Photoisomerization cyclooctadiene

Optically active benzene(poly)carboxamides and benzene(poly)carboxy-lates were used by Inoue and co-workers as sensitizers for the geometrical photoisomerization of (Z)-cyclooctene and (Z,Z)-cyclooctadienes in various solvents at different temperatures. Under energy-transfer conditions, enantiomeric excesses up to 64% ee in unpolar solvents like pentane were reported. The use of polar solvents diminished the product ee s due to the intervention of a free or solvent-separated radical ion pair generated through the electron transfer from the substrate to the excited chiral sensitizer (Scheme 58) [105-109]. 

Enantiodifferentiating photoisomerization of (Z,Z)-l,3-cyclooctadiene (60ZZ) is sensitized by chiral benzene(poly)carboxylates." Upon sensitization with hexa-(-)-menthylbenzenehexacarboxylate in pentane at -40°C, the ( ,Z)-isomer (60EZ) is obtained in 18% ee. Photoisomerization of (Z,Z)-l,5-cyclooc-tadiene (69ZZ) sensitized by (-)-menthyl benzoate affords (-)-( ,Z)-isomer (69EZ) in only 1.6% eei ... 

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