Supramolecular immobilization of a perfluoro-tagged Pd-catalyst with dendritic architectures and application in Suzuki reactions

Article abstract of DOI:10.1002/adsc.200505079

A new supramolecular complex of a perfluoro-tagged palladium phosphine catalyst to a dendritic core-shell architecture with a perfluoroalkyl shell was used as recoverable catalyst for Suzuki couplings. This homogeneous complex can also serve as a model for related catalysts adsorbed on fluorous silica gel.

Full text of DOI:10.1002/adsc.200505079

FULL PAPERS
Supramolecular Immobilization of a Perfluoro-Tagged
Pd-Catalyst with Dendritic Architectures and Application in
Suzuki Reactions
a
b
b,
Abel Garcia-Bernab e´ , Carl Christoph Tzschucke, Willi Bannwarth, *
Rainer Haag *
a
c,
Departamento de Termodin a´ mica Aplicada, Universidad Polit e´ cnica de Valencia, camino de Vera s/n,46022 Valencia,
Spain
b
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 21, 79104 Freiburg,
Germany
E-mail: willi.bannwarth@organik.chemie.uni-freiburg.de
Institut für Chemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany
c
Fax: (þ49)-30-838-53357; e-mail: haag@chemie.fu-berlin.de
Received: February 7, 2005; Accepted: May 13, 2005
Abstract: A new supramolecular complex of a per- Keywords: dendrimers; fluorinated ligands; ligand re-
fluoro-tagged palladiumphosphine catalyst to a den-
cycling; palladium catalysis; supramolecular complex;
dritic core-shell architecture with a perfluoroalkyl Suzuki coupling
shell was used as recoverable catalyst for Suzuki cou-
plings. This homogeneous complex can also serve as a
model for related catalysts adsorbed on fluorous silica
gel.
Introduction
catalysts in dendritic architectures by supramolecular
interactions. In these cases, the linkage between the
[
10]
Dendritic core-shell architectures have attracted in- transition metal complexes and the dendritic support
creasing interest for their ability to act as unimolecular is based on, e.g., ionic interactions in combination with
nanocarriers for catalysts, pharmaceuticals and other multiple hydrogen bonds and allows a high degree of
[1]
guest molecules. More recently, such nanostructures flexibility.
[
2,3]
have also been prepared with perfluorinated shells al-
lowing easy recovery of products and recycling of cata-
lytic nanoparticles in alternative reaction media, such
[
4]
as scCO and perfluorinated solvents.
2
For many applications the difficult accessibility of
dendrimers via costly multistep syntheses represents a
[5]
major problem. Hyperbranched polymers that can
be obtained in one reaction step fromthe polymeriza-
tion of AB monomers are currently being discussed
m
[
6]
as possible alternatives. In contrast to the perfectly
[
3,7]
branched glycerol dendrimers (DB¼100%),
hyper-
branched polyglycerols (PG) 1 are randomly branched,
but well-defined dendritic structures with a degree of
branching of approx. 60%. They are prepared in a
one-step process and are readily available on a kilogram
scale with relatively low molecular weight distributions
[8]
(
typically<2).
For the covalent immobilization and the recycling of
homogeneous catalysts dendritic polymers have been
used by many groups. However, only a few examples
have been reported for the non-covalent anchoring of
[
9]
Adv. Synth. Catal. 2005, 347, 1389–1394
DOI:10.1002/adsc.200505079
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Abel Garcia-Bernab e´ et al.
Yet another approach for the non-covalent immobili- Results and Discussion
zation of catalysts has been presented by one of our
groups who used interactions between fluorous phase For the preparation of the dendritic core-shell-type ar-
silica (FPS) and perfluoro-tagged palladiumcomplexes
chitecture, we esterified hyperbranched polyglycerol 1
ꢀ
1 [8]
for the immobilization and recycling of catalysts in or- (M ¼5000 g mol
)
with a perfluorinated carboxylic
n
[11]
ganic solvents. In this heterogeneous systemthe effi-
acid by azeotropic distillation (Scheme 1). This fluoro-
cient catalysis, the catalytic properties and the quantita- philic nanocarrier was prepared on a multigram scale
tive catalyst recovery by simple filtration were demon- and was easily purified with a strongly basic anion ex-
ꢀ
strated for various CꢀC coupling reactions. Due to the change resin (4 mmol OH /g) to yield 85% of PG-per-
heterogenic nature of this systemit is quite difficult to
analyze the catalytic process in detail. Therefore, a ho- from C NMR (inverse gated) was 47%.
mogeneous analogue of this catalytic system should al- Then the perfluoro-tagged palladiumphosphine com-
fluoroester 2. The degree of functionalization obtained
13
[
12]
low us to get more insight into the active catalytic species plex 3, meanwhile commercially available (Fluka),
and the supramolecular interactions by NMR tech- was immobilized in the perfluorinated shell of the den-
niques.
dritic polyglycerol ester to obtain the supramolecular ar-
Recently, we have reported on the syntheses of novel chitecture 4 simply by dissolution of both components in
dendritic architectures with perfluorinated shells de- DMF with ultrasonication (Scheme 2).
rived fromthe covalent modification of perfect glycerol
The strong interaction between the perfluorinated
dendrimers, hyperbranched polyglycerols and hyper- chains in the supramolecular complex 4 was observed
[
3]
19
branched polyethyleneimines. These polymers are by F NMR spectroscopy (Figure 1). The spectrum
soluble in several perfluorinated solvents and can en-
capsulate ions, polar dyes and metal nanoparticles.
However, we were not able to performcatalytic reac-
tions in the presence of these systems. This is probably
due to complexation and inactivation of Pd by the pres-
ence of chelating S- or N-ligands in these dendritic poly-
mers. Thus, we herein present a nitrogen- and sulfur-free
dendritic structure based on a dendritic architecture
built frompolyglycerol core and a perfluoroalkyl shell.
This system was used as supramolecular support to im-
mobilize a perfluoro-tagged catalyst and study its cata-
lytic activity in Suzuki coupling reactions as well as the
complex stability.
Scheme 1. Synthesis of dendritic polyglycerol perfluoroalkyl
ester 2 fromhyperbranched polyglycerol 1; a) pTSA cat.,
BTF, azeotropic distillation, 3 days.
Scheme 2. Supramolecular interaction between perfluoro-tagged palladium complex 3 and polyglycerol ester 2.
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Immobilization of a Perfluoro-Tagged Pd-Catalyst
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shows an isolated CF signal at ꢀ83.31 ppm. Upon com-
3
plex formation a significant amount of the CF signal un-
3
dergoes a high-field shift of 1.70 ppmto ꢀ81.61 ppm.
This shift of the CF signal is characteristic for the partial
3
intercalation of perfluoroalkyl chains and has also been
observed for other supramolecular aggregates as well as
for the self-aggregation of perfluoroalkyl-modified per-
[13]
fect dendrimers. It is noteworthy for the catalytic ac-
tion of the immobilized catalyst that the fluorous-fluo-
rous interaction is temperature dependent and is re-
duced significantly at higher temperatures (Figure 2).
This will enhance the flexibility of the ligand and hence
increase its catalytic activity. This temperature-depend-
ent aggregation might also explain the excellent results
of the heterogeneous systemconsisting of perfluoro-tag-
Figure 2. Temperature-dependence of the CF -group integral
of the complex 4 at ꢀ83.31 ppmin DMF.
3
[
11]
ged Pd catalysts adsorbed on fluorous silica gel. In this
case we now have a strong indication for the formation
of a homogeneous catalytic species, which might be to the perfluoro-tagged catalyst itself at different cata-
due to the temperature-dependent desorption of the lyst concentrations (Table 1). For these studies a 1:1mo-
catalyst fromthe solid support at elevated temperature
lar ratio of polyglycerol ester 2/perfluoro-tagged Pd cat-
(
808C), while a strong insoluble complex is present at alyst 3 was used.
roomtemperature.
Initially, we tried to use a mixture of dimethoxyethane
We also obtained TEM images of the supramolecular (DME) and water as solvent systemsimilar to the hetero-
[
11]
aggregate 4 (Figure 3). The image shows spherical parti- geneous FSG-system. However, the supramolecular
cles with a diameter of approx. 3ꢁ2 nm. This corre- catalyst 4 was not soluble in this mixture. We only ob-
sponds to the size expected for single molecules of the served homogeneous reaction conditions when using
dendritic perfluoroalkyl ester 2, however, a micellar as- DMFas a solvent. An advantage of this selective solubil-
sembly of a few particles cannot be excluded due to the ity is that the supported catalyst 4 can easily be recycled
error of the TEM measurement. But the particles are by precipitation in a mixture of DME/water (10% HCl)
certainly small enough to be homogeneously dissolved 2:1, whereas the substrate and product remain soluble
in solution.
in this mixture. The precipitated catalyst complex 4
The catalytic activity of immobilized perfluoro-tagged was filtered and re-used for three consecutive Suzuki
palladiumcomplex 4 was now studied in homogeneous coupling reactions. Each reaction series was performed
Suzuki coupling reactions (Scheme 3) and compared at three different catalyst loadings 0.1, 0.5 and 1 mol %
19
Figure 1. F NMR spectra of supramolecular complex 4 in DMF at 208C.
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Abel Garcia-Bernab e´ et al.
DMF, but the yields without support 2 are significantly
lower than the reactions of the supramolecular assembly
4
. This is probably due to the fact that the Pd complex 3
alone is not soluble in DMF, whereas the supramolecu-
lar assembly 4 is soluble. It is also noteworthy that a cat-
alyst loading of 0.1 mol % is sufficient for a single run,
however, for multiple use higher concentrations of the
complex 4 are needed (Table 1). The high catalytic activ-
ity of this supramolecular complex 4 can be explained by
its relative small particle size (3ꢁ2 nm) and the homo-
geneous reaction conditions. Thus, the dendritic support
serves to molecularly disperse the Pd complex in the re-
action mixture.
With regard toour earlier experimentswith perfluoro-
tagged Pd complexes adsorbed on fluorous silica gel, the
supramolecular complex 4 can be used as a homogene-
[11]
ous model system to explain the observed reactivity.
We assume that the way these two catalytic systems
act is similar and in both cases the release of the per-
fluoro-tagged Pd-catalyst occurs at high temperature,
while a strong complex is present at room temperature.
Figure 3. TEM image of the supramolecular complex 4. The Therefore, also the fluorous solid phase acts in the same
bar in the TEM image is equivalent to 20 nm.
way as the fluorous dendritic polymer, namely to dis-
perse the perfluoro-tagged catalyst and bring a large
portion of it into homogeneous contact with the reaction
mixture, in which the heterogeneous FPS remains in-
soluble.
Conclusion
In summary, we have immobilized a perfluoro-tagged
pramolecular palladium complex 4. a) 0.1, 0.5 and 1 mol % palladiumcatalyst on a dendritic polyglycerol ester
Scheme 3. Suzuki coupling reaction performed with the su-
catalyst (see Table 1), Na CO , DMF, 808C, 20 h.
with a perfluorinated shell 2 and investigated its catalyt-
ic activity in Suzuki couplings. The supported catalyst 4
can be separated fromthe product by simple precipita-
2
3
(
Table 1). The pure product can easily be obtained by tion using a mixture of DME/water. The dendritic sup-
extraction of the soluble phase with chloroform/water. port allows for a better solubility of the perfluoro-tagged
After concentration of the chloroformlayer the conver- palladiumcomplex in organic solvents and increases the
sion yield was determined from the H NMR spectra by yield of the reaction significantly. Also, the recycling and
integration of the methyl signals. multiple use of this supramolecular catalyst 4 is straight-
We have also tested the catalytic activity of the per- forward. This demonstrates a new concept for dissolving
fluoro-tagged palladiumcatalyst without the dendritic perfluoro-tagged catalysts in the usual organic solvents
1
support 2 under the same conditions (Table 1). While and for allowing their separation and recovery by pre-
Gladyzs et al. demonstrated that it is possible to use flu- cipitation. Furthermore, this supramolecular dendritic
orous catalysts in the absence of fluorous solvents pro- assembly serves as a valuable soluble model for the in-
vided the perfluoro-tagged catalyst is sufficiently solu- teraction of perfluoro-tagged catalysts with insoluble
[
14]
ble at the reaction temperature,
in our case, per- supports such as fluorous silica gel and clearly reveals
fluoro-tagged palladiumis also catalytically active in
the ligand diffusion from the complex at elevated tem-
Table 1. Results of consecutive Suzuki couplings.
Catalyst loading [mol%] Yield [%] (1st run) Yield [%] (2nd run) Yield [%] (3rd run) Yield [%] without support 2
0
0
1
.1
.5
>99
>99
>99
29
>99
>99
3
97
>99
26
29
55
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Immobilization of a Perfluoro-Tagged Pd-Catalyst
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[8]
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(
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2
General Procedure for Suzuki Coupling Reactions
A 0.001 M solution of perfluoro-tagged catalyst 3 (10 mg,
0.003 mmol) and the dendritic perfluoroester 2 (60 mg,
0.003 mmol) in 3 mL of absolute DMF was prepared by ultra-
sonification for 30 min. Then one equivalent 4-bromoaceto-
phenone, 1.1 equivalent of phenylboronic acid and 2.5 equiva-
lent of sodiumcarbonate were added under argon. The reac-
tion mixture was heated to 808C for 20 hours. The solvent
was evaporated and the residue was dissolved again with a mix-
ture of dimethoxyethane (DME) and water (10% HCl) (2:1).
The insoluble part was filtered, washed with DME/water (2:1)
and recycled for the next run (recovery yield ca. 50%). The
combined DME/water layers were concentrated in vacuum
and extracted three times with chloroform-water. Evaporation
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