ASYMMETRIC SELENOXIDE ELIMINATION LEADING TO CHIRAL ALLENIC SULFONES

Asymmetric oxidation of some aryl vinyl selenides (3) with Sharpless (A-C), modified Sharpless (D) or David oxidants (E-G) resulted in the formation of chiral allenic sulfones 5 of up to 42% enantiomeric excess (ee) via double asymmetric induction, i.e., asymmetric oxidation to selenoxide followed by its asymmetric elimination. The nature of aryl group of the selenides 3 showed a remarkable effect upon the ee of the product: o-nitrophenyl group (a) gave the highest ee, followed by o,p-dinitrophenyl (b), o-(trifluoromethyl)phenyl (e), and o-methoxyphenyl (f) groups, while phenyl (e) and 2-pyridyl (d) groups were not effective at all. The effects of aryl groups were kinetically analyzed by comparing the rate constants of both steps (k1 for oxidation step and k2 for elimination step) which were determined by H-1-NMR analysis of the concentration of 3, the intermediate selenoxides 4, and 5. As a result, it was disclosed that the ratio of these rate constants (k1/k2) Was closely related to the ee of the product; the smaller the ratio is, the larger the ee becomes.