moiety encapsulated in the cavity, if excited, cannot transfer the excitation energy
to the substrate existing in the bulk solution. However, once a substrate (e.g., 57Z)
is included in the cavity, the arene moiety will be relocated to make a space for the
incoming guest but still stay near the CD portal to keep the van der Waals contacts
and the π-π interaction with the included guest (Fig. 14, middle), which should
greatly reduce the rotational and conformational freedoms and facilitate the energy
transfer inside the chiral cavity. It is to be emphasized that the build-in sensitizer is
temporarily masked or inactivated in the absence of the guest to prevent the
energy-transfer to free substrates in the bulk solution, but becomes active to
promote the asymmetric photoisomerization once the substrate is introduced to
the chiral cavity. This intelligent device, if perfectly functions, achieves an ideal
on/off switching of the chiral supramolecular photosensitization, and the fundamental concept should be useful in constructing a variety of catalytic supramolecular photochirogenic systems.
The supramolecular photosensitization of 57Z with 6-O-benzoyl-β-CD 59b
affords 57E in modest ee’s of up to 11%, while the size-mismatched α- and γ-CD
analogues 59a and 59j give much lower ee’s [51]. The enantioselectivity of 57E is
significantly improved to 46% ee by introducing a methoxy group at the metaposition of the benzoyl moiety, i.e., 59d. In contrast, the regioisomeric ortho- and
para-methoxy-substituted 59c and 59e and meta-methoxycarbonyl-substituted 59 g
give 57E in much lower 4%, À11%, and À2% ee, respectively [54]. This result
reveals the critical roles of the substituent introduced to the sensitizer moiety in the
supramolecular photosensitization, and also indicates the possibility of further
enhancing the enantioselectivity by more sophisticated chemical modifications as
well as the necessity for elucidating a clearer picture of the supramolecular complexation and photosensitization mechanisms.
The asymmetric photoisomerization of 57Z with chiral sensitizer performed in
isotropic media is often critically affected by the entropy-related environmental
variants, such as temperature, solvent, and pressure [59–63]. On the contrary, the
enantioselectivity observed for the supramolecular photoisomerization of 57
included and sensitized by the above β-CD derivatives is much less sensitive to
temperature, which has been attributed to the restricted motions of the substrate and
the sensitizer moiety in the CD cavity. In contrast, the enantioselectivity for the
supramolecular photoisomerization of 57Z with permethylated β-CD derivative 59i
turned out to be a critical function of temperature, which has been rationalized by the
flexible skeleton of permethylated β-CD due to the loss of the hydrogen-bonding
network on the secondary rim of CD.
The photoisomerization of (Z,Z)-1,3-cyclooctadiene 58ZZ sensitized by chiral
alkyl benzene(poly)carboxylates, such as phthalates, pyromellitate, and mellitate,
has been investigated in isotropic media to afford 58EZ in low ee’s of up to 17%
even at À40
C [64]. Naphthalene-modified α-, β-, and γ-CDs 60a–60c employed as
chiral sensitizing hosts do not improve the enantioselectivity of the photochirogenic
reaction, affording 58EZ in only 2–5% ee [65]. Recently, Yang and coworkers have
demonstrated that [4]rotaxane 61, composed of a γ-CD wheel and a chromophoric
biphenyl axle end-capped with cucurbit[6]urils (Fig. 14), can co-include one 58ZZ
18 Supramolecular Photochirogenesis with Cyclodextrin
527
to the substrate existing in the bulk solution. However, once a substrate (e.g., 57Z)
is included in the cavity, the arene moiety will be relocated to make a space for the
incoming guest but still stay near the CD portal to keep the van der Waals contacts
and the π-π interaction with the included guest (Fig. 14, middle), which should
greatly reduce the rotational and conformational freedoms and facilitate the energy
transfer inside the chiral cavity. It is to be emphasized that the build-in sensitizer is
temporarily masked or inactivated in the absence of the guest to prevent the
energy-transfer to free substrates in the bulk solution, but becomes active to
promote the asymmetric photoisomerization once the substrate is introduced to
the chiral cavity. This intelligent device, if perfectly functions, achieves an ideal
on/off switching of the chiral supramolecular photosensitization, and the fundamental concept should be useful in constructing a variety of catalytic supramolecular photochirogenic systems.
The supramolecular photosensitization of 57Z with 6-O-benzoyl-β-CD 59b
affords 57E in modest ee’s of up to 11%, while the size-mismatched α- and γ-CD
analogues 59a and 59j give much lower ee’s [51]. The enantioselectivity of 57E is
significantly improved to 46% ee by introducing a methoxy group at the metaposition of the benzoyl moiety, i.e., 59d. In contrast, the regioisomeric ortho- and
para-methoxy-substituted 59c and 59e and meta-methoxycarbonyl-substituted 59 g
give 57E in much lower 4%, À11%, and À2% ee, respectively [54]. This result
reveals the critical roles of the substituent introduced to the sensitizer moiety in the
supramolecular photosensitization, and also indicates the possibility of further
enhancing the enantioselectivity by more sophisticated chemical modifications as
well as the necessity for elucidating a clearer picture of the supramolecular complexation and photosensitization mechanisms.
The asymmetric photoisomerization of 57Z with chiral sensitizer performed in
isotropic media is often critically affected by the entropy-related environmental
variants, such as temperature, solvent, and pressure [59–63]. On the contrary, the
enantioselectivity observed for the supramolecular photoisomerization of 57
included and sensitized by the above β-CD derivatives is much less sensitive to
temperature, which has been attributed to the restricted motions of the substrate and
the sensitizer moiety in the CD cavity. In contrast, the enantioselectivity for the
supramolecular photoisomerization of 57Z with permethylated β-CD derivative 59i
turned out to be a critical function of temperature, which has been rationalized by the
flexible skeleton of permethylated β-CD due to the loss of the hydrogen-bonding
network on the secondary rim of CD.
The photoisomerization of (Z,Z)-1,3-cyclooctadiene 58ZZ sensitized by chiral
alkyl benzene(poly)carboxylates, such as phthalates, pyromellitate, and mellitate,
has been investigated in isotropic media to afford 58EZ in low ee’s of up to 17%
even at À40
C [64]. Naphthalene-modified α-, β-, and γ-CDs 60a–60c employed as
chiral sensitizing hosts do not improve the enantioselectivity of the photochirogenic
reaction, affording 58EZ in only 2–5% ee [65]. Recently, Yang and coworkers have
demonstrated that [4]rotaxane 61, composed of a γ-CD wheel and a chromophoric
biphenyl axle end-capped with cucurbit[6]urils (Fig. 14), can co-include one 58ZZ
18 Supramolecular Photochirogenesis with Cyclodextrin
527