Supramolecular enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by capped γ-cyclodextrins:: Critical control of enantioselectivity by cap rigidity
Supramolecular enantiodifferentiating photocyclodimerization of 2-anthracenecarboxylate mediated by capped γ-cyclodextrins:: Critical control of enantioselectivity by cap rigidity
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DOI：
10.1021/jo800533y
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发表时间：
2008-08-01
影响因子：
3.6
通讯作者：
Inoue, Yoshihisa
中科院分区：
文献类型：
--
作者：
Yang, Cheng;Mori, Tadashi;Inoue, Yoshihisa
合成了一系列用封端和非封端芳香基团修饰的γ-环糊精（CD）来介导2-蒽甲酸（AC）的对映分化[4 + 4]光环二聚反应。通过圆二色性、紫外-可见光和核磁共振光谱研究了这些 γ-CD 与 AC 的络合行为，揭示了在所有情况下稳定的 1:2 主客体络合物的形成。具有桥接 A 和 D 葡萄糖单元的联苯基的加帽 γ-CD 在所检查的加帽和非加帽 γ-CD 中最严格地限制了所包含的 AC 分子。由加帽的γ-CD介导的AC光环二聚作用显着提高了头对头光二聚体3的产率和对映体过量(ee)。顺头对尾光二聚体2的ee和绝对构型主要取决于加帽的刚性。因此，柔性封端和边缘取代的γ-CD提供了大约40% ee的中等ee的2，而具有刚性联苯帽的γ-CD提供了-58% ee的反足2。有趣的是，由于对映体分化过程中较大的微分熵变化，由灵活加帽的γ-CD介导的2的ee对温度变化高度敏感。相比之下，熵效应似乎在带有刚性封端的γ-CD的AC的光环二聚化中没有发挥重要作用。由天然和大多数修饰的γ-CD介导的对映分化光环二聚反应获得的微分焓和熵变化给出了极好的熵-熵补偿图，联苯封端的γ-CD除外，表明在严格封端的环糊精腔内显着不同的对映分化机制的操作。
A series of gamma-cyclodextrins (CDs) modified with capping and noncapping aromatic group(s) were synthesized to mediate the enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthracenecarboxylic acid (AC). The complexation behavior of these gamma-CDs with AC was studied by circular dichroism, UV-vis, and NMR spectroscopy to reveal the formation of stable 1:2 host-guest complexes in all cases. The capped gamma-CD with a biphenyl group bridging the A and D glucose units was shown to confine the included AC molecules most strictly among the capped and noncapped gamma-CDs examined. Photocyclodimerization of AC mediated by capped gamma-CDs considerably improved the yield and enantiomeric excess (ee) of the head-to-head photodimer 3. The ee and the absolute configuration of syn-head-to-tail photodimer 2 critically depended on the rigidity of capping. Thus, the flexibly capped and rim-substituted gamma-CDs afforded 2 in moderate ee's of around 40%, whereas gamma-CD with a rigid biphenyl cap gave the antipodal 2 in -58% ee. Interestingly, the ee of 2 mediated by flexibly capped gamma-CDs was highly sensitive to the temperature variation as a consequence of large differential entropy changes in the enantiodifferentiation process. In contrast, the entropy effect does not appear to play a significant role in the photocyclodimerization of AC with rigidly capped gamma-CDs. The differential enthalpy and entropy changes obtained for the enantiodifferentiating photocyclodimerization mediated by native and most of the modified gamma-CDs gave an excellent enthalpy-entropy compensation plot with an exception of the biphenyl-capped gamma-CD, indicating the operation of significantly different enantiodifferentiation mechanism within the rigidly capped cyclodextrin cavity.