Manganese‐Salen Complexes as Oxygen‐Transfer Agents in Catalytic Epoxidations − A Density Functional Study of Mechanistic Aspects

The mechanism of the Jacobsen−Katsuki epoxidation has been investigated by application of density functional theory; the results of a series of calculations for simplified model systems of different spin states are presented. In the chosen computational approach, the epoxidation of ethylene with a cationic five‐coordinate model catalyst is predicted to occur through a radical intermediate, similarly to the reaction mechanism calculated for the corresponding neutral six‐coordinate species. Although the radical intermediate shows a small energetic preference for the quintet state over the triplet state, the computed reaction profile does not suggest that two‐state reactivity involving spin change plays a major role during the oxygen‐transfer step. Comparative orbital analysis of the cationic and the neutral complexes elucidates the role of a ligand trans to the oxo group. A π‐donor trans to the forming OR − ligand in the radical intermediate causes a relative destabilization of a possible quintet occupation, thus conferring spin rigidity to the six‐coordinate species derived from the neutral catalyst. A reaction pathway resulting in rotational collapse might involve a spin‐crossing process. The ligand framework of the tetra‐chelating N,O ligand in the radical intermediate exhibits a considerable amount of ligand folding. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)