Highly Efficient Camphor-Derived Oxaziridines for the Asymmetric Oxidation of Sulfides to Chiral Sulfoxides

Chiral N-sulfonyl-oxaziridines derived from 8-camphorsulfonic acid and fenchone have been evaluated as asymmetric oxidizing agents for the conversion of sulfides to chiral sulfoxides. There is no correlation between the redox potentials nor the 17O NMR chemical shifts of the oxaziridines and their relative oxidation rates, nor with the enantiomeric excesses achieved, indicating that steric effects are responsible for their behaviour. The results are consistent with an attack of one sulfur lone pair at the oxaziridine oxygen in such a way that both sulfur lone pairs lie in the plane of the oxaziridine ring. The most efficient oxaziridines, the camphorlactone-sulfonyloxaziridine [(4aS,9 aR)-10,10-dimethyl-6,7-dihydro-4 H-4 a,7-methano-oxazirino[3,2-j]oxepino[3,4-c]isothiazol-9(5H)-one 3,3-dioxide] and the 3-endo-bromocamphorsulfonyloxaziridine [(4aS,8 S,8 aR)-8-bromo-9,9-dimethyl-5,6,7,8-tetrahydro-4 H- 4a,7-methano-oxazirino-2,1-benzisothiazole 3,3-dioxide] allow the preparation of chiral sulfoxides with up to 85% enantiomeric excess.