Preparation and Characterization of Optically Active Schiff Base-Oxovanadium(IV) and -Oxovanadium(V) Complexes and Catalytic Properties of These Complexes on Asymmetric Oxidation of Sulfides into Sulfoxides with Organic Hydroperoxides

Abstract

Abstract Oxovanadium(V) complexes, VVO (Schiff base)Y (Y=ClO4, Cl, NO3), with quadridentate Schiff base ligands derived from optically active 1,2-diamines and salicylaldehyde or its derivatives were prepared by oxidizing the corresponding VIVO (Schiff base) with CeIV in acetonitrile followed by the addition of HY. The complexes were characterized by their 1H NMR, IR, UV-Vis, and circular dichroism spectra, and electrochemistry. Reversible reduction potentials (V5+/V4+) obtained by cyclic voltammetry for a series of oxovanadium(V) complexes with Schiff base ligands derived from 5-substituted salicylaldehydes showed a linear dependence on the Hammett parameter. Both the vanadium(IV) and the vanadium(V) complexes catalyze asymmetric oxidation of sulfides into the corresponding sulfoxides with organic hydroperoxides. The oxovanadium(IV) complex catalysts gave much better asymmetric induction than the oxovanadium(V) complex catalysts and in most cases the enantiomeric excess (ee) ranged between 20 and 40%. In the reaction of VIVO (Schiff base) with an organic hydroperoxide the complex is finally converted into the oxovanadium(V) species, and a dark brown intermediate is suggested to be the catalytically active species.