Henry Rzepa's Blog Chemistry with a twist

March 5, 2017

Stable “unstable” molecules: a crystallographic survey of cyclobutadienes and cyclo-octatetraenes.

Cyclobutadiene is one of those small iconic molecules, the transience and instability of which was explained theoretically long before it was actually detected in 1965.[1] Given that instability, I was intrigued as to how many crystal structures might have been reported for this ring system, along with the rather more stable congener cyclo-octatetraene. Here is what I found.



  1. L. Watts, J.D. Fitzpatrick, and R. Pettit, "Cyclobutadiene", Journal of the American Chemical Society, vol. 87, pp. 3253-3254, 1965. http://dx.doi.org/10.1021/ja01092a049

March 21, 2013

To be cyclobutadiene, or not to be, that is the question? You decide.

Filed under: Interesting chemistry — Tags: , , , — Henry Rzepa @ 1:12 pm

A quartet of articles has recently appeared on the topic of cyclobutadiene.[1],[2],[3],[4]. You will find a great deal discussed there, but I can boil it down to this essence. Do the following coordinates (obtained from a (disordered) previously published[5] x-ray refinement) correspond to a van der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide, or do they instead represent a covalent interaction between these two components resulting in a compound with the chemical name 2-oxabicyclo[2.2.0]hex-5-en-3-one (i.e. not a cyclobutadiene)?



  1. H.S. Rzepa, "A Computational Evaluation of the Evidence for the Synthesis of 1,3-Dimethylcyclobutadiene in the Solid State and Aqueous Solution", Chemistry - A European Journal, vol. 19, pp. 4932-4937, 2013. http://dx.doi.org/10.1002/chem.201102942
  2. M. Shatruk, and I.V. Alabugin, "Reinvestigation of “Single-Crystal X-ray Structure of 1,3-dimethylcyclobutadiene”", Chemistry - A European Journal, vol. 19, pp. 4942-4945, 2013. http://dx.doi.org/10.1002/chem.201103017
  3. Y. Legrand, D. Dumitrescu, A. Gilles, E. Petit, A. van der Lee, and M. Barboiu, "A Constrained Disorder Refinement: “Reinvestigation of “Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene” by M. Shatruk and I. V. Alabugin”", Chemistry - A European Journal, vol. 19, pp. 4946-4950, 2013. http://dx.doi.org/10.1002/chem.201203234
  4. Y. Legrand, D. Dumitrescu, A. Gilles, E. Petit, A. van der Lee, and M. Barboiu, "Reply to A Computational Evaluation of the Evidence for the Synthesis of 1,3-Dimethylcyclobutadiene in Solid State and Aqueous Solution-Beyond the Experimental Reality", Chemistry - A European Journal, vol. 19, pp. 4938-4941, 2013. http://dx.doi.org/10.1002/chem.201203235
  5. Y. Legrand, A. van der Lee, and M. Barboiu, "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix", Science, vol. 329, pp. 299-302, 2010. http://dx.doi.org/10.1126/science.1188002

September 18, 2011

Some fun with no-go areas of chemistry: cyclobutadiene.

Organic chemistry has some no-go areas, where few molecules dare venture. One of them is described by a concept known as anti-aromaticity. Whereas aromatic molecules are favoured species, their anti-equivalent is avoided. I previously illustrated this (Hückel rule) with cyclopropenium anion. Now I take a look at cyclobutadiene, for which the π-system is said to be iso-electronic (where two electrons in a double bond have replaced the carbanion lone pair).


August 8, 2010

Reactions in supramolecular cavities – trapping a cyclobutadiene: ! or ?

Cavities promote reactions, and they can also trap the products of reactions. Such (supramolecular) chemistry is used to provide models for how enzymes work, but it also allows un-natural reactions to be undertaken. A famous example is the preparation of P4 (see blog post here), an otherwise highly reactive species which, when trapped in the cavity is now sufficiently protected from the ravages of oxygen for its X-ray structure to be determined. A colleague recently alerted me to a just-published article by Legrand, van der Lee and Barboiu (DOI: 10.1126/science.1188002) who report the use of cavities to trap and stabilize the notoriously (self)reactive 1,3-dimethylcyclobutadiene (3/4 in the scheme below). Again sequestration by the host allowed an x-ray determination of  the captured species!


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