Ultrasound-assisted synthesis of symmetrical biaryls by palladium-catalyzed detelluration of 1,2-diarylditellanes

Article abstract of DOI:10.1016/j.tetlet.2009.12.028

An ultrasound-assisted synthesis of functionalized symmetrical biaryls with electron-withdrawing or electron-donating substituents is described and illustrated by the palladium-catalyzed detelluration of 1,2-diarylditellanes. This procedure offers easy access to symmetrical biaryls in short reaction time and the products are achieved in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2009.12.028

Tetrahedron Letters 51 (2010) 863–867
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Ultrasound-assisted synthesis of symmetrical biaryls by palladium-catalyzed
detelluration of 1,2-diarylditellanes
*
Fateh V. Singh, Hélio A. Stefani
Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, São Paulo, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
An ultrasound-assisted synthesis of functionalized symmetrical biaryls with electron-withdrawing or
electron-donating substituents is described and illustrated by the palladium-catalyzed detelluration of
1,2-diarylditellanes. This procedure offers easy access to symmetrical biaryls in short reaction time
and the products are achieved in good to excellent yields.
Received 2 November 2009
Revised 5 December 2009
Accepted 7 December 2009
Available online 11 December 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
Detelluration reaction
Biaryls
Diarylditellurides
Biaryl scaffolds are important structural motifs for both syn-
thetic and medicinal purposes. These scaffolds are also common
structural features found in a large number of biologically impor-
tant natural products.1 Aside from their occurrence in complex
natural products and pharmaceutical agents,2 these compounds
are also applied as chiral ligands,3 liquid crystal materials,4 and or-
ganic conductors.5
this method requires phosphine or phosphate ligands and harsh
reaction conditions.21 In the past few years, Yamamoto et al. re-
ported an efficient method to obtain symmetrical biaryls through
OMe
OMe
O
O
Several synthetic biaryl compounds have been reported to have
diverse biological activities6 such as antidiabetic,7 anticancer,8 anti-
O
O
O
O
CO2Me
CO2Me
CO2Me
CH2OH
bacterial,9 and
c
-secretase inhibitory activity.10 Some symmetric
and asymmetric biaryls such as
a-DDB (methyl 4,40-dimethoxy-
O
O
5,6,50,60-dimethylenedioxy biphenyl-2,2-dicarboxylate) and bicyc-
lol (Fig. 1) are used as leading hepatoprotective agents.11 Recently,
some biaryl scaffolds have been identified as a new class of antile-
ishmanial agents.12
OMe
OMe
α-DDB
Bicyclol
Figure 1. Structures of biologically active biaryl scaffolds.
Aryl–aryl bond formation for the preparation of symmetrical
and unsymmetrical biaryl compounds is one of the most useful
and important tools in modern organic chemistry. The synthesis
of some unsymmetrical biaryl compounds has been achieved by
metal-catalyzed13 and non-metal-catalyzed14 approaches in the
past few years. Symmetrical biaryls are traditionally obtained
using the Ullmann reaction15 and some other methods.16 In the
past decade, the synthesis of symmetrical biaryl scaffolds has con-
tinued using metal-assisted homocoupling of aryl halides,17 boro-
nic acids,18 aryl Grignard reagents,19 and arene diazonium salts.20
Wong and Zhang reported the synthesis of these systems through
the palladium-catalyzed homocoupling of aryl boronic acids, but
Table 1
Study of the catalyst effect on detelluration of 1,2-diphenylditellane 1a
Entry
Catalysta
Yieldb (%)
1
2
3
4
5
6
7
8
—
PdCl2
nr
95
82
20
62
46
nr
nr
Pd(AcO)2
Pd(dba)
Pd(BzCN)2
PdCl2(PEPPSI)
Fe(acac)3
Cu(OAc)2
Reaction conditions: diphenylditelluride (1 equiv), catalyst (10 mol %), Na2CO3
(2 equiv), Ag2O (2 equiv), MeOH, irradiate in ultrasonic bath.
a
10 mol % of catalyst was used.
The yields were determined by GC analysis.
* Corresponding author. Tel.: +55 11 3091 3654; fax: 55 11 3815 4418.
E-mail address: hstefani@usp.br (H.A. Stefani).
b
0040-4039/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2009.12.028

864
F. V. Singh, H. A. Stefani / Tetrahedron Letters 51 (2010) 863–867
Table 2
the homocoupling of aryl boronic acids, but this method suffers
from low yields with electron-withdrawing functionalities as aryl
boronic acids.22
Study of the effects of additive and base on the detelluration of 1,2-diphenylditellane
1a
Entry
Base
Additive (equiv)
Pd(PPh3)4 (mol %)
Yielda (%)
Organotellurium compounds have undergone remarkable
development as intermediates in synthetic organic chemistry.
Organotellurium compounds have been used instead of halogens
as electrophilic partners in some palladium-catalyzed cross-cou-
pling reactions.23,24 Recently, we reported the synthesis of sym-
metrical biaryls25 through the palladium-catalyzed homocoupling
of n-butyl aryltellurides, which proceeds in a few minutes using
ultrasonic waves as a source of energy. The ultrasound effects are
attributed to a physical process called cavitation.26
Herein, we report a new protocol for the synthesis of symmetri-
cal biaryls using a palladium-catalyzed detelluration reaction of
functionalized 1,2-diarylditellanes with ultrasonic waves as a
source of energy. The strength of the procedure lies in the formation
of a C–C bond and the introduction of electron-donor or -acceptor
functionalities into the products.
1
2
3
4
5
6
7
8
9
10
11
12
13
—
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
AgOAc (2)
—
AgOAc (2)
Ag2O (2)
CuI (2)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (10)
PdCl2 (8)
nr
Na2CO3
Cs2CO3
NaHCO3
DIPEA
Et3N
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
95
92
72
79
84
nr
95
98
35
55
78
70
Ag2O (2)
Ag2O (2)
Ag2O (2)
PdCl2 (10)
Reaction conditions: diphenylditelluride (1 equiv), PdCl2 (10 mol %), base (2 equiv),
additive (2 equiv), MeOH, irridiate in ultrasonic bath.
a
The yields were determined by GC analysis.
The approach to prepare biaryl compounds 2a–i was based on a
palladium-catalyzed detelluration reaction of functionalized 1,2-
diarylditellanes 1a–i. The parent precursors 1,2-diarylditellanes
1a–j were conveniently prepared in high yields through the Grig-
nard reaction of aryl halides followed by the addition of tellurium
and oxidation by air.27
Table 3
Study of the solvent effect on the detelluration of 1,2-diphenylditellane 1a
Entry
Solvent
Yielda (%)
Initially, we optimized the conditions for the detelluration of
functionalized 1,2-diarylditellanes 1. In order to find appropriate
conditions for the detelluration reaction of 1,2-diarylditellanes,
1,2-diphenylditellane 1a was selected as a model substrate and a
variety of conditions were screened, as described in Tables 1–3.
The reactions were monitored by TLC or GC.
1
2
3
4
5
MeOH
MeCN
THF
1,4-Dioxane
Toluene
98
38
35
17
23
Reaction conditions: diphenylditelluride (1 equiv), PdCl2 (10 mol %), Na2CO3
(2 equiv), Ag2O (2 equiv), solvent, irradiate in ultrasonic bath.
Initially, we surveyed palladium catalysts for this detelluration
reaction. We attempted reactions with some palladium (Table 1,
a
The yield was determined by GC analysis.
Table 4
Detelluration of functionalized 1,2-diarylditellanes 1a–j
R4
R2
R1 R4
R4
PdCl2, Na2CO3
R3
Te
Te
R3
R3
R3
Ag2O, MeOH
) ) )
R2
R1
R4 R1
R2
R1
R2
2
Entry
a
Diarylditellurides (1)
Biaryls (2)
Reaction time (min)
30
Yielda (%)
95
Te
Te
2a28
Br
Cl
Te
Te
Br
Br
b
c
30
30
25
92
88
82
Br
Cl
2b29
Te
Te
Cl
Cl
2c29
F
Te
Te
F
F
d
F
2d28
F3C
F3C
CF3
CF3
e
Te
Te
45
84
2e25
Me
Me
Me
Me
Te
Te
f
45
30
78
83
Me
2f29
Te
Te
Me
Me
g
Me
2g29

F. V. Singh, H. A. Stefani / Tetrahedron Letters 51 (2010) 863–867
865
Table 4 (continued)
Entry
Diarylditellurides (1)
Biaryls (2)
Reaction time (min)
30
Yielda (%)
MeO
Me
Te
Te
MeO
Me
OMe
h
i
92
OMe
2h28
Me
Me
Me
Me
Te
Te
40
60
82
Me
Me
Me
Me
2i29
Me
Me
Me Me
Me
Te
Te
Me
j
—
Me
Me Me
Me
Reaction conditions: diarylditelluride (1 equiv), PdCl2 (10 mol %), Na2CO3 (2 equiv), Ag2O (2 equiv), MeOH, irridiate in ultrasonic bath.
a
Isolated yields.
entries 2–6) and non-palladium (Table 1, entries 7 and 8) catalysts,
but the reaction only worked with palladium catalysts. Pd(II) and
(0) species were used in these detelluration reactions, but the best
results were obtained with Pd(II) species (Table 1, entries 2, 3 and
5). Two equivalents of Ag2O as an additive were used, with 2 equiv
of sodium carbonate and methanol as the solvent. The reaction was
irradiated for 30 min in an ultrasound bath. The best result was ob-
tained with PdCl2 (Table 1, entry 2).
The next step was the determination of the best base and the
necessity of an additive in the reaction. Initially, we used sodium
carbonate as a base in the presence of Ag2O, and the desired com-
pound was observed in 95% yield (Table 2, entry 2). We attempted
the same reaction with some other inorganic bases such as cesium
carbonate and sodium bicarbonate and observed a slight decrease
in the yield of the desired biaryl compound (Table 2, entries 3 and
4). When the same reaction was performed with organic bases
such as DIPEA and triethylamine, the desired compound was iso-
lated in 79% and 84% yields (Table 2, entries 5 and 6). We also per-
formed the reaction without using any base, but no reaction was
observed (Table 2, entry 1).
To observe the influence of the additive, we performed the same
reaction with three different additives: AgOAc, Ag2O, and CuI. The
desired biphenyl product 2a was observed in 95%, 98%, and 35%
yields, respectively (Table 2, entries 8–10). To establish the correct
stoichiometry of the reaction, we performed it with 1 equiv of
Ag2O and the desired product was observed in 55% yield (Table
2, entry 11), while the intermediate diphenyltellane was also ob-
served in 45% yield. No reaction was observed in the absence of
an additive (Table 2, entry 7).
In order to determine the best solvent for the biaryl formation,
we performed different reactions with different types of solvents.
The best result was achieved with a polar protic solvent (metha-
nol), which furnished the desired product in 98% yield (Table 3, en-
try 1). The optimization results with different solvents are
described in Table 3.
Finally, to observe the effect of ultrasonic waves in this reaction,
we attempted the same reaction under reflux conditions. The reac-
tion was completed in 6 h and desired compound 2a was isolated
in 70% yield (Table 2, entry 13). The role of Ag2O can be attributed
to the removal of phosphine ligands from the catalyst or from one
Table 5
Detelluration reaction of dinaphthylditellurides 3a–c
R
R
PdCl2, Na2CO3
Ag2O, MeOH
Te Te
R
) ) )
R
4
3
Entry
a
Dinaphthylditellurides (3)
Binaphthyls (4)
Reaction time (min)
60
Yielda (%)
Te
Te
72
4a25
Te
Te
b
c
30
35
81
85
4b28
OMe
OMe
MeO
Te
Te
MeO
4c25
Reaction conditions: dinaphthylditelluride (1 equiv), PdCl2 (10 mol %), Na2CO3 (2 equiv), Ag2O (2 equiv), MeOH, irridiate in ultrasonic bath.
a
Isolated yields.

866
F. V. Singh, H. A. Stefani / Tetrahedron Letters 51 (2010) 863–867
of the catalytic intermediates formed in the course of the
reaction.13a
Acknowledgments
The catalyst loadings were analyzed, and the best result was
obtained with 10 mol % of PdCl2, which provided a 98% yield.
During the optimization studies for biphenyl 2a, was observed
that a reaction mixture of 1.0 equiv of 1,2-diphenylditellane 1a,
2.0 equiv of Ag2O, 2.0 equiv of sodium carbonate, and 10 mol %
of PdCl2 in methanol irradiated under ultrasonic waves for
30 min were the best conditions for the synthesis of biphenyl
2a. After optimizing the conditions for the synthesis of biphenyl
2a, we synthesized a series of functionalized biaryl compounds
(2a–i) using the optimized conditions in 78–95% yields (see Table
4). Interestingly, the reaction proceeded nicely with both elec-
tron-withdrawing and electron-donating substituents in the biar-
yl architecture. When we attempted the same reaction with more
hindered 1,2-dimesitylditellane 1j under similar reaction condi-
tions, we isolated the dimesityltellane instead of corresponding
biaryl derivative 2,20,4,40,6,60-hexamethylbiphenyl 2j. All of the
synthesized compounds were characterized by spectroscopic
analysis.30
The authors are grateful to FAPESP (07/51466-9, 07/59404-2)
and CNPq for financial support.
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Ar1
Ar2
Ar1
Te
Te
+
Te
Pd
Te
Ar2
Ar1
Ar2
C
(
)
B
(
)
Te Pd (II)
Te
Pd(0)
A
(
)
Ag+
Ar1
Ar2
Ar1
Pd(II)
Te
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Pd (0)
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D
(
)
Pd(0)
Ar1
Ar2
Pd Te
Ar1 Ar2
Te +
E
(
)
Figure 2. Possible catalytic cycle of detelluration reaction of 1,2-diarylditellurides.

F. V. Singh, H. A. Stefani / Tetrahedron Letters 51 (2010) 863–867
867
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30. General experimental procedure for biaryls 2a–i and 4a–c: A suspension of 1,2-
diphenylditellane 1a (0.204 g, 0.5 mmol), PdCl2 (0.09 g, 10 mol %), sodium
carbonate (0.106 g, 1 mmol), and silver oxide (0.232 g, 1.0 mmol) in 5 mL of
methanol was irradiated in a water bath of an ultrasonic cleaner (Bransonic
Ultrasonic Cleaner) at room temperature for 30 min. After the completion of
reaction, the reaction mixture was diluted with ethyl acetate (20 mL). The
organic layer was washed with saturated solution of NH4Cl (2 ꢀ 10 mL) and
water (2 ꢀ 10 mL), dried over MgSO4, and concentrated under vacuum. The
crude product was purified by flash silica column chromatography using
hexane as an eluent and characterized as biphenyl28 2a: white solid; mp 70–
72 °C; 1H NMR (300 MHz, CDCl3) d 7.21–7.44 (m, 6H, ArH), 7.65 (d, J = 7.2 Hz,
4H, ArH); 13C NMR (75.5 MHz, CDCl3) d 127.6, 128.76, 141.29; GC–MS (%) 154
(100), 153 (44), 152 (29), 76 (32). 4,40-Dibromobiphenyl29 2b: white solid; mp
160–162 °C; 1H NMR (300 MHz, CDCl3) d 7.41 (d, J = 8.2 Hz, 4H, ArH), 7.52 (d,
J = 8.2 Hz, 4H, ArH); 13C NMR (75.5 MHz, CDCl3) d 121.93, 128.48, 132.00,
138.91; GC–MS (%) 312 (66), 152 (100), 151(25), 76 (94). 1,10-Binaphthyl25 4a:
white solid; mp 138–140 °C; 1H NMR (300 MHz, CDCl3) d 7.20 (t, J = 7.8 Hz, 2H,
ArH), 7.36–7.52 (m, 4H, ArH); 7.65–7.76 (m, 6H, ArH); 8.18 (d, J = 8.4 Hz, 2H,
ArH); 13C NMR (75.5 MHz, CDCl3) d 122.93, 126.25, 126.78, 127.19, 127.42,
128.03, 128.40, 129.99, 132.09, 134.56; GC–MS (%) 254 (90), 253 (100), 252
(80), 250 (25), 126 (98), 125 (47).
25. Singh, F. V.; Stefani, H. A. Synlett 2008, 3221.
26. (a) Margulis, M. A. High Energ. Chem. 2004, 38, 135; (b) Mason, T. J. Chem. Soc.
Rev 1997, 26, 443.
27. General experimental procedure for diarylditellurides 1a–j and 3a–c: Mg (0.240 g,
20 mmol) and catalytic amounts of I2 were heated under N2 in a two-necked
round-bottomed flask. A solution of aryl halide (10 mmol) in dry THF (20 mL)
was added dropwise and the mixture stirred for 1 h at room temperature.
Activated elemental Te (2.54 g, 20 mmol) was added, and after 1 h the system
was opened to allow oxidation. The reaction mixture was diluted with ethyl
acetate, a solution of NH4Cl was added dropwise, and stirred for 2 h. The
reaction was monitored by TLC and GC. The reaction mixture was extracted
with ethyl acetate (30 mL) and washed with a solution of brine. The organic
layer was dried over MgSO4, filtered over Celite, and concentrated under
vacuum. Finally, the red solid crystalline compounds 1a–j and 3a–c were
obtained without using column chromatography.
31. Barton, D. H. R.; Ozbalik, N.; Ramesh, M. Tetrahedron Lett. 1988, 29, 3533.