The Heck arylation of mono- and disubstituted olefins catalyzed by palladium supported on alumina-based oxides

Article abstract of DOI:10.1016/j.apcata.2010.11.041

Palladium catalysts containing Pd(0) or Pd(II) supported on alumina-based mixed oxides prepared by alkoxide sol-gel method (Al₂O₃, Al₂O₃-ZrO₂, Al₂O₃- ZrO₂-Eu₂O₃, Al₂O₃-MgO, Al₂O₃-CeO₂, Al₂O₃-Fe ₂O₃) are very effective in the Heck coupling of bromobenzene with butyl acrylate in DMF solvent. In the presence of an excess of bromobenzene, butyl cinnamate (1) was formed as the main product after 4 h. With the same catalysts, good results, up to 97% yield of ethyl β-phenylcinnamate (3), were also obtained in the coupling of bromobenzene with ethyl cinnamate. The three most active catalysts, Pd(0)/Al ₂O₃, Pd(II)/Al₂O₃, and Pd(II)/Al₂O₃-Fe₂O₃, were applied in the Heck cross-coupling of cinnamates with different bromobenzene and iodobenzene derivatives, leading to β-arylcinnamate products. The highest yield was observed when iodoanisole and cinnamic aldehyde were used as substrates. The formation of Pd(0) nanoparticles in the Heck reaction carried out with supported Pd(II) catalyst precursors was confirmed by TEM measurements.

Full text of DOI:10.1016/j.apcata.2010.11.041

Applied Catalysis A: General 393 (2011) 195–205
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Applied Catalysis A: General
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The Heck arylation of mono- and disubstituted olefins catalyzed
by palladium supported on alumina-based oxides
Ewa Mieczyn´ skaa, Andrzej Gnieweka, Iweta Pryjomska-Raya,
Anna M. Trzeciaka,∗, Hanna Grabowskab, Mirosław Zawadzkib
a Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium catalysts containing Pd(0) or Pd(II) supported on alumina-based mixed oxides prepared by
alkoxide sol–gel method (Al2O3, Al2O3–ZrO2, Al2O3–ZrO2–Eu2O3, Al2O3–MgO, Al2O3–CeO2, Al2O3–Fe2O3)
are very effective in the Heck coupling of bromobenzene with butyl acrylate in DMF solvent. In the pres-
ence of an excess of bromobenzene, butyl cinnamate (1) was formed as the main product after 4 h. With
the same catalysts, good results, up to 97% yield of ethyl ␤-phenylcinnamate (3), were also obtained
in the coupling of bromobenzene with ethyl cinnamate. The three most active catalysts, Pd(0)/Al2O3,
Pd(II)/Al2O3, and Pd(II)/Al2O3–Fe2O3, were applied in the Heck cross-coupling of cinnamates with differ-
ent bromobenzene and iodobenzene derivatives, leading to ␤-arylcinnamate products. The highest yield
was observed when iodoanisole and cinnamic aldehyde were used as substrates. The formation of Pd(0)
nanoparticles in the Heck reaction carried out with supported Pd(II) catalyst precursors was confirmed
by TEM measurements.
Received 10 October 2010
Received in revised form
18 November 2010
Accepted 29 November 2010
Available online 4 December 2010
Keywords:
Heck reaction
Nanostructured alumina-based oxides
Palladium nanoparticles
Supported palladium
␤-Arylcinnamate
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
A variety of supports have been used for P-ligand-less palladium
catalysts designed for the Heck reaction, such as montmorillonite
The Heck cross-coupling reaction is recognized as one of the
most important synthetic pathways to arylated olefins, important
components for agrochemicals, pharmaceuticals, and cosmetics
production.
Typical palladium catalysts used in the Heck reaction usually
contain phosphorus ligands [1–11]; however, P-ligand-free sys-
tems containing Pd(OAc)2, PdCl2, or Pd(0) colloids have also been
demonstrated recently to be highly active [5–21]. Phosphorus lig-
ands are very often moisture and air sensitive, which creates
problems in the catalytic system. In addition, there is a strong ten-
dency to avoid application of these ligands because of their possibly
negative impact on the natural environment.
K-10 [22–24], alumina [25–28], MgLaO [29], zeolites [30–34],
oxides of Ti, Zn, and Zr [35–39], MgO [38–40], Si/Al [41,42], hydrox-
yapatite [43], layered double hydroxide (LDH) [44], chitosan [45],
and modified silica [46–48]. Pd/C has also been studied intensively
[49–52].
While in many cases high activity of these catalysts has been
observed in the arylation of acrylates or styrene, very few studies
have focused on their use in the synthesis of diarylated functional-
ized olefins such as ␤-arylcinnamates [53]. Typically, homogeneous
systems containing phosphanes or tetraalkylammonium salts have
been found more efficient for the preparation of diarylated func-
tionalized olefins [54–59].
Immobilized palladium species, preferably phosphorus-free,
present a very attractive alternative to soluble palladium catalysts.
Their main advantage is easier removal from the reaction mixture
and possibility of recycling. In addition, the immobilized palladium
catalysts containing very simple palladium precursors do not form
any decomposition compounds that could contaminate the final
organic product.
Thus, we decided to test palladium catalysts supported on dif-
ferent alumina-based oxides in order to find an efficient catalyst
for the synthesis of diarylated functionalized olefins. Previously,
we have studied palladium catalysts supported on Al2O3 in the
Heck reaction of butyl acrylate with bromobenzene in [Bu4N]Br
medium [26]. Under such conditions, using an excess of bromoben-
zene, it was possible to obtain in one step the diarylated product,
␤-phenylcinnamate. It was also shown that the supported palla-
dium catalysts act as a source of soluble Pd(II) complexes and Pd(0)
nanoparticles, both catalytically active [26].
In this paper, we present studies of the catalytic activity of Pd(II)
and Pd(0) immobilized on different alumina-based mixed oxides
∗
Corresponding author.
E-mail address: ania@wchuwr.pl (A.M. Trzeciak).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.11.041

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E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
in the Heck reaction leading to mono- and diarylated olefin deriva-
tives. The mixed metal oxides used as the supports for palladium
nanoparticles might exhibit several advantages over usually used
Al2O3. Alumina–zirconia oxides obtained by the sol–gel technique
[60] are characterized by good thermal and mechanic stability
[61–63]. Additionally, the Al2O3–ZrO2 oxide containing 1% of Eu2O3
was also prepared [60] and tested as the catalyst support. It was
expected that under the reaction conditions, Eu3+ could be reduced
to Eu2+, possessing strong reducing properties and consequently
facilitating stabilization of the catalytically active Pd(0) nanoparti-
cles. Cerium dioxide is probably one of the most well investigated
oxides among the rare-earth metal oxides and Al2O3–CeO2 is a very
suitable combination for preparing supports for immobilization of
transitional metals. Ceria has various functions: it stabilizes alu-
mina, keeps high surface area [64,65] and prevents the sintering
of precious metals thus stabilizing their dispersed state [66]. The
introduction of MgO to Al2O3 has effect on increasing hardness of
the sol–gel–gel derived materials [67,68]. Similarly, the addition of
Fe2O3 influences the structure of alumina improving its mechanical
strength [69].
containing 25 mg of Pd. After 72 h, the solution was decanted, and
the Pd(II)-containing catalyst was washed three times with water
and dried.
Palladium content was estimated by the ICP method, after min-
eralization of a weighted sample with aqua regia. The obtained
palladium loading was from 1.3 to 1.9 wt.% in the final catalysts.
2.3. Preparation of supported Pd(0) catalysts
First, 25 cm3 of an aqueous solution containing 0.5 cm3 of
N2H4·H2O (80%) was heated to 60 ◦C. When the temperature was
stabilized, the support containing Pd(II) was introduced to the
hydrazine solution. Pd(II) was reduced to Pd(0) nanoparticles in
1 min, however the heating was continued for next 10 min. Finally,
the obtained catalyst was washed with water and then dried in
vacuo.
Palladium content was estimated by the ICP method, after min-
eralization of a weighted sample with aqua regia. The obtained
palladium loading was from 1.3 to 1.9 wt.% in the final catalysts.
Using these alumina-based oxides as the supports for palladium,
we expected to estimate the influence of the second metal oxide
on the catalytic activity. The application of different supports for
palladium catalysts could also allow us to formulate more general
conclusions about the catalytic activity of heterogeneous systems
in the synthesis of diarylated functionalized olefins.
2.4. XRD and TEM measurements
The particle size and morphology of the catalyst samples were
characterized using a FEI Tecnai G2 20 X-TWIN electron microscope
(TEM) operating at 200 kV and providing 0.25 nm resolution. Spec-
imens for TEM studies were prepared by putting a droplet of a
colloidal suspension on a copper microscope grid covered with a
perforated carbon film followed by evaporating the solvent under
IR lamp for 15 min. The mean particle diameter and size distribu-
tions were calculated by counting at least 100 particles from the
enlarged micrographs. The size distribution plots were fitted by
means of a log-normal curve approximation.
The as-prepared and heat treated samples were characterized
for their phase purity and crystallinity by X-ray powder diffraction
(XRD) using DRON-3 diffractometer (Ni-filtered CuK␣ radiation); a
scan rate of 0.5 deg/min was used to record the patterns in the 2ꢀ
range of 20–80◦.
Textural properties of sample samples (surface area, pore
size distribution and pore volume) were determined by nitrogen
adsorption–desorption isotherms at liquid nitrogen temperature
by using an automatic volumetric apparatus (FISONS Sorptomatic
1900). Prior to measurements the samples were degassed for some
hours at 300 ◦C and 10−3 Torr. The adsorption of nitrogen was fol-
lowed until near saturation (P/P0 = 0.95), then the desorption was
followed until the closure of the hysteresis loop. The adsorption
data were interpreted by the application of a conventional BET
method for determination of the surface area SBET (m2/g). The total
pore volume Vp (cm3/g) adsorbed near saturation was read from
the adsorption isotherm. The pore size distribution was analyzed
following the Dollimore–Heal method, which was applied to the
desorption branch of each of the isotherms. The mean pore size R
(nm) for each sample was read from the corresponding pore size
distribution curve.
2. Experimental
2.1. Preparation of alumina-based supports
The nanostructured Al2O3–ZrO2 support containing 10% of ZrO2
was prepared from AlO(OH) and ZrO(NO3)·xH2O [60]. In the first
step hydrolysis of Al(OiPr)3 at 90 ◦C with continuous stirring gave
AlO(OH). The suspension of AlO(OH) was added to the solution of
ZrO(NO3)·xH2O in water containing HNO3 ([Zr]:[NO3] = 1:4) and
the mixture was heated at 90 ◦C for 72 h. The resulted transpar-
ent sol was condensed to obtain gel which was extruded in a
wire form (2 mm in diameter), dried and calcined at 500 ◦C for 4 h.
The obtained Al2O3–ZrO2 system is a porous material of 0.3 cm3/g
total pores volume, characterized by a narrow pore distribution
and developed specific surface BET equal to 220 m2/g. XRD and
TEM studies of Al2O3–ZrO2 samples have shown the presence of
␥-Al2O3 phase whereas ZrO2 most probably exists in an amor-
phous or highly dispersed form. The Al2O3–ZrO2–Eu2O3 support
wasobtained byimpregnationofAl2O3–ZrO2 with an aqueous solu-
tion of Eu(NO3)3 for 24 h, followed by drying and calcination at
500 ◦C for 4 h [60].
Nanostructured supports consisting of Al2O3 as the main com-
ponent and Mg, Fe(III) or Ce(IV) oxides were prepared by the
same procedure using one-step aqueous sol–gel technique. The
aluminium precursor, AlO(OH), was produced through hydrolysis
of 102 g of solid Al(OiPr)3 for 15 min in 600 cm3 of distilled water
heated at 90 ◦C with continuous stirring. Thus obtained AlO(OH)
was then peptized by slow addition of 2.86 cm3 of 65% nitric acid.
Next, an aqueous solution containing the calculated amount of the
respective nitrate (Mg, Fe or Ce) was added. The mixture was heated
under reflux at 90 ◦C for 72 h and the resulted sol was concentrated
by evaporation to obtain gel which was extruded in a wire form
(2 mm in diameter), dried and calcined at 500 ◦C for 4 h yielding
Al2O3–2%CeO2, Al2O3–10%Fe2O3 or Al2O3–10%MgO.
2.5. Heck reaction procedure
The Heck reaction of bromobenzene and butyl acrylate
was carried out in
a
50 cm3 Schlenk tube with magnetic
stirring. The reagents: catalyst containing 1.4 × 10−5 mol of pal-
ladium, PhBr 0.46 cm3 (4.36 × 10−3 mol), olefin (CH2 CHC(O)OBu)
1.89 × 10−3 mol, NaHCO3 (or other base, 4.76 × 10−3 mol) and DMF
(5 cm3), were introduced to the Schlenk tube under an Ar atmo-
sphere. The reaction was carried out at 140 ◦C for 4 h. Afterwards,
diethyl ether (10 cm3) was added, and the sample was quenched
with 10 cm3 of water. The mixture was extracted with 2 portions
of diethyl ether (10 cm3 each), and an ether solution was analyzed
2.2. Preparation of supported Pd(II) catalysts
First, 0.8 g of the support was impregnated, with stirring, in
10 cm3 of an aqueous acidic solution (CHCl = 0.09 mol/dm3) of PdCl2,

E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
197
by GC (Hewlett Packard 8452A) with mesitylene as the internal
standard.
MS: m/z (%) m/z 214(100), 199(84), 171(26), 156(11), 139(5),
128(16), 107(5)
The Heck reaction of an aryl halide (2.0 × 10−3 mol) with ethyl
cinnamate (or cinnamic aldehyde, 1.89 × 10−3 mol) was carried out
in a similar way. The organic products were identified by 1H NMR
and MS (GC-MS, Hewlett Packard 8452A) data. The product yields
and substrate (olefin) conversion were determined by GC analysis.
For recycling experiments, only a solid catalyst was used. It was
filtered off after the first reaction, washed with diethyl ether, dried
and transferred to the Schlenk tube containing the organic sub-
strates, base, and solvent.
3. Results and discussion
3.1. Characterization of alumina-based oxides
XRD diffraction patterns of alumina and a set of the alumina-
based mixed oxides have been recorded. The peaks in the
diffractogram of alumina (Fig. 1a) at 37.6◦, 45.8◦ and 66.6◦ are very
close to those reported for crystalline ␥-Al2O3 (JCPDS no. 10-0425)
and might be attributed to the (3 3 1), (4 0 0) and (4 4 0) planes,
respectively. These signals are also observed in the diffractograms
of the Al2O3–10%Fe2O3 (Fig. 1d) and Al2O3–2%CeO2 (Fig. 1b) sam-
ples. Moreover, two very weak diffraction peaks at around 33.3◦
and 35.8◦ in the first case, and a broad peak at 28.6◦ in the sec-
ond case may be observed indicating the formation of ␣-Fe2O3
and CeO2 phases according to JCPDS file no. 13-0534 and 34-0394,
respectively. The large width of these peaks might be accounted for
the presence of very small particles. These results also reveal that
probably no solid solutions are formed and these mixed oxides are
composed of highly dispersed ␣-Fe2O3 and CeO2 particles. The XRD
results obtained for the Al2O3–10%MgO sample (Fig. 1c) suggest
the formation of alumina solid solution: the characteristic signals
of ␥-Al2O3 are shifted towards lower 2ꢀ positions; for instance,
Al2O3 (4 4 0) reflection at 66.6◦ is moved to 66.1◦. No MgO phase
is detected indicating that all amount of MgO (10%) was incorpo-
rated into the structure of alumina to form a typical solid solution.
According to literature [71,72], MgO content up to 30% may form
solid solutions with alumina without presence of MgO phase. Gen-
erally, broad reflections that are observed for both alumina and
the alumina-based solid solutions indicate fine particle nature of
the obtained materials. The average Al2O3 crystallite size is about
4 nm, as determined from the X-ray line broadening using Scher-
rer’s formula.
Organic products 1–12 were identified by 1H NMR (Bruker 300
and 500 MHz) and MS data. Products 1, 2 [26] and 5, 7, 8 and 9 [70]
were characterized by comparison with their reported data.
2.6. (3) Ethyl ˇ-phenylcinnamate
MS: m/z (%) 252(M+, 86), 223(24), 207(100), 178(90), 152(21),
105(24), 77(21), 51(17) up to 3% of (E+Z) ethyl ␣-phenylcinnamates
was also found:
MS: m/z (%) 252(M+, 76), 207(14), 179(100), 152(14), 135(66),
107(48), 77(14), 51(10).
MS: m/z 252(M+, 72), 207(24), 179(100), 152(14), 135(62),
107(45), 77(14), 51(10).
2.7. (4)-Ethyl ˇ-(4-formylphenyl)cinnamate
E: 1H NMR (CDCl3) ı, ppm: 1.18 (t, J = 7.1 Hz, 3H, CH3), 4.01
(q, J = 7.1 Hz, 2H, CH2), 6.5 (s, 1H, vinyl), 7.27–7.28 (m, 2H, aryl),
7.45–7.46 (m, 3H aryl), 7.5 (d, J = 8.3 Hz, 2H, aryl) 7.8(d, J = 8.3 Hz,
2H, aryl), 10.07 (s, 1H, CHO).
MS: m/z (%) 280(M+, 96), 251(27), 235(94), 208(54), 178(100),
152(17), 105(23), 77(21), 51(17).
Z: 1H NMR (CDCl3) ı, ppm: 1.28(t, J = 7.1 Hz, 3H, CH3), 4.4(q,
J = 7.1 Hz, 2H, CH2), 6.51(s, 1H, vinyl), 7.27–7.28 (m, 2H, aryl),
7.45–7.46 (m, 3H aryl), 7.5 (d, J = 8.3 Hz, 2H, aryl) 7.8(d, J = 8.3 Hz,
2H, aryl), 10.12(s, 1H, CHO).
Taking into account the adsorption–desorption isotherms with
corresponding pore size distributions (shown in Fig. 2) and the
calculated values of both the specific surface area (SBET) and
mean pore diameter (R), listed in Table 1, it could be noted that
all the prepared samples are mesoporous in nature. The nitro-
gen adsorption–desorption isotherms are of Type IV and exhibit
the characteristic H2 hysteresis loop for mesoporous materials
according to IUPAC classification. However, the textural prop-
erties of the prepared samples are different, influenced by the
MS: m/z (%) 280(M+, 79), 251(21), 235(77), 208(48), 178(100),
152(17), 105(23), 77(19), 51(15) up to 3% of (E+Z) ethyl ␣-(4-
formylphenyl)cinnamates was also found:
MS: m/z (%) 280(M+, 100), 235(11), 207(63), 178(89), 152(11),
135(74), 107(37), 77(11), 51(8).
MS: m/z (%) 280(M+, 100), 235(16), 207(58), 178(74), 152(11),
135(58), 107(32), 77(11), 51(8).
2.8. (10)-ˇ-(4-Methyl)cinnamic aldehyde
E: MS: m/z (%) 222(40), 221(52), 208(17), 207(100), 189(8),
179(27), 178(41), 165(9), 152(7), 115(24), 102(19), 89(10), 77(6).
Z: MS: m/z (%) 222(42), 221(52), 208(16), 207(100), 189(7),
179(27), 178(41), 165(9), 152(7), 115(24), 102(19), 89(10), 77(6).
d
c
2.9. (11)-ˇ-Phenylcinnamic aldehyde
MS: m/z (%) 208(76), 207(100), 179(31), 178(47), 152(10),
131(10), 105(7), 102(36), 77(12), 51(24).
b
a
2.10. (12) ˇ-(4-Methoxyphenyl)cinnamic aldehyde
E: MS: m/z (%) 238(M+, 100), 223(16), 207(38), 178(15), 165(48),
135(19), 132(20), 102(38), 89(17), 63(13), 39(8).
CeO2
Fe2O3
MgO
10
20
30
40
50
60
70
80
Z: MS: m/z (%) 238(M+, 100), 223(15, 207(42), 178(17), 165(52),
135(19), 132(20), 102(40), 89(17), 63(13), 39(8) up to 10% of 4,4ꢀ-
dimethoxybiphenyl was also formed as a result of homo-coupling
of iodoanisole
2θ [deg]
Fig. 1. XRD diffractograms of alumina (a) and mixed oxides: Al2O3–2%CeO2 (b),
Al2O3–10%MgO (c), Al2O3–10%Fe2O3 (d) and comparison with ASTM data.

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E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
400
350
300
250
200
150
100
50
eter is moved to higher values and centered at 7.8 or 9.5 nm
0,030
0,025
0,020
0,015
0,010
0,005
0,000
for the Al2O3–10%MgO and Al2O3–2%CeO2 samples, respectively.
The addition of a metal nitrate to aluminium hydroxide dur-
ing sol–gel preparation leads to a decrease of the BET specific
surface area of the resulting alumina-based mixed oxides: 210,
170 and 158 m2/g for the Al2O3–10%Fe2O3, Al2O3–2%CeO2 and
Al2O3–10%MgO samples, respectively, in comparison to 218 m2/g
for the pure ␥-alumina. The drop of SBET on the order of 4–27%
(especially in case of Al2O3–10%MgO or Al2O3–2%CeO2) and shift to
a larger pore diameter show that the introduced metal oxides con-
siderably modify the textural properties of ␥-alumina. Additionally,
a variation in the total pore volume with the sample composi-
tion is also observed. The Al2O3–10%MgO and Al2O3–10%Fe2O3
oxides showed a lower volume (∼0.2 cm3/g) in comparison to the
pure ␥-alumina (∼0.3 cm3/g), while the total pore volume observed
for the Al2O3–2%CeO2 mixed oxide increased to ∼0.5 cm3/g.
Therefore, cerium oxide, which does not form a bulk solid solu-
tion with Al2O3, significantly modifies the porosity of ␥-alumina
(Table 1).
a
d
c
a
b
b
2
10
20
Pore size, nm
d
c
0
0.0
0.2
0.4
0.6
0.8
1.0
relative pressure P/PO
Fig. 2. N2 adsorption–desorption isotherms and corresponding pore size distribu-
tions of alumina (a) and mixed oxides: Al2O3–2%CeO2 (b); Al2O3–10%MgO (c) and
Al2O3–10%Fe2O3 (d).
3.2. Characterization of supported Pd(II) and Pd(0) catalysts
Table 1
The morphology of the Pd(II)/Al2O3 catalyst precursor was char-
acterized by Z-contrast imaging with high angle annular dark field
(HAADF) scanning transmission electron microscopy (STEM). The
obtained micrograph (Fig. 3) reveals that palladium is not uni-
formly dispersed on the surface of the support. It forms islands with
an average diameter of about 10–15 nm. Very often these islands
are observed directly on the surface of rather thin fragments of
Al2O3, not necessarily in the pores of the support. High resolution
TEM confirmed that the adsorbed Pd(II) species are amorphous, no
crystalline structure could be observed. Most probably they form a
very thin layer which is easily dissolved during the catalytic reac-
tion.
Textural properties of the supports in comparison to the pure alumina.
Sample
SBET (m2/g)a
R (nm)b
Vp (cm3/g)c
Al2O3
210
218
182
170
210
158
4.8
2.2
2.7
9.5
3.8
7.8
0.31
0.27
0.25
0.51
0.21
0.23
Al2O3–10%ZrO2 [60]
Al2O3–10%ZrO2–1%Eu2O3 [60]
Al2O3–2%CeO2
Al2O3–10%Fe2O3
Al2O3–10%MgO
a
Specific surface area (according BET method).
BJH method desorption pore diameter.
Total pore specific volume (at P/Po = 0.99).
b
c
Pd(0) catalysts supported on alumina-based oxides have been
obtained from supported Pd(II) using hydrazine as the reducing
agent. TEM investigations have been performed on three selected
catalysts supported on Al2O3, Al2O3–Fe2O3 and Al2O3–CeO2. The
obtained micrographs evidenced the presence of Pd(0) in form of
nanoparticles in all of the tested samples. The nanoparticles size
is practically the same for Pd(0) catalysts supported on Al2O3 and
Al2O3–Fe2O3. The observed particle diameter is in range 4–15 nm
and the average size is 7–8 nm. Slightly bigger nanoparticles have
been obtained on Al2O3–CeO2 support, the average Pd(0) diameter
is about 12 nm.
addition of another metal oxide to ␥-alumina. It is expected that
the textural properties of the mixed oxides result both from the
number of hydroxyl groups on the surface and the interactions
between all the components added during the preparation pro-
cess. Pore size distributions (inset of Fig. 2) are monomodal for
all the samples. The Al2O3–10%Fe2O3 mixed oxide is character-
ized by a slightly shifted (compared to pure alumna) sharp curve
centered at 3.8 nm, while considerably broad pore size distri-
butions are observed for the other oxides (Al2O3–10%MgO and
Al2O3–2%CeO2). It should be noted that the mean pore size diam-
Fig. 3. TEM bright field image (left) and corresponding HAADF STEM micrograph (right) of the Pd(II)/Al2O3 catalyst precursor.

E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
199
[Pd]
Br
+
CO2Bu
+
DMF, (base)
140oC, 4h
CO2Bu
CO2Bu
trans
1
2
Scheme 1.
3.3. Catalytic activity of Pd supported on alumina-based oxides in
Heck reaction
100
75
The Heck reaction was carried out in DMF as
a sol-
vent, with an excess of bromobenzene versus butyl acrylate
([PhBr]:[CH2 CHCO2Bu] = 2.3). Under such conditions, it was pos-
sible to obtain two products, butyl cinnamate
1 (only the
trans-isomer is formed) and butyl phenylcinnamate 2, in one stage
(Scheme 1).
In the reaction carried out with Pd(0)/Al2O3 without any base,
71% of the monoarylated product 1 was obtained. The effect of dif-
ferent bases on the composition of the reaction products is shown
in Table 2. In most cases, 1 was the main product; only in the pres-
ence of NaHCO3 2 was found with 29% yield. The best results were
obtained with NaHCO3 and NaOAc; therefore, these two bases were
applied in further experiments.
50
25
Kinetic studies performed for the Pd(0)/Al2O3 catalyst in DMF
in the presence of both bases, NaHCO3 and NaOAc, showed that
product 2 was formed only when the conversion of the substrates
to product 1 had reached 100% (Fig. 4). Thus, it was possible to
obtain product 1 with high selectivity breaking the reaction after
an appropriate length of time.
0
0
100
1
200
time (min)
300
2
400
To examine whether a similar feature is also characteristic
for Pd(II) catalysts a competition experiment was performed, in
which an equimolar mixture of ethyl cinnamate and butyl acry-
late was used as the substrate. This reaction, catalyzed with
Pd(II)/Al2O3–Fe2O3, started without any induction period however
Fig. 4. The yield of products 1 and 2 formed in the Heck reaction catalyzed by
Pd(0)/Al2O3. [Pd] (1.4 × 10−5 mol); PhBr (4.36 mmol); butyl acrylate (1.89 mmol);
mesitylene as the internal standard (0.715 mmol); NaHCO3 (4.76 mmol); DMF
(5 ml); 140 ◦C; Ar.
only the acrylate was converted to product 1. After 4 h 92% of butyl
acrylate reacted, whereas ethyl cinnamate remained completely
unreacted.
Table 2
Catalytic activity of Pd(0)/Al2O3 in the Heck reaction in DMF with different bases
(reaction products 1 and 2 see Scheme 1).
Figs. 5 and 6 illustrate the catalytic activity of the supported
Pd(0) and Pd(II) catalysts. In all cases, the monoarylated product 1
Base
Olefin conversion (%)/product yield 1:2
Pd(0)/Al2O3
71
71:0
–
100
80
100
71:29
NaHCO3
NaHCO3
100
95:5
a
60
65
65:0
40
HCOONa
Bu3N
2
1
20
0
52
52:0
100
97:3
NaOAc
Et3N
62
62:0
83
83:0
Cs2CO3
Fig. 5. Results of the Heck reaction with different Pd(0) supported catalysts (reac-
tion products 1 and 2 see Scheme 1. [Pd] (1.4 × 10−5 mol); PhBr (4.36 mmol); butyl
acrylate (1.89 mmol); mesitylene as the internal standard (0.715 mmol); NaHCO3
(4.76 mmol); DMF (5 ml); 140 ◦C; 4 h; Ar.
[Pd] (1.4 × 10−5 mol); PhBr (4.36 mmol); butyl acrylate (1.89 mmol); mesitylene as
the internal standard (0.715 mmol); base (4.76 mmol); DMF (5 ml); 140 ◦C; 4 h; Ar.
130 ◦C.
a

200
E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
100
80
60
40
20
0
2
1
Fig. 6. Results of the Heck reaction with different Pd(II) supported catalysts (reac-
tion products 1 and 2 see Scheme 1. [Pd] (1.4 × 10−5 mol); PhBr (4.36 mmol); butyl
acrylate (1.89 mmol); mesitylene as the internal standard (0.715 mmol); NaHCO3
(4.76 mmol); DMF (5 ml); 140 ◦C; 4 h; Ar.
Fig. 7. Results of the Heck reaction with different Pd(0) and Pd(II) supported
catalysts according to Scheme 2. [Pd] (1.4 × 10−5 mol); PhBr (2.0 mmol); ethyl cin-
namate (1.89 mmol); mesitylene as the internal standard (0.715 mmol); NaHCO3
(4.76 mmol); DMF (5 ml); 140 ◦C; 4 h; Ar.
dominated; however, formation of 2 in amounts of up to 29% was
observed in some reactions catalyzed by Pd(0). The effect of the
kind of support on the Heck reaction yield appeared to be not very
significant, and the presence of ca. 10% of Fe2O3 or MgO as well as 2%
of CeO2 had no significant impact on the reaction yield, which was
over 80% in almost all cases. Regardless of the kind of the support,
Pd(0) was in most cases more productive than Pd(II), especially in
respect to the yield of 2. It can be concluded that the reduction of
palladium before the Heck reaction to the form of supported Pd(0)
nanoparticles has a positive effect on the reaction course because
both types of catalysts, containing Pd(II) or Pd(0), originated from
the same precursor.
To check whether the supported catalysts are also active in the
synthesis of diarylated functionalized olefins under the same con-
ditions, the coupling of ethyl cinnamate with bromobenzene was
selected as a test reaction (Scheme 2 and Fig. 7). Surprisingly, in this
reaction, better results were obtained for immobilized Pd(II) used
without earlier pre-treatment. The highest yield, 95% and 87%, was
found for Pd(II)/Al2O3 and Pd(II)/Al2O3–Fe2O3. A relatively good
result, 76% yield, was also obtained for Pd(0)/Al2O3.
These three catalysts were selected for further studies oriented
towards synthesis of ␤-arylcinnamates in reactions of iodo- or
bromobenzene derivatives containing activating or deactivating
substituents in the phenyl ring. In these experiments, ethyl cin-
namate or cinnamic aldehyde was used as the olefinic substrate
(Scheme 3). To obtain comparable results, all the experiments were
performed over the same length of time, 4 h. The coupling products
were obtained with moderate-to-excellent yield for all three of the
catalysts studied. Differences between the catalysts were small;
however, slightly higher yield of the products was observed for
Pd(II)/Al2O3–Fe2O3 in reactions with NaHCO3 as the base. When
NaOAc was used instead, the yield of the product decreased, but at
the same time the E:Z selectivity increased. Only in the reaction of
4-bromobenzaldehyde with ethyl cinnamate, did the application
of NaOAc lead to an increase in both yield and the E:Z ratio. The
highest yields of ethyl ␤-arylcinnamates were obtained in the reac-
tions of ethyl cinnamate with bromobenzene and with iodoanisole
(Table 3).
Consequently, excellent results (100% yield) were also obtained
in the reaction of iodoanisole with cinnamic aldehyde. On the other
Br
[Pd]
+
DMF, (base)
CO2Et
CO2Et
140oC, 4h
3
Scheme 2.
R
[Pd]
X
+
+
DMF, (base)
140oC, 4h
R1
R1
R1
R
R
Z-
E-
Scheme 3.

E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
201
Table 3
Results of the Heck reaction according to Scheme 3 – formation of diarylated olefins with Pd(0) and Pd(II) supported catalysts.
X
R
R1
Product no.
Base
Yield (%)a (E:Z)
Pd(0)/Al2O3
Pd(II)/Al2O3
Pd(II)/Al2O3–Fe2O3
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
I
I
I
Br
I
H
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CHO
3
4
4
5
6
7
7
8
8
7
8
8
8
9
9
10
11
12
76
95
44 (71:29)
60 (83:17)
51 (63:37)
8
71 (60:40)
53 (78:22)
76 (57:43)
66 (73:27)
65 (71:29)
84 (72:28)
65 (75:25)
100 (64:36)
20 (37:63)
36 (57:43)
53 (60:40)
72
87
44 (71:29)
60 (81:19)
54 (62:38)
7
82 (60:40)
30 (83:17)
85 (57:43)
35 (78:22)
68 (70:30)
90 (73:27)
58 (75:25)
100 (65:35)
22 (36:64)
45 (55:45)
67 (63:37)
62
4-CHO
4-CHO
4-C(O)Me
2-Me
4-Me
4-Me
4-OMe
4-OMe
4-Me
4-OMe
4-OMe
4-OMe
C6H4
NaHCO3
NaOAc
NaHCO3
NaHCO3
NaHCO3
NaOAc
NaHCO3
NaOAc
NaHCO3
NaHCO3
NaOAc
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
37 (72:28)
36 (63:37)
7
77 (60:40)
72 (71:29)
82 (72:28)
CO2Et
CO2Et
CHO
CHO
CHO
8 (36:64)
36 (55:45)
C6H4
4-Me
H
Br
Br
Br
4-OMe
42 (63:37)
54 (75:25)
[Pd] (1.4 × 10−5 mol); ArX (2.0 mmol); ethyl cinnamate or cinnamic aldehyde (1.89 mmol); mesitylene as the internal standard (0.715 mmol); base (4.76 mmol); DMF (5 ml);
140 ◦C; 4 h; Ar.
a
Conversion to the coupling product determined by GC.
hand, bromobenzene derivatives substituted in position 2 do not
form any coupling products under the conditions applied. Only for
2-bromotoluene a cross-coupling product was obtained with the
yield up to 8%.
The E:Z values obtained with supported palladium catalysts are
very similar to those reported for homogeneous catalytic systems
containing Pd(OAc)2/PPh3/[Bu4N]Br [59]. When we used such a
catalytic system for the coupling of 4-iodoanisole with ethyl cin-
namate, 100% yield of the product was obtained after 4 h with
E:Z = 70:30.
the third one. The yield of product 2 also decreased at the same
time.
In our earlier studies of the Heck reaction in molten [nBu4N]Br
as the reaction medium, we compared the results for the recycling
of a Pd/Al2O3 catalyst carried out in two different ways [26]. In the
first procedure, the reaction products were extracted with diethyl
ether after the first run, and for the second run, the catalyst was
used together with [nBu4N]Br, which contained soluble palladium
compounds eluted from the support in the first run. Using this pro-
cedure, we obtained ca. 100% of butyl acrylate conversion even in
the third run (Table 4) [26]. When the product separation proce-
dure was changed and only the heterogeneous Pd/Al2O3 catalyst,
separated from the reaction mixture and carefully washed with
diethyl ether, was used for the second run, the observed conversion
of butyl acrylate dropped to ca. 10% already in the third run [26].
Thus, these results indicated that soluble palladium species, like
anionic Pd(II) complexes of [PdPhX3]− type or Pd(0) nanoparticles
stabilized with ammonium salt [16,45,55], contributed remarkably
to the catalytic activity of Pd(0)/Al2O3 when the reaction was per-
formed in [nBu4N]Br. It is worth to note, that the results of the
Heck reaction are practically independent on the kind of Pd precur-
sor used (Pd/Al2O3, PdCl2(PhCN)2, Pd/PVP) in [nBu4N]Br as reaction
medium [16,17,26].
3.4. Recycling of Pd supported on alumina-based oxides in Heck
reaction
For the reuse studies, the catalyst was separated from the reac-
tion mixture by filtration immediately after the catalytic reaction,
washed with diethyl ether, and dried. In such a procedure, the
presence of any soluble palladium species leached from the cat-
alyst was minimized. Two series of experiments were performed
with Pd(0)/Al2O3: without any base and with NaHCO3 using bro-
mobenzene and butyl acrylate as the substrates (Table 4). In the
first case, only product 1 was formed, and its amount decreased
from 71% in the first reaction to 42% in the third one. The presence
of a base contributed to a higher yield of the products with some
decrease in the subsequent runs, from 100% in the first to 76% in
The procedure used in this paper is similar to that described
above as the second one, however, the product yields obtained
Table 4
Recycling test of different palladium catalysts of Heck reaction (Scheme 1) performed in DMF and [nBu4N]Br as reaction medium.
Catalyst
Base/solvent
Olefin conversion (%)/product yield 1: 2
Run 1
Run 2
Run 3
Pd(0)/Al2O3
–/DMF
71
67
42
71: 0
100
77: 23
100
17: 83
100
67: 0
92
88: 4
100
22: 78
100
42: 0
76
76: 0
100
46: 54
10
Pd(0)/Al2O3
NaHCO3/DMF
NaHCO3/[Bu4N]Bra
NaHCO3/[Bu4N]Brb
Pd(0)/Al2O3 [26]
Pd(II)/Al2O3 [26]
1: 99
4: 96
[Pd] (1.4 × 10−5 mol); PhBr (4.36 mmol); butyl acrylate (1.89 mmol); mesitylene as internal standard (0.715 mmol); NaHCO3 (4.76 mmol); DMF (5 ml) or [nBu4N]Br (0.75 g);
140 ◦C; 4 h; Ar.
a
Pd/Al2O3 catalyst was used in the next run together with [Bu4N]Br phase [26].
Pd/Al2O3 catalyst was separated, washed, dried and used in the next run [26].
b

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E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
Table 5
Diameters of Pd(0) nanoparticles supported on various alumina-based oxides observed in TEM before and after the Heck reaction.
Sample
Nanoparticles size (nm)a
Typical diameter (nm)b
Size distributionc
Pd(0)/Al2O3 before reaction
Pd(0)/Al2O3 after reaction
4–15
4–15
7
7
–
d = 8.3 nm
FWHM = 4.8 nm
Pd(II)/Al2O3 after reaction
4–15
9
d = 9.8 nm
FWHM = 6.3 nm
Pd(0)/Al2O3–Fe2O3 before reaction
Pd(0)/Al2O3–Fe2O3 after reaction
4–16
4–16
8
8
–
d = 8.8 nm
FWHM = 4.3 nm
Pd(II)/Al2O3–Fe2O3 after reaction
8–32
18
d = 19.9 nm
FWHM = 10.1 nm
Pd(0)/Al2O3–CeO2 before reaction
Pd(0)/Al2O3–CeO2 after reaction
Pd(II)Al2O3–CeO2 after reaction
5–20
5–20
5–20
12
12
15
–
–
–
a
Total range of observed nanoparticle diameters.
Most characteristic nanoparticle size in a sample.
Log-normal fit of nanoparticle size distribution.
b
c
in DMF are much higher than those reported previously for tetra-
butylammonium bromide [26]. Thus, participation of the leached
palladium forms acting as homogeneous catalysts seems to be
smaller in the Heck reaction carried out in DMF than in molten
tetrabutylammonium salts.
Palladium leaching in the studied systems was estimated on the
basis of Pd content in the supported catalysts separated after the
Heck reaction. ICP analyses performed immediately after hot filtra-
tion showed that up to 25% of palladium leached in case of the Pd(0)
catalyst precursors and even 50% when Pd(II) precursors had been
Fig. 8. TEM micrographs showing structure of Pd(0)/Al2O3 after the Heck reaction (a) and Pd(0) nanoparticles formed in situ from Pd(II)/Al2O3 precursor under the Heck
reaction conditions (b).

E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
203
Fig. 9. TEM micrographs showing structure of Pd(0)/Al2O3–Fe2O3 after the Heck reaction (a) and Pd(0) nanoparticles formed in situ from Pd(II)/Al2O3–Fe2O3 precursor under
the Heck reaction conditions (b).
used. Similarly, the XRD measurements also showed higher con-
tent of supported palladium in the Pd(0)/Al2O3 catalyst recovered
after the Heck reaction.
catalytic efficiency. Similarly, it has been recently found that the
immobilized Pd(II) is more active than soluble Pd(II) complexes in
the Heck reaction of cyclohexene with bromobenzene [73].
To discuss further the role of the leached palladium in the cat-
alytic process a hot filtration test was performed for the Heck
reaction catalyzed with Pd(II)/Al2O3. The reaction mixture was fil-
tered after 1 h of reaction, when the yield of product was 45%.
Next, the solution was heated (140 ◦C) for 3 h without catalyst and
during that time the yield increased to 54%. In the reference sam-
ple, not filtrated, 84% of 1 was found after 4 h. Thus, it might be
assumed, in agreement with reports of other authors [18–20,46],
that the leached palladium contains catalytically active molecular
Pd species and catalytically inactive Pd clusters, both remaining in
equilibrium in the reaction mixture. The real contribution of these
forms in our systems is under studies.
It is well documented in the literature that the concentration of
soluble palladium leached from the support decreases remarkably
at the end of a catalytic reaction as a result of redeposition [14,28].
To check the level of Pd readsorption in our systems a sample of
Pd(II)/Al2O3 was analyzed immediately after the Heck coupling of
ethyl cinnamate with bromobenzene. After that process 47% of pal-
ladium was found on the support. The same sample of the catalyst
was left in the post-reaction mixture at room temperature for next
2 days and after that time the palladium content was estimated
again. The ICP analysis gave practically the same result, 46.6% of
the initial amount of palladium. Thus, it might be concluded that
Pd leaching is irreversible in those systems.
To estimate the role of soluble palladium species in the Heck
coupling of ethyl cinnamate with bromobenzene a comparative
experiment was performed under the same conditions with sol-
uble Pd(OAc)2 used as the catalyst. The yield of the product with
1.4 × 10−5 mol Pd was 32% after 4 h and it was remarkably lower
than with the supported Pd(II) catalysts. Interestingly, when an
analogous reaction catalyzed with Pd(OAc)2 was carried out in the
presence of pure Al2O3 support, the yield of 1 increased to 58% after
4 h indicating the positive effect palladium immobilization on the
3.5. TEM studies of Pd supported on alumina-based oxides
TEM method was used to characterize the supported palladium
catalysts before and after the Heck reaction. The observed range of
the nanoparticle size and average diameter (Table 5) is practically
the same for the as-prepared supported Pd(0) catalyst precursors
and the Pd(0) catalysts recovered after the Heck reaction carried
out in DMF as the solvent (Scheme 1).

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E. Mieczyn´ska et al. / Applied Catalysis A: General 393 (2011) 195–205
The Pd(0) nanoparticles supported on Al2O3 are spherical and
[5] J. Tsuji, New Perspectives for the 21st Century, 2nd ed., John Wiley and Sons
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none of them show typical crystal outlines (Fig. 8a). They have sim-
ilar diameters (the maximum of the size distribution is located at
8.3 nm), do not indicate any aggregation and are quite uniformly
dispersed on the surface of the alumina support. The Pd(0) particles
obtained on the Al2O3–Fe2O3 support (Fig. 9a) are slightly larger
(the average size is 8.8 nm). However, most of them are aggre-
gates composed of several discernible particles in quasispherical
arrangements. For both of the supports the Pd(0) size distributions
are practically symmetric with FWHM of 4.3 and 4.8 nm, respec-
tively. In both cases Pd(0) nanoparticles recovered after the Heck
reaction were practically the same as the freshly prepared and no
redispersion of the Pd(0) nanoparticles was observed. Contrast-
ingly, in the Heck reaction carried out in molten [nBu4N]Br the
change of the size of alumina-supported Pd(0) particles was very
significant [26]. This fact suggests that under the conditions chosen
in this work the process is more heterogeneous, although partici-
pation of soluble palladium forms cannot be ruled out.
Pd(0) nanoparticles are also formed in situ from the supported
Pd(II) precursors under the Heck reaction conditions. Thus obtained
particles are generally several nanometers larger (especially in case
of the Al2O3–Fe2O3 support) when compared to the Pd(0) cata-
lysts prepared by reduction with N2H4·H2O (Table 5). However, not
only is the average nanoparticle diameter larger but also there are
some more significant morphological differences. Pd(0) nanoparti-
cles obtained from the supported Pd(II) precursors (Figs. 8b and 9b)
are mostly characterized by very well defined geometrical shapes
with many corners and edges. Comparing to the Pd(0) catalysts,
it is clear from the histograms that the size distributions are also
broader, with less defined maxima and somewhat asymmetric,
shifted towards larger particles. Analogous effect is also observed
for the Pd catalysts supported on the Al2O3–CeO2 mixed oxide.
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4. Conclusions
Palladium catalysts supported on the alumina-based mixed
oxides have been used in the Heck reaction leading to mono- and
diarylated olefins. Their catalytic activity in the cross-coupling of
aryl halides with cinnamic ester or aldehyde is similar to that
reported for homogeneous systems with phosphane or tetraalky-
lammonium salt co-catalysts. Good-to-excellent yields of the
coupling products were obtained in 4 h, without any additives.
As confirmed by TEM studies, the supported Pd(II) was reduced
to Pd(0) during reaction forming nanoparticles that probably act
as catalytically active species. Although participation of soluble
palladium forms in the catalytic process cannot be excluded,
recyclability of the investigated systems was confirmed. Higher
palladium leaching was observed for Pd(II) than in case of the
immobilized Pd(0) catalysts, which often consist of nanoparticles
that are sterically constrained in the pores of the support [60]. The
high level of Pd leaching allows to predict that the catalysts studied
are not suitable for longer uses.
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