Selective heck arylation of cyclohexene with homogeneous and heterogeneous palladium catalysts

Article abstract of DOI:10.3390/molecules15042166

Palladium catalysts containing Pd(II) supported on Al²O ₃ and alumina-based mixed oxides, Al₂O³- ZrO₂, Al₂O₃-CeO₂, and Al ₂O₃-Fe₂O₃, are very effective in the Heck coupling of iodobenzene with cyclohexene in DMF solution. The best results, up to 81% of monoarylated products with a selectivity to 4-phenylcyclohexene (3) close to 90% were obtained with KOH as a base. The catalytic activity of palladium supported on aluminabased oxides was compared with that of homogeneous precursors, such as Pd(OAc)₂ and PdCl ₂(PhCN)₂, used in [Bu₄N]Br as the reaction medium. Under such conditions homogeneous systems were more selective and produced up to 60% of monoarylated products with a selectivity to 3 close to 60%. Copyright

Full text of DOI:10.3390/molecules15042166

Molecules 2010, 15, 2166-2177; doi:10.3390/molecules15042166
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Selective Heck Arylation of Cyclohexene with Homogeneous and
Heterogeneous Palladium Catalysts
Ewa Mieczyńska and Anna M. Trzeciak *
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland;
E-Mail: gosia@wchuwr.pl (E.M.)
∗ Author to whom correspondence should be addressed; E-Mail: ania@wchuwr.pl;
Tel.: +48 71 3757253; Fax: +48 71 3282348.
Received: 27 January 2010; in revised form: 10 March 2010 / Accepted: 23 March 2010 /
Published: 25 March 2010
Abstract: Palladium catalysts containing Pd(II) supported on Al2O3 and alumina-based
mixed oxides, Al2O3-ZrO2, Al2O3-CeO2, and Al2O3-Fe2O3, are very effective in the Heck
coupling of iodobenzene with cyclohexene in DMF solution. The best results, up to 81% of
monoarylated products with a selectivity to 4-phenylcyclohexene (3) close to 90% were
obtained with KOH as a base. The catalytic activity of palladium supported on alumina-
based oxides was compared with that of homogeneous precursors, such as Pd(OAc)2 and
PdCl2(PhCN)2, used in [Bu4N]Br as the reaction medium. Under such conditions
homogeneous systems were more selective and produced up to 60% of monoarylated
products with a selectivity to 3 close to 60%.
Keywords: palladium; Heck coupling; heterogenized catalysts; cyclohexene
1. Introduction
The Heck coupling, a C–C bond formation process also described as olefin arylation, is one of the
most important palladium-catalyzed reactions [1–6]. Arylated olefins, the final products of the Heck
reaction, have very broad application in the synthesis of pharmaceuticals, agrochemicals, and natural
products. Therefore, very intensive research has been addressed to the development of new, simple and
efficient catalytic systems for the Heck coupling.

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Typical palladium catalysts for the Heck reaction contain phosphines; however, recently,
phosphine-free catalytic systems, economical and environmentally friendly, have been receiving
increasing attention [7–16].
It seems most likely that in phosphine-free systems a key role is played by Pd(0) nanoparticles
formed in situ from Pd(II) precursors [4,9,14,15]. However, in the absence of any stabilizing agents
such catalysts can be deactivated as a result of the formation of inactive “palladium black”.
Tetraalkylammonium salts or inorganic supports are known as good stabilizers of palladium, often
used in catalytic systems to prevent their deactivation [8,11,17–20].
There are also many examples of application in Heck coupling of palladium supported on inorganic
oxides of Al, Ti, Zn, Zr, Mg [21–26], on silica [27,28] as well on zeolites [29,30]. In most cases
reactions of aryl halides with acrylates, acrylonitrile, styrene or terminal alkenes were studied [31].
Mechanistic studies performed in these systems led to the conclusion that the main catalytic role is
played by soluble palladium species formed as a result of leaching from the support and re-adsorbed in
the end of the catalytic process [32].
The Heck reaction of cyclic olefins, like cyclohexene, cyclopentene, or 2,3-dihydrofuran, present an
attractive pathway to create molecules containing stereogenic centers [20,33–40]. The difficulty of
these reactions has to do with their regioselectivity, because a mixture of different products, mono- and
diarylated olefins, is always formed. Studies of the Heck coupling of cyclohexene with 4-
bromoacetophenone have revealed the high catalytic activity of soluble palladium species, such as
palladacycles or Pd(OAc)2/PPh3, which produce 89% and 79% of monoarylated olefins. Surprisingly
only very few reports deal with application of heterogeneous systems in Heck coupling of
cycloalkenes. For example, different heterogeneous palladium catalysts, i.e. Pd/C, Pd/SiO2, Pd/MgO,
Pd/Al2O3, have been used in Heck coupling of 4-bromoacetopheneone with cyclohexene and cyclo-
pentene [33]. In contrast to homogeneous systems, heterogeneous catalysts were active not only in the
Heck coupling but also in dehalogenation of the substrate leading to the formation of acetophenone as
the predominant product [33].
In this paper, we present the results of our studies on the Heck coupling of iodobenzene with
cyclohexene performed in homogeneous and heterogeneous phosphine-free systems. We chose
palladium supported on alumina-based oxides as heterogeneous catalysts which shown already good
catalytic performance in other cross-coupling reactions [41,42].
2. Results and Discussion
2.1. Homogeneous catalysts
The Heck reaction of cyclohexene with iodobenzene is presented on Figure 1. According to
literature data, three monoarylated products, 1, 2, and 3, may be formed, with 4-phenylcyclohexene (3)
being the most thermodynamically stable [33]. Besides monoarylated products, diarylated ones can
also be formed, and Figure 2 shows their five possible isomers.

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Figure 1. The Heck reaction of cyclohexene with iodobenzene.
[Pd]
+
I
+
+
base
2
1
3
Figure 2. Diarylated products formed in the Heck reaction of cyclohexene with iodobenzene.
4
5
6
7
8
Two palladium complexes, Pd(OAc)2 and PdCl2(PhCN)2, were tested in the Heck reaction under
selected reaction conditions (140 ºC, 4 h) in DMF as solvent and produced less than 15% of products.
The most plausible explanation for such low activity is a fast deactivation of the catalyst caused by its
reduction to palladium black. The successful Heck coupling procedures of cycloalkenes with aryl
halides reported up to now consisted in application of phosphorus ligands, preventing formation of
inactive black palladium [33–40]. Therefore, in subsequent experiments molten [Bu4N]Br was used as
the reaction medium. A positive effect of tetrabutylammonium salt was demonstrated earlier in other
palladium catalyzed systems [11,20,43] and it was expected that also in the studied systems [Bu4N]Br
would stabilize reduced palladium species (i.e. nanoparticles) formed in situ.
Figures 3 and 4 present the correlations between the total yield of the Heck reaction and the amount
of palladium for both of the systems studied. Characteristically, in a case of Pd(OAc)2 the dependence
was non-linear: the yield increased with an increase in the amount of palladium and then decreased.
This type of concentration dependence had been observed earlier in other Heck reactions and
interpreted as an indication of the participation of Pd(0) nanoparticles in the reaction course [44]. The
formation of Pd(0) nanoparticles can be considered in the systems studied in agreement with XPS
results that shown, that when PdCl2(PhCN)2 reacted with NaHCO3 in the presence of [Bu4N]Br ca.
50% of palladium was reduced to Pd(0) [22]. Most probably, Pd(0) nanoparticles stabilized by
[Bu4N]Br were formed, similar to these described for Pd-chitosan nanocomposite [45]. In the presence
of [Bu4N]Br and PhI palladium nanoparticles are dissolved forming soluble complexes of Pd(II), also
catalytically active. A special role in the catalytic process is played by soluble complexes containing
aryl ligands, e.g. [Bu4N]2[Pd(Ar)X3], key intermediates in the catalytic cycle [8].

Molecules 2010, 15
Figure 3. The yield of mono- and diarylated cyclohexenes obtained in the Heck reaction
2169
catalyzed by Pd(OAc)2 in [Bu4N]Br (140 ºC, 4 h, NaHCO3).
100
80
60
di
40
mono
20
0
0.03 0.06 0.18 0.35 0.39 0.66 0.70 0.92
Pd % mol
Figure 4. The yield of mono- and diarylated cyclohexenes obtained in the Heck reaction
catalyzed by PdCl2(PhCN)2 in [Bu4N]Br (140 ºC, 4 h, NaHCO3).
100
80
60
di
40
mono
20
0
0.09 0.10 0.19 0.30 0.39 0.40 0.78
Pd % mol
From the dependences shown in Figures 3 and 4, it is possible to estimate the optimal range of the
palladium amount in both systems. For Pd(OAc)2, the highest yields of coupling products were
obtained when the amount of palladium was 0.35–0.39% mol. For PdCl2(PhCN)2, the range of
applicability was wider, 0.30–0.78% mol, and an effect of the amount of palladium was less
significant.
The yield of monoarylated products was similar for both catalysts, the maximum being 60% and the
average amount of diarylated products being ca. 18%. In Table 1 there are collected results illustrating
an effect of base on the reaction yield and selectivity. In the presence of NaOAc the yield of diarylated
products increased to ca. 30%, whereas amount of monoarylated cyclohexenes reminded close to 50%.
When KOH was used as a base 60–67% of monoarylated products were formed. As expected, product

Molecules 2010, 15
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3 was the main one in the fraction of monoarylated cyclohexenes and the highest selectivity to 3 was
noted for KOH as a base.
Table 1. Results of the Heck coupling of iodobenzene with cyclohexene catalyzed by
Pd(OAc)2 and PdCl2(PhCN)2 in [Bu4N]Br.
PhI
conversion
(%)
Yield
1+2+3
(%)
55
Selectivity
1:2:3
Yield
Biphenyl
(%)
Catalyst
Base
4+5+6+7+8
(%)
16
27
23
17
30
20
Pd(OAc)2
NaHCO3
NaOAc
KOH
74
16:20:64
17:21:62
13:3:84
4
90
53
9
100
67
10
11
15
20
PdCl2(PhCN)2 NaHCO3
81
53
19:19:62
18:28:54
15:5:80
NaOAc
88
53
KOH
100
60
Reaction conditions: 2.68 × 10-3 mol of PhI, 8.77 × 10-3 mol of cyclohexene, 0.53% mol Pd, 2.5 ×
10-3 mol of NaHCO3, 5 cm3 of DMF or 2.3 × 10-3 mol of [Bu4N]Br, 140 ºC, 4 h.
2.2. Heterogeneous catalysts
For further studies, we selected heterogenized palladium catalysts obtained by impregnation of
PdCl2 on alumina-based oxides. Alumina and three different alumina-based oxides containing 10% of
ZrO2, or Fe2O3 or 2% of CeO2, were used as supports. The presence of the second metal oxide
modifies the physicochemical properties of alumina, which can influence the catalytic activity. For
example, an effect of CeO2 on acid-base properties of alumina was observed [46] and an increase of
the hydrothermal stability of mixed Al2O3-ZrO2 oxides was noted [47,48]. It was also expected, that
application of four different alumina-based oxides as supports for palladium catalysts would allow to
formulate some more general conclusions about the catalytic activity of heterogeneous systems in the
arylation of cyclohexene.
First, the effect of the solvent was studied for one selected catalyst, Pd/Al2O3 + CeO2, at different
palladium concentrations (Table 2). When the amount of palladium increased from 0.26% mol to
0.35% mol, both the conversion of iodobenzene and the yield of monoarylated products 1–3 increased
in DMF as solvent. The maximum yield of products 1–3 was 46% with selectivity to 3 of 74%. In
these reactions, the yield of diarylated olefins and biphenyl remained at a similar level, 4–9%. When
instead of DMF molten [Bu4N]Br was used as the reaction medium, the conversion increased to
79–91%, but selectivity decreased remarkably. The monoarylated products were obtained with a yield
of ca. 40%, whereas the amount of diarylated olefins increased to ca. 30% and that of biphenyl to
16%. It is interesting to note that selectivity in the fraction of monoarylated products also changed and
product 2 became the main one, whereas 3 predominated in reactions performed in DMF.
In agreement with earlier results an intensive leaching of palladium from the support can be
proposed for the catalytic system in [Bu4N]Br, leading to formation of anionic species of the type
[Bu4N]2[PdX4], similar to these present in a homogeneous system. However, the different composition
of final products of Heck coupling formed with homogeneous and heterogeneous palladium precursors
in tetrabutylammonium salt can indicate that supported palladium is also involved in the reaction

Molecules 2010, 15
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course. XRD and TEM analyses of one selected catalyst Pd/Al2O3-Fe2O3 performed after Heck
reaction confirmed the presence of Pd(0) nanoparticles of diameter ca. 17 nm (XRD) or 15–50 nm
(TEM) (Figures 5 and 6).
Figure 5. XRD pattern of Al2O3-Fe2O3 support and Pd/Al2O3-Fe2O3 after Heck reaction.
Figure 6. TEM of the Pd/Al2O3-Fe2O3 after Heck reaction.
The effect of PPh3 on the Heck coupling was also studied for comparison; however, it can be
described as an inhibiting one, lowering the yield of monoarylated products (Table 1). Such an
inhibiting effect can be explained to some extent by the reducing properties of PPh3. However PPh3
can also occupy active sites on the surface of the catalyst making it less accessible for coordination of
reactants.

Molecules 2010, 15
Table 2. Results of the Heck coupling of iodobenzene with cyclohexene catalyzed by
2172
Pd/Al2O3+CeO2 in DMF and in [Bu4N]Br.
PhI
Yield
1+2+3
(%)
31
Selectivity
1:2:3
Yield
Pd%
mol
[PPh3]:[Pd]
Biphenyl
(%)
Solvent
conversion
4+5+6+7+8
(%)
39
79
43
91
64
85
27
32
(%)
4
0.26
0.26
0.35
0.35
0.53
0.53
0.35
0.35
DMF
16:13:71
16:47:38
15:12:73
13:49:38
14:10:76
13:57:39
18:12:70
18:5:77
5
15
6
[Bu4N]Br
DMF
32
31
5
33
[Bu4N]Br
DMF
45
30
10
29
4
16
12
16
5
42
[Bu4N]Br
DMF
40
5
17
DMF
10
22
4
5
Reaction conditions: 2.68 × 10-3 mol of PhI, 8.77 × 10-3 mol of cyclohexene, 2.5 × 10-3 mol of
NaHCO3, 5 cm3 of DMF or 2.3 × 10-3 mol of [Bu4N]Br, 140 ºC, 4 h.
Further studies were continued in DMF as a solvent offering a higher selectivity in Heck coupling.
Four palladium catalysts supported on different mixed-oxides used without any additives, produced up
to 50% of monoarylated products. To check whether any improvement of selectivity was possible,
different bases were tested for the catalyst Pd/Al2O3 (Table 3). With Cs2CO3 and K2CO3, an increase in
biphenyl yield was noted, even up to 29%. Much better results were found when KOH or NaOH was
used, KOH being superior to NaOH if the total yield of monoarylated cyclohexenes and selectivity to
product 3 are taken into account. A similar improvement in yield and selectivity was also noted for
other palladium catalysts supported on Al2O3 + ZrO2 and Al2O3 + Fe2O3, which gave 78% and 81% of
monoarylated products when KOH was used as a base. Moreover, selectivity to 3 exceeded 80%
Table 3. Results of the Heck coupling of iodobenzene with cyclohexene catalyzed by Pd
supported on alumina-based oxides in DMF.
PhI
Yield
1+2+3
(%)
41
Selectivity
1:2:3
Yield
Biphenyl
(%)
Catalyst
Base
conversion
4+5+6+7+8
(%)
66
89
64
90
63
98
58
57
86
90
88
(%)
9
Pd/Al2O3 + ZrO2
Pd/Al2O3 + CeO2
Pd/Al2O3 + Fe2O3
Pd/Al2O3
NaHCO3
KOH
17:3:80
13:3:84
14:10:76
14:4:82
15:6:79
11:1:88
16:8:76
16:7:77
15:12:73
17:4:79
12:0:88
14
7
78
4
NaHCO3
KOH
42
10
18
7
12
15
8
67
NaHCO3
KOH
47
81
12
7
5
NaHCO3
NaOAc
K2CO3
NaOH
KOH
37
14
8
43
7
48
9
29
15
11
53
22
12
65
Reaction conditions: 2.68 × 10-3 mol of PhI, 8.77 × 10-3 mol of cyclohexene, 0.53% mol
Pd, 2.5 × 10-3 mol of NaHCO3, 5 cm3 of DMF, 140 ºC, 4 h.

Molecules 2010, 15
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3. Experimental
3.1. Preparation of alumina-based supports
Alumina-based oxides were prepared by the sol-gel technique [41,42]. The proportion of ZrO2,
Fe2O3 and CeO2 to Al2O3 was equal 10:100, 10:100 and 2:100. Specific surface area estimated
according to BET method was ca. 200 m2/g.
3.2. Preparation of supported Pd(II) catalysts
First, the support (0.8 g) was impregnated, with stirring, in an aqueous acidic solution (8 cm3,
CHCl = 0.09 mol/dm3) of PdCl2, containing 40 mg of Pd. After 72 h, the solution was decanted, and the
Pd(II)-containing catalyst was washed three times with water and dried. The palladium content was
estimated by the ICP method after mineralization of the weighted sample with aqua regia. The amount
of palladium was from 1.3 to 1.9 wt % in the final catalysts.
3.3. Heck reaction procedure
The Heck reactions of iodobenzene with cyclohexene were carried out in a 50 cm3 Schlenk tube
with magnetic stirring. The reagents: the catalyst, PdCl2(PhCN)2 (2.5 × 10-6–2.09 × 10-5 mol) or
Pd(OAc)2 (8.46 × 10-7–2.01 × 10-5 mol) or Pd supported on alumina-based oxide (7.0 × 10-6,
9.4 × 10-6 or 1.41 × 10-5 mol), PhI (0.3 cm3, 2.68 × 10−3 mol), cyclohexene (0.5 cm3, 8.77 × 10−3 mol),
base (NaHCO3, NaOAc, K2CO3, KOH, NaOH, 2.5x10-3 mol), and the solvent, [Bu4N]Br (0.75 g, 2.3 ×
10−3 mol) or DMF (5 cm3), were introduced to the Schlenk tube under an N2 atmosphere. The tube was
closed, and the reaction was carried out at 140 ºC for 4 h. Afterwards, organic products were separated
by extraction with diethyl ether (twice with 10 cm3) and analyzed by GC–MS (Hewlett Packard
8452A) with dodecane (0.1 cm3, 4.4x10-4 mol) as internal standard. For reactions performed in DMF,
diethyl ether (15 cm3) was added, and a sample was quenched with water (10 cm3). Organic products
were extracted from the water phase with diethyl ether (15 cm3), and a combined ether solution was
analyzed by GC. Products 1, 2, and 3 were identified by comparison of their MS spectra with literature
data [49]. Five isomers of diarylated cyclohexenes were found in the GC, and their composition
was confirmed by MS (m/z = 234, M+).
4. Conclusions
The Heck coupling of iodobenzene with cyclohexene performed with Pd(OAc)2 and PdCl2(PhCN)2
as catalyst precursors in molten [Bu4N]Br salt resulted in a maximum yield of monoarylated products
amounting to ca. 60%. The soluble palladium species, [Bu4N]2[PdX4] complexes, and Pd(0)
nanoparticles stabilized by tetrabutylammonium salt were most probably responsible for the observed
reactivity. The same precursors used in DMF presented a very low catalytic activity. In contrast,
palladium catalysts containing Pd(II) supported on Al2O3 and alumina-based mixed oxides, Al2O3-
ZrO2, Al2O3-CeO2, Al2O3-Fe2O3, were quite effective in DMF, producing up to 46% of monoarylated
products, with 4-phenylcyclohexene (3) being the predominant one. The same catalysts used in
[Bu4N]Br presented a lower selectivity and produced 3-phenylcyclohexene (2) and 4-phenyl-

Molecules 2010, 15
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cyclohexene (3) in comparable amounts, with a slight excess of 2. Different results obtained in with
homogeneous and heterogeneous precursors can be explained by the contribution in the reaction
course of insoluble, supported palladium species besides of soluble ones. The most attractive and
promising results, up to 81% of monoarylated products, were obtained with heterogenized palladium
catalysts in DMF, with KOH as the base. Excellent selectivity to 3 was also noted under such
conditions.
Acknowledgements
Financial support of the Polish Ministry of Science and Higher Education (N164/COST/2008) and
COST D40 are gratefully acknowledged. The authors are grateful to Hanna Grabowska and Mirosław
Zawadzki (Polish Academy of Sciences, Wrocław) for preparation of alumina-based oxides, to Ms
Agnieszka Boroń for testing of homogeneous catalysts, to Mr Marek Hojniak (Faculty of Chemistry,
University of Wrocław) for GC and GC-MS analyses and to Mr Tomasz Borkowski and dr Wojciech
Gil (Faculty of Chemistry, University of Wrocław) for XRD and TEM analyses.
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Sample Availability: Samples of the compounds are available from the authors.
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