Synthesis and photoisomerization of diarylcyclobutenes

Article abstract of DOI:10.1055/s-0030-1258435

Symmetrically and unsymmetrically substituted diarylcyclobutenes are synthesized in 20-70% yields from alkyne precursors via cobalt-catalyzed [2+2] cycloadditions. The reactions proceed under mild conditions and provide access to differently substituted diarylethene derivatives. All the diarylcyclobutene products undergo reversible photoisomerization upon irradiation with UV/Vis light. The ring-closed isomers show different thermal stabilities towards reisomerization with half-lives ranging from 9 to 300 hours. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1258435

PAPER
905
Synthesis and Photoisomerization of Diarylcyclobutenes
S
ynthesis and
e
Photoisome
t
rizatio
e
n
of
D
iarylc
r
yclobutenes Raster,a Stefan Weiss,a Gerhard Hilt,b Burkhard König*a
a
Fakultät für Chemie und Pharmazie, Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
Fachbereich Chemie, Philipps-Universität Marburg, 35037 Marburg, Germany
Fax +49(941)9431717; E-mail: Burkhard.koenig@chemie.uni-regensburg.de
b
Received 6 January 2011
and allows the simple preparation of symmetrically and
unsymmetrically substituted derivatives. The photochro-
mic properties of the new compounds were investigated.
Abstract: Symmetrically and unsymmetrically substituted diaryl-
cyclobutenes are synthesized in 20–70% yields from alkyne precur-
sors via cobalt-catalyzed [2+2] cycloadditions. The reactions
proceed under mild conditions and provide access to differently
substituted diarylethene derivatives. All the diarylcyclobutene
products undergo reversible photoisomerization upon irradiation
with UV/Vis light. The ring-closed isomers show different thermal
stabilities towards reisomerization with half-lives ranging from 9 to
300 hours.
R1
R3
1a: (i), 1b,1c: (ii)
R1
R1
I
(iii), (iv)
R2
R2
R2
3a R1 = H; R2 = Ph
S
S
S
1a R1 = H; R2 = Ph; R3 = Br
2a R1 = H; R2 = Ph
1 = H; R2 = Me
2b R1 = H; R2 = Me
3b R
Key words: diarylethene, photoswitchable, cyclobutene, photo-
chromic reactions, [2+2] cycloaddition
1b R1 = H; R2 = Me; R3 = H
1c R1, R2
=
; R3 = H
3c R1, R2
=
2c R1, R2
=
R1
R3
Reversible photoswitchable compounds are interesting
for applications in optoelectronic data storage, as active
parts of organic polymers with switchable refractive indi-
ces,1 or as photochromic moieties used to control biologi-
cal processes.2 One recent example is a diarylethene-
based photoswitchable inhibitor for the enzyme, human
carboanhydrase.3 With some diarylethenes both isomers
show high fatigue resistance and good thermal stability,
which is important for a variety of applications. However,
the synthesis of derivatives of higher complexity is chal-
lenging, as known routes are either not versatile enough,
or the required starting materials are difficult to handle.4
The most convenient routes for diarylethene synthesis
were reported by Feringa5 and Irie.6 The key step in the
Feringa route is a McMurry7coupling reaction using the
corresponding diarylketone precursors to afford a five-
membered ring system. However, the synthesis of unsym-
metric diarylethenes via this approach is limited, as suit-
able diarylketone precursors are difficult to obtain. The
Irie route uses octafluorocyclopentene as the starting ma-
terial, which is substituted with the appropriate lithiated
aryl compounds giving fluorinated cyclopentenes. Oc-
tafluorocyclopentene is volatile and therefore difficult to
handle; another disadvantage is the formation of mono-
substituted side products.8 Our goal was to establish a new
route for the synthesis of symmetrically and unsymmetri-
cally substituted diarylethenes. The described approach
uses diarylalkynes, obtained via Sonogashira coupling,
and a cobalt-catalyzed [2+2]-cycloaddition reaction to
give the photochromic diarylethenes. This reaction se-
quence leads to diarylethenes bearing a cyclobutene moi-
ety,9–16 a group of compounds not widely studied so far,
R4
R2
(v)
I
R3
S
S
R4
5a R1, R3 = H; R2, R4 = Ph
5b R1, R3 = H; R2 = Ph , R4 = Me
S
4a R3 = H, R4 = Ph
5c R1 = H , R2 = Ph; R3, R4
=
4b R3 = H, R4 = Me
5d R1, R2 = R3, R4
=
4c R3, R4
=
R4
R3
S
H
H
(vi)
O
N
H
H
O
Me
S
O
R1
N
R2
Me
O
7a R1, R3 = H; R2, R4 = Ph
6
7b R1, R3 = H; R2 = Ph; R4 = Me
7c R1 = H; R2 = Ph; R3, R4
=
R4
R3
S
8
(vi)
S
R1
R2
9a R1, R3 = H; R2, R4 = Ph
9b R1, R3 = H; R2 = Ph; R4 = Me
Scheme 1 (i) n-BuLi, Et2O, I2; (ii) I2, HIO3, H2O, AcOH, CCl4; (iii)
Me3SiC≡CH, Pd(PPh3)2Cl2, Ph3P, CuI, Et3N, THF; (iv) K2CO3,
MeOH; (v) Pd(PPh3)2Cl2, Ph3P, CuI, Et3N, THF; (vi) Co(dppp)Br2,
ZnI2, Zn, CH2Cl2.
SYNTHESIS 2011, No. 6, pp 0905–0908
x
x
.
x
x
.2
0
1
1
Advanced online publication: 11.02.2011
DOI: 10.1055/s-0030-1258435; Art ID: Z09511SS
© Georg Thieme Verlag Stuttgart · New York

906
P. Raster et al.
PAPER
Symmetrically and unsymmetrically substituted diaryl- a loss of about 50% of the initial absorption intensity due
ethenes 7a–c, 9a and 9b, with a central cyclobutene moi- to photodecomposition was observed after 10 irradiation
ety, were prepared as shown in Scheme 1. Thiophene cycles. Figure 1 shows the changes in the absorption in-
derivatives 1a–c were converted into iodoarenes 2a–c by tensity of compound 7a, at 312 nm, over 10 photoisomer-
bromine–iodine exchange using n-butyllithium and iodine ization cycles.
(for 1a), or aromatic substitution with iodine and iodic
acid (for 1b and 1c). Both reactions yielded the desired
products in about 70% yield. Palladium-catalyzed
Sonogashira coupling of compounds 2 with ethynyltri-
methylsilane, followed by cleavage of the trimethylsilyl
protecting group with potassium carbonate gave alkynes 3
in good yields. Using a second Sonogashira cross-cou-
pling reaction, terminal alkynes 3 were reacted with
thiophenes 4 to yield diarylalkynes 5 as precursors for the
subsequent cobalt-catalyzed [2+2] cycloaddition. This re-
action was carried out under very mild conditions using
the cobalt catalyst previously reported by Hilt.17,18 At
room temperature, zinc was used as the reducing agent to
generate the active catalytic cobalt(I) species. Maleimide
derivative 6 and cyclopentene (8) were used as the alkene
components in the cycloaddition reaction. The yield of the
reaction strongly depends on the nature of the diaryl-
alkyne and its aryl substitution pattern. Cycloaddition re-
actions of alkenes 6 and 8 with diarylalkynes 5a and 5b
Figure 1 Photoisomerization and bleaching of compound 7a as
monitored by changes in the absorption at 312 nm
gave the corresponding products 7a, 7b, 9a and 9b in
yields of 65–70%, while benzothienyl substituted diaryl-
alkynes 5c and 5d each reacted sluggishly, and only prod-
uct 7c was isolated in a poor 20% yield.
All the synthesized diarylethenes underwent reversible
photochromic ring-closing reactions in dichloromethane
on irradiation with UV light (320 nm), and ring-opening
in the presence of visible light. The absorption properties
and thermal stabilities of the ring-closed forms of com-
pounds 7a–c, 9a and 9b are summarized in Table 1.
Table 1 Photophysical Data of Compounds 7a–c, 9a and 9b
Compd
l
max (nm) (e⋅104) lmax (nm)
Thermal stability at r.t.
(closed form) [t1/2 (h)]
(open form)
287 (4.1)
288 (3.3)
265 (1.5)
287 (4.1)
288 (3.3)
7a
7b
7c
9a
9b
534
495
490
534
495
8.9
301.3
133.0
8.9
Figure 2 ORTEP representation of the X-ray crystal structure of
compound 7a
The structure of compound 7a was determined by X-ray
crystal structure analysis (Figure 2). The conjugated
thiophene rings are twisted due to steric interactions be-
tween the methyl substituents. The cyclobutene ring of the
[2.2.1]bicycloheptene unit is exo configured, while the
imide group adopts an endo orientation.
300.1
The ring-closed diarylethenes 7 and 9 can switch back to
the ring-open isomers in the presence of visible light, and
undergo a slow thermal cycloreversion reaction in the
dark. The thermal ring-opening of related four-membered
ring systems has been described in the literature.9 The
half-lives of the ring-closed isomers depend on the aro-
matic stabilization energies gained upon conversion into
the ring-open isomer, and therefore on the thiophene
substitution pattern.19 The thermal stabilities range from
t1/2 = 8.9 hours for compound 7a to t1/2 = 300 hours for the
unsymmetrically substituted derivative 7b. The photo-
isomerization reactions were performed several times and
In summary, photoisomerizable diarylcyclobutenes were
obtained from diarylalkynes via a cobalt-catalyzed [2+2]-
cycloaddition reaction. The efficiency of the ring-closing
reaction depends on the structure of the diarylalkyne pre-
cursor, and product yields of up to 70% were obtained.
The products undergo reversible photochromic reactions
in dichloromethane by alternating irradiation with UV
light and visible light. A specific advantage of the present-
ed synthetic route is the simpler access to diarylethenes
bearing two different aryl substituents.
Synthesis 2011, No. 6, 905–908 © Thieme Stuttgart · New York

PAPER
Synthesis and Photoisomerization of Diarylcyclobutenes
907
Table 2 Physical Properties, IR and MS Data for Compounds 3c, 5b–d, 7a–c, 9a and 9b
Compd Mp
(°C)
IR (CHCl3) cm–1
Formula
Mass
(found)
Mass
(calcd)
3c
5b
5c
5d
7a
7b
7c
9a
9b
174–176 2925, 2854, 1458, 1377, 1122, 933, 815
161–163 2337, 1605, 1301, 748, 539, 496
C11H8S
172.0342 172.0347
308.0690 308.0693
344.0695 344.0693
318.0539 318.0537
C19H16S2
C22H16S2
C20H14S2
168–170 2911, 2336, 2197, 1595, 837, 681
155–157 3053, 2360, 1740, 720, 620, 557
223–225 2353, 2132, 1533, 1442, 1291, 1030, 883, 773, 631, 537, 498
C34H29NO2S2 547.1638 547.1640
C29H27NO2S2 485.1489 485.1483
C32H27NO2S2 521.1490 521.1483
a
2927, 2361, 2253, 2168, 1769, 1695, 1598, 1434, 1136, 997, 631
a
3061, 2933, 2360, 2169, 1942, 1697, 1596, 1433, 1229, 1032, 831, 686, 420
a
2361, 2170, 1740, 1369, 1218, 631, 540
C29H26S2
C24H24S2
438.1474 438.1476
376.1327 376.1319
a
2360, 2339, 2207, 2171, 2116, 2032, 1740, 1669, 1437, 1374, 1220, 834
a Product obtained as a sticky oil.
Table 3 Yields and NMR Spectroscopic Data for Compounds 3c, 5b–d, 7a–c, 9a and 9b
Compd Yield (%)1H NMR (ppm)
13C NMR (ppm)
3c
70
2.72 (s, 3 H), 3.48 (s, 1 H), 7.30–7.41 (m, 2 H), 7.86 (d, J = 8.0 Hz, 1 H), 15.4, 82.3, 111.5, 114.8, 122.0, 122.4, 124.5,
7.77 (d, J = 8.0 Hz, 1 H)
124.7, 137.4, 140.0, 146.6
5b
75
2.41 (s, 3 H), 2.52 (s, 3 H), 2.58 (s, 3 H), 6.68 (s, 1 H), 7.22 (s, 1 H),
7.24–7.30 (m, 2 H), 7.34–7.40 (m, 2 H), 7.52–7.57 (m, 1 H)
14.5, 14.7, 15.2, 85.6, 86.7, 119.4, 121.1, 125.2,
125.6, 127.1, 127.5, 128.9, 134.0, 135.9, 140.2,
140.9, 142.2
5c
72
2.64 (s, 3 H), 2.71 (s, 3 H), 7.23–7.42 (m, 5 H), 7.54–7.58 (m, 2 H), 7.73 14.8, 15.4, 84.8, 89.1, 116.0, 120.9, 122.1, 122.4,
(d, J = 7.7 Hz, 1 H), 7.86 (d, J = 7.4 Hz, 2 H)
124.5, 124.7, 125.2, 125.6, 127.6, 129.0, 133.9,
137.5, 139.9, 140.4, 142.6, 144.4
5d
7a
74
70
2.77 (s, 6 H), 7.30–7.48 (m, 4 H), 7.93 (d, J = 7.6 Hz, 2 H), 7.75 (d,
J = 7.9 Hz, 2 H)
15.5, 87.7, 116.0, 122.1, 122.4, 124.5, 124.8,
137.6, 139.8, 144.5
1.50 (d, J = 10.7 Hz, 1 H), 2.11 (d, J = 10.7 Hz, 1 H), 2.27 (s, 6 H),
2.80–2.85 (m, 2 H), 2.96 (s, 2 H), 3.01 (s, 3 H), 3.27–3.30 (m, 2 H), 7.09 127.4, 128.9, 133.7, 134.0, 134.2, 136.7, 140.4,
(s, 2 H), 7.20–7.28 (m, 2 H), 7.30–7.38 (m, 4 H), 7.48–7.53 (m, 4 H) 177.8
14.9, 24.4, 34.7, 37.5, 44.0, 48.1, 122.8, 125.4,
7b
7c
65
20
1.42 (d, J = 10.6 Hz, 1 H), 2.02 (d, J = 10.6 Hz, 1 H), 2.12 (s, 3 H), 2.21 14.6, 14.8, 15.1, 24.4, 34.6, 37.5, 43.9, 48.2, 77.3,
(s, 3 H), 2.32 (s, 3 H), 2.74 (s, 2 H), 2.82–2.90 (m, 2 H), 2.94 (s, 3 H), 122.9, 125.1, 125.4, 127.3, 128.9, 132.5, 133.3,
3.22–3.30 (m, 2 H), 6.50 (s, 1 H), 7.01 (s, 1 H), 7.16–7.20 (m, 1 H),
7.25–7.30 (m, 2 H), 7.43–7.47 (m, 2 H)
133.9, 134.1, 134.7, 134.9, 136.0, 136.5, 140.2,
177.8
1.28 (d, J = 10.7 Hz, 1 H), 1.54 (d, J = 10.7 Hz, 1 H), 2.10 (s, 3 H), 2.42 14.7, 15.5, 24.4, 34.9, 37.3, 37.8, 43.8, 45.3, 47.9,
(s, 3 H), 2.66 (d, J = 5.3 Hz, 1 H), 3.01 (d, J = 5.4 Hz, 1 H), 3.03–3.05 48.3, 122.0, 122.5, 123.8, 124.3, 125.4, 127.3,
(m, 4 H), 3.20–3.21 (m, 1 H), 3.25 (dd, J = 5.4, 9.2 Hz, 1 H), 3.34 (dd, 128.0, 128.8, 132.6, 133.6, 133.9, 137.1, 137.8,
J = 5.4, 9.2 Hz, 1 H), 6.99 (s, 1 H), 7.21–7.32 (m, 5 H), 7.39–7.40 (m, 137.9, 138.8, 139.3, 140.3, 177.6, 177.8
2 H), 7.51–7.56 (m, 1 H), 7.73–7.76 (m, 1 H)
9a
9b
65
70
1.35–1.47 (m, 2 H), 1.74–1.88 (m, 4 H), 2.29 (s, 6 H), 3.94 (d, J = 6.5 14.8, 23.4, 26.8, 46.9, 123.2, 125.4, 127.1, 128.8,
Hz, 2 H), 7.18 (s, 2 H), 7.21–7.26 (m, 2 H), 7.32–7.37 (m, 4 H), 7.51– 134.3, 134.9, 135.6, 135.8, 139.9
7.54 (m, 4 H)
1.30–1.42 (m, 2 H), 1.68–1.84 (m, 4 H), 2.20 (s, 3 H), 2.26 (s, 3 H), 2.38 14.5, 14.7, 15.2, 23.3, 26.8, 30.3, 46.8, 77.0, 123.3,
(s, 3 H), 3.40–3.47 (m, 2 H), 6.59 (s, 1 H), 7.13 (s, 1 H), 7.20–7.37 (m, 125.3, 125.4, 127.1, 128.8, 133.6, 133.9, 134.4,
3 H), 7.49–7.53 (m, 2 H)
134.8, 135.3, 135.5, 135.7, 136.0, 139.7
solvents (analytical grade) were purchased from commercial suppli-
ers and used without further purification. Melting points were ob-
tained using a Lambda Photometrics Optimelt MPA100 apparatus
(Lambda Photometrics, Harpenden, UK), and are not corrected. IR
spectra were obtained using a Varian Biorad FT-IR Excalibur FTS
3000 spectrometer. 1H NMR spectra were recorded at 300 MHz on
The following compounds were prepared according to literature
methods: 3-bromo-2-methyl-5-phenylthiophene (1a),20 2-methyl-
benzo[b]thiophene (1c),21 3-iodo-2-methyl-5-phenylthiophene
(2a),19 3-iodo-2-methylbenzo[b]thiophene (2c)20 and 1,2-bis(2-
methyl-5-phenylthiophene-3-yl)ethyne (5a).21 All other reagents
were obtained from commercial sources. Unless otherwise noted,
Synthesis 2011, No. 6, 905–908 © Thieme Stuttgart · New York

908
P. Raster et al.
PAPER
a Bruker Avance 300 spectrometer, or at 600 MHz on a Bruker
Avance III Br600 with a cryogenic probe head (Bruker, Karlsruhe,
Germany). 13C NMR spectra were recorded at 75 MHz on a Bruker
Avance 300 spectrometer. The NMR spectra were recorded in
CDCl3 as solvent and chemical shifts are reported in ppm. UV/Vis
spectra were recorded using a Varian Cary BIO 50 UV/Vis/NIR
spectrophotometer (Varian Inc., CA, USA). Mass spectra were ob-
tained using Finnigan SSQ 710A (EI), Finnigan MAT 95 (CI) or
Finnigan MAT TSQ 7000 (Thermo FINNIGAN, USA) (ES/LC–
MS)] instrumentation. Thin layer chromatography (TLC) was per-
formed on alumina plates coated with silica gel (Merck silica gel 60
Diarylcyclobutenes 9a and 9b; General Procedure
In a Schlenk tube, [1,3-bis(diphenylphosphino)propane]cobalt(II)
bromide [Co(dppp)Br2] (64 mg, 0.1 mmol, 20 mol%), anhyd ZnI2
(64 mg, 0.2 mmol, 40 mol%) and Zn powder (13 mg, 0.2 mmol, 40
mol%) were dissolved in anhyd CH2Cl2 (2 mL) under an N2 atm.
The alkyne 5a or 5b (0.5 mmol) and cyclopentene (8) (0.045 mL,
0.5 mmol) were added and the mixture stirred for 24 h at r.t. CH2Cl2
(10 mL) and H2O (10 mL) were then added to the reaction mixture
and the organic layer was separated. Subsequently, the water layer
was extracted CH2Cl2 (2 × 10 mL) and then the organic layers were
combined and dried over MgSO4. The solvent was removed and the
residue was purified by preparative HPLC (9a) or by silica gel flash
chromatography (9b: PE–EtOAc, 9.75:0.25). Physical and analyti-
cal data for compounds 9a,b are given in Tables 2 and 3.
F
245, thickness 0.2 mm). Column chromatography was accom-
plished with Merck Geduran SI 60 silica gel as the stationary phase.
Petroleum ether (PE) refers to the fraction boiling in the 70–90 °C
range.
Supporting Information for this article is available online at
3-Ethynyl-2-methylbenzo[b]thiophene (3c)
http://www.thieme-connect.com/ejournals/toc/synthesis.
A mixture of THF–Et3N (48 mL, 2:1) was degassed for 10 min and
then 3-iodo-2-methylbenzo[b]thiophene (2c) (5.3 g, 19.3 mmol),
Me3SiC≡CH (5.57 mL, 38.7 mmol), Pd(PPh3)2Cl2 (54 mg, 0.4
mol%), Ph3P (40 mg, 0.8 mol%) and CuI (29 mg, 0.6 mol%) were
added under an N2 atm. The resulting soln was heated at 65 °C for
12 h. The reaction mixture was cooled, Et2O (40 mL) and H2O (40
mL) were added and the organic layer was separated. Subsequently,
the water layer was extracted with Et2O (2 × 40 mL). Then the or-
ganic layers were combined, dried over MgSO4 and the solvent was
removed. The residue was dissolved in MeOH (220 mL), K2CO3
(2.97 g, 21.1 mmol) was added and the resulting soln was stirred for
2 h at r.t. The reaction mixture was filtered, the solvent was re-
moved and the residue was purified by silica gel flash chromatogra-
phy (PE; Rf = 0.6). Physical and analytical data for compound 3c are
given in Tables 2 and 3.
Acknowledgment
Financial support from the University of Regensburg and the
Deutsche Forschungsgemeinschaft is acknowledged. We thank
Prof. Henri Brunner for helpful advice.
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Diarylcyclobutenes 7a-c; General Procedure
In a Schlenk tube, [1,3-bis(diphenylphosphino)propane]cobalt(II)
bromide [Co(dppp)Br2] (64 mg, 0.1 mmol, 20 mol%), anhyd ZnI2
(64 mg, 0.2 mmol, 40 mol%) and Zn powder (13 mg, 0.2 mmol, 40
mol%) were dissolved in anhyd CH2Cl2 (2 mL) under an N2 atm.
The alkyne 5a–c (0.5 mmol) and alkene 6 (0.5 mmol) were added
and the mixture stirred for 24 h at r.t. CH2Cl2 (10 mL) and H2O (10
mL) were then added to the reaction mixture and the organic layer
was separated. Subsequently, the water layer was extracted with
CH2Cl2 (2 × 10 mL) and then the organic layers were combined and
dried over MgSO4. The solvent was removed and the residue was
purified by silica gel flash chromatography (7a and 7b: PE–EtOAc,
1:1; Rf ~0.5) or by preparative HPLC (7c). Physical and analytical
data for compounds 7a–c are given in Tables 2 and 3. The X-ray
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