542
M. M. Heravi et al. ■Montmorillonite K-10 Catalyzed Knoevenagel Condensation
opment of a continuous microwave reaction for
lene compounds such as malonitrile and ethylcya-
noacetate in the presence of montmorillonite K- organic synthesis [16, 17] this work may have more
10 under microwave irradiation in solvent free
conditions. These reactions lead to completion in and low cost of the method may lead to environ-
industrial application where the absence of solvent
mental and cost advantages.
2-10 min to produce olefinic products in 45-85%
yield (Table). In the case of ketones, the condensa-
tion of cyclohexanone and 2 -methyl-cyclohexa-
none readily occurred (entries 7 and 8 ), but the
Experimental
Melting points were determined with a Reichert
hindered 2 ,6 -dimethylcyclohexanone failed to re- hot plate melting point apparatus. All compounds
are known and exhibit satisfactory spectroscopic
data. Yields refer to isolated products purified by
column chromatography. Montmorillonite K-10
was dried and simultaneously activated in a micro-
wave oven for 3 min at 780 W prior to use. Micro-
wave irradiation was carried out in a National
oven model 6755 at 900 W. For safety reasons all
the experiments with microwave ovens should be
performed in an efficient hood in order to avoid
contact with vapors. If a tall beaker is used and
act. Reactions were more difficult with aromatic
ketones. However we were able to isolate the
products from the reaction of acetophenone and
benzophenone with malonitrile in moderate yields
(entries 9 and 10).
In order to evaluate the synergy between dry
media and microwave irradiation in this reaction
several experiments were tried. The irradiation of
benzaldehyde and malonitrile without clay was un-
successful and the aldehyde remained practically the irradiation sequence is interrupted with a 60
sec cooling there is little vaporization and very
high conversion can be observed.
unchanged after irradiation. Similarly, when mont-
morillonite K-10 was used as catalyst in methylene
chloride solution (1 0 h reflux), the corresponding
alkene was not obtained. Although clay in dry me-
dia catalyzed the reaction at room temperature,
yield was only 25% after 2 h of reaction, increase
in temperature using classical heating (1 h, 120 °C)
in the absence of solvent gave a moderate yield
(40%) of the corresponding alkene. Only in the
case of dry media completed with microwave irra-
diation, taking advantage of synergy between both
methodologies, the Knoevenagel condensation
proceeded efficiently.
Preparation of Alkenes by Knoevenagel
Condensation
General Procedure: Equimolar amounts (10
mmol) of carbonyl compounds and active methy-
lene compound (malonitrile or ethylcyanoacetate)
were dispersed on montmorillonite K-10 (5 g dried
for 3 min in the microwave oven just before use)
in a tall beaker. This mixture is then exposed to
microwave irradiation for the appropriate time
(Table). Then it was taken up into CH2C12 and the
clay was separated by filtration and the solvent
evaporated under reduced pressure. Pure products
could be isolated by column chromatography
using hexane-ethylacetate (9:1) as eluent.
In conclusion, this methodology, which associ-
ates dry heterogeneous media with microwave ir-
radiation, appears easy, fast and clean to perform
the Knoevenagel reaction. Furthermore, by devel-
Table. M ontm orillonite K-10 assisted K noevenagel condensation under microwave irradiation in solventless system.
Entry
Carbonyl compound
R
M.p., b.p.
[°C]
Lit
Time
[min]
Yield
[%]
1
2
3
4
5
6
7
8
9
Benzaldehyde
CN
C 0 2Et
CN
C 0 2Et
CN
C 0 2Et
CN
CN
CN
CN
3
5
5
5
8
8
5
5
10
10
85
74
68
62
68
62
68
72
45
42
83 (m.p.)
51 (m.p.)
164 (m.p.)
96 (m.p.)
159 (m.p.)
168 (m.p.)
150 (b.p., 10 m mHg)
160 (b.p.. 10 mmHg)
92 (m.p.)
[18]
[18]
[19]
[19]
[18]
[18]
[18]
[20]
[21]
[22]
p-Brom obezaldehyde
p-N itrobezaldehyde
Cyclohexanone
2-M ethylcyclohexanone
A cetophenone
10
Benzophenone
138 (m.p.)
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Download Date | 7/1/15 2:48 PM
Heravi, Majid M.
