Chemistry Reference
In-Depth Information
the large quantity of data, by the well-known
complexity of solvent effects in the reaction
[67,128-130]. More interestingly, the materials were
capable of the more demanding aldol reaction (see
below), giving good to excellent yields of condensa-
tion products in the reaction between benzaldehyde
and a series of acetophenones (p
K
-ca. 20) and
related compounds at 150°C. Similarly Michael addi-
tions of diethyl malonate (p
K
-ca. 13) to enones were
possible using the more strongly basic Cs-exchanged
materials.
Improvements to these catalysts included the Cs
2
O
impregnated versions [76] and further improve-
ments to the stability of the catalysts were made by
the use of binary caesium-lanthanum oxides sup-
ported on MCM-41 [75].
Simple amine bases include aminopropyl-
substituted materials, dimethylaminopropyl and
aminoethylaminopropyl equivalents. These materi-
als are mild bases that are capable of a range of reac-
tions, such as Michael, Knoevenagel and related
reactions involving carbon acids with p
K
a
values
typically of <13.
The simplest of these systems is the aminopropyl-
substituted system, derived from the cheap 3-
aminopropyl trimethoxysilane. Both grafted and
one-pot variations have been prepared and evalu-
ated as catalysts for the Knoevenagel reaction.
Laspéras
et al
. [131] (Fig. 7.31) and Brunel [47]
have published details of the catalytic behaviour of
aminopropyl groups grafted onto MCM-41-type
materials. These materials then were utilised in
the Knoevenagel reaction of benzaldehyde and
ethyl cyanoacetate, this time in dimethylsulfoxide as
solvent. Conversions and yields were very good, and
reaction times were around 5-10 times faster than
the metal-substituted MCMs discussed previously. It
was shown that activity was proportional to loading
over a fairly wide range and it was proposed that
the active site was in fact the imine, possibly explain-
ing the faster reaction rate. The related piperidine
catalyst (formed by displacement of chlorine from
chloropropyl-substituted MCMs by piperidine) was
significantly less active. The catalysts could be
recovered and reused after activation.
Macquarrie has prepared a range of corresponding
materials using the one-pot co-condensation method
[34,67-69] and neutral amines as templates. These
materials are very efficient catalysts for the Knoev-
enagel reaction, allowing the formation of conden-
sation products from ketones such as cyclohexanone
and dialkyl ketones in high yields and selectivities
(Fig. 7. 32).
As can be seen, turnover numbers are good and
are 4-5 times higher than those obtained for the
grafted aminopropyl silicas (based on amorphous
chromatographic silica) [67,130]. Turnover numbers
increase further when a second polarity-modifying
group is added (catalyst
3
) by co-condensation of
tetraethoxysilane, aminopropyl trimethoxysilane
and phenyl trimethoxysilane in one pot. An addi-
tional benefit of this bifunctional catalyst system
is that the rate is increased substantially purely
by modifying the polarity of the system. Post-
functionalisation of the aminopropyl material by
silylation actually caused a reduction in activity in
this case, possibly by pore size reduction, although
this is also found in the phenyl-substituted materi-
als; an alternative explanation may be silylation of
the NH
2
group.
A very surprising result indeed is the almost com-
plete lack of activity of the catalyst in the reaction
between benzaldehyde and ethyl cyanoacetate,
which is the standard (and often only) reaction used
to characterise many such catalysts. These results
indicate that the nature of the catalyst is significantly
different to the grafted materials, which is confirmed
Fig. 7.31
Catalysts used in the Knoevenagel condensation by
Laspéras
et al
. [131].
