August 22, 2019, 01:10:54 AM
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Topic: alpha-Racemization in Aldol Addition/Condensation with asymmetric carbonyls  (Read 119 times)

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Offline pretysmitty

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So if you added asymmetric aldehydes/ketones with a chiral center via aldol addition, will the product form a racemic mixture AND be dependent on kinetic/thermodynamic conditions? I haven't found an example mechanism online, but it would really help to see one.

What I'm having trouble with is adding the second reagent to the enol form of the first reagent. The enol form has a double bond which allows for the 2nd reagent to attack from either side (leading to R or S). The keto form retains the stereo-configuration of the original reagent, so you won't see any change in configuration.

Also to confirm: when you have an asymmetric addition, is the main thing controlling the outcome kinetic/thermodynamic control? I know that the preferred alpha carbon will be the one with the most hydrogens, but is the only other thing that matters (save for rare exceptions) kinetic/thermodynamic control?

Offline kriggy

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Can you show as an example of the reaction you mean, its bit hard to follow your post. Anyway,

a) if the electrophile is chiral, then you are likely get a mixture of diastereomers - the chiral center on the electrophile doesnt change a configuration. If you are lucky, you might get single product
b) if the nucleophile is chiral aldehyde, then it doesnt matter because you form planar enolate and lose the chirality. Leading to a mixture of diastereomers
c) If the nucleophile is chiral its ketone and you generate the enolate on the other side of the ketone, then you should get a mixture of diastereomers as well but depending on the conditions you can get a single product (see evans auxiliary)

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