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Topic: Grignard reagents with ketones  (Read 2554 times)

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Offline xshadow

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Grignard reagents with ketones
« on: July 19, 2018, 06:07:54 AM »
Hi...I don ' t understand the strereochemistry of this reaction,for example if I have:

"R1COR2" +  R3MgX

I'll get:
R1R2R3COH

But C has a stereogenic center....so which enantiomer will we get?

R or S on carbon? Or both?

Thanks

Offline zarhym

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Re: Grignard reagents with ketones
« Reply #1 on: July 19, 2018, 06:14:11 AM »
This will be a SN2 reaction. Normally,  the strereochemistry of the center depends on the structure of R1 2 and 3. 
If your R1 2 and 3 are not very bulky group, very likely you are going to get a R/S mixture. The Nu can attack from both side of CO plane.

Offline sjb

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Re: Grignard reagents with ketones
« Reply #2 on: July 19, 2018, 08:53:32 AM »
This will be a SN2 reaction.

So what's the leaving group?

Normally,  the strereochemistry of the center depends on the structure of R1 2 and 3. 
If your R1 2 and 3 are not very bulky group, very likely you are going to get a R/S mixture. The Nu can attack from both side of CO plane.

It doesn't really depend on the bulk (as such) of the reactants, but yes, if there are enantiomers involved you may get diastereoselectivity.

Online Babcock_Hall

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Re: Grignard reagents with ketones
« Reply #3 on: July 19, 2018, 01:42:48 PM »
@OP,

Consider two cases:  One, R1 and R2 are identical.  Two, the identities of the two R groups are not identical.  How are these cases different from the point of view of stereochemistry?

Offline clarkstill

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Re: Grignard reagents with ketones
« Reply #4 on: July 20, 2018, 05:30:28 AM »
Assuming R1 and R2 are different, the ketone is prochiral https://en.wikipedia.org/wiki/Prochirality

The grignard reagent can attack either prochiral face of the ketone, with attack on each face leading to each of the two possible enantiomers.

If R1 and R2 contain no stereogenic centres, and no other chiral catalysts or reagents are used, then the transition state energy for attacking each prochiral face is identical, so the rate of forming both the (R) and (S) products should be equal. As a result, you will get a racemic mixture of the two enantiomeric products.

Offline zarhym

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Re: Grignard reagents with ketones
« Reply #5 on: July 21, 2018, 04:02:18 AM »
This will be a SN2 reaction.

So what's the leaving group?


It's Nu attack to the carbonyl carbon.
I guess I blur the definition of SN2 reaction over time.
Thanks.

Offline wildfyr

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Re: Grignard reagents with ketones
« Reply #6 on: July 21, 2018, 08:12:16 AM »
It is not an SN2, there is no direct leaving group

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