Silica-supported LiHSO4 as a highly efficient, heterogeneous and reusable catalytic system for the solvent-free synthesis of β-enaminones and β-enamino esters

Article abstract of DOI:10.1007/BF03245861

A highly efficient, simple and green procedure for the synthesis of β-enaminones and β-enamino esters is described. The reaction of aromatic and aliphatic amines with β-dicarbonyl compounds using catalytic amount of silica-supported LiHSO₄ (LiHSO₄/SiO₂) under solvent-free conditions at 80 °C affords the title compounds in high to excellent yields and in short reaction times.

Full text of DOI:10.1007/BF03245861

J. Iran. Chem. Soc., Vol. 7, No. 1, March 2010, pp. 69-76.
JOURNAL OF THE
Iranian
Chemical Society
Silica-Supported LiHSO4 as a Highly Efficient, Heterogeneous and Reusable Catalytic
System for the Solvent-Free Synthesis of ꢀ-Enaminones and ꢀ-Enamino Esters
A. Hasaninejada,*, A. Zareb,*, M.R. Mohammadizadeha and M. Shekouhya
aDepartment of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
bDepartment of Chemistry, Payame Noor University (PNU), Iran
(Received 18 November 2008, Accepted 4 January 2009)
A highly efficient, simple and green procedure for the synthesis of ꢀ-enaminones and ꢀ-enamino esters is described. The
reaction of aromatic and aliphatic amines with ꢀ-dicarbonyl compounds using catalytic amount of silica-supported LiHSO4
(LiHSO4/SiO2) under solvent-free conditions at 80 °C affords the title compounds in high to excellent yields and in short reaction
times.
Keywords: LiHSO4/SiO2, Solvent-free, ꢀ-Enaminone, ꢀ-Enamino ester, ꢀ-Dicarbonyl compound, Green chemistry
INTRODUCTION
which makes synthesis simpler, saves energy, and prevents
solvent waste, hazards, and toxicity [3]. Consequently, a safe
catalysis in combination with solventless conditions would
provide for a more suitable technique to facilitate the so-called
“ideal synthesis”. In this paper, we wish to introduce silica-
supported LiHSO4 as a new catalyst for organic synthesis. The
In recent years, the use of solid-supported catalysts has
gained considerable attention both in industrial and academia
research due to their unique properties such as enhanced
reactivity as well as selectivity, straightforward workup,
recyclability of the catalyst and the eco-friendly reaction
conditions [1,2]. Silica gel is one of the most interesting solid-
supports because it has surface properties that suggest that
very rich organic reactions may be at work. SiO2 is an
-
strong oxophilicity of Li+ as well as H+ of HSO4 powerfully
activate oxygen-containing electrophiles for nucleophilic
attack [4].
ꢀ-Enaminone and ꢀ-enamino ester derivatives are
significant because they have been extensively applied as
synthons for the synthesis of various biologically active
compounds, such as anti-epileptic [5], antibacterial [6a,b],
anti-inflammatory [6b], anticonvulsant [6b,c], antitumor
[6b,d], and other trapiutic agents [7]. Therefore, there is a
great deal of interest in the synthesis of this class of
compounds. The direct condensation of amines with ꢀ-
dicarbonyl compounds has been used as a useful synthetic
route toward ꢀ-enaminones and ꢀ-enamino esters [8-23].
Several catalysts have been applied to achieve this
transformation, including NaAuClO4 [8], Bi(TFA)3 [9],
inexpensive,
reusable,
commercially
available
and
environmentally benign which, accompanied with different
catalysts, has been used in various organic transformations [2].
Nevertheless, most of the existing processes in organic
synthesis involve toxic and volatile organic solvents as
reaction media which are environmentally unacceptable from
the green chemistry view point. One of the most effective
techniques to solve this problem is solvent-free conditions
*Corresponding author. E-mail: ahassaninejad@yahoo.com;
abdolkarimzare@yahoo.com

Hasaninejad et al.
O
NH2
O
Sc(OTf)3 [10], Zn(ClO4)2.6H2O/MgSO4 [11], Yb(OTf)3 [12],
LiHSO4/SiO2 (20 mol%)
CeCl3.7H2O [13], [EtNH3]NO3 [14], ZrOCl2.8H2O [15],
HClO4.SiO2 [16], I2 [17], silica chloride [18], LaCl3.7H2O
[19], Bi(TFA)3/tetrabutylammonium bromide [20], heteropoly
acids [21], ceric(IV) ammonium nitrate [22] and
phoaphotungstic acid [23]. There are also some synthetic
approaches to ꢀ-enaminones including condensation of methyl
ketones with dimethylformamide dimethylacetal [24],
cyclization of amino acids [25], aminolysis of dithioacetals
[26], reductive cleavage of isoxazoles [27] and substitution of
the imidoylbenzotriazoles with trimethylsilyl (TMS) enol
ethers [28]. However, these reported methods are associated
with one or more of the following drawbacks: (i)
unsatisfactory yield, (ii) long reaction time, (iii) low
selectivity, (iv) no generality, (v) the use of non-available and
expensive reagents, (vi) tedious work-up procedure, and (vii)
lack of agreement with the green chemistry protocols.
Therefore, it seems highly desirable to find an efficient,
simple, general, green, and cheap protocol for the synthesis of
ꢀ-enaminones and ꢀ-enamino esters.
+
80 °C, Solvent-free
4 min
N
H
94%
O
Scheme 1
LiHSO4 (2.080 g) as a white powder. The HCl gas produced
was trapped with NaOH solution.
General Procedure for the Preparation of
LiHSO4/SiO2 Catalytic System
A mixture of LiHSO4 (0.5 g) and SiO2 (9.5 g) was ground
vigorously to give LiHSO4/SiO2 catalytic system as a white
powder (10 g).
General Procedure for the Synthesis of ꢀ-Enaminone
and ꢀ-Enamino Ester Derivatives
To a mixture of amine (1 mmol) and ꢀ-dicarbonyl
compound (1 mmol) in a 10 ml round-bottomed flask
connected to a reflux condenser was added LiHSO4/SiO2
(0.416 g, 20 mol%) and the resulting mixture was stirred in an
oil-bath (80 °C). After completion of the reaction (monitored
by TLC), the reaction mixture was cooled to room
temperature. To the resulting mixture, was added EtOAc (15
ml), stirred for 5 min and filtered to separate the catalyst. The
solvent of the filtrate was evaporated and the crude product
was purified by recrystallization from EtOH/H2O (1/1). The
recycled catalyst was washed by EtOAc, dried and re-used.
In this paper, we report a new, clean and highly efficient
method for the synthesis of ꢀ-enaminones and ꢀ-enamino
esters via ꢀ-enamination of ꢀ-dicarbonyl compounds using
LiHSO4/SiO2 as
a new, non-toxic, readily producible,
heterogeneous and reusable catalytic system (Scheme 1).
EXPERIMENTAL
Chemicals and Apparatus
Selected Spectral Data of the Products
All chemicals were purchased from Merck or Fluka
Chemical Companies. Plate silica gel, (Mesh 60, 15-40 μm)
was used as support. All known compounds were identified by
4-(5,5-Dimethyl-3-oxocyclohex-1-enylamino)phenyl
1
acetate (Table 3, entry 4). White solid; H NMR (CDCl3): ꢁ
1
(ppm) 1.13 (s, 6H), 2.10 (s, 2H), 2.24 (s, 2H), 2.76 (s, 3H),
5.73 (s, 1H), 7.02 (d, J = 7.1 Hz, 2H), 7.12 (s, 1H), 7.84 (d, J =
7.1 Hz, 2H); 13C NMR (CDCl3): ꢁ (ppm) 26.4, 28.1, 32.8,
43.6, 50.3, 100.6, 115.8, 121.2, 140.3, 143.2, 158.8, 196.8,
198.4; Anal. Calcd. for C16H19NO3: C, 70.31; H, 7.01; N, 5.12.
Found: C, 70.14; H, 7.13; N, 5.02.
comparison of their melting points and H NMR data with
1
those in the authentic samples. The H NMR (250 MHz) and
13C NMR (62.5 MHz) were run on a Bruker Avance DPX-250,
FT-NMR spectrometer. Microanalysis was performed on a
Perkin-Elmer 240-B microanalyzer. Melting points were
recorded on a Stuart Scientific Apparatus SMP3 (UK) in open
capillary tubes.
3-(Furan-2-ylmethylamino)-5,5-dimethylcyclohex-2-
1
enone (Table 3, entry 9). White solid; H NMR (CDCl3): ꢁ
(ppm) 1.05 (s, 6H), 1.90 (s, 2H), 1.97 (s, 2H), 4.23 (s, 2H),
4.88 (s, 1H), 5.06 (s, 1H), 6.25 (d, J = 7.7 Hz, 1H), 6.31 (m,
1H), 7.34 (d, J = 5.6 Hz, 1H); 13C NMR (CDCl3): ꢁ (ppm)
General Procedure for the Preparation of LiHSO4
A mixture of LiCl (0.848 g, 20 mmol) and H2SO4 (2.002 g,
20 mmol) was stirred under N2 gas for 120 min to give
70

Silica-Supported LiHSO4 as a Highly Efficient Catalytic System
27.3, 31.5, 40.1, 43.3, 50.2, 96.3, 107.8, 110.5, 142.3, 149.8;
165.6, 196.8; Anal. Calcd. for C13H17NO2: C, 71.21; H, 7.81;
N, 6.39. Found: C, 71.10; H, 7.68; N, 6.47.
effect of different molar ratios of LiHSO4/SiO2 on ꢀ-
enamination of dimedone (1 mmol) with aniline (1 mmol)
(Scheme 1) under solvent-free conditions at a range of 25-100
ºC. The results are summarized in Table 1. As Table 1
indicates, the best results were observed when the reaction was
performed in the presence of 20 mol% of LiHSO4/SiO2 at 80
ºC.
The efficiency and capacity of the solvent-free procedure,
in comparison to solution condition, was also studied. For
thispurpose, a mixture of aniline (1 mmol), dimedone (1
mmol) and LiHSO4/SiO2 (20 mol%) was stirred in some
solvents (10 ml) at 80 ºC or under reflux conditions (Table 2).
As it can be seen from Table 2, the solvent-free method
afforded higher yield and shorter reaction time.
(Z)-4-(3-Oxo-1,3-diphenylprop-1-enylamino)phenyl
1
acetate (Table 3, entry 18). H NMR (CDCl3): ꢁ (ppm) 2.49
(s, 3H ), 5.28 (s, 1H), 6.77 (d, J = 7.5 Hz, 2H), 7.39-7.51 (m,
8H), 7.97 (d, J = 7.5 HZ, 2H), 8.10 (d, J = 7.0 Hz, 2H), 12.88
(s, 1H); 13C NMR (CDCl3): ꢁ (ppm) 26.1, 99.0, 113.8, 121.6,
127.4, 128.9, 129.3, 130.1, 130.8, 131.8, 132.2, 135.5, 139.4,
144.1, 160.0, 190.3, 196.8; Anal. Calcd. for C23H19NO3: C,
77.29; H, 5.36; N, 3.92. Found: C, 77.08; H, 5.24; N, 4.06.
RESULTS AND DISCUSSION
To optimize the reaction conditions, we first studied the
After optimization of the reaction conditions, different
Table 1. Effect of Different Molar Ratios of LiHSO4/SiO2 and Temperature on the Reaction of
Aniline with Dimedone under Solvent-free Conditions
Entry Molar Ratio of LiHSO4/SiO2 (%)
Temperature (°C)
Time (min) Yield (%)a
1
2
3
4
5
6
7
8
9
5
10
20
30
20
20
20
20
20
80
80
80
80
25 (r.t.)
50
70
90
100
45
12
4
37
85
94
91
Trace
18
83
94
89
3
120
60
15
4
3
aIsolated yield.
Table 2. Comparative the ꢀ-Enamination of Dimedone with Aniline using
LiHSO4/SiO2 in Solution Conditions vs. the Solvent-free Method
Entry Solvent
Temperature (°C)
Time (min)
Yield (%)a
1
2
3
4
5
6
Solvent-free
80
4
60
60
60
60
94
61
54
35
24
-
EtOH
CH2Cl2
CH3CN
THF
Reflux
Reflux
Reflux
Reflux
80
H2O
120
aIsolated yield.
71

Hasaninejad et al.
aromatic and aliphatic amines were condensed with some ꢀ-
electron-releasing substituents or halogens on the aromatic
ring of aromatic amines had no significant effect on the
reaction results (Table 3, entries 2-5, 12, 18 and 20-22);
however, electron-withdrawing substituents slightly decreased
dicarbonyl compounds to furnish the desired ꢀ-enaminones
and ꢀ-enamino esters in high to excellent yields and in short
reaction times (Table 3). It was observed that the presence of
Table 3. Synthesis of ꢀ-Enaminones and ꢀ-Enamino Esters using LiHSO
4/SiO2 under Solvent-Free Conditions
at 80 °C
Time
(min)
Yield
(%)a
M.p. °C
(Lit.)
Entry
1
Amine
Product
184-186
(185) [29]
NH2
NH2
4
94
O
NH2
OMe
126-128
(129-131) [30]
2
8
89
N
H
OMe
O
O
MeO
AcO
Br
192-194
MeO
AcO
Br
NH2
3
3
7
95
92
93
87
95
93
94
89
(195) [29]
N
H
NH2
4
225-227
219-220
182-184
N
H
O
NH2
5
6
N
H
O
NH2
NH2
NH2
6
10
4
NC
N
H
NC
O
128-129
(125) [29]
7
N
H
O
O
O
8
5
118-119
149-151
117-119
N
H
N
O
N
O
NH2
9
6
N
H
NH2
10
7
N
H
72

Silica-Supported LiHSO4 as a Highly Efficient Catalytic System
Table 3. Continued
47-49
NH2
11
10
8
91
NH
O
(47) [31]
Me
O
66-68
(65-67) [32]
Me
NH2
12
13
93
85
NH
O
111-114
(112-114) [33]
HN
NH O
NH2
20
H2N
109-111
NH
O
NH2
NH2
14
12
91
(108) [12]
NH
O
61-63
(59) [34]
15
16
12
30
94
87
O
HN
NH O
182-184
(180) [9]
NH2
H2N
100-103
NH
O
NH2
17
18
30
40
87
84
(99-101) [35]
AcO
NH
O
AcO
NH2
149-150
Oil
NH2
19
20
21
22
23
27
18
24
33
25
88
91
89
87
86
NH
O
(Oil) [33]
OEt
O
MeO
Me
Cl
45-47
(44.5-45) [31]
MeO
Me
Cl
NH2
NH
OEt
Oil
(Oil) [33]
NH2
NH2
NH
NH
O
O
OEt
OEt
57-58
(53-54) [32]
NH2
NH
O
Oil
(Oil) [33]
OEt
aIsolated yield.
73

Hasaninejad et al.
Table 4. The Condensation of Aniline with Dimedone in
the Presence of Recycled LiHSO4/SiO2
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Entry
Cycle
1st use
2nd use
3rd use
4th use
5th use
Time (min)
Yield (%)a
1
2
3
4
5
4
5
94
94
93
90
89
8
10
15
aIsolated yield.
the yields and increased the reaction times (Table 3, entry 6).
The condensation of aliphatic amines as well as diamines with
ꢀ-dicarbonyl compounds was also efficiently performed
(Table 3, entries 7-10, 13, 15, 16 and 23).
Ease of recycling of the catalyst is the most significant
advantage of our method. For the reaction of aniline with
dimedone no significant loss of the product yield was
observed when LiHSO4/SiO2 was reused after four times of
recycling (please see Table 4).
CONCLUSIONS
In summary, we have introduced a new and highly
efficient catalyst for ꢀ-enamination of ꢀ-dicarbonyl
compounds. The significaant aspect of our methodology are
efficiency, generality, high yield, short reaction time, ease of
preparation of the catalyst, low cost, cleaner reaction profile,
ease of product isolation, simplicity, potential for recycling of
the catalyst, and finally agreement with the green chemistry
protocols.
ACKNOWLEDGEMENTS
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Soc. 5 (2008) 96; b) G.H. Imanzadeh, A. Khalafi-
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A. Parhami, J. Iran. Chem. Soc. 4 (2007) 229; c) A.
Zare, A. Hasaninejad, A.R. Moosavi Zare, A. Khalafi-
Nezhad, A. Parhami, J. Iran. Chem. Soc. 5 (2008) 617;
d) G.H. Imanzadeh, A. Zare, A. Khalafi-Nezhad, A.
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We appreciate Persian Gulf University and Payame Noor
University (PNU) Research Councils for the financial support
of this work.
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