Microwave-assisted efficient synthesis of 2-arylbenzo[b]furans and 2-ferrocenylbenzo[b]furans from readily prepared propargylic alcohols and o-iodophenols

Article abstract of DOI:10.1007/s12039-011-0137-9

A simple, efficient and expeditious method for synthesis of 2-arylbenzo[b]furans and 2- ferrocenylbenzo[b]furans from readily prepared propargylic alcohols, o-iodophenols and silica gel with the catalyst of PdCl₂(PPh₃)₂ (2 mol%)/CuI (2mol%) and microwave-promoted Sonogashira coupling/cyclization reaction is developed. The methodology can produce good to excellent yields. In addition, this method can also be completed in one-pot with iodobenzene, 2-methyl-3-butyn-2-ol and 2-iodo-4-methylphenol as reactants. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0137-9

c
J. Chem. Sci. Vol. 123, No. 5, September 2011, pp. 697–702. ꢀ Indian Academy of Sciences.
Microwave-assisted efficient synthesis of 2-arylbenzo[b]furans
and 2-ferrocenylbenzo[b]furans from readily prepared
propargylic alcohols and o-iodophenols
LIANSHENG WU, XIAOKANG SHI, XIAOYUN XU, FEN LIANG and GUOSHENG HUANG∗
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
e-mail: hgs2368@163.com
MS received 14 September 2010; revised 18 May 2011; accepted 27 June 2011
Abstract. A simple, efficient and expeditious method for synthesis of 2-arylbenzo[b]furans and 2-
ferrocenylbenzo[b]furans from readily prepared propargylic alcohols, o-iodophenols and silica gel with the
catalyst of PdCl2(PPh3)2 (2 mol%)/CuI (2 mol%) and microwave-promoted Sonogashira coupling/cyclization
reaction is developed. The methodology can produce good to excellent yields. In addition, this method can also
be completed in one-pot with iodobenzene, 2-methyl-3-butyn-2-ol and 2-iodo-4-methylphenol as reactants.
Keywords. Benzo[b]furan; o-iodophenol; propargylic alcohols; microwave irradiation.
1. Introduction
their substrate preparation needs tedious operation and
expensive reagents, long reaction time or low yield.
Microwave (MW)-promoted reactions are widely
known and can be utilized as an alternative energy
source for organic reactions ordinarily fulfilled by heat-
ing since it was found to be able to accelerate a
wide variety of transformations.10 Kabalka group has
reported microwave-enhanced Sonogashira coupling
and cyclization reactions on alumina,10c but his method
needs expensive trimethylsilyl-acetylene as monopro-
tected acetylene derivative, large amount of catalyst,
and low yield.
Benzo[b]furans and their derivatives are ubiquitous in
the realms of pharmacologically active agents and iso-
lated natural products which have important biologi-
cal properties.1 For example, they are widely used as
antitumor drugs,2 inhibitors of 5-lipoxygenase,3 blood
coagulation factor Xa inhibitors,4 and antimicrobial.5
Known methods for the synthesis of benzo[b]furan
and its derivatives commonly employ the intramolec-
ular cyclization of a suitably substituted benzene.1,6
Benzo[b]furans are the focus of many recent reports
on palladium-based cross-coupling reactions with cop-
per iodide as a co-catalyst. In most of these cases
the process is accomplished through a tandem Sono-
gashira coupling/5-endo-dig cyclization starting from
either o-alkynylphenols or o-iodophenols. These one-
pot palladium-based protocols have advantages over the
multi-step traditional methods in terms of functional
group tolerability and yields of the desired benzofuran.
Recently, the Venkataraman group,7 Buchwald group,8
and others9 have been interested in developing copper-
catalysed cross-coupling reactions; copper-based meth-
ods have an economic attractiveness. However, these
methods have several drawbacks in terms of different
reaction conditions, they need expensive acetylenes, or
2. Experimental
NMR spectra were performed on a Mercury 4N-PEG-
300 (1H: 300 MHz; 13C: 75 MHz) spectrometer, using
CDCl3 as a solvent and TMS as the internal standard.
IR spectra recorded on Nicolet Nexus 670 FT-TR spec-
trophotometer as KBr pellets or KBr film. Mass spec-
tra were recorded by the EI method on a HP 5998 mass
spectrometer.
2.1 5-Methyl-2-phenylbenzo[b]furan (3a); Typical
procedure
All reactions were performed on a 0.5 mmol scale rel-
ative to 1a. A round-bottom side-arm flask (10 mL)
containing PdCl2(PPh3)2 (0.01 mmol), CuI (0.01 mmol)
∗For correspondence
697

698
Liansheng Wu et al.
was subjected to the Schlenk-line procedures of evac- 2.4 5-Chloro-2-ferrocenylbenzo[b]furan (3t)
uation and purging of argon for three cycles. 2-iodo-4-
methylphenol (1a, 0.5 mmol), 2-methyl-4-phenylbut-3-
Red solid; mp 159–160◦C; IR (KBr) ν: 3110, 3089,
2919, 2851, 1603, 1462, 1438, 1260, 1159, 1059, 1009,
935, 910, 875, 800, 693, 478 cm−1; 1H NMR (300 MHz,
CDCl3): δ = 4.13 (s, 5H), 4.39 (t, J = 1.8 Hz, 2H), 4.77
(d, J = 1.8 Hz, 2H), 6.56 (s, 1H), 7.20 (dd, J = 2.4 Hz,
9.0 Hz, 1H), 7.38 (d, J = 8.7 Hz, 1H), 7.46 (s, 1H);
13C NMR (75 MHz, CDCl3): δ = 66.37, 69.53, 69.69,
74.27, 99.08, 111.62, 119.44, 123.14, 128.27, 130.95,
152.94, 158.80; FAB-MS: m/z = 336 [M+]; Anal.Calcd
for C18H13ClFeO: C, 64.23; H, 3.89. Found: C, 64.15;
H, 3.76.
yn-2-ol (2a, 0.5 mmol), t-BuOK (1.5 mmol), silica gel
(200 mg) and DMSO (2 ml) were successively added,
and then the mixture was stirred magnetically at room
temperature for 1 min to mix thoroughly. The flask was
then exposed to microwave irradiation (intermittently at
1 min intervals; 110◦C) for 2 min. After cooling to r.t,
the crude product was extracted with ethyl acetate (3 ×
10 mL). The combined organic phases were washed
with brine (2 × 5 mL), dried over anhydrous MgSO4
and concentrated in vacuo. The residue was subjected
to flash column chromatography with hexanes/EtOAc
(10/1) as eluent to obtain the desired 3a (92.6 mg, 89%
yield).
2.5 5-Tert-butyl-2-ferrocenylbenzo[b]furan (3u)
1
White solid; mp: 131◦C (lit11 131◦C); H NMR
Red solid; mp 138–139◦C; IR (KBr) ν: 3089, 2954,
(300 MHz, CDCl3): δ = 7.82 (d, J = 7.5 Hz, 2 H),
7.45–7.30 (m, 5 H), 7.06 (d, J = 8.1 Hz, 1 H), 6.93
(s, 1 H), 2.43 (s, 3 H); 13C NMR (75 MHz, CDCl3):
δ = 153.97, 153.31, 132.48, 130.60, 129.18, 128.72,
128.40, 125.50, 124.82, 120.73, 110.62, 101.08, 21.31;
MS (EI): m/z = 208 [M+]; Anal. Calcd for C15H12O: C,
86.51; H, 5.81. Found: C, 86.57; H, 5.78.
2924, 2861, 1607, 1478, 1447, 1361, 1268, 1160, 1077,
1
1005, 924, 876, 807, 748, 643, 483 cm−1; H NMR
(300 MHz, CDCl3): δ = 1.38 (s, 9H), 4.13 (s, 5H), 4.37
(s, 2H), 4.77 (s, 2H), 6.56 (s, 1H), 7.29 (d, J = 7.8 Hz,
1H), 7.38 (d, J = 8.1 Hz, 1H), 7.49 (s, 1H); 13C NMR
(75 MHz, CDCl3): δ = 31.88, 34.67, 66.26, 69.29,
69.77, 75.20, 99.97, 110.01, 116.27, 121.08, 129.18,
145.70, 152.88, 156.95; FAB-MS: m/z = 358 [M+];
Anal Calcd for C22H22FeO: C, 73.76; H, 6.19. Found:
C, 73.71; H, 6.10.
2.2 5-Methyl-2-ferrocenylbenzo[b]furan (3r)
Red solid; mp 144–145◦C; IR (KBr) ν: 3107, 2919,
2861, 1606, 1482, 1445, 1267, 1078, 1008, 931, 804,
2.6 5,7-Dimethyl-2-ferrocenylbenzo[b]furan (3v)
1
731, 479 cm−1; H NMR (300 MHz, CDCl3): δ =
Red solid; mp 134–135◦C; IR (KBr) ν: 3096, 3014,
2920, 2859, 1607, 1439, 1412, 1320, 1279, 1204,
1137, 1106, 1079, 1007, 933, 867, 844, 819, 747, 510,
2.45 (s, 3H), 4.13 (s, 5H), 4.37 (t, J = 1.8 Hz, 2H),
4.77 (t, J = 1.8 Hz, 2H), 6.56 (s, 1H), 7.06 (d, J =
7.8 Hz, 1H), 7.30 (s, 1H), 7.36 (d, J = 8.1 Hz, 1H);
13C NMR (75 MHz, CDCl3): δ = 21.32, 66.19, 69.19,
69.61, 75.21, 99.41, 110.28, 119.85, 124.31, 129.64,
132.11, 153.01, 157.01; FAB-MS: m/z = 316 [M+];
Anal. Calcd for C19H16FeO: C, 72.18; H, 5.10. Found:
C, 72.20; H, 5.21.
1
484 cm−1; H NMR (300 MHz, CDCl3): δ = 2.40 (s,
3H), 2.52 (s, 3H), 4.11 (s, 5H), 4.38 (s, 2H), 4.79
(s, 2H), 6.58 (s, 1H), 6.82 (s, 1H), 7.16 (s, 1H); 13C
NMR (75 MHz, CDCl3): δ = 15.03, 21.27, 66.27, 69.13,
69.67, 75.63, 99.76, 117.29, 120.37, 125.54, 129.07,
132.08, 152.03, 156.58; FAB-MS: m/z = 330 [M+];
Anal Calcd for C20H18FeO: C, 72.75; H, 5.49. Found:
C, 72.58; H, 5.55.
2.3 2-Ferrocenylbenzo[b]furan (3s)
Red solid; mp 124–125◦C; IR (KBr) ν: 3085, 2920,
2851, 1606, 1458, 1438, 1253, 1164, 1104, 1076, 1006,
933, 812, 746, 479 cm−1; 1H NMR (300 MHz, CDCl3):
δ = 4.13 (t, J = 2.7 Hz, 5H), 4.37 (t, J = 1.5 Hz, 2H),
4.78 (t, J = 1.2 Hz, 2H), 6.62 (s, 1H), 7.18–7.24 (m,
2H), 7.47–7.51 (m, 2H); 13C NMR (75 MHz, CDCl3):
δ = 66.29, 69.26, 69.65, 75.08, 99.66, 110.79, 119.95,
122.71, 123.14, 129.57, 154.61, 156.98; FAB-MS: m/z
= 302 [M+]; Anal.Calcd for C18H14FeO: C, 71.55; H,
4.67. Found: C, 71.59; H, 4.60.
3. Results and discussion
Previously, András Kotschy group has reported tan-
dem Sonogashira coupling using the cheap reagent
2-methyl-3-butyn-2-ol.12 An elegant procedure devel-
oped by Chow, realizing the simultaneous removal of
acetone and the Sonogashira coupling of the released
terminal acetylene with a second aryl halide under
phase transfer conditions.13 Very recently our team

Microwave-assisted efficient synthesis of 2-arylbenzo[b]furans and 2-ferrocenylbenzo[b]furans
699
has developed an efficient one-pot synthesis of 2-aryl- less effectively (entries 4–6). From these evidence, we
substituted benzo[b]furans from easily prepared aryl found silica gel to be a particular important reagent in
iodides, o-iodophenols and the inexpensive reagent 2- this system. Silica gel apparently acts as a temperature
methyl-3-butyn-2-ol.14 To extend the scope of this pro- moderator. Without silica gel, the liquid reactants can
tocol, we report here a simple, efficient and expeditious react uncontrollably. The effect of the base on the cou-
method for the preparation of 2-arylbenzo[b]furans pling/cyclization reaction was examined with t-BuOK
via PdCl2(PPh3)2/CuI co-catalysed coupling reaction as the most effective one (entries 6, 12–16). Decreasing
of o-iodophenols and propargylic alcohols under the or increasing the reaction time did not result in a higher
microwave irradiation.
yield (entries 17 and 18). On the basis of these control
Our initial study began with the reaction of 1 equiv. experiments, we chose the PdCl2(PPh3)2/CuI, t-BuOK,
of 2-iodo-4-methylphenol (1a, 0.5 mmol), 1 equiv. of silica gel, DMSO system as the standard protocol
2-methyl-4-phenylbut-3-yn-2-ol (2a), and 3 equiv. of t- to synthesize 2-arylbenzo[b]furans from o-iodophenols
BuOK with 2 mol% of PdCl2(PPh3)2 and 2 mol% of and propargylic alcohols.
CuI as the co-catalyst in DMF under argon, the reac-
Under the optimized reaction conditions above, a
tion vessel was then exposed to microwave irradiation series of 2-arylbenzo[b]furans can be prepared in good
for 2 min in an unmodified household microwave oven to excellent yields (table 2, entries 1–8). It can be seen
(Midea W7022J, 700 W). The desired product 3a was that the yield decreases with the increasing electron
isolated in a 21% yield (table 1, entry 1). Further opti- withdrawing of the functional group (table 2, entries
mization of the reaction conditions revealed that DMSO 3–5, 8). The presence of ortho group also gives low
was more effective than other solvents, such as DMF, yield of the product (table 2, entry 7). According to the
toluene, MeOH, EtOH, THF, CH3CN (entries 1–3, 6– results in table 2 (table 2, entries 9 and 10, 12 and 13), it
11). Silica gel played an important role in this system. can be seen that the position of aryl’s functional group
Other additives, such as alumina, 4 Å MS, were tested has a distinct effect on the yield. It is likely that the
Table 1. Reaction of 2-iodo-4-methylphenol with 2-Methyl-4-phenylbut-3-yn-2-ol under various conditions.
I
OH
PdCl2(PPh3)2,CuI,Base
solvent, MW
+
OH
O
3a
1a
2a
Entry
Additive
Base
Solvent
Time (min)
Yield (%)a
b
1
2
3
4
5
6
7
8
–
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
KOH
K2CO3
Et3N
NaOAc
K3PO4
t-BuOK
t-BuOK
DMF
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
5
21
38
11
67
58
–
–
DMSO
toluene
DMSO
DMSO
DMSO
DMF
MeOH
EtOH
THF
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Aluminac
4Å MSd
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
Silica gel
89
56
n.r.e
n.r.
n.r.
n.r.
48
9
10
11
12
13
14
15
16
17
18
n.r.
18
n.r.
12
78
80
aIsolated yield after column chromatography, bno additive was used.
cThe amount of additives is 200 mg for all the reactions. d4Å molecular sieves (freshly activated)
en.r. = no reaction

700
Liansheng Wu et al.
Table 2. Synthesis of 2-Arylbenzo[b]furans and 2-ferrocenylbenzo[b]furans.a
2 mol% PdCl2(PPh3)2,
2 mol% CuI,
3.0 eq t-BuOK, SiO2
I
R2
OH
R1
+
R1
R2
DMSO, MW
OH
O
1
2
3
Yieldc
b
Entry
R1
R2
Product
1
2
3
4
5
6
7
8
4-CH3
H
4-Cl
4-NO2
4-Br
4 -tert-butyl
4,6-dimethyl
4-Phenyl
4-CH3
–
–
–
–
–
–
–
–
–
H
Phenyl
–
–
–
–
–
–
–
4-CH3Phenyl
2-CH3Phenyl
3,4-dimethylPhenyl
4-CH3OPhenyl
2-CH3OPhenyl
4-ethoxyPhenyl
4-ClPhenyl
3-ClPhenyl
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
89
82
69
55
52
81
68
63
93
67
79
90
56
69
73
76
79
88
74
76
73
69
trace
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
2-ClPhenyl
Fcd
–
4-Cl
–
–
–
–
3t
4-tert-butyl
4,6-dimethyl
4-NO2
3u
3v
3w
aThe reaction was carried out with 1 (0.5 mmol), 2 (0.5 mmol), SiO2 (200 mg), t-BuOK (3 equiv) and DMSO
(2 ml) in the presence of PdCl2(PPh3)2 (2 mol%)/CuI (2 mol%) promoted by microwave for 2 min under argon.
b4-Substituted-o-iodophenols were synthesized from readily available phenols following the method reported by
Edgar and Falling.15
cIsolated yields after column chromatography.
dFc = ferrocene
steric hindrance of the ortho group led to correspond- ate, which greatly simplifies the operational steps, but
ing yield decrease. Upon further studies, we success- the desired product 3a was obtained with lower yield
fully coupled a variety of o-iodophenols with 2-methyl- (64%) comparatively. To prove the benefit of the use
4- ferrocenylbut-3-yn-2-ol in good to excellent yields, of alcohol instead of alkyne,13 we employed pheny-
but 2-iodo-4-nitrophenol was ineffective (table 2, lacetylene instead of 2a to synthesize 3a under our
entries 18–23). Finally, we applied our synthetic pro- optimized reaction conditions. We found the homo-
tocol to synthesize 3a in one-pot (scheme 1). This coupling product 4 was a prominent side reaction
one-pot process needs no isolation of the intermedi- product (scheme 2). In contrast, because of a better
I
PdCl2(PPh3)2, CuI,
Et3N, r.t.
OH
Ph
OH
PhI
+
O
PdCl2(PPh3)2, CuI,
t-BuOK, SiO2, DMSO, MW
3a, 64% yield
Scheme 1. One-pot sequential synthesis of 5-methyl-2-phenylbenzo[b]furan (3a).

Microwave-assisted efficient synthesis of 2-arylbenzo[b]furans and 2-ferrocenylbenzo[b]furans
701
2 mol% PdCl2(PPh3)2,
2 mol% CuI,
3.0 eq t-BuOK, SiO2
I
+
Ph
+
OH
DMSO, MW
O
1a
3a, 46% yield
4, 30% yield
Scheme 2. Coupling-cyclization reaction between 2-iodo-4-methylphenol and phenylacetylene.
O
t-BuOK
CuI
(i)
R2
C
C
OH
R2
C
CH
R2
C
C
Cu
2
(ii)
R2
C
C
Cu
PdCl2(PPh3)2
+
CuCl
(Ph3P)2Pd
C
CR2
+
2
(Ph3P)2Pd0
R2C
C
C
CR2
+
I
(Ph3P)2Pd0
R1
OH
(iii)
1
Pd
I
C
CR2
R1
R1
OH
OH
A
C
R2
C
C
Cu
CuI
C
CR2
Pd
C
CR2
R1
R1
-
O
OH
B
R1
R2
O
3
Scheme 3. Plausible mechanism.
selectivity toward the formation of the cross-coupling gives a σ-arylpalladium(II) complex (A) which then
product, the benzo[b]furans were obtained with little trans-metallates with cuprous acetylide to generate the
contamination.
arylalkynylpalladium(II) species (B). Through reduc-
A plausible mechanism for the formation of 2- tive elimination of Pd0 then affords acyclic products,
substituted benzofurans by a single-step palladium- e.g., 2-alkynylphenols (C). The latter on cyclization in
catalysed reaction of o-iodophenol with propargylic the presence of t-BuOK where the phenoxide ion made
alcohols is illustrated in scheme 3. Removal of acetone an attack on the triple bond resulted in the formation of
from propargylic alcohols can release terminal acety- the benzofurans.
lene derivatives in step (i). The formation of Pd0 from
the interaction of bis(triphenylphosphine) palladium(II)
chloride and cuprous acetylide as shown in step (ii). We
have found evidence in favour of this from this work
4. Conclusions
on the dimerisation of monosubstituted alkynes to 1,4-
disubstituted 1,3-diynes by palladium–copper catalysis.
Oxidative addition of o-iodophenol to a Pd0 complex
In summary, we have developed a microwave-
promoted, simple, and efficient method for synthe-
sis of various substituted benzo[b]furans starting from

702
Liansheng Wu et al.
o-iodophenols and propargylic alcohols. Reagent 2-
methyl-4-arylbut-3-yn-2-ols are more easily prepared
than aryl acetylenes to synthesize benzo[b]furans. The
most attractive features of this method were short reac-
tion time, good yields, low-cost and easy preparation.
7. (a) Gujadhur R and Venkataraman D 2001 Synth.
Commun. 31 2865; (b) Gujadhur R, Venkataraman D
and Kintigh J T 2001 Tetrahedron Lett. 42 4791; (c)
Gujadhur R K, Bates C G and Venkataraman D 2001
Org. Lett. 3 4315; (d) Bates C G, Gujadhur R K and
Venkataraman D 2002 Org. Lett. 4 2803; (e) Bates C G,
Saejueng P, Murphy J M and Venkataraman D 2002 Org.
Lett. 4 4727
Acknowledgement
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Chem. Soc. 119 10539; (b) Kiyomori A, Marcoux J F
and Buchwald S L 1999 Tetrahedron Lett. 40 2657; (c)
Klapars A, Antilla J C, Huang X H and Buchwald S L
2001 J. Am. Chem. Soc. 123 7727; (d) Wolter M, Klapars
A and Buchwald S L 2001 Org. Lett. 3 3803; (e) Kwong
F Y, Klapars A and Buchwald S L 2002 Org. Lett. 4
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We thank the State Key Laboratory of Applied Organic
Chemistry for financial support.
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