A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH) review as achieving the former, and here I take a (sideways) look at one of their predictions.
Compound 286 was imagined by them as a π2+ π2+ π2+ π2 cycloaddition sustaining four antarafacial bond formations across the four bonds labelled a. To really see this is true, you have to look at a 3D model (if you can see this without a model, you must have a superb stereochemical brain) and inspect how the bonds form with respect to any of the four alkene units. What other patterns are perceptible?
Well, here is the calculated transition state for the reaction. It actually has C2 symmetry and so is a chiral geometry.
Of the four forming σ-bonds, the surprise is that two of them are really very short (1.751Å) and these involve the butadiene, and the other two are much longer (2.142Å) and involve the alkenes. The former really are short enough to call a Möbius homo-aromatic bond!Sadly, as the IRC below shows, the free energy activation barrier for the (endothermic) reaction is not thermally accessible, and so there would be little point to encourage anyone to try to synthesise this species. But it certainly shows that even hypothetical reactions like this can have interesting features worth learning about.
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Does the barrier drop significantly if one of the alkene moieties is replaced by an allyl cation moiety [thus retaining the 10 e- circuit]?
I hope it isn't a problem to submit a comment 7 years after the original post. I just found it.
Re: 7 years; its no problem at all. I have indeed noticed that unlike many a blog topic relating to "social media", chemistry tends to have a rather longer relevant "shelf life".
Regarding replacing ethene with a 3-carbon two electron unit, its an interesting suggestion. The coordinates for the TS are hyperlinked above to a data repository, so the "pain of entry" should be lower than it would normally be. If I have time I might give it a go, but also encourage others to try this sort of "experiment" out.