Vanadium(IV) acetylacetonate catalyzed stereoselective synthesis of β-enaminoesters and β-enaminones

Article abstract of DOI:10.1016/j.tetlet.2012.11.051

An efficient and stereoselective procedure has been described for the synthesis of a series of β-enaminoesters and β-enaminones by vanadium(IV) acetylacetonate [VO(acac)₂] catalyzed reaction of β-ketoesters and 1,3-diketones with both aliphatic and aromatic amines. X-ray crystallographic studies of some representative compounds corroborate two types of structural geometry formed by inter-molecular as well as intra-molecular hydrogen bonds.

Full text of DOI:10.1016/j.tetlet.2012.11.051

Tetrahedron Letters 54 (2013) 436–440
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Vanadium(IV) acetylacetonate catalyzed stereoselective synthesis
of b-enaminoesters and b-enaminones
Rajibul A. Laskar a, Naznin A. Begum a, Mohammad Hedayetullah Mir b, Shahzad Ali c, Abu T. Khan c,
⇑
a Bio-Organic Chemistry Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India
b Department of Chemistry, Aliah University, DN 41, Salt Lake, Kolkata 700 091, India
c Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and stereoselective procedure has been described for the synthesis of a series of b-enamino-
esters and b-enaminones by vanadium(IV) acetylacetonate [VO(acac)2] catalyzed reaction of b-ketoesters
and 1,3-diketones with both aliphatic and aromatic amines. X-ray crystallographic studies of some rep-
resentative compounds corroborate two types of structural geometry formed by inter-molecular as well
as intra-molecular hydrogen bonds.
Received 26 August 2012
Revised 10 November 2012
Accepted 12 November 2012
Available online 24 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
This work is dedicated to Professor Mihir K.
Chaudhuri, Vice-chancellor, Tezpur
University on the occasion of his 65th
birthday.
Keywords:
b-Enaminoesters
b-Enaminones
b-Ketoesters
1,3-Diketones
VO(acac)2
Hydrogen bonding
X-ray crystallography
b-Enaminoesters and b-enaminones are the building blocks for
an important class of molecules synthesized from the b-dicarbonyl
compounds.1–4 They are the precursors for a variety of versatile
biologically active molecules like taxol, peptides, and alkaloids.1–8
Chiral ligands for diastereoselective synthesis can also be obtained
from the optically active enaminones.9 Moreover, the b-enamino-
esters and b-enaminones are significant intermediates for the for-
reported recently. In spite of their applicability, these methods
suffer from drawbacks like prolonged reaction time,2,15 high
temperature,19 formation of amides as side products, expensive
catalysts,12,17,21 high catalyst loading,20,22,26 and the use of
hazardous solvents for example, benzene. Thus, a search for a
new catalyst and simple procedure is of practical importance.
Vanadium acetylacetonate [VO(acac)2] has been proven as a
remarkable reagent in various organic syntheses due to its wide
spectrum of applicability and profound reactivity. This low cost re-
agent is convenient to handle due to extremely low toxicity.28a
Moreover, it is soluble in organic solvents. The catalytic activity
of VO(acac)2 in the epoxidation of alkenes and geraniol, oxidation
of dialkyl disulfides, and selective aerobic oxidation of activated
alcohols into acids or aldehydes is well-known in the litera-
ture.28–30 Recently, the use of VO(acac)2 as catalyst has been re-
ported in the oxidation of b-dicarbonyl compounds,31 olefination
mation of b-aminoacids and
c-aminoalcohols. The major
advantage of these compounds is their stability under simulated
physiological pH conditions and low toxicity.10 Numerous methods
for their syntheses are reported in the literature.1,11–14 The classical
among them, is the condensation of amines and 1,3-dicarbonyl
compounds where water is removed azeotropically by refluxing
in aromatic solvents. Conversion with catalysts like Al2O3,2
SiO2,15 montmorillonite K-10,16 NaAuCl4,17 Zn(ClO4)2ꢀ6H2O,18
AcOH under ultrasound,19 Zn(OAc)2ꢀ2H2O,20 Bi(OTf)3,21 I2,22 Sc(OT-
f)3,23a HClO4ꢀSiO2,23b ionic liquid [EtNH3]NO3 and [Hmin]+Tfaꢁ,25
of a,
a0-divinyl ketones through catalytic Meyer–Schuster rear-
24
CeCl3ꢀ7H2O26 and ZrOCl2ꢀ8H2O,27a H2SO4ꢀSiO2,27b ceric ammonium
rangement,32a synthesis of benzimidazoles,32b and synthesis of car-
bon nanospheres.33 Very recently, we have demonstrated that a
combination of VO(acac)2, hydrogen peroxide, and sodium iodide
is a good system for cleavage of dithioacetals of sugars into alde-
hyde sugars34a and iodination of various organic substrates.34b To
nitrate (CAN),27c LiHSO4/SiO2,27d and -proline27e have also been
L
⇑
Corresponding author.
E-mail address: atk@iitg.ernet.in (A.T. Khan).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.051

R. A. Laskar et al. / Tetrahedron Letters 54 (2013) 436–440
437
Table 1
the best of our knowledge, VO(acac)2 catalyzed synthesis of
Optimization of reaction condition using VO(acac)2 catalysta
b-enaminoesters and b-enaminones from b-ketoesters and
1,3-diketones has not yet been reported. Herein, we report an
easy and selective procedure for the synthesis of a series of
b-enaminoesters and b-enaminones from b-ketoesters and
1,3-diketones respectively using 10 mol % of VO(acac)2 as catalyst
as shown in Scheme 1.
Entry Catalyst
used
Catalyst amount Reaction
Product Yieldb (%)
in (mol %)
time (min)
1
2
3
4
5
6
No Catalyst
—
1
2
5
10
15
3 days
90
50
30
15
3a
3a
3a
3a
3a
3a
55
65
74
90
93
92
VO(acac)2
VO(acac)2
VO(acac)2
VO(acac)2
VO(acac)2
For our study, the catalyst VO(acac)2 was prepared by following
the literature procedure.35 For optimization of the reaction condi-
tions, a mixture of methyl acetoacetate (2.4 mmol) and benzyl
amine (2.0 mmol) was stirred at room temperature without adding
any catalyst. After 3 days of continuous stirring, the desired prod-
uct 3a was obtained in 55% yield along with unreacted starting
materials. Next, the same mixture was stirred at room temperature
in the presence of 1 mol % of vanadyl acetylacetonate and the prod-
uct 3a was isolated in 65% yield (Table 1, entry 2). Likewise, similar
reactions were examined with 2 mol %, 5 mol %, 10 mol %, and
15 mol % successively and we have noted that 10 mol % catalyst
is a sufficient amount for complete conversion as well as to obtain
the best yield (Table 1, entries 3–6). However, the same transfor-
mation can be achieved using 5 mol % catalyst, but it requires long-
er duration. The product 3a was characterized by recording 1H
NMR, 13C NMR spectra, and elemental analysis.
15
a
The reactions were carried out with methyl acetoacetate (2.4 mmol) and benzyl
amine (2.0 mmol).
b
Isolated yield.
ꢀ
reveal that 3t belongs to the triclinic space group P1 with Z = 4
whereas 3v belongs to the monoclinic space group P21/n with
Z = 4. Crystal structure analysis shows that 3t forms two indepen-
dent discrete molecules because of intra-molecular hydrogen
bonding interactions (Oꢀ ꢀ ꢀH = 1.93 Å, Oꢀ ꢀ ꢀN = 2.696 Å and
Oꢀ ꢀ ꢀH = 1.74 Å, Oꢀ ꢀ ꢀN = 2.698 Å) leading to the formation of quasi-
aromatic ring. On the other hand, 3v forms 1D polymer via inter-
molecular hydrogen bonding interactions through C@Oꢀ ꢀ ꢀH–N
bonds (Oꢀ ꢀ ꢀH = 1.99 Å, Oꢀ ꢀ ꢀN = 2.810 Å, \Oꢀ ꢀ ꢀH–N = 159°). The
hydrogen-bonded ring formation could not be possible in the case
of 3v due to the structural constraint as shown in Figure 1a.
The 1H NMR spectra of the products show two different kinds of
chemical shifts of NH protons. Notably, the NH proton of b-enami-
none (entry 3v and 3w) (derived from dimedone and benzylamine
or dimedone and p-ethyl aniline) was found at 4.80 ppm and
5.95 ppm which supports the formation of E-isomer. In this case
the intermolecular hydrogen bond causes the compound to become
a 1D-zigzag hydrogen-bonded polymeric form as evident from
the X-ray crystallographic structure of Figure 1a. The downfield shift
of the NH proton is in the range of d value 8.9–12.5 ppm which
indicates the predominant formation of the Z-b-enaminoesters or
Z-b-enaminones. The intra-molecular hydrogen bonding plays the
key role in maintaining the geometry of the molecule intact and
responsible for higher d values because of the formation of
quasi-aromatic ring which is evident from Figure 1b.
After optimizing the reaction conditions,36 the reaction of a
wide variety of b-ketoesters (1b–j) was examined with benzyl
amine using 10 mol % of VO(acac)2 at room temperature and the
desired products 3b–j were obtained in good yields (Table 2, en-
tries b–j). Similarly, various b-ketoesters (1k–s) and aromatic
amines were scrutinized in the presence of 10 mol % catalyst under
identical reaction conditions and the products (3k–s) were isolated
in moderate to good yields. It is worth-while to mention that elec-
tron-rich aromatic amines take a shorter reaction time and provide
good yields as compared to aromatic amine having electron-
withdrawing substituents. Encouraged by these results, various
1,3-diketones (1t–x) were treated with different aromatic amines
and benzyl amine using the same amount of catalyst under similar
reaction conditions and the results are given in Table 2.
Interestingly, acyclic b-ketoesters and 1,3-diketones result
Z-b-enaminoesters and Z-b-enaminones with 100% stereoselectivity,
respectively, whereas cyclic 1,3-diketones give exclusively
E-b-enaminones as shown in Scheme 1.
From Table 2, it can be seen that the nucleophilic benzylamine
reacts faster with a variety of b-ketoesters and 1,3-diketones to
give b-enaminoesters and b-enaminones in excellent yields as
compared to 2-methoxyaniline (entry 1l) and 2,6-dimethylaniline
(entry 1o). A plausible mechanistic pathway has been outlined in
Scheme 2.
Again in the case of unsymmetrical diketone, benzoyl acetone
(entries 3t–u), the amine always attacked the keto group posi-
tioned a- to the methyl group and in all cases; this is evident from
the 1H NMR spectra. The methyl group exhibited a distinctive sin-
glet at 2.07 ppm, instead of 2.22 ppm which is a characteristic peak
of the methyl group of –COCH3.
Stereoselective syntheses of enaminones and enaminoesters
using various catalysts have been well-studied.21,23b,27c However,
it provides further scope to design a particular substrate to obtain
stereoselective products unequivocally. The major advantage of
our procedure is the stereoselective formation of the products ow-
ing to the intermolecular and intramolecular hydrogen-bonding.
To shed further light on the geometry of the compounds, the
molecular structures of 3t and 3v were confirmed by a single crys-
tal X-ray analysis (Fig. 1).37 X-ray crystallographic experiments
O
VO(acac)2
O
O
NH2
(10 mol %)
NH
O
N
H
R
+
+
R
r.t., solvent-free
condition
R
Not formed
1
2
3
4
R = OMe, OEt, OCMe3, OPh, OBz, Ph etc.
VO(acac)2
(10 mol %)
NH2
+
N
H
O
r.t., solvent-free
condition
O
O
1
2
3
Scheme 1.

438
R. A. Laskar et al. / Tetrahedron Letters 54 (2013) 436–440
Table 2
VO(acac)2 catalyzed formation of b-enaminoesters and b-enaminones
Entry
a
b-Ketoester or 1,3-diketone (1)
Amine (2)
Time (min)
15
Producta (3)
NH
Yieldb (%)
93
O
O
O
O
O
O
O
O
O
O
NH2
NH2
NH2
NH2
NH2
NH2
NH2
O
O
O
O
O
O
OMe
OMe
OEt
NH
NH
NH
NH
NH
b
c
d
e
f
15
15
20
20
20
92
91
89
90
88
OEt
CMe3
OCMe3
O
O
O
O
C10H21
C10H21
C16H33
O
O
C16H33
O
O
O
O
O
O
NH
O
O
O
g
25
10
93
90
O
NH2
h
OEt
O
N
H
OEt
O
O
O
NH2
NH2
NH
O
O
i
j
15
20
92
91
O
O
Ph
O
O
O
O
NH
O
Ph
Ph
NH2
OMe
k
l
60
55
76
89
NH
O
O
O
O
O
OMe
OMe
OEt
O
O
O
O
O
O
O
O
OMe
OMe
OMe
Me
OMe
NH
NH2
OMe
OEt
OMe
NH
NH2
NH2
m
n
55
60
60
85
87
87
OMe
NH
OCMe3
OMe
CMe3
O
Me
NH
NH2
Me
o
Me
OMe

R. A. Laskar et al. / Tetrahedron Letters 54 (2013) 436–440
439
Table 2 (continued)
Entry
b-Ketoester or 1,3-diketone (1)
Amine (2)
Time (min)
65
Producta (3)
Yieldb (%)
O
O
O
O
O
O
O
O
Me
Me
NH2
OEt
p
q
r
89
88
92
NH
Me
O
Me
OEt
Me
Me
Me
NH2
Me
OCMe3
65
60
NH
Me
O
O
CMe3
O
Me
NH2
Me
Ph
NH
Me
Ph
NH2
O2N
OMe
s
t
90
15
60
62
86
88
O2N
NH
O
OMe
O
O
O
O
NH
O
NH2
Ph
Ph
Ph
OMe
OMe
NH
NH2
u
O
Ph
NH2
v
15
89
N
O
O
O
O
H
NH2
Et
w
x
25
20
81
83
Et
O
O
O
O
N
H
NH2
N
O
H
a
The reactions were performed using b-ketoester (1.2 mmol) or 1,3-diketone (1.2 mmol), and aromatic amine (1 mmol) or benzylamine (1 mmol).
Isolated yield.
b
This method is applicable for the preparation of the enaminones
with a linear long chain and bulky b-keto esters (e.g., entries
1d–g), cyclic b-ketoesters (e.g., entry 1h), and 1,3-diketones (e.g.,
entries 1t–x). While both aromatic and benzyl amine reacted
to give the products, aromatic amines react comparably slow.
The comparison of our protocol with the existing methods of
synthesizing b-enaminoesters and b-enaminones in the presence
of various catalysts has been shown in Table 3.
acac
O
O
O
VO(acac)2
O
O
V
O
O
R
1
R
H2N
In conclusion, we report a simple, efficient, and cost effective
protocol for the formation of b-enaminones from b-ketoesters as
well as b-diketones by their reaction with both aromatic and
aliphatic amines. The reaction occurs in the presence of the
catalytic amount of vanadium(IV) acetylacetonate at room
temperature under solvent free condition. With the help of
X-ray crystallographic data, we have also identified two different
types of structural geometries for b-enaminoesters and b-enami-
nones which explain the upfield and the downfield chemical
shifts of NH-proton in the 1H NMR spectra of the corresponding
compounds.
2
H2O
NH
NH
O
OVO(acac)
R
R
3
Z-
acac
Scheme 2.
VO(acac)2

440
R. A. Laskar et al. / Tetrahedron Letters 54 (2013) 436–440
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Catalyst used
Amount used (mol %)
Time (min)
Yield (%)
ap-Toluenesulfonic acid
aH2SO4ꢀSiO2
aHClO4ꢀSiO2
aCAN
10
10 h
5
5
87
95
96
88
89
99
97
98
90
92
400 mg/mmol
50 mg/mmol
20
10
20
5
50 mg/mmol
10
10
8
aVO(acac)2
bI2
15
30
5
5
2 h
15
bBi(TFA)3
bHClO4ꢀSiO2
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bSc(OTf)3
bVO(acac)2
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Acknowledgements
33. Ndwandwe, S.; Tshibangu, P.; Dikio, E. D. Int. J. Electrochem. Sci. 2011, 6, 749.
34. (a) Khan, A. T.; Ali, S.; Sidick Basha, R.; Khan, M. M.; Lal, M. Carbohydr. Res. 2011,
346, 2629; (b) Khan, A. T.; Ali, S. Bull. Chem. Soc. Jpn. 2012, 85, 1239.
35. Kantam, M. L.; Neelam, B.; Reddy, C. V.; Chaudhuri, M. K.; Dehury, S. K. Catal.
Lett. 2006, 95, 19.
S.A. and A.T.K. acknowledge the DST for providing single
XRD-facility in the Department of Chemistry, IIT Guwahati. R.A.L.
and N.A.B. are thankful to DST-FIST and UGC-SAP program for
financial support to the Department of Chemistry, Visva-Bharati
and also thanks to Dr. Adinath Majee, Department of Chemistry,
Visva-Bharati for his kind help in recording the IR spectra. We
are extremely grateful to the referees for their valuable comments
and suggestion.
36. General procedure for the preparation of b-enaminoesters and b-enaminones: Into
a mixture of aromatic amine or benzyl amine (1 mmol) and b-ketoesters or 1,3-
diketones (1.2 mmol) in
a 25 ml round bottomed flask vanadium(IV)
acetylacetonate (0.026 g, 0.10 mmol) was added and it was kept for stirring
at room temperature. The completion of the reaction was monitored by TLC.
The product was extracted by aqueous MeOH or by CH2Cl2 (20 mL). The pure
product was obtained either by fractional crystallization using aqueous MeOH
(for solid product) or by directly passing through a silica gel (60–120 mesh)
column using 1:9 EtOAc/hexane as eluent (for liquid product).
ꢀ
37. Crystal data: 3t: Triclinic space group P1, a = 10.1740(5), b = 11.0786(5),
Supplementary data
c = 14.1398(7) Å,
V = 1424.39(12) Å3, Z = 4,
R1 = 0.0759, wR2 = 0.2380,
a
= 86.629(3),
q
b = 84.875(3),
c = 63.836(3)°,
calcd = 1.172 g cmꢁ3
GOF = 0.979
,
l
= 0.073 mmꢁ1
I > 2 (I),
,
T = 296 K,
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2012.11.051.
for
r
Mok -ray
a
(k = 71,073 pm). 3v: Monoclinic space group P21/n, a = 5.9283(8),
b = 19.514(3), c = 11.7648(16) Å, b = 102.341(8)°, V = 1329.6(3) Å3, Z = 4,
q
calcd = 1.146 g cmꢁ3
,
l
= 0.071 mmꢁ1
,
T = 296 K, R1 = 0.1255, wR2 = 0.2968,
References and notes
GOF = 0.942 for I > 2
r
(I), Mok -ray (k = 71,073 pm). Crystallographic data have
a
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 828349 and 828350. A copy of
the data can be obtained free of charge from CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK or e-mail: deposit@ccdc.cam.ac.uk.
1. Ferraz, H. M. C.; Oliveira, E. O.; Payret-Arrua, M. E.; Brandt, C. A. J. Org. Chem.
1995, 60, 7357.
2. Valduga, C. J.; Braibante, H. S.; Braibante, E. F. J. J. Heterocycl. Chem. 1998, 35,
189.