Heck reactions catalyzed by Pd(0)-PVP nanoparticles under conventional and microwave heating

Article abstract of DOI:10.1016/j.apcata.2011.09.014

Pd(0) nanoparticles stabilized by polyvinylpyrrolidone (Pd-PVP) with a diameter of 3-6 nm in ethanol catalyzed Heck coupling of iodobenzene with different alkenes under microwave heating. Products were obtained in good yields (62-99%) and good selectivity to the E-isomers. Microwave heating proved to be superior to conventional heating, providing products in higher yields and selectivities in short times (12 min).

Full text of DOI:10.1016/j.apcata.2011.09.014

Applied Catalysis A: General 408 (2011) 47–53
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Heck reactions catalyzed by Pd(0)-PVP nanoparticles under conventional and
microwave heating
Daniela de L. Martinsa,∗, Heiddy M. Alvarezb, Lúcia C.S. Aguiarc, Octavio A.C. Antunesc
a Universidade Federal Fluminense (UFF), Centro de Estudos Gerais, Instituto de Química, Campus do Valonguinho, Outeiro de São João Batista s/n, Centro,
Niterói, RJ 24020-141, Brazil
b Departamento de Ciências Exatas, Universidade Estadual de Feira de Santana, km 03 BR 116 – Campus Universitário da UEFS – Feira de Santana, BA 44 031-460, Brazil
c Instituto de Química, UFRJ, CT Bloco A, 6◦ Andar, Cidade Universitária, Rio de Janeiro, RJ 21945-970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 February 2011
Received in revised form 8 September 2011
Accepted 10 September 2011
Available online 16 September 2011
Pd(0) nanoparticles stabilized by polyvinylpyrrolidone (Pd-PVP) with a diameter of 3–6 nm in ethanol
catalyzed Heck coupling of iodobenzene with different alkenes under microwave heating. Products were
obtained in good yields (62–99%) and good selectivity to the E-isomers. Microwave heating proved to
be superior to conventional heating, providing products in higher yields and selectivities in short times
(12 min).
© 2011 Elsevier B.V. All rights reserved.
This paper is dedicated to the memory of
Professor Octavio A.C. Antunes
(Universidade Federal do Rio de Janeiro).
Keywords:
Nanoparticles
Heck coupling
Palladium
PVP
Polyvinylpyrrolidone
Microwave
1. Introduction
assembling important pharmaceuticals, natural products and mul-
tifunctional derivatives.
Nanoscience is a new interdisciplinary scientific field which
develops and advances rapidly nowadays [1,2]. Miniaturization to
nanometric scale in the range of 1–100 nm gives rise to qualitative
changes in the physicochemical properties of the systems under
study [3]. Nanoparticles are believed to have properties which
lie somewhere between those of single-particle species and bulk
materials [4]. An important field of application for nanoparticles is
that of catalysis due to their large surface area [5,6]. In this regard
the unique properties of these systems can lead to interesting appli-
cations [7,8].
Palladium colloids can be used as catalysts with high reactiv-
ity in C–C bond formation [9–11] reactions as the Heck and Nolley
[12–15], Kumada and co-workers [16], Negishi [17], Suzuki and co-
workers [18,19], Hatanaka and Hiyama [20], Sonogashira et al. [21]
and Stille [22] reaction. These methodologies constitute an impor-
tant tool for organic synthesis and have become a natural way of
The palladium(0)-catalyzed arylation and vinylation of alkenes
with aryl or vinyl halides, known as the Heck reaction, is a method-
ology of increasing versatility for the formation of new C–C bonds
[23–25]. Although homogeneous palladium catalysts are usually
effective in this reaction, their uses often bring about the typical
problems associated to all homogeneous catalysts, that is, sep-
aration of the catalyst from the reaction products and catalyst
recovery. One of the major drawbacks of the early homogeneous
systems was the precipitation of palladium black with limitation
of the lifetime of the active species. Addition of phosphine-ligands
improves the stability of catalytic systems but, from an economical
or an environmental perspective, development of phosphine-free
systems is desirable [13]. Palladium(0) nanoparticles have been
found to be involved in many phosphine-free reactions [26].
Actually, metal nanoparticles hold promises as recoverable and
recyclable catalysts [6].
Polymers as polyvinylpyrrolidone (PVP) are efficient stabilizers
for metal nanoparticles [10,11,19,27]. PVP is a cheap linear poly-
mer which potential to stabilize palladium(0) nanoparticles was
already reported. Pd-PVP nanoparticles have proven their effective-
ness to catalyze C–C coupling reactions and the results reported
∗
Corresponding author. Tel.: +55 21 2629 2228; fax: +55 21 2629 2144.
E-mail addresses: deluna@vm.uff.br, danideluna2@yahoo.com.br
(D. L. Martins).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.09.014

48
D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
between arylation and hidroarylation in Heck couplings catalyzed
by Pd-PVP nanoparticles.
2. Experimental
2.1. Materials
Scheme 1. Heck reactions catalyzed by Pd-PVP.
The reagents used in the Heck reactions were purchased
from commercial sources and used without further purification.
Polyvinylpyrrolidone (40,000 Da) was used as stabilizer for the pal-
ladium(0) nanoparticles.
in the literature indicate that the method used for the prepara-
tion of nanoparticles play a pivotal role in their catalytic activity.
Whether nanoparticles act as an inert reservoir of active, soluble
molecular palladium or as surface-active catalysts is a point of con-
tinuous debate in the literature. Recently, Lee et al. reported an
elegant operand XAS study which revealed nanoparticles are sta-
ble to metal leaching throughout the reaction, with surface density
Pd defect sites directly implicated in the catalytic cycle [28].
The majority of the reports of the literature, however, consist
in their application in the Suzuki reactions. Little attention has
been given to their application in Heck couplings. Trzeciak and
co-workers [29] employed a two step procedure (obtainment of
H2PdCl4 acidic solution followed by the reduction of this com-
plex by pyrogallol in the presence of PVP) to obtain PVP-stabilized
nanoparticles (medium diameter = 19.8 nm) and applied this cat-
alytic system to the mono and di-arylation of n-butyl acrylate with
bromobenzene. These authors employed the TBAB (tetrabutylam-
monium bromide) as additive at 130 ◦C under N2 atmosphere and
conventional heating.
2.2. Preparation of the catalyst [34]
PVP (2.5 g) with an average molecular weight of 40,000 Da was
added to methanol (150 mL) and stirred until the total solubiliza-
tion of the polymer. To this solution, it was added Pd(OAc)2 (0.05 g)
and the mixture was immersed in a pre-heated oil bath at 85 ◦C
under constant magnetic stirring for 3.0 h. Temperature was con-
trolled to be maintained between 85 and 90 ◦C. Next, methanol
was removed in a rotary evaporator and the palladium(0)-PVP cat-
alyst was transferred quantitatively to a 25 mL volumetric flask
using ethanol as solvent. This solution of Pd(0)-PVP in ethanol was
directly used as the catalyst for the Heck reactions.
2.3. Heck reaction under microwave irradiation
Reactions were carried out with a single mode cavity Discover
Microwave synthesizer (CEM Corporation, NC, USA) producing
continuous irradiation at 2455 MHz and infrared temperature con-
trol system. Microwave experiments were carried out in sealed
tubes with an efficient magnetic stirring (which avoids prob-
lems of non-homogeneity in temperature) and with simultaneous
cooling. In a typical experiment, a tube equipped with a mag-
netic stirrer was charged with K2CO3 (2.0 mmol), ethanol (2.0 mL),
iodobenzene (1.0 mmol), ethyl acrylate (1.3 mmol) and 0.22 mL of
Pd(0)-PVP solution (0.1% of Pd with respect to the iodobenzene
mass). This tube was sealed and the content was subjected to
focused microwave irradiation at 300 W for 12 min, with 120 ◦C
as the general final temperature. Then, the reaction mixture was
cooled to room temperature and diluted with 8.0 mL of diethyl
ether and stirred until the nanoparticles and the inorganics have
precipitated. Organic phase was analyzed by GC-MS. Afterward
the remaining precipitate was washed with diethyl ether. The
combined organic phases were filtered through celite, dried over
Na2SO4 and the solvent was removed by rotary evaporation to give
the crude product. Products obtained were analyzed by GC-MS, 1H-
and 13C-NMR spectroscopy.
Evangelisti et al. [30] employed
a metal vapor synthesis
(MVS) technique to prepare PVP-stabilized palladium(0) nanopar-
ticles (1.0–3.5 nm). These researchers employed this catalyst in
the Heck couplings between aryl halides and n-butyl acrylate
(acrylate/ArX = 2) at 75–125 ◦C, under argon atmosphere, under
conventional heating, with N(n-Pr)3 as base in NMP.
Durap et al. [31] employed methanol and NaBH4 as the
palladium reducing agents with K2PdCl4 as the Pd(II) source
in the presence of PVP under refluxing conditions to obtain
Pd-PVP nanoparticles (diameter = 4.5 nm). The catalyst thus pre-
pared was employed in the Heck coupling between styrene
(styrene/ArBr = 1.5) and bromobenzenes (bromobenzene, bro-
moanisole, 4-bromotoluene) (DMF, K2CO3, 120 ◦C, inert atmo-
sphere). E-stilbenes were obtained in 45 min in yields ranging from
34% to 100%.
The use of microwave as alternative energy source to input
chemical reactions has received considerable amount of attention
in the last years [32,33]. Microwave irradiation has been applied
successfully to different types of reactions and in many cases,
increased yields, reduced reaction times and enhanced selectivity
of the reaction compared to conventional heating.
In the previous reports where Pd-PVP nanoparticles were
applied to the Heck couplings only the conventional heating was
employed with styrene or n-butyl acrylate [29–31]. No consider-
ations have been made about the occurrence of hydroarylation
processes and the influence of the solvent in the selectivity of the
coupling.
In this work we present results demonstrating the quite good
efficiency and selectivity of Pd(0) nanoparticles stabilized by
polyvinylpyrrolidone (Pd-PVP) as catalyst in the Heck reaction
in the absence of phosphine ligands or phase transfer cata-
lysts, Scheme 1. A straightforward approach was employed in
the preparation of the palladium nanoparticles. Different alkenes
(monosubstituted and disubstituted) were employed in the cou-
plings with iodobenzene, under conventional heating. As far as we
concern microwave irradiation was not employed to accelerate the
Heck couplings catalyzed by Pd-PVP nanoparticles. Furthermore,
in the present work, we present results concerning the selectivity
2.4. Heck reaction under conventional heating
A 10 mL round-bottomed flask equipped with a reflux condenser
was filled with K2CO3 (2.0 mmol), ethanol (2.0 mL), iodobenzene
(1.0 mmol), ethyl acrylate (1.3 mmol) and 2.2 mL of Pd(0)-PVP solu-
tion (1.0% of Pd with respect to the iodobenzene mass). The flask
was placed in an oil bath, and the mixture was stirred and heated
at 80 ◦C. After a reaction time of 4 h, the flask was stirred until the
precipitation of nanoparticles and inorganic. The organic phase was
analyzed by GC-MS.
2.5. Heck reaction with a pre-heated oil bath at 120 ◦C
A sealed tube equipped with a magnetic stirrer and charged
with the reaction mixture (see Section 2.2) was immersed into
a pre-heated oil bath at 120 ◦C for 12 min, after which this

D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
49
Table 1
mixture was treated the same manner described for the represen-
tative procedures under microwave heating.
Heck reaction between iodobenzene (1) and styrene (2a) under conventional
heating.
Entry
Conditionsa
Yield%b
2.6. Methods for the catalyst analysis
1
2
3
4
5
K2CO3, room temperature, 48 h
K2CO3, 80 ◦C, 4 h
20
93
84
85
84
Transmission electron microscopy (TEM) was performed with a
JEOL-1210 microscope operating at 80 kV. Two drops of the Pd(0)-
PVP solution was placed on a copper grid (300 mesh) coated by a
piolophorm film and dried for 24 h at 60 ◦C. The nanoparticle size
distribution for each sample was determined by counting the size
of approximately 200 palladium nanoparticles from TEM images
obtained from different places on the TEM grids.
KOAc, 80 ◦C, 4 h
NaHCO3, 80 ◦C, 4 h
K3PO4, 80 ◦C, 4 h
a
Conditions: 1% Pd-PVP, K2CO3 (2.0 mmol), ethanol (2.0 mL), iodobenzene
(1.0 mmol); 2a (1.2 mmol), 80 ◦C, 4 h, round-bottomed flask equipped with a reflux
condenser.
b
GC-MS yield to E-stilbene related to iodobenzene, normalized areas.
2.7. Methods for identification and quantification of substrate
and product
1H and 13C-NMR spectra were recorded on a Brucker AMX-200
spectrometer operating at 200 MHz (1H) and 50 MHz (13C) in CDCl3
with TMS as an internal standard. The analyses by GC-MS were per-
formed on a Shimadzu GCMS-QP2010Plus. The separation of the
compounds was achieved on RTx5MS (30 m × 0.25 mm × 0.25 ␮m).
Helium was used as a carrier gas and the injection split ratio was
1:20. Separation was achieved using the following temperature
program: 20 ◦C/min from 100 (hold time = 2 min) to 260 ◦C (hold
time = 2 min). Injector and detector temperatures were 260 ◦C and
ion source temperature was 200 ◦C. The ration of E- and Z-isomers of
the coupling products were determined by 1H-NMR analysis. Yields
to the E products were based on the consumption of iodobenzene.
E-stilbene 3a as a white solid – dH (200 MHz CDCl3); dC (50 MHz
CDCl3) 7.54 (4H, d, J 7.1 Hz), 7.42–7.27 (6H, m), 7.23 (2H, s); dC
(50 MHz CDCl3) 137.4, 128.8, 127.7, 126.6; m/z (EI) 180 (100%), 165
(53%), 152, 102, 89 (38%), 76, 63, 51
E-ethyl cinnamate 3c as a colourless oil – dH (200 MHz CDCl3)
7.61 (1H, d, J 16 Hz), 7.50–7.41 (2H, m), 7.36–7.29 (3H, m), 6.36 (1H,
d, J 16 Hz), 4.25–4.14 (2H, q), 1.30–1.16 (3H, t); dC (50 MHz CDCl3);
m/z (EI) 176, 148, 147, 131 (100%), 103 (58%), 77, 51
E-t-butylcinnamate 3d as a colourless oil – dH (200 MHz CDCl3)
7.52 (1H, d, J 16 Hz) 7.47–7.42 (2H, m), 7.32–7.19 (3H, m), 6.3(1H,
d, J 16 Hz) 1.47 (9H, s); dC (50 MHz CDCl3); m/z (EI) 204, 189, 147
(100%), 131 (65%), 103, 77, 57 (43%)
E-cinnamamide 3b as a white solid – dH (200 MHz CDCl3) 7.71
(1H, d, J 15.7 Hz), 6.47 (1H, d); dC (50 MHz CDCl3); m/z (EI) 147, 146
(100%), 131 (50%), 103 (82%), 77, 51, 44
Fig. 1. TEM image of the Pd(0)-PVP nanoparticles.
3. Results and discussion
The procedure reported by Bradley et al. [34] was followed
in preparing the palladium colloidal solution stabilized by PVP.
Pd(OAc)2 was employed as the palladium source and methanol as
the reducing agent in the presence of varying amounts of PVP as
stabilizer. After refluxing for 2–3 h, methanol was removed using
a rotary evaporator and the residue was dissolved in ethanol. The
solution thus prepared had a dark brown colour and was stable for
weeks at room temperature. Following this methodology we could
obtain nanoparticles with a particle size between 3 and 6 nm as
revealed by TEM analysis (Fig. 1).
Li et al. have also employed the Pd-PVP nanoparticles, prepared
according to Bradley’s methodology, in the Sonogashira couplings
[35]. The optimized conditions employed by Li et al. (2.0 mmol
K2CO3, 1% Pd-PVP, ethanol, 80 ◦C) were utilized in the present work
as the initial conditions for the coupling between iodobenzene (1)
and styrene (2a) (Scheme 2, Table 1: entry 2). Excellent result was
obtained employing such conditions (93%, entry 2). Since the nature
of the base is of great importance in the Heck couplings, other bases
were also tested in the Heck coupling between iodobenzene and
Scheme 2. Heck reaction between iodobenzene and styrene under conventional
heating.
styrene (Table 1, entries 3–5), using the same temperature and cat-
alyst loading with ethanol as solvent. Reactions under conventional
heating (Tables 1 and 2) were performed in rounded-bottomed
flask coupled to a reflux condenser.
Reactions with potassium carbonate (Table 1, entry 2) and
sodium bicarbonate (entry 4) as bases were cleaner than the reac-
tions carried out with potassium acetate (entry 3) and phosphate
(entry 5). The use of K2CO3 provided better yields to E-stilbene than
NaHCO3; indeed in the later case a greater amount of stilbene iso-
mers was obtained, indicating that double bond isomerization can
occur (entry 4). The reaction was also performed at room temper-
ature (entry 1) but, as it can be observed; poor yield was obtained

50
D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
Scheme 3. Heck reaction between iodobenzene and alkenes under conventional heating.
Scheme 4. Heck reaction between acrylonitrile (2e) and iodobenzene (1) catalyzed by Pd-PVP under conventional heating.
Scheme 5. Heck reaction between iodobenzene (1) and n-butyl methacrylate (8) and ␣-styrene under conventional heating (80◦ C, 4 h).
even after 48 h (monitored by GC/MS), indicating that higher tem-
peratures are necessary for the oxidative addition to occur.
After reactions, diethyl ether was added to the reaction mix-
ture in order to precipitate the palladium nanoparticles and all the
inorganics. Crude coupling products were obtained from combined
organic phases, after removing solvents under reduced pressure.
In an attempt to extend the application of our system under
the optimized conditions for styrene (2.0 mmol K2CO3, 1% Pd-PVP,
ethanol, 80 ◦C; Table 1, entry 2 → 3a, 93%), the coupling of iodoben-
zene (1) with other alkenes (2b-e) was also carried out (Scheme 3,
Table 2).
As shown in Table 2, good yields to E-alkenes were achieved in
4 h of reaction, under conventional heating at 80 ◦C, for substrates
2a-d.
The reaction of iodobenzene and acrylonitrile (2e) (entry 5),
however, led to a mixture of products: Heck products (monoary-
lated 3e and the diarylated alkene (4e), reduced products (5e, 6e)
and the Michael adduct (7e) derived from ethanol (Scheme 4).
The unusual selectivity towards reduced Heck products
(hydroarylation) when acrylonitrile (2e) was employed in Heck
coupling reactions was also observed by Antunes and co-workers.
These authors employed 2-hydroxypropyl-␣-cyclodextrins-
capped palladium nanoparticles as catalysts.
Under thermal conditions, reaction of iodobenzene and n-butyl
methacrylate (8) yielded a mixture 1:1 of the isomers E and Z-9
together with butyl 2-benzylacrylate (9-gem; Scheme 5). Employ-
ing ␣-styrene as coupling partner also resulted in a mixture 1:1 of
E/Z isomers.
Heck coupling reactions catalyzed by Pd-PVP nanoparticles also
proceeded satisfactorily under microwave irradiation (Table 3,
Scheme 6)
Table 2
Heck reaction between iodobenzene (1) and alkenes (2a-e) under conventional
heating.
Entrya
R
Yield %b
Product
1
2
3
4
5
Ph
93
3a
3b
3c
3d
3e
CONH2
CO2Et
CO2t-Bu
CN
>99 (70)c
78
>99 (77)c
22 (30)d
a
Conditions: 1% Pd-PVP, K2CO3 (2.0 mmol), ethanol (2.0 mL), iodobenzene
(1.0 mmol); alkene (1.2 mmol), 80 ◦C, 4 h, round-bottomed flask equipped with a
reflux condenser.
b
GC-MS yield to the E-alkene related to iodobenzene, normalized areas.
Product isolated yield.
GC-MS Yield to 3-phenylpropionitrile.
c
Scheme 6. Heck reaction between iodobenzene and alkenes under microwave irra-
diation.
d

D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
51
Table 3
Heck reaction between iodobenzene and alkenes under microwave irradiation.
Entrya
R
T (◦C)
Yield (3a-e) %b
Yield (5a-e) (%)c
1
2
3
4
5
6
7
8
9
Ph
Ph
80
120
80
120
80
120
80
120
120
82
–
1.0
–
15
33
7.0
31
1.0
44
70 (1.0)c
<1.0
Scheme 7. Michael reaction between ethyl acrylate and ethanol under microwave
heating.
CONH2
CONH2
CO2Et
CO2Et
CO2t-Bu
CO2t-Bu
CN
62 (<1.0)c
45
83 (12)c
22
300
72 (11)c
38 (<1.0)c
Temperature
Power
250
a
..... Pd(0)-PVP (0.1%)
200
Conditions: 0.1% Pd-PVP, K2CO3 (2.0 mmol), ethanol (2.0 mL), iodobenzene
(1.0 mmol); alkene (1.2 mmol), 120 ◦C, 300 W, sealed tube, 12 min.
b
GC-MS yield to E-alkene product.
GC-MS yield to E-alkene using a sealed tube equipped with magnetic stirring
c
150
100
50
immersed in a pre-heated oil bath for 12 min.
Gratifyingly, upon decreasing the catalyst loading from 1% to
0.1%, an excellent level of conversion could still be achieved at 80 ◦C
(entry 1) in 12 min. We have also observed in a general manner,
that better yields to E-alkenes were obtained when the tempera-
ture of the reaction was increased from 80 to 120 ◦C. No appreciable
yield was achieved for the coupling of iodobenzene with acrylamide
when reaction temperature was 80 ◦C (Entry 3, Table 3). At this
temperature, almost only biphenyl was formed (23% yield) as the
product of the homocoupling reaction of iodobenzene. Increasing
reaction temperature to 120 ◦C resulted in a dramatic increase of
the yield of E-cinnamamide – 62% (entry 4). The corresponding
hydroarylated product (5b: 15%) was also observed for the reaction
with acrylamide at this temperature.
Yields to the corresponding coupling products apparently fol-
low the alkene (2a-e) reactivities. It can be observed from the data
shown in Table 3 that ethyl acrylate (entry 5) is more reactive than
t-butyl acrylate (entry 7) for steric reasons and acrylamide (entry 3)
is a less electron deficient alkene than acrylates. Acrylonitrile (entry
9) is a very reactive alkene from the electronic viewpoint, as well
as from the stereo reason. Heck reaction between iodobenzene and
acrylates (entries 5 and 7, Table 3) and acrylonitrile (entry 9) also
furnished greater amounts of the reduced product compared with
the coupling with acrylamide (entry 3) and styrene (entry 1). In the
case of the acrylates, yields to the reduced products decrease with
increasing temperature (entries 5 versus 6 an 7 versus 8). Heck reac-
tion between iodobenzene and ethyl acrylate was carried out under
microwave irradiation with simultaneous coupling and without
cooling. No appreciable change in reaction yields or selectivities
were observed comparing these two reaction conditions.
Kappe and co-workers [33] applied microwave heating to the
Heck reaction between iodobenzene and butyl acrylate, in the pres-
ence of Pd/C as catalyst. Under the employed conditions [5% Pd/C,
Et3N, MeCN, 180 ◦C], moderate (62%, 10 min of irradiation) and
excellent (>99%, 20 min of irradiation) were obtained. Compared
to the use of Pd/C as a heterogeneous catalyst (5% Pd, 10 min), the
palladium nanoparticles developed in the present work (0.1% Pd,
12 min) allow not only the use of the catalyst in lower loading, but
also better conversions to the coupling products (62–83%).
Michael adducts of ethanol and olefins were formed when
acrylates and acrylamide were used as coupling partners, under
microwave irradiation. Control experiments were carried out in
order to investigate if palladium was acting as a Lewis acid cata-
lyst (Scheme 7). It was found that Michael products were formed
even in the absence of the palladium nanoparticles, as well as in
the absence of added K2CO3.
0
0
200
400
600
800
1000
Time (s)
Fig. 2. Potency and heating profiles for microwave-assisted Michael reactions.
the heating profile in the presence of the catalyst while the heavy
ones represent the heating profile in the absence of the Pd/PVP
nanoparticles. It can be observed, when the catalyst is present, the
reaction mixture reaches rapidly the temperature limit (120◦ C)
which is maintained by irradiating the mixture at 200 W. When the
nanoparticles are absent, however, even at 300 W the limit temper-
ature (120◦ C) is not reached. This result indicates that the mixture
which contains the nanoparticles has a great capacity of transform-
ing radiation in heat because the nanoparticles are good microwave
absorbers.
When coupling reactions were carried out in sealed tubes in pre-
heated oil baths at 120 ◦C for 12 min, in order to compare the results
with reactions under microwave irradiation at this same tempera-
ture (entries 2, 4, 6, 8 and 9), poor yields to the coupling products
were obtained. These results demonstrated the beneficial effect of
the microwave irradiation over Pd-PVP catalyzed Heck reactions. It
is important to emphasize that careful must be taken when com-
paring microwave heating experiments with the oil bath ones, since
heating profiles under microwave heating could not be reproduced
in the oil bath experiments. For example, heterogeneous palladium
catalysts are great microwave absorbers leading to higher super-
ficial temperatures that are not reproduced under conventional
heating. It can be the case, in the present work if, as evidenced by Lee
et al. [28], Pd-PVP catalyzed C–C bond forming reactions are hetero-
geneous in nature, since there are higher superficial temperatures
due to preferential heating of the supported-catalyst in compari-
son with the solvent. As it could be observed by the heating profiles
presented in Fig. 2, Pd-PVP nanoparticles are good microwave
absorbers. Besides that, ionic bases, not soluble in ethanol, are
strong microwave absorbers as well, leading to superior temper-
atures on their surfaces. Both effects could have contributed to the
better results found in coupling reactions under microwave irradia-
tion. Suzuki couplings reactions catalyzed by Pd-PVP nanoparticles
are also accelerated under microwave heating [19].
As mentioned before, reduced products were also found in
the Heck coupling reactions catalyzed by Pd(0)-PVP nanoparti-
cles. Antunes and co-workers disclosed their results concerning
the influence of the solvent and the type of palladium catalyst on
the selectivity Sonogashira versus hydroarylation in the coupling
In Fig. 2, we can observe the difference between the heating pro-
files of the test reactions performed with the aim of knowing if the
nanoparticles were acting as Lewis catalysts in Michael reactions
between the ethyl acrylate and ethanol. Dashed lines represent

52
D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
Table 4
Heck reaction between 2-iodotoluene and ethyl acrylate with different palladium
nanoparticles and solvents.
Entry
Conditions of
preparing the
Pd(0)-PVP
Solvent
Yield
Yield E-
alkene
(%)
Reduced
product (%)
(3a-e) %b
1
2
3
4
5
6
7
2.5 g PVP, 30 min
2.5 g PVP, 120 min
2.5 g PVP, 180 min
1.2 PVP, 120 min
0.6 PVP, 120 min
2.5 g PVP, 120 min
2.5 g PVP, 120 min
EtOH
EtOH
EtOH
EtOH
EtOH
t-BuOH
MECN
63
63
55
47
37
23
59
74
74
83
80
66
12
21
8
19
31
–
>99
92
–
aConditions: 0.1% Pd-PVP, K2CO3 (2.0 mmol), ethanol (2.0 mL), 2-iodotoluene
(1.0 mmol); alkene (1.2 mmol), 120 ◦C, 300 W, sealed tube, 5 min.
b
GC-MS yield to E-alkene product.
cGC-MS yield to E-alkene using a sealed tube equipped with magnetic stirring
immersed in a pre-heated oil bath for 12 min.
Fig. 3. Particle size distribution for Pd(0)-PVP produced when 2.5 g of PVP was
employed.
Scheme 8. Catalytic cycle for hydroarylation in Sonogashira couplings.
Fig. 4. Particle size distribution for Pd(0)-PVP produced when 0.6 g of PVP was
employed.
reaction between iodobenzene and phenylacetylene [36]. These
authors reported that the selectivity Sonogashira versus hydroary-
lation is influenced by the nature of the catalyst and that
hydroarylation decreases when supported catalysts such as Pd/C
are employed. In the same work [36], Antunes and co-workers also
proposed, in agreement with other reports in the literature as that
of Wu et al. [37] and Hierso et al. [38], that the palladium catalyst
may act as hydrogen transfer catalyst. In this case, the solvent can be
of great importance as a hydride source (Scheme 8). Based on these
previous works, we decided to prepare the Pd(0)-PVP nanoparti-
cles under different reaction conditions in order to investigate if
catalysts with different activity and selectivity could be produced
and how the selectivity hydroarylation versus Heck reactions could
be influenced (Table 4). Different amounts of the stabilizing agent
PVP and different reaction times were applied to the synthesis of
the palladium nanoparticles (Table 4, entries 1–5). Methanol was
employed as the reducing agent for the Pd(II) precursor in all the
cases. In order to investigate the influence of the solvent on the
selectivity of the coupling reactions, we performed the Heck reac-
tions between 2-iodotoluene and ethyl acrylate (Scheme 9) under
microwave heating with ethanol, t-butanol and acetonitrile (entries
2, 6 and 7).
As it can be observed from the data presented in Table 4, the
time of reaction employed to prepare the nanoparticles seems not
to influence the conversions to products (entries 1–3). However,
the amount of PVP employed as the capping agent influenced the
conversion to the products as well as the selectivity Heck versus
hydroarylation (entries 2, 4 and 5). Decreasing amounts of PVP in
the catalysts resulted in decreasing conversions and slower reac-
tions. Superior selectivity in favour of hydroarylated products were
observed as decreasing amounts of PVP were employed. When 2.5 g
of PVP was employed (entry 2), around 70% of the particles were in
the range of 3–4 nm of diameter (Fig. 3) and when 0.6 g of PVP was
employed, around 90% of the particles were in this range (Fig. 4).
Smaller particles have a greater amount of palladium atoms on
the surface able to coordinate with the solvent to form palladium
hydride species, thus favouring the hydroarylated products when
minor amounts of PVP are employed to synthesize the catalysts.
Scheme 9. Heck coupling between 2-iodotoluene and ethyl acrylate under microwave heating.

D.L. Martins et al. / Applied Catalysis A: General 408 (2011) 47–53
53
In order to highlight the influence of the solvent on the selec-
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Acknowledgements
Financial support from CAPES, CNPq, FAPERJ and FAPESB, Brazil-
ian Support Foundations is gratefully acknowledged.