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

Abstract

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 CNOH to yield the radical cations [CNOH]˙⁺, which exist in prototropic equilibria with the neutral iminoxyl radicals [CNO]˙ (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 [CNOCH₃]˙⁺. At low concentrations of 2a and high dose rates the 2a radicals [CNO]˙ decayed bimolecularly via unstable dimers to the aldehyde CO, 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 α-amino substitution.