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 benzenetetracarboxylates at −40 to −80 °C. 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 °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 C5−C8 cycloalkenes, the origin
of the highly efficient enantiodifferentiation and a detailed mechanism for the enantiodifferentiating
photoisomerizations are discussed.