Enantiodifferentiating photoisomerization of cyclooctenes

Figure 2 Pressure switching of product chirality in enantiodifferentiating photoisomerization of cyclooctene 47 sensitized by (— )-menthyl pyromellitate 45a in pentane at 25°C.

Inoue, Y., Yamasaki, N., Yokoyama, T., Tai, A., Highly Enantiodifferentiating Photoisomerization of Cyclooctene by Congested and or Triplex forming Chiral Sensitizers, J. Org. Chem. 1993, 58, 1011 1018. 

Tsuneishi, H., Hakushi, T, and Inoue, Y., Singlet- versus triplet-sensitized enantiodifferentiating photoisomerization of cyclooctene remarkable effects on spin multiphcity upon optical yield, /. Chem. Soc., Perkin Trans. 2, 1601-1605, 1996. 

Inoue, Y., Matsuyama, E., and Wada, T, Pressure and temperature control of product chirality in asymmetric photochemistry. Enantiodifferentiating photoisomerization of cyclooctene sensitized by chiral benzenepolycarboxylates,/. Am. Chem. Soc., 120, 10687-10696, 1998. 

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. 

It is well documented that direct or sensitized photolysis of (Z)-cyclooctene 34Z leads to geometrical isomerization to planar chiral (f )-isomer 34E (Scheme 1.7). Due to the comprehensive work by Inoue et al., the enantiodifferentiating photoisomerization of 34Z sensitized by opticaUy active benzenecarboxylates has become a benchmark reaction for evaluating chiral photosensitizing system. The first supramolecular enantiodifferentiating photoisomerization of 34Z was examined in direct photolysis of its j8-CD complex... 

Enantiomeric (E)-cyclooctene (20E) was first resolved in 1963 through its diastereomeric platinum(II) complex. Synthesis of optically active 20E has been the subject of intensive study since 1968. The first preparation involves the treatment of enantiopure (E)-cyclooctane-l,2-thiocarbonate with triisooctyl phosphate or of (E)-cyclooatane-l,2-trithiocarbonate with l,3-dibenzyl-2-methyl-l,3,2-diazaphospholi-dine. Following analogous synthetic routes, enantiomeric (E)-cycloheptene (18E) can be produced and trapped by 2,5-diphenyl-3,4-isobenzofuran as an optically active adduct. In 1973, the circular dichroism spectrum of enantiopure 20E vapor was recorded in the vacuum UV region down to 150 nm. The first enantiodifferentiating Z-E photoisomerization of cyclooctene sensitized by chiral benzenecarboxylates appeared in 1978. Transfer of chiral information from sensitizer to substrate occurs within the exciplex intermediate. ... 

Enantiodifferentiating Z-E photoisomerization of cyclooctene (20Z) sensitized by chiral polyalkyl benzenepolycarboxylates has been extensively studied some of the results are summarized in Table 16.3. The crucial enantiodifferentiating step is the rotational relaxation of prochiral 20Z to enantiomeric perpendicular singlets within the exciplex of the chiral sensitizer with 20Z. In order to elucidate the detailed mechanism and to obtain higher enantiomeric excess (ee), the steric and electronic effects of the sensitizer and its chiral substituent as well as the involvement of the intra/intermolecular triplex have been studied extensively. Thus, the sensitizations with bulky 8-phenylmenthyl and 8-cyclohexylmenthyl 1,2,4,5-benzenetetracarboxylates afford good ees of up to 50% even at room temperature. The ee is improved at lower temperatures to reach 64% at -89°C. In contrast, chiral triplet sensitizers afford much lower ees, which are not appreciably improved even at low temperatures (at least in this system). The enantiodifferentiating photoisomerization can also be effected by chiral aromatic amides, phos-phoryl esters, - and phosphoramides. ... 

Shi, M. and Inoue, Y., Enantiodifferentiating photoisomerization of (Z)-cyclooctene sensitized by chiral Cj-symmetric phosphoramide, Awst. /. Chem., 54, 113-115, 2001. 

Wada, T, Sugahara, N., Kawano, M., and Inoue, Y, First asymmetric photochemistry with nucleosides and DNA enantiodifferentiating Z-E photoisomerization of cyclooctene, Chem. Lett., 1174-1175, 2000. 

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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