Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, 80309, USA.
Current address: Dept. of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel.
Angew Chem Int Ed Engl. 2018 Feb 12;57(7):1821-1825. doi: 10.1002/anie.201709966. Epub 2018 Jan 18.
The planarity of the second stable conformer of 1,3-butadiene, the archetypal diene for the Diels-Alder reaction in which a planar conjugated diene and a dienophile combine to form a ring, is not established. The most recent high level calculations predicted the species to adopt a twisted, gauche structure owing to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the double bonds. The structure cis-1,3-butadiene is unambiguously confirmed experimentally to indeed be gauche with a substantial dihedral angle of 34°, in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche- and trans-butadiene provides an opportunity to examine the effects of conjugation and steric interactions.
1,3-丁二烯的第二个稳定构象的平面性尚未确定，1,3-丁二烯是 Diels-Alder 反应中典型的二烯，在 Diels-Alder 反应中，一个平面共轭二烯和一个双烯亲合体结合形成一个环。最近的高精度计算预测，由于内部末端氢原子之间的空间相互作用，该物质会采取扭曲的 gauche 结构，而不是由双键共轭所倾向的平面 cis 结构。实验明确证实 cis-1,3-丁二烯实际上是 gauche 构型，具有相当大的二面角 34°，与理论非常吻合。观察到两个隧道成分表明分子在两个等效的对映异构体之间容易进行互变。gauche-和 trans-丁二烯的实验确定结构的比较提供了一个检验共轭和空间相互作用影响的机会。
