Wittig reaction question

[aajjeee]

I have done a Wittig reaction from cinnamaldehyde and Benzyl triphenyl phosphonium to obtain diphénylbutadiène (i might be using the french notation i have joined the reaction) and i am supposed to obtain a mix of trans-cis and trans-trans product, but while drawing them, i found 2 molecule each that would fit the description.

my question is: is it true that only the top 2 (in the drawing) are found , and how can i distinguish the 1st & 4th and the 2nd & 3rd.

I also added the mechanism which i drew, i find the 4th one to come more naturally, but the proffessor said his source found it to almost only give trans-trans product

(edit) well i think there may be rotation just after the aldehyde is attacked, so only the most stable of the 2 possibilities of trans trans or cis-trans is formed

and i would think that this is why you only observe 2 of the 4, But i am still wondering how you would differentiate them.

Edited April 1, 2017 by aajjeee

[Elite Engineer]

The trans-trans product will be majority/ most abundant product because it is more stable. However there will always be minor products as well due to "adequate" energy conditions and certain bonding angles that happen to form bonds just right..that being said your professor is correct to say that the trans-trans product is found exclusively by itself, or at least with some cis-trans product present. This because the trans-trans and cis-trans product likely makes up more than 97-99% with < 2% to 1% being the cis-cis minor products. At the < 2% to 1% range is it helpful or even necessary to pay attention to the "other" products that were formed? Yes, they're there...but mostly irrelevant.

 

As far as differentiating the products, you'd have to separate therm..im guessing chiral chromatography or diasteremoization?

Edited April 2, 2017 by Elite Engineer

[aajjeee]

but why is it that in general wittig reactions lead to the cis product (the original attak is done in the most stable way, but the rotation to create the 4 member ring intermediate brings them closer) but this one sould lead to the trans product.

(sidenote, i actually got pretty close to a racemic mix, with 56% trans-trans 44% cis trans.

when i asked how i would differenciate them, i was asking nomenclature-wise (the 2nd and 3rd products are both trans-trans-diphénylbutadiène in my book, but they clearly are different)