Site-selective Suzuki-Miyaura cross-coupling reactions of 2,3,4,5-tetrabromofuran

Article abstract of DOI:10.1039/c0ob00695e

Suzuki-Miyaura reactions of 2,3,4,5-tetrabromofuran allow a convenient and site-selective synthesis of mono-, di- and tetraarylfurans which are not readily available by other methods.

Full text of DOI:10.1039/c0ob00695e

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Site-selective Suzuki–Miyaura cross-coupling reactions of
2,3,4,5-tetrabromofuran†
Munawar Hussain,a Rasheed Ahmad Khera,a Nguyen Thai Hunga and Peter Langer*a,b
Received 8th September 2010, Accepted 22nd October 2010
DOI: 10.1039/c0ob00695e
Suzuki–Miyaura reactions of 2,3,4,5-tetrabromofuran allow
a convenient and site-selective synthesis of mono-, di- and
tetraarylfurans which are not readily available by other
methods.
coworkers reported a versatile approach to highly functionalized
furans by domino Sonogashira coupling/cyclocondensation re-
actions and related transformations.11 The tendency of furans to
readily undergo lithiation and electrophilic reactions at positions
C-2 or C-5 and to undergo acid-mediated decomposition makes
the selective synthesis of highly substituted furans a rather
demanding task. Although many strategies are known, they are
often not applicable to the selective synthesis of highly substituted
furans.
Substituted furans correspond to one of the most important
classes of five-membered heterocycles and have great significance
in medicinal, agricultural and material chemistry. In addition, they
are widely found in natural products which include, for example,
the cembranolides lophotoxin, kallolides, pukalide, the cytotoxic
glanvillic acids A and B, the Plakorsins A–C, rosefuran and several
other prominent molecules.1–3 Substituted furans constitute an
omnipresent molecular entity of several classes of biologically
active compounds and are present in commercially important
pharmaceuticals, flavor and fragrance additives (insect and fish
antifeedants).1c,4 In addition, furans are key synthetic intermedi-
ates which have been widely used in the synthesis of various cyclic
and acyclic target molecules.4 The development of new synthetic
approaches to polysubstituted furans thus represents an important
area of research in organic chemistry.5–8
Two main strategies for the synthesis of substituted furans
can be distinguished: a) the functionalization of existing furan
derivatives and b) the assembly of the furan system by means of
cyclization reactions of acyclic precursors.9 Classic examples of the
first strategy mainly rely on electrophilic substitution reactions. A
variety of classic furan syntheses are based on cyclocondensation
reactions (e.g., the Feist–Benary reaction or other methods). An
attractive approach to mono- and disubstituted furans relies
on transition metal-catalyzed cycloisomerization reactions of
unsaturated acyclic precursors, such as allenyl ketones, alkynyl
ketones, or epoxides.5 In this context, the work of Marshall and
coworkers and of Hashmi and coworkers have demonstrated great
utility. Recently, tetra-substituted furans have been prepared from
alkynes by a tandem process of palladium-catalyzed oxidation and
Lewis acid-catalyzed cyclization.4,10 Nakano and co-workers have
reported the synthesis of tetra-substituted furans by palladium-
catalyzed reactions of 3-furancarboxylic acids.10a Mu¨ller and
The low stability of furans, in particular under aerobic and
acidic conditions, makes their cross-coupling reactions more frag-
ile and the product isolation more difficult than in the thiophene
series. Palladium(0)-catalysed cross-coupling reactions of 2,3-
dibromofuran and substituted derivatives have been previously
studied. In this context, Negishi, Stille, Suzuki and Sonogashira
coupling reactions and nucleophilic aromatic substitution reac-
tions have been reported.12 These reactions proceed with excellent
site-selectivity in favour of carbon atoms C-2 and C-5. Bellina, Su-
likowski and Rossi and their coworkers reported site-selective tran-
sition metal-catalyzed reactions of several dibromofuranones.13
2,3,4,5-Tetrabromofuran represents an interesting substrate for
transition metal-catalyzed cross-coupling reactions as all four
carbon atoms are halogenated. To the best of our knowl-
edge, palladium(0)-catalysed cross-coupling reactions of 2,3,4,5-
tetrabromofuran have not been reported to date. Recently, we have
reported Suzuki-Miyaura reactions of tetrabromothiophene and
-selenophene.14 Herein, we report our preliminary results related
to Suzuki–Miyaura reactions of 2,3,4,5-tetrabromofuran. These
reactions provide a convenient and site-selective approach to a
variety of aryl-substituted furans which are not readily available
by other methods. Due to the unstable nature of furans compared
to thiophenes, the reactions of 2,3,4,5-tetrabromofuran reported
herein are much more prone to side-reactions and need much
more optimization than the corresponding reactions of 2,3,4,5-
tetrabromothiophene.
2,3,4,5-Tetrabromofuran (1) was prepared following a literature
procedure.15 It must be prepared in highly pure, crystalline
form and should be stored at -20 ◦C since slightly impure or
oily material is considerably less stable. The reaction of 1 with
arylboronic acids 2a–j (4.4 equiv.) afforded the stable 2,3,4,5-
tetraarylfurans 3a–j (Scheme 1, Table 1). The products were
isolated in good to excellent yields for both electron-rich and
electron-poor arylboronic acids. The reaction conditions were
systematically optimized for derivatives 3c, 3f, and 3i which are
aInstitut fu¨r Chemie, Universita¨t Rostock, Albert-Einstein-Str. 3a, 18059
Rostock, Germany. E-mail: peter.langer@uni-rostock.de; Fax: +49 381
4986412
bLeibniz-Institut fu¨r Katalyse e. V. an der Universita¨t Rostock, Albert-
Einstein-Str. 29a, 18059 Rostock, Germany
† Electronic supplementary information (ESI) available: Supplementary
information. See DOI: 10.1039/c0ob00695e
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Table 1 Synthesis of tetraarylfurans 3a–j
Table 3 Synthesis of 2,5-diaryl-4,5-dibromofurans 4a–e
2,3
Ar
% (3)a
2
4
Ar
% (4)a
a
b
c
d
e
f
C6H5
4-MeC6H4
4-EtC6H4
4-tBuC6H4
3-ClC6H4
92
90
92
92
80
85
89
82
98
76
e
a
b
c
d
e
f
3-ClC6H4
87
91
85
89
88
0b
g
h
k
l
4-(CF3)C6H4
3-(CF3)C6H4
4-(MeO)C6H4
4-ClC6H4
4-FC6H4
m
2-Naphthyl
g
h
i
4-(CF3)C6H4
3-(CF3)C6H4
4-(MeO)C6H4
3,5-Me2C6H3
a Yields of isolated products. b Decomposition.
j
a Yields of isolated products.
Scheme 2 Synthesis of 4a–e. Conditions: i, 2e, g, h, k, l (2.0 equiv.),
Pd(PPh3)4 (2 mol%), aq. K2CO3 (2 M), toluene/dioxane (4 : 1), 80 ◦C, 3 h.
reaction times and catalyst systems. Arylboronic acids 2e, g, h,
k, m were selected for the optimization studies based on their
electron-withdrawing and -donating nature and steric effects.
During the optimization, we have found that the temperature did
not have an important influence on the yield of 4 and on the
regioselectivity provided that exactly 2.0 equiv. of the boronic acids
were used. Using dioxane as the solvent, Pd(PPh3)4, Pd(PPh3)2Cl2
and Pd(OAc)2, in the presence of Cy3P or XPhos, were studied as
the catalysts in the reactions of 1 with boronic acids 2e, g, h, k.
All these reactions resulted in the formation of complex mixtures
of mono-, di-, tri- and tetraarylfurans. In case of 2m, a reduced
product formed by loss of a bromine atom was formed. The use
of different bases (2 M aqueous solution of K2CO3 or the use
of K3PO4 or Cs2CO3 in organic solvents) and a decrease of the
reaction temperature did not allow to solve the problems related
to the site-selectivity. The reaction suffered from low conversions
when the solvents toluene and DME were used. The employment
of THF as the solvent, using Pd(PPh3)4 and 2 M K2CO3, resulted
in the formation of complex mixtures for different reaction times
(3–8 h) and temperatures (60–80 ◦C).
Scheme 1 Synthesis of 3a–j. Conditions: i, 2a–j (4.4 equiv.), Pd(PPh3)4
(3 mol-%), aq. K2CO3 (2 M), dioxane, 80 ◦C, 5 h.
derived from arylboronic acids containing electron-donating and
electron-withdrawing substituents (Table 2).
The best yields were obtained when Pd(PPh3)4 or Pd(PPh3)2Cl2
(3 mol-%) were used as the catalyst (dioxane, 80 ◦C, 5 h) (entries
1–4, Table 2). Excellent yields were obtained when an aqueous
solution of K2CO3 (2 M) or when K3PO4 were employed as the
base. The yields dropped when Pd(OAc)2 (3 mol-%) in the presence
of XPhos or (Cy)3P were employed (entries 5–7, Table 2). In
conclusion, the application of the reaction conditions given in
entry 3 of Table 2 allowed to prepare the products in excellent
yields. It was also noted that electron-poor arylboronic acids
provided slightly lower yields than electron-rich arylboronic acids.
This can be explained by the lower nucleophilicity of electron-poor
boronic acids.
The Suzuki–Miyaura reaction of 1 with arylboronic acids 2e,
g, h, k–m (2.0 equiv.), in the presence of Pd(PPh3)4, gave the
2,5-diaryl-3,4-dibromofurans 4a–e (Scheme 2, Table 3). During
the synthesis of inhibitors of B-Raf kinase, Andrew and co-
workers studied site-selective Suzuki–Miyaura reactions of 2,3-
dibromofuran. These reactions, which were carried out in a
DME/H2O/K2CO3 system, proceeded in rather low yields.10a
The application of these conditions to Suzuki reactions of 1
proved to be unsuccessful. Therefore we decided to optimize the
reaction conditions methodically for different solvent systems,
While the use of a single solvent was unsuccessful for the
regioselective synthesis of 2,5-diaryl-3,4-dibromofurans 4a–e, the
use of solvent mixtures allowed to address the problem. We
selected dioxane/toluene as a solvent system to control the
solubility of the boronic acids. Pd(PPh3)4 and 2 M K2CO3 were
again used as the catalyst and base, respectively. While the use
of a 4 : 1 dioxane/toluene mixture again provided mixtures of
products, the use of a 3 : 2 and 1 : 1 dioxane/toluene mixture
Table 2 Optimization of the synthesis of tetraarylfurans
Entry
Conditions
% (3c)a
% (3f)a
% (3i)a
1
2
3
4
5
6
7
Pd(PPh3)2Cl2 (3 mol-%), aq. K2CO3 (2 M)
Pd(PPh3)2Cl2 (3 mol-%), K3PO4
Pd(PPh3)4 (3 mol-%), aq. K2CO3 (2 M)
Pd(PPh3)4 (3 mol-%), K3PO4
Pd(OAc)2 (3 mol-%), XPhos (6 mol-%), aq. K2CO3 (2 M)
Pd(OAc)2 (3 mol-%), (Cy)3P (6 mol-%), aq. K2CO3 (2 M)
Pd(OAc)2 (3 mol-%), (Cy)3P (6 mol-%), K3PO4
90
85
92
88
10
65
50
82
75
85
78
5
45
43
96
88
98
92
15
69
55
a Yields of isolated products; all reactions were carried out in dioxane (80 ◦C, 5 h).
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Table 4 Synthesis of 2-aryl-3,4,5-tribromofurans 5a–e
Table 5 Synthesis of unsymmetrical tetraaryfurans 6a–e
2
5
Ar
% (5)a
6
Ar1
Ar2
% (6)a
e
g
h
i
a
b
c
d
e
3-ClC6H4
87
91
85
89
88
a
b
c
d
e
3-ClC6H4
C6H5
C6H5
4-MeC6H4
4-tBuC6H4
3,5-Me2C6H3
88
4-(CF3)C6H4
3-(CF3)C6H4
4-(MeO)C6H4
4-ClC6H4
3-(CF3)C6H4
3-(CF3)C6H4
3-(CF3)C6H4
3-(CF3)C6H4
86b
93
93b
88b
l
a Yields of isolated products.
a Yields of isolated products. b Prepared by a one-pot procedure from 1.
showed better results for boronic acids 2g, h, k. In fact, the
desired products 4 were formed as the major products among a
complex mixture of other products. Gratifyingly, the employment
of a 1 : 4 dioxane/toluene mixture afforded exclusively 2,5-biaryl-
3,4-dibromofurans 4a–d which could be isolated in excellent
yields (85–91%). It is worth to note that the use of a 1 : 4 diox-
ane/toluene mixture of solvents allowed excellent site-selectivities
to be obtained even when the reactions were carried out at reflux
(110 ◦C).
The Suzuki–Miyaura reaction of 1 with arylboronic acids 2e,
g–i, l (1.0 equiv.) afforded the 2-aryl-3,4,5-tribromofurans 5a–e
(Scheme 3, Table 4). The stoichiometry (employment of exactly
1.0 equiv. of the arylboronic acid) played an important role. The
best yields (85–91%) were obtained when the conditions developed
for the synthesis of products 4a–e were employed (vide supra), i.e.
the use of Pd(PPh3)4 (2 mol%) as the catalyst, the use of an aqueous
solution of K2CO3 (2 M) as the base, and the use of a 4 : 1 mixture
of toluene and dioxane.
Scheme 5 Synthesis of 6b, d–e. Conditions: i, boronic acid 2h (2.0 equiv.),
Pd(PPh3)4 (3 mol%), aq. K2CO3 (2 M), toluene/dioxane (4 : 1), 80 ◦C, 3 h,
ii, boronic acid 2a, d, j (2.2 equiv.), 80 ◦C, 3 h.
The unsymmetrical tetraarylfuran 7 was prepared by Suzuki–
Miyaura reaction of 2-aryl-3,4,5-tribromofuran 5c with 4-tert-
butylarylboronic acid (2d) in dioxane at 80 ◦C for 5 h
(Scheme 6).
Scheme 3 Synthesis of 5a–e. Conditions: i, 2e, g–i, l (1.0 equiv.), Pd(PPh3)4
(2 mol%), aq. K2CO3 (2 M), toluene/dioxane (4 : 1), 80 ◦C, 3 h.
Scheme 6 Synthesis of 7. Conditions: i, 2c (3.3 equiv.), Pd(PPh3)4
(2 mol%), aq. K2CO3 (2 M), dioxane 80 ◦C, 5 h.
The unsymmetrical tetraarylfurans 6a, c were synthesized by
Suzuki reactions of 2,5-diaryl-3,4-dibromofurans 4a, c with 2a,
b (Scheme 4, Table 5). During the synthesis of 6a, c, no issue
of site-selectivity had to be addressed. Hence, the reactions
could be successfully carried out in dioxane and the use of a
toluene/dioxane mixture was not necessary.
The structures of all products of this report were established
by 2D NMR techniques (NOESY, HMBC) or by X-ray crystal
structure analyses.
In conclusion, we have studied the synthesis of mono-, di- and
tetraarylfurans by the first Suzuki–Miyaura reactions of 2,3,4,5-
tetrabromofuran. The use of a binary solvent system toluene-
dioxane played an important role during the optimization of
the site-selectivity of the reactions. The products reported are
not readily accessible by other methods. All reactions proceed
with excellent site-selectivity in favour of position 2 and 5 which
are more electron-deficient than positions 3 and 4 and, thus,
more rapidly undergo an oxidative addition with the palladium(0)
catalyst.
Scheme 4 Synthesis of 6a, c. Conditions: i, 2a, b (2.0 equiv.), Pd(PPh3)4
(2 mol%), aq. K2CO3 (2 M), dioxane (4 : 1), 80 ◦C, 3 h.
A one-pot strategy to accomplish the synthesis of the unsym-
metrical tetraarylfurans 6b, d, e was also studied. The Suzuki–
Miyaura reaction of 1 with arylboronic acid 2h (2.0 equiv.) in
toluene/dioxane (4 : 1), separation of the solution in three equal
portions and subsequent addition of arylboronic acids 2a, 2d or
2j afforded products 6b, 6d and 6e in high yields, respectively
(Scheme 5, Table 5).
Acknowledgements
Financial support by the State of Vietnam (MOET scholarship for
N. T. H.), by the DAAD (scholarship for R. A. K.), by the State
of Mecklenburg–Vorpommern (scholarship for M. H.) and by the
Higher Education Commission of Pakistan (scholarship for M.
H.) is gratefully acknowledged.
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R. Whitehead, I. Ghiviriga, R. M. Walczak and D. L. Wright, J. Org.
Chem., 2004, 69, 3726; (l) M. Aso, A. Ojida, G. Yang, O.-J. Cha, E.
Osawa and K. Kanematsu, J. Org. Chem., 1993, 58, 3960; (m) W. Xin,
L. L. Yan, C. WeiXing and L. Jing, Sci. China, Ser. B: Chem., 2009, 52,
1220.
6 D. M. X. Donnelly, M. J. Meegan in Comprehensive Heterocyclic
Chemistry, C. W. Bird and G. W. H. Cheeseman, ed., Vol. 4, Part 3,
pp. 657–712, Pergamon Press, Oxford, 1984.
7 (a) W. Kreiser, Nachr. Chem. Tech. Lab, 1981, 29, 118; (b) B. H.
Lipshutz, Chem. Rev., 1986, 86, 795.
8 N. H. C. Wong, Pure Appl. Chem., 1996, 68, 335.
9 (a) For the synthesis of furans by cyclization reactions developed from
our laboratory, see for a review: E. Bellur, H. Feist and P. Langer,
Tetrahedron, 2007, 63, 10865; (b) See also: G. Mroß, E. Holtz and P.
Langer, J. Org. Chem., 2006, 71, 8045; (c) E. Bellur and P. Langer,
Synthesis, 2006, 480; (d) E. Bellur and P. Langer, J. Org. Chem., 2005,
70, 10013; (e) E. Bellur, H. Go¨rls and P. Langer, Eur. J. Org. Chem.,
2005, 2074.
References
1 (a) For reviews of furan syntheses, see: X. L. Hou, Z. Yang, H. N. C.
Wong In Progress in Heterocyclic Chemistry, Vol. 15, G. W. Gribble
and T. L. Gilchrist, ed., Pergamon, Oxford, 2003, 167–205; (b) A. T.
Merritt and S. V. Ley, Nat. Prod. Rep., 1992, 9, 243; (c) A. Padwa,
M. Ishida, C. L. Muller, S. S. Murphree J. Org. Chem. 1992, 57,
1170. and references therein; (d) M. V. Sargent and F. M. Dean In
Comprehensive Heterocyclic Chemistry, Vol. 3, C. W. Bird and G. W. H.
Cheeseman, ed., Pergamon Press, Oxford UK, 1984, 599–656; (e) F. M.
Dean In Advances in Heterocyclic Chemistry, Vol. 31, A. R. Katritzky,
ed., Academic Press, New York, 1983, 237–344; (f) Natural Products
Chemistry, Vol. 1–3, K. Nakanishi, T. Goto, S. Ito, S. Natori and
S. Nozoe, ed., Kodansha, Ltd., Tokyo, 1974; (g) W. Friedrichsen in
Comprehensive Heterocyclic Chemistry, A. R. Katritzky, C. W. Rees and
E. F. V. Scriven, ed., vol. 2, Elsevier, 1996, p. 359–363, and references
cited therein; (h) B. Ko¨nig in Science of Synthesis, Vol. 9, Thieme,
Stuttgart, 2001, p. 183–285.
2 (a) L. A. Paquette and P. C. Astles, J. Org. Chem., 1993, 58, 165;
(b) B. H. Lipshutz, Chem. Rev., 1986, 86, 795; (c) Plakorsin A–C: S.
Al-Busafi and R. C. Whitehead, Tetrahedron Lett., 2000, 41, 3467;
(d) Y.-C. Shen, C. V. S. Prakash and Y.-H. Kuo, J. Nat. Prod., 2001,
64, 324; (e) Glanvillic acid A and B: D. E. Williams, T. M. Allen, R.
V. Soest, H. W. Behrisch and R. J. Andersen, J. Nat. Prod., 2001, 64,
281.
3 W. Fenical, R. K. Okeeda, M. M. Basnadurraga, P. Culver and R. S.
Jacobs, Science, 1981, 212, 1512.
4 (a) A. Wang, H. Jiang and Q. Xu, Synlett., 2009, 6, 932; (b) S. A.
Look, M. T. Burch, W. Fenical, Z. Qi-tai and J. Clardy, J. Org. Chem.,
1985, 50, 5741; (c) M. G. Missakian, B. J. Burreson and P. J. Scheuer,
Tetrahedron, 1975, 31, 2513.
5 (a) J. A. Marshall and X.-J. Wang, J. Org. Chem., 1991, 56, 960; (b) B.
M. Trost and M. C. McIntosh, J. Am. Chem. Soc., 1995, 117, 7255;
(c) P. Wipf, L. T. Rahman and S. R. Rector, J. Org. Chem., 1998, 63,
960; (d) J. A. Marshall and E. D. Robinson, J. Org. Chem., 1990, 55,
3450; (e) J. A. Marshall and X. Wang, J. Org. Chem., 1992, 57, 3387;
(f) J. A. Marshall and W. J. DuBay, J. Org. Chem., 1993, 58, 3602; (g) J.
A. Marshall and G. S. Bartley, J. Org. Chem., 1994, 59, 7169; (h) J.
A. Marshall and C. A. Sehon, J. Org. Chem., 1995, 60, 5966; (i) A. S.
K. Hashmi, T. L. Ruppero, T. Kno¨fel and J. W. Bats, J. Org. Chem.,
1997, 62, 7295; (j) B. Gabriele, G. Salerno, F. De Pascali, M. Costa
and G. P. Chiusoli, J. Org. Chem., 1999, 64, 7693; (k) J. B. Sperry, C.
10 (a) A. K. Takle, M. J. Bamford, S. Davies, R. P. Davis, D. K. Dean,
A. Gaiba, E. A. Irving, F. D. King, A. Naylor, C. A. Parr, A. M. Ray,
A. D. Reith, B. B. Smith, P. C. Staton, J. G. A. Steadman, T. O. Stean
and D. M. Wilson, Bioorg. Med. Chem. Lett., 2008, 18, 4373; (b) S. F.
Kirsch, Org. Biomol. Chem., 2006, 4, 2076; (c) H. Kawai, S. Oi and Y.
Inoue, Heterocycles, 2006, 67, 101; (d) R. C. D. Brown, Angew. Chem.,
Int. Ed., 2005, 44, 850; (e) A. Jeevanandam, A. Ghule and Y.-C. Ling,
Curr. Org. Chem., 2002, 6, 841.
11 (a) A. S. Karpov, E. Merkul, T. Oeser and T. J. J. Mu¨ller, Eur. J. Org.
Chem., 2006, 2991; (b) A. S. Karpov, E. Merkul, T. Oeser and T. J.
J. Mu¨ller, Chem. Commun., 2005, 2581; (c) R. U. Braun and T. J. J.
Mu¨ller, Synthesis, 2004, 14, 2391.
12 (a) S. Schro¨ter, C. Stock and T. Bach, Tetrahedron, 2005, 61, 2245; (b) T.
Bach and L. Kru¨ger, Eur. J. Org. Chem., 1999, 2045; (c) C. Stock, F.
Ho¨fer and T. Bach, Synlett, 2005, 511.
13 (a) G. A. Sulikowski, F. Agnelli and R. M. Corbett, J. Org. Chem.,
2000, 65, 337; (b) F. Bellina, C. Anselmi, S. Viel, L. Mannina and R.
Rossi, Tetrahedron, 2001, 57, 9997; (c) F. Bellina, C. Anselmi and R.
Rossi, Tetrahedron Lett., 2001, 42, 3851; (d) R. Rossi, F. Bellina and E.
Raugei, Synlett, 2000, 1749.
14 (a) T. T. Dang, T. T. Dang, N. Rasool, A. Villinger and P. Langer,
Adv. Synth. Catal., 2009, 351, 1595; (b) T. T. Dang, A. Villinger and P.
Langer, Adv. Synth. Catal., 2008, 350, 2109.
15 C. W. Shopee, J. Chem. Soc., Perkin Trans. 1, 1985, 45.
This journal is
The Royal Society of Chemistry 2011
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