Trapped optically active (E)-cycloheptene generated by enantiodifferentiating Z-E photoisomerization of cycloheptene sensitized by chiral aromatic esters

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.