[chr147]

I understand that s-cis dienes have both double bonds on the same side (in respect to the single bond in between them) and s-trans double bonds are on either side (in respect to the single bond in between them). However, if a few dienes are all s-trans, can steric effects play a role in how fast they react when they try to rotate into s-cis for a diels alder?

Thanks in advance!

[mmman]

I'm not sure if im qualified to answer this but I am learning the same stuff right now and I can tell you how I understand it. So let's take the example of (2E,4E)-2,4-hexadiene vs. (2E,4Z)-2,4-hexadiene. For the (2E,4E) version to get into it's s-cis conformation it costs less energy than for the (2E,4Z) because of the methyl-hydrogen van der waals repulsion in the (2E,4Z) version vs. just the hydrogen-hydrogen interaction in the (2E,4E) version. In other words the equilibrium constant between the two conformations is larger for the (2E,4E) version. Therefore at any given time you will have more of the (2E,4E) s-cis conformer to react making it the faster reacting isomer.

Attached is a pretty good picture Loudon gives.

[Dan]

However, if a few dienes are all s-trans, can steric effects play a role in how fast they react when they try to rotate into s-cis for a diels alder?

I think I have observed this effect experimentally.

I was trying to develop a formal 4+1 cycloaddition reaction a while back (sorry, can't give details for IP reasons), and found that it only worked well in cases where the substitution patterns biased the system towards s-cis conformation through 1,3-allylic strain (similar to that shown in mmman's) post.