Observation of oxygenated intermediates in functional mimics of aminophenol dioxygenase.

Aminophenol dioxygenases (APDO) are mononuclear nonheme iron enzymes that utilize dioxygen (O) to catalyze the conversion of o-aminophenols to 2-picolinic acid derivatives in metabolic pathways. This study describes the synthesis and O reactivity of two synthetic models of substrate-bound APDO: [Fe(Tp)(APH)] (1) and [Fe(Tp)(APH)] (2), where Tp = hydrotris(3,5-dimethylpyrazole-1-yl)borate, APH = 4,6-di-tert-butyl-2-aminophenolate, and APH = 4-tert-butyl-2-aminophenolate. Both Fe(II) complexes behave as functional APDO mimics, as exposure to O results in oxidative CC bond cleavage of the o-aminophenolate ligand. The ring-cleaved products undergo spontaneous cyclization to give substituted 2-picolinic acids, as verified by H NMR spectroscopy, mass spectrometry, and X-ray crystallography. Reaction of the APDO models with O at low temperature reveals multiple intermediates, which were probed with UV-vis absorption, electron paramagnetic resonance (EPR), Mössbauer (MB), and resonance Raman (rRaman) spectroscopies. The most stable intermediate at -70 °C in THF exhibits multiple isotopically-sensitive features in rRaman samples prepared with O and O, confirming incorporation of O-derived atom(s) into its molecular structure. Insights into the geometric structures, electronic properties, and spectroscopic features of the observed intermediates were obtained from density functional theory (DFT) calculations. Although functional APDO models have been previously reported, this is the first time that an oxygenated ligand-based radical has been detected and spectroscopically characterized in the ring-cleaving mechanism of a relevant synthetic system.