Pyrene-tagged dendritic catalysts noncovalently grafted onto magnetic Co/C nanoparticles: An efficient and recyclable system for drug synthesis

Article abstract of DOI:10.1002/anie.201209969

Hold on to your palladium! Phosphines have been grafted on magnetic Co/C nanoparticles through π-π interactions. The resulting Pd complexes showed high activity for Suzuki couplings and the system involving a dendritic ligand was recyclable, allowing the preparation of the drug Felbinac over 12 consecutive runs with minimal Pd leaching. After extraction with CH ₂Cl₂, Felbinac met the requirements of the pharmaceutical industry (<5 ppm Pd). Copyright

Full text of DOI:10.1002/anie.201209969

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DOI: 10.1002/anie.201209969
Synthetic Methods
Pyrene-Tagged Dendritic Catalysts Noncovalently Grafted onto
Magnetic Co/C Nanoparticles: An Efficient and Recyclable System for
Drug Synthesis**
Michel Keller, Vincent Colliꢀre, Oliver Reiser, Anne-Marie Caminade,* Jean-Pierre Majoral,*
and Armelle Ouali*
The reuse of catalysts is highly desirable for economic and
ecological reasons, and to this day constitutes an important
challenge. Along these lines, magnetic nanoparticles (MNP)
are increasingly recognized as appealing supports for catalytic
systems in the development of more efficient and green
processes.[1] Contrary to conventional supports, such as
polymers or silica, which require time-consuming precipita-
tion and filtration steps,[2] their separation can easily be
achieved by magnetic decantation. Moreover, MNPs are
mechanically robust and can be easily agitated during
a reaction by application of an external magnetic field.[3]
Generally, MNPs are stabilized by coating the magnetic
core with polymer or silica shells, which are used for the
covalent immobilization of homogeneous catalysts.[1] Mag-
netic carbon-coated nanoparticles[4] have also been used as
reusable supports for the covalent immobilization of cata-
lysts;[5] however, their graphene-like shell offers the unique
possibility for non-covalent catalyst attachment by p–p
stacking,[6] a concept that was recently demonstrated with
pyrene-tagged[7] Pd monomeric complexes. Covalently immo-
bilized dendronized catalysts, which possess an anchoring site
and branches terminating with active sites, represent another
promising strategy for recoverable catalysts, as they were
shown to afford enhanced surface functionalization[8] and
better catalytic activity than corresponding monomeric cata-
lysts.[8a,d] Combining both strategies, we planned to graft
pyrene-tagged dendritic Pd-phosphine catalysts onto Co/C
MNPs by p–p stacking, and to evaluate the activity and
recyclability of the resulting composites in Suzuki reactions.
Pyrene derivative 3 was prepared in high yield (91%)
from commercial 1 and tyramine 2 (Scheme 1). 3 was allowed
to react with N3P3Cl6 and Cs2CO3 to afford 4-G0 in 81% yield.
The growth of this dendron was achieved by using the
À
reactivity of the P Cl bonds towards phenolic group of 4-OH-
Scheme 1. Synthesis of pyrene (Pyr)-tagged phosphines 6-G0, 6-G1, and
9: a) H2N-(CH2)2-C6H4-OH (2), DMF, EDC, HOBt, 0–208C (91%);
b) N3P3Cl6 (2 equiv), Cs2CO3, THF, À78 to 208C (81%); c) H(O)C-
C6H4-OH, THF, 208C (90%); d) H2NNMeP(S)Cl2, CHCl3, 258C; e) HO-
C6H4-PPh2 (5), Cs2CO3, THF, 208C (90%); f) conditions (e) (70%);
g) H(O)C-C6H4-PPh2 (8), MeOH, 208C (70%). DMF=dimethylforma-
mide, EDC=N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide,
HOBt=1-hydroxybenzotriazole.
C6H4-CHO (step c) and further condensation of the alde-
hydes with H2NNMeP(S)Cl2 to afford 1st generation dendron
4-G1 (step d). The Cl-terminated dendrons 4-G0 and 4-G1
were reacted with phosphine 5[9a] to yield pyrene-tagged
multivalent phosphines 6-G0 and 6-G1 (steps e and f).
Incorporation of pyrene labels on the surface of dendrimers
has been previously achieved, but 6-G0 and 6-G1 constitute
the first examples of phosphorous dendrons with pyrene
cores.[10] Moreover, the monomeric pyrene-tagged phosphine
9 was prepared by condensation between 7 and phosphine
8.[9b]
[*] Dr. M. Keller, V. Colliꢀre, Dr. A.-M. Caminade, Dr. J.-P. Majoral,
Dr. A. Ouali
LCC-CNRS, 205 Route de Narbonne, BP 44099
31077 Toulouse Cedex 4 (France)
Dr. M. Keller, V. Colliꢀre, Dr. A.-M. Caminade, Dr. J.-P. Majoral,
Dr. A. Ouali
Universitꢁ de Toulouse; UPS, INPT; LCC
31077 Toulouse (France)
The ability of pyrene-tagged phosphines 9, 6-G0, and 6-G1
(Schemes 1 and 2) to interact with graphene layers through p–
p stacking was next investigated.[11] The loading was found to
decrease as the size of the ligand increased, and loadings of
0.15, 0.1, and 0.03 mmol of pyrene tags per gram of NP were
found for 9, 6-G0, and 6-G1, respectively (Table 1).[12] Partial
desorption of the phosphines from the surface of the NPs was
observed when heating the suspension to 608C, resulting in
a decrease of the loading of 6-G0 from 0.1 to 0.05 mmolgÀ1
E-mail: caminade@lcc-toulouse.fr
majoral@lcc-toulouse.fr
armelle.ouali@lcc-toulouse.fr
Prof. Dr. O. Reiser
Institut fꢂr Organische Chemie, Universitꢃt Regensburg
Universitꢃtsstrasse 31, 93053 Regensburg (Germany)
[**] We thank CNRS and ANR Globucat for funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209969.
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Scheme 2. Structures of monomeric (9) and dendritic (6-G0 and 6-G1)
phosphine ligands.
Table 1: Binding of 9, 6-G0, and 6-G1 onto NPs through p-stacking, as
measured by comparative elemental analysis.[11]
Entry
Pyrene tag
Loading[a]
Phosphine density[b]
1
2
3
4
5
9
0.15
0.10
0.15
0.50
0.30
0.25
0.50
Figure 1. Pd-catalyzed couplings of ArBr and PhB(OH)2 in the presence
6-G0
6-G1
6-G0
6-G0
of pyrene-tagged phosphines and Co/C MNPs. Reaction conditions:
Pd(OAc)2 (0.005 mmol), Co/C MNPs, phosphine ligands (0.005 mmol
for 9, 0.001 mmol for 6-G0, 0.0005 mmol for 6-G1; Pd/phosphine
ratio=1:1), ArBr (1 mmol), ArB(OH)2 (1.14 mmol), Na2CO3 (3 mmol),
THF/H2O (2:5; 7 mL). Bar graph shows yields determined by GC
analysis using 1,3-dimethoxybenzene as an internal standard.
0.03
0.05[c]
0.10[d]
[a] Loading values are given as mmol of pyrene tag per gram of NP.
[b] Phosphine density values are given as mmol per gram of NP.
[c] Loading of 6-G0 at 608C in THF/H2O (2:5). [d] Loading of 6-G0 after
heating to 608C for 14 h and cooling down to 208C in THF/H2O (2:5).
generation increased. The absence of a negative dendritic
effect observed here may be due to the presence of the pyrene
moiety, which could enhance the performance of the coupling
by creating favorable interactions with aryl substrates.[19]
Recycling experiments were attempted in the case of
ligands 9, 6-G0, and 6-G1 in the preparation of 10a from PhBr
and PhB(OH)2 (Figure 2). Using 9 and 6-G1, a steady
(entry 4).[7] However, this desorption is completely reversible,
at room temperature the phosphine is again completely
absorbed onto the NPs (entry 5). Such reversible desorption
from MNPs at 608C, bridging homogeneous and heteroge-
neous processes, can be seen as a major advantage for
catalysis compared to traditional covalent grafting, as all
functional groups are chemically accessible.[13,14] Moreover,
this constitutes the 1st catalytic system grafted by p-inter-
actions in a THF/water mixture, a medium often more
suitable for organic reactions than pure water.
Monomeric phosphine 9 and dendrimers 6-G0 and 6-G1,
which possess five and ten ligands, respectively, on their
surface, were tested as ligands for Pd in Suzuki couplings
(Figure 1).[15,16] Nanocatalyts based on different organic and
inorganic supports have been reported for this versatile and
[2,17,18]
Figure 2. Recycling experiments in the coupling of PhBr and PhB(OH)2
using immobilized catalysts bearing 9, 6-G0, or 6-G1 as the ligand. For
experimental conditions, see Figure 1. Bar graph shows yields deter-
mined by GC analysis using 1,3-dimethoxybenzene as an internal
standard.
À
powerful method of C C bond formation.
To our
knowledge, only a few examples of dendronized catalysts
supported on MNPs have been reported, and all of them are
covalently grafted.[8a,c,d,g]
The coupling of boronic acids with various aryl bromides
was performed in the presence of Co/C MNPs, Pd(OAc)2
(0.5 mol%), ligands 9, 6-G0, and 6-G1 (1 phosphine moiety
per Pd), and Na2CO3 in a THF/water (2:5) mixture (Figure 1).
Good to excellent yields of products 10a–f (70–98%) were
obtained, even in the cases of electron-rich (R = 4-OMe: 10d,
4-Me: 10c) or bulky (R = 2-Me: 10e) substrates. These
conditions are competitive in terms of Pd loading,[18] espe-
cially compared to MNP-supported catalysts involving cova-
lently immobilized dendronized Pd complexes, which require
2.4–5 mol% of metal.[8a,d,g] For each substrate, monomeric and
dendritic ligands conferred about the same activity to Pd
(Figure 1). Interestingly, in reported Suzuki couplings involv-
ing dendritic phosphines that are missing the pyrene groups,
regardless of whether they are grafted onto MNPs[8a,d] or
not,[17a,j] the catalytic activity was found to decrease as the
decrease in yield was observed from runs one to four.
However, the catalytic system based on 6-G0 displayed better
recycling properties: yields of 10a remaining unchanged
(95%) over four runs. This may be because 6-G0 has the
highest phosphine density (Table 1) of the three, which may
increase the stability of the resulting Pd catalyst and contrib-
ute to its robustness. In the absence of pyrene-tagged ligands,
the catalyst could not be recovered.
HRTEM, scanning transmission electron microscopy
(STEM) and energy dispersive X-ray (EDX) spectra were
recorded on MNP functionalized by Pd complexes involving
dendritic phosphine 6-G0 before (Figure 3a) and after 5
catalytic cycles (b).[11] In both cases, the MNP were found to
be surrounded by gray shells (area 1), which are not visible on
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Table 3: Analysis of Pd leaching into crude mixtures during the
preparation of 10a from PhBr and PhB(OH)2.[a]
Run
Pd [ppm]
Yield of 10a [%]
1
2
5
10
274
110
35
9
895
95
95
95
95
94
Pd(OAc)2/6-G0 (no MNP)
[a] The ligand used for all runs was 6-G0. For reaction conditions, see
Figure 1.
Figure 3. STEM images of Co/C MNPs functionalized by Pd–6-G0
complexes before catalysis (a) and after 5 runs (b). Dark regions
(areas 2) contain Co (core), whereas the gray shells (areas 1) mainly
contain Pd and ligand.
274 ppm (ca. 14% of introduced Pd) after the 1st run,
110 ppm (ca. 6%) after the 2nd, 35 ppm (ca. 2%) after the 5th
run and 9 ppm (ca. 0.5%) after the 10th run, while maintain-
ing yields of 95%. These results point to a non-specific
binding of Pd outside the ligand; Pd that is removed in the
extraction procedures of the first two runs. It is of particular
note that significantly reduced Pd leaching was observed with
6-G0 on MNPs versus the non-recoverable catalyst consisting
of only Pd(OAc)2 and 6-G0: the non-recoverable catalyst
system produced three-fold more Pd contamination than its
NMP-immobilized analogue.
naked Co/C MNPs.[4,7] EDX spectra recorded in both cases
show the presence of Pd and ligand in these light regions,
whereas the dark areas (area 2) mainly contain Co and
correspond to the MNP cores, as expected. These results
support effective grafting of the Pd/dendritic phosphine
complexes onto the Co/C MNP through p–p-stacking.
The recyclability of the catalyst bearing 6-G0 was next
tested in the preparation of Felbinac 10g, a commercial non-
steroidal anti-inflammatory drug currently used to treat
muscle inflammation and arthritis.[20] 10g could be isolated
quantitatively (Table 2, run 1) under conditions competitive
In summary, we report here the successful combination of
several strategies to arrive at novel, recyclable Pd catalysts.
Using a magnetic support enables the facile agitation and
recovery of the catalyst by simply applying an external
magnetic field, which avoids the need for precipitation or
filtration steps. Moreover, attaching a dendritic ligand onto
the MNP surface allows up to five times higher loading
(0.5 mmolgÀ1 active sites) than the previously reported direct
functionalization of catalysts onto the NP.[7] Finally, immobi-
lization of the ligand by reversible p–p interactions allows the
catalysis to proceed in the homogeneous phase at elevated
temperatures (608C), which was herein demonstrated for the
first time in an organic solvent/water mixture rather than in
pure water, thus extending the applicability of such catalysts
to non-water-soluble substrates. The corresponding Pd com-
plexes were tested in Suzuki couplings and displayed good
activity for the preparation of various biaryls. In terms of
recyclability, a dendritic effect was observed and the most
efficient catalyst (bearing ligand 6-G0) could be recovered by
straightforward magnetic decantation and reused at least 12
times without loss in activity. In terms of catalytic perfor-
mance, the overall TON and the ease of recovery are two
strengths of the reported catalyst over other immobilized
systems.[2,17,18] Although the 6-G0-based catalyst does not
completely eliminate Pd leaching into the crude product,
Felbinac (10g), which was prepared over multiple runs, met
the specification limits for metal residues in the pharmaceut-
ical industry (< 5 ppm Pd) after a straightforward extraction
with CH2Cl2, and without requiring tedious chromatographic
purification.[22] The extension of catalyst applications is now
underway.
Table 2: Recycling of the immobilized catalyst.[a]
Run
1
Product
Yield [%][b]
100 (98)[c]
2–11
12
10g
10b
100
98 (94)
[a] For reaction conditions, see Figure 1. [b] Yields determined by GC
analysis using 1,3-dimethoxybenzene as an internal standard. Yield of
isolated products given in parentheses. [c] Contamination levels of the
recovered product: crude: 111 ppm Pd, after extraction with CH2Cl2:
<5 ppm Pd and <0.005 ppm Co.
to those reported for its preparation, which involve much
higher Pd loadings.[21] The catalyst could be reused ten times
without loss of activity (runs 2–11). A 12th run was also
successful, even when changing the substrate used, with 10b
being obtained quantitatively (overall TON = 2400). Pd
leaching was measured by ICP-MS after run 1, and 111 ppm
of Pd was detected in the crude (ca. 6% of introduced Pd);
extraction with CH2Cl2 allowed for the isolation of 10g with
less than 5 ppm of Pd contamination, which meets the
requirements of the pharmaceutical industry.[22] No traces of
Co were detected. The evolution of Pd leaching over the
cycles was also investigated for the synthesis of 10a (Table 3).
A substantial drop in Pd leaching was observed in the crude:
Experimental Section
Catalytic tests and recycling: Pd(OAc)2 (0.005 mmol), Co/C NP
(10 mg for 9, 2 mg for 6-G0, 1 mg for 6-G1), and ligand (0.005 mmol of
9, 0.001 mmol of 6-G0, 0.0005 mmol of 6-G1) were added under argon
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to a Radley tube carousel equipped with a magnetic stir bar. THF
(2 mL) and H2O (5 mL) were then added. After 1 h at room
temperature, ArBr (1 mmol), ArB(OH)2 (1.14 mmol) and Na2CO3
(3 mmol) were introduced. The tube was closed, stirred, and heated at
608C for 14 h. After cooling to room temperature, the mixture was
sonicated for 30 min at 208C. Magnetic decantation with a Nd magnet
was then performed and the MNPs were washed 5 times with THF/
H2O (2:5) mixture (5 mL). After concentration of the liquid phase
under vacuum, 1,3-dimethoxybenzene (1 mmol, 130 mL), CH2Cl2
(15 mL) and H2O (15 mL) were added. After 2 extractions with
CH2Cl2, the organic layers were combined, filtered through a plug of
Celite and analyzed by GC or isolated by column chromatography.
The solid was directly used for a new catalytic run. THF (2 mL), H2O
(5 mL), ArBr (1 mmol), ArB(OH)2 (1.14 mmol) and Na2CO3
(3 mmol) were introduced under argon. The flask was closed, stirred,
and heated at 608C for 14 h.
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[12] Dendrimers without pyrenes are not able to interact with NPs.
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Received: December 13, 2012
Revised: January 10, 2013
Published online: February 20, 2013
Keywords: catalyst recycling · dendrimers ·
.
magnetic nanoparticles · palladium · Suzuki coupling
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