A possible mechanism for enantioselectivity in the chiral epoxidation of olefins with [Mn(salen)] catalysts

The origin of enantioselectivity in the Jacobsen-Katsuki reaction has been investigated by applying density functional calculations in combination wit h molecular mechanics methodologies. The calculations suggest that a high e nantiomeric excess is connected to three specific features: 1) a chiral dii mine bridge. which induces folding of the salen ligand(H(2)salen= bis(salic ylidene)ethylenediamine). and hence the formation of a chiral pocket; 2) bu lky groups at the 3,3'-positions of the salen ligand, which cause a prefere ntial approach from the side of the aromatic rings; and 3) pi conjugation o f the olefinic double bond, which confers regioselectivity and, consequentl y, enantioselectivity. In combination with experimental studies, the model also provides a rationale for the decrease in ee values when one of these c omponents is missing.