Optically active (E,Z)-1,3-cyclooctadiene: First enantioselective synthesis through asymmetric photosensitization and chiroptical property

Article abstract of DOI:10.1021/ja963160c

Enantiodifferentiating photoisomerizations of (Z,Z)-1,3-cyclooctadiene (1ZZ) to the chiral E,Z-isomer 1EZ were performed at varying temperatures in the presence of some benzenepoly-, naphthalene(di)-, and anthracenecarboxylates. Of these chiral sensitizers, (-)-menthyl benzenehexacarboxylate afforded the highest enantiomeric excesses (ee's) up to 10% and 18% in pentane at 25 and -40°C, respectively. In contrast, the use of polar solvents greatly diminished the product's ee, suggesting intervention of a radical ionic rather than exciplex intermediate in these solvents. Optically pure 1EZ is shown to possess anomalously high specific rotation and circular dichroism as a simple diene chromophore: [α]²⁵(D) 1380°(CH₂Cl₂), Δε 12.8 M^-1 cm^-1 (λ(ext) 228 nm, cyclohexane). The chiroptical properties observed not only correct the previous data but also present the conclusive evidence for the recent theoretical predictions.

Full text of DOI:10.1021/ja963160c

472
J. Am. Chem. Soc. 1997, 119, 472-478
Optically Active (E,Z)-1,3-Cyclooctadiene: First
Enantioselective Synthesis through Asymmetric
Photosensitization and Chiroptical Property
Yoshihisa Inoue,* Hiroshi Tsuneishi, Tadao Hakushi, and Akira Tai
Contribution from the ERATO Photochirogenesis Project, Department of Molecular Chemistry,
Faculty of Engineering, Osaka UniVersity, Yamada-oka, Suita 565, Japan, Department of Applied
Chemistry, Himeji Institute of Technology, Shosha, Himeji 671-22, Japan, and Department of
Material Science, Himeji Institute of Technology, Kamigori, Hyogo 678-12, Japan
X
ReceiVed September 9, 1996
Abstract: Enantiodifferentiating photoisomerizations of (Z,Z)-1,3-cyclooctadiene (1ZZ) to the chiral E,Z-isomer 1EZ
were performed at varying temperatures in the presence of some benzenepoly-, naphthalene(di)-, and anthracene-
carboxylates. Of these chiral sensitizers, (-)-menthyl benzenehexacarboxylate afforded the highest enantiomeric
excesses (ee’s) up to 10% and 18% in pentane at 25 and -40 °C, respectively. In contrast, the use of polar solvents
greatly diminished the product’s ee, suggesting intervention of a radical ionic rather than exciplex intermediate in
these solvents. Optically pure 1EZ is shown to possess anomalously high specific rotation and circular dichroism
as a simple diene chromophore: [R]25D 1380° (CH2Cl2), ∆ꢀ 12.8 M cm (λext 228 nm, cyclohexane). The chiroptical
properties observed not only correct the previous data but also present the conclusive evidence for the recent theoretical
predictions.
-1
-1
Introduction
ones have been revealed to racemize spontaneously at room
11
temperature. In this context, (E,Z)-cyclooctadienes are intrigu-
ing as borderline cases. The E,Z-isomer of nonconjugated 1,5-
cyclooctadiene is thermally as stable as the parent (E)-
E-Isomers of medium-sized cycloalkenes have long aroused
much theoretical, spectroscopic, synthetic, and physicochemical
interests because of their constrained, chiral molecular structures
1
3
cyclooctene, whereas conjugated (E,Z)-1,3-cyclooctadiene
1
-19
as well as enhanced thermal and photochemical reactivities,
(
1EZ) is relatively stable only up to 60 °C but cyclizes to
arising from the distorted π orbital and the hindered rope-
jumping motion of the methylene chain bridging the double
bond. However, only eight-membered cyclic E-isomers have
been demonstrated unequivocally to be chiral with moderate
14b
bicyclo[4.2.0]oct-7-ene at 90 °C with a lifetime of 50 min.
In spite of the thermal and photochemical syntheses of 1EZ
1
7
18
reported by Cope et al. and Liu, its chiroptical properties
8
19
have not been investigated experimentally or theoretically
thermal stability at ambient temperatures, and their chiroptical
properties have been well documented.8
-10,12,16
until recently, probably due to the thermal instability and the
lack of convenient enantioselective synthetic methods. Isaksson
On the other
hand, smaller-sized (E)-cycloalkenes, which should be chiral,
8
are merely known as unstable transient species,15 while the larger
et al. reported that racemic 1EZ can be resolved optically by
chiral HPLC over triacetylcellulose to give optically active 1EZ
of >90% enantiomeric excess (ee). This sample was reported
X
Abstract published in AdVance ACS Abstracts, January 1, 1997.
(
(
(
(
1) Allinger, N. L.; Sprague, J. T. J. Am. Chem. Soc. 1972, 94, 5734.
2) Ermer, O.; Lifson, S. J. Am. Chem. Soc. 1973, 95, 4121.
3) Johnson, R. P.; DiRico, K. J. J. Org. Chem. 1995, 60, 1074.
4) Yaris, M.; Moscowitz, A.; Berry, R. S. J. Chem. Phys. 1968, 49,
2
0
to show large specific rotation and circular dichroism: [R]
D
-
1
-1
-649° (c 0.017, ethanol), ∆ꢀ -8.83 M cm (λext 230.5 nm,
8
ethanol). Since the two CD peaks at 230 and <190 nm gave
3
150.
the same (negative) sign inconsistent with the prediction of the
(
(
5) Bach, R. D. J. Chem. Phys. 1970, 52, 6423.
6) Robin, M. B.; Taylor, G. N.; Kuebler, N. A.; Bach. R. D. J. Org.
2
0
diene helical rule, they attempted to determine the absolute
configuration of 1EZ by CNDO/S-CI calculation and tentatively
assigned the (-)-form as possessing the P helicity or R
Chem. 1973, 38, 1049.
(
7) Mason, M. G.; Schnepp, O. J. Chem. Phys. 1973, 59, 1092.
(8) Isaksson, R.; Roschester, J.; Sandstr o¨ m, J.; Wistrand, L. J. Am. Chem.
8
configuration. This assignment was recently confirmed by
Soc 1985, 107, 4074.
9) Cope, A. C.; Ganellin, C. R.; Johnson, H. W., Jr.; Van Auken, T.
V.; Winkler, J. S. J. Am. Chem. Soc. 1963, 85, 3276.
(
Bouman and Hansen through more accurate ab initio RPA
19
calculation.
(10) Cope, A. C.; Mehta, A. S. J. Am. Chem. Soc. 1964, 86, 5626.
Besides the chromatographic optical resolution mentioned
above, no chemical or photochemical synthetic approaches to
the enantiomeric 1EZ have been reported so far. In this aspect,
the enantiodifferentiating photoisomerization of (Z,Z)-1,3-cy-
clooctadiene (1ZZ) sensitized by a chiral sensitizer provides
us with a convenient synthetic access to optically active 1EZ,
since the photosensitization of (Z)-cyclooctene with chiral
aromatic esters has been shown to afford its E-isomer in good
(
11) Cope, A. C.; Banholzer, K.; Keller, H.; Pawson, B. A.; Whang, J.
J.; Winkler, H. J. S. J. Am. Chem. Soc. 1965, 87, 3644.
(
(
12) Cope, A. C.; Pawson, B. A. J. Am. Chem. Soc. 1965, 87, 3649.
13) Cope, A. C.; Hecht, J. K.; Johnson, H. W., Jr.; Keller, H.; Winkler,
H. J. S. J. Am. Chem. Soc. 1966, 88, 761.
14) (a) Shumate, K. M.; Neuman, P. N.; Fonken, G. J. J. Am. Chem.
(
Soc. 1965, 87, 3996. (b) Bromfield, J. J.; MaConaghy, J. S., Jr. Tetrahedron
Lett. 1969, 3723.
(15) Inoue, Y.; Ueoka, T.; Kuroda, T.; Hakushi, T. J. Chem. Soc., Chem.
Commun. 1981, 1031. Inoue, Y.; Hagiwara, S.; Daino, Y.; Hakushi, T. J.
Chem. Soc., Chem. Commun. 1985, 1307.
(
2
1,22
optical yields up to 64%.
16) Tsuneishi, H.; Hakushi, T.; Tai, A.; Inoue, Y. J. Chem. Soc., Perkin
Trans. 2 1995, 2057.
(20) Moscowitz, A.; Charney, E.; Weiss, U.; Zeffer, H. J. Am. Chem.
Soc. 1961, 83, 4661. Lighter, D. A.; Bouman, T. D.; Gawronski, J. K.;
Gawronska, K.; Chappuis, J. L.; Crist, B. V.; Hansen, A. E. J. Am. Chem.
Soc. 1981, 103.
(
(
(
17) Cope, A. C.; Bumgardner, C. L. J. Am. Chem. Soc. 1956, 78, 2812.
18) Liu, R. S. H. J. Am. Chem. Soc. 1967, 89, 112.
19) Bouman, T. D.; Hansen, A. E. Croatia Chem. Acta 1989, 62, 227.
S0002-7863(96)03160-5 CCC: $14.00 © 1997 American Chemical Society

Optically ActiVe (E,Z)-1,3-Cyclooctadiene
J. Am. Chem. Soc., Vol. 119, No. 3, 1997 473
(33 400), 241.8 (21 300), 315.2 nm (6480 M- cm-1); H NMR (CDCl
1
1
)
Scheme 1
3
δ 0.85 (d, J ) 6.8 Hz, 6H), 0.93 (d, J ) 6.8 Hz, 6H), 0.97 (d, J ) 6.6
Hz, 6H), 0.92-1.00 (m, 2H), 1.12-1.24 (m, 4H), 1.54-1.63 (m, 4H),
.87 (m, 4H), 2.01 (m, 2H), 2.24 (m, 2H), 5.07 (d-t, J ) 4.4, 11.0
Hz, 2H), 7.64 (d-d, J ) 3.4, 6.6 Hz, 2H), 8.04 (s, 2H), 8.80 (d-d, J
1
)
3.4, 6.6 Hz, 2H). (-)-Dimenthyl 1,8-naphthalenedicarboxylate
9a): mp 161.5-162.5 °C; [R]25
-20.9° (c 0.97, CHCl ); UV (pentane)
max (ꢀ) 219.8 (31 500), 293.2 nm (6900 M cm ); IR (KBr) ν 2950,
(
λ
D
3
-
1
-1
-
1
2
850, 1070, 1510, 1460, 1380, 1280, 1200, 1140, 1080, 990, 770 cm ;
H NMR (CDCl ) δ 0.86 (d, J ) 6.8 Hz, 6H), 0.97 (m, 2H), 0.99 (d,
In this study, we performed the enantiodifferentiating geo-
metrical photoisomerization of 1ZZ sensitized by a variety of
optically active benzenepoly-, naphthalene(di)-, and anthracene-
carboxylates in order to reveal the (chir)optical properties of
the optically active 1EZ obtained and also to elucidate the
electronic and steric effects of the conjugating double bond upon
the enantiodifferentiation process in the excited state.
1
3
J ) 6.3 Hz, 6H), 0.99 (d, J ) 7.1 Hz, 6H), 1.14 (m, 2H), 1.24 (m,
2H), 1.54 (m, 2H), 1.65 (m, 2H), 1,76 (m, 4H), 2.25 (m, 2H), 2.34 (m,
2H), 4.90 (d-t, J ) 11.0, 4.4 Hz, 2H), 7.51 (m, 2H), 7.94 (m, 4H).
Photolysis. All irradiations were carried out in a temperature-
controlled water (25 °C) or methanol (-40 to -74 °C) bath. The light
sources employed were a conventional 300-W high-pressure mercury
lamp for irradiations at 25 °C and an equivalent lamp fitted with a
transparent Pyrex vacuum sleeve designed for the low-temperature
irradiation (Eikosha). A solution (3 or 300 mL), containing 1ZZ (0.2
M), optically active sensitizer 2-10 (10 mM), and cyclooctane (10
mM) added as an internal standard, was irradiated at >300 nm under
an argon atmosphere in a Pyrex tube (1-cm i.d.) placed near the lamp
surface or in an annular Pyrex vessel surrounding the lamp, the whole
system being immersed in a cooling bath. In most cases, irradiations
were continued until the apparent photostationary states were reached,
frequently monitoring the isomer composition of irradiated solution
on GC.
Product Isolation. In preparative runs using an annular vessel (300
mL), 1EZ produced was selectively extracted from the photolysate with
three 30-mL portions of 20% aqueous silver nitrate at <5 °C. The
combined aqueous extract was washed with two 25-mL portions of
pentane and then added dropwise with vigorous stirring into a
concentrated aqueous ammonia solution at 0 °C. The resulting mixture
was extracted with three 25-mL portions of pentane. The combined
pentane extract was dried over sodium sulfate and concentrated at a
reduced pressure (50-100 Torr), and then the residue obtained was
subjected to bulb-to-bulb distillation in vacuo to give chemically pure
1EZ. In analytical scale runs with 3-mL solutions, practically the same
procedures, except for the scale, were employed to obtain the pentane
solution of pure 1EZ for chiral GC analysis.
Experimental Section
General. Melting points were measured with a YANACO MP-21
apparatus and are uncorrected. H NMR spectra were obtained on a
1
JEOL GX-400 spectrometer in chloroform-d. Infrared spectra were
obtained on a JASCO IR-810 instrument. Electronic absorption and
fluorescence spectra were recorded on JASCO Ubest-50 and FP-777
instruments, respectively; fluorescence spectra are not corrected for the
instrument response function. Optical rotations were determined at 589
nm in a thermostated conventional 10-cm cell, using a Perkin-Elmer
polarimeter model 243B. Circular dichroism spectra were recorded in
the vapor and solution phases on a JASCO J-720 spectrometer.
Fluorescence lifetimes were measured with 0.1-0.01 mM solutions
of 3 or 6 in aerated pentane and/or acetonitrile by means of the time-
correlated single-photon-counting method on a Horiba NAES-550
instrument equipped with a pulsed H
2
light source. The radiation from
the lamp was made monochromatic by a 10-cm monochromator, and
the emission from sample solution was detected through an appropriate
filter (UV-33, -35, or -39).
Gas chromatographic analyses of the geometrical isomers 1ZZ and
1
EZ in photolyzed solutions were performed on a 1-m packed column
of 40% â,â′-oxydipropionitrile at 65 °C with a Shimadzu 6A instrument.
Enantiomeric excesses of 1EZ isolated through the selective extraction
with aqueous silver nitrate were determined by gas chromatography
over a 30-m chiral capillary column (SUPELCO â-DEX 120) at 45
Results and Discussion
°
C, using a Shimadzu GC-14B instrument. It may be interesting to
note that, in order to attain good enantiomeric separation and high
reproducibility in the chiral GC analysis, the amount of injected sample
should be kept in a certain range. In our case using a Shimadzu
integrator C-R6A, the best reproducibility (within an error of (0.5%
ee) was obtained when the integrated area of the enantiomer peak was
kept in the range 30 000-100 000.
Materials. Solvent pentane was stirred over concentrated sulfuric
acid until the acid layer no longer turned yellow, washed with water,
neutralized with aqueous sodium bicarbonate, dried over magnesium
sulfate, and then distilled fractionally. Methanol and acetonitrile were
fractionally distilled from magnesium turnings and diphosphorus
pentaoxide, respectively.
Geometrical Photoisomerization. Our previous investiga-
tions on the photosensitized enantiodifferentiating isomerization
16,21
of cyclooctene
clearly indicate that the intervention of a
singlet exciplex, in which substrate interacts intimately with
chiral sensitizer for a certain period, is essential in order to
accomplish effective enantiodifferentiation in the excited state.
However, virtually no attempt has been reported on the singlet
photosensitization of 1,3-cyclooctadiene (1), in contrast to
its well-documented photochemical behavior upon direct
2
6-29
18
irradiation
and triplet photosensitization.
We first investigated the photosensitized geometrical isomer-
(
-)-Menthyl, (-)-bornyl, and/or (-)-1-methylheptyl arenecarboxy-
izations of 1 with some benzenepolycarboxylates (2 and 3),
lates employed as chiral sensitizers were synthesized in pyridine from
the corresponding alcohols and acid chlorides and purified by repeated
recrystallization from methanol or ethanol, according to the procedures
21,22,30
which are efficient singlet sensitizers for cyclooctene
and
31
1,5-cyclooctadiene. The structures of the sensitizers employed
are illustrated in Chart 1, and the E/Z ratios obtained are listed
in Tables 1 and 2. Possessing a lower singlet energy than
cyclooctene, the conjugated diene 1 was sensitized not only with
benzene(poly)carboxylates but also with naphthalene(di)- and
anthracenecarboxylates 4-10 to give moderate E/Z ratios, which
21-25
reported previously.
6a): mp 93.5-94.5 °C; [R]
(-)-Dimenthyl 1,4-naphthalenedicarboxylate
25
(
2
1
D
-88.1° (c 1.51, CHCl
3
); IR (KBr) ν
950, 2870, 1720, 1590, 1520, 1460, 1380, 1250, 1190, 1140, 1100,
030, 990, 960, 920, 840, 780, 660 cm- ; UV (pentane) λmax (ꢀ) 212.6
1
(21) Inoue, Y.; Yamasaki, N.; Yokoyama, T.; Tai, A. J. Am. Chem. Soc.
1
989, 111, 6480; J. Org. Chem. 1992, 57, 1332.
22) Inoue, Y.; Yamasaki, N.; Yokoyama, T.; Tai, A. J. Org. Chem.
993, 58, 1011.
23) Yamasaki, N.; Inoue, Y.; Yokoyama, T.; Tai, A.; Ishida, A.;
Takamuku, S. J. Am. Chem. Soc. 1991, 113, 1933.
24) Inoue, Y.; Okano, T.; Yamasaki, N.; Tai, A. J. Photochem.
Photobiol. A 1992, 66, 61.
25) Inoue, Y.; Yamasaki, N.; Shimoyama, H.; Tai, A. J. Org. Chem.
993, 58, 1785.
(26) Srinivasan, R. J. Am. Chem. Soc. 1962, 84, 4141.
(27) Dauben, W. G.; Cargill, R. L. J. Org. Chem. 1962, 27, 1910.
(28) Nebe, W. J.; Fonken, G. J. J. Am. Chem. Soc. 1969, 91, 1249.
(29) Inoue, Y.; Daino, Y.; Hagiwara, S.; Nakamura, H.; Hakushi, T. J.
Chem. Soc., Chem. Commun. 1985, 804.
(30) Inoue, Y.; Takamuku, S.; Kunitomi, Y.; Sakurai, H. J. Chem. Soc.,
Perkin Trans. 2 1980, 1672.
(
1
(
(
(
(31) Goto, S.; Takamuku, S.; Sakurai, H.; Inoue, Y.; Hakushi, T. J. Chem.
Soc., Perkin Trans. 2, 1980, 1678.
1

474 J. Am. Chem. Soc., Vol. 119, No. 3, 1997
Inoue et al.
Chart 1
Figure 1. Fluorescence spectra of hexamethyl mellitate (3, R ) Me;
0.1 mM) excited at 290 nm in pentane (upper traces) and in acetonitrile
(
lower traces) in the absence/presence of (Z,Z)-1,3-cyclooctadiene (1ZZ)
of varying concentrations: (a) 0, (b) 2.5, (c) 5, (d) 10, (e) 20, (f) 40,
g) 60, and (h) 100 mM.
(
Table 1. Enantiodifferentiating Photoisomerization of
(
(
Z,Z)-1,3-Cyclooctadiene (1ZZ) Sensitized by Optically Active
Poly)alkyl Arene(poly)carboxylates in Pentane at 25 °C
irradiation conversion, yield,
sensitizer
time, h
%
%
E/Z
% ee/op
a
-0.3c
2
2
2
3
3
a
b
c
a
b
12
29.9
17.7
18.7
29.6
e
12.8
20.5
22.7
12.7
16.8
9.2
11.0
21.1
20.0
9.8
19.8
29.8
19.8
16.3
19.1
10.4
8.2
8.3
4.4
4.5
7.8
8.9
9.1
9.8
4.6
6.8
10.0
9.5
5.9
10.6
8.7
10.6
6.8
0.272
0.132
0.101
0.119
0.044
0.052
0.098
0.115
0.104
0.118
0.050
0.077
0.127
0.119
0.065
0.132
0.124
0.132
0.081
b
d
86
0.1
b
d
95
0.1c
b
37
-10.1
b
d
117
-0.5c
a
9
-1.5
b
d
3
4
5
6
c
a
a
a
180
-0.9
a
c
1
1
-2.5
b
a
b
a
b
a
b
b
a
b
a
d
69
0.4c
1
0
0.3
d
29
0.8c
1
0
-0.3
d
87
0.9c
1
4
1.2
d
7
8
a
a
72
72
0.6d
0.2c
1
3
-0.3
d
9
1
a
0a
72
24
0.3
Figure 2. Fluorescence spectra of (-)-dimenthyl 1,4-naphthalenedi-
carboxylate (6a; 0.01 mM) excited at 320 nm in pentane in the absence/
presence of (Z,Z)-1,3-cyclooctadiene (1ZZ) of varying concentra-
tions: (a) 0, (b) 5, (c) 10, (d) 20, (e) 40, (f) 60, (g) 100, (h) 200, and
(i) 400 mM.
-1.1c
a
Reaction scale: 3 mL. b Reaction scale: 300 mL. Percent ee
c
d
determined by capillary GC. Percent op calculated from specific
rotation. Not determined.
e
the fluorescence-quenching experiments with some representa-
tive sensitizers. As exemplified in Figures 1 and 2, the
fluorescence of methyl and (-)-menthyl mellitates 3 and (-)-
menthyl 1,4-naphthalenedicarboxylate (6a) in aerated pentane
and/or acetonitrile was quenched efficiently by the substrate 1ZZ
up to 400 mM concentration.
As can be seen from Figure 1a, as the fluorescence intensity
decreases gradually by adding quencher 1ZZ into a pentane
solution of 3 (R ) Me), a new weak emission attributable to
are of similar magnitude. In all cases, the major course of
photolysis was the Z-E isomerization as was the case with
direct photolysis2
6-29
and triplet sensitization. The chemical
18
yields of 1EZ based on the consumed 1ZZ are moderate to
high (40-70%) in most cases even upon prolonged irradiations
or at low temperatures, as shown in Table 2.
Sensitization Mechanism. In order to elucidate the excited
state(s) involved in the photosensitization process, we performed

Optically ActiVe (E,Z)-1,3-Cyclooctadiene
J. Am. Chem. Soc., Vol. 119, No. 3, 1997 475
Table 2. Temperature and Solvent Effects upon Product’s ee in Enantiodifferentiating Photoisomerization of (Z,Z)-1,3-Cyclooctadiene (1ZZ)
Sensitized by Optically Active (Poly)alkyl Arene(poly)carboxylatesa
sensitizer
solvent
pentane
temperature, °C
irradiation time, h
conversion, %
yield, %
E/Z
% ee
2
a
25
12
12
37
29.9
31.5
29.6
21.1
12.4
3.8
12.8
20.2
22.7
24.5
16.8
19.7
20.0
24.5
29.8
17.0
19.1
15.2
8.3
12.8
6.4
1.6
4.5
11.1
8.9
16.0
9.8
10.5
9.5
10.4
8.7
0.272
0.223
0.119
0.162
0.073
0.017
0.052
0.139
0.115
0.212
0.118
0.131
0.119
0.138
0.124
0.136
-0.3
1.2
-10.1
-17.6
-1.3
0.2
-1.5
-5.8
-2.5
-1.2
0.3
-0.3
1.2
0.6
-0.3
-0.5
-
40
25
40
25
25
25
74
25
74
25
40
25
40
25
40
b
3
a
pentane
-
8
14
20
9
10
11
11
10
10
14
12
13
12
c
3
CH CN
MeOH
c
3
3
4
6
8
b
pentane
-
-
-
-
-
c
pentane
pentane
pentane
pentane
a
a
a
11.2
a
Reaction scale: 3 mL, unless noted otherwise. b Reaction scale: 300 mL. c Sensitizer concentration less than 1 mM, due to low solubility.
Table 3. Fluorescence Quenching of Aromatic Esters with
(
Z,Z)-1,3-Cyclooctadiene (1ZZ)a
k
M
q
τ,
q
k ,
-1
-1
s-1
sensitizer
solvent τ, nsb
pentane 1.1 26.9 2.4 × 10
CH
M
1
1
9
9
0
0
hexamethyl mellitate
(
3, R ) Me)
3
CN 0.6 20.1 3.4 × 10
c
(
(
-)-hexamenthyl mellitate (3a) pentane 0.7
3.68 5.3 × 10
4.62 1.2 × 10
-)-dimenthyl 1,4-
pentane 4.0
naphthalenedicarboxylate (6a)
a
Measured in aerated solvents at 18 °C. b Determined independently
by single-photon-counting technique. c Methyl mellitate gives a usual
single fluorescence peak, while the menthyl ester 3a is known to exhibit
unusual dual fluorescence behavior induced by steric hindrance (see
ref 23). The listed values were determined for the relaxed state emission
that appears at a longer wavelength (372 nm).
It is noted that the fluorescence quenching by 1ZZ occurs
9
-1 -1
Figure 3. Stern-Volmer plots for fluorescence quenching of hexa-
methyl mellitate by 1ZZ in pentane (O) and acetonitrile (b), (-)-
hexamenthyl mellitate in pentane (9), and (-)-dimenthyl 1,4-naph-
thalenedicarboxylate (4) in pentane.
with high efficiency at rates much faster than 10 M s . In
particular, the fluorescence of methyl mellitate was quenched
by 1ZZ near the diffusion-controlled rate both in pentane (2.4
10
-1 -1
10
-1 -1
×
10
M
s ) and in acetonitrile (3.4 × 10
M
s ), while
the bulky menthyl groups in 3a and the lower singlet energy of
naphthalenedicarboxylate 6a evidently decelerate the quenching
exciplex intermediate emerges at a longer wavelength, ac-
companying the isoemissive point at 480 nm; see the inset. The
net exciplex fluorescence, obtained by the spectrum subtraction,
showed maxima at ∼450 nm. Similar fluorescence behavior
was observed with 3a (R ) Men) upon quenching by 1ZZ in
pentane.
In contrast, the quenching profile of 3 (R ) Me) in acetonitrile
differs considerably. In the polar solvent, the fluorescence
intensity decreases similarly upon addition of 1ZZ, but no
emission appears at longer wavelengths (Figure 1b). The
efficient quenching and the lack of exciplex emission jointly
indicate that the exciplex, if formed, is not emissive at all or,
more probably, the spontaneous electron transfer from 1ZZ to
9
9
-1 -1
process to 5.3 × 10 and 1.2 × 10 M s , respectively. It is
also emphasized that the exciplex emission observed upon
quenching of mellitates 3 (R ) Me or Men) in pentane is
missing in acetonitrile, probably owing to the facile formation
of a nonemissive radical ion pair upon quenching. Although
no further spectroscopic search for the transient ionic species
was made, the sudden drops of the product’s ee in polar solvents
described below indicate experimentally the loss of intimate and
long-lived interactions between chiral sensitizer and substrate.
These results demonstrate unequivocally that the sensitization
of 1ZZ with arene(poly)carboxylates proceeds in the singlet
21,22
manifold as in the case with cyclooctene.
The lower singlet
3, generating a radical ion pair, is the dominant process in polar
energy of 1ZZ compared with cyclooctene will accelerate the
singlet energy transfer in pentane, and virtually the same
sensitization/enantiodifferentiation scenario operative in the
cyclooctene case may be applicable to the present case at least
in pentane. However, in sharp contrast to the negligible solvent
effects upon ee is the cyclooctene case, the effect of polar
solvents is much more drastic for the conjugated diene 1ZZ.
This is presumably because the lower ionization potential of
acetonitrile. The fluorescence of naphthalenedicarboxylate 6a
was also quenched efficiently by 1ZZ in pentane without
producing any new emission (Figure 2).
21
According to the conventional Stern-Volmer treatment of
the quenching data,21 the relative fluorescence intensity F0/F
2
1
was plotted as a function of the concentration of added 1ZZ to
give a good straight line for each sensitizer examined, as shown
in Figure 3. From the Stern-Volmer constant (kqτ) obtained
as a slope of the plot and the fluorescence lifetime (τ) determined
independently by the single-photon-counting technique, we can
calculate the quenching rate constant (kq) for each sensitizer
3
2
32
1
ZZ (8.68 eV), compared to that of cyclooctene (8.82 eV),
(
32) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T.
Energetics of Gaseous Ions; American Institute of Physics: New York,
(Table 3).
1977.

476 J. Am. Chem. Soc., Vol. 119, No. 3, 1997
Inoue et al.
facilitates the electron transfer producing solvent-separated or
free radical ion pair in polar solvents.
Table 4. Specific Rotations of Strained (E)-Cyclooctenes and
(
E,Z)-Cyclooctadienesa
Enantiodifferentiation. Optical purity (op) and/or enantio-
meric excess (ee) of 1EZ isolated from the photolysate were
determined respectively by measuring the product’s optical
rotation and integrating the enantiomer peaks separated on chiral
GC. The product’s op or ee values obtained in the enantio-
differentiating photosensitizations at 25 °C with several chiral
arenecarboxylates are summarized in Table 1. Of these
sensitizers examined, only benzenehexacarboxylates 3 afforded
moderately optically active 1EZ of 2-10% ee in pentane at 25
cycloalkene
[R]
D
(solvent)
ref
(E)-cyclooctene
(E)-1-methylcyclooctene
E,Z)-1,5-cyclooctadiene
E,Z)-1,3-cyclooctadiene (1EZ)
426 (CH Cl )
106 (CH
152 (CH
2
2
2
2
2
2
b
c
d
e
f
Cl
Cl
)
)
(
(
1380 (CH
2 2
Cl )
8
95 (EtOH)
a
Measured at 20-25 °C, unless stated otherwise. b Reference 10.
c
Reference 16. d Reference 13; measured at 0 °C. e This work. Ref-
f
erence 8; sample used was >90% ee.
°
C, whereas the other sensitizers, including benzenetetracar-
boxylate 2 and naphthalene(di)- and anthracenecarboxylates
-10, gave poor ee’s, less than 1% even in pentane. One should
4
be cautious however in evaluating the enantiodifferentiating
ability of a chiral sensitizer, since the product’s ee has amply
been shown to depend significantly upon the irradiation tem-
perature especially in the asymmetric photosensitization with
sterically congested benzenepolycarboxylates.21,22
Temperature and Solvent Effects. To explore the temper-
ature effect upon ee, the enantiodifferentiating photosensitiza-
tions with 2a, 3a-c, 4a, 6a, and 8a were performed in pentane
at two different temperatures. As shown in Table 2, the ee
values exhibit relatively small temperature dependences in most
cases. This indicates that the enthalpic, as well as entropic,
q
q
differences (∆∆H and ∆∆S ) for the enantiodifferentiating
process producing (R)- and (S)-1EZ are not as great as in the
2
1,22
cyclooctene case.
a was improved considerably from 10.1% at 25 °C to 17.6%
at -40 °C, and similar enhancement of ee was observed with
b.
In several cases, the product chirality is switched indeed
Nonetheless, the ee value obtained with
3
3
within the temperature range employed or expected to be
inverted by changing the temperature. These results clearly
demonstrate that, although the ee’s obtained are not particularly
high in the present cases, the entropy term still plays an essential,
but less important, role in the enantiodifferentiating process
within exciplex intermediates for the reasons discussed below.
However, in view of the high ee’s and the drastic temperature
21,22
effect observed in the cyclooctene case,
these low ee’s and
minimal entropic contributions are totally unanticipated. The
conjugated diene 1 was expected to form a tighter exciplex in
nonpolar solvents by virtue of the increased charge transfer (C-
T) interaction, which guarantees in principle more intimate
interactions within the exciplex, but the results are opposite.
The increased C-T character of the intervening exciplex is
evident from the remarkable solvent effects upon the exciplex
fluorescence and the product’s ee. The low ee’s and the missing
exciplex emission in polar solvents are reasonably interpreted
in terms of the facile formation of a solvent-separated or free
Figure 4. Absorption spectra of (Z,Z)- and (E,Z)-1,3-cyclooctadiene
(1ZZ, solid line; 1EZ, dashed line): (a) vapor (saturated at 23 °C)
and (b) cyclohexane solution ([1ZZ] ) 0.099 mM; [1EZ] ) 0.111
mM).
Hence, the high C-T character itself does not immediately mean
low ee’s especially in nonpolar solvents, for which other factor-
(
s) must be responsible.
Examining the crystallographic structures33 and CPK molec-
•+
•-
radical ion pair (1 ‚‚‚3a ), components of which are separated
from each other and therefore not expected to contribute to the
chiral recognition.
ular models of some mellitates 3 (R ) Me, t-Bu, and Men), we
found that placing the 1ZZ molecule over bulky alkyl mellitate
causes significant steric hindrance and the intimate face-to-face
interaction becomes more difficult to attain than in the cy-
clooctene case. Experimentally, the following observations are
indicative of more severe steric hindrance upon interaction of
excited mellitate with 1ZZ rather than (Z)-cyclooctene: (1) the
rate of fluorescence quenching by 1ZZ dramatically decreases
by a factor of 4.5 by replacing the sensitizer’s methyl with a
bulky menthyl group (Table 3) and (2) in spite of the much
lower singlet energy of 1ZZ, the quenching rate constant for
It is inappropriate however to postulate such a solvent-
separated or free radical ion pair in nonpolar pentane. Judging
from the moderate enantiodifferentiations attained in pentane,
diene 1 is evidently located in proximity to the excited chiral
sensitizer, forming an exciplex, but the stereochemical interac-
tion between the components does not appear to be as intimate
as in the cyclooctene case. In contrast, the photosensitizations
of cyclooctene under analogous conditions never show such a
9
-1 -1
1
ZZ (5.3 × 10 M s ) is of a magnitude similar to that for
drastic solvent effect and give comparable ee’s in polar and
9 -1 -1 23
nonpolar solvents.21 This is also the case even in the photo-
(Z)-cyclooctene (3.4 × 10 M s ). A similar unfavorable
sensitization of cyclooctene with triplex-forming sensitizers
(33) Yasuda, M.; Kuwamura, G.; Nakazono, T.; Shima, K.; Inoue, Y.;
2
2
possessing electron-donating phenyl group(s) in the side arm.
Yamasaki, N.; Tai, A. Bull. Chem. Soc. Jpn. 1994, 67, 505.

Optically ActiVe (E,Z)-1,3-Cyclooctadiene
J. Am. Chem. Soc., Vol. 119, No. 3, 1997 477
Table 5. Theoretical and Experimental (Chir)optical Properties of
(
E,Z)-1,3-Cyclooctadiene (1EZ)a
transition
N f V N f V
f π f π
70%) (50%)
relative intensity
1
(N f V /N f V )
entry
naturea
1
2
2
π
2
3
*
π
1
3
*
(
∆ b
E
calc.c
exp.d
5.97
5.49
7.12
6.77
(∆0.45)
0.12
g
(∆0.48)
fe
calc.c
exp.d
0.17
0.12
0.71
g
2.5
2.3
f
Rh
calc.d
c
-35
-70
-86
-161
f
exp.
a
Major contribution assigned by Bouman and Hansen (ref 19).
b
c
Excitation energy in eV. Reference 19; calculated by ab initio RPA
calculation. This work; measured in vapor phase. e Oscillator strength.
d
f
Reference 8; corrected for the underestimated absorption coefficient,
see text. g Not determined. h Rotatory strength in 10 cgs.
-40
of relevant (E)-cyclooctene derivatives, specific rotations of
which are listed in Table 4. The [R]D value of parent
(E)-cyclooctene (426°) is exceptionally high for a simple alkene
in comparison with the moderate [R]D values reported for (E)-
16
isomers of 1-methylcyclooctene (106°) and 1,5-cyclooctadiene
1
3
(
1
152°). In this context, the [R]D value of 1EZ as high as
380°, reminiscent of the tremendously high specific rotations
3
5
of helicenes, may be attributable to the helical arrangement
of the p orbitals in this highly twisted conjugated diene.
Absorption and circular dichroism spectra of the same sample
(10.1% ee) were measured both in cyclohexane solution and in
vapor phase. As can be seen from Figure 4, both geometrical
isomers 1ZZ and 1EZ exhibit the absorption maxima (λmax) at
almost identical wavelengths around 228 nm in solution and
219 nm in vapor phase. The comparable λmax for 1ZZ and 1EZ
may seem rather exceptional, since highly constrained E-isomers
Figure 5. Circular dichroism spectra of 1ZZ (solid line) and/or 1EZ
(10.1% ee; dashed line); the same samples and conditions as in Figure
4
.
6
36
of cyclooctene and 1-methylcyclooctene absorb at longer
wavelengths than the Z-isomers do. However, the diffused and
less-intensive absorption profile of 1EZ is characteristic of a
strained (E)-cycloalkene. Unexpectedly, the measured absorp-
effect upon ee of increased steric hindrance in the substrate has
been reported in the enantiodifferentiating photoisomerization
16
of 1-methylcyclooctene sensitized by benzenepolycarboxylates.
In these cases, the substrates cannot form any intimate exciplex
but give rise to a more distant exciplex to afford lower ee’s.
We deduce therefore that there exist fairly critical steric, as well
as electronic, requirements for both substrate and sensitizer
structures to accomplish highly efficient enantiodifferentiation
in the excited state. For a better enantiodifferentiating photo-
sensitization system, our efforts should be directed toward a
more compact substrate that forms an intimately interacting
exciplex probably with high C-T character.
-
1
-1
tion coefficient of 1EZ, ꢀ 3625 M cm (λmax 229.5 nm,
cyclohexane) turned out to be significantly greater than the
-
1
-1
reported value: ꢀ 2630 M cm (λmax 230.5 nm, cyclohex-
ane).
1
7
As shown in Figure 5, the CD spectrum of (-)-1EZ (10.1%
ee) in cyclohexane exhibits one negative Cotton effect peak at
-
1
-1 37
2
28 nm (∆ꢀ -1.29 M cm ), in formal agreement with the
8
previous work, while the measurement in vapor phase reveals
another yet larger negative peak at 183 nm in addition to the N
f V1 band at 226 nm. Although this new peak was predicted
Chiroptical Properties of (E,Z)-1,3-Cyclooctadiene (1EZ).
The enantiodifferentiating photosensitization of 1ZZ provided
us with a direct access to optically active 1EZ, chiroptical
(
34) This is much greater than the value reported previously by Isaksson
properties of which are of much theoretical and spectroscopic
20
8
et al.: [R] D -649° (c 0.017, ethanol). The reason(s) for this discrepancy
will be discussed later in ref 37.
interest.4
-8,19
We carried out the preparative scale photolysis
(
(
35) Newman, M. S.; Lednicer, D. J. Am. Chem. Soc. 1956, 58, 4765.
36) Tsuneishi, H.; Inoue, Y.; Hakushi, T.; Tai, A. J. Chem. Soc., Perkin
of 1ZZ in the presence of (-)-menthyl mellitate 3a. Isolation
through the selective extraction with aqueous silver nitrate gave
chemically pure 1EZ (>99.5% by GC). The isolated sample
was immediately subjected to the measurement of optical
rotation and also to the GC analysis over a chiral capillary
column, affording the specific rotation of [R]25D -139.6° (c 1.07,
CH2Cl2) and the enantiomeric excess of 10.1%. Repeated
measurements with the same sample and independent experi-
ments gave practically identical values within the experimental
errors of (5° in [R]D or (0.5% in ee. Using the specific
rotation (-139.6°) and ee value (10.1%), we can determine the
specific rotation of optically pure (-)-1EZ as -1380° (CH2-
Cl2).34
Trans. 2 1993, 457.
37) From the ee and ∆ꢀ values obtained above, we can evaluate the ∆ꢀ
(
-
1
-1
value of optically pure 1EZ as 12.8 M cm . Again, this ∆ꢀ value is
-
1
-1
much greater than that (∆ꢀ 8.83 M cm in ethanol) reported previously
8
by Isaksson et al., although their CD spectrum is essentially superimposable
to ours except for the intensity. However, using the corrected absorption
coefficient obtained above, Isaksson’s ∆ꢀ value can be recalculated to be
-
1
-1
1
2.2 M cm (EtOH). In spite of the different solvent used, this value is
well within the experimental error, taking into account the reported ee
8
(
>90%). Similarly Isaksson’s specific rotation may also be corrected for
the absorption coefficient, since they determined the concentration from
the absorbance. However, the corrected [R]D value for the >90% ee sample
8
is only 895° (EtOH), which is much smaller than ours, i.e., 1380° (CH2-
Cl2). Since the ∆ꢀ values are comparable in cyclohexane and ethanol, we
have no reasonable explanation for this discrepancy, although at least part
of the difference may be attributed to the solvent effect.
It may be interesting to compare the chiroptical properties

478 J. Am. Chem. Soc., Vol. 119, No. 3, 1997
Inoue et al.
theoretically,19 its precise location and relative intensity have
not been elucidated experimentally. The theoretical and ex-
perimental (chir)optical properties of 1EZ are summarized in
Table 5. The calculated transition energies for the N f V1
and N f V2 bands are in good agreement with the experimental
value obtained here, taking into account the general tendency
193 nm is assigned to the N f V2 transition, although some
Rydberg character would be overlapping.
1
9,35,38
Acknowledgment. This work was supported by grants from
the Ministry of Education, Science, Sports, and Culture of Japan,
the Nagase Science and Technology Foundation, the Sumitomo
Foundation, and the Asahi Glass Foundation.
1
9
of overestimation by ca. 0.5 eV in the ab initio calculations.
Furthermore, the observed relative intensity of the rotatory
strengths, i.e., R(N f V2)/R(N f V1) ) 2.3, is in excellent
agreement with the predicted value (2.5). Then, the peak at
JA963160C
(
38) Robin, M. B. Higher Excited States of Polyatomic Molecules;
Academic: New York, 1975; Vol. 2, p 176.