DBU-Based Dicationic Ionic Liquids Promoted Esterification Reaction of Carboxylic Acid with Primary Chloroalkane Under Mild Conditions

Article abstract of DOI:10.1007/s10562-017-2184-4

Abstract: A series of DBU-based (DBU = 1, 8-diazabicyclo [5.4.0] undec-7-ene) dicationic ionic liquids with different anions were successfully synthesized by the reaction of DBU and 1, 6-dichlorohexane. Therein, the dicationic ionic liquid 1, 1′-(hexane-1, 6-diyl)bis(1, 8-diazabicyclo [5.4.0]undec-7-enium) dichlorine (IL-1) was successfully employed as an efficient catalyst in esterification reaction of carboxylic acids with primary chloroalkanes under mild conditions without any additional basic reagents. Moreover, the optimum catalyst could be efficiently reused for five times without any significant change on the catalytic effect. The improved protocol is not only efficient, but also green and pollution-free.

Full text of DOI:10.1007/s10562-017-2184-4

Catal Lett
DOI 10.1007/s10562-017-2184-4
DBU-Based Dicationic Ionic Liquids Promoted Esterification
Reaction of Carboxylic Acid with Primary Chloroalkane Under
Mild Conditions
Yang Liu · Guijie Mao · Hongyan Zhao · Jinyi Song1,2 · Huatao Han1,2
1
,2
1
1
1,2
1,2
·
,2
1,2
Zhuofei Li · Wenyi Chu · Zhizhong Sun
Received: 12 June 2017 / Accepted: 25 August 2017
©
Springer Science+Business Media, LLC 2017
Abstract A series of DBU-based (DBU=1, 8-diazabicy-
clo [5.4.0] undec-7-ene) dicationic ionic liquids with dif-
ferent anions were successfully synthesized by the reaction
of DBU and 1, 6-dichlorohexane. Therein, the dicationic
ionic liquid 1, 1′-(hexane-1, 6-diyl)bis(1, 8-diazabicyclo
of carboxylic acids with primary chloroalkanes under mild
conditions without any additional basic reagents. Moreo-
ver, the optimum catalyst could be efficiently reused for five
times without any significant change on the catalytic effect.
The improved protocol is not only efficient, but also green
and pollution-free.
[
5.4.0]undec-7-enium) dichlorine (IL-1) was successfully
employed as an efficient catalyst in esterification reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2184-4) contains supplementary
material, which is available to authorized users.
*
Wenyi Chu
wenyichu@hlju.edu.cn
*
Zhizhong Sun
sunzz@hlju.edu.cn
1
2
School of Chemistry and Materials Science, Heilongjiang
University, Harbin 150080, People’s Republic of China
Key Laboratory of Chemical Engineering Process
and Technology for High-efficiency Conversion,
College of Heilongjiang Province, Harbin 150080,
People’s Republic of China
Vol.:(0
1
123456789)
3

Y. Liu et al.
Graphical Abstract
X
N
Cl
N
N
Cl
Cl
N
N
N
N
N
N
Cyclohexane, reflux, 8h
N
Cl
X
IL-2~4
IL-1
O
O
IL-1 /70°C
+
R'
Cl
EtOH-H O
2
R
O
R'
R
OH
20 esterification products
Keywords DBU-based dicationic ionic liquids · Halides ·
Esterification reaction · Carboxylic acid · Primary
chloroalkane
efficient catalytic activity. Compared with the monocationic
ILs, the dicationic ILs showed better designability because
different types of substrates could be used to form corre-
sponding cationic part and different functional groups could
be also introduced into the spacer [16–19], meanwhile, the
anionic moiety might be consisted of different organic and
inorganic components. So the dicationic ionic liquids exhib-
ited a better application prospect [20]. Some applications of
the dicationic ILs achieved better reaction effect in the ester-
ification of carboxylic acids with primary chloroalkanes and
the corresponding experimental results also indicated the
ionic liquids’ capability on solubility besides catalytic activ-
ity [21–24]. However, these catalytic systems still required
the aid of basic reagents. So the requirement of basic dica-
tionic ILs with high catalytic performance is obvious.
DBU as superbase was successfully synthesized into dif-
ferent monocationic type and dicationic type ionic liquids.
Various property studies proved the advantages and unique-
ness of DBU-based ionic liquids [25, 26]; meanwhile effi-
cient catalytic activity was observed in different nucleophilic
reactions as Knoevenagel condensation reaction [27], aza-
Michael addition reaction [28], etc. During the investigation
of this work, a series of DBU-based dicationic ILs were
synthesized and applied to develop a facile, efficient and
recyclable strategy for esterification of carboxylic acids and
primary chloroalkanes under mild conditions without any
additional basic reagents. The synthesized dicationic ILs not
only acted as a catalyst, but also acted as a solubilizing agent
for starting materials and products. Moreover, applicability
and recyclability of the optimum catalytic system was also
studied.
1
Introduction
Esters are widely used in chemical fields such as fragrances,
polyester, plasticizer pharmaceutical [1, 2] natural products
[
3, 4] and etc. Therefore, esterification reaction has become
one of the most frequently used reactions in organic chem-
istry. Wherein, in the presence of base, the reaction of car-
boxylic acids with halogenated alkanes is one of the main
esterification methods. Compared with bromoalkanes and
iodoalkanes, the utility of chloroalkane could reduce the
cost of reaction and achieve the objective of atom economy
besides low reaction activity. In addition, different kinds of
bases such as K CO [5], CsF [6], triethylamine (Et N) [7],
2
3
3
hexamethylenetetramine (HMTA) [8], tetramethyl- ammo-
nium hydroxide (TMAH) [9] have been tested and reported.
Although these methods have showed efficient catalytic
effect, some problems still need to be solved as environmen-
tal pollution, toxicity of base, high boiling point solvents as
well as harsh reaction conditions. Therefore, it is necessary
to develop an efficient and green strategy for esterification.
Some studies have showed that the ionic environment
from ionic liquids (ILs) is helpful for the formation of reac-
tive intermediates for different nucleophilic reactions [10,
1
1]. ILs are used as important reaction media for their vari-
ous unique properties such as high thermal stability, nonvol-
atility, low vapour pressure, miscibility and reusability [12,
1
3]. These specific properties are dependent on their struc-
ture and compositions of ions, hence ILs are also mentioned
as “designer solvents and/or catalysts” [14, 15]. The utility
of ionic liquids not only greatly reduced the use of different
hazardous and polluting organic solvents but also improved
1
3

DBU-Based Dicationic Ionic Liquids Promoted Esterification Reaction of Carboxylic Acid with…
2
Experimental
2.2 Synthesis of DBU-Based Dicationic ILs
2
.1 General
The specific synthetic procedures of DBU-based dicationic
ILs were executed according to Scheme 1 and the details
were as follows [29–31].
All reagents and chemicals purchased from Aladdin or
Alfa Company was of analytical grade and used without
purification. And 1-ethyl-1,8-diazabicyclo [5.4.0]undec-
2.2.1 Synthesis of 1, 1′‑(hexane‑1, 6‑diyl)bis(1,
8‑diazabicyclo [5.4.0]undec‑7‑enium) dichlorine
(IL‑1)
7
-enium chloride ([EtDBU]Cl), 1-butane-1,8-diazabicyclo
[
5.4.0]undec-7-enium chloride ([BuDBU]Cl), 1-hexane-
1
,8-diazabicyclo [5.4.0]undec-7-enium chloride ([HDBU]
Cl), [1,1′-(ethyl-1,4-diyl)-bis(1,8- diazabicyclo[5.4.0]
undec-7-ene)]dichlorine ([C (DBU) ]Cl), [1,1′-(butane-
DBU (68.88 mmol, 10.30 mL) and 1,6-dichlorohexane
(34.44 mmol, 5.00 mL) were charged with 40 mL cyclohex-
ane in a 100 mL two necked flask and stirred at room tem-
perature, then raised to reflux temperature for 8 h in water
bath under nitrogen atmosphere. White solid precipitate was
formed. When the reaction was over, the white solid was
collected by filtration, washed with ethyl acetate (3×10 mL)
2
2
1
,4- diyl)-bis(1,8-diazabicyclo[5.4.0]undec-7-ene)]
dichlorine([C (DBU) ]Cl),1-hexy-l-(1,8-diazabicyclo[5.4.0]
4
4
undec-7-enium)tetrafluoroborate ([HDBU]BF ), tetrabutyl
4
-
ammonium acetate ([TBA][Ac]), 1-butyl-3-methylimidazo-
lium proline ([bmim][Pro]) and 1,1′-Hexane-1,6-diylbis(3-
methylpyridinium)dichlorine ([C (Mpy) ]Cl) were obtained
and dried under vacuum oven at 50 °C for 5 h. White solid;
6
2
1
from commercial sources. The esterification reactions were
monitored by TLC (silica gel). All the NMR spectra of
synthesized ILs were characterized using D O or CDCl as
Yield 93%; H NMR (400 MHz, D O) δ = 3.61–3.53 (m,
2
4H), 3.45 (dq, J =12.7, 6.1 Hz, 12H), 2.87–2.70 (m, 4H),
2.01 (t, J = 5.9 Hz, 4H), 1.66 (dtd, J = 18.4, 14.1, 12.0,
2
3
1
3
solvent at room temperature on a Bruker Avance-400 spec-
trometer. High resolution mass spectra were recorded on
6.7 Hz, 16H), 1.38–1.28 (m, 4H); C NMR (100 MHz,
D O) δ = 166.4, 54.6, 53.4, 48.8, 46.9, 28.1, 27.84, 27.8,
2
II
−1
ThermoScientific XSeries ICP-MS mass spectrometer. The
25.6, 25.5, 22.8, 19.7; FT-IR (KBr, cm ): 2927, 2863,
elemental analysis test was accomplished on a SPECTRO
ARCOS elemental analyzer operation. And thermogravimet-
ric analyses (TGA) of synthesized ILs was accomplished on
Scinco TGA N-100 instrument with a heating rate of 10 °C/
1617, 1467; HRMS calcd for (M-2Cl)/2: 194.1777; found:
194.1781. Elem. anal. calc. (%) for C H Cl N : C, 62.73;
2
4
44
2
4
H, 9.65; Cl, 15.43; N, 12.19; found: C, 62.71; H, 9.66; Cl,
15.41; N, 12.22.
−1
min under nitrogen atmosphere. The pH values of the solu-
tions of the catalytic system were tested using METTLER
TOLEDO SevenCompact™ pH meter. All the esterification
products were purified by column chromatography and char-
2.2.2 Synthesis of 1, 1′‑(hexane‑1, 6‑diyl)bis(1,
8‑diazabicyclo[5.4.0] undec‑7‑enium) double
tetrafluoroborate (IL‑2)
1
acterized by H-NMR spectroscopy.
Sodium tetrafluoroborate (21.76 mmol, 2.39 g) was dis-
solved in 20 mL methanol and IL-1 (10.88 mmol, 5 g) was
added slowly into the flask under stirring. Then the mixture
was refluxed for 8 h and sodium bromide was filtered off.
Removed methanol on a rotary evaporator and dried the
X
N
Cl
N
N
Cl
2)-4)
1
) Cl
N
N
N
N
N
Cyclohexane, reflux, 8h
N
N
X
Cl
IL-2~4
IL-1
Reagents and conditions:
X:
ILs No.:
IL-2
2
3
4
)=NaBF , MeOH, reflux, 8h;
2)=BF
4
4
2
)=Litf N, Water, rt,12h;
3)=Tf N
IL-3
2
)=KOH, DCM, rt, 10h.
4)=OH
IL-4
Scheme 1 Synthesis of DBU-based dicationic ILs
1
3

Y. Liu et al.
obtained IL-2 under vacuum oven at 60 °C for 3 h. White
2.2.4 Synthesis of 1, 1′‑(hexane‑1, 6‑diyl)bis(1,
1
solid; Yield 87%; H NMR (400 MHz, D O) δ=3.66–3.53
8‑diazabicyclo[5.4.0]undec ‑7‑enium) double
2
(
m, 4H), 3.46 (dq, J = 12.5, 6.0 Hz, 12H), 2.88–2.72 (m,
hydroxyl (IL‑4)
4
1
H), 2.01 (t, J=5.9 Hz, 4H), 1.67 (dq, J=21.9, 8.2, 7.2 Hz,
6H), 1.42–1.26 (m, 4H); 13C NMR (100 MHz, D2O)
After dissolving IL-1 (10.88 mmol, 5 g) in 20 mL dichlo-
romethane, potassium hydroxide (21.76 mmol, 1.22 g) was
added into the solution. Then the mixture was kept stirring
for 10 h at room temperature. The ion exchange process
was completed; the mixture was filtered to remove the inor-
ganic salt. After that, the residue obtained from removing
dichloromethane was washed with ether and dried at 70 °C
under vacuum for 5 h. White paste, Yield 83%; 1H NMR
(400 MHz, D2O) δ=3.61–3.54 (m, 4H), 3.44 (dt, J=12.7,
5.0 Hz, 12H), 2.90–2.68 (m, 4H), 2.01 (d, J=5.9 Hz, 4H),
δ=166.4, 54.6, 53.4, 48.8, 46.9, 28.1, 27.9, 27.8, 25.6, 25.5,
−
1
2
2.8, 19.7; FT-IR (KBr, cm ): 2920, 2861, 1617, 1057;
HRMS calcd for (M-2BF )/2: 194.1777; found: 194.2007.
4
Elem. anal. calc. (%) for C H B F N : C, 51.27; H, 7.89;
2
4
44
2
8
4
N, 9.96; found: C, 51.26; H, 7.90; N, 9.67.
2
.2.3 Synthesis of 1, 1′‑(hexane‑1, 6‑diyl)bis(1,
‑diazabicyclo[5.4.0] undec ‑7‑enium) double
bis(triflic) imide (IL‑3)
8
1
.75–1.59 (m, 16H), 1.32 (d, J = 3.9 Hz, 4H); 13C NMR
Add bis (trifluoromethane) sulfonamide lithium salt
21.76 mmol, 6.25 g) into 30 mL deionized water. And
(100 MHz, D2O) δ = 166.4, 54.6, 53.4, 48.8, 46.9, 28.1,
−
1
(
27.8, 27.8, 25.6, 25.5, 22.8, 19.7; FT-IR (KBr, cm ): 3370,
2936, 2854, 1616; HRMS calcd for (M-2OH)/2: 194.1777;
found: 194.1777. Elem. anal. calc. (%) for C H N O : C,
IL-1 (10.88 mmol, 5 g) was added slowly into the flask.
Then the mixture was kept stirring for 12 h and new pre-
cipitate was formed. The target dicationic ionic liquid was
filtrated, washed with deionized water and dried under a
vacuum oven at 70 °C for 6 h. White solid, Yield 83%; 1H
2
4
46
4
2
68.20; H, 10.97; N, 13.26; found: C, 68.19; H, 10.98; N,
13.25.
NMR (400 MHz, CDCl ) δ=3.81–3.25 (m, 16H), 2.80 (d,
2.3 Typical Procedure for Esterification Reaction
3
J=24.4 Hz, 4H), 2.09 (d, J=19.3 Hz, 4H), 1.85–1.29 (m,
2
5
1
0H); 13C NMR (101 MHz, CDCl ) δ=166.7, 121.5, 118.3,
Carboxylic acids (1.00 mmol), primary chloroalkanes
(1.20 mmol) and IL-1 (0.30 mmol) were added respec-
tively into a two necked flask equipped with 6 mL 50%
aqueous ethanol solution under stirring, then raised the sys-
tem temperature to 70 °C for a needed time in water bath.
The progress of the reaction was monitored using thin layer
3
5.3, 54.4, 49.1, 47.1, 28.5, 28.2, 28.0, 26.0, 25.8, 22.9,
−
1
9.8; FT-IR (KBr, cm ): 2948, 2865, 1620, 617; HRMS
calcd for (M-2Tf N)/2: 194.1777; found: 194.1777. Elem.
2
anal. calc. (%) for C H F N O S : C, 35.44; H, 4.67; N,
2
8
44 12
6
8 4
8
.86; S, 13.51; found: C, 35.43; H, 4.66; N, 8.87; S, 13.52.
Table 1 Evaluation of the optimal catalysts and reaction temperature in the esterification reaction
Entry
Catalyst
Amount of catalyst (mol%)
T (°C)
Yield (%)a
1
2
3
4
5
6
7
8
9
–
–
70
70
70
70
70
70
70
70
50
60
80
–
IL-1
IL-2
IL-3
IL-4
IL-1
IL-1
IL-1
IL-1
IL-1
IL-1
30
30
30
30
10
20
40
30
30
30
92
84
86
88
68
81
92
74
86
92
1
1
0
1
Reaction conditions: benzoic acid (1.00 mmol), 1-chlorobutane (1.20 mmol), 50% aqueous ethanol solution (6 mL), reaction time 2 h
a
Isolated yields
1
3

DBU-Based Dicationic Ionic Liquids Promoted Esterification Reaction of Carboxylic Acid with…
Table 2 Evaluation of the optimum solvent system in the esterifica-
chromatography or distillation. The yields varied from 67
to 93% (Table 1).
tion reaction
Entry
Solvent
Yield (%)a
1
2
3
4
5
6
7
H O
56
64
73
81
87
92
92
2
3
Results and Discussion
10% aqueous ethanol
20% aqueous ethanol
30% aqueous ethanol
40% aqueous ethanol
50% aqueous ethanol
60% aqueous ethanol
3
.1 Catalytic Study
Initially, the esterification reaction of benzoic acid with
1
-chlorobutane was carried out as model reaction to evalu-
ate the optimal IL catalyst as well as the reaction conditions
and corresponding experimental results were summarized in
Table 1. The experimental results indicated that there was
no formation of product without catalyst (Table 1, entry 1).
And good yields were obtained with DBU-based dicationic
ILs as catalysts. It confirmed the efficient catalytic activity
and solubilization of these catalysts. Compared with these
synthesized catalysts, IL-1 seemed to be with better catalytic
activity and gave a high yield of 92% (Table 1, entries 2–5).
After the optimum catalyst was confirmed, the amount of
catalyst and the most appropriate reaction temperature was
further explored. When the amount of IL-1 was increased
from 10.0 to 40.0 mol%, a high yield of 92% was obtained at
Reaction conditions: benzoic acid (1.00 mmol), 1-chlorobutane
(
1.20 mmol), 30 mol% of IL catalyst, solvent (6 mL), reaction time
2
h at 70 °C
a
Isolated yields
chromatography (TLC). When the reaction was over, the
reactor was cooled down to room temperature. The mixture
was diluted with water (10 mL) and extracted with ethyl
acetate (3 × 5 mL), the extract was dried over anhydrous
Na SO , filtered, and concentrated in a rotary evaporator
2
4
to collect target product. Meanwhile, the reborn catalyst
could be applied in the next cycle was after removing the
solvent and dried at 80 °C under vacuum for 6 h. All the
esterification products were further purified by column
3
0.0 mol%, while a higher amount of catalyst didn’t gave a
higher yield (Table 1, entries 2, 6–8). Similarly, a high yield
was obtained at 70 °C when the reaction temperature was
Table 3 Comparison results of the optimum catalyst with other catalysts
Entry
Catalyst
Amount of catalyst (mol%)
Yield (%)a
1
2
3
4
5
6
7
8
9
IL-1
30
30
60
60
60
30
30
60
60
60
30
92
74
61
64
67
85
89
57
47
42
61
DBU
([EtDBU]Cl)
([BuDBU]Cl)
[HDBU]Cl
[C (DBU) ]Cl
2
2
[C (DBU) ]Cl
4
4
[HDBU]BF4
[TBA][Ac]
1
1
0
1
[bmim][Pro]
[C Mpy) ]Cl
6
(
2
Reaction conditions: benzoic acid (1.00 mmol), 1-chlorobutane (1.20 mmol), 50% aqueous ethanol solution (6 mL), 70 °C reaction time 2 h,
1
1
-ethyl-1,8-diazabicyclo[5.4.0]undec-7-enium chloride ([EtDBU]Cl), 1-butane-1,8-diazabicyclo[5.4.0]undec-7-enium chloride ([BuDBU]Cl),
-hexane-1,8-diazabicyclo[5.4.0] undec-7-enium chloride ([HDBU]Cl), [1,1′-(ethyl-1,4-diyl)-bis(1,8-diazabicyclo[5.4.0] undec-7-ene)]dichlo-
rine ([C2(DBU)2]Cl), [1,1′-(butane-1,4-diyl)-bis(1,8-diazabicyclo[5.4.0]undec-7-ene)] dichlorine ([C4(DBU)4]Cl), 1-hexyl-(1,8-diazabicy-
clo [5.4.0]undec-7-enium)tetrafluoroborate ([HDBU]BF4), tetrabutylammonium acetate ([TBA][Ac]), 1-butyl-3-methylimidazolium proline
(
[bmim][Pro]). and 1,1′-hexane-1,6-diylbis (3-methylpyridinium) dichlorine ([C6(Mpy)2]Cl
Isolated yields
a
1
3

Y. Liu et al.
Table 4 Esterification of carboxylic acids with chlorinated hydrocarbons
Entry
R
R′
Products
Time (h)
Yield (%)a
b
1
2
3
4
5
6
7
8
9
Ph–
Ph–
Ph–
Ph–
Ph–
Ph–
4–NO – C H –
4–NO – C H –
3–Cl– C H –
3–Cl– C H –
2–HO– C H –
3,5–NO – C H –
PhCH=CH–
PhCH=CH–
PhCH=CH–
PhCH=CH–
4–HOOC– C H –
4–HOOC– C H –
–CH3
PhCOOCH CH
2
2
2
2
2
2
2
2
94, 96 , 97c
2
3
–CH CH
PhCOO(CH ) CH
93
92
52
29
89
95
93
87
84
67
77
90
88
87
84
79
77
63
62
67
65
2
3
2 2
3
–CH CH CH
PhCOO(CH ) CH
2
2
3
2 3
3
–CH(CH3)2
–(CH3)3
Ph–
PhCOOCH CH(CH )
2
3 2
PhCOOC(CH3)3
PhCOOCH Ph
2
–CH3
4–NO – C H COOCH CH
2
6
4
2
6
4
2
3
–CH CH CH
4–NO – C H COO(CH ) CH
3
2
6
4
2
2
3
2
6
4
2 2
–CH3
3–Cl– C H COOCH CH
3
2
2
6
4
6
4
2
10
11
12
13
14
15
16
17
18
19
20
21
22
–CH CH CH
3–Cl– C H COO(CH ) CH
3
6
4
2
2
3
3
6
4
2 3
–CH CH
2–HO– C H COO(CH ) CH
2.5
2.2
2.2
2.2
2.2
2.2
3
3
4
4
3.8
4.2
6
4
2
6
4
2 2
3
–CH3
–CH3
3,5–NO – C H COOCH CH
2
6
3
2 6 3 2 3
PhCH=CHCOOCH CH
2 3
PhCH=CHCOO(CH ) CH
PhCH=CHCOO(CH ) CH
PhCH=CHCOOCH Ph
–CH CH
2
3
2 2
3
3
–(CH ) CH
2
2
3
2 3
Ph–
–CH3
2
d
d
e
e
1,4– C H (COOCH CH )
6
4
6
4
2
3 2
–CH CH CH
1,4– C H (COOCH CH CH )
3 2
6
4
2
2
3
6
4
2
2
Ph–
Ph–
–CH3
HOOCCH2–
–CH Cl
PhCOO(CH ) OCOPh
2
2 2
–(CH ) Cl
PhCOO(CH ) OCOPh
2
5
2 6
–CH CH CH
CH COO(CH ) CH
2
2
3
3
2 3
3
d
–CH3
CH (COOCH CH )
2 2 3 2
All reactions were carried out on carboxylic acid (1.00 mmol), primary chloroalkane (1.20 mmol), IL-1 (0.30 mmol), 50% aqueous ethanol solu-
tion (6 mL) at 70 °C
a
Isolated yields
The yield from bromoethane
The yield from iodoethane
The scale of chloroethane and chlorobutane was 2.20 mmol
b
c
d
e
The scale of benzoic acid was 2.00 mmol
9
8
8
7
7
6
5
5
4
4
8
2
6
0
4
8
2
6
raised from 50 to 80 °C, and the yield had no increase at a
higher temperature (Table 1, entries 2, 9–11).
Following, the optimum solvent system was investigated
and the corresponding results were shown in Table 2. It
could be observed that there was a significant increase on
yield with addition of ethanol (Table 2, entries 1–7). A high
yield of 92% was obtained in 50% aqueous ethanol solution,
while the yield had no further increase in higher content of
aqueous ethanol (Table 2, entries 6 and 7).
40
1
2
3
4
5
Run
Under optimum reaction conditions, some other catalysts
were screened and the corresponding experimental results
were summarized in Table 3. It could be found that the recy-
clable catalyst IL-1 has shown much better catalytic activity
than DBU (Table 3, entries 1 and 2). Then, different type
Fig. 1 Recyclability of the optimum catalytic system
1
3

DBU-Based Dicationic Ionic Liquids Promoted Esterification Reaction of Carboxylic Acid with…
Table 5 The pH values
Recycle time
pHa
0
1
2
3
4
5
of every time in recycle
experiments
8.59
8.59
8.59
8.57
8.45
8.21
Reaction conditions: benzoic acid (1.00 mmol), 1-chlorobutane (1.20 mmol), 50% aqueous ethanol solution
(
6 mL), 70 °C reaction time 2 h
pH values were determined at 25 °C with the solution system containing IL-1 of different recycle time
a
DBU-based ionic liquids catalysts were tested and the results
confirmed a high catalytic performance of IL-1 (Table 3,
entries 1, 3–8). Meanwhile, the dicationic ionic liquids have
showed better catalytic ability than monocationic ones.
Moreover, some other traditional basic ionic liquids were
found to be inferior to DBU-based ionic liquids on nucleo-
philic reaction especially esterification reaction (Table 3,
entries 1, 9–11).
Thence, the protocol showed good catalytic activity for the
esterification of different carboxylic acids and primary chlo-
roalkanes under optimum reaction conditions.
3.2 Recyclability Test of the Optimum Catalytic System
A good recyclability of the catalytic system could not only
make the reaction process much greener, but also realize
economic feasibility and sustainable development. The
recyclability test was studied using the model esterification
reaction of benzoic acid with 1-chlorobutane under optimum
conditions. When the reaction was over, the mixture was
diluted with water (10 mL) and extracted with ethyl acetate
(3×5 mL). And the catalyst was recovered after removing
the solvent and dried at 80 °C under vacuum for 6 h. Then
it could be treated as reborn catalyst and used in next run.
As shown in Fig. 1, the yield of target product under opti-
mum conditions didn’t show significant change even after
five runs, which indicated that the catalyst had almost no
loss and the catalytic effect was less affected in recyclable
catalytic system. Therefore, IL-1 could be used as a stable
and efficient catalyst for the esterification of the carboxylic
acid and primary chloroalkane.
Since the optimum reaction conditions were determined,
a variety of aromatic or aliphatic acids with different pri-
mary chloroalkanes were chosen as substrates for further
study and the experimental results were presented in Table 4.
Primarily, the experiments of aromatic acids with different
substituents were executed. The reactions of benzoic acid
gave good yields, whereas there was only a slight decrease
of the yield with an increase in the length of chain alkanes
(
Table 4, entries 1–4). Compared to iodoethane (97%) and
bromoethane (96%), chloroethane gave a slightly low yield
(
94%) (Table 4, entry 1). While compared to n-chlorobu-
tane (92%), the yields of sec-butyl chloride and tert-butyl
chloride decreased obviously to 52% and 29%, respectively
(
Table 4, entries 3–5). The esterification reactions of sub-
stituted benzoic acids with –NO , –OH and –Cl were also
2
executed (Table 4, entries 7–12). It could be found the yields
of nitrobenzoate (95%) and butyl nitrobenzoate (93%) were
higher than the corresponding benzoic acid esterification
products because of the presence of electron withdrawing
groups (Table 4, entries 7, 8). Meanwhile, relatively lower
yields of 3-chloro benzoic acid esterification products were
obtained and the yields of ethyl chlorobenzoate and butyl
chlorobenzoate were 87% and 84%, respectively (Table 4,
entries 9, 10). Similar yields of 2-hydroxy and 3, 5-dini-
tro benzoic acid esterification products were obtained in
Moreover, the pH values of the recycle experiments were
investigated and the results indicated pH value didn’t change
too much during the recyclability test (Table 5).
4 Conclusions
In conclusion, several DBU-based dicationic ILs with dif-
ferent anions were successfully synthesized and screened
catalytic activity in esterification reaction of carboxylic acids
with primary chloroalkanes under mild conditions without
addition of any basic additives. The optimal protocol using
IL-1 as catalyst showed good catalytic effect with various
substrates compared to traditional methods, and the after-
treatment was simple. The catalyst was easy to be recycled,
and the yields did not change significantly after reusing for
five times.
6
7 and 77%, respectively (Table 4, entries 11, 12). Then,
good catalytic activity was also demonstrated in the esteri-
fication of cinnamic acid and the yields of two terephthalic
acid esterification products were 79 and 77%, respectively
(
Table 4, entries 13–18). Besides, the reactions of benzoic
acid with 1, 2-dichloroethane and 1, 6-dichlorohexane were
proceed smoothly, and the desired diesters were obtained
in the yields of 63 and 62%, respectively (Table 4, entries
Acknowledgements We are grateful for the financial support from
the Research Project of the Natural Science Foundation of Heilongjiang
Province of China (No. B201208).
1
9 and 20). Similarly, the aliphatic esterification products
butyl acetate (67%) and diethyl malonate (65%) were also
achieved in moderate yields (Table 4, entries 21 and 22).
1
3

Y. Liu et al.
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