Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
J Am Chem Soc. 2020 Apr 8;142(14):6493-6498. doi: 10.1021/jacs.0c01430. Epub 2020 Mar 31.
The synthesis of conjugated Möbius molecules remains elusive since twisted and macrocyclic structures are low-entropy species sporting their own synthetic challenges. Here we report the synthesis of a Möbius macrocycle in 84% yield via alkyne metathesis of 2,13-bis(propynyl)[5]helicene. MALDI-MS, NMR spectroscopy, and X-ray diffraction indicated a trimeric product of twofold symmetry with / configurations in the helicene subunits. Alternatively, a threefold-symmetric structure was determined by DFT calculations to be more thermodynamically stable, illustrating remarkable kinetic selectivity for this alkyne metathesis cyclooligomerization. Computational studies provided insight into the kinetic selectivity, demonstrating a difference of 15.4 kcal/mol between the activation barriers for the / and / diastereodetermining steps. Computational (ACID and EDDB) and experimental (UV-vis and fluorescence spectroscopy and cyclic voltammetry) studies revealed weak conjugation between the alkyne and adjacent helicene groups as well as the lack of significant global aromaticity. Separation of the and enantiomers was achieved via chiral HPLC at the analytical scale.
由于扭曲的大环结构是熵值较低的物种，具有自身的合成挑战，因此共轭 Möbius 分子的合成仍然难以捉摸。在这里，我们报告了通过 2,13-双（丙炔基）[5]螺苯的炔烃复分解反应以 84%的产率合成 Möbius 大环。MALDI-MS、NMR 光谱和 X 射线衍射表明，在螺苯亚基中具有 /构型的二倍对称三聚体产物。或者，通过 DFT 计算确定了三倍对称的结构，该结构在热力学上更稳定，说明了该炔烃复分解环齐聚反应具有显著的动力学选择性。计算研究提供了对动力学选择性的深入了解，表明 /和 /非对映选择性决定步骤的活化能垒之间存在 15.4 kcal/mol 的差异。计算（ACID 和 EDDB）和实验（UV-vis 和荧光光谱以及循环伏安法）研究表明，炔烃和相邻螺苯基团之间存在弱共轭，并且缺乏显著的整体芳香性。通过手性 HPLC 在分析规模上实现了 和 对映异构体的分离。
