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

Rubén Mas-Ballesté, Larry Que

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Abstract

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)FeIII(OOH) (CH3COOH)]2+ and [(TPA)FeIVO(NCCH 3)]2+ intermediates can be observed. However, neither is the active epoxidizing species. In fact, [(TPA)FeIVO(NCCH 3)]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.

Original languageEnglish (US)
Pages (from-to)15964-15972
Number of pages9
JournalJournal of the American Chemical Society
Volume129
Issue number51
DOIs
StatePublished - Dec 26 2007

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