β-alanine-DBU: A highly efficient catalytic system for knoevenagel-doebner reaction under mild conditions

Article abstract of DOI:10.1002/cjoc.201180455

A mild and efficient Knoevenagel-Doebner reaction from malonic acid and a wide range of aldehydes was catalyzed by a catalytic system consisting of β-alanine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), affording the corresponding (E)-α,β-unsaturated carboxylic acids in good to excellent yields and with high stereoselectivity. The advantage of the method is that the reaction could proceed smoothly at ambient temperature so that it can tolerate a variety of functional groups and avoid unnecessary side reactions. Copyright

Full text of DOI:10.1002/cjoc.201180455

FULL PAPER
DOI: 10.1002/cjoc.201180455
β-Alanine-DBU: A Highly Efficient Catalytic System for
Knoevenagel-Doebner Reaction under Mild Conditions
Zhu, Lingjiana(祝令建)
Sheng, Chunquana(盛春泉)
Lei, Ningb(雷宁)
Miao, Zhenyuan*,a(缪震元)
Zhuang, Chunlina(庄春林)
Zhang, Wannian*,a(张万年)
Yao, Jianzhonga(姚建忠)
a School of Pharmacy, Second Military Medical University, Shanghai 200433, China
b Department of Quality Assurance, Lian Yun Gang JARI Pharmaceutical Co., Ltd, Lianyungang, Jiangsu 222000,
China
A mild and efficient Knoevenagel-Doebner reaction from malonic acid and a wide range of aldehydes was
catalyzed by a catalytic system consisting of β-alanine and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), affording
the corresponding (E)-α,β-unsaturated carboxylic acids in good to excellent yields and with high stereoselectivity.
The advantage of the method is that the reaction could proceed smoothly at ambient temperature so that it can toler-
ate a variety of functional groups and avoid unnecessary side reactions.
Keywords Knoevenagel-Doebner reaction, amino acids, DBU, aldehydes, catalysis
Introduction
method for Knoevenagel-Doebner reaction meeting the
requirement of moderate conditions. Therefore, the de-
velopment of a mild and efficient catalyst for this reac-
tion with cheap or commercially available reagents
would extend the application scope of Knoevenagel-
Doebner reaction in organic synthesis.
In connection with our ongoing project aiming at the
development of novel antitumor homocamptothcins
(hCPTs) containing unstable lactam rings and lactone
rings, we designed a route to introduce α,β-unsaturated
carboxylic acid to the structure of hCPT.[19] Due to the
unstable factors, the influence of various catalysts on
the reaction of formyl substituted hCPT and malonic
acid was studied under ambient temperature. The pre-
liminary experiments revealed that the combination of
β-alanine and DBU (1,8-diazabicyclo[5.4.0]undec-7-
ene) gave the most promising results.
Herein, we report the novel, simple and facile cata-
lytic approach to α,β-unsaturated carboxylic acids in
high yields by subjecting the easily accessible aldehydes
to malonic acid at ambient temperature. Firstly, to verify
the superiority of the catalyst combination, a series of
acid-base catalysts, including amino acids, simple or-
ganic bases, and their combination, were screened for
the Knoevenagel-Doebner reaction of benzaldehyde
with malonic acid in ethanol at room temperature, and
the results are depicted in Table 1.
The synthesis of α,β-unsaturated carboxylic acids
from aldehydes is a very common carbon-carbon double
bond forming reaction which is highly meaningful in
organic chemistry.[1-4] The α,β-unsaturated carboxylic
acids represent a relatively large family of organic acids,
which are important reagents in organic synthesis both
as intermediates and final products. For instance, they
have been used to prepare bioactive compounds such as
the antibacterial reutericyclin[5] or terahydromyricoid.[6]
Also, they can be found in many natural product struc-
tures.[7,8] Due to their extensive applications, the
α,β-unsaturated carboxylic acids are synthesized on a
commercial scale. Among the various methods, the
Knoevenagel-Doebner reaction is widely recognized as
the main method to construct the carbon-carbon double
bond, which is necessary to provide the α,β-unsaturated
carboxylic acids.[9,10]
However, Knoevenagel-Doebner reactions often re-
quire severe refluxing condition,[11-16] which limits their
application to the structures containing the typical reac-
tive functional groups such as alcohols, amines, lactams,
lactones, carboxylic acids, etc. It is widely accepted that
an ideal structure-oriented synthesis should consist of
skeleton-constructing reactions under facile conditions
while avoiding unnecessary functional group changes.[17]
To the best of our knowledge, except for the method
reported by Kemme et al.,[18] there is no good catalytic
*
E-mail: zhangwnk@hotmail.com or miaozhenyuan@hotmail.com; Tel.: 0086-021-81871243
Received February 11, 2011; accepted August 24, 2011.
Chin. J. Chem. 2012, 30, 139—143
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
139

Zhu et al.
FULL PAPER
Table 1 Catalyst screening for the Knoevenagel-Doebner reac-
tion
The effect of solvent on the reaction was also inves-
tigated in the presence of 100 mol% β-alanine-2 mol%
DBU under the same conditions (Table 2). We found
that the alcohols were in favor of high yields (Entries
1—3, Table 2). On the contrary, low yields were ob-
served in other organic solvents (Entries 4—7, Table 2).
Comparatively, ethanol was considered as the enjoyable
solvent for its high efficiency and easy treatment.
Table 2 Solvent screening for the Knoevenagel-Doebner reac-
tion
Entry
Catalysta
Time/h Yieldb/% for 2a Yieldb/% for 3
Entry
Solvent
EtOH
Time/h
8
Yielda/% for 2a
1
2
3
4
5
6
7
8
9
None
72
72
72
72
72
72
8
0
Trace
0
0
0
Piperidine
Pyridine
1
2
3
4
5
6
7
97
92
0
MeOH
C3H7OH
CH3CN
THF
24
β-Alanine
Glycine
40
10
25
97c
75
85
50
10
90
93
85
40
40
5
24
88
72
65
GABA
10
0
72
50
β-Alanine-DBU
Alanine-DBU
GABA-DBU
CH2Cl2
PhCH3
72
35
36
24
36
72
24
12
0
72
Trace
a Isolated yields.
0
10 Glycine-DBU
11 DBU
0
The reaction of benzaldehyde with malonic acid was
then used as the model substrate, and the loading
amount of β-alanine and DBU was assessed for their
effect on the reaction efficiency (Table 3). When the
β-alanine loading was increased from 10 mol% to 50
mol%, the yield increased from 15% to 90% and the
reaction time was also reduced greatly from 96 h to 24 h
(Entries 1—3, Table 3). At a constant β-alanine loading
of 50 mol%, the reaction efficiency could be enhanced
by a little increase of DBU (Entry 4, Table 3), while a
further increase of DBU or β-alanine did not improve
the reaction significantly (Entries 5 and 6, Table 3).
Thus, the combination of 50 mol% of β-alanine and 1
mol% of DBU was optimal to ensure high reaction effi-
ciency while maintaining a reasonable reaction time.
0
12 β-Alanine-TEA
13 β-Alanine-DABCO
0
0
14 β-Alanine-piperazine 24
15 β-Alanine-AcONa 48
0
0
a
Unless otherwise indicated, all the reactions were carried out in
ethanol and the loadings of amino acid and base were 100 mol%
and 2 mol%, respectively. Determined by HPLC. Isolated
yield.
b
c
Results and Discussion
The piperidine and pyridine,[20,21] efficient organo-
catalysts for general Knoevenagel-Doebner reactions,
showed no catalytic activity at room temperature (En-
tries 2 and 3, Table 1). For β-alanine, a reported efficient
catalyst for the Knoevenagel-Doebner reaction under
refluxing condition,[22] only moderate amount of prod-
uct was produced in EtOH (Entry 4, Table 1). However,
the combination of the commercially available amino
acids with DBU could significantly improve the cata-
lytic efficiency, affording the condensation products 2a
in moderate to excellent yields (Entries 7—10, Table 1).
Especially, when β-alanine-DBU was used as the cata-
lyst, the yield was increased remarkably to 97%. Fur-
thermore, catalyst DBU only showed weak catalytic
activity (Entry 11, Table 1). Combinations of β-alanine
with various bases, such as triethylamine (TEA),
DABCO (1,4-diazabicyclo[2.2.2]octane), piperazine,
and sodium acetate, were also screened. Most of them
were as effective as DBU (Entries 12—15, Table 1).
Therefore, the β-alanine-DBU should be an agreeable
acid-base catalyst for Knoevenagel-Doebner reaction
under mild conditions.
Table 3 The effect of catalyst loading on the Knoevenagel-
Doebner reaction
Entry β-alanine/mol% DBU/mol% Time/h Yielda/% for 2a
1
2
3
4
5
6
10
30
0.1
0.5
0.8
1
96
72
24
8
15
35
90
97
96
97
50
50
50
2
9
100
2
8
a Isolated yields.
In order to demonstrate the generality and utility of
our methodology, we designed and synthesized a series
of α,β-unsaturated carboxylic acids under the optimal
reaction conditions, namely using 50 mol% of β-alanine
and 1 mol% of DBU as catalyst, ethanol as solvent and
reaction at room temperature. As shown in Table 4, all
the reactions could proceed smoothly, affording the
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Chin. J. Chem. 2012, 30, 139—143

β-Alanine-DBU: A Highly Efficient Catalytic System for Knoevenagel-Doebner Reaction
corresponding (E)-α,β-unsaturated acids in good to ex-
cellent yields. It is obvious that the new developed
method is applicable to quite a wide scope of aldehydes,
including aromatic aldehydes with electron-donating
groups (Entry 3, Table 4) and moderate electron-with-
drawing (Entries 7—10, Table 4) and heteroaromatic
aldehydes (Entries 11 and 12, Table 4). Additionally,
many functional groups including free phenolic hy-
droxyl group (Entry 2, Table 4), acetal (Entry 4, Table
4), ester (Entry 5, Table 4) and amide (Entry 6, Table 4)
were tolerated.
Based on the experimental results (Entries 4 and 7,
Table 1), we proposed that DBU was involved in the
process of decarboxylation under these reaction condi-
tions. Hence, we reasoned that an alternative mecha-
nism which goes through two rate-limiting transition
states is possible under our present conditions as de-
picted in Scheme 1.[23] The first step is a classical Kno-
evenagel condensation which is catalyzed by β-alanine.
In this process, the aldehyde condenses with malonic
acid via the initial formation of highly electrophilic
iminium ion transition state T1 which facilitates the
formation of diacid II, while other amino acids can not
form this six-membered ring transition state. When II
was treated with DBU, a smooth decarboxylation takes
place and affords α,β-unsaturated acid III. The E
stereochemistry of the double C＝C bond of the α,β-
unsaturated acid was assigned on the basis of NMR data.
We speculated that DBU functioned as a Lewis basic
catalyst which undergoes conjugate addition to diacid II
followed by decarboxylation via the indicated transition
state T2 (Scheme 1).
Table 4 The Knoevenagel-Doebner reaction between various
aldehydes and malonic acid
Entry
Aldehyde
Yielda/%
97
1
2a
2b
2
3
4
5
85
90
92
93
2c
2d
2e
Conclusions
In conclusion, we have developed a mild and effi-
cient method for the synthesis of (E)-α,β-unsaturated
acids from various aldehydes and malonic acid cata-
lyzed by β-alanine-DBU in ethanol. Compared to the
established methods, this novel catalytic system has
several advantages, such as a wide scope of substrates,
mild reaction conditions, simple work-up, and tolerance
to many functional groups. Future studies in our group
will focus on exploring the application scope of this
method.
6
7
8
95
96
95
2f
Experimental
2g
2h
General information
All reagents and solvents were reagent grade or pu-
rified by standard methods before use. The melting
points were determined using an electrothermal appara-
1
tus and are uncorrected. H and 13C NMR spectra were
recorded at 500 MHz with a Bruker instrument, and
reported with TMS as internal standard and DMSO-d6
as solvent. TLC analysis was carried out on silica gel
plates GF254 (Qindao Haiyang Chemical, China).
9
97
98
90
91
60
2i
10
11
12
2j
General procedure for the preparation of (E)-α,β-
unsaturated acids
2k
2l
To a round bottomed flask with magnetic stirring bar,
aldehyde (0.05 mol) and β-alanine (0.025 mol, 50 mol%)
was dissolved in anhydrous EtOH (30 mL). The malonic
acid (5.7 g, 0.055 mol) followed by DBU (5×10－4 mol,
1 mol%) were added. The mixture was stirred at room
temperature for a certain period of time until the alde-
hyde was consumed. After cooling, 3 mol/L HCl (25 mL)
was poured into the reaction mixture. The precipi-
13
2m
a Isolated yields.
Chin. J. Chem. 2012, 30, 139—143
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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141

Zhu et al.
FULL PAPER
Scheme 1 A proposed mechanism for Knoevenagel-Doebner reaction catalyzed by β-alanine-DBU
tation was filtered off and washed by water and cold
ethanol to give the target compound 2 and then analyzed
by 1H NMR and 13C NMR.
Obtained in 92% yield; white solid; m.p. 243—244 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 6.07 (s, 2H), 6.34 (d,
J＝15.9 Hz, 1H), 7.50 (d, J＝15.9 Hz, 1H), 6.94 (d, J＝
8.1 Hz, 1H), 7.15 (dd, J＝1.6, 8.1 Hz, 1H), 7.36 (d, J＝
1.6 Hz, 1H), 12.21 (s, 1H); MS (ESI) m/z: 191 (M＋－1).
Anal. calcd for C10H8O4: C 62.50, H 4.20; found C
62.54, H 4.21.
Chromatogram
The analysis was performed on an Agilent-1100 LC
system (Agilent Technologies, Palo Alto, CA, USA).
The mobile phase consisted of 0.2% (V/V) acetic acid
(A) and methanol (B), respectively. The sample was
separated on a TSKgel ODS-100V C18 column (3.0 μm,
150 mm×4.6 mm I.D., TOSOH Co., Tokyo, Japan) at a
column temperature of 25 ℃ and the flow rate of 1.0
mL/min with a constant gradient: 35% B. The detection
wavelength was at 285 nm and the injection volume was
10 μL.
(E)-3-(4-Acetoxyphenyl)acrylic
acid
(2e)
Obtained in 93% yield; white solid; m.p. 206—208 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 2.08 (s, 3H), 6.53 (d,
J＝16 Hz, 1H), 6.79 (d, J＝8.6 Hz, 2H), 7.73 (d, J＝16
Hz, 1H), 7.74 (d, J＝8.6 Hz, 2H), 12.37 (s, 1H); MS
(ESI) m/z: 207 (M＋＋1). Anal. calcd for C11H10O4: C
64.07, H 4.89; found C 64.04, H 4.90.
(E)-3-(4-Acetamidophenyl)acrylic
acid
(2f)
Obtained in 95% yield; white solid; m.p. 258—259 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 2.06 (s, 3H), 6.39 (d,
J＝15.9 Hz, 1H), 7.51 (d, J＝15.9 Hz, 1H), 7.61 (m,
Ar-H, 4H), 10.11 (s, 1H), 12.24 (s, 1H); 13C NMR
(DMSO-d6, 500 MHz) δ: 24.5, 117.6, 119.3, 129.2,
129.4, 141.6, 114.0, 168.1, 169.0; MS (ESI) m/z: 206
(M＋＋1). Anal. calcd for C11H11NO3: C 64.38, H 5.40;
found C 64.35, H 5.39.
Spectroscopic data of the products 2
(E)-3-Phenylacrylic acid (2a) Obtained in 97%
1
yield; white solid; m.p. 133 ℃; H NMR (DMSO-d6,
500 MHz) δ: 6.53 (d, J＝16 Hz, 1H), 7.41—7.69 (m,
5H, Ar-H), 7.59 (d, J＝16 Hz, 1H), 12.37 (s, 1H); MS
(ESI) m/z: 149 (M＋＋1). Anal. calcd for C9H8O2: C
72.96, H 5.44; found C 72.93, H 5.42.
(E)-3-(3,4-Dichlorophenyl)acrylic
acid
(2g)
(E)-3-(4-Hydroxyphenyl)acrylic
acid
(2b)
Obtained in 96% yield; white solid; m.p. 217—219 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 6.65 (d, J＝16 Hz,
1H), 7.55 (d, J＝16 Hz, 1H), 7.78 (d, J＝8.3 Hz, 1H),
7.88 (dd, J＝1.8, 8.3 Hz, 1H), 8.06 (d, J＝1.8 Hz, 1H),
13.41 (s, 1H); MS (ESI) m/z: 215 (M＋－1). Anal. calcd
for C9H6Cl2O2: C 49.80, H 2.79; found C 49.82, H 2.79.
Obtained in 85% yield; white solid; m.p. 212—213 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 6.28 (d, J＝15.9 Hz,
1H), 6.78 (d, J＝8.6 Hz, 2H), 7.48 (d, J＝15.7 Hz, 1H),
7.51 (d, J＝8.5 Hz, 1H), 12.37 (s, 1H); MS (ESI) m/z:
165 (M＋＋1). Anal. calcd for C9H8O4: C 65.85, H 4.91;
found C 65.81, H 4.92.
(E)-3-(4-Bromophenyl)acrylic
acid
(2h)
(E)-3-(4-Methoxyphenyl)acrylic
acid
(2c)
Obtained in 95% yield; white solid; m.p. 262—264 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 6.56 (d, J＝15.6 Hz,
1H), 7.56 (d, J＝15.6 Hz, 1H), 7.61 (dd, J＝1.8 Hz, 6.7
Hz, 2H), 7.65 (dd, J＝1.7, 6.7 Hz, 2H), 12.44 (s, 1H);
MS (ESI) m/z: 225 (M－1). Anal. calcd for C9H7BrO2:
C 47.61, H 3.11; found C 47.63, H 3.10.
Obtained in 90% yield; white solid; m.p. 171—173 ℃;
1H NMR (DMSO-d6, 500 MHz) δ: 3.79 (s, 3H), 6.37 (d,
J＝15.9 Hz, 1H), 6.97 (d, J＝8.7 Hz, 2H), 7.54 (d, J＝
15.9 Hz, 1H), 7.63 (d, J＝8.7 Hz, 2H), 12.19 (s, 1H);
MS (ESI) m/z: 177 (M＋－1). Anal. calcd for C10H10O3:
C 67.41, H 5.66; found C 67.44, H 5.65.
(E)-3-(2-Bromophenyl)acrylic acid (2i) Obtained
(E)-3-(1,3-Benzodioxol-5-yl)acrylic
acid
(2d)
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 139—143

β-Alanine-DBU: A Highly Efficient Catalytic System for Knoevenagel-Doebner Reaction
1
in 97% yield; white solid; m.p. 217—218 ℃; H NMR
(DMSO-d6, 500 MHz) δ: 6.57 (d, J＝15.3 Hz, 1H), 7.37
(m, 1H), 7.43 (t, J＝7.5 Hz, 1H), 7.71 (d, J＝7.9 Hz,
1H), 7.84 (d, J＝15.3 Hz, 1H), 7.90 (d, J＝7.5 Hz, 1H),
12.37 (s, 1H); MS (ESI) m/z: 227 (M＋＋1). Anal. calcd
for C9H7BrO2: C 47.61, H 3.11; found C 47.64, H 3.10.
(E)-3-(3-Nitrophenyl)acrylic acid (2j) Obtained
Science Foundation of China (Nos. 30930107,
30973640), the Key Project of Science and Technology
of Shanghai (No. 09431901700), the Major Special
Project for the Creation of New Drugs (No.
2009ZX09301-011) and Shanghai Leading Academic
Discipline Project (No. B906).
1
in 98% yield; yellow solid; m.p. 199—201 ℃; H
References
NMR (DMSO-d6, 500 MHz) δ: 6.75 (d, J＝16.2 Hz,
1H), 7.70 (t, J＝8.1 Hz, 1H), 7.73 (d, J＝16.5 Hz, 1H),
8.18 (d, J＝8.1 Hz, 1H), 8.23 (dd, J＝2.2, 8.2 Hz, 1H),
8.52 (s, 1H), 12.60 (s, 1H); 13C NMR (DMSO-d6, 500
MHz) δ: 122.7, 123.2, 124.8, 130.7, 134.4, 136.6, 141.9,
148.7, 167.5; MS (ESI) m/z: 192 (M＋－1). Anal. calcd
for C9H7NO4: C 55.96, H 3.65; found C 55.93, H 3.66.
(E)-3-(2-Furyl)acrylic acid (2k) Obtained in 90%
yield; white solid; m.p. 141 — 143 ℃ ; 1H NMR
(DMSO-d6, 500 MHz) δ: 6.16 (d, J＝15.9 Hz, 1H), 6.62
(m, 1H), 6.92 (d, J＝3.3 Hz, 1H), 7.35 (d, J＝15.9 Hz,
1H), 7.82 (d, J＝1.1 Hz, 1H), 12.37 (s, 1H); MS (ESI)
m/z: 139 (M＋＋1). Anal. calcd for C7H6O3: C 60.87, H
4.38; found C 60.84, H 4.37.
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1
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91% yield; brown solid; m.p. 146—148 ℃; H NMR
(DMSO-d6, 500 MHz) δ: 6.17 (d, J＝15.7 Hz, 1H), 7.14
(dd, J＝3.6, 5.1 Hz, 1H), 7.50 (d, J＝3.4 Hz, 1H), 7.70
(d, J＝5.0 Hz, 1H), 7.73 (d, J＝15.7 Hz, 1H), 12.36 (s,
1H); 13C NMR (DMSO-d6, 500 MHz) δ: 117.9, 128.9,
129.9, 132.1, 137.1, 139.3, 167.6; MS (ESI) m/z: 153
(M＋－1). Anal. calcd for C7H6O2S: C 54.53, H 3.92;
found C 54.50, H 3.93.
(E)-4-Methyl-2-pentenoic acid (2m) Obtained in
60% yield; brown oil; 1H NMR (DMSO-d6, 500 MHz) δ:
1.00 (d, J＝6.5 Hz, 6H), 2.45 (m, 1H), 5.70 (m, 1H),
6.80 (m, 1H), 12.11 (s, 1H); MS (ESI) m/z: 113 (M＋－
1). Anal. calcd for C6H10O2: C 63.14, H 8.83; found C
63.18, H 8.81.
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[19] This part of the work has not been reported.
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Acknowledgement
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This work was supported by the National Natural
(E1102111 Zhao, X.; Lu, Z.)
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