Cyclooctadienes anodic oxidation

When using a carbon electrode, the anodic oxidation of conjugated dienes (7) such as isoprene, pipe-rylene, cyclopentadiene and 1,3-cyclohexadiene in methanol or acetic acid mainly gives oxidative 1,4-addition products (8 equation 13). For example, 1,3-cyclohexadiene gives l,4-dimethoxycyclohex-2-ene (9) in 47% yield (equation 14). 1,3-Cyclooctadiene, in a similar experiment, yields a considerable amount of the allylically substituted product. 

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. 

Some typical results are shown in Table 2. The table shows that oxidation of conjugated dienes such as isoprene, piperylene (1,3-pentadiene), cyclopentadiene and 1,3-cyclohexadiene with a carbon anode in methanol or in acetic acid containing tetraethylammonium p-toluenesulfonate (EtjNOTs) as the supporting electrolyte yields mainly 1,4-addition products2. 1,3-Cyclooctadiene yields a considerable amount of the allylically substituted product. 

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See also in sourсe #XX -- [ , , ]

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