Chemistry Reference
In-Depth Information
shown in Scheme 16.1 were used, mainly those built from the cyclotriphosphazene
core (series
7-G
n
) suitably functionalized on the surface, depending on the
properties desired [32]. Several properties that will be displayed in the following
paragraphs are also known for other types of dendrimers. However in some cases,
and especially concerning some biological properties, these phosphorus dendrimers
display very unique properties.
16.3 ORGANOMETALLIC DERIVATIVES OF PHOSPHORUS
DENDRIMERS AND THEIR USE AS CATALYSTS
The use of dendrimers as catalysts was recognized very early, with the aim of
combining both the advantages of homogeneous catalysts (solubility) and hetero-
geneous catalysts (easy recovery), without their disadvantages. The first example
dates back to 1994, using organic dendrimers [33]. Several reviews have covered this
topic [34], but only very few concern phosphorus dendritic catalyst [35]. In many
cases phosphorus derivatives (phosphines) are used to complex the metal, but the
dendritic backbone does not contain phosphorus in their structure. In this chapter, we
will focus only on the case of dendritic catalysts built from phosphorus-containing
dendrimers. The first example in this field also dates back to 1994, when the palladium
complex of the phosphorus dendrimers
2-G
n
was used for the electrocatalyzed
reduction of CO
2
to CO [7]. Later on, the rhodium complexes of the dendrimers
3-G
n
were found efficient catalysts for olefin hydrogenation in a 1:200 metal-to-
substrate ratio. The efficiency of the dendrimer was found similar to that of
monomeric complexes, but the advantages of these dendritic catalysts are their easy
separation and reuse [8].
We have also contributed to this field, but in first attempts weweremainly interested
in organometallic chemistry and not in catalysis; several phosphorus dendrimers
having metallic derivatives as end groups [36] or in their internal structure [37] were
synthesized for this purpose. When using them later on as catalysts, we generally
choose to graft the catalytic complexes at the periphery, as terminal groups of
dendrimers. It must be emphasized that in all cases, the molar percentage of catalyst
that is indicated is the molar percentage of metal, with onemetal per ligating site on the
dendrimer. In this way it is possible to compare directly the efficiency of a monomer
and of a dendrimer. In our first example of catalysis, wewanted to compare the catalytic
efficiency depending on the location of the catalytic sites. For this purpose, we grafted
amino diphosphine ligands either at the surface of dendrimers or at the core of
dendrons. The RuH
2
(PPh
3
)
2
complex
27-G
3
was used for Knoevenagel condensations
and diastereoselective Michael additions. In the latter case, the dendron
28-G
3
having
one RuH
2
(PPh
3
)
2
complex at the core was also tested. No difference in the diastereos-
electivity was observed when compared to the dendrimer, and to the monomer
(Figure 16.3). The PdCl
2
complex of the third-generation dendrimer shown in
Figure 16.3 was used as catalyst in Stille couplings. In all these cases, it was possible
to recover and reuse twice the dendritic catalyst, without any significant loss of activity
in marked contrast with the behavior of the corresponding monomer [38].
