Origins of Aryl Substituent Effects on the Stereoselectivities of Additions of Silyl Enol Ethers to a Chiral Oxazolinium Ion

Density functional theory calculations are reported that reveal the role of aromatic interactions in the additions of aryl-substituted silyl enol ethers to a chiral oxazolinium ion. Aryl-substituted silyl enol ethers give the opposite diastereomer of the adduct than do aliphatic silyl, enol ethers, due to a combination of attractive cation-pi and CH-pi interactions, reduced steric repulsion, and lower torsional strain in the more "crowded" transition state.