Fe(TPA)-catalyzed alkane hydroxylation. Metal-based oxidation vs radical chain autoxidation

Catalytic alkane functionalization by the Fe(TPA)/(t)BuOOH system (with [Fe(TPA)Cl₂]⁺ (1), [Fe(TPA)-Br₂]⁺ (2), and [Fe₂O(TPA)₂(H₂O)₂]⁴⁺ (3) as catalysts; TPA = tris(2-pyridylmethyl)amine) has been investigated in further detail to clarify whether the reaction mechanism involves a metal- based oxidation or a radical chain autoxidation. These two mechanisms can be distinguished by the nature of the products formed, their dependence on O₂ (determined from argon purge and ¹⁸O₂ labeling experiments), and the kinetic isotope effects associated with the products. The metal-based oxidation mechanism is analogous to heme-catalyzed hydroxylations and would be expected to produce mostly alcohol with a large kinetic isotope effect. The radical chain autoxidation mechanism entails the trapping of substrate alkyl radicals by O₂ to afford alkylperoxy radicals that decompose to alcohol and ketone products in a ratio 1:1 or smaller via Russell termination steps. Consistent with the latter mechanism, alcohol and ketone products were observed in a ratio of 1:1 or less, when catalysts 1, 2, or 3 were reacted with alkane and 150 equiv of (t)BuOOH; these product yields were diminished by argon purging, demonstrating the participation of O₂ in the reaction. However, when the 3-catalyzed oxidation was carried out in the presence of a limited (<20 equiv) amount of (t)BuOOH or CmOOH, the sole product observed was alcohol; k(H)/k(D) values of 10 were observed, consistent with a metal- based oxidation. To reconcile these apparently conflicting results, a mechanistic scheme is proposed involving the formation of an alkylperoxyiron(III) intermediate which can oxidize either the substrate (metal-based oxidation) or excess ROOH (to generate alkylperoxy radicals that initiate a radical chain autoxidation process), the relative importance of the two mechanisms being determined by the concentration of ROOH.