Iminoxyl radicals and stable products from the one-electron oxidation of 1-methylindole-3-carbaldehyde oximes

The radical intermediates and the stable products formed on one-electron oxidation of 1-methylindole-3-carbaldehyde oxime 2a were compared with those of 1-methylindole-3-carboxamidine oxime 4a in aqueous solution. The dibromide radical anion generated radiolytically by pulse radiolysis reacted with both 2a and 4a dropC=NOH to yield the radical cations [dropC=NOH](.+), which exist in prototropic equilibria with the neutral iminoxyl radicals [>C=NO](.) (pK(a) = 3.53 +/- 0.03 and 5.01 +/- 0.01 at ionic strength 0.05 M, respectively). This was confirmed by the observed primary salt-effect which accelerated the decay of the radical cations but not the iminoxyl radicals. Methylation of the N-hydroxyimino function in both 2a and 4a precluded deprotonation of the corresponding radical cations [dropC=NOCH3](.+). At low concentrations of 2a and high dose rates the 2a radicals [dropC=NO](.) decayed bimolecularly via unstable dimers to the aldehyde dropC=O, with higher concentrations and lower dose rates favouring the chain-catalysed isomerisation of the N-hydroxyimino moiety. Radicals from 4a decay bimolecularly to form unstable dimers which degrade to produce an amide,, nitrile and carboxylic acid. The observed differences in-the oxidation chemistry of 2a and 4a probably reflect the enhanced stabilisation of iminoxyl radicals through alpha -amino substitution.