Iron-Catalyzed Enantioselective Epoxidations with Various Oxidants: Evidence for Different Active Species and Epoxidation Mechanisms

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Abstract

Iron complexes with chiral bipyrrolidine-derived aminopyridine (PDP) ligands are among the most efficient Fe-based bioinspired catalysts for regio- and stereoselective oxidation of C-H and C=C moieties with hydrogen peroxide. Besides hydrogen peroxide, other oxidants (peroxycarboxylic acids and organic hydroperoxides) can be effectively used. In this work, we have examined the mechanistic landscape of the Fe(PDP) catalyst family with various oxidants: H2O2, organic hydroperoxides, and peracids. The combined EPR spectroscopic, enantioselectivity, Hammett, Z-stilbene epoxidation stereoselectivity, and 18O-labeling data witness that the same oxoiron complexes [(L)FeV=O(OC(O)R)]2+ are the actual epoxidizing species in both the catalyst systems (L)Fe/H2O2/carboxylic acid and (L)Fe/AlkylOOH/carboxylic acid. On the contrary, in the systems (L)Fe/R2C(O)OOH (R2 = CH3 or 3-Cl-C6H4), in the presence or in the absence of carboxylic acid, the epoxidation is predominantly conducted by the acylperoxo-iron(III) intermediates [(L)FeIII(OOC(O)R2)]2+, in a concerted fashion.

Original languageEnglish
Pages (from-to)60-69
Number of pages10
JournalACS Catalysis
Volume7
Issue number1
DOIs
Publication statusPublished - 6 Jan 2017

Keywords

  • asymmetric epoxidation
  • bioinspired catalysis
  • enantioselectivity
  • EPR
  • iron
  • mechanism
  • NONHEME IRON
  • OLEFIN EPOXIDATION
  • COMPLEXES
  • H2O2
  • ONE-ELECTRON OXIDATION
  • ASYMMETRIC EPOXIDATION
  • METHYLENIC SITES
  • C-H OXIDATION
  • SELECTIVITY
  • FE

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