Reversible diastereoselective photocyclization of a diarylethene in a single-crystalline phase

Article abstract of DOI:10.1021/ja001350o

Optically active photochromic (S)- and (R)-1-(2-methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-penten-1-yl)-5-phenyl-3- thienyl]-3,3,4,4,5,5-hexafluorocyclopentenes ((S)-1a and (R)-1a) were synthesized. X-ray crystallographic measurement showed that (S)-1a a

Full text of DOI:10.1021/ja001350o

J. Am. Chem. Soc. 2000, 122, 9631-9637
9631
Reversible Diastereoselective Photocyclization of a Diarylethene in a
Single-Crystalline Phase
Tetsuhiro Kodani, Kenji Matsuda, Taro Yamada, Seiya Kobatake, and Masahiro Irie*
Contribution from the Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, and CREST, Japan Science
and Technology Corporation, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
ReceiVed April 18, 2000. ReVised Manuscript ReceiVed July 19, 2000
Abstract: Optically active photochromic (S)- and (R)-1-(2-methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-penten-
1-yl)-5-phenyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentenes ((S)-1a and (R)-1a) were synthesized. X-ray
crystallographic measurement showed that (S)-1a and (R)-1a single crystals adopted orthorhombic chiral space
group P212121 and the open-ring isomers were packed in the crystals in only one conformer of the possible
two atropisomers. Upon irradiation with 366 nm light (S)-1a and (R)-1a underwent reversible photocyclization
reactions both in solution and in the single-crystalline state, and in the crystalline phase almost only one
closed-ring diastereomer was produced, while the diastereoselection was not observed in solution. The dominant
diastereomer produced from (S)-1a was determined to be (S,R,R)-1b by X-ray crystallographic analysis.
Introduction
Scheme 1. Photochromism of Diarylethene Derivativesa
Photochromism is defined as a reversible photoisomerization
between two forms having different absorption spectra.1 So far
various types of photochromic compounds, such as spiroben-
zopyrans, azobenzenes, fulgides, and diarylethenes, have been
developed. Among the compounds, diarylethenes show char-
acteristic reactivities. Both isomers are thermally stable and
some of them undergo photochromic reactions even in the
single-crystalline state.2
a The photogenerated closed-ring isomers have two enantiomers,
(R,R) and (S,S).
crystalline phase and direct observation of the reaction process
in the single crystals by X-ray crystallography.
Diarylethenes undergo cyclization/cycloreversion photochro-
mic reactions. The photochemical conrotatory cyclization
produces two enantiomers (R,R and S,S) originating from two
asymmetric carbon atoms. The photocyclization in solution, in
general, results in the formation of two enantiomers in equal
amounts. Even when a chiral substituent is introduced into the
diarylethene, enrichment of one of the diastereomers hardly takes
place.3 Enrichment of one of the enantiomers or diastereomers
in photochemical reactions requires chiral environments,4 such
as cavities of optically active host molecules5 or crystals with
chiral space groups.6 Among various methods of enantio- or
diastereoselection, photoreactions in chiral crystals are of general
use. Here we report on diastereoselection in the photocyclization
reaction of (S)-1a and (R)-1a with a chiral substituent in the
Results and Discussion
Photochromic Reactions in Solution. It is a necessary
condition to prepare crystals with chiral space groups for the
enrichment of one of the enantiomers or diastereomers in
crystalline photoreactions. Although we tried to prepare such
crystals by recrystallization of achiral diarylethenes with
substituted thiophene and/or benzothiophene aryl groups from
various solutions, we failed. Therefore, we decided to prepare
chiral crystals by introducing a chiral substituent to a di-
arylethene. We chose 1,2-bis(2-methyl-5-phenylthiophen-3-yl)-
3,3,4,4,5,5-hexafluorocyclopentene, which is known to show
single-crystalline photochromism,7 as the diarylethene and
3-methyl-1-penten-1-yl group as the chiral substituent.
(1) (a) Brown, G. H. In Photochromism; Wiley-Interscience: New York.
1971. (b) Du¨rr, H.; Bouas-Laurent, H. Photochromism, Molecules and
Systems; Elsevier: Amsterdam, 1990.
(5) (a) Gerdil, R.; Barchietto, G.; Jefford, C. W. J. Am. Chem. Soc. 1984,
106, 8004. (b) Weisinger-Lewin, Y.; Vaida, M.; Popovitz-Biro, R.; Chang,
H. C.; Mannig, F.; Frolow, F.; Lahav, M.; Leiserowitz, L. Tetrahedron 1987,
43, 1449. (c) Aoyama, H.; Miyazaki, K.; Sakamoto, M.; Omote, Y.
Tetrahedron, 1987, 43, 1513. (d) Pushkara Rao, V.; Turro, N. J. Tetrahedron
Lett. 1989, 30, 1464. (e) Toda, F. Acc. Chem. Res. 1995, 28, 480. (f)
Sundarababu, G.; Leibovitch, M.; Corbin, D. R.; Scheffer, J. R.; Rama-
murthy, V. Chem. Commun. 1996, 2159.
(6) (a) Wegner, G. Pure Appl. Chem. 1977, 49, 443. (b) Addadi, L.;
Lahav, M. J. Am. Chem. Soc. 1978, 100, 2838. (c) Hasegawa, M.; Chem.
ReV. 1983, 83, 507. (d) Ramamurthy, V. Photochemistry in Organized and
Constrained Media; VCH: New York, 1991. (e) Suzuki, T.; Fukushima,
T.; Yamashita, Y.; Miyashi, T. J. Am. Chem. Soc. 1994, 116, 2793. (f)
Sakamoto, M. Chem. Eur. J. 1997, 3, 684. (g) Leibovitch, M.; Olovsson,
G.; Scheffer, J. R.; Trottoer, J. J. Am. Chem. Soc. 1998, 120, 12755.
(7) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000,
122, 4871.
(2) (a) Irie, M.; Uchida, K.; Eriguchi, T.; Tsuzuki, H. Chem. Lett. 1995,
899. (b) Irie, M.; Lifka, T.; Uchida, K. Mol. Cryst. Liq. Cryst. 1997, 81,
297. (c) Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985. (d)
Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am. Chem.
Soc. 1999, 121, 2380. (e) Kobatake, S.; Yamada, M.; Yamada, T.; Irie, M.
J. Am. Chem. Soc. 1999, 121, 8450. (f) Yamada, T.; Kobatake, S.; Muto,
K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 1589. (g) Irie, M. Chem. ReV.
2000, 100, 1685.
(3) Yamaguchi, T.; Uchida, K.; Irie, M. J. Am. Chem. Soc. 1997, 119,
6066.
(4) For reviews of photochemical asymmetric photoreactions, see: (a)
Rau, H. Chem. ReV. 1983, 83, 535. (b) Inoue, Y. Chem. ReV. 1992, 92,
741. (c) Feringa, B. L.; van Delden, R. A. Angew. Chem., Int. Ed. 1999,
38, 3418.
10.1021/ja001350o CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/23/2000

9632 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Scheme 2. Photochromism of (S)-1a and (R)-1aa
Kodani et al.
a (a) the photogenerated closed-ring isomers of (S)-1a have two diastereomers, (S,R,R)-1b and (S,S,S)-1b. (b) The photogenerated closed-ring
isomers of (R)-1a have two diastereomers, (R,S,S)-1b and (R,R,R)-1b.
Scheme 3a
a Reagents and conditions: (a) ethylene glycol, p-toluenesulfonic acid monohydrate, benzene; (b) Pd(PPh3)4, Na2CO3, phenylboronic acid, THF,
H2O; (c) n-BuLi, 3-(2,3,3,4,4,5,5-heptafluorocyclopent-1-en-1-yl)-2-methyl-5-phenylthiophene, THF; (d) pyridinium p-toluenesulfonate, acetone;(e)
Mg, (R)- or (S)-methylbutylbromide, THF; and (f) DMSO.
The synthesis of the chiral diarylethenes (S)-1a and (R)-1a
was performed according to Scheme 3. The 3-methyl-1-penten-
1-yl substituent was introduced at the 2-position of the thiophene
ring by Grignard reaction of (S)-(+)- and (R)-(-)-methylbutyl-
bromide with aldehyde 6.
At first, we examined photoreactivity of (S)-1a in solution.
Upon irradiation with 366 nm light, (S)-1a underwent a
photochromic reaction in hexane solution. Figure 1a shows the
absorption spectral change of compound (S)-1a (λmax 288 nm,
ꢀmax 33000 M-1 cm-1) in hexane by photoirradiation. Upon
irradiation with 366 nm light, the colorless solution of (S)-1a
turned blue, in which the absorption maximum was observed
at 580 nm (ꢀmax 14000 M-1 cm-1). The color change is due to
the formation of the closed-ring isomer of (S)-1a.2c,g The
conversions by irradiation with 366 and 313 nm light from the
open- to the closed-ring isomers were 87 and 91%, respectively.
Upon irradiation with 578 nm light for 3 min, the spectrum
returned back to the original. The photochromic performance
of (R)-1a was identical with that for compound (S)-1a in hexane
solution as shown in Figure 1b.
X-ray Crystallographic Analysis of (S)-1a and (R)-1a.
Colorless rectangular-shaped single crystals of (S)-1a were
obtained from a mixed solution of hexane and chloroform. The
crystal structure was determined by X-ray crystallographic
analysis (Figure 2 and Table 1). An (S)-1a single crystal adopted
an orthorhombic chiral space group P212121. The structure was
determined by direct methods and successive Fourier synthesis.
Full-matrix least-squares refinement of positional and thermal
parameters led to the final convergence with R1 ) 0.0464 and
wR2 ) 0.1205. The distance of C(1)-C(10), which are reactive
carbon atoms, was 3.56 Å, which is short enough for the reaction

Photocyclization of a Diarylethene in a Single-Crystalline Phase
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9633
Photochromic Reactions in the Crystalline Phase. (S)-1a
and (R)-1a underwent photochromic reactions in the single-
crystalline phase. Upon irradiation with 366 nm light, the single
crystals turned blue and the blue color disappeared by irradiation
with visible light (λ > 500 nm). The coloration/decoloration
cycles could be repeated many times while keeping the crystal
shape. To know the origin of the blue color, the blue-colored
crystals were dissolved into hexane, and the absorption spectrum
was measured. The absorption maxima were the same as those
of the closed-ring isomers shown in Figure 1. In addition a
characteristic methyl signal was observed at 2.16 ppm due to
1
the closed-ring isomer in the H NMR spectrum of the blue
colored product (see Experimental Section). These results
indicate that the blue color of the crystals is due to the closed-
ring isomer.
The color of the crystal was observed under polarized light.
Figure 3 shows the absorption anisotropy and the polar plot.
At a certain angle, the crystal had a deep blue color (Figure
3a). When the crystal was rotated as much as 90°, the color
almost disappeared (Figure 3b). The absorption maximum
shifted from 645 to 620 nm by rotating the sample. The shift is
ascribed to the contribution of the short axis electronic transition,
as shown in Figure 3d. The changes of the blue-color intensity
and the absorption maximum by rotating the crystal sample
indicate that the closed-ring isomers are regularly oriented in
the crystal (Figure 3c,d). In other words, the photochromic
reaction took place in the crystal lattice.2
Figure 1. (a) Absorption spectra of (S)-1a in hexane (c ) 7.5 × 10-6
M): open-ring isomer (solid line), closed-ring isomer (dotted line), and
in the photostationary state under irradiation with 366 nm light (dashed
line). (b) Absorption spectra of (R)-1a in hexane (c ) 8.0 × 10-6 M):
open-ring isomer (solid line), closed-ring isomer (dotted line), and in
the photostationary state under irradiation with 366 nm light (dashed
line).
Asymmetric Induction. Diastereoselectivity in the cycliza-
tion process was examined in solution as well as in the
crystalline phase. The photoirradiated sample was analyzed with
a reversed-phase HPLC column (Mightysil RP-18 GP, CH3-
CN/H2O ) 75:25 volume ratio). The monitoring wavelength
used was 580 nm, because only the closed-ring isomer of (S)-
1a has absorption at the visible wavelength. It was possible to
separate the two diastereomers with the conventional reversed-
phase HPLC column. The closed-ring isomers produced in
hexane, CH3CN, and THF solutions by irradiation with 366 nm
light were a mixture of equal amounts of two diastereomers,
(S,R,R)-1b and (S,S,S)-1b. There was not diastereomeric excess
in all solvents studied here as shown in Figure 4a.
Table 1. Crystal Data and Structure Refinement for (S)-1a,
(S)-1a′, and (R)-1a
(S)-1a
(R)-1a
(S)-1a′
formula
C32H26F6S2
588.65
orthorhombic
P212121
formula weight
crystal system
space group
unit cell dimension
a, Å
9.3904 (16) 9.3680 (10) 9.399 (2)
11.864 (19) 11.8571(12) 11.866 (3)
The ratio of the two diastereomers dramatically changed in
the crystalline phase reaction, as shown in Figure 4b,c. A 500
W super high-pressure mercury lamp was employed as the light
source for the photocyclization of (S)-1a or (R)-1a in the
crystalline phase. The irradiation wavelengths of light were 366,
405, and 435 nm. The longer wavelength of light increased the
conversion. Table 2 shows the relation between conversion and
d.e. (diastereomeric excess) value.
b, Å
c, Å
25.019 (4)
2787.3 (8)
4
1.403
1.048
24.971 (3)
2773.7(5)
4
1.410
1.041
24.972 (6)
2784.9 (12)
4
1.404
0.891
volume, Å3
Z
density (calcd), g/cm3
goodness-of-fit on F
final R indices [I > 2σ(I)], 0.0464
0.0462
0.0660
R1 )
wR2 (all data), wR2 )
0.1205
0.1149
123 K
0.1781
temperature
The reaction was completely diastereoselective when the
conversion was less than 6%. The d.e. slightly decreased when
the conversion increased over 10%. The slight decrease of
selectivity is attributable to distortion of the crystal lattice as
the reaction proceeds.
to take place in the crystalline phase.8 The open-ring isomers
were packed in only one conformer of the possible two
atropisomers in the crystal. The packing of molecules viewed
normal to the (011) plane, which is the wide face of the crystal,
is shown in Figure 3d. Crystal polymorphism was not observed.
The rectangular single crystals were also obtained from a
mixed solution of hexane and chloroform of (R)-1a. The
molecular configuration of (R)-1a in the crystal was the mirror
image of (S)-1a as shown in Figure 2. Full-matrix least-squares
refinement of positional and thermal parameters led to the final
convergence with R1 ) 0.0462 and wR2 ) 0.1149. As expected,
the molecular structure of (R)-1a was identical with that of
(S)-1a.
X-ray Crystallographic Analysis of the Photocyclization
Reaction in the Single-Crystalline Phase. To know the details
of the diastereoselection in the crystalline phase, direct observa-
tion of the reaction process by X-ray crystallography was carried
out.2f To detect the reaction process by X-ray crystallography,
it is required to increase the conversion higher than 5%. Linearly
polarized 440 nm light was used as the light source to penetrate
the light into the crystal bulk. At 440 nm (S)-1a has the
absorption tail and the absorption of the closed-ring isomer is
minimum as shown in Figure 3. The direction of polarization
of irradiation light was selected parallel to the a-axis and normal
(8) Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433.

9634 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Kodani et al.
Figure 2. ORTEP drawing of the absolute configuration of (a) (S)-1a and (b) (R)-1a.
Figure 3. (a, b) Polarized absorption spectra of the colored crystal on the (011) face (see text); (c) polar plot at 645 nm; and (d) packing of the
molecules viewed normal to the (011) face. Arrows denote the direction of transition moment of absorption at 645 nm.
to the (011) face so that the photogenerated blue color intensity
becomes minimum as shown in Figure 3b. Irradiation was
continued for 73 h. The colorless crystal turned deep blue. This
deep blue crystal was denoted by (S)-1a′.
X-ray crystallographic analysis data of (S)-1a′ are shown in
Table 1. The unit cell parameters were almost identical even
after irradiation. The coordinates of the open-ring isomer (S)-
1a were used for the initial model for the refinement. After the

Photocyclization of a Diarylethene in a Single-Crystalline Phase
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9635
Figure 4. Diastereoselectivity of photoirradiated sample of (S)-1a and
(R)-1a by HPLC analysis at 580 nm (Mightysil RP-18 GP, CH3CN/
H2O ) 75:25 volume ratio): (a) photocyclization reaction in hexane
solution; (b) photocyclization reaction in the single-crystalline phase
of (S)-1a (conversion ) 13.2%); and (c) photocyclization reaction in
the single-crystalline phase of (R)-1a (conversion ) 6.0%).
Figure 6. ORTEP drawing of (a) (S)-1a and (b) photogenerated closed-
ring isomer (S,R,R)-1b. Hydrogen atoms are omitted for clarity.
Irradiation wavelength: 440 nm. Irradiation time: 73 h.
Table 2. Asymmetric Induction in the Crystalline Phase
Photochromism of (S)-1a and (R)-1a
conversion, %a
d.e., %b
1.7c,e
2.9c,f
>99
>99
>99
97.2
96.7
95.0
photogenerated in a conrotatory mode. Electron density peaks
corresponding to two carbons at reaction points also appeared
(0.35 and 0.30 e‚Å-3). These peaks were assigned as the closed-
ring isomer (S,R,R)-1b. The displacement factors of the
photogenerated closed-ring isomer were refined isotropically.
Using the model, the full-matrix least-squares refinement was
well converged (R1 ) 0.0660 for the data with I > 2σ(I), wR2
) 0.1781 for all data). Judging from the structure of the closed-
ring isomer, it can be concluded that only one diastereomer was
formed in the crystal. According to the atom occupancy factor,
the conversion from (S)-1a to (S,R,R)-1b was about 8%. The
final molecular structure of the closed-ring isomer (S,R,R)-1b
and the structure of the open-ring isomer (S)-1a are shown in
Figure 6. The structure change indicates that the topochemically
controlled cyclization reaction took place in the crystalline phase.
The closed-ring isomer (b) of Figure 6 was produced from the
open-ring isomer (a) by the minimal conrotatory rotation of the
two thiophene rings. Upon irradiation with visible light (λ >
500 nm), the peaks of (S,R,R)-1b disappeared.
6.0d,g
9.3c,g
11.9c,g
13.2c,g
a Determined by HPLC using Superspher Si60 (MERCK). b Deter-
mined by HPLC using Mightysil RP-18 GP (KANTO CHEMICALS).
c (S)-1a. d (R)-1a. e Irradiation with 366 nm light. f Irradiation with 405
nm light. g Irradiation with 435 nm light.
Conclusions
Optically active (S)- and (R)-1-(2-methyl-5-phenyl-3-thienyl)-
2-[2-(3-methyl-1-penten-1-yl)-5-phenyl-3-thienyl]-3,3,4,4,5,5-
hexafluorocyclopentenes ((S)-1a and (R)-1a) underwent pho-
tochromic reactions both in hexane solution and in the single-
crystalline state. Although equal amounts of two diastereomers
of the closed-ring isomers were produced in solution by
irradiation with 366 nm light, the diastereoselective photocy-
clization reaction took place in the single crystal. The X-ray
crystallography proved that the reaction was topochemically
regulated by the crystal lattice.
Figure 5. Difference Fourier electron density map of (S)-1a photoir-
radiated with 440 nm light for 73 h. (S,R,R)-1b was generated
topochemically (conversion ) 8%).
first least-squares refinement, the difference Fourier electron
density map (Figure 5) showed the existence of two quite high
electron density peaks (0.99 and 0.94 e‚Å-3) comes from the
sulfur atoms of the photogenerated closed-ring isomer. The
distance between the two sulfur atoms was 3.68 Å, which is
close to the distance of the closed-ring isomer of 1,2-bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene, 3.65 Å.7 The
positions are close to those expected for the closed-ring isomer
Experimental Section
1
A. Materials. H NMR spectra were recorded on a Varian Gemini
200 spectrometer (200 MHz). Tetramethylsilane was used as internal
standard. UV-vis spectra were recorded on a Hitachi U-3500 spec-

9636 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Kodani et al.
trophotometer. Mass spectra were obtained by JEOL JMS-HX110A
instruments. Melting points are not corrected.
methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-hydroxy-1-pentyl)-5-phen-
yl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene (210 mg, 0.35 mmol)
in 10 mL of DMF was heated at 170 °C for 13 h.9 The mixture was
cooled to room temperature and then water was added. The resultant
mixture was extracted with ether and the organic extract was washed
with brine and dried (MgSO4). Finally, purification of the crude product
by preparative thin-layer chromatography (hexane) and recrystallization
afforded dithienyletene (S)-1a (80 mg, 39%) as slightly yellow
All reactions were performed under an atmosphere of dry argon
unless otherwise specified. All reactions were monitored by thin-layer
chromatography carried out on 0.2-mm E. Merck silica gel plates (60F-
254). Column chromatography was performed on silica gel (E. Merck,
70-230 mesh).
3,5-Dibromo-2-(2,5-dioxolanyl)thiophene (3). A solution of 3,5-
dibromothiophene-2-carbaldehyde (2) (13.0 g, 47 mmol), ethylene
glycol (2.8 mL, 5.2 mmol), and p-toluenesulfonic acid monohydrate
(0.1 g, 0.49 mmol) in benzene (100 mL) was refluxed for 13 h with a
Dean-Stark condenser. The reaction mixture was poured into aqueous
sodium bicarbonate, extracted with ether, washed with aqueous sodium
bicarbonate and water, dried (MgSO4), and concentrated. Dioxolane 3
(14.3 g, 96%) was obtained as an oily product: 1H NMR (CDCl3) δ
3.97-4.17 (m, 4 H), 6.06 (s, 1 H), 6.94 (s, 1 H); HRMS (FAB) m/z
312.852 ([M + H]+), calcd for C7H7Br2O2S 312.853.
3-Bromo-2-(2,5-dioxolanyl)-5-phenylthiophene (4). A mixture of
Pd(PPh3)4 (2.1 g, 1.8 mmol), dibromo compounds 3 (5.8 g, 18 mmol),
and THF (120 mL) was stirred for 0.5 h; phenylboronic acid (2.4 g, 20
mmol) and a suspension of Na2CO3 (9.6 g, 91.0 mmol) in 60 mL of
water were subsequently added. The mixture was stirred and refluxed
for 7 h and allowed to cool to room temperature. Ether was added,
and the organic layer was collected and the water layer was extracted
with ether. The organic layers were dried over MgSO4, and the solvents
were evaporated. Purification of the crude product by column chro-
matography (silica gel, CHCl3/hexane ) 1:1) afforded phenylthiophene
derivative 4 (3.1 g, 55%) as an oily product:1H NMR (CDCl3) δ 4.01-
4.23 (m, 4 H), 6.14 (s, 1 H), 7.15 (s, 1 H), 7.31-7.71 (m, 5 H); HRMS
(FAB) m/z 310.973 ([M + H]+), calcd for C7H7Br2O2S 310.974.
1
crystals: mp 116.7-117.2 °C; H NMR (CDCl3) δ 0.62 (t, J ) 6.2
Hz, 3 H), 0.78 (d, J ) 10 Hz, 3 H), 1.10-1.23 (m, 2 H), 1.78-1.92
(m, 1 H), 1.94 (s, 3 H), 5.85-5.99 (m, 2 H), 7.21-7.65 (m, 12 H);
UV-vis (hexane) λmax (ꢀ) 288 (33000); HRMS (FAB) m/z 588.1378
([M]+), calcd for C32H26F6S2 588.1380.
Closed-Ring Isomer (1:1 Mixture of (S,R,R)-1b and (S,S,S)-1b).
The closed-ring isomer (1:1 mixture of (S,R,R)-1b and (S,S,S)-1b) was
separated by HPLC using silica gel columns (Superspher Si60 MERCK)
in normal phase (hexane/AcOEt ) 97:3). The retention time of open-
ring isomer was 7.0 min and that of the closed-ring isomer was 8.6
min (flow rate ) 0.7 mL/min): 1H NMR (CDCl3) δ 0.69-0.74 (m, 3
H), 0.86-0.89 (m, 3 H), 1.24-1.27 (m, 3 H), 2.16 (s, 3 H), 5.49-
5.63 (m, 1 H), 6.43-6.49 (m, 1 H), 6.64 (s, 1 H), 6.76 (s, 1 H), 7.40-
7.59 (m, 10 H); UV-vis (hexane) λmax (ꢀ) 274 (17000), 310 (20000),
580 (14000).
(R)-1-(2-Methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-penten-1-
yl)-5-phenyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene ((R)-1a).
The synthetic procedure was same for (S)-1a except (R)-(-)-methyl-
butylbromide10 was used instead of (S)-(+)-methylbutylbromide: mp
1
117-118 °C; H NMR (CDCl3) δ 0.62 (t, J ) 6.2 Hz, 3 H), 0.78 (d,
J ) 10 Hz, 3 H), 1.10-1.23 (m, 2 H), 1.78-1.92 (m, 1 H), 1.94 (s, 3
H), 5.85-5.99 (m, 2 H), 7.21-7.65 (m, 12 H); HRMS (FAB) m/z
588.1382 ([M]+), calcd for C32H26F6S2 588.1380.
1-(2-Methyl-5-phenyl-3-thienyl)-2-[2-(2,5-dioxolanyl)-5-phenyl-
3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene (5). To a well-stirred
solution of monobromo compound 4 (1.5 g, 4.8 mmol) in anhydrous
THF (50 mL) under Ar at -78 °C was added dropwise a solution of
n-BuLi (3.2 mL, 5.3 mmol) and stirring was continued for 1.5 h at
-78 °C. Then, a solution of 3-(2,3,3,4,4,5,5-heptafluorocyclopent-1-
en-1-yl)-2-methyl-5-phenylthiophene (1.9 g, 5.3 mmol) was added
dropwise. The mixture was stirred for 5 h and allowed to warm to
room temperature and water was added. The resultant mixture was then
extracted with ether and the organic extract was washed with brine
and dried (MgSO4). The solvent was removed. Column chromatography
(silica gel, CHCl3/hexane ) 1:1) afforded dithienylethene 5 (1.2 g, 43%)
as an oily product: 1H NMR (CDCl3) δ 1.99 (s, 3H), 3.78-3.99 (m,
4 H), 5.37 (s, 1 H), 7.26-7.63 (m, 12 H); HRMS (FAB) m/z 579.090
([M + H]+), calcd for C29H21F6O2S2 579.088.
1-(2-Methyl-5-phenyl-3-thienyl)-2-(2-formyl-5-phenyl-3-thienyl)-
3,3,4,4,5,5-hexafluorocyclopentene (6). A solution of dithienylethene
5 (2.7 g, 4.7 mmol) in wet acetone (40 mL) containing pyridinium
tosylate (2.3 g, 9.2 mmol) was refluxed for 12 h. The mixture was
cooled to room temperature and water was added. The resultant mixture
was then extracted with ether and the organic extract was washed with
brine and dried (MgSO4). The solvent was removed in vacuo. Finally,
purification of the crude product by recrystallization afforded slightly
yellow crystals of dithienylethene 6 (2.2 g, 95%): mp 132-133 °C;
1H NMR (CDCl3) δ 2.00 (s, 3 H), 7.21-7.69 (m, 12 H), 9.56 (s, 1 H);
HRMS (FAB) m/z 535.063 ([M + H]+), calcd for C27H17F6OS2 535.062.
(S)-1-(2-Methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-penten-1-
yl)-5-phenyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene ((S)-1a).
To a stirred solution of (S)-(+)-methylbutylbromide (0.24 mL, 1.9
mmol) in 15 mL of anhydrous THF was added a powder of Mg (45
mg, 1.9 mmol) under Ar at room temperature. After 1 h, dithienylethene
6 (300 mg, 0.6 mmol) was added and stirring was continued for 1 h at
room temperature. The resultant mixture was then extracted with ether
and the organic extract was washed with brine and dried (MgSO4).
The solvent was removed. Finally, purification of the crude product
by preparative thin-layer chromatography (silica gel, CHCl3/hexane )
1:1)afforded1-(2-methyl-5-phenyl-3-thienyl)-2-[2-(3-methyl-1-hydroxy-
1-pentyl)-5-phenyl-3-thienyl]-3,3,4,4,5,5-hexafluorocyclopentene
(170 mg, 50%) as an oily product. A stirred solution of 1-(2-
B. Photochemical Measurements. Solvents used for physical
measurement were spectroscopic grade and purified by distillation
before use. Absorption spectra were measured on a spectrophotometer
(Hitachi U-3500). Absorption spectra in the single-crystalline phase
were measured using a Leica DMLP polarizing microscope connected
with a Hamamatsu PMA-11 detector. The polarizer and analyzer were
set parallel to each other.
Photoirradiation was carried out using a USHIO 500 W super high-
pressure mercury lamp and a USHIO 500-W xenon lamp. Mercury
lines of 366, 405, 435, and 578 nm were isolated by passing the light
through a combination of a Toshiba band-pass filter or a cutoff filter
and a monochromator (Ritsu MC-20L).
HPLC was performed on a Hitachi L-7100 pump coupled with a
Hitachi L-7400 UV detector. Silica gel columns (Superspher Si60
MERCK) in normal phase (hexane/AcOEt ) 97:3) were used to
separate the closed-ring isomers. Silica gel columns (Mightysil RP-18
GP KANTO chemicals) in reversed phase (CH3CN/H2O ) 75:25) were
used to analyze the diastereomers.
C. Crystallography. The data collection was performed on a Bruker
SMART1000 CCD-based diffractometer (50 kV, 40 mA) with Mo KR
radiation. The data were collected as a series of ω-scan frames, each
with a width of 0.3°/frame. The crystal-to-detector distance was 5.118
cm. Crystal decay was monitored by repeating the 50 initial frames at
the end data collection and analyzing the duplicate reflections. Data
reduction was performed using SAINT software, which corrects for
Lorentz and polarization effects, and decay. The cell constants were
calculated by global refinement. The structure was solved by direct
methods using SHELXS-8611 and refined by full least-squares on F2
using SHELXL-97.12 The positions of all hydrogen atoms were
calculated geometrically and refined by the riding model. The disordered
part was refined isotropically.
(9) Traynelis, V. J.; Hergenrother, W. L.; Livingston, J. R.; Valicenti, J.
A. J. Org. Chem. 1962, 27, 2377.
(10) Tius, M. A.; Gu, X.; Truesdell, J. W.; Savariar, S.; Crooker, P. P.
Synthesis 1988, 36.
(11) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(12) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; Universita¨t Go¨ttingen: Go¨ttingen, 1997.

Photocyclization of a Diarylethene in a Single-Crystalline Phase
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9637
Photoirradiation for the X-ray crystallographic analysis was carried
out with a USHIO 500-W Xe lamp. Linearly polarized monochromatic
440 nm light was obtained by passing light through a cutoff filter, a
monochromator (Ritsu MC-20L) and a Gram-Tompson polarizer. For the
photocyclization reaction, the (011) face was irradiated with the linearly
polarized 440 nm light. The direction of polarization of irradiation light
was selected parallel to the a-axis and normal to the (011) face.
Irradiation was continued for 73 h. For the cycloreversion reaction,
the crystal was exposed to nonpolarized 578 nm light for 1 h.
Acknowledgment. This work was supported by CREST
(Core Research for Evolutional Science and Technology) of
Japan Science and Technology Corporation (JST).
Supporting Information Available: Tables of X-ray struc-
tural data for (S)-1a, (R)-1a, and photoirradiated (S)-1a (PDF).
An X-ray crystallographic file in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA001350O