The stoichiometric asymmetric epoxidation of E-beta-methylstyrene with cationic chromium-salen oxo complexes was studied. It was found that enantiomeric excess (ee) could be strongly affected (+/-20% ee) by the presence of compounds containing extended pi-electron systems. In certain cases, such additives appeared to stabilize the active oxidant slowing reaction. Unsubstituted and methyl-substituted imidazoles were found to be beneficial additives but imidazoles with aromatic substituents were very detrimental. These results are interpreted as providing support for Katsuki's views on the importance of pi-interactions in the analogous manganese-salen catalyzed epoxidations. Compounds containing S=O and C=O bonds also affected enantioselectivity but to a lesser extent. Phosphine sulfides an borane could survive contact with the oxidant depending on the salen substitution pattern and in the latter case the enantioselectivity was raised. A phosphorus ylide was found to stabilize a Cr(V) oxo species, approximately doubling its lifetime. (C) 2002 Elsevier Science B.V All rights reserved.