Molybdenum-catalyzed olefin epoxidation: Ligand effects

We synthesized substituted pyrazolylpyridine ligands to examine their donor properties by spectroscopic (IR, NMR) and computational (AM1) methods. The influence of the substitution patterns on spectroscopic and thermodynamic Features of molybdenum oxobisperoxo complexes [(L-L)MoO(O-2)(2)] (L-L = 2-(1-alkyl-3-pyrazolyl)pyridine/pyrazine) correlates with the activities of the complexes in catalytic olefin epoxidation reactions. This further proof for the relation between the Lewis acidity and the catalytic activity of epoxidation catalysts supports a reaction mechanism in which the peroxo complex activates the oxidizing agent (H2O2, ROOH) instead of directly transferring an oxygen atom from a eta(2)-peroxo ligand to the olefin.