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How does the axial ligand of cytochrome p450 biomimetics influence the regioselectivity of aliphatic versus aromatic hydroxylation?

De Visser, S. P ; Sharif University of Technology | 2009

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  1. Type of Document: Article
  2. DOI: 10.1002/chem.200802234
  3. Publisher: 2009
  4. Abstract:
  5. The catalytic activity of highvalent iron-oxo active species of heme enzymes is known to be dependent on the nature of the axial ligand trans to the iron-oxo group. In a similar fashion, experimental studies on iron-oxo porphyrin biomimetic systems have shown a significant axial ligand effect on ethylbenzene hydroxylation, with an axial acetonitrile ligand leading to phenyl hydroxylation products and an axial chloride anion giving predominantly benzyl hydroxylation products. To elucidate the fundamental factors that distinguish this regioselectivity reversal in iron-oxo porphyrin catalysis, we have performed a series of density functional theory calculations on the hydroxylation of ethylbenzene by [FeIV=O(Por+)L] (Por = porphyrin; L = NCCH3 or Cl-), which affords 1-phenylethanol and p-ethylphenol products. The calculations confirm the experimentally determined product distributions. Furthermore, a detailed analysis of the electronic differences between the two oxidants shows that their reversed regioselectivity is a result of differences in orbital interactions between the axial ligand and iron-oxo porphyrin system. In particular, three high-lying orbitais Π*xz,Π*yz and a 2u), which are singly occupied in the reactant complex, are stabilised with an anionic ligand such as Cl-, which leads to enhanced HOMO-LUMO energy gaps. As a consequence, reactions leading to cationic intermediates through the twoelectron reduction of the metal centre are disfavoured. The aliphatic hydroxylation mechanism, in contrast, is a radical process in which only one electron is transferred in the rate-determining transition state, which means that the effect of the axial ligand on this mechanism is much smaller. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
  6. Keywords:
  7. Bioinorganic chemistry ; Dioxygen ligands ; Oxygenation ; 1-Phenylethanol ; Active species ; Aliphatic hydroxylation ; Anionic ligands ; Aromatic hydroxylation ; Axial ligand ; Biomimetic systems ; Catalytic activity ; Cationic intermediates ; Chloride anions ; Cytochrome p450 ; Density functional theory calculations ; Electronic differences ; Experimental studies ; Fundamental factors ; Heme enzyme ; Heme proteins ; High-valent irons ; HOMO-LUMO energy gap ; Ligand effects ; Metal centres ; Orbital interaction ; Product distributions ; Radical process ; Transition state ; Two-electron reduction ; Acetonitrile ; Biochemistry ; Chlorine compounds ; Density functional theory ; Dyes ; Ethanol ; Ethylbenzene ; Hemoglobin ; Hydroxylation ; Iron analysis ; Oxygenators ; Porphyrins ; Regioselectivity ; Sulfur compounds ; Ligands ; Benzene derivative ; Article ; Chemical model ; Chemical structure ; Chemistry ; Metabolism ; Oxidation reduction reaction ; Stereoisomerism ; Benzene Derivatives ; Biomimetics ; Catalysis ; Cytochrome P-450 Enzyme System ; Heme ; Iron ; Models, Chemical ; Molecular Structure ; Oxidation-Reduction ; Stereoisomerism ; Thermodynamics
  8. Source: Chemistry - A European Journal ; Volume 15, Issue 22 , 2009 , Pages 5577-5587 ; 09476539 (ISSN)
  9. URL: https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/chem.200802234