Palladium supported on poly(N-vinylimidazole) or poly(N-vinylimidazole-co-N-vinylcaprolactam) as a new recyclable catalyst for the Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.jorganchem.2007.06.056

A new catalytic system based on the palladium supported on poly(N-vinylimidazole) or poly(N-vinylimidazole-co-N-vinylcaprolactam) was investigated. High efficiency of the catalyst along with its recycling ability was demonstrated in the Mizoroki-Heck reaction.

Full text of DOI:10.1016/j.jorganchem.2007.06.056

Journal of Organometallic Chemistry 692 (2007) 4402–4406
www.elsevier.com/locate/jorganchem
Note
Palladium supported on poly(N-vinylimidazole)
or poly(N-vinylimidazole-co-N-vinylcaprolactam) as a new
recyclable catalyst for the Mizoroki–Heck reaction
a,
b
c
c
*
Irina P. Beletskaya , Alexey R. Khokhlov , Elena A. Tarasenko , Vladimir S. Tyurin
a
Chemistry Department, M.V. Lomonosov Moscow State University, Leninskye Gory, 119992 Moscow, Russia
b
Physics Department, M.V. Lomonosov Moscow State University, Leninskye Gory, 119992 Moscow, Russia
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, 31 Leninsky Prospect, 119991 Moscow, Russia
c
Received 11 April 2007; received in revised form 24 June 2007; accepted 28 June 2007
Available online 4 July 2007
Abstract
A new catalytic system based on the palladium supported on poly(N-vinylimidazole) or poly(N-vinylimidazole-co-N-vinylcaprolac-
tam) was investigated. High efficiency of the catalyst along with its recycling ability was demonstrated in the Mizoroki–Heck reaction.
ꢀ
2007 Elsevier B.V. All rights reserved.
Keywords: Mizoroki–Heck reaction; Recycling the catalyst; Polymer supported catalyst; Palladium; Polyvinylimidazole
1
. Introduction
catalysts. The latter consists of metal or metal complex
immobilized on the surface of inorganic material [8,16–21]
or polymer [8,22–30]. Usually such catalysts are less active
and gradually lose their activity because of leaching. Major
advances were achieved by the application of soluble poly-
mers which possess groups capable of bonding metal. The
activity of these catalytic systems is similar to that of homo-
geneous transition metal complexes, and they can be easily
separated from products by changing temperature or sol-
vent [31–33]. Normally phosphine group binds to the metal
in such systems. Recently, N-donor molecules have been
applied as ligands in palladium catalyzed reactions [34–41]
because it allows to avoid toxic and readily oxidative unsta-
ble phosphines, and it has become possible to use this type
of bonding with a polymer matrix [42–46].
The Mizoroki–Heck reaction is one of the widely used
methods for the C–C bond formation, thus it is not surpris-
ing that the development of efficient catalysts allowing a
wide range of reagents to react goes on for many years
[
1–7]. The problem mostly concerned by researchers is to
combine high efficiency of the catalyst with the possibility
of its easy separation from the reaction products [8]. The
latter is especially important in the synthesis of biologically
active compounds. Recovering of the catalyst is not less
important taking into account high price of the palladium,
normally used in the reaction.
Homogeneous palladium complexes with phosphine
ligands [1,9] and ‘‘ligandless’’ complexes in the presence of
additives stabilizing palladium clusters [10–15] are usually
applied in the Mizoroki–Heck reaction. The major draw-
backs of homogeneous catalysts are difficult separation
from reaction products and impossibility of recycling.
These problems do not arise in the case of heterogeneous
2. Results and discussion
Here, we report a new catalytic system with PdCl2
bonded to polymers such as poly(N-vinylimidazole) (PVI)
or poly(N-vinylimidazole-co-N-vinylcaprolactam) (PVI-
PVC) (30 mol% content of the latter monomer in the
copolymer). The catalyst was investigated in a model
*
Corresponding author. Tel./fax: +7 095 939 3618.
E-mail address: beletska@org.chem.msu.ru (I.P. Beletskaya).
0
022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.06.056

I.P. Beletskaya et al. / Journal of Organometallic Chemistry 692 (2007) 4402–4406
4403
Table 1
Reaction of phenyl iodide with n-butyl acrylate
a
b
c,d
Yield (%)
Entry
Polymer
2
Molar ratio PdCl :polymer
Base
T (ꢁC)
Time (h)
c
1
2
3
4
5
6
7
8
PVI
PVI-PVC
PVI
PVI
PVI-PVC
PVI
PVI-PVC
PVI-PVC
PVI-PVC
PVI-PVC
1:5
1:5
1:5
1:5
1:5
1:10
1:10
1:50
1:5
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
120
120
120
100
100
120
120
120
120
120
2
2
24
8
8
4
4
17
2
35
99
d
c
99
c
39
c
93
c
5
d
97
68
d
3
c
9
n-Bu
3
N
99
e
c
1
0
a
b
c
1:5
K
2
CO
3
2
65
Molar ratio of the reagents PhI:n-butyl acrylate:base:‘‘Pd’’ = 1.0:1.2:2.0:0.01.
To monomeric unit.
1
By H NMR.
d
e
By GLC.
.1% ‘‘Pd’’.
0
(
entry 10), yield after 2 h at 120 ꢁC drops to 68%, but the
reaction remains stereoselective.
To clarify the morphology of the catalyst transmission,
electron microscopy (TEM) studies were carried out.
TEM images of the catalyst revealed the presence of palla-
dium nanoparticles of 8–10 nm size with the even distribu-
tion inside the polymer matrix. XRD pattern additionally
proved the existence of the palladium metal core in the par-
ticles (see Fig. 1).
Then we investigated the possibility of recycling the cat-
alyst. After the reaction ran to completion, the polymer
supported catalyst was precipitated by diethyl ether, sepa-
rated from the solution containing the product, washed
and used in the next reaction cycle with a new portion
of reagents. Five consecutive cycles of the reaction showed
that the catalyst did not lose its activity (Table 2) and
could be completely recycled. We did not observe the pal-
ladium black formation in the reaction even after the fifth
cycle.
Scheme 1.
Mizoroki–Heck reaction with a number of substrates of
different reactivity. The results of the reaction of phenyl
iodide with n-butyl acrylate in DMF in the presence of
1
mol% of ‘‘Pd’’ and K CO as a base are presented in
2 3
Table 1 (see Scheme 1).
Obviously the reaction proceeds faster using the copoly-
mer rather than the homopolymer. It completes after 2 h at
20 ꢁC to produce n-butyl trans-cinnamate in a quantitative
yield with the PVI-PVC copolymer-Pd system as the cata-
lyst and gives just 35% yield of the product with the homo-
polymer case (entries 1 and 2). Nevertheless prolonged time
of the reaction (24 h) allows to complete the reaction with
the latter catalyst (entry 3). Decreasing temperature
increases the reaction time (entries 4 and 5). Increasing
polymer/palladium ratio also increases the reaction time
1
The scope of applications of the catalytic system was
investigated in the reaction with a number of aryl iodides
and aryl bromides (see Scheme 2). In all cases the product
was formed in high yield and with good stereoselectivity
(Table 3). The reaction of n-butyl acrylate with such inac-
tive substrate as phenyl bromide need the temperature to
(
entries 6–8). The change of the base K CO to n-Bu N does
2 3 3
not influence the reaction (entry 9). Ten fold decrease of the
palladium content to 0.1 mol% slows down the reaction
2
Fig. 1. Transmission electron micrographs of the catalytic system based on PdCl : PVI–PVC = 1:5 after the reaction of phenyl iodide with n-butyl acrylate
(
Table 1, entry 2) showing palladium nanoparticles (a) and the structure of the polymeric matrix (b), XRD pattern (c).

4404
I.P. Beletskaya et al. / Journal of Organometallic Chemistry 692 (2007) 4402–4406
Table 2
a
Catalyst recycling in the reaction of PhI with n-butyl acrylate
b
Cycle
Yield of the product (%)
c
c
Molar ratio
Molar ratio
PdCl
2
:PVI-PVC = 1:5
2
PdCl :PVI-PVC = 1:10
1
2
3
4
5
a
99
99
91
94
98
97
99
96
92
92
Scheme 4.
Molar ratio of the reagents PhI:n-butyl acrylate:K
.0:1.2:2.0:0.01. Reaction conditions: DMF, 120 ꢁC, 2 h in the case of
PdCl :PVI-PVC = 1:5 and 4 h in the case of PdCl :PVI-PVC = 1:10.
2 3
CO :‘‘Pd’’ =
1
2
2
b
By GLC.
c
To monomeric unit.
Scheme 5.
ture and longer time are needed to obtain a reasonable
yield of trans-stilbene, while stereoselectivity becomes
lower due to the formation of the second product 1,1-diph-
Scheme 2.
enylethene. However, the addition of the n-Bu NBr
improves the result (see Scheme 4).
Surprisingly, the reaction of styrene with p-acetylphenyl
bromide, which is normally a less active substrate com-
pared to phenyl iodide, proceeds much smoother (see
Scheme 5).
4
Table 3
Reactions of n-butyl acrylate with aryl halides
a
b
ArX
Reaction time (h)
Yield of the product (%)
6
p-MeOC H
4
I
4
86
93
91
95
95
94
64
84
p-MeC
p-MeC(O)C
p-MeO CC
p-MeC(O)C
p-NCC Br
6
H
4
I
3
2
4
2
6
H
4
I
I
2
6
H
4
3. Conclusion
6
H
4
Br
6
H
4
2
C
C
C
6
6
6
H
H
H
5
5
5
Br
Br
Br (+100%
NBr)
8 (at 140 ꢁC)
24 (at 140 ꢁC)
8
In conclusion we investigated a new recyclable catalytic
system comprising palladium(II) chloride and poly(N-viny-
limidazole) and poly(N-vinylimidazole-co-N-vinylcaprolac-
tam). The high efficiency and stability of the system was
shown in the model Mizoroki–Heck reaction with a num-
ber of active (aryl iodides and n-butyl acrylate) and less
active (aryl bromides and styrene) substrates. The catalyst
can be used multiple times in repeating cycles and its activ-
ity was shown to be constant.
c
83
n-Bu
4
a
2 3 2
Molar ratio of the reagents ArX:n-butyl acrylate:K CO :PdCl :PVI-
PVC = 1.0:1.2:2.0:0.01:0.05. Reaction conditions: DMF, 120 ꢁC.
b
Isolated yield.
By H NMR.
c
1
be increased to 140 ꢁC, but the addition of tetrabutylam-
monium bromide allows to decrease the temperature back
to 120 ꢁC.
4. Experimental
The recycling of the catalyst in the reaction with aryl
bromide was investigated and no loss in activity was
noticed after five consecutive cycles (see Scheme 3).
The reaction with less active styrene proceeds consider-
ably slower compared to n-butyl acrylate. Higher tempera-
4
.1. General procedure
Most of reagents were supplied by Lancaster. Solvents
were purified by standard technique. All reactions were car-
ried out under argon atmosphere and were monitored by
thin-layer chromatography (TLC) using silica gel 60 F254
plates (Merck). Column chromatography was performed
on Merck silica gel 60 (0.040–0.063 mm) for the isolation
of products and on Alfa Aesar silica gel (60–200 nm) for
1
qualitative GLC and H NMR analysis samples.
GLC analysis was performed on chromatograph Hew-
lett Packard 5890 Series II Plus with capillary column
HP-1701 (15 m length, 0.32 mm diameter, 0.25 lm phase
layer thickness) using hexadecane as an internal standard.
Scheme 3.

I.P. Beletskaya et al. / Journal of Organometallic Chemistry 692 (2007) 4402–4406
4405
1
H NMR spectra were obtained using Bruker AMX-400
K CO (3.0 mmol), DMF (7 mL) were mixed in a reaction
2 3
1
spectrometer at 400 MHz in CDCl . Qualitative
NMR analysis was performed using dimethyl fumarate as
an internal standard.
H
flask and n-butyl acrylate or styrol (1.8 mmol) was added.
The reaction mixture was stirred at 120–140 ꢁC. After cool-
ing to ambient temperature, the reaction mixture was
poured in water (20 ml) and extracted with CH Cl
3
2
2
4
4
.2. Polymers preparation
(3 · 15 mL). The combined organic phases were dried over
Na SO , filtered and evaporated in vacuum. The products
2
4
.2.1. Poly(N-vinylimidazole)
Argon was bulbed through a solution of N-vinylimidaz-
were isolated by flash chromatography using petrol ether
or CH Cl or their mixture as eluents. The structure of
2
2
1
13
ole in water (the volumetric concentration of the monomer-
0%) and AIBN as the initiator (1%) for 20 min, then the
products was determined by H and C NMR.
3
reaction mixture was maintained during 140 h at 50 ꢁC.
After cooling to ambient temperature, the mixture was
diluted in 15 times with water, subjected to dialysis and lio-
philic drying. The yield was >80%. Molar mass was deter-
mined by the sedimentation analysis. MM = 75300.
Acknowledgements
This work was supported by Russian Academy of
Sciences (Program N 4 ‘‘Creation and investigation of
macromolecules and macromolecular structures of new
generations’’), US Civilian Research and Development
Foundation in cooperation with the Federal Agency of
Science and Innovation of the Russian Federation (joint
Grant RUC1-2802-MO-06). Authors thank Dr. Sergey
Abramchuk for acquiring TEM images, Dr. Alexander
Kashin for providing GLC.
4.2.2. Copolymer of N-vinylimidazole and
N-vinylcaprolactam (30%)
Argon was bulbed through a solution of monomers in
ethanol (the general volumetric concentration of the mono-
mers À30%) and AIBN as the initiator (0.1%) for 20 min,
then the reaction mixture was maintained during 48 h at
5
0 ꢁC. After cooling to ambient temperature, the mixture
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