Illusory molecular expression of "Penrose stairs" by an aromatic hydrocarbon.
Illusory molecular expression of "Penrose stairs" by an aromatic hydrocarbon.
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DOI：
10.1002/anie.201102210
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发表时间：
2011-06
影响因子：
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通讯作者：
Waka Nakanishi;Taisuke Matsuno;J. Ichikawa;H. Isobe
中科院分区：
文献类型：
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作者：
Waka Nakanishi;Taisuke Matsuno;J. Ichikawa;H. Isobe
用二维线条画的形式表达三维物体是一项重要的技能，特别是在科学和技术领域。[1]自从发明了象征性的和巧妙的表示苯作为一个六边形，化学物质的线条画是最不可或缺的手段来描绘分子。[2]然而，正如彭罗斯和彭罗斯指出的那样，线描有时会通过表现不可能的物体来欺骗和娱乐观众。[1，3]彭罗斯楼梯，在一个连续的圆圈中不断下降的楼梯，由MC Müller在他著名的石版画《上升和下降》中以艺术形式表达。[4]我们发现，这样一个虚幻的结构出现在一个具有独特的卡拉科尔拓扑结构的分子中。[5]值得注意的是，分子彭罗斯阶梯，环双[4]螺旋烯1，欺骗我们的方式不同于彭罗斯最初的建议。1在溶液中的动力学行为已被观察到显示共价和非共价化学的可能组合，以探索独特的分子拓扑结构在未来。我们通过二氟烯烃2的分子内环化反应制备螺烯亚基来开始1的合成。[6]正如我们以前报道的，[7]二氟烯烃与卤代苯基容易发生骨架重排，但所需的二卤代[4]螺烯3在温和的环化条件下得到。因此，2在08 ℃下用幻酸（FSO 3 H·SbF 5）进行Friedel-Crafts型环化，并通过用四氟硼酸三苯甲基酯（Ph 3CBF 4）脱氢进一步转化为3，而不分离环化的中间体（方案1）。3的核磁共振谱分析表明，2经硅胶柱层析纯化后，以53%的产率得到了化合物3，但它也受到了微量的副产物的污染，包括苯并[c]芘和未环化的化合物。[8]因此，3（CA。60%纯度，NMR分析）与[Ni（cod）2]、1，5-环辛二烯和2，2 ′-联吡啶混合，在通过重结晶和GPC纯化后，以26%的产率（从2分离的产物在3个步骤后的产率）得到1（方案1）。作为副产物，还分离出少量的3和未环化的污染物之间的交叉偶联产物。[9]虽然偶联反应也发生在相应的铜酸盐的氧化偶联反应中，[10]尽管需要通过金属化逐步转化的强大程序，1的产率并没有提高。通过单晶的X射线晶体学分析明确地建立了1的分子结构。[11]具有相同螺旋构型的两个螺旋烯亚基偶联在一个分子中，并且晶体由两种对映异构体的混合物组成（见下文）。考虑到亚基的螺旋手性和键的轴向手性，[12]我们可以将对映异构体的绝对构型分别指定为（P，P，R，R）和（M，M，S，S）（P，P，R，R-形式见图1）。在每个螺旋烯亚基中，中间螺旋的递减距离测量为0.95和0.63，末端的面间角为238和239 °。
Conveying three-dimensional objects in the form of twodimensional line drawings is an important skill, especially in scientific and technological fields.[1] Ever since the invention of the symbolic and ingenious representation of benzene as a hexagon, line drawings of chemical substances are among the most indispensable means to depict molecules.[2] Line drawings, however, can sometimes deceive and amuse the viewers by representing impossible objects, as pointed out by Penrose and Penrose.[1, 3] The Penrose stairs, endlessly descending stairs in a continuous circle, were expressed in art by MC Escher in his famous lithograph, Ascending and Descending.[4] We found that such an illusory structure emerged from a molecule with unique caracole topology.[5] Notably, the molecular Penrose stairs, cyclobis [4] helicene 1, deceive us in a manner different from the original proposal of Penrose. The dynamic behavior of 1 in solution has been observed to show a possible combination of covalent and noncovalent chemistry to explore unique molecular topologies in future. We started the synthesis of 1 by preparing the helicene subunit by an intramolecular cyclization reaction of difluoroalkene 2.[6] As we reported previously,[7] difluoroalkene with halogenated phenyl groups was prone to a skeletal rearrangement, but the desired dihalo [4] helicene 3 was obtained under milder cyclization conditions. Thus, 2 was subjected to a Friedel–Crafts type cyclization with magic acid (FSO3H· SbF5) at 08C and was further converted to 3 by dehydrogenation with trityl tetrafluoroborate (Ph3CBF4) without isolation of the cyclized intermediate (Scheme 1). NMR spectroscopical analysis of 3 showed that the compound was obtained in 53% yield from 2 after purification by silica gel column chromatography; however, it was also contaminated with a trace amount of byproducts including chrysene and uncyclized compounds.The convergent homocoupling of subunit 3 proceeded smoothly by Ni-promoted biaryl coupling to afford cyclobis [4] helicene 1.[8] Thus, 3 (ca. 60% purity, NMR analysis) was mixed with [Ni (cod) 2], 1, 5-cyclooctadiene, and 2, 2’-bipyridine to give 1 in 26% yield (yield of isolated product after 3 steps from 2) after purification by recrystallization and GPC (Scheme 1). As a byproduct, a small amount of a crosscoupling product between 3 and the uncyclized contaminant was also isolated.[9] Although the coupling reaction also took place with oxidative coupling of the corresponding cuprate,[10] the yield of 1 was not improved despite the formidable procedure that requires a stepwise conversion by metalation. The molecular structure of 1 was established unequivocally by X-ray crystallographic analysis of a single crystal.[11] Two helicene subunits with the same helical configuration are coupled in one molecule, and the crystal is composed of a mixture of two enantiomers (see below). Taking into account the helical chirality of the subunits and the axial chirality at the linkages,[12] we can assign the absolute configurations of the enantiomers as (P, P, R, R) and (M, M, S, S), respectively (P, P, R, R-form shown in Figure 1). In each helicene subunit, the decrement distances of the middle helix measure 0.95 and 0.63, and the interplanar angles at the terminus are 238 and