Chemistry Reference
In-Depth Information
out using dimethylaminopropyl-functionalised
silica-organic hybrids [77]. The tertiary amine func-
tionalised materials were substantially better cata-
lysts than primary or secondary materials (possibly
due to their reduced ability to be a Michael nucle-
ophile themselves) and the hybrid system was much
more efficient and faster than a silica-based material
of the same loading. Increased loadings led to a
further enhancement in rate, and catalysts could be
recycled and reused with little loss in activity being
seen until the sixth use. Jacobs' guanidine catalyst
(Fig. 7.25, catalyst A) was shown to be active in the
Michael addition of ethyl cyanoacetate and diethyl
malonate (p
K
values of 11 and 13, respectively) to
acceptors such as enones and acrylates. Yields were
excellent and reactions were rapid under mild con-
ditions [110].
Corma's supported quaternary hydroxide [145]
effectively catalysed the Michael reaction of keto-
esters, diesters and ethyl cyanoacetate with enones,
although in some cases significant amounts of
double addition were noted.
from these areas is that excellent levels of activity
can be achieved with heterogeneous systems but
that a great deal of care must be exercised in the
design of the catalyst and the reaction conditions.
Indeed, it is clear that the heterogenisation (on a
highly polar, polarising, hydrogen-bonding, wet
surface) of a carefully and thoroughly designed cat-
alyst that has been optimised for use in a very dry,
low-polarity organic solvent is unlikely to deliver
immediately a similarly efficient material. On the
other hand, if these factors can be taken into
account, the steric limitations imposed by a tightly
controlled reaction space (such as a very regular
pore) are exactly what most homogeneous catalysts
try to achieve by the incorporation of bulky groups.
Thus, with the correct design of catalyst in the right
pore system, very selective catalysts should be possi-
ble. Up until recently, the only highly regular pore
systems available were in the zeolites, and these
were too small to be of general use as enantioselec-
tive hosts. Nevertheless, Corma
et al
. demonstrated
that such pore systems could be very effiicient as
hosts for highly efficient enantioselective catalysts
[151-154].
Laspéras
et al
. [58,155,156] have carried out a
detailed study on the utility of supported ephedrines
as catalysts for the alkylation of benzaldehyde with
diethylzinc, a reaction that goes through an N,O-
ZnR chelate (Fig. 7.37). Their initial work displayed
similar reactivity/activity trends to systems based on
amorphous silica [157,158] and homogeneous ver-
sions [155]. They found that the optimum catalyst
required that the ephedrine molecules were spaced
by the use of non-reactive alkyl spacers attached to
the surface, that there was direct interaction of
the ephedrine hydroxyl with the silica surface mak-
ing alternative less-enantioselective centres) and
that the surface itself plays a role in activating the
3.5 Enantioselective catalysis
This represents one of the most challenging areas of
catalysis, especially for heterogeneous systems. Few
examples of heterogeneous enantioselective catalysts
have been demonstrated; only a small proportion of
those that are known approach acceptable levels of
enantioselectivity. Polymer-supported catalysts and
modified noble metal hydrogenation catalysts are
two major areas of heterogeneous enantioselective
catalysis where good results have been obtained after
several years of considerable efforts. Both of these
areas are outside the scope of this review, but ex-
cellent reviews are available for those interested
[146-150]. One of the prime lessons to be learnt
Fig. 7.37
Surface structures in a
supported ephedrine catalyst.
