Highly efficient and unique enantiodifferentiating Z-E photoisomerization of cyclooctene (1) was achieved through singlet photosensitization with chiral polyalkyl benzenepolycarboxylates. Conversion dependence of optical yields and an attempted kinetic resolution of racemic (E)-isomer IE unequivocally showed that the enantiodifferentiating step is not the initial quenching of chiral sensitizer with enantiomeric IE but the rotational relaxation of planar 1Z to twisted singlet cyclooctene (1p) within the singlet exciplex with chiral sensitizer. Fluorescence quenching of some benzenetetracarboxylates with 1Z and IE provides further evidence for the intervention of an exciplex intermediate, showing a new emission at a longer wavelength, and also for the insignificant enantiodifferentiation in the quenching step, affording nearly diffusion-controlled quenching rate constants around 1010 s"1 for IE. This novel enantiodifferentiating'photosensitizing system via exciplex, in combination with low-temperature irradiation, not only affords the highest optical yields up to 53% but also exhibits unusual temperature-switching behavior of product chirality especially with the o-dicarboxylate sensitizers. The activation parameters obtained in the temperature-dependence study indicate that the temperature-switching phenomenon is attributed to the unequal activation entropies, or frequency factors, for the enantiodifferentiating relaxations of 1Z to (R)-and (S)-1 P within the exciplex, for which the dynamic structural changes in the relaxation process may be responsible.

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Photosensitized enantiodifferentiating cis-trans isomerizations of 1,2-diphenylcyclopropane (1), 1,2bis(4-methoxyphenyl)cyclopropane (2), and 2,3-diphenyloxirane (3) were performed in the presence of optically active alkyl arenecarboxylates as electron-accepting photosensitizers. Several photosensitizers possessing different chromophores and chiral groups were examined in polar and nonpolar solvents, and optical purities (op) up to 10% for It and 1% for 2t and 3t were obtained. The product's op varied drastically with solvent polarity and, in an extreme case, typical polar and nonpolar solvents gave antipodal products. Mechanistic investigations revealed that the enantiodifferentiation occurs in the quenching of the excited chiral sensitizer by racemic substrates and that full equilibrium is not established between the enantiomeric ambident intermediates, probably owing to their short lifetimes.Considerable effort has been devoted to the study of enantiodifferentiating photosensitized isomerizations1-10 ever since the first such isomerization, that of trons-1,2diphenylcyclopropane (1), was reported by Hammond and Cole.3 4® They employed (+)-(A)-l-(l-(JV-acetylamino)-

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