Chemistry Reference
In-Depth Information
FIGURE 16.5
Monomer versus generation 1 dendrimer for catalyzing Stille couplings.
As shown above, the activity measured for the dendrimers was comparable or
slightly higher than that of the monomers in many cases, but no general rules can be
deduced concerning the efficiency of a dendritic catalyst. We have synthesized and
complexed iminophosphine derivatives, in particular the monomeric palladium
complex
31-G
1
. Comparison of their efficiency
for various Stille couplings was really puzzling (see Figure 16.5). Measurements of
the rate of conversion by
1
H NMR show that in case (a) the monomer is more efficient
than the dendrimer, in case (b) the monomer and the dendrimer have the same
efficiency, and in case (c) the dendrimer is more efficient. However, the real difference
between
31
and the first-generation dendrimer
31
and
31-G
1
is that the latter can be recovered and reused, but not
the former [40].
Another
32-G
3
) derived from
(2S)-2-amino-1-(diphenylphosphinyl)-3-methylbutane was complexed
example of
iminophosphine
(compound
in situ
by
3
-C
3
H
5
)Cl]
2
. This catalyst was used in asymmetric allylic alkylations of rac-
(E)-diphenyl-2-propenyl acetate and pivalate. This reaction necessitates also a base
(BSA) and an acetate salt; better results were obtained with LiOAc than with KOAc.
The percentage of conversion, the yield of isolated 2-(1,3-diphenylallyl)-malonic acid
dimethyl ester, and its enantiomeric excess were measured in each case, and were
found good to very good (ee from90 to 95%) (Figure 16.6). Furthermore, the dendritic
catalyst can be recovered and reused at least two times, with almost the same
efficiency [41].
The last example of phosphine dendrimers that we have synthesized was obtained
by grafting the water-soluble PTA ligand (1,3,5-triaza-7-phosphaadamantane)
as terminal groups of generations 1-3. Their ruthenium complexes (compounds
33-G
n
) were used as catalysts in aqueous media for two types of experiments. A
slightly positive dendritic effect on the regioselectivity was observed for hydration
of alkynes, and a large positive dendritic effect was observed in the biphasic
[Pd(
Z
