Iron-promoted ortho-And/or ipso-hydroxylation of benzoic acids with H 2O2

Olga V. Makhlynets, Parthapratim Das, Sonia Taktak, Margaret Flook, Rubén Mas-Ballesté, Elena V. Rybak-Akimova, Lawrence Que

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85 Scopus citations


Regioselective hydroxylation of aromatic acids with hydrogen peroxide proceeds readily in the presence of iron(II) complexes with tetradentate aminopyridine ligands [FeII(BPMEN)-(CH3CN) 2](ClO4)2 (1) and [FeII(TPA)- (CH3CN)2](OTf)2 (2), where BPMEN=N, N'-dimethyl-N, N'-bis(2-pyridylmethyl)-1,2-ethylenediamine, TPA=tris-(2- pyridylmethyl)amine. Two cis-sites, which are occupied by labile acetonitrile molecules in 1 and 2, are available for coordination of H2O 2 and substituted benzoic acids. The hydroxylation of the aromatic ring occurs exclusively in the vicinity of the anchoring carboxylate functional group: ortho-hydroxylation affords salicylates, whereas ipso-hydroxylation with concomitant decarboxylation yields phenolates. The outcome of the substituent directed hydroxylation depends on the electronic properties and the position of substituents in the molecules of substrates:3-substituted benzoic acids are preferentially ortho-hydroxylated, whereas 2-and, to a lesser extent, 4-substituted substrates tend to undergo ipso-hydroxylation/decarboxylation. These two pathways are not mutually exclusive and likely proceed via a common intermediate. Electron-withdrawing substituents on the aromatic ring of the carboxylic acids disfavor hydroxylation, indicating an electrophilic nature for the active oxidant. Complexes 1 and 2 exhibit similar reactivity patterns, but 1 generates a more powerful oxidant than 2. Spectroscopic and labeling studies exclude acylperoxoiron(III) and FeIV=O species as potential reaction intermediates, but strongly indicate the involvement of an FeIII-OOH intermediate that undergoes intramolecular acid-promoted heterolytic O-O bond cleavage, producing a transient iron(V) oxidant.

Original languageEnglish (US)
Pages (from-to)13171-13180
Number of pages10
JournalChemistry - A European Journal
Issue number47
StatePublished - Dec 7 2009
Externally publishedYes


  • Aromatic hydroxylation
  • Hydrogen peroxide
  • Iron
  • Oxidation
  • Regioselective hydroxylation

ASJC Scopus subject areas

  • General Chemistry
  • Catalysis
  • Organic Chemistry


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