Ionic liquid-catalyzed internal redox esterification reaction

Article abstract of DOI:10.1080/00397911.2011.632702

The internal redox esterification of α,β-unsaturated aldehydes and alcohols was carried out using different ionic liquids (ILs) as catalysts and reaction solvents. The basic ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim]OAc), exhibited the best activity for this reaction. The influences of the amount of ionic liquid catalyst and reaction time on yield of saturated ester have been investigated. The results showed that ionic liquid anions have a crucial effect on the redox esterification of α,β- unsaturated aldehydes and alcohols. The nucleophilic carbenes generated in situ from the ionic liquid cation were believed to be actual active species for this reactions.

Full text of DOI:10.1080/00397911.2011.632702

Synthetic Communications1, 43: 1287–1298, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.632702
IONIC LIQUID–CATALYZED INTERNAL REDOX
ESTERIFICATION REACTION
Yinyin Yu, Li Hua, Wenwen Zhu, Yu Shi, Ting Cao,
Yunxiang Qiao, and Zhenshan Hou
Key Laboratory for Advanced Materials, Research Institute of Industrial
Catalysis, East China University of Science and Technology, Shanghai, China
GRAPHICAL ABSTRACT
Abstract The internal redox esterification of a,b-unsaturated aldehydes and alcohols was
carried out using different ionic liquids (ILs) as catalysts and reaction solvents. The basic
ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim]OAc), exhibited the best
activity for this reaction. The influences of the amount of ionic liquid catalyst and reaction
time on yield of saturated ester have been investigated. The results showed that ionic liquid
anions have a crucial effect on the redox esterification of a,b-unsaturated aldehydes and
alcohols. The nucleophilic carbenes generated in situ from the ionic liquid cation were
believed to be actual active species for this reactions.
Keywords Basic ionic liquids; carbene; redox esterificartion; a,b-unsaturated aldehydes
INTRODUCTION
Metal-free-catalyzed processes are interesting alternatives to classical organic
transformations because they are often more economical and environmentally
friendly. The protonation of a,b-unsaturated aldehydes is an important method
for the formation of saturated esters starting from a,b-unsaturated aldehydes giving
=
a C C double bond and a carbonyl group, which are interesting building blocks for
the synthesis of natural products and pharmaceuticals. These reactions are usually
catalyzed by deprotonation of azolium salts such as thiazolium, imidazolium, and
Received May 16, 2011.
Address correspondence to Zhenshan Hou, Key Laboratory for Advanced Materials, Research
Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237,
China. E-mail: houzhenshan@ecust.edu.cn
1287

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Y. YU ET AL.
triazolium salts in the presence of bases such as NaOH, NaH,
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), K2CO3, MeONa, and Et3N.[1–4]
Several processes for the redox esterification reaction have been put forward,
including protonation of a nucleophilic homoenolate where the resulting activated
carbonyl unit was trapped by a nucleophile, which contributed to good to excellent
yields of a broad scope of the saturated esters by addition an excess of phenol as a
Brønsted acid.[5] The choice of the bases was found to be critical in this type of reac-
tion.[6] The weaker bases can allow protonation of the homoenolate and subsequent
generation of activated carboxylates. When diisopropylethylamine (DIPEA) was
used in the reaction of a,b-unsaturated aldehydes with an alcohol, the initially gen-
erated homoenolate was protonated more rapidly than the carbon–carbon bond
formation of the homoenolate. Additionally, the solvent polarity also played an
important role in generating homoenolate equivalents from a,b-unsaturated alde-
hydes.[7] For example, weak to middle polar solvents such as tetrahydrofuran,
(THF), CH2Cl2, and PhMe favored forming homoenolate equivalents to obtain
saturated esters.
The room-temperature ionic liquids are a kind of environmentally friendly cat-
alysts[8–10] and reagents,[11,12] because of their adjustable physical and chemical
properties, and get attention from scholars from various fields such as synthesis,[13]
catalysis, separation, and electrochemistry. Interest in basic ionic liquids is driven by
internal alkalinity, which suggests that basic ionic liquids could substitute for inor-
ganic bases. There are two main types of application of basic ionic liquids: alkaline
catalyst and specific adsorbent agent that uses the basic center to realize the separ-
ation process. Until now most research into catalytic properties of basic ionic liquids
has been focused on hydroxide-based ionic liquids ([bmim]OH), as reported by
Mehnert,[14] which catalyze many types of reaction including the Mannich reac-
tion,[15] Michael addition reaction,[16–19] Knoevenagel reaction,[20] and Markovnikov
addition reaction.[21]
In addition to the hydroxyl anion ionic liquids, carboxylate-based ionic
liquids have been recognized because the thermal and chemical stability of
carboxylate-based ionic liquids are greater than those of [bmim]OH. The basic ionic
liquid 1-butyl-3-methylimidazolium (L)-lactate ([bmim]lactate) not only catalyzed
the Adler–Diels reaction between ethyl acrylate and cyclopentadiene (in which rate
enhancements and selectivities were similar to those of reactions performed in lith-
ium perchlorate–diethyl ether mixtures) but also was separated and reused more
easily.[22] O-Acetylation of alcohols and saccharides using a dicyanamide ionic liquid
as a catalyst was a rapid, clean, and mild approach.[23] Furthermore, the basic ionic
liquid is not only an excellent solvent for a range of hydroxylated compounds but
also an effective base catalyst for O-acetylation. Lewis basic ionic liquids such as
1-butyl-4-aza-1-azaniabicyclo[2.2.2]octane bromide ([C4DABCO]Br) as the catalyst
[24]
also can bring about activation of CO2 and be a recyclable and efficient catalyst
for selective synthesis of 5-aryl-2-oxazolidinones from aziridines and CO2 without
utilization of any organic solvent or additive, and high conversion and chemo-
and regioselectivity were attained.
In this study, several neutral ionic liquids and basic ionic liquids were synthe-
sized and used in the redox esterification between a,b-unsaturated aldehydes and
alcohols. Obviously, ionic liquids are much cheaper and more stable than carbene

IONIC LIQUID CATALYSIS
1289
catalysts given by azolium salts in the presence of a strong base.[25,26] Furthermore,
basic ionic liquids can act as not only a catalyst but also a solvent and base promoter
in the present system. The results showed that by using 1-butyl-3-methylimidazolium
acetate ([bmim]OAc) as catalyst for redox esterification, high conversion of a,b-
unsaturated aldehydes and good yield of saturated ester were obtained. The reaction
mechanism is also discussed in this work.
RESULTS AND DISCUSSION
Effect of Ionic Liquid on Redox Esterification Reaction
Ionic liquids were prepared according to the procedure reported in the litera-
ture.[27] In initial efforts to evaluate different conditions, we employed cinnamalde-
hyde and ethanol as model substrates. The reactions were studied using
cinnamaldehyde (1 mmol) and ethanol (3 mmol) in ionic liquid (0.5 mmol) without
any other base additives (Scheme 1).
Table 1 shows the effect of ionic liquid catalysts on redox esterification reaction.
Ionic liquids such as 1-butyl-3-methylimidazolium acetate ([bmim]OAc), 1-butyl-3-
methylimidazolium dicyanamide ([bmim]N(CN)2), 1-butyl-3-methylimidazolium lac-
tate
([bmim]lactate),
and 1-butyl-3-methylimidazolium dimethylglycinate
([bmim]DMG) afforded greater activity than other ionic liquids studied in this work
(Table 1, entries 1–7, 9). In particular, [bmim]OAc (Table 1, entry 4) exhibited excel-
lent performance for redox esterification of a,b-unsaturated aldehyde. Nevertheless,
when we utilized the strong basic ionic liquid [bmim]OH as a catalyst, the liquid
extracts did not contain the desired products after reaction, and the high-molecular-
weight products were obtained possibly because the basicity of [bmim]OH was so
strong so that side reaction became dominant. In addition, pure imidazolium hydrox-
ide is unstable under reaction conditions unless stabilized by the presence of a proton
donor such as a protic solvent (e.g., water, methanol, etc.).[28] In contrast, [bmim]
[OAc] is a relatively stable ionic liquid,[29] suggesting that the equilibrium proposed
in Scheme 2 can be effectively tuned by appropriate choice of anions.[30]
As shown in Table 1 (entries 10 and 11), the reaction did not occur in pure
water or ethanol without ionic liquids, which indicated that the ionic liquid catalyst
was absolutely necessary for this reaction. Moreover, it can be seen that the cations
and especially anions of the ionic liquids have significant effects on redox esterifi-
cation of cinnamaldehyde. The neutral ionic liquids such as [bmim]PF6 and
[bmim]Cl did not afford any desired products (Table 1, entries 1 and 2). When the
2-position proton in imidazilium cation was substituted by the methyl group, the
reaction did not happen as well even though the anion was same (Table 1, entries
Scheme 1. Redox esterification reaction between cinnamaldehyde and ethanol.

1290
Y. YU ET AL.
Table 1. Comparison of different catalysts for reduction esterification of cinnamaldehyde with ethanol in
ionic liquidsa
Entry
Catalyst=solvent
t (h)
Conversion (%)
Selectivity (%)
1
2
[bmim]PF6
[bmim]Cl
2
2
0
0
0
0
3
[bmmim]OAc
[bmim]OAc
[bmim]N(CN)2
[bmim]Lactate
[bmim]DMG
[bmim]DMG
[bmim]OHb
H2O
2
0
0
4
2
99
50
15
17
94
—
0
100
30
100
100
100
—
0
5
2
6
2
7
2
8
12
2
9
10
11
2
Ethanol
2
0
0
aAll reactions were carried out with cinnamaldehyde and ethanol in ionic liquids. Reaction conditions:
cinnamaldehyde=ethanol ¼ 1:3 (molar radio), ionic liquids 0.5 mmol, cinnamaldehyde 1.0 mmol, T ¼ 70 ꢀC.
bHigh-molecular-weight side products were produced.
3 and 4). This probably indicates that the substrate cinnamaldehyde had a strong
interaction with the proton on the 2-position of imidazolium ring.[31] It was also
demonstrated that [bmim]OAc ionic liquid can afford greater conversion and yield
than other basic ionic liquids such as [bmim]N(CN)2, [bmim]lactate, and
[bmim]DMG (Table 1, entries 5–8). It suggests that the basicity of ionic liquids
had a significant impact on the conversion of the umpolung reaction of cinnamalde-
hyde. From previous reports,[32] we knew that the basicity of ionic liquids decreased
in the following order: [bmim]OH > [bmim]DMG > [bmim]OAc > [bmim]
N(CN)2 > [bmim]Cl > [bmim]PF6. The reason for the high reactivity of [bmim]OAc
for this reaction is that [bmim]OAc is Lewis basic ionic liquid[33] and might promote
the formation of a nucleophilic carbene catalyst in the process of redox esterification
reaction as displayed in Scheme 2. On the other hand, dimethylglycine anion could
not facilitate the carbene formation, possibly because of its steric hindrance. More-
over, strong Brønsted basic ionic liquid [bmim]OH showed no reactivity for the
redox esterification reaction. It is known that a weak base is more favorable than
a strong base for the imidazolium or triazolium salt–catalyzed umpolung reaction,
which is in good agreement with our results.[6] In summary, the results showed
Scheme 2. Possible acid–base equilibrium between the constitutive ions of a 1,3-dialkylimidazolium ionic
liquid.

IONIC LIQUID CATALYSIS
1291
Table 2. Effects of the amounts of [bmim]OAc on the reactiona
Entry
IL=sub.
Yield (%)
1
2
3
4
1:10
1:4
1:2
1:1
0
25
99
99
aAll reactions were carried out with cinnamaldehyde and ethanol in ionic
liquid. Reaction conditions: cinnamaldehyde=ethanol ¼ 1:3 (molar radio),
ionic liquids 0.5 mmol, T ¼ 70 ꢀC, t ¼ 2 h.
that [bmim]OAc was an excellent catalyst and solvent for redox esterification of
cinnamaldehyde.
The efficiency of the reaction was also affected by the added amount of
[bmim]OAc (Table 2). The conversion of cinnamaldehyde increased with the molar
ratio of [bmim]OAc to cinnamaldehyde, and the conversion of cinnamaldehyde
was obtained up to 99% when the molar ratio was 1:2, but more amounts of the
IL catalyst did not increase the yield noticeably (Table 2, entries 3, 4).
The reaction kinetics was plotted as shown in Fig. 1. The conversion of cinna-
maldehyde and the yield for the saturated ester were increased with reaction time by
using [bmim]OAc, [bmim]N(CN)2, and [bmim]DMG ionic liquids as catalysts and
reaction solvents. The conversion achieved 96% at 70 ꢀC after reaction for only 1 h
with [bmim]OAc, which was much greater than that using other basic imidazolium
ionic liquid catalysts such as [bmim]N(CN)2 and [bmim]DMG ionic liquids, indicat-
ing that the [bmim]OAc was a very efficient catalyst for this reaction. No obvious
increasing in the yield of ester was obtained when the reaction time was more than
1 h. The advantages of the ionic liquid catalyst [bmim]OAc are that it is much
Figure 1. Time profile of redox esterification between cinnamaldehyde and ethanol catalyzed by [bmim]OAc,
[bmim]N(CN)2, and [bmim]DMG. Reaction conditions: ethanol (3 mmol), cinnamaldehyde (1mmol), IL
catalyst (0.5 mmol), T ¼ 70 ꢀC.

1292
Y. YU ET AL.
cheaper and simpler than the previous reported carbene catalysts given by azolium
salts in the presence of a strong base.[5]
Survey of Different a,b-Unsaturated Aldehydes
Encouraged by these interesting results, we proceeded to examine the ability of
other aldehydes to act as substrates and react with different alcohols. As shown in
Table 3, with [bmim]OAc as the catalyst, most of the aldehydes can be converted
efficiently into the corresponding saturated ester under mild conditions.
The electron-deficient cinnamaldehyde afforded the best reactivity among the
aldehydes studied, possibly because the addition of a nucleophilic carbene catalyst
Table 3. Results of reduction esterification of a,b-unsaturated aldehydes with different alcohols using
[bmim]OAc as catalyst and solventa
t
(h)
Conversion Selectivity
(%)
Entry
1
Aldehydes
Alcohols
Product
(%)
Ethanol
1
1
4
4
97
100
2
3
4
Methanol
97
98
96
100
100
100
n-Propanol
Iso-propanol
5
6
Cyclohexanol
Ethanol
4
6
85
85
100
90
7
Ethanol
1
15
0b
aAll reactions were carried out with a,b-unsaturated aldehyde with alcohol in [bmim]OAc. Reaction
conditions: aldehyde=ethanol ¼ 1:3 (molar radio), ionic liquids 0.5 mmol, a,b-unsaturated aldehyde
1 mmol, T ¼ 70 ꢀC.
bThe isomer of citral(carane 4,5-epoxide) was detected as a side product.

IONIC LIQUID CATALYSIS
1293
to the carbonyl compound (cinnamaldehyde) became more favorable than nonaro-
matic aldehydes (Table 3, entries 1, 6, and 7), which was very similar to the previous
reports.[5–7] However, the citral was converted to its isomer (carane 4,5-epoxide)
(Table 3, entry 7), which indicated that [bmim]OAc catalyst could not activate citral
to form the appreciate intermediate. On the other hand, when the same cinnamalde-
hyde reacted with different alcohols, the reaction afforded a satisfying yield of
saturated esters, but the activity decreased with longer alkyl chains of alcohols,
which implied that the larger steric hindrance may result in lower reactivity
(Table 3, entry 5).[5]
Reaction Mechanism
It has been reported that anions with high basicity (e.g., acetate) enable proton
transfer in imidazolium-based ionic liquids, providing an imidazole-2-ylidene deriva-
tive (N-heterocyclic carbene, NHC) and an acid (e.g., acetic acid) as shown in
Scheme 1.[30] Two neutral species are either attached to each other by a hydrogen
bond (NHC–acetic acid complex) or are dissociated, and the fine-tuning of the car-
bene concentration in the imidazolium-based ionic liquids is possible by using coun-
teranions with different basicity. According to the previous investigation,[30] the
NHC–acetic acid complex could be produced in the present ionic liquid catalyst sys-
tem. As a result, aldehydes and alcohols will then promote the decomposition of
Scheme 3. Proposed mechanism to generate homoenolate equivalent from a,b-unsaturated aldehyde.

1294
Y. YU ET AL.
Table 4. Reusability results of [Bmim]OAca
Run
Conversion (%)
1
2
3
97
90
30
aAll reactions were carried out with cinnamalde-
hyde and ethanol in ionic liquids. Reaction con-
ditions: cinnamaldehyde=ethanol ¼ 1:3 (molar
radio), ionic liquids 0.5 mmol, Cinnamaldehyde
1 mmol, T ¼ 70 ꢀC, t ¼ 1 h.
NHC–acetic acid complex and thus form free carbene and acetic acid. As we all
know, the acetic acid can act as an electrophile and the nucleophile carbenes are
the actual active species for the redox esterification reaction.
On the basis of this discussion, a plausible mechanism is depicted in Scheme 3.
Reaction between the NHC produced from proton transfer in the ionic liquid and
a,b-unsaturated aldehyde gave rise to a zwitterionic structure I, which involved a for-
mal 1,2-proton shift to generate an extended enediamine intermediate. The normal
nucleophilic character of the carbonyl carbon was then extended to the a-position,
and in the presence of an electrophile (E-X, CH3COO-H) should produce enol IV.
After tautomerization of IV to activated ester V, the attack of a suitable nucleophile
and the regeneration of the nucleophilic catalyst completed the catalytic cycle.
Because the ionic liquids could be separated from the reaction mixture by add-
ing diethyl ether and reused directly as solvent and catalyst, the reusability of the
ionic liquid for the reaction was examined, and the results were shown in Table 4.
The results showed that the ionic liquid catalyst could be reused three times,
although a continuous decrease in catalytic activity was observed. The main reason
for the decreasing activity was the loss of ionic liquid in the consecutive runs.
EXPERIMENTAL
Reagents were obtained from commercial sources. Commercially available
reagents were further purified before use. Products are all known compounds
and identified by gas chromatography–mass spectrometry (GC-MS), and their
physical and spectra data were compared with those reported in the literature.
1
The H NMR (400-MHz) spectra were recorded on a Bruker Avance-500 NMR
spectrometer.
Synthesis of 1-Butyl-3-methylimidazolium Chloride [bmim]Cl and
1-Butyl-2,3-dimethylimidazolium Chloride [bmmim]Cl
[bmim]Cl was prepared and purified by using the procedure described in the
literature.[27] The reaction of 1-methylimidazole with an excess of 1-chlorobutane
was performed in ethyl acetate at 70 ꢀC for 48 h. The solid product of [bmim]Cl
was washed with ethyl acetate and then filtered. The residual solvent was removed
by rotary evaporation, and the resulting product was dried at 60 ꢀC under vacuum

IONIC LIQUID CATALYSIS
1295
for 24 h. 1H NMR (400 MHz, CDCl3): d ¼ 0.96 (t, 3H, J ¼ 7.30 Hz, CH3), 1.40
(m, 2H, CH2), 1.91 (m, 2H, CH2), 4.13 (s, 3H, CH3), 4.35 (t, 2H, J ¼ 7.30 Hz,
CH2), 7.47 (s, 1H, imid-H), 7.62 (s, 1H, imid-H), 10.67 (s, 1H, 2-imid-H).
1-Butyl-2,3-dimethylimidazolium chloride [bmmim]Cl was prepared using
similar procedures described previously, and gave a white crystal.[34] 1H NMR
(400 MHz, CDCl3): d ¼ 0.95 (t, 3H, J ¼ 7.30 Hz, CH3), 1.37 (m, 2H, CH2), 1.78
(m, 2H, CH2), 2.74 (s, 3H, CH3), 3.99 (s, 3H, CH3), 4.19 (t, 2H, J ¼ 7.40 Hz,
CH2), 7.58 (d, 1H, J ¼ 1.98 Hz, imid-H), 7.87 (d, 1H, J ¼ 1.98 Hz, imid-H).
Synthesis of 1-Butyl-3-methylimidazolium Acetate [bmim]OAc
These ILs were synthesized and purified by using the procedure described in
Refs. 34. Briefly, CH3CO2Na was added to a solution of [bmim]Cl in ethanol. The
mixture was stirred for 24 h, and the solid NaCl was removed by filtration. After
removing ethanol by evaporation under reduced pressure, the viscous liquid [bmi-
m]OAc was dissolved in CH2Cl2, filtered to remove residual salt, and then dried
under vacuum for 24 h at 60 ꢀC to afford a slightly yellow liquid. 1H NMR
(400 MHz, CDCl3): d ¼ 0.96 (t, 3H, J ¼ 7.40 Hz, CH3), 1.40 (q, 2H, CH2), 1.91 (q,
2H, CH2), 2.61 (s, 3H, CH3), 4.13 (s, 3H, CH3), 4.35 (t, 2H, J ¼ 7.40 Hz, CH2),
7.54 (s, 1H, imid-H), 7.66 (s, 1H, imid-H), 10.20 (s, 1H, 2-imid-H).
Other ionic liquids were prepared using the procedures described previously.[34]
1-Butyl-3-methylimidazolium Hexafluorophosphate [bmim]PF6
1H NMR (400 MHz, CDCl3): d ¼ 0.99 (t, 3H, J ¼ 7.30 Hz, CH3), 1.36 (m, 2H,
CH2), 1.86 (m, 2H, CH2), 3.94 (s, 3H, CH3), 4.16 (t, 2H, J ¼ 7.30 Hz, CH2), 7.10 (s,
1H, imid-H), 7.25 (s, 1H, imid-H), 8.68 (s, 1H, 2-imid-H).
1-Butyl-3-methylimidazolium Dicyanamide [bmim]DCA
1H NMR (400 MHz, CDCl3): d ¼ 0.99 (t, 3H, J ¼ 7.40 Hz, CH3), 1.41 (m, 2H,
CH2), 1.92 (m, 2H, CH2), 4.03 (s, 3H, CH3), 4.26 (t, 2H, J ¼ 7.40 Hz, CH3), 7.30 (s,
1H, imid-H), 7.45 (s, 1H, imid-H), 9.17 (s, 1H, 2-imid-H).
1-Butyl-3-methylimidazolium Lactate [bmim]Lactate
1H NMR (400 MHz, CDCl3): d ¼ 0.96 (t, 3H, J ¼ 7.40 Hz, CH3), 1.35 (d, 3H,
J ¼ 1.98 Hz, Lac-b-CH3), 1.40 (m, 2H, CH2), 1.90 (m, 2H, CH2), 4.05 (s, 3H,
NCH3), 4.17 (t, 2H, J ¼ 7.40 Hz, NCH2), 4.28 (q, 1H, Lac-a -CH), 7.49 (s, 1H,
imid-H), 7.58 (s, 1H, imid-H), 9.72 (s, 1H, 2-imid-H).
1-Butyl-3-methylimidazolium Dimethylglycine [bmim]DMG
1H NMR (400 MHz, CDCl3): d ¼ 0.95 (t, 3H, J ¼ 7.3 Hz, CH3), 1.39 (m, 2H,
CH2), 1.89 (m, 2H, CH2), 2.97 (s, 3H, CH3), 3.70 (s, 2H, CH2), 4.11 (s, 3H,
N-CH3), 4.32 (t, 2H, J ¼ 7.3 Hz, CH2), 7.45 (s, 1H, imid-H), 7.59 (s, 1H, imid-H),
9.71 (s, 1H, 2-imid-H).

1296
Y. YU ET AL.
General Procedure for Redox Esterification of Cinnamaldehyde
and Ethanol with Ionic Liquids
In a typical experiment, [bmim]OAc (0.5 mmol) was charged into a 25-ml
Schlenk flask with a magnetic stirrer and a reflux condenser followed by the addition
of cinnamaldehyde (1 mmol). The mixture was stirred for 5 min at room temperature
following addition of ethanol (3 mmol). The mixture was heated to 70 ꢀC and stirred
for 1 h. After reaction for a desired time, followed by cooling to ambient tempera-
ture, the products were isolated by liquid–liquid extract with diethyl ether. The liquid
extract of the reaction was removed and analyzed by GC-MS. The remaining cata-
lyst was washed with diethyl ether three times, the solvent was removed under vac-
uum, and the catalyst was used in the next run.
CONCLUSION
In summary, it has been demonstrated that basic ionic liquids could behave as
the efficient catalyst and solvent for the redox esterification reaction between a,b-
unsaturated aldehydes and alcohols without any additional bases. The ionic liquid
anions are found to affect the catalytic activity significantly. Lewis base ionic liquid
[bmim]OAc afford the best catalytic activity, in comparison with other ionic liquids
such as [bmim]Cl, [bmim]PF6, [bmim]N(CN)2, [bmmim]OAc, [bmim]lactate, and
[bmim]DMG. The N-heterocyclic carbenes derived in situ from 1-butyl-3-methyl-
imidazolium acetate are active species for generating homoenolate species from
a,b-unsaturated aldehydes. These nucleophilic intermediates can be protonated by
acetic acid, and the resulting activated carbonyl unit is trapped with an alcohol
nucleophile to afford a saturated ester. Further investigation of the recycling of
the ionic liquid catalyst for this reaction are being actively pursued in our laboratory.
ACKNOWLEDGMENTS
The authors are grateful for support from the National Natural Science Foun-
dation of China (No. 20773037, No. 21073058), the Research Fund for the Doctoral
Program of Higher Education of China (No. 20100074110014), and the Commission
of Science and Technology of Shanghai Municipality (No. 07J14023), China.
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