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 Pichon (Associate Editor, Nature Chemistry)