Epoxides, stilbene-like

Like the trichloromethyl peroxide radical, peroxothio compounds can perform even nucleophilic oxygenation of substrates that are inert to Oj" in aprotic solvents. For example, stilbene is not changed in dry benzene containing 18-crown 6-ether and KOj. In the presence of diphenylsul-fide, however, the interaction takes place and results in the formation of stilbene epoxide. According to Oae et al. (1981), stilbene initially gives PhCH(00 )CH Ph anion-radical adduct. Abstraction of O from the adduct leads to stilbene epoxide with 40% yield (Oae et al. 1981). 

Acenaphthene oxide is known to be unstable under acidic conditions and is therefore isolated in low yields when prepared by common methods. The highly strained acenaphthene was smoothly epoxidized in quantitative yield at —30°C in CH3CN by using 51 . C -stilbene was epoxidized with 51 at —35 °C in CH3CN to a mixture of the trans-(70%) and c/3-epoxides (30%) (equation 79). This result shows that the peroxysulfonyl intermediate must be a radical like the acylperoxy radical ArC(0)00 ° and the phenyl nitroso oxide radical PhNOO . ... 

FIGURE 1.28 Likely mechanism of the isomerization in the Jacobsen-Katsuki epoxidation of c/s-stilbene. 

If the molecule is chiral, decide whether or not it is enantiomerically pure. You will often have to know more about the reaction than is evident in the scheme. For instance, fran.v-stilbene oxide 56 is always chiral but may or not be racemic. It is not evident in the scheme below and it could easily be either. It all depends how it was made. If only achiral starting materials and reagents were used, the epoxide 56 must be racemic. If asymmetry was present somewhere, it may be a single enantiomer. But had we put optically pure stilbene oxide 56 into the reaction then we could expect to get an optically pure product 57 at the end. Amino acid 54 is likely to be enantiomerically pure anyway (as are other compounds from the chiral pool) and if we were in any doubt it has been drawn as a single enantiomer. In other situations we need to know the context to be sure. 

This formation was linear as a function of time and cyclohexanol was only detected as a very minor product. Cyclooctane was similarly oxidized with the formation of cyclooctanone as a major product. Other iron(III)porphyrins involving different axial ligands, like Fe(TDCPP)Q and Fe(TPP)Cl, were much less efficient and less selective. Irradiation of Fe(TDCPP)OH in the presence of 02 and cis-stilbene failed to give any formation of stilbene epoxide, and cyclohexene was mainly oxidized on its allylic position with formation of cyclohex-2-enol and cyclohex-2-enone under such conditions. Finally, the intermediate formation of OH radicals was detected by spin trapping experiments during cyclohexane oxidation by 02 with photochemically-activated Fe(TDCPP)OH. 

Cis-stilbene is not epoxidized and alkenes containing allylic C-H bonds like cyclohexene are mainly oxidized at allylic positions (Fig.6). Therefore, it is... 

Triphenylsilylpotassium was intensively studied by Gilman before he made the silyllithium reagent, and he showed that it reacted with a variety of electrophilic substances in addition to carbonyl compounds and epoxides, such as silyl halides, pyridine, azo- and azoxybenzene, thiocyanates, and benzonitrile, as well as with far less electrophilic substances like stilbene, anthracene, diphenyl sulfide, and diphenyl sulfone. ... 

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