Efficient preparation of photoswitchable dithienylethene-linker-conjugates by palladium-catalyzed coupling reactions of terminal alkynes with thienyl chlorides and other aryl halides

Article abstract of DOI:10.1002/asia.200900503

Three photochromic dithie-nylethene-linker-conjugates with an adamantane core containing different spacer lengths and footprint areas with carboxylic anchoring groups are synthesized. The synthetic routes start either from the ethynylene-linker 5 or the iodo-substituted linker 8. Reaction conditions for the final Sonogashira coupling step between ethynylenelinker 5 with the chloro-substituted dithienylethene 4 in the presence of [PdCl ₂(CH₃CN)₂]/X-Phos and Cs₂CO ₃ or K3PO4 are optimized using 2-chloro-5-methylthiophene (9) and triethylsily-lacetylene or triisopropylsilylacetylene (10a,b) as model compounds. Experimental conditions are found to suppress the activation of the C(sp)-Si bond in TIPS-acetylene 10 b, a reaction leading to a subsequent cross-coupling reaction to form by-product 12. Furthermore, activation of the C(sp)-Si bond in the presence of the fluorinated backbone of the chloro-substituted di-thienylethene 4 can also be prevented. The photochromic properties of the conjugate 3 and its precursor dithienyl-ethene 7b are also investigated. 2010 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim.

Full text of DOI:10.1002/asia.200900503

FULL PAPERS
DOI: 10.1002/asia.200900503
Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates
by Palladium-Catalyzed Coupling Reactions of Terminal Alkynes with
Thienyl Chlorides and Other Aryl Halides
Marc Zastrow,[a] Sujatha Thyagarajan,[b] Saleh A. Ahmed,[a] Paul Haase,[a]
Sabine Seedorff,[a] Dmitri Gelman,[c] Josef Wachtveitl,[d] Elena Galoppini,[b] and
Karola Rꢀck-Braun*[a]
In memory of Herbert Schumann
Abstract: Three photochromic dithie-
nylethene-linker-conjugates with an
adamantane core containing different
spacer lengths and footprint areas with
carboxylic anchoring groups are syn-
thesized. The synthetic routes start
either from the ethynylene-linker 5 or
the iodo-substituted linker 8. Reaction
conditions for the final Sonogashira
coupling step between ethynylene-
linker 5 with the chloro-substituted di-
[PdCl
G
bond in TIPS-acetylene 10b, a reaction
leading to a subsequent cross-coupling
reaction to form by-product 12. Fur-
or K3PO4 are optimized using 2-chloro-
5-methylthiophene (9) and triethylsily-
lacetylene or triisopropylsilylacetylene
(10a,b) as model compounds. Experi-
mental conditions are found to sup-
À
thermore, activation of the C(sp) Si
bond in the presence of the fluorinated
backbone of the chloro-substituted di-
thienylethene 4 can also be prevented.
The photochromic properties of the
conjugate 3 and its precursor dithienyl-
press the activation of the C(sp) Si
Keywords: cross-coupling
dium · photochromism · thiophene ·
tripodal ligands
AHCTUNGTRENGNeUN thene 7b are also investigated.
thienylethene
4 in the presence of
Introduction
[a] M. Zastrow, Dr. S. A. Ahmed,+ P. Haase, S. Seedorff,
Prof. Dr. K. Rꢀck-Braun
A major challenge in developing functional modules for mo-
lecular electronics, photovoltaics, or biosensor technology is
tuning and controlling energy and electron transfer between
dye adsorbates and semiconductor and metal substrates.[1–4]
A detailed understanding of the elementary processes on
the surfaces, and their photoinduced dynamics requires a ra-
tional molecular design of model compounds.[5–7] It has been
demonstrated that many properties of molecular adsorbate
systems already depend upon the surface-immobilization
strategy.[4,8] For example, the formation of aggregates strong-
ly depends upon the orientation of single dye molecules and
their lateral interaction on the metal or semiconductor sur-
face. Small and large footprint tripodal linkers with three
anchoring groups have been used by us and others to bind
dyes,[9–11] Ru-bpy,[6] and other metal complexes,[12] as well as
photoswitches[3,13–15] to the surface of metal-oxide nanoparti-
cles and metals. Such systems allow control of the dye–sur-
face distance by varying the spacer length.[16] Tripodal link-
ers are also useful for preventing unwanted aggregate for-
mation, as the coverage of the surface is controllable by the
size of the footprint area. The syntheses of tripodal linkers
Institut fꢀr Organische Chemie
Technische Universitꢁt Berlin
Strasse des 17. Juni 135, 10623 Berlin (Germany)
Fax : (+49)30-314-28625
E-mail: krueck@chem.tu-berlin.de
[b] S. Thyagarajan, Prof. E. Galoppini
Department of Chemistry
Rutgers University
73 Warren Street, 07102 Newark (USA)
[c] Dr. D. Gelman
Department of Organic Chemistry
Hebrew Institute of Jerusalem
Givat Ram, 91904 Jerusalem (Israel)
[d] Prof. Dr. J. Wachtveitl
Institut fꢀr Physikalische und Theoretische Chemie
Goethe-Universitꢁt Frankfurt am Main
Max-von-Laue Str. 7, 60438 Frankfurt (Germany)
[+] Permanent address:
Chemistry Department
Faculty of Science
Assiut University, 71516 Assiut (Egypt)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900503.
1202
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Chem. Asian J. 2010, 5, 1202 – 1212

involve Pd-catalyzed cross-coupling reactions as key
steps.[13,17,18] Generally, small linker units derived from
1,3,5,7-tetraphenyladamantane having the three binding
groups attached to the para position of the phenyl residues,
already contain an alkyne spacer for the attachment of a
chromophore. Then, the chromophore-linker-conjugate is
prepared by a Sonogashira coupling reaction of the ethynyl-
substituted tripodal linker system with an iodo- or bromo-
substituted chromophoric unit.[9,13,14] More recently, linker
systems with larger footprints were obtained by an alterna-
tive synthetic route, whereby the iodo-substituted linker pre-
cursor is coupled with the ethynyl-substituted chromo-
phore.[18] We followed both synthetic routes towards the
preparation of the challenging tripodal dithienylethene-
linker-conjugates 1, 2, (Scheme 1) and 3 (Scheme 2) contain-
ing different spacer lengths and footprint areas, and bearing
carboxylic acid anchoring groups for the attachment of pho-
toswitches to semiconductor nanoparticles. Such photochro-
mic systems are useful for studying the dynamics of light-
controllable interfacial electron transfer. Thus, in a module
containing a dye-diarylethene-semiconductor triad, reversi-
ble modulation of electronic interactions by light could open
access towards new applications. Previously, related con-
cepts were successfully demonstrated in solution for photo-
switchable donor–bridge–acceptor systems.[19–23] We herein
report the synthesis of dithienylethene-linker-conjugates for
long-term studies of the ultrafast dynamics of the electron
transfer in dye-diarylethene-semiconductor triad systems.
In view of the recent development of powerful Sonoga-
shira coupling methods that allow cross-coupling of the less
reactive aryl chlorides by using electron-rich phosphines,[24]
and copper-free conditions to avoid homocoupling,[25] the
Scheme 1. Structures of the dithienylethene-linker-conjugates 1 and 2 and
design of their synthesis starting from the ethynylene-linker 5 and the
chloro- and bromo-substituted dithienylethenes 4 and 6.
Abstract in German: Beschrieben wird die Synthese von
drei photochromen Dithienylethen-Linker-Konjugaten, bes-
tehend aus einer zentralen Adamantaneinheit, Carbonsꢁure-
Ankergruppen, sowie variierenden “Spacerlꢁngen” und un-
terschiedlichen Fußabdrꢀcken. Die synthetischen Routen zu
diesen Verbindungen beginnen entweder ausgehend von
dem Ethinylen-Linker 5 oder von dem Iod-substituierten
Linker 8. Fꢀr die finale Sonogashira-Kupplung wurden
dabei Reaktionsbedingungen zwischen dem Ethinylen-
Linker 5 und dem Chlor-substituierten Dithienylethen 4 in
Gegenwart von [PdCl2ACHTUNGTRNE(NUG CH3CN)2]/X-Phos und Cs2CO3 oder
Scheme 2. Synthetic pathway toward dithienylethene-linker-conjugate 3
with the ethynylene-substituted dithienylethene 7b and the iodo-substi-
tuted linker 8.
K3PO4 verwendet, die anhand von Modellstudien mit 2-
Chlor-5-methylthiophen (9) und Triethylsilylacetylen oder
Triisopropylsilylacetylen (10a,b) optimiert wurden. Dabei
wurden experimentelle Bedingungen gefunden, um die Ak-
tivierung der C(sp)-Si-Bindung von TIPS-Acetylen 10b zu
unterdrꢀcken, welche ansonsten zu dem Nebenprodukt 12
fꢀhrte. Darꢀber hinaus konnte die Aktivierung der C(sp)-Si-
Bindung in Anwesenheit des fluorierten Rꢀckgrats des
chlor-substituierten Dithienylethen 4 verhindert werden.
Schließlich wurden die photochromen Eigenschaften des
Konjugates 3 und der Vorlꢁuferverbindung 7b untersucht.
chloro-substituted dithienylethene 4[26] and the ethynyl-sub-
stituted linker system 5[6] were used as starting materials for
the synthesis of the linker-conjugate 1 (Scheme 1).
The general and efficient palladium-catalyzed method de-
veloped by Gelman and Buchwald involves the use of
[PdCl2ACHTNUGTRENUNG(CH3CN)2] and X-Phos (2-dicyclohexylphosphino-
2’,4’,6’-triisopropylbiphenyl) in acetonitrile or dioxane at 70–
908C, in combination with inorganic bases, such as Cs2CO3
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K. Rꢀck-Braun et al.
FULL PAPERS
or K3PO4.[25] In these studies, copper salts were found to en-
hance the rate of oligomerization of the alkyne and to sup-
press the coupling process. To prevent this side reaction,
either copper-free conditions were employed or the alkyne
was added slowly to the reaction mixture. However, thio-
phene is a p-electron-rich heterocycle. Even though halo-
genated thiophenes have been frequently employed in cross-
coupling reactions,[27] most of the cross-coupling protocols
were developed for iodo-substituted or activated bromo-
and chloro-substituted thiophenes, that is, substituted with
electron-withdrawing groups to facilitate the oxidative addi-
tion step. Differences in reactivity between 2-halo- and 3-
halothiophenes were not found to be very pronounced. For
example, Erker et al. thoroughly investigated the palladium-
catalyzed cyanation of thiophene halides employing Pd2
Table 1. Model Sonogashira coupling reactions with thiophene 9 and
TES- or TIPS-acetylene 10a,b.
Entry Acetylene Base
Solvent
t [h] Yield [%][a] 11a/c
1
2
3
4
5
6
7
8
9
10a
10a
10b
10b
10b
10b
10b
10b
10b
10b
Cs2CO3 CH3CN
Cs2CO3 CH3CN
Cs2CO3 CH3CN
Cs2CO3 DMF
17[b]
4
76[c,d]
63[c,e]
83[c]
30[c,g]
33[c,h]
54[c]
50[c]
92[i]
2.5[b]
18[f]
17[f]
18[f]
ACHTUNGTRENNUNG(dba)3 and dppf as catalyst system in the presence of Zn
Cs2CO3 dioxane
Cs2CO3 toluene/DMF
powder and Zn(CN)2 in DMA at 808C or 1208C.[28] Gener-
ally, mild reaction conditions for bromo- and chloro-substi-
tuted thiophenes are still rare, especially for compounds
containing electron-releasing groups.[29,30] More recently,
electron-rich 2-methyl-substituted iodo- and bromothio-
phenes were successfully employed in Suzuki–Miyaura reac-
tions[31] or Heck–Mizoroki coupling procedures at elevated
temperature (80-1408C).[32] Sonogashira reaction methods
for a range of alkynes with halothiophenes were thoroughly
investigated by Santelli employing 2-iodo-, 2-bromo-, and 3-
Cs2CO3 toluene/CH3CN 17[f]
Cs2CO3 CH3CN
2.5[b]
24[f]
K3PO4
K3PO4
CH3CN
CH3CN
67[i]
10
24[b,j]
85[i]
[a] Yield of isolated product. [b] Complete conversion according to TLC.
[c] Ratio cat/ligand [mol%]=1:3. [d] Ratio 11a/12=77:23. [e] Ratio 11a/
12=85:15. [f] No further conversion according to TLC. [g] Ratio 11c/
14=82:18. [h] Ratio 11c/12/14=84:8:8. [i] Ratio cat/ligand [mol%]=
10:30. [j] The reaction was conducted at 808C.
bromothiophenes by using [PdACHTUNGTRNE(UNG C3H5)Cl]2/Tedicyp, K2CO3,
CuI in DMF at 1008C for 20 h.[33] Also, copper-free micro-
wave-assisted procedures were developed for aryl chlorides
and were successfully applied in the coupling of 2-chloro-
thiophenes.[34] Again, in most cases, heat was necessary to
promote the oxidative addition step to palladium, including
cross-coupling reactions involving an electron-rich N-hetero-
cyclic-carbene ligand.[32] The copper-free protocol of
Gelman and Buchwald was extended by Novꢃk and co-
workers by using Pd/C in the presence of X-Phos, and was
successfully employed for the coupling of 2-chlorothiophene
to phenylacetylene in DMA in the presence of K2CO3 at
1108C.[35] Recently, Beller reported copper-free palladium-
catalyzed Sonogashira conditions for the coupling of 3-chlor-
othiophene using N-substituted heteroaryl phosphines at
908C in toluene in the presence of Na2CO3.[36] This survey of
the literature demonstrates the challenges still associated
with the coupling of halo-substituted thiophenes. Herein, we
report on solutions for Sonogashira coupling reactions of
the chloro-substituted dithienylethene 4 for the synthesis of
conjugate 1, and the model compound 2-chloro-5-methyl-
thiophene (9) (Table 1). Methods published previously by
our groups for the key coupling reactions in the syntheses of
the tripodal dithienylethene-linker-conjugates 2 (Scheme 1)
and 3 (Scheme 2) were investigated, using common catalysts,
the alkynyl-substituted dithienylethene 7b and the iodo-sub-
stituted linker building block 8 were chosen (Scheme 2).
Results and Discussion
To find the optimal reaction conditions for the valuable
starting material 4, we carried out model studies with 2-
chloro-5-methylthiophene (9) and triethylsilylacetylene
(TES-acetylene) (10a) or triisopropylsilylacetylene (TIPS-
acetylene) (10b) (Table 1). The first coupling reaction of 9
and TES-acetylene 10a (1.3 equiv) was accomplished with
[PdCl2ACHTNUTRGNE(UNG CH3CN)2] (1 mol%), X-Phos (3 mol%), and Cs2CO3
(2.6 equiv) in dry acetonitrile (0.5m) containing <10 ppm
water at 1008C (Table 1, entry 1). The cross-coupling prod-
uct 11a was isolated in 76% yield after 17 h reaction time,
workup, and flash chromatography. The dithienylethynylene
12 (Table 1, entry 1) was also isolated (ratio 11a/12=77:23;
1H NMR spectroscopy of the crude product mixture). Exact
turnover and the formation of the hydrodehalogenated by-
product 13 was not ascertained by 1H NMR spectroscopy
because of the low boiling point of 2-methylthiophene (13)
(b.p.: 110–1138C).[37] However, the mass balance was satisfy-
ing, and by shortening the reaction times, the amount of by-
product 12 could be minimized at the expense of incomplete
turnover. Thus, after 4 h, predominant formation of product
11a (ratio 11a/12=85:15) was observed, and 11a was isolat-
ed in 63% yield by flash chromatography (Table 1, entry 2).
The formation of compound 12 can be understood as two
consecutive Sonogashira reactions, because formation of 12
for example, PdACHTUNGTRENNUNG(PPh3)4, Pd2AHCTNURTGE(GNNNU dba)3/PCAHTNGUTREN(NUGN o-tol)3, or [PdCl2
ACHTUNGTRENNUNG(PhCN)2]/ACHTUNGTRENNUNG[(tBu)3PH]BF4 with or without CuI as cocatalyst
in the presence of NEt3 or DIPA.[13,14,18]
Accordingly, the bromo-substituted precursor 6 and the
linker system 5 were selected as useful starting materials for
conjugate 2 (Scheme 1). For the synthesis of conjugate 3,
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Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates
takes place in one step without the detection and isolation
of the supposed intermediate 11b (Table 1). A coupled reac-
tion either demands the Pd-mediated activation or the un-
served. Fortunately, with K3PO4 the reaction temperature
could be decreased to 808C. After 24 h, complete consump-
tion of the starting material was monitored by TLC, and 11c
was isolated in 85% yield after flash chromatography
(Table 1, entry 10). The results summarized in Table 1 dem-
onstrate an influence of a) the reaction time and b) the sol-
vent, as the best yields were observed for short reaction
times in acetonitrile. Also, the moisture content of the bases
and the solvents, and the differences in the solubility and
the basicity of the bases, may have contributed to the reac-
tion outcome.[45–47] All these factors similarly affect a palladi-
À
wanted deprotection of the C(sp) Si bond. Sila–Sonogashira
reactions are usually run in the presence of fluoride ions.[38]
However, Pombo–Villar[39] and others[40–42] demonstrated
À
that the C(sp) SiMe3 bond is also directly activated or de-
protected by the appropriate choice of catalysts, additives
(Bu4NX, X=Br, Cl),[39,42] CuI,[39] or other reaction condi-
tions (Cs2CO3, 1508C).[40,41] Interestingly, in this case, the
À
C(sp) SiEt3 bond was previously observed to be stable
À
under similar reaction conditions, but with shorter reaction
times.[25] However, for TIPS-substituted acetylene 10b, com-
plete conversion of the starting material was observed after
2.5 h, and product 11c was isolated in 83% yield (Table 1,
entry 3). Side products were not detected. When the reac-
tion was carried out in DMF or dioxane, conversion was in-
complete (Table 1, entries 4 and 5), and the formation of a
new by-product, the homocoupled product 14 (1H NMR),
was observed. Furthermore, the
um-mediated C(sp) SiR3 activation, as well as the cleav-
age[48] of the TES-group and the TIPS-group (Table 1,
entry 5). However, in the cases examined, the deprotected
1
alkyne 11b was not detected by H NMR spectroscopy of
the crude products.
On route to the photoswitch-linker-conjugate 1, the syn-
thesis of compounds 15a,b (Scheme 3) was tested, as both
compounds are also valuable starting materials for the prep-
formation of dithienylethyny-
lene 12 was also observed (ratio
in DMF: 11c/14=82:18; ratio
in dioxane: 11c/12/14=84:8:8;
Table 1, entries 4 and 5). The
formation of the homocoupled
product 14 from the chloro-sub-
stituted thiophene 9 was previ-
ously observed in the literature
in the absence of a transmetal-
lation partner.[43]
Scheme 3. Synthesis of the ethynylene-dithienylethenes 15a,b by model Sonogashira coupling reactions of the
chloro-substituted dithienylethene 4 with the acetylenes 10a,b.
The use of a 2:1 mixture of
toluene and DMF as the reac-
tion solvent yielded the cross-
coupling product 11c in 54%
yield after 18 h without formation of by-products. Moreover,
a reaction carried out in a 2:1 mixture of toluene and aceto-
nitrile proceeded without formation of by-products, and 11c
was obtained in 50% yield (Table 1, entries 6 and 7). Ac-
tually, the yield of 11c was increased by using a higher load-
ing of the palladium catalyst (10 mol%) and X-Phos
(30 mol%), analogous to a report previously published by
Lautens and co-workers for a related catalyst precursor.[44]
Finally, in acetonitrile and in the presence of Cs2CO3 at
1008C after 2.5 h workup and flash chromatography, product
11c was isolated in 92% yield (Table 1, entry 8). Also, reac-
tions with K3PO4 were tested (Table 1, entries 9 and 10).
Therefore, similar to Cs2CO3,
aration of additional conjugates with spacer lengths different
from 1–3. Thus, the chloro-substituted dithienylethene 4
(Scheme 3) was dissolved in acetonitrile (0.5 mL, 0.2m solu-
tion) and treated with [PdCl2ACHTUNTRGNEUNG(CH3CN)2] (1 mol%), X-Phos
(3 mol%), and Cs2CO3 (2.6 equivalents) in the presence of
TES-acetylene 10a (1.3 equivalents) at 908C. After 17 h,
conversion of the chloride 4 was complete. However, bis-
thienylethynylene 16 (Scheme 3, Table 2), which was isolat-
ed in 40% yield, and the hydrodehalogenated by-product 17
were the only products obtained, and the cross-coupling
product 15a was not observed. When the concentration of
compound 4 in anhydrous redistilled acetonitrile was raised
also K3PO4, derived from
Table 2. Optimization of the Sonogashira coupling reaction of dithienylethene 4 with TES- or TIPS-acetylene
10a,b.
Sigma–Aldrich (K3PO4·1.5H2O
and K3PO4·7H2O),[45] was dried
for 5 h at 3008C. By running
the reaction at 1008C for 24 h
(TLC monitoring), product 11c
was obtained in 67% yield
(Table 1, entry 9). The forma-
Entry
Acetylene
Base
T [8C]
t [h]
Ratio[a] 15:16:17
Yield [%][b] 15
Yield [%][b] 16
1
2
3
4
10a
10b
10b
10b
Cs2CO3
Cs2CO3
K3PO4
K3PO4
90[c,d]
100[e,f]
80[e,f]
17
2.5
24
24
0:82:18
0:65:35
71:18:11
90:0:10
–
–
58
51
40
19
10
–
100[e,f]
[a] 1H NMR of the crude reaction mixture. [b] Yield of isolated product. [c] 0.2m solution. [d] Ratio cat/ligand
tion of by-products was not ob- [mol%]=1:3. [e] 0.5m solution. [f] Ratio cat/ligand [mol%]=10:30.
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K. Rꢀck-Braun et al.
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to 0.5m, the rate of conversion decreased to 22% (1H NMR
spectroscopy), and only traces of 16 were isolated. Next, the
coupling of the chloro-substituted dithienylethene 4 with
TIPS-acetylene 10b (1.3 equiv) was investigated in the pres-
ence of catalyst (10 mol%) and ligand (30 mol%) in combi-
nation with Cs2CO3 at 1008C in acetonitrile (0.5m).
After 2.5 h, complete consumption of the starting material
was detected. Surprisingly, the 1H NMR analysis after
workup, indicated that product 15b was not formed, al-
though the more stable TIPS-acetylene 10b was used. In-
stead, the dithienylethynylene 16 (Scheme 3) was deter-
mined (Table 2, entry 2) besides the hydrodehalogenated
by-product 17. Compound 16 was isolated in 19% yield.
However, when using K3PO4 instead of Cs2CO3, complete
consumption of the starting material was observed at 808C
as well as 1008C, and product 15b was isolated in 58% or
51% yields, respectively (Table 2, entries 3 and 4). In the
presence of K3PO4, only traces of the by-products 16 and 17
were detected. A possible explanation for the results ob-
65% yield after flash chromatography. Furthermore, the
bromo-substituted precursor 6 was cleanly and completely
coupled with 5 under identical conditions furnishing the con-
jugate 2 in 76% yield (Scheme 4). The challenging dithienyl-
AHCTUNGTREGeNNUN thene-linker-conjugate 3 with the large footprint was pre-
pared by following a recently published procedure.[18] Di-
thienylethene 7a was synthesized starting from literature-
known compounds as described in the experimental section.
Desilylation with NaOH in methanol at RT gave 7b[23] in
92% yield. Standard Sonogashira-coupling conditions were
employed for the coupling of 8 (0.002m) with 7b (2 equiv)
by applying a recently published procedure by Galoppini[18]
using Pd2ACHTNUTRGENNUG(dba)3 (50 mol%) and PACHTUNGTREN(NUNG o-tol)3 (300 mol%) in the
presence of triethylamine/THF at 608C for 24 h. After flash
chromatography, the yields of conjugate 3 ranged between
25 and 38%. Subsequent deprotection of the methylester
functionalities of 3 with NaOH in THF afforded the free
acid 18 in 82% yield. For comparison, we also studied the
coupling of 7b with 8 under the aforementioned reaction
conditions. Compound 8 (0.1m) dissolved in acetonitrile was
treated with 7b (1.3 equiv) in the presence of Pd-catalyst
(10 mol%), X-Phos (30 mol%), and K3PO4 at 808C for
20 h. After workup and flash chromatography, conjugate 3
was isolated in 25% yield (Scheme 5). Neither longer reac-
tion times (up to 65.5 h), nor an increase in concentration
(0.5m) changed the result. Furthermore, during scale-up by
a factor of two, the yield decreased drastically, and product
À
tained, is the C F activation at the perfluorocyclopentene
moiety when using Cs2CO3, leading to the presence of fluo-
ride ions.[49,50] Then, activation of the TIPS residue of 15b
by fluoride ions and a subsequent Sila–Sonogashira coupling
explains the formation of compound 16. Obviously, this side
reaction can be reduced by using K3PO4 instead of Cs2CO3.
The coupling of the chloro-substituted dithienylethene 4
with the sterically hindered tripodal ethynylene-linker 5 also
proceeded well when applying K3PO4 in acetonitrile at 808C
for 4 h (Scheme 4).
The predominant formation of product 1 (ratio 1:4:17=
1
78:16:6) was ascertained by H NMR spectroscopy. The tri-
podal dithienylethene-linker-conjugate 1 was isolated in
Scheme 5. Synthesis of the dithienylethene-linker-conjugate 3. Reagents
and conditions: a) NaOH, MeOH, RT, 2.5 h, 7b: 92%; b) 50 mol% Pd2
Scheme 4. Preparation of the dithienylethene-linker-conjugates 1 and 2.
N
PACHTUNGTRENNUNG
Reagents and conditions: a) 10 mol% [PdCl
G
2ACHTUNGTRENNUNG
Phos, K3PO4 (2.6 equiv), CH3CN, 808C, 4 h, 1: 65%, 2: 76%.
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Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates
3 was isolated in 7% yield. In all coupling reactions with the
iodo-substituted linker 8, establishing turnover numbers and
the yield of by-products were difficult because of the similar
spectral properties of linker 8 and product 3, which compli-
1
cate the determination of the ratios by H NMR spectrosco-
py.
Interestingly, when the commercially available K3PO4 was
dried to constant weight for the coupling of 8 with 7b under
otherwise identical conditions, the cross-coupling reaction
did not proceed. Similar unpredictable influences of the
drying procedure were previously reported for bases in pal-
ladium- as well as copper-mediated coupling reactions.[45–47]
Photochromism
All dithienylethenes described in this paper were isolated as
Figure 1. UV/Vis absorption spectrum of dithienylethene 7b (O-isomer=
solid line, pss=dotted line, c=2.5ꢄ10À5 molLÀ1) in THF pss (326 nm)
mixtures of the open-isomer (O-isomer) and the closed-
isomer (C-isomer) in a ratio of O/C-isomer ꢀ95:5. Pure
samples of the open-isomers of compounds 7b and 3 were
prepared by ring-closing with 326 nm light, followed by ring-
opening with 607 nm light (Scheme 6).
reached after 400 s.
observed. The pericyclic ring-
closure to the blue colored C-
isomer is achieved by irradia-
tion with 326 nm light. For the
C-isomer of 7b, two new
maxima at 335 nm and 600 nm
are monitored, whereas the
maxima of the C-isomer of con-
jugate 3 are found at 360 nm
and 600 nm. The absorption
band at 335 nm of compound
7b is red-shifted by 25 nm by
the tripodal system 3, and the
maximum at 600 nm is slightly
broadened. This effect can be
assigned to the extended p-
system of the linker. The clear
conversion of the isomers into
Scheme 6. Photochromism of the dithienylethene-linker-conjugate 3 and the dithienylethene 7b.
each other is indicated by iso-
sbestics points (328 nm for 7b
and at 348 nm for 3). The re-
The UV/Vis data of the open- and closed-isomers of di-
thienylethene 7b and the corresponding linker-conjugate 3
in THF at room temperature are shown in Figures 1 and 2,
respectively. The absorption maxima of the isomeric forms
of both compounds are summarized in Table 3.
The O-isomer of compound 7b shows a maximum at
308 nm, whereas for the O-isomer of the dithienylethene-
linker-conjugate 3, two maxima at 290 nm and 306 nm are
verse ring-opening was investigated by irradiation at
607 nm. The ratios of the two isomers for both compounds
in the photostationary state (pss) at 326 nm were deter-
mined by 1H NMR spectroscopy in [D8]THF. For com-
pounds 7b and 3, ratios of O/C=5:95 and O/C=11:89 were
calculated, respectively.
Table 3. Spectroscopic data of the dithienylethenes 7b and 3.
[a]
Compound
lmax O-isomer[a] (e)[b]
lmax pss[a] (e)[b]
liso
pss326 (O/C)[c]
7b
3
308 (38.8)
335 (25.1), 379[d]
(15.1), 600 (17.5)
328
348
5:95
11:89
290 (102.4), 306 (92.3), 333[d]
(41.9)
290 (91.2), 306 (71.4), 360 (33.5), 390[d]
G
1
[a] [nm]. [b] [103 cmÀ1 mÀ1]. [c] Ratios determined by H NMR spectroscopy in [D8]THF in the pss upon illumination at 326 nm. [d] Shoulder.
Chem. Asian J. 2010, 5, 1202 – 1212
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1207

K. Rꢀck-Braun et al.
FULL PAPERS
dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl (X-Phos) (97%) was
commercially available from Sigma–Aldrich. Cs2CO3 (catalogue number
441902–50G, 99%) and K3PO4 (catalogue number P5629–25G, tribasic
ꢀ98%) were purchased from Sigma–Aldrich and stored in a desiccator.
Small portions (1–2 g) were removed and heated in a Schlenk tube under
vacuum (2.0ꢄ10À2 mbar) for five hours at 3008C before use. When larger
amounts of K3PO4 were used and dried to constant mass, no conversion
to the coupling product could be monitored. Flash column chromatogra-
phy was performed on silica gel (ICN silica, 32–63 mm, 60 ꢅ, ICN Bio-
medicals GmbH). Thin-layer chromatography (TLC) was performed on
Merck plates (silica gel 60 F254). Melting points were determined with a
Bꢀchi SMP-20 or with a Leica Gallen III melting-point apparatus (uncor-
rected). 1H NMR and 13C NMR spectra were recorded on either
a
Bruker AM 400 or a Bruker DRX 500 spectrometer at room tempera-
ture; residual solvent protons were used as the internal standard {CDCl3:
d(1H)=7.26,
dG
dACTHNGUTERNNUG
(13C)=66.40}.
Chemical shifts are given in ppm relative to tetramethylsilane (TMS) and
coupling constants in Hz. Overlapping carbon atoms in 13C NMR spectra
for the compounds 1, 2, 3, 7b, and 18 were identified by standard tech-
niques, based on heteronuclear decoupling NMR spectroscopy. The reso-
nances of the carbon atoms of the fluorinated cyclopentene moiety of the
compounds 1, 2, 3, 6, 7a, 7b, 15b, 16, and 18 could not be observed be-
cause of the low intensity and extensive coupling. Low-resolution elec-
tron-impact mass spectra (EIMS, 70 eV) or fast atom bombardment mass
spectra (FAB) were recorded with a Finnigan MAT 95 SQ. ESI spectra
were recorded on an LTQ Orbitrap XL. GC-MS analysis was obtained
with an HP-6890 gas chromatograph. IR spectra were recorded with a
Nicolet Avatar 360 spectrometer as ATR. Elemental analyses were car-
ried out on a Vario El from Analytik Jena. 3,5-Dibromo-2-methylthio-
phene[52] was synthesized in 73% yield according to a procedure for the
preparation of 3-bromobenzo[b]thiophen-2-carbaldehyde.[53] 3-Bromo-2-
methylthiophen-5-ylboronic acid[54] was synthesized in 80% yield accord-
Figure 2. UV/Vis absorption spectrum of conjugate 3 (O-isomer=solid
line, pss=dashed line, c=1.0ꢄ10À5 molLÀ1
) in THF; pss (326 nm)
reached after 540 s.
Conclusions
The dithienylethene-linker-conjugates 1 and 2 have been
synthesized from the ethynylene-linker 5, and the halo-sub-
stituted dithienylethenes 4 and 6 by Sonogashira cross-cou-
pling. For the chloro-substituted dithienylethene 4, model
studies were required for the optimization of the reaction
ing to
acid.[55] The known precursors 3-bromo-2-methyl-5-(4-methoxyphenyl)-
AHCTUNGTRENNUNG
thiophene,[56] 3-bromo-2-methyl-5-[4-(trimethylsilylethynyl)phenyl]thio-
a procedure published for 2,5-dimethylthiophen-3-ylboronic
conditions in the presence of [PdCl2ACHTNUGTRNEG(UN CH3CN)2]/X-Phos and
Cs2CO3 or K3PO4 to prevent homocoupling of the ethyny-
lene-linker 5. Dithienylethene 4 and 2-chloro-5-methylthio-
phene (9) were investigated in the model Sonogashira-cou-
pling reactions with TES-acetylene and TIPS-acetylene
10a,b. A strong influence of the reaction time, the solvent,
and the base was observed. Conditions were found for the
solvent acetonitrile that allow for suppression of the activa-
phene,[57] 1-[5-(4-methoxy-phenyl)-2-methylthien-3-yl]perfluorocyclopen-
tene,[58] and 5-chloro-3-iodo-2-methylthiophene[59] were essentially pre-
pared by literature-known procedures. 1-(4-Ethynylphenyl)-3,5,7-tris-(4-
carboethoxyphenyl)adamantane (5)[6] and 1-(4-iodophenyl)-3,5,7-[(3-car-
bomethoxyphenyl)-4-ethynylphenyl]adamantane (8)[18] were synthesized
by following the published methods. Photoirradiation was carried out
using a high-pressure 200 W mercury short arc lamp (HBO, Osram) with
a 326 nm interference filter (Amko) and a 1000 W Xe-lamp (XBO,
Osram) with a 607 nm interference filter (Amko) until a photostationary
state (pss) was reached. UV/Vis spectra were measured on a Shimadzu
UV-1601 UV/Visible spectrophotometer.
À
tion of the C(sp) Si bond of TIPS-acetylene either in trans-
formations of the electron-rich thiophene 9 or in the pres-
ence of the fluorinated backbone of the chloro-substituted
dithienylethene 4. However, in transformations of dithienyl-
À
X-ray Crystallographic Analysis of 6
C28H19BrF6OS2: Mr =629.46 gmolÀ1
ACHTUNGTRENNUNG
ACHTUNGTRENNUNGethene 4, the activation of the C(sp) Si bond and hydrode-
;
halogenation could only be minimized by using K3PO4. Fi-
nally, conjugate 3 was synthesized from the corresponding
iodo-substituted linker 8 and the ethynylene-substituted di-
thienylethene 7b, and was successfully deprotected yielding
compound 18. The photochromic properties of the conjugate
3 and its precursor dithienylethene 7b have also been inves-
tigated. Currently, we are investigating the dynamics of the
electron transfer through the novel dithienylethene-linker-
conjugates reported herein.
monoclinic crystal system with space group P21/c and Z=4, a=
25.5489(7), b=9.1126(3), c=10.9269(3) ꢅ; a=g=908, b=93.171(3)8; V=
2540.07(13) ꢅ3, 1calcd. =1.646 MgmÀ3; F
ACTHNUTRGNEU(GN 000)=1264; linear absorption co-
efficient m=1.849 mmÀ1; type of diffractometer: Siemens SMART CCD;
T=150(2) K; MoKa radiation; q range 2.99–25.008; index ranges À30ꢁ
hꢁ30, À10ꢁkꢁ10, À12ꢁlꢁ12; reflections collected 16389; independ-
ent reflections 4456 (Rint =0.0468); observed reflections 346 [I>2s(I)],
reflection used for refinement 4456; program system used: SHELXS-97,
SHELXL-97, and SHELXTL; empirical absorption correction, direct
methods, full-matrix refinement at F2 with all independent reflections,
weighting Scheme SHELXL; goodness-of-fit parameter (based on F2)
S=0.862; residual densities D1max and D1min =0.598 and 0.372 eꢅÀ3. Hy-
drogen atoms: refined with riding model, refined with temperature factor
1.5 times Ueq of the carbon atoms; non-hydrogen atoms: refined aniso-
tropically. Data/restraints/parameters: 4456/6/346; R index (all data): wR2
(based on F2)=0.0654; R index (conventional) [I>2s(I)]: R1 (based on
F)=0.0330.[60]
Experimental Section
Solvents were purified according to standard procedures.[51] Anhydrous
DMF and CH3CN were purchased from Acros in AcrosSeal bottles. 5-
Chloro-2-methylthiophene (9, 95.8%) was purchased from Alfa Aesar.
Triethylsilylacetylene (10a, 96.6%), triisopropylsilylacetylene (10b,>
99%), and [PdCl2ACHTUNGTRENNUNG(CH3CN)2] (99%) were purchased from Acros, and 2-
1208
www.chemasianj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1202 – 1212

Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates
Syntheses
0.15 mmol) in MeOH (7 mL), NaOH (45.0 mg, 1.13 mmol, 7.30 equiv)
was added. After stirring for 2.5 h at ambient temperature, TLC indicat-
ed complete conversion of the starting material. Then, water (10 mL) and
CH2Cl2 (10 mL) were added to the blue reaction mixture. The aqueous
phase was separated and extracted with CH2Cl2 (10 mL). The combined
organic layers were dried (MgSO4) and evaporated in vacuo. The crude
product was purified by flash chromatography on silica gel using hexane/
CH2Cl2 (2:1) to give product 7b as a blue solid (82.0 mg, 0.14 mmol,
92%); mixture of isomers (O-isomer : C-isomer ꢀ95:5 as determined by
1H NMR). C-isomer: Rf =0.65 (hexane/CH2Cl2, 2:1); O-isomer: Rf =0.60
(hexane/CH2Cl2, 2:1); m.p.: 125–1278C; IR (ATR): n˜ =3301, 3075, 3032,
3001, 2954, 2924, 2853, 2107, 1717, 1662, 1608, 1572, 1557, 1515, 1492,
1475, 1465, 1440, 1394, 1380, 1338, 1301, 1274, 1252, 1190, 1179, 1138,
6: 1-[5-(4-Methoxyphenyl)-2-methylthienyl-3-yl]-2-[2-methyl-5-(4-bromo-
phenyl)-thien-3-yl]-3,3,4,4,5,5-hexafluorocyclopentene (6) was synthe-
sized according to a method described by Feringa and co-workers for re-
lated compounds.[61] A solution of 4 (80.0 mg, 0.15 mmol) in Et2O (2 mL)
was slowly treated with n-butyllithium (1.6m in hexane, 0.1 mL,
0.15 mmol, 1.00 equiv) at room temperature. After stirring for 1 h at this
temperature, tri-n-butylborate (41.4 mg, 0.18 mmol, 1.20 equiv) was
added in one portion, and stirring was continued for one hour. Then, 1-
bromo-4-iodobenzene (88.0 mg, 0.31 mmol, 2.00 equiv), PdACHTNUGTRNEG(UN PPh3)4
(7.8 mg, 6.7 mmol, 4.5 mol%), THF (2 mL), and Na2CO3 (20% w/w,
1 mL) were added, and the mixture was heated to reflux for 16 h. After
this time, TLC monitoring indicated complete conversion of the starting
material and the reaction mixture was quenched with water (4 mL). The
organic layer was separated and the aqueous layer was extracted with
EtOAc (3ꢄ10 mL). The combined organic layers were dried (MgSO4)
and evaporated in vacuo. The crude product was purified by flash chro-
matography on silica gel using hexane/CH2Cl2 (7:1) to give product 6 as a
blue solid (66.0 mg, 0.10 mmol, 66%); mixture of isomers (O-isomer/C-
1110, 1092, 1055, 1035 cmÀ1 1H NMR (400 MHz, CDCl3): d=7.50 (s,
;
4H), 7.49–7.45 (m, 2H), 7.31 (brs, 1H), 7.16 (brs, 1H), 6.94–6.90 (m,
2H), 3.84 (s, 3H), 3.15 (s, 1H), 1.99 (s, 3H), 1.96 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=159.7, 142.4, 142.2, 141.4, 140.4, 133.8, 132.9,
127.1, 126.3 (2C), 125.8, 125.5, 123.3, 121.6, 121.4, 114.6, 83.4, 78.5, 55.6,
14.8, 14.6 ppm; MS (70 eV): m/z (%): 574 (100) [M+], 559 (22), 544 (20),
151 (10), 69 (10); HRMS (EI): m/z (%) calcd for C30H20F6OS2: 574.0860;
found: 574.0853.
1
isomer ꢀ95:5 as determined by H NMR). For X-ray analysis, compound
6 could be separated by recrystallization from CH2Cl2 as slightly blue
crystals. C-isomer: Rf =0.29 (hexane/CH2Cl2, 7:1); O-isomer: Rf =0.25
(hexane/CH2Cl2, 7:1); m.p.: 183–1858C; IR (ATR): n˜ =3069, 3002, 2956,
2941, 2919, 2853, 2837, 1704, 1610, 1554, 1515, 1497, 1474, 1466, 1440,
1401, 1394, 1337, 1299, 1274, 1253, 1192, 1180, 1138, 1112, 1092, 1074,
General procedure for the Sonogashira cross-coupling of 2-chloro-5-
methylthiophene (9) with silylprotected acetylenes:
A thick-walled
Schlenk tube was evacuated, heated under vacuum, and refilled with
argon three times. Then, the Schlenk tube was charged with [PdCl2
ACHNUTRTGENNG(UN CH3CN)2], X-Phos, and Cs2CO3 or K3PO4 (2.60 equiv) followed by the
1054, 1036, 1009, 950, 898, 889, 819 cmÀ1 1H NMR (400 MHz, CDCl3):
;
thienyl chloride 9 (61.3 mg, 0.46 mmol) and acetonitrile (0.92 mL) under
a positive pressure of argon. The yellow suspension was degassed (5 min,
ultrasound) and then stirred for 30 min at room temperature. Afterwards,
the alkyne (0.60 mmol, 1.3 equiv) was added using a syringe, and the
tube was sealed with a teflon valve. The reaction mixture was stirred at
the desired temperature for the indicated period of time (TLC monitor-
ing). After complete consumption of the starting material, the resulting
suspension was allowed to reach room temperature, was diluted with
water (5 mL), and extracted with CH2Cl2 (3ꢄ15 mL). The combined or-
ganic layers were dried (MgSO4), concentrated in vacuo, and the residue
was purified by flash chromatography on silica gel (hexane) to provide
the desired product.
d=7.52–7.49 (m, 2H), 7.48–7.45 (m, 2H), 7.42–7.38 (m, 2H) 7.27 (brs,
1H), 7.15 (brs, 1H), 6.94–6.90 (m, 2H), 3.84 (s, 3H), 1.97 (s, 3H),
1.95 ppm (s, 3H); 13C NMR (100 MHz, CDCl3): d=159.7, 142.4, 141.9,
141.0, 140.4, 132.5, 132.3, 127.2, 127.1, 126.3, 126.2, 125.8, 123.0, 121.9,
121.4, 114.6, 55.6, 14.7, 14.6 ppm; HRMS (EI): m/z (%) calcd for
C28H19BrF6OS2: 627.9965; found: 627.9966; elemental analysis: calcd (%)
for C28H19BrF6OS2 (629.47): C 53.43, H 3.04; found: C 53.41, H 3.23.
7a: 1-[5-(4-Methoxyphenyl)-2-methylthienyl-3-yl]-2-{2-methyl-5-[4-(trime-
thylsilyl-ethynyl)phenyl]thien-3-yl}-3,3,4,4,5,5-hexafluorocyclopentene
(7a) was prepared analogously to a procedure described by Gust and co-
workers:[23]
A solution of 3-bromo-2-methyl-5-(4-trimethylsilylethynyl-
phenyl)thiophene[57] (192 mg, 0.55 mmol) in THF (4 mL) was cooled to
À788C under argon atmosphere and treated dropwise with n-butyllithium
(1.6m in hexane, 0.34 mL, 0.55 mmol, 1.00 equiv). After stirring for
45 min at À788C, a solution containing 1-[5-(4-methoxyphenyl)-2-methyl-
thien-3-yl]perfluorocyclopentene[58] (200 mg, 0.50 mmol, 0.90 equiv) in
THF (2 mL) was cooled to À788C, and added by using a cannula to the
lithiated thiophene. The resulting yellow solution was stirred for 4 h at
this temperature and then allowed to warm up to room temperature. Stir-
ring was continued for a total of 16 h until TLC monitoring indicated
complete conversion of the starting material. The reaction mixture was
quenched with HCl (20 mL, 1m) and then extracted with CH2Cl2 (3ꢄ
25 mL). The combined organic layers were washed with water (25 mL),
dried (MgSO4), and evaporated in vacuo. The crude product was purified
by flash chromatography on silica gel using hexane/CH2Cl2 (5:1) to give
compound 7a as a blue solid (230 mg, 0.36 mmol, 71%); mixture of iso-
2-(Triethylsilylethynyl)-5-methylthiophene (11a): According to the gener-
al procedure, compound 11a was obtained by the reaction of 2-chloro-5-
methylthiophene (9) (61.3 mg, 0.46 mmol) with triethylsilylacetylene
(10a) (84.2 mg, 0.60 mmol, 1.30 equiv), [PdCl2ACTHNUGTRNE(UNG CH3CN)2] (1.2 mg,
4.6 mmol, 1 mol%), X-Phos (6.6 mg, 13.8 mmol, 3 mol%), and Cs2CO3
(391 mg, 1.20 mmol, 2.60 equiv) for 17 h at 1008C in acetonitrile
(0.92 mL) to give product 11a (83.0 mg, 0.35 mmol, 76%) as a yellow oil.
Rf =0.49 (hexane); IR (ATR): n˜ =3074, 2955, 2934, 2912, 2874, 2734,
2143, 1744, 1537, 1458, 1414, 1379, 1236, 1178, 1154, 1018, 1007, 974, 797,
762, 735, 728 cmÀ1 1H NMR (400 MHz, CDCl3): d=7.03 (d, J=3.5 Hz,
;
1H), 6.59 (dq, J=1.0 Hz, 3.4 Hz, 1H), 2.45 (d, J=1.0 Hz, 3H), 1.04 (t,
J=7.9 Hz, 9H), 0.67 ppm (q, J=7.9 Hz, 6H); 13C NMR (100 MHz,
CDCl3): d=142.1, 133.0, 125.2, 121.1, 99.3, 95.5, 15.5, 7.6, 4.5 ppm; MS
(70 eV): m/z (%): 236 (30) [M+], 207 (100), 179 (76), 151 (88); HRMS
(EI): m/z (%) calcd for C13H20SSi: 236.1055; found: 236.1065.
1
mers (O-isomer/C-isomer ꢀ 95:5 as determined by H NMR). C-isomer:
2-(Triisopropysilylethynyl)-5-methylthiophene (11c): Method A: Accord-
ing to the general procedure, compound 11c was obtained by the reac-
tion of 2-chloro-5-methylthiophene (9) (61.3 mg, 0.46 mmol) with triiso-
propylsilylacetylene (10b) (109 mg, 0.60 mmol, 1.30 equiv), [PdCl2
Rf =0.77 (hexane/CH2Cl2, 2:1); O-isomer: Rf =0.72 (hexane/CH2Cl2, 2:1);
m.p.: 80–838C; IR (ATR): n˜ =3076, 3032, 3001, 2959, 2935, 2855, 2837,
2157, 1705, 1610, 1556, 1516, 1475, 1492, 1466, 1441, 1337, 1301, 1273,
1251, 1190, 1179, 1138, 1111, 1092, 1055, 988, 898, 887, 864, 843, 822 cmÀ1
;
1H NMR (400 MHz, CDCl3): d=7.47–7.45 (m, 6H), 7.30 (brs, 1H), 7.15
(brs, 1H), 6.93–6.90 (m, 2H), 3.84 (s, 3H), 1.97 (s, 3H), 1.95 (s, 3H),
0.27 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=159.7, 142.4, 142.1,
141.5, 140.4, 133.4, 132.7, 127.1, 126.3, 126.2, 125.8, 125.3, 123.1, 122.6,
121.4, 114.5, 104.8, 95.7, 55.6, 14.8, 14.7, 0.1 ppm; HRMS (EI): m/z (%)
calcd for C33H29F6OS2Si: 647.1333 [M++H]; found: 647.1332; elemental
analysis: calcd (%) for C33H28F6OS2Si (646.78): C 61.28, H 4.36; found:
C 61.40, H 4.63.
AHCTUNGTRENNUNG
2.5 h in acetonitrile (0.92 mL) to give product 11c as
a yellow oil
(118 mg, 0.43 mmol, 92%).
Method B: According to the general procedure, compound 11c was ob-
tained by the reaction of 2-chloro-5-methylthiophene (9) (61.3 mg,
0.46 mmol) with triisopropylsilylacetylene (10b) (109 mg, 0.60 mmol,
1.30 equiv), [PdCl2ACTHNUTRGNE(UNG CH3CN)2] (12.0 mg, 46.2 mmol, 10 mol%), X-Phos
1-[5-(4-Methoxyphenyl)-2-methylthienyl-3-yl]-2-[2-methyl-5-(4-ethynyl-
phenyl)-thien-3-yl]-3,3,4,4,5,5-hexafluorocyclopentene (7b):[43] To a vigo-
rously stirred solution of the TMS-protected dithienylethene 7a (100 mg,
(66.1 mg, 138 mmol, 30 mol%), and K3PO4 (255 mg, 1.20 mmol,
2.60 equiv) for 17 h at 808C in acetonitrile (0.92 mL) to give product 11c
as a yellow oil (110 mg, 0.39 mmol, 85%). Rf =0.64 (hexane); IR (ATR):
Chem. Asian J. 2010, 5, 1202 – 1212
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1209

K. Rꢀck-Braun et al.
FULL PAPERS
n˜ =3073, 2957, 2936, 2942, 2924, 2891, 2865, 2143, 1744, 1537, 1462, 1383,
Rf =0.35 (hexane/EtOAc 5:1); m.p.: 119–1228C; IR (ATR): n˜ =3523,
3059, 3038, 2978, 2929, 2902, 2853, 1714, 1609, 1571, 1553, 1509, 1475,
1464, 1443, 1366, 1338, 1275, 1253, 1188, 1179, 1143, 1104, 1019, 986, 897,
1178, 1154, 996, 883, 797, 754 cmÀ1 1H NMR (400 MHz, CDCl3): d=7.03
;
(d, J=3.6 Hz, 1H), 6.60 (dq, J=1.1 Hz, 3.6 Hz, 1H), 2.45 (d, J=1.1 Hz,
3H), 1.12 ppm (s, 21H); 13C NMR (100 MHz, CDCl3): d=141.9, 132.7,
125.2, 121.4, 100.0, 94.4, 18.8, 15.5, 11.5 ppm; HRMS (EI): m/z (%) calcd
for C16H26SSi: 278.1524; found: 278.1517; elemental analysis: calcd (%)
for C16H26SSi (278.5): C 69.00, H 9.41; found: C 68.88, H 9.41.
854, 831, 762 cmÀ1 1H NMR (400 MHz, CDCl3): d=8.05 (d, J=8.4 Hz,
;
6H), 7.56 (d, J=8.5 Hz, 6H) 7.53–7.45 (m, 6H), 7.28 (brs, 1H), 7.14 (brs,
1H), 6.93–6.90 (m, 2H), 4.39 (q, J=7.1 Hz, 6H), 3.83 (s, 3H), 2.21–2.20
(m, 12H), 1.95 (s, 3H), 1.94 (s, 3H) 1.40 ppm (t, J=7.1 Hz, 9H);
13C NMR (100 MHz, CDCl3): d=166.6, 159.7, 153.8, 149.5, 143.4, 142.5,
140.5, 131.7, 131.6, 129.9, 128.8, 127.1, 126.2, 125.6, 125.3, 125.2, 121.8,
121.2 (2C), 120.7, 114.5, 93.8, 81.7, 61.1, 55.5, 46.8 (2C), 39.7, 39.5, 14.7,
14.6, 14.5 ppm; HRMS (EI): m/z (%) calcd for C67H56F6O7S2 [M+À2H]:
1150.3372; found: 1150.3321; elemental analysis: calcd (%) for
C67H58F6O7S2 (1153.29): C 69.78, H 5.07; found: C 70.07, H 5.33.
1-[5-(4-Methoxyphenyl)-2-methylthienyl-3-yl]-2-[2-methyl-5-(triisopropyl-
silyl-ethynyl)thien-3-yl]-3,3,4,4,5,5-hexafluorocyclopentene
(15b):
A
thick-walled Schlenk tube was evacuated, heated under vacuum, and re-
filled with argon three times. Then, the Schlenk tube was charged under
a positive pressure of argon with dithienylethene 4 (50.0 mg, 98.2 mmol),
[PdCl2ACHTUNGTRENNUNG(CH3CN)2] (2.5 mg, 9.8 mmol, 10 mol%), X-Phos (14.0 mg,
29.4 mmol, 30 mol%), K3PO4 (54.1 mg, 255 mmol, 2.60 equiv), and aceto-
nitrile (0.20 mL). The bluish suspension was degassed (5 min, ultrasound)
and stirred for 30 min, before triisopropylsilylacetylene (10b) (23.2 mg,
127 mmol, 1.3 equiv) was added using a syringe. The Schlenk tube was
sealed with a teflon valve, and the resulting suspension was stirred for
24 h at 808C (TLC monitoring). The resulting brown suspension was al-
lowed to reach room temperature, was diluted with water (5 mL), and ex-
tracted with CH2Cl2 (3ꢄ15 mL). The combined organic layers were dried
(MgSO4) and evaporated in vacuo. The crude product was purified by
flash chromatography on silica gel using hexane/EtOAc (10:1) to give
product 15b as a bluish solid (37.0 mg, 56.5 mmol, 58%) besides by-prod-
uct 16; mixture of isomers (O-isomer : C-isomer ꢀ 95:5 as determined
by 1H NMR). C-isomer: Rf =0.73 (hexane/CH2Cl2, 2:1); O-isomer: Rf =
0.67 (hexane/CH2Cl2, 2:1); m.p.: 104–1078C; IR (ATR): n˜ =2956, 2943,
2865, 2146, 1709, 1610, 1572, 1554, 1515, 1473, 1463, 1441, 1420, 1337,
1300, 1273, 1253, 1192, 1178, 1137, 1112, 1090, 1051, 1037, 987, 883, 823,
Dithienylethene-linker-conjugate 2: A thick-walled Schlenk tube was
evacuated, heated under vacuum, and refilled with argon three times.
Then, the Schlenk tube was charged under a positive pressure of argon
with dithienylethene 6 (30.0 mg, 47.6 mmol), [PdCl2ACTHNUGTRNE(UNG CH3CN)2] (1.2 mg,
4.8 mmol, 10 mol%), X-Phos (6.8 mg, 14.3 mmol, 30 mol%), K3PO4
(26.1 mg, 123 mmol, 2.60 equiv), and acetonitrile (0.01 mL). The bluish
suspension was degassed (5 min, ultrasound) and stirred for 30 min
before compound 5 (42.0 mg, 61.9 mmol, 1.30 equiv) was added. After-
wards, the Schlenk tube was sealed with a teflon valve and the suspension
was stirred at 808C for 4 h (TLC monitoring). Then, the resulting suspen-
sion was allowed to reach room temperature and was diluted with water
(15 mL) and EtOAc (15 mL). The aqueous phase was separated and ex-
tracted with EtOAc (3ꢄ15 mL). The combined organic layers were dried
(MgSO4) and evaporated in vacuo. The crude product was purified by
flash chromatography on silica gel using hexane/EtOAc (5:1) to give
compound 2 as a bluish solid (45.0 mg, 36.6 mmol, 76%); mixture of iso-
mers (O-isomer : C-isomer ꢀ95:5 as determined by 1H NMR). O/C-
isomer: Rf =0.32 (hexane/EtOAc, 5:1); m.p.: 140–1438C; IR (ATR): n˜ =
2979, 2932, 2902, 2854, 1716, 1609, 1515, 1336, 1277, 1255, 1188, 1106,
735 cmÀ1 1H NMR (400 MHz, CDCl3): d=7.48–7.44 (m, 2H), 7.21 (brs,
;
1H), 7.12 (brs, 1H), 6.93–6.90 (m, 2H), 3.84 (s, 3H), 1.95 (s, 3H), 1.90 (s,
3H), 1.12ppm (s, 21H); 13C NMR (100 MHz, CDCl3): d=159.7, 143.2,
142.5, 140.5, 131.9, 127.1, 126.3, 125.6, 125.0, 122.1, 121.3, 114.5, 98.3,
96.8, 55.6, 18.8, 14.7, 14.6, 11.4 ppm; HRMS (EI): m/z (%) calcd for
C33H36F6OS2Si: 654.1881; found 654.1874; elemental analysis: calcd (%)
for C33H36F6OS2Si (654.84): C 60.53, H 5.54; found: C 60.49, H 5.80.
1019, 989, 833, 763 cmÀ1 1H NMR (400 MHz, CDCl3): d=8.05 (d, J=
;
8.8 Hz, 6H), 7.57–7.52 (m, 12H), 7.49–7.45 (m, 4H), 7.31 (brs, 1H), 7.15
(brs, 1H), 6.93–6.90 (m, 2H), 4.38 (q, J=7.1 Hz, 6H), 3.84 (s, 3H), 2.21
(s, 12H), 1.99 (s, 3H), 1.97 (s, 3H) 1.40 ppm (t, J=7.1 Hz, 9H); 13C NMR
(100 MHz, CDCl3): d=166.6, 159.7, 153.9, 149.2, 142.4, 142.0, 141.6,
140.4, 133.2, 132.3, 131.9, 130.0, 128.9, 127.1, 126.3 (2C), 125.8, 125.5,
125.24, 125.18, 123.1, 122.8, 121.4 (2C), 114.5, 90.6, 89.2, 61.1, 55.5, 46.9,
46.8, 39.8, 39.5, 14.8, 14.7, 14.5 ppm; MS (70 eV): m/z (%): 1226 (30) [M+
À2H], 712 (10) 655 (10), 549 (17), 433 (10), 368 (17), 278 (15), 214 (18),
163 (20), 139 (42), 97 (57), 71 (65), 57 (100); HRMS (EI): m/z (%) calcd
for C73H60F6O7S2 [M+À2H]: 1226.3685; found: 1226.3727.
16: 1,2-Bis{[5-(4-methoxyphenyl)-2-methylthiophen-3-yl)-2-(5-phenyl-2-
methylthiopen-3-yl]-3,3,4,4,5,5-hexafluorocyclopentene}ethyne (16) was
isolated as a blue solid (4.8 mg, 4.9 mmol, 10%); mixture of isomers (O-
1
isomer : C-isomer ꢀ95:5 as determined by H NMR). C-isomer: Rf =0.14
(hexane/CH2Cl2, 5:1); O-isomer: Rf =0.10 (hexane/CH2Cl2, 5:1);
m.p.: 160–1628C; IR (ATR): n˜ =3075, 3033, 3000, 2956, 2924, 2853, 1725,
1706, 1610, 1554, 1515, 1473, 1465, 1440, 1337, 1273, 1253, 1192, 1138,
1113, 1091, 1053, 1035, 986, 899, 822 cmÀ1
;
1H NMR (400 MHz, CDCl3):
Dithienylethene-linker-conjugate 3: Method A: A flask was charged with
d=7.47–7.45 (m, J=8.8 Hz, 4H), 7.29 (brs, 2H), 7.13 (brs, 2H), 6.93–
6.91 (m, J=8.8 Hz, 4H), 3.84 (s, 6H), 1.94 (s, 6H), 1.93 ppm (s, 6H);
13C NMR (100 MHz, CDCl3): d=159.8, 144.2, 142.6, 140.5, 132.3, 127.1,
126.3, 125.6, 125.5, 121.3, 120.9, 114.6, 86.1, 55.6, 14.7 ppm (2C); MS
(70 eV): m/z (%): 970 (100) [M+], 546 (10), 312 (22), 191 (15), 130 (20),
119 (62), 105 (70), 91 (20), 77 (18); HRMS (EI): m/z (%) calcd for
C46H30F12O2S4: 970.0937; found: 970.0933.
iodo-tripod
96.2 mmol, 2.00 equiv), Pd
8
(50.0 mg, 48.0 mmol), dithienylethene 7b (55.0 mg,
2A(dba)3 (22.0 mg, 24.0 mmol, 50 mol%), P(o-
G
ACHTUNGTRENNUNG
tol)3 (43.8 mg, 144 mmol, 300 mol%) in Et3N (7.4 mL), and dry THF
(15 mL). The resulting reaction mixture was stirred at 608C for 24 h
under argon atmosphere (TLC monitoring), then cooled to room temper-
ature and poured into water (20 mL). Then, the reaction mixture was ex-
tracted with CH2Cl2 (3ꢄ20 mL). The combined organic layers were
washed with water (10 mL), dried (MgSO4), and evaporated in vacuo.
The crude product was purified by flash chromatography on silica gel
using hexane/EtOAc (4:1) to give compound 3 as a blue solid (27.0 mg,
18.1 mmol, 38%); mixture of isomers (O-isomer : C-isomer ꢀ 95:5 as de-
Dithienylethene-linker-conjugate 1: A thick-walled Schlenk tube was
evacuated, heated under vacuum, and refilled with argon three times.
Then, the Schlenk tube was charged under a positive pressure of argon
with dithienylethene 4 (35.0 mg, 68.7 mmol), ACHTUNTRGENNUG[PdCl2ACHTUNGTRENN(GUN CH3CN)2] (1.8 mg,
1
termined by H NMR).
6.9 mmol, 10 mol%), X-Phos (9.8 mg, 20.6 mmol, 30 mol%), K3PO4
(38.0 mg, 179 mmol, 2.60 equiv), and acetonitrile (0.14 mL). The bluish
suspension was degassed (5 min, ultrasound) and stirred for 30 min
before the tripod 5 (60.7 mg, 89.3 mmol, 1.30 equiv) was added. After-
wards, the Schlenk tube was sealed with a teflon valve, and the suspen-
sion was stirred for 4 h at 808C (TLC monitoring). The resulting suspen-
sion was allowed to reach room temperature, and was diluted with water
(15 mL) and EtOAc (15 mL). The aqueous phase was separated and ex-
tracted with EtOAc (3ꢄ15 mL). The combined organic layers were dried
(MgSO4) and evaporated in vacuo. The crude product was purified by
flash chromatography on silica gel using hexane/EtOAc (5:1) to give
product 1 as a bluish solid (52.0 mg, 45.1 mmol, 65%); mixture of isomers
(O-isomer : C-isomer ꢀ95:5 as determined by 1H NMR). O/C-isomer:
Method B: A thick-walled Schlenk tube was evacuated, heated under
vacuum, and refilled with argon three times. Then, the Schlenk tube was
charged under a positive pressure of argon with iodo-tripod 8 (25.0 mg,
24.0 mmol), ACHTUNGTRNENUG[PdCl2ACHTUNGTRNE(NUGN CH3CN)2] (0.6 mg, 2.4 mmol, 10 mol%), X-Phos
(3.4 mg, 7.2 mmol, 30 mol%), K3PO4 (13.3 mg, 62.4 mmol, 2.60 equiv), and
acetonitrile (240 mL). The bluish suspension was degassed (5 min, ultra-
sound) and stirred for 30 min before the dithienylethene 7b (17.9 mg,
31.2 mmol, 1.30 equiv) was added. Afterwards, the Schlenk tube was
sealed with a teflon valve, and the suspension was stirred for 20 h at
808C (TLC monitoring). The resulting suspension was allowed to reach
room temperature, and diluted with water (5 mL) and EtOAc (5 mL).
The aqueous phase was separated and extracted with EtOAc (3ꢄ5 mL).
1210
www.chemasianj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1202 – 1212

Efficient Preparation of Photoswitchable Dithienylethene-Linker-Conjugates
The combined organic layers were dried (MgSO4) and evaporated in
vacuo. The crude product was purified by flash chromatography on silica
gel using hexane/EtOAc (4:1) to give product 3 as a bluish solid (9.0 mg,
6.0 mmol, 25%); mixture of isomers (O-isomer : C-isomer ꢀ 95:5 as de-
termined by 1H NMR). O/C-isomer: Rf =0.19 (hexane/EtOAc, 5:1);
m.p.: 111–1148C; IR (ATR): n˜ =3495, 3068, 3033, 2952, 2923, 2851, 2208,
1908, 1725, 1600, 1579, 1511, 1491, 1463, 1438, 1406, 1357, 1338, 1321,
1304, 1279, 1257, 1191, 1179, 1143, 1112, 1103, 1080, 1054, 1018, 988, 897,
[6] E. Galoppini, W. Guo, W. Zhang, P. G. Hoertz, P. Qu, G. J. Meyer, J.
Am. Chem. Soc. 2002, 124, 7801–7811.
[7] M. Myahkostupov, P. Piotrowiak, D. Wang, E. Galoppini, J. Phys.
Chem. C 2007, 111, 2827–2829.
[8] E. Galoppini, Coord. Chem. Rev. 2004, 248, 1283–1297.
[9] M. Lamberto, C. Pagba, P. Piotrowiak, E. Galoppini, Tetrahedron
Lett. 2005, 46, 4895–4899.
[10] Y. Shirai, L. Cheng, B. Chen, J. M. Tour, J. Am. Chem. Soc. 2006,
128, 13479–13489.
888, 832, 754 cmÀ1 1H NMR (400 MHz, CDCl3): d=8.22 (t, J=1.64 Hz,
;
3H), 8.00 (dt, J=1.60 Hz, 7.84 Hz, 3H), 7.71 (dt, J=1.48 Hz, 7.80 Hz,
3H), 7.57–7.41 (m, 25H), 7.32 (brs, 1H), 7.16 (brs, 1H), 6.93–6.90 (m,
2H), 3.94 (s, 9H), 3.84 (s, 3H), 2.19 (s, 12H), 1.98 (s, 3H), 1.96 ppm (s,
3H); 13C NMR (100 MHz, CDCl3): d=166.6, 159.7, 149.6, 149.5, 142.4,
142.0, 141.6, 140.4, 135.8, 133.2, 132.9, 132.3, 132.0, 131.9, 130.6, 129.3,
128.7, 127.1, 126.30, 126.27, 125.8, 125.5, 125.32, 125.30, 124.0, 123.1,
122.9, 121.4, 121.3, 121.0, 114.6, 90.7, 90.3, 89.2, 88.4, 55.6, 52.4, 47.0 (2C),
39.6 (2C), 14.8, 14.7 ppm; MS (70 eV): m/z (%): 1490 (100) [M++3H],
1412 (10), 1253 (20), 1251 (25); HRMS (ESI): m/z (%) calcd for
C94H69F6O7S2: 1487.4383; found: 1487.4373.
[11] S. Thyagarajan, E. Galoppini, P. Persson, J. M. Giaimuccio, G. J.
Meyer, Langmuir 2009, 25, 9219–9226.
[12] L. Wei, K. Padmaja, W. J. Youngblood, A. B. Lysenko, J. S. Lindsey,
D. F. Bocian, J. Org. Chem. 2004, 69, 1461–1469.
[13] S. Zarwell, K. Rꢀck-Braun, Tetrahedron Lett. 2008, 49, 4020–4025.
[14] S. Zarwell, S. Dietrich, C. Schulz, P. Dietrich, F. Michalik, K. Rꢀck-
Braun, Eur. J. Org. Chem. 2009, 2088–2095.
[15] D. Takamatsu, Y. Yamakoshi, K.-I. Fukui, J. Phys. Chem. B 2006,
110, 1968–1970.
[16] C. C. Clark, G. J. Meyer, Q. Wei, E. Galoppini, J. Phys. Chem. B
2006, 110, 11044–11046.
Dithienylethene-linker-conjugate 18: The methyl ester
3 (5.0 mg,
3.4 mmol) was dissolved in THF (0.8 mL), and NaOH (1m, 0.2 mL) was
added. The resulting solution was stirred at room temperature until TLC
monitoring indicated no further conversion of the starting material
(47 h). Then, the mixture was extracted with CH2Cl2 (3ꢄ1 mL). After-
wards, the aqueous phase was acidified with HCl (2m, 0.2 mL) and ex-
tracted with CHCl3 (3ꢄ1 mL). Subsequent evaporation of the solvent in
vacuo afforded compound 18 as a blue solid (4.0 mg, 2.8 mmol, 82%);
[17] W. Guo, E. Galoppini, G. Rydja, G. Pardi, Tetrahedron Lett. 2000,
41, 7419–7421.
[18] S. Thyagarajan, A. Liu, O. A. Famoyin, M. Lamberto, E. Galoppini,
Tetrahedron 2007, 63, 7550–7559.
[19] J. M. Endtner, F. Effenberger, A. Hartschuh, H. Port, J. Am. Chem.
Soc. 2000, 122, 3037–3046.
[20] J. H. Hurenkamp, J. J. D. de Jong, W. R. Browne, J. H. van Esch,
B. L. Feringa, Org. Biomol. Chem. 2008, 6, 1268–1277.
[21] M. Berberich, A.-M. Krause, M. Orlandi, F. Scandola, F. Wꢀrthner,
Angew. Chem. 2008, 120, 6718–6721; Angew. Chem. Int. Ed. 2008,
47, 6616–6619.
[22] T. Fukaminato, T. Umemoto, Y. Iwata, M. Irie, Chem. Lett. 2005, 34,
676–677.
[23] P. A. Liddell, G. Kodis, A. L. Moore, T. A. Moore, D. Gust, J. Am.
Chem. Soc. 2002, 124, 7668–7669.
mixture of isomers (O-isomer
:
C-isomer ꢀ95:5 as determined by
1H NMR). O/C-isomer: Rf =0.68 (EtOAc/AcOH, 80:1); m.p.: 201–
2048C; IR (ATR): n˜ =3335, 3065, 3034, 2927, 2852, 2616, 2210, 1911,
1704, 1602, 1559, 1511, 1463, 1439, 1386, 1273, 1253, 1180, 1137, 1114,
1054, 1035, 988, 833, 768, 760 cmÀ1 1H NMR (400 MHz, [D8]THF): d=
;
8.16 (s, 3H), 7.98 (d, J=7.7 Hz, 3H), 7.70 (d, J=7.8 Hz, 3H), 7.69–7.63
(m, 9H), 7.56–7.44 (m, 17H), 7.27 (brs, 1H), 6.94–6.92 (m, 2H), 3.79 (s,
3H), 2.24 (s, 12H), 2.02 (s, 3H), 2.00 ppm (s, 3H); 13C NMR (100 MHz,
[D8]THF): d=165.9, 159.9, 150.3, 150.2, 142.6, 142.0, 141.6, 140.2, 135.0,
132.9, 132.5, 131.9, 131.4, 131.3, 129.1, 128.4, 126.6, 126.0, 125.8 (2C),
125.5, 125.3 (2C), 125.2, 123.7, 123.0, 122.9, 121.0, 120.8, 120.6, 114.2,
90.5, 89.9, 88.5, 87.8, 54.6, 46.5 (2C), 39.5 (2C), 13.6, 13.5 ppm; MS
(FAB): m/z (%): 1444 (10) [M+], 413 (15), 282 (45), 242 (100); HRMS
(ESI): m/z (%) calcd for C91H61F6O7S2 [M+ÀH]: 1443.3757; found:
1443.3784.
[24] A. Kçllhofer, T. Pullmann, H. Plenio, Angew. Chem. 2003, 115,
1086–1088; Angew. Chem. Int. Ed. 2003, 42, 1056–1058.
[25] D. Gelman, S. L. Buchwald, Angew. Chem. 2003, 115, 6175–6178;
Angew. Chem. Int. Ed. 2003, 42, 5993–5996.
[26] J. Areephong, W. R. Browne, B. L. Feringa, Org. Biomol. Chem.
2007, 5, 1170–1174.
[27] S. Schrçter, C. Stock, T. Bach, Tetrahedron 2005, 61, 2245–2267.
[28] T. Erker, S. Nemec, Synthesis 2004, 23–25.
[29] K. Billingsley, S. L. Buchwald, J. Am. Chem. Soc. 2007, 129, 3358–
3366.
[30] C. Torborg, A. Zapf, M. Beller, ChemSusChem 2008, 1, 91–96.
[31] Y. Kitamura, S. Sako, T. Udzu, A. Tsutsui, T. Maegawa, Y. Mongu-
chi, H. Sajiki, Chem. Commun. 2007, 5069–5071.
[32] E. A. B. Kantchev, G.-R. Peh, C. Zhang, J. Y. Ying, Org. Lett. 2008,
10, 3949–3952.
[33] M. Feuerstein, H. Doucet, M. Santelli, Tetrahedron Lett. 2005, 46,
1717–1720.
[34] C. Yi, R. Hua, H. Zeng, Q. Huang, Adv. Synth. Catal. 2007, 349,
1738–1742.
Acknowledgements
This work was supported by the German–Israeli Foundation for Scientific
Research and Development (Grant No. 894/05, D.G. and K.R.-B.), the
Deutsche Forschungsgemeinschaft (SFB 658, B6 and Cluster of Excel-
lence 314, A4, K.R.-B.) and the Fonds der Chemischen Industrie (K.R.-
B.). Part of this work was also supported by the Alexander von Hum-
boldt-Stiftung through a research grant to Dr. Saleh A. Ahmed. E.G.
thanks ACS PRF (46663-AC10) and NSF (NIRT 030829) for funding.
[35] A. Komꢃromi, Z. Novꢃk, Chem. Commun. 2008, 4968–4970.
[36] C. Torborg, J. Huang, T. Schulz, B. Schꢁffner, A. Zapf, A. Spannen-
berg, A. Bçrner, M. Beller, Chem. Eur. J. 2009, 15, 1329–1336.
[37] W. J. King, F. F. Nord, J. Org. Chem. 1949, 14, 638–642.
[38] Y. Hatanaka, T. Hiyama, J. Org. Chem. 1988, 53, 918–920.
[39] U. S. Sørensen, E. Pombo-Villar, Tetrahedron 2005, 61, 2697–2703.
[40] H. Huang, H. Liu, H. Jiang, K. Chen, J. Org. Chem. 2008, 73, 6037–
6040.
[41] C. Yi, R. Hua, J. Org. Chem. 2006, 71, 2535–2537.
[42] D. A. Alonso, C. Nꢃjera, M. C. Pacheco, Adv. Synth. Catal. 2003,
345, 1146–1158.
[43] J. Hassan, L. Lavenot, C. Gozzi, M. Lemaire, Tetrahedron 1999, 40,
857–858.
[1] P. Piotrowiak, E. Galoppini, Q, Wei, G. J. Meyer, P. Wiewiꢆr, J.
Am. Chem. Soc. 2003, 125, 5278–5279.
[2] R. Huber, J.-E. Moser, M. Grꢁtzel, J. Wachtveitl, J. Phys. Chem. B
2002, 106, 6494–6499, and references therein.
[3] S. Wagner, F. Leyssner, C. Kçrdel, S. Zarwell, R. Schmidt, M. Wei-
nelt, K. Rꢀck-Braun, M. Wolf, P. Tegeder, Phys. Chem. Chem. Phys.
2009, 11, 6242–6248.
[4] P. Dietrich, F. Michalik, R. Schmidt, C. Gahl, G. Mao, M. Breusing,
M. B. Raschke, B. Priewisch, T. Elsꢁsser, R. Mendelsohn, M. Wei-
nelt, K. Rꢀck-Braun, Appl. Phys. A 2008, 93, 285–292.
[5] L. Dworak, V. V. Matylitsky, J. Wachtveitl, ChemPhysChem 2009,
10, 384–391.
Chem. Asian J. 2010, 5, 1202 – 1212
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1211

K. Rꢀck-Braun et al.
FULL PAPERS
[44] C. Blaszykowski, E. Aktoudianakis, D. Alberico, C. Bressy, D. G.
Hulcoop, F. Jafarpour, A. Joushaghani, B. Laleu, M. Lautens, J. Org.
Chem. 2008, 73, 1888–1897.
[45] K. Dooleweerdt, H. Birkedal, T. Ruhland, T. Skrydstrup, J. Org.
Chem. 2008, 73, 9447–9450.
[46] A. Klapars, X. Huang, S. L. Buchwald, J. Am. Chem. Soc. 2002, 124,
7421–7428.
[47] C. Meyers, B. U. W. Maes, K. T. J. Loones, G. Bal, G. L. F. Lemiꢇre,
R. A. Dommisse, J. Org. Chem. 2004, 69, 6010–6017.
[55] S. V. Shorunov, M. M. Krayushkin, F. M. Stoyanovich, M. Irie, Russ.
J. Org. Chem. 2006, 42, 1490–1497.
[56] S. H. Kawai, S. L. Gilat, R. Ponsinet, J.-M. Lehn, Chem. Eur. J. 1995,
1, 285–293.
[57] A. Osuka, D. Fujikane, H. Shinmori, S. Kobatake, M. Irie, J. Org.
Chem. 2001, 66, 3913–3923.
[58] A. Fernꢃndez-Acebes, J.-M. Lehn, Chem. Eur. J. 1999, 5, 3285–
3292.
[59] G. Engelmann, R. Stçßer, G. Koßmehl, W. Jugelt, H.-P. Welzel, J.
Chem. Soc. Perkin Trans. 2 1996, 2015–2019.
´
[48] P. J. Kocienski, Protecting Groups, Georg Thieme Verlag Stuttgart,
New York, 1994.
[60] CCDC 747599 [for 6 (O-isomer)] contains the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre at
www.ccdc.cam.ac.uk/data_request/cif.
[61] J. J. D. de Jong, L. N. Lucas, R. Hania, A. Pugzlys, R. M. Kellogg,
B. L. Feringa, K. Duppen, J. H. van Esch, Eur. J. Org. Chem. 2003,
1887–1893.
[49] N. S. Stoyanov, N. Ramchandani, D. M. Lemal, Tetrahedron Lett.
1999, 40, 6549–6552.
[50] K. Mikami, Y. Itoh, M. Yamanaka, Chem. Rev. 2004, 104, 1–16.
[51] D. D. Perrin, W. L. F. Armarego, Purification of Laboratory Chemi-
cals, 4th ed., Butterworth-Heinemann, 1996.
[52] R. Lantz, A.-B. Hçrnfeldt, Chem. Scr. 1972, 2, 9–15.
[53] W. Ried, H. Bender, Chem. Ber. 1955, 88, 34–38.
[54] S. L. Gilat, S. H. Kawai, J.-M. Lehn, Chem. Eur. J. 1995, 1, 275–284.
Received: September 28, 2009
Published online: March 25, 2010
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