Density functional calculations of 1H NMR spectra and reaction barriers at the ωB97XD/6-311G(d,p)/continuum water level do not support the claimed identification of encarcerated 1,3-dimethylcyclobutadiene in either the solid state or aqueous solution, as reported by Barboiu et al (Chem. Eur. 2011, 17, 10021). Instead, previous suggestions that the species identified in the solid state is in fact 2-oxabicyclo[2.2.0]hex-5-en-3-one (the Dewar lactone Me22) are reaffirmed. Analysis of the ground-state electronic structure of this species indicates an unusual π-anomeric effect is promoting a Dunitz-like chemical reaction pathway leading to the eventual elimination of carbon dioxide and formation of 1,3-dimethylcyclobutadiene.
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