Photosensitization, chiral induction

Photosensitized enantiodifferentiating reactions are synthetically attractive and mechanistically interesting photochemical processes. The chiral information of the sensitizer is transferred to the substrate by short-Hved interactions in the excited state (i.e., during the lifetime of an exciplex of a reaction intermediate and the chiral sensitizer that is involved in the reaction mechanism) hence, the chirahty is multiplied, and only catalytic amounts of the optically active sensitizer are required. The stabilization energy of an exciplex compared to the locally excited state and its lifetime are often found to be strongly dependent on both electronic and steric properties of its components. Chiral induction can be achieved by different stabilization energies or lifetimes of the exciplex between the sensitizer and the intermediates that lead to the enantiomeric photoproducts. The absence of other reaction pathways without intimate contact to the sensitizer and of racemization processes in the further course of the reaction mechanism is an additional requirement to ensure effective chirality transfer. 

Hoffmann, N., Buschmann, H., Raabe, G., and Scharf, H. D., Chiral induction in photochemical reactions. 15. Detection of stereoelectronic effects by temperature dependent measurements of the diastereoselectivity in the photosensitized [2-1-2]-cycloaddition. Tetrahedron, 50,11167-11186,1994. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

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. 

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. ... 

With similar chiral sensitizers enantiomeric excesses of up to 8.2% ee were observed in the photosensitized cyclodimerization of cyclohexa-1,3-diene 34 to the exo-[4-E2]-cyclodimer 44 and the a tx-[2-l-2]-cyclodimer 45. The e do-[4-1-2]-adduct and syn-[2-l-2]-adduct were obtained as minor components. Polymenthyl benzenepolycarboxylates similar to 40 yielded dimers 44 and 45 in low yield and with enantioselectivities of up to 2.5% ee at room temperature and 4.0% ee at -41°C. Protected saccharides as chiral substituents of the photosensitizer 46 enhanced the asymmetric induction in dimethyl ether up to 8.2% ee, but only in the case of the [4-E2]-dimer 44 (Scheme 15). 

The asymmetric induction of chiral saccharide substituents was more effective in the cyclodimerization of cyclohexene 47. Trans-anti-trans dimer 48 was obtained in low enantioselectivity (2.1% ee) at 25°C, but in much higher enantioselectivity (68% ee) at -78°C in pentane with the chiral sensitizer (-)-50 (Scheme 16) this represents the highest ee value reported for an enantiodilferentiating-photosensitized cyclodimerization. 

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