Reaction Coordinates of Catalytic Enantioselective Reactions

Although reaction mechanisms in asymmetric catalysis vary greatly, each must contain a step that controls the enantioselectivity, often called the enantioselectivity-determining step. This enantioselection occurs in the first irreversible step that takes place through diastereomeric transition states. Later steps in the reaction mechanism have no impact on the enantioselectivity of the process. 

Reactions with a Singie Enantioselectivity-Determining Step 

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.  

Reactions with Reversiblity Prior to the Enantioselectivity-Determining Step  

The Curtin-Hammett Principle Applied to Asymmetric Catalysis 

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