Silica sulfuric acid promotes aza-Michael addition reactions under solvent-free condition as a heterogeneous and reusable catalyst

Article abstract of DOI:10.3390/molecules14114779

A highly efficient, inexpensive, recyclable, convenient, and green protocol for chemoselective aza-Michael addition reactions of amines/thiols to α,β-unsaturated compounds using silica sulfuric acid (SSA or SiO ₂-SO₃H) was developed.

Full text of DOI:10.3390/molecules14114779

Molecules 2009, 14, 4779-4789; doi:10.3390/molecules14114779
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Silica Sulfuric Acid PromotesAza-Michael Addition Reactions
under Solvent-Free Condition as a Heterogeneous and Reusable
Catalyst
Yan Wang, Yan-Qin Yuan and Sheng-Rong Guo *
Department of Chemistry and Biology, Lishui University, Lishui, Zhejiang 323000, China
* Author to whom correspondence should be addressed; E-Mail: guosr9608@163.com.
Received: 3 November 2009; in revised form: 19 November 2009 / Accepted: 23 November 2009 /
Published: 23 November 2009
Abstract: A highly efficient, inexpensive, recyclable, convenient, and green protocol for
chemoselective aza-Michael addition reactions of amines/thiols to α,β-unsaturated
compounds using silica sulfuric acid (SSA or SiO2-SO3H) was developed. This method is
simple, convenient and the title compounds are produced in good to excellent yields.
Keywords: amines; thiols; silica sulfuric acid (SSA); Michael reaction; α,β-unsaturated
olefins
Introduction
The Michael reaction has been studied for over a century. The conjugate addition of amines to
carbon–carbon double bonds is a useful protocol in synthetic organic chemistry [1–4]. It is used
extensively in the synthesis of pharmaceutical intermediates, peptide analogues, antibiotics, and other
biologically active molecules and drugs [5–10]. In the past few years, a number of alternative
procedures have been developed for the conjugate addition of amines to α,β-unsaturated carbonyl
compounds. In particular, various Lewis acid catalyzed reactions have been reported. This reaction has
been investigated using catalysts such as lanthanum trichloride (LaCl3) [11], bromodimethylsulfonium
bromide [12], silica supported perchloric acid [13], cerium(IV) ammonium nitrate (CAN) [14,15],
β-cyclodextrin [16], zirconium(IV) chloride [17], samarium(III) triflate [18], ZrOCl2·8H2O on
montmorillonite K10 [19], N-methylimidazole [20], Cu(acac)2 /ionic liquid [21], 1,8-diazabicyclo[5.4.0]

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Molecules 2009, 14
undec-7-ene (DBU) [22], bismuth (III)triflate [23], cadmium chloride (CdCl2) [24], Y(NO3)3·6H2O
[25], and cellulose supported copper(0) [26]. Unfortunately, many of these processes suffer from
limitations, such as the use of expensive reagents, harsh conditions, relatively long reaction times, high
catalyst loading, low selectivity and tedious work-up procedures for their separation, recycling, or
disposal problems and effluent pollution. All these limitations prompted us to explore other better
catalysts, which have more efficiency and less limitations. We tried to find a catalyst that has certain
properties, such as the good thermal and mechanical stabilities of supported reagents, is easy to handle,
of low toxicity, non-corrosive, easy to separate from reaction mixtures through filtration, and feasible
for reuse.
Very recently, silica sulfuric acid (SSA) has been widely used as a reusable, heterogeneous,
inexpensive solid Brønsted acid catalyst and has received much attention [27–30] and there has been
increasingly awareness about the use of solid acids such as silica sulfuric acid (SSA) for synthesizing
organic intermediates and fine chemicals [31–34]. SSA is a strong Brønsted and Lewis acid,
presumably arising from the formation of SiO2-SO3H sites on the surface. 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. We hoped that SSA would be a
superior proton source to the standard acidic solid supports for running Michael reactions under
heterogeneous conditions, and herein we wish to report the use of SSA as a reusable solid acid catalyst
for the Michael addition reaction.
Results and Discussion
In a set of initial experiments, acrylic acid 2-phenylsulfanyl-ethyl este（r PTEA）was allowed to react
with morpholine in the presence of a varying quantities of SiO2-SO3H. The results show that an
excellent yield of the Michael adduct can be achieved by reacting a mixture of morpholine (1.2 equiv.)
and PTEA (1.0 equiv) in the presence of SiO2-SO3H (SSA, 100 mg) at room temperature (Scheme 1).
This encouraged us to exploit the generality and scope of this reaction catalyzed by SiO2-SO3H by
using other Michael acceptors with various aromatic and aliphatic amines. As shown in Table 1, the
Michael addition of various aliphatic amines and aryl amines carrying either electron-donating or
electron-withdrawing groups were successfully reacted with PTEA or PEEA to produce their
corresponding Michael adducts in high to excellent yields. The addition of primary amines such as
benzylamine resulted in only monoalkylated products; the aromatic amines showed poor reactivity
compared to the aliphatic amines. No side products were observed when using excess amines; pure
products could be obtained by removal of the catalyst by filtration followed by column
chromatography. The Michael addition of heteroaromatic imidazole with PTEA took more time
(analysis by TLC and GC) as compared with other amines and gave a low yield of product which may
be due to the weak nucleophilicity of the imidazole molecule steming from its aromaticity. The
reactions with 4-nitroaniline and 2-aminophenol did not proceed at all, even at higher temperatures
(entries 13, 14).

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Molecules 2009, 14
Scheme 1. Michael addition of aliphatic and aromatic amines with PTEA or PEEA.
O
O
SSA, r.t. 30min
solvent-free
NH
+
N
O
OCH CH S
O
2
2
S
(PTEA)
O
H
NH
+
2
0
O
N
O
SSA, 60
C
S
R
R
OCH CH S
solvent-free
O
2
2
(PEEA)
Table 1. Michael addition of amines catalyzed by SSA under solvent-free conditions.
Michael
acceptor
Time
(h)
Yield
(%) b
Entry
Amine
Product
O
1
PTEA
PTEA
PTEA
PTEA
PTEA
0.5
1.5
0.5
1.0
1.5
92
N
OCH2CH2S
NH2
H
1
H3C
H3CO
Cl
H C
3
O
2
3
4
5
88
90
96
85
N
OCH2CH2S
OCH2CH2S
NH
2
H
2
H3CO
Cl
O
N
NH2
H
3
O
N
H
OCH CH S
2 2
NH2
4
Cl
Cl
Cl
Cl
O
N
H
OCH2CH2S
NH2
5
O
NH
N
OCH CH S
2 2
6
7
PTEA
PTEA
PTEA
PTEA
PTEA
0.5
1.5
1.5
2.0
4.0
95
93
88
86
12
O
O
6
O
NH
N
OCH2CH2S
7
O
NH
N
OCH2CH2S
8
8
O
NH2
N
OCH2CH2S
9
H
9
O
NH
N
OCH2CH2S
10
N
N
10

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Molecules 2009, 14
Table 1. Cont.
Product
Michael
acceptor
Time
(h)
Yield
(%) b
Entry
Amine
SH
NH
O
11
PTEA
0.5
0.5
94
S
OCH2CH2S
2
NH2
11
HO
Et
HO
Et
O
12
13
14
PTEA
PTEA
PTEA
91
S
OCH2CH2S
SH
OH
12
No Reaction
NH2
O2N
No Reaction
NH2
O
15
16
17
PEEA
PEEA
PEEA
2.5
3.0
1.0
86
90
89
N
H
OCH CH S
2 2
NH2
13
H C
3
H3C
O
N
OCH CH S
2 2
NH2
H
14
H CO
3
H3CO
O
N
OCH CH S
2 2
NH2
H
15
O
NH
N
OCH CH S
2 2
18
PEEA
0.5
92
O
O
16
An experiment was performed to show the chemoselectivity between thiols and aniline by using
2-aminobenzenethiol (1.2 mmol) and PTEA (1.0 mmol) in the presence of SSA (0.1 g) at room
temperature. After 30 min, compound 11 was the only product (Scheme 2).
Scheme 2. Michael addition of 2-aminobenzenethiol to PTEA.
O
SH
S
O
SSA, r.t.
S
+
OCH2CH2S
O
solvent-free
NH2
NH2
Another competition reaction was also carried out by using 3-ethyl-4- hydroxythiophenol (1.2
mmol), and PTEA (1 mmol) in the presence of SSA (0.1 g) at room temperature, and compound 12
was formed as the only product (Scheme 3). From the above two experiments, we conclude that the
activity of the amines and phenols are both less than that of thiols.

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Molecules 2009, 14
Scheme 3. Michael addition of 3-ethyl-4-hydroxythiophenol to PTEA.
SH
+
O
S
O
SSA, r.t.
S
OCH2CH2S
O
HO
Et
solvent-free
HO
Et
We also examined the reaction of aniline with PTEA using different protic and aprotic solvents in
the presence of SiO2-SO3H at different temperature. The results showed that solvent-free conditions
gave a better yield of β-amino compound (Table 2).
Table 2. Michael addition reaction of PTEA with aniline in protic and aprotic solvents.
Entry
Solvent
Yield (%)
1
2
3
4
5
6
7
8
C2H5OH
H2O
72
34
78
83
41
86
86
91
CH3CO2H
ClCH2CH2Cl
Toluene
CH3CN
CH2Cl 2
Neat
The catalyst system was found to be recyclable and the reaction condition can be scaled up. In order
to test the reusability of the catalyst, a reaction of PTEA (10 mmol) and morpholine (12 mmol) was
carried out in the presence of SSA (1.0 g) and the catalyst was recovered after completion and
activated by heating at 100 °C under vacuum for 1 h. The recovered catalyst was reused for the
aza-Michael reaction of another batch of PTEA (10 mmol) and morpholine (12 mmol) giving 90%
yield of the desired product after 1 h. Again, the catalyst was recovered, reactivated and reused
repeatedly for three more consecutive times for aza-Michael reactions affording 85%, 70% and 70%
yields, respectively. From this observation, it is clear that the reaction can be scaled up and the catalyst
is reusable with a slight decrease in catalytic activity.
To compare the catalytic activity of SiO2-SO3H with ordinary silica, an identical experiment was
performed by stirring an intimate mixture of PTEA (1.0 mmol), morpholine (1.2 mmol) and silica
(240-400 mesh, 100 mg) at room temperature. No aza-Michael addition product was formed even after
24 h, which indicate that the requirement of SiO2-SO3H is key factor for the successful outcome of the
reaction. Furthermore, using of SiO2-SO3H in the aza-Michael addition reduces the cost of the
catalysts, and the reaction completed in less time. It is not very clear whether SiO2-SO3H act as a solid
acid catalyst or as a reservoir providing a small concentration of acid in solution. However, from the
experiments carried out using recovered SiO2-SO3H suggest that it acts as a solid acid in the
aza-Michael reactions.

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Molecules 2009, 14
Experimental
General
All reagents were purchased from commercial sources and used without further purification. TLC
analysis was performed with glass backed plates precoated with silica gel and examined under UV
(254 nm). NMR spectra were measured in CDCl3 with Me4Si as the internal standard on a Bruker
Advance 300 instrument at room temperature. IR spectra were recorded on Bruker FT-IR spectrometer,
absorbances are reported in cm-1. Elemental analyses were performed on a Perkin–Elmer-2400
elemental analyzer.
General procedure for the synthesis of β-amino carbonyl compounds
A mixture of aniline (1.2 mmol) and Michael acceptor PTEA (1.0 mmol or PEEA 1.0 mmol) and
SSA (0.1 g) was stirred at room temperature for 0.5–6 h. The reaction was monitored by TLC. Upon
completion, EtOAc (5 mL) was added and the SiO2-SO3H catalyst can be reused after it was removed
by filtration. The filtrate was washed with saturated NaHCO3 (aq) and brine, then dried with anhydrous
Na2SO4, and concentrated under vacuum. The residue was analyzed by NMR. In most cases, pure
product was obtained. If needed, the residue was further purified by silica gel column chromatography
using a 4:1 hexane–EtOAc mixture as eluent to afford the pure product. All of the products are new
compounds, and the structures of products were determined from their NMR, MS or elemental
analyses.
Selected spectroscopic data
1
3-Phenylaminopropionic acid 2-phenylsulfanyl-ethyl ester (1): yellow oil; H-NMR δ: 7.42 (d,
J = 7.7 Hz, 2H), 7.18-7.34 (m, 5H), 6.75 (t, J = 7.3 Hz, 1H), 6.62 (d, J = 8.0 Hz, 2H), 4.30 (t,
J = 6.8 Hz, 2H), 3.95 (br 1H), 3.46 (t, J = 6.3 Hz, 2H), 3.17 (t, J = 6.8 Hz, 2H), 2.62 (t, J = 6.3 Hz, 2H);
13C-NMR δ: 172.1, 147.5, 134.9, 130.1, 129.3, 129.1, 126.7, 117.8, 113.1, 63.0, 39.4, 33.9, 32.5; Anal.
Calcd. for C17H19NO2S: C, 67.74; H, 6.35. Found: C, 67.76; H, 6.34.
1
3-p-Tolylaminopropionic acid 2-phenylsulfanyl-ethyl ester (2): yellow oil; H-NMR δ: 7.43-7.21 (m,
5H), 7.02 (d, J = 8.2 Hz, 2H), 6.58 (d, J = 8.2Hz, 2H), 4.29 (t, J = 6.8 Hz, 2H), 3.93 (br, 1H), 3.44 (t,
13
J = 6.3 Hz, 2H), 3.16 (t, J = 6.8 Hz, 2H), 2.60 (t, J = 6.2 Hz, 2H), 2.27(s, 3H); C-NMR δ: 172.2,
145.2, 134.9, 130.1, 129.8, 129.1, 127.0, 126.7, 113.3, 62.9, 39.8, 33.8, 32.5, 20.4; Anal. Calcd. for
C18H21NO2S: C, 68.54; H, 6.71. Found: C, 68.56; H, 6.68.
1
3-(4-Methoxyphenylamino)-propionic acid 2-phenylsulfanyl-ethyl ester (3): Off-white oil; H-NMR δ:
7.23-7.42 (m, 5H), 6.80 (q, J = 2.3 Hz, J = 6.9 Hz, 2H), 6.62 (d, J = 2.3 Hz, J = 6.9 Hz, 2H), 4.29 (t,
J = 6.8 Hz, 2H), 3.76 (s, 3H), 3.43 (br, 1H), 3.40 (t, J = 6.3 Hz, 2H), 3.16 (t, J = 6.8 Hz, 2H), 2.58 (t,
J = 6.3 Hz, 2H); 13C-NMR δ: 172.2, 152.5, 141.7, 135.0, 129.8, 129.0, 126.7, 114.9, 114.6, 63.9, 55.8,
40.5, 33.9, 32.5; Anal. Calcd. for C18H21NO3S: C, 65.23; H, 6.39. Found: C, 65.15; H, 6.34.

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Molecules 2009, 14
1
3-(4-Chlorophenylamino)propionic acid 2-phenylsulfanyl-ethyl ester (4): Off-white oil; H-NMR δ:
7.41 (d, J = 1.5 Hz, 2H), 7.23-7.39 (m, J = 6.4 Hz, 3H), 7.13 (t, J = 2.2 Hz, 2H), 6.59 (t, J = 2.0 Hz,
2H), 4.29 (t, J = 6.8 Hz, 2H), 3.97 (br, 1H), 3.42 (t, J = 6.2 Hz, 2H), 3.16 (t, J = 6.8 Hz, 2H), 2.58 (t,
13
J = 6.2, 2H); C-NMR δ: 171.0, 143.9, 136.4, 129.6, 129.3, 128.9, 126.1, 125.1, 114.9, 63.6, 39.0,
37.8, 35.7; Anal. Calcd. for C17H18ClNO2S: C, 60.80; H, 5.40. Found: C, 60.83; H, 5.42.
1
3-(3,4-Dichlorophenylamino)propionic acid 2-phenylsulfanyl-ethyl ester (5): yellow oil; H-NMR δ:
7.56-7.78 (m, 3H), 7.14-7.21 (m, 3H), 6.71 (d, J = 8.2 Hz, 2H), 5.59 (br, 1H), 4.27 (t, J = 6.8 Hz, 2H),
13
3.36 (t, J = 6.3 Hz, 2H), 3.12 (t, J = 6.8 Hz, 2H), 2.60 (t, J = 6.3 Hz, 2H); C-NMR δ: 172.3, 155.8,
136.1, 134.5, 132.9, 131.4, 131.1, 130.6, 127.6, 124.2, 116.4, 63.2, 34.5 , 31.4, 22.5; Anal. Calcd. for
C17H17Cl2NO2S: C, 55.14; H, 4.63. Found: C, 55.10; H, 4.65.
1
3-Morpholin-4-ylpropionic acid 2-phenylsulfanyl-ethyl ester (6): Off-white oil; H-NMR δ: 7.42-7.38
(m, 2H), 7.28-7.34 (m, 2H), 7.25-7.22 (m, 1H), 4.27 (t, J = 6.8 Hz, 2H), 3.73-3.71 (m, 4H), 3.15 (t,
13
J = 6.8 Hz, 2H), 2.71 (t, J = 7.2 Hz, 2H), 2.38-2.46 (m, 6H); C-NMR δ: 171.9, 135.1, 129.9, 129.1,
126.6, 66.5, 63.0, 53.6, 53.1, 32.4, 31.4; Anal. Calcd. for C15H21NO3S: C, 60.99; H, 7.17. Found: C,
60.97; H, 7.16.
1
3-Diethylamino-propionic acid 2-phenylsulfanyl-ethyl ester (7): Off-white oil; H-NMR δ: 7.13-7.28
(m, 5H), 4.46 (t, J = 6.8 Hz, 2H), 3.74 (t, J = 6.3 Hz, 2H), 3.12 (t, J = 6.8 Hz, 2H), 2.78 (t, J = 6.3 Hz,
2H), 2.43 (q, J = 6.2 Hz, 4H), 1.10 (t, J = 6.2 Hz, 6H); 13C-NMR δ: 172.1, 135.6, 128.7, 126.6, 124.9,
66.5, 48.6, 46.8, 53.1, 34.8, 16.7; Anal. Calcd. for C15H23NO2S: C, 64.02; H, 8.24. Found: C, 64.04;
H, 8.25.
1
3-Piperidin-1-yl-propionic acid 2-phenylsulfanyl-ethyl ester (8): Off-white oil; H-NMR δ: 7.75-7.85
(m, 3H), 7.43-7.51 (m, 2H), 4.33 (t, J = 6.8 Hz, 2H), 3.68 (t, J = 7.2 Hz, 2H), 2.63 (t, J = 6.8 Hz, 2H),
2.44 (t, J = 7.2 Hz, 2H), 2.34 (br, 10H); 13C-NMR δ: 171.9, 135.1, 129.9, 129.1, 126.6, 66.5, 63.0, 53.6,
53.1, 32.4, 31.4, 29.8; Anal. Calcd. for C16H23NO2S: C, 65.49; H, 7.90. Found: C, 65.51; H, 7.91.
3-Benzylaminopropionic acid 2-phenylsulfanyl-ethyl ester (9): Off-white oil; 1H-NMR δ: 7.21-7.38 (m,
10H), 4.27 (t, J = 6.8 Hz, 2H), 3.81(s, 2H), 3.16 (t, J = 6.3 Hz, 2H), 2.90 (t, J = 6.8 Hz, 2H), 2.52 (t,
J = 6.3 Hz, 2H); 13C-NMR δ: 172.5, 140.1, 135.1, 130.0, 129.1, 128.5, 128.0, 127.0, 126.6, 62.8, 53.8,
44.4, 34.7, 32.4; Anal. Calcd. for C18H21NO2S: C, 68.54; H, 6.71. Found: C, 68.53; H, 6.68.
1
3-Imidazol-1-yl-propionic acid 2-phenylsulfanyl-ethyl ester (10): Off-white oil; H-NMR δ: 7.56 (s,
1H), 6.97-7.09 (m, 5H), 6.87 (d, J = 7.3 Hz, 2H), 4.36 (t, J = 6.8 Hz, 2H), 3.96 (t, J = 6.3 Hz, 2H), 3.16
13
(t, J = 6.8 Hz, 2H), 2.60 (t, J = 6.3 Hz, 2H); C-NMR δ: 170.2, 141.3, 129.9, 127.9., 126.7, 125.2,
124.8, 122.9, 65.6, 39.5, 32.4, 31.6; Anal. Calcd. for C14H16N2O2S: C, 60.85; H, 5.84. Found: C, 60.87;
H, 5.82.
3-(2-Aminophenylsulfanyl)propionic acid 2-phenylsulfanyl-ethyl ester (11): Off-white Oil; 1H-NMR δ:
7.15-7.42 (m, J = 7.3 Hz, 7H), 6.67-6.75 (m, 2H), 4.40 (br, 2H), 4.25 (t, J = 6.8 Hz, 2H), 3.15 (t,

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Molecules 2009, 14
13
J = 6.3 Hz, 2H), 2.96 (t, J = 6.8 Hz, 2H), 2.54 (t, J = 6.3 Hz, 2H); C-NMR δ: 171.7, 148.7, 136.6,
135.0, 130.2, 130.0, 129.1, 126.7, 118.5, 116.4, 115.0, 63.2, 34.5, 32.4, 29.5; Anal. Calcd. for
C17H19NO2S2: C, 61.23; H, 5.74. Found: C, 61.25; H, 5.72.
1
3-(3-Ethyl-4-hydroxyphenylsulfanyl)propionic acid 2-phenylsulfanyl-ethyl ester (12): yellow oil; H-
NMR δ: 7.15-7.43 (m, 6H), 6.69-6.72 (m, J = 8.2 Hz, 2H), 5.56 (br, 1H), 4.27 (t, J = 6.8 Hz, 2H), 3.15
(t, J = 6.3 Hz, 2H), 3.02 (t, J = 6.8 Hz, 2H), 2.54-2.66 (m, 4H), 1.24 (t, J = 7.6 Hz, 3H); 13C-NMR δ:
172.1, 153.6, 135.0, 134.1, 131.7, 131.2, 130.0, 129.1, 126.7, 124.7, 115.9, 63.2, 34.5, 32.4, 31.1, 22.9,
13.9; Anal. Calcd. for C19H22O3S2: C, 62.95; H, 6.12. Found: C, 62.92; H, 6.14.
1
3-Phenylaminopropionic acid 2-(naphthalen-2-ylsulfanyl)-ethyl ester (13): yellow oil; H-NMR δ:
7.73-7.81 (m, 4H), 7.42-7.52 (m, 4H), 7.04 (d, J = 8.1 Hz, 2H), 6.45 (d, J = 8.1 Hz, 2H), 4.29 (t,
J = 6.7 Hz, 2H), 3.91 ( br, 1H), 3.41 (t, J = 6.3 Hz, 2H), 3.24 (t, J = 6.7 Hz, 2H), 2.57 (t, J = 6.3 Hz,
13
2H); C-NMR δ: 172.0, 145.6, 132.9, 132.6, 131.7, 129.9, 128.2, 128.2, 127.9, 127.5, 127.2, 126.4,
126.2, 116.7, 112.8, 63.4, 39.5, 33.3, 32.1; Anal. Calcd. for C21H21NO2S: C, 71.76; H, 6.02. Found: C,
71.79; H, 6.00.
3-p-Tolylaminopropionic acid 2-(naphthalen-2-ylsulfanyl)-ethyl ester (14): white solid, m.p. 75-77 °C;
1H-NMR δ: 7.76-7.83 (m, 4H), 7.49 (t, J = 8.4 Hz, 3H), 7.01 (d, J = 8.1 Hz, 2H), 6.56 (d, J = 8.1 Hz,
2H), 4.32 (q, J = 6.7 Hz, 2H), 3.90 ( br, 1H), 3.41 (t, J = 6.3 Hz, 2H), 3.25 (m, J = 6.7 Hz, 2H), 2.57 (t,
13
J = 6.3 Hz, 2H), 2.51 (s, 3H); C-NMR δ: 172.2, 145.2, 133.7, 132.4, 132.0, 129.8, 128.7, 128.2,
128.1, 127.7, 127.2, 127.0, 126.7, 126.0, 113.4, 62.9, 39.8, 33.9, 32.4, 20.4; Anal. Calcd. for
C22H23NO2S: C, 72.30; H, 6.34. Found: C, 72.32; H, 6.36.
3-(4-Methoxyphenylamino)propionic acid 2-(naphthalen-2-ylsulfanyl)-ethyl ester (15): Off-white oil;
1H-NMR δ: 7.78-7.89 (m, 4H), 7.46-7.54 (m, 3H), 6.82 (d, J = 8.8 Hz, 2H), 6.63 (d, J = 8.8 Hz, 2H),
4.35 (t, J = 6.8 Hz, 2H), 3.82 (br, 1H), 3.75 (s, 3H), 3.38 (t, J = 6.3 Hz, 2H), 3.27 (t, J = 6.3 Hz, 2H),
13
2.58 (t, J = 6.8 Hz, 2H); C-NMR δ: 172.2, 152.6, 141.7, 133.8, 132.5, 132.0, 128.7, 128.0, 127.9,
127.7, 127.2, 126.7, 126.0, 115.1, 114.8, 63.1, 55.8, 40.6, 33.9, 32.3; Anal. Calcd. for C22H23NO3S: C,
69.26; H, 6.08. Found: C, 69.29; H, 6.05.
1
3-Morpholin-4-yl-propionic acid 2-(naphthalen-2-ylsulfanyl)-ethyl ester (16): yellow oil; H-NMR δ:
7.73-7.83 (m, 4H), 7.40-7.49 (m, 3H), 4.31 (t, J = 6.8 Hz, 2H), 3.67 (t, J = 4.6 Hz, 4H), 3.23 (t,
J = 6.8 Hz, 2H), 2.64 (t, J = 7.3 Hz, 2H), 2.41-2.46 (tt, J = 7.3 Hz, J = 4.6 Hz, 6H); 13C-NMR δ: 171.0,
135.1, 133.9, 128.3, 127.2, 126.5, 126.3, 126.1, 124.7, 66.5, 63.0, 53.6, 53.1, 32.4, 31.4; Anal. Calcd.
for C19H23NO3S: C, 66.06; H, 6.71. Found: C, 66.08; H, 6.73.
Conclusions
In conclusion, we have described herein the scope and limitations of the use of silica sulfuric acid
(SSA or SiO2-SO3H) as a highly efficient catalyst for Michael additions of amines under solvent-free
conditions. This process avoids the use of organic solvent to carry out the reaction, and the catalyst can

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Molecules 2009, 14
be separated from the product very easily after the reaction. Handling of the catalyst is easy, and it can
be used again.
Acknowledgements
This work was funded by the Natural Science Foundation of Zhejiang Province, P.R. China (Project
Y407240) and key project of Lishui University.
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