Asymmetric alkene epoxidation with chromium oxo salen complexes. Effect of added phosphoryl ligands

In the stoichiometric asymmetric epoxidation of E-β-methylstyrene with cationic chromium–salen oxo complexes, enantioselectivity is elevated by the addition of many different types of phosphoryl compounds. Triarylphosphine oxides are the most effective, and, amongst them, tris(3,5-dimethylphenyl)phosphine oxide gave the highest ee to date (93%). There is a ceiling effect of additive, bulky additives are ineffective, there is no substituent electronic effect within the additive and there is a saturation of its effect with increasing concentration.