The reaction of [FeII(tpa)] with H2O2 in acetonitrile and acetone--distinct intermediates and yet similar catalysis.

Antoni Mairata i Payeras, Raymond Y N Ho, Megumi Fujita, Lawrence Que

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Abstract

The reaction of [FeII(tpa)(OTf)2] (tpa=tris(2-pyridylmethyl)amine) and its related 5-Me3-tpa complex with hydrogen peroxide affords spectroscopically distinct iron(III)-peroxo intermediates in CH3CN and acetone. The reaction in acetonitrile at -40 degrees C results in the formation of the previously reported Fe(III)-OOH intermediate, the end-on hydroperoxo coordination mode of which is established in this paper by detailed resonance Raman isotope-labeling experiments. On the other hand, the reaction in acetone below -40 degrees C leads to the observation of a different peroxo intermediate identified by resonance Raman spectroscopy to be an FeIII-OOC (CH3)2OH species; this represents the first example of an intermediate derived from the adduct of H2O2 and acetone. The peroxoacetone intermediate decays more rapidly than the corresponding FeIII-OOH species and converts to an FeIV=O species by O-O bond homolysis. This decay process is analogous to that observed for [FeIII(tpa)(OOtBu)]2+ and in fact exhibits a comparable enthalpy of activation of 54(3) kJ mol(-1). Thus, with respect to their physical properties at low temperature, the peroxoacetone intermediate resembles [FeIII(tpa)(OOtBu)]2+ more than the corresponding FeIII-OOH species. At room temperature, however, the behavior of the Fe(tpa)/H2O2 combination in acetone in catalytic hydrocarbon oxidations differs significantly from that of the Fe(tpa)/tBuOOH combination and more closely matches that of the Fe(tpa)/H2O2 combination in CH3CN. Like the latter, the Fe(tpa)/H2O2 combination in acetone catalyzes the hydroxylation of cis-1,2-dimethylcyclohexane to its tertiary alcohol with high stereoselectivity and carries out the epoxidation and cis-dihydroxylation of olefins. These results demonstrate the subtle complexity of the Fe(tpa)/H2O2 reaction surface.

Original languageEnglish (US)
Pages (from-to)4944-4953
Number of pages10
JournalChemistry (Weinheim an der Bergstrasse, Germany)
Volume10
Issue number20
DOIs
StatePublished - Oct 11 2004

PubMed: MeSH publication types

  • Journal Article
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

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