Abstract
Cyclobutadiene (CBD), the smallest cyclic hydrocarbon bearing conjugated double bonds, has long intrigued chemists because of its chemical characteristics. The question of whether the molecule could be prepared at all has been answered, but the parent compound and its unperturbed derivatives have eluded crystallographic characterization or synthesis "in water". Different approaches have been used to generate and to trap cyclobutadiene in a variety of confined environments: a) an Ar matrix at cryogenic temperatures, b) a hemicarcerand cage enabling the characterization by NMR spectroscopy in solution, and c) a crystalline guanidinium-sulfonate- calixarene G4C matrix that is stable enough to allow photoreactions in the solid state. In the latter case, the 4,6-dimethyl- a-pyrone precursor, Me21, has been immobilized in a guanidinium- sulfonate-calixarene G4C crystalline network through a combination of noncovalent interactions. UV irradiation of the crystals transforms the entrapped Me21 into a 4,6-dimethyl-Dewar-b-lactone intermediate, Me22, and rectangular- bent 1,3-dimethylcyclobutadiene, Me2CBDR, which are sufficiently stable under the confined conditions at 175 K to allow a conventional structure determination by X-ray diffraction. Further irradiation drives the reaction towards Me23&Me2CBDS/CO2 (63.7%) and Me2CBDR (37.3%) superposed crystalline architectures and the amplification of Me2CBDR. The crystallographic models are supported by additional FTIR and Raman experiments in the solid state and by 1H NMR spectroscopy and ESI mass spectrometry experiments in aqueous solution. Amazingly, the 4,6-dimethyl-Dewar-b-lactone, Me22, the cyclobutadiene-carboxyl zwitterion, Me23, and 1,3-dimethylcyclobutadiene, Me2CBD, were obtained by ultraviolet irradiation of an aqueous solution of G4CACHTUNGTRENUNG{Me21}. 1,3-Dimethyl- ACHTUNGTRENUNGcyclobutadiene is stable in water at room temperature for several weeks and even up to 50 8C as demonstrated by 1H NMR spectroscopy.