Chiral manganese and iron complexes of binaphthyl Schiff bases: Syntheses, crystal structures and asymmetric epoxidation of alkenes

The chiral binaphthyl Schiff-base manganese complexes [MnL(X)] [H₂L = 2,2¹-bis(3-R²-5-R ²-2-hydroxybenzylideneimino)-1,1′-binaphthyl; R¹ = R² = Cl, X = O₂CMe 1; R¹ = Et, R² = NO₂, X = O₂CMe 2; R¹ = R² = Cl, X = acac (acetylacetonate) 3] and an iron analogue [FeL(acac)] 4 (R¹ = R² = Cl) have been prepared. The crystal structures of the racemic forms of 3·CH₂Cl₂ and 4·2H₂O exhibit non-planar cis-β configurations with dihedral angles of 85.5 and 72.1° respectively between the two naphthalene rings. Complexes (R)-1 to 3 are active catalysts for the asymmetric epoxidation of alkenes by PhIO. The oxidation of cis-β-methylstyrene by (R)-2 in toluene at -20°C gave an enantiomeric excess of 76%.