A mild and efficient procedure for asymmetric Michael additions of cyclohexanone to chalcones catalyzed by an amino acid ionic liquid

Article abstract of DOI:10.1016/j.tetasy.2008.06.022

A mild and efficient procedure for Michael additions of cyclohexanone to chalcones has been developed. In the presence of amino acid ionic liquid [EMIm][Pro] (200 mol %), cyclohexanone reacted with various chalcones to afford Michael adducts in high yield

Full text of DOI:10.1016/j.tetasy.2008.06.022

Tetrahedron: Asymmetry 19 (2008) 1515–1518
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A mild and efficient procedure for asymmetric Michael additions
of cyclohexanone to chalcones catalyzed by an amino acid ionic liquid
*
Yunbo Qian, Shiyong Xiao, Lei Liu, Yongmei Wang
Department of Chemistry, The Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild and efficient procedure for Michael additions of cyclohexanone to chalcones has been developed.
In the presence of amino acid ionic liquid [EMIm][Pro] (200 mol %), cyclohexanone reacted with various
chalcones to afford Michael adducts in high yields (85–98%) and moderate to good enantioselectivities
(16–94% ee), accompanied by an unexpected solvent-dependent inversion of the enantioselectivity.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 7 May 2008
Accepted 12 June 2008
Available online xxxx
1. Introduction
tion; this issue has not been well studied except by Wang et al.,9
who used a chiral pyrrolidinylmethylsulfonamide as a catalyst.
Over the past few years, searching for new solvents and materi-
als based on chiral ionic liquids (CILs) has become a research focus
of increasing importance; a growing number of CILs have been de-
signed for and utilized in potential applications, such as asymmet-
ric synthesis, stereoselective polymerization and chiral resolution.1
With the rapid development of CILs, these new solvents have the
potential to play a key role in enantioselective organic chemistry,
and their impact in this field is expected to be enhanced.
Among the newly developed CILs, those derived from amino
acids have attracted increasing interest in the chemical commu-
nity,2 particularly in asymmetric synthesis. Due to their convenient
chemical modification, CILs with chiral cations derived from natu-
ral amino acids can be successfully used as highly enantioselective
organocatalysts,3 while there are much less CILs modified on the
anion structure.
Herein, for the first time, we used an anion-modified CILs 1-
ethyl-3-methylimidazolium-(S)-2-pyrrolidinecarboxylic acid salt
[EMIm][Pro] as a catalyst, and have developed a mild and efficient
procedure for the asymmetric Michael additions of cyclohexanone
to chalcones. The products could be obtained in much less time,
with high yields and moderate to good enantioselectivities, accom-
panied by an unexpected solvent-dependent inversion of the
enantioselectivity which to the best of our knowledge has not been
reported before.
2. Results and discussion
Initially, encouraged by the dramatic catalytic activity of the
amino acid proline, we decided to research the properties of
[EMIm][Pro] 1 (Fig. 1) in a Michael addition. Catalyst 1 could be
easily obtained according to the literature (Scheme 1).4 N-Methyl-
imidazole was employed as a starting material to react with excess
bromoethane in ethyl acetate to give 1-ethyl-3-methylimidazo-
lium bromide in good yield (80%). Then, 1-ethyl-3-methylimidazo-
lium hydroxide could easily be obtained by an anion exchange
reaction of 1-ethyl-3-methylimidazolium bromide with 201 ꢀ 7
Since Fukumoto et al.4 succeeded in synthesizing amino acid io-
nic liquids from 20 natural amino acids, studies on the properties
of amino acid ionic liquids have intensified.5 However, their appli-
cations as solvents or catalysts have yet to be reported, which
drove us to research the properties of amino acid ionic liquid used
as chiral solvents and catalysts for asymmetric synthesis.
We chose the Michael addition reaction as a model reaction,
which constitutes as one of the most important classes of new car-
bon–carbon bond-forming reactions for the preparation of organic
target products in synthetic organic chemistry.6 There are also
publications reporting Michael addition reactions catalyzed by
CILs,3,7 all of which are modified on the cations to provide a chiral
group. Howerver, these reactions employ highly activated Michael
acceptors, such as nitroalkenes.8 Enantioselective catalytic conju-
gate addition of ketones with enones remains a challenging reac-
Styrene-DVB. Lastly, L-proline was used to neutralize 1-ethyl-3-
methylimidazolium hydroxide with the desired product CILs in
70% yield.
O-
+
N
N
1
N
H
O
Figure 1. 1-Ethyl-3-methylimidazolium-(S)-2-pyrrolidinecarboxylic acid salt
([EMIm][Pro]).
* Corresponding author. Tel./fax: +86 22 23502654.
E-mail address: ymw@nankai.edu.cn (Y. Wang).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.06.022

1516
Y. Qian et al. / Tetrahedron: Asymmetry 19 (2008) 1515–1518
a
b
+
+
N
N
N
N
N
N
Br-
-OH
c
O-
+
N
N
N
H
O
Scheme 1. Synthesis of chiral amino acid ionic liquid: 1-ethyl-3-methylimidazo-
lium-(S)-2-pyrrolidinecarboxylic acid salt ([EMIm][Pro]). Reagents and conditions:
(a) CH3CH2Br, CH3CO2C2H5, 80%; (b) 201 ꢀ 7 Styrene-DVB; (c) (S) proline, 70%.
The experiments were first conducted by screening [EMIm][Pro]
as a catalyst in various ratios with chalcone 3 to promote the
Michael reaction (Table 1). The initial conditions were with the
reaction performed in CH3OH at room temperature with cyclohex-
anone 2 and the chalone 3 in a 3.5:1 ratio. When using 10 mol %
catalyst loading (Table 1, entry 1), the Michael addition product
4a was obtained in 30% yield with moderate (23:77) dr value but
low enantioselectivity (17%). Surprisingly, when 50 mol % of 1
was added (Table 1, entry 2), the yield soared to 75% in 20 h along
with a 6:94 diastereoselectivity and a 40% ee value. Encouraged by
this result, we screened the amount of the catalyst under similar
reaction conditions, finding that 200% of 1 had the right properties
to achieve good product formation (Table 1, entry 6) with an excel-
lent yield (98%) in 4 h and a high ee value up to 86%. The addition
of more catalyst did not give better results but a decrease of the ee
value. (Table 1, entry 7–10). Figure 2 gives an outline view of the
result.
Figure 2. The effect of [EMIm][Pro] catalyst in the asymmetric Michael addition of
cyclohexanone with chalcone in CH3OH.
Table 2
Investigation of different kinds of solvents in the asymmetric Michael addition of
cyclohexanone with chalone
O
O
Ph
O
O
[EMlm][Pro] 1
Ph
+
Ph
Ph
r.t., solvent
2
3a
4a
Entry
Solvent
t (h)
15
5
15
4
4
4
4
4
Yielda (%)
drb
eeb (%)
1
2
3
4
5
6
7
8
9
Toluene
THF
50
50
45
98
90
95
70
75
89
91
11:89
8:92
27
41
6
CH2Cl2
CH3OH
C2H5OH
DMSO
BMIBF4
BMIPF6
29:71
20:80
15:85
12:88
22:78
20:80
14:86
21:79
Table 1
86
The effect of [EMIm][Pro] catalyst in the asymmetric Michael addition of cyclohex-
anone with chalcone in CH3OH
69
ꢁ78c
31
O
O
Ph
O
46
62
60
O
Cyclohexanone
[EMIm][Pro]
4
4
1
[EMlm][Pro]
r.t., CH3OH
10
Ph
+
Ph
Ph
a
Isolated yields after column chromatography.
Determined by HPLC analysis on a chiral AD-H column.
An inversed configuration determined by HPLC analysis on a chiral AD-H
b
c
2
3a
4a
column.
Entry
Cat. (mol %)
t (h)
Yielda (%)
drb
eeb (%)
1
2
3
4
5
6
7
8
9
10
50
20
20
10
4
4
4
4
4
4
4
30
75
85
86
90
98
96
93
94
96
23:77
6:94
17:83
8:92
14:86
20:80
13:87
9:91
17
40
59
64
70
86
78
75
67
59
entries 4–6) were used, accompanied by an unexpected solvent-
dependent inversion of the enantioselectivity. Determined by HPLC
analysis on a chiral AD-H column, the product obtained from
DMSO had an inverse configuration when compared with that ob-
tained from methanol or ethanol. The same reaction carried out in
ionic liquids [BMIm]BF4 and [BMIM]PF6 provided the adduct with
31% and 44% ee values, both in good yield (Table 2, entries 7 and
8). There was a small ee value decrease when using cyclohexanone
as a solvent, while a 60% ee value was obtained in 91% yield when
the catalyst 1 was used as a solvent (Table 2, entries 9 and 10).
Stimulated by the dramatic solvent effect, we tested a variety of
chalones 3 with cyclohexanone 2 to investigate the generality of
the solvent-dependence in this reaction, and the results are sum-
marized in Table 3. All reactions were performed at room temper-
ature in the presence of 200 mol % of 1 in CH3OH or DMSO. It
appeared that both electron-donating (entries 2 and 9) and elec-
tron-withdrawing (entries 3–5 and 10–12) groups led to similar
yields but lower selectivities when compared with the unsubstitut-
ed substrate. The chalcone bearing an amine group provided the
100
150
190
200
210
270
300
350
8:92
8:92
10
a
Isolated yields after column chromatography.
Determined by HPLC analysis on a chiral AD-H column.
b
In the presence of 200 mol % of 1, the addition of cyclohexanone
with chalcone was examined in different solvents. Table 2 summa-
rizes the results. A substantial change of the solvent had a signifi-
cant effect on the yield and stereoselection. When performed in
toluene, THF or dichloromethane (Table 2, entries 1–3), the reac-
tion produced 4a in moderate yield and poor selectivity. Good re-
sults were attained when methanol, ethanol or DMSO (Table 2,

Y. Qian et al. / Tetrahedron: Asymmetry 19 (2008) 1515–1518
1517
Table 3
Catalytic asymmetric Michael addition reactions of cyclohexanone 2 with chalcone 310,11
Ar1
O
O
O
Ar1
O
O
O
[EMlm][Pro] 1
[EMlm][Pro] 1
Ar2
Ar1
Ar2
Ar2
+
r.t., DMSO
r.t., CH3OH
2
3a
4
4a
Entry
Ar1
Ph
4-MeC6H4
4-ClC6H4
Ph
4-BrC6H4
Ph
2-ClC6H4
Ph
4-MeC6H4
4-ClC6H4
Ph
4-BrC6H4
Ph
Ar2
Conditionsd
t (h)
Yielda (%)
drb
eeb,c (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ph
Ph
Ph
4-ClC6H4
Ph
4-NH2C6H4
4-MeC6H4
Ph
Ph
Ph
A
A
A
A
A
A
A
B
B
B
B
B
B
B
4
4
4
4
4
8
4
4
4
4
4
4
8
4
98
90
98
97
80
87
99
95
98
94
88
90
85
99
20:80
5:95
4:96
15:85
23:77
4:96
86
37
60
23
44
94
29
1:99
12:88
16:84
10:90
8:92
10:90
4:96
ꢁ78
ꢁ72
ꢁ65
ꢁ72
ꢁ39
ꢁ91
ꢁ16
4-ClC6H4
Ph
4-NH2C6H4
4-MeOC6H4
2-ClC6H4
4:96
a
Isolated yields after column chromatography.
Determined by HPLC analysis on a chiral AD-H column.
An inverse configuration determined by HPLC analysis on a chiral AD-H column.
Conditons A: reaction performed in CH3OH; conditions B: reaction performed in DMSO.
b
c
d
adduct with excellent ee values both in CH3OH and in DMSO (en-
tries 6 and 13). We assumed that between catalyst 1 and the amine
group there was probably a strong hydrogen-bond to anchor the
substrate in a firm way (Fig. 3, State C). Substituents both at the
Ar1 and Ar2 groups on the chalcone (entries 7 and 14) resulted in
a small increase in diastereoselectivity, but an obvious decrease
in enantioselectivity occurred.
Notably, the solvent-dependent inversion of the enantioselec-
tivity took place in all reactions shown in Table 3. Products ob-
tained from DMSO mostly displayed a higher ee value than those
obtained from CH3OH, each with a reverse enatioselectivity on an
AD-H column after HPLC analysis.
With the success of the above reactions, we continued our study
by exploring the recyclability of the catalyst which is important
from an industrial point of view, especially when too much catalyst
had been used compared with the result reported. We carried out
our study by using the addition of cyclohexanone with chalcone in
CH3OH as a model reaction. After the reaction was complete, the
reaction mixture was evaporated in vacuo and then extracted with
ethyl acetate to give the ionic liquid residue. The residue was re-
used directly as a catalyst and this process was repeated five times.
It was found that the desired adduct could still be obtained with a
comparable yield but a small decrease of the ee value (Table 4).
To determine the configurations of the Michael adducts, we re-
peated the experiment in the literature9 catalyzed by proline. After
comparing the peak time on an AD-H column with our product 4
obtained from CH3OH, the relative configurations were assigned
as syn- and the absolute configurations were confirmed to be
(2S,10R).
Although attempts to detail the reaction mechanism have not
yet been undertaken, we now propose a plausible transition state,
representing the stereoselective and solvent-dependent of Michael
addition reactions of cyclohexanone with chalcones. The reaction
started with iminium activation of the cyclohexanone by the pro-
line anion, together with an electrostatic interaction between chal-
cones and the imidazolium moiety of the catalyst (Fig. 3, State A).
When DMSO was used as a solvent, the oxygen atom from the
DMSO had occupied the position of chalcone and blocked the
O
O
O
O-
O-
N+
O-
N
N
N+
N
N
N+
N
N
O
O
O-
+
SH
Ar1
H
Ar2
N
H
A
B
C
Figure 3. Proposed transition states A, B and C.

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Y. Qian et al. / Tetrahedron: Asymmetry 19 (2008) 1515–1518
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Q.; Headley, A. D. J. Org. Chem. 2006, 71, 9857–9860; (g) Ni, B.; Garre, S.;
Headley, A. D. Tetrahedron Lett. 2007, 48, 1999–2002.
Recycling study of the Michael addition catalyzed by an amino acid ionic liquid
Reuse
t (h)
Yielda (%)
drb
eeb (%)
4. Fukumoto, K.; Yoshizawa, M.; Ohno, H. J. Am. Chem. Soc. 2005, 127, 2398–2399.
5. (a) Ohno, H.; Fukumoto, K. Acc. Chem. Res. 2007, 40, 1122–1129; (b) Zhang, Z.-
F.; Li, J.-G.; Zhang, Q.-G.; Guan, W.; Yang, J.-Z. J. Chem. Eng. Data 2008, 53, 1196–
1198; (c) Yang, J.-Z.; Zhang, Q.-G.; Wang, B.; Tong, J. J. Phys. Chem. B 2006, 110,
22521–22524.
1
2
3
4
5
4
4
4
4
4
98
95
97
97
98
21:79
20:80
18:82
18:82
20:80
86
83
82
80
81
6. For recent reviews, see: (a) Berner, O. M.; Tedeschi, L.; Enders, D. Eur. J. Org.
Chem. 2002, 1877–1894; (b) Krause, N.; Hoffmann-Röder, A. Synthesis 2001,
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7. (a) Li, P.; Wang, L.; Wang, M.; Zhang, Y. Eur. J. Org. Chem. 2008, 7, 1157–1160;
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Wu, L.-Y.; Yan, Z.-Y.; Xie, Y.-X.; Niu, Y.-N.; Liang, Y.-M. Tetrahedron: Asymmetry
2007, 18, 2086–2090; (d) Ni, B.; Zhang, Q.; Headley, A. D. Green Chem. 2007, 9,
737–739; (e) Ou, W.-H.; Huang, Z.-Z. Green Chem. 2006, 8, 731–734; (f) Wang,
Z.; Wang, Q.; Zhang, Y.; Bao, W. Tetrahedron Lett. 2005, 46, 4657–4660.
8. (a) Liu, F.; Wang, S.; Wang, N.; Peng, Y. Synlett 2007, 15, 2415–2419; (b) Shen,
Z.; Mang, Y.; Jiao, C.; Li, B.; Ding, J.; Zhang, Y. Chirality 2007, 19, 307–312; (c)
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a
Isolated yields after column chromatography.
Determined by HPLC analysis on a chiral AD-H column.
b
proximity to the prochiral iminium from the side where chalcone
had attacked in the other solvents. As a result, an inverse of the
enantioselectivity was observed (Fig. 3, State B).
3. Conclusion
In conclusion, we have developed a new, mild and efficient pro-
cedure for Michael additions of cyclohexanone with chalcones. In
the presence of amino acid ionic liquid [EMIm][Pro] (200 mol %),
cyclohexanone could react with various chalcones to afford Mi-
chael adducts in high yields (85–98%) with moderate to good
enantioselectivities (16–94% ee), accompanied by an unexpected
solvent-dependent inversion of the enantioselectivity. Further
investigations of this novel transformation and the synthetic appli-
cations of the [EMIm][Pro] are underway.
10. Typical procedure: chalcone
3
(17 mg, 0.081 mmol) was added to
a vial
containing cyclohexanone (2; 30
l
L, 0.284 mmol) and catalyst 1 (37.0 mg,
0.0162 mmol) in CH3OH (0.5 mL) at room temperature. The mixture was
stirred vigorously and monitored by TLC. When the reaction was finished, the
reaction mixture was directly purified by flash silica gel chromatography (ethyl
acetate/hexane, 1:5) to afford the adduct as a white solid; yield: 25 mg (98%);
20: 80 dr (determined by HPLC) and 86% ee; Chiralpak AD-H column (i-PrOH/
hexane 10/90, flow rate 0.9 mL/min, k = 254 nm): tR = 26.3 min (minor) and
31.4 min (major).
11. All new compounds gave satisfactory analytical and spectral data. (S)-2-((R)-1-
(4-Bromophenyl)-3-oxo-3-phenylpropyl)-cyclohexanone (Table 3, entry 5)
Acknowledgements
yield: 80%. Mp 115–118 °C 1H NMR (300 MHz, CDCl3)
d (ppm): 7.91 (d,
J = 7.2 Hz, 2H), 7.52 (t, J = 7.3 Hz, 1H), 7.43 (d, J = 7.8 Hz, 2H), 7.37 (d, J = 8.4 Hz,
2H), 7.07 (d, J = 8.4 Hz, 2H), 3.70 (dt, J = 9.8, 9.8, 3.9 Hz, 1H), 3.50 (dd, J = 16.4,
3.9 Hz, 1H), 3.20 (dd, J = 16.37, 9.79 Hz, 1H), 2.69 (dt, J = 10.0, 10.0, 4.8 Hz, 1H),
2.52–2.33 (m, 2H), 2.03–1.97 (m, 1H), 1.81–1.62 (m, 4H), 1.29–1.26 (m, 1H),
13C NMR (75 MHz, CDCl3) d (ppm): 213.0, 198.4, 141.1, 136.9, 133.0, 131.6,
130.2, 128.5, 128.1, 120.4, 55.5, 43.8, 42.4, 40.6, 32.5, 28.5, 24.3; HPLC
Chiralpak AD-H column (i-PrOH/hexane 10/90, flow rate 0.9 mL/min,
k = 254 nm): tR = 22.6 min (minor) and 26.7 min (major). (S)-2-((R)-1-(4-
Phenyl)-3-oxo-3-(40-aminophenyl)-propyl)-cyclohexanone (Table 3, entry 6)
yield: 87%. Mp 153–155 °C 1H NMR (300 MHz, CDCl3) d (ppm): 8.5 (s, 1H),
7.97–7.92 (m, 2H), 7.84–7.81 (m, 2H), 7.60–7.50 (m, 2H), 7.23–7.18 (m, 3H),
3.79 (dt, J = 9.8, 9.8, 4.0 Hz, 1H), 3.63 (dd, J = 16.0, 4.0 Hz, 1H), 3.34 (dd, J = 15.9,
9.6 Hz, 1H), 2.78 (dt, J = 10.1, 9.8, 4.8 Hz, 1H), 2.60–2.38 (m, 2H), 2.04–1.96 (m,
1H), 1.84–1.76 (m, 2H), 1.64–1.58 (m, 3H), 1.34–1.30 (m, 1H), 13C NMR
(75 MHz, CDCl3) d (ppm): 213.6, 198.7, 142.0, 135.5, 134.4, 132.5, 129.9, 129.6,
128.5, 128.4, 128.2, 127.7, 126.6, 124.0, 55.9, 44.3, 42.3, 41.4, 32.5, 28.5, 24.1;
HPLC Chiralpak AD-H column (i-PrOH/hexane 10/90, flow rate 0.9 mL/min,
k = 254 nm): tR = 35.1 min (major) and 55.6 min (minor). (S)-2-((R)-1-(2-
Chlorophenyl)-3-oxo-3-(40-methoxyphenyl)-propyl)-cyclohexanone (Table 3,
entry 7) yield: 99%. Mp 122–123 °C 1H NMR (300 MHz, CDCl3) d (ppm): 7.94 (d,
J = 8.8 Hz, 2H), 7.30 (d, J = 7.8 Hz, 1H), 7.27–7.24 (m, 1H), 7.18 (t, J = 7.4, 7.4 Hz,
1H), 7.09 (t, J = 7.5, 7.5 Hz, 1H),7.436.89 (d, J = 8.8 Hz, 2H), 4.22 (dt, J = 10.0,
10.0, 3.8 Hz, 1H), 3.84 (s, 3H),3.52 (dd, J = 15.9,3.9 Hz, 1H), 3.30 (dd, J = 16.0,
9.9 Hz, 1H), 2.89 (dt, J = 10.3, 10.2, 5.0 Hz, 1H), 2.55–2.35 (m, 2H), 2.04–1.98
(m, 1H), 1.85–1.79 (m, 1H), 1.64–1.56 (m, 3H), 1.27–1.22 (m, 1H). 13C NMR
(75 MHz, CDCl3) d (ppm): 213.2, 197.1, 163.3, 139.7, 134.7, 130.5, 130.2, 129.9,
129.6, 127.6, 127.0, 113.6, 58.5, 55.4, 42.7, 42.6, 38.5, 32.6, 28.7, 24.7; HPLC
Chiralpak AD-H column (i-PrOH/hexane 10/90, flow rate 0.9 mL/min,
k = 254 nm): tR = 41.0 min (minor) and 84.1 min (major).
This work was supported financially by the National Natural
Science Foundation of China (Grant No. 20672061, China). We also
thank Nankai University State Key Laboratory of Elemento-organic
Chemistry for the support.
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