A water-compatible, highly active and reusable PEG-coated mesoporous silica-supported palladium complex and its application in Suzuki coupling reactions

Article abstract of DOI:10.1039/b605733k

A highly active heterogeneous palladium catalyst was prepared from coated mesoporous materials which contain a layer of readily available PEG with a labile coordinating ability for palladium. The aqueous suspension of the catalyst may be reused several times by simple extraction of the aqueous reaction mixture with ether to remove the product. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605733k

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A water-compatible, highly active and reusable PEG-coated
mesoporous silica-supported palladium complex and its application in
Suzuki coupling reactions{{
ab
Qing Yang, Shengming Ma,* Jixue Li, Fengshou Xiao* and Hai Xiong
a
b
b
a
Received (in Cambridge, UK) 16th December 2005, Accepted 3rd February 2006
First published as an Advance Article on the web 4th May 2006
DOI: 10.1039/b605733k
A highly active heterogeneous palladium catalyst was prepared
from coated mesoporous materials which contain a layer of
readily available PEG with a labile coordinating ability for
palladium. The aqueous suspension of the catalyst may be
reused several times by simple extraction of the aqueous
reaction mixture with ether to remove the product.
silica-supported palladium catalyst, which has shown high activity
in Suzuki coupling reactions in water.
As shown in Scheme 1, tetra(ethylene glycol) (1) was allylated in
the presence of KOH to afford alcohol 2, which was followed by
etherification with EtBr to afford allyl ether 3. Pt-catalyzed
hydrosilylation of 3 with HSi(OEt) afforded the organosilane 4,
3
which was used for grafting onto the surface of SBA-15 to support
the Pd catalyst.
One of the most successful stories of organometallic chemistry is its
application in homogeneous catalysis, demonstrating tunable
reactivity and selectivity due to the diversity of the metal(s) and
the ligand(s) in organometallic catalysts. However, after homo-
geneous organometallic catalysis, the final products are always
contaminated with metallic species due to the leaching of the
catalyst; this being a very serious problem in industry, especially
the pharmaceutical industry. Palladium-catalyzed coupling
Mesoporous silica SBA-15 was synthesized according to the
reported procedure using tri-block P123 as a template under acidic
2
4
conditions. After coating with organosilane 4, the mesoporous
2
3
3 4
silica 5 was treated with a 5 6 10 M solution of Pd(PPh ) in
THF with stirring. After 10 h, the mixture was filtered and washed
with THF to afford the supported catalyst 6 (Scheme 1). The Pd%
2
1
in this supported catalyst was 0.050 mmol g (0.53% by wt), as
1–3
reactions,
4–6
7
determined by ICP, and the P% in this supported catalyst was
Heck reactions,
Sonogashira couplings and
21
8
0.017 mmol g
(0.0522% by wt), as determined by high
Tsuji–Trost allylations have become very important organic
transformations in modern synthetic organic chemistry. However,
they usually suffer from the high loading of the palladium catalyst.
In most cases, up to 5 mol% of catalysts have to be used, which
may have a very serious negative impact on their possible
industrial applications. Thus, the development of supported
palladium catalysts showing a much higher catalytic activity
without leaching is highly desirable. Generally, Pd catalysts can be
successfully supported on organic materials such as core-shell
performance ion chromatography (HPIC), indicating that the
number of PPh units per Pd in the supported catalyst was very
3
limited (P/Pd = 1/3).
The transmission electron microscopy (TEM) images of catalyst
6, shown in Fig. 1, provide a direct observation of the morphology
and distribution of the palladium in the catalyst. Obviously, a
hexagonally packed pore structure was found in catalyst 6.
2
4
Comparing the TEM images of SBA-15 with those of 6, it can be
concluded that no bulk aggregation of the metal on the surface
could be found, indicating that the Pd was finely dispersed on the
walls of the PEG-coated SBA-15.
9
,10
11,12
block copolymers,
ligand-incorporated polymers
and den-
13
drimers. Pd catalysts can also be supported on inorganic
12,15
materials such as zeolites, carbon,
14
16
hydroxides, amorphous
17
18–22
The prepared catalyst was used to catalyze Suzuki coupling
silicates, and mesoporous silica.
23,24
mesoporous silica
We proposed that if
2
5
reactions that may have high industrial potential. Due to the
hydrophilic nature of PEG, we wished to consider the possibility of
conducting Suzuki coupling reactions in water. Among the bases
is coated with a layer of a molecule
containing functionalities (L) with labile coordinating ability, it
may be an excellent supporting material for any metallic complex
n
(MY ). In this paper, we wish to report the realization of such
a concept by the production of a highly active mesoporous
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai, 200032, P. R. China. E-mail: masm@mail.sioc.ac.cn;
Fax: (+86) 21-6416-7510
b
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun, 130012, P. R. China.
E-mail: fsxiao@mail.jlu.edu.cn; Fax: (+86) 431-5168590
{
procedures and characterization data of all new compounds. See DOI:
Electronic Supplementary Information (ESI) available: Experimental
1
{
0.1039/b605733k
This article is an updated version of DOI: 10.1039/b517862b which was
published as an Advance Article on the web on 22nd February 2006 and
retracted by the authors on 13th March 2006.
Scheme 1 Synthesis of mesoporous silica-supported catalyst 6.
Chem. Commun., 2006, 2495–2497 | 2495
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Table 2 Suzuki couplings catalyzed by SBA–Si–PEG–Pd(PPh
lower catalyst loadings
3
)
n
at
a
Catalyst (6)
loading (mol%) Time/h Yield (%)
b
Entry Halide (7)
1
2
3
4
o-NCC
o-HOC
o-HOC H I
6
H
4
Br
0.01
0.01
0.001
0.005
48
24
24
72
78 (9g)
6
H
4
I
98 (9e)
98 (9e)
66 (9c)
Fig. 1 TEM images of SBA–Si–PEG–Pd(PPh
3
)
n
6.
6
4
2 6 4
o-MeO CC H I
a
The reactions of 1.0 equiv. of 7, 1.2 equiv. of 8 and 2.0 equiv. of
tested (K
2
CO , CsCO , Et N, KOH and K PO ), K
3
3
3
3
4
3
PO
4
was the
b
K
3
PO
4
in H
2
O were conducted at 50 uC. Isolated yield.
best, affording 9a in 90% yield at 50 uC. When the reaction was
conducted at 30 uC, the yield of 9a was lower. No obvious
improvement was observed when the reaction was conducted at
a
3 n
Table 3 The reuse of SBA–Si–PEG–Pd(PPh ) 6
70 or 100 uC.
Thus, the scope of the Suzuki coupling reaction was studied by
conducting it in water at 50 uC using K PO as the base. Some
3
4
typical results are listed in Table 1. It can be concluded that (1) the
scope of the reaction is broad: both aryl halides and aryl boronic
acids with electron donating or withdrawing groups can be used;
Run
1
2
3
4
5
b
Yield (%)
Leaching of Pd
a
91
nd
92
nd
91
nd
91
nd
89
nd
(2) sterically hindered aryl halides and boronic acids can also be
c
used (Table 1, entries 4, 11 and 12); (3) in most of cases, the
reaction with 0.1 mol% catalyst can afford the products in fairly
good yields, the catalyst loading can even be reduced to 0.01 or
The reactions of 0.3 mmol of 7, 1.2 equiv. of 8, 2.0 equiv. of
PO and 1.0 mol% of 6 in H O (3 mL) were conducted at 50 uC
K
3
4
2
b
c
for 10 h. Isolated yield. After ether extraction to remove the
product, the ether was removed by evaporation, and the sample was
submitted to ICP for determination of the extent of leaching of Pd
to the organic layer. nd = not detected by ICP.
0
.001 mol% (Table 2); (4) even phenyl bromide (Table 1, entry 2)
26,27
and the very unreactive phenyl chloride with SPhos
entry 3) can be applied.
(Table 1,
It is well known that Pd(PPh ) is unstable, losing catalytic
3
4
a yield of 86% (0.01 mol% of Pd, 50 uC, 24 h). In comparison,
activity upon exposure to air. However, it was quite a surprise for
us to observe that the supported catalyst 6 is still effective even
after exposure to air for 6 weeks, and that the reaction can be
conducted in the open air without any obvious loss of activity with
when the Pd(PPh
coupling product 9a was formed in a much lower yield of 60%
1.5 mol% of Pd, 50 uC, 72 h).
3 4
) -catalyzed reaction was conducted in air, the
(
In addition, due to the hydrophilic nature of the supported
catalyst, after ether extraction to remove the product, the catalyst
in water could be reused without any obvious loss of the activity
Table 1 The SBA–Si–PEG–Pd(PPh
various substrates
3
)
n
-catalyzed Suzuki coupling of
a
(Table 3). Leaching of the Pd into the organic layer after the
reaction, which was determined by ICP, was not observed.
In conclusion, we have designed and prepared a new palladium
catalyst supported on PEG-modified mesoporous silica that is still
catalytically active after exposure to air for up to 6 weeks and
shows very high catalytic activity in Suzuki coupling reactions. Its
stability towards air may be due to the mesoporous structure. An
aqueous suspension of the catalyst may be reused several times by
simple extraction of the aqueous reaction mixture with ether to
remove the product. No leaching of the catalyst to the organic
layer was observed. Further studies in the application of this
catalyst to other organic reactions are now being conducted in our
laboratory.
1
Halide (7) R –X
2
b
Yield (%)
Entry
1
Boronic acid (8) R
C
C
C
6
H
6
H
6
H
5
5
5
I
Br
Cl
p-MeOC
p-MeOC
p-MeOC
p-MeOC
Ph
6
6
6
6
H
4
H
4
H
4
H
4
88 (9a)
87 (9a)
84 (9a)
86 (9b)
82 (9a)
95 (9c)
90 (9d)
98 (9e)
81 (9f)
83 (9g)
89 (9h)
89 (9i)
86 (9j)
c
2
d
3
4
e
2,6-Me
p-MeOC
o-MeO CC
(Z)-EtO CC
o-HOC
p-O NC
2
C
6
H
3
I
5
6
7
8
9
6
H
4
I
2
6
H
4
I
Ph
Ph
Ph
Ph
2
2
H
2
I
6
H
4
I
2
6
H
4
Br
1
0
1
2
3
o-NCC
o-NCC
6
H
4
Br
Br
Ph
1
1
1
6
H
4
o-MeOC
o-MeOC
6
H
H
4
o-MeO
o-MeO
2
CC
CC
6
6
H
H
4
I
I
6
4
Notes and references
f
2
4
3 6 4
p-CF C H
a
1 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
2
The reactions of 0.3 mmol of 7, 1.2 equiv. of 8, 2.0 equiv. of
PO and 0.1 mol% of 6 in H O (3 mL) were conducted at 50 uC
Metal-Catalyzed Cross-Coupling Reactions, ed. A. de Meijere and
F. Diederich, Wiley-VCH, Weinheim, 2004.
K
3
4
2
b
c
for 10 h. Isolated yield. Reaction was conducted at 90 uC for 10 h
using 2 equiv. of bromide with 0.5 mol% of 6. Reaction was
d
3 J. Tsuji, Palladium Reagents and Catalysts, Wiley, Chichester, 2nd edn,
004.
2
conducted using 2 equiv. of chloride at 60 uC for 10 h with 0.5 mol%
e
of 6 and 1 mol% of SPhos. 1 mol% of 6 and 2 mol% of PPh
4 R. F. Heck, Comprehensive Organic Synthesis, ed. B. M. Trost and
I. Fleming, Pergamon, Oxford, 1991, vol. 4, pp. 3881.
3
were
f
used. Reaction time 20 h.
5
A. W. Cabri and I. Candiani, Acc. Chem. Res., 1995, 28, 2.
2
496 | Chem. Commun., 2006, 2495–2497
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