Chemo/regioselective Aza-Michael additions of amines to conjugate alkenes catalyzed by polystyrene-supported AlCl3

Article abstract of DOI:10.1016/j.tet.2012.12.037

A simple and efficient procedure is presented for Aza-Michael additions of various amines with conjugate alkenes bearing electron withdrawing group catalyzed by polystyrene-supported aluminum chloride (Ps-AlCl₃) without the use of any solvents. The catalyst shows high catalytic activity for both aromatic amines and aliphatic amines. Chemoselective additions of the two types of amines with conjugate alkenes are achieved. Regioselective additions of two different amino groups in one molecule proceed smoothly. Ps-AlCl ₃ has better recyclability and can be reused several times without apparent loss of activity.

Full text of DOI:10.1016/j.tet.2012.12.037

Tetrahedron 69 (2013) 1712e1716
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chemo/regioselective Aza-Michael additions of amines to conjugate
alkenes catalyzed by polystyrene-supported AlCl3
*
Liyan Dai , Yi Zhang, Qianqian Dou, Xiaozhong Wang, Yingqi Chen
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 October 2012
Received in revised form 4 December 2012
Accepted 14 December 2012
Available online 21 December 2012
A simple and efficient procedure is presented for Aza-Michael additions of various amines with conju-
gate alkenes bearing electron withdrawing group catalyzed by polystyrene-supported aluminum chlo-
ride (Ps-AlCl3) without the use of any solvents. The catalyst shows high catalytic activity for both
aromatic amines and aliphatic amines. Chemoselective additions of the two types of amines with con-
jugate alkenes are achieved. Regioselective additions of two different amino groups in one molecule
proceed smoothly. Ps-AlCl3 has better recyclability and can be reused several times without apparent loss
of activity.
Keywords:
Chemo/regioselectivity
Aza-Michael addition
Heterogeneous catalysis
Polystyrene-supported AlCl3
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
catalytic activity for the Michael additions. Although many eco-
nomic and sustainable protocols have been employed by the re-
b
-Amino carbonyl/cyano compounds not only exist in bi-
searchers dedicated to the development of green processes for Aza-
Michael additions, some of catalysts have poor or even weak cat-
alytic effect for the additions of aromatic amines25,27e29,31,39 be-
cause their nucleophilicity is weaker than that of aliphatic amines.
Furthermore, all of reported methods above did not reveal any
catalytic capability for regioselective Aza-Michael additions.
Free AlCl3 is corrosive and sensitive to water, but Ps-AlCl3 is an
environmental-friendly, heterogeneous catalyst.40,41 Ps-AlCl3 is
nonhygroscopic, stable, and can even be used in some reactions,
which produce water. Here we aimed at Aza-Michael additions
(Scheme 1) using Ps-AlCl3 as a heterogeneous catalyst. As expected,
the catalyst showed high catalytic activity for various amines (ar-
omatic and aliphatic amines). Surprisingly, these two types of
amines reacted in different reaction temperatures, which indicated
to us that we might be dealing with the chemoselectivity of Aza-
Michael additions. As a result, at room temperature and in the
presence of Ps-AlCl3, the chemoselective additions of the two types
of amines with conjugate alkenes proceeded well and gave ali-
phatic monoadducts as the only products. Subsequently, we proved
that the regioselective additions of two different amino groups in
one molecule with conjugate alkenes were feasible with only
products of aliphatic additions. The results showed that it was one
of the easiest ways to carry out these chemo/regioselective addi-
tions avoiding cumbersome and tedious protection and depro-
tection. In addition, the catalyst has better recyclability and can be
reused several times without apparent loss of activity.
ologically natural products but also serve as synthetic precursors for
some of antibiotics, pharmaceutical intermediates, b-amino alco-
hols, and chiral auxillaries.1,2 The Aza-Michael additions of nucle-
ophiles to conjugate alkenes are one of the important
methodologies for building up carbonenitrogen bonds in the syn-
thesis of b
-amino carbonyl/cyano compounds.3 Traditionally, Lewis
acids, such as AlCl3,4 PtCl4$5H2O,5 InCl3/TMSCl,6 Bi(NO)3,7 samar-
ium iodobinaphtholate,8 boric acid,9 [Ni(PPP)(THF)](ClO4)2,10
Pd(N,N0-ppo)Cl2,11 N-heterocyclic carbenes,12 bases,13e15 and ionic
liquids16e18 were reported for the process. Recently, many novel
catalysts have been used for enantioselective Aza-Michael
additions19e22 with good to excellent yields and enantiomeric ex-
cess. However, excessive waste and a tedious workup for catalyst
separation limited the applications of these homogeneous catalysts.
Heterogeneous catalysis is a clean and environment-friendly
catalytic method. Many catalysts, such as MOF-99,23 polystyrene-
supported CuI-imidazole complex,24 biomaterial supported orga-
nocatalyst,25 silica sulfuric acid,26 phosphate impregnated titania,27
nanocrystalline copper (II) oxide,28 silica gel,29 PANIeCu,30 PAN-
IeIn,31 PSSA,32 HClO4eSiO2,33 Amberlyst-15,34 KF/Al2O3,35
b-cy-
clodextrin,36 CeCl3$7H2O/NaI/Al2O3,37 enzymes38,39 showed good
* Corresponding author. Tel./fax: þ86 571 87952693; e-mail address: dailiyan@
zju.edu.cn (L. Dai).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.037

L. Dai et al. / Tetrahedron 69 (2013) 1712e1716
1713
even when reaction temperature was raised to 70 ꢀC. The exception
was the reaction of 2-aminopyridine with methacrylate. The
product was not what we expected, as 1H NMR indicated that the
methyl group disappeared. We confirmed that Michael addition
and ammonolysis of ester occurred to form a nitrogen-containing,
six-membered ring structure, however, no reaction of 2-
aminopyridine with acrylonitrile was observed. We assumed that
the ammonolysis of the ester promoted the Michael addition,
whereas acrylonitrile could not do that, therefore, no Michael ad-
duct was produced.
Scheme 1. Aza-Michael additions of amines with conjugate alkenes catalyzed by Ps-
AlCl3.
2. Results and discussion
Ps-AlCl3 was prepared through immobilizing anhydrous alu-
minum chloride on polystyrene (8% divinylbenzene) according to
a procedure reported in literature. The capacity of Ps-AlCl3 was
0.45 AlCl3 mmol/g catalyst42 and was based on its chloride content,
which was determined by the Mohr titration method.
Table 2
Aza-Michael additions of amines with conjugate alkenesa
Entry
1
Adduct
Time (h)
Yield (%)
93
To evaluate the catalytic activity of Ps-AlCl3 in Aza-Michael re-
action, the addition between aniline and methacrylate was chosen
as a model. The effects of temperature, catalyst dosage, and reaction
time were investigated in detail and the results were summarized
in Table 1. The initial reaction was carried out using 15 mol % of Ps-
AlCl3 (with respect to the AlCl3 content in Ps-AlCl3) relative to
methacrylate under solvent-free condition at 40 ꢀC. After 4 h, the
reaction yield was 23%, increased with increasing temperature, the
optimal temperature was found to be 70 ꢀC with a yield of 92%. The
reason for requiring a higher temperature was probably due to the
weaker acidity of Ps-AlCl3 compared to anhydrous AlCl3. In addi-
tion, mesoporous pores and channels of polystyrene were un-
favorable for the molecular mass-transfer process. The catalyst
dosage was also an important factor for the addition, as 8 mol % of
Ps-AlCl3 gave a yield of 74% after 4 h while 12 mol % of Ps-AlCl3 was
sufficient to give the yield of 92% after 2.5 h. Interestingly, Ps-AlCl3
was much more efficient for the addition of morpholine with ac-
rylonitrile, only using 6 mol % of Ps-AlCl3 at room temperature after
2 h, the reaction yield was 92%. This is because the nucleophilicity
of morpholine was stronger than that of aniline.
2.5
2
3
2.4
2.5
94
92
4
5
2.4
2.2
92
91
6
7
2.1
3
92
90
Table 1
8
9
3
99b
92
Reaction conditions optimization for Aza-Michael additions
Entry
1a
2
3
4
Cat.
T (ꢀC)
Time (h)
Yield (%)
15 mol % Ps-AlCl3
15 mol % Ps-AlCl3
15 mol % Ps-AlCl3
15 mol % Ps-AlCl3
15 mol % Ps-AlCl3
12 mol % Ps-AlCl3
10 mol % Ps-AlCl3
8 mol % Ps-AlCl3
8 mol % Ps-AlCl3
6 mol % Ps-AlCl3
4 mol % Ps-AlCl3
40
50
60
70
Reflux
70
70
70
rt
4
3
23
48
75
2.5
2.5
2.5
2.5
2.5
2.5
4
2
2
3
92, 91, 89, 80, 65c
5
6
7
8
92
92
85
74
92
92
81
10
2.5
trace
9b
10
11
11
12
1.5
2
92
91
rt
rt
a
Reaction conditions (entry 1e8): aniline (11 mmol), methyl acrylate (2 mmol).
Reaction conditions (entry 9e12): morpholine (11 mmol), acrylonitrile
b
(2 mmol).
c
Ps-AlCl3 was reused for five runs.
13
14
0.8
1
93
92
The generality of the heterogeneous catalyst was also examined
in the above optimized Ps-AlCl3-catalyzed system. We used various
aromatic amines and aliphatic amines to react with methacrylate
and acrylonitrile. The results were summarized in Table 2. Most
15
1
93
additions proceeded smoothly to give the b-amino carbonyl/cyano
compounds with good to excellent yields. The electronic effects of
substrates had some impacts on additions. Aromatic amines bear-
ing electron-donating groups exhibited a bit higher reactivity than
aniline. Primary amines gave monoaddition products selectively,
and no bisaddition products appeared because of excess primary
amines and the steric effect. Aliphatic cyclic secondary amines had
higher reactivity. Moreover, the additions of sterically hindered
secondary amines, such as diisopropylamine could not proceed
16
17
2
93
91
1.5
18
2
92
(continued on next page)

1714
L. Dai et al. / Tetrahedron 69 (2013) 1712e1716
Table 3 (continued)
Table 2 (continued )
Entry
Adduct
Time (h)
1.4
Yield (%)
Entry
R1
Aliphatic amines
Adduct
Yield (%)
94
19
20
21
93
5
p-OCH3
2
93
91
6
7
p-OCH3
o-CH3
91
93
1.5
22
2
92
a
Reaction conditions: amines (11 mmol), conjugate alkenes (2 mmol), Ps-AlCl3
(0.53 g, 0.27 g for aromatic amines, and aliphatic amines, respectively), 70 ꢀC for
aromatic amines, room temperature for aliphatic amines.
8
o-CH3
o-CH3
93
91
b
The conversion is 99%, the adduct is unstable.
9
a
Based upon the different reactivity of aromatic amines and ali-
phatic amines, we investigated the chemoselective Aza-Michael
reactions in one pot (Scheme 2). A mixture (1:1) of aromatic
amines and aliphatic amines was treated with methacrylate and
acrylonitrile in the presence of Ps-AlCl3 at room temperature. As
shown in Table 3, no matter which aliphatic amines (primary
amines, secondary amines) competed with aniline or substituted
aromatic amines, aliphatic adducts were obtained exclusively with
good yields, which showed the excellent chemoselectivity of Ps-
AlCl3-catalyzed system. With these good results, we continued to
study the regioselective additions of two different amino groups in
one molecule. 4-Aminophenethylamine and 3-aminobenzyl-
methylamine, which contained both aromatic and aliphatic amino
groups, were treated with methacrylate and acrylonitrile at room
temperature in the presence of Ps-AlCl3 (Scheme 3).
Reaction conditions: aromatic amines (5 mmol), aliphatic amines (5 mmol),
methyl acrylate (2 mmol), Ps-AlCl3 (0.27 g), R. T., 2e2.5 h.
b
1.5 ml ethanol was added.
Scheme 3. Regioselective Aza-Michael additions catalyzed by Ps-AlCl3.
As shown in Table 4, conjugate alkenes reacted with both ami-
noethyl and aminomethyl groups regioselectively in high yields to
get only aliphatic adducts at room temperature.
Table 4
Regioselective Aza-Michael additions of amines with conjugate alkenesa
Scheme 2. Chemoselective Aza-Michael additions catalyzed by Ps-AlCl3.
Entry
Amine
EWG
T (ꢀC)
Yield of monoadduct (%)
1
2
3
4
1
1
2
2
COOMe
CN
COOMe
CN
rt
rt
rt
rt
86
89
88
89
Table 3
Chemoselective Aza-Michael additions of amines with conjugate alkenesa
a
Reaction conditions of monoadditions: amines (11 mmol), conjugate alkenes
(2 mmol), Ps-AlCl3 (0.27 g), rt, 2 h.
Entry
1
R1
Aliphatic amines
Adduct
Yield (%)
H
91
Furthermore, the recyclability and reusability of Ps-AlCl3 were
investigated. The Ps-AlCl3 was separated from reaction mixture by
simple filtration after completion of the first reaction, then washed
with ethyl acetate, dried, and used in the next run under the same
reaction conditions. The result showed that 89% yield of aniline was
achieved at the third operation.
2
H
93
Encouraged by the results above, a plausible mechanism for the
3
4
H
90
Aza-Michael reaction was hypothesized (Fig. 1).33 First, the
a,b-
unsaturated carbonyl compound coordinated the cationic center of
Ps-AlCl3 with the carbonyl oxygen to increase the electrophilicity of
p-OCH3
92b
the
b-carbon. Second, the nitrogen atom in amine attacked b-car-
bon forming new CeN bond, and at the same time intramolecular

L. Dai et al. / Tetrahedron 69 (2013) 1712e1716
1715
AlCl3 for aromatic amines; 0.27 g, 0.12 mmol AlCl3 for aliphatic
amines) was stirred at 70 ꢀC for aromatic amines, room tempera-
ture for aliphatic amines. After completion of the reaction, Ps-AlCl3
was filtered and washed with ethyl acetate (3ꢁ10 ml), dried, and
used in the next run under the same reaction conditions. The fil-
trate was concentrated to give the crude product, which was pu-
rified further by column chromatography (petroleum ether/ethyl
acetate or methanol/ethyl acetate).
4.3. Selected spectral data
4.3.1. 2H-Pyrido[1,2-a]pyrimidin-4(3H)-one. 1H
400 MHz): 7.70e7.68 (d, 1H), 7.59e7.54 (q, 1H), 6.96e6.94 (d,
1H), d 2.73e2.69 (t,
NMR
d
(CDCl3,
d
d
d
6.71e6.67 (q, 1H),
d
4.43e4.39 (t, J¼7.2, 2H),
Fig. 1. The proposed mechanism of Aza-Michael addition catalyzed by Ps-AlCl3.
J¼7.2, 2H). 13C NMR (CDCl3, 100 MHz):
¼174.80, 157.62, 140.15,
d
138.12, 122.71, 112.23, 50.23, 29.11. HRMS: m/z calcd for C8H8N2O:
148.0637, found: 148.0634.
proton transferred. Finally,
b-amino carbonyl/cyano compounds
were generated and dissociated from Ps-AlCl3.
On the other hand, we still compared the Ps-AlCl3 catalyst with
some heterogeneous catalysts reported in the literatures for the
Aza-Michael additions (Table 5). As shown in Table 5, in terms of
reaction time and yields, Ps-AlCl3 showed better activity.
4.3.2. Methyl 3-(4-aminophenethyl)aminopropanoate. 1H NMR
(CDCl3, 500 MHz):
3H),
(t, J¼7, 2H),
d
7.00e6.98 (d, 2H),
d
6.64e6.62 (d, 2H),
d
3.66 (s,
d
2.96e2.93 (t, J¼6.5, 2H),
d
2.89e2.86 (t, J¼7, 2H),
d 2.76e2.73
d
2.59e2.57 (t, J¼6.5, 2H). 13C NMR (CDCl3, 125 MHz):
d
¼173.14, 144.99, 129.77, 129.29, 115.64, 51.99, 51.16, 44.84, 34.87,
Table 5
33.96. HRMS: m/z calcd for C12H18N2O2: 222.1368, found: 222.1373.
Comparison of Aza-Michael addition of aniline with methacrylate catalyzed by Ps-
AlCl3 and other catalysts
4.3.3. 3-(4-Aminophenethyl)aminopropionitrile. 1H NMR (CDCl3,
Entry
Catalyst
Solvent
T (ꢀC)
T (h)
Yield (%)
500 MHz):
d 6.99e6.98 (d, 2H), d 6.64e6.63 (d, 2H), d 2.93e2.91 (t,
1
2
3
4
5
6
7
Ps-AlCl3
d
70
rt
60
50
rt
2.5
4
92
88
31
20
60
24
90
J¼6.5, 2H),
d
2.86e2.84 (t, J¼7, 2H),
d
2.71e2.68 (t, J¼7, 2H),
[DDPA][HSO4]43
Lipase B39
PIT27
H2O
Toluene
d
d
2.50e2.47 (t, J¼6.5, 2H). 13C NMR (CDCl3, 125 MHz):
¼145.01,
d
42
24
36
48
6
129.74, 129.56, 118.89, 115.64, 50.93, 45.29, 35.57, 18.93. HRMS: m/z
Nano CuO28
DBU13
DCM
CH3CN
H2O
calcd for C11H15N3: 189.1266, found: 189.1268.
rt
rt
-CD36
4.3.4. Methyl 3-(N-3-aminobenzyl-N-methyl)aminopropanoate. 1H
b
NMR (CDCl3, 400 MHz):
6.56e6.53 (q, 1H), 3.65 (s, 3H),
2.53e2.49 (t, J¼7.2, 2H),
d
7.08e7.04 (t, 1H),
3.40 (s, 2H),
2.19 (s, 3H). 13C NMR (CDCl3,
d
6.66e6.63 (t, 2H),
d
d
d
d
2.74e2.70 (t,
J¼7.2, 2H),
d
d
3. Conclusions
100 MHz):
d
¼173.02, 146.51, 139.81, 129.02, 119.14, 115,53, 113.86,
62.02, 52.62, 51.54, 41.88, 32.53. HRMS: m/z calcd for C12H18N2O2:
222.1368, found: 222.1366.
In summary, the Ps-AlCl3 was an efficient catalyst for Aza-
Michael addition of amines (aromatic and aliphatic amines) to
conjugate alkenes. The additions were carried out smoothly with
good to excellent yields under solvent-free and mild reaction
conditions. The catalyst showed good chemoselectivity and regio-
selectivity in Aza-Michael addition. After completion of the re-
action, Ps-AlCl3 could be easily separated from the reaction system
via filtration, showing better recyclability and reusability without
apparent loss of activity.
4.3.5. 3-(N-3-Aminobenzyl-N-methyl)aminopropionitrile. 1H NMR
(CDCl3, 500 MHz):
1H), 6.60e6.58 (q, 1H),
2.50e2.47 (t, J¼6.5, 2H),
d
7.11e7.08 (t, 1H),
3.48 (s, 2H),
2.29 (s, 3H). 13C NMR (CDCl3, 125 MHz):
d
6.72 (s, 1H),
d 6.69e6.67 (d,
d
d
d
2.73e2.70 (t, J¼7, 2H),
d
d
d
¼146.86, 139.37, 129.48, 119.24, 119.08, 115.58, 114.39, 62.02, 52.27,
42.06, 16.45. HRMS: m/z calcd for C11H15N3: 189.1266, found:
189.1267.
4. Experimental
4.1. General
Acknowledgements
The authors would like to thank Dr. Shichao Yu for some ben-
eficial advices, Nanjing Microspheres Hi-Efficiency Isolation Carrier
Co., Ltd for providing polystyrene. This work was supported by the
NSFC (No: 21176213), and Zhejiang Key Innovation Team of Green
Pharmaceutical Technology (No: 2010R50043).
All the chemicals used were of analytical grade and were used
without further purification. Polystyrene (8% divinylbenzene) was
purchased from Nanjing Microspheres Hi-Efficiency Isolation Car-
rier Co., Ltd. GC profiles were recorded on FuLi 9790 with a SE-54
column (30 mꢁ0.32 mmꢁ0.5
m
m). 1H and 13C NMR spectra were
Supplementary data
recorded in CDCl3 using TMS as an internal standard on a Bruker
spectrophotometer at 500/400 and 125 MHz, respectively. High
Resolution mass spectra of new compounds were recorded on
GCTPremier at 70 eV.
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.tet.2012.12.037.
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