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Unprecedented Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution
Author(s) -
Legrand YvesMarie,
Gilles Arnaud,
Petit Eddy,
van der Lee Arie,
Barboiu Mihail
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201100693
Subject(s) - cyclobutadiene , chemistry , aqueous solution , raman spectroscopy , nuclear magnetic resonance spectroscopy , molecule , crystallography , spectroscopy , sulfonate , solid state nuclear magnetic resonance , covalent bond , stereochemistry , organic chemistry , physics , nuclear magnetic resonance , quantum mechanics , optics , sodium
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 G 4 C matrix that is stable enough to allow photoreactions in the solid state. In the latter case, the 4,6‐dimethyl‐α‐pyrone precursor, Me 2 1 , has been immobilized in a guanidinium–sulfonate–calixarene G 4 C crystalline network through a combination of non‐covalent interactions. UV irradiation of the crystals transforms the entrapped Me 2 1 into a 4,6‐dimethyl‐Dewar‐β‐lactone intermediate, Me 2 2 , and rectangular‐bent 1,3‐dimethylcyclobutadiene, Me 2 CBD R , 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 Me 2 3&Me 2 CBD S /CO 2 (63.7 %) and Me 2 CBD R (37.3 %) superposed crystalline architectures and the amplification of Me 2 CBD R . The crystallographic models are supported by additional FTIR and Raman experiments in the solid state and by 1 H NMR spectroscopy and ESI mass spectrometry experiments in aqueous solution. Amazingly, the 4,6‐dimethyl‐Dewar‐β‐lactone, Me 2 2 , the cyclobutadiene‐carboxyl zwitterion, Me 2 3 , and 1,3‐dimethylcyclobutadiene, Me 2 CBD , were obtained by ultraviolet irradiation of an aqueous solution of G 4 C{Me 2 1} . 1,3‐Dimethylcyclobutadiene is stable in water at room temperature for several weeks and even up to 50 °C as demonstrated by 1 H NMR spectroscopy.