Intramolecular Oxidative O-Demethylation of an Oxoferryl Porphyrin Complexed with a Per-O-methylated β-Cyclodextrin Dimer

The intramolecular oxidation of ROCH₃ to ROCH₂ OH, where the latter compound spontaneously decomposed to ROH and HCHO, was observed during the reaction of the supramolecular complex (met-hemoCD3) with cumene hydroperoxide in aqueous solution. Met-hemoCD3 is composed of meso-tetrakis(4-sulfonatophenyl)porphinatoiron(III) (Fe^III TPPS) and a per-O-methylated β-cyclodextrin dimer having an -OCH₂ PyCH₂ O- linker (Py=pyridine-3,5-diyl). The O=Fe^IV TPPS complex was formed by the reaction of met-hemoCD3 with cumene hydroperoxide, and isolated by gel-filtration chromatography. Although the isolated O=Fe^IV TPPS complex in the cyclodextrin cage was stable in aqueous solution at 25 °C, it was gradually converted to Fe^II TPPS (t_1/2 =7.6 h). This conversion was accompanied by oxidative O-demethylation of an OCH₃ group in the cyclodextrin dimer. The results indicated that hydrogen abstraction by O=Fe^IV TPPS from ROCH₃ yields HO-Fe^III TPPS and ROCH₂^. . This was followed by radical coupling to afford Fe^II TPPS and ROCH₂ OH. The hemiacetal (ROCH₂ OH) immediately decomposed to ROH and HCHO. This study revealed the ability of oxoferryl porphyrin to induce two-electron oxidation.