Nb(salen)-Catalyzed Sulfoxidation

Abstract

Niobium(salen) complex was found to be an effective catalyst for asymmetric oxidation of various sulfides using urea˙hydrogen peroxide as terminal oxidant.

Key words

niobium - asymmetric catalysis - sulfoxidation - niobium(salen) complex

References

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5

Metallosalen complexes are usually prepared by mixing the corresponding metal ion (usually in the form of metal halide or metal alkoxide) and salen ligand under basic conditions or by mixing metal ion and a salen ligand pretreated with NaH. However, the niobium complex prepared from NbCl₃(dme) and salen ligand in these ways showed inferior catalytic activity in terms of enantioselectivity (for examples, the oxidation of methyl phenyl sulfides with these Nb-4 complexes at room temperature showed 12-46% ee).

11

Typical experiment procedure is exemplified with asymmetric oxidation of methyl phenyl sulfide with 4 as the chiral ligand: ligand 4 (10 mg, 12 µmol) and NbCl₃(dme) (2.3 mg, 8.0 µmol) were dissolved in dichloromethane (2 mL) in a glovebox, and the solution was stirred for 2 h at room temperature. After addition of MS 4 Å (ca. 20 mg), the mixture was stirred for another 30 min. The mixture was taken out of the glovebox and cooled to -10 °C under nitrogen. To this mixture were added methyl phenyl sulfide (12.0 µL, 0.10 mmol) and urea˙hydrogen peroxide adduct (UHP) (10.5 mg, 0.11 mmol) successively, and the mixture was stirred for 2 days at the temperature. The mixture was directly chromatographed on silica gel (hexane-ethyl acetate = 1:1-1:19) to give methyl phenyl sulfoxide (12.4 mg, 88%). The enantiomeric excess of the sulfoxide was determined to be 86% ee by HPLC analysis using chiral stationary phase column (Daicel Chiralcel OD-H, hexane-i-PrOH = 9:1).