francis - 16-10-2011 at 21:09
I have a question from a lab which asks:
Why does trans,trans-1,4-diphenylbutadiene undergo a Diels-Alder cycloaddition with maleic anhydride, but the cis,trans-1,4-diphenylbutadiene does
not?
I have tried explaining why I think it does not react, in this image:
I would think that, Due to the steric bulk of the Ph group, the molecule prefers the s-trans conformation. However it cannot react with the dienophil
in this conformation, because the product bond lengths would be too great (it would be strangely out of shape).
The trans,trans-1,4-diphenylbutadiene on the other hand, has an s-cis conformation where the Ph groups are both facing away from each other, so there
is less steric preference for the s-trans conformation.
Would this be correct? Is there another reason why cis,trans-1,4-diphenylbutadiene would not react with maleic anhydride?
Nicodem - 17-10-2011 at 07:53
If you draw or build a molecular model, you can see that a cis,trans isomer does not have any stable cisoid diene comformations which are required for
the Diels-Alder mechanism.
