[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.

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[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.

diastereoselective syntheses, enantioselective syntheses diastereoselective syntheses, enantioselective syntheses (incl. cis/trans-isomerism) O 0031 -034Asymmetric Alkene Epoxidation with Chromium Oxo Salen Complexes. A Systematic Study of Salen Ligand Substituents.-In the asymmetric epoxidation of E-β-methylstyrene, the title complexes with halogeno-substituents in the 3,3'-and 6,6'-positions, e.g. (I), afford the highest enantioselectivity. Use of triphenylphosphine oxide as donor ligand and of nitrate counter ion is beneficial in most cases. -(O'MAHONY, COLM P.; MCGARRIGLE, EOGHAN M.; RENEHAN, MARIE F.; RYAN, KENNETH M.; KERRIGAN, NESSAN J.; BOUSQUET, CLAUDINE; GILHEANY, DECLAN G.; Org. Lett. 3 (2001) 22, 3435-3438; Dep. Chem., Univ. Coll., Belfield, Dublin 4, Ire.; EN)