Sonogashira reactions of 2,3,4,5-tetrabromofuran: Synthesis of 2,3,4,5-tetraalkynylfurans, 2,3,5-trialkynylfurans and 2,5-dialkynylfurans

Article abstract of DOI:10.1055/s-0031-1291007

2,3,4,5-Tetrabromofuran is transformed into a variety of alkynyl-substituted furans by regioselective Sonogashira cross-coupling reactions. In this context, the first 2,3,4,5-tetraalkynylfurans and 2,3,5-trialkynylfurans were prepared. 2,3,4,5-Tetraalkynylfurans and 2,5-dialkynyl-3,4-diarylfurans show interesting fluorescence properties. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1291007

LETTER
▌1463
lSetoternogashira Reactions of 2,3,4,5-Tetrabromofuran: Synthesis of 2,3,4,5-Tetra-
alkynylfurans, 2,3,5-Trialkynylfurans and 2,5-Dialkynylfurans
Sonogashira Reactions of 2,3,4,5-Tetrabromofuran
Imran Malik,a,b Zeeshan Ahmad,a Sebastian Reimann,a,c Muhammed Nawaz,a Tamás Patonay,d Peter Langer*a,c
a
Institut für Chemie, Universität Rostock, Albert Einstein Str. 3a, 18059 Rostock, Germany
b
Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad, Pakistan
c
Leibniz-Institut für Katalyse an der Universität Rostock e.V., Albert Einstein Str. 29a, 18059 Rostock, Germany
d
Department of Organic Chemistry, University of Debrecen, Egyetem tér 1, 4032 Debrecen, Hungary
Received: 21.03.2012; Accepted: 23.03.2012
and epoxides,15 by [3+2] annulations of alkynes with alde-
Abstract: 2,3,4,5-Tetrabromofuran is transformed into a variety of
hydes,16 and by cyclizations of 1,4-diones in the presence
alkynyl-substituted furans by regioselective Sonogashira cross-cou-
of catalytic amounts of p-toluenesulfonic acid.17
pling reactions. In this context, the first 2,3,4,5-tetraalkynylfurans
and 2,3,5-trialkynylfurans were prepared. 2,3,4,5-Tetraalkynylfu-
rans and 2,5-dialkynyl-3,4-diarylfurans show interesting fluores-
cence properties.
An alternative approach to substituted furans relies on pal-
ladium-catalyzed cross-coupling reactions of brominated
furans. However, the low stability of furans, in particular
under aerobic and acidic conditions, makes their cross-
Key words: catalysis, palladium, Sonogashira reaction, furan,
regioselectivity, fluorescence
coupling reactions more fragile and the product isolation
more difficult than in the thiophene series. In recent years,
site-selective reactions18 of polybrominated furan deriva-
Furan derivatives represent important building block in
synthetic organic chemistry. Substituted furans have at-
tracted attention, due to their remarkable pharmacological
and photophysical properties.1 Furan is found in many
natural products, e.g. lophotoxin, pukalide, the cembrano-
lides, the plakorsins A–C, rosefuran, kallolide, adociacet-
ylene B, XH-14, perillene, and dendrolasin.2 The furan
derivative ranitidine (commercial name Zantac) is used
for the treatment of peptic ulcer and gastroesophageal re-
flux disease. Nitrofurantoin is used for the treatment of
urinary tract infections and nifuroxazide is suggested for
colitis and diarrhea treatment.3 A great deal of research ef-
forts on furan derivatives revealed also other biological
properties, such as anti-inflammatory, antituberculosis,
anticancer, and antifungal activities.4 Furan derivatives
have been also reported to show excellent photophysical
properties and have, thus, gained attention in material sci-
ences. In recent years, 2,5-bis(phenylethynyl)furans,5
furanyl nucleosides,6 bisfuranylethenes,7 and furopyrimi-
dine derivatives8 have been reported to show nonlinear
optical (NLO), fluorescence and photochromic properties.
tives have been studied. Negishi, Stille, Suzuki, and Sono-
gashira reactions and nucleophilic aromatic substitution
reactions have been reported for 2,3-, 2,4- and 2,5-dibro-
mofurans and proceed with excellent site selectivity in fa-
vor of position C-2.19 Bellina, Sulikowski and Rossi and
their co-workers reported site-selective transition-metal-
catalyzed reactions of several dibromofuranones.20
2,3,4,5-Tetrabromofuran represents an interesting sub-
strate for transition-metal-catalyzed cross-coupling reac-
tions as all four carbon atoms of the furan moiety are
halogenated. Recently, we have reported Suzuki–Miyaura
reactions of 2,3,4,5-tetrabromofuran which were the first
palladium(0)-catalyzed cross-coupling reactions of this
substrate.21 The synthesis of tetraarylthiophenes22 and tet-
ra(alkynyl)thiophenes23 by Suzuki and Sonogashira reac-
tions of tetrabromothiophene has been previously
reported, respectively. The synthesis of tetra(alkynyl)pyr-
roles has also been reported.24 Herein, we report what are,
to the best of our knowledge, the first Sonogashira reac-
tions of 2,3,4,5-tetrabromofuran. The reactions proceed
with very good site selectivity and provide a convenient
access to the first 2,3,4,5-tetraalkynylfurans and 2,3,5-tri-
alkynylfurans and to a variety of other derivatives. The
UV-Vis absorption and fluorescence properties of the
products were studied.
Various strategies have been developed for the synthesis
of furans. Furans can be synthesized by cyclodehydrations
of 1,4-diketones (Paal–Knorr synthesis),9 by condensa-
tions of α-halocarbonyl compounds with 1,3-dicarbonyl
compounds (Feist–Benary reaction),10 by cyclizations of
β,γ–epoxyketones,11 by cyclodehydrations of γ-hydroxy-
α,β-unsaturated ketones,12 by heteroannulation reactions
along with transition-metal-catalyzed cyclizations,13 by
cyclocondensations of phosphoranes with α-haloke-
tones,14 by base-mediated cyclizations of allenyl alcohols
2,3,4,5-Tetrabromofuran (1) was prepared following a lit-
erature procedure.25 The Sonogashira reaction of 1 with
2.4 equivalents of acetylenes 2a–d afforded the 2,5-di(ar-
ylethynyl)-3,4-dibromofuranfurans 3a–d in 71–81% yield
with very good site-selectivity (Scheme 1, Table 1).26 The
selectivity is observed as expected. The first attack occurs
at the electronically more deficient positions 2 and 5.18d
The reactions were carried out under standard conditions
using Pd(PPh3)2Cl2 (10 mol%) and CuI (5 mol%) as the
catalyst and diisopropylamine (DIPA) as the base and sol-
vent. The decrease of the catalyst loading resulted in a de-
SYNLETT 2012, 23, 1463–1466
Advanced online publication: 18.05.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1291007; Art ID: ST-2012-B0262-L
© Georg Thieme Verlag Stuttgart · New York

1464
I. Malik et al.
LETTER
crease of the yield. Attempts to carry out
a
have to be increased. The use of 10 mol% of Pd catalyst
monoalkynylation (using one equivalent of the alkyne) re- corresponds to 2.5 mol% per coupling step.
sulted in the formation of mixtures of mono- and di-
alkynylated products.
R
R
Br
R
Br
Br
Br
R
Br
Br
2a,c,e
2a–d
Br
Br
O
O
Br
R
O
Br
R
O
R
R
5a–c
1
3a–d
1
Scheme 3 Synthesis of 5a–c. Reagents and conditions: 1 (1.0 equiv),
2a,c,e (4.8 equiv), CuI (5 mol%) Pd(PPh3)2Cl2 (10 mol%), DIPA
(5 mL), 75 °C, 2 h
Scheme 1 Synthesis of 3a–d. Reagents and conditions: 1 (1.0
equiv), 2a–d (2.4 equiv), CuI (5 mol%), Pd(PPh3)2Cl2 (10 mol%),
DIPA, 60 °C, 2 h
Table 3 Synthesis of 5a–c
Table 1 Synthesis of 3a–d
2
2, 5
R
Yield (%)a of 5
2, 3
R
Yield (%)a of 3
c
a
e
a
b
c
3-(MeO)C6H4
4-t-BuC6H4
3-MeC6H4
83
81
75
a
b
c
4-t-BuC6H4
Ph
78
71
81
76
3-(MeO)C6H4
6-methoxynaphth-2-yl
a Yields of isolated products.
d
a Yields of isolated products.
The Suzuki–Miyaura reaction of 2,5-di(arylethynyl)-3,4-
dibromofurans 3a–d with (4-methoxyphenyl)boronic acid
(6a) and (4-methylphenyl)boronic acid (6b), in the pres-
ence of 10 mol% of Pd(PPh3)2Cl2 (70 °C, 2 h), resulted in
the formation of products 7a–d in 69–79% yields
(Scheme 4, Table 4). Other types of coupling reactions,
such as cyanation and formylation, are currently being
studied.
The Sonogashira reaction of 1 with 3.6 equivalents of
2c,e,f afforded the 2,3,5-tri(arylethynyl)-4-bromofurans
4a–c in 64–77% yield (Scheme 2, Table 2). The reactions
were carried out under identical conditions as discussed
for the synthesis of 3a–d.
R
Ar
Br
Ar
Br
Br
R
Br
Ar B(OH)2
Br
6a,b
2c,e,f
Br
O
O
Br
R
R
O
O
R
R
R
R
3a–d
7a–d
1
4a–c
Scheme 4 Synthesis of 7a–d. Reagents and conditions: 3a–d (1.0
equiv), 6a,b (2.0 equiv), Pd(PPh3)2Cl2 (10 mol%), K2CO3 (3 equiv),
dioxane, 70 °C, 2 h.
Scheme 2 Synthesis of 4a–c. Reagents and conditions: 1 (1.0 equiv),
2c,e,f (3.6 equiv), CuI (5 mol%) Pd(PPh3)2Cl2 (10 mol%), DIPA,
60 °C, 2 h
Table 4 Synthesis of 7a–d
Table 2 Synthesis of 4a–c
3, 7
6
R
R1
Yield (%)a
of 7
2
4
R
Yield (%)a of 4
c
e
f
a
b
c
3-(MeO)C6H4
3-MeC6H4
n-Pr
77
73
64
a
b
c
a
a
a
b
4-t-BuC6H4
Ph
4-(MeO)C6H4
4-(MeO)C6H4
4-(MeO)C6H4
76
69
79
74
3-(MeO)C6H4
a Yields of isolated products.
d
6-methoxynaphth-2-yl 4-MeC6H4
a Yields of isolated products.
The Sonogashira reaction of 1 with 4.8 equivalents of
acetylenes 2a,c,e afforded the 2,3,4,5-tetra(arylethy-
nyl)furans 5a–c in 75–83% yield (Scheme 3, Table 3).
During the optimization, the temperature had to be in-
creased from 60 °C to 75 °C. The catalyst loading did not
The UV–Vis and fluorescence spectroscopic data of vari-
ous furan derivatives, measured in chloroform at 25 °C,
are summarized in Table 5. A typical spectrum is depicted
in Figure 1. The absorption wavelengths (λabs) are in the
Synlett 2012, 23, 1463–1466
© Georg Thieme Verlag Stuttgart · New York

LETTER
Sonogashira Reactions of 2,3,4,5-Tetrabromofuran
1465
UV region (303–374 nm) and their emission wavelengths We have reported the synthesis of di-, tri- and tet-
(λem) (fluorescence) are in the UV or blue region (418– raalkynylated furans by the first Sonogashira reaction of
442 nm). Di- and trialkynylfurans 3a–d and 4a–c are not 2,3,4,5-tetrabromofuran. The synthesis of 2,3,4,5-tet-
fluorescent. In contrast, tetraalkynylfurans 5a–c possess raalkynylfurans has been reported for the first time.
excellent fluorescence properties and exhibit absorptions 2,3,4,5-Tetraalkynylfurans and 2,5-dialkynyl-3,4-diaryl-
at λabs(max) = 309, 303, 304 nm and emissions at λem(max)
=
furans show a bright fluorescence. The properties of the
425, 422, 420 nm with Stokes shift values 116, 119, 116 tetraalkynylated furans are particularly promising because
nm, respectively. 2,5-Dialkynyl-3,4-diarylfurans 7a–d of the excellent Stokes shifts. Our current studies are di-
also exhibit good fluorescence properties. While the emis- rected towards exploring the scope of the methodology,
sion wavelengths of 7a–d are in the same range as the studying the quantum yields and further optimizing the
emissions of 5a–c, the Stokes shifts of 7a–d are consider- fluorescence properties by variation of the substitution
ably lower as compared to 5a–c. Since higher Stokes pattern.
shifts generally correspond to better fluorescence proper-
ties, tetraalkynylfurans 5a–c are the best derivatives with
respect to possible technical applications. The absorptions
Acknowledgment
Financial support by the State of Pakistan (scholarships for I.M.,
Z.A. and M.N.), by the DAAD (scholarships for I.M. and M.N.) and
by the Interdisciplinary Faculty of the University of Rostock
(scholarship for S.R.) is gratefully acknowledged.
of the alkynylated products show a significant bathochro-
mic shift as compared to parent furan. Tetra(al-
kynyl)thiophenes23a and 2,5-bis(alkynyl)thiophenes23b
have been previously prepared. However, their photo-
physical properties were, to the best of our knowledge, not
studied. Therefore, a comparison is not possible. While Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
2,5-bis(alkynyl)-3,4-dicyanothiophenes have been report-
ed to form liquid crystals,23b 2,5-dialkynylfurans 3 and 7
did not show liquid crystalline properties.
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Figure 1 Absorption and emission spectra of compound 5c
Table 5 Absorption and Emission Spectroscopic Data of Alkynylat-
ed Furans (c = 10–6 mol/L, in CHCl3)
Product
λ1abs
[nm]
lg ε
λ4em
[nm]
Stokes shift
[nm]
5a
5b
5c
7a
7b
7c
7d
309
303
304
356
355
356
374
3.83
4.07
4.16
4.06
4.25
3.70
3.47
425
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116
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62
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Synlett 2012, 23, 1463–1466

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LETTER
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A suspension of tetrabromofuran (1), Pd(PPh3)2Cl2 (10
mol%), CuI (5 mol%) in diisopropylamine was degassed
three times in a pressure tube. The acetylene (1.2 equiv per
bromine atom) was added using a syringe. The mixture was
heated at the indicated temperature (60–80 °C) for 2–4 h.
The reaction mixture was filtered and the residue was
washed with CH2Cl2. The filtrate was washed with a
saturated solution of ammonium chloride (2 x 25 mL), water
(2 x 25 mL) and was subsequently dried over anhydrous
Na2SO4. The solvent was removed in vacuo. The product
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3,4-Dibromo-2,5-bis[(4-tert-butylphenyl)ethynyl]furan
(3a)
Starting with 1 (150 mg; 0.40 mmol), 4-tert-butylphenyl-
acetylene (2a) (0.16 mL, 0.94 mmol), CuI (5 mol%),
Pd(PPh3)2Cl2 (10 mol%), and diisopropylamine (5 mL), 3a
was isolated as a white solid (163 mg, 78%); mp 197–199
°C. 1H NMR (300 MHz, CDCl3): δ = 1.36 (s, 18 H, CH3),
7.40 (d, 4 H, J = 8.6 Hz), 7.51 (d, 4 H, J = 8.6 Hz). 13C NMR
(75.4 MHz, CDCl3): δ = 31.1 (CH3), 34.9, 81.5, 98.8, 109.3,
118.8 (C), 125.5, 132.3 (CH), 136.7, 152.6 (C). IR (KBr): ν
= 2952 (w), 1497 (m), 1461 (m), 1362 (m), 1266 (m), 1102
(m), 1013 (m), 923 (w), 833 (s) cm–1. GC-MS (EI, 70 eV):
m/z (%) = 536 (M+, [79Br, 79Br], 30), 538 (M+, [79Br, 81Br],
100), 540 (M+, [81Br, 81Br], 62), 523 (52), 508 (2), 493 (4),
467 (3), 350 (3), 314 (18), 299 (26), 254 (15), 226 (9).
HRMS (EI, 70 eV): calcd for C28H26Br2O (M+, [79Br, 79Br]:
536.03449; found 536.03353; calcd for C28H26Br2O (M+,
[79Br, 81Br]: 538.03245; found 538.03238; calcd for
C28H26Br2O (M+, [81Br, 81Br]: 540.03040; found 540.03176.
Synlett 2012, 23, 1463–1466
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