Asymmetric photosensitization

There are a couple of comprehensive reviews on general asymmetric photochemistry in solution [133,134] and also on asymmetric photosensitization [135,136]. An account on multidimensional control of asymmetric photoreaction by environmental factors has also appeared recently [137]. This reflects a keen interest in chiral photochemistry and photochemical asymmetric synthesis [29]. In this section, we will concentrate mostly on the asymmetric photosensitization of cycloalkenes. 

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... 

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. 

Encmtiodifferentiating photosensitization involves a prochiral or racemic substrate interacting with an optically active sensitizer via quenching (Section 2.2.2) or exciplex formation (Section 2.2.3),585 rather than in the subsequent thermal process. Asymmetric photosensitization requires only a catalytic amount of the chiral sensitizer. An example is provided in Case Study 6.2 (E Z photoisomerization). 

Since the ground-breaking discovery of enantiodifferentiating photosensitized geometrical isomerization by Hammond and Cole [47], extensive work has been carried out in this area. However, in spite of considerable efforts devoted to a variety of asymmetric photosensitizations using many different substrates and sensitizers, the op s reported have only recently begun to reach appreciable levels... 

Asymmetric photosensitization techniques can also be used to facilitate the en-antiodifferentiating polar addition of methanol to 1,1-diphenylpropene [57]. A range of chiral naphthalenecarboxylate sensitizers bearing analogous auxiliaries to those employed in the photoisomerization of cyclooctene (e.g., 20a-c) (see Scheme 7) were found to give the anti-Markovnikov product, l,l-diphenyl-2-methoxypropane in optical yields of up to 27%. 

Direct interaction of the excited state with a chiral environment seems the most attractive way to create asymmetry in photoproducts, because the chiral inductor is not destroyed during the reaction. Since the pioneering work on the asymmetric isomerization of cyclopropanes in photosensitized experiments [12] was published, recent reports have shown that high selectivities can be achieved in asymmetric photosensitizations [13]. However, the most promising results for synthetic applications involve photoreactions in rigid and organized media such as chiral monocrystals or inclusion complexes. 

Table 3. Asymmetric photosensitized cycloaddition reaction of 4 with alkenes using the provisional axial chirality...

A few comprehensive reviews on asymmetric photochemistry in solution and on asymmetric photosensitization have been published. An account of the multidimensional control of asymmetric photoreaction by environmental factors appeared recently. ... 

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. 

Inoue, Y, Tsuneishi, H., Hakushi, T., and Tai, A., Optically active (iiZ)-l,3-cyclooctadiene first enantioselective synthesis through asymmetric photosensitization and chiroptical property, /. Am. Chem. Soc., 119, 472-478, 1997. 

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. ... 

Cis/trans cyclodimer 49 is produced with almost no enantioface differentiation. The authors of the original publication suggest that dimer 48 is produced through the concerted [In,. + 2JtJ-cycloaddition of Z-cyclohexene to -cyclohexene that is initially formed by an asymmetric photosensitization process retaining its chiral information. Dimer 49, however, arises from a nonconcerted process from E-cyclo-hexene with concurrent loss of its optical activity. 

---