Oxygenative Aromatic Ring Cleavage of 2‑Aminophenol with Dioxygen Catalyzed by a Nonheme Iron Complex: Catalytic Functional Model of 2‑Aminophenol Dioxygenases

2015-02-16T00:00:00Z (GMT) by Sayanti Chatterjee Tapan Kanti Paine
2-Aminophenol dioxygenases catalyze the oxidative ring cleavage of 2-aminophenol to 2-picolinic acid using O<sub>2</sub> as the oxidant. Inspired by the reaction catalyzed by these nonheme iron enzymes, a biomimetic iron­(III)-2-amidophenolate complex, [(<i>t</i>Bu-L<sup>Me</sup>)­Fe<sup>III</sup>(4,6-di-<i>t</i>Bu-AP)]­(ClO<sub>4</sub>) (<b>1a</b>) of a facial tridentate ligand (<i>t</i>Bu-L<sup>Me</sup> = 1-[bis­(6-methyl-pyridin-2-yl)-methyl]-3-<i>tert</i>-butyl-urea and 4,6-di-<i>t</i>Bu-H<sub>2</sub>AP = 2-amino-4,6-di-<i>tert</i>-butylphenol) bearing a urea group have been isolated. The complex reacts with O<sub>2</sub> to cleave the C–C bond of 4,6-di-<i>t</i>Bu-AP regioselectively and catalytically to afford 4,6-di-<i>tert</i>-butyl-2-picolinic acid. An iron­(II)-chloro complex [(<i>t</i>Bu-L<sup>Me</sup>)­Fe<sup>II</sup>Cl<sub>2</sub>(MeOH)] (<b>1</b>) of the same ligand also cleaves the aromatic ring of 4,6-di-<i>t</i>Bu-AP catalytically in the reaction with O<sub>2</sub>. To assess the effect of urea group on the ring cleavage reaction of 2-aminophenol, two iron complexes, [(BA-L<sup>Me</sup>)<sub>2</sub>Fe<sup>II</sup><sub>2</sub>Cl<sub>4</sub>] (<b>2</b>) and [(BA-L<sup>Me</sup>)­Fe<sup>III</sup>(4,6-di-<i>t</i>Bu-AP)]­(ClO<sub>4</sub>) (<b>2a</b>), of a tridentate ligand devoid of urea group (BA-L<sup>Me</sup> = benzyl-[bis­(6-methyl-pyridin-2-yl)-methyl]-amine) have been isolated and characterized. Although the iron complexes (<b>1</b> and <b>1a</b>) of the ligand with urea group display catalytic reaction, the iron complexes (<b>2</b> and <b>2a</b>) of the ligand without urea group do not exhibit catalytic aromatic ring fission reactivity. The results support the role of urea group in directing the catalytic reactivity exhibited by <b>1</b> and <b>1a</b>.