Indoles radical benzoyloxylation

A radical ipso substitution at the 3-position of 2,3-disubstituted indoles has also been reported in their reaction with benzoyl-r-butyl nitroxide leading to (227) or, with the 2-substituted indole, the dimer (228) (cf. Section 3.05.1.4) (81CC694). In contrast with the benzoyloxylation reactions the nitroxide radical initially abstracts the hydrogen atom at the 1-position to form the indolyl radical. This mechanism is supported by the failure of the corresponding 1 -methylindole to undergo an analogous oxidation. 

Radicals such as benzyl and hydroxyl are unselective in their interaction with indoles, resulting in mixtures of products, so such reactions are of little synthetic use. On the other hand, benzoyloxylation of N-substituted indoles gives benzoates of indoxyl, i.e. it effectively oxidises the indole heterocyclic ring, via p-attack by the strongly electrophilic benzoyloxy-radical. In contrast, the weakly electrophilic radical derived from malonate reacts selectively at C-2, via an atom-transfer mechanism." "... 

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