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Catalysis Communications 10 (2009) 833–837
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Bronsted acidic ionic liquid as an efficient and reusable catalyst
for transesterification of b-ketoesters
Ziyauddin S. Qureshi, Krishna M. Deshmukh, Malhari D. Bhor, Bhalchandra M. Bhanage *
Department of Chemistry, Institute of Chemical Technology (Autonomous), University of Mumbai, N. Parekh Marg, Matunga, Mumbai – 400 019, India
a r t i c l e
i n f o
Article history:
Received 2 August 2008
Received in revised form 28 November 2008
Accepted 7 December 2008
Available online 27 December 2008
a b s t r a c t
Halogen free Bronsted acidic ionic liquid N-methyl-2-pyrrolidone hydrogen sulfate has been efficiently
used as a catalyst for the transesterification of b-ketoesters with variety of alcohols. The ionic liquid
shows high catalytic activity and reusability with good to excellent yields of the desired products.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Bronsted acidic ionic liquid
b-Ketoesters
Alcohols
N-Methyl-2-pyrrolidone hydrogen sulfate
1. Introduction
Transesterification of b-ketoesters is one of the important reaction for the synthesis of esters and have variety of applications in
pharmaceutical, agrochemical, chemical and polymer industries
[1,2]. Transesterification is an equilibrium process and several
methods have been reported for the transesterification of b-ketoesters using various bases, enzymes and antibodies [1,3]. Traditionally such transformations are carried out in presence of an
acidic or basic catalyst to promote the rate of reaction. In this regard several protic acidic [4–6], and basic catalysts [7–11], have
been reported in the literature. Solid Lewis acid catalysts such as
distannoxanes [12], InI3 [13], Mo–ZrO2 [14], Ti(IV) alkoxides [3b],
Zinc–I2 [15], and Nb2O5 [16] are also developed for the transesterification of b-ketoesters. However, many of these methods are not
greener and developing a greener protocol for this transformation
is still a challenging task.
In recent years, ionic liquids (ILs) have been considered as a
promising greener reaction media, which overcomes the disadvantages of both traditional molecular solvents and melt salts. These
ILs are non-flammable, thermally stable and exhibits negligible vapor pressure (non-volatile) and offer the potential for recyclability.
These properties make them advantageous and have found to be
widely used in catalytic and non-catalytic reactions [17]. Recent
trends in the field of ionic liquid chemistry are the catalytic applications of task-specific ILs to the organic transformations.
* Corresponding author. Tel.: +91 22 24145616; fax: +91 22 24145614.
E-mail address: [email protected] (B.M. Bhanage).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2008.12.048
Transesterification of methyl acetoacetate (MAA) with alcohols
was first reported by Ming et al. using ionic liquid-regulated sulfamic acid [18]. Bronsted acidic ionic liquid NH2SO3H [C3MIm]Cl
shows satisfactory conversion rate and selectivity for transesterification, however it suffers from disadvantages like IL is used as solvent and it is required in a large amount (i.e., 10 g of ionic liquid
and 1 g of sulfamic acid), moreover the halogen content makes it
less attractive from greener prescriptive. Hence, the review of literature suggests that there are still challenges in developing a greener protocol for transesterification of b-ketoesters.
Herein we report, a halogen free Bronsted acidic IL N-methyl-2pyrrolidone hydrogen sulfate as an efficient and reusable catalyst
for transesterification of b-ketoesters (Scheme 1). The IL has several important features over prevalent transesterification systems
such as (1) ½NMPþ HSO
4 shows better activity in catalytic amount,
(2) Simpler synthetic procedure with high reproducibility, (3) Nmethyl-2-pyrrolidone as a source of cation is more economical
and commercially available as compared to other imidazolium/
pyridinium counter parts.
2. Experimental
2.1. General
All the chemicals were received from M/s S.D. Fine Chemicals
Ltd., India and used without further purification. The reactions
were carried out in a round bottom flask fitted with a distillation
condenser for the removal of methanol/ethanol during reaction
progress.
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Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837
O
N+
H HSO4-
O
1
OR
O
2
+ R OH
O
O
OR2
+ R1OH
o
80 C
Scheme 1. Transesterification of b-ketoester using ½NMPþ HSO
4.
2.2. Synthesis of ionic liquids
In this study, several ILs were synthesized according to the procedures reported in the literature [19]. These are included 1-butyl3-methylimidazolium p-toluenesulfonic acid ([BMIm]+Tsa), 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIm]+HSO4), 1methylimidazolium hydrogen sulfate ([HMIm]+HSO4) and Nmethyl-2-pyrrolidone hydrogen sulfate ([NMP]+HSO4) (Scheme
2).
2.3. Characterization methods
The IR spectra of ionic liquids were recorded on Buck Scientific
M-500 IR Spectrometer using KBr pellets in the frequency range of
1750–1300 cm1 using pyridine as a probe for acidity measurement. The 1H NMR spectrum of the ionic liquid was recorded
immediately after dissolution of complex in D2O with TMS as reference (400 MHz).
2.4. A typical experimental procedure for transesterification of bketoesters
MAA (0.58 g, 5 mmol), 1-Butanol (0.48 g, 6.5 mmol) and
[NMP]+HSO4 (0.049 g, 5 mol%) were placed in a 25 mL round bottom flask fitted with a distillation condenser. The reaction mixture
was heated to 80 °C for desired time and then cooled to room temperature. MAA conversion as well as product formation was monitored by gas chromatography (30 m 0.32 mm1D-0.25 lm BP10). A small amount of water (2 5 mL) was added to the reaction
mixture and transesterified product was extracted in toluene
(3 10 mL). The residue was chromatographed on silica gel column, eluted with a mixture of ethyl acetate-pet ether, to afford
pure b-ketoesters. The identities of the products were confirmed
by using GC–MS (Shimadzu GC–MS QP 2010) as well as using
authentic samples.
3. Results and discussion
3.1. Catalyst characterization
The ionic liquid catalyst was characterized by spectroscopic
techniques such as IR, 1H NMR.
3.1.1. Characterization of ionic liquid using infrared spectroscopy
The Bronsted acidity of the ionic liquid was determined using
pyridine as a probe molecule by monitoring the band range of
1350–1600 cm1 arising from its ring vibration modes [20,21].
The IR spectrum of pyridine shows a band at 1480 cm1 (Fig. 1a).
N
+
H2SO4
O
N+
H HSO4O
Scheme 2. Synthesis of ionic liquid.
Fig. 1. IR spectra: (a) neat pyridine, (b) pyridine with [NMP]+HSO4, (c) pyridine
with [HMIm]+HSO4, (d) pyridine with [BMIm] +HSO4, (e) pyridine with [BMIm]+pTSA. Note: only IR of (b) shows notable peaks for Bronsted acidity at 1550 cm1.
However, slight shift was observed in wave number after mixing
pyridine with ionic liquid. The spectrum of ionic liquids shows a
band at 1550 cm1 indicating the presence of Bronsted acid sites
due to the formation of pyridinium ions (Fig. 1b–e).
3.1.2. Characterization of ionic liquid using 1H NMR
[NMP]+HSO4: 1H NMR (400 MHz, CDCl3-d1, TMS): d 1.85 (m,
2H, J = 6.1 Hz), 2.28 (t, 2H, J = 7.4 Hz), 2.61 (s, 3H, J = 6.1 Hz), 3.32
(t, 2H, J = 7.1 Hz), 8.26 (bs, 1H).
[BMIm]+HSO4: 1H NMR (400 MHz, D2O-d2, TMS): d 0.91 (t, 3H,
J = 7.5 Hz), 1.25(m, 2H, J = 7.1 Hz), 1.75 (m, 2H, J = 7.4 Hz), 3.86 (s,
3H), 4.18 (t, 2H, J = 7.0 Hz), 7.74 (m, 2H), 7.79 (s, 1H), 9.25 (s, 1H).
3.2. Influence of various catalysts
Several Bronsted acidic ionic liquids and solid acid catalysts
were tested for their catalytic activity in transesterification of bketoesters with various alcohols. The prepared ILs is viscous clear
liquid at room temperature. They are partially miscible in esters,
miscible in water and alcohol and can be conveniently separated
whereas solid acid catalysts were easily filtered. Transesterification
of MAA with 1-butanol to give butyl acetoacetate using
[NMP]+HSO4 as a catalyst was chosen as a model reaction for
study. Various ILs and solid acid catalysts were screened for the
transesterification of MAA with 1-butanol and the results are summarized in Table 1. It was found that [NMP]+HSO4 gave 80% conversion of MAA with 98% selectivity to transesterified product at
80 °C (entry 4), which indicates that it is an efficient catalyst compared to that of other Bronsted acidic ionic liquids and solid acid
catalysts (entry, 5 and 6) in terms of conversion, selectivity and
recyclability.
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Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837
Table 1
Influence of catalysts for transesterification of MAA with 1-butanola.
Entry
Catalyst
[BMIm] p-TSA
1
-
O
Conversionb (%)
Selectivityc (%)
Yieldd(%)
75
93
70
OH
81
90
73
OH
83
89
74
OH
80
98
79
OH
78
79
62
OH
35
71
25
Alcohol
b-ketoester
+
O
OH
O
O
[BMIm] +HSO4
2
O
O
O
[HMIm]+HSO4
3
O
O
O
[NMP] +HSO4
4
O
O
O
5
O
Amberlyst-15
O
O
6
O
Montmorillonite-K10
O
b,c,d
Conversion, selectivity and yield are based on GC analysis.
Reaction conditions: MAA (5 mmol), 1-butanol (6.5 mmol), IL (5 mol%), reaction time = 3 h, temp. = 80 °C.
d
Yield of transesterified product.
a
Various reaction parameters such as reaction time, temperature
and IL concentration were studied using Bronsted acidic IL
[NMP]+HSO4 as an optimized catalyst (Table 2). The results for
the transesterification of MAA with 1-butanol demonstrate that
chemical equilibrium is reached within 3 h at 80 °C (entry 1). Increase in the reaction time and temperature does not offer any significant advantage (entries 2–5). MAA was found to undergo
cyclization at higher temperature to form cyclic esters and prolonged reaction time leading to the formation of acylated esters
and ethers as a side products with a change in color of the reaction
mass (colorless to green). Lowering the temperature decreases the
conversion of MAA without affecting the selectivity performance
(entry 6). IL with various concentrations were also studied (entries
1, 7 and 8) and it was found that 5 mol% of IL is sufficient for
increasing the conversion rate and yield. The reaction was also
studied in absence of IL (entry 9) which gives 30% conversion indicating the significant role of IL as a catalyst for transesterification
reaction.
In order to have general applicability of the reaction, IL was further explored for transesterification of other b-ketoesters with a
variety of alcohols. The results were summarized in Table 3. It
was observed that IL [NMP]+HSO4 has a very high activity and
excellent conversion/selectivity were obtained. b-Ketoesters and
alcohols (1:1.2) were smoothly transesterified in high yields with
easy separation of IL and corresponding products.
A wide range of structurally varied open chain, cyclic and aromatic carboxylic esters underwent transesterification with different alcohols under mild reaction conditions. Transesterification of
MAA with higher aliphatic alcohols gave the yield of ester in the
range of 79–91% (entries 1–5); similar results were also obtained
in case of transesterification of ethyl acetoacetate EAA (entries
12–14).
Transesterification of aromatic/cyclic ester and secondary alcohols was found to be sluggish as compared to that of aliphatic esters/alcohols i.e., the transesterification of MAA and EAA gave
lower yield (entries 6–8, and 15). A series of aliphatic, cyclic and
aromatic b-ketoesters were efficiently transesterified to the desired products with good yields (entries 17–19). All the reactions
were analyzed by GC and were run till optimum conversion of bketoesters has been reached so as to have maximum yield of transeterified products. Hence, reaction time varies depending upon the
nature of the substrate. Transesterification of unsaturated alcohols
is a challenging one due to its facile decarboxylation rearrangement, i.e., Coroll rearrangement [22]. However, [NMP]+HSO4
was found to give good results in transesterification of b-ketoesters
with several unsaturated alcohols (entries 9–11 and 16).
Table 2
Influence of various parameters on transesterification of MAA with 1-butanol.
3.4. Reusability of catalyst
3.3. Influence of time, temperature and catalyst concentration
Entry
MAA: 1-butanol: IL
(mmol:mmol:mol%)
Time/
(h)
Temp./
(oC)
Conversiona
(%)
Selectivityb
(%)
Yieldc
(%)
1
2
3
4
5
6
7
8
9
5:
5:
5:
5:
5:
5:
5:
5:
5:
3
6
9
12
3
3
3
3
3
80
80
80
80
100
70
80
80
80
80
85
89
92
94
70
75
80
30
98
96
94
76
69
78
97
98
86
79
82
84
70
65
55
73
79
26
a,b,c
6.5:
6.5:
6.5:
6.5:
6.5:
6.5:
6.5:
6.5:
6.5:
5
5
5
5
5
5
2.5
10
-
Conversion, selectivity and yield are based on GC analysis.
In order to have greener protocol, recycling experiment were
conducted for Bronsted acidic IL [NMP]+HSO4. IL from the reaction
mixture was separated by adding water (2 5 mL) and the upper
organic phase was decanted. The aqueous solution containing IL
was dried in vacuum. The reactor containing the recovered IL
was then charged with MAA and 1-butanol again. The process is
followed for four successive cycles and slight decrease in the catalytic activity of IL was observed after fourth cycle Fig. 2. The slight
decrease in yield might be ascribed to the slight deactivation of
[NMP]+HSO4. Thus, [NMP]+HSO4 is found to be a recyclable catalyst for transesterification of b-ketoesters.
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Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837
Table 3
[NMP]+HSO4 catalyzed transesterification of b-ketoesters with different alcoholsa.
Entry
1
Alcohol
b-ketoester
O
O
Time (h)
Conversionb (%)
Selectivityc (%)
Yieldd (%)
3
80
98
79
OH
O
O
O
2
(CH2)3
OH
3
93
96
90
(CH2)4
OH
3
94
94
89
(CH2)5
OH
3
92
94
91
3
88
95
84
3.5
87
85
74
4
94
94
85
3.5
89
90
76
3
91
95
87
3
95
94
90
3
92
93
86
3
83
93
78
O
O
O
3
O
O
O
4
O
O
O
OH
5
O
O
O
6
OH
O
O
O
7
OH
O
O
O
8
OH
O
O
O
OH
9
O
O
O
10
OH
O
O
O
11
OH
O
O
O
OH
12
O
O
O
13
(CH2)3
OH
3
94
90
85
(CH2)5
OH
3
93
90
84
3.5
91
76
70
3
94
85
80
3.5
89
97
87
3.5
90
93
84
3.5
88
94
83
O
O
O
14
O
O
O
15
OH
O
O
O
16
OH
O
O
O
17
O
OH
O
O
19
O
O
b,c,d
OH
O
18
a
(CH2)3
O
(CH2)5
OH
Conversion, selectivity and yield are based on GC analysis.
Reaction conditions: methyl acetoacetate (5 mmol), 1-butanol (6.5 mmol), IL (5 mol%), temp. = 80 °C, all products confirmed by GC–MS.
Z.S. Qureshi et al. / Catalysis Communications 10 (2009) 833–837
Acknowledgement
100
Yield (%)
80
837
80
78
76
75
The financial assistance from Indira Gandhi Centre for Atomic
Research (IGCAR) Kalpakkam, India is kindly acknowledged.
60
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20
0
1
2
3
Recycle number
4
Fig. 2. Recyclability of catalyst: reaction conditions: MAA (5 mmol), 1-butanol
(6.5 mmol), IL (5 mol%), reaction time = 3 h, temp. = 80 °C, yield is based on GC
analysis.
4. Conclusion
In conclusion, the transesterification of b-ketoester with the
variety of alcohols is demonstrated using the IL [NMP]+HSO4 as a
novel Bronsted acid catalyst. It has several advantages; (1) The IL
[NMP]+HSO4 shows better catalytic activity and high yields, (2)
The economical aspects in the preparation of [NMP]+HSO4 makes
it more attractive, (3) The halogen free IL is more advantageous
and solvent free conditions meets the greener aspects in catalysis,
(4) [NMP]+HSO4 could be easily recycled after separation.