A simple and efficient one-pot synthesis of substituted benzo[b]furans by sonogashira coupling-5-endo-dig cyclization catalyzed by palladium nanoparticles in water under ligand- and copper-free aerobic conditions

Article abstract of DOI:10.1002/ejoc.201000980

A simple and efficient procedure for the synthesis of 2-substituted benzo[b]furan derivatives has been developed by the reaction of 2-iodophenols and arylacetylenes in water catalyzed by palladium nanoparticles generated in situ in open air. The reaction does not require any additive, ligand, or co-catalyst. The reaction occurs through Sonogashira coupling of 2-iodophenols with phenylacetylenes followed by 5-endo-dig cyclization. A series of functionalized benzo[b]furan derivatives was obtained in high yields by this procedure. A simple and efficient procedure for the synthesis of 2-substituted benzo[b]furan derivatives has been developed by the reaction of 2-iodophenols and arylacetylenes in water catalyzed by palladium nanoparticles generated in situ in open air.

Full text of DOI:10.1002/ejoc.201000980

FULL PAPER
DOI: 10.1002/ejoc.201000980
A Simple and Efficient One-Pot Synthesis of Substituted Benzo[b]furans by
Sonogashira Coupling–5-endo-dig Cyclization Catalyzed by Palladium
Nanoparticles in Water Under Ligand- and Copper-Free Aerobic Conditions
Debasree Saha,[a] Raju Dey,[a] and Brindaban C. Ranu*[a]
Keywords: Cyclization / Cross-coupling / Palladium / Nanoparticles / Water chemistry
A simple and efficient procedure for the synthesis of 2-substi-
tuted benzo[b]furan derivatives has been developed by the
reaction of 2-iodophenols and arylacetylenes in water cata-
lyzed by palladium nanoparticles generated in situ in open
air. The reaction does not require any additive, ligand, or co-
catalyst. The reaction occurs through Sonogashira coupling
of 2-iodophenols with phenylacetylenes followed by 5-endo-
dig cyclization. A series of functionalized benzo[b]furan de-
rivatives was obtained in high yields by this procedure.
area,[7] we report here a one-pot Pd nanoparticle catalyzed
reaction of 2-iodophenols and arylacetylenes in water in the
absence of any ligand or copper co-catalyst leading to ben-
zofurans (Scheme 1).
Introduction
Benzo[b]furan derivatives have received considerable at-
tention, as several molecules containing the benzofuran
moiety show remarkable activities against asthma, ulcers,
tumors, cardiovascular disease, type 2 diabetes, migraines,
etc.[1] Moreover, a wide range of natural products contain
benzofuran as their core unit.[2] Thus, this molecule has be-
come a very popular synthetic target and several methods
have been developed.[3] Most of these are based on transi-
tion-metal-catalyzed Sonogashira coupling of o-halophen-
ols and phenylacetylenes followed by cyclization. A variety
of reagents/catalysts such as N-heterocyclic carbene–palla-
dium complex/Cs2CO3 in DMSO,[3a] Pd(PPh3)2Cl2/CuI in
DMF,[3b] Pd(PPh3)2Cl2/CuI in THF,[3c] Pd(PPh3)2Cl2/CuI
Scheme 1. Synthesis of benzo[b]furans.
Results and Discussion
The experimental procedure is very simple. A mixture of
in diisopropylamine,[3d] Pd(PPh3)2Cl2/CuI in Et3N,[3e] Na2PdCl4, sodium dodecyl sulfate, 2-iodophenol, and aryl-
Pd(PPh3)2Cl2/CuI/Et2NH in DMF,[3f] 10% Pd-C/PPh3/CuI/ acetylene in water was heated to reflux in the presence of
(S)-prolinol in water,[3g] and Pd0 nanoparticles in CH3CN triethylamine for a certain period of time as required to
under sonication.[3h] Although these methods are quite ef- complete the reaction (TLC). Standard workup and purifi-
ficient, all of them except one[3g] involve hazardous organic cation by filtration through a short column of silica gel pro-
solvents and many of them require a co-catalyst and li- vided the product. Among a variety of bases investigated,
gands.
triethylamine was found to provide the best yield (Table 1,
The use of water as a reaction medium for organic reac- Entry 4).
tions has received wide acceptability[4] because of environ-
To determine the active catalytic species in this reaction,
mental concerns, its low cost, and its unique reactivity and an extract from the reaction mixture of 2-iodophenol and
selectivity that cannot be attained by organic solvents.[5] phenylacetylene after 5 h from the start of the reaction
Nevertheless, a one-pot multistep reaction in water is highly showed the formation of nanoparticles, 5–6 nm in size by
desirable in the context of green chemistry. On the other TEM (transmission electron microscopy; Figure 1). The
hand, the use of metal nanoparticles as efficient catalysts in identity of these particles as palladium was confirmed by
organic reactions has attracted considerable interest in re- EDX (energy dispersive X-ray) analysis (Figure 2), which is
cent times because of their benign character and ease of an important tool used to characterize metals and other
preparation.[6] As part of our continuing activities in this elements. The amount of Pd0 nanoparticles was optimized
to 4 mol-% for an efficient reaction. Nevertheless, Pd0
[a] Department of Organic Chemistry, Indian Association for the
nanoparticles are generated in situ by the reduction of
Cultivation of Science,
Na2PdCl4 by sodium dodecyl sulfate (SDS) in water.[7c]
SDS also acts as a stabilizer here.
Jadavpur, Kolkata 700032, India
E-mail: ocbcr@iacs.res.in
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D. Saha, R. Dey, B. C. Ranu
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Table 1. Effect of base on the synthesis of benzofurans.
aromatic ring of 2-iodophenol remained inert and did not
take part in the coupling process (Table 2, Entry 11). The
electron-withdrawing CO2Et group and electron-donating
CH3 group did not affect the reactivity of the catalyst in the
reaction. A smooth reaction with a variety of substituted
phenylacetylenes provided easy access to a library of func-
tionalized benzofurans. Several functionalities such as Cl,
Br, F, and OMe on the aromatic ring of the arylacetylenes
are compatible with this process.
Table 2. Synthesis of benzo[b]furans.
Figure 1. TEM image of Pd nanoparticles.
[a] Yield refers to that of the purified isolated product characterized
1
by spectroscopy (IR, H NMR, and 13C NMR).
In general, the reactions are very clean and high yielding.
The pure compounds are obtained by simple filtration of
the crude product through a short column of silica gel. For
comparison, this Pd0 nanoparticle catalyzed reaction in
water under aerial atmosphere is more efficient than the one
using Pd0 nanoparticles in CH3CN under sonication and
argon,[3h] as indicated by the reactions of 4-methyl-2-iodo-
phenol and 2-iodophenol with phenylacetylene (Table 2,
Entries 1 and 11), where our method provides 82 and 80%
yield, whereas the other one furnished 42 and 40% yield of
the product together with 18 and 13% yield of the 1,3-di-
yne. We have not isolated any 1,3-diyne in any of these reac-
Figure 2. EDX spectra of Pd nanoparticles on Cu grid.
A series of substituted 2-iodophenols underwent reac- tions. This clearly demonstrates the unique characteristics
tions with a variety of diversely functionalized arylacetyl- of the combination of Pd nanoparticles and water over or-
enes by this procedure to produce the corresponding substi- ganic solvents. Although earlier, a similar reaction using
tuted benzofurans. The results are summarized in Table 2. 10% Pd-C in water was reported,[3g] it uses triphenylphos-
Obviously, the reaction proceeds through Sonogashira cou- phane as ligand, CuI co-catalyst, and expensive (S)-
pling followed by subsequent 5-endo-dig cyclization. The re- prolinol, whereas our method involves only Pd nanopar-
action is highly selective with respect to halo couplings; ticles and does not require any ligand, co-catalyst, or addi-
only iodobenzene reacts. Thus, the Br group present in the tive like (S)-prolinol.
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One-Pot Synthesis of Substituted Benzo[b]furans
We believe that the reaction proceeds through a standard Table 4. Sonogashira coupling between aryl iodides and acetylene.
pathway[3h] of oxidative addition of 2-iodophenol to Pd0
followed by transmetalation, reductive elimination, and cy-
clization to give the product with regeneration of Pd0 for
the next cycle, as outlined in Scheme 2.
Scheme 2. Plausible mechanism.
We also studied the normal Sonogashira coupling[13] of
aryl iodides with aryl/alkylacetylenes by using this catalyst
system. It was found that sodium hydroxide was equally
effective as triethylamine for this reaction (Table 3, En-
try 2). Thus, sodium hydroxide, being less expensive and
easily available, was chosen as the base for these reactions.
The reactions are also accomplished at room temperature.
[a] Yield refers to that of the purified isolated product characterized
A wide range of substituted aryl iodides and phenylacetyl-
1
by spectroscopy (IR, H NMR, and 13C NMR).
enes participated in this reaction to provide the correspond-
ing substituted acetylenes. The results are reported in
Table 4. A variety of substituents such as F, Br, OCF3,
NH2, NO2, and COCH3 on the phenyl ring of the aryl
iodides are compatible with this procedure, and both elec-
tron-donating and electron-withdrawing groups are uni-
formly reactive. A heteroaryl iodide (Table 4, Entry 9) also
produced the corresponding product in high yield. Ali-
phatic acetylenes (Table 4, Entries 12–14) underwent reac-
tions by this procedure without any difficulty.
as demonstrated by a representative reaction of 2-iodo-
phenol and phenylacetylene in Figure 3. The loss of activity
of the Pd nanoparticles is due to the agglomerization of the
nanoparticles after each cycle. The catalytic efficiency of the
nanoparticles is very much dependent on the particle size.
Usually, an increase in the size of the nanoparticles results
in a decrease in the activity.[20] In this reaction, the Pd
nanoparticles after the third recycle were found to be
largely agglomerized as shown by the TEM image (Fig-
ure 4), and thus, their activity was greatly reduced.
Table 3. Optimization of base in the Sonogashira reaction.
The aqueous suspension containing the catalyst after the
Sonogashira reaction as well as cyclization to benzofuran
Figure 3. Recyclability diagram for the reaction of 2-iodophenol
was recycled for three runs with gradual loss of efficiency and phenylacetylene.
Eur. J. Org. Chem. 2010, 6067–6071
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D. Saha, R. Dey, B. C. Ranu
FULL PAPER
7.6 Hz, 2 H, 4-H), 7.95 (d, JH,H = 8.17 Hz, 1 H, 5-H), 8.20 (s, 1 H,
7-H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 14.4, 61.0,
101.3, 112.7, 120.3, 124.4, 125.3 (2 C), 126.4, 128.9 (2 C), 129.3,
129.8, 133.5, 154.3, 158.8, 166.8 ppm. HRMS: calcd. for C17H14O3
[M + Na]+ 289.0841; found 289.0845. See Table 2, Entry 8.
Representative Experimental Procedure for the Sonogashira Cou-
pling Reaction of Iodobenzene with Phenylacetylene to Produce Di-
phenylacetylene: To a mixture of Na2PdCl4 (12 mg, 0.0408 mmol)
and sodium dodecyl sulfate (144 mg, 0.5 mmol) in water (3 mL)
was added iodobenzene (204 mg, 1 mmol), phenylacetylene
(123 mg, 1.2 mmol), and NaOH (120 mg, 3 mmol). The mixture
was stirred at room temperature for 9 h (TLC). The reaction mix-
ture was extracted with ethyl acetate (3ϫ10 mL). The extract was
washed with water and brine and dried with Na2SO4. Evaporation
of the solvent left the crude product, which was purified by column
chromatography over silica gel (hexane) to afford pure diphenylace-
tylene (153 mg, 86%) as a white solid (m.p. 62 °C). The spectro-
scopic data (1H and 13C NMR) are in good agreement with those
reported for the authentic sample.[13a]
Figure 4. TEM image of agglomerized Pd nanoparticles after third
recycle.
Conclusions
We have developed a one-pot, very simple and efficient
procedure for the synthesis of functionalized benzo[b]furans
through a Sonogashira coupling–5-endo-dig cyclization cat-
alyzed by in situ generated palladium nanoparticles in water
and open air without any ligand or co-catalyst. A wide
range of substituted aryl iodides and phenylacetylenes
underwent this reaction to provide easy access to a library
of benzofurans. The simple, eco-friendly reaction condi-
tions, high yields of products, reusability of the catalyst, no
requirement for a ligand, co-catalyst, or special device, and
the superior efficiency of the reaction compared to similar
reported procedures[3g,3h] make this procedure more attract-
ive. Most significantly, better performance of Pd nanopar-
ticles in water than CH3CN[3h] is noteworthy.
Many of these products are known compounds and were identified
by comparison of their spectra with those reported earlier (see ref-
erences in Table 4). The new compounds were characterized by IR,
1H NMR, and 13C NMR spectroscopy and HRMS, which are pro-
vided below in order of their entries in Table 4.
1-Phenylethynyl-4-trifluoromethoxybenzene: White solid, m.p. 68–
70 °C. IR (KBr): ν = 3431, 1587, 1529, 1498, 1444, 1277, 1205,
˜
1165 cm–1. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 7.21 (d, JH,H
= 8.27 Hz, 2 H, 3-H), 7.36–7.38 (m, 3 H, 3Ј-H, 4Ј-H), 7.53–7.58
(m, 4 H, 2-H, 2Ј-H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
88.0, 90.3, 118.8, 120.8 (2 C), 122.2, 123.0, 128.5 (2 C), 128.7, 131.8
(2 C), 133.3 (2 C), 149.0 ppm. C15H9F3O (262.06): calcd. C 68.70,
H 3.46; found C 68.68, H 3.49. See Table 4, Entry 4).
1-Methoxy-4-(3-phenylprop-2ynyloxy)benzene: Brownish yellow so-
lid, m.p. 83 °C. IR (KBr): ν = 2958, 2926, 2910, 2855, 1512, 1441,
˜
1379, 1288, 1230, 1219 cm–1. H NMR (300 MHz, CDCl3, 25 °C):
1
Experimental Section
δ = 3.78 (s, 3 H, OCH3), 4.87 (s, 2 H, CH2), 6.87 (d, JH,H = 9 Hz,
2 H, 2-H), 6.99 (d, JH,H = 9.3 Hz, 2 H, 3-H), 7.30–7.32 (m, 3 H,
3Ј-H, 4Ј-H), 7.43–7.45 (m, 2 H, 2Ј-H) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ = 55.6, 57.4, 84.2, 86.9, 114.5 (2 C), 116.2 (2 C),
122.3, 128.2 (2 C), 128.6, 131.7 (2 C), 151.9, 154.3 ppm. HRMS:
calcd. for C16H14O2 [M+ H]+ 239.1094; found 239.1067. See
Table 4, Entry 14.
Representative Procedure for the Synthesis of Benzo[b]furan: To a
mixture of Na2PdCl4 (12 mg, 0.0408 mmol) and sodium dodecyl
sulfate (144 mg, 0.5 mmol) in water (3 mL) was added 2-iodo-
phenol (220 mg, 1 mmol), phenylacetylene (123 mg, 1.2 mmol), and
Et3N (303 mg, 3 mmol). The mixture was heated to reflux (oil bath)
for 14 h (TLC). After cooling, the reaction mixture was extracted
with ethyl acetate (3ϫ10 mL). The extract was washed with water
and brine and dried with Na2SO4. Evaporation of the solvent left
the crude product, which was purified by column chromatography
over silica gel (hexane/ether, 92:8) to afford pure 2-phenylbenzo[b]
furan (159 mg, 82%) as a white solid (m.p. 120 °C). The spectro-
scopic data (1H and 13C NMR) are in good agreement with those
reported for the authentic sample.[3h] The aqueous part containing
the catalyst was reused in three subsequent runs without appreci-
able loss of efficiency (Figure 3). This procedure was followed for
the synthesis of all the products listed in Table 2. Although the
general experimental procedure was based on a 1-mmol-scale reac-
tion, multimolar reactions produced similar results. The known
compounds were identified by comparison of their spectra with
those reported earlier (see references in Table 2).
Acknowledgments
We are pleased to acknowledge financial support from the Depart-
ment of Science & Technology, New Delhi through the award of a
J.C. Bose National Fellowship to B.C.R. (Grant no. SR/S2/JCB-11/
2008). D.S. and R.D. thank the Council of Scientific & Industrial
Research, New Delhi for their fellowships.
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Received: July 12, 2010
Published Online: September 13, 2010
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6071