Enantiodifferentiating photoisomerization

Crucially, not only temperature [148,149] but also other environmental factors, such as pressure [143], magnetic/electronic fields [150,151], and solvent [152] play vital roles in a variety of asymmetric photochemical reactions. In the enantiodifferentiating photoisomerization of (Z cyclo-octene 38Z, (7 )-(—)-( ,)-38E is produced predominantly at temperatures higher than the isoenantiodifferentiating temperature (r0), while the antipodal (S)-(+)-isomer prevails at temperatures lower than T0 even if... 

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... 

The enantiodifferentiating photoisomerization of (Z )-cycloheptene 36Z was also studied in some detail [163]. Low temperature irradiations of 36Z in the presence of chiral benzenepolycarboxylates as sensitizers give the labile (ii)-isomer 36E, which is subsequently trapped in the dark by either 1,3-diphenylisobenzofuran or osmium tetraoxide to afford the adduct 88 or the diol 89, respectively (Sch. 33). The observed photostationary-state EfZ ratios of 0.1 are comparable or slightly lower than that obtained for the singlet-sensitized photoisomerization of (Z)-cyclo-octene 38Z. The ee of... 

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... 

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

The above result unequivocally demonstrates that the seemingly weak solvation effect can play a decisive role, which controls and even switches the stereochemical outcome of the enantiodifferentiating photoisomerization. Temperature, pressure, and solvation, which function as environmental factors to control the enantiodifferentiation in the excited state, are all entropic in nature. Probably the key is the critical control of the weak interactions involved in the exciplex intermediate, as with the biological and supramolecular interactions in... 

This enantiodifferentiating photoisomerization of 69ZZ was further studied by using chiral phthalamide 49y and pyromellitamide 45y (see Scheme 8 for structure) in pentane at — 67°C affording 69EZ in 1.2% and 14.3% ee, respectively. 

The enantiodifferentiating photoisomerization of achiral ethyl ester 20a was performed in NaY zeolite supercages, which were chirally modified with ephedrine, pseudoephedrine, norephedrine, diethyl tartrate, alaninol, phenylalani-nol, valinol, menthol, and bomylamine, to give trans product 21a in relatively low ees modification with diethyl tartrate led to the highest ee of 12%, while the other inductors afforded ees less than 5%. They also investigated the diastereodifferentiating photoisomerization of chiral esters 20b-d in unmodified zeolites. Photoirradiation of menthyl, neomenthyl, isomenthyl, fenchyl, isopinocamphyl, and 2-methylbutyl 23,33-diphenyl-la-cyclopropanecarboxylates 20b-d in LiY,... 

An intriguing logical extension of such work is the construction of a supramolecular host modified with a chiral sensitizer for executing enantiodifferentiating photosensitization. Inoue and coworkers reported a novel supramolecular photochirogenic system, in which an optically active sensitizer immobilized in zeolite supercages sensitizes the enantiodifferentiating photoisomerization of an excess amount of substrate dissolved in bulk solution [90]. 

Figure 5 Pressure dependence of the product ee, in enantiodifferentiating photoisomerization of 47Z sensitized by 45a, 45b, and 45x in SCCO2 at 45°C.

Case Study 6.2 Asymmetric synthesis - enantiodifferentiating photoisomerization... 

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. 

However, this is not always the case. Since the temperature dependence of the product s op. is quite tricky in these enantiodifferentiating photoisomerization sensitized by optically active polyalkyl benzenepolycarboxylates, one should use special caution in changing irradiation temperature. For instance, (-)-tetramenthyl 1,2,4,5-benzenetetracarboxylate gives (-)-4.1d of 9.6% op. at 25 °C, but produces the antipode (+)-4.1d of 28.5% op at -90 °C. Thus, the product chirality is often switched by the irradiation temperature see reference l for full detail. 

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... 

The optical yield of photoreaction in chirally modified zeolite relies on several factors such as cation, chiral inductor and its loading level, which is usually kept at a high level to maximize the number of chiral inductors located around the photosubstrate. An exceptional example may be found in the enantiodifferentiating photoisomerization of 34Z (Scheme 1.7) sensitized by optically active (poly)alkyl benzoate derivatives immobilized... 

In this review, we will concentrate on the E-Z-photoisomerization and subsequent (photo)chemical transformations of cycloalkenes caused by direct and sensitized excitation. Valence isomerization, rearrangement, and fragmentation, observed upon irradiation of cyclopropene, cyclobutene, and cyclopen-tene, are only briefly described, as they have been extensively reviewed elsewhere. - - - Although the photochemistry of acycKc alkenes will not be included in this chapter, we will refer to the photobehavior of conjugated cycloalkenes, such as cycUc dienes, enones, and styrenes, as far as it relates to the cycloalkene functionahty. As the photochemical formation of labile (E)-cycloalkenes is one of the major topics in this chapter, the physical properties of the (E)-isomers will be tabulated and discussed in detail. Recent studies on enantiodifferentiating photoisomerization of cycloalkenes are summarized in one section in view of the rapidly growing interest in asymmetric photochemistry. ... 

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. ... 

Enantiodifferentiating photoisomerization of (Z,Z)-l,3-cyclooctadiene (60ZZ) is sensitized by chiral benzene(poly)carboxylates." Upon sensitization with hexa-(-)-menthylbenzenehexacarboxylate in pentane at -40°C, the ( ,Z)-isomer (60EZ) is obtained in 18% ee. Photoisomerization of (Z,Z)-l,5-cyclooc-tadiene (69ZZ) sensitized by (-)-menthyl benzoate affords (-)-( ,Z)-isomer (69EZ) in only 1.6% eei ... 

Recent studies on enantiodifferentiating photoisomerization reveal that the photosensitization of C Cg (Z)-cycloalkenes with optically active aromatic esters affords chiral ( )-cycloalkenes in good to excellent ees. The mechanism involves enantiodifferentiating rotational relaxation within the exciplex intermediate formed from the excited singlet state of chiral sensitizer and prochiral substrate. The exciplex structure and hence the stereochemical outcome are very sensitive not only to the internal factors, such as sensitizer energy and structure, but also to the external entropy-related factors such as temperature, pressure, and solvent. This leads to a novel idea of multidimensional control of product chirality by several environmental factors. The asymmetric photosensitization can be applied to the photochemical asymmetric synthesis and also be used as a powerful tool for exploiting the reaction mechanisms in the excited-state chemistry. 

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