Enhanced irregular photodimers and switched enantioselectivity by solvent and temperature in the photocyclodimerization of 2-anthracenecarboxylate with modified β-cyclodextrins

Article abstract of DOI:10.1016/j.jphotochem.2018.11.038

A series of primary-face- or secondary-face-modified β-cyclodextrins (CDs) 7-11 were prepared for mediating the enantiodifferentiating [4 + 4] photocyclodimerization reactions of 2-anthracenecarboxylic acid (AC). The complexation behavior of these hosts w

Full text of DOI:10.1016/j.jphotochem.2018.11.038

JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Enhanced irregular photodimers and switched enantioselectivity by solvent
and temperature in the photocyclodimerization of 2-anthracenecarboxylate
with modified β-cyclodextrins
Xueqin Weia, Xingke Yua, Yuxue Zhanga, Wenting Lianga,b, Jiecheng Jia, Jiabin Yaoa, Ming Raoa,
Wanhua Wua, Cheng Yanga,⁎
a Key Laboratory of Green Chemistry & Technology, College of Chemistry, State Key Laboratory of Biotherapy, West China Medical Center, Healthy Food Evaluation
Research Center, Sichuan University, 29 Wangjiang Road, Chengdu, 610064, China
b Institute of Environmental Sciences, Department of Chemistry, Shanxi University, Taiyuan, 030006, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of primary-face- or secondary-face-modified β-cyclodextrins (CDs) 7-11 were prepared for mediating the
enantiodifferentiating [4 + 4] photocyclodimerization reactions of 2-anthracenecarboxylic acid (AC). The
complexation behavior of these hosts with AC was studied by 1H NMR, UV–vis, circular dichroism spectroscopy
and ITC titration. The product distribution and product’s ee turned out to be a critical function of the skeleton
and periphery modifications of β-CD. The combined yields of slipped cyclodimers 5 and 6 were greatly enhanced
in the presence of modified β-CDs. In contrast to native β-CD and other β-CD derivatives, host 9, in which a
phenyltriazolyl group was introduced on the primary face of β-CD, afforded antipodal cyclodimer 6 in a much
higher enantioselectivity of 46% ee. The influence of temperature and salt additives were investigated in depth.
Intriguingly, product chirality was switched by temperature in the presence of 6 M CsCl. Under an optimized
condition, host 8 gave slipped cyclodimer 6 in 70% ee, which is the highest enantioselectivity ever reported for
the slipped cyclodimer.
Photocyclodimerization
Anthracenecarboxylic acid
Modified β-Cyclodextrins
Enantioselectivity
Chirality inversion
1. Introduction
chiral, UV-transparent and versatile in molecular recognition.
Enantiodifferentiating [4 + 4] photocyclodimerization of 2-anthra-
Photochirogenesis, a photochemical alternative to thermal asym-
metric synthesis, which has particular advantages in producing highly
strained and multicyclic chiral compounds that are not accessible by the
thermal counterparts, has attracted significant attention of photo-
chemists [1–5]. However, it remains a great challenge to achieve a high
level of stereochemical control in photochirogenesis due to the weak
and short-lived interactions commonly featured in photosubstrates at
the excited state. Supramolecular photochirogenesis [2,6–13], bene-
fiting from longer and more intimate supramolecular interactions at
both ground and excited states, has been demonstrated as an attractive
and promising strategy. A variety of supramolecular hosts, including
chirally modified zeolites [14–16], synthetic templates [17,18], cyclo-
dextrins (CDs) [19,20], biomolecules [21,22], liquid crystals [23] and
chiral gels [24,25] have been explored for mediating chiral photo-
reactions. Of the above hosts, cyclodextrin is particularly attractive and
widely used as a chiral molecular container for asymmetric photo-
reactions due to their properties such as readily availabile, inherent
cenecarboxylate is one of the best established and utilized photoreac-
tions, which has been studied comprehensively by several research
groups since the seminal work by Inoue et al [26–41]. Photoirradiation
of AC generally affords four [4 + 4] cyclodimers 1-4 (Scheme 1), of
which cyclodimers 2 and 3 are chiral. Native and modified γ-CDs have
been commonly used as chiral hosts for the supramolecular photo-
chirogenic cyclodimerization of AC [27–37,42,43]. These foregoing
studies revealed that a variety of internal and external factors, such as
the chemical structure of chiral host, temperature, solvent, pressure,
and irradiation wavelength, critically determine the enantioselective
outcomes of the photocyclodimerization.
Very recently, we found that AC stepwise forms a 1:1 complex and a
2:2 complex with β-CD, which does not merely shift the product se-
lectivity toward the anti-HT 910:9′,10′-cyclodimer
1 but un-
precedentedly activates the nonclassical reaction route to the slipped
anti- and syn-HT 5,8:9′,10′-cyclodimers 5 and 6 (Scheme 1) [38]. We
synthesized a series of β-CD derivatives that are grafted with cationic
⁎ Corresponding author.
E-mail address: yangchengyc@scu.edu.cn (C. Yang).
https://doi.org/10.1016/j.jphotochem.2018.11.038
Received 8 November 2018; Received in revised form 22 November 2018; Accepted 26 November 2018
Availableonline27November2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
Scheme 1. Photocyclodimerization of 2-Anthracenecarboxylate (AC) with Modified β-Cyclodextrins.
substituent(s) on the primary rim for facilitating slipped stacking
through electrostatic interaction, which evidently improved the slipped
5,8:9′,10′-cyclodimers 5 and 6. In this work, we prepared a series of
primary-face- or secondary-face-modified β-CDs 7-11 as chiral hosts to
control the enantiodifferentiating [4 + 4] photocyclodimerization re-
actions of AC, for the purpose to manipulate the preorganization of two
ACs in the 2:2 complex, and thus achieve a higher level of control to-
wards the slipped 5,8:9′,10′-cyclodimers 5 and 6. The affecting factors
and mechanisms for the exclusive formation of the slipped cyclodimers
with enhanced enantioselectivity was also investigated.
(20 mg) in DMF (30 mL) was stirred for 5 h at room temperature. After
neutralized by addition of dilute HCl, the solvent was removed under
vaccum. The residue was dissolved in water and chromatographed by
reversed-phase column with a gradient elution from water to 10%
aqueous EtOH (1 L) to give pure 7. Yield:50%. 1H NMR (400 MHz, D2O)
δ 5.31 (d, J = 3.4 Hz, 1 H), 5.09-5.00 (m, 6 H), 4.13-3.48 (m, 42 H). 13
C
NMR (101 MHz, DMSO-d6) δ 102.55, 102.32, 102.22, 102.18, 102.25,
102.55, 101.49, 101.44, 101.32, 95.00, 82.34, 82.30, 82.23, 81.67,
81.63, 81.61, 80.77, 78.41, 78.39, 74.73, 74.48, 73.71, 73.54, 73.37,
73.32, 73.16, 72.91, 72.88, 72.71, 72.66,72.48, 72.47, 72.30, 69.51,
60.71, 60.65, 60.56, 60.54, 60.28, 60.18, 60.16, 60.10, 55.62, 53.22.
HR-MS: m/z calcd for [M + Na]+: 1139.3490, found: 1139.3525.
2. Experimental
2.1. General methods
2.2.2. Synthesis of 8
Compound 7 (540 mg) and sodium azide (160 mg) were dissolved in
10 mL dried DMF, and the solution was stirred at 85 °C for 8 h. Then,
DMF was removed under vacuum, and the residue was dissolved in
30 mL ultrapure water. After filtration, the solution was applied on a
reversed-phase column. A gradient elution from water to 25% aqueous
EtOH was applied to give 8. Yield:18%. 1H NMR (400 MHz, D2O) δ 5.05
(t, J = 3.8 Hz, 7 H), 3.97-.90 (m, 6 H), 3.85 (m, 15 H), 3.68 (dd,
J = 10.3, 3.5 Hz, 1 H), 3.66-3.45 (m, 13 H). 13C NMR (101 MHz, DMSO-
d6) δ 102.09, 102.04, 102.04, 102.01, 101.99, 101.98, 101.95, 101.91,
101.74, 101.72, 101.71, 100.48, 100.44, 100.43, 96.84, 92.72, 90.25,
81.76, 81.53, 81.51, 81.48, 81.48, 81.46, 81.45, 81.23, 81.19, 81.18,
81.16, 81.14, 80.65, 77.50, 73.25, 73.24, 73.15, 73.06, 73.04, 72.97,
72.62, 72.56, 72.53, 72.43, 72.42, 72.08, 66.92, 66.91, 60.57, 60.20,
60.18, 60.17, 60.13, 60.11, 60.07, 60.04, 59.96, 59.95, 59.93, 59.87,
59.81, 59.78, 58.08, 55.06, 54.85, 54.84. HR-MS: m/z calcd for [M+]:
1159.3762, found: 1159.3769.
2-Anthracenecarboxylic acid and β-CD were purchased from TCI
and Aldrich, respectively, and used as received. Doubly distilled water
and HPLC grade solvents were used for photoreactions and spectral
measurements. Other solvents were purchased from Aladdin Chemical
Co., China. 1H and 13C NMR spectra were obtained with a Brucker DRX-
400 Spectrometer. HR–MS were detected by matrix-assisted laser des-
orption ionization time-of-flight mass spectrometry (MALDI-ROF MS,
Bruker, USA). UV–vis spectra were recorded on a JASCO V650 spec-
trophotometer (bandwidth = 2.0 nm). Circular dichroism spectra were
measured on a JASCO J-1500 spectropolarimeter (bandwidth = 2.0
nm). ITC data were recorded by using VP-ITC MicroCalorimeter.
Photoproducts were analyzed by a Shimadzu LC Prominence 20 HPLC
instrument equipped with UV–vis and fluorescence detectors.
2.2. Synthesis
2.2.1. Synthesis of 7
2.2.3. Synthesis of 9
A solution of 2A-naphthalenesulfonyl-β-CD (300 mg) and NaOH
6A-Deoxy-6A-azido-β-CD (580 mg) and phenylacetylene (100 mg)
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X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
Fig. 1. 1H NMR spectra of (a) AC (0.4 mM), (b) 7 (1.0 mM), and (c) a mixture of AC (2.0 mM) and 7 (4.0 mM) measured in a pD 9.0 D2O buffer solution at 20 °C.
were dissolved in dried DMF using CuI as catalyst, and the solution was
stirred at 50 °C for 24 h. The DMF was removed under vacuum, and the
residue was chromatographed by a reversed-phase column to give pure
9. Yield:91%. 1H NMR (400 MHz, DMSO + D2O(1:1)) δ 8.36 (s, 1 H),
7.73 (d, J = 7.3 Hz, 2 H), 7.42 (t, J = 7.5 Hz, 2 H), 7.34 (t, J = 7.3 Hz,
1 H), 5.04 (d, J = 3.4 Hz, 1 H), 4.88 (dd, J = 15.6, 12.4 Hz, 5 H), 4.77
(dd, J = 9.7, 3.2 Hz, 2 H), 4.54 (dd, J = 13.8, 9.2 Hz, 1 H), 3.96 (dd,
J = 29.3, 19.3 Hz, 1 H), 3.79-3.54 (m, 20 H), 3.50-3.23 (m, 18 H), 3.10
(d, J = 9.6 Hz, 1 H), 2.88 (d, J = 11.5 Hz, 1 H), 2.77 (d, J = 11.3 Hz,
1 H). 13C NMR (101 MHz, DMSO-d6) δ 147.17, 147.14, 129.92, 129.45,
128.86, 128.83, 128.61, 125.52, 122.99, 122.98, 122.95, 122.90,
119.81, 102.24, 101.94, 83.35, 83.33, 83.31, 82.40, 81.77, 81.48,
81.45, 81.44, 81.40, 81.38, 81.35, 81.32, 81.30, 81.27, 81.26, 81.25,
81.24, 81.22, 81.21, 81.21, 80.91, 80.21, 73.30, 73.25, 73.18, 73.14,
72.17, 72.14, 72.11, 72.04, 72.02, 71.90, 71.88, 60.93, 60.38, 60.37,
60.35, 60.34, 60.34, 60.31, 60.17, 60.16, 60.14, 60.10, 60.08, 59.55,
dissolved in water (2 mL), and the aqueous solution was re-precipitated
by pouring into acetone to form a white precipitate. This dissolution-
precipitation process was repeated for another two times to afford
compound 11 as a pale white solid. Yield:85% 1H NMR (400 MHz, D2O)
δ 5.17-5.04 (m, 7 H), 3.98 (dd, J = 17.4, 8.0 Hz, 9 H), 3.87 (d,
J = 12.1 Hz, 12 H), 3.70- 3.54 (m, 14 H). 13C NMR (101 MHz, DMSO-
d6) δ 157.68, 157.64, 101.84, 101.71, 101.63, 101.44, 101.42, 101.41,
97.32, 88.66, 87.72, 82.39, 82.30, 82.29, 82.27, 82.26, 81.03, 72.94,
72.91, 72.86, 72.81, 72.61, 71.91, 71.75, 71.71, 71.68, 71.61, 70.40,
70.38, 60.18, 42.08, 42.06, 42.05, 42.03, 42.01, 39.53. HR-MS: m/z
calcd for [M+H]+: 1176.4154, found: 1176.4155.
2.3. Photoreaction
Photoirradiations were performed in a temperature controlled
water/ethylene glycol bath. Solutions containing 0.2 mM AC and
5.2 mM CD derivative were irradiated at 365 nm under an argon at-
mosphere with an LED lamp with a diameter of 1 cm and an intensity of
200 mW/cm2 for 30 min.
58.51, 58.05, 51.20, 51.18. HR-MS: m/z calcd for [M + Na]+
:
1284.4130, found: 1284.4137.
2.2.4. Synthesis of 10
Compound 10 was prepared from 6A-deoxy-6A-iodo-β-CD [44] as a
starting material. 6A-deoxy-6A-iodo-β-CD (200 mg) and thiourea
(15.3 mg) were mixed in DMF (2 mL) and the solution was stirred at
room 85 ℃ for 24 h then poured dropwise onto acetone. The pre-
cipitates collected were dissolved in water (2 mL) and the aqueous so-
lution was re-precipitated by pouring into acetone. This dissolution-
precipitation process was repeated another two times to afford com-
pound 10 as a white solid. Yield:88% 1H NMR (400 MHz, DMSO + D2O
(1:1)) δ 7.51 (d, J = 8.2 Hz, 2 H), 7.20 (d, J = 8.0 Hz, 2 H), 4.98-4.78
(m, 7 H), 3.84 (t, J = 8.8 Hz, 1 H), 3.77-3.52 (m, 26 H), 3.50-3.27 (m,
15 H), 2.26 (s, 3 H).13C NMR (101 MHz, DMSO-d6) δ 171.07, 141.42,
141.38, 141.13, 140.91, 129.22, 129.08, 125.51, 125.47, 102.04,
102.03, 102.02, 101.98, 101.96, 101.94, 101.92, 81.68, 81.38, 81.37,
81.35, 81.33, 81.31, 81.29, 81.26, 81.25, 81.23, 81.14, 73.46, 73.21,
72.06, 71.99, 60.24, 60.21, 60.19, 60.18, 60.17, 60.16, 60.14, 60.12,
59.61, 38.88, 38.67, 38.46, 38.25, 38.03, 37.82, 37.61, 20.84, 20.74.
HR-MS: m/z calcd for [M + Na]+: 1216.3658, found: 1216.3649.
3. Results and discussion
3.1. Complexation studies
The complexation behavior of AC with the modified β-CDs was
studied by NMR, UV–vis, and CD spectral studies. We have demon-
strated that a 2:2 complex of AC and β-CD was formed through the
association of two 1:1 complex from the secondary rim of modified β-
CDs in aqueous solution [38]. The main driving force for the 1:1
complexation of AC with β-CD is the hydrophobic interaction, and the
inter-β-CD hydrogen-bonding and π-π stacking between ACs were
found to play an important role for the formation of the 2:2 complex.
The complexation was improved by introducing cationic groups onto
the primary rim of β-CD through additional electrostatic interaction.
The 2:2 complexation of AC with host 7 was confirmed by 1H NMR
spectroscopy examination. As illustrated in Fig. 1, all proton signals of
both AC and β-CDs showed certain extent upfield shifts, when host 7
was mixed with AC in aqueous solution. The protons H5, H8 and H9,
H10 of AC exhibited more apparent upfield than protons H1, H3, H4
and H6, H7, which can be accounted for by the stacked overlap of the
included two AC molecules in the 2:2 complex. Only one set of AC NMR
proton signals were observed, indicating that the complexation/de-
complexation equilibrium is fast at the NMR time-scale.
2.2.5. Synthesis of 11 [38]
6A-Deoxy-6A-amino-β-CDs (250 mg), 1H-pyazole carboxamidine
hydrochloride (500 g), and N,N-diisopropylethylamine (2.0 mL) were
mixed in water (2 mL) and the solution was stirred at room temperature
for 48 h and then poured into acetone. The precipitate collected were
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X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
photoreaction was calculated. As can be seen from Table 1, the popu-
lation of free AC and the 1:1 and 2:2 complexes in a solution containing
0.2 mM AC is in a ratio of 5:90:5 for native β-CD but significantly shifts
to
a 0:60:40 ratio for 9 to facilitate the subsequent photo-
cyclodimerization.
3.2. Photocyclodimerization of AC with modified β-CDs
Photoirradiation of AC in the presence of modified β-CD in aqueous
solutions in the absence or presence of CsCl was performed at 365 nm
by using an LED lamp at various temperature. The relative yield and ee
for each photodimer were determined by tandem achiral-chiral HPLC
analysis to give the results listed in Table 2.
3.2.1. Host dependence
We have demonstrated that the product distribution and enantios-
electivity in the photocyclodimerization of AC with CD is determined
by the preorientation of stacked AC pairs in the CD cavity at the ground
state, as the tightly packed complex structure and the short lifetime of
AC at the excited state does not allow the rotation or the positional
exchange of AC in the cavity of CD [27]. Thus, the formation of 2:2
complexes and their stacking model is crucial for the enantioselective
production of chiral cyclodimers in the photocyclodimerization of AC
mediated by β-CDs. To elucidate how the chemical modification on the
primary-face and secondary-face of β-CDs affect the host–guest com-
plexation, as well as the stereochemistry of photocyclodimerization of
AC, we made a comparison of the product distribution of the photo-
reaction mediated by 7-11.
Fig. 2. UV–vis spectral changes of 2-anthracenecarboxylic acid
([AC] = 0.2 mM) in aqueous solution upon gradual addition of 11 from 0 to
5.0 mM.
In water at 0.5 ℃, photocyclodimerization of AC in the presence of
native β-CD afforded cyclodimer 1 as a major product. On the contrary,
the use of hosts 7 and 8 considerably enhanced the yield of slipped
cyclodimers 5 and 6 to 74–91% under the same condition. It is noted
that both 7 and 8 are secondary-face-modified, which will partially
break the hydrogen-bonding network at the secondary face of β-CD and
thus to give
a more flexible and deformed hydrophobic cavity.
Consequently, AC molecule may not penetrate as deep into the de-
formed CD cavity as it does in the cavity of native β-CD, thus facilitating
more slipped stacking.
The relative yields of slipped cyclodimers 5 and 6 were moderately
increased to ca. 50% in the presence of cationic β-CDs 10 and 11. We
ascribe this to the electrostatic interaction between carboxylate anion
of AC and the cationic substituent that draw the included AC from the
primary rim of β-CDs to facilitate the formation of the slipped stacking.
On the other hand, the primary face-modified 6A-phenyltriazolyl-6A-
deoxy- β-CD 9 resulted in the irregular cyclodimers 5 and 6 in 91%
combined yield. This result demonstrated that an aromatic substituent
may function more effective than the electrostatic interaction in im-
proving the irregular cyclodimers, possibly due to that the large aro-
matic phenyltriazolyl group can increase the hydrophobicity of CD
cavity and improve the complexation through the π-π interaction at the
primary rim. This is confirmed by the much enhanced binding affinity
observed with 9 (Table 1).
Fig. 3. Circular dichroism spectra of 5.0 mM 7-11 in the presence of 0.2 mM AC
in aqueous solution (pH 9.0) at 20 °C.
As can be seen from Fig. 2, the addition of host 11 to an aqueous
solution of AC led to profound bathochromic shifts of the AC’s 1Bb and
1La bands with an isosbestic point at 266.5 nm, suggesting the π-π
stacking of two AC molecules co-included in two β-CD cavities. For all
β-CD derivatives 7-11, the circular dichroism spectra of AC (Fig. 3) in
the presence of a host showed a positive Cotton effect at the 1Bb band
and a very weak negative one at the 1La band. On the basis of the Sector
Rule proposed by Kajtar et al. [45], we deduce that AC penetrates into
the CD cavity, with the 1Bb transitions parallel to the CD’s longitudinal
axis.
The product’s ee turned out to be a critical function of the skeleton
and periphery modifications of β-CD. For instance, upon photoirradia-
tion of AC in water at 0.5 ℃, skeleton-modified 7, 8 and periphery-
modified 10, 11 afforded cyclodimer 5 in ca. 40% ee, while phenyl-
triazolyl-capped β-CD 9 gave the same enantiomer in 56% ee, which is
twice as much as that obtained with native β-CD. These results in-
dicated that the ground-state population of re-si and si-re complexes are
affected by skeleton and periphery modifications of β-CD. The steric
effect and the π-π interaction at the primary rim of β-CD may sig-
nificantly improve the population of certain stacked AC pairs.
Native β-CD, skeleton-modified 7 and periphery-modified 10, 11
gave nearly identical ee’s (6–9%) for 6 in water at 0.5 ℃, while the
skeleton-modified 8 led to the formation of the same enantiomer in
To quantitatively evaluate the complexation of AC with modified β-
CDs, isothermal titration calorimetry (ITC) studies were carried out to
determine the binding affinity for the 1:1 and 2:2 complexation (Fig. 4).
The association constants for stepwise 1:1 (K1) and 2:2 complexations
(K2) thus obtained were listed in Table 1. Host 7 showed a moderately
reduced K1 and an enhanced K2 relative to that of native β-CD.
Whereas, the K1 and K2 values for host 9 are much larger than that of
native β-CD, most probably due to the increased hydrophobicity of the
cavity. The cationic substituent introduced into β-CD can also greatly
improve the association constants because of the electrostatic interac-
tion between the cationic substituents of the CD and the carboxylate
anion of AC [38]. By using the association constant obtained, the po-
pulation of free and bound AC species in aqueous solutions used for
37% ee. Intriguingly, host
9 showed the opposite enantiomeric
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X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
Fig. 4. ITC data of host (a) 7 and (b) 9 complexing with AC in aqueous buffer solution (pH 9) at 20 °C.
preference to give antipodal cyclodimer 6 in a much higher 46% ee
under the same condition, indicating that the diastereoselectivity of
syn-HT precursor complex is more critically controlled by aromatic
modification on the primary face.
To more quantitatively evaluate the temperature effect on the pro-
duct enantioselectivity, the natural logarithm of the relative enantiomer
ratios of slipped cyclodimer 6 was plotted against the reciprocal tem-
perature according to the Eyring equations. As shown in Fig. 5, all the
data obtained for the native β−CD and host 7-11 showed a good linear
relationship, indicating a single enantiodifferentiation mechanism op-
erates over the temperature examined. From the slope and intercept of
plot, the differential enthalpy change (ΔΔH) and entropy change (ΔΔS)
were calculated (Table 3). As illustrated in Table 3, native β-CD and
cationic β-CDs 10, 11 gave positive ΔΔH (1.4-9.4 kJ mol−1) and rela-
3.2.2. Temperature and salt dependence
We have found that external factors, like temperature and pressure,
may play a critical role in photocyclodimerization of AC mediated by γ-
CDs. β-CD has a relatively rigid cavity, and the temperature effect is not
significant in the photosensitized isomerization cyclooctenes [46–49].
On the other hand, AC formed 2:2 complex with β-CD, which is ex-
pected to be more flexible and entropy-dependent [38]. The tempera-
ture effects on the enantiodifferentiating photocyclodimerization of AC
with modified β-CDs were examined in water at various temperature
ranging from 0.5 to 40 ℃. Indeed, both the product distribution and the
ee of 5 and 6 obtained with different hosts showed a temperature-de-
pendence. As can be seen from Table 2, upon photoirradiation in water,
the 5/1 ratio and the ee of 5 are less sensitive to temperature change
than those of 6, agreeing well with our previous investigation that the
precursor complexes of anti-HT spontaneously cyclodimerize upon ex-
citation, while the syn-HT complexes are less-reactive [38].
tively large negative ΔΔS (-20.9–31.8 J mol-1
K
−1) while skeleton-
modified 7, 8 and phenyltriazolyl-capped 9 afforded negative ΔΔH of
-5.0–8.5 kJ mol-1 and positive ΔΔS of 9.6-28.3 J mol-1 −1. This in-
K
dicated that the introduction of the electrostatic interaction did not
significantly switched the entropy preference of native β-CD for the
enantiodifferentiation of 6. On the other hand, host 7-9 have a more
flexible skeleton or a bulky aromatic substituent, which will greatly
alter stacking model of AC pairs in 2:2 complex as well as their entropy
properties. The flexible β-CD derivatives should bind ACs in a substrate-
induced-fit manner and provide us a promising tool to control the
complexation and photochemical behavior through manipulating
Table 1
Association Constants for the Complexation of AC with Native and Modified β-CDs and Calculated Populations of Free and Complexed AC Species in Solutions under
the Conditions of Photoirradiation.
host
association constanta
K2/K1
population of ACb / %
K1 (M−1
)
K2 (M−1
)
K1K2 (105 M−2
)
free
1:1 complex
2:2 complex
β-CDc
3800
33100
3080
150
1480
197
5.7
489.9
6.1
0.04
0.04
0.06
0.05
5
1
6
0
90
70
88
60
5
29
6
TMA2-β-CDc
7
9
55300
2770
1531.8
40
a
Association constants for the 1:1 and 1:2 complexation obtained by isothermal titration calorimetry at 20 ℃.
Populations calculated on the basis of the association constants, assuming that [AC] = 0.2 mM and [host] = 5.2 mM.
From reference [38].
b
c
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X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
Table 2
Photocyclodimerization of AC Mediated by Native and Modified β-CDs in PBS at Temperatures (T) in the Presence and Absence of CsCla.
host
CCsCl / M
T/°C
relative yield/%
ee/%b
5*/1
6*/2*
(1 + 5*)/
(2* + 6*)
1
2*
3*
4
5*
6*
2*
3*
5*
6*
β-CDc
0
0.5
25
50
−20
0.5
20
40
0.5
20
65
65
60
20
14
13
16
26
19
14
4
1
1
4
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
1
3
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
1
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
26
25
23
49
55
59
59
56
50
45
64
62
61
54
64
46
30
59
59
53
51
67
64
61
63
64
65
66
39
33
30
46
52
47
45
37
33
24
56
53
49
46
8
8
9
−18
e
43
47
41
−7
10
23
32
40
43
44
12
21
29
40
40
28
26
15
23
28
33
56
59
65
37
42
47
49
38
41
43
−18
−2
21
30
37
41
42
−10
6
16
24
31
−36
−16
−4
10
6
1
−2
42
37
31
21
37
17
13
70
0.4
0.39
0.38
2.5
3.9
4.5
3.8
0.8
0.6
0.5
17
8.9
6.8
3.1
7.6
3.5
1.7
74
8.1
10.0
9.2
6.3
2.2
2.3
2.6
2.9
6.4
7.3
9.2
2.1
2.2
2.3
2.6
2.6
1.5
0.9
1.5
1.6
1.6
3.3
3.2
2.5
2.0
1.7
1.8
1.9
2.0
6.4
7.3
9.2
0.9
1.6
1.9
1.9
6.1
7.4
7.7
1.8
2.4
3.1
3.3
−10
−8
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
−1
−1
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
6.0
2.1
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
6.0
31
31
28
25
18
31
41
32
31
30
28
27
41
52
40
38
36
35
24
29
33
37
36
35
33
14
12
10
52
39
34
35
14
12
12
36
30
24
23
7
0
40
6.0
−20
0.5
20
40
0.5
20
7
9
18
9
13
18
1
8
0
40
6.0
−20
0.5
20
40
0.5
20
3
67
53
31
20
11
14
9
7
6
0
0
0
1
47
55
60
2
5.0
3.6
7.1
8.7
9.6
314
212
160
95
0.8
0.6
0.5
21
2.4
2.5
2.3
0.8
0.6
0.4
6.5
3.0
1.8
1.5
9
0
−46
−47
−49
−54
−53
−51
−51
6
12
29
−39
−24
−4
7
9
20
26
−32
−17
−1
14
40
6.0
−20
0.5
20
40
0.5
20
10
0
40
6.0
−20
0.5
20
40
0.5
20
9
19
20
49
55
64
8
17
27
31
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
f
f
f
f
f
f
f
f
11
0
40
6.0c
−20
0.5
20
20
30
40
aPBS solutions (pH 9.0) of AC and β-CD in the absence or presence of 6 M CsCl were irradiated at 365 nm with a LED light.
bEnantiomeric excess determined by chiral HPLC, where the first-eluted enantiomer is given a positive sign for all of the chiral products, i.e., 2, 3, 5, and 6.
cFrom reference [38].
dYield < 0.5%.
eNot determined because of the low yield.
fLarger than 1000 because of the low yield of 2.
entropy-related factors.
product chirality. For example, host 10 afforded 5 and 6 in -18% and
-30% ee at -20 ℃, which were inverted to 39% and 7% ee at 40 ℃. For
host 11, 5 and 6 were given in -10% and -32% ee, respectively, at -20 ℃
while in 30% and 14% ee, respectively, at 40 ℃. The inversion of
product chirality with host 10 and 11 could be ascribed to the larger
differential entropies (Table 3) in the presence of 6 M CsCl. This result
demonstrated that the entropy effect could be significantly regulated by
the salt additives, and therefore provided an effective way to the con-
trol of stereochemical outcome of photochirogenesis. Notably, host 8
gave slipped cyclodimer 6 in 70% ee in aqueous 6 M CsCl solution at
-20 ℃, which is the highest value ever reported for 5,8:9′10′ anthracene
cyclodimers. The external heavy-atom effect of Cs+ which can quench
the excited uncomplexed AC in solution should be partially responsible
for this improvement. [50]
In order to study the effect of the salt additive on the photoreaction,
photoirradiation of AC with modified β-CDs was carried out in the
presence and absence of 6 M CsCl. As shown in Table 2, the combined
yields of slipped cyclodimers 5 and 6 were significantly increased upon
addition of 6 M CsCl, which can be rationalized in terms of the elec-
trostatic interaction between metal cations and the carboxylate anions
of ACs, which would move the included ACs toward the portal of the β-
CD capsule to result in more slipped structures. The electrostatic in-
teraction between re-si and si-re complexes and between re-re and si-si
complexes may not be in the same level, which leads to the ee’s en-
hancement of 6 upon addition of 6 M CsCl. The temperature effects
become more remarkable upon addition of CsCl. Lowering the tem-
perature can enhance the electrostatic interaction, and therefore the
combined yields of slipped cyclodimers 5 and 6 were increased with
cationic β-CDs 10, 11 in water and with all host examined in 6 M CsCl
solution. Indeed, in the presence of 6 M CsCl, the photoreaction showed
more apparent temperature dependence, even led to an inversion of
4. Conclusion
In this study, skeleton- or periphery-modified β-CDs 7-11 were
379

X. Wei et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)374–381
Fig. 5. Plots of the relative rate constants for
the formation of enantiomers of 6 against the
reciprocal temperature upon the photo-
cyclodimerization of AC mediated by native β-
CD (solid square), 7 (empty square), 8 (solid
circle), 9 (empty circle), 10 (solid triangle) and
11 (empty triangle) in (a) aqueous solution and
(b) 6 M CsCl solution.
Appendix A. Supplementary data
Table 3
The differential activation parameters for the formation of enantiomeric 6 in
the photocyclodimerization of AC mediated by native β-CD and 7-11 in aqu-
eous solution and 6 M CsCl solution.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.11.
038.
host
β-CD
7
CCsCl / M
ΔΔH (kJ mol−1
)
ΔΔS (kJ mol−1 K−1
)
0
6.0
0
−5.0
−10.3
2.9
−20.9
−34.6
9.6
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