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Topic: Wittig Reaction  (Read 3298 times)

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Offline Captain Teemo

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Wittig Reaction
« on: March 25, 2014, 09:00:35 AM »
Hi guys,

So I am reacting the phosphonium bromide of Ethyl 4-bromobutanoate with cyclohexanone to get 4-cyclohexylidenebutyric acid in the end.

The procedure is to use potassium tert-butoxide as the strong base to form the carbanions from the phosphonium bromide salt.
The literature says that the solution should turn deep red/orange.

However, in my case, when I added the phosphonium salt (dissolved in DMF) to a suspension of t-OBuK in DMF under Nitrogen, I don't get the deep red solution at all.

What does it suggest?
Have the carbanions been formed?


Thank you!! :)

Offline discodermolide

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Re: Wittig Reaction
« Reply #1 on: March 25, 2014, 09:03:06 AM »
Try doing the addition the other way round.
Also you can take some of your "ylide solution" and see if it reacts, check by TLC. If it doesn't then add more base.
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Offline Captain Teemo

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Re: Wittig Reaction
« Reply #2 on: March 25, 2014, 09:25:15 AM »
Thanks Disco! :D
i'll try to do that

but the thing is K-tOBu is not soluble in DMF, it makes a kind of suspension and might be hard to do the addition the other way round?

Offline discodermolide

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Re: Wittig Reaction
« Reply #3 on: March 25, 2014, 09:29:52 AM »
I think you can buy solutions in THF, check Aldrich. Also look for "instant ylide".
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Offline Captain Teemo

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Re: Wittig Reaction
« Reply #4 on: March 25, 2014, 09:33:17 AM »
Sure, thankss!! :D

Offline Eloquentia

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Re: Wittig Reaction
« Reply #5 on: March 25, 2014, 11:26:51 AM »
tBuOK should be decently soluble in DMF. The suspension you see might also be of Potassium bromide. Do you use dry enough DMF?  tBuOK is not extremely sensitive to traces of water, but the ylide sure might be.

Offline zsinger

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Re: Wittig Reaction
« Reply #6 on: March 26, 2014, 11:29:30 AM »
Disco beat me to it.  Try, if possible, to ALWAYS make the ylide first, as that is always going to be the reactive species.  Making it first usually precludes it from being messed with in situ :).  Hope this helps.
             -Zack
« Last Edit: March 26, 2014, 12:20:21 PM by zsinger »
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Offline Captain Teemo

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Re: Wittig Reaction
« Reply #7 on: March 29, 2014, 09:49:20 PM »
Thanks so much for your help guys! :)

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