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

2-Aminophenol dioxygenases catalyze
the oxidative ring cleavage
of 2-aminophenol to 2-picolinic acid using O2 as the oxidant.
Inspired by the reaction catalyzed by these nonheme iron enzymes,
a biomimetic iron­(III)-2-amidophenolate complex, [(tBu-LMe)­FeIII(4,6-di-tBu-AP)]­(ClO4) (1a) of a facial tridentate ligand (tBu-LMe = 1-[bis­(6-methyl-pyridin-2-yl)-methyl]-3-tert-butyl-urea and 4,6-di-tBu-H2AP = 2-amino-4,6-di-tert-butylphenol) bearing a
urea group have been isolated. The complex reacts with O2 to cleave the C–C bond of 4,6-di-tBu-AP
regioselectively and catalytically to afford 4,6-di-tert-butyl-2-picolinic acid. An iron­(II)-chloro complex [(tBu-LMe)­FeIICl2(MeOH)] (1) of the same ligand also cleaves the aromatic ring of 4,6-di-tBu-AP catalytically in the reaction with O2.
To assess the effect of urea group on the ring cleavage reaction of
2-aminophenol, two iron complexes, [(BA-LMe)2FeII2Cl4] (2) and [(BA-LMe)­FeIII(4,6-di-tBu-AP)]­(ClO4) (2a), of a tridentate ligand devoid of urea
group (BA-LMe = benzyl-[bis­(6-methyl-pyridin-2-yl)-methyl]-amine)
have been isolated and characterized. Although the iron complexes
(1 and 1a) of the ligand with urea group
display catalytic reaction, the iron complexes (2 and 2a) 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 1 and 1a.