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Topic: Crossed aldol produces wagner meerwein rearrangement  (Read 4929 times)

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Offline ?synthetic?

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Crossed aldol produces wagner meerwein rearrangement
« on: May 07, 2014, 06:13:33 AM »
hi, stuck on something i did, that i cant explain. when i attempted the aldol reaction of  3-(benzyloxy)-2,2-dimethylpropanal it didnt work. my old supervisor said it was because it underwent a wagner meerwein rearrangement (one of the methyl migrated). to me from the nmr i couldnt tell how he knew because it looked like a mess. how and why would this happen. i have looked at the mechanism of this rearrangement and understand it sort of but why in this case. how could i reason it for my thesis. ps the aldols that were attempted were, evan's oxazolidinone using boron triflate, oppolzer sultam using ticl4 and then a direct crossed aldol using proline and propanal. thanks in advance. he also said this was common for this type of aldehyde (one with gem methyls)??

Offline discodermolide

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #1 on: May 07, 2014, 07:43:00 AM »
Did you use a base in the mixture such as triethylamine?
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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #2 on: May 07, 2014, 03:13:50 PM »
thanks for your swify reply, yes i did! for both the aux steps. i looked at the mechanism, most ive seen so far are acid cataylsed dehydrations. if you would please i'd be grateful if you could explain to me how the base works. thanks in advance

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #3 on: May 07, 2014, 03:45:33 PM »
hi, i've a think and posted a mechanism. would this be what happens? or i am completely wrong? thanks

Offline discodermolide

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #4 on: May 07, 2014, 05:12:39 PM »
You make the enolate from the acyl Evans by treating with dibutylboron triflate followed by triethylamine. This makes the trans enolate. Then you throw in the aldehyde and let it warm up to do the aldol reaction.
Excess boron triflate so could cause rearrangements to happen so keeping the base in excess should prevent this from happening.
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Offline Dan

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #5 on: May 07, 2014, 05:20:54 PM »
hi, i've a think and posted a mechanism. would this be what happens? or i am completely wrong? thanks

Check your curly arrows, they are not consistent with the structures you have drawn. What is the triethylamine doing in the first step?

Do you have any evidence to support your proposed product?
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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #6 on: May 08, 2014, 07:33:24 AM »
Thank you discodermolide and Dan. I thought that the  base could abstract the acidic proton, leaving behind the carbocation enabling the rearrangement?? Evidence...loss of gem methyl but there's also no evidence of the hydroxyl group either, NMR messy even after purification.

Offline discodermolide

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #7 on: May 08, 2014, 07:56:46 AM »
How exactly did you carry out this experiment?
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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #8 on: May 08, 2014, 08:10:28 AM »
Basic method

evans method: oxazolidinone in DCM at 0°C, added n-BuOTf followed by Et3N, cooled to -78°C then added aldehyde.

sultam method: sultam in DCM at 0°C, added n-BuOTf followed by DIPEA, cooled to -78°C then added aldehyde followed by TICl4.

Looking back, in both cases the boron triflate and base were in excess

Offline discodermolide

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #9 on: May 08, 2014, 10:39:25 AM »
I had a side reaction in this procedure caused by the excess of boron triflate. A protecting group fell off and the thing it was protecting reacted. This to the amount of excess boron triflate. Try cutting down on this.
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Offline kriggy

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #10 on: May 08, 2014, 11:09:54 AM »
Thank you discodermolide and Dan. I thought that the  base could abstract the acidic proton, leaving behind the carbocation enabling the rearrangement?? Evidence...loss of gem methyl but there's also no evidence of the hydroxyl group either, NMR messy even after purification.

How can you have carbocation when H+ is abstracted? Or am I missing something?

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Re: Crossed aldol produces wagner meerwein rearrangement
« Reply #11 on: May 08, 2014, 11:11:28 AM »
I thought that the  base could abstract the acidic proton, leaving behind the carbocation

Proton abstraction (deprotonation) does not leave cations behind.
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