HIGHLY ENANTIODIFFERENTIATING PHOTOISOMERIZATION OF CYCLOOCTENE BY CONGESTED AND OR TRIPLEX-FORMING CHIRAL SENSITIZERS

In an effort to generate photoproducts with higher optical purities (op), two novel strategies, i.e., intra/intermolecular triplex formation and increased steric hindrance, have been employed in the enantiodifferentiating Z --> E photoisomerization of cyclooctene (1) sensitized by optically active (ar)alkyl benzene(poly)carboxylates 2-5 at temperatures ranging from 25 to -90-degrees-C. The newly synthesized benzenepolycarboxylates, possessing extremely bulky and/or electron-donating (ar)-alkyl groups, gave products with the highest op's ever reported for the enantiodifferentiating photosensitizations, not only at low temperatures (up to 64% op at -89-degrees-C) but also at ambient temperature (50% op). Both strategies to fix the sensitizer conformation and to induce more dynamic conformational changes in the exciplex/triplex intermediate are shown to function well and to give very high op's. The temperature-dependence studies also demonstrate that the temperature switching of the product chirality is not an extraordinary but rather a general phenomenon, which is attributed to the significant contribution of the entropy factor in the enantiodifferentiating process, caused by the dynamic structural change in the excited complex.