A simple, efficient and green procedure for the Knoevenagel condensation of aldehydes with N-methylpiperazine at room temperature under solvent-free conditions

Article abstract of DOI:10.1080/00397910802029364

Knoevenagel condensation of aromatic, aliphatic, and heteroaromatic aldehydes with active methylene compounds such as ethylcyanoacetate, malononitrile, and cyanoacetamide proceed very smoothly at room temperature by simply mixing the ingredients together under solvent-free conditions in the presence of N-methylpiperazine in excellent yields of the E-configured products. Copyright Taylor & Francis Group, LLC.

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A Simple, Efficient and Green
Procedure for the Knoevenagel
Condensation of Aldehydes
with N-Methylpiperazine at
Room Temperature under
Solvent-Free Conditions
Chhanda Mukhopadhyay a & Arup Datta a
a Department of Chemistry, University of Calcutta,
Kolkata, India
Version of record first published: 18 Aug 2008
To cite this article: Chhanda Mukhopadhyay & Arup Datta (2008): A Simple, Efficient
and Green Procedure for the Knoevenagel Condensation of Aldehydes with N-
Methylpiperazine at Room Temperature under Solvent-Free Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 38:13, 2103-2112
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Synthetic Communications1, 38: 2103–2112, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802029364
A Simple, Efficient and Green Procedure for the
Knoevenagel Condensation of Aldehydes with
N-Methylpiperazine at Room Temperature
under Solvent-Free Conditions
Chhanda Mukhopadhyay and Arup Datta
Department of Chemistry, University of Calcutta, Kolkata, India
Abstract: Knoevenagel condensation of aromatic, aliphatic, and heteroaromatic
aldehydes with active methylene compounds such as ethylcyanoacetate, malono-
nitrile, and cyanoacetamide proceed very smoothly at room temperature by sim-
ply mixing the ingredients together under solvent-free conditions in the presence
of N-methylpiperazine in excellent yields of the E-configured products.
Keywords: Active methylene compounds; Aldehydes; Knoevenagel condensation;
N-methylpiperazine; Solvent-free conditions
INTRODUCTION
Designing organic reactions under solvent-free conditions is one of the
most important aspects of synthetic organic chemistry. Our aim is there-
fore to protect our environment in this overcrowded world of increasing
demands by the application of novel, simple, efficient, cost-effective,
high-yielding and green methodologies. Reactions that are carried out
at room temperature top the list because no additional source of heat,
light, or sound energy is required. We therefore continuously search for
those reactions that are feasible under solvent-free conditions taking
place at room temperature (specifically using a green protocol) to give
products in excellent yields.
Received November 5, 2007
Address correspondence to Chhanda Mukhopadhyay, Department of Chem-
istry, University of Calcutta, 92 APC Road, Kolkata, 700 009, India. E-mail:
csm@vsnl.net; cmukhop@hotmail.com
2103

2104
C. Mukhopadhyay and A. Datta
Therefore, solvent-free reactions are gaining popularity[1–3] because
of very easy set-up of reaction conditions. Moreover, due to the close
proximity of the reactants, the reactions are completed within minutes
rather than taking long hours to complete. Faster reaction rates coupled
with low costs of running the reaction as no special techniques is
required, makes such reactions to gain immense importance in the
industry.
Knoevenagel Condensation is a very popular and widely carried out
reaction in Synthetic Organic Chemistry.[4] Quite a number of methods
are known to develop this reaction.[5–13] However, most of these reactions
are carried out using organic solvents that bring about environmental
waste and pollution and therefore go against the ‘‘green’’ methodology.
At the same time, some solvent-free conditions for the Knoevenagel Con-
densation are also cited which employ heterogeneous basic catalysts.[14–17]
Some homogeneous basic catalyst is also employed[18] but then the
reaction has to be carried out either in the microwave or in an organic
solvent. So, a totally ‘‘green’’ protocol for this reaction still needs to be
explored.
Here, in this paper, we report the first application of the homo-
geneous basic catalyst N-methylpiperazine, the reaction being carried
out fully at room temperature in few minutes under totally ‘‘green’’ con-
ditions without using microwave or any other organic solvent during the
reaction. The work-up is very simple, without requiring column chro-
matography and the use of volatile organic solvents for purification.
All the products are obtained in high yields by direct recrystallisation.
The overall methodology is thus highly ‘‘environmentally benign’’.
RESULTS AND DISCUSSION
A variety of aromatic (bearing different substituents in the aromatic
ring), heteroaromatic, and aliphatic aldehydes (1) were allowed to react
with either ethylcyanoacetate, malononitrile, or cyanoacetamide (2) in
equimolar ratios with 20 mol% of N-methylpiperazine under solvent-free
conditions at room temperature to produce the Knoevenagel conden-
sation products (3) in excellent yields (Scheme 1, Table 1). The reaction
takes place simply by mixing the ingredients together and keeping them
for some time. Mention must be made of the fact that the a,b-unsaturated
nitriles or their derivatives (the products of Knoevenagel condensation)
are very important precursors for novel and complex heterocyclic sys-
tems[19] and biologically active pyrimidine derivatives.[20] It was found
that 20 mol% of N-methylpiperazine was the optimum usage under
solvent-free conditions, the yields did not increase largely with higher

Knoevenagel Condensation of Aldehydes
2105
Scheme 1. Knoevenagel condensation of aldehydes with active methylene
compounds.
concentrations of the base or in the presence of solvents such as CH2Cl2,
EtOAc, THF, or MeOH (Table 2).
The reaction did not proceed at all in absence of the catalyst (entry-1,
Table 2). Maximum yield was obtained with 20 mol% of the catalyst
under solvent-free conditions. One example of concurrent Knoevenagel
and aromatic nucleophilic substitution reaction must be cited[21] with
p-fluorobenzaldehyde in boiling ethanol where the Knoevenagel Con-
densation product was obtained instead of the aromatic nucleophilic
substitution product. The analysis of the data in Table 1 indicates that
those aromatic substrates with electron withdrawing substituents like
nitro, chloro, bromo etc. reacted fast while those aromatic substrates with
electron donating groups like methoxy and hydroxy reacted slowly
because of the decrease in electrophilicity of the aldehydic carbonyl
carbon. However, all the substrates (1) reacted very well and in excel-
1
lent yields (Table 1). All the products (3) gave satisfactory H NMR,
13C NMR and IR data.
One important point to be mentioned is that all the products
obtained from ethylcyanoacetate and cyanoacetamide produced the
stereospecific E-configured olefins (obtained by analogy with the earlier
references[12,22]). In one such reference,[22] in the case of the adduct
obtained from 4-methoxybenzaldehyde and ethylcyanoacetate, a small
NOE effect was detected between the olefinic proton and the methylene
of the carboethoxy group. Thus, it is very much well established that
the Knoevenagel Condensation products with ethylcyanoacetate and
cyanoacetamide produce the E-configured olefins.
EXPERIMENTAL
A Typical Experimental Procedure for the Knoevenagel Condensation Reaction
A mixture of aldehyde (3 mmol) along with [ethylcyanoacetate
(3 mmol) or malononitrile (3 mmol) or cyanoacetamide (3 mmol)] and

2106

2107

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C. Mukhopadhyay and A. Datta
Table 2. Optimisation of N-methylpiperazine under various conditions for the
synthesis of 3e
Amout of catalyst
(mole percent)
Solvent
used (ml)
Time
(mins)
Yield
(%) of 3e
Entry
1
2
3
4
5
6
7
8
9
10
0
05
10
20
30
40
20
20
20
20
No solvent
No solvent
No solvent
No solvent
No solvent
No solvent
CH2Cl2 (10)
EtOAc (10)
THF (10)
30
30
25
10
10
10
30
30
30
35
10
55
72
93
75
70
60
55
50
52
MeOH (10)
N-methylpiperazine (20 mol%) (loading of the catalyst were done simply
by weighing and mixing the catalyst with the other two reactants) was
taken in a 50-ml conical flask and allowed to stand at room temperature
(25–30ꢀC) with occasional swirling for the specified time period (given in
Table 1). After the reaction was complete (monitored by thin-layer chro-
matography, TLC), the product was diluted with ethyl acetate (20 ml).
The solvent layer was washed successively with sodium metabisulphite
solution (20%) and brine and dried over anhydrous sodium sulphate.
After removal of the solvent, the product was purified by direct recrystal-
lization from ethyl acetate=petroleum ether. The spectroscopic data for
some selective compounds are given next.
Data
Ethyl-(E)-2-cyano-3-(4-nitrophenyl)-2-propenoate (3g) (entry 7, Table 1):
IR (KBr): 2996.5, 2224.5, 1721.2, 1609.6, 1516.0, 1346.9, 1269.5, 856.3,
and 764.5 cmꢁ1; 1H NMR (300 MHz, CDCl3) d: 8.35 (2H, d with a small
triplet splitting at the base, J ¼ 8.85 Hz and 1.89 Hz, aromatic C3-H and
=
aromatic C5-H), 8.30 (1H, s, CH), 8.13 (2H, d with a small triplet split-
ting at the base, J ¼ 8.89 Hz and 1.93 Hz, aromatic C2-H and aromatic
C6-H), 4.42 (2H, q, J ¼ 7.2 Hz, O-CH2CH3), 1.41 (3H, t, J ¼ 7.2 Hz,
O-CH2CH3); 13C NMR (75 MHz, CDCl3) d: 161.37, 151.68, 149.70,
136.88, 131.47, 124.28, 114.48, 107.37, 63.31, 14.06.
Ethyl-(E)-2-cyano-3-(3-hydroxyphenyl)-2-propenoate (3k) (entry 11,
Table 1): IR (KBr): 3436.3, 3380.8, 2224.5, 1696.6, 1589.5, 1445.3,
1376.6, 1268.5, 1222.9, 1098.1, and 767.0 cmꢁ1 1H NMR (300 MHz,
;

Knoevenagel Condensation of Aldehydes
2109
=
CDCl3) d: 8.23 (1H, s, CH), 7.61 (1H, t, J ¼ 2.01 Hz, aromatic C2-H),
7.46 (1H, td, J ¼ 7.73 Hz and 1.19 Hz, aromatic C6-H), 7.40 (1H, t,
J ¼ 7.87 Hz, aromatic C5-H), 7.10 (1H, qd, J ¼ 7.89 Hz and 1.26 Hz, aro-
matic C4-H), 4.41 (2H, q, J ¼ 7.20 Hz, ꢁOCH2CH3), 1.42 (3H, t,
J ¼ 7.20Hz, ꢁOCH2CH3); 13C NMR (75 MHz, CDCl3) d: 162.47,
156.31, 155.00, 132.66, 130.55, 124.58, 120.95, 116.21, 115.50, 103.02,
62.85, and 14.12. Anal. calcd. for C12H11NO3: C, 66.35; H, 5.10; N,
6.45. Found: C, 66.42; H, 5.05; N, 6.39%.
(E)-Ethyl-2-cyano-2-hexenoate (3q) (entry 17, Table 1): IR (neat):
3436.1, 2964.0, 2232.1, 1734.1, 1626.9, 1270.8, 1073.1, and 758.8 cmꢁ1
=
;
1H NMR (300 MHz, CDCl3) d: 7.67 (1H, t, J ¼ 7.80 Hz, CH), 4.33
(2H, q, J ¼ 7.20Hz, ꢁOCH2CH3), 2.56 (2H, q, J-7.50 Hz, C4-H), 1.62
(2H, sextet, J ¼ 7.50 Hz, C5-H), 1.39 (3H, t, J ¼ 7.20 Hz, ꢁ OCH2CH3),
1.03 (3H, t, J ¼ 7.20 Hz, C6-H); 13C NMR (75 MHz, CDCl3) d: 163.50,
161.30, 113.67, 109.97, 62.38, 33.74, 21.20, 14.06, and 13.66.
2-(2-Chlorophenylmethylene) malononitrile (3t) (entry 20, Table 1): IR
(KBr): 3048.8, 2226.4, 1582.5, 1438.6, 1287.9, 1045.6, 758.1, and
1
617.7 cmꢁ1; H NMR (300 MHz, CDCl3) d: 8.25 (1H, s, CH), 8.16
=
(1H, d, J ¼ 7.76 Hz, aromatic C3-H), 7.56 (2H, d with a small triplet split-
ting at the base, J ¼ 8.16 Hz and 1.30 Hz, aromatic C4-H and aromatic
C6-H), 7.48–7.39 (1H, m, aromatic C5-H); 13C NMR (75 MHz, CDCl3)
d: 155.96, 136.22, 134.95, 130.61, 129.40, 128.96, 127.70, 113.12, 111.82,
and 85.68.
2-(4-N, N-Dimethylaminophenyl methylene) malononitrile (3x) (entry
24, Table 1): IR (KBr): 3435.1, 2924.5, 2203.7, 1613.8, 1522.9, 1358.9,
1183.2, 814.3, and 722.1 cmꢁ1 1H NMR (300 MHz, CDCl3) d: 7.79
;
(2H, d with a small triplet splitting at the base, J ¼ 9.15 Hz and
=
2.88 Hz, aromatic C2-H and aromatic C6-H), 7.42 (1H, s, CH), 6.68
(2H, d with a small triplet splitting at the base, J ¼ 9.22 Hz and
2.82 Hz, aromatic C3-H and aromatic C5-H), 3.13 (6H, s, ꢁNMe2);
13C NMR (75 MHz, CDCl3) d: 157.97, 154.17, 133.71, 119.25, 115.91,
114.84, 111.58, 71.78, and 40.02.
(E)-2-Cyano-3-(3-bromophenyl)2-propenamide (3a0) (entry 27,
Table 1): IR (KBr): 3469.5, 3336.5, 2363.1, 2209.0, 1690.5, 1593.6,
1
1383.4, 1202.2, 781.7, and 673.2 cmꢁ1; H NMR (300 MHz, DMSO-d6)
=
d: 8.13 (1H, s, CH), 8.07 (1H, t, J ¼ 1.64 Hz, aromatic C2-H), 7.89
(1H, brd, J ¼ 7.89 Hz, aromatic C4-H), 7.80 (2H, brs, ꢁCONH2), 7.75
(1H, td, J ¼ 8.06 Hz and 1.01 Hz, aromatic C6-H), 7.49 (1H, t,
J ¼ 8.1 Hz, aromatic C5-H); 13C NMR (75 MHz, DMSO-d6) d: 162.33,
148.97, 134.70, 134.30, 132.26, 131.34, 128.78, 122.27, 116.07, and 108.38.
(2E,4E)-2-Cyano-5-phenyl-2,4-pentadienamide (3b0) (entry 28, Table
1): IR (KBr): 3419.9, 3227.0, 2211.1, 1680.2, 1571.3, 1380.2, 1163.6,
1
982.7 and 752.2 cmꢁ1; H NMR (300 MHz, DMSO-d6) d: 7.95 (1H, d,

2110
C. Mukhopadhyay and A. Datta
J ¼ 11.28 Hz, C3-vinylic H), 7.80 (2H, brs, ꢁCONH2), 7.67–7.62 (2H, m,
aromatic C2-H and aromatic C6-H), 7.45–7.38 (4H, m, vinylic C5-H and
aromatic C3-, C4-, and C5-H), 7.13 (1H, dd, J ¼ 15.37 Hz and 11.29 Hz,
C4-vinylic H); 13C NMR (75 MHz, DMSO-d6) d: 163.01, 151.71, 147.25,
135.34, 131.07, 129.59, 128.69, 123.50, 115.89, and 108.28.
CONCLUSION
In conclusion, we have developed a very simple, efficient, high yielding,
cost effective, solvent-free, ‘‘green’’ protocol for the Knoevenagel Con-
densation of aldehydes at room temperature with N-methylpiperazine
just by mixing the ingredients together and keeping them for sometime
with occasional swirling. We very much hope that this methodology will
be very much beneficial to both academicia and industry.
ACKNOWLEDGEMENT
One of the authors (A. D.) thanks the Council of Scientific and Industrial
Research (New Delhi) for his fellowship (J. R. F.). Thanks are also to
CAS Instrumentation Facility, University of Calcutta for spectral data.
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