[wereworm73]

I was wondering...If you introduced propenenitrile to trans,trans,trans,trans-2,4,6,8-decatetraene, would the dienophile prefer to react with the internal pair of pi-bonds (at carbons 4 & 6) or one of the two terminal pairs (carbons 2 & 4 or carbons 6 & 8 )?  Or is there no significant preference and a 2:1 mixture results?


Me--C      C---C       C--Me
       \\     //     \\     //         
        C---C      C---C

[movies]

You'd almost certainly get a mixture, but I don't know which would really be preferred.  If I had to guess I think I would go with either the diene on the left or the one in the middle.  The one on the right is less electron rich than the other two so it should be the least reactive.

I'm trying to think about how the orbitals would work out on that system and I guess the largest orbitals on the HOMO of the diene would be with the middle one, so maybe that would be preferred.  It might depend on the dienophile, however, so that you would get appropriate size-matching of the orbitals.

[wereworm73]

Interesting...an argument in favor of the middle pair.  I didn't think about the orbitals in the middle pair possibly being larger than the others.

Actually, I was thinking it would favor the outer pairs more (possibly making up more than 80% of the product).  It looks like only one bond (between carbons 2 & 3 or 7 & 8 ) would need to be in the s-cis conformation, and also the product would still contain conjugated bonds.  If there was a Diels-Alder reaction in the middle pair, then the product would only have isolated bonds (higher energy), and I think there would need to be at least 2 bonds in the s-cis conformation because if there was only one bond in s-cis (between carbons 4 & 5), the molecule would look something like a big, jagged V and might repel the dienophile too much (though propenenitrile is relatively small).