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.
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota, 55455, USA.
Rubén
R
Mas-Ballesté
Lawrence
L
Que
D019342:Acetic Acid / Q000737:chemistry*; D000475:Alkenes / Q000737:chemistry*; D002384:Catalysis; D004852:Epoxy Compounds / Q000737:chemistry*; D008670:Metals / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*
D000475:Alkenes; D004852:Epoxy Compounds; D008670:Metals; D016877:Oxidants; D019342:Acetic Acid