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.