Crystal Structures of [VO(sal-l-ala)(OCH3)(CH3OH)] (sal-l-ala=N-salicylidene-l-alaninate) and {[VO(sal-l-ala)]2O}2·2CH2Cl2, and the Catalytic Activity of These and Related Complexes on Asymmetric Oxidation of Methyl Phenyl Sulfide with t-Butyl Hydroperoxide

Abstract

Abstract A reddish brown oxovanadium(V) complex, [VVO(sal-l-ala)(OCH3)(CH3OH)] (2a, sal-l-ala=N-salicylidene-l-alaninate) was formed by dissolving [VIVO(sal-l-ala)(H2O)] (1a) in methanol. Treatment of 2a with wet dichloromethane yielded deep blue crystals of {[VVO(sal-l-ala)]2O}2·2CH2Cl2 (3a). The structures of 2a and 3a have been determined by X-ray analyses. Crystal data are: [VO(sal-l-ala)(OCH3)(CH3OH)] (2a), monoclinic, C2, a=19.621(6), b=6.734(1), c=12.059(4) Å, β=117.72(2)° and Z=4. {[VO(sal-l-ala)]2O}2·2CH2Cl2 (3a), orthorhombic, P212121, a=19.938(4), b=23.854(5), c=11.526(2) Å, and Z=4. In 2a, the tridentate sal-l-ala ligand and a methoxide ion occupy the basal positions, and form distorted octahedral geometry around the vanadium(V) ion together with one methanol molecule (V–O distance: 2.384(4) Å) and the oxide ion of oxovanadium(V). The tetranuclear complex (3a) consists of two μ-oxo dinuclear vanadium(V) complexes, and each vanadium(V) ion has a distorted octahedral coordination. The Schiff base coordinates equatorially to each vanadium(V) ion, of which one axial site is occupied by O2− and the other is weakly linked to the carboxylate O atom of a nearby Schiff base (V–O distance: 2.34–2.56 Å). Octahedral coordination on each vanadium(V) is achieved by sharing an equatorial O2− ligand. The brown complexes, [VO(sal-l-aa)(OCH3)(CH3OH)] (2, sal-l-aa=N-salicylidene l-amino carboxylate) catalyze the oxidation of methyl phenyl sulfide with t-butyl hydroperoxide in dichloromethane at 4 °C to give (R)-methyl phenyl sulfoxide with optical yields (e.e.) of 8–14%.