ENANTIODIFFERENTIATING Z-E PHOTOISOMERIZATION OF CYCLOOCTENE SENSITIZED BY CHIRAL POLYALKYL BENZENEPOLYCARBOXYLATES

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 1E unequivocally showed that the enantiodifferentiating step is not the initial quenching of chiral sensitizer with enantiomeric 1E 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 1E 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 10(10) s-1 for 1E. 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)-1p within the exciplex, for which the dynamic structural changes in the relaxation process may be responsible.