Trapped optically active (E)-cycloheptene generated by enantiodifferentiating Z-E photoisomerization of cycloheptene sensitized by chiral aromatic esters
Trapped optically active (E)-cycloheptene generated by enantiodifferentiating Z-E photoisomerization of cycloheptene sensitized by chiral aromatic esters
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DOI：
10.1021/ja991315l
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发表时间：
1999-11-24
影响因子：
15
通讯作者：
Inoue, Y
中科院分区：
文献类型：
--
作者：
Hoffmann, R;Inoue, Y
在手性苯四甲酸酯敏化的（Z）-异构体（1 Z）在-40 ~-80 ℃的对映异构化反应中，首次制备了高应变、光学活性的（E）-环庚烯（1 E）.在手性敏化剂的存在下低温照射1 Z，随后通过与1，3-二苯基异苯并呋喃的Diels-Alder反应或通过与OsO 4的氧化立体定向捕获光学活性光产物1 E，分别得到环加合物或反式-1，2-环庚烷。随后通过手性HPLC或GC测定两种产物的对映体过量（ee's）。的ee的产品，这是用来作为一个衡量的激发态中的手性转移的效率，被发现不仅严重依赖于所采用的手性敏化剂，但也对所采用的温度和溶剂。因此，在极端情况下，简单地通过将溶剂从二氯甲烷改变为己烷，产物的ee加倍。此外，产品的手性可以切换在一个相对较窄的温度范围内的激基复合物中间体内的对映异构化的熵项的显着贡献的结果。在-80 ℃下，在己烷中用（-）-苯四甲酸正丁酯敏化得到77%的ee值，这是对于不对称光敏化所获得的最高ee值。根据对映异构化过程的微分活化焓和微分熵以及C5-C8环烯烃的荧光猝灭实验，讨论了高效对映异构化的起源和对映异构化的详细机理.
Highly strained, optically active (E)-cycloheptene (1E) was prepared for the first time in the enantiodifferentiating geometrical photoisomerization of the (Z)-isomer (1Z) sensitized by chiral benzene-tetracarboxylates at -40 to -80 OC. Low-temperature irradiations of 1Z in the presence of the chiral sensitizer, and subsequent stereospecific trapping of the optically active photoproduct, 1E, through a Diels-Alder reaction with 1,3-diphenylisobenzofuran or by oxidation with OsO4, afforded the cycloadduct or trans-1,2-cycloheptanediol, respectively. The enantiomeric excesses (ee's) of the two products were subsequently determined by chiral HPLC or GC. The ee of the product, which was used as a measure of the efficiency of chirality transfer in the excited state, was found to depend critically not only on the chiral sensitizer employed but also on the temperature and solvent employed. Thus, the ee of the product was doubled in an extreme case simply by changing the solvent from dichloromethane to hexane. Furthermore, the product chirality could be switched over a relatively narrow range of temperature as a consequence of the significant contribution of the entropy term in the enantiodifferentiating isomerization within the exciplex intermediate. Sensitization with (-)-bornyl benzenetetracarboxylate in hexane at -80 degrees C gave an ee value of 77%, which is the highest ee ever obtained for an asymmetric photosensitization. Based on the differential activation enthalpy and entropy for the enantiodifferentiating process and the fluorescence quenching experiments with C-5-C-8 cycloalkenes, the origin of the highly efficient enantiodifferentiation and a detailed mechanism for the enantiodifferentiating photoisomerizations are discussed.