Syntheses of dithienylcyclopentene optical molecular switches

Article abstract of DOI:10.1002/1099-0690(200301)2003:1<155::AID-EJOC155>3.0.CO;2-S

Properly functionalized dithienylethenes show promise for light-induced switching processes. To prevent cis/trans isomerization from competing with conrotatory 6π-electron ring closure, the ethene segment is usually incorporated in a (perfluorinated) cyclopentene. In the present article syntheses of perhydrocyclopentene 1 and perfluorocyclopentene 2 are described, which are amenable for large-scale conversions. Both compounds have chloro substituents at the 5-position of the thiophene rings to allow further functionalization. The conversion of the chloro substituents of 1 to formyl, carboxylate, boronyl, and hydrogen groups by halogen/lithium exchange at room temperature is described, and examples are given of further elaboration of 1 and 2 by attachment, both in a symmetrical as well as unsymmetrical fashion, of additional functionality by condensation, Friedel-Crafts or Suzuki reactions. The newly prepared thienylperhydrocyclopentene derivatives show reversible photochromism if the substituents at the 5-postions allow for conjugation with the thiophene π-system. Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/1099-0690(200301)2003:1<155::AID-EJOC155>3.0.CO;2-S

FULL PAPER
Syntheses of Dithienylcyclopentene Optical Molecular Switches
Linda N. Lucas,[a] Jaap J. D. de Jong,[a] Jan H. van Esch,*[a] Richard M. Kellogg,[a,b] and
Ben L. Feringa*[a]
Keywords: Photochromism / Electrocyclic reactions / Cross-coupling / Diarylethene
Properly functionalized dithienylethenes show promise for
light-induced switching processes. To prevent cis/trans iso-
merization from competing with conrotatory 6π-electron ring
closure, the ethene segment is usually incorporated in a (per-
fluorinated) cyclopentene. In the present article syntheses of
perhydrocyclopentene 1 and perfluorocyclopentene 2 are de-
exchange at room temperature is described, and examples
are given of further elaboration of 1 and 2 by attachment,
both in a symmetrical as well as unsymmetrical fashion, of
additional functionality by condensation, Friedel−Crafts or
Suzuki reactions. The newly prepared thienylperhydrocyclo-
pentene derivatives show reversible photochromism if the
scribed, which are amenable for large-scale conversions. substituents at the 5-postions allow for conjugation with the
Both compounds have chloro substituents at the 5-position of
the thiophene rings to allow further functionalization. The
thiophene π-system.
conversion of the chloro substituents of 1 to formyl, carb- ( Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim,
oxylate, boronyl, and hydrogen groups by halogen/lithium
Germany, 2003)
The most commonly used diaryl-, and in particular dithi-
enyl-, ethenes are the diarylperfluorocyclopentenes.[7] Di-
arylmaleic anhydrides[8] and diarylmaleimides are also com-
monly used.[9] Many functionalized derivatives of these di-
arylethenes have been synthesized.[6] Although the photo-
chromic properties of these compounds are highly
attractive, the synthesis of diarylethenes is not trivial. Di-
arylperfluorocyclopentenes are prepared by a double sub-
stitution reaction on octafluorocyclopentene by a lithiated
thiophene derivative. The yields are usually moderate at
best, it is not easy to scale up the procedure, and a consider-
able amount of monosubstituted perfluorocyclopentene
product is formed. The major cause of these complications
is that octafluorocyclopentene is very volatile (bp. 26Ϫ28
°C)[10] and therefore not easy to handle. Only in one case
has a very high yield (99%) been reported.[11] Another
drawback of this route is that octafluorocyclopentene is
very expensive and not regularly available.
Introduction
Of the many diarylethenes available, dithienylethenes of
the sort illustrated in Scheme 1 show particular promise for
optical switching processes. As illustrated they undergo re-
versible ring-closure on irradiation with ultraviolet (UV) or,
in some cases, visible light. The closed intermediates are
stable to oxidation if R1 and R4 are non-hydrogen substitu-
ents, methyl groups being an excellent example. In many
cases the photochemically induced ring-closure is thermally
irreversible and the compounds can show good fatigue res-
istance.[1] These are promising features for application in
optical data storage,[2,3] molecular wires[4,5] and as molecu-
lar switches.[6]
Recently we communicated a new and rather easy syn-
thetic procedure wherein perhydro- rather than perfluoro-
cyclopentene derivatives are prepared.[12] The reactions can
be performed on a large scale and cheap(er) starting mat-
erials can be used. In this paper we present the full details
on the synthesis and derivatization of these new dithienylcy-
clopentenes as well as details of conversions with perfluoro-
cyclopentene derivatives. We also report methods for non-
symmetrical derivatization of these switches, and conclude
with preliminary data on the photochromic properties of
these compounds.
Scheme 1. Switching cycle for 3,3Ј-dithienylethenes
[a]
Laboratory of Organic Chemistry, Stratingh Institute,
University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Fax: (internat.) ϩ 31-50/363-4296
E-mail: esch@chem.rug.nl
[b]
Syncom B.V.,
Kadijk 3, 9747 AT Groningen, The Netherlands
Eur. J. Org. Chem. 2003, 155Ϫ166  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ193X/03/0101Ϫ0155 $ 20.00ϩ.50/0
155

L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa
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to the McMurry ring closure reaction without significant
decrease in yield.
Results and Discussion
This synthesis can be performed on a large scale and the
starting materials are easily accessible, but the drawback of
5 is the lack of functionality for further derivativization.
Some attempts were made to brominate the 5-methyl
groups of 5 using NBS, but no discrimination could be
achieved between the methyl groups. Later it turned out
that the photochemical reactions of these simple bis(2,5-
dialkylthienyl)cyclopentenes are in fact irreversible, which
implies that these compounds are not suitable as photo-
chromic switches.[12]
The retrosynthetic approach to functionalized dithienyl-
cyclopentenes is shown in Scheme 2. In this route, the cent-
ral cyclopentene ring is formed in the last step by a ring
closure reaction of a 1,5-diketone by a McMurry reac-
tion.[13,14] Another possibility to form this ring is by a ring-
closure
metathesis
employing
a
2,6-diaryl-1,6-
heptadiene.[15,16] The latter compound can be obtained
from the 1,5-diketone by a Wittig reaction. The generation
of substituted cyclopentenes either by a McMurry reaction
or a ring-closure metathesis are well established reactions
that can be carried out on a multigram scale. The McMurry
reaction was preferred, because it is performed in two con-
secutive steps in a ‘‘one-pot’’ procedure. As an alternative
an ‘‘instant’’ method can be used, in which active titanium
is prepared in the presence of the substrate.[17]
The second approach shown in Scheme 3 starting from
compound 7 offers access to compounds that can be more
readily functionalized. Halogens are very versatile func-
tional groups. First 2-bromo-5-methylthiophene was used
as a substrate for the FriedelϪCrafts acylation. However,
this reaction was not successful because it was found that
during the FriedelϪCrafts acylation the bromo substituent
shifted to the 3-position, followed by acylation at the more
reactive 2-position. This rearrangement (‘‘halogen dance’’)
has been observed before for bromothiophenes,[20] and iod-
ides are known to behave similarly. However, when 2-
chloro-5-methylthiophene[20,21] was subjected to
a
FriedelϪCrafts reaction[18] with AlCl3 and glutaryl dichlor-
ide in CS2 at 0 °C the desired diketone 8 was obtained in
98% yield (Scheme 3). The reasons for this high selectivity
and the resulting avoidance of the need for the separation
of isomers are not clear.[20]
Scheme 2
The key intermediates in the two synthetic routes shown
Diketone 8 was then used in a McMurry reaction[19] with
in Scheme 3 are the 1,5-diaryl-1,5-diketones 4 and 8. Many TiCl3(THF)3 and Mg in THF at 40 °C.[13] NMR analysis
procedures for the preparation of aryl ketones are known, revealed that instead of the expected product 1, the dechlor-
the most straightforward of which is a FriedelϪCrafts acyl- inated ring-closed product 9 had been formed. Catalytic
ation of the corresponding aryl compounds with a 1,5-di- dechlorination of aromatic chlorides using Grignard re-
carboxylic dichloride. Alternatively, the addition of a agents in the presence of (C2H5)2TiCl2 has also been re-
metalated aryl (thienyl) group to a 1,5-dinitrile or 1,5-dies- ported by Takahashi et al.[22] They observed that the use of
ter can be employed. The thiophene derivative to be used THF as a solvent dramatically improves the reactivity in
requires some consideration. The most reactive positions in this dehalogenation reaction. Compound 9 is not very
the thiophene molecule are the 2- and 5-positions, but in stable, and deteriorates even at 4 °C within a week.
the route envisioned in Schemes 2 and 3 the acylation must McMurry reactions can also be carried out with milder re-
occur at the 3-position, which is only possible if the 2- and ducing agents like zinc to prepare the Ti0 species in situ.[17]
5-positions are substituted. The functional groups have to When the reaction was carried out with TiCl3(THF)3 and
be compatible with the reaction conditions and, if non- Zn in THF at 40 °C, the desired diarylethene 1 could be
equivalent, must have the correct directing effects. The obtained. Later it was found that instead of TiCl3, which
FriedelϪCrafts reaction is preferred, because it saves one was suddenly removed from the commercial market, easier
step compared to an addition of a metalated aryl (thienyl) to handle TiCl4 also could be used. The synthesis of com-
group to a 1,5-dinitrile or 1,5-diester.
pound 1 can be performed on a large scale (largest scale
First, the synthesis of the simple dithienylcyclopentene 5 used to date was 1 mol) and requires only cheap starting
was carried out in order to test the viability of the proposed materials.
route (Scheme 3). The FriedelϪCrafts acylation[18] of 3 with
The photochromic switch 1 can easily be functionalized
glutaryl dichloride in CS2 using AlCl3 as a Lewis acid gave in many different ways. Compound 1 can readily undergo a
a tarry reaction product, from which the desired 1,5-dike- chlorine/lithium exchange at ambient temperature
tone 4 could be isolated by column chromatography in 40% (Scheme 4) thus providing a versatile handle for the intro-
yield. Ring closure of 4 by a McMurry reaction with Mg duction of functionality.
and TiCl3(THF)3 in THF at 40 °C[19] gave the desired 1,2-
Quenching of the doubly lithiated 1 with, for instance,
bis(2,5-dimethylthien-3-yl)cyclopentene (5) in 58% yield DMF gave the dialdehyde 10 in 52% yield (Scheme 4). The
after purification using column chromatography with hex- diacid 11 (Scheme 4) can be obtained by oxidation of 10,[23]
ane. Later it was found that unpurified 4 could be subjected or directly by adding CO2(s) to a solution of lithiated 1.[24]
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Syntheses of Dithienylcyclopentene Optical Molecular Switches
FULL PAPER
Scheme 3. Reagents: (i) glutaryl dichloride, AlCl3, CS2, 40%; (ii) Mg, TiCl3(THF)3, THF, 55%; (iii) NCS, HOAc, C6H6, 82%; (iv) glutaryl
dichloride, AlCl3, CS2, 98%; (v) Mg, TiCl3(THF)3, THF, 55%; (vi) Zn, TiCl3(THF)3, THF, 50%
Scheme 4. Reagents: (i) nBuLi, room temperature., THF; (ii) DMF, THF, 52%; (iii) AgNO3, NaOH, H2O; (iv) CO2(s), THF, 81%;
(v) (R)-(ϩ)-1-phenylethylamine, MeOH, 26%; (vi) malonitrile, piperidine, reflux, EtOH, 72%; (vii) N-methylmorpholine, 2-chloro-4,6-
dimethoxytriazine, dodecylamine, CH2Cl2, 53%; (viii) hexyl 5-chloro-5-oxovalerate, AlCl3, CS2, 54%
This compound can for instance be used to synthesize chloro-5-oxovalerate.[28] Substitution occurs exclusively at
amides 14,[25] or alkyl esters. The dialdehyde 10 is a pre- the most reactive 5-position.
cursor for the bis[(phenylethyl)imino] derivative 12.[26] Fur-
Photochromic switches with an extended aromatic system
thermore a condensation reaction with malonitrile has been are currently the focus of much attention. It would be useful
carried out to yield 13.[27] Lithiated 1 can also be quenched if an approach to diarylcyclopentene-based switches were
with a Brønsted acid to give 9. Although this compound is available, in which an aromatic moiety could readily be
not very stable, it can be used to synthesize compound 15 coupled with a dithienylcyclopentene building block. The
under standard FriedelϪCrafts conditions using hexyl 5- bis(chlorothienyl)cyclopentene switch 1 is in principle such
Eur. J. Org. Chem. 2003, 155Ϫ166
157

L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa
FULL PAPER
Scheme 5. Reagents: (i) Ni(dppp)Cl2, reflux, EtO2, 40%
a building block. The most straightforward method to ex- have been made by Fu,[37] Buchwald,[38] Nolan,[39] Beller[40]
tend the aromatic system of 1 is by a cross coupling reaction and Trudell.[41] Suzuki coupling of chlorothiophene derivat-
using organometallic reagents with organic halides and re- ives has not been reported yet, but the recently developed
lated electrophiles. Cross coupling methods[29] available for systems looked promising for our purpose. A series of reac-
synthesis of biaryls are the Kumada coupling, the Suzuki tions were carried out under the various conditions as
coupling, the Stille coupling, or aryl CϪC bond formation reported.[37Ϫ41] Unfortunately none of these attempts led
reactions mediated by organozinc reagents and aryl halides. to a cross-coupled product, and starting compound 1 was
In order to attach an aryl group to 1 it can be allowed to recovered in all cases. Apparently, also in the Suzuki reac-
react with an organometallic reagent or the corresponding tion the low reactivity of the chloroϪthienyl bond prevents
boronic acid derivative. Alternatively, 1 can be transformed the coupling reaction from taking place. The most obvious
into the organometallic compound or a boronic acid. Be- solution to this problem is to convert the chloride switch 1
cause many arylboronic acids and organometallic com- into the more reactive bromide derivative or to the boronic
pounds are commercially available, it is easier to use 1 as ester derivative. This latter approach has been applied be-
the aryl halide.
fore in the synthesis of diarylperfluorocyclopentene derivat-
The first approach to synthesize the oligoaryl switch was ives by Lehn et al.[5] It was found that 1 could also be con-
by the Kumada[30] cross coupling reaction, because of the verted into a boronic acid by means of an organolithium
good experience in our group with this reaction for the syn- reagent followed by reaction with a borate, which was then
thesis of oligothiophene derivatives.[31] First a coupling be- allowed to react with an aryl halide in a Suzuki cross coup-
tween 1 and 16 in diethyl ether with Ni(dppp)Cl2 as catalyst ling reaction.
was performed,[32] but unfortunately only a single cross
First, 1 was lithiated with nBuLi in THF at room temper-
coupling took place to give 17 in 40% yield together with a ature, and then treated with B(OBu)3 to provide the
small amount of starting material (Scheme 5). bis(boronic ester) 18 (Scheme 6). The unpurified material
Increasing the amount of catalyst to even stoichiometric was used directly in the Suzuki reaction without any
quantities did not improve this result and only the monoad- workup because it was found that it easily hydrolyses to the
duct could be obtained. Compound 17 is, of course, inter- dehalogenated switch 9 during isolation. A similar observa-
esting for the synthesis of nonsymmetric switches.
tion was made by Lehn et al.[5]
Another (phosphane)Ni complex often used in the Kum-
ada cross coupling is [Ni(PPh3)2Cl2]. On use of 1 equiv. of
this catalyst under the same conditions as described above
(Scheme 5), the starting material, the mono- and the bis(ad-
duct) were recovered from the reaction mixture in a 1:1:4
ratio. Reduction of the amount of catalyst resulted in a
lower yield of the bis(adduct), and at 10 mol % of catalyst
or less no bis(adduct) was formed at all. This route clearly
is not suitable. Most likely, the decreased reactivity of the
chloride compared to bromides or iodides is the main prob-
lem in this Kumada cross coupling reaction.[33]
As an alternative to the Kumada cross coupling the two
aryl fragments might be coupled using a Suzuki reaction.[29]
These cross coupling reactions often proceed under mild
conditions, provided the organoboron compound is activ-
ated with a suitable base. Because of their higher reactivity
Scheme 6. Reagents: (i) nBuLi, room temperature, B(OBu)3, THF;
(ii) C6H5Br, Pd(PPh3)4, Na2CO3, ethylene glycol, H2O, THF, 70%
For the Suzuki cross coupling reaction, Pd(PPh3)4 was
chiefly aryl bromides, iodides, and triflates have been used used as palladium source, Na2CO3 as base, THF as solvent
as starting materials for Suzuki reactions. There is currently and several drops of ethylene glycol were added as cosolv-
a great deal of interest in the coupling of aryl chlorides with ent. In this particular case phenyl bromide was used as aryl
arylboronic acids.[34] Although nickel catalysts are useful halide source (Scheme 6). This procedure worked well, and
for this reaction as has been demonstrated by Indolese[35] after column chromatography, isolated yields of up to 70%
and Saito et al.,[36] most studies have focussed on palladium were reached. The method described above for the Suzuki
catalysts. Recently significant breakthroughs in this area reaction to provide symmetrical 19 could also be used to
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Syntheses of Dithienylcyclopentene Optical Molecular Switches
FULL PAPER
Scheme 7. Reagents: (i) nBuLi, room temperature, B(OBu)3; (ii) 2-bromo-5-dodecylthiophene, Pd(PPh3)4, Na2CO3, ethylene glycol, reflux,
H2O, THF, 37%; (iii) tBuLi, room temperature, B(OBu)3; (iv) methyl 2-(5-bromo-2-thienyl)acetic acid, Pd(PPh3)4, Na2CO3, ethylene
glycol, reflux, H2O, THF, 32%
synthesize nonsymmetrical switches after a slight modifica- ponding diketone.[48] That same approach was used here.
tion. In this way compound 21 (Scheme 7) was synthesized. Diethyl hexafluoroglutarate (27) was synthesized[49] in
Using 1 equiv. of nBuLi and subsequent reaction with quantitative yield by a standard acid-catalysed esterification
B(OBu)3 provided the monoboronic ester 20. This product of hexafluoroglutaric acid.
was allowed to react in the Suzuki reaction with 2-bromo-
In this alternative approach[50] 2-chloro-5-methylthi-
5-dodecylthiophene[42] to provide 21. The amount of the ophene (7) was used as the starting material (Scheme 9).
disubstituted or unsubstituted compound was less than 5%. Treatment of this compound with a lithiating reagent would
Compound 21 was subjected to a second Suzuki reaction. result in lithium/halogen exchange of the chlorine atom at
In this case tBuLi was used in order to promote lithiation the 2-position instead of deprotonation at the 4-position.
because neither nBuLi nor sBuLi were effective. The same Therefore this compound was first brominated at the 4-po-
reaction sequence was repeated, this time with methyl 2-(5- sition using Br2 in chloroform to give 3-bromo-5-chloro-2-
bromo-2-thienyl)acetic acid[43] as aryl halide source to pro- methylthiophene (26). Compound 26 was then lithiated at
vide 23 in 12% overall yield starting from switch 1 Ϫ78 °C in anhydrous diethyl ether using nBuLi. Under
(Scheme 7). All the dithienylcyclopentenes described in this these conditions exclusive lithium/halogen exchange occurs
article synthesized starting form compound 1 show photo- with the bromine atom at the 3-position, and the chlorine
chromic behavior similar to that of known diarylethenes.[44] substituent is not affected. Lithiated 26 was then treated
We investigated whether dithienylperfluorocyclopentene with a solution of 27 in diethyl ether at the same temper-
switches could also be synthesized according to the route ature. After acidic workup, the 1,5-diketone 25 was ob-
developed for 1. A FriedelϪCrafts acylation of thiophene 7 tained in good yield, and other regioisomers were not
with hexafluoroglutaryl dichloride (24),[45] AlCl3 and ben- formed.
zene or toluene has been described in the literature.[46,47]
Ring closure was achieved by the McMurry coupling
Unfortunately this reaction did not work for the com- with TiCl3(THF)3 and Zn in THF at 40 °C to provide 2,
bination of 2-chloro-5-methyl-thiophene (7) and hexa- which was purified by column chromatography. It is, of
fluoroglutaryl dichloride (24), and only an undefined black course, in principle possible to use thiophene derivatives
tar was obtained (Scheme 8). The use of SnCl4 as Lewis other than 26 in this route, provided that they can be
acid did not lead to any improvement. Most likely, the acyl- lithiated exclusively at the 3-position. However, it was found
ation does not occur because the acylium intermediate is so that 3-bromo-5-chloro-2-methylthiophene is an extremely
difficult to obtain owing to the strongly electron-with- versatile intermediate for the introduction of functional
drawing fluorine atoms.
groups at a later stage at the 5,5Ј-positions of the diarylper-
The diethyl ester of hexafluoroglutaric acid (27) has been fluorocyclopentenes. Compound 2 can also easily be deriva-
used in a reaction with phenyllithium to obtain the corres- tized as described for compound 1. Also in the case of 2
Eur. J. Org. Chem. 2003, 155Ϫ166
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L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa
FULL PAPER
Scheme 8. Attempted FriedelϪCrafts acylation
Scheme 9. Reagents: (i) Br2, CHCl3, 93%; (ii) nBuLi, Ϫ78 °C, Et2O, 70%; (iii) Zn, TiCl3(THF)3, THF, 55%
the compound can readily undergo a chlorine/lithium ex- ring opening,[1] and therefore it can be assumed that the
change at ambient temperature (Scheme 4) thus providing PSS represents the closed form of the switch.
1
a versatile handle to introduce functionality. In this way the
The structure of the closed form was confirmed by H
1
diformyl compound could be obtained as described before NMR spectroscopy. The 13C and H NMR spectra of the
by Lehn.[27] Also the Suzuki reaction could be performed open form of 1Ϫ19 showed clearly that these compounds
in the way described before for 1, but 2 gave much lower have C2 symmetry. Upon irradiation of solutions of the
yields than 1 in the Suzuki reaction with phenyl bromide, open form of 10, 12, 13 and 19, a second set of peaks ap-
even when Lehn’s conditions[5] were applied.
peared which are slightly shifted with respect to those of
The photochromic behaviour of 1 and 2 and derivatives the open form indicating that a photochemical reaction has
thereof was studied by irradiation with a high-pressure mer- occurred, but splitting or doubling of the peaks was not
cury lamp at selected wavelengths, and monitored by UV/ observed, which is in agreement with the C2 symmetry of
Vis spectroscopy. Table 1 shows the absorption maxima for the closed form. The differences in chemical shift between
the open and closed forms of a number of the synthesized the open and closed form are most pronounced for the thi-
derivatives described in this article and their corresponding ophene proton and methyl group, and are summarized in
extinction coefficients. The absorption maxima of the open Table 2.
forms of 1, 5 and 9 all appear at wavelengths Ͻ 240 nm,
Irradiation of solutions of 10Ϫ19 in the PSS with visible
but extension of the switches with functional groups which light (λ Ͼ 450 nm) causes the absorption bands in the vis-
can participate in conjugation with the thienyl groups, like ible region to disappear and the absorption band in the UV
with compounds 10Ϫ19 of Table 1, results in a batho- between 250 and 340 nm was restored. Apparently, photo-
chromic shift of the absorption maxima to wavelengths be- chemical switching between the ring-open and ring-closed
tween 250 and 340 nm. Upon irradiation at wavelengths form of 10Ϫ19 is reversible. Especially the photochromic
near their absorption maxima, the solutions of compounds switching of compounds 12Ϫ19 is excellent, and at least 5
10Ϫ19 turn red to purple and two new absorption maxima photochemical switching cycles between the open and
appear in the visible region at 340Ϫ450 nm and closed form can be performed without any sign of degrada-
520Ϫ732 nm, respectively (Table 1 and Figure 1). The ap- tion. The dialdehyde switch 10 is photochemically less
pearance of these bands is characteristic for the formation stable as became clear from the Ϯ 8% decrease in intensity
of the closed form.[1] It should be noted that in all cases a of the band at 280 nm of the open form after one cycle,
photostationary state (PSS) is obtained, and the extinction which is similar to the photochemical stability of the perflu-
coefficients reported in Table 1 are those of the PSS. Due orinated dialdehyde as reported by Lehn et al.[27]
to nonzero absorption of the closed form in the UV spectral
The photochemical behavior of compounds 1, 5 and 9 is
region, both ring closure and ring opening take place after different. The open form of 1 showed a distinct absorption
photoexcitation, leading to an equilibrium situation (PSS) in the UV region after irradiation, and a clear absorption
determined by the quantum yields of ring closing and ring appeared in the visible region (λmax ϭ 444 nm). After
opening. However, quantum yields obtained for diaryleth- longer irradiation times (t Ͼ 10 min), the absorption bands
enes showed that the cyclization is more efficient than the started to decrease indicating that 1 is degrading, probably
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Syntheses of Dithienylcyclopentene Optical Molecular Switches
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Table 1. UV/Vis spectroscopic data of the open and closed (PSS) form of several compounds
[a]
Compound
R/RЈ
λmax open (ε)
λmax PSS (ε)
1
H/Cl
F/Cl
H/Me
H/H
240 (19.0)
276, 444 (1.2)
2
242 (24.9), 300 (4.8)
233 (24.0), 275[b] (8.1)
229 (21.0), 270[b] (9.2)
280 (41.0), 318 (14.9)
252 (29.7), 290[b] (9.9)
271 (54.0), 308[b] (23.6)
334 (27.4), 392 (30.3)
264 (28.7), 298[b] (9.9)
277 (33.3), 303[b] (23.1)
334 (19.3), 331 (5.71), 501 (3.9)
201 (12.8)
5
9
231[b] (10.0)
10[c]
11[d]
12
H/CHO
383 (16.6), 379 (15.8), 580 (14.7)
253 (15.0), 347 (8.7), 531 (6.8)
366 (12.7), 555 (12.4)
358 (18.3), 448 (17.9), 732 (21.8)
269 (12.9), 347 (8.0), 522 (8.9)
269 (24.8), 287 (21.2), 354 (11.1), 364 (12.0) 529 (14.6)
H/COOH
H/CHϭNCH(Me)Ph
H/CHϭC(CN)2
H/CONHC12H25
H/Ph
13[c]
14[d]
19
[a]
UV/Vis spectroscopic data are obtained for n-hexane solutions at 25 °C, unless noted otherwise, and reported in nm (λmax) and 103
cmϪ1 Ϫ1 (ε). The PSS were obtained by irradiation of solutions of the open form with a 150-W mercury lamp with a 313-nm band pass
filter (compounds 2, 10, 11, 12, 14, 19), a 405-nm band pass filter (compound 13), or without filter (1, 5, 9). Shoulder. Benzene as
solvent. Methanol as solvent.
[b]
[c]
[d]
which makes 1 unsuitable as a photochromic switch. Com-
pounds 5 and 9 do not show maxima in the visible region
after irradiation with UV light, and prolonged irradiation
also leads to degradation of the compounds. Here the per-
fluoro compound 2 behaves more favorable. Irradiation of
switch 2 showed a distinct absorption in the visible region
(λmax ϭ 501 nm) due to formation of the closed form, and
irradiation of the closed form with visible light (λ Ͼ
460 nm) caused a complete conversion into the open form.
Apparently, for the perfluoro switch 2 it is possible to switch
selectively between the open and the closed form.
A very important property of the dithienylcyclopentene
switches is the thermal irreversibility of the photochemical
cyclization, and especially the closed form of the perfluoro-
cyclopentene switches are stable up to temperatures well
above 100 °C.[1] The thermal stability of the closed forms
Figure 1. (a) UV/Vis spectra of 19 (4.8 ϫ 10Ϫ5 Ϫ1 in n-hexane) in
(PSS) of 13 and 19 were tested, because the thermal stability
of their perfluoro derivatives were known from literat-
ure.[52,27] For compound 19 it was found that the closed
form shows excellent thermal stability up to 80 °C for pro-
longed periods (Ͼ 14 h), although at 100 °C slowly reverts
to the open form. The ring-closed form of switch 13 ap-
peared to be less stable at elevated temperatures, and reverts
to the ring-open form with a half-life time of 4.27 min at
60 °C in benzene, which is in fact slower then the perfluoro-
the open form (Ϫ) and the closed form (PSS, ---)
1
Table 2. H NMR chemical shift data before and after irradiation
Compound[a] δCH3 open δCH3 closed δCH open δCH closed
10[b]
12[b]
13[b]
19[c]
2.04
1.97
2.14
1.98
2.17
1.94
2.05
2.25
7.42
6.95
7.40
7.04
6.72
7.37
6.56
6.27
cyclopentene analogues compound (t1/2 ϭ 3.3 min).[27] 1
H
NMR spectroscopy revealed the closed form only reverts to
the open form, as there is no sign of any decomposition
products in the spectra.
In summary, a facile synthetic route to diarylethenes 1
and 2 has been described. The reactions can be performed
on a large scale and cheap starting materials can be used
compared to the commonly employed syntheses. The dithi-
enylcyclopentene 1 is a highly versatile starting material for
[a] 1H NMR spectra were obtained for 1 m solutions of the com-
pounds before and after irradiation at 25 °C (δ in ppm), using
[b]
[c]
lamp and filters as described in Table 1.
[D8]Toluene as solvent.
CDCl3 as solvent.
to the compound proposed by Branda et al.[51] Apparently
the photochemical switching of 1 can take place, but the
process is not fully reversible due to degradation processes,
Eur. J. Org. Chem. 2003, 155Ϫ166
161

L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa
FULL PAPER
TiCl3(THF)3 had disappeared and then 4 (0.36 g, 1.1 mmol) was
added to the black solution. After stirring for 2 h at 40 °C, the
mixture was poured into aqueous hydrochloric acid (6 , 50 mL).
After extraction with diethyl ether (2 ϫ 50 mL), the combined di-
ethyl ether layers were washed with a saturated sodium bicarbonate
solution (2 ϫ 25 mL) and H2O (1 ϫ 25 mL), dried (Na2SO4), fil-
tered and the solvent was evaporated in vacuo to yield a brown oil
(0.28 g, 86%). Chromatography on silica gel (hexane/ethyl acetate,
9:1) afforded the product as a white solid (0.19 g, 58%), m.p.
89.0Ϫ90.6 °C. 1H NMR (300 MHz, CDCl3): δ ϭ 1.85 (s, 6 H),
a variety of dithienylcyclopentene-based compounds that
can be used as molecular switches, and both symmetrical
and nonsymmetrical derivatives are readily accessible. It ap-
pears that if the conjugation of switch 1 is increased, the
dithienylcyclopentenes shows excellent switching behavior
and little fatigue. It is foreseen that these favorable proper-
ties together with the facile synthetic procedures reported
here will strongly stimulate the application of dithienylcy-
clopentene-based photochromic switches in advanced sys-
tems like switchable molecular wires and antenna systems, 1.95Ϫ2.09 (m, 2 H), 2.34 (s, 6 H), 2.73 (t, J ϭ 7.4 Hz, 4 H), 6.41
(s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 13.8 (q), 14.9
molecular electronics, and smart materials.
(q), 22.7 (t), 38.2 (t), 125.9 (d), 131.9 (s), 134.0 (s), 134.8 (s), 135.3
(s) ppm. MS (EI): m/z ϭ 288 [Mϩ]. C17H20S2: calcd. C 70.78, H
6.99; found C 70.13, H 6.92.
Experimental Section
2-Chloro-5-methylthiophene (7): 2-Methylthiophene (100 mL, 1.03
mol) and N-chlorosuccinimide (152 g, 1.13 mol) were added to a
stirred solution of benzene (400 mL) and acetic acid (400 mL). The
suspension was stirred for 30 min at room temperature, then, after
1 h of heating at reflux, the cooled mixture was poured into a 3 
aq. NaOH solution (300 mL). The organic phase was washed with
a 3  aq. NaOH solution (3 ϫ 300 mL), dried (Na2SO4), filtered
and the solvent evaporated in vacuo to yield a slightly yellow liquid.
Purification of the product by vacuum distillation (19 Torr, 55 °C)
General Procedures: Melting points were determined with a Büchi
melting point apparatus or a Mettler FP1 melting point apparatus
1
and are uncorrected. H NMR spectra were recorded at 200 MHz,
300 MHz or 500 MHz, 13C NMR spectra were recorded at
50.3 MHz, 75.4 MHz or 125.7 MHz, and 19F NMR spectra were
recorded at 188.2 MHz or 470.3 MHz. All spectra were taken at
ambient temperature, with the residual protons from the solvent as
an internal reference and the chemical shifts are reported relative
to TMS. Mass spectrometry was performed using CIϩ, DEI or EIϩ
ionization procedures. The dithienylethenes are sometimes hard to
sublime and only by means of DEI (desorption electron ionization)
it is possible in these cases to obtain the mass spectrum. Derivatives
synthesized starting from compound 1 are light-sensitive and were
therefore exclusively handled in the dark using brown glassware,
and column chromatography was performed in yellow light.
1
afforded a colourless liquid (111 g, 84%), b.p. 55 °C (19 Torr). H
NMR (300 MHz, CDCl3): δ ϭ 2.30 (s, 3 H), 6.40Ϫ6.42 (m, 1 H),
6.58 (d, J ϭ 2.2 Hz, 1 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ
15.2 (q), 124.3 (s), 125.7 (s), 126.4 (s), 138.4 (s) ppm. MS (EI):
m/z ϭ 131 [Mϩ]. C5H5ClS: calcd. C 45.29, H 3.80; found C 45.76,
H 3.77.
1,5-Bis(5-chloro-2-methylthien-3-yl)pentane-1,5-dione (8): Under
vigorous stirring AlCl3 (48 g, 0.36 mol) was added in portions to
an ice-cooled solution of 7 (32.3 mL, 0.298 mol) and glutaryl
dichloride (25 g, 0.15 mmol) in CS2 (300 mL). After addition of
AlCl3, the reaction mixture was stirred for 2 h at room temperature.
Then ice-cold water (100 mL) was carefully added to the reaction
mixture and the water layer was extracted with diethyl ether (3 ϫ
150 mL). The combined organic phases were washed with water (1
ϫ 100 mL), dried (Na2SO4), filtered and the solvent was evapor-
ated in vacuo to yield a brown tar (53 g, 98%). This tar can be
purified by flash chromatography (hexane/ethyl acetate, 9:1) to pro-
vide a white solid (25.9 g, 48%), m.p. 82.0Ϫ85.0. For further reac-
tions it is, however, not necessary to purify this tar. 1H NMR
(200 MHz, CDCl3): δ ϭ 1.98Ϫ2.12 (m, 2 H), 2.66 (s, 6 H), 2.86 (t,
J ϭ 6.8 Hz, 2 H), 7.19 (s, 2 H) ppm. 13C NMR (125.7 MHz,
CDCl3): δ ϭ 16.0 (q), 18.1 (t), 40.4 (t), 125.2 (d), 126.7 (s), 134.7
(s), 147.6 (s), 194.7 (s) ppm. IR (Nujol): ν˜ ϭ 1675 cmϪ1 (CϭO).
MS (EI): m/z ϭ 360 [Mϩ]. C15H14Cl2O2S2: calcd. C 49.87, H 3.91;
found C 49.46, H 3.94.
Materials: Reagents and starting materials were used as supplied.
The solvents were distilled and dried before use, if necessary, using
standard methods. Aldrich silica gel grade 9385 (230Ϫ400 mesh)
was used for column chromatography. Hexyl 5-chloro-5-oxovaler-
ate,[28] 2-bromo-5-dodecylthiophene,[42] methyl 2-(5-bromo-2-thien-
yl)acetic acid[43] and diethyl hexafluoroglutarate (27)[48] were syn-
thesized according to literature procedures. The petroleum ether
used had a boiling range of 40Ϫ60 °C.
1,5-Bis(2,5-dimethylthien-3-yl)pentane-1,5-dione (4): AlCl3 (7.02 g,
52.7 mmol) and 2,5-dimethylthiophene (5.0 mL, 44 mmol) were ad-
ded to CS2 (100 mL). The mixture was heated to reflux and glutaryl
dichloride (3.71 g, 22.0 mmol) in CS2 (25 mL) was added dropwise.
After the addition of glutaryl dichloride, the reaction mixture was
refluxed for 2 h. After cooling to room temperature, cold H2O
(50 mL) was carefully added to the reaction mixture and the water
layer was extracted with diethyl ether (3 ϫ 75 mL). The combined
organic phases were washed with a saturated NaHCO3 solution (1
ϫ 50 mL) and water (1 ϫ 50 mL), dried (Na2SO4), filtered and the
solvent was evaporated in vacuo to yield a yellow solid (3.78 g,
54%), m.p. 57.0Ϫ58.2, which was used in subsequent reactions
without further purification. 1H NMR (200 MHz, CDCl3): δ ϭ
1.85 (s, 6 H), 1.95Ϫ2.09 (m, 2 H), 2.34 (s, 6 H), 2.73 (t, J ϭ 7.2,
9.6 Hz, 4 H), 6.41 (s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3):
δ ϭ 14.9 (q), 16.0 (q), 18.5 (t), 40.6 (t), 126.0 (d), 134.9 (s), 135.3
(s), 147.1 (s), 195.9 (s) ppm. IR (Nujol): ν˜ ϭ 1669 cmϪ1 (CϭO).
MS (EI): m/z ϭ 320 [Mϩ]. C17H20O2S2: calcd. C 63.72, H 6.29;
found C 64.14, H 6.33.
1,2-Bis(2-methylthien-3-yl)cyclopentene (9): TiCl3(THF)3 (1.57 g,
4.23 mmol) and Mg (0.26 g, 11 mmol) were stirred under nitrogen
in dry THF (30 mL) at 40 °C until the blue colour of TiCl3(THF)3
had disappeared, whereupon 8 (1.53 g, 4.23 mmol) was added to
the black solution. After stirring for 30 min at 40 °C, the mixture
was cooled to room temperature and poured into aqueous hydro-
chloric acid (6 , 50 mL). The resulting mixture was extracted with
diethyl ether (2 ϫ 50 mL). The combined organic layers were
washed with a saturated NaHCO3 solution (2 ϫ 25 mL) and H2O
(1 ϫ 25 mL), dried (Na2SO4), filtered and the solvent was evapor-
1,2-Bis(2,5-dimethylthien-3-yl)cyclopentene
(5):
TiCl3(THF)3 ated in vacuo to yield a brown oil (0.79 g, 72%). Chromatography
(0.42 g, 1.1 mmol) and Mg (69 mg, 2.8 mmol) were stirred under
nitrogen in dry THF (30 mL) at 40 °C until the blue colour of
of the oil on silica gel (petroleum ether) afforded the compound as
a yellow oil (0.22 g, 20%). Due to its instability further purification
162
Eur. J. Org. Chem. 2003, 155Ϫ166

Syntheses of Dithienylcyclopentene Optical Molecular Switches
was not successful. 1H NMR (200 MHz, CDCl3): δ ϭ 1.92 (s, 6 until pH ϭ 1 was reached. The precipitated product was filtered
FULL PAPER
H), 1.97Ϫ2.14 (m, 2 H), 2.79 (t, J ϭ 7.5 Hz, 4 H), 6.74 (d, J ϭ
with care and washed with 1  HCl. The residual water was azeo-
5.0 Hz, 2 H), 6.95 (d, J ϭ 5.0 Hz, 2 H) ppm. 13C NMR
tropically removed with toluene to yield an off-white solid (0.43 g,
1
(125.7 MHz, CDCl3): δ ϭ 13.9 (q), 22.9 (t), 38.3 (t), 121.0 (d), 128.0 1.2 mmol, 81%). H NMR ([D6]DMSO, 300 MHz): δ ϭ 1.91 (s, 6
(s), 134.3 (d), 136.5 (s), 143.2 (s) ppm. MS (EI): m/z ϭ 260 [Mϩ].
H, CH3), 1.95Ϫ2.05 (m, 2 H), 2.77 (t, J ϭ 7.8 Hz, 4 H), 7.40 (s, 2
H,) ppm. 13C NMR (75.4 MHz, [D6]DMSO): δ ϭ 14.3 (q), 22.3
(t), 37.9 (t), 130.4 (s), 133.8 (d), 134.3 (s), 136.4 (s), 141.7 (s), 162.6
(s) ppm. IR: ν˜ ϭ 1550, 1663, 2578, 2841, 2953 cmϪ1, MS (EI):
m/z ϭ 348 [Mϩ]. C17H16O4S2: calcd. C 58.60, H 4.63; found C
58.77, H 4.73.
1,2-Bis(5-chloro-2-methylthien-3-yl)cyclopentene (1): ‘‘Instant
method’’: A mixture of 8 (1.13 g, 3.13 mmol), TiCl3(THF)3 (2.32 g,
6.26 mmol), Zn dust (0.82 g, 7.8 mmol) and THF (30 mL) was
stirred under nitrogen at 40 °C for 1 h. The mixture was cooled to
room temperature and poured through a glass filter containing sil-
ica gel that was pretreated with petroleum ether. The silica was
rinsed with petroleum ether. After evaporation of the solvent, a
yellow solid (0.97 g, 94%) remained. Pure 1 was obtained as a white
solid (0.45 g, 44%) after purification by chromatography on silica
gel (petroleum ether), m.p. 75.6Ϫ78.0. 1H NMR (200 MHz,
CDCl3): δ ϭ 1.98 (s, 6 H),1.94Ϫ2.09 (m, 2 H), 2.71 (t, J ϭ 7.5 Hz,
4 H), 6.58 (s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 14.9
(q), 23.6 (t), 39.1 (t), 125.9 (d), 127.4 (s), 134.0 (s), 135.1 (s), 135.5
(s) ppm. MS (EI): m/z ϭ 328 [Mϩ]. C15H14Cl2S2: calcd. C 54.71,
H 4.29; found C 54.54, H 4.24.
1,2-Bis(2-methyl-5-{[(1-phenylethyl)imino]methyl}thien-3-yl)cyclo-
pentene (12): 10 (40.3 mg, 0.130 mmol) was dissolved in (ϩ)-(1R)-
1-phenylethylamine (3.4 mL of a stock solution of 1 mL of amine
in 99 mL of methanol). After 18 h of stirring at room temperature,
the solvent was removed in vacuo. The mixture was diluted with
dichloromethane and dried (Na2SO4), filtered and the solvents were
evaporated in vacuo to yield a brown oil. Chromatography on
Al2O3 (hexane/ethyl acetate/Et3N, 2:1:0.02) afforded a purple oil
(17.6 mg, 26%). 1H NMR (200 MHz, CDCl3): δ ϭ 1.55 (d, J ϭ
6.6 Hz, 6 H), 1.97 (s, 6 H), 1.94Ϫ2.10 (m, 2 H), 2.76 (t, J ϭ 7.7 Hz,
4 H), 4.45 (q, J ϭ 6.8, J ϭ 6.4 Hz, 2 H), 6.95 (s, 2 H), 7.24Ϫ7.36
(m, 10 H), 8.25(s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ
14.9 (q), 22.9 (t), 24.8 (q), 38.3 (t), 69.1 (d), 126.6 (d), 126.73 (d),
128.4(d), 131.6 (s), 134.4 (s), 135.9 (s), 138.5 (s), 139.0 (s), 145.1 (s),
152.7 (d) ppm. IR: ν˜ ϭ 1631 cmϪ1. HRMS: calcd. for C33H34N2S2
522.216, found 522.216.
1,2-Bis(5-chloro-2-methylthien-3-yl)cyclopentene (1): Due to lack of
TiCl3 the reaction was later on carried out with TiCl4. THF
(50 mL) and Zn dust (2.5 g) were placed in a three-necked flask
under nitrogen. TiCl4 (6.2 mL, 29 mmol) was added very cautiously
using a glass syringe. The solution turned yellow and was refluxed
for 45 min. Next it was cooled in an ice bath and 8 (6.90 g,
19.2 mmol) was added in portions. This mixture was refluxed for
2 h, subsequently quenched with 10% aq. K2CO3 (50 mL) and ex-
tracted with diethyl ether (4 ϫ 20 mL). The combined organic
layers were washed with H2O (1 ϫ 25 mL), dried (Na2SO4) and the
solvent was removed in vacuo. The compound was purified by col-
umn chromatography in the same way as described before in this
case to yield (3.16 g, 50%) of a white solid. The properties of 1
synthesized by this route are identical to those described for 1 pre-
pared with TiCl3(THF)3.
1,2-Bis[5-(2,2-dicyanoethenyl)-2-methylthien-3-yl]cyclopentene (13):
A
mixture of malonitrile (15 mg, 0.23 mmol), 10 (35.0 mg,
0.111 mmol) and a catalytic amount of piperidine (1 drop of a
stock solution of 1 drop of amine in 2 mL of absolute ethanol) in
absolute ethanol (1.5 mL) was heated to reflux. After 17 h, the so-
lution was cooled to room temperature and the solvent was re-
moved in vacuo. Trituration of the crude product in methanol re-
sulted in the formation of a brown/orange solid, which was isolated
by filtration in the dark and dried in vacuo (33 mg, 72%), m.p.
154Ϫ156 °C. 1H NMR (200 MHz, CDCl3): δ ϭ 2.14 (s, 6 H),
2.05Ϫ2.20 (m, 2 H), 2.82 (t, J ϭ 7.2 Hz, 4 H), 7.40 (s, 2 H), 7.63
(s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 15.4 (q), 22.8 (t),
38.3 (t), 113.1 (s), 113.9 (s), 132.1 (d), 135.2 (s), 137.7 (s), 139.0 (s),
148.8 (s), 150.3 (s) ppm. IR (KBr): ν˜ ϭ 2343 cmϪ1, 2223 cmϪ1
(CϵN), 1574 cmϪ1 (CϭCϪCN)2. MS (EI): m/z ϭ 412 [Mϩ].
HRMS: calcd. for C23H16N4S2 412.0816, found 412.0822.
1,2-Bis(5-formyl-2-methylthien-3-yl)cyclopentene
(10):
nBuLi
(7.85 mL of a 1.6  solution in hexane, 12.6 mmol) was added to a
stirred solution of 1 (1.97 g, 5.98 mmol) in anhydrous THF (20 mL)
under nitrogen at room temperature. 1 h after the addition, the re-
action mixture was quenched with anhydrous dimethylformamide
(0.97 mL, 12 mmol). The mixture was stirred then for an additional
hour at room temperature, before it was poured into aqueous HCl
(2 , 50 mL). The mixture was extracted with diethyl ether (3 ϫ
25 mL). The combined organic layers were washed with a saturated
NaHCO3 solution (2 ϫ 25 mL) and H2O (1 ϫ 25 mL), and dried
(Na2SO4), filtered and the solvents evaporated in vacuo to yield a
brown solid (1.89 g, 90%). Chromatography on silica gel (hexane/
ethyl acetate, 9:1) afforded the product as a brown/orange solid
1,2-Bis{5-[(dodecylamino)carbonyl]-2-methylthien-3-yl}cyclopentene
(14): 11 (0.20 g, 0.60 mmol) was suspended in CH2Cl2 (5 mL) and
placed in an ice bath. Subsequently N-methylmorpholine (0.13 mL,
1.2 mmol) was added and the solid material dissolved. Then 2-
chloro-4,6-dimethoxytriazine (0.20 g, 1.2 mmol) was added, and a
white precipitate was formed immediately after this addition. The
reaction mixture was stirred for 2 h at 0 °C, and then another 2
equiv. of N-methylmorpholine (0.13 mL, 1.2 mmol) were added fol-
lowed by dodecylamine (0.26 mL, 1.2 mmol). Stirring was con-
tinued for 1 h at 0 °C, and subsequently overnight at room temper-
ature. CH2Cl2 (50 mL) was added and the solution was washed
with 1  HCl (2 ϫ 20 mL), brine (1 ϫ 20 mL), a saturated aq.
bicarbonate solution (1 ϫ 20 mL) and H2O (1 ϫ 20 mL). The or-
ganic phase was dried (Na2SO4) and evaporation of the solvent
gave a solid product. After purification by column chromatography
1
(0.98 g, 52%), m.p. 116.1Ϫ117.9 °C. H NMR (200 MHz, CDCl3):
δ ϭ 2.04 (s, 6 H), 2.07Ϫ2.17 (m, 2 H), 2.83 (t, J ϭ 7.5 Hz, 4 H),
7.42 (s, 2 H), 9.74 (s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3):
δ ϭ 15.3 (q), 22.8 (t), 38.3 (t), 134.9 (d), 136.9 (s), 137.3 (s), 140.1
(s), 146.3 (s), 182.2 (s) ppm. IR (KBr): ν˜ ϭ 1662 cmϪ1 (CϭO).
MS (EI): m/z ϭ 316 [Mϩ]. HRMS: calcd. for C17H16O2S2 316.059,
found 316.061.
1,2-Bis(5-carboxy-2-methylthien-3-yl)cyclopentene (11): nBuLi
(2.4 mL of a 1.6  solution in hexane, 3.8 mmol) was added to a (CH2Cl2/MeOH, 40:1) an off-white solid was obtained (0.22 g,
1
stirred solution of 1 (0.50 g, 1.5 mmol) in anhydrous THF (15 mL)
53%), m.p. 102 °C. H NMR (300 MHz, CDCl3): δ ϭ 1.41 (t, J ϭ
under nitrogen at room temperature. After 1 h, solid CO2 (excess) 6.6 Hz, 3 H, CH3), 1.24 (m,18 H), 1.56 (m, 4 H), 1.92 (s, 6 H),
was added. After 30 min, water (15 mL) was added. The water layer
was washed with diethyl ether and then acidified with 30% HCl
1.97Ϫ2.07 (m, 2 H), 2.77 (t, J ϭ 7.5 Hz, 4 H), 3.35 (m, 4 H), 5.77
(t, 2 H, NH), 7.17 (s, 2 H) ppm. 13C NMR (75.4 MHz, CDCl3):
Eur. J. Org. Chem. 2003, 155Ϫ166
163

L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg, B. L. Feringa
δ ϭ 14.1 (q), 14.6 (q), 22.7 (t), 22.8 (t), 26.8 (t), 26.9 (t), 29.3 (t), in the Suzuki cross coupling reaction without any workup because
FULL PAPER
29.4 (t), 29.5 (t), 29.6 (t, 2ϫ), 31.9 (t), 38.4 (t), 40.0 (t), 129.3 (d), the product is deboronized during isolation.
134.4 (s), 134.7 (s), 136.2 (s), 139.5 (s), 161.8 (s) ppm. IR (neat):
1,2-Bis(2-methyl-5-phenylthien-3-yl)cyclopentene (19): Bromoben-
ν˜ ϭ 1535, 1561, 1618, 2852, 2922, 3298 cmϪ1. MS (DEI): 682 [Mϩ].
zene (1.12 mL, 9.29 mmol) was dissolved in THF (12 mL),
Pd(PPh3)4 (0.37 g, 0.30 mmol) was added, and the resulting solu-
tion was stirred for 15 min at room temperature. Then aqueous
Na2CO3 (23 mL, 2 ) and 6 drops of ethylene glycol were added.
This two-phase system was heated in an oil bath just below reflux
at a temperature of 60 °C and the solution of 18 was added drop-
wise via a syringe in a short time period of approximately 5 min.
Subsequently the mixture was refluxed for 2 h and cooled to room
temperature, after which diethyl ether (50 mL) and H2O (50 mL)
were added. The organic layer was separated and dried (Na2SO4).
After concentration, the compound was purified by column chro-
matography on silica (hexane) to yield a brown/yellowish solid
HRMS: calcd. for C41H66N2O2S2 682.457, found 682.455.
1,2-Bis[5-(5-hexyloxy-1,5-dioxopentyl)-2-methylthien-3-yl]cyclo-
pentene (15): AlCl3 (3.50 g, 26.2 mmol) was added in three portions
to a solution of 9 (3.10 g, 11.9 mmol) and hexyl 5-chloro-5-oxoval-
erate (5.60 g, 23.8 mmol) in CS2 (90 mL) at 0 °C. After addition,
the ice bath was removed and the mixture was stirred at room tem-
perature for 2 h. The mixture was then cautiously quenched with
ice-cold water and the water layer was extracted with diethyl ether
(2 ϫ 75 mL). The combined organic layers were washed with a
saturated aq. bicarbonate solution (2 ϫ 50 mL) and brine (1 ϫ
50 mL). The organic layer was dried with Na2SO4 and the solvent
evaporated to leave a dark oil. This oil was purified by column
1
(1.80 g, 70%), m.p. 85 °C. H NMR (300 MHz, CDCl3): δ ϭ 1.98
chromatography (SiO2; CH2Cl2/MeOH, 40:1) to afford a pure (s, 6 H), 2.03Ϫ2.13 (m, 2 H), 2.84 (t, J ϭ 7.5 Hz, 4 H), 7.03 (s, 2
product as colorless oil (4.01 g, 54%), 1H NMR (CDCl3, H), 7.19Ϫ7.25 (m, 2 H), 7.32 (dd, J ϭ 6.9, J ϭ 7.5 Hz, 4 H), 7.49
300 MHz): δ ϭ 0.87 (t, J ϭ 6.6 Hz, 3 H), 1.29 (s, 6 H), 1.5Ϫ1.64
(d, J ϭ 7.2 Hz, 4 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 14.4
(m, 2 H), 1.94Ϫ2.05 (m, 2 H), 1.96 (s, 6 H), 2.0Ϫ2.13 (m, 2 H), (q), 23.0 (t), 38.5 (t), 123.9 (s), 125.2 (d), 126.8 (d), 128.7 (d), 134.4
2.38 (t, J ϭ 7.2 Hz, 2 H), 2.80 (t, J ϭ 7.8 Hz, 4 H), 2.84 (t, J ϭ (s), 134.5 (s), 136.6 (s), 139.5 (s) ppm. C27H24S2 (412.6): calcd. C
7.2 Hz, 2 H), 4.05 (t, J ϭ 6.8 Hz, 4 H), 7.38 (s, 2 H) ppm. 13C
NMR (CDCl3, 125 MHz): δ ϭ 13.9 (q), 15.0 (q), 19.7 (t), 22.5 (t),
22.8 (t), 25.5 (t), 28.5 (t), 31.4 (t), 33.3 (t), 37.7 (t), 38.4 (t), 64.5
(t), 132.8 (s), 134.8 (d), 136.8 (s), 139.87 (s), 144.3 (s), 173.1 (s),
191.8 (s) ppm. HRMS: calcd. for C37H52O6S2 656.321, found
656.319.
78.60, H 5.86; found C 78.65, H 5.90.
1-(5-Chloro-2-methylthien-3-yl)-2-[5-(dibutoxyboryl)-2-methylthien-
3-yl]cyclopentene (20): 1 (0.47 g, 1.4 mmol) was dissolved in anhyd-
rous THF (10 mL) and nBuLi (0.89 mL of 1.6  solution in hexane,
1.4 mmol) was added dropwise under nitrogen using a syringe. The
mixture was stirred for 30 min at room temperature and B(OBu)3
(0.58 mL, 2.1 mmol) was added in one portion. This solution was
stirred for 1 h at room temperature and was used in the nonsym-
metrical Suzuki cross coupling reaction without further workup.
2-Thienylmagnesium Bromide (16): A solution of 2-bromothiophene
(1.00 mL, 10.3 mmol) in anhydrous diethyl ether (8 mL) was added
dropwise to magnesium turnings (0.30 g, 12 mmol) in diethyl ether
(2 mL). The mixture spontaneously started to reflux, became tur-
bid and the amount of magnesium diminished in time. After 1 h of
reflux, the Grignard reagent was ready for use in the Kumada
coupling.
1-(5-Chloro-2-methylthien-3-yl)-2-[5-(5-dodecylthien-2-yl)-2-methyl-
thien-3-yl]cyclopentene (21): 2-Bromo-5-dodecylthiophene (0.49 g,
1.4 mmol) was dissolved in THF (12 mL), Pd(PPh3)4 (53 mg, 3
mol %) was added, and this solution was stirred for 15 min at room
temperature. Then aq. Na2CO3 (6 mL, 2 ) and 8 drops of ethylene
glycol were added. This two-phase system was heated in an oil bath
just below reflux (60 °C) and the solution of 20 (1.4 mmol) in THF
(8 mL) was added dropwise via a syringe over a short time. After
that, the mixture was heated at reflux for 2 h. After cooling, diethyl
1-(5-Chloro-2-methylthien-3-yl)-2-[2-methyl-5-(thien-2-yl)thien-3-
yl]cyclopentene (17): A Grignard reagent was prepared of 2-bromo-
thiophene (0.57 mL, 4.8 mmol), Mg (0.12 g, 5.1 mmol) and diethyl
ether (5 mL) according to the procedure described for 16. At the
same time 1 (0.40 g, 1.2 mmol) was added to a suspension of
Ni(dppp)Cl2 (21 mg, 0.04 mmol) in anhydrous diethyl ether ether (100 mL) and H2O (50 mL) were added. The organic layer
(10 mL). The thienylmagnesium bromide solution was added drop- was washed with H2O (1 ϫ 50 mL), 2  HCl (2 ϫ 50 mL), brine
wise at 0 °C. The mixture turned black immediately. After addition, (2 ϫ 50 mL) and dried (Na2SO4). After evaporation of the solvent,
the mixture was refluxed for 20 h. Subsequently the reaction mix-
ture was quenched with 2  HCl at 0 °C, extracted with diethyl
ether (3 ϫ 50 mL) and dried (Na2SO4). After column chromato-
graphy (petroleum ether), 17 was obtained as a red oil (0.18 g,
the product was purified by column chromatography (petroleum
ether) on silica to yield a pink oil (0.28 g, 37%). 1H NMR (CDCl3,
300 MHz): δ ϭ 0.87 (t, J ϭ 6.3, J ϭ 6.9, 3 H), 1.20Ϫ1.34 (m, 20
H), 1.58Ϫ1.71 (m, 2 H), 1.87 (s, 3 H), 1.91 (s, 3 H), 1.97- 2.08 (m,
40%). 1H NMR (300 MHz, CDCl3): δ ϭ 1.90 (s, 3 H), 1.95 (s, 3 2 H), 2.68Ϫ2.81 (m, 4 H), 6.59 (s, 1 H), 6.62 (d, J ϭ 3.6 Hz, 1 H),
H), 2.00Ϫ2.09 (m, 2 H), 2.72Ϫ2.82 (m, 4 H), 6.62 (s, 1 H), 6.86 (s, 6.76 (s, 1 H), 6.84 (d, J ϭ 3.6 Hz, 1 H) ppm. 13C NMR (CDCl3,
1 H), 6.99 (d, J ϭ 4.2 Hz, 1 H), 7.05 (d, J ϭ 3.6 Hz, 1 H), 7.15 (d,
300 MHz): δ ϭ 14.1 (q), 14.2 (q), 22.7 (t), 22.9 (t), 29.1 (t), 29.3
J ϭ 5.1 Hz, 1 H) ppm. 13C NMR (75.4 MHz, CDCl3): δ ϭ 14.2 (t), 29.5 (t), 29.6 (t), 30.1 (t), 31.6 (t), 31.9 (t), 38.4 (t), 38.5 (t),
(q), 14.2 (q), 22.9 (t), 38.3 (t), 38.4 (t), 122.9 (d), 123.7 (d), 124.3
(d), 126.7 (d), 127.6 (d), 133.1 (s), 133.22 (s), 133.9 (s), 135.0 (s),
122.6 (d), 123.5 (d), 123.9 (s), 124.6 (d), 125.0 (s), 126.8 (d), 133.2
(s), 133.3 (s), 133.7 (s), 133.7 (s), 135.0 (s), 135.1 (s), 135.9 (s), 144.8
135.0 (s), 136.0 (s), 137.6 (s) ppm. MS (EI): 377 [Mϩ]. HRMS: (s) ppm. HRMS: calcd. for C31H41S3Cl 554.206, found 554.205.
calcd. for C19H17ClS3 376.017, found 376.018.
1-[5-(Dibutoxyboryl)-2-methylthien-3-yl]-2-[5-(5-dodecylthien-2-yl)-
1,2-Bis[5-(dibutoxyboryl)-2-methylthien-3-yl]cyclopentene (18):
1
2-methylthien-3-yl]cyclopentene (22): 21 (0.30 g, 0.55 mmol) was
dissolved in anhydrous THF (5 mL) and tBuLi (0.35 mL of 1.7 
solution in pentane, 0.61 mmol) was added dropwise under nitro-
gen at 0 °C using a syringe. The mixture was stirred for 45 min at
(1.75 g, 5.30 mmol) was dissolved in anhydrous THF (12 mL) and
nBuLi (4.5 mL of 2.5  solution in hexane, 11.2 mmol) was added
under nitrogen at room temperature in 5 portions using a syringe.
This solution was stirred for 30 min at room temperature, then room temperature and B(OBu)3 (0.22 mL, 0.83 mmol) was added
B(OBu)3 (4.3 mL, 16 mmol) was added in one portion. This reddish
solution was stirred for 1 h at room temperature and was then used
in one portion. This solution was stirred for 1 h at room temper-
ature and was used in the nonsymmetrical Suzuki cross coupling
164
Eur. J. Org. Chem. 2003, 155Ϫ166

Syntheses of Dithienylcyclopentene Optical Molecular Switches
FULL PAPER
reaction without further workup because the product is hydrolyzed
during isolation.
(1.50 g, 4.12 mmol), Zn dust (0.53 g, 8.2 mmol) and THF (25 mL)
were stirred under nitrogen at 40 °C for 1 h. The mixture was co-
oled and poured through a glass filter containing silica gel that
was pretreated with petroleum ether. The silica was rinsed with
petroleum ether. A white solid (0.49 g, 55%) was obtained after
purification by chromatography on silica gel (petroleum ether),
1-[5-(5-Dodecylthien-2-yl)-2-methylthien-3-yl]-2-[(5-{5-[(methoxy-
carbonyl)methyl]thien-2-yl})-2-methylthien-3-yl]cyclopentene (23):
Methyl 2-(5-bromothien-2-yl)acetic acid (0.13 g, 0.55 mmol) was
dissolved in THF (12 mL) and Pd(PPh3)4 (53 mg, 3 mol %) was
added, this solution was stirred for 15 min at room temperature.
Then aq. Na2CO3 (6 mL, 2 ) and 6 drops of ethylene glycol were
added. This two-phase system was heated in an oil bath just below
reflux (60 °C) and the solution of 22 (0.55 mmol) in THF (5 mL)
was added dropwise via a syringe over a short time, followed by
reflux of the mixture for 4 h. After cooling, diethyl ether (100 mL)
and H2O (50 mL) were added. The organic layer was washed with
H2O (1 ϫ 20 mL), saturated aq. NH4Cl (1 ϫ 25 mL), a saturated
aq. bicarbonate solution (1 ϫ 25 mL), brine (2 ϫ 25 mL) and dried
(Na2SO4). After concentration, the compound was purified by col-
umn chromatography (hexane/ethyl acetate, 9:1) on silica to yield
an oil (0.12 g, 32%). 1H NMR (300 MHz, CDCl3): δ ϭ 0.87 (t, J ϭ
6.3, J ϭ 6.9, 3 H), 1.20Ϫ1.34 (m, 20 H), 1.58Ϫ1.71 (m, 2 H), 1.90
(s, 3 H), 1.91 (s, 3 H), 1.98- 2.09 (m, 2 H), 2.71Ϫ2.81 (m, 4 H),
3.73 (s, 3 H), 3.78 (s, 2 H), 6.62 (d, J ϭ 3.6 Hz, 1 H), 6.78 (d, J ϭ
3.9 Hz, 1 H), 6.80 (s, 1 H), 6.83 (d, J ϭ 3.3 Hz, 1 H), 6.84 (s, 1 H),
6.87 (d, J ϭ 3.3 Hz, 1 H) ppm. 13C NMR (CDCl3, 300 MHz): δ ϭ
14.1 (q), 14.3 (q), 22.7 (t), 22.9 (t), 29.1 (t), 29.3 (t), 29.5 (t), 29.6
(t), 30.1 (t), 31.6 (t), 31.9 (t), 35.4 (t), 38.5 (t), 52.3 (q), 122.5 (d),
123.63 (d), 124.3 (d), 124.5 (d), 127.5 (d), 132.9 (s), 133.3 (s), 133.6
(s), 133.9 (s), 134.3 (s), 134.6 (s), 135.1 (s), 136.1 (s), 136.3 (s), 137.6
(s), 144.4 (s), 144.7 (s), 170.7 (s) ppm. HRMS: calcd. for
C38H48O2S4 664.254, found 664.252.
1
m.p. 132 °C. H NMR (200 MHz, CDCl3): δ ϭ 1.88 (s, 6 H), 6.88
(s, 2 H) ppm. 13C NMR (500 MHz, CDCl3): δ ϭ 14.3 (q), 110.7
(t), 117.7 (t), 123.9 (d), 125.4 (s), 127.9 (s), 135.8 (s), 140.4 (s) ppm.
19F NMR (470.3 MHz, CDCl3): δ ϭ Ϫ114.78 (dd, J ϭ 5.5, 5.0 Hz,
4 F), Ϫ136.37 (dd, J ϭ 6.2, 5.0 Hz, 2 F) ppm. C15H8Cl2F6S2
(437.3): calcd. C 41.20, H 1.84; found C 41.25, H 1.87.
Acknowledgments
The Royal Netherlands Academy of Sciences is gratefully acknow-
ledged for a fellowship for J. v. E. The authors thank Patrick van
Rijn for his contribution to the synthetic work.
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Received July 16, 2002
[O02392]
166
Eur. J. Org. Chem. 2003, 155Ϫ166