Planar-to-planar chirality transfer in the excited state. Enantiodifferentiating photoisomerization of cyclooctenes sensitized by planar-chiral paracyclophane

Article abstract of DOI:10.1021/ja203781f

Photochemical planar-to-planar chirality transfer was effected by using (R)-[10]paracyclophane-12-carboxylates as a planar-chiral sensitizer and (Z)-cyclooctene and (Z,Z)-1,5-cyclooctadiene as prochiral substrates to give a planar-chiral (E)- and (E,Z)-isomer in up to 44% and 87% enantiomeric excess, respectively, the latter of which being the highest ever reported for a sensitized photochirogenic reaction.

Full text of DOI:10.1021/ja203781f

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Planar-to-Planar Chirality Transfer in the Excited State.
Enantiodifferentiating Photoisomerization of Cyclooctenes Sensitized
by Planar-Chiral Paracyclophane
Ryo Maeda,† Takehiko Wada,†,‡ Tadashi Mori,† Shigeyuki Kono,§ Nobuhiro Kanomata,*,§,|| and
Yoshihisa Inoue*,†
†Department of Applied Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan
‡Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan
§Department of Applied Chemistry, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki 214-8571, Japan
Department of Chemistry and Biochemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
S Supporting Information
b
Scheme 1. Planar-to-Planar Chirality Transfer upon Z-to-E
ABSTRACT: Photochemical planar-to-planar chirality transfer
was effected by using (R)-[10]paracyclophane-12-carboxylates
as a planar-chiral sensitizer and (Z)-cyclooctene and (Z,Z)-1,5-
cyclooctadiene as prochiral substrates to give a planar-chiral (E)-
and (E,Z)-isomer in up to 44% and 87% enantiomeric excess,
respectively, the latter of which being the highest ever reported
for a sensitized photochirogenic reaction.
Photoisomerization of 1Z and 2ZZ Sensitized by Planar-
Chiral Paracyclophanes 3a,b and Point-Chiral Reference
Sensitizer 4a
hotochirogenesis, or chiral photochemistry, has been a target
P
of intensive studies in recent years, and a variety of molecular
and supramolecular approaches have hitherto been proposed and
examined.1 Of these strategies, asymmetric photosensitization,
requiring a minimal amount of a chiral source, is the most
straightforward, chirogen-efficient method to achieve photochir-
ogenesis. Thus, a great deal of effort has been devoted to the
study of the enantiodifferentiating photoisomerization of (Z)-
cycloalkenes2À5 sensitized by point-chiral (ar)alkyl benzene-
(poly)carboxylates to afford the planar-chiral (E)-isomers in good
enantiomeric excesses (ee's) of up to 77%, despite the short-lived
sensitizer-substrate interactions in the excited state. It was further
revealed that the product’s ee is a critical function of various entropy-
related environmental variants, such as temperature,2 pressure,3 and
solvation.4
(1E, 2EZ) (Scheme 1). We further elucidated the mechanisms of
energy and chirality transfer in the excited state and optimized
the product’s ee by manipulating environmental variants.
A solution containing 1Z or 2ZZ (5 mM) and (R)-3 (1 mM)
was irradiated at temperatures ranging from +25 to À140 °C.
The irradiated solutions were analyzed by GC first on a CBP20
column to give the E/Z ratio, and then the (E)-isomer, 1E or
2EZ, selectively extracted by aqueous silver nitrate, was analyzed
on a Supelco β-DEX 225 column to give the ee value.2À5
The photoisomerization proceeded smoothly to give the
photostationary E/Z ratios of ca. 0.01 upon prolonged irradia-
tions (Table 1), which is much smaller than those (0.1À0.4)
reported for conventional benzene(poly)carboxylate sensitizers.2À4
Notably, (À)-menthyl 2,5-dimethylbenzoate (4a), employed as an
electronically equivalent noncyclophane sensitizer, also gave a low
E/Z ratio of 0.03. It is likely therefore that the steric bulk of the 2,5-
substituents hinders the sensitization of 1Z rather than 1E (which is
more constrained and easier to be excited)2b to give the low E/Z
ratios for both 3 and 4, the former of which may have additional
hindrance caused by the conformationally locked benzylic methy-
lenes. Planar-chiral 3a and 3b, rather than point-chiral 4a, showed
better photochirogenic performance to give (R)-1E in 10% and 13%
ee, respectively, upon irradiation at room temperature, while 4a
Small- and large-sized cyclophanes attracted much attention
primarily as deformed π-systems6 and supramolecular hosts,
respectively.7 Planar-chiral cyclophanes were employed as chiral
ligands, reagents, and catalysts8À13 in the catalytic alkynylation of
aldehydes with [2.2]paracyclophane-based ligands,9 the cyclo-
propanation with bridged pyridinium ylides,10 and the biomi-
metic reduction with [10]parapyridinophanes used as bridged
NADHs.11 Optically active cyclophanes have also been applied
successfully to metal-mediated stereoselective synthesis12 and
dynamic asymmetric transformation.13
In this first study to explore the planar-to-planar chirality
transfer in the excited state, we employed methyl and isopropyl
(R)-[10](5,6)paracyclophane-12-carboxylates (3a, b)13a as pla-
nar-chiral sensitizers to effect the enantiodifferentiating photo-
isomerization of (Z)-cyclooctene (1Z) and (Z,Z)-1,5-cyclooc-
tadiene (2ZZ) to the corresponding planar-chiral (E)-isomer
Received: April 25, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja203781f J. Am. Chem. Soc. 2011, 133, 10379–10381
|

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Table 1. Enantiodifferentiating Photoisomerization of 1Z
and 2ZZ Sensitized by Paracyclophane 3 or Reference Sen-
sitizer 4a
substrate
sensitizer
solventb
temp/°C
E/Z
% ee
1Z
3a
P
25
À40
À110
25
0.007
À9.7
À24.5
À41.7
À13.2
À31.6
À42.9
À3.1
0.002
c
3b
4a
3a
M
P
0.010
0.010
0.005
0.028
0.030
À40
À110
25
À40
À110
25
À6.9
0.007
c
À13.0
À43.2
À56.1
À75.3
À87.4
À36.9
À56.9
À76.5
À84.8
Figure 1. Eyring plots for the ee of 1E (closed symbols) and 2EZ (open
symbols) produced in the enantiodifferentiating photoisomerization of
1Z and 2ZZ sensitized by 3a (circles), 3b (squares), and 4a (triangle).
2ZZ
M
c
À40
À110
À140
25
0.007
0.019
0.002
0.008
0.012
0.010
Table 2. Differential Enthalpy (ΔΔH‡) and Entropy (ΔΔS‡)
of Activation for Enantiodifferentiating Photoisomerization
of 1Z and 2ZZ Sensitized by 3 or 4
IM
M
3b
À40
À110
À140
substrate
sensitizer
solventa
ΔΔH‡
ΔΔS‡
IM
a [1Z] or [2ZZ] = 5 mM; [sensitizer] = 1 mM; irradiated for 30 min
(only for 4a) or 60 min under Ar with a 500-W high-pressure Hg lamp
fitted with a UV-27 filter. b Solvent: P, pentane; M, methylcyclohexane
(MCH); IM, a 3:1 (v/v) mixture of isopentane and methylcyclohexane.
No precipitation observed even at À140 °C. c Value not determined.
1Z
3a
3b
4a
3a
3b
P
0.44
0.44
0.15
0.91
0.78
0.98
0.95
0.35
1.4
M
P
2ZZ
M or IM
M or IM
0.89
a Solvent: P, pentane; M, methylcyclohexane; IM, a 3:1 (v/v) mixture
of isopentane and methylcyclohexane (used only for irradiation at À
140 °C).
afforded 3% ee under the same conditions. The ee of 1E gradually
increased by decreasing the irradiation temperature to give 42% ee for
3a and 43% ee for 3b at À110 °C, whereas 4a afforded 1E in 13% ee.
As shown in Figure 1, all of the ee’s obtained fell on a single straight
line upon Eyring analysis,2À5 indicating operation of a sole enantio-
differentiation mechanism over the entire temperature range em-
ployed. It is noteworthy that the temperature dependence behaviors
for methyl and isopropyl esters are very similar (Figure 1), indicating
that the peripheral modification is not an effective tool for manipulat-
ing the photochirogenic process.
Possessing an additional double bond at the transannular
position, 1,5-cyclooctadiene 2ZZ was a better substrate, affording
(R)-2EZ in 43% and 37% ee even at 25 °C upon sensitization
with 3a and 3b, respectively. Remarkably, the ee was steadily
enhanced up to 87% (for 3a) and 85% (for 3b) by reducing the
temperature down to À140 °C to give nice linear Eyring plots
(Figure 1); these ee’s are the highest ever reported for an
enantiodifferentiating photosensitized reaction.
(0À0.8 M) to give a weak emission at 398 nm with an isoemissive
point (Figure 2a, inset), suggesting the intervention of an
exciplex in the photosensitization of 1Z with paracyclophane.
Similar behavior was observed upon fluorescence quenching of
3b and 4b by 1Z. Quantitative analyses of the quenching data
gave the SternÀVolmer constants (kqτ) listed in Table 3. The
fluorescence lifetimes (τ) determined independently by using
the single-photon-counting technique allow us to calculate the
quenching rate constants (kq) for 1Z as (1.5À2.1) Â 108 MÀ1 sÀ1
(Table 3), which are 1 order of magnitude smaller than those
reported for less-hindered sensitizers, i.e. (1À3) Â 109 MÀ1 sÀ1 2b
.
The appreciably larger kq of 3.3 Â 108 MÀ1 sÀ1 for 4b may reflect the
dual accessible noncyclophane structure. These small kq values for
both cyclophanebenzoates 3 and disubstituted benzoate 4 indicate
that the ortho/meta substitution causes steric hindrance to decelerate
the quenching by 1Z.
The differential enthalpy (ΔΔH‡) and entropy (ΔΔS‡) of
activation were calculated from the slope and intercept of the
plot, according to the established procedures.2À4 The activation
parameters shown in Table 2 reveal that the consistently higher
ee’s of 1E obtained with 3, rather than 4, are attributed to the
almost tripled ΔΔH‡ and ΔΔS‡ for the cyclophane sensitizers,
while the further enhanced ee’s for 2EZ are mostly enthalpic in
origin, probably as a result of the stronger interaction of the
excited sensitizer with the more donating substrate 2ZZ, rather
than 1Z.
Cyclooctadiene 2ZZ also slowly quenched the fluorescence of
3a and 3b at (0.7À0.9) Â 108 MÀ1 sÀ1 (Table 3), which are
slightly smaller than those for 1Z. Interestingly, the exciplex
emission appeared at appreciably longer wavelengths of 417À418 nm
(Figure 2b, inset). Probably due to the higher electron-donating
ability, 2ZZ forms a better stabilized (by 3.3 kcal molÀ1) and more
intimately interacting exciplex than 1Z to afford 2EZ in higher ee’s of
up to 87%.
In this study, we have expanded the scope of photochirogen-
esis to the planar-to-planar chirality transfer in the excited state by
employing planar-chiral paracyclophane sensitizers and prochiral
cyclooctene substrates to produce planar-chiral (E)-isomers in
the highest ee’s ever reported for a sensitized photochirogenic
To better understand the excitation process, we examined the
fluorescence quenching behavior of 3a, 3b, and 4b (R = Me)
upon addition of 1Z. As shown in Figure 2a, the fluorescence of
3a at 334 nm was slowly reduced in intensity by adding 1Z
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dx.doi.org/10.1021/ja203781f |J. Am. Chem. Soc. 2011, 133, 10379–10381

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’ AUTHOR INFORMATION
Corresponding Author
kanomata@waseda.jp; inoue@chem.eng.osaka-u.ac.jp
’ ACKNOWLEDGMENT
This work was supported by Grant-in-Aid (A) 21245011 (Y.I.),
(B) 21350119 (T.W.), and (B) 23350018 (T.M.) from the Japan
Society for the Promotion of Science, by a Mitsubishi Chemical
Corporation Fund (T.M.), and also by a Waseda University Grant
for Special Research Projects 2009A-605 (N.K.).
Figure 2. Fluorescence spectra of (a) 3a (0.038 mM) in the presence of
0À0.8 M 1Z (λex 270 nm) and (b) 3a (0.027 mM) (λex 290 nm) in the
presence of 0À0.8 M 2ZZ in MCH at room temperature. Inset: exciplex
emission obtained by spectral subtraction.
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Table 3. Fluorescence Quenching of 3a, 3b, and 4b by 1Z or
2ZZ
λmax
/
kq/108
λexciplex/
sensitizer
nm
τ/ns quencher kqτ/MÀ1
M
À1 sÀ1
nm
3a
334
1.3a
1.1d
0.6g
1Zb
2ZZc
1Ze
2ZZf
1Zh
0.27
0.12
0.17
0.08
0.20
2.1
0.9
1.5
0.7
3.3
398
417
401
418
À
3b
4b
333
313
a [3a] = 0.27 mM in MCH at 25 °C. b [3a] = 0.038 mM in MCH at 25
°C. c [3a] = 0.027 mM in MCH at room temperature. d [3b] = 0.25 mM
in MCH at 25 °C. e [3b] = 0.025 mM in MCH at 25 °C. [3b] =
f
0.025 mM in MCH at room temperature. g [4b] = 0.61 mM in MCH at
room temperature. h [4b] = 0.061 mM in MCH at room temperature.
reaction. The major difference from the conventional point-
chiral photosensitization systems is the selective shielding of
one of the enantiotopic faces of chiral cyclophanes 3 by the
decamethylene bridge, which both enthalpically and entropically
renders the approach and the subsequent geometrical isomeriza-
tion of substrate 1Z or 2ZZ more enantioface-selective to give
high ee’s. The inherently low E/Z ratios obtained upon para-
cyclophane sensitization may be readily overcome by continu-
ously circulating the photolyzed solution through a column
loaded with AgNO3/SiO2 to selectively extract the (E)-isomer
from the E/Z mixture during the period of irradiation.14 Chiral
cyclophane sensitizers are more suitable for the photoisomeriza-
tion of smaller-sized cycloalkenes that afford highly strained (E)-
isomers stabilized only at low temperatures but are expected to
experience less steric hindrance upon exciplex formation giving
higher E/Z ratios. It is also interesting to note that strained (E)-
cyclooctenes 1E and 2EZ are potent inhibitors of the ripening
action of ethylene as a plant hormone and (R)-1E is more
effective than the antipode,15 rendering the present method
attractive in relevant research. The concept of planar-to-planar
chirality transfer in the excited state has established a new scope
of photochirogenesis and can be a useful and powerful tool in a
more general context of (supra)molecular photochirogenesis.
Sugawara, N.; Wada, T.; Zou, Y.; Binder, B. M. Chem. Biol. 2008, 15, 313.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and a
b
comprehensive list of the original data used in Figure 1. This
material is available free of charge via the Internet at http://pubs.
acs.org.
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dx.doi.org/10.1021/ja203781f |J. Am. Chem. Soc. 2011, 133, 10379–10381