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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on July 19, 2018, 06:07:54 AM

Title: Grignard reagents with ketones
Post by: xshadow 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

Title: Re: Grignard reagents with ketones
Post by: zarhym 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.

Title: Re: Grignard reagents with ketones
Post by: sjb on July 19, 2018, 08:53:32 AM

Quote from: zarhym on July 19, 2018, 06:14:11 AM

This will be a SN2 reaction.

So what's the leaving group?

Quote from: zarhym on July 19, 2018, 06:14:11 AM

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.

Title: Re: Grignard reagents with ketones
Post by: Babcock_Hall 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?

Title: Re: Grignard reagents with ketones
Post by: clarkstill on July 20, 2018, 05:30:28 AM

Assuming R1 and R2 are different, the ketone is prochiral https://en.wikipedia.org/wiki/Prochirality (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.

Title: Re: Grignard reagents with ketones
Post by: zarhym on July 21, 2018, 04:02:18 AM

Quote from: sjb on July 19, 2018, 08:53:32 AM

Quote from: zarhym on July 19, 2018, 06:14:11 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.

Title: Re: Grignard reagents with ketones
Post by: wildfyr on July 21, 2018, 08:12:16 AM

It is not an SN2, there is no direct leaving group