Developments in the Simmons−Smith-Mediated Epoxidation Reaction

The reaction between Et2Zn, ClCH2I, sulfide, and aldehyde furnishes terminal epoxides in high yields. The reaction occurs via a zinc carbenoid, which reacts with the sulfide to furnish an ylide, which in turn reacts with the aldehyde to give the epoxide. Chiral ligands capable of chelation to zinc [1,2-amino alcohols, amino acids, bis(oxazolines), taddols] were examined, but only low enantioselectivity was observed (up to 11% ee). A number of chiral sulfides were also examined, but again only low enantioselectivity was observed (up to 16% ee). However, linking a sulfide to a metal capable of chelation to zinc [a bis(oxazoline) bearing a sulfide at the 5 position] produced a reagent that gave up to 54% ee in the epoxidation process. The same system was applied to the preparation of terminal aziridines from imines. The optimum group on nitrogen was a sulfonyl group, although groups capable of chelation of zinc (o-methoxyphenyl) were also effective. Attempts to render the aziridination process asymmetric by using the above strategy were less successful (up to 19% ee).