Stereoselectivity

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Scheme to illustrate the transition states

Stereoselectivity is a term used in stereochemical dynamics . A reaction proceeds stereoselectively if one of several possible stereoisomers is predominantly or exclusively formed.

Transitional states

The explanation of the stereoselectivity is based on the theory of transition states . The reaction path with the lowest activation energy is preferably followed, so that a stereoisomer is preferentially formed. A simple example is the nucleophilic substitution according to the S N 2 mechanism with Walden reversal .

Sn2 mechanism

Even with the elimination of hydrogen chloride from 2-chlorobutane , the cis and trans isomers of 2-butene can be formed stereoselectively:

  • cis -2-butene

  • trans -2-butene

The extent of the stereoselectivity depends on the energetic difference between the transition states and the reaction temperature. The transition state can be influenced, among other things, by the stereochemical structure of a catalyst and the solvent used.

In the case of enantioselective reactions, one speaks of diastereotopic transition states . Diastereotopic transition states can only develop if at least one of the reaction partners is chiral ( induced stereoselectivity ). Enzymes act as stereoselective catalysts by specifically lowering the energy of a specific transition state. Sharpless epoxidation is an example of a stereoselective reaction in synthetic organic chemistry .

If several stereoisomers are theoretically conceivable as reaction products, but if only one transition state is passed through, the reaction is called stereospecific . A theoretical model for calculating the yield of stereoselective reactions was developed by Ernst Ruch and Ivar Ugi .

For specific types of reactions there are rules that predict the formation of the preferred stereoisomer. Thus, the allowed Cram usually a qualitative prediction of the stereochemical outcome of diastereoselective nucleophilic addition of organometallic compounds to a carbon-oxygen double bond in α-position a chiral center with three substituents having different space-filling.

See also

Individual evidence

  1. ^ Ernest L. Eliel: Stereochemistry of carbon compounds , Weinheim, Verlag Chemie, (1966).
  2. ↑ Ernst Ruch, Ivar Ugi: The stereochemical structure model, a mathematical model for the group-theoretical treatment of dynamic stereochemistry , in: Theoret. Chim. Acta , 1966, 4, 287-304, doi : 10.1007 / BF00528481 .
  3. ^ Donald J. Cram , Fathy Ahmed Abd Elhafez: Studies in Stereochemistry. X. The Rule of "Steric Control of Asymmetric Induction" in the Syntheses of Acyclic Systems . In: J. Am. Chem. Soc. ; 1952; 74 (23); 5828–5835, doi : 10.1021 / ja01143a007 .

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