ENANTIODIFFERENTIATING PHOTOISOMERIZATION OF 1-METHYLCYCLOOCT-1-ENE SENSITIZED BY CHIRAL ALKYL BENZENECARBOXYLATES - STERIC EFFECTS UPON STEREO DIFFERENTIATION

Enantiodifferentiating Z-to-E photoisomerizations of 1-methylcyclooct- 1-ene (2) sensitized by (-)-menthyl benzene(poly)carboxylates were per formed at varying temperatures and the steric effects of the methyl gr oup introduced to the parent cyclooctene (1) upon both isomerization a nd enantiodifferentiation processes were studied. The photostationary state 2E:2Z ratio, (E/Z)(pss), was shown to decrease dramatically with decreasing irradiation temperature and increasing steric hindrance in the sensitizer. Kinetic analyses of the sensitized photoisomerization demonstrated that the temperature- and sensitizer-dependent (E/Z)(pss ) ratios originate solely from the quenching process. The steric effec t on the (E/Z)(pss) values of the introduced methyl group is much grea ter for the (Z)-isomer (2Z) than for the (E)-isomer (2E), as compared with the cyclooctene case reported previously. The optical purities (% op) of photoproduct 2E were found to be relatively low (< 10%) in gene ral, and the increased steric hindrance induced by the methyl group do es not appear to enhance the product's optical purity. However, the te mperature dependence studies revealed that the differential activation enthalpy (Delta Delta H-double dagger) and entropy (T Delta Delta S-d ouble dagger) as functions of the number of chiral groups in the benze ne(poly)carboxylate sensitizer give a quite similar profile to those p arameters obtained for parent 1, showing uniform increases for 2 of 0. 4 and 0.15 kJ mol(-1) in Delta Delta H-double dagger and T Delta Delta S-double dagger, respectively, for all chiral sensitizers.