Nickel nanoparticles catalyzed chemoselective Knoevenagel condensation of Meldrum's acid and tandem enol lactonizations via cascade cyclization sequence

Article abstract of DOI:10.1016/j.tetlet.2011.05.032

A novel protocol has been reported wherein, polyethylene glycol (PEG) stabilized nickel nanoparticles have been used as catalyst for chemoselective Knoevenagel condensation of aromatic aldehydes and Meldrum's acid to give 5-arylidene Meldrum's acid which underwent tandem enol lactonization by Michael addition/cyclization sequence with active methylene compounds in the presence of Ni nanoparticles to give corresponding enol lactone derivatives.

Full text of DOI:10.1016/j.tetlet.2011.05.032

Tetrahedron Letters 52 (2011) 3666–3669
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Nickel nanoparticles catalyzed chemoselective Knoevenagel condensation
of Meldrum’s acid and tandem enol lactonizations via cascade
cyclization sequence
⇑
Jitender M. Khurana , Kanika Vij
Department of Chemistry, University of Delhi, Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel protocol has been reported wherein, polyethylene glycol (PEG) stabilized nickel nanoparticles
have been used as catalyst for chemoselective Knoevenagel condensation of aromatic aldehydes and
Meldrum’s acid to give 5-arylidene Meldrum’s acid which underwent tandem enol lactonization by
Michael addition/cyclization sequence with active methylene compounds in the presence of Ni nanopar-
ticles to give corresponding enol lactone derivatives.
Received 9 April 2011
Revised 5 May 2011
Accepted 8 May 2011
Available online 14 May 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Ni nanoparticles
Knoevenagel condensation
Michael additon
Ethylene glycol
Organic catalytic reactions have emerged as a powerful syn-
thetic tool for the construction of highly functionalized com-
pounds. The versatile Knoevenagel and Michael addition
reactions have numerous applications in the elegant synthesis of
fine chemicals and are reported under homogeneous, solvent-free,
ethanol–piperidene, MgO–silica grinding, etc.1 The Knoevenagel
condensation of Meldrum’s acid (2,2-dimethyl-4,6-dioxo-1,3-diox-
ane) with aromatic aldehydes furnishes the corresponding arylid-
ene derivatives,2–5 which are versatile substrates for a variety of
reactions.6–9 Enol lactones represent an important class of biolog-
ically active coumarin derivatives10,11 but only a few synthetic se-
quences are available for their synthesis.12–16 In many cases the
synthetic sequence is rather long, consequently leading to low
product yields. Therefore, there is sufficient scope for the develop-
ment of alternative strategies for the synthesis of enol lactones.
Transition metal nanoparticles, including nickel, have gained tre-
mendous importance as they exhibit interesting electrical, optical,
magnetic and chemical properties especially catalytic properties
which cannot be achieved by their bulk counterparts.17,18 Amongst
the variety of chemical methods19 available for the preparation of
transition metal nanoparticles, the modified polyol method has
been conveniently utilized for the heterogeneous nucleation and
growth of nickel nanoparticles.20 An interesting array of examples
are available wherein Ni nanoparticles have been used as a catalyst
for organic transformations and in combinatorial chemistry.21–27
Recently, we have reported the Knoevenagel condensation of alde-
hydes with barbituric acids28 as well as with o-phenylenediamine
and 2-aminothiophenol29 catalyzed efficiently by PVP (polyvinyl-
pyrrolidone)-stabilized Ni(0) nanoparticles in ethylene glycol. We
were intrigued about the behavior of nickel nanoparticles in Knoe-
venagel condensation of Meldrum’s acid with aromatic aldehydes
and their subsequent Michael additions with different active meth-
ylene compounds. We wish to report herein the first Ni nanoparti-
cles promoted synthesis of arylidene Meldrum’s acids and the
corresponding enol lactone derivatives viz., tetrahydro-3H-chro-
mene-2,5-diones, dihydropyrano[3,2-c]chromene-2,5-diones, dihy-
dropyrano[4,3-b]pyran-2,5-diones and 4-aryl-3,4-dihydrobenzo[g]
chromene-2,5,10-triones.
In this Letter, we have reported a simple, easy and highly
efficient protocol for the formation of 5-arylidene-2,2-dimethyl-
[1,3]dioxane-4,6-diones (3) by the Knoevenagel condensation of
R'
OH
CHO
O
O
O
R'
CHO
O
O
1p-q
R
1a-o
O
O
PEG-Ni nps R
PEG-Ni npsO
EG, r.t.
O
2
COOH
O
3a-o
3p: R'= H
3q: R'= OMe
EG, r.t.
⇑
Corresponding author. Tel.: +91 11 27667092; fax: +91 11 27666518.
E-mail addresses: jmkhurana@chemistry.du.ac.in, jmkhurana1@yahoo.co.in (J.M.
Scheme 1. PEG-stabilized Ni nanoparticles catalyzed Knoevenagel condensation of
Khurana).
Meldrum’s acid with aromatic aldehydes.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.032

J. M. Khurana, K. Vij / Tetrahedron Letters 52 (2011) 3666–3669
3667
200
(111)
150
100
50
(200)
(222)
0
30
40
50
60
70
80
2 Theta (degrees)
(a)
(b)
Figure 1. (a) TEM image of PEG–Ni nanoparticles; (b) X-ray diffraction data of PEG–Ni nanoparticles.
Table 1
PEG stabilized Ni nanoparticles mediated Knoevenagel condensation of aromatic
aldehydes and Meldrum’s acid in ethylene glycol
Table 2
PEG–Ni nanoparticles catalyzed tandem enol lactonizations via Michael additions by
active methylenes (4) on arylidene Meldrum’s acids (3)
Entry ArCHO (1)
Product Mp (°C) Obs. (L)
Time
Yield
(3)
(min) (%)
Entry ArCHO
4
Product Yield Time
Mp (°C) Obs.
(L)
a
4-(CH3)2NC6H4
3a
170 (175)4
10
95
(1)
(%)
(min)
(1a)
1
4-(CH3)2NC6H4
(1a)
4-CH3C6H4 (1b) 4a 5b
4a 5a
94
10
135 (136–138)15a
b
c
d
e
f
4-CH3C6H4 (1b)
Piperonyl (1c)
4-HOC6H4 (1d)
2-Furanyl (1e)
C6H5CH@CH (1f)
4-(HO)-3-
3b
3c
3d
3e
3f
150 (149–152)1e
160 (167–169)4
190 (195)4
10
10
10
10
12
10
89
92
97
87
90
93
2
3
4
5
6
93
91
83
88
90
10
10
15
12
10
204
Piperonyl (1c)
4-HOC6H4 (1d)
2-Furanyl (1e)
C6H5CH@CH
(1f)
4a 5c
4a 5d
4a 5e
4a 5f
150 (151–153)15a
166 (166–168)15a
86–88
92 (97)5b
109 (109)5a
g
3g
134 (136–138)4
152 (158–160)15b
(CH3O)C6H3 (1g)
4-CH3OC6H4 (1h) 3h
h
i
120–122 (126)4
10
12
97
95
7
8
9
4-(HO)-3-
(CH3O)C6H3 (1g)
4-CH3OC6H4
(1h)
4b 5g
4b 5h
4b 5i
92
89
84
10
10
12
124–126 (127–
129)15a
112–114
3,4-(CH3O)2C6H3
(1i)
3-HOC6H4 (1j)
4-ClC6H4 (1k)
C6H5 (1l)
3,4,5-(CH3O)3C6H2 3m
(1m)
2,5-(CH3O)2C6H3
(1n)
2-Br-piperonyl
(1o)
2-HOC6H4 (1p)
2-(HO)-3-
3i
158–160 (156–158)2
j
k
l
3j
3k
3l
170–172 (175)5a
156–158 (157–158)2
84 (85)4
10
15
10
15
93
75
79
91
3,4-
124
(CH3O)2C6H3
(1i)
m
150–152 (155–156)2
10
11
12
13
3-HOC6H4 (1j)
4-ClC6H4 (1k)
C6H5 (1l)
3,4,5-
4b 5j
4c 5k
4c 5l
4c 5m
87
85
89
90
12
15
10
10
—
n
o
3n
120 (120–121)5c
145 (-)
10
10
93
85
108–110
120–122
158–160
3o
(CH3O)3C6H2
(1m)
p
q
3p
3q
188 (190–192)5c
215 (218–219)5d
5
5
88
90
14
2,5-
4c 5n
86
12
90–92
(CH3O)C6H3 (1q)
(CH3O)2C6H3
(1n)
15
16
2-Br-piperonyl
(1o)
4-CH3OC6H4
(1h)
Piperonyl (1c)
2,5-
(CH3O)2C6H3
(1n)
4-ClC6H4 (1k)
4-ClC6H4 (1k)
4-HOC6H4 (1d)
2,5-
(CH3O)2C6H3
(1n)
Piperonyl (1c)
4-ClC6H4 (1k)
4-HOC6H4 (1d)
Piperonyl (1c)
3,4-
4c 5o
4d 6a
91
92
10
10
112–114
132–134
aromatic aldehydes with Meldrum’s acid in the presence of PEG-
stabilized Ni nanoparticles in ethylene glycol at room temperature.
Subsequently, Michael additions of a variety of active methylene
compounds viz., cyclohexane-1,3-diones (4a–c), 4-hydroxycouma-
rin (4d), 4-hydroxy-6-methyl-pyran-2-one (4e) and 2-hydroxy-
1,4-naphthoquinone (4f) with 5-arylidene Meldrum’s acid in the
presence of PEG-stabilized Ni nanoparticles followed by tandem
enol lactonization led to an interesting array of products, namely,
4-aryl-4,6,7,8-tetrahydro-3H-chromene-2,5-diones (5), 4-aryl-3,4-
dihydropyrano[3,2-c]chromene-2,5-diones (6), 4-aryl-7-methyl-
3,4-dihydropyrano[4,3-b]pyran-2,5-diones (7), and 4-aryl-3,
4-dihydrobenzo[g]chromene-2,5,10-triones (8) in quantitative
yields.
Nickel nanoparticles were prepared by modified polyol pro-
cess20 and were stabilized with PEG-4000 rather than polyvinyl
pyrrolidone as reported. Reduction of Ni2+ salt precursor
(NiCl2ꢀ6H2O) with sodium borohydride in the presence of PEG-
4000 in ethylene glycol led to the formation of highly monodi-
spersed Ni nanoparticles as given in Eq. 1.
17
18
4d 6b
4d 6c
89
91
7
10
154–156
160
19
20
21
22
4d 6d
4e 7a
4e 7b
4e 7c
78
80
92
90
15
12
7
186–188
194
236–238
154
10
23
24
25
26
27
4e 7d
4f 8a
4f 8b
4f 8c
4f 8d
93
79
86
89
91
10
10
10
10
10
146
222 (224–227)16
240
204–206 (208–211)16
188–190
(CH3O)2C6H3
(1i)
3,4,5-
(CH3O)3C6H2
(1m)
28
4f 8e
86
15
200 (203–206)16

3668
J. M. Khurana, K. Vij / Tetrahedron Letters 52 (2011) 3666–3669
OH
O
R1
R2
O
O
O
(4e)
O
R1
R2
O
O
O
O
(4)
a: R1=CH3, R2=CH3
b: R1=H, R2=CH3
c: R1=CH3, R2=H
O
Ar
O
(5)
Ar
(7)
O
O
O
PEG-Ni nps
EG, r.t.
PEG-Ni nps
EG, r.t.
OH
Ar
O
OH
O
O
O
O
O
O
O
(3a-o)
O
O
O
O
(8)
Ar
(4f)
(4d)
O
(6)
Ar
Scheme 2. PEG-stabilized Ni nanoparticles catalyzed tandem enol lactonizations via Michael addition.
2Ni2þðegÞ þ BHꢁðsÞ þ 2H2OðegÞ þ 2nPEGðegÞ
mene-2,5-dione (5a, 94%) obtained by Michael addition followed
by a concomitant loss of acetone and carbon dioxide molecule. In
the absence of Ni nanoparticles, this reaction was incomplete even
after 24 h, thereby justifying the involvement of Ni nanoparticles.
Subsequently, other 5-arylidene Meldrum’s acids (3) also under-
went tandem enol lactonization with various cyclic 1,3-diketones
(4a–c) to furnish the corresponding 4-aryl-4,6,7,8-tetrahydro-3H-
chromene-2,5-diones (5a–o). Reactions of 5-arylidene Meldrum’s
acids (3) with other active methylene compounds namely, 4-
hydroxycoumarin (4d), 4-hydroxy-6-methylpyran-2-one (4e) and
2-hydroxy-1,4-naphthoquinone (4f) in the presence of nickel
nanoparticles also underwent rapid and tandem enol lactonization
involving Michael addition to give 4-aryl-3,4-dihydropyrano[3,2-c]
chromene-2,5-diones (6a–d), 4-aryl-7-methyl-3,4-dihydropyr-
ano[4,3-b]pyran-2,5-diones (7a–d) and 4-aryl-3,4-dihydro-
benzo[g]chromene-2,5,10-trione (8a–e) in high yields (Scheme 2,
Table 2).30
In conclusion, we have reported a highly efficient and novel
protocol for the Knoevenagel condensation of aromatic aldehydes
with Meldrum’s acid followed by the tandem enol lactonization
with a wide variety of active methylene compounds. PEG-stabi-
lized Ni nanoparticles proved to be a highly promising catalyst
for the same and an interesting array of compounds were obtained
in quantitative yields in short time span.
4
! 2NiðPEGÞ ðsÞ þ 2H2ðgÞ þ 4HþðegÞ þ BOꢁðegÞ
ð1Þ
n
2
The metal dispersion so obtained was characterized by TEM
analysis which revealed the formation of coated Ni nanoparticles
with an average diameter of 8 nm (Fig. 1a). Metallic nature of
nanoparticles was confirmed by X-ray diffraction data (Fig. 1b).
In continuation to our endeavor with the utilization of Ni nano-
particles as catalyst in organic synthesis, the above sample of Ni
nanoparticles was used as such for the reactions. A reaction of 4-
N,N-dimethylaminobenzaldehyde (1a, 1 equiv) was attempted
with Meldrum’s acid (1 equiv) in the presence of varying amounts
of Ni nanoparticles dispersion in ethylene glycol. The best result
was obtained using 2 mL of ethylene glycol dispersion of Ni nano-
particles at room temperature (wherein 2 mL of the dispersion
contains 0.0235 wt % of Ni:I). 95% of 5-(4-dimethylaminobenzyli-
dene)-2,2-dimethyl[1,3]dioxane-4,6-dione (3a) was obtained in
10 min. Subsequently, reactions of other aldehydes with Mel-
drum’s acid also gave the corresponding 5-arylidene derivatives
of Meldrum’s acid (3a–o) (Scheme 1).30 The reaction showed selec-
tivity toward electron donating substituents leading to the alkene
formation. Electron withdrawing substituents, on the contrary,
gave incomplete reactions along with mixture of products. The
reactions of 2-hydroxy arylaldehydes (1p and 1q) did not afford
the corresponding alkenes but yielded the corresponding 2-oxo-
2H-chromene-3-carboxylic acid (3p and 3q) (Scheme 1, Table 1).
The involvement of Ni nanoparticles was confirmed since a
reaction of 1a with Meldrum’s acid in ethylene glycol alone (con-
taining traces of PEG) at room temperature in the absence of Ni
nanoparticles was incomplete even after 6 h. Similar results were
obtained when the reaction was repeated with Ni2+ salt as well
as NaBH4 alone in ethylene glycol at room temperature. The reac-
tion was also attempted with Ni powder (size <150 micron) in eth-
ylene glycol. The reaction was only 54% complete even after
stirring for 24 h unlike complete reactions in 10–15 min in the
presence of Ni nanoparticles. One of the most crucial results came
from the reaction of 1a with Meldrum’s acid in the presence of iso-
lated Ni nanoparticles (obtained from the dispersion by the polyol
method) which were washed repeatedly with abs. ethanol fol-
lowed by dispersal in ethylene glycol. The reaction was found to
be complete in 15 min and 87% of 3a was isolated.
Acknowledgment
K.V. is thankful to CSIR, New Delhi, India for the award of Junior
and Senior Research Fellowship.
References and notes
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to act as suitable catalyst for Michael addition reactions involving 3
and a variety of active methylene compounds. A reaction of 3a with
dimedone (4a) in the presence of Ni nanoparticles at room temper-
ature was complete in 10 min and the isolated product was identi-
fied to be the enol lactone derivative, that is, 4-(4-
dimethylaminophenyl)-7,7-dimethyl-4,6,7,8-tetrahydro-3H-chro-
9. Huang, X.; Xie, L. Synth. Commun. 1986, 16, 1701.
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J. M. Khurana, K. Vij / Tetrahedron Letters 52 (2011) 3666–3669
3669
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dimethyl[1,3]dioxane-4,6-dione (3o): Mp: 145 °C; IR (KBr, cmꢁ1
)
m
max: 2926,
1727, 1594, 1504, 1477, 1416, 1380, 1328; 1H NMR (300 MHz, CDCl3) d: 1.81 (s,
6H), 6.09 (s, 2H, OCH2,), 7.14 (s, 1H, Ar–H), 7.57 (s, 1H, Ar–H), 8.65 (s, 1H); 13
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A. J. G. J. Colloid. Interf. Sci. 2007, 311, 461. Preparation of Ni nanoparticles: Nickel
nanoparticles were synthesized in ethylene glycol from NiCl2.6H2O (2ꢂ10-4M)
and NaBH4 by the modified polyol process20 and polyethylene glycol (PEG-
4000) was used as the stabilizing agent. Our results are in accordance with the
reported one and match satisfactorily with the conventional protocol.
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C
NMR (75 MHz, CDCl3) d: 27.7, 102.7, 104.7, 111.7, 114.6, 121.7, 125.3, 147.1,
152.3, 156.4, 162.6. [M+H]+354, 356 [M+2]+.
4-(4-Hydroxy-3-methoxyphenyl)-4,6,7,8-tetrahydro-3H-chromene-2,5-dione
(5i): Mp: 124 °C; IR (KBr, cmꢁ1
) mmax: 3336, 2956, 2918, 1782, 1644, 1516,
1460, 1448, 1425, 1380; 1H NMR (400 MHz, CDCl3) d: 2.1 (m, 2H), 2.43–2.45
(m, 2H), 2.64–2.66 (m, 2H), 2.90–2.91 (m, 2H), 3.85 (s, 3H, OCH3), 4.23–4.25
(m, 1H), 5.55 (s, 1H, OH), 6.58–6.60 (m, 1H, Ar–H), 6.70–6.71 (m, 1H, Ar–H),
6.78–6.80 (m, 1H, Ar–H). 13C NMR (75 MHz, CDCl3) d: 20.6, 27.3, 33.4, 36.3,
36.7, 55.8, 109.6, 114.6, 118.6, 132.4, 144.9, 146.7, 166.0, 167.1, 196.4.
[M+H]+289.
24. Alonso, F.; Riente, P.; Yus, M. Synlett 2008, 1289.
4-(2,5-Dimethoxyphenyl)-3,4-dihydropyrano[3,2-c]chromene-2,5-dione (6c):
25. Alonso, F.; Riente, P.; Yus, M. Eur. J. Org. Chem. 2008, 4908.
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27. Sapkal, S. B.; Shelke, K. F.; Shingate, B. B.; Shingare, M. S. Tetrahedron Lett. 2009,
50, 1754.
Mp: 160 °C; IR (KBr, cmꢁ1
) mmax: 2922, 1776, 1731, 1649, 1504, 1375, 1237;
1H NMR (400 MHz, DMSO-d6) d: 2.69–2.74 (m, 1H), 3.43–3.47 (m, 1H), 3.57 (s,
3H, OMe), 3.60 (s, 3H, OMe), 4.31–4.33 (m, 1H), 6.68–6.87 (m, 3H, Ar–H), 7.41–
7.43 (m, 2H, Ar–H), 7.64–7.68 (m, 1H, Ar–H), 7.81–7.83 (m, 1H, Ar–H); 13C NMR
(100 MHz, DMSO-d6) d: 31.3, 36.0, 55.8, 56.6, 106.6, 111.2, 112.2, 116.0, 116.8,
123.9, 124.7, 128.3, 132.8, 151.7, 153.2, 154.0, 156.2, 164.1, 174.0; [M]+336.
4-(4-Chlorophenyl)-7-methyl-3,4-dihydropyrano[4,3-b]pyran-2,5-dione (7a):
28. Khurana, J. M.; Vij, K. Catal. Lett. 2010, 138, 104.
29. Khurana, J. M.; Sneha; Vij, K. Synth. Commun. 2011, article in press.
30. General procedure for the synthesis of 5-arylidene Meldrum’s acids (3): Aldehyde
(1, 2 mmol) was added to a well stirred dispersion of nickel nanoparticles in
ethylene glycol [2 mL of dispersion (0.0235 wt % of Ni)/0.1 g of 1] in a 25 mL
R.B. flask. To it, Meldrum’s acid (2, 2 mmol) was added and the contents were
stirred at room temperature till a solid product separates out. Completion of
the reaction was monitored by TLC using ethyl acetate: petroleum ether
(40:60) as the eluent. All the reactions were invariably complete in 10 min.
Upon completion, the reaction mixture was diluted with water, the solid
product was filtered, washed with water, dried and recrystallised from ethanol.
5-Arylidene Meldrum’s acids were obtained as identified by their mp and
spectral data.
Mp: 194 °C; IR (KBr, cmꢁ1 max: 2863, 1771, 1737, 1317, 1202; 1H NMR
) m
(400 MHz, DMSO-d6) d: 1.53–1.85 (m, 5H), 3.95 (s, 1H), 7.52 (m, 2H, Ar–H),
7.97 (m, 2H, Ar–H), 8.30 (s, 1H); 13C NMR (100 MHz, DMSO-d6) d: 26.8, 30.5,
40.1, 90.6, 100.5, 128.4, 130.9, 133.5, 134.5, 156.0, 163.2, 168.7, 205.9;
[M]+290.
4-(3,4-Dimethoxyphenyl)-3,4-dihydrobenzo[g]chromene-2,5,10-trione (8d):
Mp: 188–190 °C; IR (KBr, cmꢁ1 2924, 1674, 1587, 1337; 1H NMR
) mmax:
(400 MHz, DMSO-d6) d: 2.84–2.87 (m, 1H), 3.33- 3.37 (m, 1H), 3.64 (s, 3H,
OCH3), 3.69 (s, 3H, OCH3), 4.46–4.48 (m, 1H), 6.67–6.69 (m, 1H, Ar–H), 6.76–
6.78 (m, 1H, Ar–H), 6.87–6.88 (m, 1H, Ar–H), 7.83–7.85 (m, 2H, Ar–H), 7.93–
7.94 (m, 1H, Ar–H), 8.04–8.05 (m, 1H, Ar–H). 13C NMR (100 MHz, DMSO-d6) d:
34.0, 39.0, 55.5, 55.6, 111.1, 118.3, 125.1, 126.0, 131.0, 131.7, 134.3, 134.6,
148.3, 149.1, 152.0, 165.7, 177.2, 182.8. [M+1]+365.
General procedure for the enol lactonization of 3 with active methylenes (4) to
yield the corresponding enol lactone derivatives (5–8): In a typical experiment, 5-
arylidene Meldrum’s acid (3, 2 mmol) was added to a well stirred dispersion of