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 degrees C with 0.5 mol % catalyst and a 1:1.5 olefin/H2O2 ratio in a 1:2 CH3CN/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)FeIII(OOH)(CH3COOH)2+ and (TPA)FeIVO(NCCH3)2+ intermediates can be observed. However, neither is the active epoxidizing species. In fact, (TPA)FeIVO(NCCH3)2+ is shown to form in competition with substrate oxidation. Consequently, it is proposed that epoxidation is mediated by (TPA)FeV(O)(OOCCH3)2+, generated from O-O bond heterolysis of the (TPA)FeIII(OOH)(CH3COOH)2+ intermediate, which is promoted by the protonation of the terminal oxygen atom of the hydroperoxide by the coordinated carboxylic acid.