Abstract
The stoichiometric asymmetric epoxidation of E-β-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 π-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 Katasuki’s views on the importance of π-interactions in the analogous manganese–salen catalyzed epoxidations. Compounds containing SO and CO 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.
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