Comment on "Single-crystal X-ray structure of 1,3-dimethylcyclobutadiene by confinement in a crystalline matrix".

Legrand et al. (Reports, 16 July 2010, p. 299) reported the experimental observation of square-planar and rectangular-bent geometries of 1,3-dimethylcyclobutadiene (Me(2)CBD) confined within a crystalline matrix. However, we found no evidence for the Me(2)CBD formation. We argue that the experimental x-ray density data are better attributed to the bicyclic β-lactone intermediate where carbon dioxide is covalently bound to cyclobutadiene.