Highly efficient Michael addition reaction of amines catalyzed by silica-supported aluminum chloride

Article abstract of DOI:10.1080/00397910802499559

Aliphatic and aromatic amines undergo smooth nucleophilic addition to α,β-unsaturated compounds in the presence of a catalytic amount of silica-supported aluminum chloride at 60°C and under solvent-free conditions to produce the corresponding β-amino compounds in excellent yields. This method is simple and convenient and works efficiently under mild conditions. This catalyst can used again without losing its activity three times. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802499559

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Highly Efficient Michael
Addition Reaction of Amines
Catalyzed by Silica-Supported
Aluminum Chloride
Mohammad R. Saidi a , Yaghoub Pourshojaei a &
Fezzeh Aryanasab a
a Department of Chemistry, Sharif University of
Technology, Tehran, Iran
Available online: 25 Feb 2009
To cite this article: Mohammad R. Saidi, Yaghoub Pourshojaei & Fezzeh Aryanasab
(2009): Highly Efficient Michael Addition Reaction of Amines Catalyzed by Silica-
Supported Aluminum Chloride, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 39:6, 1109-1119
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Synthetic Communications1, 39: 1109–1119, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802499559
Highly Efficient Michael Addition Reaction of
Amines Catalyzed by Silica-Supported
Aluminum Chloride
Mohammad R. Saidi, Yaghoub Pourshojaei, and Fezzeh Aryanasab
Department of Chemistry, Sharif University of Technology, Tehran, Iran
Abstract: Aliphatic and aromatic amines undergo smooth nucleophilic addition
to a,b-unsaturated compounds in the presence of a catalytic amount of silica-
supported aluminum chloride at 60 ꢀC and under solvent-free conditions to
produce the corresponding b-amino compounds in excellent yields. This method
is simple and convenient and works efficiently under mild conditions. This
catalyst can used again without losing its activity three times.
Keywords: Amines, Michael addition, silica-supported aluminum chloride, a,b-
unsaturated olefins
INTRODUCTION
Interest in developing environmentally benign and solvent-free reactions
has increased dramatically. Consequently, operationally simple catalytic
processes, which circumvent the use of toxic materials, have become
a powerful tool in constructing an organic transformation.[1–5] One
problem for using solvent-free reactions is high catalyst loading, which
is generally required to effect the desired transformations in a reasonable
time scale.
The Michael reaction is one of the most important carbon–carbon
bond-forming reactions. A variety of Michael acceptors, such as a,
b-unsaturated ketones, aldehydes, esters, and nitrils, can be used in this
Received July 2, 2008.
Address correspondence to Mohammad R. Saidi, Department of Chemistry,
Sharif University of Technology, Tehran, Iran. E-mail: saidi@sharif.edu
1109

1110
M. R. Saidi, Y. Pourshojaei, and F. Aryanasab
reaction, which can be readily transformed into a range of different func-
tionalities.[6] A large number of methods have been reported quite
recently for 1,4-conjugate addition to electron-deficient olefins.
This reaction has been investigated using catalysts such as lanth-
anum trichloride (LaCl3),[6] bromodimethylsulfonium bromide,[7] silica-
supported perchloric acid,[8] cerium(IV) ammonium nitrate (CAN),[9,10]
b-cyclodextrin,[11] zirconium(IV) chloride,[12] samarium(III) triflate,[13]
ZrOCl2 ꢁ 8H2O on montmorillonite K10,[14] N-methylimidazole,[15] amber-
lyst-15,[16] 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),[17] bismuth (III)
triflate,[18] cadmium chloride (CdCl2),[19] Y(NO3)3 ꢁ 6H2O,[20] and cellulose-
supported copper(0).[21] However, there are various limitations with the
reported methodologies, especially when using aromatic amines, such as
long reaction times, low yields for aromatic amines, use of halogenated sol-
vents, difficulty in recovery of high-boiling solvents, high temperatures,
requirement of special efforts for preparation of the catalysts, use of costly
catalysts, and moderate yields.
In the past few years, the use of silica-supported aluminum chloride
as a reusable, heterogeneous, inexpensive solid Brønsted acid catalyst has
been widely used and received much attention.[22–26] Aluminum chloride
is a very active Lewis acid that is soluble in many organic solvents and
produces unwanted side reactions. By using silica-supported aluminum
chloride, the high reactivity of this active Lewis acid is under control.
Silica-supported aluminum chloride is a strong Brønsted and Lewis acid,
presumably arising from the formation of SiOAlCl2 sites on the sur-
face.[27] This heterogeneous catalyst can be easily separated from the
reaction media, has greater selectivity, and is recyclable, easier to handle,
more stable, nontoxic, and insoluble in organic solvents.
RESULTS AND DISCUSSION
Because of our interest in studying the Michael reaction under solvent-
free and environmentally benign methods,[28–32] herein we report a mild,
easy, and catalytic process for Michael addition of aromatic amines at
60 ꢀC and aliphatic amines at room temperature in excellent to quantita-
tive yields using silica-supported aluminum chloride as catalyst. At first,
we examined the reaction of aniline with methyl acrylate and 0.2 g of
silica-supported aluminum chloride at room temperature under solvent-
free conditions. Partial conversion took place within 4 h, leading to
Michael adduct. When the temperature was raised to 60 ꢀC, complete
conversation took place after 4 h (Scheme 1).[33] By using 0.1 g of silica-
supported aluminum chloride, the reaction was not complete after 4 h at
60 ꢀC. When 0.3 g of catalyst was used, some polymeric materials were

Michael Addition of Amines
1111
Scheme 1. Michael addition of aromatic and aliphatic amines with methyl
acrylate.
also formed as by-products. In the next step, we used piperidine with
methyl acrylate and 0.2 g of catalyst at room temperature under solvent-
free conditions. After 2 h, the reaction was complete with quantitative
yields of the Michael adduct.
These results encouraged us to exploit the generality and scope of this
reaction by using other Michael acceptors with various aromatic and ali-
phatic amines under catalytic amounts of silica-supported aluminum chlor-
ide. The reaction proceeded well with aromatic and aliphatic amines. As
shown in Table 1, by this method, Michael addition of various aliphatic
amines and aryl amines carrying either electron-donating or electron-
withdrawing substituents were successfully reacted with a,b-unsaturated
olefins to produce their corresponding Michael adducts in high to excellent
yields. By using this method, Michael addition of 2,4,6-trimethylaniline and
imidazole proceeded effectively with a,b-unsaturated compounds, but in
low yields of products. Longer reaction times did not improve the yields
of the products. The reaction with 4-nitroaniline did not proceed at all, even
at higher temperatures. Michael addition with 2-naphthylamine in the pre-
sence of 0.2 g of catalyst gave quantitative yield after 20h at 60ꢀC.
Methyl vinyl ketone (MVK) undergoes Michael addition under these
reaction conditions. When aromatic amine was used with MVK, a mono-
adduct was formed within shorter reaction time (Table 1, entry 19;
mixture of 96% of mono- and 4% bis-adducts were formed after 4 h).
In the case of 4-chloroaniline and 3,4-dichloroaniline, by using 1.5
equivalent of MVK, less than 10% of bis-adduct was formed after 4 h.
When 3 equivalents of MVK was used, bis-adduct was formed with a
longer reaction time (Table 1, entries 20 and 24).
We also examined the reaction of aniline with methyl acrylate using
different protic and aprotic solvents in the presence of silica-supported
aluminum chloride at 50–60 ꢀC. The results showed that solvent-free
conditions gave a better yield of b-amino compound (Table 2).
Silica-supported aluminum chloride catalyst can be used three times
without losing its activity. The recovered silica-supported aluminum

1112
M. R. Saidi, Y. Pourshojaei, and F. Aryanasab
Table 1. Michael addition of amines catalyzed by silica-supported aluminum
chloride under solvent-free conditions
Time Yield
(h)
Entry
1
Amine
Michael acceptor
Product
(%)
PhNH2
4.0 Quant.
2
1.5 Quant.
3
4
N. R
4.0
5.5
8.0
97
85
50
5
6
7
2.0 Quant.
2.0 Quant.
8
9
4.0
52
10
2.0 Quant.
(Continued )

Michael Addition of Amines
1113
Table 1. Continued
Time Yield
(h)
Entry
11
Amine
Michael acceptor
Product
(%)
PhNH2
6.0
75
63
12
13
14
1.5
6.5
7.0
90
60
15
16
2.0
5.0
86
40
17
18
2.0 72.5
10.0
15
19
20
PhNH2
4.0 Quant.a
7.0 Quant.a,b
(Continued )

1114
M. R. Saidi, Y. Pourshojaei, and F. Aryanasab
Table 1. Continued
Time Yield
(h)
Entry
21
Amine
Michael acceptor
Product
(%)
2.0
90
22
5.0
44
23
24
2.0 77.5
8.0 Quant.b,c
25
20.0 Quant.
a96% mono-substitution and 4% bis-substitution products.
b100% bis-substitution.
cThree equivalents of Michael acceptor were used.
chloride was tested in the reaction of aniline and methyl acrylate. The
catalyst was still active when it was used for the third time, but the yield
of the product decreased each time (Table 3).
A competition reaction was also carried out by using aniline
(1 mmol), morpholine (1 mmol), and methyl acrylate (1 mmol) in the
presence of silica-supported aluminum chloride at room temperature.
After 3 h, compound 6 was formed as major product with small amount
of 1 (Scheme 2).
The silica-supported aluminum chloride catalyst was prepared
according to the reported procedure in the literature.[6]

Michael Addition of Amines
1115
Table 2. Michael addition reaction of methyl acrlylate with aniline in protic and
aprotic solvent
Solvent
Yield (%)
C2H5OH
H2O
ClCH2CH2Cl
PhCH3
CH3CN
28
50
89
55
83
Table 3. Activity of the recovered silica-supported aluminum chloridea
Run
Yield (%)
1
2
3
97
69
65
a1 mmol aniline, 1.5 mmol methy1 acrylate, and 0.2 g silica-supported alumi-
num chloride were used each time.
Scheme 2. Competition reaction of aniline and morpholine with methyl acrylate.

1116
M. R. Saidi, Y. Pourshojaei, and F. Aryanasab
CONCLUSION
In conclusion, silica-supported aluminum chloride is among the most
efficient catalysts for Michael addition of amines, especially aromatic
amines, with different Michael acceptors under solvent-free conditions.
This process avoids the use of organic solvent to carry out the reaction,
and the catalyst can be separated from the product very easily after the
reaction. Handling of the catalyst is easy, and it can be used again.
EXPERIMENTAL
General Procedure for the Synthesis of Compounds 1
Aniline (1 mmol), methyl acrylate (1.5 mmol), and 0.2 g of catalyst
were placed into a small flask (or a test tube) and stirred at 60 ꢀC
for 4 h. The progress of the reaction was monitored with thin-layer
chromatography (TLC). Then ethyl acetate (10 mL) was added, and
the solution was filtered. The solid material was washed with 5 mL
ethyl acetate, and the combined organic filtrates were dried over
Na2SO4. The solvent was removed under reduced pressure, and the
residue was analyzed by its NMR spectra. In most cases, pure product
was obtained. If needed, the residue was separated by silica gel (Merck
80–240 mesh) column chromatography using a 4:1 n-hexane–EtOAc
mixture as eluent to afford the pure product. All of the products are
known, and the structure of the products were determined from their
NMR and IR spectra and by comparison with those reported in the
literature.
Selected Spectroscopic Data
Compound 5: 1H NMR (500 MHz, CDCl3, CCl4) d: 2.20 (s, 9H), 2.64 (t,
J ¼ 6.1 Hz, 2H), 3.24 (t, J ¼ 6.1 Hz, 2H), 3.49 (br, 1H), 3.74 (s, 3H), 6.80
(s, 2H).
Compound 6: 1H NMR (500 MHz, CDCl3, CCl4) d: 2.39 (t,
J ¼ 4.4 Hz, 4H), 2.42 (t, J ¼ 7.2 Hz, 2H), 2.60 (t, J ¼ 7.2 Hz, 2H), 3.61
(t, J ¼ 4.4 Hz, 4H), 3.63 (s, 3H).
Compound 10: 1H NMR (500 MHz, CDCl3, CCl4) d: 2.77 (t,
J ¼ 6.6 Hz, 2H), 3.28 (t, J ¼ 6.6 Hz, 2H), 3.60 (br, 1H), 6.90–7.48 (m, 5H).
Compound 12: 1H NMR (500 MHz, CDCl3, CCl4) d: 2.65 (t,
J ¼ 6.1 Hz, 2H), 3.53 (t, J ¼ 6.1 Hz, 2H), 4.00 (br, 1H), 6.55 (d, J ¼ 8.3 Hz,
2H), 7.14 (d, J ¼ 8.3 Hz, 2H).

Michael Addition of Amines
1117
1
Compound 19: H NMR (500 MHz, CDCl3, CCl4) d: 2.20 (s, 6H),
2.71 (t, J ¼ 6.8 Hz, 4H), 3.58 (t, J ¼ 6.8 Hz, 4H), 6.60 (d, J ¼ 7.4 Hz,
2H), 7.18 (d, J ¼ 7.4 Hz, 2H).
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