Ultrasound-assisted synthesis of unsymmetrical biaryls by Stille cross-coupling reactions

Article abstract of DOI:10.1016/j.ultsonch.2011.08.013

We describe herein an efficient method for the synthesis of unsymmetrically-substituted biphenyls using a sonochemical variation of the Stille coupling, whose results have also been compared with the conventional silent reaction. Ultrasound significantly enhances this useful organometallic transformation affording products in higher yields and in shorter reaction times than non-irradiated reactions. The scope has been explored with a selection of arylstannanes as precursors and, remarkably, no by-products resulting from homo-coupling could be detected.

Full text of DOI:10.1016/j.ultsonch.2011.08.013

Ultrasonics Sonochemistry 19 (2012) 410–414
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Ultrasound-assisted synthesis of unsymmetrical biaryls by Stille
cross-coupling reactions
Claudia E. Domini , Gustavo F. Silbestri , Beatriz Fernández Band , Alicia B. Chopa 2
⇑
⇑
,1
1
Instituto de Química del Sur (CONICET-UNS), Departamento de Química, Universidad Nacional del Sur, Avenida Alem 1253, Bahía Blanca B8000CPB, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2011
Received in revised form 16 August 2011
Accepted 18 August 2011
Available online 4 October 2011
We describe herein an efficient method for the synthesis of unsymmetrically-substituted biphenyls using
a sonochemical variation of the Stille coupling, whose results have also been compared with the conven-
tional silent reaction. Ultrasound significantly enhances this useful organometallic transformation afford-
ing products in higher yields and in shorter reaction times than non-irradiated reactions. The scope has
been explored with a selection of arylstannanes as precursors and, remarkably, no by-products resulting
from homo-coupling could be detected.
Keywords:
Ultrasound
Ó 2011 Elsevier B.V. All rights reserved.
Stille cross-coupling
Arylstannanes
1
2
1
. Introduction
one of the groups of the stannane [R Sn(R )
3
] is transferred. Finally,
a reaction of ‘‘reductive elimination’’ leads to the coupled product
1
3
The Stille reaction involves the coupling of an organotin com-
(R –R ), thus regenerating the initial complex [Pd(0)Ln]. Numerous
catalysts are commercially available and they promote the coupling
reaction efficiently. Among them are tetrakis (triphenylphosphine)
2
pound with a sp -hybridized organic halide catalyzed by palla-
dium. This transformation has become a useful synthetic tool for
carbon–carbon bond formation and can also be extended to
numerous organic electrophiles [1–3] (Eq. (1)).
palladium(0) [Pd(PPh
[Pd (dba) ], bis(acetonitrile)dichloro palladium(II) [Pd(CH
Cl ], bis(triphenylphosphine)dichloropaladium(II) [Pd(PPh
and [1,1-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
Pd(dppf)Cl ].
It is well known that chemical and mechanical effects supplied
3
)
4
], tris(dibenzylideneacetonedipalladium(0)
CN)
Cl ],
2
3
3
2
2
3
)
2
2
Pdð0ÞLn
1
2
3
1
3
2
R SnðR Þ þ R X
!
R —R þ ðR Þ SnX
ð1Þ
3
3
[
2
In general, the reaction is carried out in an inert atmosphere using
anhydrous degassed solvents although there are few examples
working under air atmosphere. The process was discovered in the
late 1970s and it still represents a versatile reaction in contempo-
rary organic synthesis [4–6].
The commonly accepted mechanism for this reaction is shown
in Scheme 1 and follows the general principles of other cross-cou-
pling reactions catalyzed by transition metals [7].
by ultrasound come from cavitation. Growth and implosion of
microbubbles in liquids will induce high temperatures and pres-
sures inside such cavities, while shock waves at the interface and
bulk liquid are largely responsible for enhanced mass and energy
transfers. Ultrasonic irradiation constitutes a convenient way to
accelerate and improve numerous organic and organometallic
reactions by virtue of the effects mentioned above [8–11].
The use of ultrasound in Stille reactions remains essentially
ignored. However, there are a series of recent examples illustrating
the advantages of microwave irradiation for this type of reactions.
This subject has been reviewed by Dallinger and Kappe [12], who
focus on MW-assisted reactions in aqueous media, while Cravotto
et al. [13] describes Stille and other Pd-catalyzed couplings under
MW in a variety of solvents and conditions.
The catalytically active species is Pd(0), although a pre-catalyst
based on Pd(II) can also be used; Pd(II) is rapidly reduced to Pd(0)
by consuming the starting stannane.
The initial step of the catalytic cycle corresponds to the ‘‘oxida-
tive addition’’ of the organic halide to Pd(0), leading to the coordin-
3
atively saturated intermediate [R Pd(II)Ln-X]. The second step is a
‘
‘transmetalation’’ reaction, often the rate-limiting step, in which
Our purpose in this paper is the synthesis of biphenyls cata-
lyzed by palladium using Stille reaction under ultrasound. This
method provides high performance in short reaction times at mod-
erate temperatures with a reduced amount of catalyst. This proto-
col is much more benign and effective than conventional syntheses
and it benefits from the multiple effects of cavitation.
⇑
Corresponding authors. Tel.: +54 291 4595101/3535; fax: +54 291 4595106
C.E. Domini), tel.: +54 291 4595101/3537; fax: +54 291 4595187 (G.F. Silbestri).
E-mail addresses: cdomini@criba.edu.ar (C.E. Domini), gsilbestri@uns.edu.ar (G.F.
(
Silbestri).
1
Member of CONICET.
Member of CIC.
2
1
350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2011.08.013

C.E. Domini et al. / Ultrasonics Sonochemistry 19 (2012) 410–414
411
Pd(II)
after prolonged irradiation. An increment of the cycles, from 4 to 7,
produced an increment on the final temperature up to 90 °C and,
under these conditions, cross-coupling took place giving the de-
sired product in 41% yield, along with some homo-coupling prod-
uct. In order to determine if an increment of the temperature
would improve the yields of the reaction, we placed an external
bath (70 °C); after 2 min of irradiation the temperature of the dis-
solution reached 130 °C, the catalyst decomposed and the yield of
the desired product decreased to 22%.
With these results in hand, we decided to work under nitrogen
atmosphere (entries 4–8). With pleasure we noticed that, in all
cases, the reaction gave the desired cross-coupling product and
even traces of homo-coupling product were not detected (GC/
EM). The temperature is an essential factor to be considered,
being 90 °C (oil bath) the optimum working temperature (entry
1
2
R Sn(R )
3
1
1
R -R
Pd(0)Ln
R -R3
1
3
R -X
Oxidative
Addition
Reductive
Elimination3
1
3
R -Pd(II)Ln-R
R -Pd(II)Ln-X
XSn(R2)3
R Sn(R )
3
1
2
5
). Thus, working at lower (70 °C) or higher (100 °C) tempera-
Transmetalation
tures, under equal irradiation conditions, the desired biaryl was
obtained in lower yields (entries 4 and 6). Experiment 7 shows
that a lowering of the acoustic power to 300 W (50% output) pro-
duced, also, a sharp decreased in reaction yield. Moreover, when
the reaction was carried out without an oil bath (entry 8), the
solution temperature reached 100 °C and, under these conditions,
the catalyst decomposed after 20 min, giving only 70% yield of
biaryl.
Scheme 1. Catalytic cycle.
2
. Results and discussion
Our initial exploration started with the cross-coupling reaction
of (para-methoxyphenyl) trimethylstannane with bromobenzene
as a model system. The transformation was conducted in DMF, a po-
lar solvent suitable for both reagents, and using Pd(PPh ) Cl as
3 2 2
As seen in Table 2, no reaction occurs without catalysis, working
under the optimal reaction conditions defined. Moreover, either
Pd(0)(dba) or Pd(II)(PhCN) Cl were not suitable catalysts as there
3
2
2
catalyst (Eq. (2)). The results obtained are summarized in Table 1.
Reaction times were defined according with the presence of
black Pd (decomposition) in the reaction mixture. A series of
parameters have been assessed searching for optimal conditions,
such as the power output, temperature (oil bath), reaction time,
and working under air or inert atmosphere. The latter parameter
proved to be critical, since in the reactions carried out under air
the catalyst rapidly decomposes (2–10 min) and, also, homo-cou-
pling secondary product was detected. Entries 1–3 show the re-
sults obtained working in an open system under air; thus, the
process was ineffective working at 420 W (70% output power)
and 4 cycles where the temperature of the solution reached 50 °C
was no reaction in the first case and the biaryl was obtained in only
3 2 2
19% yield in the second one, being Pd(PPh ) Cl the appropriate
catalyst.
The scope of this sonochemical reaction was evaluated by the
reaction of bromobenzene with ortho-, meta- and para-substituted
trimethylstannyl derivatives (Table 3). In all cases, the reactions
were performed at the optimal conditions of temperature, catalyst,
and ultrasonic power previously found (ArSn:PhBr, 1:1.2, DMF,
2 mol% of Cl Pd(PPh ) with respect to arylstannane; power/cycle:
2 3 2
70/7; in an oil bath (90 °C); 30 min).
Yields ranged between 57% and 97% within 30 min, whereas
the conventional thermal method required, approximately, 20 h
to get comparable results [14]. As expected, the presence of an
(
from room temperature), and no product could be observed even
Table 1
a
Optimal conditions for the synthesis of para-methoxybiphenyl.
Br
MeO
SnMe
3
OMe
Cl Pd(PPh ) (2mol%)
2
3 2
US
c
Yield (%GC)d
0
Entry
Power ultrasound/cyclesb
Temperature (°C)
Bubbling gas
Time (min)
1
2
3
4
5
6
7
8
70/4
70/7
70/7
70/7
70/7
70/7
50/7
70/7
50
90
Air
Air
Air
120
10
2
30
30
30
70
20
e
41
22
f
g
130
h
70
N
N
N
N
N
2
2
2
2
2
34
97
83
42
70
h
90
h
100
h
90
100
a
Relation ArSn:PhBr = 1:1.2, DMF, 2 mol% of Cl
With a pulse setting of ‘‘10’’ the reaction is sonicated without interruption whereas with a pulse setting, for example, of ‘‘5’’ the reaction is sonicated for 5 s and then
2
Pd(PPh
3 2
) with respect to arylstannane.
b
sonication stops for 5 s.
c
From room temperature, unless otherwise stated.
Quantified by GC, using the external standard method. Formation of the homo-coupling product was not observed under N atmosphere.
2
Together with 9% of homo-coupling product.
From 70 °C (oil bath).
Together with 6% of homo-coupling product.
Constant temperature (oil bath).
d
e
f
g
h

4
12
C.E. Domini et al. / Ultrasonics Sonochemistry 19 (2012) 410–414
Table 2
Comparison with different catalysts.a
Br
MeO
SnMe
3
OMe
[
Pd] (2mol%)
US
Entry
Pd catalyst
Yieldb
1
2
3
4
–
0c
Cl
Pd
Pd(PhCN)
2
Pd(PPh
3
)
2
97
c
2
(dba)
3
0
2
Cl
2
19
a
Relation ArSn:PhBr = 1:1.2, DMF, 2 mol% of Cl
cycle: 70/7; 90 °C (oil bath); 30 min.
2
Pd(PPh
3
)
2
with respect to arylstannane; power/
b
Quantified by GC, using the external standard method.
Arylstannane was recovered.
c
Table 3
Cross-coupling reactions of arylstannanes and bromobenzene.
SnMe3
Br
Cl Pd(PPh ) 2mol%
Z
2
3 2,
Z +
method A or B
Entry
1
Z
Product
Classical method A yield (%)a (20 h)
Ultrasonic method B yield (%)a,b (30 min)
MeO
7
3c
76
o-OMe
OMe
2
3
m-OMe
p-OMe
87c
75c
87
97
OMe
Me
Cl
4
5
m-Me
m-Cl
52d
65
57
72
a
b
c
Quantified by GC, using the external standard method.
Formation of the homo-coupling product was not observed.
See Ref. [14].
See Ref. [2].
d
electron-donating group (–OMe) at para-position led to better re-
sults than other substitution patterns. Remarkably, the chromato-
graphic analysis did not detect the presence of the Wurtz-type
homo-coupling product, which represents a key advantage of
the ultrasonic procedure. The latter results would be attributed
to the acceleration caused by ultrasound, thus avoiding formation
of side products. It is interesting to note that two equivalents of
stannane are inevitably reduced per equivalent of Pd(II) and,
accordingly, the Stille reaction usually generates a small percent-
age of homo-coupling products [15].
As it is shown in Table 4, with regard to the organotin reagent,
aryl trimethylstannanes were slightly more reactive than the
corresponding tributylstannyl derivatives, working at optimal
conditions.
From a mechanistic viewpoint, further studies will be required
to elucidate the exact role of sonication. At first glance, the ob-
served acceleration would be ascribed to enhanced mass and en-
ergy transfers induced by the cavitational collapse. Like in other
organometallic reactions, the existence of coordinative unsatu-
rated species as intermediates could also be invoked. This would

C.E. Domini et al. / Ultrasonics Sonochemistry 19 (2012) 410–414
413
Table 4
Comparison of the reactivity of (trimethyl)- versus tributylstannanes.
a
SnR3
Br
Cl Pd(PPh ) 2mol%
Z
2
3 2,
Z +
DMF / 90° C / US / 30 min
Entry
R
Z
Product
Ultrasonic method B yield (%)b,c (30 min)
1
2
Me
Bu
o-OMe
m-OMe
m-Me
76
64
MeO
3
4
Me
Bu
87
82
OMe
Me
5
6
Me
Bu
57
44
7
8
Me
Bu
p-OMe
97
80
OMe
a
b
c
Relation ArSn:PhBr = 1:1.2, DMF, 2 mol% of Cl
Quantified by GC, using the external standard method.
Formation of the homo-coupling product was not observed.
2 3 2
Pd(PPh ) with respect to arylstannane; power/cycle: 70/7; 90 °C (oil bath); 30 min.
be a sonication-sensitive step in agreement with type I reactions as
formulated previously [16,17].
was charged with a mixture of bromobenzene (1.20 mmol) and
PdCl (PPh (2 mol%) suspended in dry DMF (20 mL). (para-
2
3 2
)
Methoxy phenyl) trimethylstannane (1.00 mmol) was added
dropwise via syringe and the mixture was stirred at 80 °C (oil
bath) for 20 h (monitoring the disappearance of the stannane by
TLC). After the resulting black precipitates were filtered through
a silical gel pad, water (50 mL) was added and then the solution
was extracted with ethyl ether (3 ꢀ 20 mL). The combined organ-
3
. Conclusions
In conclusion, this work shows our preliminary results with
ultrasonically-induced Stille cross-coupling reactions. This varia-
tion leads to products in moderate to good yields within 30 min
at 90 °C and without Wurtz-type dimerization products. In con-
trast, the conventional thermal method requires 20 h at 80 °C
4
ic layers were dried over MgSO and concentrated under reduced
pressure. The residue was purified by dry column vacuum chro-
matography (DCVC) [18] on silica gel (hexane:EtOAc, 9:1–6:4)
to give 0.134 g (0.730 mmol, 73%) of para-methoxybiphenyl as a
[
14] and the desired product is always accompanied by homo-cou-
pling products between 10% and 15% yield. Further improvements
and mechanistic studies are currently under way.
1
white solid. m.p.: 88–90 °C; H NMR: d 7.49–7.43 (m, 4H),
7
.34–7.30 (m, 2H), 7.24–7.17 (m, 1H), 6.92–6.86 (m, 2H); 3.71
1
3
(
1
s, 3H); C NMR: d 158.9 (C), 140.6 (C), 133.6 (C), 128.4 (CH),
27.9 (CH), 126.5 (CH), 126.4 (CH), 114.0 (CH), 55.1 (CH ); MS
4
. Experimental
3
+
+
(m/z, relative intensity): 184 (83, M ), 169 (51, M -15), 141
4.1. General methods
(
98), 115 (100) [14].
A Cole Parmer 4710 series ultrasonic homogenizer operating at
0 kHz (600 W) provided the high intensity ultrasound. This con-
sists of an ultrasonic generator equipped with a probe that emits
the sound vibration in the solution through a titanium alloy bar
4
.2.2. Ultrasonic irradiation (B)
To a solution of 0.02 mmol of Pd catalyst in 13 mL of DMF,
mmol of arylstannane and 1.2 mmol of bromobenzene were
2
1
added and the reaction mixture was exposed to ultrasonic irradia-
tion at 90 °C (oil bath) during 30 min (monitoring the disappear-
ance of the stannane by TLC). After the resulting black
precipitates were filtered through a silical gel pad, water (50 mL)
was added and the solution was extracted with ethyl ether
(
25 mm diameter) dipped into the top of the liquid in a two-necked
round-bottomed Pyrex flask (volume 25 mL) equipped with a ther-
mometer and a nitrogen inlet.
NMR spectra were recorded on a Bruker ARX 300 (300.1 MHz
1
13
for H, 75.5 MHz for C) using CDCl
nal reference. Mass spectra were obtained with a GC/MS instru-
ment (HP5-MS capillary column, 30 m ꢀ 0.25 mm ꢀ 0.25 m)
equipped with 5972 mass selective detector operating at 70 eV
EI). Program: 50 °C for 5 min with increase 10 °C/min–250 °C.
3 4
as solvent and SiMe as inter-
(
3 ꢀ 20 mL). The combined organic layers were dried over MgSO
and concentrated under reduced pressure to give 0.178 g
0.970 mmol, 97%) of para-methoxybiphenyl as a white solid.
4
l
(
(
5
. Product characterization
4
4
.2. General procedure for the synthesis of para-methoxybiphenyl
Meta-methoxybiphenyl: Colorless oil. 1H NMR: d 7.50–7.43 (m,
.2.1. Classical method (A)
A two-necked, 50 mL round-bottomed flask fitted with a reflux
2H), 7.34–7.26 (m, 2H), 7.25–7.17 (m, 2H), 7.10–6.99 (m, 2H);
1
3
6.77 (m, 1H), 3.71 (s, 3H); C NMR: d 159.6 (C), 142.3 (C), 140.7
condenser, nitrogen inlet, magnetic stirrer and rubber septum,
(C), 129.3 (CH), 128.3 (CH), 126.9 (CH), 126.7 (CH), 119.2 (CH),

4
14
C.E. Domini et al. / Ultrasonics Sonochemistry 19 (2012) 410–414
1
12.5 (CH), 112.2 (CH), 54.8 (CH
3
); MS (m/z, relative intensity): 184
[3] J.K. Stille, The palladium-catalyzed cross-coupling reactions of organotin
reagents with organic electrophiles, Angew. Chem. Int. Ed. Engl. 25 (1986)
08–524.
+
+
(
70, M ), 154 (32, M -30), 141 (66), 139 (36), 115 (100) [14].
5
1
Ortho-methoxybiphenyl: Colorless oil. H NMR: d 8.02–7.86 (m,
[
4] G. Kerric, E. Le Grognec, F. Zammattio, M. Paris, J.-P. Quintard, Use of polymer-
supported phenyltin for the creation of aryl–aryl or aryl–heteroaryl bonds via
Stille cross-coupling reactions, J. Organometal. Chem. 695 (2010) 103–
2
H), 7.76–7.62 (m, 5H), 7.45–7.16 (m, 3H) 4.12 (s, 3H); 13C NMR:
d 156.0 (C), 138.1 (C), 131.2 (C), 130.4 (CH), 129.1 (CH), 128.1
CH), 127.5 (CH), 126.4 (CH), 121.8 (CH), 120.4 (CH), 110.9 (CH),
110.
(
1
1
[
[
5] E. Flöistrup, P. Goede, R. Strömberg, J. Malm, Synthesis of estradiol backbone
mimics via the Stille reaction using copper(II) oxide as co-reagent, Tetrahedron
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6] A. Pal, R. Ghosh, N.N. Adarsh, A. Sarkar, Pyrazole-tethered phosphine ligands
for Pd(0): useful catalysts for stille kumada and hiyama cross-coupling
reactions, Tetrahedron 66 (2010) 5451–5458.
+
3
08.9 (CH), 55.1 (CH ); MS (m/z, relative intensity): 184 (75, M ),
41 (70); 139 (30), 115 (100) [14].
Meta-methylbiphenyl: Colorless oil. 1H NMR: d 7.48–7.45 (m,
H), 7.30–7.27 (m, 2H), 7.24–7.19 (m, 2H), 7.07–7.02 (m, 3H),
2
2
1
3
[7] For a mechanistic investigation: P. Espinet, A.M. Echavarren, The mechanisms
of the stille reaction, Angew. Chem. Int. Ed. 43 (2004) 4704–4734.
.35 (s, 3H); C NMR: d 138.4 (C), 136.8 (C), 136.6 (C), 129.4
); MS (m/z, rel-
ative intensity): 168 (100, M ); 167 (57, M -1); 152 (36); 139 (20);
15 (32) [19].
(
CH), 128.8 (CH), 128.1 (CH), 127.5 (CH), 21.5 (CH
3
[
8] J.L. Luche (Ed.), Synthetic Organic Sonochemistry, Plenum Press, New York,
1998.
+
+
[
9] T.J. Mason, J.P. Lorimer, Applied Sonochemistry. The Uses of Power Ultrasound
in Chemistry and Processing, Wiley-VCH, New York, 2002.
1
1
Meta-chlorobiphenyl: Colorless oil. H NMR: d7.21–7.80 (m, 9H);
C NMR: d 140.2 (C), 135.2 (C), 135.1 (C), 130.3 (CH), 129.3 (CH),
27.7 (CH), 127.6 (CH), 125.7 (CH); MS (m/z, relative intensity):
[
10] T.J. Mason, D. Peters, Practical Sonochemistry, 2nd ed., Ellis Horwood, London,
2002.
[11] G. Cravotto, P. Cintas, Power ultrasound in organic synthesis: moving
cavitational chemistry from academia to innovative and large-scale
applications, Chem. Soc. Rev. 35 (2006) 180–196.
1
3
1
1
+
+
+
88/190 (3/1, 100, M ); 152 (95, M -Cl); 151 (34, M -HCl); 126
(
20) [20].
[
12] D. Dallinger, C.O. Kappe, Microwave-assisted synthesis in water as solvent,
Chem. Rev. 107 (2007) 2563–2591.
[
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promoted by ultrasound and/or microwave irradiation, Curr. Org. Chem. 12
Acknowledgments
(
2008) 1588–1612.
This work was partially supported by CONICET, CIC, ANPCYT
and the Universidad Nacional del Sur, Bahía Blanca, Argentina.
The authors would like to thank Prof. Dr. Pedro Cintas Moreno by
his commentaries and Veronica Philippi for her revision of the Eng-
lish grammar and vocabulary.
[14] A.B. Chopa, G.F. Silbestri, M.T. Lockhart, Strategies for the synthesis of bi- and
triarylic materials starting from commercially available phenols, J.
Organometal. Chem. 690 (2005) 3865.
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5
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[19] T. Tu, H. Mao, C. Herbert, M. Xua, K.H. Dötz,
[
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ultsonch.2011.08.013.
1
A
pyridine-bridged bis-
benzimidazolylidene pincer nickel(II) complex: synthesis and practical
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