Aromatic compounds Mannich reaction using economical acidic ionic liquids based on morpholinium salts as dual solvent-catalysts

Article abstract of DOI:10.1080/00397910903470509

Economical acidic ionic liquids containing an alkanesulfonic acid group in a morpholinium cation were found to be effective catalysts for the Mannich reactions of various kinds of aromatic aldehydes, acetophenone and aromatic amines at mild reaction conditions. The satisfactory results were obtained with short reaction time, good yields of the Mannich bases and simplicity in the experimental procedure. These ionic liquids could be recycled and reused for up to five times without considerable decreasing in the catalytic activity. Copyright

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Aromatic Compounds Mannich Reaction
Using Economical Acidic Ionic Liquids
Based on Morpholinium Salts as Dual
Solvent-Catalysts
Caibo Yue a
a School of Chemistry and Chemical Engineering, Anhui University of
Technology, Maanshan, P.R. China
Published online: 09 Nov 2010.
To cite this article: Caibo Yue (2010): Aromatic Compounds Mannich Reaction Using Economical Acidic
Ionic Liquids Based on Morpholinium Salts as Dual Solvent-Catalysts, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:24, 3640-3647
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Synthetic Communications1, 40: 3640–3647, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903470509
AROMATIC COMPOUNDS MANNICH REACTION
USING ECONOMICAL ACIDIC IONIC LIQUIDS BASED
ON MORPHOLINIUM SALTS AS DUAL
SOLVENT-CATALYSTS
Caibo Yue
School of Chemistry and Chemical Engineering, Anhui University of
Technology, Maanshan, P.R. China
Economical acidic ionic liquids containing an alkanesulfonic acid group in a morpholinium
cation were found to be effective catalysts for the Mannich reactions of various kinds of
aromatic aldehydes, acetophenone and aromatic amines at mild reaction conditions. The
satisfactory results were obtained with short reaction time, good yields of the Mannich
bases and simplicity in the experimental procedure. These ionic liquids could be recycled
and reused for up to five times without considerable decreasing in the catalytic activity.
Keywords: Catalysts; economical ionic liquids; Mannich reaction; solvent-free conditions
INTRODUCTION
Mannich reaction is one of the most classical and important carbon-carbon
bond-forming reactions in organic synthesis for the preparation of ß-amino carbonyl
compounds, which in turn are important synthetic intermediates for various phar-
maceuticals and natural products.[1–3] The conventional catalysts for the classical
three-component Mannich reaction of ketones, aldehydes and amines involve mainly
mineral and organic acids like proline,[4] p-dodecylbenzenesulfonic acid,[5] HBF4,[6,7]
and some Lewis acids,[8–10] which often suffer from the disadvantages of harsh reac-
tion conditions, toxicity and difficulty, long reaction times in the process. Nowadays,
some new synthetic methods for the Mannich reaction have been developed to
improve and modify this reaction by microwave[11] or ultrasound irradiation.[12]
Meanwhile, it has been noted that this reaction could also be promoted by a
wide array of catalysts, such as rare earth perfluorooctane,[13–15] ionic liquids,[16–19]
salts,[20–22] proline,[23–25] and others.[26–28] However, most of these catalysts also have
many disadvantages, that is expensive, requiring atmosphere sensitive reagents, regu-
latory constraints and so on. As a result, development of the cheap, green catalysts
and methodologies remain an ever challenging objective.
Received April 29, 2009.
Address correspondence to Dr. Caibo Yue, School of Chemistry and Chemical Engineering, Anhui
University of Technology, Maanshan 243002, China. E-mail: yuecaibo@eyou.com
3640

MANNICH REACTIONS OF AROMATIC COMPOUNDS
3641
In recent years, with increasing environmental concerns and the regulatory
constraints faced in the industry, more attention has been paid to the synthesis of
task-specific ionic liquids, which have functional groups and their applications acted
as both catalyst and solvent in chemical research.[29–34] However, traditional
task-specific ionic liquids with the cation of imidazole are expensive, which hinders
their applications in the chemical industry. Therefore, the synthesis and application
of economical task-specific ionic liquids are still in demand. In our previous work, in
order to extend the scope and decrease the cost of ionic liquids, some relatively
economical task-specific ionic liquids have been synthesized and their applications
in Knoevenagel condensation reaction, esterification reactions have also been inves-
tigated, respectively.[35,36]
Herein, we have discovered that some cheap Brønsted acidic ionic liquids that
bear an SO3H group in a morpholinium cation, and subsequently used them as cata-
lyst and solvent for the one-pot three-component Mannich reaction of aromatic
aldehydes, aromatic amines and acetophenone under relatively mild conditions.
RESULTS AND DISCUSSION
The Mannich reaction among benzaldehyde, acetophenone and aniline was
chosen as a model reaction to test the catalytic acitivities of these three brønsted
acidic ionic liquids, [MPA][TFA], [MPA][HSO4] and [MMPA][PTSA] and no other
solvents were added as initial explorations. The results are summarized in Table 1. It
was demonstrated that no ß-Amino carbonyl compound was observed in the absence
Table 1. Mannich reaction of aromatic aldehydes, acetophenone and aromatic amines catalyzed by acidic
ionic liquids
Entry
R1
R2
Ionic liquid (mol%)
None
[MPA][HSO4]
Time (h)
Yields (%)
1
2
H
H
12
3
0
No reaction
H
H
3
H
H
[MPPA][CF3COO] (25)
[MMPPA][PTSA] (25)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (5)
[MPPA][CF3COO] (10)
[MPPA][CF3COO] (15)
[MPPA][CF3COO] (20)
[MPPA][CF3COO] (30)
[MPPA][CF3COO] (50)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (25)
[MPPA][CF3COO] (25)
2
89
78
82
67
75
82
86
87
87
91
93
62
93
92
70
4
H
H
2
5a
6
H
H
2
H
H
4
7
H
H
H
H
3.5
2.5
2
8
9
H
H
10
11
12
13
14b
15
16
17
H
H
2
H
H
2
p-CH3
p-CH3O
p-NO2
H
H
1.5
1
H
H
5
p-CH3O
p-CH3
p-NO2
1.5
2
H
H
6
Reaction condition: 10 mmol aromatic benzaldehyde, 10 mmol acetophenone and 10 mmol aromatic
amine; temperature ¼ 25 ꢀC. Two drops of water (3.33 mmol) was added to the reaction mixtures.
aTwo drops of water was not added.
b15 mmol acetophenone and 15 mmol aniline was added to stir well.

3642
C. YUE
of ionic liquids used as dual solvent and catalyst. [MPA][TFA] or [MMPA][PTSA]
could catalyze this reaction. However, [MPA][HSO4] reacted readily with aniline
due to its strong acidity and no Mannich product was obtained (entry 2, Table 1).
In addition, a small amount of water could promote the yield of product in the
Mannich reaction catalyzed by these two acidic ionic liquids. This experimental
phenomenon was in accord with the result from the literature.[17] On comparing
the results obtained by using [MPA][TFA] and [MMPA][PTSA], it is observed that
the catalytic activity of [MPA][TFA] is higer than that of [MMPA][PTSA].
To optimize the reaction conditions, we examined the effect of the amount of
acidic ionic liquid on yield of the Mannich base using the Mannich reaction of ben-
zaldehyde, acetophenone and aniline catalyzed by [MPA][TFA] as a model reaction.
From the data of Table 1, we can see that the amount of the acidic ionic liquid
affected the yield of product significantly. The yield of the product increased with
the amount of [MPA][TFA] when ionic liquid was not enough in the reactor, but
did not change considerably as the amount of [MPA][TFA] more than 25 mol%.
The main reason could be that the acidic active sites increase with the amount of
[MPA][TFA], which enhances the reaction rate. The optimized conditions were
extended to a series of aromatic aldehyde acceptors and aromatic amines to explore
the generality of this catalytic system and the results are also summarized in Table 1.
In most cases, the Mannich bases were obtained in good yields. However, when one
of three-component was an aliphatic compound, such as propionaldehyde, butyral-
dehyde, butanone, acetone, propylamine and ethylamine, the Mannich reaction did
not take place in the presence of the catalyst even if the reaction time was prolonged.
Furthermore, the yield of the product was very low when cyclohexanone was using
as a ketone donor.
As can also be known from Table 1, the substitutional group of the reactants
was crucial to the yield of Mannich bases. So far as aromatic aldehydes were con-
cerned, the yield of Mannich base for the Mannich reaction using p-methylbenzalde-
hyde as a aldehyde donor was higher than that using benzaldehyde, but lower than
that using p-methoxybenzaldehyde (entries 12, 13, and 14, Table 1). Moreover, in
case of aromatic amines, it is noteworthy that the reactivity order of aromatic amines
was p-methoxyaniline > p-methylaniline > aniline > p-nitroaniline (entries 15, 16,
and 17, Table 1). The main reason could be that the exist of electron-donating group
could be favorable to increasing its nucleophilic ability which determined the
Mannich reaction speed. Unfortunately, the reaction using cyclohexyl amine or
Table 2. Reusing of the acidic ionic liquid [MPA][TFA]a
Entry
Run
Yields (%)
1
2
3
4
5
6
Fresh
89
86
85
87
84
86
1
2
3
4
5
a10 mmol benzaldehyde, 10 mmol aniline, and 10 mmol aceto-
phenone, 2.5 mmol [MPA][TFA], temperature ¼ 25 ꢀC, time ¼ 2 h.

MANNICH REACTIONS OF AROMATIC COMPOUNDS
3643
2,4-xylidene as an amine compount did not take place even after prolonging the reac-
tion time up to 24 h under this optimized conditions as examined through TLC.
The recycling of the acidic ionic liquid, [MPA][TFA] was also investigated. In
the model reaction among benzaldehyde, acetophenone and aniline, the results listed
in Table 2 showed that the [MPA][TFA] could be reused for five times without
lowering of the catalytic activity. Meanwhile, no large difference was observed
compared the IR of the reused ionic liquid with that of fresh ionic liquid. Therefore,
the structure of the ionic liquid did not change during the process of the Mannich
reaction.
CONCLUSION
In summary, an efficient protocol for Mannich reaction of aromatic aldehydes,
aromatic amines and aromatic ketones used economical acidic ionic liquids based on
morpholinium salts as potentical catalysts was developed. This method avoids the
use of acid or basic catalysts and environmentally unfavorable volatile organic sol-
vents (VOCs). The most attractive part of this work is that the cation was the cheap
morpholinium sultone, not triphenyl phosphonium sultone or immidazolium sul-
tone. Therefore, we believe that this work would have potential application in green
chemical industry.
EXPERIMENTAL
1H NMR were measured on a Bruker DRX500 (500 MHz) spectrometer in
D2O with TMS as internal standard. The IR spectra were run on a Nicolete
spectrometer (KBr). Melting points were taken on a hot-plate microscope appara-
tus and were not corrected. All yields refer to isolated products after purification.
All chemicals (AR grade) were commercially available and used without further
purification.
Synthesis of Morpholinium Propane Sulfonate (MPS) and
N-Methylmorpholinium Propane Sulfonate (MMPS)
To a solution of morpholine or N-methylmorpholine (0.10 mol) in anhydrous
acetone was added 1,3-propane-sulfone (0.10 mol) in portions within 30 min, and
then the mixture was stirred for 3 h at room temperature (25 ꢀC). A white precipitate
thus was formed, and subsequently filtered and washed with anhydrous acetone at
0 ꢀC. The white solid product was dried in vacuo at 60 ꢀC for 48 h to give morpholi-
nium propane sulfonate or N-methylmorpholinium propane sulfonate. The yield of
morpholinium propane sulfonate was 96%, m.p. 236–238 ꢀC and the yield of
N-methylmorpholinium propane sulfonate was 87%, m.p. > 280 ꢀC. MPS:
1H NMR (500 MHz, D2O): d ¼ 4.12 (s, 1H, -NH), 3.72–4.00 (m, 4H, -CH2-
O-CH2-), 3.32–3.54 ðm; 2H; ꢁCH2ꢁSOꢁ3 Þ, 3.12–3.26 (m, 4H, -CH2-Nþ-CH2-),
2.75–2.92 (t, 2H, -Nþ-CH2-), 2.06–2.12 (m, 2H, -CH2-), MMPS: 1H NMR
(500 MHz, D2O): d ¼ 3.95 (m, 4H, -CH2-O-CH2-), 3.52–3.55 (m, 2H,
ꢁCH2ꢁSOꢁ3 ), 3.40–3.48 (m, 4H, -CH2-Nþ-CH2-), 3.13 (s, 3H, -CH3), 2.89–2.91
(t, 2H, -Nþ-CH2-), 2.14–2.17 (m, 2H, -CH2-).

3644
C. YUE
Scheme 1. Synthesis approach of MPS and MMPS.
Synthesis of Acidic Ionic Liquids, [MPA][TFA](Morpholinium
Propanesulfonic Acid Ammonium Trifluoroacetate),
[MPA][HSO4](Morpholinium Propanesulfonic Acid Ammonium
Hydrogen Sulfate), and [MMPA][PTSA](N-Methylmorpholinium
Propanesulfonic Acid Ammonium P-Toluenesulfonate)37
Equal molar of MPS or MMPS and sulfuric acid (96%), trifluoroacetic acid or
fluoboric acid (40%) were mixed and stirred at specific temperature for specific time.
Then the combined solution was evaporated in vacuo at 85 ꢀC for 2 h to remove the
water. The product was washed with diethylꢁether three times and then dried under
vacuum at 60 ꢀC for 48 h. The [MPA][X] (X ¼HSO4ꢁ CF3COOꢁ) and [MMPA][X]
ꢁ
=
(X PTSA ) were obtained in 89%, 84%, and 86%, respectively. Since these brønsted
acidic ionic liquids were synthesized in accordance with the procedures previously
reported in the literature, no further characterization was done.
Typical Procedure for Mannich Reaction Among Aromatic
Aldehydes, Acetophenone, and Aromatic Amines
In a typical reaction, to a mixture of aromatic aldehyde (10 mmol), acetophe-
none (10 mmol) and aromatic amine (10 mmol) was added acidic ionic liquids
(2.5 mmol) as catalyst and solvent. The mixture were stirred at room temperature
(25 ꢀC) in a 50 ml round-bottomed flask equipped with a magnetic stirrer. After a
certain time the reaction mixture became viscous and close to solid. Then the solid
was washed with water (5 ml ꢂ 3) and recrystallized from anhydrous ethanol. The
products was vacuum dried for 6 h at 50 ꢀC and identified by 1H NMR and physical
data (m.p.) with those reported in the open literature. The unreacted starting
Scheme 2. Synthesis of economic ionic liquids.

MANNICH REACTIONS OF AROMATIC COMPOUNDS
3645
Scheme 3. Mannich reactions in ionic liquids.
materials in the filtrate were separated from the ionic liquids by extracting with ether.
The left water layer containing ionic liquids was vacuum dried at 80 ꢀC for 4 h to
remove water and reused in the next run.
ACKNOWLEDGMENTS
The author gratefully acknowledges SRTP of Anhui University of Technology
(project no. 08028); the Key Project of the Scientific Research Foundation sponsored
by the Education Department of Anhui Province, China (project no. KJ2010A339);
and China Postdoctoral Science Foundation funded project (project no.
20100470153) for financial support.
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