Stereoselectivity

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

Transition states

The explanation of the stereoselectivity occurs in the theory of transition states. The reaction pathway with the lowest activation energy is preferably run, so that one stereoisomer is formed preferentially. A simple example is the nucleophilic substitution by SN2 mechanism with Walden inversion.

Also in the elimination of hydrogen chloride from 2 -chlorobutane, the cis - and trans-isomers of 2-butene can occur stereoselectively:

Trans-2- butene

The degree of stereoselectivity depending on the energy difference between the transition state and the reaction temperature. The transition state can be influenced among others by the stereochemical structure of the catalyst used and the solvent used.

For enantioselective reactions then one speaks of diastereotopic transition states. Diastereotopic transition states can develop only if at least one of the reactants is chiral (induced stereoselectivity ). Enzymes act by selectively lowering the energy of a specific transition state as stereoselective catalysts. An example of a stereoselective reaction of organic synthetic chemistry is the Sharpless epoxidation.

Are more stereoisomers theoretically conceivable as reaction products, but only a transition state is passed through, is called the reaction stereospecifically. A theoretical model calculating the yield of stereoselective reactions have been developed by Ernst Ruch and Ivar Ugi.

Rules exist to predict the formation of the preferred stereoisomer For special types of reaction. To allow the Cram's rule, a qualitative prediction of the stereochemical outcome of a diastereoselective nucleophilic addition of organometallic compounds to a carbon- oxygen double bond, which has a chiral center with three substituents of different space filling in α - position.