Asymmetric epoxidation Atom transfer reactions

Asymmetric cyclopropanation, 57, 1 Asymmetric dihydroxylation, 66 2 Asymmetric epoxidation, 48, 1 61, 2 Asymmetric reduction, 71, 1 Asymmetric Strecker reaction, 70, 1 Atom transfer preparation of radicals, 48, 2... 

Asymmetric reactions that can exhibit this type of behavior include atom and group transfer reactions, such as the asymmetric oxidation of sulfides, some asymmetric epoxidations of olefins, " asymmetric aziridination of olefins, - and as)rmmetric cyclo-propanation of olefins. In the asymmetric oxidation of sulfides, a non-racemic, cliiral, low-valent metal complex is oxidized, in this case by iodosobenzene, to generate a highly reactive 0x0 intermediate. The 0x0 is then transferred directly to the sulfur to form the sulfoxide in the enantioselectivity-determining step. A representative example is illustrated in Equation 14.12 that involves a chiral salen-based catalyst. ... 

For substituted epoxides (i.e. R H) nucleophilic attack of monomer by the active anion takes place at the least sterically-hindered CH2 carbon atom to give the head-to-tail structures shown above. Also, the configuration of the asymmetric carbon atom (—CHR—) in the monomer is retained in the polymer. Chain transfer to monomer can be a significant side reaction if the substituent group possesses hydrogen atoms on the a-carbon atom, e.g. for propylene oxide... 

Both the oxygen atoms of the hydroperoxide anion interact with the highly electrophilic metal ion. This weakens the 0-0 bond and facilitates transfer of the non-carbon-bonded oxygen atom to the alkene in a concerted manner. As we will see in Section 8.5.3, this general mechanism with some modifications also operates in asymmetric epoxidation reactions, where organic hydroperoxides are used. 

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