Entangled palladium nanoparticles in resin plugs

Article abstract of DOI:10.1039/b711978j

Palladium nanoparticles were entrapped within resin plugs and used in a range of ligand-free cross-coupling reactions; the convenient modular format of the resin plug enhanced resin handling and allowed the catalysts to be easily recovered and multiply reused. The Royal Society of Chemistry.

Full text of DOI:10.1039/b711978j

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Entangled palladium nanoparticles in resin plugs{
Romain Najman,a Jin Ku Cho,a Andrew F. Coffey,b John W. Daviesb and Mark Bradley*a
Received (in Cambridge, UK) 3rd August 2007, Accepted 11th September 2007
First published as an Advance Article on the web 4th October 2007
DOI: 10.1039/b711978j
with succinyl chloride (Scheme 1). Controlling the amount of
Palladium nanoparticles were entrapped within resin plugs and
used in a range of ligand-free cross-coupling reactions; the
convenient modular format of the resin plug enhanced resin
handling and allowed the catalysts to be easily recovered and
multiply reused.
Pd(OAc)2 added allowed Pd-resin plugs with various metal
loadings to be prepared (0.8 to 98 mmol/plug). Palladium levels
in the plugs were determined by inductively coupled plasma atomic
emission spectroscopy (ICP-AES), see ESI for details.{
Analysis of the resulting plugs (Fig. 1) revealed a core-shell
structure, with black resin beads loaded with palladium being
located predominantly at the edges of the plug [Fig. 1(b)]. Within
the beads themselves [Fig. 1(c) and 1(d)] microscopic analysis
showed a palladium nanoparticle gradient with tiny nanoparticles
observed at the edge of the beads [Fig. 1(e) and 1(f)], presumably
due to rapid reduction at the edge. Transmission electron
microscopy (TEM) gave an average size for the palladium particles
as approximately 7 nm at the edge, moving to progressively larger
nanoparticles towards the middle of the bead itself (approximately
25–70 nm), where various ‘‘shapes’’ (e.g. triangular, cubic,
rhomboidal, diamond, trapezium, pentagon) were observed for
the palladium particles [Fig. 1(g–m)], suggesting that the polymer
scaffolds could offer an interesting approach to the control of
nanoparticle architectures. The thickness of the whole palladium-
filled layer inside a bead was around 30 mm.
The interest in transition metal-catalysed reactions in organic
synthesis and specifically in palladium-catalysed cross-couplings
continues to grow.1 Considering both environmental and
economic issues, there is a need for palladium catalysts that can
be easily recovered, generate highly pure products and that
generate few catalyst-based contaminants. Over recent years efforts
have been expended in the area of palladium immobilisation, with
a number of insoluble supports developed to address the concerns
of recovery and reusability. Traditionally, palladium metal has
been adsorbed onto inorganic supports such as silica or alumina,2
but more recently polymer-supported palladium catalysts have
been devised via immobilisation of a ligand and subsequent metal
coordination to form an anchored complex.3 However, this
technique involves the use of expensive ligands and problems of
leaching associated with these supported homogeneous catalysts
have been reported. Other methods have been developed by
Akiyama and Kobayashi4 and Ley et al.5 to encapsulate palladium
catalysts by ‘‘polymer incarceration’’ and interfacial polymerisa-
tion respectively. Pre-formed resin beads offer an attractive
alternative, with advantages associated with their commercial
availability and mechanical stability as well as a substantial
knowledge associated with their use. Following studies on
polymer-supported catalysts,6 we wish to report here the
entanglement of palladium nanoparticles into resin plugs (resin
beads sintered and embedded within inert high-density polyethyl-
ene to give a modular cylindrical support)7 and the catalytic
activity of these ‘‘cross-linked(XL)-Pd plugs’’ in Suzuki–Miyaura,8
Sonogashira–Hagihara9 and Heck–Mizoroki reactions.10
The ability of the palladium-loaded plugs to catalyse C–C bond
forming reactions and their recyclability were assessed in 3 major
classes of couplings, i.e. the Suzuki–Miyaura, Sonogashira–
Hagihara and Heck–Mizoroki reactions (Table 1). The results
show that resin plug-entangled palladium nanoparticles were good
catalysts for the three cross-coupling reactions, giving good yields
and purities. Moreover, they could be reused multiple times with
no major loss of activity, with recovery by simple filtration and
washing and with no precautions taken to prevent exposure to
oxygen or moisture. Their efficiency in cross-coupling reactions
was further investigated using a broader range of substrates of
varying electron density (Table 2).{
The palladium-captured cross-linked-resin plugs were readily
prepared via a 3-step procedure:11 (1) palladium loading (achieved
by heating a mixture of the Polymer Laboratories aminomethyl-
styrene resin plugs and palladium acetate in 1,4-dioxane at 80 uC);
(2) palladium nanoparticle generation by 10% hydrazine-mediated
reduction in 1,4-dioxane and (3) cross-linking and entanglement
aSchool of Chemistry, The University of Edinburgh, The King’s
Buildings, West Mains Road, Edinburgh, UK EH9 3JJ.
E-mail: Mark.Bradley@ed.ac.uk; Fax: +44 (0)131 650 6453;
Tel: +44 (0)131 650 4820
bPolymer Laboratories Ltd., Essex Road, Church Stretton, UK
SY6 6AX. Fax: +44 (0)1694 722 171; Tel: +44 (0)1694 723 581
{ Electronic supplementary information (ESI) available: Detailed experi-
mental procedures of XL-Pd resin plug preparation and TEM analysis,
Suzuki–Miyaura, Sonogashira–Hagihara and Heck–Mizoroki couplings,
characterisation data of the products. See DOI: 10.1039/b711978j
Scheme 1 Synthesis of XL-Pd resin plugs.
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Table 1 Cross-linked-Pd plugs in Suzuki, Sonogashira and Heck
cross-couplings and their recyclability
Recycle
1
2
3
4
Isolated yield (%)
Suzukia
92
73
92
96
74
83
87
73
81
92
72
Sonogashiraa
Heckb
79c
a
Reaction conditions: substrate [X = Br, R = NO2] (1.0 mmol),
phenylboronic acid or phenylacetylene (1.5 mmol), K2CO3
(3.0 mmol), XL-Pd plug (9.8 mol%), DMF (4 mL), air, 80 uC.
b
Reaction conditions: substrate [X = I, R = H] (1.0 mmol), styrene
(1.5 mmol), K2CO3 (1.5 mmol), XL-Pd plug (0.08 mol%), DMF
c
(5 mL), air, 115 uC. The amount of Pd in solution at the 5th run
was determined by ICP-AES to be 0.37 ppm.
Table 2 Pd plugs catalytic activity in Suzuki, Sonogashira and Heck
cross-couplings of aryl iodidesa
Isolated yield (%)
Entry
X
R
Suzukib
Sonogashirab
Heckc
1
2
3
4
5
a
I
I
I
I
I
OMe
CF3
NO2
Me
91
81
99
91
86
77
87
92
88
85
91
95
97
97
95
H
Reaction conditions: aryl iodide (1.0 mmol), coupling partner
(1.5 mmol), K2CO3 (1.5 mmol), XL-Pd plug, DMF (5 mL), air,
b
115 uC. Phenylboronic acid or phenylacetylene, 2.8 mol% of XL-
Pd plug. Styrene, 0.6 mol% of XL-Pd plug.
c
Fig. 1 (a) Optical image of a cross-linked (XL)-Pd containing resin plug;
(b) optical image of the top of a XL-Pd resin plug; (c) optical microscope
image (low magnification) of a single bead from within the plug that has
been sliced; (d–m) TEM images showing: (d). the interior of a sliced bead;
(e) outer layer (640 000); (f) magnified view of outer layer (6400 000);
(g) inner layer (640 000); (h) magnified view (6400 000) of inner layer
showing triangular; (i) cubic; (j) rhomboidal; (k) diamond; (l) trapezium;
and (m) pentagon shapes of palladium particles.
Table 3 Various Heck reactions with XL-Pd plugsa
Entry
X
R
R9
Isolated yield (%)
1
2
3
4
5
6
7
a
I
I
I
I
I
I
Br
H
H
Ph
92
97
93
79
97
91
4
C5H4N
CO2Me
CO2 Bu
C5H4N
Ph
Ph
Results were very consistent with reactions achieved with high
yields and purities regardless of the substituents on the aromatic
ring and again with all reactions run in air. Pd resin plugs with low
Pd loadings were evaluated in Heck–Mizoroki reactions using
substituted alkenes and haloarenes as summarised in Table 3.
Substrates such as 4-iodoanisole underwent coupling reactions
with acrylic esters (Table 3, entries 3 and 4) and aryl alkenes
(Table 3, entries 5 and 6) in excellent yields, however the
corresponding bromo derivatives proved difficult to couple to
phenylacetylene under these conditions (Table 3, entry 7).
OMe
OMe
OMe
OMe
OMe
t
Reaction conditions: aryl halide (1.0 mmol), alkene (1.5 mmol),
K2CO3 (1.5 mmol), XL-Pd plug (0.08 mol%), DMF (5 mL), air,
115 uC.
preparation of the nanoparticles also led to some unusual shapes
of palladium nanoparticle such as trapeziums and pentagons,
presumably as the result of isolation mediated by the polymer
within the resin bead during nanoparticle growth.
We would like to thank the EPSRC for funding and Dr Anton
Page (Southampton General Hospital) for providing microscopic
analysis.
In summary, palladium nanoparticles, entangled in a convenient
modular resin format, were prepared. The resin plug-supported Pd
catalysts were successfully employed to perform Suzuki–Miyaura,
Sonogashira–Hagihara and Heck–Mizoroki cross-couplings with-
out the need of any precautions to avoid oxygen exposure and
could be recycled without a major drop in efficiency. The
5032 | Chem. Commun., 2007, 5031–5033
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M. L. Kantam and B. Sreedhar, J. Am. Chem. Soc., 2002, 124,
14127.
Notes and references
3 (a) F.-T. Luo, C. Xue, S.-L. Ko, Y.-D. Shao, C.-J. Wu and Y.-M. Kuo,
Tetrahedron, 2005, 61, 6040; (b) Y. Uozumi and Y. Nakai, Org. Lett.,
2002, 4, 2997; (c) C. A. McNamara, F. King and M. Bradley,
Tetrahedron Lett., 2004, 45, 8239.
4 R. Akiyama and S. Kobayashi, J. Am. Chem. Soc., 2003, 125, 3412.
5 C. Ramarao, S. V. Ley, S. C. Smith, I. M. Shirley and N. DeAlmeida,
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8 (a) N. Miyaura, T. Yanagi and A. Suzuki, Synth. Commun., 1981, 11,
513; (b) F. Bellina, A. Carpita and R. Rossi, Synthesis, 2004, 2419.
9 (a) K. Sonogashira, Y. Thoda and N. Hagihara, Tetrahedron Lett.,
1975, 16, 4467; (b) R. R. Tykwinski, Angew. Chem., Int. Ed., 2003, 42,
1566.
{ Typical procedure for palladium-catalysed chemistries using XL-Pd resin
plug. Synthesis of 4-methoxybiphenyl via Suzuki–Miyaura cross-coupling. To
a solution of 4-methoxyphenyl iodide (1.0 mmol) in DMF (5 mL) was
added phenylboronic acid (183 mg, 1.5 mmol, 1.5 equiv.), K2CO3 (207 mg,
1.5 mmol, 1.5 equiv.) and 1 Pd plug catalyst (30 mmol, 3 mol%). The
reaction mixture was heated at 115 uC under magnetic stirring. The Pd plug
was removed from solution with tweezers, washed with DCM (10 6 6 mL)
and dried in the vacuum oven at room temperature for 48 h. The reaction
mixture was diluted in DCM (100 mL) and washed with 1 M HCl (pH 7).
The aqueous phase was extracted with DCM (3 6 50 mL). The combined
organic phases were dried over MgSO4, filtered and concentrated under
reduced pressure. The residue was purified by column chromatography on
silica gel to yield the final product as a white solid (167 mg, 91%).
1 For reviews, see: (a) K. C. Nicolaou, P. G. Bulger and D. Sarlah,
Angew. Chem., Int. Ed., 2005, 44, 4442; E. Negishi and L. Anastasia,
Chem. Rev., 2003, 103, 1979; (b) C. A. McNamara, M. J. Dixon and
M. Bradley, Chem. Rev., 2002, 102, 3275.
2 (a) C. M. Crudden, M. Sateesh and R. Lewis, J. Am. Chem. Soc.,
2005, 127, 10045; (b) B. M. Choudary, S. Madhi, N. S. Chowdari,
10 (a) T. Mizoroki, K. Mori and A. Ozaki, Bull. Chem. Soc. Jpn., 1971, 44,
581; (b) R. F. Heck and J. P. Nolley, Jr., J. Org. Chem., 1972, 37, 2320;
(c) I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009.
11 J. K. Cho, R. Najman, T. W. Dean, O. Ichihara, C. Muller and
M. Bradley, J. Am. Chem. Soc., 2006, 128, 6276.
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