COMMUNICATION
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Sustainable Mizoroki–Heck reaction in water: remarkably high activity
of Pd(OAc) immobilized on reversed phase silica gel with the aid of an
ionic liquid
2
a
Hisahiro Hagiwara,* Yoshitaka Sugawara, Takashi Hoshi and Toshio Suzuki
a
b
b
Received (in Cambridge, UK) 22nd February 2005, Accepted 18th April 2005
First published as an Advance Article on the web 5th May 2005
DOI: 10.1039/b502528a
Palladium acetate immobilized on reversed phase amorphous
silica gel with the aid of an ionic liquid, [bmim]PF , was highly
powder as well as pellets were effective in water and exhibited the
same catalytic activity (entries 4 and 5). Compared to the reaction
in n-dodecane, the reaction was apparently accelerated in water,
since the reaction completed at a lower reaction temperature in a
6
efficient in the promotion of the Mizoroki–Heck reaction in
pure water without a ligand up to the sixth re-use, in 95%
3
21
shorter period of time.
average yield with TON and TOF 1,600,000 and 71,000 (h ),
respectively.
However an attempt to recycle the catalyst resulted in the
unexpected removal of the ionic liquid layer from the silica gel into
the aqueous layer to result in leaching of the Pd. Water entered
between the ionic liquid layer and the silica gel surface, probably
due to hydrophilic nature of the latter.
The Mizoroki–Heck reaction is undoubtedly one of the most
powerful metal-catalyzed C–C bond forming reactions. Tremen-
dous amounts of results have been accumulated and new findings
1
are still increasing. One of the current interests in this reaction is
In order to solve this problem, we focused on immobilization of
the development of a catalyst of high performance as well as
sustainable and environmentally benign reaction conditions.
We previously reported on the immobilization of Pd/C in an
2
Pd(OAc) on reversed phase silica gels, among which hexylated
(HEX), aminopropylated (NAP), and N,N-diethylaminopropyl-
ated silica gel (NDEAP) were employed (Scheme 1). These silica
ionic liquid, [bmim]PF , for a recyclable Mizoroki–Heck reaction
6
2
system. Subsequently reported was the very facile and economic-
gels were easily obtained by grafting with the corresponding silane
7
coupling reagents. Pd(OAc)
was successfully immobilized on
2
ally friendly immobilization of Pd(OAc)
2
in silica gel pores with the
. The catalyst immobilized on the
silica gel was highly effective in n-dodecane at 150 uC to realize a
5% average yield and TON 68,000 up to the 6th use.
The hydrophobic nature of [bmim]PF led us to investigate the
each of the reversed phase silica gels except HEX, with the aid
of [bmim]PF according to the same procedure as reported
3
aid of an ionic liquid, [bmim]PF
6
6
3
21
previously. The amount of Pd loading was 0.4 y 0.5 mmol g
by weight gain.
9
Reactivity of these immobilized catalysts was tested in the
reaction of iodobenzene and cyclohexyl acrylate and representative
results were compiled in Table 2. The reaction was sluggish, as
shown in entries 1 and 2, compared to the catalyst immobilized on
normal phase silica gel (entries 4 and 5, Table 1), and NDEAP-Pd
gave a better result. The less bulky amino group of NAP-Pd may
coordinate to the Pd to block the active site more than the
N,N-diethylamino group of NDEAP-Pd. As expected, sustain-
ability of the immobilized catalyst was improved as shown in the
recycle use in entries 3 and 4. The catalyst was easily re-used after
decantation of the upper layer followed by washing with diethyl
6
4
Mizoroki–Heck reaction in water as a solvent, employing the
same immobilized catalyst. The major advantage in carrying out
the reaction in water is its non-flammable, inexpensive, non-toxic
nature. There is no need to desiccate substrates prior to the
reaction. Other advantages are the high cohesive energy density
2
3
(
c.e.d. 5 550 cal cm ), high dielectric constant and high internal
pressure, which might facilitate bimolecular reactions involving
5
ionic intermediates. Furthermore, use of a heterogeneous catalyst
is expected to increase Mizoroki–Heck reaction rates by adsorp-
tion of substrates on the catalyst.
Initially, an optimum base was investigated at 100 uC in the
reaction of iodobenzene and cyclohexyl acrylate employing
Table 1 Examination of base in the reaction of iodobenzene and
cyclohexyl acrylate in the presence of immobilized catalyst on normal
phase silica gel
Pd(OAc) immobilized on normal phase silica gel with the aid of
2
[bmim]PF . Soon after heating, the colour of the catalyst turned
6
a
e
from brown to black, which suggested the formation of Pd(0). The
Entry
Base
Time (h)
Yield (%)
reaction began as a triphasic system and terminated as a biphasic
system with the aqueous and organic layers homogeneous. The
product was partitioned into diethyl ether, purified by medium
pressure LC and identified to be the trans isomer by NMR
spectroscopy. Inorganic bases gave lower yields as shown in
b
1
K
K
3
Et N
n-Bu
n-Bu
2
CO
PO
3
18
2
5.5
4
46
32
2
100
97
c
d
2
3
4
c
3
b
4
3
N
N
c
5
3
3
a
The reaction was carried out with 0.05 equiv. of the catalyst and
Table 1, entries 1 and 2. Especially, silica gel pellets dissolved in 2 h
6
when K PO4 was employed (entry 2). Among organic bases,
b
2
equiv. of base in water at 100 uC. The catalyst immobilized on
21
c
3
silica gel powder (Pd loading: 0.25 mmol g ) was used. The
1
2
n-Bu N gave better results. The catalysts immobilized on silica gel
3
catalyst immobilized on silica gel pellets (Pd loading: 0.2 mmol g )
d e
was used. The silica gel pellets dissolved. Isolated yields of pure
product based on iodobenzene.
*hagiwara@gs.niigata-u.ac.jp
2
942 | Chem. Commun., 2005, 2942–2944
This journal is ß The Royal Society of Chemistry 2005
Hagiwara, Hisahiro
