3,4-Bis(5-iodo-2-methylthien-3-yl)-2,5-dihydrothiophene: A powerful synthon for the preparation of photochromic dithienylethene derivatives

Article abstract of DOI:10.1055/s-2006-933108

A dithienylethene synthon, 3,4-bis-(5-iodo-2-methylthien-3-yl)-2,5- dihydrothiophene, was prepared and used as a starting material for the preparation of several photochromic dithienylethene derivatives via Suzuki cross-coupling reactions. The dithienylethene synthon was stable once stored in the dark at room temperature. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-933108

LETTER
737
3,4-Bis(5-iodo-2-methylthien-3-yl)-2,5-dihydrothiophene: APowerfulSynthon
for the Preparation of Photochromic Dithienylethene Derivatives
A
P
owerful Syntho
a
n for the
P
n
reparation of Photo
X
chromic Dithieny
i
e
D
erivative
s
, De Xing Zeng, Yi Chen*
Technical Institute of Physics and Chemistry, The Chinese Academy of Science, Beijing 100101, P. R. of China
Fax +86(10)64879375; E-mail: yichen@mail.ipc.ac.cn
Received 21 November 2005
dithienylethene derivatives is still attracting considerable
Abstract: A dithienylethene synthon, 3,4-bis-(5-iodo-2-methylth-
ien-3-yl)-2,5-dihydrothiophene, was prepared and used as a starting
material for the preparation of several photochromic dithien-
ylethene derivatives via Suzuki cross-coupling reactions. The
attention. We reported here the design and synthesis of a
power synthon 3,4-bis-(5-iodo-2-methylthien-3-yl)-2,5-
dihydrothiophene (2a), which was stable at room temper-
dithienylethene synthon was stable once stored in the dark at room ature in the dark and could be used to prepare photochro-
temperature.
mic diethienylethene derivatives via Suzuki cross-
coupling reaction.5 Although the preparation of diaryl-
Key words: dithienylethene, photochromism, Suzuki coupling
ethene derivatives employing the Suzuki coupling reac-
tion has been studied before,6 the diarylethene boronic
acid reagent, obtained from the lithiation of halogenated
Photochromic dithienylethenes are one of the most prom-
diarylethene with n-BuLi/THF followed by treatment
ising organic photochromic compounds for photoelec-
with B(OBu)3, was found to be unstable and easily hydro-
tronic applications because of their fatigue resistance and
thermal irreversible properties.1 The design and synthesis
of photochromic dithienylethenes have always been an in-
tensely studied research area because of their widespread
use in photon device applications such as memory media2
and optical switching.3 Although many methodologies for
the preparation of dithienylethene derivatives have been
reported,4 the development of facile synthetic routes to
lyzed. As a result, the diarylethene derivatives could only
be obtained in poor yields. The use of our synthon 2a,
could alleviate this problem and enables us to prepare a
series of dithienylethene derivatives 3a–9a.
Compound 2a was obtained in 63% yield by halogen ex-
change of the known dichloride 1a9 with iodine in the
presence of n-BuLi.10 Monoiodinated and dechlorinated
S
S
S
S
S
S
S
S
S
8a (78%)
3a (81%)
B(OH)2
7a (82%)
B(OH)2
B(OH)2
S
S
S
1) n-BuLi
B(OH)2
2) I2
S
S
Cl
Cl
I
S
S
I
S
S
9a (87%)
1a
2a (63%)
(HO)2B
OMe
S
B(OH)2
O2N
B(OH)2
S
S
S
NO2
S
O2N
S
S
S
S
S
S
S
OMe
MeO
4a (84%)
6a (80%)
5a (88%)
Scheme 1
SYNLETT 2006, No. 5, pp 0737–0740
1
7.
0
3.
2
0
0
6
Advanced online publication: 09.03.2006
DOI: 10.1055/s-2006-933108; Art ID: W09205ST
© Georg Thieme Verlag Stuttgart · New York

738
N. Xie et al.
LETTER
dithienylethene by products were also detected and isolat- colorless. Similar results were obtained when other di-
ed in 16% and 10% yields, respectively. Compound 2a thienylethenes were irradiated with UV light and visible
was stable at ambient temperature in the dark and exhibit- light. The photophysical data of the ring-open and ring-
ed photochromic behavior in solution.
closed isomers are listed (Table 1).
Treatment of 2a with various commercially available
boronic acid reagents using the Suzuki coupling reaction
afforded compounds 3a–8a in 78–88% yield (Scheme 1).
Compound 9a, on the other hand, was prepared from
fluorene. Hence, fluorene was brominated at the 2-posi-
tion by NBS in distilled propylene carbonate, followed by
dimethylation at the 9-position with iodomethane in
DMSO. 9,9-Dimethylfluorene-2-boronic acid, obtained
from 2-bromo-9,9-dimethylfluorene by n-BuLi lithiation
followed by treatment with B(OBu)3, was coupled to our
synthon 2a using Pd(PPh3)4 as the catalyst to produce
compound 9a in 87% yield. The structural identities of
compounds 2a–9a were characterized by spectroscopic
methods.11,12
S
S
UV
VIS
S
S
S
S
7a
7b
Scheme 3
Carbon–carbon bond forming reactions between synthon
2a and other organometallic reagents such as an organo-
magnesium and organostannane reagent were also ex-
plored (Scheme 2). Synthon 2a was efficient for both
Kumada7 and Stille coupling reactions.8 Hence, treatment
of 2a with anisylmagnesium bromide using Ni(dppp)Cl2
as a catalyst gave dithienylethene derivative 4a in 76%
yield. Alternatively, compound 2a reacted with anisyl-
stannane in the presence of Pd(PPh3)4 furnished the same
derivative 4a in 81% yield. In comparison to other report-
ed protocols, the advantages of our method presented in
this paper are: 1) better stability of our synthon, 2) excel-
lent product yield, and 3) wider applicability to various
coupling reactions.
Figure 1 Absorption changes of 7a (5 × 10–5 mol/L, MeCN) with
UV light (313 nm) irradiation
Table 1 UV-VIS Data of Compounds in Acetonitrile
Compd
l
max (nm, open)
lmax (nm, closed)
2
3
4
5
6
7
8
9
271
296
300
270
320
320
306
335
470
524
528
524
542
545
503
553
S
S
MgBr
Ni(dppp)Cl2
+
S
S
I
I
S
S
OMe
OMe
MeO
MeO
2a
4a (76%)
S
S
SnBu3
OMe
Pd(PPh3)4
+
S
S
I
I
S
S
OMe
2a
4a (81%)
Scheme 2
The fatigue resistant experiments were carried out in the
presence of a low-pressure mercury lamp (30 W) and a
xenon light (500 W), equipped with different wavelength
filters, as light sources for photocoloration and photo-
bleaching, respectively. The experiments showed that
compound 6a possessed excellent fatigue resistance; no
degradation was detected after 10 cycles of UV/Vis
radiation. For compounds 5a, 7a, and 9a, only a 5% de-
crease in absorption (optical density) was detected after
10 cycles. There was about a 10% decrease in absorption
for compounds 3a and 8a, and about 20% decrease in ab-
sorption for 4a, after 10 cycles. In addition, preliminary
All prepared dithienylethene derivatives underwent ring-
opening and ring-closing photoisomerization in solution
under UV/Vis irradiation. The photochromism of 7a in
acetonitrile was studied (Scheme 3 and Figure 1). Upon
irradiation with UV light, the absorption band at 320 nm,
which could be attributed to the ring-open isomer 7a, de-
creased in intensity; at the same time, two new bands at
370 nm and 545 nm, which corresponded to the ring-
closed isomer 7b, appeared. The two new bands at 545 nm
and 370 nm gradually disappeared when the sample was
subjected to visible light (>400 nm) irradiation; this pro-
cess was accompanied by a change of color from violet to
Synlett 2006, No. 5, 737–740 © Thieme Stuttgart · New York

LETTER
A Powerful Synthon for the Preparation of Photochromic Dithienylethene Derivatives
739
(7) (a) Tamao, K.; Sumitani, K.; Kumada, M. J. Am. Chem. Soc.
1972, 94, 4374. (b) Juang, H.; Nolan, S. P. J. Am. Chem.
Soc. 1999, 121, 9889. (c) Bohm, V. P. W.; Weskamp, T.;
Gstottmayr, C. W. K.; Herrmann, W. A. Angew. Chem. Int.
Ed. 2000, 39, 1602.
(8) (a) Andersen, N. G.; Keay, B. A. Chem. Rev. 2001, 101,
997. (b) Powell, D. A.; Maki, T.; Fu, G. C. J. Am. Chem. Soc.
2005, 127, 510. (c) Dubbaka, S. R.; Vogel, P. J. Am. Chem.
Soc. 2003, 125, 15292. (d) Gallagher, W. P.; Maleczka, R.
E. Jr. J. Org. Chem. 2005, 70, 841.
investigations showed that all compounds were thermally
stable at ambient temperature.
In summary, a photochromic synthon, 3,4-bis-(5-iodo-2-
methylthien-3-yl)-2,5-dihydrothiophene 2a, was prepared
and used in the preparation of photochromic dithien-
ylethene derivatives. All prepared photochromic deriva-
tives exhibited ring-opening and ring-closing photo-
isomerization under UV/Vis irradiation with good fatigue
resistance.
(9) Chen, Y.; Zeng, D. X.; Fan, M. G. Org. Lett. 2003, 5, 1435.
(10) Preparation of Synthon 2a: To a cooled (–50 °C) solution
of 3,4-bis-[2,2¢-dimethyl-5,5¢-(dichlorodithienyl)]-2,5-
dihydrothiophene 1a9 (2 mmol, 0.7 g) in THF (50 mL), n-
BuLi was added dropwise (1.6 M in hexane; 3.6 mL) under
nitrogen. The mixture was stirred for 0.5 h at –50 °C and
then cooled to –78 °C. To the cooled mixture (–78 °C) I2 (6
mmol, 1.5 g) was added in THF (5 mL), the reaction mixture
was stirred for 1 h, then the temperature was allowed to rise
to –20 °C. H2O (50 mL) was then added to the residue and
the product was extracted with Et2O (3 × 20 mL). The
combined organic phase was washed with a solution of
NaHSO3 (30%; 30 mL), H2O (30 mL), a sat. solution of
NaCl (30 mL), and dried over MgSO4. After evaporation of
the solvent, the crude product was purified by flash column
chromatography (PE–EtOAc, 20:1). Yield: 63%; mp > 200
°C. MS (EI): m/z (%) = 530 (M+, 100), 276 (20), 71 (31), 43
(48). HRMS: m/z calcd for C14H12I2S3 [M+]: 530.2468;
found: 530.2461.
(11) Boronic Acid Reagents: The corresponding boronic acid
reagents for the preparation of diarylethene compounds 3a–
8a were purchased from ACROS. The corresponding
boronic acid reagent for preparation of diarylethene
compound 9a was synthesized as follows. A mixture of NBS
(5.34 g, 30 mmol) and fluorene (4.98 g, 30 mmol) in distilled
propylene carbonate (40 mL) was warmed in a 60 °C bath for
2 min. As the solution warmed the mixture turned orange
and the solid dissolved. After stirring for 30 min at r.t. the
color turned to yellow, and a negative starch-iodide test was
obtained. The solution then was poured into H2O (0.5 L), the
precipitate was filtered, and crystallization from EtOH gave
6.7 g of 2-bromofluorene (91%). To a solution of DMSO (50
mL) and NaOH (4.0 g, 0.1 mol) was added 2-bromofluorene
(2.45 g, 0.01 mol), the mixture was stirred for 1 h at r.t..
Iodomethane (7.1 g, 0.05 mol) in DMSO (10 mL) was then
added to the above solution. The mixture was stirred for 4 h
at r.t. When no 2-bromofluorene was detected by TLC
anymore, the mixture was poured into ice water (100 mL)
and the product was extracted with CHCl3 (3 × 20 mL). The
combined organic phase was washed with a sat. solution of
NaCl and dried over MgSO4. After evaporation of the
solvent, the crude product was purified by flash column
chromatography (PE) to afford 2-bromo-9,9-dimethyl-
fluorene in 80% yield. To a solution of 2-bromo-9,9-
dimethylfluorene (1.36 g, 5 mmol) in THF (100 mL), n-BuLi
(1.6 M in hexane; 8 mL) was added slowly at –50 °C. The
mixture was stirred for 30 min at –50 °C, followed by slow
addition of B(OBu)3 (1 mL) in THF (10 mL) at –78 °C. The
mixture was stirred for 5 h, during this time the temperature
was increased slowly to 0 °C. After no 2-bromo-9,9-
dimethylfluorene was detected by TLC, the mixture was
poured into HCl solution (1 M; 100 mL), the product was
extracted with CHCl3 (3 × 20 mL). The combined organic
phase was washed with a sat. solution of NaCl and dried over
MgSO4. After evaporation of the solvent, the crude product
was purified by flash column chromatography (PE–EtOAc,
2:1) to afford 9,9-dimethylfluorene-2-boronic acid in 60%
Acknowledgment
This work was supported by the National Science Foundation of
China (No. 60277001 and No. 60337020).
References and Notes
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m/z calcd for C26H20N2O4S3 [M+]: 520.6520; found:
520.6526. Anal. Calcd for C26H20N2O4S3: C, 59.98; H, 3.87;
N, 5.38; S, 18.48. Found: C, 59.91; H, 3.82; N, 5.41; S,
18.54.
yield. 1H NMR (300 MHz, CDCl3): d = 8.34 (t, 1 H, J1 = 7.8
Hz, J2 = 7.9 Hz), 7.92 (d, 1 H, J = 7.7 Hz), 7.87–7.73 (m, 2
H), 7.53–7.40 (m, 3 H), 2.64 (s, 2 H), 1.64 (s, 3 H), 1.52 (s,
3 H).
(12) Diarylethene Derivatives 3a–9a (Suzuki Coupling
Reaction); General Procedure: To a solution of 2a (270
mg, 0.5 mmol) in anhyd THF (15 mL), Pd(PPh3)4 (40 mg, 3
mol%) and a solution of K2CO3 solution (2.5 M; 2 mL) were
added The resulting solution was stirred for 10 min at
ambient temperature. To the resulting solution was added the
corresponding boronic acid reagent (1.5 mmol) under
nitrogen at r.t. The resulting solution was refluxed under
nitrogen and the reaction was monitored by TLC. After the
starting material 2a disappeared, H2O (20 mL) was added to
the residue, and the product was extracted with Et2O (3 × 20
mL). The combined organic phase was washed with H2O (30
mL) and a sat. solution of NaCl (30 mL), respectively, and
dried over MgSO4. After evaporation of the solvent, the
crude product was purified by flash column chromatography
(PE–EtOAc).
6a: Yield: 80%; mp 123–125 °C. 1H NMR (300 MHz,
CDCl3): d = 7.19 (d, 2 H, J = 6.0 Hz), 7.07 (d, 2 H, J = 2.8
Hz), 6.99 (t, 2 H, J1 = 3.8 Hz, J2 = 3.7 Hz), 6.90 (s, 2 H), 4.17
(s, 4 H), 1.99 (s, 6 H). 13C NMR (400 MHz, CDCl3): d =
137.3, 135.2, 134.0, 133.7, 133.1, 127.8, 124.1, 124.0,
123.3, 42.8, 14.3. MS (EI): m/z (%) = 442 (M+, 100), 381
(50), 190 (22), 171 (23), 127 (30), 45 (18). HRMS: m/z calcd
for C22H18S5 [M+]: 442.7142; found: 442.7146. Anal. Calcd
for C22H18S5: C, 59.69; H, 4.10; S, 36.21. Found: C, 59.72;
H, 4.15; S, 36.28.
7a: Yield: 82%; mp > 200 °C. 1H NMR (300 MHz, CDCl3):
d = 7.62–7.56 (m, 12 H), 7.45 (t, 4 H, J1 = 7.4 Hz, J2 = 7.5
Hz), 7.36 (d, 2 H, J = 7.2 Hz), 7.01 (s, 2 H), 4.23 (s, 4 H),
2.04 (s, 6 H). 13C NMR (400 MHz, CDCl3): d = 140.9, 140.5,
140.0, 135.8, 134.4, 133.1, 128.8, 127.8, 127.5, 127.4,
126.9, 125.7, 123.5, 43.0, 14.5. MS (EI): m/z (%) = 582 (M+,
85), 581 (M+ – 1, 100), 529 (21), 235 (26), 174 (81), 157
(56), 145 (50), 130 (84), 78 (38), 53 (71). HRMS: m/z calcd
for C38H30S3 [M+]: 582.8530; found: 582.8534. Anal. Calcd
for C38H30S3: C, 78.31; H, 5.19; S, 16.50. Found: C, 78.38;
H, 5.16; S, 16.58.
3a: Yield: 81%; mp 165–167 °C. 1H NMR (300 MHz,
CDCl3): d = 7.51 (d, 4 H, J = 6.0 Hz), 7.35 (t, 4 H, J1 = 7.2
Hz, J2 = 7.6 Hz), 7.26 (d, 2 H, J = 7.1 Hz), 7.04 (s, 2 H), 4.21
(s, 4 H), 2.01 (s, 6 H). 13C NMR (400 MHz, CDCl3): d =
140.4, 135.7, 134.4, 134.2, 133.1, 128.9, 127.5, 125.4,
123.5, 43.0, 14.4. MS (EI): m/z (%) = 430 (M+, 100), 369
(40), 121 (36), 77 (8), 59 (10). HRMS: m/z calcd for
C26H22S3 [M+]: 430.6578; found: 430.6581. Anal. Calcd for
C26H22S3: C, 72.51; H, 5.15; S, 22.34. Found: C, 72.48; H,
5.17; S, 22.29.
8a: Yield: 78%; mp > 200 °C. 1H NMR (300 MHz, CDCl3):
d = 8.09 (d, 2 H, J = 8.2 Hz), 7.85 (dd, 4 H, J1 = 8.3 Hz,
J2 = 8.2 Hz), 7.51–7.34 (m, 8 H), 6.96 (s, 2 H), 4.27 (s, 4 H),
2.27 (s, 6 H). 13C NMR (400 MHz, CDCl3): d = 138.0, 136.2,
134.0, 133.9, 133.6, 132.2, 131.6, 128.5, 128.4, 128.3,
127.9, 126.6, 126.0, 125.6, 125.2, 42.7, 14.3. MS (EI):
m/z (%) = 529 (M+ – 1, 100), 513 (15). HRMS: m/z calcd for
C34H26S3 [M+]: 530.7774; found: 530.7779. Anal. Calcd for
C34H26S3: C, 76.94; H, 4.94; S, 18.12. Found: C, 76.89; H,
4.98; S, 18.20.
4a: Yield: 84%; mp 174–175 °C. 1H NMR (300 MHz,
CDCl3): d = 7.43 (d, 4 H, J = 8.5 Hz), 6.91 (s, 2 H), 6.87 (d,
4 H, J = 8.5 Hz), 4.19 (s, 4 H), 3.83 (s, 6 H), 1.99 (s, 6 H).
13C NMR (400 MHz, CDCl3): d = 159.0, 140.2, 134.6, 134.2,
133.0, 127.1, 126.7, 122.5, 114.3, 55.4, 43.0, 14.3. MS (EI):
m/z (%) = 490 (M+, 32), 429 (13), 215 (16), 199 (18), 71
(58), 43 (100). HRMS: m/z calcd for C28H26O2S3 [M+]:
490.7094; found: 490.7090. Anal. Calcd for C28H26O2S3: C,
68.54; H, 5.34; S, 19.60. Found: C, 68.61; H, 5.29; S, 19.68.
5a: Yield: 88%; mp > 200 °C. 1H NMR (300 MHz, CDCl3):
d = 8.31 (s, 2 H), 8.10 (d, 2 H, J = 8.0 Hz), 7.78 (d, 2 H,
J = 7.7 Hz), 7.52 (t, 2 H, J1 = 7.0 Hz, J2 = 7.2 Hz), 7.15 (s, 2
H), 4.22 (s, 4 H), 2.01 (s, 6 H). 13C NMR (400 MHz, CDCl3):
d = 148.7, 137.8, 137.6, 135.7, 134.8, 133.4, 131.0, 129.9,
125.2, 121.7, 119.9, 43.0, 14.5. MS (EI): m/z (%) = 520 (M+,
28), 518 (100), 488 (59), 197 (23), 166 (20), 59 (18). HRMS:
9a: Yield: 87%; mp > 200 °C. 1H NMR (300 MHz, CDCl3):
d = 7.70 (t, 4 H, J1 = 7.9 Hz, J2 = 8.0 Hz), 7.52–7.44 (m, 4
H), 7.42 (t, 2 H, J1 = 7.7 Hz, J2 = 7.8 Hz), 7.35–7.31 (m, 4
H), 7.11 (s, 2 H), 4.24 (s, 4 H), 2.08 (s, 6 H), 1.48 (s, 12 H).
13C NMR (400 MHz, CDCl3): d = 154.4, 153.8, 140.9, 138.7,
138.6, 135.5, 134.5, 133.2, 130.5, 127.3, 127.1, 124.5,
123.5, 122.6, 120.4, 120.0, 119.6, 46.9, 42.9, 27.2, 14.5. MS
(EI): m/z (%) = 664 (M+ + 2, 60), 661 (M+ – 1, 100), 471
(24), 46 (28). HRMS: m/z calcd for C44H38S3 [M+]:
662.9822; found: 662.9827. Anal. Calcd for C44H38S3: C,
79.71; H, 5.78; S, 14.51. Found: C, 79.65; H, 5.81; S, 14.58.
Synlett 2006, No. 5, 737–740 © Thieme Stuttgart · New York