Diastereoselective photochromism of a bisbenzothienylethene governed by steric as well as electronic interactions

Article abstract of DOI:10.1021/ja029535d

Introduction of an asymmetric center to C-2 of one of the benzothiophene rings of bisbenzothienylperfluorocyclopentene results in a highly diastereoselective photochromic system. The stereogenic center bears a hydrogen atom, a methyl group, and a methoxymethoxy group. The steric as well as the electronic repulsions gave an 87-88% diastereomer excess in various solvents at room temperature with 80-85% conversion to the colored form. The enantioselective synthesis was also carried out. Upon photoirradiation in hexane, a change in optical rotation at 820 nm, where neither the open form nor the colored form absorbs light, was observed repeatedly. Copyright

Full text of DOI:10.1021/ja029535d

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Published on Web 05/23/2003
Diastereoselective Photochromism of a Bisbenzothienylethene Governed by
Steric as Well as Electronic Interactions
Yasushi Yokoyama,*,† Hidenori Shiraishi,† Yutaka Tani,† Yayoi Yokoyama,‡ and
Yoshitaka Yamaguchi†
Department of AdVanced Materials Chemistry, Graduate School of Engineering, Yokohama National UniVersity,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501 Japan, and Department of Home Economics, Faculty of Home
Economics, Tokyo Kasei Gakuin UniVersity, 2600 Aihara-cho, Machida, Tokyo 194-0292 Japan
Received December 1, 2002; E-mail: yyokoyam@ynu.ac.jp
Diarylethenes are known as one of the important classes of
thermally irreversible photochromic compounds.1 The photochromic
reaction of diarylethenes is, like another important class of thermally
irreversible fulgides,2 based on the photochemically induced con-
rotatory 6π-electrocyclization of the 1,3,5-hexatriene moiety, which
takes a C2-symmetric helical structure because of the steric
congestion. As a result, the photocyclization reaction produces a
racemic mixture of the colored closed-form (C-form), possessing
two new stereogenic carbon atoms.
With this expectation in mind, we designed and synthesized 1O
as depicted in Scheme 1, possessing a hydrogen atom, a methyl
group, and a methoxymethoxyl (MOMO) group on the stereogenic
carbon atom on C-2 of one of the benzothiophenes.
Scheme 1. Synthesis of Racemic 1Oa
If some intermolecular or intramolecular stereoregulating force
works on a diarylethene molecule to make one of the enanitiomeric
helical structures more stable than the other, then the ratio of the
stereoisomers of the C-form molecules becomes biased. If such a
stereoselective photochromic reaction is possible, it can be a quite
useful switch of functions for biological as well as materials
sciences.2 Indeed, there have been reported a number of stereo-
selective photochromic reactions.3 As for diarylethenes, several
attempts have been reported so far. However, they were effective
only at low temperature,4 in crystals at low conversion,5 or in
reactions that required some additives.6
a (a) BuLi, CH3CHO, 84%, then CH3OCH2Cl, NaH, 90%; (b) I2, HIO3,
38%; (c) BuLi, perfluorocyclopentene, 85%; (d) 3-lithio-2-methylbenzo-
thiophene, 37%.
We here report 1O, a highly diastereoselective photochromic
bisbenzothienylethene possessing only one stereogenic center in
the molecule, which works efficiently at room temperature, in
various solvents, with high conversion, and without any additives.
Our strategy of stereoregulation is to use allylic 1,3-strain7 as
well as electronic repulsion between the substituents. Allylic strain
has been used in various aspects of organic synthesis.8 When it is
applied to the terminal part of the ring-closing hexatriene of a
diarylethene, the smallest substituent, such as hydrogen, faces the
bulky hexafluorocyclopentene that locates cis to the stereogenic
carbon atom. Accordingly, two different substituents (medium sized
M and largest L) hang down in the rather wide space as shown in
Chart 1. The other benzothiophene would take the position close
In the 1H NMR spectrum of 1O in CDCl3 at room temperature,
four sets of doublets of the secondary methyl group of the
conformational isomers (δ 0.23, 1.00, 1.45, and 1.53, in the ratio
of ca. 65:15:15:5) were observed. These were generated from the
combination of anti-parallel and parallel conformations of the
hexatriene moiety and the position of the second benzothiophene
group (i.e., close to M or close to L). The predominant conforma-
tional isomer showed the doublet methyl peaks at δ 0.23, suffering
a strong magnetic anisotropy, suggesting that the allylic strain
controlled conformation mentioned above might be the most stable.
When racemic 1O was irradiated with 313-nm light in hexane
at room temperature, two diastereomeric 1C were formed. An
HPLC analysis proved that the diastereomer excess (de) was 87%,
with the conversion ratio to the C-form of 80%. It returned to 1O
completely by 519-nm light irradiation. In other solvents such as
ethyl acetate (88% de, 85% conversion) and toluene (88% de, 85%
conversion), the de values were similarly high. The changes in
absorption spectra during 313-nm light irradiation to give the
photostationary state (pss) and during 519-nm light irradiation to
the pss solution in hexane are available as Supporting Information.
Optical resolution of 1C was successfully done with an HPLC
equipped with Daicel OD-H column. Although all trials to obtain
the single crystals of 1C were unsuccessful because of its rather
amorphous-like nature, crystals suitable for X-ray crystallographic
analysis of enantiomerically pure 1O were obtained after visible-
light irradiation to one of the resolved enantiomer of 1C. Its ORTEP
drawing is shown in Figure 1.9
Chart 1
to M rather than close to L because of steric repulsion, resulting in
the diastereomerically biased ratio of the helical conformers. If L
has heteroatoms, their electronic repulsion with the sulfur atom on
the second benzothiophene would make the conformer ratio more
biased.
† Yokohama National University.
‡ Tokyo Kasei Gakuin University.
9
7194
J. AM. CHEM. SOC. 2003, 125, 7194-7195
10.1021/ja029535d CCC: $25.00 © 2003 American Chemical Society

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
C O M M U N I C A T I O N S
If the change in optical rotation at the wavelength outside the
absorption of both isomers is large, then we can detect the state of
the photochromic system without inducing the photoreaction.3a The
change in specific optical rotation values by photochromic reactions
is examined for (S)-1O in hexane at 820 nm, where both isomers
have no absorbance. It was -144° for the open form, whereas it
was -50° at pss. As the O-form may take the helical structure, the
value is larger than that of the planar C-form possessing three
asymmetric carbon atoms. As the diarylethene prepared from the
CBS-oxazaborolidine route showed +137° for the O-form and +48°
for pss, it was proved that it is the enantiomer of the resolved 1.
In conclusion, we have succeeded to obtain a highly diastereo-
selective photochromic diarylethene system only by introducing one
asymmetric center, employing the steric as well as the electronic
interactions. The synthesis includes enantioselective preparation of
1O. By photoirradiation, a change in optical rotation at 820 nm,
where neither 1O nor 1C absorbs light, was observed repeatedly.
Figure 1. ORTEP drawing of (S)-1O (30% probability). Numbers shown
in the figure correspond to the explanations in the text below.
The crystal structure of 1O revealed that (1) the allylic strain is
nicely working around the relevant double bond on the benzo-
thiophene, so that the hydrogen atom on the stereogenic center is
facing the perfluorocyclopentene ring; (2) the second benzo-
thiophene resides close to the methyl group; (3) the hexatriene
moiety takes the s-cis-cis-s-cis (i.e., anti-parallel) conformation
so that the photochemical ring closure can occur easily; (4) the
absolute configuration of the stereogenic center of this enantiomer
was determined to be S by the Flack parameter9 (0.1(1)) of the
X-ray analysis.
Acknowledgment. We thank The Ministry of Education,
Science, Sports, and Culture of Japan for financial support through
Grant-in-Aids for Scientific Research on Priority Areas (A)
“Molecular Synchronization for Design of New Materials System”
and “Fundamental Science and Technology of Photofunctional
Interfaces”. The authors thank Dr. Kimiko Kobayashi, RIKEN,
Wako, Japan, for helpful discussions on the results of X-ray
crystallographic analysis.
To avoid the optical resolution during the synthesis, we tried to
synthesize optically enriched 1O by the enantioselective reduction
of a ketone to generate the asymmetric center. We employed (S)-
2-methyl-CBS-oxazaborolidine reduction10 of 2-acetylbenzothiophene,
which is known to give the R configuration from aromatic alkyl
ketones. The reduction gave (R)-2-hydroxyethylbenzothiophene in
100% yield and 96.5% ee (by HPLC) for 800-900 mg of starting
ketone (Scheme 2). Completion of the synthesis gave (R)-1O (vide
infra).
Supporting Information Available: Crystallographic data of (S)-
1O, change in absorption spectra during photoreactions, and experi-
mental details of the synthesis of 1O with characterization data. This
material is available free of charge via Internet at http://pubs.acs.org.
References
(1) Irie, M. Chem. ReV. 2000, 100, 1685-1716.
(2) Yokoyama, Y. Chem. ReV. 2000, 100, 1717-1739.
(3) (a) Yokoyama, Y.; Uchida, S.; Yokoyama, Y.; Sugawara, Y.; Kurita, Y.
J. Am. Chem. Soc. 1996, 118, 3100-3107. (b) Yokoyama, Y.; Sagisaka,
T.; Yamaguchi, Y.; Yokoyama, Y. Chem. Lett. 2000, 220-221. (c)
Yokoyama, Y.; Okuyama, T.; Yokoyama, Y.; Asami, M. Chem. Lett. 2001,
1112-1113. (d) Miyashita, A.; Iwamoto, A.; Kuwayama, T.; Shitara, H.;
Aoki, Y.; Hirano, M.; Nohira, H. Chem. Lett. 1997, 965-966.
(4) Yamaguchi, T.; Uchida, K.; Irie, M. J. Am. Chem. Soc. 1997, 119, 6066-
6071.
Scheme 2. Synthesis of (R)-1Oa
(5) (a) Kodani, T.; Matsuda, K.; Yamada, T.; Kobatake, S.; Irie, M. J. Am.
Chem. Soc. 2000, 122, 9631-9637. (b) Matsuda, K.; Yamamoto, S.; Irie,
M. Chem. Lett. 2001, 7291-7293.
(6) Murguly, E.; Norsten, T. B.; Branda, N. R. Angew. Chem., Int. Ed. 2001,
40, 1752-1755.
(7) (a) Johnson, F. Chem. ReV. 1968, 68, 375-413. (b) Hoffmann, R. W.
Chem. ReV. 1989, 89, 1841-1860.
(8) (a) Yokoyama, Y.; Kawashima, H.; Masaki, H. Chem. Lett. 1989, 453-
456. (b) Yokoyama, Y.; Kawashima, H.; Kohno, M.; Ogawa, Y.; Uchida,
S. Tetrahedron Lett. 1991, 32, 1479-1482. (c) Yokoyama, Y.; Tsuchikura,
K. Tetrahedron Lett. 1992, 33, 2823-2824.
(9) See the Supporting Information.
(10) (a) Corey, E. J.; Bakshi, R. K.; Shibata, S. J. Am. Chem. Soc. 1987, 109,
5551-5553. (b) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998,
37, 1986-2012.
a (a) (S)-2-Methyl-CBS-oxazaborolidine, THF, -23 °C, 100%, 96.5%
ee; (b) Br2, methyloxirane, THF, then, CH3OCH2Cl, NaH, THF, 87%; (c)
BuLi, THF, then A, 61%.
JA029535D
9
J. AM. CHEM. SOC. VOL. 125, NO. 24, 2003 7195