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Topic: Is this phosphonium ylide giving ~100% E product?  (Read 5574 times)

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Offline Altered State

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Is this phosphonium ylide giving ~100% E product?
« on: November 02, 2013, 02:17:49 PM »


I have this phosphonium ylide, and it reacts with an aldehyde (propanal) to undergo wittig reaction.

I know that stabilized ylides give E product, and this is kinda stabilized but the only examples I saw in textbooks or notes, cover alkenes stabilized by carbonyl groups, aromatic ring resonance... And even in some of those cases, the reaction doesen't yield 100% E alkene (e.g. 90:10), and this is logical because wittig reaction is stereoselective, not stereospecific.
What do you think about this case? Maybe 90:10 E:Z? Or lower E %? Maybe even more Z product than E?

Offline zsinger

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #1 on: November 02, 2013, 03:27:51 PM »
NEVER 100 Percent.  Maybe close.  Study Horner-Wadsworth Modification……..might help your understanding a bit better.
"The answer is of zero significance if one cannot distinctly arrive at said place with an explanation"

Offline Altered State

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #2 on: November 02, 2013, 03:57:58 PM »
NEVER 100 Percent.  Maybe close.  Study Horner-Wadsworth Modification……..might help your understanding a bit better.

Yes, I know that, never 100%, but what I want to know is what would be the case for this particular kind of allylic phosphonium ylides. What would be major product, E or Z?

HWE reaction involves a particular kind of stabilized ylide, so the reaction yields E alkene as major product, but this is not the case...

Offline Altered State

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #3 on: November 04, 2013, 07:12:33 AM »
Anyone has a clue on this?

Offline AlphaScent

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #4 on: November 04, 2013, 10:08:41 AM »
I think your question is too specific.  I atleast cannot for sure predict the outcome of that particular reaction.  The Wittig reaction is a great reaction but can be limited in its use.  The addition of a methylene group is how most often the Wittig is employed.  Most will use Horner-Wadsworth for strictly E conversion.  In your case I would look at Schlosser modification.  This maybe a better way for you to convert to the diene you would like.

Also, you would most likely do a Witting using the conjugated aldehyde.
If you're not part of the solution, then you're part of the precipitate

Offline Altered State

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #5 on: November 04, 2013, 10:52:50 AM »
I think your question is too specific.  I atleast cannot for sure predict the outcome of that particular reaction.  The Wittig reaction is a great reaction but can be limited in its use.  The addition of a methylene group is how most often the Wittig is employed.  Most will use Horner-Wadsworth for strictly E conversion.  In your case I would look at Schlosser modification.  This maybe a better way for you to convert to the diene you would like.

Also, you would most likely do a Witting using the conjugated aldehyde.

Yes, there may be few better ways to obtain that kind of dienes.

Maybe I have not made myself clear enough, I'm not trying to yield a certain compound, I'm trying to predict the result of that reaction.

The whole problems is:
First Ph3P reacts with 3-bromoprop-1-ene, and then deprotonation with n-BuLi to get the ylide.
Finally we make it react with propanal to yield an alkene.

I'm expected to explain stereochemistry of the final product, and even when I know why stabilized ylides yield mostrly E product and unstabilized ones Z product, can't tell if this yilide, being not as stable as, for example, an α-carbonyl ylide, but way more stable than a simple alkane-ylide, would yield mostly E or if it would be something like E:Z = 60/40, or maybe mostly Z...

Maybe it's still a very specific question, in that case, I guess an explanation like this would work.
Wish I had acces to a lab to try it out :(
Running some experiments would make it clear.

Offline AlphaScent

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #6 on: November 08, 2013, 12:13:40 PM »
Ahh ok, I apologize.

I believe this will proceed to a major product of a Z alkene.  When a ylide is stabilized it is by either lithium salts added to the reaction (SChlosser) or because a electron-withdrawing group is present to stabilize the ylide.  I do not think that an alkene alone is electron with drawing enough to allow for strictly E conversion. 

A phenyl group being present does allow for E, so it stands to reason my statement above could be wrong.   I do not speak in absolute truths Mr. State, but that is my reasoning to your question. 



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Offline Altered State

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Re: Is this phosphonium ylide giving ~100% E product?
« Reply #7 on: November 08, 2013, 04:49:28 PM »
I do not speak in absolute truths Mr. State, but that is my reasoning to your question.

And so is mine, thanks for feedback.

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