A basic ionic liquid as catalyst and reaction medium: A rapid and simple procedure for Aza-Michael addition reactions

Article abstract of DOI:10.1002/ejoc.200600999

A fast, mild, and quantitative procedure for Michael addition reactions between various amines and α,β-unsaturated carbonyl compounds and nitriles in the presence of an easily accessible basic ionic liquid - 3-butyl-1-methylimidazolium hydroxide, [bmIm]OH - as both catalyst and reaction medium has been developed. For large-scale reactions the products could be directly distilled from the ionic liquid, allowing the use of organic solvents to be avoided totally. The ionic liquid could be reused at least eight times with consistent activity and was stable during the reaction process. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600999

FULL PAPER
DOI: 10.1002/ejoc.200600999
A Basic Ionic Liquid as Catalyst and Reaction Medium: A Rapid and Simple
Procedure for Aza-Michael Addition Reactions
[a]
[a]
[a]
[a]
[a]
Jian-Ming Xu, Qi Wu, Qing-Yi Zhang, Fu Zhang, and Xian-Fu Lin*
Keywords: Michael addition / Ionic liquids
A fast, mild, and quantitative procedure for Michael addition
ucts could be directly distilled from the ionic liquid, allowing
reactions between various amines and α,β-unsaturated car- the use of organic solvents to be avoided totally. The ionic
bonyl compounds and nitriles in the presence of an easily
accessible basic ionic liquid – 3-butyl-1-methylimidazolium
hydroxide, [bmIm]OH – as both catalyst and reaction me-
dium has been developed. For large-scale reactions the prod-
liquid could be reused at least eight times with consistent
activity and was stable during the reaction process.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
[
6a,6b]
Introduction
aluminate, as a catalyst in Friedel–Crafts acylations,
a
[
7]
number of ionic liquids containing stable acidic or neutral
The aza-Michael reaction is an important reaction in or-
ganic chemistry, especially for the synthesis of C-N hetero-
[
8]
anions have been developed and successfully applied to
catalyze many types of reactions. Relevant reports about
ionic liquids containing stable basic anions, however, were
relatively scarce: supported choline hydroxide has been used
as a basic catalyst for aldol condensation reactions between
several ketones and aldehydes, while the basic ionic liquid
bmIm]OH has been successfully applied to catalyze
Michael additions of active methylene compounds to conju-
[
1]
cycles
containing the β-aminocarbonyl functionality,
which not only constitutes a component of biologically
active natural products but also serves as an essential inter-
mediate in the synthesis of β-amino ketones, β-amino acids,
and β-lactam antibiotics.[ Such aza-Michael reactions have
generally been promoted by harsh bases or strong acids,
which give rise to environmentally hazardous residues and
undesirable by-products.[ With the goal of avoiding the
typical disadvantages resulting from the presence of such
catalysts, a large number of alternative procedures have
been developed in the past few years. Unfortunately,
though, many of these procedures require long reaction
times, rigorous reaction conditions, large excesses of rea-
gents, and/or highly damaging chemicals. In many cases the
catalyst and excess reagents have not been recoverable,
which has strongly limited industrial applications. The de-
velopment of an environmentally benign and simple pro-
cedure for aza-Michael addition has thus become particu-
larly fascinating and remains a great challenge.
[
9]
2]
[
[
10]
gated ketones, carboxylic esters, and nitriles.
We found
3]
that [bmIm]OH could also be used as an efficient catalyst
for Markovnikov additions of N-heterocycles and vinyl es-
ters, while Ranu and co-workers have recently employed
it in Knoevenagel condensations of aliphatic and aromatic
carbonyl compounds. In view of these elegant discoveries
of catalytic activity in basic ionic liquids, we envisaged that
[
11]
[4]
[
12]
Room temperature ionic liquids have attracted increasing
attention as the green, high-tech reaction media of the fu-
[
5]
ture, while it has also been reported that ionic liquids con-
taining imidazolium cations can act as powerful media in
some catalytic organic reactions not only for the facilitation
of catalyst recovery but also for the acceleration of the reac-
tion rate and improvement of selectivity.[6] Since the first
successful use of an ionic liquid, dialkylimidazolium chloro-
[
a] Department of Chemistry, Zhejiang University
Hangzhou 310027, People’s Republic of China
Fax: +86-571-87952618
E-mail: llc123@css.zju.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Scheme 1. Michael additions between amines and α,β-unsaturated
carbonyl compounds.
1798
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 1798–1802

Basic Ionic Liquid as Catalyst and Reaction Medium
FULL PAPER
basic ionic liquids might also catalyze aza-Michael ad- smoothly in [bmIm]OH without the need for any other cat-
ditions efficiently, and here we report for the first time the alyst, to provide the corresponding 1,4 adducts in high
use of [bmIm]OH both as a novel and recyclable reaction yield. The products obtained could be isolated by simple
medium and as an efficient catalyst for Michael additions extraction with diethyl ether, and the ionic liquid could be
between amines and α,β-unsaturated compounds to afford further washed with diethyl ether and reused several times
the corresponding 1,4-adducts in high yields at room tem- without further purification. All the Michael adducts ob-
1
13
perature (Scheme 1).
tained were characterized by IR, H NMR, and C NMR
spectroscopy. Interestingly, no by-product generated from
1,2-addition, polymerization, hydrolytic reaction, or any
other reaction was detected by TLC and GC-MS.
Results and Discussion
Basic ionic liquids were synthesized by anion exchange Table 2. [bmIm]OH-Catalyzed Michael reactions between amines
with the corresponding imidazolium chlorides by the litera- and α,β-carbonyl compounds or nitriles.[
a]
[13]
ture methods, with a slight modification, and were char-
1
13
acterized by IR, H NMR, and C NMR spectroscopy. We
first compared the catalytic effects of different ionic liquids
for aza-Michael addition reactions, and the results are sum-
marized in Table 1. When the basic ionic liquid [bmIm]OH
was used to catalyze the reaction between piperidine and
methyl acrylate, a quantitative yield was obtained in 10 min
(Table 1, Entry 1), but only moderate yields were obtained
for some neutral ionic liquids such as 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([bmIm]PF ) and 1-bu-
6
tyl-3-methylimidazolium tetrafluoroborate ([bmIm]BF4)
(Table 1, Entries 4 and 5). The result was quite interesting,
showing that ionic liquids with longer cation carbon chain
lengths exhibited relatively higher Michael addition activity
(Table 1, Entries 1 and 2). Some blank experiments to dem-
onstrate the catalytic capabilities of [bmIm]OH were also
carried out. When the reaction was carried out in some mo-
lecular solvents such as THF and DMSO in the absence
of [bmIm]OH, it was observed that the addition reaction
between piperidine and methyl acrylate produced the corre-
sponding product in less than 30% yield in 48 h (Table 1,
Entries 6 and 7). Interestingly, when a small amount (5%)
of [bmIm]OH was added to neutral ionic liquids such as
[
bmIm]BF and [bmIm]PF , the reaction could then also
4 6
achieve 93% and 91% yields, respectively. All these results
showed that the ionic liquid [bmIm]OH was playing a sig-
nificant catalyst role during the reaction process, as well as
serving as the reaction medium.
Table 1. Aza-Michael reaction between piperidine and methyl
acrylate under different reaction conditions.[
a]
Entry
Solvent
Time
Yield [%][b]
1
2
3
4
5
6
7
[bmIm]OH
[emIm]OH
[bdmim]OH
10 min
10 min
10 min
10 min
10 min
2 d
98
94
95
60
55
15
30
[
a] Reaction conditions: amines (1 mmol), α,β-unsaturated com-
pounds (1.2 mmol) in [bmIm]OH (1 mL) at 25 °C. [b] Isolated
yield. [c] The bis adduct (5%) was formed.
[bmIm]BF
4
[bmIm]PF
THF
6
The Michael addition between piperidine and methyl
acrylate proceeded smoothly at ambient temperature and
DMSO
2 d
[
(
a] Reaction conditions: piperidine (1.0 mmol), methyl acrylate an almost quantitative yield was obtained after 10 min
1.2 mmol), solvent (1 mL) at room temperature. [b] Isolated yield.
(
Table 2, Entry 1). As the carbon chain length of the
alcohol moiety increased, the reactivity decreased (Table 2,
With optimal conditions to hand, we next examined the Entries 1–3). Apart from acrylate ester, acrylonitrile and
generality of this strategy for other substrates, and the re- methyl vinyl ketone could also provide quantitative yields
sults are summarized in Table 2. All the reactions proceeded in short reaction times (Table 2, Entries 4 and 5).
Eur. J. Org. Chem. 2007, 1798–1802
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1799

J.-M. Xu, Qi Wu, Q.-Y. Zhang, Fu Zhang, X.-F. Lin
FULL PAPER
Sterically hindered Michael acceptors such as methyl a significant role in the catalysis. We thus speculated that
methacrylate and methyl crotonate were tested under the catalysis by the basic ionic liquid [bmIm]OH may attribut-
same conditions (Table 2, Entries 12 and 13). Both ac- able to the following factors. Firstly, amines exhibited
ceptors reacted readily with piperidine to give the corre- higher nucleophilicity in ionic liquids than in organic sol-
[
15]
sponding 1,4 adducts in high yields with relatively longer vents.
On the other hand, the hydroxide anion of the
reaction times. In general, secondary amines gave higher ionic liquid [bmIm]OH might assist in the formation of nu-
yields than the primary amines, while in the case of benzyl- cleophilic anions generated from amines, which should in-
amines the bis adducts (5%) were also formed (Entry 10). crease the nucleophilicity of amines further, as has been
13
Sterically hindered amines were found to be less active and demonstrated by a C NMR experiment in our former
[
11]
gave only moderate yields (Table 2, Entry 11).
work.
The [bmIm]OH remained intact during the reaction pro-
In order to demonstrate the scope of the practical appli-
1
cability of this methodology, the Michael reaction between cess according to H NMR analysis of the recovered ionic
piperidine and methyl acrylate was carried out on a larger liquid. The aza-Michael addition reactions were carried out
scale (200 mmol) in [bmIm]OH (5 mL). The reaction was under ambient conditions and were capable of going to
complete in 15 min (TLC) and the excess methyl acrylate completion in 10–20 minutes, which might suppress side re-
was evaporated off and recycled. The product was distilled actions due to carbene formation by imidazolium-based
[
16]
directly from the reaction vessel under reduced pressure, to ionic liquids in the presence of strong bases.
This basic
provide the corresponding 1,4-adduct in 99% yield, while ionic liquid [bmIm]OH was thus stable under this pro-
the remaining [bmIm]OH was reused without any ad- cedure.
ditional operation. The reaction was repeated eight times
and the results are shown in Figure 1. It can be seen that
the ionic liquid [bmIm]OH remained consistent activity Conclusions
during the reaction. This process totally avoided the use of
The basic ionic liquid [bmIm]OH provides a convenient,
organic solvents and almost quantitative yields were ob-
clean, efficient, and recyclable catalytic medium for Michael
addition reactions between various amines and α,β-unsatu-
tained. Consequently, it was a green synthetic method and
suitable for industrial application.
rated compounds, thus substituting for volatile organic sol-
vents and toxic catalysts. Furthermore, the products can be
distilled directly from the ionic liquid in large-scale reac-
tions, thus totally avoiding the use of any organic solvent.
The ionic liquid can be reused for at least eight runs with-
out loss of activity, thanks to which it exhibits industrial
potential. Furthermore, the ionic liquid was stable during
the reaction process. More applications (forming C–O, C–
S, and C–C bonds) of the Michael addition reactions pro-
moted by this task-specific ionic liquid [bmIm]OH are un-
der development in our laboratories.
Experimental Section
Materials and General Methods: 1H and 13C NMR spectra were
recorded in CDCl with a Bruker AVANCE DMX 500 spectrome-
3
Figure 1. Recycling of [bmIm]OH in the large-scale Michael ad-
dition between piperidine and methyl acrylate. Reaction conditions:
piperidine (200 mmol), methyl acrylate (240 mmol) in [bmIm]OH
ter at 500 MHz and 125 MHz, respectively. Chemical shifts are re-
ported in ppm (δ), relative to tetramethylsilane (TMS) as the in-
ternal standard. IR spectra were measured with a Nicolet Nexus
FTIR 670 spectrophotometer. All chemicals were obtained from
commercial suppliers and were used without further purification.
Solvents for column chromatography were distilled before use.
(5 mL) at room temperature in 15 min.
The role of [bmIm]OH in the Michael addition was wor-
thy of exploration. It had been reported that acidic C2–H
components in ionic liquids could interact with carbonyl General Procedure for the Synthesis of Basic Ionic Liquid: [bmIm]-
groups and influence reaction results,[
vestigate the role of acidic C2–H in our reaction system
some control experiments were performed. When the
ionic liquid 1-butyl-2,3-dimethylimidazolium hydroxide
14]
OH was prepared by passing the corresponding imidazolium halide
[bmIm]Cl) through a column filled with anion-exchange resin, as
described in the literature. The ionic liquid was characterized by
so in order to in-
(
[5]
1
13
1
IR, H NMR, and C NMR spectroscopy. H NMR (500 MHz,
CDCl ): δ = 10.24 (s, 1 H), 7.62 (t, J = 1.7 Hz, 1 H), 7.50 (t, J =
.7, 1 H), 4.34 (t, J = 7.4 Hz, 2 H), 4.12 (s, 3 H), 3.32–3.25 (brs, 1
H), 1.93–1.87 (m, 2 H), 1.40–1.36 (m, 2 H), 0.96 (t, J = 7.4 Hz, 3
3
([bdmIm]OH) was used to catalyze the Michael addition
1
between piperidine and methyl acrylate, a 95% isolated
yield was obtained in 10 min (Table 1, Entry 3). This result
13
3
H) ppm. C NMR (125 MHz, CDCl ): δ = 137.2, 123.8, 122.1,
was comparable with that obtained in [bmIm]OH, indicat- 49.7, 36.7, 32.1, 19.4, 13.4 ppm. IR (neat): ν˜ = 3422, 3079, 1571,
–
1
ing that the acidic C2–H unit in [bmIm]OH was not playing 1169 cm . These values were in good agreement with those re-
800 Eur. J. Org. Chem. 2007, 1798–1802
1
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Basic Ionic Liquid as Catalyst and Reaction Medium
FULL PAPER
ported.[10] This ionic liquid is highly stable under ambient condi-
tions and can be used in reactions without any difficulty, as recently
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9
2
General Procedures for Michel Additions between Amines and Con-
jugated Alkenes Promoted by [bmIm]OH: The amine (1 mmol) and
the α,β-unsaturated carbonyl compound (1.2 mmol) were added to
4
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Synthesis of 3a on a Larger Scale (0.2 mol): Piperidine (200 mmol)
and methyl acrylate (240 mmol) were added to [bmIm]OH (5 mL)
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J.-M. Xu, Qi Wu, Q.-Y. Zhang, Fu Zhang, X.-F. Lin
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Revised Version Received: December 11, 2006
Published Online: February 15, 2007
5297.
1802
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Eur. J. Org. Chem. 2007, 1798–1802