Microwave-assisted Suzuki reaction catalyzed by Pd(0)-PVP nanoparticles

Article abstract of DOI:10.1016/j.tetlet.2010.09.145

Suzuki cross-coupling reaction was successfully carried out in ethanol utilizing a palladium colloidal solution stabilized by polyvinylpyrrolidone (PVP). High isolated yields (75-97%) to biaryls were obtained using different bases, aryl halides, and aryl boronic acids with a small loading of the palladium catalyst. Pd(0)-PVP nanoparticles with 3-6 nm of medium diameter were prepared from Pd(OAc)₂ in the presence of the stabilizer PVP using methanol as the reducing agent.

Full text of DOI:10.1016/j.tetlet.2010.09.145

Tetrahedron Letters 51 (2010) 6814–6817
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Microwave-assisted Suzuki reaction catalyzed by Pd(0)–PVP nanoparticles
Daniela de Luna Martins , Heiddy M. Alvarez , Lúcia C. S. Aguiar c
a,
⇑
b
a
Núcleo de Pesquisas de Produtos Naturais, Universidade Federal do Rio de Janeiro (UFRJ), Cidade Universitária, CCS, Bloco H, Rio de Janeiro, RJ 21941-590, Brazil
Departamento de Ciências Exatas, Universidade Estadual de Feira de Santana, km 03, BR 116, Campus da UEFS, Feira de Santana, BA 44031-460, Brazil
Intittuto de Química, UFRJ, Cidade Universitária, CT, Bloco A, Lab. 617, Rio de Janeiro, RJ 21941-909, Brazil
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Suzuki cross-coupling reaction was successfully carried out in ethanol utilizing a palladium colloidal
solution stabilized by polyvinylpyrrolidone (PVP). High isolated yields (75–97%) to biaryls were obtained
using different bases, aryl halides, and aryl boronic acids with a small loading of the palladium catalyst.
Received 31 August 2010
Accepted 27 September 2010
Available online 8 October 2010
Pd(0)–PVP nanoparticles with 3–6 nm of medium diameter were prepared from Pd(OAc)
of the stabilizer PVP using methanol as the reducing agent.
2
in the presence
Ó 2010 Elsevier Ltd. All rights reserved.
Transition-metal nanoparticles have attracted a great deal of
attention in the last few years; their preparation, structure deter-
Teranishi,24 for instance, prepared palladium nanoparticles from
the reduction of H PdCl with alcohols as reducing agents of the
2 4
Pd(II) species in the presence of PVP as the stabilizer. These authors
reported that the amount of PVP, the kind, and concentration of the
alcohol in the solvent play an important role on the control of the
mean diameter of monodispersed Pd nanoparticles.
1
,2
mination, and applications are topics of current interest.
An
important field of application for nanoparticles is that of catalysis
due to their large surface area.3 In this regard the unique proper-
ties of these systems can lead to interesting applications in differ-
–6
7
,8
ent types of reactions which include hydrogenations,
couplings,
C–C
Microwave-assisted heating under controlled conditions has
9
–11
12
13
25
Fischer–Tropsch, oxidations etc. Metallic nano-
been proven as an invaluable technology for organic synthesis
particles have been considered at the frontier between homoge-
neous and heterogeneous catalysis14 allowing for a very high
activity under mild conditions. Progress in nanocatalysis has been
made in efficiency and selectivity of the reactions as well as in the
recovery and reusability of the catalyst.
and their application in several cases has lead to acceleration of
reactions, improvement of yields, and selectivities. Our research
26
group has already demonstrated the huge potential of this tech-
nique applied to palladium-catalyzed C–C bond couplings.2
In this work, soluble Pd(0) nanoparticles stabilized by PVP were
utilized as a catalyst in Suzuki reactions under microwave heating
without phosphine ligands (Scheme 1).
0c,27
Palladium chemistry has emerged as one of the most prominent
tools for the C–C bond formation with an increasing number of pa-
15
pers devoted to the cross-coupling reactions like Kumada, Suzu-
Pd(0)–PVP nanoparticles were prepared following the proce-
1
6
17
18
19
20
28,29
ki,
Stille,
Negishi,
and Hiyama
or to the Heck
and
dure reported by Bradley et al.
2
using Pd(OAc) as the palladium
2
1
Sonogashira couplings. The Suzuki cross-coupling reactions of
arylboronic acids and aryl halides provide an effective synthetic
route to biaryls, which are useful as precursors to pharmaceuticals,
polymers, liquid crystals etc.22 Although homogeneous palladium
catalysis are usually effective in this reaction, their use often brings
about the typical problems associated with all homogeneous cata-
lysts, that is, separation of the catalyst from the reaction products
and catalyst recovery. Water soluble polymers like polyvinylpyr-
rolidone (PVP) are effective stabilizers for nanoparticles and their
utilities as stabilizers for palladium nanoparticles have been dem-
source and methanol as the reducing agent in the presence of pol-
yvinylpyrrolidone as the stabilizer. After refluxing for 3 h, metha-
nol was removed under reduced pressure and the residue was
dissolved in ethanol or in acetonitrile. The brown solutions thus
prepared were stable for months at room temperature without
precipitation. Following this methodology, nanoparticles with a
particle size ranging from 3 to 6 nm were obtained (Fig. 1). For
comparison, Pd(0)–PVP nanoparticles were also prepared accord-
2
4
ing to the Miyake et al. methodology, in which ethanol was used
as the reducing agent in the presence of PVP as the stabilizer and
an acid solution of H PdCl was employed as the palladium source.
2 4
23
onstrated in some opportunities in the literature.
Properties of nanocatalysts are size dependent and it is well
documented in the literature that the stability and size of nano-
scaled colloidal particles strongly depend upon the method fol-
lowed as well as the experimental conditions. Miyake and
Our initial investigation on the evaluation of the catalytic activ-
ity of Pd(0)–PVP nanoparticles started with the cross-coupling
Pd(0)-PVP, K CO ,
2
3
Ar X + Ar B(OH)
Ar -Ar
1 2
1
2
2
EtOH, 300W, 120º C
⇑
Corresponding author. Tel.: +55 21 2562 7256; fax: +55 21 2562 7140.
E-mail address: danideluna2@yahoo.com.br (D. de Luna Martins).
Scheme 1. Pd(0)–PVP-catalyzed Suzuki reactions under MW.
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.145

D. de Luna Martins et al. / Tetrahedron Letters 51 (2010) 6814–6817
6815
Table 1
Suzuki reaction between iodobenzene and phenylboronic acid
Ya (%)
TOF (h )
À1
Conditions
3
1
2
3
4
0.1% Pd(0)–PVP, K
0.1% Pd(0)–PVP, K
0.025% Pd(0)–PVP, K
2
CO
CO
3
, EtOH, 12 min
, MeCN, 12 min
>99
96
>99
>99
2.6 Â 10
2.5 Â 10
1.0 Â 10
2.6 Â 10
3
4
4
2
3
2
CO
3
, EtOH, 12 min
0.01% Pd(0)–PVP, K CO
2
3
, EtOH, 12 min
3
b
(
1.3 Â 10 )
4
5
5
5
6
7
0.005% Pd(0)–PVP, K
2
CO
CO
3
, EtOH, 12 min
, EtOH, 7 min
>99
99
99
5.2 Â 10
4.4 Â 10
4.4 Â 10
0.005% Pd(0)–PVP, K
2
3
c
0.005% Pd(0)–PVP , K
2
CO
3
, EtOH, 7 min
2 2 3
Conditions: iodobenzene (1.0 mmol); PhB(OH) (1.1 mmol), K CO (2.0 mmol), % Pd
3
2
w/w with respect to iodobenzene, 2.0 mL EtOH, P = 300 W, 120 °C, sealed tube.
a
GC–MS yield with respect to iodobenzene, normalized areas.
Conventional heating, 4 h, 80 °C.
b
c
Prepared according to the Miyake and co-workers methodology (H
2 4
PdCl ,
ethanol as reducing agent, PVP as stabilizer).
tion.30 Palladium black precipitation was not observed at the end
of the reaction when the Pd(0)–PVP prepared according to the
Bradley and co-workers’ methodology was employed. When the
nanoparticles prepared according to Miyake and co-workers’24
procedure were utilized however, palladium black precipitation
was observed at the end of the reaction under microwave heating.
Similar observations were made by El-Sayed and co-workers when
they employed this catalyst under conventional heating. 31 These
authors attributed this precipitation to the low stability of these
nanoparticles under high temperatures. The Pd–PVP nanoparticles
employed in this work (PVP/Pd = 101, 3–6 nm in ethanol) dis-
played a superior stability at the same time as excellent activities
could be achieved with this catalyst.
Figure 1. TEM image of the Pd(0)–PVP nanoparticles.
between phenylboronic acid and iodobenzene under microwave
irradiation (Scheme 2, Table 1).
2
7
Antunes and co-workers also employed the Miyake and co-
workers nanoparticles in the Suzuki coupling under microwave
irradiation (0.2 mol % Pd–PVP, K CO , 15 mL of 40% EtOH/water,
Our research group has previously reported that the solvent in
which the nanoparticles were dissolved after the evaporation of
methanol had a great influence on the coupling under conventional
heating since Suzuki reaction was much faster in ethanol than in
2
3
2
6
220 W). These researchers obtained biphenyl quantitatively in
40 min of irradiation (TOF = 750/h). In the present work, using
the same catalyst (Table 1, entry 7), biphenyl was obtained in
excellent yield (99%) with 0.005% (mol %) of palladium in 7 min
3
0
acetonitrile. Under microwave heating, however, the use of both
acetonitrile and ethanol led to a good yield of the coupling product
5
(
Table 1, entries 1 and 2). High turnovers and turnover frequencies
of irradiation (4.4 Â 10 /h).
could be obtained for the catalytic system using low loadings of
palladium (0.005%, entries 5 and 6). Turnover frequencies were
remarkably improved by employing dielectric heating when com-
In the majority of the reaction time, potency was maintained
between 60 and 70 W and temperature in 120 °C. The temperature
and potency profile for the reactions under microwave irradiation
are shown in Figure 2.
3
0
pared to those obtained under conventional heating (entry 4).
Under conventional heating with the Pd(0)–PVP prepared accord-
The excellent results obtained in the cross-coupling of iodoben-
zene with phenylboronic acid under microwave heating encour-
ing to the Bradley and co-workers methodology,28 quantitative
3
0
yield to biphenyl was achieved in 4 h using 0.01% of the catalyst.
In the present work it can be observed that 0.005% of palladium
and 7 min of microwave irradiation were enough for the obtain-
ment of biphenyl in 99% yield. Catalytic activity of the pre-formed
Pd(0)–PVP colloids in ethanol were comparable to that observed
1
1
1
1
1
80
60
40
20
00
Temperature (ºC)
Power (W)
2
for Pd(OAc) in ethanol and palladium could be easily recovered
by the addition of ethyl ether to the reaction mixture in order to
precipitate the PVP-stabilized palladium nanoparticles as well as
the inorganics.
Pd(0)–PVP prepared according to Miyake and co-workers’24
8
6
4
2
0
0
0
0
0
procedure (ethanol/water, PVP, H
catalyst under microwave irradiation with ethanol as the solvent
entry 7, Table 1) in the same conditions utilized in the Suzuki
coupling with Pd(0)–PVP prepared according to the Bradley and
2 4
PdCl ) was also employed as a
(
2
8
co-workers’
2
procedure (methanol, Pd(OAc) , PVP) (entry 6).
Excellent yield (99%) to biphenyl was obtained in 7 min of the reac-
-20
Pd source, base,
0
100
200
300
400
Time (s)
500
600
700
800
PhI + PhB(OH)2
Ph-Ph
solvent, 300W, 120º C
Figure 2. Temperature and potency profile for the microwave-assisted Suzuki
Scheme 2. Suzuki reaction between iodobenzene and phenylboronic acid.
reactions catalyzed by Pd(0)–PVP.

6
816
D. de Luna Martins et al. / Tetrahedron Letters 51 (2010) 6814–6817
aged us to further screen other aryl halides and boronic acids as
coupling partners (Table 2, Scheme 3).
Pd(0)-PVP, K CO ,
2 3
Ar I + Ar B(OH)
1
Ar -Ar2
1
2
2
C H OH, 300W, 120º C
2
5
Very good yields were achieved in the cross-coupling reactions
catalyzed by Pd(0)–PVP under microwave irradiation with several
Scheme 3. Suzuki coupling between aryl halides and arylboronic acids catalyzed by
3
2
aryl iodides. Electron-deficient iodides (entries 5 and 6, Table 2)
and the electron-rich ones afforded the corresponding biphenyls in
very good yields. Good results were also obtained in the presence
of an electron withdrawer like fluorine in the aryl boronic acid.
Reaction between 2-furanboronic acid and iodobenzene afforded
the corresponding biaryl in poor yield (entry 7, Table 2). Smith
Pd(0)–PVP.
pling reaction with phenylboronic acids under microwave heating
(Scheme 4, Table 3). Yields of these reactions reflected the reactiv-
ity of the aryl bromides toward the oxidative addition. These re-
sults were consistent with the previous Letter by Smith et al.
33
3
3
et al. have reported that transmetalation is the limiting rate step
of the reaction between boronic acids and iodoarenes. As in this
case a large amount of biphenyl (17%) was detected, it is probable
that the palladium(II) complex, generated after the quick oxidative
addition of iodobenzene to the catalytic active palladium(0) spe-
cies, became susceptible to the homocoupling, because the furanyl
group was not readily transferred to this complex.
Although a good isolated yield (90%) was achieved in the cou-
pling of phenylboronic acid with 4-iodoanisol (entry 2), the same
was not observed for the reaction between this boronic acid and
iodoanilines (entries 10 and 11). The amino group deactivated
the aryl iodide to a greater extent than did the methoxy group.
These results can be attributed to the higher basicity of the nitro-
gen atom compared to oxygen. Furthermore, amines are good li-
gands for transition metals and can inhibit the cycle steps where
available sites are necessary. Increasing the catalyst loading from
that oxidative addition is the rate limiting step for the coupling
of aryl bromides. Good yields were achieved when bromobenzene
(
87%) and 4-bromoacetophenone (79%) were used as electrophiles
with 0.025% of palladium. However, the coupling with the electron
rich 4-bromoanisol furnished the corresponding 4-methoxybi-
phenyl in poor yield (43%).
After the reaction, product could be easily isolated by precipi-
tating the nanoparticles together with the inorganics present in
the reaction mixture by the addition of diethyl ether.
In summary, microwave irradiation improved the reaction rates
of the Suzuki cross-couplings between arylboronic acids and aryl
halides catalyzed by Pd(0)–PVP. The corresponding biaryls could
be obtained in good yields (75–97%) and short times (12 min) from
aryl iodides and in 43–87% yield from the aryl bromides. Products
could be easily isolated precipitating the nanoparticles and the
inorganics present in the reaction mixture by adding diethyl ether.
As far as we are aware, our system exhibits significant advantages
0
.005% to 0.025% (entry 11, Table 2), 2-phenylaniline could be ob-
tained in 80% yield.
Attempts to further investigate the scope and limitations of the
Pd(0)–PVP catalysts, aryl bromides were also employed in the cou-
4
5
as remarkably higher turnover numbers (up to 10 to 10 /h in
1
2 min  750/h in 40 min) were attained for the coupling of aryl
iodides and bromides.
Table 2
Microwave-assisted Suzuki reaction with aryl iodides
Entry
1
Boronic acid
Aryl halide
PhI
Product
Ph–Ph
Yieldb (%) (Y)
PhB(OH)
PhB(OH)
2
83 (>99)
2
2
MeO
I
MeO
90 (>99)
3
PhB(OH)
2
97 (99)
I
4
5
PhB(OH)
PhB(OH)
2
2
I
92 (>99)
85 (>99)
O N
I
NO2
O
2
O
6
7
PhB(OH)
2
94
I
O
O
B(OH)
PhI
34b
2
8
9
(HO) B
F
PhI
PhI
F
94 (>99)
75 (>99)
<0.01
2
(HO) B
OH
2
OH
1
1
0
1
PhB(OH)
PhB(OH)
2
2
H N
I
H N
2
2
NH2
I
NH2
80b,c
a
b
c
Reaction conditions: Ar1I (1.0 mmol), Ar
GC–MS yield.
2 2 2 3
B(OH) (1.1 mmol), K CO (2.0 mmol), 0.005% Pd(0)–PVP, EtOH (2.0 mL), sealed tube, 300 W, 12 min. Isolated yields.
0
.025% of Pd(0)–PVP.

D. de Luna Martins et al. / Tetrahedron Letters 51 (2010) 6814–6817
6817
1
2
9. Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1988, 53, 918–920.
Pd(0)-PVP, K CO ,
2
3
0. (a) Heck, R. F.; Nolley, J. P. J. Org. Chem. 1972, 37, 2320–2322; (b) Beletskaya, I.
P.; Cheprakov, A. V. Chem. Rev. 2000, 100, 3009–3066; (c) Martins, D. L.;
Alvarez, H. M.; Aguiar, L. C. S.; Antunes, O. A. C. Lett. Org. Chem. 2007, 4, 253–
Ar Br + PhB(OH)2
Ar -Ph
1
1
EtOH, 300W, 120º C
2
55.
1. (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467–
470; (b) Chinchilla, R.; Nájera, C. Chem. Rev. 2007, 107, 874–922.
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Scheme 4. Pd(0)–PVP-catalyzed Suzuki reactions under MW.
2
4
2
Table 3
Suzuki couplings between aryl bromides and phenylboronic acid
23. (a) Roy, P. S.; Bagchi, J.; Bhattacharya, S. K. Transit. Met. Chem. 2009, 34, 447–
453; (b) Zhang, Y.; Yu, J.; Liu, H. J. Colloid Interface Sci. 2007, 313, 503–510.
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Product
Yielda (%)
87
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1
Ar Br
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Br
Appukkuttan, P.; Eycken, E. V. Eur. J. Org. Chem. 2008, 1133–1155.
Ph
1
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9, 3895–3898.
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29. 2.5 g of PVP (40,000 Da) was added to 150 mL of methanol and stirred until the
total solubilization of the polymer. To this solution, Pd(OAc) was added and
4
2
2
3
MeO
O
Br
Br
MeO
O
Ph
Ph
43
79
2
the resulting mixture was immersed in a pre-heated oil bath at 85 °C, under
constant stirring for 3.0 h. Next, methanol was removed in a rotary evaporator
and the Pd(0)–PVP nanoparticles were transferred quantitatively to a 25 mL
volumetric flask.
N
Br
N
Ph
3
0. Martins, D.L; Alvarez, H. M.; Aguiar, L. C. S. Suzuki reactions catalyzed by
Pd(0)–PVP nanoparticles under conventional heating and ultrasound
irradiation—the Letter has been submitted for reviewing.
4
60
3
3
1. Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385–2388.
2. In a typical experiment, a pyrex tube equipped with a magnetic stirrer was
Conditions: aryl iodide (1.0 mmol), arylboronic acid (1.1 mmol), K
2 3
CO (2.0 mmol),
ethanol (2 mL), 0.01% Pd with respect to the mass of the aryl bromide.
2 3
charged with 0.276 g of K CO (2.0 mmol), 2.0 mL of ethanol, 0.22 mL of Pd(0)–
a
GC–MS yield with respect to the consumption of the aryl halide by normali-
zation of the areas.
PVP solution (0.1% Pd with respect to the iodobenzene mass), iodobenzene,
(1.0 mmol) and phenylboronic acid (1.1 mmol). This tube was sealed and the
content was subjected to the focused microwave irradiation at 300 W for
12 minutes (2.0 min ramping + 10 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 precipitation. Organic phase was analyzed by GC–MS (GCMS-
QP2010PluesShimadz). Afterward, the mixture was passed through a Celite
Acknowledgments
The authors acknowledge CAPES, CNPq, FAPERJ and FAPESB,
Brazilian Support Foundations for financial support.
pad, washed with ether, dried over Na
reduced pressure. Products were analyzed by GC–MS and H RMN.
2 4
SO and the solvent evaporated at
1 34
3
3. Smith, G. B.; Dezeny, G. C.; Huges, D. L.; King, A. O.; Verhoeven, T. R. J. Org.
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4. Biphenyl: white solid m/z (EI) 154 (100), 153 (40), 128 (5), 102 (4), 76 (28), 51
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1
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14. Astruc, D.; Lu, F.; Aranzas, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852–7872.
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16. (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513–519; (b)
Suzuki, A. J. Organomet. Chem. 1999, 576, 147–168.
27.29, 127.07, 26.84; nmax (KBr) 3060, 3032, 1684, 1599, 1452, 1419, 1357,
235, 803, 758, 697, 589. 4-phenylacetophenone: white solid m/z (EI) 196
(
(
1
50), 181 (100), 152 (60), 76 (75), 43 (36). 4-hydroxybiphenyl: white solid d
H
3
200 MHz CDCl ) 7.59–7.28 (7H, m), 6.93 (2H, d, J 10 Hz); m/z (EI) 170 (100),
1
1
7. (a) Stile, J. K. J. Am. Chem. Soc. 1979, 101, 4992–4998; (b) Stille, J. K. Angew.
Chem., Int. Ed. Engl. 1986, 98, 508–524.
8. Negishi, E. J. Am. Chem. Soc. 1976, 98, 6729–6731.
41 (35), 115 (35). 2-phenylpyridine: m/z (EI) 155 (100), 127 (19), 77 (21), 63
(
5), 51 (15).