Photochirogenesis

Asaoka, S., Wada, T., and Inoue, Y. (2003) Microenvironmental polarity control of electron-transfer photochirogenesis. Enantiodifferentiating polar addition of... 

Photochirogenesis, 1st International Symposium on asymmetric photochemistry. Osaka, Japan, Sept 4-6, 2001. 

Griesbeck AG, Meierhenrich UJ (2002) Asymmetric photochemistry and photochirogenesis. Angew Chem Int Ed 41 3147-3154... 

In the modified zeolite sensitization study, a significant enhancement of product ee was reported to occur by the introduction of a chiral sensitizer into the zeolite supercages that otherwise give an almost racemic product [47]. Further supramolecular approaches to the enantiodifferentiating photosensitization will lead to more solid conclusions on the mechanisms and factors that control supramolecular photochirogenesis and open a new channel to asymmetric supramolecular photochemistry in the near future see Chap. 9. 

II. SUPRAMOLECULAR PHOTOCHIROGENESIS WITH CHIRALLY MODIFIED ZEOLITES... 

Achiral cis-1,2-diphenylcyclopropane photoisomerizes to the chiral trans isomer upon singlet- or triplet-photosensitized irradiation [64-67], It is expected that the reactant and chiral inductor immobilized in a zeolite supercage interact intimately with each other to afford more efficient photochirogenesis. Ramamurthy and coworkers reported that the enantio- and diastereodifferentiating photoisomeriza-tions of ris-2p,3p-diphenyl-la-cyclopropanecarboxylates 20 (Scheme 7) in chirally modified zeolite supercages lead to the corresponding chiral trans isomer 21 [68]. 

The most successful supramolecular photochirogenesis with chirally modifiea zeolites is the enantiodifferentiating photocyclization of tropolone derivatives in[ zeolites modified with (— )-ephedrine or (— )-norephedrine, giving bicyclo[3.2.0J heptadienes in high ees. It is interesting to compare this result with the highlj ... 

The photoisomerization of (Z)-cyclooctene (30) (Scheme 12) to the (E)-isomer (31) was sensitized by enantiopure alkyl benzenecarboxylates immobilized in zeolite to give modest ees. The use of an antipodal sensitizer pair of (R)-and (S)-1 -methyIheptyl benzoates, 32d and 32e, yielded enantiomeric 31 in — 5% and +5% ee, respectively, while the same sensitizers gave practically racemic 31 upon irradiation in homogeneous solutions. This small, but apparent, enhancement of the product ee observed upon irradiation in modified zeolite supercages is likely to arise from the decreased conformational freedom of the adsorbed sensitizer, the hindered approach of 30 to the sensitizer, and/or the different exciplex structure in confined media. In this context, it is interesting to examine the effect of temperature on the supramolecular photochirogenesis in modified zeolites and to compare the results with those obtained in the homogeneous phase. Such an examination will reveal the distinctly different role of entropy in confined media, which should be clarified in a future study. 

In this study, they demonstrated for the first time that chirally modified zeolites not only function as supramolecular photosensitizing media but also enhance the original enantiodifferentiating ability of the chiral photosensitizer. This newly developed methodology should not be restricted to this particular system but be readily expandable to a wide variety of supramolecular photochirogenesis reactions. 

III. SUPRAMOLECULAR PHOTOCHIROGENESIS WITH NATIVE AND MODIFIED CYCLODEXTRINS... 

In the early 1980s, pioneering work on photochirogenesis using CDx was ... 

The observed photobehavior of the benzaldehyde-CDx complexes in the solid state is unique and completely different from that of these complexes in aqueous solution and also from that of benzaldehyde 36 in organic solvents. The substantial formation of 4-benzoylbenzaldehyde 38 upon irradiation in (3- and y-CDx cavities indicates that these medium-sized CDx s provide the radical pair within a fairly spacious supercage environment, thus allowing the para-rearrangement (Scheme 13). The formation of practically racemic 37 upon irradiation of the y-CDx complex may also be attributed to the looser orientation of benzaldehyde 36 in the y-CDx cavity than in the (3-CDx cavity. It was thus demonstrated that the chiral hydrophobic cavity of native cyclodextrins not only modifies the photoreactivity of the included guest but also functions as a chiral supramolecular environment for photochirogenesis, albeit resulting in only modest ee%. 

This result indicates that the entropy factor plays a minor role in this supramolecular photochirogenesis, as is exactly the case with the solid-state photochemistry, but is in sharp contrast to the decisive role of entropy in the conventional (nonsu-pramolecular) counterpart performed in isotropic media, where an inversion of product chirality by temperature variation occurs [83-89]. 

IV. SUPRAMOLECULAR PHOTOCHIROGENESIS WITH SYNTHETIC CHIRAL TEMPLATES... 

Zandomeneghi and coworkers demonstrated for the first time the usefulness of BSA in supramolecular photochirogenesis. They reported the highly enantioselec-tive photodecomposition of racemic binaphthol 69 and ketoprofen 70 in the presence of BSA. [141-143,145]. 

From these results, they claimed that this newly developed function nucleosides and ds-DNA should not be restricted to this particular system could be expanded to a variety of supramolecular photochirogenesis. 

Yoshihisa Inoue is Professor of Chemistry, Osaka University, Japan, and Director of the ICORP Entropy Control Project, JST, Japan. The coauthor or coeditor of more than 300 scientific publications and books, he holds several patents. Professor Inoue headed the ERATO Photochirogenesis Project (1996-2001) and the ICORP Entropy Control Project (2002-2007), both supported by JST. He is recipient of the Japanese Photochemistry Association Award (1998) and the Molecular Chirality Award (2003). Professor Inoue received the B.S. (1972) and Ph.D. (1977) degrees in chemistry from Osaka University, Japan, and worked with N. J. Turro as a postdoctoral fellow at Columbia University (1978-1979). 

In this chapter we will review the recent advances of supramolecular photochirogenesis in various confined media, excluding micelles, chiral solvents, liquid crystals, metal complexes, polymer matrices, clays, and crystals. Micelles are a typical supramolecular assembly with an internal hydrophobic core which shows a unique boundary effect, e.g., enhanced radical recombination of geminate radical pairs produced by ketone photolysis [26], but essentially no asymmetric photoreaction has hitherto been reported in micelles. Photochemical asymmetric induction in chiral solvents [27,28] and chiral liquid crystals [29,30] have been known... 

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