Fe(TPA)-Catalyzed Alkane Hydroxylation. Metal-Based Oxidation vs Radical Chain Autoxidation

Abstract

Catalytic alkane functionalization by the Fe(TPA)/tBuOOH 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 tBuOOH; 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 tBuOOH or CmOOH, the sole product observed was alcohol; kH/kD 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.