Thermal decomposition product of 1,2-bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene

Article abstract of DOI:10.1016/j.tetlet.2017.10.017

1,2-Bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene shows photochromism in n-hexane at room temperature. We have identified a new compound produced by the thermal decomposition in a mixture of cis- and trans-decahydronaphthalene above 100 °C in the a

Full text of DOI:10.1016/j.tetlet.2017.10.017

Tetrahedron Letters 58 (2017) 4447–4449
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Tetrahedron Letters
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Thermal decomposition product of 1,2-bis(2-methyl-1-benzofuran-3-yl)
perfluorocyclopentene
a
a
a
b
Tadatsugu Yamaguchi a, , Tsuyoshi Nakagawa , Toru Ozeki , Mitsuhiro Fukuda , Masakazu Morimoto ,
⇑
Shizuka Takami c
a Hyogo University of Teacher Education, 942-1 Shimokume, Kato, Hyogo 673-1494, Japan
b Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan
c Department of Environmentals Engineering, Niihama National College of Technology, 7-1 Yagumo-cho, Niihama, Ehime 792-8580, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
1,2-Bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene shows photochromism in n-hexane at room
temperature. We have identified a new compound produced by the thermal decomposition in a mixture
of cis- and trans-decahydronaphthalene above 100 °C in the absence of light.
Ó 2017 Elsevier Ltd. All rights reserved.
Received 21 August 2017
Revised 2 October 2017
Accepted 6 October 2017
Available online 6 October 2017
Keywords:
Photochromism
Diarylethene
Benzofuran
Decomposition product
X-ray crystallography
Diarylethene derivatives are organic photochromic compounds
with photofatigue resistance and thermal irreversibility.1–3 1,2-Bis
(2-methyl-1-benzothien-3-yl)perfluorocyclopentene shows pho-
tochromism in n-hexane.4 We have reported that 1,2-bis(2-alkyl-
with visible light. The amount of this yellow product increased
with time duration of heating at 100 °C. We were able to determine
the structure of 1b by conducting the following experiments.
1-benzofuran-3-yl)perfluorocyclopentenes
also
show
pho-
tochromism in n-hexane with photofatigue resistance and thermal
irreversibility.5 They are effective in bringing out certain proper-
ties, which cannot be achieved with some benzothiophene com-
pounds, such as photochromism in the single-crystal phase.6,7 In
a previous paper, we reported that 1,2-(2-methyl-1-benzofuran-
3-yl)perfluorocyclopentene (1a) is
a
fatigue-resistant pho-
tochromic compound that undergoes thermally irreversible pho-
tochromism in n-hexane at room temperature or in toluene at
70 °C for 3 h.5 Recently, the thermal stability of diarylethenes at
high temperatures has been actively studied.8,9 When we exam-
ined the thermal decay of compound 1a at 100 °C in the mixture
of cis- and trans-decahydronaphthalene (decalin), we found its
thermal decomposition product 1b.
When a colorless ring-opened form of 1a was dissolved in dec-
alin and heated at 100 °C in an electric dryer in a dark place, a yel-
low product different from the ring-closed form (1c) was obtained.
This yellow product does not lose its color even when irradiated
Compound 1a (440 mg) was heated in decalin at 120 °C for 48 h
in air to synthesize a yellow pyrolysis product with a yield of 66
mg. The pyrolysis product was purified by TLC and GPC, resulting
in one large single crystal weighing 33.1 mg. This crystal was dis-
⇑
Corresponding author.
E-mail address: tyamagu@hyogo-u.ac.jp (T. Yamaguchi).
https://doi.org/10.1016/j.tetlet.2017.10.017
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.

4448
T. Yamaguchi et al. / Tetrahedron Letters 58 (2017) 4447–4449
solved in deuterochloroform (CDCl3). Its structure was determined
by 1H NMR and 13C NMR spectroscopies. From the results,10 it was
found that it has a symmetric structure similar to the ring-closed
form. The MS (EI) spectral data from this product suggest that
the mass of the yellow product was greater than that of the ring-
closed structure (1c) by the mass of one oxygen atom.
From the chloroform solution in which the large single crystal
with a mass of 33.1 mg was dissolved, a crystal (1b) was obtained
for X-ray crystal structural analysis.11 An ORTEP diagram obtained
by X-ray crystal structural analysis is shown in Fig. 1. The oxygen
atom cross-linked the carbon atoms bearing the two methyl
groups in the benzofuran moieties of the ring-open form. The
cross-linked central ring had a cycloheptadiene structure, which
is unique in this diarylethene series. This compound is a product
of oxidation in air. In addition, the results of elemental analysis
and HR-MS (FAB) also supported this chemical structure. This
structure of the decomposition product is different from that of
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene pre-
viously reported.12
Table 1 shows the reaction condition, the yield of 1b, and the
recovery of 1a in decalin. The amount of the decomposition pro-
duct (1b) increased with the time duration of heating at 100 °C.
The yield of 1b reached about 11% after heating in decalin at
100 °C for 72 h. We conducted the reactions in oxygen-free condi-
tion. No decomposition products 1b were present in the reactions
using nitrogen bubbled decalin solution (entry 9 and 10 in Table 1).
The recovery of 1a was a 100% at 100 °C for 72 h.
formalism variant (IEFPCM) for all the calculations. Frequency was
also calculated for the three structures to confirm whether the
structure obtained is energy-minimized at the stationary point.
The absorption spectra of the three structures were obtained by
calculating singlet-excited states using the TD-DFT method in the
range from 200 nm to 600 nm. The difference in the calculated
absorption spectra between wB97XD and CAM-B3LYP was found
to be insignificant (see Supplementary data).
Fig. 3 shows the wavelengths of the absorption maximum and
estimated results corresponding to the ring-opened form (kmax
=
297.62 nm) (1a), the ring-closed form (1c), and the decomposition
product (1b) using CAM-B3LYP as a functional with 6-311+G(d,p)
as the basis set. The correspondence of the magnitude of the molar
extinction coefficient at the absorption maximum of the products
(kmax = 414.82 nm) (1b) to that of the ring-closed form (kmax
=
442.87 nm) (1c) was noted, but the division of the two absorption
bands of 1b cannot be estimated in the visible region. Results of
measurement of X-ray crystal structure and spectroscopic analysis
of one large single crystal show the absence of structural isomers
indicating that transition cannot be simply simulated by calcula-
tion. It may be appropriate to assign the microstructure in the vis-
ible bands to the decomposition product.
The absorption spectra of 1a (ring-opened form), 1c (ring-
closed form), and 1b (decomposition product) in n-hexane are
compared with the molar extinction coefficient on the vertical axis
(Fig. 2). The absorption of the decomposition product (1b) is at the
wavelength slightly shorter than the absorption maximum wave-
length of 469 nm (
(1c). The wavelengths of the absorption maximum were at 454 nm
454, 31,000 MÀ1 cmÀ1) and 432 nm in n-hexane. The decomposi-
e ) of the ring-closed form
469, 14,400 MÀ1 cmÀ1 5
(e
tion product (1b) has a large molar extinction coefficient and two
absorption bands in the visible region.
To determine the difference between 1b and 1c in the absorp-
tion band, their optimized structures were estimated by
calculation.
Time-dependent density functional theory (TD-DFT) calcula-
tions were performed using Gaussian 09W, Rev.D.01.13 We first
obtained optimized structures of 1a, 1b, and 1c using wB97XD or
CAMÀB3LYP as a functional with 6-311+G(d,p) as the basis set.
Recently, the use of long range corrected functionals such as
wB97XD and CAM-B3LYP has produced precise geometries as well
as better prediction of physicochemical properties than that of
B3LYP.14 The effect of solvation in n-hexane was realized using
the polarizable continuum model (PCM) with the integral equation
We synthesized the benzofuran derivative 2a,15 which has phe-
nyl moieties at the reactive carbon atoms to attempt that prevent
from any oxidation. Compound 2a has an absorption maximum at
277 nm (
e
277, 3.42 Â 104 MÀ1 cmÀ1) in n-hexane. Upon irradiation
with visible light, a visible absorption band was observed at 482
nm. It indicated the generation of the ring-closed form 2c (
e
,
482
1.80 Â 104 MÀ1 cmÀ1). The color of 2c disappeared after irradiation
with visible light (k > 440 nm). In a comparative experiment in
decalin at 100 °C for 48 h, no product of the decomposition product
(2b) was observed at all for the compound 2a containing phenyl
substituents at the reaction site (see Supplementary data).
There is a possibility that a decomposition product is formed
with a benzofuran derivative during reactions with an alkyl sub-
stituent at high temperatures in air. However, 1b is hardly pro-
duced by heating at least at 70 °C for 3 h even when 1a is
prepared in a solution. A sample of crystalline 1a obtained 15 years
ago5 by its purification for elemental analysis in a single crystal
state does not contain a yellow decomposition product. Therefore,
1a can be used without any problem if it is stored at room temper-
ature in the dark.
In conclusion, we confirmed by X-ray crystallography the struc-
ture of the decomposition products gradually formed in decalin
solution of 1,2-bis(2-methylbenzofuran-3-yl)perfluorocyclopen-
tene (1a) in air at 100 °C for 24 h or more. The decomposition pro-
duct had a ring-closed form and had a structure in which the
intermediate between the furan moieties were cross-linked at
Fig. 1. ORTEP diagram of 1b. The hydrogen atoms are omitted for clarity.

T. Yamaguchi et al. / Tetrahedron Letters 58 (2017) 4447–4449
4449
Table 1
Effects of time and temperature on thermal decomposition reaction.
Entry
Temp(°C)
Time(h)
Yield 1b (%)
Recovery 1a (%)
1
2
3
4
5
6
7
8
80
80
72
96
24
48
72
24
48
72
48
72
1
3
2
6
11
3
9
14
0
0
99
97
98
94
89
97
91
86
100
100
100
100
100
120
120
120
100
100
9a
10a
a
Oxygen-free conditions.
Fig. 3. Simulated absorption spectra of 1a (dashed line), 1b (solid line), and 1c
(dotted line) in n-hexane obtained using Gaussian 09W with CAM-B3LYP as a
functional with 6-311+G(d,p) as the basis set.
Fig. 2. Absorption spectra of 1a (dashed line), 1b (solid line), and 1c (dotted line) in
n-hexane.
the reaction carbon atoms attached to the methyl groups. The
cross-linked central ring had a cycloheptadiene structure.
5. Yamaguchi T, Irie M. J Org Chem. 2005;70:10323–10328.
6. Yamaguchi T, Irie M. Eur J Org Chem. 2006;3105–3111.
7. Yamaguchi T, Irie M. J Photochem Photobio A. 2006;178:162–169.
8. Shoji H, Kobatake S. Chem Commun. 2013;49:2362–2364.
Acknowledgments
9. Tanaka K, Kitagawa D, Kobatake S. Tetrahedron. 2016;72:2364–2368.
10. 1b: colorless crystals; mp. 147–148 °C, 1H NMR (400 MHz, CDCl3) d 1.46(s, 6H),
7.05(d, J = 8.4 Hz, 2H), 7.13(dt, J = 7.2 Hz, 1.2 Hz, 2H), 7.49(dt, J = 7.6 Hz, 1.2 Hz,
2H), 7.92(d, J = 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3) d 23.30, 23.31, 109.58,
111.69, 120.52, 122.83, 126.38, 126.43, 126.47, 126.53, 134.71, 148.64, 161.71.
MS (FAB) Calcd. for C23H14F6O3 452.0847: Found: m/z 452.0855(M+). Anal.
Calcd. for C23H14F6O3: C 61.07, H 3.12%. Found: C 61.36, H 3.24%.
11. Crystal data for 1b: C23H14F6O3, MW = 452.34, monoclinic, space group P21/c, a
This work was supported by a Grant-in-Aid for Science Research
(16K00962) from the Japan Society for Promotion Science.
A. Supplementary data
= 11.7629(2) Å, b = 9.1891(2) Å, c = 18.1562(3) Å,
a = 90°, b = 95.3928(10)°, c =
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2017.10.017.
90°, V = 1953.83(6) Å3, Z = 4, Dc = 1.538 g cmÀ3, R1 = 0.0578 for 3054 observed
reflections with I > 2r (I) from 3423 unique reflections. CCDC deposition
number: 1553827.
12. Irie M, Lifka T, Uchida K, Kobatake S, Shindo Y. Chem Commun. 1999;747–748.
13. Frisch MJ, Trucks GW, Schlegel HB, et al. Gaussian 09W, Revision
D.01. Wallingford, CT: Gaussian Inc; 2009.
14. Chen YT, Chen KHC, Fukuda M. J Comput Chem Jpn. 2017;16:1–16.
15. 2a: Colorless crystals; mp. 160–161 °C; 1H NMR (200 MHz) d 6.89–7.05(m,
10H), 7.10–7.20(m, 8H), 7.26–7.30(m, 2H). Anal. Calcd. for C33H18F6O2: C 70.72,
H 3.24%. Found: C 70.46, H 3.43%. MS (FAB) Calcd. for C33H18F6O2 560.1211.
Found: m/z 560.1205(M+).
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