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

Abstract

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%.