Formation of hydroxyl spin adducts via nucleophilic addition-oxidation to 5,5-dimethyl-1-pyrroline N-oxide (DMPO)

Abstract

The formation of the hydroxyl radical spin adduct of DMPO, HO-DMPO., has been studied by EPR spectroscopy under conditions where the hydroxyl radical cannot be involved. One method was the photosensitized oxidation of DMPO to its radical cation, followed by reaction of the latter with water. This procedure worked well with sensitizers Sens (quinones) having triplet states with E°(TSens*/Sens.-) > 1.7-1.8 V (SCE). The second method employed nucleophilic addition of water to DMPO, followed by reaction of the intermediate hydroxylamine by mild thermal oxidants (quinones, FeIII) with redox potentials in the range -0.5 to 0.5 V. The observation of HO-DMPO. in this reaction was dependent on the acidity of the medium, since the relatively slow rate of formation of HO-DMPO. must be compensated for by a correspondingly longer half-life, τ1/2. This is decreased by increasing acidity, and a lower limit of τ1/2 was estimated at 1-1.5 min. The use of buffered neutral or slightly alkaline solutions or ethyl acetate as solvent satisfied this requirement. The nucleophilic mechanism, sometimes denoted the Forrester-Hepburn mechanism, was also applicable to the reaction between normal alcohols and DMPO, giving rise to RO-DMPO.. A sterically demanding alcohol (t-BuOH) or electrophilic alcohols (2,2,2-trifluoroethanol or 1,1,1,3,3,3-hexafluoropropan2-ol) required photochemical activation for reaction to occur.