On 8th August this year, I posted on a fascinating article that had just appeared in Science[cite]10.1126/science.1188002[/cite] in which the crystal structure was reported of two small molecules, 1,3-dimethyl cyclobutadiene and carbon dioxide, entrapped together inside a calixarene cavity.
Snapshot of a Strained Ring Benzene and cyclobutadiene possess diametrically opposed properties. The former, a hexagonal hydrocarbon with a geometry perfectly suited to its bonding arrangement, exhibits unusual stability. The latter, with its two fewer carbons tightly squeezed into the right angles of a 4-membered ring, rapidly forms a dimer to relieve its considerable geometric and electronic strain. Monomeric cyclobutadiene was first isolated in substantial quantity by confining it within a molecular shell, but it has eluded full structural characterization. Legrand et al. (p. 299 ) have now found a host lattice that stabilizes a dimethyl-substituted derivative of the molecule sufficiently to allow analysis of its structure and bonding motif by x-ray diffraction.
Legrand et al . (Reports, 16 July 2010, p. 299) reported on the photolytic reaction of an α-pyrone confined in a crystalline matrix. Their structural analysis invoked four products: activated precursor, isomeric Dewar β-lactone, and square and rectangular isomers of 1,3-dimethylcyclobutadiene. The reported x-ray data, however, suggest that all observed structures correspond to only one distinct species, the Dewar β-lactone.
Scheschkewitz and Alabugin et al . suggest that photolysis under confinement in a crystalline matrix of 4,6-dimethyl-α-pyrone does not yield the crystal structure of 1,3-dimethylcyclobutadiene (Me 2 CBD) as we reported, but rather that of a 4,6-dimethyl-β-lactone intermediate. We provide arguments that the square-planar Me 2 CBD S / CO 2 complex and the rectangular-bent Me 2 CBD R molecule are stabilized under confinement by the guanidinium-sulfonate-calixarene host matrix used in our study.
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.