based on the structure, models for the observed enantioselectivity in many binap-based lewis acid-catalyzed reactions have been proposed. for example, in the palladium-catalyzed enantioselective diels-alder reaction shown below, the dienophile is thought to coordinate the metal center at the equatorial sites. thus the equatorial phenyl group on phosphorus obstructs the "si"-face, resulting in excellent enantioselectivity. a very similar model was used to rationalize the outcome of a nickel-catalyzed asymmetric enolate alkylation reaction, where the substrate also bears an auxiliary that allows it to chelate onto the metal. on the other hand, a copper(i)-catalyzed hetero-ene reaction is thought to proceed through a tetrahedral intermediate, offering an alternative mode of stereoinduction by changing the metal center.