Henry Rzepa's Blog

According to Herges[cite]10.1021/ci00017a011[/cite],[cite]10.1039/c2cc34026g[/cite] the mechanism of single-step (concerted) reactions can be divided into three basic types; linear (e.g. substitution, elimination etc), pericyclic (e.g. Diels Alder) and a third much rarer, and hence very often overlooked type that was named coarctate. This is based on the topology of bond redistribution patterns, an explicit real example[cite]bhsn4t[/cite] illustrating:

It happens that this reaction bears a close similarity to epoxidation using peracid, the characteristic feature being that the central (spiro) atom has two bonds forming to it and two bonds breaking from it in both reactions.^†,‡ I had noted for the latter reaction that in fact the bond redistribution, although concerted, was asynchronous. This asynchrony was represented by the green arrows preceding the blue ones (or vice-versa for the reverse reaction).

So here I decided to investigate if the same might be true of the coarctate reaction shown above (ωB97XD/6-311G(d,p)/SCRF=water.[cite]10.6084/m9.figshare.787693[/cite]

The transition state is indeed interestingly asynchronous. The O-O bond (shown green above) is clearly the first to break; neither of the C-C bonds has really started to do so at the transition state. But the process remains resolutely concerted.

 

The IRC above shows clearly that the reaction has a room-temperature barrier (i.e. it is a very facile process). But missing really from this process is any hidden intermediate either (there is the merest hint at IRC = -2). So this reaction is interesting for

1. its classification apart from the normal two types of organic mechanism, as a coarctate type
2. Its asynchrony in the bond redistributions
3. but this asynchrony not resulting in any hidden intermediates.

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^† Another example was the topic of this post.

^‡ One can contrive an even higher-order reaction (thus far un-named) in which (formally) three bonds break and three bonds form at a single atom.

Tags: Ozonolysis

This entry was posted on Wednesday, September 4th, 2013 at 2:52 pm and is filed under reaction mechanism. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.