Debating cyclobutadiene

Mar 26, 2019
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The notoriously strained and unstable cyclobutadiene ring is currently creating quite a stir. In a recent Science report – which we highlighted here – Mihail Barboiu’s group described the preparation of a calixarene-based matrix, in which they encapsulated a dimethyl-a-pyrone. UV irradiation converted the pyrone precursor to a bicyclic ß-lactone intermediate, which was in turn converted by photolysis into carbon dioxide and dimethyl-cyclobutadiene — the strained ring being stable enough to be observed directly by X-ray crystallography, both in its square-planar and rectangular-bent geometries.

The X-ray analysis, however, has not convinced David Scheschkewitz, nor Igor Alabugin’s group, who responded in the form of technical comments. These articles, published in Science last week, along with a response from Barboiu’s team, are part of a lively discussion which started a couple of months ago on Henry Rzepa’s blog — see here, now here and, Rzepa announces, also in a soon-to-be published article.

In a nutshell, the main point of the discussion is whether the X-ray data does show two separate species — dimethyl-cyclobutadiene and carbon dioxide — confined together within the host matrix, or whether the strong van der Waals interactions reported between the two species are in fact covalent (or partially ionic-covalent). In the latter case, it would thus be the lactone (suggested by Barboiu and colleagues to be an intermediate) that is really observed.

Considerations on possible bond lengths and angles; X-ray diffraction analysis of disordered species; calculations about whether the UV irradiation does (or can) sufficiently excite the lactone; difficulties in interpreting absorption data for confined species, where the host matrix may (or may not) play a crucial role — the arguments invoked by all parties involved certainly make for a very interesting debate.

Anne

Anne Pichon (Associate Editor, Nature Chemistry)


Anne Pichon

Senior Editor, Nature Chemistry, Springer Nature

Anne received a broad training in chemistry at the National Graduate School of Chemistry in Montpellier, France. She then focused on inorganic and supramolecular chemistry and obtained her MPhil and PhD degrees from the Queen's University Belfast, UK, investigating porous coordination polymers for host–guest applications. After an internship with Nature Reviews Drug Discovery, Anne moved to John Wiley and Sons in 2007 as an assistant editor of the Society of Chemical Industry journals. She joined Nature Chemistry in October 2008, and was initially based in Tokyo where she also worked on other publishing projects with Nature Asia-Pacific. In April 2013, Anne relocated to the London office and now works full time on the journal.

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