Phosphotungstic acid catalysed synthesis of β-enamino compounds under solvent-free conditions

Article abstract of DOI:10.3184/030823407X273488

A convenient eco-friendly procedure has been developed for the synthesis of β-enaminones and β-enamino esters by reacting 1,3-dicarbonyl compounds with amines in the presence of catalytic amounts of phosphotungstic acid (H₃PW₁₂O₄₀,1 mol%). The reaction proceeds smoothly at room temperature under solvent-free conditions and gives the corresponding β-enamino compounds in high to excellent yields.

Full text of DOI:10.3184/030823407X273488

696 RESEARCH PAPER
DECEMBER, 696–698
JOURNAL OF CHEMICAL RESEARCH 2007
Phosphotungstic acid catalysed synthesis of β-enamino compounds
under solvent-free conditions
Geng-Chen Li*
School of Material Science and Engineering, Shijiazhuang Railway Institute, Shijiazhuang 050043, China
A convenient eco-friendly procedure has been developed for the synthesis of b-enaminones and b-enamino esters
by reacting 1,3-dicarbonyl compounds with amines in the presence of catalytic amounts of phosphotungstic acid
(H3PW12O40, 1 mol%). The reaction proceeds smoothly at room temperature under solvent-free conditions and gives
the corresponding b-enamino compounds in high to excellent yields.
Keyword: 1,3-dicarbonyl compounds, amines, enaminones, enamino esters, phosphotungstic acid, solvent-free conditions
56
Recently, the use of solid acids as heterogeneous catalysts
has attracted much attention and has become an area of
active studyin chemistry.1,2 As part of these studies, the use
of heteropoly acids (HPAs) which are strong acids, harmless
to the environment and highly stable toward humidity and
have flexibility in modifying the acid strength has found
attention.3-7 HPAs are green and are efficient bifunctional
catalysts. Their acidity is significantly higher than that of
traditional mineral acids and inorganic acids. Furthermore,
HPAs are capable of protonating and activating some
substrate.8 In particular, the Keggin-type HPAs such as
H3PW12O40 (PW), H3PMo12O40 (PMo) or H4SiW12O40
(SiW) are the most efficient catalysts for a variety of catalytic
processes for industrial application.9 PW is considered to
be the strongest heteropoly acid in the Keggin series. It has
been reported to be an efficient catalyst for many important
organic transformations, including the intramolecular
rearrangement of benzyl phenyl ether to 2-benzyl phenol,9
the Beckmann rearrangement,10 the Fries rearrangement,11 the
synthesis of 1,1-diacetates,12 b-acetamido ketones,13 diaryl
sulfoxides,14 diisobornyl ether,15 1,3-dioxolane derivatives,16
5-alkoxycarbonyl-4-aryl-3,4-dihydropyrimidin-2-ones,17
quinaldines and lepidines,18 the diacetal of pentaerythritol,19 3,
4-dihydropyrimidin-2(1H)-ones20anda-aminophosphonates.21
It also catalyses the N-t-butoxycarbonylation of amines,22 the
chemoselective oxathioacetalisation of carbonyl compounds,23
trimethylcyanosilylation reactions of aldehydes and ketones,24
and the Michael addition reaction of thiols to a,b-unsaturated
ketones.25
b-Enamino compounds have been extensively used as
intermediates in organic synthesis.26-29 In particular, they
have been utilised as synthons for the synthesis of various
biologically active heterocyclic compounds having anti-
inflammatory, antitumor, antibacterial, and anticonvulsant
activities30-31 and as intermediates for the preparation of
b-enaminoacids, g-enaminoaclohols, and b-enamino esters.32
Due to its wide range of utility in the pharmaceutical
industry, the enamination of β-dicarbonyl compounds with
various amines has become an important transformation
and consequently several methods have been developed for
the synthesis of these compounds. Among them, the most
simple and straightforward conventional method is the
azeotropic removal of water by refluxing an amine and 1,3-
dicarbonyl compounds in an aromatic solvent.33 Several
improved procedures have been subsequently reported using
catalyst systems, including the use of protic acids,34 Lewis
acids such as Zn(ClO4)2·6H2O,35 CeCl3·7H2O,36 NaAuCl4,37
Bi(OTf)3,38 InBr3,32 CoCl2·6H2O,39 CAN,40-41 Yb(OTf)3,42,43
SnCl4·5H2O,44 ZrCl4,45 ZrOCl2·8H2O,46 Zn(Ac)2·2H2O47 and
Sc(OTf)3,48 I2,49 and solid acids such as montmorillonite K10,50
silica chloride,51 silica gel,52 natural clays,53 and sulfated
zirconia.54 Recently, [EtNH3]NO3,55 and HClO4·SiO2 have
also been used to promote this transformation. However, some
of these methodologies have not been entirely satisfactory,
with disadvantages such as low yields, prolonged reaction
time, harsh reaction conditions, use of harmful organic
solvents, and a requirement of an excess of the catalysts and of
special apparatus. Thus, the development an efficient, practical
and environmentally benign synthetic method to overcome
the limitations is still an important experimental challenge.
Herein, we wish to report a novel and high yielding solvent-
free method for the preparation of b-enamino compounds
using a catalytic amount of phosphotungstic acid (Scheme 1).
We initially studied the reaction of aniline and ethyl
acetoacetate as a benchmark reaction in the presence of
1 mol% of PW at room temperature under solvent-free
conditions. To our delight, the reaction occurred to afford
ethyl 3-(phenylamino)but-2-enoate (3f) in 95% yield when
the reaction mixture was allowed to stir for 45 min (Table 1,
entry f). Further studies established that 1 mol% of catalyst
was necessary to promote this reaction. In the absence of
catalyst, the model reaction was run and only 30% of the
product could be obtained even with stirring for 24 h (Table 1,
entry g). An increase in the amount of PW to more than
1 mol% showed no substantial improvement in the yield,
whereas the yield was reduced by decreasing the amount of
PW to 0.1 mol%. Reactions in solvents such as acetonitrile,
tetrahydrofuran, ethanol, dichloromethane, ethyl acetoacetate
and dimethylformamide gave lower yields of the desired
product even at prolonged reaction times. So, we choose the
reaction to be proceeded under solvent-free conditions.
Having established the optimised experimental conditions,
the scope of the reaction was then explored and several
representative results are summarised in Table 1. As shown
in Table 1, the present methodology worked efficiently with
a wide variety of substrates. In general, primary and benzylic
amines reacted with a broad range of structurally diverse
1,3-dicarbonyl compounds to afford the corresponding
b-enaminones or b-enamino esters in high yields in short
times. However, anilines with an electron-withdrawing group
(1l and 1y) retarded the progress of reaction and afforded low
yields of the products. It was also found that the substituted
groups (1h and 1w) on the ortho position in the aniline
influenced the reaction rates. Moreover, the optically active
(R)-( + )-a-methyl benzyl amine (1d) was converted into the
R2
R2
O
O
N
phosphotungstic acid (1 mol%)
Solvent-free, r.t.
R1NH2
R3
+
R3
H
O
R1
R4
R4
1
2
3
* Correspondent. E-mail: gengchenli@sina.com.cn
Scheme 1
PAPER: 07/4912

JOURNAL OF CHEMICAL RESEARCH 2007 697
Table 1 Synthesis of b-enaminones and b-enamino esters catalysed by phosphotungstic acid
Entry
R1
R2
R3
R4
Time/min
Yield/%a
Ref.
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
CH3(CH2)3
C6H11
H2C=CHCH2
(R)-PhCH(CH3)
PhCH2
Ph
Ph
OMe
OEt
OMe
OMe
OEt
OEt
OEt
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
30
15
15
93
92
91
90
92
95
30b
94
93
93
91
72
54
54
39
39
54
94
93
95c
92
95
92
95
80
78
82
39
39
39
39
19
39
39
54
40
39
H
15
H
15
H
18
H
45
H
24 h
60
2-Me–C6H4
4-Me–C6H4
4-OEt–C6H4
4-iPr–C6H4
4-Br–C6H4
PhCH2
OMe
OEt
H
H
40
OEt
H
40
OEt
H
40
OEt
H
360
90
45
OCH2CH2
OCH2CH2
OCH2CH2
OEt
Me
Me
Me
(CH2)3
(CH2)3
H
Ph
75
91
4-OMe–C6H4
Ph
60
92
90
93
4-OMe–C6H4
CH3(CH2)3
H2C=CHCH2
H2NCH2CH2CH2
PhCH2CH2
Ph
OEt
90
92
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
15
54
39
39
39
54
54
54
54
54
s
t
Me
H
15
Me
H
15
u
v
w
x
y
z
Me
H
15
Me
H
15
2-Me–C6H4
4-Me–C6H4
2-Br–C6H4
Ph
Me
H
20
Me
H
12
Me
H
480
120
120
Ph
H
aa
4-OEt–C6H4
Ph
H
aYield refer to isolated products, products were characterised by IR, 1H NMR and elemental analysis.
bNo catalyst.
cTwo equivalents of acetylacetone were used.
corresponding b-enaminoester (3d) without any racemisation
or inversion confirmed by measuring 3d’s optical rotation.
When 1,3-diaminopropane (1t) was used, two equivalents
of acetylacetone were used and the product was formed with
two enaminone groups (3t). It should be pointed out that
when 1,3-diketones with two different substituents, such as
1-benzoylacetone, reacted with amines the regioselective
amination of the aliphatic carbonyl group (2z and 2aa) was
observed. From linear 1,3-diketones and 1,3-ketoesters
we always obtained the corresponding b-enaminones and
b-enamino esters having a (Z)-configuration of the carbon-
carbon double bond due to the formation of intramolecular
hydrogen bonding between oxygen atom of carbonyl and NH
reside, as determined by 1H NMR analysis (dH > 8.2 for NH)
and comparison with the chemical shifts of vinylic protons of
similar Z-enaminones.32
Recycling of the catalyst was also investigated. After
completion of the benchmark reaction, the catalyst was filtered
off, washed with diethyl ether and activated at 100°C for 2 h
and reused in another reaction with the same substrates. There
was no significant change in the activity after two cycles (95,
93 and 90% of product after three runs).
In order to show the merit of phosphotungstic acid in
comparison with other recently reported catalysts for the
synthesis of 3f as a model reaction, we have tabulated some of
results in Table 2. It is clear from Table 2, that phosphotungstic
acid is an equally efficient but a much cheaper and reusable
catalyst.
and rendering this methodology highly suitable for industrial
applications.
Experimental
IR spectra were obtained as KBr pellets for samples solid and as
thin films for liquid samples with a Thermo Nicolet FT-IR200
spectrometer. NMR spectra were recorded on a Bruker AV 300
spectrometer in CDCl3 using TMS as an internal standard. The
melting points are uncorrected and were recorded on a WRR melting
point instrument. Elemental analyses were performed on a PE 2400
CHNS/O Analyser.
General procedure for the synthesis of b-enaminones or
b-enamino esters: A mixture of ethyl acetoacetate (1.30 g, 10 mmol)
with aniline (0.93 g, 10 mmol), and H3PW12O40 (0.29 g, 0.1 mol) was
stirred at room temperature. The progress of reaction was followed
by TLC. After completion of reaction, as indicated by TLC, the
reaction mixture was extracted with diethyl ether (3 × 10 ml) and
the catalyst was filtered off. The combined ether extract was treated
with saturated sodium bicarbonate and dried over anhydrous Na2SO4,
concentrated under reduced pressure and the resulting product was
purified by silica gel column chromatography (20% ethyl acetate in
n-hexane as eluent) to afford pure ethyl 3-phenylamino-but-2-enoate
(1.95 g, 95%). The filtered catalyst was repeatedly washed with
diethyl ether and reused.
Selective spectroscopic and analytical data for AA'XX' systems in
1H NMR J* = J23 + J25.
Methyl (R)-3-(1-phenyl-ethylamino)but-2-enoate (3d):39Acolourless
liquid, [a]D20: –540 (c 1.15, EtOH); IR (neat): 3280, 2973, 2929,
1653, 1608, 1494, 1446, 1378, 1266, 1054, 764, 700 cm-1. 1H NMR
(CDCl3, 300 MHz) d ppm: 1.52 (d, J = 6.6 Hz, 3H), 1.78 (s, 3H), 3.67
(s, 3H), 4.49 (s, 1H), 4.65 (q, J = 6.6 Hz, 1H), 7.20–7.38 (m, 5H),
9.00 (br s, NH). 13C NMR (CDCl3, 75 MHz) d ppm: 19.7, 25.0, 50.0,
52.9, 82.9, 125.5, 127.2, 128.8, 150.0, 161.6, 180.0. Anal. Calcd. for
C13H17NO2: C, 71.21; H, 7.81; N, 6.39. Found: C, 71.5; H, 7.8; N, 6.5.
Ethyl 3-(4-isopropylphenylamino)but-2-enoate (3k): A yellow oil;
IR (neat): 3262, 2962, 2932, 1655, 1620, 1519, 1362, 1332, 1278,
1162, 1058, 758, 668 cm-1. 1H NMR (CDCl3, 300 MHz) d ppm: 1.28
(t, J = 7.2 Hz, 3H), 1.39 (d, J = 7.2 Hz, 6H), 1.98 (s, 3H), 2.94 (m,
1H) 4.22 (q, J = 7.2 Hz, 2H), 5.12 (s, 1H), 7.02 (m, J* = 8.4 Hz, 2H),
7.18 (m, J* = 8.4 Hz, 2H), 10.28 (brs, NH). 13C NMR (CDCl3, 75
MHz) d ppm: 14.5, 20.0, 23.8, 33.5, 58.5, 85.5, 124.4, 126.8, 137.0,
145.6, 159.0, 170.0. Anal. Calcd for C15H21NO2: C, 72.84; H, 8.56;
N, 5.66. Found: C, 72.7; H, 8.45; N, 5.75.
In summary, an environmentally friendly procedure
for the synthesis of b-enaminones and b-enamino esters
through phosphotungstic acid–catalysed condensation of
1,3-dicarbonyl compounds and amines has been developed.
This method has several unique merits, such as simple
experimental procedure, solvent-free conditions, short
reaction times, high yields of products and chemo- and
stereoselectivities. In addition, the catalyst can be easily
recovered and reused, providing thereby eco-friendly and
economic advantages over previously reported protocols
PAPER: 07/4912

698 JOURNAL OF CHEMICAL RESEARCH 2007
Table 2 Synthesis of ethyl 3-(phenylamino)but-2-enoate (3f) in the presence of different catalysts
Catalyst/solvent
Catalyst load
Time
Yield/%
Ref.
InBr3/solvent-free
1 mol%
5 mol%
10 mol%
5 mol%
5 mol%
1 mol%
2 mol%
1 mol%
2 mol%
5 mol%
5 mol%
20 mol%
10 mg
10 min
4 h
35 min
1 h
15 min
60 min
60 min
40
94
95
76
64
95
92
95
95
93
86
95
79
95
95
32
Zn(ClO4)2·6H2O/CH2Cl2
CeCl3·7H2O/solvent-free
Bi(OTf)3/H2O
35
36
38
CoCl2·6H2O/solvent-free
CAN/solvent-free
Yb(OTf)3/solvent-free
ZrCl4/solvent-free
ZrOCl2·8H2O/solvent-free
Zn(OAc)2·2H2O/CH2Cl2
Sc(OTf)3/solvent-free
I2/solvent-free
Silica gel/solvent-free
Phosphotungstic acid/solvent-free
39
40
43
45
50 min
2 days
60 min
3 min
35 h
46
47
48
49
52
1 mol%
45 min
This work
Ethyl 3-(4-bromophenylamino)-but-2-enoate (3l):45 A pale yellow
solid; m.p. 52–53°C. IR (KBr) 3276, 2978, 1648, 1610, 1580, 1480,
1385, 1261, 1169, 854, 790 cm-1. 1H NMR (CDCl3, 300 MHz) d ppm:
1.28 (t, J = 7.2 Hz, 3H), 2.00 (s, 3H), 4.16 (q, J = 7.2 Hz, 2H), 4.72 (s,
1H), 6.96 (m, J* = 8.4 Hz, 2H), 7.45 (m, J* = 8.4 Hz, 2H), 10.40 (br
s, NH). 13C NMR (CDCl3, 75 MHz) d ppm: 14.6, 20.3, 59.0, 118.0,
125.8, 132.3, 138.6, 162.4, 170.5. Anal. Calcd for C12H14BrNO2: C,
50.72; H, 4.97; N, 4.93; Found: C, 50.85; H, 4.8; N, 5.1.
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solid, m.p. 85–86°C; IR (KBr) 3415, 2980, 1600, 1505, 1475, 1433,
1372, 1322, 820, 745 cm-1. 1H NMR (CDCl3, 300 MHz) d ppm: 1.44
(t, J = 6.9 Hz, 3H), 2.08 (s, 3H), 4.05 (q, J = 6.9 Hz, 2H), 5.88 (s,
1H), 6.80 (m, J* = 8.7 Hz, 2H), 7.10 (m, J* = 8.7 Hz, 2H), 7.43–7.93
(m, 5H), 13.00 (br s, NH). 13C NMR (CDCl3, 75 MHz) d ppm: 14.9,
20.3, 63.8, 93.5, 114.9, 126.6, 127.2, 128.2, 130.8, 131.5, 140.0,
157.2, 163.2, 188.5. Anal. Calcd. for C18H19NO2: C, 76.84; H, 6.81;
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