Ramamurthy and coworkers investigated the β-CD-mediated chiral photoisomerizations of prochiral cis-1,2-diphenylcyclopropanes (1), pyridones (3), and
tropolones (5) to the trans-isomer (2) and the valence isomers (4 and 6) (Fig. 2,
Eqs. 1–3) [14–16]. They demonstrated that β-CD forms a stable host-guest complex
with 1 both in solution and in the solid state, which photoisomerizes to optically
active 2 in the CD cavity upon direct irradiation [14]. Thus, the photolyses of β-CD
complexes of 3 and 5 in the solid state afford the corresponding cyclization products
4 and 6 in enantiomeric excesses (ee’s) of up to 60% for 4b and 33% for 6b. In sharp
contrast, the solution-phase photolyses of 3 and 5 in the presence of excessive β-CD
lead to the formation of 4 and 6 in very low ee’s. Interestingly, the ee of 4b obtained
in the solid-state photolysis of the β-CD complex of 3b is sensitive to moisture,
being reduced from 60% to 26% with decreasing water content of the host-guest
complex in the solid state from 9% to 2% [15]. However, the presence of other
solvents (such as n-hexane) does not appreciably influence the product’s ee. The
higher ee obtained for the more moist complex was ascribed to the hydrogenbonding interactions of water molecules with CD and/or substrate in the host-guest
complex, which may constrain the orientational and conformational freedoms of the
substrate in the complex [15]. The solid-state photocyclization of the complex of 5a
with α-, β-, and γ-CD yields 6a in 5%, 28%, 0% ee, respectively, indicating that only
the host size-matched to the guest, i.e., β-CD in this particular case, can afford
significant ee [16].
Eycken and coworkers found that para-substituted 4-phenoxybutenes 7a,b
(Fig. 2, Eq. 4) form the corresponding 1:1 complexes with β-CD, which precipitate
from hot aqueous solution [17]. Photoirradiation of the complexes in the solid state
affords polycyclic product 8a,b. On the other hand, meta-isomers 9a,b (Fig. 2, Eq. 5)
form the corresponding 1:2 host-guest complexes with β-CD, and photocyclize to
give a mixture of regioisomeric 10a,b and 11a,b upon irradiation in the solid state.
The highest ee reported for 10a is 17%, while the other products are produced in
lower ee’s. The enantioselectivity observed is ascribed to the selective blockage of
one of the enantiotopic faces of the aromatic ring upon complexation with β-CD,
which facilitates the preferential attack of the vinyl from the open face.
Mori et al. investigated the effects of complexation of aromatic ester 12 with
β-CD on the subsequent photochemical decarboxylation. They found that the photodecarboxylation of (S)-12 in solution proceeds concertedly in a cheletropic manner
through the spiro-lactonic transition state (Fig. 2, Eq. 6) to afford 13 with complete
memory of the original chirality [22–26]. When racemic 12 is photolyzed in the
presence of β-CD, (R)-13 is obtained in 14% ee, suggesting that (R)-12 undergoes
faster photodecarboxylation in the cavity of β-CD.
18.3 CD-Mediated Bimolecular Chiral Photochemical Reactions
α-, β-, and γ-CD possess hydrophobic cavities of different sizes. α-CD binds only a
small linear molecule or moiety, such as alkyl chain, in its narrow cavity (5–6 Å in
diameter) [13b], while β-CD usually includes one medium-sized molecule, such as
18 Supramolecular Photochirogenesis with Cyclodextrin
513
tropolones (5) to the trans-isomer (2) and the valence isomers (4 and 6) (Fig. 2,
Eqs. 1–3) [14–16]. They demonstrated that β-CD forms a stable host-guest complex
with 1 both in solution and in the solid state, which photoisomerizes to optically
active 2 in the CD cavity upon direct irradiation [14]. Thus, the photolyses of β-CD
complexes of 3 and 5 in the solid state afford the corresponding cyclization products
4 and 6 in enantiomeric excesses (ee’s) of up to 60% for 4b and 33% for 6b. In sharp
contrast, the solution-phase photolyses of 3 and 5 in the presence of excessive β-CD
lead to the formation of 4 and 6 in very low ee’s. Interestingly, the ee of 4b obtained
in the solid-state photolysis of the β-CD complex of 3b is sensitive to moisture,
being reduced from 60% to 26% with decreasing water content of the host-guest
complex in the solid state from 9% to 2% [15]. However, the presence of other
solvents (such as n-hexane) does not appreciably influence the product’s ee. The
higher ee obtained for the more moist complex was ascribed to the hydrogenbonding interactions of water molecules with CD and/or substrate in the host-guest
complex, which may constrain the orientational and conformational freedoms of the
substrate in the complex [15]. The solid-state photocyclization of the complex of 5a
with α-, β-, and γ-CD yields 6a in 5%, 28%, 0% ee, respectively, indicating that only
the host size-matched to the guest, i.e., β-CD in this particular case, can afford
significant ee [16].
Eycken and coworkers found that para-substituted 4-phenoxybutenes 7a,b
(Fig. 2, Eq. 4) form the corresponding 1:1 complexes with β-CD, which precipitate
from hot aqueous solution [17]. Photoirradiation of the complexes in the solid state
affords polycyclic product 8a,b. On the other hand, meta-isomers 9a,b (Fig. 2, Eq. 5)
form the corresponding 1:2 host-guest complexes with β-CD, and photocyclize to
give a mixture of regioisomeric 10a,b and 11a,b upon irradiation in the solid state.
The highest ee reported for 10a is 17%, while the other products are produced in
lower ee’s. The enantioselectivity observed is ascribed to the selective blockage of
one of the enantiotopic faces of the aromatic ring upon complexation with β-CD,
which facilitates the preferential attack of the vinyl from the open face.
Mori et al. investigated the effects of complexation of aromatic ester 12 with
β-CD on the subsequent photochemical decarboxylation. They found that the photodecarboxylation of (S)-12 in solution proceeds concertedly in a cheletropic manner
through the spiro-lactonic transition state (Fig. 2, Eq. 6) to afford 13 with complete
memory of the original chirality [22–26]. When racemic 12 is photolyzed in the
presence of β-CD, (R)-13 is obtained in 14% ee, suggesting that (R)-12 undergoes
faster photodecarboxylation in the cavity of β-CD.
18.3 CD-Mediated Bimolecular Chiral Photochemical Reactions
α-, β-, and γ-CD possess hydrophobic cavities of different sizes. α-CD binds only a
small linear molecule or moiety, such as alkyl chain, in its narrow cavity (5–6 Å in
diameter) [13b], while β-CD usually includes one medium-sized molecule, such as
18 Supramolecular Photochirogenesis with Cyclodextrin
513