Asymmetric alkene epoxidation with chromium oxo salen complexes. A systematic study of salen ligand substituents.

[structure: see text]. In the stoichiometric asymmetric epoxidation of E-beta-methylstyrene with cationic chromium-salen oxo complexes, enantioselectivity is increased by halo-substitution at the 3,3'- and 6,6'-positions and decreased at the 4,4'- and 5,5'-positions on the salen rings. Addition of triphenylphosphine oxide significantly increases selection with 3,3'- or 5,5'-substituents but not with 4,4'- or 6,6'-substituents. Use of nitrate counterion is beneficial in most cases. The results are discussed with respect to the mode of stereoselection in metal-salen epoxidations.

High enantioselectivities in an (E)-alkene epoxidation by catalytically active chromium salen complexes. Insight into the catalytic cycle.

[structure: see text]. The epoxidation of (E)-beta-methylstyrene mediated by an oxochromium salen complex yields the epoxide in 92% ee in stoichiometric mode, the highest ee yet reported for a metal-mediated epoxidation of an (E)-alkene. The effect of added donor ligands, previously substantial, has reached a ceiling with this complex. In catalytic mode a slightly reduced ee and higher yield is obtained, indicating both the presence of a second oxidation cycle and that the major oxidant reacts with its reduced form.