A Possible Mechanism for Enantioselectivity in the Chiral Epoxidation of Olefins with [Mn(salen)] Catalysts

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Abstract
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.