Electro-organic reactions. Part IX. Mechanism of the Kolbe reaction; stereochemistry of reaction of anodically generated cyclohexyl radicals and cations
The products of anodic oxidation, under comparable conditions in methanol, have been characterised and identified for a series of 4-substituted cyclohexanecarboxylates (substituents cis- and trans-t-butyl and trans-phenyl) and cis-2-phenylcyclohexanecarboxylate. The stereoisomeric bicyclohexyls (one-electron oxidation) from the 4-cis-and -trans-t-butyl- and -trans-phenyl compounds, and cis- and trans-1-methyl-4-t-butylcyclohexane (from cross coupling with acetate) are formed in the statistically expected proportions. This is compelling evidence that cyclohexyl radicals are not strongly adsorbed at the anode. The stereoisomeric dimers from cis-2-phenylcyclo-hexylcarboxylate are formed in non-random proportions, which supports the view that substitution by aromatic species encourages adsorption. Further oxidation to cyclohexyl cations is also encouraged by 2- and 4-phenyl substitution, and comparison with the results of solvolysis experiments shows that this is also a surface effect.