Ultrasound-assisted synthesis of symmetrical biaryls by palladium-catalyzed homocoupling of aryl n-butyl tellurides

Article abstract of DOI:10.1055/s-0028-1087244

An ultrasound-assisted synthesis of symmetrical biaryls with electron-withdrawing or -donating substituents is described and illustrated by palladium-catalyzed homocoupling reaction of aryl tellurides. This procedure offers easy access to biaryls in short

Full text of DOI:10.1055/s-0028-1087244

LETTER
3221
Ultrasound-Assisted Synthesis of Symmetrical Biaryls by Palladium-
Catalyzed Homocoupling of Aryl n-Butyl Tellurides
U
ltrasound-As
a
siste
d
Synt
t
hesis
o
e
f
S
ymmetri
h
ca
l
Biaryls Veer Singh,a Hélio A. Stefani*a,b
a
Departamento de Farmácia, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, São Paulo, SP - Brazil
Deprtamento de Biofísica, Universidade Federal de São Paulo, São Paulo, SP - Brazil
Fax +55(11)38154418; E-mail: hstefani@usp.br
b
Received 11 June 2008
Recently, Buchwald’s chiral biaryl phosphine ligands
with triisopropyl moieties (XPhos and tert-butyl XPhos)
have emerged as versatile chiral ligands for palladium-
catalyzed asymmetric syntheses.8 Several biaryl com-
Abstract: An ultrasound-assisted synthesis of symmetrical biaryls
with electron-withdrawing or -donating substituents is described
and illustrated by palladium-catalyzed homocoupling reaction of
aryl tellurides. This procedure offers easy access to biaryls in short
reaction time, and the products are achieved in good to excellent pounds have recently been reported as potent glucagon re-
ceptor antagonists for the treatment of diabetes.9 The
yields.
biaryl Bay-27-9955, functionalized with two isopropyl
units, has been reported to inhibit glucagon from the hu-
man glucagon receptor with an IC50 value of 110 nM.10 In
addition, some of the biaryl scaffolds have been identified
as new class of antileishmanial agents.11
Key words: homocoupling reaction, symmetrical biaryls, aryl
n-butyl tellurides
The demand for biaryl scaffolds for both synthetic and
medicinal purposes has increased dramatically during the
past few decades. These scaffolds are common structural
features found in many biologically important natural
products such as michellamine A (I) and secalonic acid
(II, Figure 1).1 Besides their great diversity in complex
natural products and pharmaceutical agents,2 these com-
pounds are fascinating and challenging research objects in
the material and polymer sciences.3,4 Axially chiral biar-
yls are useful as versatile auxiliaries for asymmetric syn-
thesis,5 as chiral phases for chromatography,6 and
important substrates for chiral liquid crystalline materi-
als.7
The construction of biaryl axis can be achieved either by
intermolecular- or by intramolecular cross-coupling of
two similar or dissimilar aromatic rings in the presence of
organometallic complexes. Palladium-catalyzed cross-
coupling reactions between the electrophilic compounds
ArX (X being mainly Cl, Br, I, and OTf) and organome-
tallic species ArM (M being Mg, Zn, Sn, and B) are on the
verge of becoming truly general process for the construc-
tion of biaryl systems. Recently, the synthesis of unsym-
metrical biaryls by the palladium-catalyzed cross-
coupling of potassium aryltrifluoroborate salts and aryl
tellurides was reported.12
Symmetrical biaryls were traditionally obtained by the
Ullmann reaction13 and some other methods.14 In current
decade the synthesis of symmetrical biaryl systems has
been emphasized around the metal-assisted homocou-
pling of aryl halides,15 boronic acids,16aryl Grignard re-
agents,17 and arene diazonium salts.18 Wong and Zhang
reported the synthesis of these systems by the palladium-
catalyzed homocoupling of aryl boronic acids but re-
quired phosphine or phosphate as ligands and harsh reac-
tion conditions.19 In the past decade some symmetrical
biaryls have been reported by palladium-catalyzed cou-
pling reactions of diaryltellurium(IV) dichlorides,20a aryl-
tellurium(IV) trichlorides,20b and telluronium salts.20c
HO
COOMe
Me
O
O
OH
Me
OH
OH
O
O
Me
HN
MeOOC
OH
Me
Me
OH
secalonic acid (II)
OH OMe
OMe OH
HO
Me
Me
Recently, organotellurium compounds have attained re-
markable development as synthons and intermediates in
synthetic organic chemistry. In the current decade, the or-
ganotellurium compounds have been used instead of halo-
gens as electrophilic partners in some palladium-
catalyzed cross-coupling reactions.21,22
NH
OH Me
michellamine A (I)
Figure 1 Structures of michellamine A (I) and secalonic acid (II)
Herein, we report a new protocol for the synthesis of biar-
yl systems by palladium-catalyzed homocoupling of func-
tionalized aryl n-butyl tellurides using ultrasonic waves as
a source of energy.23 The strength of the procedure lies in
SYNLETT 2008, No. 20, pp 3221–3225
Advanced online publication: 24.11.2008
DOI: 10.1055/s-0028-1087244; Art ID: S05908ST
© Georg Thieme Verlag Stuttgart · New York
1
6
.1
2
.2
0
0
8

3222
F. V. Singh, H. A. Stefani
LETTER
the formation of the C–C bond and introduction of elec- The role of Ag2O can be attributed to the removal of phos-
tron-donor or -acceptor functionalities into their molecu- phine ligands of the catalyst or from one of the catalytic
lar structure.
intermediates formed in the course of the reaction.12 The
catalyst loadings were analyzed, and the best result was
afforded with 8 mol% of Pd(PPh3)4, in 85% yield.
The approach to prepare biaryl compounds 2a–h was
based on palladium-catalyzed homocoupling reaction of
functionalized aryl n-butyl tellurides 1a–h. The parent Finally, to observe the effect of ultrasonic waves in this
precursors aryl n-butyl tellurides 2a–h were conveniently reaction, we put the same reaction under reflux condi-
prepared in high yields by Grignard reaction of aryl ha- tions, and the reaction was completed in 10 hours while
lides followed by addition of tellurium and butylation by the desired compound 2a was isolated in 52% yield.
n-bromobutane.12
During the optimization studies for biphenyl 2a, it was ob-
Initially, we paid our attention on the determination of the served that the reaction mixture of n-butyl phenyl telluride
optimal conditions for the homocoupling of aryl tellurides 1a (1.0 equiv), Ag2O (1.0 equiv), sodium carbonate (2.0
1. Toward this end, n-butyl phenyl telluride 1a was cho- equiv), and of Pd(PPh3)4 (8 mol%) in methanol, irradiated
sen as model substrate and a variety of conditions was under ultrasonic waves for 45 minutes, were the best con-
screened (Table 1). The reactions were monitored by TLC dition for the synthesis of biphenyl 2a. After achieving the
or GC.
best condition for the synthesis of the desired compound
2a, we synthesized a series of functionalized biaryl com-
pounds 2a–h using the optimized conditions in 59–85%
yields (see Table 2). Unfortunately, this reaction was not
working with more hindered aryl n-butyl telluride 1i. All
the synthesized compounds were characterized by spec-
troscopic analysis.24
First of all, we determinated the palladium catalyst. The
Pd(II) and (0) species were used in the homocoupling re-
actions and the best result was reached with Pd(PPh3)4
(Table 1, entry 4). It was used Ag2O as additive, triethyl-
amine as base, and methanol as solvent, and the reaction
was irradiated for 45 minutes in an ultrasound bath. The
product was obtained in 64% isolated yield.
To generalize this approach, n-butyl naphthyl tellurides
3a–c were prepared from functionalized bromo naphtha-
lene using Grignard reaction, followed by the addition of
tellurium and n-butylation, and attempted the same reac-
tion under similar reaction conditions, which yielded bi-
naphthyl 4a–c in good yields (see Table 3).
Table 1 Study of Catalyst Effect on Homocoupling of n-Butyl
Phenyl Telluride 1a
Entry
Catalysta
–
Yield (%)b
1
2
3
4
5
n.r.c
15
In summary, the ultrasound-assisted synthesis of func-
tionally symmetrical biaryls by homocoupling reaction of
easily accessible aryl tellurides was demonstrated. This
methodology has the flexibility of introducing electron-
donor or -acceptor functionalities in the biaryl architec-
tures. Further applications of our methodology for the
synthesis of biaryls are currently in progress.
PdCl2
Pd(OAc)2
Pd(PPh3)4
Pd(acac)2
28
64
n.r.
a Catalyst (8 mol%) was used.
b Isolated yields.
Acknowledgment
c n.r. = no reaction.
The authors are grateful to FAPESP (07/51466-9, 07/59404-2) and
CNPq (154976/2006-7) for the financial support.
The next step was the determination of the best base and
the necessity of an additive in the reaction. Initially, an or-
ganic base, such as triethylamine, in the presence of Ag2O
was used, and the desired compound was isolated in 64%
yield. When the same reaction was performed with an in-
organic base, such as sodium carbonate, the desired com-
pound was achieved in 85% yield. Furthermore, to check
the effect of base, we performed the same reaction without
base, but no reaction was observed.
References
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desired product 2a was isolated in 53% yield with
AgOAc. Furthermore, to establish the stoichiometry of
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an additive.
Synlett 2008, No. 20, 3221–3225 © Thieme Stuttgart · New York

LETTER
Ultrasound-Assisted Synthesis of Symmetrical Biaryls
3223
Table 2 Homocoupling Reaction of Functionalized n-Butyl Phenyl Tellurides 1a–h
R2
R1 R4
R1
R2
Na2CO3, Pd(PPh3)4
R3
R3
R3
TeBu
Ag2O, MeOH
ultrasound
R2
R4
R4
R1
1
2
Entry
1
Aryl telluride 1
Product 2
Reaction time (min) Yield (%)
TeBu
45
60
45
45
85
71
78
78
1a
2a
Cl
Cl
Cl
TeBu
2
3
4
1b
2b
MeO
1c
TeBu
MeO
2c
OMe
Me
Me
Me
TeBu
TeBu
1d
2d
Br
Br
Br
5
6
60
90
78
59
1e
2e
TeBu
Me
Me Me
1f
2f
TeBu
7
8
60
82
62
F3C
F3C
CF3
1g
2g
F
TeBu
F
F
120
Me
Me Me
Me Me
1h
Me
1i
2h
Me
2i
Me
9
16 h
n.r.a
TeBu
Me
Me
Me Me
a n.r. = no reaction.
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Synlett 2008, No. 20, 3221–3225 © Thieme Stuttgart · New York

3224
F. V. Singh, H. A. Stefani
LETTER
Table 3 Homocoupling Reaction of n-Butyl Naphthyl Tellurides 3a–c
R
TeBu
Na2CO3, Pd(PPh3)4
Ag2O, MeOH
ultrasound
R
R
4
3
Entry
Naphthyl telluride 3
Product 4
Reaction time (min) Yield (%)
TeBu
1
90
21
3a
4a
4b
TeBu
2
3
60
60
82
79
3b
OMe
TeBu
MeO
MeO
3c
4c
(11) Singh, F. V.; Vatsyayan, R.; Roy, U.; Goel, A. Bioorg. Med.
Chem. Lett. 2006, 16, 2734.
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2003, 36, 731.
(22) (a) Zeni, G.; Perin, G.; Cella, R.; Jacob, R. G.; Braga, A. L.;
Silveira, C. C.; Stefani, H. A. Synlett 2002, 975. (b) Braga,
A. L.; Lüdtke, D. S.; Vargas, F.; Donato, R. K.; Silveira,
C. C.; Stefani, H. A.; Zeni, G. Tetrahedron Lett. 2003, 44,
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Cho, C.-S.; Ohe, K.; Uemura, S. J. Organomet. Chem. 1996,
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(24) General Experimental Procedure for Biaryls 2a–h and
4a–c
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A suspension of aryl telluride (1a, 0.135 g, 0.5 mmol),
Pd(PPh3)4 (0.45 g, 8 mmol), Na2CO3 (0.106 g, 1 mmol) and
Ag2O (0.116 g, 0.5 mmol) in MeOH (3 mL) was irradiated
in a water bath of an ultrasonic cleaner for 45 min. Then, the
reaction was diluted with EtOAc (30 mL). The organic layer
was washed with sat. solution of NH4Cl (2 × 10 mL) and
H2O (2 × 10 mL), dried over MgSO4, and concentrated
under vacuum. The crude product was purified by flash
silica column chromatography using hexane as eluent and
characterized as biphenyl 2a.
(16) Xu, Z.; Mao, J.; Zhang, Y. Catal. Commun. 2008, 9, 97; and
references cited therein.
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(18) Robinson, M. K.; Kochurina, V. S.; Hanna, J. M.
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Nishiyama, H. Synlett 2006, 1027.
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Synlett 2005, 2230.
Compound 2a: white solid; mp 70–72 °C. 1H NMR (300
MHz, CDCl3): d = 7.13–7.28 (m, 6 H, ArH), 7.45 (d, J = 7.2
Hz, 4 H, ArH). 13C NMR (75.5 MHz, CDCl3): d = 122.6,
126.95, 130.11, 141.61. GC-MS (%): 154 (100), 153 (57),
152 (39), 76 (44).
Compound 2b: white solid; mp 146–148 °C. 1H NMR (300
MHz, CDCl3): d = 7.38 (d, J = 8.0 Hz, 4 H, ArH), 7.45 (d,
J = 8.0 Hz, 4 H, ArH). 13C NMR (75.5 MHz, CDCl3): d =
Synlett 2008, No. 20, 3221–3225 © Thieme Stuttgart · New York

LETTER
128.00, 129.12, 133.52, 138.20. GC-MS (%): 222 (100), 152
Ultrasound-Assisted Synthesis of Symmetrical Biaryls
3225
130.34, 131.80, 134.95. GC-MS (%): 290 (100), 271 (24),
201 (28), 152 (19), 89 (21).
(63), 93 (21), 75 (47).
Compound 2c: white solid; mp 172–174 °C. 1H NMR (300
MHz, CDCl3): d = 3.84 (s, 6 H, 2 OMe), 6.95 (d, J = 8.4 Hz,
4 H, ArH), 7.47 (d, J = 8.4 Hz, 4 H, ArH). 13C NMR (75.5
MHz, CDCl3): d = 54.89, 113.70, 127.28, 133.04, 158.24.
GC-MS (%): 214 (100), 199 (87), 171 (21), 128 (16).
Compound 2d: white solid; mp 122–124 °C. 1H NMR (300
MHz, CDCl3): d = 2.33 (s, 6 H, 2 Me), 6.96 (d, J = 7.6 Hz,
4 H, ArH), 7.64 (d, J = 7.6 Hz, 4 H, ArH). 13C NMR (75.5
MHz, CDCl3): d = 20.93, 127.28, 129.94, 137.88, 138.76.
GC-MS (%): 182 (68), 167 (100), 165 (45), 152 (19), 89
(21).
Compound 2h: colorless oil. 1H NMR (300 MHz, CDCl3):
d = 2.02 (s, 6 H, 2 Me), 6.86–7.05 (m, 6 H, ArH). 13C NMR
(75.5 MHz, CDCl3): d = 22.99, 114.48, 117.77, 118.89,
133.34, 139.93, 160.12, 163.37. GC-MS (%): 218 (92), 203
(100), 201 (52), 183 (60).
Compound 3a: white solid; mp 138–140 °C. 1H NMR (300
MHz, CDCl3): d = 7.20 (t, J = 7.8 Hz, 2 H, ArH), 7.36–7.52
(m, 4 H, ArH), 7.66–7.76 (m, 6 H, ArH), 8.18 (d, J = 8.4 Hz,
2 H, ArH). 13C NMR (75.5 MHz, CDCl3): d = 122.94,
126.26, 126.78, 127.19, 127.42, 128.02, 128.40, 129.99,
132.09, 134.56. GC-MS (%): 254 (90), 253 (100), 252 (80),
250 (25), 126 (98), 125 (47).
Compound 2e: white solid; mp 160–162 °C. 1H NMR (300
MHz, CDCl3): d = 7.41 (d, J = 8.2 Hz, 4 H, ArH), 7.48 (d,
J = 8.2 Hz, 4 H, ArH). 13C NMR (75.5 MHz, CDCl3): d =
128.23, 129.05, 133.76, 138.45. GC-MS (%): 312 (66), 152
(89), 76 (100).
Compound 3b: white solid; mp 180–182 °C. 1H NMR (300
MHz, CDCl3): d = 7.44–7.58 (m, 6 H, ArH), 7.66–7.83 (m,
6 H, ArH), 8.00 (s, 2 H, ArH). 13C NMR (75.5 MHz, CDCl3):
d = 119.37, 125.84, 126.45, 126.57, 127.42, 128.80, 129.15,
129.49, 131.39, 134.06. GC-MS (%): 254 (100), 252 (34),
126 (25). Compound 3c: white solid; mp >250 °C. 1H NMR
(300 MHz, CDCl3): d = 3.85 (s, 6 H, 2 OMe), 7.03 (s, 2 H,
ArH), 7.10 (d, J = 9.0 Hz, 2 H, ArH), 7.44 (d, J = 9.0 Hz,
2 H, ArH), 7.50–7.60 (m, 4 H, ArH), 7.85 (s, 2 H, ArH).
13C NMR (75.5 MHz, CDCl3): d = 55.11, 105.56, 116.83,
119.57, 128.18, 128.29, 129.40, 129.45, 129.81, 132.85,
157.69. GC-MS (%): 314 (100), 299 (29), 271 (29), 228
(25), 157 (25).
Compound 2f: colorless oil. 1H NMR (300 MHz, CDCl3):
d = 2.08 (s, 6 H, 2 Me), 7.02 (t, J = 8.4 Hz, 2 H, ArH), 7.18
(t, J = 8.4 Hz, 4 H, ArH), 7.50 (d, J = 8.0 Hz, 2 H, ArH).
13C NMR (75.5 MHz, CDCl3): d = 22.70, 124.74, 127.02,
127.10, 130.62, 132.13, 137.65. GC-MS (%): 182 (77), 167
(100), 166 (22), 165 (48).
Compound 2g: colorless oil. 1H NMR (300 MHz, CDCl3):
d = 7.38 (t, J = 7.8 Hz, 2 H, ArH), 7.58 (d, J = 7.8 Hz, 2 H,
ArH), 7.70 (d, J = 7.8 Hz, 2 H, ArH), 7.80 (s, 2 H, ArH).
13C NMR (75.5 MHz, CDCl3): d = 121.39, 122.71, 123.80,
Synlett 2008, No. 20, 3221–3225 © Thieme Stuttgart · New York

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