A general and efficient method for synthesis of enaminones and enamino esters catalyzed by NbCl5 under solvent-free conditions

Article abstract of DOI:10.1007/s00706-012-0783-8

A general and simple procedure was 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 niobium pentachloride. The reaction proceeds smoothly at room temperature under solvent-free conditions and leads to chemo- and regioselective formation of enamine derivatives in high to excellent yields.

Full text of DOI:10.1007/s00706-012-0783-8

Monatsh Chem (2013) 144:191–196
DOI 10.1007/s00706-012-0783-8
ORIGINAL PAPER
A general and efficient method for synthesis of enaminones
and enamino esters catalyzed by NbCl5 under solvent-free
conditions
•
Yu-Heng Liu Ping Wang Gui-Tian Cheng
•
Received: 16 October 2011 / Accepted: 28 April 2012 / Published online: 30 May 2012
Ó Springer-Verlag 2012
Abstract A general and simple procedure was 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 niobium pentachloride.
The reaction proceeds smoothly at room temperature under
solvent-free conditions and leads to chemo- and regiose-
lective formation of enamine derivatives in high to
excellent yields.
such as CoCl2ꢀ6H2O [11], Cu(NO3)2ꢀ3H2O [12], InBr3 [13],
ZrCl4 [14], ZrOCl2ꢀ8H2O [15], SnCl4ꢀ5H2O [16], NH4
Ce(NO3)5 [17, 18], Ni(OAc)2 [19], Zn(OAc)2ꢀ2H2O [20], I2
[21, 22], B2O3/Al2O3 [23], SbCl3/SiO2 [24], HClO4/SiO2
[25], P2O5/SiO2 [26], silica chloride [27], sulfamic acid [28],
L-proline [29], phosphotungstic acid [30], and Ag [31] or Cu
nanoparticles [32] have been used to promote this transfor-
mation. Despite these advances, there were always one or
more drawbacks with these procedures, such as the use of
expensive or less readily available reagents, rigorous reac-
tion conditions, longer reaction times, unsatisfactory yields,
low selectivity, or the use of toxic solvents that limit these
methods to small-scale synthesis. Thus, searching for a new
catalyst and procedure is still of practical importance.
Recently, for environmental and economic reasons,
attention has been focused on catalytic reactions under
solvent-free conditions [33, 34]. NbCl5 has been consid-
ered as an effective Lewis acid catalyst for a variety of
organic transformations, such as trimethylsilyl protection
of hydroxyl groups [35], synthesis of a-aminophosphonates
[36], quinoxaline derivatives [37], diaminotriarylmethanes
[38], bis(indolyl)alkanes [39], 1,1-diacetates [40], and 1,5-
benzodiazepine derivatives [41]. As part of our continuing
interest in the development of new synthetic methodologies
[42–47], herein, we wish to report a mild and efficient
method for the chemo- and regioselective examination of
1,3-dicarbonyl compounds in the presence of catalytic
amounts of NbCl5 (Scheme 1).
Keywords 1,3-Dicarbonyl compounds ꢀ b-Enaminones ꢀ
b-Enamino esters ꢀ Niobium pentachloride
Introduction
b-Enaminones and b-enamino esters have been extensively
used as key intermediates in organic synthesis [1–6]. In
particular, they have been employed as synthons of a wide
variety of heterocycles [7], pharmaceutical compounds
having anti-epileptic [8], molluscicidal and larvicidal
activities [9], and naturally occurring alkaloids [10]. The
conventional approach for the preparation of b-enaminones
and b-enamino esters is direct condensation of 1,3-dicar-
bonyl compounds with amines in refluxing aromatic solvent
with azeotropic removal of water. In recent years, it has been
reported that the use of catalysts enables the reaction to be
performed under milder conditions. A variety of catalysts
Y.-H. Liu (&)
The College of Preclinical Medicine, Hebei Medical University,
Results and discussion
Shijiazhuang 050017, China
e-mail: liu2795478@163.com
The Lewis acid-catalyzed condensation of amines with b-
ketoesters or b-diketones has recently been described.
Vohra and coworkers reported that the reaction of
P. Wang ꢀ G.-T. Cheng
Hebei Chemical and Pharmaceutical Vocational Technology
College, Shijiazhuang 050026, China
123

192
Y.-H. Liu et al.
R1
R3
acetylacetone and aniline was performed in CH2Cl2 in the
presence of Zn(OAc)2ꢀ2H2O and MgSO4; the correspond-
ing product (Table 1, 3a) was obtained in 86 % yield after
2 days [20]. When this reaction was carried out in the
presence of NbCl5 under solvent-free conditions at room
temperature, the desired product was formed in 96 % yield
within 5 min. The effect of the amount of catalyst on the
O
O
NbCl5 (1 mol%)
R3
Solvent-free, r.t.
O
HN
R1NH2
+
R2
R2
2
3
Scheme 1
Table 1 Synthesis of b-enaminones and b-enamino esters catalyzed by NbCl5
Entry
Amine
R2
R3
Product
Time/min
Yield/%a
M.p. (lit. m.p.)/°C
1
PhNH2
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
3a
5
5
96
95
96
94
95
93
81
82
80
75
72
94
85
95
93
75
72
93
80
95
82
96
90
94
80
84
95
93
92
96
93
94
96
92
85
96
95
94
96
95
49–50 (49–50 [34])
38–39 (39–40 [34])
58–59 (59–60 [34])
50–51 (51–52 [34])
41–43 (41–42 [13])
39–40 (40–41 [13])
39–40 (40–42 [13])
60–62 (61–62 [34])
72–73 (71–72 [34])
48–50 (49–51 [49])
62.3–62.8 (62–63 [48])
Oil (oil [32])
2
2-MeC6H4NH2
4-MeC6H4NH2
2-MeOC6H4NH2
4-MeOC6H5NH2
4-EtOC6H5NH2
3-ClC6H4NH2
4-ClC6H4NH2
2-BrC6H4NH2
4-BrC6H4NH2
3-CF3C6H4NH2
Furan-2-amine
1-Naphthylamine
CH3(CH2)3NH2
H2C=CHCH2NH2
Morpholine
Me
3b
3
Me
3c
5
4
Me
3d
15
4
5
Me
3e
6
Me
3f
5
7
Me
3g
120
110
300
150
360
6
8
Me
3h
9
Me
3j
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
a
Me
3i
Me
3k
Me
3l
Me
3m
3n
60
3
61–63 (62–64 [23])
Oil (oil [13])
Me
Me
3o
5
Oil (oil [13])
Me
3p
120
120
5
45–47 (46–47 [13])
45–46 (46–47 [13])
Oil (oil [13])
Piperidine
Me
3q
(R)-PhCHMeNH2
PhNH2
Me
3r
CH2CH2CH2
CH2CH2CH2
Ph
3s
50
10
60
7
179–180 (178–179 [26])
Oil (oil [26])
CH3(CH2)3NH2
PhNH2
3t
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
3u
96–97 (95–97 [13])
46–47 (45–46 [13])
28–30 (29–30 [11])
57–58 (56–57 [34])
60–61 (61–62 [13])
48–50 (44–48 [49])
36–37 (37–38 [13])
Oil (oil [13])
PhNH2
OMe
3v
2-MeOC6H4NH2
4-MeC6H4NH2
4-ClC6H4NH2
4-BrC6H4NH2
PhCH2NH2
OMe
3w
3x
20
6
OMe
OMe
3y
150
200
4
OMe
3z
OMe
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
3aj
3ak
3al
3am
3an
Cyclohexanamine
H2C=CHCH2NH2
PhNH2
OMe
5
OMe
5
Oil (oil [13])
OEt
5
Oil (oil [13])
PhNH2
OCH2CHMe2
OCH2CH2OMe
OCH2CH=CH2
OMe
20
20
20
20
20
10
30
20
12
10
Oil
PhNH2
Oil
PhNH2
Oil
PhNH2
Oil
4-H2NC6H4NH2
H2N(CH2)3NH2
H2N(CH2)6NH2
H2N(CH2)6NH2
H2N(CH2)2NH2
H2N(CH2)2NH2
Me
Me
Me
Me
Me
Me
Me
185–187 (186–188 [23])
51–52 (52–53 [13])
82–84
Me
Me
OMe
71–73
OCH2CHMe2
OCH2CH=CH2
52–54
81–83
Isolated yields
123

A general and efficient method for synthesis of enaminones and enamino esters
193
yield was investigated. It was found that there was no
obvious difference between 1 mol% and 2 mol% catalyst
loading. The use of lower catalyst loading (0.1 mol%)
required a longer time period (1 h) to afford a comparable
result. So we inferred that 1 mol% of NbCl5 was sufficient
for this reaction. This reaction was also examined in var-
ious solvents, such as dichloromethane, acetonitrile,
ethanol, and methanol, in the presence of 1 mol% of
NbCl5. The results showed that the presence of additional
solvents made the reaction rate lower. Therefore, we chose
to perform this reaction under solvent-free conditions in the
presence of 1 mol% NbCl5 at room temperature.
could also be concluded that anilines bearing electron-
withdrawing groups required longer time and gave lower
yields. The use of other b-ketoesters such as ethyl aceto-
acetate, isobutyl acetoacetate, 2-methoxyethyl acetoacetate,
and allyl acetoacetate was also effective. The alkoxy (–R2)
moiety present had little influence on the reaction, and
generally high yields were obtained.
The method was found to be chemoselective. Since a
keto carbonyl group is more electrophilic than an ester
group, the amine attacks only at the ketone carbonyl for
b-ketoesters. The Z selectivity in the products derived from
1
acyclic diketones and b-ketoesters was confirmed by H
Splendidly, the scope of this protocol was extensively
broad and versatile, spanning a wide range of amines and
1,3-dicarbonyl compounds. As shown in Table 1, a series
of aromatic amines containing both electron-withdrawing
and donating substituents were treated with acetylacetone,
and the corresponding b-enaminones were obtained in
good to excellent yields. Aromatic amines with electron-
withdrawing groups such as -Cl, -Br, and -CF3 showed
weaker reactivity than those containing electron-neutral or
electron-donating groups. It should be noted that ortho-
substituted anilines such as 2-methoxyaniline (Table 1,
entry 4) and 2-bromoaniline (Table 1, entry 9) gave lower
yields than the others, which may be due to the steric
hindrance effect. The mildness of the reaction conditions
permits furan-2-amine to undergo the reaction successfully
to produce the desired product in high yield without for-
mation of any polymerization product (Table 1, entry 12).
Aliphatic amines and allylic amine were shown to be
reactive to undergoing the title reaction, and a shorter
reaction time was required compared to aromatic amines. It
is noteworthy that optically active (R)-1-phenylethanamine
was converted successfully into the corresponding b-
enaminone (Table 1, entry 18) without any racemization or
inversion by measuring its optical rotation and comparing
with the literature value [13]. The reactions worked also
well when secondary amines like morpholine and piperi-
dine were utilized. In addition, cyclic 1,3-dicarbonyl
compounds such as cyclohexane-1,3-dione underwent
similar reactions, leading to the corresponding b-enami-
none in high yields (Table 1, entries 19 and 20). In the case
of unsymmetrical diketones such as 1-phenylbutane-1,3-
dione, the regiochemistry was controlled by the more
reactive carbonyl group, which underwent the attack by
aniline to give exclusively 1-phenyl-3- (phenylamino)but-
2-en-1-one (3u).
NMR spectra. The proton of the –NH group appearing at a
lower field (d [ 8.6 ppm) indicated the formation of an
intramolecular bond, which stabilized the products. How-
ever, the b-enaminones derived from cyclic diketones such
1
as 1,3-cyclohexadione displayed H NMR spectra having
the signals for the non-hydrogen bonded proton of the –NH
group in the region of d = 4.5-6.5 ppm, thus indicating
the E configuration [10].
Encouraged by the above interesting results, we also
attempted to prepare bis-enaminones or bis-enamino esters
to further broaden the scope. To our delight, the reaction of
diamines with two equiv. of 1,3-dicarbonyl compounds
resulted in the expected products (Table 1, 3aj–3an) in
high yield. Moreover, the reaction of acetylacetone and
aniline was carried out on a scale of 100 mmol. As
expected, the reaction proceeded nicely to afford the
desired product in 92 % yield in 10 min.
Finally, in order to show the merit of the present work
we compared this method with the reported results in the
literature (Table 2), which showed that NbCl5 is the most
efficient catalyst with respect to the reaction time, the
amount of catalyst, and yield of product. In addition, the
toxicity of niobium salts is not high (LD50 orally in rats
725 mg Nb/kg) [50].
Conclusion
We developed a simple and efficient method of synthesis of
b-enaminones and b-enamino esters by the reaction of 1,3-
dicarbonyl compounds with amines in the presence of
catalytic amounts of NbCl5 under solvent-free conditions.
The mild conditions, short times, simple workup, high
yield, and chemo- and regioselectivity are the salient fea-
tures of this procedure.
With the successful enamination of diketones, we further
studied the reaction of amines and methyl acetoacetate
under similar conditions. It was found that the corre-
sponding b-enamino esters were obtained in high to
excellent yields. The different reactivities of the anilines
were dependent on the substituents on the benzene ring. It
Experimental
All solvents and chemicals were obtained commercially
and were used as received. Melting points were determined
123

194
Y.-H. Liu et al.
Ref.
Table 2 Comparison of the
efficiency of NbCl5 with
reported catalysts for
the synthesis of 4-(phenyl-
amino)pent-3-en-2-one (3a)
Catalyst/solvent
Catalyst load
Time
Yield/%
CoCl2ꢀ6H2O
Cu(NO3)2ꢀ3H2O/solvent-free
InBr3/solvent-free
5 mol%
15 min
10 min
10 min
12 min
10 min
15 min
9 min
2 days
3 min
2 h
95
92
94
96
95
95
98
86
79
87
98
91
85
95
[11]
[12]
[13]
[14]
[15]
[16]
[19]
[20]
[22]
[23]
[25]
[27]
[29]
[30]
10 mol%
1 mol%
ZrCl4/solvent-free
1 mol%
ZrOCl2ꢀ8H2O/solvent-free
SnCl4ꢀ5H2O/solvent-free
Ni(OAc)2/solvent-free
Zn(OAc)2ꢀ2H2O/CH2Cl2
I2/solvent-free
2 mol%
2 mol%
5 mol%
5 mol%
20 mol%
15 % (w/w)
50 mg/mol
10 % (w/w)
5 mol%
B2O3/Al2O3/solvent-free
HClO4ꢀSiO2/solvent-free
Silica chloride/solvent-free
L-Proline/solvent-free
14 min
5 min
4 h
Phosphotungstic
acid/solvent-free
1 mol%
15 min
Ag nanoparticles/MeOH
Cu nanoparticles/MeOH
NbCl5/solvent-free
20 mol%
10 mol%
1 mol%
8 h
90
92
96
[31]
2.5 h
5 min
[32]
This work
786, 752, 698 cm-1
;
1H NMR (500 MHz, CDCl3):
by using an X-4 apparatus. IR spectra were recorded using
a Bruker-TENSOR 27 spectrometer. NMR spectra were
taken with a Bruker DRX-500 spectrometer at 500 MHz
(1H) and 125 MHz (13C) using CDCl3 as the solvent.
Elemental analyses were obtained on a Vario EL III
CHNOS elemental analyzer, and the results obtained
agreed favorably with calculated values.
d = 0.96 (d, J = 7.0 Hz, 6H), 1.92–1.98 (m, 1H), 1.99 (s,
3H), 3.88 (d, J = 7.0 Hz, 2H), 4.72 (s, 1H), 7.08 (d,
J = 7.5 Hz, 2H), 7.15 (t, J = 7.5 Hz, 1H), 7.32 (t,
J = 7.5 Hz, 2H), 10.38 (br s, 1H, NH) ppm; 13C NMR
(125 MHz, CDCl3): d = 19.3, 20.4, 26.1, 69.2, 86.2, 124.5,
125.0, 129.2, 139.5, 159.0, 170.6 ppm.
2-Methoxyethyl 3-(phenylamino)-2-butenoate
(3af, C13H17NO3)
Typical experimental procedure for the preparation
of enaminones or enamino esters
IR (KBr):vꢀ = 3,257, 3,188, 3,034, 2,927, 1,651, 1,616,
1,597, 1,585, 1,501, 1,440, 1,385, 1,359, 1,271, 1,230,
1,165, 1,064, 1,028, 983, 786, 752, 698 cm-1; H NMR
1
A mixture of amine (5 mmol), 1,3-dicarbonyl compound
(5 mmol), and NbCl5 (0.05 mmol) in a round-bottom flask
was stirred with a magnetic stirrer at room temperature.
When the two substrates were not liquids, the mixture was
ground with a pestle in the mortar. After completion of the
reaction as monitored by thin-layer chromatography
(TLC), the mixture was diluted with 10 cm3 ethyl acetate
and washed with 20 cm3 water, and the aqueous layer was
then extracted with ethyl acetate (2 9 10 cm3). The com-
bined organic layer was dried over MgSO4 and
concentrated under vacuum to give the product in almost
pure form. Further purification was carried out by recrys-
tallization from hexane–ether if the product was solid or by
short column chromatography on silica gel eluting with
ethyl acetate/petroleum ether (2:8 v/v).
(500 MHz, CDCl3): d = 1.96 (s, 3H), 3.37 (s, 3H), 3.59 (t,
J = 7.5 Hz, 2H), 4.24 (t, J = 7.5 Hz, 2H), 4.75 (s, 1H),
7.05 (d, J = 7.5 Hz, 2H), 7.12 (t, J = 7.5 Hz, 1H), 7.29 (t,
J = 7.5 Hz, 2H), 10.38 (br s, 1H, NH) ppm; 13C NMR
(125 MHz, CDCl3): d = 20.2, 58.9, 61.8, 70.9, 85.8,
124.3, 125.0, 129.0, 139.2, 159.2, 170.0 ppm.
Allyl 3-(phenylamino)-2-butenoate (3ag, C13H15NO2)
IR (KBr):vꢀ = 3,257, 3,188, 3,033, 2,928, 1,660, 1,614,
1,597, 1,585, 1,504, 1,441, 1,385, 1,354, 1,269, 1,228,
1,161, 1,053, 1,029, 995, 786, 752, 698 cm-1; H NMR
1
(500 MHz, CDCl3): d = 2.02 (s, 3H), 4.64 (d, J = 5.5 Hz,
2H), 4.77 (s, 1H), 5.24 (dd, J = 10.5, 1.5 Hz, 1H), 5.36
(dd, J = 17.5, 1.5 Hz, 1H), 5.96–6.04 (m, 1H), 7.10 (d,
J = 7.5 Hz, 2H), 7.17 (t, J = 7.5 Hz, 1H), 7.34 (t,
J = 7.5 Hz, 2H), 10.40 (br s, 1H, NH) ppm; 13C NMR
(125 MHz, CDCl3): d = 20.3, 63.6, 85.7, 117.2, 124.5,
125.1, 129.1, 133.4, 139.3, 159.4, 170.0 ppm.
Isobutyl 3-(phenylamino)-2-butenoate (3ae, C14H19NO2)
IR (KBr):vꢀ = 3,202, 2,960, 2,937, 1,651, 1,622, 1,596,
1,583, 1,504, 1,384, 1,271, 1,230, 1,161, 1,055, 1,008, 908,
123

A general and efficient method for synthesis of enaminones and enamino esters
195
Methyl 3-(phenylamino)-2-pentenoate (3ah, C12H15NO2)
IR (KBr):vꢀ = 3,312, 2,978, 2,947, 1,654, 1,612, 1,595,
1,583, 1,501, 1,379, 1,267, 1,228, 1,164, 1,037, 1,003, 895,
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786, 750, 698 cm-1
;
1H NMR (500 MHz, CDCl3):
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3.69 (s, 3H), 4.74 (s, 1H), 7.09 (d, J = 7.5 Hz, 2H), 7.17 (t,
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1,568, 1,431, 1,356, 1,290, 1,222, 1,103, 1,034, 984,
1
765 cm-1; H NMR (500 MHz, CDCl3): d = 1.39–1.42
(m, 4H), 1.59 (quin, J = 7.0 Hz, 4H), 1.90 (s, 6H), 1.99 (s,
6H), 3.22 (q, J = 7.0 Hz, 4H), 4.95 (s, 2H), 10.86 (br s,
2H, NH) ppm; 13C NMR (125 MHz, CDCl3): d = 18.8,
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White solid; IR (KBr):vꢀ = 3,276, 2,936, 2,860, 1,647,
1,593, 1,427, 1,265, 1,226, 1,163, 1,111, 1,049, 932,
1
781 cm-1; H NMR (500 MHz, CDCl3): d = 1.39 (quin,
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d = 19.4, 26.5, 30.3, 42.8, 49.8, 81.5, 162.0, 170.9 ppm.
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Diisobutyl 3,30-(1,2-ethanediyldiimino)bis(2-butenoate)
(3am, C18H32N2O4)
White solid; IR (KBr):vꢀ = 3,267, 2,966, 1,643, 1596,
1496, 1367, 1265, 1226, 1,168, 1,114, 1,055, 1,006,
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787 cm-1 1H NMR (500 MHz, CDCl3): d = 0.92 (d,
;
J = 6.5 Hz, 12H), 1.91 (s, 6H), 2.28–2.34 (m, 2H), 3.36 (t,
J = 6.0 Hz, 4H), 3.80 (d, J = 6.5 Hz, 4H), 4.51 (s, 2H),
8.64 (br s, 2H, NH) ppm; 13C NMR (125 MHz, CDCl3):
= 19.3, 28.0, 43.9, 68.8, 71.5, 83.6, 161.3, 170.7 ppm.
Diallyl 3,30-(1,2-ethanediyldiimino)bis(2-butenoate)
(3an, C16H24N2O4)
White solid; IR (KBr):vꢀ = 3,271, 3,086, 2,883, 1,649,
1,589, 1,497, 1,259, 1,228, 1,172, 1,111, 1,053, 997,
;
783 cm-1 1H NMR (500 MHz, CDCl3): d = 1.92 (s,
6H), 3.36 (d, J = 6.0 Hz, 4H), 4.54 (d, J = 6.0 Hz, 4H),
4.55 (s, 2H), 5.19 (dd, J = 10.5, 1.5 Hz, 2H), 5.29 (dd,
J = 17.5, 1.5 Hz, 2H), 5.90–5.98 (m, 2H), 8.64 (br s, 2H,
NH) ppm; 13C NMR (125 MHz, CDCl3): d = 19.3, 43.8,
63.4, 83.3, 117.1, 133.5, 161.7, 170.1 ppm.
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Acknowledgments The financial support of this work by the
National Natural Science Foundation of China (21072042) and Nature
Science Foundation of Hebei Province (B2011205031) is gratefully
acknowledged.
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