Simple access to sol-gel precursors bearing fluorescent aromatic core units

Article abstract of DOI:10.1002/ejoc.201200076

Di-, tri-, and tetrathienyl-substituted polycyclic aromatic fluorophores were prepared from different aryldi-, aryltri-, or aryltetrahalides by a simple and fast Suzuki coupling. The reaction was optimized for the synthesis of the desired materials on mul

Full text of DOI:10.1002/ejoc.201200076

FULL PAPER
DOI: 10.1002/ejoc.201200076
Simple Access to Sol–Gel Precursors Bearing Fluorescent Aromatic Core Units
Maximilian Hemgesberg,[a] Dominik M. Ohlmann,[a] Yvonne Schmitt,[a]
Monique R. Wolfe,[b] Melanie K. Müller,[a] Benjamin Erb,[a] Yu Sun,[a] Lukas J. Gooßen,*[a]
Markus Gerhards,*[a] and Werner R. Thiel*[a]
Keywords: Cross-coupling / Palladium / Fluorescence / Silicon
Di-, tri-, and tetrathienyl-substituted polycyclic aromatic flu-
orophores were prepared from different aryldi-, aryltri-, or
aryltetrahalides by a simple and fast Suzuki coupling. The
reaction was optimized for the synthesis of the desired mate-
porous organosilica syntheses. They were converted into a
variety of new trimethoxysilyl arenes by using a very ef-
ficient Pd-mediated C–Si cross-coupling, which was also ex-
tended to the corresponding thienyl bromides by using a di-
rials on multigram scale. The coupled products were con- meric PdI catalyst. All compounds were characterized by 1H
verted into the corresponding iodides through iodination
with N-iodosuccinimide. The iodides turned out to be versa-
tile starting materials for applications, such as periodic meso-
NMR, 13C NMR, 29Si NMR, and ATR-IR spectroscopy and
HRMS.
new organic–inorganic hybrid materials, we took an interest
in the idea of producing highly fluorescent compounds
bearing (trialkoxysilyl)thiophene moieties as subunits for
gelation.[7]
Introduction
The development of organic and organic–inorganic hy-
brid materials and their application in energy conversion,
for example, solar cells[1] and light-emitting thin films,[2] has
recently received widespread attention in the scientific com-
munity. Covalently embedding functionalized organic moie-
ties into an inorganic framework opens up a whole new
field in material sciences. Aromatic and heteroaromatic
compounds have been of interest for fundamental research
in the past. Moreover, some of them have recently received
major attention due to their combination of advantageous
optical, electronic, chemical, and/or physical properties.[3]
Recent advances in polymer chemistry particularly focus on
thiophenes containing π-donor–acceptor pairs suitable for
producing photoluminescent and energy-harvesting macro-
molecules.[4] These might be interesting for the development
of new organic light-emitting diodes (OLEDs), organic
field-effect transistors (OFETs), and polymeric optical fi-
bers (POFs).[5] Experimental as well as theoretical ap-
proaches have shown that the fluorescence properties and
MO energy gaps of aryl-bridged dithiophenes (2-thienyl)-
(Ar)-(2-thienyl) can be tuned by modification of their aro-
matic core.[6] As our group is dedicated to the synthesis of
Results and Discussion
C–C Couplings
Intrigued by the progress in the field of polymer synthe-
sis, we designed our reaction pathway accordingly: In a first
step, a catalytic C–C bond formation would give us the 2-
thienyl arenes that should be easy to isolate and purify, for
example, by recrystallization, due to their planar structure
and thermal stability. Subsequent Wohl–Ziegler iodination
or bromination by using N-halosuccinimides would result
in the formation of the 5-halo-2-thienyl arenes, the latter
ones being the ideal candidates for Pd-mediated C–Si cou-
pling.
Starting with the C–C coupling, we intended to avoid
both Stille-type coupling of aryl halides with the rather ex-
pensive and toxic 2-(tributylstannyl)thiophene[8] as well as
Kumada-type[9] or Negishi-type[10] couplings by using air-
and moisture-sensitive Grignard or zinc organyls, respec-
tively (Scheme 1).
Therefore, the first key step in the synthesis of the desired
compounds was the Suzuki–Miyaura-type coupling of thio-
phene-2-boronic acid with different polycyclic aromatic hal-
ides 1–18a to yield the corresponding thienyl-substituted
products 1–15b and 18b. As many of the starting materials
given in Figure 1 are scarcely available, we invested some
time in finding the appropriate synthetic methods to obtain
suitable amounts of the desired aryl halides.
[a] Fachbereich Chemie, Technische Universität Kaiserslautern
and Research Centre OPTIMAS
Erwin-Schrödinger-Straße 54, 67663 Kaiserslautern, Germany
Fax: +49-631-2054676
E-mail: thiel@chemie.uni-kl.de
goossen@chemie.uni-kl.de
gerhards@chemie.uni-kl.de
[b] Department of Chemistry, Yale University
New Haven, CT 06520-8107, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200076.
2142
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Sol–Gel Precursors Bearing Fluorescent Aromatic Core Units
Suzuki–Miyaura couplings of aromatic and heteroarom-
atic halides with boronic acids have already been intensely
studied. The reaction conditions have been optimized in
terms of solvent, temperature, catalyst loading, and reac-
tion time, turning the Suzuki coupling into a versatile and
environmentally benign process.[12]
Our first goal was to apply a literature procedure to our
polycyclic aromatic substrates. In order to efficiently per-
form batch reactions on a 20-mmol scale, we designed sev-
eral criteria for the reaction set up which comprise (1) fast
conversion of the starting materials (less than 1 h); (2) the
ability to work under non-inert conditions; (3) the use of
common solvents such as alcohol/water mixtures; (4) a sim-
ple workup.
The reaction variants we took from published prepara-
tions, for example, by using n-butanol/H2O (7:1)/K3PO4,[13]
failed to give the products within the desired reaction time
due to incomplete conversion. Appealed by the concept of
“naked” Pd catalysts,[14] we tried to establish a new pro-
cedure which would meet our expectations. Employing
Pd(PPh3)4 as a comparatively cheap Pd source and potas-
sium carbonate as a base (scheme in Table 1), we identified
a 9:1 mixture of ethoxyethanol/water to be the ideal solvent
to obtain the dithienyl derivatives in moderate to excellent
yields. Using other solvents such as THF/H2O (1:1) gave
poorer yields.
Scheme 1. Possible access to dithienyl-substituted arenes by dif-
ferent transition-metal-catalyzed coupling reactions. Pathways
marked with dashed lines have not been explored in this work.
Table 1. Scope of the newly developed Suzuki coupling protocol
for the synthesis of thienyl-substituted aromatics.[a]
Product
Yield [%][b]
Product
Yield [%][b]
1b
2b
3b
4b
5b
6b
7b
8b
98
74
9b
87
97
81
97
95
44
85
90
10b
11b
12b
13b
14b
15b
18b
83[c]
58
73
89
98[d]
85
[a] Reaction conditions: Aryl dihalide (20 mmol), 2-thienylboronic
acid (44 mmol), K2CO3 (88 mmol), Pd(PPh3)4 (0.08 mmol), ethoxy-
ethanol/H2O (135:15 mL), 130 °C, 15 min. [b] Isolated yield of the
product after recrystallization and workup of the mother liquor.
[c] Using NEt3 (80 mmol) and K2CO3 (8 mmol) as base. [d] Reac-
tion on 1-mmol scale, 1.0 mol-% Pd, diglyme/ethoxyethanol/H2O
(20:20:5 mL), 150 °C, 4 h.
Figure 1. Starting materials for Suzuki–Miyaura coupling reac-
tions.
We could not produce larger quantities of 1,8-dibro-
mopyrene due to poor selectivity in the dibromination of
pyrene and the tedious workup by fractionated crystalli-
zation. Although noted otherwise in the literature, we could
After careful optimization, we developed a protocol for
not accomplish a large-scale synthesis of 2,7-dibromo- the efficient conversion of the aryl halides, requiring a cata-
pyrene starting from the reduced species THP (6a).[11] How- lyst loading as low as 0.2 mol-% per reactive moiety. Only
ever, all other compounds were isolated in suitable yields in the case of very poorly soluble tetrabromopyrene 7a was
without problems.
it necessary to apply forcing conditions, using diglyme/
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L. J. Gooßen, M. Gerhards, W. R. Thiel et al.
FULL PAPER
ethoxyethanol/H2O (4:4:1) and higher catalyst loadings of As we assumed a certain, yet limited, stability of Pd-
1.0 to 1.5 mol-%. The results of our work on the new cou- (PPh3)4 to be decisive for our reaction system, this outcome
pling variant are summarized in Table 1.
was in coherence with our expectations. Although the iso-
Notably, the reaction does not require inert conditions, lated yields of these reactions have been further improved
as it proceeds very rapidly and, to our best knowledge, by optimization of the reaction set up and workup, we con-
mostly produces no significant amounts of interfering by- sider the method given above as being the far more advan-
products. In terms of conversion, our results match the out- tageous one, especially when compared to the methods pre-
come of microwave irradiation experiments carried out by sented in the literature.[17]
others, as the products started to precipitate within a few
Testing the versatility of our method, we were able to
minutes.[15] In systematic studies, we discovered that replac- readily obtain 13b and 18b, which, to the best of our knowl-
ing potassium carbonate with organic bases such as NEt3 edge, have not been prepared by Suzuki coupling before.
did not lower the conversion; however, it slowed down However, we experienced some minor drawbacks: The pro-
the reaction rate. Base-sensitive 2,5-diiododimethyl cedure could not be applied to N-heterocycles, such as 3,6-
terephthalate (3a) was therefore successfully coupled in the dibromo-9-methyl-9H-carbazole[18] or 3,8-dibromo-1,10-
presence of an excess amount of NEt3 and substoichi- phenanthroline,[19] giving only trace amounts of product
ometric quantities of K2CO3. We further observed that the and otherwise yielding an inseparable black tar. 9,10-Di-
reaction mostly proceeded smoothly at temperatures be- bromoanthracene[20] and p-dibromoveratrole were unreac-
tween 100 and 130 °C.
tive under the given conditions, as they were recovered in
When exploring the substrate reactivity, we found elec- about 80–85% yield.
tron-poor arenes such as the biphenyl derivatives 5a, 6a,
Similarly, all attempts to synthesize dithienyl-substituted
and 8a, ketone 11a, and sulfoxide 12a to be the most reac- quinoxaline 16b or benzothiadiazole 17b through Suzuki
tive ones. A significant turnover was detected for those coupling failed, as we mostly recovered only larger amounts
compounds even below 100 °C. Arenes bearing electron- of the unchanged dihalide. Compound 17a has been re-
rich functional groups such as nitrogen- or sulfur-contain- ported to undergo Suzuki[21] and Sonogashira reactions,[22]
ing heterocycles (i.e., 13a and 14a, respectively) needed to but did not react to 17b even when prolonging the reaction
be heated above 130 °C to react smoothly. In the case of time up to 24 h under inert conditions. The reason for all
14a, the decrease in reactivity might have led to the lower these incompatibilities may lie in the electron-rich aromatic
yield that was observed. This tendency is supported by the cores of both the dihalides and thiophene-2-boronic acid,
outcomes of the couplings of 1a to 4a: relatively electron- making it nearly impossible for Pd0 to undergo the common
deficient dithienyl arenes featuring phenylene (1a) and cycle of oxidative addition and reductive elimination. Other
bis(carboxymethyl)phenylene (3a) moieties were obtained in structural aspects, such as the diimino moieties of 16a and
higher yields than electron-richer xylenes (2a) and pyridines 17a, may also contribute to this behavior, requiring a highly
(4a). Contrary to the reactions’ dependency on the struc- reactive catalyst and/or thienyl species.
ture of the arenes, there was no significant difference be-
A complementary approach to the desired product
tween aryl bromides and aryl iodides.
through decarboxylative cross-coupling of 17a with potas-
A principal driving force of the reaction can be seen in sium 2-thienyl carboxylate at elevated temperatures[23] by
the poor solubility of the products in the polar solvent mix- using quinoline was not successful either. Hence, 16b and
ture caused by the planarity and the extensive π-stacking of 17b were prepared by Stille coupling by using thienyl tri-n-
the emerging large aromatic molecules.
butylstannane and Pd(PPh3)2Cl2
as
a
catalyst
We successfully isolated 1,4-dithienylbenzene (1b) in ex- (Scheme 2).[24]
cellent yields of around 98% after carrying out the reaction
at 1 to 20-mmol scales and in a moderate yield of 51% at
50-mmol scale. When further increasing the amount of the
dihalide up to 100 mmol, yields tended to be no longer re-
producible but were very low in general, as no sufficient
heating could be applied within the short reaction time
given. As Pd0 species generated in situ seem to be most
reactive within the very first minutes of the conversion,
proper heating of the reaction vessel might be crucial to
rapidly generate the required amounts of catalyst while at
the same time ensuring that the boronic acid does not un-
Scheme 2. Stille coupling towards thienyl-substituted diimino de-
rivatives. Yields for the isolated product are given.
dergo unwanted side reactions, such as protodeboryl-
ation.[16]
Several 10-mmol test reactions using Pd(OAc)2 or immo-
The yield of the resulting crystalline crude products ob-
bilized Pd EnCat 40 as metal sources and PPh3 as ligand tained by Stille coupling are comparable to those of the
under otherwise unchanged conditions furnished desired above-mentioned Suzuki reactions; however, the prepara-
compound 1b, however, in lower isolated yields of 22% for tion of larger amounts of the thiophene-containing mole-
the homogeneous and 6% for the heterogeneous version. cules was laborious, as it was hard to completely remove
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Sol–Gel Precursors Bearing Fluorescent Aromatic Core Units
the tin organyls from the crude reaction mixtures by or more distinct absorption bands (i.e., for 11b, 12b, 16b,
recrystallization. Further experiments will be conducted to and 17b). Additionally, a large aromatic system or heterocy-
solve this problem, for example, by treatment with clic systems lead to a redshift in the absorption. The same
EDTA[25] or adsorption on silica.
effect is observed by insertion of a C=O group. Thus, it
can be concluded that different absorption regions can be
obtained through systematic control of the structural arran-
gements.
Fluorescence Data of the Dithienyl Arenes
The emission spectra of these substances show a great
variety (Figure 3) even if most substances emit in the range
from 380 to 600 nm. In general, the emission spectra are
less structured than one would expect from the absorption
spectra, and variation of the excitation wavelength does not
result in any changes in the emission spectra due to the
fluorescence from the lowest excited state.
Oligothiophenes are not only known for having semicon-
ducting properties but also for their strong luminescence
when exposed to UV light. Some of the thienyl arenes pre-
sented herein have already proven to be useful in energy
harvesting and light conversion applications (see the Sup-
porting Information). We were therefore pleased to detect
blue to orange fluorescence during the Suzuki coupling. For
a selection of samples the absorption and fluorescence spec-
tra are shown in Figures 2 and 3, and the corresponding
absorption and emission maxima are given in Table 2. The
spectra of the remaining compounds are given in the Sup-
porting Information.
Figure 3. Emission spectra of 1b, 11b, 12b, 16b, and 17b solvated
in CH2Cl2. Excitation wavelengths: 1b: λex = 272 nm, 11b: λex
=
321 nm, 12b: λex = 305 nm, 16b: λex = 335 nm, 17b: λex = 275 nm.
The influence of the structure on the emission is similar
to that discussed for the absorption properties. As well as
for the absorption spectra, the position of the emission can
be controlled by the binding motif between the aromatic or
heterocyclic system. A further method to get the emission
to the desired region is the use of heteroatoms or functional
groups (e.g., carbonyl, ester, or amino groups) whereupon
the influence of the carbonyl group on the emission proper-
ties can be weakened by inserting another functional group.
The influence of the functional groups can also be analyzed
by the Stokes shift of the compounds, and here again a
large Stokes shift is observed for compounds containing the
Figure 2. Absorption spectra of 1b, 11b, 12b, 16b, and 17b solvated
in CH2Cl2.
Table 2. Absorption and emission maxima and Stokes shift.
Substance
λabs,max [cm–1]
λem,max [cm–1]
Stokes shift
[cm–1]
1b
30675
32637
29360
36630
32510
26110
16892
22989
18450
17699
4565
15745
6371
18090
14811
11b
12b
16b
17b
The shown absorption spectra (Figure 2) cover a region above-mentioned functional groups.
from 230 up to 550 nm with a strong structured shape. It
Two of the substances, 16b and 17b, were iodinated, and
can be observed that the structure of the absorption spectra the spectra are shown in Supporting Information. It is
depends both on the binding motifs between the aromatic found that the iodination leads to a redshift in the absorp-
or heterocyclic systems and the use of different functional tion spectra and emission spectra. Additionally, for the
groups.
iodinated substances a stronger structured absorption spec-
A single bond between the aromatic systems seems to trum is found and an additional emission band at a lower
lead to a less-structured spectrum (i.e., for 1b), and annu- wavelength. For compound 7b (a pyrene derivative), an ex-
lation of the aromatic systems results in an overlap of two cimer was also detected.
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L. J. Gooßen, M. Gerhards, W. R. Thiel et al.
FULL PAPER
Thus, by means of the absorption and emission spectra
Contrary to the mild reaction conditions and the mini-
of the 20 substances the influence of binding motifs as well mal excess of NIS employed in most of the aforementioned
as heteroatoms or functional groups on the absorption and syntheses, the tetraiodination of 7b was achieved only by
emission properties can be discussed, and with this infor- heating the starting material in pure acetic acid together
mation the spectroscopic properties of the dyes can be with an excess amount of NIS. Most of the poorly soluble
adapted to the designated application.
iodides were recrystallized from DMF, DMSO, or, in case
of 7b, large amounts of o-dichlorobenzene. However, the
yields were lowered significantly (approx. 20% overall loss)
by each recrystallization step. Comparing the NMR spectra
of some crystalline samples, which showed minor impuri-
ties, to the ones of the crude products, we assume that in
many cases there is no need for further purification. The
byproducts are most likely to be generated under the harsh
conditions necessary for recording the spectra ([D6]DMSO
at 100–120 °C).
Phenothiazine derivatives are readily oxidized to form
stable paramagnetic radical cations.[26] Therefore, the only
arylthiophene that could not be iodinated with NIS was
15b, yielding a deep purple precipitate which we were not
able to re-reduce to the free amine, neither by simple
recrystallization from DMF/H2O nor by using ascorbic
acid.[27] Other attempts to prepare 15c by metalation with
nBuLi and addition of I2 at low temperatures or by electro-
philic iodination by using ICl and potassium acetate at
25 °C failed.[28]
Iodination of Arylated Thiophenes
As thiophene preferably reacts with radical species at its
acidic 2- and 5-positions, we could easily synthesize iodides
1c–14c and 16c–18c by modifying the procedure of Cao et
al.[24] and by replacing NBS with NIS (Table 3). The pres-
ence of glacial acetic acid in the reaction mixture was iden-
tified to be crucial for rapid conversion of the starting mate-
rial; however, a 1:1 mixture of HOAc/chloroform as re-
ported was not required. Due to the very low solubilities of
most products, they were obtained as precipitates from the
hot reaction mixture during or shortly after the dissolving
of the iodination agent. Only in some cases, such as for the
more-soluble 2,5-bis(5-iodothien-2-yl)-1,4-dimethylbenzene
(2c) did the chloroform have to be removed before filtering
to obtain a precipitate.
Table 3. Scope of the selective iodination of aryl-substituted thienyl
halides in the 5-position by using N-iodosuccinimide (NIS).[a]
The crystal structure of diiodide 16c is shown in Fig-
ure 4. Crystals of suitable quality were obtained from a
dichloromethane solution of the crude product by slow
evaporation of the solvent over weeks. Selected bond
lengths and angles prove that the dithienylarenes exhibit a
quasilinear and rigid structure. The planarity of the aro-
matic system, which might be dependent on the substitution
of the central phenylene unit, is only slightly broken in the
case of 16c (for further details on the X-ray structure analy-
sis, see the Supporting Information).
Product
Yield [%][b]
Product
Yield [%][b]
1c
2c
3c
4c
5c
6c
7c
8c
9c
90
80
84
71
80
10c
11c
12c
13c
14c
16c
17c
18c
84
89
76
70
96
93
98
96
98
99[c]
87
73
Figure 4. Molecular structure of 16c in the solid state determined
by X-ray single-crystal diffraction (ball-stick model). Selected bond
lengths [Å]: C3–C7 1.46, C6–C8 1.48, C4–N1 1.36, C7–S1 1.74,
C9–C10 1.42, C12–C13 1.40, C13–I1 2.07. Selected angles [°] and
dihedral angles [°]: C8–S2–C16 90.9, C4–N1–C9 117.4, C2–C1–C6
122.0, C6–C8–C14 123.9, S2–C16–I2 119.6; C4–C3–C7–S1 8.3,
C5–C6–C8–S2 12.6.
[a] Reaction conditions: Dithienyl arene (5 mmol), NIS
(10.3 mmol), CHCl3/HOAc (20:5 mL), 100 °C, 5 min. [b] Isolated
yield. [c] Reaction performed on 1-mmol scale, NIS (8.00 mmol),
pure HOAc, 130 °C, 4 h.
In most cases, the substitution of the thienyl protons with
iodine, as expected, led to an immediate quenching of the
intense (violet to orange) fluorescence observed in the reac-
tion mixtures. Only for compounds 16c and 17c did we find
a reduced yet still remarkable emission of light, which could
be attributed to different fluorescence properties of those
C–Si Couplings
Although being a versatile tool for the synthesis of non-
electron-rich arenes (see spectra in the Supporting Infor- commercially available aryltrialkoxysilanes, the palladium-
mation).
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Sol–Gel Precursors Bearing Fluorescent Aromatic Core Units
aryl halides and trialkoxysilanes HSi(OR)3 (R = Me, Et) Table 4. C–Si cross-coupling reactions of 5-iodothien-2-yl-func-
tionalized aromatics to the corresponding bis(trimethylsilyl) deriva-
has not yet been exceedingly covered in the literature. Since
its discovery in the late 1990s, this σ-bond metathesis-type
tives.[a]
reaction[29] was further developed by Manoso and De-
Shong[30] as well as by Masuda et al., who also presented
a rhodium-mediated version.[31] The main reason for the
comparatively minor attention this reaction has received up
to now might be its rather limited scope of starting materi-
als, comprising only several aryl iodides and electron-rich
aryl bromides, and its tendency to give considerable
amounts of undesired byproducts, especially by reductive
proto-dehalogenation.
Furthermore, the reaction products are moisture sensi-
tive and can be purified only to a very limited extent. The
variety of functional groups and the high molecular weight
Product
Yield [%][b]
Product
Yield [%][b]
of the aromatics presented here make it even more difficult
to pursue common purification methods, for example, bulb-
to-bulb distillation. As we initially wanted to synthesize un-
known 4,7-bis[5-(trimethoxysilyl)thien-2-yl]benzo[c][1,2,5]-
thiadiazole (17d), we focused on developing suitable condi-
tions for the coupling of 17c with trimethoxysilane or tri-
ethoxysilane.
We based our initial screening on the protocol of Ma-
noso and DeShong, employing NMP as a polar aprotic sol-
vent and Pd(dba)2/Johnphos (di-tert-butylphosphanylbi-
phenyl) as catalyst.[30] In contrast to the literature, aqueous
workup of the obtained solutions was not feasible, because
1d
2d
3d
4d
5d
6d
98
98
91
85
92
98
8d
70
88
69
97
94
98
11d
13d
16d
17d
18d
[a] Reaction conditions: Di- or trithienyl halide (1 mmol), Pd(dba)2
(0.40 mmol), JohnPhos (0.80 mmol), HSi(OMe)3 (1 mmol), DIPEA
(3 mmol) per halide moiety, DMF (5 mL), 80 °C, 30 min. [b] Iso-
1
lated yield (90 to 95% purity according to H NMR).
Having developed an efficient method for the synthesis
extraction with pentane and washing with deionized water of trimethoxysilylarenes from thienyl iodides, we sought the
yielded only inseparable mixtures of polymerized material. activation of the much cheaper thienyl bromides. Because
Furthermore, the use of triethoxysilane yielded at best the the corresponding aryl bromides turned out to be even less
dehalogenated starting materials. Changing the solvent to soluble in DMF than the aryl iodides, no conversion was
DMF and/or altering the reaction temperatures showed no observed after 2 h at 60 °C under otherwise unchanged con-
beneficial effect. A test reaction with 16c performed in ditions. A stepwise increase in the temperature and/or the
quinoline as basic solvent and using Pd(dppf)Cl2·CH2Cl2 reaction time up to 120 °C and 16 h furnished only the de-
as the catalyst had a similar outcome, as 16b was recovered halogenated species. The use of more electron-rich and
in 78% yield.
more sterically demanding phosphanes such as XPhos[32]
Finally, we replaced triethoxysilane with trimethoxysil- and BrettPhos[33] did not alter these results. However, we
ane, which led to a decisive improvement in reactivity. By were finally able to convert 1,4-bis(5-bromothien-2-yl)benz-
carrying out the reaction in DMF and at modest tempera- ene (1e) into desired silylated product 1d in quantitative
tures, full conversion of the aryl iodides was observed yield and with an excellent selectivity by employing the
within 2 h at 60 °C or within 30 min at 80 °C (Table 4). This highly active PdI dimer (tBu3PPdBr)2 (Scheme 3).
way, the amount of catalyst was reduced to only 0.2 mol-%
Pd per thiophene moiety, which corresponds to less than
10% of the figures given in the literature.[30] After a simple
workup, which included filtering off the inorganic salts and
removing the oxidized ligand by adsorption on Na2SO4, the
desired sol–gel precursors were reproducibly isolated in ex-
cellent to almost quantitative yields. When scaling up the
synthesis of 1d to 20 mmol, we noticed that the reaction
became clearly exothermic during the first minutes and fur-
ther proceeded without any change in product distribution
or purity.
Because the obtained sol–gel precursors formed highly
viscous waxes, they tended to trap the solvent inside their
matrix. Furthermore, they readily polymerize when heated
or when stored for a longer time. The outcome of the ex-
periments given in Table 4 nevertheless indicates a very
broad applicability of the optimized procedure.
Scheme 3. Access to trimethoxysilyl-substituted thiophenes from
their corresponding bromides by using a highly active dimeric PdI
catalyst.
This catalyst has been reported to facilitate C–C, C–N,
and C–S bond formations,[34] but it has never been used for
Eur. J. Org. Chem. 2012, 2142–2151
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2147

L. J. Gooßen, M. Gerhards, W. R. Thiel et al.
FULL PAPER
the coupling of aryl halides with trialkoxysilanes. The low plore the possible use of these silicates in optics, electronics,
catalyst loadings of only 0.1 mol-% per halide moiety out- sensors, and heterogeneous catalysis, for example, as super-
weigh the relatively high costs of this Pd dimer. Experi- hydrophobic acidic materials for the esterification of fatty
ments are under way to explore the applicability of this pro- acids.
tocol to the synthesis of trialkoxysilanes from other aryl
and heteroaryl halides.
In the long run, the quasilinear dithienyl halides and si-
lanes described above might be useful in the synthesis of
organic linkers for metal–organic frameworks (MOFs) by
Pd-catalyzed carboalkoxylation,[35] serve as organic build-
ing blocks in Hiyama cross-couplings,[36] or help to develop
new high-performance hybrid polymers.[37]
Conclusions
We have presented access towards fluorescent sol–gel
precursors incorporating various functional groups by
using a simple and reliable three-step synthesis, covering a
considerable scope of readily available polycyclic aromatic
compounds. The Suzuki coupling we introduced herein fea-
tures an optimized workup which enabled us to produce
larger amounts of thiophene-substituted arenes with no
need for column chromatography.
We further accomplished the iodination of thienyl arenes,
although still encountering problems for redox-sensitive
phenothiazines. The iodides did not only to some extent
show some interesting luminescence properties themselves
but could also most conveniently be converted into the de-
sired (trimethoxysilyl)thienyl arenes. The C–Si bond forma-
tion proceeds quantitatively with all tested substrates, al-
though the isolated yields, in some cases, might slightly vary
depending on the workup. We therefore could accomplish
our goal of using the special reactivity of the thiophene
moieties to generate a variety of new arylsilanes.
Because the new bis(trimethoxysilylthienyl) arenes we de-
scribed are rather sensitive when exposed to air or in solu-
tion and might also be able to react with silica gel and
quartz surfaces, we currently are undertaking great efforts
to determine their luminescence properties. The UV/Vis and
fluorescence spectra will then be compared to those of the
non-silylated arenes. We may already point out that their
fluorescence in solution most likely does not significantly
differ from the data presented herein. However, some of the
above-mentioned compounds feature a different fluores-
cence behavior in the solid state, which will be investigated
with respect to the method of deposition on several sur-
faces. Figure 5 gives a first impression on the different fluo-
rescence intensities and colors of the silanes that are ob-
served in solution.
Experimental Section
General: Compounds 1a, 4a, 5a, and 6a and all other chemicals
necessary for the syntheses of the Suzuki reaction substrates as well
as for the reaction sequences presented herein were purchased from
Sigma–Aldrich, Acros Organics, Maybridge, and VWR and used
as received. Dry DMF was supplied by Acros Organics, whereas
THF, pentane, toluene, Et2O, and CH2Cl2 were dried prior to use
1
according to standard procedures.[38] Liquid phase H NMR, 13C
NMR, and 29Si NMR spectra were recorded with two Bruker Spec-
trospin devices with a resonance frequency of 400 or 600 MHz,
151 or 101 MHz, and 76 MHz for the 1H, 13C, and 29Si nuclei,
respectively. The spectra are internally referenced to SiMe4. The
infrared spectra with a resolution of Ϯ2 cm–1 were recorded by
using a PerkinElmer FT-ATR IR 1000 spectrometer equipped with
a diamond coated ZnSe-window. High-resolution mass spectra
were obtained in EI+ mode with a Waters GCT Premier spectrome-
ter. MALDI-ToF measurements were conducted with a Bruker
Daltonics Ultraflex spectrometer with CHCA (α-cyano-4-hy-
droxycinnamic acid) being the matrix compound. The UV/Vis ab-
sorption and fluorescence of the precursor were measured by using
a Perkin–Elmer Lambda 900 and a Horiba Jobin–Yvon Fluorolog
3–22 τ in steps of 0.1 and 1.0 nm, respectively; the SiO2 cuvettes
used had a width of 1.0 cm.
Absorption and Emission Spectra: All substances were diluted in
dichloromethane purchased from Merck in Uvasol quality. The
concentration of the solutions used for the absorption spectra was
in the range of 10–5 molL–1, for the fluorescence measurements in
the range of 10–6 molL–1. Absorption spectra were recorded with
a Perkin–Elmer Lambda 950 UV/Vis/NIR double beam spectro-
photometer, emission spectra were recorded with a Jobin–Yvon
FluoroMax2 spectrophotometer. The cuvettes used for absorption
measurements were cylindrical quartz cuvettes with a path length
of 1 cm, for emission measurements a 1 cmϫ1 cm quartz cuvette
was used.
X-ray Structure Analyses: Crystal data and refinement parameters
for compound 16c are collected in Table 5. The structure was
solved by direct methods (SHELXS97), completed by subsequent
difference Fourier syntheses, and refined by full-matrix least-
squares procedures. For 16c, a semiempirical absorption correction
(Multiscan) was carried out. All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were placed in calculated positions and refined by using a riding
model.
Figure 5. Appearance of (trimethoxysilylthienyl)silane solutions in
DCM under normal and UV light (λex = 254/366 nm; from left to
right: 6d, 13d, 16d, and 11d).
CCDC-864482 (for 16c) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
We might soon be able to create sol–gel precursors carry-
ing nearly every desired functional group and will then pro-
ceed with the sol–gel synthesis of new hybrid organosilica
Syntheses of the Suzuki Reaction Substrates: The full procedures
and xerogels, characterize their molecular structure, and ex- are only given if appropriate.
2148
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 2142–2151

Sol–Gel Precursors Bearing Fluorescent Aromatic Core Units
2,8-Dijododibenzo[b,d]furan (9a):[43] Prepared in 38% yield from di-
benzo[b,d]furan as colorless crystals. 1H NMR (400 MHz, CDCl3):
δ = 8.12 (d, 2 H), 7.66 (dd, 2 H), 7.25 (d, 2 H) ppm. HRMS: calcd.
for C12H6I2O 419.8508; found 419.8550.
Table 5. Summary of the crystallographic data and details of data
collection refinement.
16c
Empirical formula
Formula weight [gmol–1]
Crystal size [mm3]
T [K]
C16H8I2N2S2
546.16
0.14ϫ0.10ϫ0.06
150(2)
2,8-Dibromodibenzo[b,d]thiophene (10a):[44] Prepared in 34% yield
from 12a as colorless crystals. The yield was limited due to difficult-
ies with the phase separation after quenching the reaction with
1
H2O. H NMR (400 MHz, CDCl3): δ = 7.82 (dd, 2 H), 7.68–7.61
λ [Å]
1.54184
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
monoclinic
C2/c
34.1122(17)
5.7372(2)
18.5596(9)
90
118.376(7)
90
3195.8(2)
8
(m, 2 H), 7.53 (t, 2 H) ppm. HRMS: calcd. for C12H6Br2S
339.8557; found 339.8585.
2,7-Diiodo-9H-fluoren-9-one (11a):[45] Prepared in 68% yield from
9H-fluoren-9-one as orange crystals. 1H NMR (600 MHz, CDCl3):
δ = 7.95 (d, 2 H), 7.83 (dd, 2 H), 7.25 (d, 2 H) ppm. HRMS: calcd.
for C13H6I2O 431.8508; found 431.8488.
2,8-Dibromodibenzo[b,d]thiophene-5,5-dioxide (12a):[44] Prepared in
two steps with a total yield of 72% from dibenzo[b,d]thiophene as
colorless crystals. 1H NMR (600 MHz, 100 °C, [D6]DMSO): δ =
8.15 (s, 2 H), 8.09 (d, 2 H), 8.00–7.94 (m, 2 H) ppm. HRMS: calcd.
for C8H8I2 371.8455; found 371.8456.
γ [°]
V [Å3]
Z
ρcalcd. [gcm–1]
2.270
33.362
μ [mm–1]
θ-range [°]
2.94–62.67
9434
2551 [Rint. = 0.0416]
2551/0/199
0.0337/0.0812
0.0356/0.0823
1.077
Reflections collected
Independent reflections
Data/restraints/parameters
Final R indices [IϾ2σ(I)][a]
R indices (all data)
GooF[b]
2,7-Dibromo-10-methylacridin-9(10H)-one (13a):[2,18,46] Prepared in
three steps with a total yield of 56% from N-phenylanthranilic acid
1
as yellow crystals. H NMR (400 MHz, [D6]DMSO): δ = 8.42 (s, 2
H), 7.93 (d, 2 H), 7.80 (d, 2 H), 3.93 (s, 3 H) ppm. HRMS: calcd.
for C12H6Br2O2S 364.9051; found 364.9049.
3,6-Dibromo-9H-thioxanthen-9-one (14a):[47] Prepared in 60% yield
from 9H-thioxanthen-9-one as bright yellow crystals. 1H NMR
(600 MHz, [D6]DMSO): δ = 8.51 (s, 2 H), 7.88 (d, 2 H), 7.74 (d, 2
H) ppm. HRMS: calcd. for C13H6Br2OS 367.8506; found 367.8511.
Δρmax/min [eÅ–3]
0.591/–2.126
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|, ωR2 = [Σω(Fo – Fc ) /Σω(Fo)2]1/2
.
2
2 2
2
2 2
[b] GooF = [Σω(Fo – Fc ) /(n – p)]1/2
.
2,5-Diiodo-1,4-dimethylbenzene (2a):[39] Prepared in 75% yield from
p-xylene as colorless crystals. 1H NMR (400 MHz, CDCl3): δ =
7.67 (s, 2 H), 2.36 (s, 6 H) ppm. HRMS: calcd. for C8H8I2 357.8716;
found 357.8709.
2,7-Dibromo-10-methyl-10H-phenothiazine (15a):[48] Prepared in
two steps with a total yield of 42% from 10H-phenothiazine as
colorless crystals. Bromination of the N-methylated derivative using
NBS was done in DMF for 1 h at 80 °C and 24 h at 25 °C (similar
to 13a), followed by quenching with H2O and recrystallization from
EtOH/CHCl3. Even samples pure in NMR can be dark purple in
color due to minor radical cation impurities. H NMR (400 MHz,
CDCl3): δ = 7.31–7.20 (m, 2 H), 6.65 (d, 1 H), 3.31 (s, 2 H) ppm.
HRMS: calcd. for C13H9Br2ONS 368.8822; found 368.8817.
5,8-Dibromoquinoxaline (16a):[49] Prepared in two steps with a total
yield of 39% from 17a as golden crystals. 1H NMR (400 MHz,
CDCl3): δ = 9.04 (s, 2 H), 8.02 (s, 1 H) ppm. HRMS: calcd. for
C8H4Br2N2 285.8721; found 285.8727.
4,7-Dibromobenzo[c][1,2,5]thiadiazole (17a),[50] Prepared in two
steps with a total yield of 71% from o-phenylenediamine as grayish
crystals. 1H NMR (400 MHz, CDCl3): δ = 7.75 (s, 2 H) ppm.
HRMS: calcd. for C6H2Br2N2S 291.8305; found 291.8318.
2,5-Diiododimethyltherephthalate (3a):[40] Prepared in three steps
with a total yield of 16% from 2a. The yield was limited due to the
laborious workup of the thick MnO2 slurry obtained at the end of
the oxidation steps. Esterification of 2,5-diiodotherephthalic acid:
SOCl2 (2.3 equiv., 2.97 g, 25.0 mmol, 1.81 mL) was added dropwise
to MeOH (100 mL). The solution was stirred at 25 °C for 2 h. The
acid (4.60 g, 11.0 mmol) was added. The resulting suspension was
heated to 70 °C under reflux conditions for a further 8 h. The
formed solid was filtered at –18 °C, dried in the vacuum, and
recrystallized from MeOH/CH2Cl2 (1:1) to yield 3a (2.80 g,
6.30 mmol, 57%) as colorless crystals. 1H NMR (400 MHz,
CDCl3): δ = 8.35 (s, 2 H), 3.97 (s, 6 H) ppm. HRMS: calcd. for
C10H8I2O4 445.8512; found 445.8500.
1
1,3,6,8-Tetrabromopyrene (7a):[41] Prepared in 98% yield from py-
rene as a colorless solid. The crude product, being very poorly solu-
ble in all available solvents except for boiling PhNO2, was used
without purification and only characterized by HRMS: calcd. for
C16H6Br4 513.7203; found 513.7189.
Tri(4-iodophenyl)amine (18a):[51] Prepared in 61% yield from tri-
1
phenylamine as colorless crystals. H NMR (400 MHz, CDCl3): δ
= 7.56 (d, 6 H), 6.83 (d, 6 H) ppm. HRMS: calcd. for C18H12I3
622.8104; found 622.8079.
Standard Suzuki Coupling Procedure: Thiophene-2-boronic acid
(5.63 g, 44.0 mmol, 2.20 equiv.) was mixed with the aromatic di-
halide (20.0 mmol) and K2CO3 (12.1 g, 88.0 mmol, 4.40 equiv.) in
ethoxyethanol/deionized water (9:1, 150 mL). Pd(Ph3)4 (94.0 mg,
8ϫ10–5 mol, 0.4 mol-%) was added to the stirred suspension,
which was then heated rapidly to 130 °C and maintained under
reflux conditions for 15 min. The resulting solution darkened and
mostly became black very shortly after reaching the boiling point,
the crude product precipitating either directly or on cooling of the
2,7-Dibromo-4,5,9,10-tetrahydropyrene (6a):[11] Prepared in two
steps with a total yield of 32% from pyrene as colorless crystals.
The yield was limited due to several recrystallization steps in both
of the stages. H NMR (400 MHz, CDCl3): δ = 7.22 (s, 4 H), 2.83
(s, 8 H) ppm. HRMS: calcd. for C16H12Br2 363.9286; found
363.9286.
1
3,6-Diiodo-9H-fluorene (8a):[42] Prepared in 57% yield from 9-H-
fluorene as colorless crystals. 1H NMR (400 MHz, CDCl3): δ =
7.80 (s, 2 H), 7.62 (d, 2 H), 7.41 (d, 2 H), 3.75 (s, 2 H) ppm. HRMS: reaction mixture to 25 °C. Deionized water (200 mL) was added at
calcd. for C13H8I2 417.8716; found 417.8707. room temperature to precipitate the main part of the product and
Eur. J. Org. Chem. 2012, 2142–2151
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L. J. Gooßen, M. Gerhards, W. R. Thiel et al.
FULL PAPER
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Acknowledgments
We gratefully appreciate funding by Nanokat and the Collaborative
Research Centre SFB/TRR 88 “3MET”. For Ph.D. fellowships, we
thank the Konrad-Adenauer-Stiftung (M. H.) and the Stiftung der
Deutschen Wirtschaft (D. M. O.). Y. Sch. thanks the Carl-Zeiss-
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Received: January 23, 2012
Published Online: February 22, 2012
Eur. J. Org. Chem. 2012, 2142–2151
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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