Enantioselective Synthesis of Triple Helicenes by Cross-Cyclotrimerization of a Helicenyl Aryne and Alkynes via Dynamic Kinetic Resolution
Enantioselective Synthesis of Triple Helicenes by Cross-Cyclotrimerization of a Helicenyl Aryne and Alkynes via Dynamic Kinetic Resolution
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DOI：
10.1021/jacs.0c01723
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发表时间：
2020-06-03
影响因子：
15
通讯作者：
Kamikawa, Ken
中科院分区：
文献类型：
--
作者：
Yubuta, Ayaka;Hosokawa, Tomoka;Kamikawa, Ken
通过钯催化的对映选择性交叉环三聚反应，合成了具有光学活性的三重螺旋烯（TH-1）。当分别使用4a和（S）-QUINAP作为炔和手性配体时，获得高达98：2的对映体比率。通过单晶X-射线衍射分析，TH-1a的绝对立体化学为（M，P，M）。基于高效液相色谱分析，对高温下对映体纯TH-1a的外消旋动力学进行了研究。外消旋反应的活化能为29.1 kcal mol（-1）。密度泛函理论计算表明，钯催化的对映选择性交叉环三聚反应是通过五元钯配体6a与两个[5]螺烯的动态动力学拆分进行的. 6a的几种最初形成的立体异构体最终通过[5]螺烯基部分的转化异构化成最稳定的钯配体中间体（M，P，M）-6a。然后，将4插入6a中以形成（M，P，M）-12 a，接着还原消除，导致以立体选择性方式形成（M，P，M）-TH-1a。用圆二色性和圆偏振发光研究了TH-1a的光学性质。
Optically active triple helicenes (TH-1) were prepared via a palladium-catalyzed enantioselective cross-cyclotrimerization of two helicenyl arynes 5, which are generated in situ from 3, with dialkyl acetylenedicarboxylate 4. Enantiomeric ratios of up to 98:2 were obtained when using 4a and (S)-QUINAP as the alkyne and chiral ligand, respectively. The absolute stereochemistry of TH-1a was revealed to be (M,P,M) by a single-crystal X-ray diffraction analysis. Kinetic studies of the racemization of enantiomerically pure TH-1a at elevated temperatures were conducted based on a high-performance liquid chromatography analysis. The activation energy for the racemization was found to be 29.1 kcal mol(-1). Density functional theory calculations revealed that the palladium-catalyzed enantioselective cross-cyclotrimerization reactions proceed via the dynamic kinetic resolution of a five-membered palladacycle 6a with two [5]helicenes. Several initially formed stereoisomers of 6a eventually isomerize into the most thermodynamically stable palladacycle intermediate (M,P,M)-6a by inversion of the [5]helicenyl moiety. Then, the insertion of 4 into 6a to form (M,P,M)-12a, followed by a reductive elimination, leads to the formation of (M,P,M)-TH-1a in a stereoselective manner. The optical properties of TH-1a were studied by circular dichroism and circularly polarized luminescence.