Zimmerman-Traxler model

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The Zimmerman-Traxler model is used to explain the stereochemical course of organic reactions.

definition

If new stereocenters are formed in organic reactions when a CC bond is formed, the prediction of the stereoselectivity, i.e. the prediction of which stereoisomer is preferentially formed, is of particular importance. For this purpose, the model of a 6-part transition state, the Zimmerman-Traxler transition state, is used. The atoms directly involved in the reaction are arranged and held in a 6-membered ring in a chair conformation. The largest substituentis in an equatorial position, which energetically favors the state. The other substituents are arranged in such a way that the steric hindrance from the largest substituent is as low as possible. The bonds are broken and formed according to the reaction mechanism. Generally, the bonds move one position within the 6-ring. The product formed is then mapped back to the Lewis notation .

Use in aldol addition

The aldol is a classic example of using the Zimmerman-Traxler model.

E / Z enolate

Here an aldehyde reacts with an enolate . Two stereocenters are built up so that four stereoisomers can arise. A six-membered transition state is formed from the enolate, the aldehyde carbonyl group and the cation of the enolate, which coordinates to the aldehyde oxygen atom. The aldehyde can approach the enolate in four ways: The aldehyde can approach the enolate from the front or rear. In the absence of a stereocenter ( substrate control ), there is no differentiation between the two half-spaces, so that both approaches are energetically the same and a racemate results.

Arrangement of the Z-enolate
Arrangement of the E-enolate

Furthermore, the radical R of the aldehyde can be either axial or equatorial. In the axial position, there is a higher 1,3-diaxial tension , since the two radicals R of enolate and aldehyde come close to one another. Thus, the two possible transition states (Z 1 , Z 2 or E 1 , E 2 ) are energetically different and a reaction preferably occurs via the more favorable (equatorial position). The result is diastereoselectivity , with the ( Z ) -enolate preferentially producing the syn -aldol ( B ) and the ( E ) -enolate the anti- aldol ( A ).

literature

  • R. Brückner, "Reaction Mechanisms", Spektrum Verlag