Photosensitization enantiodifferentiating

This class of photosensitized enantiodifferentiations includes a variety of irreversible photodecomposition and photorearrangement reactions induced by energy or electron transfer from chiral sensitizers to racemic substrates. 

C. Photosensitized enantiodifferentiating isomerization of a cyclic C=C double bond... 

A review of photochemistry carried out in ionic liquids and a monograph dealing with a variety of aspects of photochromism have been published Reviews have also highlighted the photosensitized enantiodifferentiating isomerization of cycloalkenes other than cyclooctene and the area of asymmetric photochemistry in general Mechanisms for the photosensitized isomerization of alkenes have been discussed and calculations have been carried out to assess the electron-transfer processes in the tetramethylethene-tetracyanoethene system ... 

TABLE 16.3 Photosensitized Enantiodifferentiating Isomerization of (Z)-Cyclooctene (20Z) (continued)... 

Inoue, Y., Yokoyama, T, Yamasaki, N., and Tai, A., Temperature switching of product chiraHty upon photosensitized enantiodifferentiating ds-trans isomerization of cyclooctene, /. Am. Chem. Soc., Ill, 6480-6482,1989. 

Wada, T, Shikimi, M., Inoue, Y, Lem, G., and Turro, N.L, First photosensitized enantiodifferentiating isomerization by optically active sensitizer immobilized in zeolite supercages,/. Chem. Soc., Chem. Commun., 1864-1865, 2001. 

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. 

There has been considerable interest in various photocycloaddition reactions over the last years which not only broadened the number of useful photochemical applications but also revealed further mechanistic insight into these reactions [76,77]. Among these reactions, reports focusing on either the [2 -h 2] or the [4 -I- 2] cycloaddition, are numerous. Also the efforts toward the enantiodifferentiating photosensitization in photocyclization reactions have to be mentioned [78],... 

Since alcohols are less effective as hydrogen donors than amines, a PET photoaddition can occur only when the oxidized component of the reaction is the alkene. Furthermore, if the photosensitizer is chiral, the polar addition would occur in an enantiodifferentiating manner to some degree. Thus, the photoaddition of 2-propanol to 1,1-diphenylpropene, when sensitized by chiral naphthalene(di)carbox-ylates, formed the anti-Markovnikov photoadduct with enantiomeric excesses of up to 58% [53]. Unfortunately, the reaction is far from attracting synthetic interest as the yields are still too low. 

Inoue, Y. (2004) Enantiodifferentiating photosensitized reactions, in Chiral Photochemistry (Molecular and Supramolecular Photochemistry), Vol. 11... 

As a consequence of the restricted jump-rope rotation around the trans double bond, (i j-cyclo-octene 38E is chiral. Optically active (E)-cyclo-octene has long been known, but the conventional multistep synthesis is rather tedious [138-140]. In contrast, direct-preparation of optically active (Ej-cyclo-octene through asymmetric photosensitization is an attractive alternative. The first enantiodifferentiating Z-E photoisomerization of cyclo-octene 38Z sensitized by simple chiral alkyl benzenecarboxylates was reported in 1978 to give low enantiomeric excesses (ee s) of <6% [141] a variety of systems and conditions have been examined since then to raise the product ee. For an efficient transfer of chiral information... 

Photoisomerization of (Z)-l-methylcyclo-octene 59aZ sensitized by chiral benzene(poly)carboxylates suffers significantly from steric effects. The photosensitization of 59aZ with bulky sensitizers leads to photostationary E/Z ratios and product ee s much lower than those obtained for less-hindered (Z)-cyclo-octene 38Z [158]. Enantiodifferentiating photoisomeri-zations of (Z,Z)-l,3-cyclo-octadiene and (Z,Z)-l,5-cyclo-octadiene (71Z and 78Z) have also been studied by using chiral benzene(poly)carboxylates at various temperatures [159]. Photosensitization of 71Z with hexa-(—)-menthyl benzenehexacarboxylate affords the (E,Z)-product 71E in 18% ee in pentane at —40 °C. In the photoisomerization of 78Z in pentane sensitized by (—)-menthyl benzoate, the (—j- Zj-isomer 78E is only slightly favored giving a 2% ee [126]. 

The smaller-sized cyclohexene and cycloheptene have also been subjected to enantiodifferentiating photoisomerization, although the corresponding ( )-isomers are short-lived transient species. Photosensitization of (Z)-cyclohexene 31Z with chiral benzene(poly)carboxylates affords trans-anti-trans-, cis-trans-, and cis-anti-cis-cyclodimers 34 (Sch. 32). Interestingly, of the former two chiral products, only the trans-anti-trans isomer is optically active and its ee reaches up to 68%, whilst the cis-trans isomer is totally racemic under a variety of irradiation conditions, for which two competing, concerted, and stepwise cyclodimerization mechanisms are responsible. Thus, the enantiodifferentiating photoisomerization of 31Z to the optically... 

E, evaluated from that of the adduct 88 is higher than that of (Fj-cydo-octene 38E obtained under the comparable conditions, and is sensitive to the solvent polarity and reaction temperature. Upon sensitization with tetra-(—)-bornyl 1,2,4,5-benzenetetracarboxylate in hexane at — 80 °C, the ee of 36E goes up to 77% ee, which is the highest value reported for the enantiodifferentiating photosensitized isomerization to-date. 

Recent studies on the enantiodifferentiating photosensitization reveal that the singlet sensitization with appropriate enantiopure compounds leads to good-to-excellent enantiodifferentiating photoisomerization of cyclohexene, cycloheptene, and cyclo-octene through the corresponding... 

The title topic has been reviewed frequently in the general context of chiral photochemistry [1,3], and hence we will only briefly refer to the earlier works and put more emphasis on the latest results in this chapter devoted solely to enantiodifferentiating photosensitized reactions. 

The first successful enantiodifferentiating photosensitization was achieved ii 1965 for the geometrical photoisomerization of 1,2-diphenylcyclopropane 20, ii... 

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. 

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