Aromatic hydroxylation in a copper bis(imine) complex mediated by a micro-eta2:eta2 peroxo dicopper core: a mechanistic scenario |
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Authors: | Sander Ole Henss Anja Näther Christian Würtele Christian Holthausen Max C Schindler Siegfried Tuczek Felix |
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Affiliation: | 1. Institut für Anorganische Chemie, Christian Albrechts Universit?t Kiel, Max‐Eyth Strasse 2, 24098 Kiel (Germany), Fax: (+49)?431?880?152?0;2. Institut für Anorganische und Analytische Chemie, Justus‐Liebig‐Universit?t Gie?en, Heinrich‐Buff‐Ring 58, 35392 Gie?en (Germany), Fax: (+49)?641?993?414?9;3. Institut für Anorganische und Analytische Chemie, Johann Wolfgang Goethe‐Universit?t Frankfurt, 60438 Frankfurt am Main (Germany), Fax: (+49)?798?294?17 |
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Abstract: | Detailed mechanistic studies on the ligand hydroxylation reaction mediated by a copper bis(imine) complex are presented. Starting from a structural analysis of the CuI complex and the CuII product with a hydroxylated ligand, the optical absorption and vibrational spectra of starting material and product are analyzed. Kinetic analysis of the ligand hydroxylation reaction shows that O2 binding is the rate-limiting step. The reaction proceeds much faster in methanol than in acetonitrile. Moreover, an inverse kinetic isotope effect (KIE) is evidenced for the reaction in acetonitrile, which is attributed to a sterically congested transition state leading to the peroxo adduct. In methanol, however, no KIE is observed. A DFT analysis of the oxygenation reaction mediated by the micro-eta2:eta2 peroxo core demonstrates that the major barrier after O2 binding corresponds to electrophilic attack on the arene ring. The relevant orbital interaction occurs between the sigma* orbital of the Cu2O2 unit and the HOMO of the ligand. On the basis of the activation energy for the rate-limiting step (18.3 kcal mol(-1)) this reaction is thermally allowed, in agreement with the experimental observation. The calculations also predict the presence of a stable dienone intermediate which, however, escaped experimental detection so far. Reasons for these findings are considered. The implications of the results for the mechanism of tyrosinase are discussed. |
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Keywords: | copper hydroxylation N ligands O?O activation reaction mechanisms |
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