School of Engineering and Science - Jacobs University

N
N
P
Fe P(C 6H 5 ) 2
O
P
P
N
O O
P(C 6 H 5 ) 2
Quiphos
BPPFA: X= (CH 3 ) 2 N
Diop
O
O PPh 2
(C 6 H 5 ) 2 P
P(C 6 H 5 ) 2
O PPh 2
P(C 6 H 5 ) 2
P(C 6 H 5 ) N
2
O
X
BPPM: X=(CH 3 ) 3 COCO
PPCP
Synphos
Fig. 1-14. Some chiral ligands used in homogeneous asymmetrical catalysis.
In spite of the fact that chiral homogeneous catalysts demonstrate excellent
enantioselectivity, due to the separation criterion mentioned above, the disadvantage
this type of catalysts appears. In fact, these soluble catalyst are more difficult to
separate and handle that the heterogeneous ones. One promising strategy to combine
the best properties of the two catalyst types in the heterogenization or immobilization of
active metal complexes on supports or carriers, which may be separated by filtration or
precipitation. However we leave this approach out of the current thesis, but keeping it
in EU-COST project, please see below. Consequently, emphasis has to be given to the
design of stable heterogeneous catalysts that are capable of high enantioselectivity.
Heterogeneous asymmetric catalysis
To date there have been numerous approaches to the design of heterogeneous
asymmetric catalysts, since Schwab and coworkers first demonstrated that Cu and Ni
could be supported on chiral silica surfaces [38, 39] and that the resulting catalysts
could give low enantioselection in the dehydration of butan-2-ol.
There are several strategies for the design of chiral heterogeneous catalysts, the three
mentioned below, lead to stable catalysts that allow high enantioselectivity.
• Modification of metal interfaces with chiral molecules;
• Tethering of active homogeneous catalysts to a three-dimensional structure;
• Electrostatic interaction as a means of immobilization.
An overview of the about three strategies can be found in the work of Hutchings [40],
whereas the current thesis focuses more on modification of the surface of the metal
nanoclusters by chiral molecules.
In contrast to homogeneous enantioselective catalysis the heterogeneous one is rather
young and has a limited success in applications. Several factors contributed to this
situation: a) the more complex structure of the heterogeneous catalyst surface on which
coexist centers with different catalytic activity and selectivity, which can lead to
undesired secondary reactions, b) an increased difficulty to create an effective
asymmetric environment and to accommodate it with the multitude of reactions that are
interesting to be carried out under enantioselective restrictions.
X
9