Heck reactions of iodobenzene and methyl acrylate with conventional supported palladium catalysts in the presence of organic and/or inorganic bases without ligands

Article abstract of DOI:10.1002/(SICI)1521-3765(20000303)6:5<843::AID-CHEM843>3.0.CO;2-G

The vinylation of iodobenzene with methyl acrylate has been studied with several supported palladium catalysts in N-methylpyrrolidone in the presence of triethylamine and/or sodium carbonate. The reaction can be performed in air without any solubilizing or activating ligands. It was found that significant amounts of palladium leach out into the solvent and these dissolved Pd species essentially catalyze the reaction. It is interesting, however, that almost all the palladium species in the solution can redeposit onto the surface of the supports after the reaction has been completed (at 100% conversion of iodobenzene). Thus, the catalysts were recyclable without loss of activity. The use of both inorganic and organic bases is very effective in the promotion of the palladium redeposition as well as in the enhancement of the reaction rate. For Heck reactions with bromobenzene and chlorobenzene it was found that the use of triethylamine and sodium carbonate increases the selectivity of the Heck coupling product (benzene is also produced for these two substrates), but the mixed bases do not affect the overall rate of reaction as much.

Full text of DOI:10.1002/(SICI)1521-3765(20000303)6:5<843::AID-CHEM843>3.0.CO;2-G

FULL PAPER
Heck Reactions of Iodobenzene and Methyl Acrylate with Conventional
Supported Palladium Catalysts in the Presence of Organic and/or Inorganic
Bases without Ligands
Fengyu Zhao, Bhalchandra M. Bhanage, Masayuki Shirai, and Masahiko Arai*[a]
Abstract: The vinylation of iodoben-
zene with methyl acrylate has been
studied with several supported palladi-
um catalysts in N-methylpyrrolidone in
the presence of triethylamine and/or
sodium carbonate. The reaction can be
performed in air without any solubiliz-
ing or activating ligands. It was found
that significant amounts of palladium
leach out into the solvent and these
dissolved Pd species essentially catalyze
the reaction. It is interesting, however,
that almost all the palladium species in
the solution can redeposit onto the
surface of the supports after the reaction
has been completed (at 100% conver-
sion of iodobenzene). Thus, the catalysts
were recyclable without loss of activity.
The use of both inorganic and organic
bases is very effective in the promotion
of the palladium redeposition as well as
in the enhancement of the reaction rate.
For Heck reactions with bromobenzene
and chlorobenzene it was found that the
use of triethylamine and sodium carbo-
nate increases the selectivity of the Heck
coupling product (benzene is also pro-
duced for these two substrates), but the
mixed bases do not affect the overall
rate of reaction as much.
Keywords: base effects ´ catalysts ´
Heck vinylation ´ leaching ´ palla-
dium
important problem of the dissolution of palladium species
into the solvent (leaching). Djakovich and Koehler have very
recently given quantitative data for this palladium leaching
from zeolite-supported palladium catalysts;[6i] however, no
other significant data has been reported. Augustine and
OꢀLeary showed that in their Heck arylation, which used a
carbon-supported palladium catalyst, the same reaction
products were obtained as those with commonly used
homogeneous catalysts.[6f] Shmidt and Mametova used a
silica-supported palladium catalyst for Heck reactions and
indicated that the reaction proceeded in the liquid phase.[6k]
Mehnert et al. have recently prepared a new type of hetero-
geneous palladium catalyst on mesoporous MCM-41 material,
and it has been shown to be active and recyclable for Heck
reactions.[6j] It is still felt, however, that a more complete
investigation should be carried out on the usefulness of
conventional supported metal catalysts for Heck reactions.
The preparation of these catalysts is facile and some catalysts
are commercially available; therefore, their use would be
appreciated if they were effective Heck catalysts.
Introduction
The Heck reaction has received considerable attention in
recent years as it offers a versatile method for the generation
of new carbon ± carbon bonds.[1] The reaction is normally
carried out with a palladium catalyst, a phosphine ligand, and
a base in a homogeneous mode of operation. However, the
reaction suffers from severe limitations that have so far
precluded its wide-spread industrial application.[2] Relatively
large amounts of catalysts are needed for reasonable con-
versions and catalyst recycling is often hampered by the early
precipitation of palladium black. A few approaches have been
described to improve homogeneous catalyst systems by means
of sterically hindered phosphine ligands,[3a, 3b] a large excess of
coordinating ligands,[3a, 3c] and high-pressure conditions.[3d, 3e]
In view of the practical and industrial applications, the use
of heterogeneous catalysts may be promising. There are
several reports on the use of free colloidal palladium
particles[4] and palladium catalysts dispersed on various
supporting materials in the presence[5] and absence[6] of
ligands. The latter catalysts may be concerned with an
In the present work, we have used several ordinary
supported palladium catalysts without any ligands for the
Heck vinylation of iodobenzene with methyl acrylate in N-
methylpyrrolidone (NMP). The reactions were conducted
under various conditions to examine the factors that affect the
rate of reaction as well as the extent of palladium leaching. It
was found that the supported palladium leaches out into the
[a] Dr. M. Arai, F. Zhao, Dr. B. M. Bhanage, Dr. M. Shirai
Institute for Chemical Reaction Science
Tohoku University
Katahira, Aoba-ku, Sendai 980-8577 (Japan)
Fax : (‡ 81)22-217-5631
E-mail: marai@icrs.tohoku.ac.jp
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M. Arai et al.
FULL PAPER
Table 1. Supported Pd catalysts used in the present work.[a]
solvent and that the reaction is mainly catalyzed with these
dissolved palladium species. It is interesting to note, however,
that the dissolved palladium species can redeposit onto the
support after completion of the reaction so that the catalysts
are recyclable without loss of activity. It is also interesting that
the use of a mixed base of sodium carbonate and triethyl-
amine not only increased the rate of reaction but also
promoted the redeposition of the palladium species. To our
knowledge, no work has yet been reported on the usefulness
of such mixed bases for Heck reactions. Brief studies have also
been made to compare the less reactive bromo- and chloro-
substrates with iodobenzene.
Catalyst
Surface area
of support [m2 g
Reduction
temperature [8C] of Pd dispersion
Degree
1
]
1 wt% Pd/SiO2
10 wt% Pd/C
1 wt% Pd/C
300
893
1181
570
450
200
200
450
0.91 (1.0 nm)[b]
0.18 (5.0 nm)[b]
0.54 (1.7 nm)[b]
0.85 (1.1 nm)[b]
1 wt% Pd/SM(Mg)
[a] The reduction was carried out for 3 h at the given temperatures. [b] Estimates
of Pd particle sizes from an approximate expression of diameter (in nm) ˆ 0.9/
(degree of metal dispersion).[13]
Results and Discussion
The chief experiments were carried out with iodobenzene and
the results on its vinylation with methyl acrylate are discussed
in detail in the following. The results with bromo- and chloro-
substrates are also reported briefly. The catalysts used in the
present work are given in Table1.
Figure 1. Time-conversion profiles of Heck reactions for reduced, cal-
cined, or untreated 1% Pd/SiO2 samples. Reaction conditions: 758C,
catalyst 0.20 g (including 0.0188 mmol Pd), NMP 30 mL, iodobenzene and
methyl acrylate 5 mmol each, triethylamine 5 mmol.
Reaction profiles and palladium leaching: The overall rate of
the Heck vinylation of iodobenzene with methyl acrylate as
well as the leaching of palladium have been found to depend
on several factors. Under the
conditions used, trans-methyl
cinnamate was produced in
100% selectivity. Figure 1
shows time-conversion curves
at 758C for 1% Pd/SiO2 sam-
ples that were either reduced
with H2 at 4508C for 3 h, cal-
cined in air at 4508C for 3 h, or
were not treated before the
reaction. The reduced sample
was more active than the other
two samples, which had similar
activities. Figure 2 shows the
conversion, the amount of Pd
that leached out into the NMP
solvent, and the amount of Pd
remaining on the support dur-
ing the reaction. For the sam-
ples examined, the rate of re-
action increased as the amount
of leached Pd increased. This
indicates that the Heck reac-
tions are mainly homogeneous-
ly catalyzed by Pd species dis-
solved in the solution, in ac-
cordance with the previous
result of Shmidt and Mameto-
va.[6k] Marreto-Rosa et al. stud-
ied the carbonylation of allyl
Figure 2. The conversion (left) and amount of Pd leached out into NMP or remaining on the support (right) as a
function of time for a) untreated, b) reduced, and c) calcined 1% Pd/SiO2 samples. The amounts of Pd on the
support are relative values with respect to the initial amounts of Pd present in the catalysts. Reaction conditions:
758C, catalyst 0.20 g (including 0.0188 mmol Pd), NMP 30 mL, iodobenzene and methyl acrylate 5 mmol each,
triethylamine 5 mmol.
ethers with a charcoal-support-
ed Pd catalyst.[7] They believed
that the reaction proceeded
through homogeneous catalytic
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Chem. Eur. J. 2000, 6, No. 5

Heck Reactions
843±848
Table 2. Effects of bases on the activity and Pd leaching of supported Pd catalysts.[a]
Catalyst
Amount
Base
Time [h]
Conversion [%]
Pd leached
Pd on support
into NMP [ppm]
after reaction [%]
1% Pd/SiO2
1% Pd/SiO2
1% Pd/SiO2
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
1% Pd/C
1% Pd/SM(Mg)
0.15 g (0.0141 mmol Pd)
0.15 g (0.0141 mmol Pd)
0.15 g (0.0141 mmol Pd)
0.08 g (0.0752 mmol Pd)
0.08 g (0.0752 mmol Pd)
0.08 g (0.0752 mmol Pd)
0.08 g (0.0752 mmol Pd)
0.15 g (0.0141 mmol Pd)
0.15 g (0.0141 mmol Pd)
TEA
Na2CO3
TEA/Na2CO3
TEA
Na2CO3
1
4
100
100
100
100
47
100
100
100
100
23
34
4
0.49
0.24
0.91
9.6
7.5
9.6
9.8
0.91
0.87
0.5
0.5
0.5
4
0.5
0.5
0.5
9
60
10
4
4
6
Na2CO3
TEA/Na2CO3
TEA/Na2CO3
TEA/Na2CO3
[a] Pd/SiO2 and Pd/SM(Mg) were reduced at 4508C for 3 h and Pd/C at 2008C for 3 h before the reaction; reaction conditions: 1408C, NMP (30 mL),
iodobenzene (5 mmol), methyl acrylate (5 mmol), triethylamine (TEA, 5 mmol), sodium carbonate (5 mmol). When both bases were used, the amounts of
TEA and sodium carbonate were 5.0 mmol and 5.0 mmol, respectively.
Table 3. Effect of inorganic (sodium carbonate ) and organic (triethylamine) bases on the Pd leaching of 10% Pd/C catalyst.[a]
Inorganic base [mmol]
Organic base [mmol]
Time [h]
Conversion [%]
Pd leached
Pd on support
initial
final
into NMP [ppm]
after reaction [ppm]
5.0
5.0
2.5
2.5
2.5
1.25
1.25
0
2.5
2.5
0
0
0
0
0
0
0
0
4.0
0.5
0.5
0.5
0.5
1.0
0.5
0.5
0.5
100
100
100
100
100
80
100
100
100
10
4
4
4
4
60
6
4
9.6
9.8
9.8
9.8
9.8
7.5
9.8
9.8
9.7
5.0
1.0
2.5
5.0
2.3
3.0
1.0
7.5
0
8
[a] Reaction conditions: 1408C, catalyst 0.08 g (including 0.0752 mmol Pd), NMP (30 mL), iodobenzene (5 mmol), and methyl acrylate (5 mmol).
pathways. For the allylation of aniline with allyl acetate with a
Pd/C catalyst and triphenylphosphine, Bergbreiter and Chen
also reported that the reaction was catalyzed by soluble
palladium complexes.[8] Furthermore, Figure 2 also shows that
the reduced Pd species can leach out more easily to result in a
higher rate of reaction, compared with oxidized Pd species on
the calcined and untreated samples. It can be seen from
Figure 2 that the amount of Pd dissolved in the solution
decreased at higher conversion levels, which indicates the
redeposition of Pd on the surface of SiO2. Similar trends were
also observed with the other catalysts.
Therefore, it can be stated that Pd is leached from the
support during the reaction and it can redeposit onto the
surface of the support after completion of the reaction.
Similar redeposition of Pd has recently been observed by
Jayasree et al. for the synthesis of 2-arylpropionic acids by
carbonylation with a supported Pd catalyst in the presence of
phosphine ligands.[9]
The influence of the ratio of organic:inorganic bases on Pd
leaching was examined with 10% Pd/C catalyst. Table 3 gives
the levels of Pd leaching into the solvent and that remaining
on the support after 100% conversion of iodobenzene at
1408C. When the two bases were used together, sodium
carbonate was mainly consumed and triethylamine had no
effect on the Pd leaching, which was 4 ppm. When the amount
of sodium carbonate was 1.25 mmol, smaller than that of the
substrates, the level of Pd leaching was somewhat larger,
6 ppm. Table 4 shows the influence of the amount of
substrates at constant amount of the two bases at 1408C.
The amount of Pd dissolved in the solution was very high
when unreacted iodobenzene remained. In contrast, it is lower
by one order of magnitude when iodobenzene was completely
consumed and it was independent of the amount of methyl
acrylate.
Palladium leaching and redeposition: The amount of Pd
which has leached into the solvent and that which has
remained on support after the completion of Heck reactions
are given in Table 2 for several catalysts at 1408C. For 1% Pd/
SiO2 and 10% Pd/C catalysts, the use of a mixed base of
triethylamine and sodium carbonate was very effective in
reducing the amount of Pd remaining in the solution. The
residual amount of Pd in the solution decreased from 23 or
34 ppm (for TEA and Na2CO3 alone, respectively) to 4 ppm
(mixed base) and from 9 or 10 ppm (for TEA and Na2CO3
alone, respectively) to 4 ppm (mixed base) for the former and
latter catalysts, respectively. That is, for 1% Pd/SiO2 and 10%
Pd/C catalysts, 91% and 98% of the initial metal loading can
be recovered, respectively. For the other two catalysts, the
levels of Pd leaching were also found to be very low when
triethylamine and sodium carbonate were used together. The
Pd leaching and redeposition may depend on the state of Pd
dispersion (particle size and metal ± support interactions) and/
or the surface properties of support.
Effects of temperature and supports on Pd redeposition:
Figure 3 shows the changes of the concentration of Pd
dissolved in the solution and the amount of Pd on the support
as a function of time at different temperatures for the 10%
Pd/C catalyst. Some time was needed to heat the reaction
mixture, and the Heck reaction proceeded during the heating.
The reaction was complete in 30 min after the reaction
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M. Arai et al.
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Table 4. Effects of the substrate concentrations on the Pd leaching of 10% Pd/C catalyst.[a]
Iodobenzene [mmol]
initial final
Methyl acrylate [mmol]
Iodobenzene
conversion [%]
Pd leached
into NMP [ppm]
Pd on support
after reaction [%]
initial
final
5.0 3.5
1.2
2.0
6.0
10.0
10.0
21.0
0
0
1.0
5.0
0
34
40
100
100
100
100
50
30
4
5
5
8.0
8.8
9.8
9.8
9.8
9.8
5.0
5.0
3.0
0
5.0
0
10.0
20.0
0
0
1.0
4
[a] Reaction conditions: 1408C, catalyst 0.08 g (including 0.0752 mmol Pd), NMP (30 mLI, triethylamine (1 mmol), sodium carbonate (2.5 mmol), reaction
time 30 min.
Figure 3. The changes in the concentration of Pd species in NMP (open
symbols) and the amount of Pd present on support (solid symbols) as a
function of time at different temperatures for the 10% Pd/C catalyst. The
amounts of Pd on the support are relative values with respect to the initial
amount of Pd present in the catalyst. Reaction conditions: catalyst 0.08 g
(including 0.0752 mmol Pd), NMP 30 mL, iodobenzene and methyl
acrylate 5 mmol each, triethylamine 1 mmol, sodium carbonate 2.5 mmol.
mixture reached the desired temperatures of 90, 110, and
1608C. The time given in Figure 3 is that which had elapsed
after the desired temperatures were attained. The residual
amount of Pd in the solution at 30 min was small and it
decreased more rapidly to a very low level when the temper-
ature was higher, that is, the redeposition of Pd on the support
is faster at higher temperatures.
In Figure 4, three supports are compared with respect to the
Figure 4. The changes in the concentration of Pd species in NMP (solid
line) and the amount of Pd present on the support (dashed line) as a
function of time at 1608C for Pd catalysts on different supports: a) 10% Pd/
C; b) 1%Pd/SiO2; c) 1%Pd/SM(Mg). The amounts of Pd on the support
are relative values with respect to the initial amounts of Pd present in the
catalysts. Reaction conditions: 10% Pd/C catalyst 0.08 g (including
0.0752 mmol Pd), 1% Pd/SiO2 and 1% Pd/SM(Mg) 0.15 g (including
0.0141 mmol Pd), NMP 30 mL, iodobenzene and methyl acrylate 5 mmol
each, triethylamine 1 mmol, sodium carbonate 2.5 mmol.
redeposition of Pd at 1608C. The redeposition depends on the
nature of supports used, and it proceeds more rapidly on the
carbon support than the others. Previously, Augustine and
OꢀLeary used several supporting materials for Pd and they
indicated the effect of their acid/base properties on the
regioselectivity for the Heck arylation of n-butyl vinyl ether
and benzoyl chlorides.[6g] Djakovich and Koehler reported the
influence of the type of zeolite on the leaching of Pd.[6i]
important from a practical point of view. The ordinary
supported Pd catalysts are thermally stable at 1608C or
above, at which temperature active organometallic Pd com-
plexes cannot be used.
Catalyst recycling: The same 10% Pd/C catalyst was used for
several reactions, each of which was conducted up to 100%
conversion under the following conditions: catalyst 0.08 g,
iodobenzene and methyl acrylate 5 mmol each, triethylamine
1 mmol, sodium carbonate 2.5 mmol, NMP 30 mL, temper-
ature 1608C, time 30 min. After each run, the reaction
mixture was centrifuged and the liquid mixture was decanted.
The residual catalyst was again mixed with fresh solvent,
substrates, and bases. The catalyst could be reused for four
runs while retaining the same activity and selectivity as well as
without loss of the active Pd species. These results are
Nature of the reaction processes: The present Heck reaction
system is homogeneous even though the heterogeneous
catalysts are used, in accordance with the previous result of
Shmidt and Mametova.[6k] The results observed may be
explained in part by a homogeneous Heck reaction mecha-
nism.[1] The reaction is catalyzed mainly with the Pd species in
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Chem. Eur. J. 2000, 6, No. 5

Heck Reactions
843±848
the liquid phase. It is easier for the Pd from the H2-reduced
catalyst to leach into the solvent compared with the unre-
duced and calcined catalysts (Figures 1 and 2). In addition,
active Pd0 species may be more easily formed in the solution
with the first catalyst than with the last two. The NMP solvent
and triethylamine could be involved in the formation of such
active species. For homogeneous Heck reactions in NMP, the
rate of reaction is higher in the absence of a triphenylphos-
phine ligand than in the presence of this ligand. This suggests
the formation of active Pd0 complexes with the NMP
molecules.[10] Triethylamine is a reducing agent for the
formation of active Pd0 species, and thus increases the rate
of reaction. The aryl halide can react with Pd to form a [Pd ±
PhI] complex; this would also promote Pd leaching.
After the completion of the reaction, the Pd species can
redeposit onto the supports and the catalysts are recyclable.
The residual amount of Pd species in the solvent is much less
when triethylamine and sodium carbonate are used at the
same time. The latter is mainly involved in the Heck reaction
while the former enhances the rate of reaction (Table 2). The
Pd redeposition is influenced by triethylamine in the solution
while it is not influenced by sodium carbonate. Interactions
may exist between triethylamine and dissolved Pd species to
produce some complexes which prevent the redeposition of
Pd onto the support. In addition, there is a possibility that the
base adduct (HEt3NI) formed is adsorbed by the supports[6f]
and then hinders the Pd redeposition. The amount of Pd
species in the solution is large when iodobenzene is present.
The formation of a [Pd ± PhI] complex would also hinder the
redeposition of the Pd species.
Unfortunately, for these substrates, the dominating reaction
was dehalogenation so that the main product was benzene.
Although the selectivity of the Heck coupling product is low,
the presence of sodium carbonate did increase the yield. The
amounts of Pd which leached into the solvent were larger for
the mixed bases and sodium carbonate than those for
triethylamine, for which the Pd in the solution is very low so
that there was very little Heck coupling. Thus, it can be
assummed that the Heck coupling is homogeneously cata-
lyzed by the dissolved Pd species, while the dehalogenation
reaction occurs on the surface of the Pd particles. The results
given in Table 5 indicate that the overall rate of reaction is not
low, but further efforts are needed to improve the Heck
selectivity. The effects of different combinations of other
inorganic bases (such as potassium carbonate and caesium
carbonate) and triethylamine have been investigated in our
laboratory. The basic reaction mechanism would be the same
for the three substrates, but the relative rates of the
elementary steps involved may depend on the kinds of
substrate and base used. This may result in the differences in
the effects of the mixed bases for iodo-, bromo-, and chloro-
substrates. In their recent work on a heterogeneous Pd
catalyst, Mehnert et al. have reported that the Heck coupling
selectivity is 82% at 39% conversion for bromobenzene with
styrene and 40% at 16% conversion for chlorobenzene with
n-butyl acrylate at 1708C.[6j]
Conclusions
The following conclusions are drawn from the present results
on the Heck reactions of iodobenzene and methyl acrylate in
NMP with organic and inorganic bases with supported Pd
catalysts without any ligands in air (points 1 ± 7) as well as for
the Heck reactions with bromobenzene and chlorobenzene
(points 8 and 9).
1) The reaction is homogeneous and catalyzed by Pd species
dissolved in NMP under the reaction conditions used.
2) The reduced Pd species are more easily leached out into
the solvent compared with the oxidized species. This
results in a higher rate of reaction.
Application to bromo and chloro substrates: For practical
purposes, it is important to use the less reactive but cheaper
bromo- and chloro-substrates, rather than iodo-substrates.
Efforts are still being made towards the activation of these
substrates in homogeneous[11] and heterogeneous[6j] catalytic
reactions. The supported Pd catalysts are stable at high
temperatures, which may be necessary for Heck reactions of
these less reactive substrates. Therefore, we briefly examined
the effect of the organic and inorganic mixed bases on the
Heck vinylation reaction of bromobenzene and chloroben-
zene with methyl acrylate at 1608C (Table 5). For either
bromobenzene or chlorobenzene, the overall rate of reaction
is promoted by the use of the mixed bases, similar to the case
of iodobenzene; however, this effect is not so remarkable.
3) The Pd species in solution can redeposit onto the supports
only after the reaction is complete.
4) The Pd redeposition is promoted by the use of a mixed
base of triethylamine and sodium carbonate.
Table 5. Results of Heck reactions of bromobenzene/chlorobenzene and methyl acrylate with 10% Pd/C catalyst.[a]
Substrate
Base
Time [h]
Conversion [%]
Selectivity[b] [%]
Pd leached
Pd on support
into NMP [ppm]
after reaction [%]
bromobenzene
TEA
TEA/Na2CO3
Na2CO3
TEA
TEA/Na2CO3
Na2CO3
12
12
12
18
18
18
75
96
88
55
69
59
7
14
22
0
3
4
1
40
40
2
42
42
9.9
8.4
8.4
9.9
8.3
8.3
chlorobenzene
[a] The catalyst was reduced at 2008C for 3 h before reaction. Reaction conditions: 1608C, catalyst: 0.08 g (including 0.0752 mmol Pd), NMP (30 mL),
bromobenzene/chlorobenzene (5 mmol), methyl acrylate (5 mmol), triethylamine (TEA, 5 mmol), sodium carbonate (5 mmol). When both the bases were
used, the amounts of TEA and sodium carbonate were 1.0 mmol and 2.5 mmol, respectively.[b] Selectivity ˆ methyl cinnamate/(methyl cinnamate ‡
benzene).
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M. Arai et al.
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5) The Pd redeposition depends on temperature and support
used.
6) The catalysts, in particular 10 wt%Pd/C, can be effectively
recycled without loss of Pd species.
Acknowledgements
The authors appreciate the financial support from Japanese Society for the
Promotion of Science for BMB to join this research.
7) The supported catalysts are thermally stable and therefore
reactions can be conducted at the relatively high temper-
ature of 1608C.
8) The use of the mixed bases does not change the overall rate
of reaction very much.
9) The presence of sodium carbonate increases the selectivity
of the Heck coupling product (for these two substrates,
dehalogenation predominantly occurs to produce more
benzene than methyl cinnamate).
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Experimental Section
Catalyst: A commercial sample of 10% Pd on activated carbon was
purchased from Wako Pure Chemicals Industries. Other 1% Pd catalysts
were prepared by ion exchange, with [Pd(NH3)4Cl2] as the precursor, with
SiO2 (Aldrich Chemical Company, Davisil grade 646, 35 ± 60 mesh),
activated carbon (Nippon Chromato Works, 80 ± 100 mesh), and a magne-
sium-containing smectite-like porous material (denoted by SM(Mg))
prepared by a hydrothermal method in our laboratory.[12a,b] The Pd/C
samples were reduced under a flow of hydrogen at 2008C for 3 h or at
4508C for 3 h; these reduced samples were stored in a desiccator. The
details of preparation procedures for these catalysts is described else-
where.[12a,c] The surface areas of the supports were determined by the BET
nitrogen adsorption method. The areas of exposed palladium atoms were
measured by hydrogen adsorption at room temperature (Table 1).
Heck reactions: The Heck reactions were carried out in a 100 ml autoclave
under an ambient atmosphere. Commercial (Wako Pure Chem. Ind.)
iodobenzene (or bromobenzene, chlorobenzene), methyl acrylate, NMP
solvent, triethylamine, and sodium carbonate were used. The reaction was
monitored at various time intervals by analyzing the reaction mixture with
a gas chromatograph (GC, Yanaco G3800, Silicon OV-1, 6 m) equipped
with a flame ionization detector. Iodobenzene, methyl acrylate, and
triethylamine were mixed with NMP solvent (30 mL) and a small quantity
of this mixture was first analyzed by GC. The liquid mixture was placed in
the autoclave and then the catalyst and sodium carbonate (if necessary)
were added. The reaction mixture was heated to the desired reaction
temperature, while vigorously agitating with a magnetic stirrer. During the
reaction, small quantities of the reaction mixture were sampled and
analyzed by GC. The conversion was determined from the amount of
iodobenzene consumed and error limits for the %-conversion data were
Æ3.0% on account of the reproducibility of the data for repeated runs.
After the reaction was complete, the mixture was left to stand for ꢀ15 min.
The solid materials precipitated and the liquid phase, which included NMP,
triethylamine, substrates, products, and HI-base adducts, was separated by
decantation. The residual solid mixture (including catalyst and sodium
carbonate) and a small quantity of liquid were washed with acetone a few
times and then dried in air. The catalyst and inorganic base were separated
by size, the former being fine powder, and the weight of the residual sodium
carbonate was measured. Within experimental error, the weight of the
catalyst did not change after the reaction. The amount of Pd leaching out
into the solution was measured by atomic absorption spectroscopy. Under
the conditions used, the concentrations of Pd determined included errors of
less than Æ0.5 ppm.
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Received: June 11, 1999 [F1846]
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