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Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: Babcock_Hall on September 10, 2021, 10:21:54 AM

Title: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Babcock_Hall on September 10, 2021, 10:21:54 AM

Whenever we do this reaction using an alkyl aldehyde, we see a small amount of a side product that elutes more quickly than the main product.  It gives a positive reaction to permanganate stain.  IIRC when I took an NMR one time, I saw signals in the vinyl region, but I did not analyze the signals at the time (I may do so in the near future).  I do not recall seeing a side produce when an aromatic aldehyde is used.  The base in our Horner Wadsworth Emmons reactions is typically DBU.  I imagine that our aldehyde is somehow doing an elimination, but the oxidation state must be accounted for, and I don't have a good mechanism thought out.  As long as this is a minor reaction, I doubt that it is costing us much in yield, but I want to think ahead in case it does in the future.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: rolnor on September 10, 2021, 03:18:19 PM

Its a little tricky to follow your thoughts whithout the structure of your aldehydes, I have to little input.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Babcock_Hall on September 10, 2021, 03:44:19 PM

The aromatic aldehyde is  4-formyl pyridine. Our aliphatic aldehydes have a protected carboxylic acid at the far end of a straight chain.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Guitarmaniac86 on September 10, 2021, 05:12:15 PM

How sure are you that you are not getting the other isomer? It could be that, but I do not think it accounts for it eluting earlier... I would expect both the E and Z isomers to practically co elute unless you run a really slow gradient.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: rolnor on September 11, 2021, 02:12:20 AM

The Z isomer could form cyclic internal hydrogen-bond wich gives a less polar product.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Babcock_Hall on September 13, 2021, 10:44:58 AM

It could be the cis-isomer.  One expected peak in the H-1 NMR spectrum is not accounted for.  However, there is very little of this product, and I might have missed it.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: rolnor on September 13, 2021, 02:54:21 PM

If you look at it, can it form a 6-membered ring with a hydrogen bond? You have carbonyl and NH2-protons?

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Babcock_Hall on September 14, 2021, 11:39:50 AM

One of the amide hydrogen atoms may be overlapping with a hydrogen from the protecting group.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: rolnor on September 15, 2021, 12:45:12 AM

If the PG containes a carbonyl  this can bind to the amide hydrogen? Six membered ring?

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: Babcock_Hall on September 15, 2021, 10:17:27 AM

I can form an 8-membered ring involving a hydrogen bond.

Title: Re: Horner Wadsworth Emmons reactions of alkyl vs. aryl aldehydes
Post by: rolnor on September 15, 2021, 11:02:05 AM

Thats not very stable so less likely to give high Rf on TLC.