Perepoxides

Oxygen donors like peroxy acids, ozone, and pyridine IV-oxides cause carbon-carbon cleavage, perhaps by formation of a perepoxide (43 Scheme 30) (81JCS(P1)1871). Other oxidants have also been reported to react with oxiranes (64HC( 19-1)228). 

The interaction between the HOMO of alkenes and the LUMO of singlet oxygen O (Scheme 4) is the most favored in the perepoxide structure (Scheme 13). This suggests... 

After 28 years the perepoxide quasi-intermediate was supported by a two-step no intermediate mechanism [71, 72]. The minimum energy path on the potential energy surface of the reaction between singlet molecular oxygen ( A and dg-teramethylethylene reaches a vaUey-ridge inflection point and then bifurcates leading to the two final products [73]. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

A perepoxide intermediate [77] or a peroxy diradical intermediate [78-81] have been proposed. 

Cycloaddition reactions can occur with retention of configuration in the pseudoexcitation band (Sect 1.1) whereas [2jt H-2jtJ reactions are symmetry-forbidden in the delocalization band. Experimental evidence is available for the stereospecific [2-1-2] cycloaddition reactions between A and olefins with retention of configuration (Scheme 14) [82]. A perepoxide intermediate was reported to be trapped in the epoxide form [83] in the reaction of adamantylideneadamantane with singlet oxygen affording dioxetane derivatives [84]. 

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

The geometrical structure of the perepoxide quasi-intermediate was suggested to play critical roles in determining diverse selectivities of the reactions of O with substituted olefins [92]. ... 

Hydrogen bonding to the pendant (tailing) oxygen (Scheme 21) in the perepoxide quasi-intermediates controls the facial/diastereoselectivty of the ene reactions of... 

Scheme 20 HOMO-LUMO interaction in the perepoxide quasi-intermediate for the cis-effect and the regioselectivity (percent) of the hydrogen abstractions...

The reactivity order of alkenes is that expected for attack by an electrophilic reagent. Reactivity increases with the number of alkyl substituents.163 Terminal alkenes are relatively inert. The reaction has a low AHl and relative reactivity is dominated by entropic factors.164 Steric effects govern the direction of approach of the oxygen, so the hydroperoxy group is usually introduced on the less hindered face of the double bond. A key mechanistic issue in singlet oxygen oxidations is whether it is a concerted process or involves an intermediate formulated as a pcrcpoxide. Most of the available evidence points to the perepoxide mechanism.165... 

Fig. 12.13. Perepoxide transition structures from Z-2-butene and 2,3-dimethyl-2-butene. Reproduced from J. Am. Chem. Soc., 125, 1319 (2003), by permission of the American Chemical Society.

Fig. 12 Sodium complexed perepoxides III and IY and isotope effect negating zwitterion V as an intrazeolite intermediate.

A striking feature of the intrazeolite singlet oxygen ene reaction is the rate enhancement often observed in these reactions.57,67 This rate enhancement is nicely accounted for by stabilization of the incipient perepoxide as depicted by Model C in Fig. 12. This rate enhancement can be used to promote the ene reaction at the expense of other reaction modes.68 An interesting example of this was reported by Stratakis and Rabalakos.69 Photooxygenations of alkenylarenes, 11 and 12 (Fig. 15) are dominated by [2 + 2] and [4 + 2] reactions in solution but react predominately by the ene mode in intrazeolite reactions. 

The reaction of cis- and frans-stilbene oxides with phenylphosphonothioic dichloride in the presence of magnesium gives cis- and fra/ts-stilbene and (7).13 Phenylphosphinidene sulphide is postulated as being an intermediate. The zwitterion (8) bears a remarkable similarity to the controversial perepoxides which are thought to be intermediates in the reaction of singlet oxygen with alkenes. 

In 1999, Clennan and Sram reported a study of the photo-oxidations of a series of tetrasubstituted alkenes (Fig. 5) in methylene blue-doped zeolite Y [11], The ene regiochemistries are very sensitive to the size of the allylic substituent, R, in solution. The A/B ratio increases from 0.49 to 2.4 as the substituent, R, is changed from methyl to ferr-butyl. This phenomenon has been attributed [12] to a sterically induced lengthening of the carbon-2 oxygen bond in the perepoxide intermediate I and subsequent preferred opening of this long bond (Fig. 5). 

The model in Fig. 6, specifically the intrazeolite presence of a perepoxide, is supported by an isotope effect of 1.04 0.02 for the photo-oxidation of Z-2,3-dimethyl-l,l,l,4,4,4-hexadeutero-2-butene (Fig. 7). This isotope effect is completely inconsistent with an open zwitterion (Fig. 7), which would be expected to collapse to the hydroperoxide with a significant discrimination for hydrogen abstraction (/ch/ d > ) ... 

Figure 8 Products from diastereomeric perepoxides formed in intrazeolite photooxygenation of 2.

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

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