Supramolecular Catalysts by Encapsulating Palladium Complexes within Dendrimers

Article abstract of DOI:10.1021/ja038455m

Dendrimers are well-defined and highly branched macromolecules. By utilizing their capsular architectures, dendrimers encapsulating various catalytically active species can be prepared, which often bring about unique catalysis. Treatment of the alkylated

Full text of DOI:10.1021/ja038455m

Published on Web 01/21/2004
Supramolecular Catalysts by Encapsulating Palladium Complexes within
Dendrimers
Masahiko Ooe, Makoto Murata, Tomoo Mizugaki, Kohki Ebitani, and Kiyotomi Kaneda*
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka UniVersity,
1
-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received September 11, 2003; E-mail: kaneda@cheng.es.osaka-u.ac.jp
1
Dendrimers are of great interest as new materials for applications
in many areas such as catalysts.2 A characteristic feature of
dendrimers is the presence of internal cavities, which can host
3
4
organic molecules and metal nanoparticles. Using this property,
the catalytic active species can be introduced in the interior spaces
of dendrimers to construct nanoreactors, which have received
4
considerable attention because of their unique catalysis. Recently,
we have reported on the substrate-specific hydrogenation of olefins
based on the hydrogen bonds between the tertiary amino groups
inside a poly(propylene imine) (PPI) dendrimer and substrates.4
This success has provided us with a new strategy for the design of
dendrimer-encapsulated metal catalysts that utilize the inner nano-
environment of the dendrimers. In this communication, we describe
the novel immobilization of a Pd complex catalyst within the cavity
a
Figure 1. Proposed structure of dendrimer-encapsulated Pd complex.
Table 1. Heck Reaction Catalyzed by Dendrimer-Encapsulated
Pd Complexesa
5
of dendrimers through ionic bonds. The dendrimers encapsulating
Pd complexes act as unique nanoreactors for Heck reactions and
allylic aminations. Moreover, the dendritic nanoreactor could readily
recovered using thermomorphic system. To date, there are only a
few examples of the positive dendritic effects on the activity of
catalysts, in which the active sites are incorporated inside the
dendrimers.6
PPI dendrimer was chosen as a nanoreactor because of the high
density of tertiary amino groups in its interior. Peripheral amino
groups on the third to fifth generation of the PPI dendrimer 1-3
were modified with decanoyl chloride and 3,4,5-triethoxybenzoyl
chloride,4b respectively, to give the alkylated (1a-3a) and arylated
entry
ArX
PhI
PhI
PhI
PhI
PhI
PhI
PhI
1-C10H7I
PhI
PhI
PhI
1-C10H7I
R
dendrimers
P/Pd
yield (%)b
1c
2
3
4
5
6
7
8
9
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-CO2Bu
-Ph
1a
1a
2a
3a
3a
3a
3a
3a
none
3a
3a
3a
1
2
2
1
2
3
4
1
1
1
1
1
72
59
89
92
89
37
19
83
<1
38
99
99
c,d
10
e
1
1
1
2
(1b) dendrimers. 4-Diphenylphosphinobenzoic acid 4 was used as
e
-Ph
the phosphine ligand to fix the Pd complex inside the dendrimers
through ionic bonds between the carboxyl group and an internal
a
[PdCl(C3H5)]2 (0.005 mmol), dendrimer (0.01 mmol), toluene (10 mL),
amino group of the dendrimer.
aryl halide (1.0 mmol), olefin (1.4 mmol), KOAc (1.4 mmol), 2 h.
1
b
Determined by GC analysis. c Decomposition of Pd complexes was
8
H NMR studies in d -toluene of 1a showed that the addition of
observed. d PPh3 was used instead of 4 as a ligand. For 12 h.
e
4
caused downfield shifts of the R-methylene protons of the tertiary
amino groups, from the initially observed shift at 2.29 to 2.34-
.58 ppm, in ratios of 4/1a ) 1-6. These shifts can be explained
acrylate, while the reaction hardly occurred in the absence of the
dendrimer under the present conditions (entry 9). The reaction rates
increased with increasing generations of the dendrimers (entries 2,
3, and 5); the generation effect was most pronounced between the
third and fourth generations. In the case of 3a, the reaction rates
decreased as the molar ratio of 4/Pd was increased (entries 4-7).
Notably, an active Pd species could be encapsulated to prevent its
decomposition even in a 4/Pd ratio of 1 (entry 4), while the use of
a resulted in the precipitation of Pd metals (entry 1). Moreover,
using PPh instead of 4 also gave a poor result (entry 10). Generally,
the stabilization of homogeneous Pd(0) active species requires the
addition of excess phosphine ligands. The fifth generation den-
drimer acts as an effectiVe nanoreactor with high catalytic actiVity
and stability for the Pd(0) species in a low P/Pd ratio. It can be
reasoned that the dense amino groups inside the dendrimer provide
a polar nanoenvironment around the active Pd species and
coordinate to the Pd species, which leads to the high catalytic
activity and the stability for the encapsulated Pd complex.
2
7
as protonation of the tertiary amino groups of the dendrimer.
Similarly, 13C{ H} and P{ H} NMR spectra also showed interac-
tions between 1a and 4. [PdCl(C
1
31
1
8
3
5
H )]
2
was added to the above
mixture of 1a and 4 to form the dendrimer-encapsulated Pd
complex. 31P{1H} NMR spectrum of the resulting Pd complex (P/
Pd ) 1) showed a characteristic singlet at 23.1 ppm, whereas metal-
free 4 exhibited a single resonance at -3.8 ppm. The R-methylene
proton signal of the tertiary amino group remained at about 2.40
ppm during the metal complexation. It can be said that the Pd
complex is encapsulated through ionic bonds between the carboxyl
group of 4 and amino groups within the dendrimer as shown in
Figure 1. The local structure around Pd has been well characterized
using Pd K-edge EXAFS data.8
1
3
9
Initially, as shown in Table 1, the potential ability of the
dendrimers encapsulating Pd complexes as nanoreactors was
investigated in the Heck reaction.9 The dendritic Pd complexes
efficiently catalyzed the Heck reaction of iodobenzene with n-butyl
,10
1604
9
J. AM. CHEM. SOC. 2004, 126, 1604-1605
10.1021/ja038455m CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Scheme 1. Heck Reaction of p-Diiodobenzene with Olefins Using
Pd Complexes -
Recycling the dendritic catalysts has been attempted by solvent
a
d
17
5a,18
precipitation and membrane filtration,
which often resulted
in some losses of the catalytic activities. The solubility of the
dendrimers can be tuned by their surface modifications with keeping
their inner nanoenvironment intact.1 Modification of the PPI
dendrimer with 3,4,5-triethoxybenzoyl chloride instead of decanoyl
chloride afforded dendrimer 1b which was not soluble in aliphatic
hydrocarbons such as heptane, but possessed high solubility in polar
solvents such as DMF. In the allylic amination of cinnamyl methyl
carbonate with piperidine, the thermomorphic two phases of DMF
and heptane became homogeneous during the reaction and then
a
[
PdCl(C3H5)]2 (0.005 mmol), 3a (0.01 mmol), 4 (0.01 mmol), toluene
b
(
10 mL), KOAc (1.4 mmol). [PdCl(C3H5)]2 (0.005 mmol), 4 (0.02 mmol),
DMF (10 mL), KOAc (1.4 mmol). Determined by LC analysis at a complex
consumption of olefins. Yields are given in parentheses.
c
d
13,19
could be readily separated by cooling the reaction mixtures.
The DMF phase containing the dendritic nanoreactor could be
recycled after decantation of the heptane phase. The highly catalytic
activity was retained during reuse experiments; yields of N-
cinnamylpiperidine in the heptane phase were 46% (first), 99%
Table 2. Allylic Amination Using Dendrimer-Encapsulated Pd
Complexesa
(second), 99% (third), and 99% (fourth).
In summary, we have demonstrated that the Pd complexes could
yield (%)b
be encapsulated within the PPI dendrimers through ionic interac-
tions. The specific nanoenvironment created by the dense amino
groups inside the dendrimers can provide high catalytic activity
and stability for the Pd complexes. The employment of a thermo-
morphic system allowed the recycling of the dendritic Pd catalysts.
entry
dendrimers
P/Pd
L/B (−)
1
2
3
4
1a
2a
3a
3a
3a
3a
none
1a
none
2
2
1
2
2
4
2
2
2
78
67
36
52
>99
19
83
35
33
8.5
8.5
9.0
9.0
9.0
9.0
5.1
4.8
4.1
c
5
6
7
Supporting Information Available: Experimental details and
curve-fitting analysis (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
d
8
d
9
a
References
[PdCl(C3H5)]2 (0.0025 mmol), dendrimer (0.005 mmol), toluene (10
mL), cinnamyl methyl carbonate (1.0 mmol), morpholine (1.2 mmol), 30
min. Determined by GC analysis. For 12 h. PPh3 was used instead of
(
1) For books on dendrimers, see: (a) Fr e´ chet, J. M. J., Tomalia, D. A., Eds.
Dendrimers and Other Dendritic Polymers; J. Wiley & Sons: New York,
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b
c
d
4
as a ligand.
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can be applied to other Heck reactions. For example, 1-iodonaph-
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yields of n-butyl 3-(1-naphthyl)acrylate and 1-styryl-naphthalene,
respectively (entries 8 and 12). Moreover, in the reaction of
p-diiodobenzene with n-butyl acrylate, n-butyl p-iodocinnamate as
a monosubstituted product was obtained in 92% selectivity using
the nanoreactor, whereas a nearly equimolar mixture of mono- and
disubstituted products was obtained without the dendrimer. A
similar phenomenon was also observed using styrene instead of
n-butyl acrylate. As summarized in Scheme 1, these results strongly
support that the Heck reaction occurs inside the dendrimers.
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also act as unique nanoreactors for allylic amination. The results
of allylic amination of cinnamyl methyl carbonate with morpholine
at 20 °C are summarized in Table 2.11 The reaction rates increased
with decreasing generations of the dendrimers (entries 1, 2, and
(
3) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W.
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(
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(
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(
8) See Supporting Information for details.
(
9) For a recent reviews on Heck reaction, see: Beletskaya, I. P.; Cheprakov,
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(
10) For a recent example of the Heck reaction using dendrimer-encapsulated
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(
11) For a recent review on allylic amination, see: Johannsen, M.; Jorgensen,
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4
), which was also observed in the hydrogenation of olefins using
(
4a
our dendrimer-encapsulated Pd nanoparticles. The surface conges-
tion of the higher-generation dendrimers can suppress the penetra-
tion of substrates into the dendrimers. However, such filter effect
(
(
on the reaction rates was not observed in the above Heck
reaction.1
2,13
Molar ratios between linear and branched products
(15) For description of solvents affecting the regioselectivity in allylic
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(
(
L/B) were 8.5, 8.5, and 9.1 with 1a, 2a, and 3a, respectively
entries 1, 2, and 4), whereas the reaction in the absence of the
(
16) Oosterom, G. E.; Haaren, R. J.; Reek, J. N. H.; Kamer, P. C. J.; van
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1
2.2 (acetone), and 14.1 (DMSO).15 Regioselectivity was controlled
(
16
by the local polarity around the Pd species within the dendrimers.
Using PPh instead of 4 led to a low L/B value, with or without
the dendrimers (entries 8 and 9).
(
3
JA038455M
J. AM. CHEM. SOC.
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VOL. 126, NO. 6, 2004 1605