Is cramming necessary for chemistry

Cram's rule
Felkin-Ahn model

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The empirically established models allow the prediction of the selectivity of nucleophilic attacks on carbonyl centers.

The Bürgi-Dunitz angle of 103 ° describes the direction of attack of a nucleophile:

D. J. Cram postulated the following transition state (TS), which allows the nucleophile to attack with the Bürgi-Dunitz angle:

In the Cram-ÜZ the nucleophile attacks from the sterically less hindered side - that is anti to the large group and across the smallest substituent. In the anti-Cram-ÜZ the attack of the nucleophile takes place anti to the middle group (by-products). Anti-cram and cram products can be separated (for example using GC) because they are diastereomers:

-CH3 e.g. from CH3MgI

H. Felkin and N. T. Ahn extend the model to include electron withdrawing groups (EWG, electron withdrawing group). This is where the nucleophile comes into play anti to the EWG (electrostatic repulsion between EWG and Nu!):

If there is a complexing group (X) in the electrophile, a Cram-Chelat-ÜZ is run through together with strong Lewis acids (M, metals) - a pre-organization takes place before the attack:

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