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Fig. 15
Heterogeneous Brønsted base catalysis of the Knoevenagel condensation of ethyl
cyanoacetate with benzaldehyde using a cross-linked H
II
assembly
LLC networks containing catalytic headgroups have also been shown to be
useful for heterogeneous Lewis acid catalysis. The Sc(III)-exchanged cross-
linked H
II
phase of a taper-shaped sulfonate-functionalized LLC monomer
hasbeenshowntobeabletocatalyzetheMukaiyamaaldolandMannichre-
actions [115] with enhanced diastereoselectivity. This Sc(III)-functionalized
H
II
network affords condensation products with syn-to-anti diastereoselect-
ivity ratios of 2-to-1, whereas Sc(III) catalysts in solution or supported on
amorphous polymers show no reaction diastereoselectivity at all.
More recently, cross-linked LLC phases containing strong Brønsted acid
groups have been shown to catalyze heterogeneous esterification reactions
with higher product selectivity than non-ordered acid resins containing the
same catalytic acid groups. The cross-linked H
II
phase of a 5 : 1 (mol
mol)
blend of a sulfonic acid-functionalized LLC monomer and an l-analine-
functionalized LLC monomer was found to catalyze the esterification of
1-hexanoic acid and benzyl alcohol with higher product selectivity than com-
mercial, non-ordered sulfonic acid resin catalysts (e.g., Amberlyst, Nafion)
under the same conditions (Fig. 16) [116].
This LLC acid resin displayed more than an order of magnitude higher
selectivity for the desired ester product over dibenzyl ether side-product
compared to amorphous Amberlyst-15, Nafion NR-50, and also an isotropic
analog of the LLC acid resin. Control experiments revealed that a large
component of the enhanced selectivity is due to the regular nanostructure
present in the LLC acid resin, which is believed to afford a much more uni-
form local acid microenvironment for reactions to occur. This same LLC
acid resin was subsequently found to catalyze the synthesis of dioctyl phtha-
late (DOP) from phthalic anhydride and 2-ethyl-1-hexanol in toluene with
a slightly higher overall yield of DOP than amorphous Amberlyst-15 [117].
Experiments revealed that the LLC acid resin is a much better catalyst for the
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