Reductant-directed formation of PS-PAMAM-supported gold nanoparticles for use as highly active and recyclable catalysts for the aerobic oxidation of alcohols and the homocoupling of phenylboronic acids

Article abstract of DOI:10.1039/c2cc31948a

Polystyrene-polyamidoamine-supported gold nanoparticles were prepared using a reductant-directed formation strategy. The resulting catalysts exhibited excellent activities in the aerobic oxidation of benzyl alcohols and the homocoupling of phenylboronic acids under mild conditions and can be recycled at least 14 times without significant loss of activity. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31948a

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COMMUNICATION
Reductant-directed formation of PS–PAMAM-supported gold nanoparticles
for use as highly active and recyclable catalysts for the aerobic oxidation of
alcohols and the homocoupling of phenylboronic acidsw
Jie Zheng, Shengyue Lin, Xianhao Zhu, Biwang Jiang,* Zhen Yang* and Zhengying Pan*
Received 16th March 2012, Accepted 2nd May 2012
DOI: 10.1039/c2cc31948a
Polystyrene–polyamidoamine-supported gold nanoparticles were
prepared using a reductant-directed formation strategy. The
resulting catalysts exhibited excellent activities in the aerobic
oxidation of benzyl alcohols and the homocoupling of phenyl-
boronic acids under mild conditions and can be recycled at least 14
times without significant loss of activity.
polymerization step (Scheme 1b).7 Herein, we present a new
synthetic strategy, in which reductants are immobilized onto the
polymer chain and then metal ions are reduced in solution to
form polymer-supported nanoparticles (Scheme 1c).
Gold nanoparticles (AuNPs) are powerful catalysts that are
used in various types of reactions, such as alcohol oxidation,
CO oxidation and Ullmann reactions.8 Polyamidoamine
(PAMAM) dendrimers have been successfully used as sup-
ports to disperse and stabilize transition metals.4c,9 Using the
aforementioned reductant-directed formation method, we
successfully synthesized PAMAM-supported AuNPs for use
as highly active and recyclable catalysts.
Transition metal nanoparticles have been widely applied in
molecular recognition,1 non-linear optical2 and biological
applications,3 and the utility of these nanoparticles in organic
synthesis has attracted immense attention.4 Because of their
intrinsic nano-size-related physicochemical properties, high
surface-to-volume ratio and ease of recycling, polymer-supported
transition metal nanoparticles allow the use of milder reaction
conditions and are environmentally friendly, low-cost alter-
natives to bulk metal reagents. Numerous studies have demon-
strated that the properties of transition metal nanoparticles are
sensitive to their methods of formation.5 Currently, the general
approach for the synthesis of nanoparticle–polymer complexes
is to complex metal cations with polymer supports and to
perform a subsequent chemical reduction step (Scheme 1a).6
Another approach for the formation of nanoparticles is based
on the simultaneous or sequential addition of monomers, a
reductant and a transition metal salt solution followed by a
Polystyrene (PS) beads (5% w/w divinylbenzene) were
chosen as the polymer support. As shown in Scheme 2, after
the beads were functionalized with free amino groups, a
generation three PAMAM (G3 PAMAM) dendrimer was
constructed by performing three cycles of the following
sequential reactions: (1) a Michael-type addition to methyl
acrylate to produce b-amino propionate esters and (2) amino-
lysis of the resulting ester with ethylenediamine. In the last
cycle, N,N-dimethylethylenediamine was used in the aminolysis
reaction. Subsequently, the tertiary amino groups in G3
PAMAM were acidified, and sodium borohydride was
complexed with the polymer beads in DMF. During this
process, the discharge of gas was observed. Next, a potassium
tetrachloroaurate solution was added dropwise to the dendro-
nized polymer beads in aqueous solution to afford polymer-
supported gold nanoparticles. The polymer beads changed
their color from white to purplish black. However, if the
Scheme 1 Strategies for the formation of polymer-supported transition
metal nanoparticles (B = N, O, S, P, etc.).
Key Laboratory of Chemical Genomics, School of Chemical Biology
and Biotechnology, Shenzhen Graduate School, Peking University,
Xili University Town, PKU Campus Building F, Shenzhen 518055,
China. E-mail: panzy@pkusz.edu.cn, zyang@pku.edu.cn,
bwjiang@pkusz.edu.cn
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31948a
Scheme 2 Preparation of polymer-supported AuNP catalysts.
Chem. Commun., 2012, 48, 6235–6237 6235
c
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Table 1 Oxidation of various benzyl alcohols catalyzed by polymer-
supported AuNPsa
Entry
R1
R2
Time (h)
Yieldb (%)
Fig. 1 SEM images of polymer-supported AuNPs: (a) external and
(b) internal regions of a polymer bead; (c) TEM image of a slice of a
polymer bead (scale bar, 50 nm).
1c
2c
3c
4c,d
5
6
7
8
9
10
11
12
13
14
15
16
MeO
MeO
MeO
MeO
EtO
Me
i-Pr
H
Cl
Br
NO2
MeO
Me
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
6
12
61
77
16
16, 2nd–14th runs
>99
>99
>99
>99
>99
>99
>99
>99
13
>99
>99
>99
>99
>99
acidification step was not conducted, nanoparticles did not
form. Although the exact nature of the reductant is the subject
of further studies, it is expected that [polymer–R2N - BH3] is
formed after the protonated ammonium salt reacts with sodium
borohydride, which then reduces the gold salt to form AuNPs.10
Scanning electron microscopy (SEM) analysis demonstrated
that gold nanoparticles formed on both the external and
internal surfaces of the polymer beads (Fig. 1a and b). Trans-
mission electron microscopy (TEM) study indicated that most
of the gold nanoparticles were approximately spherical, and the
diameters of the nanoparticles ranged from 5 to 10 nm (Fig. 1c).
Before reacting the G3 PAMAM polymer beads with the gold
salt solution, inductively coupled plasma (ICP) analysis of the
beads showed that the nitrogen content was 4.86% w/w and the
boron content was 0.87% w/w. After the AuNPs were formed,
the gold content was 18.6% w/w. Energy-dispersive X-ray
spectroscopy (EDS) was performed on the cross-section of a
bead, and a clear Au band was observed (Fig. 2a). In addition,
backscattered electron (BSE) analysis of the bead’s surface
showed that the distribution of AuNPs was uniform (Fig. 2b).
As shown in Fig. 2c, the X-ray diffraction (XRD) pattern of the
PS–PAMAM–AuNP beads revealed four main peaks corres-
ponding to the (111), (200), (220), and (311) Bragg reflections of
Au; these peaks are consistent with those of gold nanocrystals.11
The formation of the AuNPs was also evidenced by the band at
570 nm in the UV-Vis spectrum (Fig. 2d).12
16
16
16
16
16
16
16
32
32
32
32
32
Cl
Br
a
Reaction conditions: substrates (0.5 mmol), DCM (1 mL), water
(1 mL), K2CO3 (1.5 mmol), catalyst (0.007 mmol), rt, O2 (1 atm).
b
c
Determined by GC-MS analysis. Catalyst: 2.8 mol%. Experi-
d
ments consisted of 2 to 14 runs.
friendly solvents.13 Thus AuNPs prepared using our novel
strategy were tested for the aerobic oxidation of alcohols.
Using p-methoxy benzyl alcohol as the substrate, aerobic
oxidation was carried out in the presence of PS–PAMAM–
AuNPs and K2CO3 in a (1 : 1) dichloromethane–water mix-
ture under atmospheric O2 at room temperature (Table 1).15
The yields of the corresponding benzaldehydes were 61%
after 6 hours, 77% after 12 hours and >99% after 16 hours
(entries 1–3). Interestingly, Gn-PAMAM–AuNPs (generation
n = 0, 1, 2) provided yields of only 73%, 50% and 39%,
respectively. The catalyst was easily recovered by simple
centrifugation, and its reactivity was maintained for at least
14 runs (entry 4). After 14 cycles, the catalyst’s gold content
was 17.3% w/w showing only a small loss of gold from the
original 18.6% w/w. As requested by a reviewer, repetition
experiments with a shorter reaction time were also conducted.
With the same reaction conditions used as in entries 1–3,
except that the reaction time was fixed at 5 hours Æ 5 minutes,
yields of 1st to 10th runs were 55%, 54%, 54%, 55%, 54%,
53%, 53%, 53%, 53%, and 52%, respectively, demonstrating
the very high stability and reproducibility of the catalyst.
AuNPs were also prepared using the classical route
(Scheme 1a), and tested under the same conditions as those
used for entry 4. Although a >99% yield was obtained in the
first run, in a sharp contrast, the yield of the second run was
only 62%. Thus, our reductant-directed approach produced
more stable catalysts than the classical route.
The selective oxidation of alcohols is a fundamental and
challenging transformation in organic synthesis. An attractive
approach is the direct oxidation of alcohols using air or
molecular oxygen (O2) and recyclable catalysts in environmentally
The oxidation of several primary and secondary benzylic
alcohols with the AuNP catalyst was also investigated.
Primary alcohols gave quantitative yields (entries 5–10) except
when a strong electron-withdrawing group (NO2) was at the
para-position, a mere 13% yield was obtained (entry 11).
Oxidation reactions of secondary alcohols required longer
Fig. 2 (a) EDS analysis of a cross-section of a hemispherical polymer
bead;14 (b) BSE image of the distribution of gold along the surface of
the polymer bead; (c) XRD pattern and (d) UV-Vis absorption
spectrum of polymer-supported AuNPs (see ESIw for full-scale graphs).
c
6236 Chem. Commun., 2012, 48, 6235–6237
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Table 2 Homocoupling of various phenylboronic acids catalyzed by
polymer-supported AuNPsa
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Entry
R
Yield (2)b
Yield (3)b
1
2
3
4
5
6
7
H
99
Trace
95
Trace
Trace
95
2-Me
3-Me
4-Me
2-OMe
3-OMe
4-OMe
Trace
96
98
Trace
94
98
Trace
Trace
a
Reaction conditions: substrate (0.5 mmol), water (2 mL), K2CO3
b
(1.5 mmol), catalyst (0.007 mmol), rt. Isolated yields.
reaction times; however, quantitative yields were still achieved
(entries 12–16).
Lastly, we examined the catalytic ability of the AuNPs in
the homocoupling of phenylboronic acids in air using water as
a solvent.16 Notably, this reaction can be carried out at room
temperature. In 24 hours, unsubstituted and meta/para-sub-
stituted phenylboronic acids afforded >90% yield of biphenyl
products, and phenols were produced in >90% yield from
ortho-substituted phenylboronic acids. Clearly, steric factors
played a major role in the selection of the primary reaction
pathway (Table 2).
In summary, we developed a novel strategy for the produc-
tion of gold nanoparticles on PAMAM-grafted polystyrene–
divinylbenzene beads by immobilizing a reductant onto
polymer microspheres. The resulting catalyst is highly efficient
and stable for the aerobic oxidation of primary and secondary
benzyl alcohols and for the homocoupling of phenylboronic
acids under mild conditions. Moreover, the catalyst is easily
recoverable and can be reused fourteen times without signifi-
cant loss of activity. In contrast, the AuNP catalyst prepared
using the conventional approach (Scheme 1a) exhibited a
dramatic loss of catalytic activity in the repetition experiment.
In conclusion, PAMAM functionalization followed by reduc-
tant complexation and transition metal salt reduction is an
effective method for the synthesis of immobilized transition
metal nanoparticle catalysts.
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attributed to the back scattering probe.
The authors gratefully acknowledge the financial support
from Science, Industry, Trade and Information Technology
Commission of Shenzhen Municipality: JC201005270281A(ZP),
ZYB200907080082A(BJ), JC200903160366A(BJ).
15 Esters could also be formed under similar gold-catalyzed oxidation
conditions, see: (a) A. Abad, P. Cpncepcion, A. Corma and
H. Garcia, Angew. Chem., Int. Ed., 2005, 44, 4066;
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Chem. Commun., 2012, 48, 6235–6237 6237