Oxygen-promoted coupling of arylboronic acids with olefins catalyzed by [CA]2[PdX4] complexes without a base

Article abstract of DOI:10.1016/j.molcata.2015.07.003

Abstract An efficient method was developed for the oxidative Heck reaction between arylboronic acids and olefins catalyzed by anionic palladium complexes of the type [CA]_x[PdCl₄] (x = 1,2) and [CA]₂[Pd₂Cl₆] (CA = imidazolium or pyridinium cation). Molecular oxygen was employed as an environmentally benign oxidant to regenerate Pd(II) species during the reaction. The elaborated protocol could be applied to the coupling of different arylboronic acids with both electron-rich and electron-poor olefins. The catalyst system was further employed for the one-pot oxidative Heck reaction followed by the Heck coupling to build conjugated compounds in good yield. Mass spectrometry (ESI-MS) and UV-vis spectroscopy was used to identify palladium-containing complexes in the reactions. A plausible reaction mechanism of the oxidative Heck reaction was proposed.

Full text of DOI:10.1016/j.molcata.2015.07.003

Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Oxygen-promoted coupling of arylboronic acids with olefins
catalyzed by [CA]2[PdX4] complexes without a base
E. Silarska, A.M. Trzeciak∗
University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method was developed for the oxidative Heck reaction between arylboronic acids and
olefins catalyzed by anionic palladium complexes of the type [CA]x[PdCl4] (x = 1,2) and [CA]2[Pd2Cl6]
(CA = imidazolium or pyridinium cation). Molecular oxygen was employed as an environmentally benign
oxidant to regenerate Pd(II) species during the reaction. The elaborated protocol could be applied to the
coupling of different arylboronic acids with both electron-rich and electron-poor olefins. The catalyst
system was further employed for the one-pot oxidative Heck reaction followed by the Heck coupling to
build conjugated compounds in good yield.
Mass spectrometry (ESI–MS) and UV–vis spectroscopy was used to identify palladium-containing
complexes in the reactions. A plausible reaction mechanism of the oxidative Heck reaction was proposed.
© 2015 Elsevier B.V. All rights reserved.
Received 22 March 2015
Received in revised form 24 June 2015
Accepted 1 July 2015
Available online 9 July 2015
Keywords:
Oxidative Heck reaction
Palladium
Ionic liquids
Arylboronic acids
Oxygen
1. Introduction
Accordingly, it has been proposed that palladium(II) is the catalyt-
ically active species, and the application of Cu(OAc)2 as an oxidant
Structural motifs of arylated olefins are present in natural prod-
ucts, pharmaceuticals, and other organic materials [1–8]. One of
the standard tools used for the preparation of arylated olefins is
the Heck reaction, which is the palladium-catalyzed coupling of an
aryl halide and an olefin [1–8]. Despite its high potential and many
applications, this reaction has some drawbacks such as harsh reac-
tion conditions, the requirement of an inert atmosphere, and the
generation of hydrogen halide as a byproduct [1–10]. In addition to
the conventional Heck reaction, olefins can also be arylated by the
use of nucleophilic organometallic reagents such as aryl derivatives
of silicon [11–14], antimony [15], and tin [16–20]. Unfortunately,
these organometallic reagents and their byproducts are highly toxic
and difficult to separate from the final product. Therefore, another
method, the oxidative Heck reaction, based on the coupling of aryl-
boronic acid and an olefin (Fig. 1) has received increasing attention.
The advantages of this procedure consist in the elimination of a
halide from the reaction mixture and very often milder conditions
than in the original Heck reaction.
regenerating the catalytic amount of Pd(II) from Pd(0) has been
described [21–25]. Later, protocols for the oxidative Heck reac-
tion with silver salts as oxidants have been developed [26–28].
Promising results have also been obtained in systems containing
Pd(OAc)2 and quinine [29,30] or DDQ (2,3-dichloro-5,6-dicyano-
1,4-benzoquinone) [31] as well as using Ni(acac)2 and TEMPO [66].
In 2003, Jung discovered that O2 could act as an efficient oxidant
of palladium in the catalytic cycle [32], and in 2006 he published the
base-free oxidative Heck-type reaction with improved selectivity
[33]. Up to now, only a few palladium(II) systems have been used in
the oxidative Heck reaction without a base [34–36,41]. On the other
hand, in order to get satisfactory results in the oxidative Heck-type
reaction with simple palladium precursors and O2 as an oxidant, the
employment of nitrogen [36–39] or phosphorous ligands [40,42] is
necessary. The oxidative coupling of arenes with olefins has also
been performed under O2 [43] and promising results have been
noted with air used instead of O2 [31,39].
A relatively small number of premade palladium(II) com-
plexes have been employed for the Heck-type reaction under
an O2 atmosphere [34,36,44]. For example, a palladium com-
plex with N,N-dimethylethanolamine [36], a dimeric tridentate
NHC-amidate-alkoxidepalladium(II) [34] and imidazole-based SPO
(secondary phosphine oxide ligands) complexes [65] have been
successfully applied in the asymmetric Heck-type reaction.
In this paper, we present the studies of the oxidative Heck-
type reaction catalyzed by anionic palladium complexes of the
The oxidative Heck reaction in a stoichiometric version was
first described by Heck in 1975 [6]. Investigations of the reaction
mechanism have shown that an oxidant is required for the effi-
cient catalytic coupling of the olefin with arylboronic acid [21–37].
∗
Corresponding author. Fax: +48 71 3282348.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
http://dx.doi.org/10.1016/j.molcata.2015.07.003
1381-1169/© 2015 Elsevier B.V. All rights reserved.

2
E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
HECK REACTION
X
R
[Pd]
R
HX
+
+
base,
N2
X= Cl, Br, I
OXIDATIVE HECK REACTION
B(OH)2
R
R
[Pd]
R
+
base, oxidant
+
Fig. 1. Scheme of Heck and oxidative Heck reactions.
[CA]x[PdCl4] and [CA]2[Pd2Cl6 ]
x=1,2
[CA] =
NCH6C3
C3H6CN
+
N
+
N
+
C4H9
CH2OC3H7
H9C4
+
N
N
N
N
N
[bmpy] [PdCl ]
(9)
2
4
[bcpim] [PdCl ]
(2)
[dmiop]2[PdCl4]
(3)
(1)
2
4
[bmim]2[PdCl4]
[bmim] [Pd Cl ]
[bmpy] [Pd Cl ]
(9a)
6
2
2
(1a)
6
2
2
+
N
C3H6CN
+
N
+
N
+
N
N
N
[cppy]2[PdCl4]
[cppy]2[Pd2Cl6]
(10)
N
(10a)
(4)
[HIMes]2[PdCl4]
(5)
[HSIMes]2[PdCl4]
[HIPr]2[PdCl4]
[HIPr]2[Pd2Cl6]
(6)
(6a)
CH2OCH2CH2CH2OCH2
N
N
CH2OCH2CH2CH2OCH2
N
+
N
+
N
N
+
+
N
N
(8)
[mdim][PdCl4]
Fig. 2. Anionic palladium(II) complexes.
(7)
[dmdim][PdCl4]
type [CA]x[PdCl4] and [CA]2[Pd2Cl6] (x = 1 or 2) and [CA]2[Pd2Cl6]
(IL = imidazolium or pyridinium cation) (Fig. 2).
2. Results and discussion
Considering the very good catalytic activity of these complexes
in methoxycarbonylation [45] and in the Suzuki reaction [46,47] in
which PhB(OH)2 was used as a substrate, we also expected good
results in the oxidative Heck-type reaction.
Air-stable anionic Pd(II) complexes can be easily prepared using
any imidazolium or pyridinium salt. In addition, the presence of
[CA]+ cations of different steric hindrance can influence the cat-
alytic reaction course, as it had already been observed earlier
[46,47]. On the other hand, the use of ionic liquids in a catalytic
reaction can be controversial because of their potential toxicity
[67,68].
2.1. Optimization of oxidative Heck reaction conditions
In the initial step of our studies, we tested different palladium
precursors and oxidants in a model reaction of butyl acrylate with
phenylboronic acid at 130 ◦C over 4 h (Fig. 3). In reactions per-
formed under base-free conditions, one product, butyl cinnamate,
was selectively formed. In contrast, in reactions catalyzed by cat. 9
in the presence of K2CO3, selectivity was lower and a side product,
butyl 3-phenylpropanoate, was found besides the major product
(Table 1, entry 6, 7). A similar negative influence of the base on reac-
tion selectivity has also been reported for other catalytic systems
[26,64].
Three different oxidants were employed in test reactions,
namely air, O2 and Cu(OAc)2. In almost all cases, independently
of the kind of palladium precursor used, O2 gave the best results
providing the highest yield of the product. It should be underlined
here that all the experiments were performed under low pressure,
1 atm only from a baloon. The highest yield, 83% was achieved
after 4 h with cat. 9 (Table 1, entry 5), and the prolongation of
the reaction time to 24 h resulted in a yield increase to 98%. Two
other palladium precursors containing a chloride ligand, PdCl2 and
We decided to test a group of anionic palladium complexes with
the aim of elaborating an efficient and simple catalytic procedure
that could be applied for a variety of substrates. To that aim, not
only conventional heating but also microwave energy was used.
B(OH)2
[Pd], oxidant
time,T, DMF
COOBu
COOBu
+
Fig. 3. Oxidative Heck-type reaction of phenylboronic acid and n-butyl acrylate.

E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
3
Table 1
Table 2
Optimization of the oxidative Heck reaction with different palladium complexes and
oxidants.
Oxidative Heck reaction in different solvents.
conversion [%]a
solvent
2-propanol
4
2-propanol/ H2O
1
H2O
22
THF
5
DMSO
26
NMP
60
Reaction conditions: butyl acrylate (1 mmol), phenylboronic acid (1.5 mmol), cat.9
(2 mol%), solvent (5 mL), O2 atmosphere, 130 ◦C, 4 h.
conversion [%]*
a
Estimated by GC.
Entry
Oxidant
catalyst
air
O2
Cu(OAc)2
Time [h]
2.3. Effect of the solvent
1
2
3
4
5
6
7
PdCl2
PdCl2(cod)
Pd2dba3
Pd(OAc)2
Cat.9
Cat.9/K2CO3
Cat.9/K2CO3
18
10
15
21
60
63
23
33
30
44
10
65
4
4
4
4
4
4
24
Not only temperature but also the kind of solvent influenced the
yield of butyl cinnamate. The reaction studied can be efficiently
performed in DMF, and a good yield, 60%, was also obtained in
NMP (Table 2). Other tested solvents were not very suitable for
the efficient formation of the product. Interestingly, 22% of butyl
cinnamate was formed in water in which cat. 9 was completely
insoluble.
On the other hand, 2-propanol and a 2-propanol/water mixture
made it possible to obtain only traces of the product, while these
solvents worked very well in the Suzuki–Miyaura cross-coupling
catalyzed by anionic palladium complexes [46,47].
21
83
50
14/16a)
20/29a)
22/30a)
78/80a)
21/38a)
32/43a)
Reaction conditions: butyl acrylate (1 mmol), phenylboronic acid (1.5 mmol), [Pd]
(2 mol%), DMF (5 mL), 130 ◦C, 4 h, Cu(OAc)2 (2 mmol).
a) Conversion to butyl cinnamate/total conversion.
*
Estimated by GC.
PdCl2(cod) (Table 1, entry 1, 2) also formed good amounts of butyl
cinnamate, 60% and 63%. Similarly as was noted earlier by other
authors, Pd(OAc)2 worked efficiently in the presence of Cu(OAc)2
[11] (Table 1, entry 4); however, with other oxidants, the results
were not satisfactory. Air appeared to be an insufficient oxidant,
providing 21% of the product in the best case.
A relatively low yield obtained with Pd2(dba)3 as the catalyst
precursor (Table 1, entry 3) indicated a preference for Pd(II) over
Pd(0) in the reaction studied. In particular, Pd2(dba)3 was not very
efficient, while it worked very well in the oxidative Heck reaction
performed with the addition of a base [37]. According to the litera-
ture [48], Pd2(dba)3 can be considered a mixture of a Pd(0) complex
and Pd(0) nanoparticles, and most probably those forms were not
suitable for catalysis under the applied conditions Table 1.
2.4. Effect of the catalyst amount
Studies of the concentration effect resulted in finding the opti-
mal amount of the catalyst, which was estimated to be 1–2% mol.
An increase in the product yield with the increase in the catalyst
amount (Fig. 5) warranted the conclusion that the reaction is cat-
alyzed by a soluble Pd(II) species rather than by Pd(0) nanoparticles,
which is the preferred catalyst for many cross-coupling reactions
[1–10]. When Pd(0) nanoparticles are involved in the catalytic pro-
cess, non-linear dependence of the product yield versus palladium
amount is expected, with characteristic yield decrease at higher
palladium concentrations [49]. This was explained by agglomer-
ation of Pd(0) nanoparticles leading to decrease of their activity
at higher concentration. Actually, it can be assumed that such
non-linear yield dependence on the Pd amount indicated on the
important role of Pd(0) nanoparticles in the reaction [49]. In the
studied system, the participation of Pd(0) nanoparticles in the
reaction course seems to be rather limited because even a high
concentration of palladium did not result in reaction inhibition.
It should be noted here that the dimeric cat. 9a gave better
results in all experiments than the monomeric cat.9 when the same
palladium amount was used. One possible explanation could be
splitting of the dimers under reaction conditions with formation
coordinatively unsaturated species of higher activity. Alternatively,
activation of substrates on two, situated closely palladium centers
could accelerate the reaction course.
2.2. Effect of temperature and reaction time
The optimization of the reaction conditions, first temperature,
was performed for cat. 9. The temperature dependence, presented
on Fig. 4, shows the absence of an induction period in the stud-
ied reaction, and consequently, ca. 10% of the product was formed
after 5 min. It can, therefore, be supposed that the eventual trans-
formation of the palladium precursor to a catalytically active form
is relatively fast. As expected, a higher temperature facilitated the
formation of the products and the highest yield, 83%, was obtained
at 130 ◦C. At 70 ◦C, the yield was 45% after 4 h, and it increased only
to 55% after 24 h.
Fig. 4. Kinetic profile of the oxidative Heck reaction in different temperatures.
Fig. 5. Effect of palladium amount on oxidative Heck reaction course.

4
E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
Table 3
tion of that could be too slow formation of palladates under in situ
conditions.
The oxidative Heck reaction catalyzed by [CA]x[PdCl4] or [CA]2[Pd2Cl6] complexes.
conversion [%]a
2.7. Effect of substrates: olefins and arylboronic acids
[CA]+
[CA]2[PdCl4]
[CA]2[Pd2Cl6]
98
PdCl2(cod) + [CA]Cl
bmim
dmiop
bcpim
HIMes
HSIMes
HIPr
dmdim
mdim
bmpy
cppy
74
54
72
88
79
78
65
40
83
72
38
42
30
63
48
24
21
19
36
40
The oxidative Heck-type reactions were next performed using
different olefins and three catalysts cat. 9, cat. 9a, and cat. 1a
(Table 4). Methyl acrylate, styrene, and allylbenzene reacted very
well producing the expected products with a high selectivity. In
reactions with cyclic olefins as substrates, the selectivity was lower,
and two products were formed for cyclohexene-3-on (Table 4, entry
2) and for cyclohexene (Table 4, entry 4). The saturated ketone,
found in the reaction of cyclohexene-3-on, was formed via conju-
gate addition reaction involving intermediate form in which ketone
coordinated to palladium via Pd-O bond [69]. The isomerization of
the arylated cycloheptene resulted in the formation of four prod-
ucts in comparable amounts (Table 4, entry 6). Only a small increase
in the product yield was achieved after the prolongation of the reac-
tion time to 7 h. Under the applied conditions, cyclopentene did not
react with phenylboronic acid.
86
91
80
Reaction conditions: butyl acrylate (1 mmol), phenylboronic acid (1.5 mmol), [Pd]
(2 mol%), DMF (5mL), O2 atmosphere, 130 ◦C, 4 h.
a
Estimated by GC.
2.5. Effect of the O2 concentration
In order to estimate the scope and the limitations of the elabo-
rated procedure, a series of arylboronic acids were used for coupling
with butyl acrylate (Table 5) in the presence of cat. 9 and O2 as an
oxidant. For comparison, reactions with Cu(OAc)2 instead of O2 as
an oxidant were also performed. Moreover, in one series of exper-
iments, microwave induction was used instead of conventional
heating.
In all cases, reactions with O2 gave better results than those with
Cu(OAc)2, confirming the good applicability of our procedure. Reac-
tions performed under microwave irradiation in an O2 atmosphere
also gave good results already after 3 h. It should be pointed out
that the successful application of microwaves was possible because
the amount of O2 present in the Schlenk tube was suitable for the
reaction.
High yields of the coupling products were obtained using
boronic acid with an electrodonor group (Table 5, entries 3–7). The
presence of a -Br substituent in position 2 or 4 of the phenyl ring
did not influence the yield of the product when O2 was used as an
oxidant (Table 5, entries 8 and 9). On the other hand, a reaction with
Cu(OAc)2 4-bromophenylboronic acid offered better performance
than with 2-bromophenylboronic acid Table 5.
In order to get more information about the catalyst stability
and the applicability of the system, a two-step reaction in one-pot
was performed. First, using the oxidative Heck procedure, (E)-butyl
3-(2-bromophenyl) acrylate was obtained. Next, the base K2CO3
and a new portion of butyl acrylate were added to the reaction
mixture. The second step, performed according to the Heck mech-
anism, resulted in the formation of a (2E,2ꢀE)-3,3ꢀ-(1,2-phenylene)
bis[2-propenoic acid] dibutyl ester with 80% yield (Scheme 1).
In an analogous experiment, (E)-butyl 3-(4-bromophenyl) acry-
late reacted with 62% yield.
After the selection of O2 as the preferred oxidant, we optimized
the procedure of its introduction to the reactor. When the cat-
alytic reaction was performed with a balloon filled with O2 and
connected to the Schlenk tube, the results were often not fully
reproducible. Most probably, it was caused by the transfer of some
amount of the reactants to the balloon at the reaction temperature
(130 ◦C). Further experiments showed that the yield of butyl cin-
namate remarkably increased when O2 was bubbled through the
solvent (DMF) before the reaction. For an efficient catalytic process,
it was enough to use the solvent saturated with O2 and to perform
the reaction in the Schlenk tube filled with O2. The volume of O2 in
the Schlenk tube was sufficient for the catalytic reaction, and the
balloon was not needed. It was also checked that 5 min of saturation
of DMF with O2 already guaranteed the formation of the product
with 80% yield. The prolongation of the saturation time to 10 or
60 min in fact had no further influence on the reaction course.
On the other hand, when DMF not saturated with O2 was used,
the reaction yield was lower, and only 59% of the product was
formed. When DMF was saturated with N2 instead of O2, the yield
was only 9%.
The elaborated experimental procedure, involving the use of a
solvent saturated with O2 and Schlenk tube filled with O2 is quite
simple. Thus, the reaction can be performed without the balloon.
2.6. Catalytic activity of different Pd(II) complexes
An optimized procedure was next used for testing the catalytic
activity of all the synthesized anionic complexes. Accordingly,
reactions were performed at 130 ◦C, using 2 mol% [Pd] in DMF sat-
urated with O2. The results shown in Table 3 evidence the good or
very good catalytic performance of monomeric as well as dimeric
precursors. It should be underlined that the catalytic results are
slightly influenced by the kind of cation present in the anionic
complexes. In particular, complexes containing imidazolium or
pyridinium cations performed good. It is interesting because only
imidazolium complexes were active in the Suzuki–Miyaura cross-
coupling while pyridinium cations showed a strong inhibiting
effect [45]. The highest yield was noted for dimeric complexes
[bmim]2[Pd2Cl6] (cat. 1a) (98%) and for [bmpy]2[Pd2Cl6] (cat. 9a)
(91%). The best monomeric catalysts, cat. 9 ([bmpy]2[PdCl4]) and
cat. 4 ([HIMes]2[PdCl4]), formed 83% and 88% of butyl cinnamate.
Thus, the steric hindrance of the cation had positive influence on
the product yield. Interestingly, a reaction catalyzed by PdCl2(cod)
and ionic liquids ([CA]Cl) in all cases produced significantly worse
results than premade anionic complexes. One possible explana-
It can, therefore, be concluded that cat. 9 after an oxidative
Heck reaction is still active and, in particular, can be applied in the
Heck cross-coupling with a very good result. The first coupling is
very selective, without the formation of the Heck product. The final
product was formed with good yield without the separation of an
intermediate cinnamate.
2.8. UV–vis monitoring of O2 activation
The performed studies clearly showed that O2 played an impor-
tant role in the catalytic cycle. We selected UV–vis spectroscopy as
a tool to monitor the reaction of [bmpy]2[PdCl4] with O2 at 130 ◦C.
The first spectrum showed a weak band at 336 nm characteristic
of a [PdCl4]2− anion (Fig. 6). When the solution was heated under
an O2 atmosphere, this band disappeared, and a new band was

E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
5
Table 4
The oxidative Heck reaction of phenylboronic acids with various olefins.
conversion [%]a
Cat.9
Cat.9a
Cat.1a
Entry
1
Olefin
Product
4 h
7 h
68
4 h
4 h
65
48
79
65
78
46
2
52
27
88
20
93
34
98
18
98
3
4
36
38
19
26
15
94
18
89
4
11
60
5
6
85
12
9
12
12
13
23
19
10
17
114
13
24
19
13
23
19
7
0
0
0
0
Reaction conditions: olefin (1 mmol), phenylboronic acid (1.5 mmol), [Pd] (2 mol%), DMF (5 mL), O2 atmosphere, 130 ◦C,4 h.
a
Estimated by GC.
BuOOC
B(OH)2
Br
Br
COOBu
COOBu
+ butyl acrylate
K2CO3, N2, 4h
cat.9,
O2
COOBu
+
DMF, 4h
80%
90%
B(OH)2
Br
COOBu
COOBu
+ butyl acrylate
K2CO3, N2, 4h
cat.9,
O2
COOBu
+
DMF, 4h
Br
BuOOC
89%
62%
Scheme 1. One-pot oxidative Heck reaction and Heck reaction.

6
E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
Table 5
The oxidative Heck reaction of butyl acrylate with arylboronic acids.
Entry
1
Ar-B(OH)2
Product
O2
83
Cu(OAc)2
50
MWa)
85
2
3
80
49
50
16
81
47
4
5
80
49
0
79
50
23
6
7
50
80
36
51
51
80
8
9
90
89
31
51
96
89
10
11
12
82
0
15
0
83
0
37
25
43

E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
7
Table 5 (Continued)
Entry
13
Ar-B(OH)2
Product
O2
80
Cu(OAc)2
46
MWa)
80
Reaction conditions: butyl acrylate (1 mmol), arylboronic acid (1.5 mmol), cat.9 (2 mol%), DMF (5 mL), O2 atmosphere, 130 ◦C, 4 h.
a) 3 h reaction.
Table 6
ESI–MS(−) data of palladium species obtained in the reaction of cat.9 + O2 and
cat.9 + substrates + O2 in DMF.
Cat.9
Cat.9 + PhB(OH)2 + butyl acrylate
[m/z]
[m/z]
[PdCl3]-
[PdCl2]
[PdCl]-
212.8
177.8
141.9
[HPdCl(butyl acrylate)(Ph)]-
[HPdCl2(butyl acrylate)]-
[PdCl3]-
347.0
306.9
212.8
177.8
141.9
[PdCl2]
[PdCl]-
Reaction conditions: butyl acrylate (1 mmol), phenylboronic acid (1.5 mmol), cat.9
(2 mol%), DMF (5 mL), O2 atmosphere, 130 ◦C, 4 h.
The analysis of the second solution, containing cat. 9, PhB(OH)2
and butyl acrylate, made it possible to identify fragments in which
substrates were coordinated to palladium. Interestingly, the simul-
taneous coordination of butyl acrylate and the phenyl group to the
same palladium ion was confirmed. In all palladium species, a Cl−
anion was also present indicating its important role in the catalytic
cycle.
Fig. 6. UV–vis absorption spectra monitoring the reaction of cat. 9 with O2 at 130 ◦
in DMF.
C
formed at 400 nm, with the intensity increasing over time. Similar
changes were also observed for the sample heated in air, although
the absorption increase was slower.
In the spectrum of the solution of cat. 9a saturated with O2, a
new band appeared at 408 nm confirming the formation of a new
species.
2.10. Mercury test
The mercury test is often used in order to distinguish between
the reaction pathway with Pd(0) nanoparticles and a truly homo-
geneous reaction catalyzed by soluble complexes [60,61]. Using
this test, we aimed to prove the dominating role of Pd(II) anionic
species in the catalytic process. In this case, the reaction should
be not affected by Hg(0). Fig. 7 presented results obtained in four
different experiments in which Hg(0) was added to the reaction
mixture after 5, 10, 20 or 30 min. Surprisingly, the oxidative Heck-
type reaction was strongly inhibited by a 500-fold excess of Hg(0)
as illustrated in Fig. 7.
In contrast, practically no changes in the UV–vis spectrum were
noted when a solution of cat. 9 in DMF was heated under N2. The
formation of a new band was also inhibited in the presence of
substrates, and, for example, no effect was observed when butyl
acrylate was present in the solution. It can be anticipated that O2
will compete with olefin for place in the coordination sphere of
palladium.
As far as we know, the activation of O2 on palladate complexes
has not been described before. On the other hand, it has been shown
by experimental and theoretical methods that Pd(0) complexes
with phosphines or chelating nitrogen ligands react with O2 prefer-
ably forming ␩2-peroxo Pd(II) complexes [50–56]. In the reaction
of a Pd(I) dimer with O2, the formation of a bridging peroxo ligand
has been proposed [57]. Scarce examples of reactions of Pd(II) com-
plexes with O2 leading to Pd(IV) complexes have been published
[58,59]. In our system, however, the observed changes in UV–vis
spectrum do not indicate the presence of Pd(IV).
The observed inhibiting effect could not be explained by
the formation of an amalgam with Pd(0) nanoparticles catalyz-
ing the reaction, because, according to the TEM analysis of the
2.9. ESI–MS measurements
ESI–MS spectra were measured in DMF solutions heated to
130 ◦C in an O2 atmosphere (Table 6). In all cases, the signals origi-
nating from palladium-containing fragments were observed in the
MS(−) part of the spectrum while only the [bmpy]+ cation was
observed in the MS(+) region. The spectrum of cat. 9 showed sig-
nals typical for anionic complexes, composed of palladium and Cl−
ligands.
Fig. 7. Mercury test in the oxidative Heck reaction.

8
E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
[CA]2[Pd2Cl6], easily prepared and stable in the presence of oxygen
O2
B(OH)2
X
X
and moisture, were used as catalysts for this reaction. The appli-
cation of oxygen (1 atm), instead of copper or silver salts, as a
reoxidant of palladium simplified the procedure.
L2Pd
R
According to the designed procedure, electron-donating and
electron-withdrawing arylboronic acids and olefins reacted over
4 h or 3 h with the use of microwaves, giving the desired product in
high yields.
H
X
XB(OH)2
L2Pd
R
A one-pot reaction composed of oxidative Heck and normal
Heck reactions was successfully applied for the synthesis of double
substituted arenes. It was also confirmed that after the first step,
the palladium catalyst was still active and could be used under the
Heck conditions.
X
R
L2Pd
L
Pd X
R
R
Using ESI–MS and UV–vis methods, some palladium(II) interme-
diates were identified. In particular, the ability of anionic complexes
to activate O2 was confirmed for the first time.
R
L = solvent
R
Fig. 9. Plausible mechanism of the oxidative Heck-type reaction.
4. Experimental
All reactants were obtained from Aldrich, Fluka or Merck in “for
synthesis” quality or higher, and were used as received without
further purification or drying.
Ionic liquids: [dmiop]Cl, [dmdim]Cl2 and [mdim]Cl2 were
obtained from prof. J. Pernak, Technical University Poznan´ .
+ O2
OOH
X
L2Pd
+ HX
- H2O2
H
X
X
X
L2Pd
L2Pd
+ 2 HX
- H2O2
+ O2
- HX
O
O
L2Pd0
Palladium
[dmiop]2[PdCl4],
[HSIMes]2[PdCl4],
complexes
[bcpim]2[PdCl4],
[HIMes]2[PdCl4],
[bmim]2[PdCl4],
[mioe]2[PdCl4],
[HIPr]2[PdCl4],
[dmdim][PdCl4],
L2Pd
Fig. 8. Possible reactions of the Pd-H intermediate with O2.
[mdim][PdCl4], [bmpy]2[PdCl4] were obtained according to
methods described in the literature [45–47].
post-reaction mixture, Pd(0) nanoparticles were not present in a
measurable amount.
4.1. Synthesis of palladium complexes
An alternative explanation of the observed mercury action could
be the redox reaction of Pd(II) with Hg(0) resulting in the formation
of Pd(0) and, in consequence, in the elimination of the catalytically
active palladium form. A similar reaction was described for Pt(II)
[62]. In order to prove such a hypothesis, the stoichiometric reac-
tion of cat.9 and Hg(0) was performed. As expected, after 30 min.
of warming, the solution became colorless and the Hg metal disap-
peared. Thus, the reduction of Pd(II) by Hg(0) is responsible for the
retardation of the catalytic reaction.
4.1.1. [cppy]2[PdCl4] (cppy = 1-(3-cyanopropyl) pyridinium
cation)
0.36 g (2.0 mmol) of [cppy]Cl was added to the solution of
0.28 g (1.0 mmol) PdCl2(cod) in hot CH3CN (5 mL). The mixture was
heated for 15 min, until the yellow solution became red. The solu-
tion was cooled to room temperature and solvent was removed
under reduced pressure. The product was precipitated by addi-
tion of diethyl ether (2–3 mL). Product yield: 85%; anal. calcd. for
PdCl4C18H22N4: C 39.8, H 4.1, N 10.3%; found: C 39.1, H 3.7, N 10.0%;
1H NMR (CD3CN): ı = 8.98 (t, J = 7.8, 2H, CH2), 8.59 (d, J = 7.8, 4H, Ar-
H), 7.36 (t, J = 7.8, 4H, Ar-H), 4.04 (m, J = 7.8, 4H, CH2), 2.14 (m, J = 7.8,
4H, CH2), 1.87 (t, J = 7.8, 4H, CH2); 13C NMR (CD3CN):ı = 148 (CAr-H),
146 (CAr-H), 130 (CAr-CH3), 119 (-CN), 53 (CH2), 25 (CH2), 15 (CH2).
2.11. Reaction scheme
A plausible mechanism for the oxidative Heck-type reaction in
the studied system, inspired by literature examples [31,36,44], is
shown in Fig. 9.
The most critical step in this mechanism is the elimination of
the hydrido-palladium intermediate, [HPdCl3]−, to recover a Pd(II)
species suitable for the next catalytic cycle. Two feasible pathways
of the Pd-H transformation can be proposed [51,63]. The first one
consists in the reductive elimination of HCl followed by the Pd(0)
oxidation with O2, while the second one involves the insertion of O2
into a Pd-H bond, leading to a Pd-OOH or Pd-OH fragment (Fig. 8).
We did not observe the formation of black palladium during the
catalytic reaction and, in addition, Pd(0) was not active under our
reaction conditions. Therefore, it can be proposed that recovering
the catalyst consists in the reaction of a Pd-hydrido species with
O2, enabling the formation of L2PdCl2 suitable for the next catalytic
cycle.
4.1.2. [cppy]2[Pd2Cl6]
0.18 g (1.0 mmol) of [cppy]Cl was added to the solution of
0.28 g (1.0 mmol) PdCl2(cod) in hot CH3CN (5 mL). The mixture was
heated for 30 min, until the yellow solution became red. The solu-
tion was cooled to room temperature and solvent was removed
under reduced pressure. The product was precipitated by addi-
tion of diethyl ether (2 mL). Product yield: 66%; anal. calcd. for
Pd2Cl6C18H22N4: C 30.0, H 3.1, N 7.8%; found: C 29.6, H 3.0, N
7.8%;1H NMR (CD3CN): 8.97 (t, J=7.05, 4H, Ar-H), 8.65 (d, J = 7.0,
4H,Ar-H), 7.40 (t, J = 7.05, 2H, Ar-H), 4.11 (t, J = 6.9, 4H, CH2), 2.16 (m,
J = 6.9, 4H, CH2), 1.87 (t, J = 6.8, 4H, CH2); 13C NMR (CD3CN):ı = 149
(CAr-H), 145 (CAr-H), 132 (CAr-CH3), 119 (-CN), 51 (CH2), 23 (CH2),
15 (CH2).
3. Conclusions
4.1.3. [bmpy]2[Pd2Cl6]
The complex was obtained according to the procedure given
for [cppy]2[Pd2Cl6] using 0.28 g (1.0 mmol) of PdCl2(cod) and
0.18 g (1.0 mmol) of [bmpy]Cl; yield: 95%; anal. calcd. for
An efficient catalyst system was developed for the oxidative
Heck-type reaction between arylboronic acids and various olefins
under base- and ligand-free conditions. Monomeric and dimeric
anionic palladium complexes of the type [CA]x[PdCl4] (x = 1,2) and
C20H32N2Pd2Cl6: C 33.1, H 4.4, N 3.9%; found: C 32.8, H 3.7, N
3.8%;1H NMR (CD3CN): ı = 8.71, 8.45 (d, J = 7.49, 8H, Ar-H), 4.57

E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
9
(t, J = 7.50, 4H, CH2), 2.50 (m, J = 7.25, 4H, CH2), 2.27 (m, J = 7.25,
4H, CH2), 2.0 (s, 6H, CH3), 1.12 (t, J = 7.30, 6H, CH3); 13C NMR
(CD3CN):ı = 161(CH3-C=) 145 (CAr); 129 (CAr); 61 (N-CH2); 33 (Ar-
CH3); 21, 19(-CH2-), 14(-CH3).
obtained by sampling each of the four experiments at different
times.
4.5. One-pot reaction of oxidative Heck and Heck reaction
4.1.4. [bmim]2[Pd2Cl6]
The solid substrates: phenylboronic acid (1.5 mmol, 0.184 g)
and palladium complex (0.02 mmol) were weighted and placed in
the 50 mL Schlenk tube in the atmosphere of oxygen. Next, olefin
(1 mmol) and 5 ml of the solvent saturated with oxygen were added
with a pipette. The reactor was closed with a rubber plug and the
reaction mixture was stirred in 130 ◦C. After 4 h the reaction was
stopped and 2 mmol of K2CO3 and 1.0 mmol of butyl acrylate were
added. In a nitrogen atmosphere the reaction mixture was stirred at
130 ◦C by next 4 h. After that time, the reactor was cooled down and
the organic products were extracted with 20 mL of diethyl ether. For
better phase separation 1 ml of water was added; 0.15 mL of mesity-
lene was added as an internal standard. The organic products were
analyzed using the GC–MS method.
The complex was obtained according to the procedure given
for [cppy]2[Pd2Cl6] using 0.28 g (1.0 mmol) of PdCl2(cod) and
0.18 g (1.0 mmol) of [bmim]Cl; yield: 95%; anal. calcd. for
C
16H30N4Pd2Cl6: C 27.3, H 4.3, N 7.9%; found: C 27.1, H 3.9, N 7.7%;
1H NMR(CD3CN) = 8.9 (s, 2H, N CH N); 7.6 (s, 2H, CH CH); 7.5 (s,
2H, CH CH); 4.2(t, J = 7.3, 4H, -N-CH2); 3.9 (s, 6H, N CH3); 1.8(m,
J = 7.5, 4H, -CH2-); 1.4 (m, 4H, J = 7.5,
-CH2-); 1.0(t, 6H, J = 7.5, -CH3); 13C NMR(CD3CN); =138
(N CH N); 126 ( CH CH ); 122(CH CH); 51 (N -CH3); 34
(
N CH2 ); 31,29 (-CH2-); 13(-CH3).
4.1.5. [HIPr]2[Pd2Cl6] (HIPr= 1,3-bis(2,6-diisopropylphenyl)
imidazolinium cation)
The complex was obtained according to the procedure given for
[cppy]2[Pd2Cl6] using 0.28 g (1.0 mmol) of PdCl2(cod) and 0.39 g
(1.0 mmol) of [HIPr]Cl; yield: 95%; anal. calcd. for C54H78N4Pd2Cl6:
C 53.8H 6.2, N 4.7%; found: C 53.0, H 3.3, N 4.3%;1H NMR (CDCl3):
8.36 (s, 2H, CH), 7.78 (s, 8H, CH2-CH2), 7.50 (t, J = 7.0, 4H,Ar-H), 7.12
(d, J = 6.9, 8H,
4.6. Physical data for the following products:
All compounds are determined by MS analysis and obtained
results were compared with these published in the references
[70–82].
Ar-H), 2.87 (m, J = 6.9, 8H, CH), 1.20 (d, J = 6.9, 48H, CH3),
13C NMR(CD3CN): ı = 140(N CH N); 137(CAr-H); 132(CAr-CH3);
127(CAr-CH); 124(CH2-CH2); 117(-CH-); 27(CH3); 22 (CH3).
(E)-Butyl cinnamate
MS: m/z(%) = 41 (14), 51 (21), 103 (53), 131 (100), 148 (71), 204
([M+] 14) 70 − 75)
4.2. Measurements
1H and 13C NMR spectra were measured in CD3CN on Bruker
500 spectrometers. UV–vis spectra were measured using Hewlett
Packard 8452A Diode Array Spectrophotometer. Electrospray ion-
ization mass (ESI–MS) spectra for synthesized compounds were
recorded on a Bruker apex ultra FT-ICR. The products of the catalytic
experiments were analyzed with a GC–MS (Hewlett Packard 8452A
instrument). Experiments with microwaves heating were prepared
in microwave oven (Anton Paar, Monowave 300, monomode, 850W
max). Power of reactor is automatically adjusted depending on the
reaction temperature.
Butyl (E)-3-(naphthalen-1-yl) acrylate
MS: m/z(%) = 41 (22), 76 (22), 153(100), 181 (41), 198 (13), 254
([M+] 48) 72)
4.3. Typical procedure for the oxidative Heck reaction
The oxidative Heck reaction was carried out in a 50 mL Schlenk
tube. The solid substrates: phenylboronic acid (1.5 mmol, 0.184 g)
and palladium complex (0.02 mmol) were weighted and placed in
the Schlenk tube which was evacuated and filled with oxygen. Next,
olefin (1 mmol) and 5 mL of the solvent saturated with oxygen
(30 min) were added with a pipette in the atmosphere of oxy-
gen. The reactor was closed with a rubber plug and the reaction
mixture was stirred in 130 ◦C. After the given reaction time, the
reactor was cooled down and the organic products were extracted
with 20 mL of diethyl ether. For better phase separation 1 mL of
water was added; 0.15 mL of mesitylene was added as an internal
standard. The organic products were analyzed using the GC–MS
method.
Butyl (E)-3-(2-methoxynaphthalen-1-
yl) acrylate
MS: m/z(%) = 41(16), 82 (13), 167 (100), 195 (28), 268 ([M+] 42)
72)
Butyl (E)-3-(2-methylnaphthalen-1-yl)
acrylate
MS: m/z(%) = 152 (27), 167 (100), 195 (20), 268(23) 72)
4.4. Hg(0) test
The oxidative Heck reaction was carried out as described above.
After the specified reaction time, mercury (Hg/Pd = 500) was added
and reaction was continued. Experimental data points have been
(E)- Butyl 3-(3-vinylphenyl) acrylate
MS: m/z(%) = 128 (51), 157(55), 174 (100), 230 ([M+] 33) 71)

10
E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
MS: m/z(%) = 51 (32), 77 (45), 103 (69), 131 (100), 162 ([M+] 59)
71,73−75)
(E)- Butyl 3-(4-vinylphenyl)
acrylate
MS: m/z(%) = 77 (14), 128 (54), 157 (58), 174 (100), 230, ([M+]
1-Phenyl-1-cyclohexene
MS: m/z(%) = 77 (23), 91 (35), 115 (52), 129 (100), 143 (52), 158
34) 71)
([M+] 100) 76)
(E)-
Butyl
3-(4-tert-
butylphenyl) acrylate
MS: m/z(%) = 189 (39), 204 (16), 245 (100), 261 ([M+] 33) 73)
3-Phenyl-1-cyclohexene
MS: m/z(%) = 104 (100), 158 ([M+] 22) 78)
(E)-Butyl 3-(2-bromophenyl) acrylate
MS: m/z(%) = MS: m/z(%)=102 (79), 209 (45), 226 (100), 282([M+]
(E)-1,3-Diphenyl-2-propene
MS: m/z(%) = 115 (62), 179 (44), 194 ([M+] 100) 78)
26) 75)
(E)-Butyl 3-(4-bromophenyl)
1-Phenylcycloheptene
MS: m/z(%) = 81 (46), 91 (65), 104 (100), 115 (46), 129 (85), 172
([M+] 63) 79)
acrylate
MS: m/z(%) = 102 (100), 209 (54), 226 (84), 282([M+] 21) 74,75)
Butyl (E)-3-(4-acetylphenyl)
3-Phenylcycloheptene
acrylate
MS: m/z(%) = 91 (54), 104 (100), 117 (82), 144 (36), 172 ([M+] 32)
MS: m/z(%) = 42 (85), 102 (24), 131 (33), 175 (100), 191 (39), 231
79)
(92), 246 ([M+] 18) 72,74,75)
(E)-Methyl 4-(3-butoxy-3-
oxoprop-1-en-1-yl) benzoate
4-Phenylcycloheptene
MS: m/z(%) = 91 (68), 104 (82), 118 (71), 129 (100), 144 (57), 172
([M+] 75) 79)
MS: m/z(%) = 175 (83), 206 (100), 262 ([M+] 21) 72,75)
(E)-Stilbene
MS: m/z(%) = 76 (20), 89 (24), 165 (46), 180 ([M+] 100) 74,75)
1,4-di(2-n-
butoxycarbonyl-(E)-vinyl) benzene
MS: m/z(%) = 41 (23), 127 (26), 218 (100), 257 (43), 330 ([M+] 24)
80)
2-Phenyl-2-cyclohexen-1-one
MS: m/z(%) = 115 (75), 144 (100), 172 ([M+] 68) 77)
1,2-di(2-n-butoxycarbonyl-(E)-
vinyl) benzene
MS: m/z(%) = 41 (76), 127 (76), 172 (100), 229 (27), 330 ([M+] 3)
2-Phenylcyclohexanone
80)
MS: m/z(%) = 104 (62), 117 (79), 131 (100), 174 ([M+] 74) 81,82)
(E)- Methyl cinnamate

E. Silarska, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 1–11
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