Marked effect of aromatic solvent on unfolding rate of helical ethynylhelicene oligomer.
Marked effect of aromatic solvent on unfolding rate of helical ethynylhelicene oligomer.
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DOI：
10.1021/ja0478882
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发表时间：
2004-10
影响因子：
15
通讯作者：
Hiroki Sugiura;Yasuko Nigorikawa;Yuto Saiki;Keiichi Nakamura;M. Yamaguchi
中科院分区：
文献类型：
--
作者：
Hiroki Sugiura;Yasuko Nigorikawa;Yuto Saiki;Keiichi Nakamura;M. Yamaguchi
采用双向Sonogashira偶联和脱保护的方法，高收率合成了具有光学活性的无环乙基螺旋烯低聚物。它们在氯仿中的CD光谱显示出少于7个螺旋的低聚物与其更高的同源物之间的较大差异，表明后者形成螺旋结构，前者形成无规螺旋结构。在室温下，螺旋七聚体在氯仿中逐渐展开成随机线圈结构。用CD测定了几种芳香族溶剂中的展开速率，发现速率常数k高度依赖于芳香族取代基的类型：在碘苯和三氟甲基苯之间，k相差7个数量级。在卤代苯中的反应速率依次为：碘苯、溴苯、氯苯、氟苯、氟苯、间二氟苯；在烷基苯中的反应速率依次为：苯乙烯、苯乙炔、乙苯、甲苯、苯；在杂原子取代芳烃中的反应速率依次为：硫苯甲醚、苯乙腈、苯乙酯、苯甲酸乙酯、苯乙酯、苯乙酯、苯乙酯、三氟甲基苯。对数k值与芳烃的绝对硬度eta具有良好的相关性，并且在软芳烃中观察到较高的展开率。蒸汽压渗透法研究表明，七聚体在苯、氟苯和三氟甲基苯中的螺旋结构为二聚体，而在氯仿和甲苯中的无序线圈结构为单体。当将无规线圈结构的氯仿溶液浓缩到小体积时，可以再生成螺旋结构。
Optically active acyclic ethynylhelicene oligomers were synthesized in high yields by a two-directional method involving Sonogashira coupling and deprotection. Their CD spectra in chloroform exhibited large differences between the oligomers with less than seven helicenes and their higher homologues, which indicated the formation of helical structures for the latter and random coil structures for the former. The helical heptamer gradually unfolded to a random coil structure in chloroform at room temperature. The unfolding rate was examined by CD in several aromatic solvents as well, and the rate constant k was found to be highly dependent on the type of aromatic substituent: k differed by seven orders of magnitude between iodobenzene and trifluoromethylbenzene. Several features of the rates are notable: The reaction rates in halobenzenes were in the order of iodobenzene > bromobenzene > chlorobenzene > benzene > fluorobenzene > m-difluorobenzene, those in alkylbenzenes were styrene > phenylacetylene > ethylbenzene > toluene > benzene, and those in heteroatom-substituted arenes were thioanisole > benzonitrile > anisole > ethyl benzoate > benzene > trifluoromethylbenzene. The log k values exhibited good correlation with the absolute hardness, eta, of the arenes, and higher unfolding rates were observed in the soft arenes. Vapor pressure osmometry studies indicated that the helical structure of the heptamer is dimeric in benzene, fluorobenzene, and trifluoromethylbenzene, while the random coil structure of the heptamer is monomeric in chloroform and toluene. When a chloroform solution of the random coil structure was concentrated to a small volume, the helical structure could be regenerated.