A highly efficient ultrasound-promoted synthesis of 2,3-disubstituted benzo[b]furans via intramolecular C-C bond formation in ionic liquid[bmim]BF4 at room temperature

Article abstract of DOI:10.1039/c2ra22355d

An efficient ultrasound-promoted synthesis of 2,3-disubstituted benzo[b]furans in the ionic liquid [bmim]BF₄ at room temperature is reported. 5-exo-dig carbanion-yne intramolecular cyclization is mediated using anhydrous K₃PO₄

Full text of DOI:10.1039/c2ra22355d

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A highly efficient ultrasound-promoted synthesis of 2,3-
disubstituted benzo[b]furans via intramolecular C–C
bond formation in ionic liquid[bmim]BF4 at room
temperature3
Cite this: RSC Advances, 2013, 3, 540
Nisha Yadav, Mohd. Kamil Hussain, Mohd. Imran Ansari, Puneet K. Gupta
and Kanchan Hajela*
An efficient ultrasound-promoted synthesis of 2,3-disubstituted benzo[b]furans in the ionic liquid
[bmim]BF4 at room temperature is reported. 5-exo-dig carbanion–yne intramolecular cyclization is
mediated using anhydrous K3PO4 as a mild, inexpensive base under atmospheric conditions giving the title
benzo[b]furans in excellent yields. Ionic liquid [bmim]BF4 has been used both as a reaction medium, as well
as a catalyst for the C–C bond formation.
Received 23rd July 2012,
Accepted 30th October 2012
DOI: 10.1039/c2ra22355d
www.rsc.org/advances
minimization
compared
with
traditional
methods.
Introduction
Sonochemistry (use of ultrasound in chemistry) offers the
synthetic chemist the advantage of chemical activation
through cavitation and provides a source of energy, which
can be used to enhance a variety of chemical reactions. The
strong acceleration of the reaction rate by ultrasonic irradia-
tion is based on the cavity effect and hot spot formation.20a,b
The cavity effect is defined as the phenomenon of formation,
growth and eventual collapse of small bubbles within a liquid.
As these bubbles are small and rapidly collapse, the
temperature and pressure within the bubble during a collapse
is very high, thus, each cavitation bubble acts as a micro-
reactor that offers the opportunity of speeding up reactions to
take place in an absolutely safe manner. In addition, the
reactions are carried out at room temperature under atmo-
spheric conditions. In order to avoid the disadvantages of
volatile and toxic organic solvents, ionic liquids (ILs) have
shown great promise as non-volatile solvents and are being
increasingly explored as an eco-friendly medium for clean
synthesis. They are easy to recycle and possess no effective
vapour pressure.21,22 This communication is the first report of
a highly efficient and environmentally benign protocol for the
ultrasound-assisted synthesis of a variety of 2,3-disubstituted
benzo[b]furans in high yields. The intramolecular C–C bond
formation leading to the heterocycle is catalyzed by a mild
base potassium phosphate23–25 in the ionic liquid [bmim]BF4,
(1-butyl 3-methyl imidazolium tetrafluoroborate) under ultra-
sound irradiation at room temperature. Ionic liquid
[bmim]BF4 has been used both as a reaction medium, as well
as the catalyst for the intramolecular carbanion–yne cycliza-
tion (Scheme 1).
Intramolecular cyclizations forming C–C bonds have become
one of the most efficient methods for the synthesis of carbocyclic
and heterocyclic compounds.1a,b A number of methodologies
using transition metal catalyzed annulations,2–4 intramolecular
Heck reaction,5,6 carbocyclization of gem-dihaloalkenes,7a–c
sequential Sonogashira coupling8 etc. have been utilized for the
formation of diverse heterocyclic compounds. 2,3-functionalized
benzo[b]furans have been widely studied as they constitute
important units in many natural products and biologically active
compounds.9,10 Their potential in chemistry and biology has
consistently stimulated the search for new methods for their
synthesis. In the literature, there are various methods for
benzo[b]furan synthesis via C–O bond formation,11–16 but very
few are available via C–C bond formation.17,18 Moreover, many of
these methods use very expensive reagents, specific PTCs (phase
transfer catalysts), harsh reaction conditions and tedious work-
up procedures. These drawbacks prompted us to develop a mild
and greener approach towards the synthesis of 2,3-disubstituted
benzo[b]furans at room temperature using ultrasonication (US)
as the energy source.
Ultrasound-assisted organic synthesis as a green approach
is a powerful technique employed for organic reactions
leading to higher yields, shorter reaction times, milder
conditions and higher analytical purity.19a–d It is considered
a processing aid in terms of energy conservation and waste
Medicinal and Process Chemistry Division, CSIR – Central Drug Research Institute,
Lucknow-226001, India. E-mail: kanchan_hajela@cdri.res.in;
Fax: (+91)-522-2623405, 2623938; Tel: (+91)-522-2612411-18, PABX, 2463/2334
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
3
c2ra22355d
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carbonate in dichloromethane completely changed the course
of the reaction, it was complete in 45 min and benzo[b]furan
was isolated in 80% yield (entry 8). Use of strong bases like
KOH or NaOH formed complex mixtures of unidentifiable
compounds from which the desired compound could not be
isolated. Use of moderate bases like Na2CO3 or K2CO3 gave
either low yields or negligible formation of benzo[b]furans
(entry 9, 10). These observations led to the belief that for
solvents basicity is more important than polarity. Since the use
of strong bases led to complex mixtures, it was thought to
study the effect of a mild base synergistically with a moderately
basic solvent. Anhydrous potassium phosphate has been
utilized as an auxiliary mild base in homogeneous metal-
catalyzed reactions, particularly in the presence of base-
sensitive functional groups in a number of C–C bond forming
reactions.26–28 In this analogy, we conducted our reaction
using potassium phosphate in DMF and surprisingly the
desired benzo[b]furan 2a was isolated in 81% yield (entry 11).
Thus, on comparing the results, K3PO4 was found to be the
most efficient base and upon examining the influence of the
amount of anhydrous K3PO4 on the reaction, it was found that
approximately 2.5–3.0 equiv. of base was necessary for the
completion of the reaction in less time. A change of solvent to
THF or DMSO did not have any significant effect on the
reaction. In order to avoid the use of toxic DMF and applying a
greener approach, the same reaction was performed in ionic
liquid [bmim]BF4. The reaction was almost quantitative in 30
min, as observed by TLC (thin layer chromatography) and 2,3-
disubstituted benzo[b]furan 2a was isolated in .95% yield
through column chromatography (entry 12). To investigate the
use of other ionic liquids, as different combinations of anions
and cations lead to different ionic liquids with different
polarities, the reaction was tried in [bmim]PF6 (1-butyl
3-methyl imidazolium hexafluorophosphate), [bmim]OH (1-
butyl 3-methyl imidazolium hydroxide) and [bpy]BF4 (N-butyl
pyridinium tetrafluoroborate) (entry 13–15). It was found that
the reaction worked best in [bmim]BF4. In the case of
[bmim]PF6 the product was isolated in .80% yield, but the
time needed for the completion of the reaction was prolonged
to 1 h, presumably due to the hydrophobic nature of this ionic
liquid.29 In our case, since the reaction is conducted under
atmospheric conditions it is more compatible in hydrophilic
ionic liquid, [bmim]BF4. As compared to other cation-based
ILs, such as pyridinium, thiazolium, pyrazolium etc.,imidazo-
lium cations exhibit higher ionic conductivities and lower
viscosity.30 As shown in the mechanism of our reaction
(Scheme 4) the imidazolium cation binds to the carbanion
formed and stabilizes the intermediates (i & ii). This binding
of the imidazolium cation is presumably facilitated due to the
availability of a hydrogen bond donor site at C-2 of the
imidazolium ring flanked by two N-atoms resulting in the
strong solute solvent interactions and acceleration of the
reaction rate. This is further supported by the results obtained
using the pyridinium cation-based ionic liquid that lacks a
H-donor site and thus affords the product in lower yields with
increased reaction time (entry 15).
Scheme 1 Synthesis of 2,3-disubstituted benzo[b]furans: K3PO4 mediated
intramolecular C–C bond formation in ionic liquid.
Results and discussion
The study was initiated by conducting the intramolecular
cyclization of simple 1-phenyl-2-(2-(phenylethynyl)phenoxy)
ethanone 1a using t-BuOK (potassium tertiary butoxide) as a
base in THF under ultrasound irradiation at room tempera-
ture. To our delight the reaction was neat and completed in 30
min yielding the desired benzo[b]furan 2a in 80% yield
(Table 1, entry 1). Encouraged by this result we decided to
choose 1a as a template for the screening of different solvents
and bases for the optimization of reaction conditions. A
change of solvent from THF to DMF or DMSO prolonged the
reaction time to 1 h (entry 2 and 3). Changing the base to
cesium carbonate and conducting the reaction in THF,
acetonitrile or acetonitrile–water mixture gave no reaction
(entry 4–6). Addition of a catalytic amount of PTC, (tetrabutyl
ammonium bromide or iodide) in conjunction with cesium
carbonate again failed to initiate any reaction (entry 7).
However, to our surprise, the addition of a catalytic amount
of DABCO (1,4-diazabicyclo[2.2.2]octane) along with cesium
Table 1 Synthesis of 2,3-disubstituted benzo[b]furans under ultrasonic irradia-
tion. Optimization of reaction conditions, solvents and bases
a
Entry Base
Solvent
Method/Time Yield (%) (2a)
1
2
t-BuOK
t-BuOK
THF
DMF
USb/30 min
US/1 h
80
90
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
t-BuOK
Cs2CO3
Cs2CO3
DMSO
THF
CH3CN
CH3CN/H2O US/5 h
CH3CN/H2O US/5 h
US/1 h
US/5 h
US/5 h
90
N. Rc
N. R
N. R
N. R
80
Cs2CO3
Cs2CO3/PTCd
Cs2CO3/DABCOe DCM
US/45 min
CH3CN/H2O US/2 h
Na2CO3 or K2CO3 CH3CN/H2O US/5 h
KOH or NaOH
C. Mf
N. R
81
96
80
84
68
C. M
40
85
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
DMF
US/1 h
[bmim]BF4 US/30 min
[bmim]OH US/1 h
[bmim]PF6 US/1 h
[bpy] BF4
US/1 h
[bmim]BF4 50 uC/1 h
[bmim]BF4 RTg/4 h
[bmim]BF4 MW/15 min
a
b
c
2.5–3 equivalent of bases were used. Ultrasonic irradiation. No
d
e
reaction. 10 mol % of phase transfer catalyst were used. 1 : 1
ratio of base and DABCO were used. Complex mixture. Room
temperature.
f
g
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To explain the promotional effect of ultrasonic irradiation,
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the same reaction was performed in [bmim]BF4 under
conventional heating and stirring at 50 uC. Though the
reaction was complete in 1 h, it showed a complex mixture
(Table 1, entry 16) from which the desired compound could
not be isolated. Similarly, when the reaction was carried out at
room temperature, it was incomplete even after 4 h of stirring
(entry 17). Thus, having accomplished the optimization of
reaction conditions, it was thought to compare the reaction
under another green condition i.e., microwave irradiation
(MW), the reaction was complete in a shorter time of 15 min,
but the cyclized product was isolated in 85% (entry 18). The
results are summarised in Table 1.
Scheme 2 C–C bond formation in ester analogues.
compared to that adjacent to a carbonyl group. Also, the
compounds formed were benzo[b]furan isomers and not
benzo[b]furans because due to the lack of a-hydrogen, the
molecules could not be aromatized to benzofurans. The results
are shown in Table 3.
Next, equipped with the optimized experimental condi-
tions, substrate generality of the reaction was investigated
under the same reaction conditions to study the effect of
substituents R1 and R2 present on the aryl rings (Table 2).
Presence of an electron-donating methoxy or electron-with-
drawing halogen group on either acetylenic aryl ring or
alkoxyaryl ring facilitated formation of benzofurans in
excellent yields (entries 1b–f). The reactions were neat and
products could be easily isolated/purified by simple filtration
through a short column of silica gel. Even dihalo and
trimethoxy groups were well tolerated (entries 1g–i). When
aryl acetylene was replaced with trimethylsilyl acetylene, the
reaction again progressed well with subsequent desilylation of
the TMS group during the cyclization process (entry 1j–m).
Further modifications in the molecular structure were
carried out by subsequently replacing the hydrogens at the
alpha position to the carbonyl carbon by one and two ester
groups, respectively (Scheme 2).
In the analogy of carbanion-yne cyclization, we thought of
extending the same reaction protocol to carbanion–ene
molecular framework and study the cyclization of (E)-2-(5-
methoxy-2-styrylphenoxy)-1-phenylethanone 3. In this case the
reaction was complete in 4 h. A white solid was isolated (80%)
by purification through column chromatography along with a
phenolic compound. Spectral analyses confirmed the structure
of 4 as 2-aroyl-3-benzyl-6-methoxy-2,3-dihyrobenzofuran and 5
as (E)-2-styrylphenol (Scheme 3).The results are again contrary
to a published report wherein chroman derivatives are formed
through 6-endo-trig cyclization using t-BuOK as a base with
2-phenylvinylphenoxy and 2-phenylallylphenoxy substrates
under reflux conditions.31 With the above observations, we
also tried the cyclization of 3 by changing the mild base,
anhydrous K3PO4, to t-BuOK. No benzo[b]furan was formed,
instead the phenacyl group was completely knocked out
forming (E)-2-styrylphenol as the sole product. Interestingly,
if the same reaction was performed under conventional
heating at 80 uC in K3PO4/[bmim]BF4, it resulted in substan-
tially low yields of 4 (40%) and increased formation of 5
(.50%). This may be explained due to the decreased activity of
the olefinic double bond (styryl substrate), as compared to the
more reactive alkyne substrate requiring more time for
cyclization to benzo[b]furan, whereas, strong basic conditions
or heating facilitates the hydrolysis of the phenacyl group. The
reason for 5-exo-dig cyclization in our study taking precedence
over 6-endo-trig cyclization may be due to subsequent
stabilization of the carbanion formed with the aryl rings
In this case the reaction time was increased up to 45 min.
This increase in reaction time is presumably due to the
decreased acidity of a-hydrogens adjacent to an ester group as
Table 2 Synthesis of 2,3-disubstituted benzo[b]furans. The effect of substituent
groups
Entry
R1
R2
Product
Yield (%)a
Table 3 Synthesis of 2,2,3-trisubstituted benzo[b]furan isomers
1a
1b
1c
1d
1e
1f
1g
1h
1i
Ph
4-MeOPh
Ph
4-MeOPh
Ph
4-BrPh
3,4,5-OMePh
3,4-ClPh
3,4-ClPh
Ph
Ph
Ph
4-MeOPh
4-MeOPh
4-BrPh
44-MeOPh
Ph
2a
2b
2c
2d
2e
2f
2g
2h
96
95
96
96
95
96
94
95
97
97
94
94
93
Entry
R1
R2
R3
Product
Yields (%)a
Ph
4-MeOPh
Si(Me)3
Si(Me)3
Si(Me)3
Si(Me)3
2i
87
85
82
80
1j
1k
1l
2j(R2 = H)
2k(R2 = H)
2l(R2 = H)
2m(R2 = H)
1n
1o
1p
1q
CO2Et
CO2Et
CO2Et
CO2Et
Ph
Ph
CO2Et
CO2Et
Ph
4-MeOPh
Ph
2n
2o
2p
2q
4-MeOPh
4-BrPh
3,4,5-OMePh
4-MeOPh
1m
a
a
Isolated Yields.
Isolated yields.
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80 uC for reuse in the next reaction. The same process is
adopted after each reaction and the recovered ionic liquid was
successively used for four consecutive cycles without any
significant loss in the efficiency of the solvent. Moreover, the
new reaction procedure was not found to be sensitive to the
quality and type of the reagents or solvents which were used,
such as those commercially available without further purifica-
tion. All of the reactions were carried out under atmospheric
conditions and therefore, in the hydrophilic ionic liquid
[bmim]BF4 no extra precaution was needed for the exclusion of
moisture.
Based on these results a possible mechanism is postulated
for the intramolecular cyclization, as shown in Scheme 4.
Abstraction of an a-proton by potassium phosphate results in
carbanion generation (i), which is stabilized by the imidazo-
lium cation of the ionic liquid (ii). The nucleophilic attack by
the anion on alkynyl carbon induces intramolecular 5-exo-dig
cyclization forming the intermediate (iii). Subsequent proton
transfer leads to aromatization affording the stabilized 2,3-
disubstituted benzo[b]furan 2a in excellent yields.
Scheme 3 Cyclization of styrylphenoxy phenyl ethanones.
attached to double or triple bonds under mild basic condi-
tions.
The reaction protocol as described above is highly efficient,
involves the use of an inexpensive base K3PO4 and employs an
ultrasonic cleaning bath as the cheapest source of ultrasonic
irradiation. Furthermore, a very small amount of ionic liquid
(1 ml mmol21) is required for the reaction, that works both as
a solvent, as well as a catalyst. The cost analysis of the reaction
protocol as compared to other reported methods makes it
quite economical due to the recyclability of the ionic liquid.
The recycling procedure involves filtration of the reaction
mixture to remove the residual base and washing with diethyl
ether (15 ml 6 3). The product along with the impurities is
isolated from the diethyl ether layer by drying over anhydrous
Na2SO4 and evaporation under reduced pressure. The ionic
liquid thus separated was dried under a vacuum overnight at
Conclusion
In conclusion, we have developed a convenient and highly
efficient ultrasound-promoted synthesis of 2,3-disubstituted
benzo[b]furans in ionic liquid at room temperature and under
mild basic conditions. The reaction is tolerant to both
electron-donating, as well as electron-withdrawing substitu-
ents, and can be used to access 2,3-functionalized benzo[b]fur-
ans in good to excellent yields through either carbanion–yne or
carbanion–ene carboannulation.
Experimental
General procedure for the synthesis of 2a
To a mixture of compound 1a (312 mg, 1 mmol) in 1 ml of
ionic liquid [bmim]BF4, anhydrous potassium phosphate (636
mg, 3 mmol) was added and the reaction mixture irradiated
under ultrasonic condition for 30 min. After completion of the
reaction, as monitored by TLC, the product was extracted with
diethyl ether (15 ml 6 3) leaving behind the ionic liquid that
was further used in another reaction cycle. The combined
organic layer was finally washed with water, dried over Na2SO4
and concentrated under reduced pressure to give a solid
residue. The pure product 2a was isolated by simple filtration/
purification over a small column of silica gel, using EtOAc/
hexane, as a white solid, m.p. 110–112 uC. 1H NMR (300
MHz,CDCl3) d 8.15–8.13 (d, J = 7.1 Hz ,2H), d 7.66–7.48 (m,
6H), 7.40–7.38 (d, J = 7.1 Hz, 2H), 7.31–7.18 (m, 4H), 4.57 (s,
2H). 13C NMR (50 MHz, CDCl3) d 185.9, 154.5, 148.4, 139.7,
137.8, 132.8, 129.2, 128.8, 128.6, 128.5, 128.4, 128.2, 126.4,
123.6, 122.2, 112.4, 30.5. IR (KBr, cm21): 1641, 1550, 1262,
1218, 1018, 769. ESI-MS: (m/z) = 313 [M + H]+.
Scheme 4 Proposed mechanism for K3PO4/[bmim]BF4 mediated C–C bond
formation/cyclization.
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Acknowledgements
The authors, N. Yadav, M. K. Hussain, M. I. Ansari would like
to acknowledge their thanks to CSIR, New Delhi, India, for
Senior Research Fellowships and P. K. Gupta for Junior
Research fellowship. Furthermore, all are grateful to the
Sophisticated Analytical Instrument Facility (SAIF) CDRI,
Lucknow, India, for providing spectral analysis/data of pure
compounds and the Ministry of Health and Family Welfare for
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