2
14
CORMA ET AL.
the carbon supporting the carbonyl group. In the case of
benzaldehyde, this polarization leads to a quicker attack of
the carbon by carbanion-type species. At the same time,
in the case of the diethyl malonate or acetophenone, the
increase in the positive charge on the carbon of the car-
bonyl group enhances the acidity of the hydrogens in the α-
position of the carbonyl group, allowing easier abstraction
by the catalyst. Calculations of the density of the positive
charge on the hydrogen atoms attached to the methylenic
group of a protonated diethyl malonate species give a value
of 0.356, clearly superior to that of the neutral species
CONCLUSIONS
A proton sponge (DMAN) was functionalized and
grafted onto amorphous silica and onto pure-silica MCM-
1. It was shown that, in the homogeneous phase, DMAN
4
requires the presence of polar solvents to reach high re-
action rates. However, supported DMAN gives better in-
trinsic activity in the absence of a solvent and when polar
solvents are not used. The activity of the supported cata-
lysts is directly related to the polarity of the support, which
is higher in the case of MCM-41. In this sense the sup-
port acts as a polar solvent that can stabilize the transition-
state charged complex. The support can also preactivate
the reagents by interaction of the carbonyl groups with the
weakly acidic silanol groups. This preactivation step allows
PS-MCM-41 catalyst to abstract protons with a higher pKa
than that of the DMAN. Our results demonstrate that the
activity of homogeneous proton-sponge catalysts could be
strongly improved by grafting them onto MCM-41. The re-
sulting organic–inorganic catalysts are stable against deac-
(
0.302). This increase in the acidity of the protonated di-
ethyl malonate could explain why the basic sites existing on
PS-MCM-41 abstract the proton and generate carbanion-
type species with molecules such as diethylmalonate and
ꢀ
2
-hydroxyacetophenone which then react with the acti-
vated benzaldehyde.
Catalyst Deactivation and Recycling
To determine whether leaching and/or deactivation of tivation and leaching.
the catalyst occurs, PS-MCM-41 was reused several times
for the Knoevenagel reaction of benzaldehyde with ethyl
cyanoacetate(2a). Thereactionswerecarriedoutinethanol
solvent at 313 K using a 1.1% molar ratio of PS-MCM-41
catalyst. When the reaction was complete, the solid was
filtered and thoroughly washed with CH2Cl2 and reused
in a second and third cycle. As shown in Fig. 7, a small
decrease in activity was observed after the first recycling,
and no further loss was observed when the small amount of
catalyst lost during the process was considered.
ACKNOWLEDGMENTS
The authors thank the Spanish CICYT for financial support (Projects
MAT2000-1392 and MAT2000-1678-C02-02), Mr. J. A. Esteban for as-
sisting with the purification and characterization of precursors, and
Dr. G. Sastre for theoretical calculations.
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FIG. 7. Knoevenagel condensation of benzaldeyde with ethyl cyano-
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DMAN with respect to 2a) at 313 K in ethanol as solvent: ꢀ, first cycle; ꢁ,
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