On the nature of the active intermediates in iron-catalyzed oxidation of cycloalkanes with hydrogen peroxide and peracids

Add time:08/24/2019    Source:sciencedirect.com

Iron complexes of the PDP family [((S,S)-PDP)FeII(OTf)2] (1) and [((S,S)-PDP*)FеIII(μ-OH)2FеIII((S,S)-PDР*)](ОTf)4 (3), (S,S)-PDР = N,N′-bis(2-pyridylmethyl)-(S,S)-2,2′-biрyrrоlidine, (S,S)-PDР* = N,N′-bis(3,5-dimеthyl-4-mеthоxypyridyl-2-mеthyl)-(S,S)-2,2′-biрyrrоlidine, and of the TPA family [(TPА)FeII(CH3СN)2](СlO4)2 (4) and [(TPА*)FеIII(μ-OH)2FеIII(TPА*)](ОTf)4 (5), TPА = tris(2-pyridylmethyl)amine, TPA* = tris(3,5-dimеthyl-4-mеthoxyрyridyl-2-methyl)аmine, catаlyze the selеctive hydrоxylation of alkаnes with hydrogen peroxide and peroxycarboxylic acids as terminal oxidants. The nature of the active species of these catalytic systems has been evaluated by combined EPR spectroscopic and catalytic studies. To this end, the catalytic systems Fe complex/oxidant/RCOOH (catalyst: 1, 3, 4, 5; oxidant: H2O2, CH3CO3H, m-chloroperoxybenzoic acid = m-CPBA; RCOOH: acetic acid = AA, 2-ethylhexanoic acid = EHA), exhibiting EPR spectra of iron-oxo and/or iron-acylperoxo intermediates, have been systematically studied in the chemoselective oxidation of cyclohexane and regioselective oxidation of adamantane. In the latter case, high yield of oxidation products (up to 67 TN per Fe atom, or 67%) and high regioselectivity (3°/2° up to 41) were observed. Depending on the nature of the catalyst, oxidant and catalytic additive, various iron-oxygen intermediates have been observed in the catalytic systems studied. Iron(V)-oxo intermediates have been suggested to be the major active species of C–H hydroxylation by the systems catalyst/H2O2/RCOOH and catalyst/CH3CO3H/RCOOH. In contrast to the catalytic systems with H2O2 and CH3CO3H as oxidants, in the systems relying on m-CPBA, the contribution of iron-acylperoxo intermediates into the oxidation may be significant.

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