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a-Arylpropanoic Acid
177
ruthenium[3] or enzymatic resolutions.[8] Although each method is
satifactory, in cost or simplicity they cannot be compared to the
cinchona-modified Pd-catalyzed hydrogenations.
Note that Fe2O3 is inactive for the reaction and Pd and Pd/Fe2O3
exhibit relative high active but no chiral product produced, the cinchona-
modified Pd/Fe2O3 catalyst show both active and enantioselective
in the heterogeneous hydrogenation, indicating that cinchona is the
origin of asymmetric induction for the asymmetric hydrogenation of
atropic acid.
Taking into account the fact that the different preparation methods
of the catalysts have been applied, it was necessary to find the most
suitable strategy to prepare the efficiency catalyst.
In our experiment three preparation methods were adopted and the
results with or without CN cinchona-modified are listed in Table 1. The
synthetic methods are as follows: catalyst(I) was prepared by simply
mixing Pd and Fe2O3. Catalyst(II) was prepared by using PdCl2 instead
of Pd, reduced by HCHO in the mixture of Fe2O3 and NaOH. Using
Fe(Ac)3Á6H2O and PdCl2 as starting material, catalyst(III) was prepared
by co-precipitation from the mixture solution of starting material.
For catalyst(I), the Pd active center is just on the surface of Fe2O3
and no or little interaction can be formed between Pd and Fe or O
atom, results in the ee of the hydrogenation product is less than 3%
(Table 1, Entry 3). In the second methodology much Pd is ‘‘dispersed’’
on the surface of Fe2O3 and exist a little interaction of Pd with Fe or O
atom, the ee of the product is up to 10% (Table 1, Entry 4). For
catalyst(III) the ee’s up to 15% (Table 1, Entry 5). When modified
with CN, the reaction rates and the optical yield are greatly increased
(Table 1, Entries 6–8).
For the model reaction, the influence of substrate, solvent, and modi-
fier concentration were also investigated. According to the ee data the
enantioselectivity seems to be strongly solvent dependent. It was found
that the application of CN provides the higher optical yields in methanol.
In our opinion this significant solvent dependence is a result of the
weaker absorption of atropic acids in the case of benzene (6ꢀ electrons
vs. 2ꢀ electrons). While using methanol the substrate can form an ester
with the solvent as pointed out for atropic acid. However, the increase is
more pronounced in benzene than in methanol, for which only a slight
increase was observed.
The reaction rates of the modified and the unmodified system were
compared in methanol. The modified reaction all take place at
higher rates than the unmodified one as Table 1 show, indicating a