Synthesis and characterization of nonsymmetric cyclopentene-based dithienylethenes

Article abstract of DOI:10.1021/jo201966g

Nonsymmetric cyclopentene-based dithienylethenes, containing both thienyl and benzothienyl units, have been synthesized for the first time, employing intramolecular McMurry reaction as a key transformation to target compounds 10 and 16. Photochromic properties of these compounds were examined in toluene and acetonitrile solutions, PMMA layers, solid films, and crystal phase. To explain an unprecedented single-crystalline photochromism phenomena of 10, X-ray crystallographic analysis was performed, revealing a significant influence of strong S???S interatomic interactions on the intramolecular distance between two photochemical reaction centers in the molecule. Compound 16 could be further synthetically modified in a step-by-step manner, thus serving as potential key intermediate to various complex bifunctional photochromic molecules.

Full text of DOI:10.1021/jo201966g

Article
pubs.acs.org/joc
Synthesis and Characterization of Nonsymmetric Cyclopentene-
Based Dithienylethenes
Vasily A. Migulin,*,† Michael M. Krayushkin,† Valery A. Barachevsky,‡ Olga I. Kobeleva,‡
Tatyana M. Valova,‡ and Konstantin A. Lyssenko§
†N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect, 47, 119991 Moscow, Russian
Federation,
‡Photochemistry Center, Russian Academy of Sciences, Novatorov str., 7a, 119421 Moscow, Russian Federation
§A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Str., 28, 119991 Moscow,
Russian Federation
S
* Supporting Information
ABSTRACT: Nonsymmetric cyclopentene-based dithienyle-
thenes, containing both thienyl and benzothienyl units, have
been synthesized for the first time, employing intramolecular
McMurry reaction as a key transformation to target
compounds 10 and 16. Photochromic properties of these
compounds were examined in toluene and acetonitrile
solutions, PMMA layers, solid films, and crystal phase. To explain an unprecedented single-crystalline photochromism
phenomena of 10, X-ray crystallographic analysis was performed, revealing a significant influence of strong S···S interatomic
interactions on the intramolecular distance between two photochemical reaction centers in the molecule. Compound 16 could be
further synthetically modified in a step-by-step manner, thus serving as potential key intermediate to various complex bifunctional
photochromic molecules.
solubility in organic solvents, high reactivity in the solid state,
etc.3,5 Most of the attention in this field has been drawn to
dithienylethenes with perfluorocyclopentene bridge, which
resulted in numerous syntheses and exploration of physical
and chemical properties of these compounds in both solution
and solid state.6 Possibility of a sequential anionic substitution
of fluorines in perfluorocyclopentene allows syntheses of either
symmetric7 or nonsymmetric8 target dithienylethenes. At the
same time, dithienylethenes with a cyclopentene bridge are
much less studied; however, they also display attractive
photochromic properties and could be considered as a
promising alternative to dithienylperfluorocyclopentenes. Stud-
ies of physical properties of both perfluoro- and perhydrocy-
clopentene connected dithienylethenes have been performed in
a comparative mode in various solutions, showing no major
dissemblance in their photochromic behavior.9 On the other
hand, no cyclopentene-based dihetarylethenes are known to
date that display photochromic properties in a single-crystalline
phase, while among their perfluorocyclopentene-based ana-
logues only a handful reveal this unique ability.10 This feature is,
however, extremely valuable due to thermal stability of the
photoinduced form, high fatigue resistance, increased sensi-
tivity, and rapid response of photochromic single crystals of
dithienylethenes, extending application of these compounds in
switching, optical memory, and display.5,11
INTRODUCTION
■
Organic photochromic materials have attracted growing interest
due to their employment in construction of various optical
devices such as photoswitches,1 actinometers and dosimeters,2
and especially in the field of optoelectronics.3 Recent progress
in the laser and information technologies makes photochromic
materials promising for use as recording media in devices for
recording, storage, and processing of optical information.3,4 For
this purpose, dihetarylethenes of a general formula 1 are most
commonly being used, which undergo a reversible intercon-
version under irradiation from the open A form to the cyclic B
form (Scheme 1).
Scheme 1. Photochromic Interconversion of
Dihetarylethenes under Irradiation
Among a variety of photochromes 1, diarylethenes
containing two thiophene-based moieties (dithienylethenes, X
= S) are the most promising because of superior thermal
stability of their cyclic isomers, high fatigue resistance, rapid
response to the effects of radiation, maximum remoteness of
absorption bands of open A and cyclic B forms, sufficient
Received: September 22, 2011
Published: November 30, 2011
© 2011 American Chemical Society
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Construction of symmetric dithienylethenes is usually
accomplished by intramolecular condensation of the corre-
sponding symmetric 1,5-diketones under McMurry conditions
(Scheme 2).12
Friedel−Crafts conditions with glutaric anhydride,21 although
analogous aryl-substituted 5-ketoacids are well documented.22
The latter could be transformed to 5-ketoacid chlorides;
however, depending on structural characteristics or employed
reagents normal 4 and pseudo 5 forms have been reported for
this reaction.23,24 Pseudo acid chlorides easily give substituted
corresponding enol lactones 6 that appear to be major side-
reaction products (Chart 2).
Scheme 2. General Synthesis of Dithienylethenes via
McMurry Reaction
Chart 2. Reported Products of Substituted 5-Ketoacid to
Acid Chloride Conversion
Both thienyl 213 and benzothienyl 314,15 (Chart 1)
containing photochromes with cyclopentene as a bridge unit
have been synthesized by this approach.
We decided to test this approach on a simple object, such as
2-methylbenzothiophene 7 (Scheme 3). The obtained 5-
ketoacid 8 was converted to acid chloride under mild
conditions followed by SnCl4-promoted acylation of 2,5-
dimethylthiophene. The McMurry cyclization of nonsymmetric
diketone 9 proceeded smoothly to give dithienylethene 10 in
excellent yield.
Chart 1. Known Symmetric Simple Cyclopentene-Based
Dithienylethenes
Encouraged by these results, we initiated research for a
bifunctional nonsymmetric photochrome. 2-Chloro-5-methyl-
thiophene 11 and 5-bromo-2-methylbenzothiophene 12
appeared to be most practical starting material for this purpose
(Scheme 4). Thus, acylation of 11 with glutaric anhydride gave
acid 13 in good yield. Unfortunately, the conversion of 13 to
the acid chloride followed by the second Friedel−Crafts
reaction turned out to be rather difficult. Both small scale loads
and further upscaling of the target diketone furnished only
moderate yields for the two-step procedure. An attempt of
approaching 14 starting from 12 was also accomplished. 5-
Ketoacid 15 was obtained in high yield; however, the following
two-step transformation gave a moderate yield as well. It is
known that pseudo form of acid chloride 5 dominates when the
substituent has an electron-withdrawing nature,23 which
apparently turned out to be an issue in our case. Thus, the
presence of either a chlorine in the thiophene ring or a bromine
in the benzothiophene fragment significantly affected the
fraction of linear 5-ketoacid chloride. It was found that mild
reagents and conditions should be applied for the acid chloride
conversion step, such as oxalyl dichloride in dichloroethane,
while upon using SOCl2 only traces of the desired product were
detected. AlCl3 was necessary to be used in the Fridel−Crafts
acylation step, since SnCl4 was not potent enough for less
active 11 and 12, as compared to 2,5-dimethylthiophene, giving
no desired product. Yet, the McMurry cyclization of diketone
14 proceeded efficiently, giving the target photochrome in high
yield.25
Further transformations of the synthesized molecules were,
however, rather limited due to their restrained reactivity. Thus,
only formylation16 and acetylation17 of benzothiophene
fragments of 3 (R = H) were performed with moderate yields,
whereas either simultaneous or consecutive lithiation of
chlorides in 2 (R = Cl) have been the only means of
constructing bigger molecules based on this skeleton, usually
via Suzuki coupling.18 Despite such lack of variety in
modification methods, hundreds of new molecules have
recently been synthesized from 2 (R = Cl) and studied. This
includes a variety of both symmetric19 and nonsymmetric20
sophisticated photochromic molecular systems.
The given examples illustrate the growing need in developing
other functionalized thienyl-based photochromic scaffolds
having a cyclopentene bridge. Ideally, the target structure
should be nonsymmetric, with different reactivity of the present
functional groups, so it could be easily modified in a step-by-
step manner. In this article, we report a novel synthetic
approach to nonsymmetric cyclopentene-based photochromes
having both thienyl- and benzothienyl- fragments. This
approach includes synthesis of nonsymmetric 1,5-diketone
followed by intramolecular condensation under McMurry
conditions. An unsubstituted representative of this new class
of dithienylethenes with a cyclopentene bridge unit displays
photochromic properties in a single crystal, which is, to our
knowledge, the first example of crystalline photochromism for
cyclopentene-connected dihetarylethenes. Another compound
having chemically active halogens on both thienyl and
benzothienyl moieties can serve as a key intermediate to
various complex bifunctional molecules.
To confirm the desired possibility of consequent substitution
of halogen atoms in 16, we chose palladium/BINAP-catalyzed
amination under Buchwald conditions.26 First, chemical
transformation under such mild conditions should not have
affected the chlorine substituent in the thiophene ring; second,
a variety of amines could be used in this reaction yielding
photochromes of various structures, especially those having two
different functionalities. For this purpose, we chose two
heterocyclic amines bearing another functional group, 1-
RESULTS AND DISCUSSION
■
Synthesis. 1,5-Dicarbonyl compounds are indispensable
intermediates in synthesis of cyclopentene-based photochromes
via the McMurry reaction. To date there are only a few
examples in the literature of acylations of thiophenes under
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Scheme 3. Synthetic Route to Photochrome 10
Scheme 4. Synthetic Routes to Photochrome 16
methylpiperazine and hydroxyl-protected 4-piperidinemethanol
(Scheme 5).
dithienylethene 16 could be successively functionalized at
both ends of the molecule, thus serving as an intermediate
building block in construction of complicated photochromic
molecules.
Scheme 5. Catalytic Amination of 16
Spectral and Kinetic Characteristics. Spectral and
kinetic studies of synthesized compounds 10 and 16 were
performed in solutions (toluene, acetonitrile, C = 2 × 10−4 M),
poly(methyl methacrylate) (PMMA) layers, and solid films
obtained by slow evaporation of the corresponding toluene
solutions.
Typical photoinduced spectral changes of photochromes 10
and 16 in acetonitrile solution are presented in Figure 1. As can
be seen, for both compounds absorption spectra of the initial
open form (curves 1) and photoinduced closed form (curves 2)
are very alike, showing no critical dependence between the
spectral properties and structural substituents.
Analogously, photocyclization under UV irradiation in the
toluene solution for both photochromes 10 and 16 resulted in
appearance of an absorption band located in the short
wavelength visible spectral region. In this solvent, introduction
of mediocre electron acceptors such as chlorine and bromine
into thienyl and benzothienyl fragments, respectively, led to a
The desired products were obtained with excellent yields
under these conditions. Further lithiation of 2-chlorothienyl
unit with t-BuLi is a trivial procedure that has numerous
examples in the literature.13,27 Thus, the reported here
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Figure 1. Absorption spectra of dithienylethenes 10 (blue) and 16
(red) in acetonitrile before (1) and after (2) UV irradiation.
Figure 2. Kinetic curves for 10 (blue) and 16 (red) in acetonitrile for
processes of photocoloration under UV irradiation (I = 12 mW/cm2)
and photobleaching under visible light (I = 190 mW/cm2).
slight bathochromic spectral shift (5 nm) of absorption bands
for both open and closed forms for compound 16, as compared
to 10 (Table 1).
It is worth mentioning that dithienylethenes 10 and 16
displayed thermal irreversibility in either solution, polymer, or
solid film. The value of its photoinduced optical density in the
photoequilibrium state remained unchanged within 24 h in the
dark at room temperature.
Thus, compounds 10 and 16 display standard photochromic
properties in both solution and solid state typical for this class
of dithienylethenes. Introduction of halogens into thiophene
and benzothiophene rings does not significantly influence their
spectral or kinetic behavior.
Solid State Properties. Finding a photochromic com-
pound that would display photochromism in a crystalline phase
was never a trivial task. Not only must two substituents at the
reaction centers in the photochromic molecule acquire
antiparallel conformation in the crystal for such photo-
cyclizations to proceed, but also the distance between these
centers should be shorter than an allowed limit of ∼4.2 Å
calculated by Irie.28 Despite the fact that quite a few
perfluorocyclopentene-based photochromes displaying single-
crystalline photochromism are known, there are no examples in
the literature to date of cyclopentene-based dihetarylethenes
that possess such properties in a crystal state. Our earlier
attempts to obtain crystals of 3 (R = Cl) displaying
photocromic activity under UV irradiation were also fruitless.15
Crystallization of 10 from hexane furnished colorless
transparent crystals of different size. To our content, these
crystals quickly acquired deep orange color under UV
irradiation, decolorizing within a minute under a visible light
exposure (Figure 3). Multiple repetitions of this procedure
Table 1. Spectral and Kinetic Characteristics for
a
Dithienylethenes 10 and 16
phot
compound
system
λA, nm
DA
λB, nm ΔDB
10
solution in toluene
solution in acetonitrile
solid film
300 sh
300 sh
300 sh
300 sh
305 sh
305 sh
305 sh
305 sh
0.3
0.2
0.3
0.7
0.2
0.2
0.9
0.6
445
440
450
450
450
440
455
450
0.05
0.03
0.04
0.12
0.04
0.02
0.08
0.13
PMMA film
16
solution in toluene
solution in acetonitrile
solid film
PMMA film
a
λA and λB are wavelengths of absorption band maxima for open and
cyclic forms, respectively; DA is a value of optical density at the
phot
absorption maximum of the open form; ΔDB
is a value of the
photoinduced optical density at the absorption maximum of the cyclic
form in the photoequilibrium state; sh stands for absorption shoulder.
Very similar absorption spectra for open and cyclic forms for
these compounds were also obtained by UV irradiation of
acetonitrile solutions, PMMA, and solid films.
In all cases, the absorption band of the open form for 16 was
shifted to the long wavelength spectral region as compared to
10. At the same time, replacement of toluene by polar
acetonitrile resulted in small hypsochromic spectral shifts. The
opposite displacement induced by molecular packing was
observed for these compounds in polymer and solid films.
Both compounds are characterized by comparable sensitivity
to UV irradiation in solution (Table 1). In either toluene or
acetonitrile the values of photoinduced optical density
(ΔDBphot) are analogous for 10 and 16, while the values of
optical density for the open form (DA) for these two
compounds remain identical. On the other hand, with the DA
growth, and as a result with an increase of absorbed activating
UW irradiation, as it happens in PMMA films containing either
Figure 3. Photographs of crystal of 10 before and after photo-
irradiation with UV light.
compound or in the solid film of 16, a consequent increase in
phot
the ΔDB
value was observed accordingly.
influenced neither the intensity of orange color nor the
swiftness of decolorization. At the same time, UV-irradiated
crystals of 10 could be stored at room temperature in a dark
place for months without any noticeable loss of color. These
experiments suggested high fatigue resistance of 10 and high
thermal stability of the closed form 10B in the single-crystalline
state.
Reversible photoinduced transformations of photochromes
10 and 16 between open and cyclic forms (Scheme 1) is
demonstrated by kinetic curves for photocoloration under UV
irradiation followed by photobleaching under visible light
(Figure 2) with all measurements being carried out at the
maximum of the absorption band of the cyclic form. As could
be seen, structural differences between these compounds had a
very little effect on the photobleaching process.
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In order to understand the nature of packing of molecules of
10 in a crystal lattice, X-ray analysis was performed. High
quality single crystals were grown from hexane by slow
evaporation.
The X-ray diffraction (XRD) analysis showed that molecular
geometry of 10 has been significantly affected by crystal
packing effects. Initially, we attempted to obtain the diffraction
data at 100 K, but at this temperature and up to 150 K all single
crystals had cracking upon mounting on the goniometric head.
At 150 K, however, we managed to X-ray a crystal of 10 and
found that this compound had crystallized in space group P21/c
with two independent molecules A and B in a unit cell. Despite
an apparent pseudosymmetry, which caused the doubling of the
crystallographic axis a, two independent molecules displayed
some significant differences in molecular geometry;29 the latter
is crucial for the photocyclization process. Although the bond
lengths distribution and even the conformation (dihedral angles
between the central ring and the heterocycles) for molecules A
and B were almost identical, the difference in the intra-
molecular distances between the reactive centers C(2′) and
C(2″) in A and B (3.839(2) and 3.699(2) Å, respectively)
attained 0.140(2) Å. As could be seen, both values are smaller
than the mentioned above limit of ∼4.2 Å, while antiparrallel
conformation of C(2′)−C(8′) and C(2″)−C(8″) bonds was
observed for both A and B molecules in the crystal (Figure 4).
Figure 5. Fragment of the crystal packing of 10 at 150 K. Carbon
atoms of the independent molecule A are shown by larger balls.
with crystallographic axis a becoming two times smaller. The
main geometric parameters of 10 at 173 K are similar to those
measured at 150 K, but the C(2′)···C(2″) separation is equal to
3.776(2) Å while the interatomic distance for the above S···S
contact is 3.709(2) Å. In other words, the stronger the
intermolecular S···S binding is, the longer is the distance
between the reactive centers in a molecule of 10.
Hence, we can conclude that in the temperature range 150−
293 K the molecular arrangement in a crystal and thus the
intramolecular separation between the two reactive centers
govern the photocyclization reaction in solid. Moreover, the
significant variation of the C(2′)···C(2″) distance as a
consequence of the intermolecular S···S interaction clearly
indicates the flexibility of 10, which could further contribute to
the photocyclization in the solid state.
Conclusion. A new synthetic approach to nonsymmetric
cyclopentene-based photochromes has been designed and
accomplished. Compounds 10 and 16 containing both thienyl
and benzothienyl units have been synthesized as representatives
of this new class of dithienylethenes. Their syntheses included
formation of a corresponding nonsymmetric 1,5-diketone,
which underwent intramolecular McMurry condensation. It
was shown that compound 16 could be consequently
transformed into different bifunctional photochromic objects
by step-by-step substitution of halogens. In the first step
bromide in the benzothiophene ring was substituted by
different amines having other functional groups in the molecule
via catalytic amination, followed by lithiation of chloride in the
thiophene ring. Spectral and kinetic studies of synthesized
compounds were performed in toluene and acetonitrile
solutions, PMMA layers, and solid films and showed standard
photochromic properties typical for dithienylethenes. Absorp-
tion spectra and kinetic curves recorded for 10 and 16 showed
almost no structural influence on photochromic properties for
these compounds. At the same time, compound 10 displayed
photochromism in a single-crystalline phase, which is the first
example of this phenomenon to date for cyclopentene-based
dihetarylethenes. X-ray analysis of 10 discovered the close
relation between the intermolecular S···S interactions and the
intramolecular distance between two reaction centers.
Figure 4. General view of one of the independent molecules at 150 K
with representation of atoms via thermal ellipsoids (p = 50%).
Hydrogen atoms are omitted for clarity.
Two different C(2′)···C(2″) distances for molecules A and B,
mentioned above, are likely to be governed by crystal packing
effects, especially by shortened intermolecular contact
S(1″)···S(2″) (3.643(1) Å), responsible for assembling
molecules A into chains. The analogous interaction between
molecules B is much weaker (S···S = 3.808(2) Å). At the same
time, in both cases these S···S contacts are characterized by the
pronounced directionality with the S(1′)···S(1″)−C(7″A) angle
being close to 160° and thus attributing to the n(S)−σ*(C−S)
charge transfer.30 As a consequence of the above differences in
the molecular geometry and intermolecular interactions, the
crystal of 10 at 153 K is formed by alternating layers (parallel to
the crystallographic plane bc) that contain either A or B
molecules (Figure 5).
EXPERIMENTAL SECTION
General. All reactions were carried out under argon atmosphere.
Solvents were dried by the usual methods and then distilled prior to
use. NMR spectra were obtained at 300 MHz for H NMR and at 75
■
All of these differences between the independent molecules
A and B persist only up to 173 K. Starting from this
temperature and up to room temperature, the number of
independent molecules in the unit cell decrease to one (Z′ = 1)
1
MHz for 13C NMR with chemical shift values in the spectra being
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7.30−7.43 (m, 2H), 7.73 (d, 1H, J = 7.8 Hz), 8.10 (d, 1H, J = 8.1 Hz);
13C NMR (75 MHz, CDCl3) δ 16.8, 19.0, 33.0, 42.3, 121.6, 123.4,
124.3, 125.1, 132.6, 137.2, 138.1, 148.2, 179.5, 197.9. Anal. Calcd for
C14H14O3S: C, 64.10; H, 5.38; S, 12.22. Found: C, 64.09; H, 5.38; S,
12.25.
Table 2. Crystal Data and Structure Refinement Parameters
for 10 at 150 and 173 K
temperature
empirical formula
150 K
173 K
C20H20S2
324.48
1-(2,5-Dimethylthiophen-3-yl)-5-(2-methylbenzo[b]-
thiophen-3-yl)pentane-1,5-dione (9). Carboxylic acid 8 (630 mg,
2.4 mmol) was added in portions to a stirred solution of oxalyl
chloride (2.4 g, 18.9 mmol) and catalytical amount of dimethylforma-
mide in dry 1,2-dichloroethane (10 mL) at 0 °C under argon
atmosphere. The formed solution was stirred at rt for 12 h. Solvents
were evaporated, and the residue was dried in vacuo to give 670 mg of
dark solid. It was dissolved in dry dichloromethane (10 mL), and 2,5-
dimethylthiophene (540 mg, 4.8 mmol) was added. Tin(IV) chloride
(1.9 g, 7.3 mmol) was then added in a dropwise manner within 30 min
to the formed solution at −5 °C under argon atmosphere. The
suspension was stirred at rt for 16 h, quenched with ice, and filtered,
and the aqueous layer was additionally extracted with chloroform (3 ×
10 mL). The combined organic extract was washed with aqueous
sodium carbonate and brine and then dried with anhydrous sodium
sulfate. After evaporation of the solvent the residue was subjected to
silica gel flash column chromatography (dichloromethane). Fractions
containing 9 were combined and evaporated. Crystallization of the
residue from ether/hexanes provided 460 mg (54%) of beige solid: mp
formula weight
crystal color, habit
crystal system
space group
Z (Z′)
colorless, prism
monoclinic
P21/c
8(2)
4(1)
a, Å
16.6382(16)
13.2364(14)
15.3904(16)
97.490(2)
3360.5(6)
1.283
8.3268(8)
13.2672(11)
15.4463(13)
97.4242(17)
1692.1(3)
1.274
b, Å
c, Å
β, deg
V, Å3
dcalc (g cm−1
)
linear absorption, μ (cm−1
)
3.11
3.09
F(000)
1376
688
2θmax, deg
58
58
reflections measured
19368
13205
independent reflections (Rint)
8908 (0.0522)
6507
4476 (0.0200)
3669
observed reflections [I > 2σ(I)]
parameters
final R(Fhkl): R1
wR2
403
202
1
95−98 °C; H NMR (300 MHz, CDCl3) δ 2.18 (tt, 2H, J = 7.0, 7.0
0.0448
0.0407
Hz), 2.39 (s, 3H), 2.66 (s, 3H), 2.77 (s, 3H), 2.92 (t, 2H, J = 7.0 Hz),
3.05 (t, 2H, J = 7.0 Hz), 6.99 (s, 1H), 7.29−7.42 (m, 2H), 7.73 (d, 1H,
J = 8.0 Hz), 8.11 (d, 1H, J = 8.1 Hz); 13C NMR (75 MHz, CDCl3) δ
14.8, 15.9, 16.8, 18.8, 40.5, 42.7, 121.6, 123.5, 124.2, 125.0, 125.9,
132.9, 135.0, 135.3, 137.3, 138.2, 147.2, 147.8, 195.7, 198.6; HRMS
(m/z) [M + H]+ calcd for C20H20O2S2, 357.0977; found 357.0974.
3-(2-(2,5-Dimethylthiophen-3-yl)cyclopent-1-enyl)-2-
methylbenzo[b]thiophene (10). Titanium(IV) chloride (490 mg,
2.6 mmol) was added in a dropwise manner to a well-stirred
suspension of zinc dust (510 mg, 7.8 mmol) in dry tetrahydrofuran
(15 mL) at −10 °C under argon atmosphere. The suspension was
stirred at rt for 1 h, then dry pyridine (200 mg, 2.6 mmol) was added
followed by addition of 9 (460 mg, 1.3 mmol) in one portion. The
suspension was refluxed for 9 h, and after this time TLC analysis
showed no starting material left in the reaction mixture. Several drops
of water were added with stirring, and the suspension was filtered
through a silica plug. After evaporation of the filtrate the residue was
subjected to silica gel flash column chromatography (hexanes) to give
360 mg (86%) of white powder. Colorless crystals of 10 could be
obtained by crystallization from hexane, and high quality single crystals
suitable for X-ray diffraction were grown from hexane by slow
0.1283
0.1254
GOF
Δρmax, Δρmin (e Å−3
0.993
0.941
)
0.424/−0.361
0.356/−0.279
referenced to the solvent. Elemental analyses were carried out with a
Perkin−Elmer CHN analyzer (2400 series II). High resolution mass
spectra (HR MS) were measured on the Bruker micrOTOF II
instrument using electrospray ionization (ESI).31 The measurements
were done in a positive ion mode (interface capillary voltage −4500 V)
or in a negative ion mode (3200 V); mass range from m/z 50 to m/z
3000 Da; external or internal calibration was done with Electrospray
Calibrant Solution (Fluka). A syringe injection was used for solutions
in acetonitrile, methanol, or water (flow rate 3 μL/min). Nitrogen was
applied as a dry gas; interface temperature was set at 180 °C. Spectral
and kinetic studies were conducted with Cary 50 bio (Varian)
spectrophotometer using a quartz cell with a path length of 0.2 cm.
Concentration of the photochromic compounds in solution was 2 ×
10−4 M. Toluene and acetonitrile from Aldrich were used as solvents.
Polymer solid films were prepared by casting the corresponding
chloroform solution (1 mL) of poly(methyl methacrylate) (PMMA)
(100 mg) and dithienylethene (0.5 mg) into an open poly(ethylene
terephthalate) (PET) box followed by evaporation in the dark. Solid
films were obtained by evaporation of the corresponding toluene
solutions (C = 10−3 M) from a quartz plate. The slightly opaque film
obtained from 10 was unevenly surfaced due to its polycrystalline
origin. Compound 16 formed a smooth amorphous transparent film
that retained its transparency throughout the measurements.
5-(2-Methylbenzo[b]thiophen-3-yl)-5-oxopentanoic Acid
(8). To a solution of 2-methylbenzo[b]thiophene (4.2 g, 28.4
mmol) and glutaric anhydride (3.9 g, 34.1 mmol) in dry dichloro-
methane (40 mL) at −15 °C was added aluminum(III) chloride (9.4 g,
70.9 mmol) in portions within 1 h under argon atmosphere. The
formed suspension was stirred at −5 to 0 °C for 2 h and then at rt for
5 h. The reaction mixture was quenched with ice, and the aqueous
layer was additionally extracted with chloroform (4 × 30 mL). The
combined organic extract was dried with anhydrous sodium sulfate.
After evaporation of the solvent the residue was crystallized from
dichloromethane/hexanes to give 2.1 g of gray powder. Filtrate was
evaporated, and the residue was subjected to silica gel flash column
chromatography (ethyl acetate/hexanes) to give additionally 1.8 g of 8.
Combination of two portions provided 3.9 g (53%) of gray solid: mp
108−110 °C; 1H NMR (300 MHz, CDCl3) δ 2.13 (tt, 2H, J = 7.1, 7.1
Hz), 2.52 (t, 2H, J = 7.1 Hz), 2.77 (s, 3H), 3.04 (t, 2H, J = 7.1 Hz),
1
evaporation: mp 97−98 °C; H NMR (300 MHz, CDCl3) δ 1.89 (s,
3H), 2.20 (s, 3H), 2.21 (m, 2H), 2.37 (s, 3H), 2.67−2.91 (broad m,
2H), 2.99−3.15 (m, 2H), 6.45 (s, 1H), 7.28−7.39 (m, 2H), 7.63 (d,
1H, J = 7.5 Hz), 7.78 (d, 1H, J = 7.5 Hz); 13C NMR (75 MHz,
CDCl3) δ 14.3, 14.5, 15.0, 23.6, 37.8, 38.0, 121.8, 122.3, 123.2, 123.7,
125.7, 130.9, 132.3, 132.9, 134.7, 134.8, 135.5, 137.8, 138.3, 139.7.
Anal. Calcd for C20H20S2: C, 74.03; H, 6.21; S, 19.76. Found: C, 74.16;
H, 6.21; S, 19.49.
5-(5-Chloro-2-methylthiophen-3-yl)-5-oxopentanoic Acid
(13). To a solution of 2-chloro-5-methylthiophene27 (15.3 g, 115
mmol) and glutaric anhydride (15.7 g, 138 mmol) in dry
dichloromethane (200 mL) at −5 °C was added aluminum(III)
chloride (38.2 g, 287 mmol) in portions within 1.5 h under argon
atmosphere. The reaction mixture was stirred at 0 °C for 2 h and then
at rt for 12 h. Ice was added, and the aqueous layer was additionally
extracted with chloroform (3 × 100 mL). The combined organic layer
was washed with brine, dried with anhydrous sodium sulfate, and
evaporated. The crude product was purified with silica gel flash column
chromatography (ethyl acetate/hexanes) to give 17 g (60%) of beige
solid: mp 79−80 °C; 1H NMR (300 MHz, CDCl3) δ 2.02 (tt, 2H, J =
7.1, 7.1 Hz), 2.48 (t, 2H, J = 7.1 Hz), 2.65 (s, 3H), 2.87 (t, 2H, J = 7.1
Hz), 7.18 (s, 1H), 11.12 (s, 1H); 13C NMR (75 MHz, CDCl3) δ 15.9,
18.7, 32.9, 40.2, 125.3, 126.6, 134.7, 147.8, 179.4, 194.4. Anal. Calcd
337
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The Journal of Organic Chemistry
Article
for C10H11ClO3S: C, 48.68; H, 4.49; Cl, 14.37; S, 13.00. Found: C,
48.61; H, 4.53; Cl, 14.29; S, 12.92.
(3.7 g, 19.5 mmol) was added in a dropwise manner to a well-stirred
suspension of zinc dust (4.8 g, 73.8 mmol) in dry tetrahydrofuran (50
mL) at −10 °C under argon atmosphere. The suspension was refluxed
for 1 h and then cooled to rt, and dry pyridine (1.5 g, 19 mmol) was
added followed by addition of 14 (2.3 g, 5 mmol) in one portion. The
suspension was refluxed for 8 h, and after this time TLC analysis
showed no starting material left in the reaction mixture. Water (1.3 g,
72.2 mmol) was added with stirring; the precipitate was filtered and
washed with hot ethyl acetate/hexanes 1/1 solution (3 × 35 mL).
After evaporation of the combined filtrate the residue was subjected to
silica gel flash column chromatography (hexanes) to give 1.6 g (75%)
of colorless gum: 1H NMR (300 MHz, CDCl3) δ 1.80 (s, 3H), 2.14 (s,
3H), 2.15 (m, 2H), 2.61−2.90 (broad m, 2H), 2.85−3.02 (m, 2H),
6.51 (s, 1H), 7.33 (dd, 1H, J = 8.4, 1.8 Hz), 7.57 (d, 1H, J = 8.4 Hz),
7.60 (d, 1H, J = 1.8 Hz); 13C NMR (75 MHz, CDCl3) δ 14.3, 14.7,
23.4, 37.7, 37.9, 118.0, 123.2, 124.8, 125.0, 126.3, 126.4, 129.9, 133.4,
133.7, 134.4, 136.7, 137.3, 137.8, 140.9; HRMS (m/z) [M + H]+ calcd
for C19H16BrClS2, 424.9617; found 424.9624.
Synthesis of 1-(5-Bromo-2-methylbenzo[b]thiophen-3-yl)-5-
(5-chloro-2-methylthiophen-3-yl)pentane-1,5-dione (14) from
13. Carboxylic acid 13 (570 mg, 2.3 mmol) was added in portions to
a stirred solution of oxalyl chloride (2.9 g, 22.8 mmol) and a catalytical
amount of dimethylformamide in dry 1,2-dichloroethane (10 mL) at
−5 °C under argon atmosphere. The formed solution was stirred at rt
for 4 h. Solvents were evaporated, and the residue was dried in vacuo
to give 605 mg of dark oily solid. It was dissolved in dry
dichloromethane (10 mL), and 5-bromo-2-methylbenzo[b]-
thiophene32 (570 mg, 2.5 mmol) was added. Aluminum(III) chloride
(670 mg, 5 mmol) was then added in portions to the formed solution
at −5 °C under argon atmosphere. The suspension was stirred at rt for
16 h, quenched with ice, and filtered, and the aqueous layer was
additionally extracted with chloroform (3 × 10 mL). The combined
organic extract was washed with aqueous sodium carbonate and brine
and then dried with anhydrous sodium sulfate. After evaporation of the
solvent the residue was subjected to silica gel flash column
chromatography (dichloromethane). Fractions containing 14 were
combined and evaporated to give 330 mg (31%) of brown solid: mp
142−145 °C; 1H NMR (300 MHz, CDCl3) δ 2.17 (tt, 2H, J = 6.9, 6.9
Hz), 2.67 (s, 3H), 2.79 (s, 3H), 2.91 (t, 2H, J = 6.9 Hz), 3.02 (t, 2H, J
= 6.9 Hz), 7.19 (s, 1H), 7.42 (dd, 1H, J = 8.5, 1.9 Hz), 7.58 (d, 1H, J =
8.5 Hz), 8.37 (d, 1H, J = 1.9 Hz); 13C NMR (75 MHz, CDCl3) δ 15.9,
17.2, 18.5, 40.5, 42.5, 119.6, 122.6, 125.3, 126.6, 126.7, 127.5, 132.1,
134.8, 135.7, 140.0, 147.6, 149.7, 194.7, 197.5; HRMS (m/z) [M +
H]+ calcd for C19H16BrClO2S2, 454.9536; found 454.9527.
5-(5-Bromo-2-methylbenzo[b]thiophen-3-yl)-5-oxopenta-
noic Acid (15). To a solution of 5-bromo-2-methylbenzo[b]-
thiophene32 (4 g, 17.6 mmol) and glutaric anhydride (2.4 g, 21.1
mmol) in dry dichloromethane (35 mL) at −15 °C was added
aluminum(III) chloride (6.3 g, 47.5 mmol) in portions within 1 h
under argon atmosphere. The reaction mixture was stirred at −5 to 0
°C for 2 h and then at rt for 5 h. The solvent was evaporated, and the
solid was washed on filter with excess of water. After drying beige solid
was extracted with hot ethyl acetate (50 mL) and hot chloroform (3 ×
40 mL). After evaporation of solvents from the combined filtrate the
residue was refluxed in ether and filtered to give 4.6 g (76%) of beige
powder after drying. Analytically pure sample could be obtained by
crystallization from ethyl acetate as white crystals: mp 170−173 °C;
1H NMR (300 MHz, DMSO-d6) δ 1.89 (tt, 2H, J = 7.2, 7.1 Hz), 2.32
1-(3-(2-(5-Chloro-2-methylthiophen-3-yl)cyclopent-1-enyl)-
2-methylbenzo[b]thiophen-5-yl)-4-methylpiperazine (17). Di-
thienylethene 16 (460 mg, 1.1 mmol), tris(dibenzyledeneacetone)-
dipalladium (0) (20 mg, 2 mol %), BINAP (41 mg, 6 mol %), sodium
tert-butoxide (220 mg, 2.3 mmol), and 1-methylpiperazine (160 mg,
1.6 mmol) were sequentially placed into a Schott tube under argon
atmosphere. Toluene (5 mL) was then added, washing all the solids
from the walls of the tube. The reaction mixture was stirred at 80 °C
for 20 h, and after this time TLC analysis showed no starting material
left in the reaction mixture. Ethyl acetate (5 mL) was added, and the
formed suspension was filtered. After evaporation of the filtrate the
residue was subjected to silica gel flash column chromatography
1
(hexanes/ethyl acetate) to give 368 mg (77%) of brown gum: H
NMR (300 MHz, CDCl3) δ 1.78 (s, 3H), 2.16 (m, 2H), 2.20 (s, 3H),
2.39 (s, 3H), 2.62 (m, 4H), 2.68−2.98 (m, 4H), 3.19 (m, 4H), 6.61 (s,
1H), 6.92 (d, 1H, J = 2.3 Hz), 6.97 (dd, 1H, J = 8.7, 2.3 Hz), 7.57 (d,
1H, J = 8.7 Hz); 13C NMR (75 MHz, CDCl3) δ 14.3, 14.8, 23.4, 37.8,
37.9, 46.1, 50.3, 55.2, 109.4, 115.2, 122.0, 124.7, 126.4, 130.3, 130.4,
133.4, 134.5, 134.6, 136.1, 136.5, 140.0, 148.8; HRMS (m/z) [M +
H]+ calcd for C24H27ClN2S2, 443.1377; found 443.1369.
4-(tert-Butyldimethylsilanyloxymethyl)piperidine. 4-Piperidi-
nemethanol (500 mg, 4.4 mmol), tert-butylchlorodimethylsilane (660
mg, 4.4 mmol), and imidazole (590 mg, 8.7 mmol) were dissolved in
dimethylformamide (2.5 mL), and the solution was kept at rt for 16 h.
After dilution with water the mixture was extracted with tert-butyl
methyl ether (3 × 10 mL). To the well-stirred combined organic
extract was added water followed by acetic acid (1.3 g, 22 mmol).
Aqueous layer was separated, neutralized with sodium bicarbonate, and
then extracted with tert-butyl methyl ether (3 × 15 mL). The
combined organic extract was dried with anhydrous sodium sulfate and
evaporated. Bulb-to-bulb distillation of the residue provided 520 mg
(t, 2H, J = 7.2 Hz), 2.80 (s, 3H), 3.02 (t, 2H, J = 7.1 Hz), 7.51 (dd,
1H, J = 8.5, 1.5 Hz), 7.92 (d, 1H, J = 8.5 Hz); 8.36 (d, 1H, J = 1.5 Hz);
12.07 (s, 1H); 13C NMR (75 MHz, DMSO-d6) δ 16.9, 19.2, 32.8, 42.1,
118.8, 123.7, 126.0, 127.1, 131.3, 135.5, 139.9, 150.6, 174.3, 197.4.
Anal. Calcd for C14H13BrO3S: C, 49.28; H, 3.84; Br, 23.42; S, 9.40.
Found: C, 49.27; H, 3.96; Br, 23.29; S, 9.34.
Synthesis of 1-(5-Bromo-2-methylbenzo[b]thiophen-3-yl)-5-
(5-chloro-2-methylthiophen-3-yl)pentane-1,5-dione (14) from
15. Carboxylic acid 15 (2 g, 5.9 mmol) was added in portions to a
stirred solution of oxalyl chloride (6.4 g, 50 mmol) and a catalytical
amount of dimethylformamide in dry 1,2-dichloroethane (25 mL) at
−15 °C under argon atmosphere. The formed suspension was slowly
warmed and stirred at rt for 5 h. Solvents were evaporated, and the
residue was dried in vacuo to give 2.1 g of brown solid. It was dissolved
in dry dichloromethane (30 mL), and 2-chloro-5-methylthiophene27
(1 g, 7.5 mmol) was added. Aluminum(III) chloride (2.3 g, 17.4
mmol) was then added in portions to the formed solution at −15 °C
under argon atmosphere. The suspension was stirred at rt for 16 h,
quenched with ice, and filtered, and the aqueous layer was additionally
extracted with chloroform (3 × 20 mL). The combined organic extract
was washed with aqueous sodium carbonate and brine and then dried
with anhydrous sodium sulfate. After evaporation of the solvent the
residue was subjected to silica gel flash column chromatography
(dichloromethane) to give 370 mg (14%) of light-brown solid with
analytical data identical to that of the earlier obtained sample (from
13).
1
(52%) of light yellow liquid: H NMR (300 MHz, CDCl3) δ 0.05 (s,
6H), 0.90 (s, 9H), 1.05−1.18 (m, 2H), 2.59 (m, 1H), 2.70 (m, 2H),
2.81 (s, 1H), 2.60 (m, 2H), 3.09 (m, 2H), 3.42 (d, 2H, J = 6.3 Hz);
13C NMR (75 MHz, CDCl3) δ −5.5, 18.3, 25.8, 30.0, 39.0, 46.3, 68.3;
HRMS (m/z) [M + H]+ calcd for C12H27NOSi, 230.1935; found
230.1930.
4-(tert-Butyldimethylsilanyloxymethyl)-1-(3-(2-(5-chloro-2-
methylthiophen-3-yl)-cyclopent-1-enyl)-2-methylbenzo[b]-
thiophen-5-yl)piperidine (18). Dithienylethene 16 (424 mg, 1
mmol), tris(dibenzyledeneacetone)dipalladium (0) (15 mg, 1.6 mol
%), BINAP (20 mg, 3.2 mol %), sodium tert-butoxide (300 mg, 3
mmol), and 4-(tert-butyldimethylsilanyloxymethyl)piperidine (458 mg,
2 mmol) were sequentially placed into a Schott tube under argon
atmosphere. Toluene (5 mL) was then added, washing all the solids
from the walls of the tube. The reaction mixture was stirred at 80 °C
for 20 h, and after this time TLC analysis showed no starting material
left in the reaction mixture. Ethyl acetate (5 mL) was added, and the
formed suspension was filtered. After evaporation of the filtrate the
residue was subjected to silica gel flash column chromatography
1
(hexanes/ethyl acetate) to give 495 mg (87%) of yellow gum: H
5-Bromo-3-(2-(5-chloro-2-methylthiophen-3-yl)cyclopent-1-
enyl)-2-methylbenzo[b]thiophene (16). Titanium(IV) chloride
NMR (300 MHz, CDCl3) δ 0.09 (s, 6H), 0.93 (s, 9H), 1.43 (m, 2H),
338
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The Journal of Organic Chemistry
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1.63 (m, 1H), 1.76 (s, 3H), 1.89 (m, 2H), 2.15 (m, 2H), 2.22 (s, 3H),
2.68 (m, 2H), 2.70−2.99 (m, 4H), 3.52 (d, 2H, J = 6.4 Hz), 3.59 (m,
2H), 6.61 (s, 1H), 6.91 (d, 1H, J = 2.2 Hz), 6.99 (dd, 1H, J = 8.7, 2.2
Hz), 7.55 (d, 1H, J = 8.7 Hz); 13C NMR (75 MHz, CDCl3) δ −5.4,
14.3, 14.9, 18.3, 23.4, 25.9, 29.1, 37.8, 38.0, 38.5, 51.0, 51.4, 68.0,
109.9, 116.2, 121.9, 124.8, 126.5, 130.1, 130.5, 133.5, 134.6, 134.7,
136.0, 136.3, 139.9, 149.7; HRMS (m/z) [M + H]+ calcd for
C31H42ClNOS2Si, 572.2238; found 572.2239.
X-ray Crystallography. High quality single crystals of 10 were
grown by slow evaporation from hexane. All diffraction data were
collected on a Bruker SMART APEX II CCD diffractometer [λ(Mo
Kα) = 0.71072 Å, ω-scans] for the same single crystal at 150 and 173
K. The substantial redundancy in data allows empirical absorption
correction to be performed with SADABS, using multiple measure-
ments of equivalent reflections. The structures were solved by direct
methods and refined by the full-matrix least-squares technique against
F2 in the anisotropic-isotropic approximation. The positions of
hydrogen atoms were calculated from geometrical point of view and
refined within the riding model. All calculations were performed with
the SHELXTL software package.33 Crystal data and structure
refinement parameters are listed in Table 2.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra for compounds 8−10 and
13−18 and crystallographic data in CIF format for 10 at 150
and 173 K. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: vmiguli@mail.ru.
■
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