Cyclooctadienes radical cations

In this context, the striking difference of the standard 70 eV El mass spectra of the isomeric cyclooctadienes may be mentioned here (see below)87. Whereas the radical cations of the stereoisomeric 1,5-cyclooctadienes, containing two bis-allylic C—C bonds, give the products of the two-fold allyl cleavage as the base peak ([C4H6]+ /[M]+ s=ss 10 1), the cis,cis-l,4 and cis.cis-].3-cyclooctadicnc ions are reluctant to do so (IC4II511 / [M]+ 1 3). Clearly again, 1,2- and 1,3-H shifts cannot efficiently compete with dissociation of the bis-allylic C—C bond. 

Cation-radicals, stabilized in zeolites, are excellent one-electron oxidizers for alkenes. In this bimolecular reaction, only those oxidizable alkenes can give rise to cation-radicals, which are able to penetrate into the zeolite channels. From two dienes, 2,4-hexadiene and cyclooctadiene, only the linear one (with the cylindrical width of 0.44 nm) can reach the biphenyl cation-radical or encounter it in the channel (if the cation-radical migrates from its site toward the donor). The eight-membered ring is too large to penetrate into the Na-ZSM-5 channels. The cyclooctadiene can be confined if the cylindrical width is 0.61 nm, however the width of the channels in Na-ZSM-5 is only 0.55 nm. No cyclooctadiene reaction with the confined biphenyl cation-radical was detected despite the fact that, in solution, one-electron exchange between cyclooctadiene and (biphenyl) proceeds readily (Morkin et al. 2003). 

In route A, one electron is removed fiom cme double bond to generate a cation radical, and subsequent transannular reaction of the cation radical with the other double bond forms a new carbon-carbon tend. On the other hand, in route B, allylic substitution or oxidative addition at one double bond takes place without intramolecular interaction between the double bonds. As exemplified by the anodic oxidation of 4-vinylcyclohexene (11) in methanol (equation 16), such dienes as 4-vinylcyclohexene, limonene and 1,5-cyclooctadiene yield only products via route B. 

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