Pd/MOx materials synthesized by sol-gel coprecipitation as catalysts for carbon-carbon coupling reactions of aryl bromides and chlorides

Article abstract of DOI:10.1002/ejic.200800846

In order to control the interaction of palladium species with oxide supports, a sol-gel coprecipitation synthesis has been developed for Pd/MO _x. The resulting Pd/MO_x catalysts are characterized by a rather strong interaction between

Full text of DOI:10.1002/ejic.200800846

FULL PAPER
DOI: 10.1002/ejic.200800846
Pd/MO Materials Synthesized by Sol-Gel Coprecipitation as Catalysts for
x
Carbon–Carbon Coupling Reactions of Aryl Bromides and Chlorides
Wolfgang Kleist,[a][‡] Jae-Kyu Lee, and Klaus Köhler*[a]
[a]
Keywords: C–C Coupling / Heck reaction / Palladium / Sol-gel processes / Supported catalysts
In order to control the interaction of palladium species with
oxide supports, a sol-gel coprecipitation synthesis has been
active substrates like bromobenzene and aryl chlorides. In
this way we are able to synthesize supported catalysts that
facilitate a controlled release of soluble Pd species under re-
action conditions that are required for the activation of aryl
developed for Pd/MO
x x
. The resulting Pd/MO catalysts are
characterized by a rather strong interaction between the
2
+
highly dispersed Pd species and the alumina and silica lat- bromides and chlorides. The Pd/MO
x
catalysts show excel-
tices. During coupling reactions of the Heck type the catalyti- lent activity in the Heck reaction of bromobenzene (TON up
–
1
cally active species is generated by partial dissolution of Pd
from the support surface. Due to the strong bonding of Pd to
the support this dissolution occurs to significant extent only
at comparatively high temperatures (Ն160 °C). These tem-
to 10000; TOF up to 5000 h ) and 4-chloroacetophenone
–
1
(TON up to 9300; TOF up to 1550 h ).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
peratures are exactly necessary for the activation of less-re- Germany, 2009)
Introduction
in a high dispersion which facilitates an efficient in-situ re-
0
duction (Pd is required for the oxidative addition of the
A large number of detailed investigations on Heck-type aryl halide as the first step of the catalytic cycle) and dissol-
[
15]
coupling reactions (Scheme 1) in the presence of supported ution under typical reaction conditions.
Beside an opti-
catalysts within the last years led to an eminent improve- mized synthetic procedure for the catalyst material, espe-
[
1–5]
ment of the mechanistic understanding in this area.
could be shown independently by different groups that the catalytic reaction was found to be essential.
It cially the right choice of reaction conditions during the
reaction is catalyzed by soluble Pd species that are tempo-
It emerged to be crucial that the dissolution and re-pre-
rarily dissolved from the surface of the support.[
6–12]
In the cipitation equilibriums are controlled effectively at any time
original catalyst Pd has to be provided as an effective pre- during the catalytic reaction. Here it is important that the
cursor species from which the active form can be generated right amount of dissolved (= active) Pd species is provided
in situ. Optimized procedures for the synthesis of such in the reaction solution. While a too low Pd concentration
highly active Pd catalysts have been reported in the litera- might be not sufficient to achieve high reaction rates, ag-
[
13,14]
In general, these materials contain Pd2+ species glomeration (and consequently deactivation) processes
ture.
Scheme 1. Heck reaction of aryl halides and styrene.
might occur if too much Pd is leached from the catalyst
support simultaneously.
One of the most important parameters that influences
these dissolution/re-precipitation processes is the reaction
temperature. Higher reaction temperatures can increase the
total amount of dissolved Pd species, but they also acceler-
[
[
a] Department of Chemistry, Technische Universität München,
Lichtenbergstr. 4, 85747 Garching, Germany
Fax: +49-89-289-13183
E-mail: klaus.koehler@ch.tum.de
‡] Present Address: Department of Chemistry and Applied Biosci-
ences, Institute for Chemical and Bioengineering, ETH Zurich,
Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland
Eur. J. Inorg. Chem. 2009, 261–266
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
261

W. Kleist, J.-K. Lee, K. Köhler
FULL PAPER
ate deactivation of active species by Pd black formation. Al-1-100-red” and “Pd-Al-1-450-red”, etc.), the other was
Higher reaction temperatures might be needed, however, for used without further treatment. Detailed information on
the activation of certain substrates. While aryl iodides and the thermal treatment of the catalyst series can be found in
activated aryl bromides can be activated easily by a great Table 1.
variety of different Pd catalysts at reaction temperatures be-
low 100 °C (due to their lower C–X binding energies),
the activation of less reactive aryl bromides and especially
[
16,17]
Table 1. Nomenclature used and thermal treatments of the various
palladium-metal oxide catalysts prepared by combined sol-gel co-
precipitation. Samples denoted as “100” were dried in air atmo-
chlorides requires in general harsher reaction conditions sphere at a temperature of 100 °C for 2 h, those denoted as “450”
and higher temperatures.[ Consequently, an efficient cata- at a temperature of 450 °C for 6 h. All of these catalysts were di-
18]
vided again into two portions: one was used without further treat-
lyst for the activation of such less-reactive substrates should
2
ment; the other was treated in a H stream at 200 °C for 2 h (de-
noted as “red”).
facilitate a controlled release of the supported Pd into the
reaction solution at these higher reaction temperatures avo-
iding agglomeration and deactivation of Pd in solution. The
Catalyst
Thermal treatment
2
Reduction (H stream)
development of such materials being more robust and appli- Pd/Al-1-100
100 °C, 2 h
100 °C, 2 h
450 °C, 6 h
450 °C, 6 h
100 °C, 2 h
100 °C, 2 h
450 °C, 6 h
450 °C, 6 h
100 °C, 2 h
100 °C, 2 h
450 °C, 6 h
450 °C, 6 h
–
Pd/Al-1-100-red
200 °C, 2 h
–
200 °C, 2 h
–
200 °C, 2 h
–
200 °C, 2 h
–
cable for a broader range of experimental conditions in par-
Pd/Al-1-450
ticular for higher temperatures would be of particular inter-
Pd/Al-1-450-red
est for practical application of supported palladium cata-
lysts.
In this work we present a new strategy to cope with this
challenge. A sol-gel coprecipitation synthesis procedure has
been developed for metal oxide supported Pd catalysts. It
Pd/Al-3-100
Pd/Al-3-100-red
Pd/Al-3-450
Pd/Al-3-450-red
Pd/Si-100
Pd/Si-100-red
200 °C, 2 h
–
200 °C, 2 h
results in a stronger Pd-support interaction (compared to Pd/Si-450
conventionally prepared supported catalysts) and leaching Pd/Si-450-red
of Pd species into reaction solution starts at 160 °C only.
Such high temperatures have to be applied for an efficient
conversion of less-reactive substrates like bromobenzene Table 1) on the activity of the resulting catalysts was studied
The influence of these different reaction parameters (see
[
19]
and aryl chlorides.
Since leaching and stabilization of by comparing the catalytic activity of the different systems
dissolved Pd species against Pd black formation may be in- in the Heck reaction of bromobenzene and styrene. First,
fluenced by ammonium halides, a variety of these salts have the influence of the pre-treatment (atmosphere) and calci-
been tested as additives/promoters in the catalytic reaction nation temperature was investigated in order to get infor-
too.
mation on the role of the oxidation state of Pd (0 or +2).
For this purpose the four catalysts of the Pd/Al-1 series
were tested in the Heck coupling of bromobenzene and sty-
rene (Pd amount: 0.01 mol-% based on bromobenzene). Af-
ter 2 hours at 160 °C the reaction was stopped and the mix-
Results and Discussion
To achieve a strong Pd/support interaction, the Pd cata- tures were analyzed by GLC and GC/MS (Figure 1). The
lysts were prepared by combined sol-gel (metal alkoxides) best results were achieved with the unreduced catalyst
i
and coprecipitation (palladium salt) procedures. Al(O Pr) , calcined at 450 °C (Pd/Al-1-450; 99% conversion, 90%
3
which was used as the precursor for Pd/Al O catalysts, was yield of E-stilbene 3). The unreduced catalyst calcined at
2
3
dissolved in 2-propanol at elevated temperature (65 °C). 100 °C (Pd/Al-1-100; 65% conversion, 60% yield) and the
Precipitation of the metal oxide was induced by controlled reduced catalyst calcined at 450 °C (Pd/Al-1-450-red; 61%
addition of water and finally a solution of PdCl in hydro- conversion, 56% yield) provided similar results. Only the
2
chloric acid. After a reaction time of one and three days, reduced catalyst calcined at 100 °C (Pd/Al-1-100-red; 6%
respectively (see Exp. Sect.), two different Pd/Al O materi- conversion and yield) showed low activity.
2
3
0
2+
als (denoted as “Pd/Al-1” and “Pd/Al-3”) were obtained.
Although Pd is the catalytically active species, Pd on
In the case of Pd/SiO catalysts a different procedure had the catalyst surface exhibits a higher activity compared to
2
0
to be applied. A preformed gel, which was obtained by ad- Pd . This is explained by the fact that Pd species can be
dition of water to a solution of Si(OEt) in ethanol, was dissolved easier when a single (pre-reduced) Pd surface
0
4
dissolved again in 1-butanol. Coprecipitation of Pd/SiO2 atom is bound to a palladium oxide or hydroxide surface
0
(
“Pd-Si”) was then achieved by addition of a solution of compared to metallic Pd . Dissolution is then induced by
[
4,15]
PdCl in hydrochloric acid. Note that these different pro- an oxidative addition of an aryl halide molecule.
An-
2
cedures had to be applied in order to obtain an incorpora- other conclusion that can be drawn from this comparison
tion of the Pd into the outermost layers of the MO lattice is that higher calcination temperatures also lead to higher
x
for both types of materials. The raw materials were divided catalytic performance. Calcination at high temperatures
in two portions each which were calcined at 100 (“Pd/Al-1- causes a stronger interaction between Pd and the catalyst
100”, etc.) or 450 °C (“Pd/Al-1-450”, etc.). Each of these support. As a consequence, Pd dissolution is hampered and
two fractions was split again and one part was treated at occurs only at higher temperatures. In the case of Pd/Al-1-
2
+
0
2
00 °C for 2 hours in H stream to reduce Pd to Pd (“Pd/ 450 a controlled Pd leaching is possible, whereas the weaker
2
262
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Eur. J. Inorg. Chem. 2009, 261–266

Supported Palladium Catalysts for C–C Coupling Reactions
Figure 2. Comparison of the different Pd-metal oxide (SiO
2 2 3
, Al O )
catalyst series; reaction conditions: 10 mmol bromobenzene,
Figure 1. Influence of thermal treatment on the activity of the Pd-
metal oxide catalysts; reaction conditions: 10 mmol bromobenzene,
1
1
5 mmol styrene, 12 mmol NaOAc, 10 mL NMP, 0.01 mol-% Pd,
60 °C, 2 h, argon atmosphere.
1
1
5 mmol styrene, 12 mmol NaOAc, 10 mL NMP, 0.01 mol-% Pd,
60 °C, 2 h, argon atmosphere.
ized Pd. Therefore a very high Pd dispersion (and stronger
Pd-support interaction) is postulated in these catalysts. In
Pd-support interaction in Pd/Al-1-100 leads to a higher Pd contrast, the presence of larger crystalline Pd species can
leaching. Therefore in the latter case agglomeration of Pd be observed by X-ray powder diffraction investigations
particles and loss in activity are observed. Synergy of high for the catalysts of the Pd/SiO series. The high catalytic
2
2
+
temperature calcination and presence of Pd leads to ex- activity of Pd/Si-450 is explained by the presence of crystal-
tremely active catalysts, while the combination of the unde- line PdO particles, whereas a lower activity was observed
0
sired properties (weakly bound Pd ) causes a dramatic de- for the other three catalysts that exhibit signals from
0
crease in activity.
metallic Pd .
In Figure 2 two modified preparation routes for Pd/
Investigations on the influence of the reaction tempera-
Al O catalysts are compared and the influence of the ox- ture on the catalyst activity were performed using several
2
3
idic support (Al O vs. SiO ) is investigated. All catalysts of the catalysts. The results for the coupling of bromoben-
2
3
2
were tested in the same reaction and under the same condi- zene and styrene with Pd/Al-3-100-red as catalyst at dif-
tions mentioned above. First, the differences between the ferent temperatures are shown in Table 2. While only 31%
Pd/Al-1 and the Pd/Al-3 series are discussed. Both catalyst conversion are reached after 2 hours at 140 °C catalyst per-
series were prepared using different conditions during the formance increases significantly at 160 °C (84% conver-
coprecipitation process: While coprecipitation of Pd/Al-1 sion). For almost complete conversion in the same time a
catalysts was performed at room temp. for 1 day, Pd/Al-3 temperature of 180 °C is needed. This temperature depen-
catalysts were stirred at 65 °C for 3 days. The latter pro- dence is consistent with the concept of a strong Pd-support
cedure causes, again, a stronger interaction between Pd and interaction. Therefore high temperatures are necessary to
the support (or even partial incorporation of Pd into the release Pd from the surface and generate catalytically active
Al O lattice). This is demonstrated by the fact that Pd can species in solution.
2
3
be removed completely from the Al-1 surface by washing
with acetonitrile at room temp., whereas no dissolution
from Al-3 catalysts is observed. The Pd/Al-3 series shows
Table 2. Influence of the reaction temperature on the catalytic ac-
tivity and selectivity in the Heck reaction of bromobenzene with
styrene; reaction conditions: 10 mmol bromobenzene, 15 mmol sty-
qualitatively the same influence of thermal treatment as the rene, 12 mmol NaOAc, 10 mL NMP, 0.01 mol-% Pd (Pd/Al-3-100-
red), 2 h, argon atmosphere.
Pd/Al-1 catalysts discussed above. However, catalysts of the
Al-3 series exhibit higher activity. Complete conversion of Temperature [°C]
% Conversion
% Yield of 3
bromobenzene is observed for both unreduced Al-3 and the
1
1
40
60
31
84
92
31
83
91
reduced Al-3-450-red catalysts. Even Al-3-100-red which
0
contains comparably weakly bound Pd (an undesired com- 180
bination) reaches 84% conversion (compared to 6% for Al-
1-100-red).
Kinetic investigations were performed to study the corre-
Pd/SiO2 catalysts show similar tendencies concerning lation of the progress of the reaction and the content of
thermal treatment. Only small differences in activity are ob- leached Pd in solution. In Figure 3 the results for the reac-
served for the first three catalysts (100, 100-red and 450) of tion of bromobenzene and styrene in presence of 0.025 mol-
the Pd/Al-1 and the Pd/Si series. This means that the influ- % Pd/Al-3-450 are shown. The reaction was performed
ence of the support is negligible and the differences can be simultaneously in 14 pressure tubes; after the respective re-
explained by small variations in Pd dispersion. Pd/Al O3 action time each reaction was stopped and one sample was
2
catalysts do not show XRD signals from metallic or oxid- taken to determine the conversion via GLC and another to
Eur. J. Inorg. Chem. 2009, 261–266
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
263

W. Kleist, J.-K. Lee, K. Köhler
FULL PAPER
quantify the Pd content in solution via AAS. A slightly ele- tion and washing. While complete conversion in the reac-
vated catalyst concentration of 0.025 mol-% was used to tion of bromobenzene with styrene is achieved in the first
minimize fluctuations of Pd concentrations in the different run, in the second run a slight decrease to 95% conversion
tubes.
is observed. This marginal loss in activity of the catalyst
can be attributed to a partial reduction of Pd2 to Pd and
to a lowered Pd dispersion after the re-deposition of the
dissolved Pd species at the end of the first run. However,
the catalysts retain a high activity in recycling experiments.
In the first part of the investigations we showed that a
high content and dispersion of Pd2 at the surface of the
support is crucial for the generation of highly active cata-
lysts. As a second main condition the presence of a strong
Pd-support interaction was found to be necessary for the
generation of dissolved Pd species in the right concentration
at the right reaction temperature. The Pd/Al-3-450 catalyst
exhibits an extremely high activity (TON up to 10000; TOF
+
0
+
–1
up to 5000 h ) in the reaction of bromobenzene with sty-
rene due to a combination of these positive properties. The
Figure 3. Correlation of the progress of the Heck reaction with Pd required reaction temperature of 160 °C was quite high for
leaching (during the reaction): 10 mmol bromobenzene, 15 mmol
reactions of aryl bromides; however, the catalyst released
styrene, 12 mmol NaOAc, 10 mL NMP, 0.025 mol-% Pd/Al-3-450,
the optimum amount of Pd at these elevated temperatures.
160 °C, argon atmosphere.
For the reaction of industrially important aryl chlorides, in
Figure 3 shows a conversion curve that is typical for het- general, harsher conditions are necessary. Due to an in-
erogeneously catalyzed Heck reactions. During an induc- creased C–X bond energy quite high temperatures are re-
tion period (first 15 minutes) no conversion is observed and quired to insert Pd into the aryl–X bond in the oxidative
Pd dissolution is initiated. After a critical Pd concentration addition step. Therefore Pd/Al-3-450 was chosen as the cat-
in solution is reached the reaction proceeds very fast. Ne- alyst in the reaction of 4-chloroacetophenone with styrene.
arly complete conversion of bromobenzene is achieved in
Reactions of 4-chloroacetophenone were performed
the next 30 minutes. The concentration of leached Pd in- using 0.01 mol-% Pd and the same reaction conditions as
creases and reaches a maximum in the same timeframe. At described for reactions of bromobenzene with the only dif-
the end of the reaction most of the dissolved Pd is re-de- ference of 6 hours reaction time (compared to 2 hours for
posited onto the support and only a small amount remains bromobenzene). In spite of this extended reaction time, only
in solution. Figure 3 shows a clear correlation between the poor conversion (14%; Table 3, entry 1) was obtained. It is
reaction progress and Pd leaching. The same qualitative be- widely accepted that addition of salts like tetra-n-butylam-
havior has also been observed for corresponding reactions monium bromide N(C H ) Br exhibits a promoting effect
4
9 4
[
2,15,20,21]
in the presence of “conventionally” prepared supported Pd in Heck reactions.
The influence of different tetra-
catalysts. However, the applied reaction temperatures for n-alkylammonium halides was studied and the results ob-
the successful coupling of bromobenzene and styrene are tained are presented in Table 3. A comparison of the four
different. While in the presence of a “normal” Pd/TiO cat- different tetra-n-butylammonium halides (entries 2–5)
2
alyst 140 °C were sufficient,[ an elevated reaction tem- showed that the bromide salt had the strongest promoting
perature of 160 °C had to be applied in this work using the effect (although in this case only 1 mmol was applied, com-
sol-gel catalysts. Note that in both cases a maximum of ca. pared to 10 mmol of the fluoride, chloride and iodide). By
11]
3
5% of the total Pd amount was found in solution in the addition of 1 mmol N(C H ) Br the conversion of 4-chloro-
4 9 4
period between 20 and 40 minutes, which is exactly the ti- acetophenone could be raised to 93%. Addition of the cor-
meframe in which complete conversion of the substrates is responding chloride and iodide yielded a comparable low
achieved. When we compare the results from Figure 3 with conversion of 26%. Note that this conversion is still twice
kinetic studies of the reaction of 4-chloroacetophenone and as high as without additional salt. The exceptionally high
[
12]
styrene,
one can clearly see that in this case exactly the conversion obtained by using the fluoride salt (51%) may
same reaction temperature of 160 °C is necessary to gener- be explained by the fact that in this case a trihydrate salt
ate a sufficient Pd concentration in solution. All these ob- was applied. The promoting effect of water in the reaction
[
13,14]
servations confirm the “quasi-homogeneous” reaction mixture is also a well known fact.
mechanism that has been proven in various similar studies differently substituted tetra-n-alkylammonium bromides
of several groups.
A comparison of
[
6–12,15]
showed only marginal influence of the length of the alkyl
Investigations on the recovery and reuse of the catalysts chains (Table 3, entries 4, 6–9). In all different cases conver-
show that Pd/MO can be easily filtered off and used for sions of 88 to 94% could be obtained. There are only a few
x
further experiments after washing with dichloromethane. In heterogeneous catalysts known up to now that exhibit such
–1
these experiments a higher Pd concentration of 0.1 mol-% high TON (8600–9300) and TOF (1450–1550 h ) in the
[
11,12,22–25]
was used to minimize losses of Pd during catalyst separa- reaction of an aryl chloride.
In the reaction of
264
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Eur. J. Inorg. Chem. 2009, 261–266

Supported Palladium Catalysts for C–C Coupling Reactions
2 3
nonactivated aryl chlorides like chlorobenzene only conver- A second charge of Pd/Al O catalysts (Pd/Al-3 series) is prepared
the same way, but the mixture is stirred for 3 days at 65 °C after
the addition of the PdCl solution. After evaporation of the solvent
four different catalysts are prepared by thermal treatment (see
Table 1).
sions lower than 10% were observed. A further optimiza-
tion of the variety of reactions parameters like base or sol-
vent is necessary for improvements in this complex reaction
system.
2
2 4 2 4
Pd/SiO catalysts are prepared from Si(OEt) and PdCl . Si(OEt)
Table 3. Substituted ammonium halides as promoters in the Heck
reaction of 4-chloroacetophenone with styrene; reaction condi-
tions: 10 mmol 4-chloroacetophenone, 15 mmol styrene, 12 mmol
(22.2 mL, 100.0 mmol) is dissolved in ethanol (250 mL) and heated
to 80 °C. Water (30 mL) is added dropwise under vigorous stirring
and the reaction mixture is stirred for 3 h at 80 °C and another 10
h at room temp. Afterwards the solvent is evaporated and the re-
sulting colorless gel is dissolved in 1-butanol (60 mL) and heated
NaOAc, 1 mmol NR
4
X, 10 mL NMP, 0.01 mol-% Pd (Pd/Al-3-
450), 6 h, 160 °C, argon atmosphere.
–1
NR % Conversion % Yield of 3 TON TOF [h ]
4
X
2
to 80 °C. A solution of PdCl (102.0 mg, 0.58 mmol) in hydrochlo-
ric acid (60 mL) is added dropwise under vigorous stirring and the
mixture is stirred for 2.5 h at 80 °C and another 24 h at room temp.
After evaporation of the solvent four different catalysts (Pd/Si
series) are prepared from the raw material (see Table 1).
None
14
51
26
93
26
88
93
94
91
11
37
24
81
10
71
85
81
85
1370
5180
2420
8900
2430
8680
9180
9310
9010
230
860
400
1480
410
1450
1530
1550
1500
O[a]
F·3H
2
N(C
N(C
N(C
N(C
N(C
N(C
N(C
4
H
4
H
4
H
4
H
6
H
7
H
8
H
9
9
9
9
)
)
)
)
4
4
4
4
[a]
Cl
Br
[a]
I
Typical Procedure for Heck Reactions: Catalytic reactions are per-
formed in sealed pressure tubes after 5 minutes of purging with
argon. Reactants and solvents are used without additional drying.
13
15
17
4
) Br
4
) Br
4
) Br
N(C12
H
25 4
) Br
Aryl halide (10 mmol), styrene (15 mmol), sodium acetate
[a] 10 mmol NR
4
X.
4
(12 mmol), NR X (1 mmol, only for reactions of 4-chloroacetophe-
none), and Pd catalyst (0.01 mol-% Pd based on aryl halide) are
dissolved/suspended in 1-methyl-2-pyrrolidone (NMP, 10 mL) and
heated to 160 °C for 2 h (bromobenzene) or 6 h (4-chloroaceto-
phenone).
Conclusions
Filtered samples are extracted with water/CH
MgSO . Products are identified by GC/MS. Conversion and yields
are quantified by GLC using diethylene glycol dibutyl ether as in-
2 2
Cl and dried with
In this work the synthesis of heterogeneous Pd/MO cat-
alysts via combined sol-gel coprecipitation routes has been
x
4
presented. Since it is known that in heterogeneously cata- ternal standard (∆rel = Ϯ 5%).
lyzed Heck reactions the active Pd species are generated by
The Pd content in solution during the reaction was determined as
dissolution of Pd from the catalyst surface, the control of
this leaching process is essential. Coprecipitation of Pd and
the metal oxide leads to a comparably strong Pd-support
follows: After the desired reaction time, the pressure tube was re-
moved from the oil bath. 5 mL of the reaction mixture were sam-
pled and immediately filtered into a 10 mL flask. Subsequently, all
interaction and as a consequence quite high temperatures liquid components were carefully evaporated by thermal treatment
are necessary to dissolve Pd and generate the catalytically in a heating block. After cooling, the solid residue was treated with
active species. However, the same high reaction tempera-
tures are, in general, necessary to activate non-activated aryl
H
0
2
SO
.5 mL) was added and the resulting mixture was refluxed for 24
h. Subsequently, again HNO (65%, 0.5 mL) was added and the
mixture is refluxed for 24 h and cooled down to room temp. After-
wards, HCl (37%, 5 mL) and an aqueous solution of LaCl (10%,
mL) were added. The mixture was diluted with bidestilled water
4 3
(98%, 2 mL) and heated to reflux for 1 h. HNO (65%,
3
bromides and aryl chlorides. Pd/MO catalysts prepared by
x
optimized sol-gel methods exhibit a high dispersion of Pd2+
3
and they release exactly the right amount of Pd into the
reaction mixture at these elevated temperatures. Therefore
they were found to be highly active in Heck reactions of
5
to a total volume of 50 mL. The palladium content was determined
by using a flame atomic absorption spectrometer Varian SpectrAA
–1
bromobenzene (TON up to 10000; TOF up to 5000 h )
and 4-chloroacetophenone (TON up to 9300; TOF up to
400.
–
1
1
550 h ). Kinetic investigations confirm the postulated
Acknowledgments
“quasi-homogeneous” reaction mechanism by showing a
clear correlation of the reaction rate and the Pd content in
solution.
Manfred Barth and his co-workers (Mikroanalytisches Labor, TU
München) are gratefully acknowledged for performing the Pd trace
analysis.
Experimental Section
[
1] A. Biffis, M. Zecca, M. Basato, J. Mol. Catal. A 2001, 173,
249–274.
Preparation of Catalysts: Pd/Al
Al(OiPr) and PdCl . Al(OiPr) (10 g, 49.0 mmol) is suspended in
-propanol (300 mL) and heated to 65 °C. Water (15 mL) and, sub-
sequently, a solution of PdCl (42.5 mg, 0.24 mmol) in hydrochloric
2 3
O catalysts are prepared from
[
2] M. T. Reetz, J. G. de Vries, Chem. Commun. 2004, 1559–1563.
3
2
3
[3] N. T. S. Phan, M. van der Sluys, C. W. Jones, Adv. Synth. Catal.
2
2
006, 348, 609–679.
4] K. Köhler, S. S. Pröckl, W. Kleist, Curr. Org. Chem. 2006, 10,
585–1601.
5] V. Polshettiwar, A. Molnár, Tetrahedron 2007, 63, 6949–6976.
2
[
[
acid (30 mL) are added dropwise under vigorous stirring. The mix-
ture is stirred for 24 h at room temp. and afterwards the solvent is
1
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