Synthesis of enaminones in aqueous media using catalytic dilute HCl

Article abstract of DOI:10.1080/00397911003797775

(Chemical Equation Presented) A green and convenient approach to the synthesis of β-enaminones from aromatic amines and 5-substituted-1,3- cyclohexanedione in the presence of dilute hydrochloric acid (30 mmol=L) as a catalyst in solvent-free media is described. This method provides several advantages such as environmental friendliness, low cost, good yields, and simple workup procedure. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911003797775

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Synthesis of Enaminones in Aqueous
Media Using Catalytic Dilute HCl
Bin Cui a , Rui-Hua Wang a , Li-Zhuang Chen a , Yan Jin a & Guang-
Fan Han a
a School of Material Science and Engineering, Jiangsu University of
Science and Technology , Zhenjiang , Jiangsu , China
Published online: 03 Mar 2011.
To cite this article: Bin Cui , Rui-Hua Wang , Li-Zhuang Chen , Yan Jin & Guang-Fan Han (2011)
Synthesis of Enaminones in Aqueous Media Using Catalytic Dilute HCl, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:7, 1064-1070, DOI:
10.1080/00397911003797775
To link to this article: http://dx.doi.org/10.1080/00397911003797775
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Synthetic Communications1, 41: 1064–1070, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003797775
SYNTHESIS OF ENAMINONES IN AQUEOUS MEDIA
USING CATALYTIC DILUTE HCl
Bin Cui, Rui-Hua Wang, Li-Zhuang Chen, Yan Jin, and
Guang-Fan Han
School of Material Science and Engineering, Jiangsu University of Science
and Technology, Zhenjiang, Jiangsu, China
GRAPHICAL ABSTRACT
Abstract A green and convenient approach to the synthesis of b-enaminones from aromatic
amines and 5-substituted-1,3-cyclohexanedione in the presence of dilute hydrochloric acid
(30 mmol=L) as a catalyst in solvent-free media is described. This method provides several
advantages such as environmental friendliness, low cost, good yields, and simple workup
procedure.
Keywords b-Enaminones; solvent-free; 5-substituted-1,3-cyclohexanedione
INTRODUCTION
b-Enaminones play a unique role in organic chemistry. They are important
precursors for the synthesis of various heterocyclic and biologically active analogs[1]
that have anticonvulsant,[2] anti-inflammatory,[3] and antitumor[4] activities. They are
also the intermediates for the synthesis of several amino acids,[5] aminols,[6]
peptides,[7] and alkaloids.[8]
Received August 1, 2009.
Address correspondence to Guang-Fan Han, School of Material Science and Engineering, Jiangsu
University of Science and Technology, Zhenjiang, Jiangsu 212003, China. E-mail: gf552002@
yahoo.com.cn
1064

SYNTHESIS OF ENAMINONES
1065
Recently, the synthesis of b-enaminones has received much attention. Various
methods have been reported for the synthesis of b-enaminones; the conventional
method for the synthesis of enaminones is the azeotropic removal of water by reflux-
ing an amine with 1,3-diketone in an aromatic solvent.[9] Several improved methods
for the preparation of b-enaminones have been reported to utilize SiO2=micro-
waves,[10]
NaAuClO4,[11]
montmorillonite
K-10,[12]
Zn(ClO4)2 ꢀ 6H2O,[13]
CeCl3 ꢀ H2O,[14] Bi(TFA)3,[15] iodine,[16] HClO4 ꢀ SiO2,[17] and ionic liquid.[18]
However, most of the methods suffer from certain drawbacks such as long reac-
tion times, unsatisfactory yields, expensive catalysts, hazardous solvents, complex
processes, poor selectivity, co-occurrence of several side reactions, and need for
chromatography to purify adducts. In this article, we report a simple procedure
and highly efficient synthesis of b-enaminones starting from 5-substituted-1,3-
cyclohexanedione and aromatic amines using dilute hydrochloric acid (30 mmol=L)
as a reusable, inexpensive, and efficient catalyst in solvent-free media.
RESULTS AND DISCUSSION
The synthetic pathway is shown in Scheme 1. The b-enaminones were obtained
by reaction of 5-substituted-1,3-cyclohexanedione 1 with aromatic amines using
dilute hydrochloric acid (30 mmol=L) as catalyst in solvent-free media. The 1H
NMR, mass spectrometry (MS), and infrared (IR) data shown in the experimental
section were in accordance with the chemical structures of the target compounds.
1
In the H NMR spectrum of compound 3a, the broad single proton peak at d
6.88 was the characteristic absorption proton peak of the N-H. The structures of
these compounds were further supported by their IR spectra. Typical absorption
ꢁ1
bands at 3312 cmꢁ1 (N-H), 1685 cmꢁ1 (C O), and 1446 cm (C-N) were observed.
=
As shown in Table 1, a series of aromatic amines 2 was reacted with
5-substituted-1,3-cyclohexane 1 in the presence of dilute hydrochloric acid
(30 mmol=L) in solvent-free media at 70–80 ꢂC, and the reactions proceeded
smoothly to afford the corresponding products 3 in good yields. All aromatic amines
containing either electron-withdrawing groups (such as halide, cyano) or
Scheme 1. R1 ¼ H, Me; R2 ¼ H, Me, Ph, 4-OMePh; R3 ¼ H, 4-Cl, 2-CN, 4-OMe.

1066
B. CUI ET AL.
Table 1. Synthesis of b-enaminones
Entry
R1
R2
R3
Mp (ꢂC)
Yield (%)
3a
3b
3c
3d
3e
3f
CH3
CH3
CH3
CH3
H
CH3
CH3
CH3
CH3
Ph
H
4-Cl
212–213
189–190
207–208
238–239
240–241
236–238
205–207
198–199
211–213
218–219
221–223
228–229
170–172
164–166
84
93
93
88
89
95
88
89
85
84
90
91
92
91
2-CN
4-OCH3
H
H
Ph
Ph
4-Cl
3g
3h
3i
H
2-CN
4-OCH3
H
H
Ph
Ph
H
3j
H
Ph
Ph
4-Cl
3k
3l
H
2-CN
4-OCH3
H
H
Ph
H
3m
3n
H
H
H
2-CN
electron-donating groups (such as methoxy) were employed and reacted well to give
the corresponding products 3 in good to excellent yields under the same reaction
conditions, so we conclude that the electronic nature of the substituents on the
aromatic ring has no obvious effect on the yields.
Taking the reaction of 2-cyanoaniline as an example, we investigated the effects
of the amount and concentration of the catalyst, which played crucial roles in the
success of the reaction in terms of the rate and yields. For example, the reaction
could not be carried out without the catalyst. It was found that the best conditions
were solvent-free at 70–80 ꢂC and a ratio of 5-substituted-1,3-cyclohexanediones
(mmol)=aromatic amine (mmol)=30 mmol=L HCl (mL) of 1:1:4. The yields of the
product show a downward trend with increasing amounts or concentrations of cata-
lyst. The results are summarized in Table 2.
The catalyst can be handled conveniently and readily removed from the reac-
tion mixture after each reaction. Thus, the process was environmentally benign.
Table 2. b-Enaminone 3c synthesis catalyzed by various amounts and
concentrations of hydrochloric acid
Concentration of HCl
(mmol=L)
Amount of HCl
(mL)
Time
(min)
Yield
(%)
Entry
1
2
15
15
0
40
20
40
80
100
40
40
40
40
0
80
60
30
30
30
30
30
30
30
0
50
68
93
88
85
78
63
56
41
3
30
30
4
5
30
30
6
7
60
90
8
9
10
120
300

SYNTHESIS OF ENAMINONES
1067
After the reaction, the mixture was cooled to room temperature, the solid was
filtered off, and the recovered filtrate containing the acid catalyst was reused for
the subsequent reaction. The catalyst was activated with only a gradual decrease
in its activity. For example, the reaction of aniline and 5,5-dimethyl-1,3-
cyclohexanedione afforded the corresponding b-enaminone 3a in 84, 78, and 71%
yields over three cycles.
CONCLUSION
In summary, we have described a highly efficient, environmentally friendly,
and convenient method for the synthesis of enaminones by treatment of
5-substituted-1,3-cyclohexanedione with aromatic amines in the presence of dilute
hydrochloric acid (30 mmol=L) as the catalyst in solvent-free media where the cata-
lyst could be recovered readily and reused in good to excellent yeilds. Moreover, the
procedure offers several advantages including good yields, lowcost, operational sim-
plicity, cleaner reactions, and minimal environmental effects. We expect that the pro-
cedure can be utilized for large-scale ecofriendly preparation of these compounds.
EXPERIMENTAL
Melting points were determined on an electrothermal apparatus, and the tem-
perature was not calibrated. Microanalysis was performed on the Perkin-Elmer 2400
Microanalytical Service. IR spectras were recorded as thin films on KBr using a
Perkin-Elmer 1700 spectrophotometer. The NMR spectra were recorded on a
Bruker ARX-300 spectrometer. Sample solutions were prepared in CDCl3 or
dimethylsulfoxide (DMSO) containing tetramethylsilane (TMS) as an internal
reference. Mass spectra were recorded on a JMS-DX300 at 70 eV.
All chemical reagents were commercially available and purified with standard
methods before being used. Solvents were dried in routine ways and redistilled.
5-Substituted-1,3-cyclohexanediones 1 were obtained from aromatic aldehyde,
acetone, and dimethyl molanate according to the literature[19] method with slight
modification.
General Procedure
A mixture of 5-substituted-1,3-cyclohexane (1,5 mmol), aromatic amine
(2,5 mmol), and 30 mmol=L dilute hydrochloric acid (40 ml) was stirred at
70–80 ꢂC for 30 min. Then the mixture was cooled to room temperature; the solid
was filtered off and washed with water. The crude products were purified by recrys-
tallization with ethanol (95%).
Data of Compounds
Compound 3a. 1H NRM (CDCl3, 300 MHz): 1.08 (s, 6H, CH3), 2.23 (s, 2H,
60-H), 2.36 (s, 2H, 40-H), 4.97 (s, 1H, vinyl-H), 6.88 (brs, 1H, N-H), 7.01–7.22 (m,
5H, Ph-H); IR (KBr) n: 3312, 1685, 1619, 1446 cmꢁ1; MS (70 eV) m=z (%): 216.13

1068
B. CUI ET AL.
(Mþ þ 1, 100). Anal. calcd. for C14H17NO: C, 78.10; H, 7.96; N, 6.51. Found: C,
78.07; H, 7.97; N, 6.49.
Compound 3b. 1H NRM (CDCl3, 300 MHz): 1.09 (s, 6H, CH3), 2.22 (s, 2H,
60-H), 2.42 (s, 2H, 40-H), 5.39 (s, 1H, vinyl-H), 5.55 (brs, 1H, N-H), 7.11–7.34 (m,
4H, Ph-H); IR (KBr) n: 3318, 1679, 1620, 1510 cmꢁ1; MS (70 eV) m=z (%): 250.09
(Mþ þ 1, 100). Anal. calcd. for C14H16ClNO: C, 67.33; H, 6.46; N, 5.61. Found:
C, 67.28; H, 6.50; N, 5.59.
Compound 3c. 1H NRM (CDCl3, 300 MHz): 1.10 (s, 6H, CH3), 2.23 (s, 2H,
60-H), 2.41 (s, 2H, 40-H), 5.47 (s, 1H, vinyl-H), 6.96 (brs, 1H, N-H), 7.21–7.63 (m,
4H, Ph-H); IR (KBr) n: 3320, 2240, 1688, 1611, 1506 cmꢁ1; MS (70 eV) m=z (%):
241.13 (Mþ þ 1, 100). Anal. calcd. for C15H16N2O: C, 74.97; H, 6.71; N, 11.66.
Found: C, 75.00; H, 6.72; N, 11.63.
Compound 3d. 1H NRM (CDCl3, 300 MHz): 1.11 (s, 6H, CH3), 2.21 (s, 2H,
60-H), 2.43 (s, 2H, 40-H), 3.77 (s, 3H, OCH3), 5.33 (s, 1H, vinyl-H), 6.42 (brs, 1H,
N-H), 7.19–7.61 (m, 4H, Ph-H); IR (KBr) n: 3310, 1691, 1621, 1511 cmꢁ1; MS
(70 eV) m=z (%): 246.14 (Mþ þ 1, 100). Anal. calcd. for C15H19NO2: C, 73.44; H,
7.81; N, 5.71. Found: C, 73.39; H, 7.83; N, 5.69.
1
Compound 3e. H NRM (CDCl3, 300 MHz): 2.59 (m, 3H, 40-H þ 60-H), 2.88
(m, 1H, 40-H), 3.43 (m, 1H, 50-H), 5.68 (s, 1H, vinyl-H), 6.56 (brs, 1H, N-H),
7.23–7.57 (m, 10H, Ph-H); IR (KBr) n: 3315, 1692, 1617, 1508 cmꢁ1; MS (70 eV)
m=z (%): 264.10 (Mþ þ 1, 100). Anal. calcd. for C18H17NO: C, 82.10; H, 6.51; N,
5.32. Found: C, 82.06; H, 6.53; N, 5.29.
1
Compound 3f. H NRM (CDCl3, 300 MHz): 2.54 (m, 3H, 40-H þ 60-H), 2.71
(m, 1H, 40-H), 3.26 (m, 1H, 50-H), 5.60 (s, 1H, vinyl-H), 6.49 (brs, 1H, N-H),
7.09–7.53 (m, 9H, Ph-H); IR (KBr) n: 3317, 1682, 1623, 1511 cmꢁ1; MS (70 eV)
m=z (%): 298.11 (Mþ þ 1, 100). Anal. calcd. for C18H16ClNO: C, 72.60; H, 5.42;
N, 4.70. Found: C, 72.57; H, 5.46; N, 4.67.
1
Compound 3g. H NRM (CDCl3, 300 MHz): 2.56 (m, 3H, 40-H þ 60-H), 2.82
(m, 1H, 40-H), 3.42 (m, 1H, 50-H), 5.63 (s, 1H, vinyl-H), 6.53 (brs, 1H, N-H),
7.19–7.75 (m, 9H, Ph-H); IR (KBr) n: 3321, 2235, 1692, 1623, 1514 cmꢁ1; MS
(70 eV) m=z (%): 289.09 (Mþ þ 1, 100). Anal. calcd. for C19H16N2O: C, 79.14; H,
5.59; N, 9.72. Found: C, 79.09; H, 5.62; N, 9.70.
1
Compound 3h. H NRM (CDCl3, 300 MHz): 2.55 (m, 3H, 40-H þ 60-H), 2.69
(m, 1H, 40-H), 3.22 (m, 1H, 50-H), 3.72 (s, 3H, OCH3), 5.59 (s, 1H, vinyl-H), 6.48 (brs,
1H, N-H), 7.16–7.69 (m, 9H, Ph-H); IR (KBr) n: 3309, 1701, 1620, 1509 cmꢁ1; MS
(70 eV) m=z (%): 294.14 (Mþ þ 1, 100). Anal. calcd. for C19H19NO2: C, 77.79; H,
6.53; N, 4.77. Found: C, 77.71; H, 6.57; N, 4.81.
1
Compound 3i. H NRM (CDCl3, 300 MHz): 2.35 (m, 1H, 60-H), 2.57 (m, 2H,
40-H þ 60-H), 2.78 (m, 1H, 40-H), 3.37 (m, 1H, 50-H), 3.70 (s, 3H, OCH3), 5.56 (s, 1H,
vinyl-H), 6.59 (brs, 1H, N-H), 7.08–7.65 (m, 9H, Ph-H); IR (KBr) n: 3322, 1711,
1618, 1510 cmꢁ1; MS (70 eV) m=z (%): 294.17 (Mþ þ 1, 100). Anal. calcd. for
C19H19NO2: C, 77.79; H, 6.53; N, 4.77. Found: C, 77.69; H, 6.58; N, 4.79.

SYNTHESIS OF ENAMINONES
1069
1
Compound 3j. H NRM (CDCl3, 300 MHz): 2.40 (m, 1H, 60-H), 2.54 (m, 2H,
40-H þ 60-H), 2.65 (m, 1H, 40-H), 2.75 (m, 1H, 50-H), 3.73 (s, 3H, OCH3), 5.59 (s, 1H,
vinyl-H), 6.52 (brs, 1H, N-H), 6.99–7.58 (m, 8H, Ph-H); IR (KBr) n: 3320, 1692,
1621, 1512 cmꢁ1; MS (70 eV) m=z (%): 328.10 (Mþ þ 1, 100). Anal. calcd. for
C19H18ClNO2: C, 69.62; H, 5.53; N, 4.27. Found: C, 69.58; H, 5.61; N, 4.29.
1
Compound 3k. H NRM (CDCl3, 300 MHz): 2.39 (m, 1H, 60-H), 2.53 (m,
2H, 40-H þ 60-H), 2.64 (m, 1H, 40-H), 2.73 (m, 1H, 50-H), 3.76 (s, 3H, OCH3), 5.49
(s, 1H, vinyl-H), 6.57 (brs, 1H, N-H), 7.17–7.54 (m, 8H, Ph-H); IR (KBr) n: 3318,
2241, 1695, 1623, 1515 cmꢁ1; MS (70 eV) m=z (%): 319.13 (Mþ þ 1, 100). Anal. calcd.
for C20H18N2O2: C, 75.45; H, 5.70; N, 8.80. Found: C, 75.47; H, 5.67; N, 8.77.
1
Compound 3l. H NRM (CDCl3, 300 MHz): 2.39 (m, 1H, 60-H), 2.53 (m, 2H,
40-H þ 60-H), 2.64 (m, 1H, 40-H), 2.73 (m, 1H, 50-H), 3.76 (s, 3H, OCH3), 5.49 (s, 1H,
vinyl-H), 6.57 (brs, 1H, N-H), 7.17–7.54 (m, 8H, Ph-H); IR (KBr) n: 3318, 1693,
1624, 1512 cmꢁ1; MS (70 eV) m=z (%): 324.15 (Mþ þ 1, 100). Anal. calcd. for
C20H21NO3: C, 74.28; H, 6.55; N, 4.33. Found: C, 74.21; H, 6.58; N, 4.35.
1
Compound 3m. H NRM (CDCl3, 300 MHz): 2.07 (m, 2H, 60-H), 2.39 (m,
2H, 50-H), 2.58 (m, 2H, 40-H), 5.58 (s, 1H, vinyl-H), 6.62 (brs, 1H, N-H),
7.21–7.65 (m, 5H, Ph-H); IR (KBr) n: 3262, 1591, 1526, 1451 cmꢁ1; MS (70 eV)
m=z (%): 188.21 (Mþ þ 1, 100). Anal. calcd. for C12H13NO: C, 76.98; H, 7.00; N,
7.48. Found: C, 76.95; H, 7.08; N, 7.51.
1
Compound 3n. H NRM (CDCl3, 300 MHz): 1.99 (m, 2H, 60-H), 2.33 (m,
2H, 50-H), 2.50 (m, 2H, 40-H), 5.56 (s, 1H, vinyl-H), 6.81 (brs, 1H, N-H),
7.12–7.30 (m, 4H, Ph-H); IR (KBr) n: 3254, 2235, 1588, 1520, 1449 cmꢁ1; MS
(70 eV) m=z (%): 213.25 (Mþ þ 1, 100). Anal. calcd. for C12H13NO: C, 73.57; H,
5.70; N, 13.20. Found: C, 73.45; H, 5.83; N, 13.15.
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