Perepoxide intermediate

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. 

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). 

A great deal of work has been focused on whether the ene reaction proceeds through a concerted or a stepwise mechanism. The initially proposed synchronous pathway was challenged by a biradical , zwitterionic or a perepoxide intermediate. Kinetic isotope effects in the photooxygenation of tetrasubstituted, trisubstituted and cis-disubstituted alkenes supported the irreversible formation of an intermediate perepoxide,... 

The intermolecular kinetic isotope effect for the competition of 20 with its deuteriated analogue 22 in chloroform was negligible (Scheme 10, h/ d = 1-00 0.02). Like in other trisubstituted alkenes, this result was interpreted in terms of irreversible formation of a perepoxide intermediate. 

The cw effect , observed also in trisubstituted enol ethers , may as well be rationalized by a similar mechanism (Scheme 12). In this case, the electron-donating group that stabilizes the partially charged double bond carbon of the perepoxide in the syn transition state is the alkoxy moiety. Therefore, the favourable interactions of the singlet oxygen with the phenyl or alkoxy substituents direct the orientation of the perepoxide intermediate. 

In transition state TS2, leading to the minor perepoxide intermediate PE2 in a limiting step, the oxygen is placed syn to the ester group, and the net dipole moment is expected to be larger than that in transition state TSi where the oxygen has an anti orientation with respect to the ester group. TS2 is therefore more polar than TSi, and expected to be stabilized better by polar solvents than TS]. Consequently, the ratio 79a/79b decreases with increase of solvent polarity. 

To verify this mechanistic possibility, the solvent dependence of the ene products derived from the photooxygenation of the isomeric a,-unsaturated esters 93-E and 93-Z was examined (Scheme 23). For 93-E, the two ene products are formed from two different perepoxides. When the oxygen atom of the perepoxide intermediate is placed syn to the ester group, 93b is produced, whereas 93a is formed from the opposite case. For isomer 93-E, the expected solvent effect was found (93a/93b = 85/15 in CCI4 or benzene, and 70/30 in DMSO). On the other hand, for the isomer 93-Z, both products are formed from the same intermediate (the perepoxide oxygen is placed anti to the ester functionality), and no solvent dependence on the ene products was found (93a/93b = 95/5 in CCI4 and 93/7 in DMSO). 

PercarboxyUc acids, determination, 699 Perepoxide intermediate ene reaction, 832-3, 835, 845, 846 cndo-perepoxide intermediate, 859 nucleophiUc substimtion cychzation, 235-6 Peresters... 

The retention of olefin geometry in the oxidation of cis and trans alkenes408 suggests a one-step concerted [2 + 2]-cycloaddition process, suprafacial in the alkene, and antarafacial in singlet oxygen.363 The strong solvent dependence, however, observed in many cases, points to a stepwise mechanism involving a perepoxide intermediate 403,409... 

Today, the perepoxide intermediate proposed earlier by Sharp [35] seems to be the most popular mechanism that found support from results derived from kinetic isotope effects, stereochemical studies, trapping experiments [36-38], and theoretical calculations [39 43],... 

The perepoxide intermediate was established by using Stephenson s tetram-ethylethylene-isotope effect test [46,47], For example, substantial intramolecular isotope effects were found in reactions of 102 with cis-related methyl and deuteriomethyl groups in tetramethylethylenes and cis-2-butene, but only a small or negligible isotope effect was observed with trans groups (Scheme 2). 

Although no direct experimental evidence was reported for a perepoxide intermediate, episulfoxide [51], an analogous compound of perepoxide, has been prepared and rearranged at 25°C to the allylic sulfenic acid ... 

These results were completely consistent with a rate-determining attack on the double bond to give an intermediate in which the stereochemistry about the olefinic bond is retained. Such a pathway is shown in Scheme 4. In such an hypothesis, an equivalent attack on the upper and the lower olefin faces would be expected, leading to equal yields of the two intermediates. Subsequent decomposition of these perepoxide intermediates would give the observed stereoiso-... 

In Scheme 4, the perepoxide intermediate is used for illustrative purposes. An exciplex between the olefin and oxygen with similar stereochemical requirements with that of a perepoxide might also accommodate these results. 

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