H and Mo(CO)6 paralleled quite closely those for epoxidations with organic peracids.435... 

The peracid methods invariably open the epoxide with reversion of configuration, i.e. trans-diol formation. Aryl substituents, however, are converted to the cw-diols with retention of configuration.118-120 Olefins which have been hydroxylated by means of in situ percarboxylic acid techniques include cyclohexene (65-73%),121 dodecane (91 %)122 and oleic acid (99%).123 Chlorestrol has been frans-hydroxylated with performic acid in high yield (91%).124... 

Recent literature refers to the stereoselective and asymmetric epoxidation of allylic alcohols with organoaluminium peroxides. PhaSiOOH epoxidizes olefins with a stereoselectivity similar to that with peracid. Reports have been made of a-substituted hydroperoxides (acids, esters, ketones, amides, and nitriles) as effective epoxidizing reagents and the application of hexachloroacetone, tetrachloracetone, and hexafluoroacetone hydroperoxide, as well as the HaOa-Vilsmeier reagent system. ... 

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]. 

Very recently we have developed a new, easier, and selective metal-free NHPTcatalyzed aerobic epoxidation of primary olefins [19] based on the in situ generation of peracetic acid from acetaldehyde. In this chapter, we will discuss the reaction mechanism in order to explain the significant differences in selectivity with respect to the epoxidation by peracids and we will show preliminary successful results in the synthesis of propylene oxide. 

Another example where an enzyme is used to mediate a completely different reaction than it normally does is the lipase-catalyzed formation of peracids by reaction of H202 with a carboxylic acid [43]. As illustrated in Fig. 15 this allows for the one step epoxidation of olefins by in situ formation of RC03H. The carboxylic acid can be used in catalytic amounts providing an overall epoxidation with H202. By a suitable choice of carboxylic acid the reaction can be carried out in a two-phase system. The scope of such novel transformations must be enormous. 

The reaction of olefins with peracids involves firstly the formation of an epoxide, reaction (5.1) in acidic media the epoxide may suffer ring-opening to yield the mono-ester of a glycol, reaction (5.2). ... 

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