Enantiodifferentiating Z-E Photoisomerization of Cyclooctene Sensitized by Chiral Polyalkyl Benzenepolycarboxylates

Article abstract of DOI:10.1021/jo00031a008

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 10¹⁰ 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 53percent but also exhibits unusual temperature-switching behavior of product chirality especially with the o-dicarboxylate sensitizer.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.