High photostationary state trans/cis ratios upon aromatic ester-sensitized photoisomerization of cyclooctene
✍ Scribed by Noritsugu Yamasaki; Yoshihisa Inoue; Taizo Yokoyama; Akira Tai
- Publisher
- Elsevier Science
- Year
- 1989
- Tongue
- English
- Weight
- 245 KB
- Volume
- 48
- Category
- Article
- ISSN
- 1010-6030
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✦ Synopsis
Upon sensitized photoisomerization of cis-cyclooctene, aromatic esters possessing electron-withdrawing group(s) gave unusually high photostationary tram/&
ratios up to 0.6. This is an improved photochemical route for preparing the highly strained truns isomer.
We have demonstrated that the direct and triplet-sensitized photoisomerizations of cyclooctene give distinctly different photostationary
translcis ratios, (t/c),, in the solution phase 11, 23. As expected for the singlet intermediate involved, direct irradiation at 185 nm affords a (t/c), ratio close to unity, i.e. 0.96, which has been utilized in the photochemical synthesis of this highly strained trans isomer in spite of the fact that vacuum-UV light sources are not commonly available [ 33. On the other hand, the triplet sensitization leads to much lower (t/c)-ratios of 0.02 -0.22, which increase with increasing triplet energy (ET) of the sensitizer, and the highest (G%ss of 0.22 is obtained for benzene sensitization. The threshold ET to effect cis-to-trans photoisomerization appears to be about 79 kcal mol-' [ 11.