Molybdenum-Catalyzed Olefin Epoxidation: Ligand Effects

We synthesized substituted pyrazolylpyridine ligands to examine their donor properties by spectroscopic (IR, NMR) and computational (AM 1) methods. The influence of the substitution patterns on spectroscopic and thermodynamic features of molybdenum oxobisperoxo complexes [(L–L)MoO(O2)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 π2‐peroxo ligand to the olefin. From the spectroscopic properties of substituted pyrazolylpyridine ligands, which are closely related to the isomer ratios of the derived molybdenum complexes A and B, we can draw conclusions about catalytic activity in olefin epoxidation. There is a clear correlation between the substitution patterns of the ligands and the Lewis acidity of the complexes. These results are supported by theoretical investigations; in addition, we have obtained a further insight into the process of oxygen transfer from the peroxidic reagent to an olefinic bond.