Search

Reaction of pale-green high spin mer-tris(picolinato)iron(III) with t-butylhydroperoxide in pyridine gives rise to an EPR signal for the t-butylperoxyl radical via a short lived purple intermediate which itself decays eventually to a yellow-brown high spin iron(III) product. The kinetics of the corresponding reaction with hydrogen peroxide have been studied in regard to the dependence of the hydrogen peroxide and picolinic acid concentration on the rate of the initial stages. The results support two rate-determining initial steps (minutes) involving the formation of a short lived purple high spin hydroperoxo- or t-butylperoxoiron(III) intermediate (λmax = 530 nm, ε ~ 1,000 dm³ mol-1 cm-1 for the hydroperoxo species) via pre-equilibrium loss of one picolinic acid which then undergoes homolytic Fe-O bond cleavage to give iron(II) and hydroperoxyl (t-butylperoxyl) radical resulting in eventual formation of deep yellow-brown solution which undergoes further complex UV-visible changes over a period of several hours. During this latter timescale these solutions are able to carry out Gif-type catalytic oxygenation of cyclohexane to cyclohexanone in the presence of H2O2 (or O2/pyH+/Zn powder) a process which however is completely inhibited in the presence of small amounts of dimethylsulfoxide, an efficient scavenger of the hydroxyl radical. Bis(picolinato)copper(II) was found to be a poor oxygenation catalyst, a finding consistent with its inability to generate the hydroxyl radical via Cu(I) under the same conditions. These results confirm that generation of hydroxyl radicals (via reaction of H2O2 with iron(II) or any other suitably reactive lower valent state) is central to the oxygenation chemistry carried out by these solutions. [ABSTRACT FROM AUTHOR]

Abstract: Catalytic oxidation of cyclohexene (organic layer) at 25°C in a stirred two phase system with fresh [Fe(OH2)6](ClO4)3/H2O2 in the pH range 2.0–4.0 (I=0.1M, NaClO4/HClO4) is complete within a 2-minute sampling time before deactivating in contrast to previous reports in the literature. Cyclohexene epoxide is a significant product alongside the more dominant allylic oxidation products; 2-cyclohexen-1-ol and 2-cyclohexen-1-one. Both epoxide and the accompanying allylic products appear over the same time period suggesting that they derive from the same catalytic species. The pH dependence of oxidation product yield (optimizing at ~pH 3) suggests that the active catalyst derives from reactions involving [Fe(OH2)5OH]2+ (pKa 2.54) and is most likely the hydroperoxo ion [Fe(OH2)5OOH]2+. Yields of cyclohexene epoxide are independent of the presence of dioxygen suggesting that an activated form of H2O2 ([Fe(OH2)5OOH]2+) is responsible. Reduction in the relative yield of the two allylic oxidation products in the absence of dioxygen suggests that they derive from 2-cyclohexen-1-peroxy radicals resulting from iron(II)-promoted Fenton chemistry following homolytic Fe O or O O cleavage on [Fe(OH2)5OOH]2+. A mechanism for the epoxide formation is proposed involving H3O+-assisted heterolytic O O cleavage on [Fe(OH2)5OOH]2+ accompanying O atom transfer to the cyclohexene double bond. Accompanying catalysis of H2O2 decomposition persists over several hours. For the first time in a Fenton-like system evidence has been obtained for different timescales for the catalysis of oxidation reactions versus catalysis of H2O2 decomposition. [Copyright &y& Elsevier]