Palladium N-methylimidazolium supported complexes as efficient catalysts for the Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2006.02.023

Different N-methylimidazolium supported ligands have been easily synthesized. The palladium complexes derived from those materials can be used for the catalysis of the Heck reaction giving excellent yields, selectivities and very good TON and TOF values. The supported Pd-pincer complexes show an increased stability, and provide a clear improvement in the recovery and reuse for the supported catalysts.

Full text of DOI:10.1016/j.tetlet.2006.02.023

Tetrahedron Letters 47 (2006) 2311–2314
Palladium N-methylimidazolium supported complexes as
efficient catalysts for the Heck reaction
´
Belen Altava, M. Isabel Burguete, Eduardo Garcıa-Verdugo, Naima Karbass,
Santiago V. Luis,* Amrit Puzary and Victor Sans
Departamento de Quı´mica Inorga´nica y Orga´nica, UAMOA, Universidad Jaume I/CSIC, Campus del Riu Sec,
Avenida Sos Baynat, s/n, E-12071 Castello´n, Spain
Received 5 December 2005; revised 2 February 2006; accepted 3 February 2006
Available online 20 February 2006
Abstract—Different N-methylimidazolium supported ligands have been easily synthesized. The palladium complexes derived from
those materials can be used for the catalysis of the Heck reaction giving excellent yields, selectivities and very good TON and TOF
values. The supported Pd-pincer complexes show an increased stability, and provide a clear improvement in the recovery and reuse
for the supported catalysts.
Ó 2006 Elsevier Ltd. All rights reserved.
Soluble and insoluble polymer-supported catalysts have
received much attention in organic cross-coupling reac-
tions due to the industrial interest of the resulting prod-
ucts and the need for the search of recoverable catalysts
with long-term stability.1 Although homogeneous cata-
lysts often have higher activities in such transforma-
tions, heterogeneous catalysts possess significant
advantages from a practical point of view as well as in
terms of sustainable green chemistry for the ease of
product purification and the potential for recycling.
Stable transition metal carbene complexes are gaining
importance in organic transformations since its discov-
ery.4 Thus for instance, the metal complexes of N-het-
erocyclic carbene (NHC) have been reported as a class
of moisture- and air-stable catalysts with remarkable
activity in carbon–carbon bond forming reactions.5
The strong metal–carbon bond in such complexes pre-
vents the possible dissociation of the metal centre, there-
by making them thermally and oxidatively stable. Thus
the NHC complexes are potential candidates to be used
as catalysts for many important coupling reactions such
as the Heck and Suzuki coupling reactions.6
The Heck coupling reaction is one of the most impor-
tant methods for carbon–carbon bond formation in gen-
eral organic synthesis, synthesis of intermediates for
pharmaceuticals and in the preparation of conducting
polymers.2 Palladium species have largely been used as
catalysts for C–C bond forming reactions in allylic sub-
stitutions and Heck coupling of aryl halides with ole-
fins.3 In such reactions, the recovery and reuse of the
catalyst is highly important from an economic point of
view, due to the high cost of palladium compounds,
but also for the strict limitations to the palladium con-
tent in final products, particularly for pharmaceutical
applications.
In order to provide the potential for easy recycling, the
synthesis of analogous heterogeneous systems is impor-
tant. Progress towards this goal include the preparation
and use of NHC metal complexes supported either on
inorganic or organic supports.7–10 Here we report on
the preparation of polymer-supported methylimidazo-
lium salts prepared via the covalent anchoring of
1-methylimidazolium moieties onto PS-DVB gel type
resins. The catalytic behaviour of the corresponding
resin-bound Pd-complexes was investigated for the Heck
reaction.
The synthesis of the supported ligands was accom-
plished according to Scheme 1. The reaction of a Merri-
field resin (1% DVB-cross-linked, 1.19 mmol Cl/g) with
neat methylimidazole led to 1 in quantitative yields. The
transformation was followed using different analytical
Keywords: Supported catalysts; Polymer; Heck reaction; Facilitated
chemical synthesis.
*
Corresponding author. Tel.: +34 964728239; fax: +34 964728214;
e-mail: luiss@qio.uji.es
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.02.023

2312
B. Altava et al. / Tetrahedron Letters 47 (2006) 2311–2314
v)
i)
X-
Cl-
OH
N
N
N
Cl
N
N
5
OH
-
1
2a X-=BF4
ii)
-
2b X-=CF3SO3
-
2c X-=SbF6
iv)
Cl-
N
i)
Cl-
N
CO2Me
CO2Me
N
N
iii)
iv),i)
N
O
Cl-
N
Cl-
N
O
N
N
6
3
4
Scheme 1. Reagents and conditions: (i) methylimidazole, 90 °C; (ii) dimethyl 5-hydroxyisophthalate NaH, tBu4NI, THF, reflux, 48 h; (iii) LiBH4 in
THF, B(OMe)3, THF, reflux, 24 h; (iv) SOCl2, CH2Cl2, reflux, 3.5 h; (v) diethanolamine, DMF, Et3N.
tools, in particular, FT-Raman spectroscopy and the
NBP test.11,12 The combined use of both techniques
allowed us to observe that the reaction time required
to achieve a quantitative conversion of the C–Cl groups
is much shorter than previously reported for related sys-
tems (30 min vs 12–78 h).9 Polymers 2a–c were prepared
by exchange of the chloride anion with NaBF4,
CF3SO3H and NaSbF6, respectively.13 The pincer
ligands were obtained by initial reaction of a Merrifield
resin with dimethyl 5-hydroxyisophthalate or diethanol-
amine to give 3 and 5, respectively. The diester 3 was
converted into the corresponding diol by treatment with
lithium borohydride and the diol was easily transformed
into the dichloride by reaction with thionyl chloride.
The reaction of dichloride with N-methyl imidazole
afforded the di-imidazolium salt 4. A similar procedure
allowed us to prepare the modified resin 6. The poly-
mer-supported Pd species were synthesized according
to the literature procedures by heating the correspond-
ing resin-bound imidazolium salt with Pd(OAc)2 in the
presence of sodium carbonate for 2 h in DMF/H2O
(1:1, v/v) at 50 °C.9a,10 Table 1 summarizes the function-
alization degrees either for the imidazolium salts (EA) or
the Pd complexes (ICP-MS). The pincer ligand showed a
ratio NHC/Pd of ca. 2:1, while monodentate ligands led
to ratios ranging from 2 to 8 depending on the nature of
the anions.
X
R
R'
Base
X-/Pd
+
R'
R
N
N
Scheme 2.
At 130 °C the conversion is complete within 15–20 min
with only 0.02 mol % of catalyst loading, while the reac-
tion at 90 °C needs relatively longer reaction times for
completion using the same substrate/catalyst ratio. No
other differences related to the change in temperature
were found.
Figure 1 shows a graph of the yield versus time for the
reactions catalyzed by the corresponding Pd-supported
complexes (0.02 mol %) derived from 1, 2a–c and
Pd(OAc)2 at 90 °C in DMF using NEt3 as the base.
The reaction was highly selective for all the catalysts,
leading to the trans-methyl cinnamate isomer exclu-
sively. The reaction rates for the supported catalysts
were slower than that of the homogeneous reaction cat-
alyzed by Pd(OAc)2. One of the most notable features
observed was the strong influence of the nature of the
anion (see Fig. 1). The reaction rates for Pd-polymers
The reaction of iodobenzene with methyl acrylate
(Scheme 2, R = H, R0 = CO2Me) was selected as the
benchmark Heck reaction to be tested with the different
Pd-supported systems.14 The reactions were initially per-
formed at two different temperatures, 90 and 130 °C, in
the presence of air and without prior solvent distillation.
100
80
60
40
20
0
Table 1. Functionalization degrees of the supported ligands and
catalysts
Entry Polymer meq. NHC/ga meq. Pd/gb meq. NHC/
meq. Pd
1
2
3
4
5
6
1
0.93
0.89
0.84
0.83
0.84
1.40
0.17
0.30
0.40
0.10
0.41
0.71
5.5
3.0
2.1
8.3
2.0
1.9
2a
2b
2c
4
0
50
100
150
time (min)
6
Figure 1. Yields of trans-methylcinnamate obtained for catalysts
derived from
(0.02%).
a Determined by elemental analysis.
b Determined by ICP-MS.
1,
2a,
2b,
2c,
Pd(OAc)2

B. Altava et al. / Tetrahedron Letters 47 (2006) 2311–2314
2313
ꢀ
2c and 2a (SbF6 and BF4ꢀ) were faster than those ob-
Polymers 4 and 6 were designed to form more stable and
robust Pd-pincer complexes.15 Catalysts derived from
bi-dentate ligands 4 and 6 showed lower reaction rates
than the Pd-1 catalyst and longer induction periods
(Fig. 2). Nevertheless, a significant improvement in sta-
bility was achieved with the catalyst derived from 4. This
catalyst was effective for at least five cycles without los-
ing any activity and leading to a TON value of 25,000
(Table 3).
served for Pd-1 and Pd-2b. This can be related to the rela-
tive coordination ability of the different anions and
with the formation and stability of diverse complex
forms in the presence of these contraanions. Similar
effects have been reported for homogeneous carbene
complexes in ionic liquids.16 The presence of different
anions could either stabilize the complex or give rise to
the formation of precipitated palladium particles.
The influence of the catalyst/substrate ratio was also
studied (Table 2). Polymer 2c proved to be an efficient
catalysts leading to good TON (11,600) and TOF
(12,000) values even for very low catalysts/substrate
ratio (0.005 mol % Pd). The participation of Pd species
in a solution is revealed by the significant leaching
observed when the reactions were carried out at 90 °C.
For polymer 2c (see Table 3) the first and second run
The study of the Pd complex prepared from the sup-
ported pincer ligand 4 as a catalyst for the Heck reaction
was extended to include different substrates. Results are
summarized in Table 4. The reaction of iodobenzene
with different alkenes led to excellent yields in all cases
(see entries 1–3 in Table 4) using Et3N as the base and
DMF as the solvent. A much lower reaction rate was
observed in the absence of DMF, using a solventless
Et3N/PhI (2:1) medium (see entry 11).
produced
a
quantitative transformation of the
substrates, but conversion decreased dramatically after
the third use.
Reaction of p-bromoacetophenone and p-nitrobromo-
benzene with methyl acrylate gave the corresponding
product in 66% and 100% yield, respectively (entries 9
Table 2. Effect of catalyst/substrate ratio
Entry
Polymer
Pd (mol %)
TONa
TOFb
100
80
60
40
20
0
1
2
3
2c
2c
2c
0.02
0.008
0.005
4550
9750
11600
4800
9750
12000
a TON after 1h, all experiments were carried out in parallel, using three
different new flasks and stir bars.
b TOF calculated for 80% conversion.
Table 3. Stability study for supported Pd catalysts
Entry Polymer
Yield (%)
Run 1 Run 2 Run 3 Run 4 Run 5
1
2
3
2ca
6b
4b
99
99
99
99
99
99
20
51
99
12
—
99
—
—
99
0
40
80
120
160
200
time (min)
a 90 °C.
Figure 2. Yield (%) for trans-methylcinnamate obtained using catalysts
derived from resins 1, 4, 6.
b 130 °C.
Table 4. Results obtained for the Heck reaction (Scheme 2) with different substrates using the Pd catalyst derived from 4
Entry
R
X
R0
Polymer
Solvent
Base
T (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
H
H
H
I
I
I
Ph
CO2Me
CN
4
4
4
4
4
4
4
4
4
4
4
DMF
DMF
DMF
Et3N
Et3N
Et3N
90
90
90
24
2.5
24
48
18
2.5
24
24
24
24
2.5
100a
100a
70a,b
0
100c
100c,d
100c
28c
66e
0
10f
OCH3
NO2
NO2
NO2
COCH3
COCH3
H
Br
Br
Br
Br
Br
Br
Br
I
CO2Me
CO2Me
CO2Me
Ph
CO2Me
CO2Me
CO2Me
CO2Me
DMF/H2O (5:1)
DMF/H2O (5:1)
DMF/H2O (5:1)
DMF/H2O (5:1)
DMF/H2O (5:1)
DMF/H2O (8:1)
DMF/H2O (5:1)
—
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
Et3N
130
130
130
130
130
130
130
90
9
10
11
H
a Yields calculated by GC and 1H NMR.
b Selectivity 85:15 trans/cis.
c Yield calculated by 1H NMR.
d 0.2% catalyst was used.
e Isolated yield.
f Solventless reaction 2:1 Et3N/PhI.

2314
B. Altava et al. / Tetrahedron Letters 47 (2006) 2311–2314
T.; Cetinkaya, B. Appl. Organometal. Chem. 2005, 19,
and 5). A quantitative transformation was also achieved
with p-nitrobromobenzene and styrene (entry 7). When
the less active bromobenzene and p-metoxybromoben-
zene were used as the substrates, only traces of product
were observed (entries 4 and 9).
´
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In summary, we have prepared polymer-supported
methyl imidazolium salts that can be used as efficient
recoverable Pd-supported catalysts for the Heck reac-
tion. The product recovery and purification is simple.
Furthermore, dry solvents or inert atmosphere were
not required to achieve excellent TON and TOF values.
The supported pincer complex derived from 4 showed a
very good activity being recyclable without any loss of
activity for at least five catalytic cycles.
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Acknowledgements
´
11. Altava, B.; Burguete, M. I.; Garcıa-Verdugo, E.; Luis,
S. V.; Vicent, M. J. Tetrahedron 2001, 8675–8683.
12. The NBP test is a sensitive colorimetric method developed
by our group for the analysis of the presence of chlorine in
chloromethylated polymers, either in gel-type (Merrifield)
or monolithic resins (Macroporous). The resin is immersed
Financial support from the Spanish Ministerio de Cien-
´
cia y Tecnologıa (CTQ 2005-08016) Bancaixa-UJI
(P1B2004-13) and Generalitat Valenciana (GRUPOS
04/031) is gratefully acknowledged. E.G. and V.S. thank
in
a solution of NBP [4-(4-nitrobenzyl)pyridine] in
´
MCYT for personal financial support (Ramon y Cajal
CH2Cl2/DMF with 5% triethylamine (TEA). Depending
on the loading of the chloride in the resin, the polymer
develops a colour ranging from pink to violet. Galindo, F.;
Altava, B.; Burguete, M. I.; Gavara, R.; Luis, S. V. J.
Comb. Chem. 2004, 6, 859–861.
and PPQ programs).
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acetone/H2O (for NaBF4 or NaSbF6) or CH2Cl2 (for
CF3SO3H). Then, 3 equiv of the correspondent salt/acid
were added, and the suspension was stirred for 48 h at rt.
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washed with water. Finally, the resulting polymer was
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