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owing to numerous pharmacological applications, for example,
as anticancer and antihyperglycemic agents.[22] A high yield of
acetalized product was observed using [Cu3(btc)2] as catalyst at
room temperature in methanol.[18a] MIL-101 was used as cata-
lyst for the self-condensation of acetone to give mesityl oxide
as the product.[23] The catalytic activity of UiO-66 and UiO-
66(NH2) was tested for the cross-aldol reaction between ben-
zaldehyde and heptanal for the formation of jasminaldehyde
under solvent-free conditions.[24]
Table 2. Solvent effect on Claisen–Schmidt reactions of p-nitrobenzalde-
hyde with acetone catalyzed by compounds 1, 2 and 2a.[a]
Compound
Solvent
Product type
Isolated Selectivity
yield
[wt%]
[wt%]
1
2
2a
2a
2a
2a
THF
b-aldol product
b-aldol product
b-aldol product
b-aldol product
22[b]
28[b]
69[b]
78[c]
40[c]
46[c]
100
100
100
100
100
100
THF
THF
THF
THF/water (9:1) b-aldol product
no solvent b-aldol product
Among various magnesium compounds that are used as
mediators or catalysts for several organic reactions,[25] magnesi-
um oxide is a versatile catalyst for carbon–carbon bond-forma-
tion reactions.[26] Magnesium oxide nanoparticles have also re-
cently been employed in the catalytic aldol condensation reac-
tion.[27] It was proposed that the surface ꢀOH and O2ꢀ of these
oxide crystallites are expected to trigger the carbon–carbon
bond-formation reaction.[27] However, little attention has been
paid to using magnesium carboxylates in CꢀC bond-formation
reactions owing to their limited synthetic procedure and hy-
groscopic nature.[28] Recently two 3D porous magnesium car-
boxylate framework materials have been synthesized through
a hydrothermal route and their activity in heterogeneous aldol
reaction was tested in the presence of triethylamine in
THF.[11,19a] Two 2D magnesium carboxylate compounds have
also been reported that catalyzed the aldol reaction under het-
erogeneous base-free conditions.[19b,c] Sometimes calcined
MOFs are a superior catalyst in aldol reactions to their parent
compounds.[19a,c] Here, the catalytic activity of 1, 2 and 2a has
been studied in a Claisen–Schmidt reaction under heterogene-
ous base-free conditions (Scheme 1). Compound 2 is the first
[a] Reaction conditions: p-nitrobenzaldehyde (2 mmol), acetone (3 mmol),
solvent (3 mL), catalyst (5 mg), temperature (0–58C). Yields were isolated
after 6 h of reaction. [b] Reactions were performed under open atmos-
phere. [c] Reactions were performed under an inert atmosphere.
reaction temperature, the b-aldol product transformed into
benzylidene ketone (condensed product). Generally, it is easier
to obtain the condensed product than the b-aldol product.
Therefore, we were interested in isolating the yield in b-aldol
form. The catalytic reactions were performed in THF, THF/water
and solventless conditions (Table 2). The best result was ob-
tained in THF. For 1 and 2, it did not matter whether the reac-
tion was carried out in the open or under an inert atmosphere.
The same yield was obtained in both cases. In the case of 2a,
however, a small change in the yield of the product was no-
ticed between the open atmosphere and inert atmosphere
(Table 2). The yield obtained under inert atmosphere was
higher than that in open atmosphere. In an open atmosphere,
the absorption of water molecules from the atmosphere will
result in the deactivation of some active sites and can cause
low yield of the product. To further explore the versatility of
the calcined species as a selective catalyst for the aldol reac-
tion and the effect of substituents on the aldol reaction, differ-
ent types of aldehydes and ketones were used as substrates in
the THF medium under an inert atmosphere (Table 3). In all the
above conditions aldehydes were converted to their respective
b-aldols as the sole product. In this study we noticed that b-
aldol products did not undergo further transformation to form
unsaturated carbonyl compounds. Generally the b-aldol prod-
uct undergoes dehydration to give a conjugated enone in the
Claisen–Schmidt reaction.[30] The yield of the b-aldol product
decreases from p-nitrobenzaldehyde to m-nitrobenzaldehyde
through o-nitrobenzaldehyde. It might be realized that the
nitro substituent at the ortho and para position affords both
a negative inductive effect and a negative mesomeric effect,
which increases electrophilicity of the C=O group of nitrobenz-
aldehydes, but substitution at the meta position affords only
the negative inductive effect. Between p-nitrobenzaldehyde
and o-nitrobenzaldehyde, more steric crowding at the ortho
position might lead to low conversion for the ortho variety. An
aldol-condensation reaction catalyzed by nonporous crystalline
magnesium oxide showed 75% conversion for p-nitrobenzal-
dehyde and acetone in 24 hours under heterogeneous condi-
tions.[27] In the case of chloro-substituted benzaldehydes, the
yield of the b-aldol product decreases from o-chlorobenzalde-
Scheme 1. Claisen–Schmidt reaction under base-free conditions.
report of a 3D magnesium-containing MOF that catalyzes the
Claisen–Schmidt reaction under heterogeneous base-free con-
ditions.
Catalytic reactions were performed in the presence of an
excess amount of ketone for the effective use of aldehyde.[29]
Reactions occur at room temperature and were performed in
THF. The performance of catalysts 1 and 2 was very similar
(Table 2). However, when calcined species (2a) were employed
as catalyst, the yield of the reaction increased remarkably
(Table 2). It can be anticipated that after 2 loses the coordinat-
ed water on heating, the metal centres of the framework com-
pound become coordinatively unsaturated. These metal sites
are now accessible to the reactant molecules and give a high
yield of product. But in the case of 1, it was not stable in the
reaction medium after dehydration and therefore catalytic ac-
tivity of calcined 1 was not measured. With an increase in the
&
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Sen, Rupam
