[Babcock_Hall]

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.

[rolnor]

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

[Babcock_Hall]

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

[Guitarmaniac86]

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.

[rolnor]

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

[Babcock_Hall]

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.

[rolnor]

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

[Babcock_Hall]

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

[rolnor]

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

[Babcock_Hall]

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

[rolnor]

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