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 with molecular mechanics methodologies. The calculations suggest that a high enantiomeric excess is connected to three specific features: 1) a chiral diimine bridge, which induces folding of the salen ligand(H₂salen = bis(salicylidene) ethylenediamine), and hence the formation of a chiral pocket: 2) bulky groups at the 3,3′-positions of the salen ligand, which cause a preferential approach from the side of the aromatic rings; and 3) π conjugation of the olefinic double bond, which confers regioselectivity and, consequently, enantioselectivity. In combination with experimental studies, the model also provides a rationale for the decrease in ee values when one of these components is missing.