Adaptive Supramolecular Nanosystems Group, Institut Européen des Membranes, Ecole Nationale Supérieure de Chimie de Montpellier-Université Montpellier II-UMR-CNRS 5635, Place Eugène Bataillon CC047, 34095 Montpellier Cedex 5, France.
Science. 2010 Jul 16;329(5989):299-302. doi: 10.1126/science.1188002.
Cyclobutadiene (CBD), the smallest cyclic hydrocarbon bearing conjugated double bonds, has long intrigued chemists on account of its strained geometry and electronic instability, but the parent compound and its unperturbed derivatives have thus far eluded crystallographic characterization. In this work, we immobilize a precursor, 4,6-dimethyl-alpha-pyrone, in a guanidinium-sulfonate-calixarene (G4C) crystalline network that confines the guest through a combination of CH-pi and hydrogen-bond interactions. Ultraviolet irradiation of the crystals transforms the entrapped 4,6-dimethyl-alpha-pyrone into a 4,6-dimethyl-beta-lactone Dewar intermediate that is sufficiently stable under the confined conditions at 175 kelvin to allow a conventional structure determination by x-ray diffraction. Further irradiation pushes the reaction to completion, enabling the structure determination of 1,3-dimethylcyclobutadiene Me2CBD. Our data support experimental observation of square-planar (Me2CBD(S)) and rectangular-bent (Me2CBD(R)) geometries in the G4C host matrix. The hydrogen-bonded, dissociated carbon dioxide coproduct interacts more strongly with Me2CBD(S) than with Me2CBD(R).
环丁二烯（CBD）是带有共轭双键的最小环状烃，由于其扭曲的几何形状和电子不稳定性，长期以来一直引起化学家的兴趣，但母体化合物及其未受干扰的衍生物迄今仍未能通过晶体学特征来描述。在这项工作中，我们将前驱体 4,6-二甲基-α-吡喃酮固定在胍-磺酸盐-杯芳烃（G4C）的晶体网络中，通过 CH-π 和氢键相互作用来限制客体。晶体的紫外线辐照将包封的 4,6-二甲基-α-吡喃酮转化为 4,6-二甲基-β-内酰胺 Dewar 中间体，在 175 开尔文的受限条件下，其稳定性足以允许通过 X 射线衍射进行常规结构测定。进一步的辐照将反应推至完成，使 1,3-二甲基环丁二烯 Me2CBD 的结构测定成为可能。我们的数据支持在 G4C 主体基质中实验观察到正方形平面（Me2CBD(S)）和矩形弯曲（Me2CBD(R)）几何形状的实验观察。与 Me2CBD(R)相比，氢键解离的二氧化碳副产物与 Me2CBD(S)相互作用更强。
