Polyethylene glycol (PEG-400) as an efficient and recyclable reaction medium for the synthesis of 2-aroylbenzofurans

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Polyethylene Glycol (PEG-400) as an
Efficient and Recyclable Reaction
Medium for the Synthesis of 2-
Aroylbenzofurans
Sanhu Zhao a b , Xiaoju Wang a & Liwei Zhang a
a Institute of Molecular Science , Shanxi University , Taiyuan ,
030006 , Shanxi , P. R. China
b Department of Chemistry , Xinzhou Teachers University , Xinzhou ,
034000 , Shanxi , P. R. China
Published online: 16 Aug 2013.
To cite this article: Sanhu Zhao , Xiaoju Wang & Liwei Zhang (2013) Polyethylene Glycol (PEG-400)
as an Efficient and Recyclable Reaction Medium for the Synthesis of 2-Aroylbenzofurans, Organic
Preparations and Procedures International: The New Journal for Organic Synthesis, 45:5, 421-428,
DOI: 10.1080/00304948.2013.816214
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Organic Preparations and Procedures International, 45:421–428, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2013.816214
Polyethylene Glycol (PEG-400) as an Efficient
and Recyclable Reaction Medium for the Synthesis
of 2-Aroylbenzofurans
Sanhu Zhao,1,2 Xiaoju Wang,1 and Liwei Zhang1
1Institute of Molecular Science, Shanxi University, Taiyuan 030006, Shanxi,
P. R. China
2Department of Chemistry, Xinzhou Teachers University, Xinzhou 034000,
Shanxi, P. R. China
Benzofuran derivatives are highly valuable molecular motifs due to their excellent prop-
erties, such as pharmaceutical,1 antifungal,2,3 antitumor4 and other bioorganic properties.5
Recent research efforts have focused on the synthesis of these molecules and a variety of
methodologies have been developed. To our knowledge, typical approaches of synthesis of
benzofuran derivatives mainly include: (1) intramolecular palladium-catalyzed O-arylation
of enols,6 (2) annulation of a furan ring onto a pre-existing benzene ring,7 and (3) catalyzed
cyclization-coupling.8,9 The first two approaches generally involve multi-step reactions
while the third one often requires the use of expensive transition-metals; moreover, they
always produce limited types of benzofuran derivatives. Compared with the above routes,
the Rap-Stoermer reaction of o-hydroxybenzaldehyde appears to be a versatile straight-
forward approach for the synthesis of various benzofuran derivatives.10–13 Traditionally, it
is carried out under basic conditions at reflux in organic solvents. Recently, new catalysts
such as 4-(dimethylamino)-pyridine14 and KF/AI2O3 and methods such as micro-wave
15
irradiation,16 ultrasonic irradiation,17 solid state synthesis,18 have been used in this reaction;
some even proceed in good to excellent yields. However, there is still a need to develop
efficient, convenient and safe general method for the synthesis of benzofurans derivatives
that avoids the use of harmful solvents.
Over the past twenty years, polyethylene glycol (PEG-400)19 and its analogues have
become more popular as alternate reaction media, due to their interesting properties such
as hydrophilicity and thermal stability; PEG-400 is inexpensive, easy to handle, non-toxic,
bio-compatible and bio-degradable.20 Several organic transformations such as substitution
reactions,21 oxidations and reductions,22 the Heck,23 Suzuki cross-coupling24 and Wacker
reactions25 have been reported using PEG-400 as a recyclable medium. In the context of our
studies aimed at the development of environmentally sustainable processes.26–29 Herein we
Received March 21, 2013; in final form April 15, 2013.
Address correspondence to L. Zhang, Shanxi University, Institute of Molecular Science,
Taiyuan 030006, P. R. China. E-mail: lwzhang@sxu.edu.cn
421

422
Zhao, Wang, and Zhang
report PEG-40 as an efficient and recyclable solvent medium for the rapid Rap-Stoermer
reaction between salicylaldehydes with various α-haloketones (Scheme 1).
Scheme 1
Initially, the Rap-Stoermer reaction of 2-hydroxybenzaldehyde with 2-bromo-1-(4-
chlorophenyl)ethanone was used as model reaction to assess the viability of the method.
When the reaction was conducted in PEG-400 in the presence of K2CO3 at room temper-
ature, only 36% yield of the product was obtained (Table 1, Entry 1), and it was noted
that PEG-400 remains in a sticky state at room temperature, and thus unable to dissolve
the reactants. The reaction was then carried out at 100◦C and gave he highest yield (92%)
in a short reaction time (Table 1, Entry 3). However, when the reaction temperature was
Table 1
Optimization of Conditionsa
Entry
Solvent
Base
Time (min)
Yield (%)b
1c
2d
3
4
5
6
7
8
9
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
acetone
acetonitrile
toluene
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
60
60
25
25
25
60
60
60
60
60
36
78
92
32
38
8
47
21
71
56
DMF
THF
10
aReaction conditions: 2-hydroxybenzaldehyde (0.26 ml, 2.5 mmol), 2-bromo-1-(4-chlorophenyl)
ethanone (0.59 g, 2.5 mmol), base (7.5 mmol), solvent (5 ml), reaction temperature 100◦C, under
nitrogen; b Isolated yield; c reaction temperature is 25◦C; d reaction temperature is 70◦C.

Synthesis of 2-Aroylbenzofurans
423
beyond 120◦C, undesired side-products were formed and low yields were obtained. Under
the same reaction conditions, the use of Na2CO3 and Cs2CO3 gave poor yields(Table 1, En-
tries 4 and 5). As a comparison, the same reaction in the common solvents such as acetone,
acetonitrile, toluene, THF and DMF gave only 8–71% yields of the product after 60 min
(Table 1, Entries 6–10). These results clearly demonstrate that the Rap-Stoermer reaction of
2-bromo-1-(4-chlorophenyl)-ethanone with 2-hydroxybenzaldehydecan be greatly acceler-
ated in PEG-400 and represents an improvement on earlier methods with respect to reaction
time and increased yields. In order to evaluate the possibility of recycling the solvent, the
initial reaction of 2-hydroxybenzaldehyde with 2-bromo-1-(4-chlorophenyl)ethanone in
PEG-400 was reexamined. The reaction mixture was extracted with diethyl ether and the
insoluble PEG-400 layer was used again for three additional reactions. The yields of the
initial and the three subsequent runs were essentially the same (91–93%.).
On the basis of these results, the generality of the Rap-Stoermer reaction in PEG-400
was investigated for the synthesis of various benzofuran derivatives by using a variety of
α-bromoacetophenones with 2-hydroxybenzaldehyde (Table 2, Entries 1–8). In general,
all reactions proceeded very smoothly and cleanly to afford the benzofuran derivatives
in high yields (84–92%). As shown in Table 2, α-bromoacetophenones (3d, 3f and 3h),
bearing electron-withdrawing groups (Br, Cl and NO2) at the para position gave the de-
sired products in excellent yields (91, 92 and 84%) in short reaction time (20–30 min),
and also the α-bromoacetophenones (3c, 3a and 3g) bearing electron-donating groups
(Me, MeO and EtO) at the para position gave the desired products in high yields (84,
92 and 90%). Results show that the substitution groups did not play significant role in
governing the reactivity of the substrate. When substituted 2-hydroxybenzaldehydes such
as 2-hydroxy-4-methoxybenzaldehyde and 2,4-dihydroxybenzaldehyde were employed for
the Rap-Stoermer reaction (Table 2, Entries 9–12), the relatively lower yields (76–85%)
were obtained. To explore the scope of this method further, α-bromopropanone (Table 2, En-
tries 13 and 14) and some α−chloroacetophenones (Table 2, Entries 15–17) were also used
in this reaction. Under the optimized reaction conditions, α-chloroacetophenones and α-
bromopropanone react efficiently and result in corresponding benzofurans with good yields
(83–87%). Compared to α-bromoacetophenone, the reaction of α-chloroacetophenone with
2-hydroxybenzaldehydes gave a lower yield.
In conclusion, we have developed an efficient and facile method for the synthesis
of 2-aroylbenzofurans by treatment of α-haloacetophenones with salicylaldehydes using
PEG-400 as a recyclable medium without the addition of any additive or organic co-solvent.
The mild reaction conditions, less expensive of reaction medium, operational simplicity,
and high yields are the advantages of the protocol.
Experimental Section
1H NMR and 13C NMR spectra were acquired in CDCl3 on a DRX300 NMR spectrometer
and Bruker Avance 500-MHz spectrometer. 1H and 13C NMR chemical shifts are reported δ
relative to internal Me4Si or residual of CHCl3. Melting points were measured on WRS-1B
digital melting point meter and are uncorrected. Elemental analyses were performed by a
Vario EL instrument. All chemical reagents were obtained from commercial suppliers and

424
Zhao, Wang, and Zhang
Table 2
Preparation of 2-Aroylbenzofurans using PEG-400a
Time
Entry Cmpd (min)
Yield
(%)b
mp (◦C)
(lit. mp.)
1H NMR(δ)
13C NMR (δ)
1
3a
20
92
95.5–95.7
(95) 33
8.11 (d, J = 7.9 Hz, 2H), 182.8, 163.6, 155.8,
7.71 (d, J = 7.9 Hz,
1H), 7.62 (d, J =
8.4 Hz, 1H), 7.51 (s,
1H), 7.47 (t, J =
152.7, 131.9, 129.8,
127.9, 127.1, 123.8,
123.2, 115.5, 113.9,
112.5, 55.5
7.8 Hz, 1H), 7.31 (t,
J = 7.5 Hz, 1H), 7.01
(d, J = 8.5 Hz, 2H),
3.89 (s, 3H)
2
3b14,c
30
15
20
87
84
91
202.1–202.5 8.40 (d, J = 8.7 Hz, 2H), 182.3, 156.3, 151.7,
8.23 (d, J = 8.7 Hz,
2H), 7.77 (d, J =
150.2, 142.1, 130.5,
129.2, 126.8, 124.4,
123.7, 117.3, 112.6
7.9 Hz, 1H), 7.65 (d, J
= 8.5 Hz, 1H), 7.62
(s, 1H), 7.56 (t, J =
8.3 Hz, 1H), 7.38 (t,
J = 7.5 Hz, 1H)
3c
59.6–60.4
(60–62)38
7.97 (d, J = 8.1 Hz, 2H), 184.1, 155.8, 152.1,
7.72 (d, J = 7.9 Hz,
143.4, 134.6, 129.7,
129.3, 128.1, 126.8,
124.1, 123.1, 116.0,
112.6, 21.8
1H), 7.63 (d,
J = 8.4 Hz, 1H), 7.51
(s, 1H), 7.48 (t, J =
7.8 Hz, 1H), 7.32 (t, J
= 8.0 Hz, 3H), 2.45 (s,
3H)
4
3d30,c
112.3–112.7 7.93 (d, J = 8.5 Hz, 2H), 183.1, 156.0, 152.1,
7.72 (d, J = 7.9 Hz,
1H), 7.67 (d, J =
135.8, 131.8, 131.0,
128.6, 128.1, 126.9,
124.1, 123.4, 116.5,
112.6
8.5 Hz, 2H), 7.62 (d, J
= 8.4 Hz, 1H), 7.53 (s,
1H), 7.51 (d, J =
7.2 Hz, 1H), 7.33 (t,
J = 7.5 Hz, 1H)
5
6
3e
3f
25
20
89
92
88.5–88.9
(89) 32
8.03 (d, J = 7.8 Hz, 2H), 184.1, 156.1, 152.1,
7.71 (d, J = 7.9 Hz,
1H), 7.62 (t, J =
7.5 Hz, 2H), 7.51 (dd,
J = 15.8, 10.4 Hz,
4H), 7.32 (t, J =
7.5 Hz, 1H)
137.3, 132.7, 129.7,
128.4, 126.8, 124.0,
123.2, 116.7, 112.7
149.5–150.3 8.03 (d, J = 8.6 Hz, 2H), 155.9, 152.2, 139.3,
(152) 30
7.73 (d, J = 8.0 Hz,
1H), 7.63 (d, J =
135.4, 133.7, 130.9,
129.0, 128.4, 126.8,
124.1, 123.4, 116.5,
112.6
8.5 Hz, 1H), 7.55 (s,
1H), 7.52 (d, J =
6.9 Hz, 3H), 7.41 (t,
J = 7.7 Hz, 1H), 7.34
(t, J = 7.2 Hz, 1H)

Synthesis of 2-Aroylbenzofurans
425
Table 2
Preparation of 2-Aroylbenzofurans using PEG-400a (Continued)
Time
Yield
(%)b
mp (◦C) (lit.
Entry Cmpd (min)
mp.)
1H NMR(δ)
13C NMR (δ)
7
3g c
25
90
93.7–94.2
8.09 (d, J = 8.6 Hz, 2H), 182.8, 163.1, 155.8,
7.70 (d, J = 7.8 Hz,
1H), 7.61 (d, J =
131.9, 129.6, 127.9,
127.0, 123.8, 123.1,
115.4, 114.3, 112.4,
63.8, 14.7
8.3 Hz, 1H), 7.50 (s,
1H), 7.45 (d, J =
7.8 Hz, 1H), 7.30 (t,
J = 7.5 Hz, 1H), 6.98
(d, J = 8.6 Hz, 2H)
8
3h14,c
25
84
135.4–136.2 8.94 (s, 1H), 8.48 (d, J = 181.5, 156.4, 151.5,
8.2 Hz, 1H), 8.41 (d,
J = 7.7 Hz, 1H), 7.77
(d, J = 7.9 Hz, 2H),
7.65 (d, J = 5.1 Hz,
2H), 7.55 (t, J =
148.1, 138.5, 134.5,
129.9, 128.6, 127.2,
126.5, 124.3, 123.4,
122.6, 116.9, 112.6
7.8 Hz, 1H), 7.37 (t, J
= 7.5 Hz, 1H)
9
3i
3j
25
30
85
84
104.8–105.3 8.01 (d, J = 5.1 Hz, 2H), 183.9, 161.2, 157.6,
(105–106) 34
7.99–7.52 (m, 4H),
7.46 (s, 1H), 7.51 (s,
1H), 7.11 (d, J =
151.8, 137.5, 132.6,
129.2, 128.4, 123.6,
120.3, 117.3, 114.5,
95.6, 55.7.
1.8 Hz, 1H), 6.98–6.95
(m, 1H), 3.87 (s, 3H)
10
150.1–150.5 8.07 (d, J = 9.0 Hz, 2H), 182.4, 163.3, 160.9,
(149) 34
7.57 (d, J = 8.7 Hz,
1H), 7.45 (s, 1H), 7.10
(d, J = 1.5 Hz, 1H),
7.02–6.94 (m, 3H),
3.90 (s, 3H), 3.89 (s,
3H)
157.4, 152.2, 131.7,
130.1, 123.4, 120.4,
116.2, 114.3, 113.7,
95.6, 55.7, 55.5.
11
12
13
3k
3l
40
40
25
76
82
85
160.2–160.8 8.00–7.96 (m, 2H), 7.51 182.7, 164.9, 164.2,
(161–162) 35
(s, 1H), 7.67–7.62 (m,
2H), 7.59–7.43 (m,
4H), 7.07 (t, J =
4.8 Hz, 2H), 5.36 (s,
1H)
135.4, 134.1, 129.3,
129.0, 128.5, 128.0,
115.7, 114.7, 108.6,
101.6.
201.6–202.7 7.95 (d, 2H, J = 1.8 Hz), 184.5, 165.1, 164.3,
(201–203) 35
7.45 (t, 1H, J =
164.2, 135.4, 131.8,
130.4, 127.1, 115.7,
114.1, 113.8, 108.6,
101.6, 55.5.
4.2 Hz), 6.95–7.04 (m,
3H), 6.40 (d, 1H, J =
2.4 Hz), 5.30 (s, 1H),
3.89 (s, 3H),
3m
75.4–76.7 (76) 7.84–7.81 (m, 1H),
188.7, 156.1, 151.5,
128.9, 126.8, 124.4,
123.7, 113.9, 112.4,
26.5
36
7.79–7.71 (m, 1H),
7.62(s, 1H), 7.51–7.45
(m, 1H), 6.99 (t, J =
8.7 Hz, 1H), 2.67 (s,
3H).
(Continued on next page)

426
Zhao, Wang, and Zhang
Table 2
Preparation of 2-Aroylbenzofurans using PEG-400a (Continued)
Time
Entry Cmpd (min)
Yield
(%)b
mp (◦C) (lit.
mp.)
1H NMR(δ)
13C NMR (δ)
14
3n
25
87
98.1–99.5
(97–98) 37
7.79 (s, 1H), 7.63 (d,
J = 8.9 Hz, 1H), 7.56
(d, J = 8.9 Hz, 1H),
7.43 (d, J = 9.6 Hz,
1H), 4.07 (s, 3H), 2.57
(s, 3H).
186.9, 152.9, 146.2,
145.0, 129.9, 123.7,
115.5, 113.9, 109.3,
55.8, 22.2
15
3o
30
84
88.2–89.1
(89) 32
8.03 (d, J = 7.8 Hz, 2H), 184.4, 156.0, 152.3,
7.71 (d, J = 7.9 Hz,
1H), 7.62 (t, J =
7.5 Hz, 2H), 7.50 (dt,
J = 14.6, 8.0 Hz, 4H),
7.32 (t, J = 7.5 Hz,
1H)
137.3, 133.0, 129.5,
128.6, 128.4, 127.0,
124.0, 123.3, 116.5,
112.5.
16
3p
25
86
95.6–96.2
(95) 33
8.11 (d, J = 7.9 Hz, 2H), 183.2, 163.8, 155.8,
7.71 (d, J = 7.9 Hz,
1H), 7.62 (d, J =
8.4 Hz, 1H), 7.51 (s,
1H), 7.47 (t, J =
131.9, 129.8, 127.9,
127.1, 123.8, 123.2,
115.5, 113.8, 112.4,
77.3, 77.1, 76.8,
55.5.
7.8 Hz, 1H), 7.31 (t,
J = 7.5 Hz, 1H), 7.01
(d, J = 8.5 Hz, 2H),
3.89 (s, 3H).
17
3q14, c
30
83
202.5–202.9 8.40 (d, J = 8.7 Hz, 2H), 182.3, 156.2, 151.7,
8.23 (d, J = 8.7 Hz,
2H), 7.77 (d, J =
150.2, 142.1, 130.5,
129.2, 126.8, 124.4,
123.6, 117.3, 112.6.
7.9 Hz, 1H), 7.65 (d,
J = 8.5 Hz, 1H), 7.62
(s, 1H), 7.56 (t, J =
7.8 Hz, 1H), 7.38 (t,
J = 7.5 Hz, 1H)
aRection conditions: 2-hydroxybenzaldehyde (0.26 ml, 2.5 mmol), α-bromoacetophenone
(2.5 mmol), K2CO3 (1.04 g, 7.5 mmol), PEG-400 (5 ml), reaction temperature 100 oC, un-
b
c
der nitrogen; Isolated yield; Compounds 3b: Anal. Calcd for C15H9NO4: C 67.42; H 3.39;
N 5.24. Found: C 67.35; H 3.37; N 5.22. 3d: Anal. Calcd for C15H9BrO2: C 59.83; H 3.01.
Found: C 59.85; H 2.91. 3g: Anal. Calcd for C17H14O3: C 76.68; H 5.30. Found: C 76.55; H
5.40. 3h: Anal. Calcd for C15H9NO4: C 67.42; H 3.39; N 5.24. Found: C 67.39; H 3.40; N
5.15. 3q: Anal. Calcd for C15H9NO4: C 67.42; H 3.39; N 5.24. Found: C 67.38%; H 3.40%;
N 5.21%.
used without further purification, α-bromoacetophenones were prepared following reported
procedures.31
General Procedure for the Rap-Stoermer Reaction in PEG-400
To a stirred mixture of 2-hydroxybenzaldehyde (0.26 ml, 2.5 mmol) and α-bromoace
tophenone (2.5 mmol) in PEG-400 (5 ml), was added solid K2CO3 (1.04 g, 7.5 mmol).
The resulting reaction mixture was heated to 100◦C in an oil-bath under a nitrogen

Synthesis of 2-Aroylbenzofurans
427
atmosphere; the progress of reaction was monitored using thin layer chromatography
(TLC) until 2-hydroxybenzaldehyde was consumed. Then the reaction mixture was cooled
to r.t. and extracted with Et2O (3 × 20 ml), the organic phase was washed with brine
(2 × 20 ml), dried over anhydrous Na2SO4, and the solvent was evaporated in vacuo to yield
the nearly pure object product. Further purification may be achieved by recrystallization
from ethanol. All the products were identified by melting point and NMR spectroscopy. The
spectroscopic and physical properties of the products were compared with those available
in the literature,14,32,33 new compounds were also characterized by combustion analysis.
Acknowledgment
We thank the Executive Editor of OPPI, Professor Jean-Pierre Anselme, for his helpful
suggestions and careful English corrections of our work. We are also grateful for finan-
cial support for this work from the Key Project of Ministry of Science and Technology
(2011ZX09201-201-06), the Scientific and Technologial Innovation Programs of Higher
Education Institutions in Shanxi (No. 2012022) and the Natural Science Foundation of
Shanxi Province (No. 2012011006-2).
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