Cyclooctene, chiral induction

Finally, the catalytic activity of chiral polymer poly(+)(MASBF)4PFeCl bearing four chiral SBF units in meso position of the porphyrin ring was investigated. However, this first example of chiral porphyrin based on SBF scaffold does not display any enantioselectivity (ee < 5 %) and even displays a low epoxidafion yield of 30 % [73]. Different alkenes such as styrene, 4-chlorostyrene, 4-methylslyrene or cyclooctene were tested but no chiral induction was observed. This might be due to the structure of the SBF unit, which leads in both monomer and polymer to a large chiral cavity and thus no asymmetric synthesis was observed. 

Hammond and Cole reported the first asymmetric photosensitized geometri-r cal isomerization with 1,2-diphenylcyclopropane (Scheme 2) [29]. The irradiation of racemic trans-1,2-diphenylcylcopropane 2 in the presence of the chiral sensitizer (R)-N-acetyl-1 -naphthylethylamine 4 led to the induction of optical activity in the irradiated solution, along with the simultaneous formation of the cis isomer 3. The enantiomeric excess of the trans-cyclopropane was about 1% in this reaction. Since then, several reports have appeared on this enantiodifferentiating photosensitization using several optically active aromatic ketones as shown in Scheme 2 [30-36]. The enantiomeric excesses obtained in all these reactions have been low. Another example of a photosensitized geometrical isomerization is the Z-E photoisomerization of cyclooctene 5, sensitized by optically active (poly)alkyl-benzene(poly)carboxylates (Scheme 3) [37-52]. Further examples and more detailed discussion are to be found in Chap. 4. 

Ti(0-Z50-Pr)4 complexes on polystyrene with chiral groups are active in asymmetric epoxidation of geraniol by TBHP to (iS, -2,3-epoxygeraniol [145]. The enantiomeric excess reaches 66%. Recycling of the catalyst leads to a small decrease of asymmetric induction (loss of one-third of activity after five regeneration cycles). There are sparse data on epoxidation of cyclohexene and cyclooctene in the presence of immobilized titanocene, zirconocene and hafhocene [3]. 

Since the first report on the asymmetric photosensitization (7.7% ee) of the isomerization of tra s-1,2-diphenylcyclopropane by Hammond and Cole, attention has been focused on enantiodifferentiating photosensitized isomerization reactions. The observed asymmetric induction was limited, until Inoue et al. achieved remarkable enantiomeric excesses of up to 64% ee in the photoisomerization of Z-cyclooctene to the optically active -cyclooctene, sensitized by chiral benzenepolycarboxylates at -89°C. Valuable insights into the mechanism (e.g., the entropy influence) were gained from the temperature and pressure dependence of the observed enantioselectivities. ... 

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