Analysis of active species in catalytic hydroxylation of benzene with hydrogen peroxide in the presence of Fe (III) complex of beta-cyclodextrin-protocatechuate by electron spin resonance spectroscopy

The Electron Spin Resonance (ESR) spin-trapping technique using 5,5-dimethy l-1-pyrroline N-oxide (DMPO) as a spin trap was used to detect oxygen radic als as an active species in the hydroxylation of benzene with hydrogen pero xide catalyzed by the protocatechuate-Fe(III)complex. The electronic proper ty 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 inten sity of DMPO-OH was shown at pH of 2.5 where the hydroxylation was most act ive in the presence of the Fe complex of beta -cyclodextrin-protocatechuate (beta -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 int ensities 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 dire ctly formed in the ternary complex from hydrogen peroxide, beta -CD-6-PA an d iron ion at pH of 2.5. It will be similar to a resonance hybrid of an o-q uinone-iron oxide radical proposed for the mechanism of the Hamilton reacti on.