Palladium-catalyzed cross-couplings of functionalized 2-bromobenzofurans for atom-economic synthesis of 2-arylbenzofurans using triarylbismuth reagents

Article abstract of DOI:10.1016/j.tetlet.2012.03.059

The palladium catalyzed, atom-economic synthesis of various functionalized 2-arylbenzofurans was achieved through cross-coupling reaction of 2-bromobenzofurans with triarylbismuth reagents. The palladium catalytic protocol is very efficient to furnish various cross-coupled functionalized 2-arylbenzofurnas in high yields using triarylbismuth reagents with three aryl couplings as multi-coupling organometallic nucleophiles in one-pot operation. All the coupling reactions were completed in 1 h short reaction time involving three couplings from triarylbismuths under heating condition.

Full text of DOI:10.1016/j.tetlet.2012.03.059

Tetrahedron Letters 53 (2012) 2662–2666
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Palladium-catalyzed cross-couplings of functionalized 2-bromobenzofurans
for atom-economic synthesis of 2-arylbenzofurans using triarylbismuth
reagents
⇑
Maddali L. N. Rao , Dheeraj K. Awasthi, Jalindar B. Talode
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The palladium catalyzed, atom-economic synthesis of various functionalized 2-arylbenzofurans was
achieved through cross-coupling reaction of 2-bromobenzofurans with triarylbismuth reagents. The
palladium catalytic protocol is very efficient to furnish various cross-coupled functionalized 2-arylbenzo-
furnas in high yields using triarylbismuth reagents with three aryl couplings as multi-coupling organome-
tallic nucleophiles in one-pot operation. All the coupling reactions were completed in 1 h short reaction
time involving three couplings from triarylbismuths under heating condition.
Received 10 February 2012
Revised 16 March 2012
Accepted 20 March 2012
Available online 24 March 2012
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
2-Bromobenzofuran
Triarylbismuths
Palladium catalysis
Cross-couplings
Atom-economic
Benzofurans are important class of biologically active oxygen
heterocycles and are present in many natural products.1,2 Some
of these compounds have been known to exhibit anti-inflamma-
tory,3 antitumor,4 anticancer,4,5 lipoxygenase inhibitors6, and
anti-fungal7 properties. 2-Aryl substituted benzofuran skeletons
are immensely present in the natural products such as cicefuran,7
ailanthoidol,8 eupomatinoids family,9 obovaten10 etc. with several
medicinal applications.
Transition metal mediated cross-coupling methods provide effi-
cient routes for the synthesis of functionalized benzofurans11
which compliment the traditional synthetic methods.12 For exam-
ple, Widdowson and co-workers have synthesized Moracin M
through cross coupling of 2-stannylbenzofuran with a 5-iodoresor-
cinolbis(triisopropylsilyl) ether.13 Similarly, Lee and co-workers
have reported the synthesis of Ailanthoidol by Stille cross-coupling
of 2-bromobenzofuran with aryltin reagent under palladium cata-
lyzed conditions.14 This coupling methodology generally provides
the desired ease and flexibility in terms of choosing the appropriate
organometallic reagent15 as nucleophilic partner in combination
with a suitable organic electrophile. Thus, various coupling reac-
tions of heteroaryl organometallic nucleophile couplings such as
2-benzo-furanylboronic acids,15a,b 2-benzofuranyltrifluorobo-
rates,15c or 2-benzofuranyldimethylsilanolate15d etc. with aryl
halides have been reported under palladium catalysis.
However, some of these heteroaryl organometallic nucleophiles
suffer from the problems of stability for efficient couplings.
However, the alternate approach involving 2-halobenzofuran
couplings16 with aryl organometallic nucleophiles is advantageous
as the corresponding 2-halobenzofurans and aryl organometallic
reagents are easily available with long shelf lives. For example,
very recently Lautens and co-workers reported the preparation of
2-bromobenzofurnas using intramolecular cyclizations of ortho-
hydoxy-gem-dibromovinyl benzenes under metal catalyzed condi-
tions.17 Another method of similar cyclization utilizing TBAF was
also been reported.18 With this background, it was of interest to ex-
plore the potential reactivity of triarylbismuth reagents for the
synthesis of 2-arylbenzofurans under palladium catalyzed condi-
tions. This is in light of the recent surge in the utility of triarylbis-
muths as non-toxic, atom-economic, and multi-coupling reagents
involving three aryl couplings from each triarylbismuth unit under
metal catalyzed conditions.19 Triarylbismuths are stable com-
pounds and can be easily prepared by the known procedures.20
Herein, we report the atom-economic synthesis of 2-aryl-
benzofurans under palladium catalyzed couplings of 2-bromoben-
zoufrans using triarylbismuths as multi-coupling organometallic
nucleophiles.
The coupling reaction of 2-bromobenzofuran was studied with
triphenylbismuth as organometallic nucleophile under different
conditions to establish a viable palladium protocol for this reaction.
The screening was conducted with 3.3 equiv of 2-bromobenzofuran
with 1 equiv of triphenylbismuth as our aim was to cross-couple
three phenyl groups from bismuth reagent to obtain 3 equiv of
⇑
Corresponding author. Tel./fax: +91 512 2597532.
E-mail address: maddali@iitk.ac.in (Maddali L. N. Rao).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.059

M. L . N. Rao et al. / Tetrahedron Letters 53 (2012) 2662–2666
2663
Table 1
2-phenylbenzofuran stoichiometrically. Hence, the investigation
was carried out with different catalysts, bases, solvents, and temper-
ature conditions to check their efficacy to deliver the optimum pro-
tocol with high cross-coupled product yield of 2-phenylbenzofuran
(Table 1).
Screening conditionsa,b,c,d,e,f
[Pd]
Br
BiPh3
Ph
+
base, solvent
temp, 1 h
O
O
We have initially employed the routinely used catalyst precur-
sor and phosphine ligand such as Pd(OAc)2 and triphenylphosphine
for this study with different bases in N-methylpyrrolidone (NMP)
solvent under heating condition (Table 1, entries 1–5). This attempt
furnished varied amounts of cross-coupled product yields. For
example, the coupling reaction with K2CO3, KOAc, and Cs2CO3
yielded 76%, 71%, and 96% respectively. The reactions with Na2CO3
and K3PO4 gave 53–54% yields. Formation of biphenyl as a minor
side product from triphenylbsimuth was observed and this is a
known reactivity of triarylbismuth to give homo-coupling product
under metal catalysis.21 From this study, Cs2CO3 base clearly
emerged as a better choice with excellent yield. This prompted us
to check the efficacy of different palladium catalysts (Table 1, en-
tries 6–9). In this investigation, although PdCl2(PPh3)2 and
Pd(PPh3)4 fared reasonably well with 70% and 72% yields, other cat-
alysts were found to be ineffective. Thus, further investigation in
different solvents was carried out with Cs2CO3 base and Pd(OAc)2/
PPh3 system (Table 1, entries 10–13). However, this study did not
provide any better outcome as the solvents such as N,N-dimethy-
lacetamdie (DMA), N,N-dimethylformamide (DMF), THF, and 1,4-
dioxane delivered yields up to 22–53%. Furthermore, use of differ-
ent amounts of Cs2CO3 base in four and 2 equiv along with no base
condition as control experiments provided 85%, 73%, and 19%
yields, respectively (Table 1, entries 14–16). This also established
that 3 equiv of Cs2CO3 is sufficient to obtain high cross-coupling
yield (Table 1, entry 5). Additional check at varied temperature
conditions between 30,40 and 60 °C found to be not suitable for
effective coupling (Table 1, entries 17–19). Further study without
catalyst and ligand did not furnish the desired product (Table 1, en-
(1 equiv)
(3.3 equiv)
(3 equiv)
S. no.
Catalyst/ligand
Base
Solvent
Temp (oC)
Yield (%)
1
2
3
4
5
6
7
8
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
PdCl2(PPh3)2
Pd(PPh3)4
PdCl2(MeCN)2
PdCl2(PhCN)2
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
Pd(OAc)2/4 PPh3
None
K2CO3
KOAc
Na2CO3
K3PO4
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
DMA
DMF
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
60
40
30
90
76
71
53
54
96
70
72
0
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
No Base
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
9
0
10
11
12
13
14
15
16
17
18
19
20
53
40
25
22
85c
73d
19
49
5e
THF
1,4-Dioxane
NMP
NMP
NMP
NMP
NMP
NMP
NMP
1e
0
a
Conditions: BiPh3 (1 equiv), 2-bromobenzofuran (3.3 equiv), base (3 equiv), Pd
catalyst (0.1 equiv), PPh3 (0.4 equiv), solvent (3 mL), 90 °C, 1 h.
b
Isolated yields.
Reaction with 4 equiv of Cs2CO3.
Reaction with 2 equiv of Cs2CO3.
GC conversion.
c
d
e
f
Homo-coupled biphenyl from BiPh3 was formed as minor side product. The
amount varied with respect to the degree of the cross-coupling product.
Table 2
a,b,c,d
Cross-couplings of 2-bromobenzofuran with different BiAr3
Pd(OAc)2 (0.1 equiv)
Br
PPh3 (0.4 equiv)
Ar
BiAr3
+
O
O
Cs2CO3 (3 equiv)
NMP, 90 oC, 1 h
(1 equiv)
(3.3 equiv)
(3 equiv)
Entry
1
BiAr3
2-Arylbenzofuran
Yield (%)
96
Bi
2.1
2.2
2.3
O
O
3
Bi
Bi
Me
3
Me
Cl
F
2
3
94
92
Cl
3
O
Bi
Bi
F
3
4
5
6
2.4
2.5
2.6
90
88
O
O
O
OMe
OMe
OEt
3
Bi
OEt
92
3
(continued on next page)

2664
M. L . N. Rao et al. / Tetrahedron Letters 53 (2012) 2662–2666
Table 2 (continued)
Entry
BiAr3
2-Arylbenzofuran
Yield (%)
86
Cl
Cl
7
2.7
2.8
Bi
O
3
OMe
OMe
8
80
Bi
O
3
a
Reaction conditions: 2-bromobenzofuran (0.825 mmol, 3.3 equiv), BiAr3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv), PPh3 (0.1 mmol,
0.4 equiv), Cs2CO3 (0.75 mmol, 3 equiv) and NMP (3 mL), 90 °C, 1 h.
b
Isolated yields calculated considering all three aryl groups of BiAr3 for coupling. Thus, 0.75 mmol of the product correspond to 100% yield.
c
Homo-coupled bi-aryls from triarylbismuths formed in all the reactions in minor amounts.
d
All products were characterized by 1H NMR, 13C NMR, IR, and ESI-HRMS data and in comparison with the literature data.
Table 3
a,b,c,d
Cross-couplings of substituted 2-bromobenzofurans with different BiAr3
R
R
Pd(OAc)2 (0.1 equiv)
PPh3 (0.4 equiv)
R = H, Ac, Br, Cl;
R'= H, Cl, OMe
Ar
Br
BiAr3
+
O
O
(1 equiv)
Cs2CO3 (3 equiv)
NMP, 90 oC, 1 h
R'
(3 equiv)
R'
(3.3 equiv)
Entry
1
BiAr3
2-Bromobenzofuran
2-Arylbenzofuran
Yield (%)
91
Ac
Ac
Bi
3.1
3.2
Br
3
O
O
Ac
Bi
Me
3
Me
2
’’
88
O
Ac
Bi
Bi
Bi
Bi
Bi
Cl
3
Cl
3
4
5
’’
’’
’’
3.3
3.4
3.5
83
89
88
O
Ac
F
3
F
O
Ac
OMe
3
OMe
O
Ac
OEt
3
OEt
6
7
’’
3.6
3.7
93
87
O
Cl
Cl
Br
3
O
O
Cl
Cl
Cl
Bi
Bi
Me
3
Me
Cl
F
8
’’
’’
’’
3.8
86
81
83
O
Cl
Cl
Cl
3
9
3.9
O
Cl
Cl
Bi
F
3
10
3.10
O
Cl

M. L . N. Rao et al. / Tetrahedron Letters 53 (2012) 2662–2666
2665
Table 3 (continued)
Entry
BiAr3
2-Bromobenzofuran
2-Arylbenzofuran
Yield (%)
Cl
Bi
OEt
OEt
Me
11
12
’’
3.11
3.12
88
90
O
3
Cl
Br
Br
Bi
Br
O
O
3
Br
Bi
Bi
Me
13
14
15
16
’’
’’
’’
’’
3.13
3.14
3.15
3.16
87
85
88
91
O
3
Br
Cl
Cl
O
3
Br
Bi
Bi
F
F
O
3
Br
OMe
OMe
O
3
Br
Bi
Bi
OEt
3
OEt
17
18
’’
3.17
3.18
93
86
O
3
Br
O
O
OMe
OMe
Me
Cl
F
Bi
Bi
Me
19
20
21
22
’’
’’
’’
’’
’’
3.19
3.20
3.21
3.22
3.23
83
80
84
81
86
O
3
OMe
Cl
O
3
OMe
Bi
F
O
3
OMe
OMe
Bi
Bi
OMe
OEt
O
3
OMe
OEt
23
O
3
OMe
a
Reaction conditions: 2-bromobenzofuran (0.825 mmol, 3.3 equiv), BiAr3 (0.25 mmol, 1 equiv), Pd(OAc)2 (0.025 mmol, 0.1 equiv), PPh3 (0.1 mmol, 0.4 equiv), Cs2CO3
(0.75 mmol, 3 equiv) and NMP (3 mL), 90 °C, 1 h.
b
Isolated yields calculated considering all three aryl groups of BiAr3 for coupling. Thus, 0.75 mmol of the product correspond to 100% yield.
c
Homo-coupled bi-aryls from triarylbismuths formed in all the reactions in minor amounts.
d
All products were characterized by 1H NMR, 13C NMR, IR, and ESI-HRMS data and in comparison with the literature data.
try 20). From this systematic investigation, the coupling conditions
involving BiPh3 (1 equiv), 2-bromobenzofuran (3.3 equiv),
Pd(OAc)2 (0.1 equiv), PPh3 (0.4 equiv), and Cs2CO3 (3 equiv) in
NMP at 90 °C for 1 h evolved as an optimized protocol to obtain
high cross-coupled yield (Table 1, entry 5).
the facile reactivity of both 2-bromobenzofuran and triphenylbis-
muth to yield high cross-coupling product under the established
protocol conditions. The observed coupling reactivity is superior in
comparison to the literature methods reported for such couplings
with other organometallic reagents.14,16
The coupling reactivity of 2-bromobenzofuran witnessed with
BiPh3 as a model reaction is heartening from the view point that
the overall coupling conditions involve only 1 h reaction time
comprising three aryl couplings from bismuth reagent. This denoted
Encouraged by this, we studied the coupling reactivity using
triarylbismuth reagents and 2-bromobenzofuran under the estab-
lished protocol conditions to understand the general coupling abil-
ity of electronically different triarylbismuth reagents. As given in

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Table 2, the reactivity of different triarylbismuths demonstrated
high coupling ability with overall 80–96%, cross-coupled yields.22
Both electronically rich and deficient triarylbismuth reagents
demonstrated high reactivity with three aryl transfers for cross-
coupling purpose in a short reaction time and is an important point
to note in these couplings.
Furthermore, the generality of these couplings with different
2-bromobenzofurans and triarylbismuths was established under
optimized conditions22 and the results are given in Table 3.
The coupling reactions of variously substituted 2-bromobenzofu-
rans with electronically different groups such as 5-acetyl, 5,7-di-
chloro, 5-bromo, and 7-methxoy reacted well to furnish the
corresponding functionalized 2-arylbenzofuans in combination
with different triarylbismuths in excellent yields. The coupling
reactions with 2,5-dibromobenzofuran reacted regioselectively
at 2-position. The reactivity of 2-bromo-5,7-dichlorobenzofuran
was also found to be similar and coupling underwent at 2-posi-
tion furnishing the corresponding 2-arylbenzofuran in high
yields.
The cross-coupling reactivity demonstrated in this study is
highly productive and led to the formation of 2-arylbenzofurans
in excellent yields in 1 h short reaction time. This is in sharp con-
trast to the reactivity earlier reported using aryl organometallic re-
agents. For example, the coupling reaction involving one C–C bond
formation using 2-bromobenzouran and aryltin reagents required
heating at 145 °C for 5 h.14 Similar couplings using aryl boronic
acids16b,c reacted under reflux conditions and long hours some
times. The coupling reaction of 2-bromobenzofuran was reported
with n-Bu3Bi at 110 °C which needed 18 h for one alkyl coupling
from trialkylbismuth reagent.23 The present aryl coupling method
with triarylbismuths is advantageous from various counts such as
three aryl couplings in atom-economic manner in one-pot opera-
tion, faster reactivity in shorter reaction times, high coupling
yields, and utilization of non-toxic bismuth reagents as organome-
tallic nucleophiles under palladium catalyzed conditions.
In summary, we have demonstrated atom-economic synthesis
of 2-arylbenzofurans from the cross-coupling reaction of 2-bro-
mobenzfurans with triarylbismuth reagents under palladium
catalytic protocol conditions. This methodology with several advan-
tages is expected to provide a facile platform in synthetic organic
chemistry for the synthesis of a plethora of 2-arylbenzofurans in
high yields.
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3955; (d) Rao, M. L. N.; Jadhav, D. N.; Venkatesh, V. Eur. J. Org. Chem. 2009,
4300–4306. and references cited therein.
We greatfully acknowledge the financial support received from
the Department of Science and Technology (DST), New Delhi for
this work through Green Chemistry program (DST No. SR/S5/GC-
11/2008). D.K.A. and J.B.T. thank the University Grants Commission
(UGC) New Delhi, respectively for research fellowships.
20. Organobismuth Chemistry; Suzuki, H., Matano, Y., Eds.; Elsevier: Amsterdam,
2001.
21. Barton, D. H. R.; Ozbalik, N.; Ramesh, M. Tetrahedron 1988, 44, 5661–5668.
22. Representative cross-coupling procedure: In a hot oven-dried Schlenk tube
under N2 atmosphere were added Ph3Bi (0.25 mmol, 110 mg, 1.0 equiv), 2-
bromobenzofuran (0.825 mmol, 163 mg, 3.3 equiv), Cs2CO3 (0.75 mmol,
244 mg, 3.0 equiv), Pd(OAc)2 (0.025 mmol, 5.6 mg, 0.1 equiv), PPh3
(0.1 mmol, 26 mg, 0.4 equiv), and NMP (3 mL) solvent. The resulting mixture
was stirred in preheated oil bath at 90 °C for 1 h. After the reaction is over, the
mixture was cooled, quenched with dil HCl and extracted with ethyl acetate.
The combined organic extract was washed with water, brine, and dried over
MgSO4 and concentrated. The crude was subjected to silica gel column
chromatography (230–400 mesh) using petroleum ether as the eluent to
obtain the pure 2-phenylbenzofuran (2.1) as a white solid (140 mg, 96%). The
product was characterized by spectroscopy and in comparison with the
literature data.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.03.
059.
References and notes
1. Donnelly, D. M. X.; Meegan, M. J. I. Comprehensive Heterocyclic Chemistry In
Katritzky, A. R., Ed.; Pergamon: New York, NY, 1984. Vol. 4.
23. Gagnon, A.; Albert, V.; Duplessis, M. Synlett 2010, 2936–2940.