Coarctate reactions as a third fundamental organic-mechanistic type.

According to Herges[1],[2] 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[3] 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.[4]

Click for 3D

Click for 3D

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.

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.


  1. R. Herges, "Coarctate transition states: the discovery of a reaction principle", Journal of Chemical Information and Computer Sciences, vol. 34, pp. 91-102, 1994.
  2. B.S. Young, R. Herges, and M.M. Haley, "Coarctate cyclization reactions: a primer", Chemical Communications, vol. 48, pp. 9441, 2012.
  3. C. Berger, C. Bresler, U. Dilger, D. Geuenich, R. Herges, H. Röttele, and G. Schröder, "A Spontaneous Fragmentation: From the Criegee Zwitterion to Coarctate Möbius Aromaticity", Angewandte Chemie International Edition, vol. 37, pp. 1850-1853, 1998.<1850::AID-ANIE1850>3.0.CO;2-B
  4. Henry S. Rzepa., "Gaussian Job Archive for C4H6O3", 2013.


2 Responses to “Coarctate reactions as a third fundamental organic-mechanistic type.”

  1. Totgia says:

    Coarctate reactions is really a new concept. I’m still having hard time to understand it 🙁

  2. […] Chemistry with a twist « Coarctate reactions as a third fundamental organic-mechanistic type. […]

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