New aspects of Knoevenagel condensation and Michael addition reactions on alkaline carbonates

Article abstract of DOI:10.1246/cl.2000.574

The Knoevenagel condensation of malononitrile with benzaldehyde on K₂CO₃, Rb₂CO₃ and Cs₂CO₃ gave the condensation product benzylidenemalononitrile but the reaction proceeded to the hydrogen

Full text of DOI:10.1246/cl.2000.574

5
74
Chemistry Letters 2000
New Aspects of Knoevenagel Condensation and Michael Addition Reactions
on Alkaline Carbonates
María A. Aramendía, Victoriano Borau, César Jiménez,* José M. Marinas, and Francisco J. Romero*
Department of Organic Chemistry, Faculty of Sciences, Córdoba University, Avda. San Alberto Magno s/n, E-14004 Córdoba, Spain
(Received November 25, 1999; CL-990998)
The Knoevenagel condensation of malononitrile with benz-
aldehyde on K CO , Rb CO and Cs CO gave the condensa-
reactor outlet with a VG Sensorlab mass spectrometer.
The variation of the concentration of the reaction products
with time revealed the following sequence of catalytic activity:
CaCO , SrCO < Li CO < BaCO < Na CO < K CO <
2
3
2
3
2
3
tion product benzylidenemalononitrile but the reaction proceed-
ed to the hydrogenated product benzylmalononitrile. Also, the
Michael addition of malononitrile to certain double bonds
occurs in the presence of K CO .
3
3
2
3
3
2
3
2
3
8
Rb CO < Cs CO . Thus, the activity increased with increasing
2
3
2
3
radius of the cation in the carbonate (i.e. with increasing elec-
tronegativity). A similar sequence was previously found for X
and Y zeolites exchanged with lithium, sodium, potassium and
2
3
4
Basic catalysts are receiving much attention lately as they
facilitate a variety of organic reactions useful in fine chemistry
processes.1 The Knoevenagel reaction has been shown to take
cesium. However, if after the condensation product benzyli-
denemalononitrile 1 was formed, the reaction was allowed to
proceed, the hydrogenated product benzylmalononitrile 2 was
obtained. Product 2 was only formed in the presence of K CO ,
-3
4
,5
place on various basic solids. With some catalysts, a Michael
addition reaction occurs as well. Both steps involve the cata-
lyst’s basic sites and have been shown to produce surface car-
banions.
2
3
Rb CO or Cs CO , but not in that of the other carbonates.
2
3
2
3
Alkaline carbonates have recently started to be used as cat-
alysts and other types of agents in organic syntheses. Thus,
6
Cs CO forms cesium oxalate from a mixture of CO and CO .
2
3
2
Also, Ullman syntheses for diaryl ethers are catalysed by
An analysis of the reaction mixture during the additional
7
K CO in the presence of CuI as co-catalyst. Alkaline carbon-
time revealed the presence of tetracyanoethylene (TCNE) in the
medium, which suggests that the hydrogen needed for the
reduction comes from malononitrile. Also, the solid, which had
so far remained unaltered as a carbonate, underwent a major
2
3
ates can also catalyse Knoevenagel reactions, where the rate of
formation of product has been found to be highly correlated
8
with carbonate alkalinity. This paper reports results obtained
in the condensation of malononitrile (MLN) and benzaldehyde
transformation and released CO during the reaction.
Skarzewski and Zon reported on the oxidative coupling of
2
9
(BZ) when the reaction is allowed to proceed beyond the time
required for complete conversion to the condensation product.
Also, results of the addition of malononitrile to double bonds in
the presence of K CO are reported.
dimethyl malonate to tetramethyl ethene-1,1,2,2-tetracarbox-
ylate using a cerium ammonium nitrate/magnesium oxide cata-
lyst. The reaction does not take place in the absence of MgO.
They proposed a reaction scheme where malonate ions formed
on the MgO surface react with malonyl radicals produced at an
oxidizing surface site. Other instances of oxidative coupling for
2
3
The following commercially available alkaline and alkaline
earth carbonates were used: Li CO , Na CO , K CO , Rb CO ,
2
3
2
3
2
3
2
3
Cs CO , CaCO , SrCO , and BaCO . Prior to use, all solids
2
3
3
3
3
were subjected to the following stepwise calcination pro-
gramme: 1 h at 200 ºC, 1 h at 300 ºC, 1 h at 400 ºC, and 1 h at
5
00 ºC. The TGA and XRD patterns obtained after calcination
showed that none of the carbonates had decomposed over the
temperature range studied. Also, their S never exceeded 2
BET
2
−1
m g . Knoevenagel reactions were conducted in a flask that
was fitted with a reflux condenser and filled with a mixture of
1
5.88 mmol of freshly distilled benzaldehyde, 47.46 mmol of
malononitrile, and 24 mL of 1,4-dioxane solvent. After heating
o
to 90 C, 0.546 g of the carbonate was added (t = 0). Michael
addition reactions were carried out in a similar way, by using an
equimolar mixture (15.88 mmol) of malononitrile and one of
the following alkenes: trans-β-methylstyrene, methyl trans-cin-
namate, trans-β-nitrostyrene, and diethyl fumarate. Portions of
the reaction mixture were subsequently withdrawn periodically
for gas chromatographic analysis using an SPB-5 60 m × 0.25
mm ID phenyl silicone capillary column and raising the temper-
o
o
–1
ature from 150 to 280 C at 15 C min . The identity of each
1
reaction product was confirmed by H-NMR and MS. The
gases released during the reaction were monitored by fitting the
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
575
unsubstituted malonates on I /KF/Al O and by electrochemical
2
2
3
means have also been reported. In an experiment with K CO ,
2
3
we mixed the reactants under different conditions and analysed
the gases released (Figure 1). As can be seen, addition of fur-
ther compound 1 to the reaction medium caused CO produc-
2
tion to increase considerably, concomitantly with an increase in
the concentration of compound 2 in the reaction medium. At the
same time, the solid was gradually converted into the oxide and
dissolved in the reaction medium as the hydrogenation pro-
gressed. Alkaline and alkaline earth carbonates which only gave
benzylidenemalononitrile did not decompose. Figure 2 shows
the variation of the selectivity towards reaction products (100 ×
conversion towards a product / percentage overall conversion of
benzaldehyde) with time. At the end, product 1 had been fully
converted into 2.
On the other hand, we considered the application of these
carbonates to other processes. Some preliminary results are
given in Table 1. As can be seen, malononitrile can be added to
certain double bonds in the presence of K CO . Interestingly,
2
3
electron-withdrawing groups favor the reaction. Thus, no reac-
tion was observed with cyclohexene or trans-β-methylstyrene.
The regioselectivity achieved in the reaction suggests a Michael
1
2
addition mechanism.
The authors are grateful to the Spanish DGICyT for finan-
cial support to their Project PB97-0446 and the Consejería de
Educación y Ciencia de la Junta de Andalucía. The NMR and
MS Service of the University of Córdoba is also gratefully
acknowledged for assistance in the realization of this work.
References
1
H. Hattori, Chem. Rev., 95, 537 (1995).
2
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3
4
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(
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carried out at a malononitrile/benzaldehyde ratio of 1:1, the
mole ratio of compounds 1 to 2 was 1.52, 1.85 and 2.50 (at t =
5
6
7
8
9
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1
20 min) in the presence of K CO , Rb CO and Cs CO ,
2 3 2 3 2 3
respectively.
The fact that the hydrogenation step occurred with the K,
Rb and Cs carbonates appears to be related to the weaker M–O
bond of these compounds (a result of the larger size of their
metal ions).10 Probably, surface M–OH and M–H species are
formed by decomposition of the carbonates, which would be
similar to those proposed by Kibby and Hall for the dehydro-
10 W. A. Hart, O. F. Beumel, Jr., and T. P. Whaley, “The
Chemistry of Lithium, Sodium, Potassium, Rubidium,
Cesium and Francium,” Pergamon Press, Oxford (1975).
11 C. L. Kibby and W. K. Hall, J. Catal., 31, 65 (1973).
12 J. March, “Advanced Organic Chemistry: Reactions,
Mechanisms, and Structure,” 3rd ed., Wiley, New York
(1985).
1
1
genation of alcohols over hydroxyapatite. In this case, the
hydrogen is abstracted from the alcohol and can be subsequent-
ly transferred to a carbonyl compound. The presence of TCNE
suggests that a dicyanocarbene occurs as an intermediate
species.