Analysis of Active Species in Catalytic Hydroxylation of Benzene with Hydrogen Peroxide in the Presence of Fe(III)Complex of β-Cyclodextrin-Protocatechuate by Electron Spin Resonance Spectroscopy
Hidetake SAKURABA*, Sachiko IRIUCHIJIMA, Toshiki TAKEZUTSUMI and Masaaki HORII
Department of Industrial Chemistry, Kanto Gakuin University; 4834 Matsuura, Kanazawa-ku, Yokohama-shi 236-8501 Japan
The Electron Spin Resonance (ESR) spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap was used to detect oxygen radicals as an active species in the hydroxylation of benzene with hydrogen peroxide catalyzed by the protocatechuate-Fe(III)complex. The electronic property of the substituted group of the catechol derivatives affected the signal intensities of DMPO-OH radical (hydroxyl radical trapped) in the pH range of 1-6, and it showed two dependent patterns on pH. The lowest signal intensity of DMPO-OH was shown at pH of 2.5 where the hydroxylation was most active in the presence of the Fe complex of β-cyclodextrin-protocatechuate (β-CD-6-PA). The signal strength, however, reached its maximum at the pH of 1 where the reaction was almost inactive. These changes in signal intensities of DMPO-OH in the pH range of 1-6 revealed that the OH radical is not a main active species in this hydroxylation system. The active species of the hydroxylation through the non-hydroxyl radical process would be directly formed in the ternary complex from hydrogen peroxide, β-CD-6-PA and iron ion at pH of 2.5. It will be similar to a resonance hybrid of an o-quinone-iron oxide radical proposed for the mechanism of the Hamilton reaction.
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