Highly enantioselective sulfoxidation with vanadium catalysts of Schiff bases derived from bromo- and iodo-functionalized hydroxynaphthaldehydes

Two series of chiral Schiff bases, 4a−e and 5a−e, prepared from the condensation of the mono-, di-, tribromohydroxynaphthaldehyde or monoiodohydroxynaphthaldehyde with chiral amino alcohols, were used in combination with VO(acac)2 for the asymmetric oxidation of aryl methyl sulfides using H2O2 as terminal oxidant. Among these Schiff bases, dibromo-functionalized 4d and iodo-functionalized 5e gave high yields (91–93%) with good enantioselectivities (80–82% ee) for the oxidation of thioanisole in dichloromethane. The asymmetric oxidation of thioanisole in toluene using these Schiff bases gave methyl phenyl sulfoxide in satisfactory isolated yields (48–62%) with high enantioselectivities (91–94% ee), which were further improved by a modified procedure with the ee value up to 98% in 62% yield. The oxidations of other aryl methyl sulfides in toluene with dibromo- and iodo-functionalized Schiff bases 5d and 5e as ligands using the modified procedure afforded the corresponding sulfoxides in 55–67% isolated yields with 95–99% ee.