Carbonium ion stabilisation

Electron donating a-substituents favour the non-Kolbe reaction but the radical intermediates in these anodic processes can be trapped during co-electrolysis with an alkanoic acid. Anodic decarboxylation of sugar uronic acids leads to formation of the radical which is very rapidly oxidised to a carbonium ion, stabilised by the adjacent ether group. However, in the presence of a tenfold excess of an alkanoic acid, the radical intermediate is trapped as the unsymmetrical coupling product [101]. Highly functionalised nucleotide derivatives such as 20 will couple successfully in the mixed Kolbe reaction [102], Other examples include the co-electrolysis of 3-oxa-alkanoic acids with an alkanoic acid [103] and the formation of 3-alkylindoles from indole-3-propanoic acid [104], Anodic oxidation of indole-3-propanoic acid alone gives no Kolbe dimer [105],... 

Non-Kolbe reactions are often favoured by skeletal reaiTangements which generate a more stable carbonium ion. Reaction of the cyclic ketal 22 is driven by formation of a carbonium ion stabilised by the oxygen substituent [114]. Reactions of nor-bomanecarboxylic acids are driven by the norbomane carbonium ion rearrangement [115, 116], Oxidation of adamant-1-ylacetic acid in methanol affords 1-methoxyhomoadamantane via a skeletal rearrangement [117],... 

The alkoxy ir -allylic derivatives underwent alkoxyl exchange very readily when treated with the appropriate alcohol containing a little mineral acid (10" M)(e.g. methoxy-ethoxy and vice versa). These reactions are believed to go via an intermediate carbonium ion stabilised by the delocalised electron system of the tt-allylic group. Methoxy tt-allylic complexes derived from 2,5-dimethylhexa-2,4-diene, cyclohepta-1,3-diene and cycloocta-1,3-diene on heating lost methanol irreversibly to give a8-unsaturated tr-allylic complexes e.g. cyclohepta-1,3-diene first gave a tt-methoxycycloheptenyl and then a TT -cycloheptadienyl complex. 

ApSimon, j. W., and J. M. Rosenfield Methyl Migration by Epoxide Cleavage. The Eflfect of Carbonium Ion Stabilisation by a Neighbouring Double Bond on the Direction of Migration on Cleavage of 9p,l lp-Epoxy-4,4-dimethylandrost-5-ene-3,17-dione. Chem. Commun. 1970, 1271. 

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See also in sourсe #XX -- [ , , ]

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