Bisarylindenols: Fixation of conformation leads to exceptional properties of photochromism based on 6π-electrocyclization

Article abstract of DOI:10.1039/c2cc35793c

Thermally irreversible photochromic 1-tert-butyl-substituted 2,3-bisthiazolylindenol has been synthesized. It showed perfect diastereoselectivity and high ring-closing quantum yield with high conversion ratio to the closed form. The collaborative interact

Full text of DOI:10.1039/c2cc35793c

View Online / Journal Homepage / Table of Contents for this issue
Featuring the research of Prof. Yasushi Yokoyama et al.
from the Photoresponsive Materials Chemistry Group,
Department of Advanced Materials Chemistry,
Yokohama National University, Yokohama, Japan.
As featured in:
Bisarylindenols: fixation of conformation leads to exceptional
properties of photochromism based on 6π-electrocyclization
Photochromic 1-t-butyl-2,3-bisthiazolyl-1-indenol was synthesized.
Upon 313-nm light irradiation in hexane, it showed 98% conversion
to the closed form with 100% diastereoselectivity as well as a
ring-closing quantum yield of 85%. The collaborative interaction
between two sets of intramolecular hydrogen bonds and the
steric restriction of a t-butyl group worked efficiently to fix its
conformation in favor of cyclization in a diastereoselective manner.
See Yasushi Yokoyama et al.,
Chem. Commun., 2012, 48, 11838.
www.rsc.org/chemcomm
Registered Charity Number 207890

Dynamic Article Links
ChemComm
View Online
Cite this: Chem. Commun., 2012, 48, 11838–11840
www.rsc.org/chemcomm
COMMUNICATION
Bisarylindenols: fixation of conformation leads to exceptional properties
of photochromism based on 6p-electrocyclizationw
Hatsune Ogawa,a Kazuya Takagi,a Takashi Ubukata,a Akiko Okamoto,b
Noriyuki Yonezawa,b Stephanie Delbaerec and Yasushi Yokoyama*a
Received 10th August 2012, Accepted 6th September 2012
DOI: 10.1039/c2cc35793c
Thermally irreversible photochromic 1-tert-butyl-substituted
2,3-bisthiazolylindenol has been synthesized. It showed perfect
diastereoselectivity and high ring-closing quantum yield with
high conversion ratio to the closed form. The collaborative
interaction of two intramolecular hydrogen bonds and the steric
restriction fixed the conformation in favour of cyclization in a
highly diastereoselective manner.
obey Woodward–Hoffmann rules to proceed in a conrotatory
manner with an s-cis–cis–s-cis configuration of the hexatriene
moiety to yield a cyclohexadiene skeleton with two stereogenic
sp3 carbon atoms, it is a good template to attain high stereo-
selectivity as well as high efficiency of the photochemical
reactions. We have already achieved high stereoselectivity of
up to 100% in cyclization reactions with several diarylethenes8j–l
with the aim of avoiding ground-state conformation of the
compound which would generate a minor stereoisomer. A high
quantum yield for the photocyclizations has also been achieved on
a different but related system by fixing the molecular conformation
in favor of cyclization.10b We report here that fixation of the
conformation can induce not only a high quantum yield for
photocyclization but also high stereoselectivity.
We have recently synthesized 1,10b an ethylene acetal of
2,3-bisthiazolylindenone 2, which showed a thermally irreversible
photochromic reaction to give its closed form 1C with a cyclization
quantum yield of 0.81. In the most stable conformation of 1 as
obtained by DFT calculations, we found that two sets of CH–N
interactions between two hydrogen atoms on the indenone acetal
part and two nitrogen atoms on the thiazole rings fix the molecule
in antiparallel conformation,12 which favors photocyclization.
As these interactions are obviously weak, we considered the
introduction of a stronger interaction in this photochromic
system to enhance the controlling ability of the conformation.
As 2 has a carbonyl group, nucleophilic alkylation will generate
the tertiary hydroxy group which can form a strong hydrogen
bond with the nitrogen atom on the nearest thiazole ring (3).
In addition, as the carbon atom bearing the hydroxy group in 3
is now stereogenic, the photochemical ring closure of 3 to give
3C will generate a pair of diastereomers. Judging from the close
proximity of the hydroxy group and the nitrogen atom, the
most favorable conformation will predominantly generate one
of the diastereomers (3Cmajor) (Scheme 1).
In contrast to ground-state synthetic organic reactions,1 it is not
easy to lead a photochemical reaction to one of several possible
stereoisomers without undergoing other incidents such as emission
of fluorescence, intersystem crossing, or thermal de-excitation,
since they proceed by way of excited states having substantially
higher energy. Photochemical reactions are, thus, less controllable
than the corresponding ground-state reactions.2
High photon economy (large quantum yield) as well as the
high stereoselectivity are required when photoreactions occur
in biological systems3 since a large quantum yield saves the
living cells from prolonged exposure to light, and the high
stereoselectivity creates a simple system with regard to the
different interactions of the photo-generated isomers of the
molecules and the biological chiral environment.
We have been carrying out investigations on photochromism
based on 6p-electrocyclization such as in fulgides4 and diaryl-
ethenes.5 There are several important properties for photochromic
compounds that the compounds should possess, such as high
fatigue durability,6 facile modulation of the absorption spectra
by accessible structural modifications,7 high stereoselectivity for
photochemical ring closure,8,9 and high photoreaction quantum
yields.10,11 As 6p-electrocyclization reactions of diarylethenes
a Graduate School of Engineering, Yokohama National University,
Tokiwadai, Hodogaya, Yokohama 240-8501, Japan.
E-mail: yyokoyam@ynu.ac.jp; Fax: +81 45-339-3934;
With these expectations in mind, we have synthesized four
bisthiazolylindenols, 3a–3d, and examined their diastereoselectivity
and quantum yields for photocyclization.
Tel: +81 45-339-3934
b Department of Organic and Polymer Materials Chemistry, Tokyo
University of Agriculture and Technology, Naka-machi, Koganei
184-8588, Japan. E-mail: yonezawa@cc.tuat.ac.jp
c Universite´ Lille Nord de France, UDSL, UMR CNRS 8516, BP83,
59006, Lille, France. E-mail: stephanie.delbaere@univ-lille2.fr
w Electronic supplementary information (ESI) available: Synthesis,
analysis of photoreactions by HPLC, 1H and 13C NMR spectra of
bisarylindenols, X-ray crystallographic analysis of 3b and DFT calcu-
lation results of 3b. CCDC 882504. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2cc35793c
Synthesis of bisthiazolylindenols 3a–3d was carried out by
nucleophilic addition of the corresponding alkyl- or phenyllithium
reagents to bisthiazolylindenone 2.10b Fortunately, X-ray crystallo-
graphic analysis of 3b successfully revealed its structure in the
crystalline state (see ESIw). The notable characteristics are the
existence of a hydrogen bond between the hydroxy group and
c
11838 Chem. Commun., 2012, 48, 11838–11840
This journal is The Royal Society of Chemistry 2012

View Online
Fig. 1 The change in absorption spectra of 3c by 313 nm light
irradiation in hexane. Concentration/mol dmÀ3: 0.450 Â 10À4 (hex-
ane). Light intensity/mW cmÀ2: 0.153. Irradiation time/min: 0, 0.25,
0.5, 1, 2, 4, 8, 16.
Scheme 1 Synthesis of the indenol family and their conformation-
controlled photochromism.
Table 1 Diastereomer ratio, conversion ratio to the closed form, and
quantum yields of photocyclization of indenols in solutiona
the nitrogen atom (OH–N distance: 215.7 pm), and a positive
hydrogen bond-like interaction between the phenyl hydrogen on
the indenol moiety and the other nitrogen atom (C(indenol Ph)-
H–N distance: 274.3 pm). These two interactions fix the conforma-
tion of 3b in favor of cyclization.
Indenol
Hexane
3a (Me)
3b (Ph)
3c (t-Bu)
3d (MOM)
94.9/5.1
97.6%
0.74
99.1/0.9
96.6%
0.78
499.9/o0.1
97.9%
0.85
90.8/9.2
98.4%
0.56
The presence of two conformers, hydrogen bonding and
non-hydrogen bonding, for 3b and 3c may be verifiable from
1H NMR. However, they showed only one set of signals in
several deuterated solvents (cyclohexane-d12, tetrahydrofuran-d8
and acetonitrile-d3; corresponding to the solvents used for the
photoreactions) at room temperature. IR data of the broad OH
stretching vibrations also did not provide any evidence of multiple
conformations.
Diethyl ether
CH3CN
75.8/24.2
97.6%
0.65
71.6/28.4
93.0%
0.49
92.0/8.0
97.3%
0.70
86.7/13.3
92.9%
0.60
99.8/0.2
97.0%
0.75
99.7/0.3
94.9%
0.64
74.6/25.4
97.2%
0.43
75.6/24.4
96.2%
0.44
a
Upper numbers: ratio of diastereomers. Middle numbers: conversion
percentage to the closed form at the photostationary state of 313 nm
light irradiation. Lower numbers: quantum yield for major closed
form generation by 313 nm light irradiation.
The indenols were then subjected to photochromic reactions
by irradiation with 313 and 514 nm light in hexane, diethyl
ether and acetonitrile (with different polarities) to see the effect
of the intramolecular hydrogen bonds. Typical absorption
spectral changes of 3c in hexane upon irradiation of 313 nm light
are shown in Fig. 1, and the diastereoselectivities, conversion
ratios to the closed form and quantum yields of the photocycliza-
tion of 3a–3d are summarized in Table 1.
The diastereoselectivity,13 shown in Table 1, is apparently
higher when the alkyl/aryl substituent is larger. This is because
the steric repulsion is enhanced for the less stable conforma-
tion when the substituent is large. This effect is the largest for
3c with a t-Bu group. When the photoreaction was carried out
in hexane, the minor diastereomer was not observed, although
tiny amounts were detected in diethyl ether and acetonitrile.
This is because the solvent molecules did not interfere with the
intramolecular hydrogen bond in hexane. As for 3d possessing
a methoxymethyl (MOM) group, it exhibited lower diaster-
eoselectivity than 3a having a methyl group, although the
MOM group is larger than the methyl group. This is inter-
preted in terms of a partial replacement of the hydrogen bond
(OH–N) by the other hydrogen bond (OH–OMe) forming a
five-membered ring. The latter hydrogen bond enables the
rotation of the single bond between the ‘‘right’’ thiazole ring
and the indenol part so that diastereoselectivity is significantly
lowered.
heteroatoms with the lone pair on the solvent molecules
interfered with the formation of intramolecular OH–N hydrogen
bonds so that the fixation of the conformation was not so
complete.
For 3c, the influence of the difference in solvent polarity is
the smallest (practically 100/0 in hexane, 99.8/0.2 and 99.7/0.3
in ether and acetonitrile, respectively) since the bulky t-Bu
group prevented 3c from adopting the less stable conformation,
irrespective of the solvent polarity. And yet, the least polar
hexane gave the highest diastereoselectivity.
Similar tendencies were observed for the cyclization quantum
yield of the indenols (Table 1). As the alkyl/aryl substituent
becomes larger, the cyclization quantum yield to form the major
closed form becomes larger. For 3c possessing a t-Bu group, it
showed the largest quantum yield of 3cCmajor generation.
In contrast, 3d, possessing a functional group capable of
forming another hydrogen bond, showed rather low cyclization
quantum yields. These results can be interpreted as follows: the
replacement of the OH–N hydrogen bond with the OH–OMe
hydrogen bond increases the amount of molecules with parallel
conformation12 which cannot cyclize in a conrotatory manner
upon light irradiation.
It should be noted that the conversion ratios of the open
forms to the closed forms upon 313 nm light irradiation are
exceptionally high for all of the indenols examined (Table 1)
Among the three solvents employed, hexane always led to
the highest diastereoselectivity. In ether or acetonitrile,
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11838–11840 11839

View Online
due to the large ring-closing quantum yields. As their ring-
opening reactions proceed entirely upon 514 nm light irradiation,
photochromism of the indenol family is a highly efficient system
in light of the switching of the molecular structure.
4 Y. Yokoyama, Chem. Rev., 2000, 100, 1717.
5 M. Irie, Chem. Rev., 2000, 100, 1685.
6 M. Irie, T. Lifka, K. Uchida, S. Kobatake and Y. Shindo, Chem.
Commun., 1999, 747.
7 (a) Y. Yokoyama, T. Tanaka, T. Yamane and Y. Kurita, Chem.
Lett., 1991, 1125; (b) K. Uchida and M. Irie, Chem. Lett., 1995,
969.
Judging from the three properties, i.e., the diastereoselectivity,
quantum yield of photocyclization, and conversion ratio to the
closed form, 3c having a t-Bu group can be considered the best
among all the compounds examined. With the collaboration
between the intramolecular OH–N and CH–N hydrogen bonds
and the sterically demanding t-Bu group, generation of the less
stable conformation was almost entirely prevented and the
molecules were fixed in the more stable antiparallel conformation,
which led to high diastereoselectivity. In addition, fixation of the
conformation also resulted in an increase in the cyclization
quantum yield to the highest level ( Z0.8) for 6p-electrocycliza-
tion of thermally irreversible photochromic compounds in
solution known so far.10,11 The stereogenic carbon atom on the
indenol part of 3c induced the helically chiral arrangement of two
thiazolyl groups by the two sets of hydrogen bonds. Upon UV
irradiation, when 3cCmajor is produced, two new stereogenic
carbon atoms are formed. As the nature of photochromic
compounds, the conversion between the helical chirality and the
point chirality of 3c can be repeated on demand with the alternate
irradiation of UV- and visible-light.
8 Representative publications from our group: (a) Y. Yokoyama,
S. Uchida, Y. Yokoyama, Y. Sugawara and Y. Kurita, J. Am.
Chem. Soc., 1996, 118, 3100; (b) Y. Yokoyama, S. Uchida,
Y. Yokoyama, T. Sagisaka, Y. Uchida and T. Inada, Enantiomer,
1998, 3, 123; (c) Y. Yokoyama, T. Sagisaka, Y. Yamaguchi,
Y. Yokoyama, J. Kiji, T. Okano, A. Takemoto and S. Mio, Chem.
Lett., 2000, 220; (d) Y. Yokoyama, T. Okuyama, Y. Yokoyama
and M. Asami, Chem. Lett., 2001, 1112; (e) Y. Yokoyama,
H. Shiraishi, Y. Tani, Y. Yokoyama and Y. Yamaguchi,
J. Am. Chem. Soc., 2003, 125, 7194; (f) Y. Yokoyama,
Chem.–Eur. J., 2004, 10, 4389; (g) M. Kose, M.
Shinoura, Y. Yokoyama and Y. Yokoyama, J. Org. Chem.,
2004, 69, 8403; (h) T. Okuyama, Y. Tani, K. Miyake and
Y. Yokoyama, J. Org. Chem., 2007, 72, 1634; (i) Y. Tani,
T. Ubukata, Y. Yokoyama and Y. Yokoyama, J. Org. Chem.,
2007, 72, 1639; (j) Y. Yokoyama, T. Shiozawa, Y. Tani and
T. Ubukata, Angew. Chem., Int. Ed., 2009, 48, 4521;
(k) T. Shiozawa, M. K. Hossain, T. Ubukata and Y. Yokoyama,
Chem. Commun., 2010, 46, 2262; (l) Y. Yokoyama, T. Hasegawa
and T. Ubukata, Dyes Pigm., 2011, 89, 223–229.
9 Representative publications from other groups: (a) T. Yamaguchi,
K. Uchida and M. Irie, J. Am. Chem. Soc., 1997, 119, 6066;
(b) T. Kodani, K. Matsuda, T. Yamada, S. Kobatake and
M. Irie, J. Am. Chem. Soc., 2000, 122, 9631; (c) K. Matsuda,
S. Yamamoto and M. Irie, Tetrahedron Lett., 2001, 42, 7291;
(d) S. Yamamoto, K. Matsuda and M. Irie, Org. Lett., 2003,
5, 1769; (e) T. Yamaguchi, K. Nomiyama, M. Isayama and M. Irie,
Adv. Mater., 2004, 16, 643; (f) M. Morimoto, S. Kobatake and
M. Irie, Chem. Commun., 2008, 335; (g) M. Takeshita and
T. Yamato, Angew. Chem., Int. Ed., 2002, 41, 2156;
(h) M. Takeshita and T. Yamato, Chem. Lett., 2004, 844;
(i) M. Takeshita and H. Jin-nouchi, Chem. Commun., 2010,
46, 3994; (j) J. J. D. de Jong, L. N. Lucas, R. M. Kellogg,
J. H. van Esch and B. L. Feringa, Science, 2004, 304, 278;
(k) K. Uchida, M. Walko, J. J. D. de Jong, S. Sukata,
S. Kobatake, A. Meetsma, J. van Esch and B. L. Feringa, Org.
Biomol. Chem., 2006, 4, 1002.
In conclusion, we have synthesized 3c, a fascinating compound
which shows both perfect diastereoselectivity and an extremely
high ring-closing quantum yield with a high conversion ratio to
the closed form in hexane. In this compound, the collaborative
interaction between two sets of intramolecular hydrogen
bonds and the steric restriction of a t-Bu group worked very
efficiently to fix its conformation in favor of cyclization in a
diastereoselective manner. These properties are highly beneficial
when photochromic compounds are applied to biological systems
for the switching of organic functions, or as probes for material
flow within cells.
10 Representative publications from our group: (a) J. Kiji, T. Okano,
H. Kitamura, Y. Yokoyama, S. Kubota and Y. Kurita, Bull.
Chem. Soc. Jpn., 1995, 68, 616; (b) K. Morinaka, T. Ubukata
and Y. Yokoyama, Org. Lett., 2009, 11, 3890.
We are grateful to Professor Keiichi Noguchi, Tokyo University
of Agriculture and Technology, for technical assistance and helpful
discussions on X-ray crystallographic analysis. This work was
supported by the Ministry or Education, Culture, Sports, Science
and Technology (MEXT) of Japan with Grants-in-Aid for Scien-
tific Research on Priority Area ‘‘New Frontiers in Photochromism’’
(471), and by Japan Society for the Promotion of Sciences (JSPS)
with Grant-in-Aid for Scientific Research (B) (23350096).
11 Representative publications from other groups: (a) S. Kawai,
T. Nakashima, Y. Kutsunugi, H. Nakagawa, H. Nakano and
T. Kawai, J. Mater. Chem., 2009, 19, 3606; (b) S. Fukumoto,
T. Nakashima and T. Kawai, Angew. Chem., Int. Ed., 2011,
50, 1565; (c) S. Fukumoto, T. Nakashima and T. Kawai, Eur. J.
Org. Chem., 2011, 5047; (d) T. Nakashima, R. Fujii and T. Kawai,
Chem.–Eur. J., 2011, 17, 10951; (e) S. Fukumoto, T. Nakashima
and T. Kawai, Dyes Pigm., 2012, 92, 868; (f) R. T. F. Jukes,
V. Adamo, F. Hartl, P. Belser and L. De Cola, Inorg. Chem., 2004,
43, 2779; (g) A. R. Santos, R. Ballardini, P. Belser, M. T. Gandolfi,
V. M. Iyer and L. Moggi, Photochem. Photobiol. Sci., 2009,
Notes and references
1 (a) Asymmetric Synthesis — The Essentials, ed. M. Mathias and
S. Brase, Wiley-VCH, Weinheim, 2nd edn, 2007; (b) E. J. Corey
¨
8, 1734; (h) S. Aloıse, M. Sliwa, Z. Pawlowska, J. Rehault,
´
¨
and A. Guzman-Perez, Angew. Chem., Int. Ed., 1998, 37, 388;
(c) T. Satyanarayana, S. Abraham and H. B. Kagan, Angew.
Chem., Int. Ed., 2009, 48, 456.
2 A. Albini, in Methods and Reagents for Green Chemistry: An
Introduction, ed. P. Tundo, A. Perosa and F. Zecchini, John Wiley
& Sons, Inc., Hoboken, NJ, USA, 2007, pp. 65–75.
3 (a) G. Mayer and A. Heckel, Angew. Chem., Int. Ed., 2006, 45, 4900;
(b) I. Willner, Acc. Chem. Res., 1997, 30, 347; (c) M.-Q. Zhu,
G.-F. Zhang, C. Li, M. P. Aldred, E. Chang, R. A. Drezek and
A. D. Q. Li, J. Am. Chem. Soc., 2011, 133, 365; (d) A. A. Beharry,
L. Wong, V. Tropepe and G. A. Wooley, Angew. Chem., Int. Ed.,
2011, 50, 1325; (e) T. Stafforst and D. Hilvert, Angew. Chem., Int.
J. Dubois, O. Poizat, G. Buntinx, A. Perrier, F. Maurel,
S. Yamaguchi and M. Takeshita, J. Am. Chem. Soc., 2010,
132, 7379; (i) G. Luchita, M. V. Bondar, S. Yao,
I. A. Mikhailov, C. O. Yanez, O. V. Przhonska, A. E. Masunov
and K. D. Belfield, ACS Appl. Mater. Interfaces, 2011, 3, 3559;
(j) J. Massaad, J.-C. Micheau, C. Coudret, R. Sanchez, G. Guirado
and S. Delbaere, Chem.–Eur. J., 2012, 18, 6568.
12 K. Uchida, Y. Nakayama and M. Irie, Bull. Chem. Soc. Jpn., 1990,
63, 1311.
13 We determined the diastereoselectivity with HPLC at the absorp-
tion maximum wavelength of the closed form. Although not
carried out here, selecting isosbestic points of the compounds in
the UV region for HPLC analyses is also a clear and valid method.
Ed., 2010, 49, 9998D. Vomasta, C. Hogner, N. R. Branda and
¨
B. Konig, Angew. Chem., Int. Ed., 2008, 47, 7644.
¨
c
11840 Chem. Commun., 2012, 48, 11838–11840
This journal is The Royal Society of Chemistry 2012