Iron-catalyzed olefin epoxidation in the presence of acetic acid: Insights into the nature of the metal-based oxidant

The iron complexes [(BPMEN)Fe(OTf)₂] (1) and [(TPA)Fe(OTf) ₂] (2) [BPMEN = N,N′-bis-(2-pyridylmethyl)-N,N′-dimethyl- 1,2-ethylenediamine; TPA = tris-(2-pyridylmethyl)amine] catalyze the oxidation of olefins by H₂O₂ to yield epoxides and cis-diols. The addition of acetic acid inhibits olefin cis-dihydroxylation and enhances epoxidation for both 1 and 2. Reactions carried out at 0°C with 0.5 mol % catalyst and a 1:1.5 olefin/H₂O₂ ratio in a 1:2 CH ₃CN/CH₃COOH solvent mixture result in nearly quantitative conversions of cyclooctene to epoxide within 1 min. The nature of the active species formed in the presence of acetic acid has been probed at low temperature. For 2, in the absence of substrate, [(TPA)Fe^III(OOH) (CH₃COOH)]²⁺ and [(TPA)Fe^IVO(NCCH ₃)]²⁺ intermediates can be observed. However, neither is the active epoxidizing species. In fact, [(TPA)Fe^IVO(NCCH ₃)]²⁺ is shown to form in competition with substrate oxidation. Consequently, it is proposed that epoxidation is mediated by [(TPA)Fe^V(O)(OOCCH₃)]²⁺, generated from O-O bond heterolysis of the [(TPA)Fe^III(OOH)(CH₃COOH)] ²⁺ intermediate, which is promoted by the protonation of the terminal oxygen atom of the hydroperoxide by the coordinated carboxylic acid.