Evidence for the Formation of an Iron(III)-Indolenylperoxo Intermediate During Indole Dioxygenation: Similarities with Heme Dioxygenases

Abstract

Evidence for an iron(III)-indolenylperoxo (OOIn–) complex, [FeIII(OOIn)(TPP)(OCH3)]− (TPP = 5,10,15,20-tetraphenylporphyrinato2–) trapped at −78 °C, was obtained. The formation of this species was initiated by reaction of either FeIII(TPP)(Cl) in the presence of excess O2, OCH3– as axial ligand and base, and 3-methylindole, or from reaction of preformed [FeIII(O2•–)(TPP)(OCH3)]− with 3-methylindole under the same basic conditions. The new species was characterized by UV–vis, X-band EPR, and Mössbauer spectroscopies. Interestingly, the EPR spectra of [FeIII(OOIn)(TPP)(OCH3)]− exhibited two overlapping low-spin (S = 1/2) rhombic signals (A: g = [2.31, 2.20, 1.93]; B: g = [2.24, 2.15, 1.96]), whose ratios are sensitive to the amount of MeOH in the solvent mixture MeOH/toluene. The two sets of EPR signals (A and B) were assigned as arising from the presence or absence of hydrogen bonding between the OCH3– axial ligand and the solvent MeOH. This assignment was supported by analysis of ligand field splitting parameters derived from the EPR data. Addition of 1-methylimidazole (1-MeIm) to [FeIII(OOIn)(TPP)(OCH3)]− at −78 °C leads to conversion of the two overlapping rhombic EPR signals into a new, single S = 1/2 rhombic spectrum with g = [2.31, 2.18, 1.94] and a sharper Mössbauer spectrum. These data are consistent with substitution of OCH3– axial ligand by 1-MeIm to give FeIII(OOIn)(TPP)(1-MeIm). Thermal decay of the iron(III)-indolenylperoxo complex gives the ring-opened, dioxygenated product N-(2-acetylphenyl)formamide (75% by GC-MS). The capability of [FeIII(OOIn)(TPP)(OCH3)]− to convert into dioxygenated product indicates that iron indolenylperoxide is a possible intermediate of the indole dioxygenation process. This work has implications for the proposed mechanisms of tryptophan/indoleamine 2,3-dioxygenase (TDO/IDO) and demonstrates a feasible pathway for dioxygenation mediated by a single iron porphyrin complex.