Pd/TEMPO-catalyzed electrooxidative synthesis of biaryls from arylboronic acids or arylboronic esters

Article abstract of DOI:10.1016/j.tet.2009.08.004

A facile electrooxidative method for synthesizing biaryls from arylboronic acids or arylboronic esters is described. In the presence of a catalytic amount of Pd(OAc)₂ and TEMPO, the electrooxidation of arylboronic acids or arylboronates gave the corresponding biaryls in moderate to excellent yields.

Full text of DOI:10.1016/j.tet.2009.08.004

Tetrahedron 65 (2009) 8384–8388
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Pd/TEMPO-catalyzed electrooxidative synthesis of biaryls from
arylboronic acids or arylboronic esters
,
a
a
b
b
a,
*
Koichi Mitsudo a , Takuya Shiraga , Daisuke Kagen , Deqing Shi , James Y. Becker , Hideo Tanaka
*
a Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka,
Okayama 700-8530, Japan
b Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile electrooxidative method for synthesizing biaryls from arylboronic acids or arylboronic esters is
described. In the presence of a catalytic amount of Pd(OAc)2 and TEMPO, the electrooxidation of aryl-
boronic acids or arylboronates gave the corresponding biaryls in moderate to excellent yields.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 24 July 2009
Accepted 5 August 2009
Available online 7 August 2009
1. Introduction
2. Results and discussion
Biaryls are key skeletons of functional materials and pharma-
ceutical compounds. Various methods for preparing biaryl com-
pounds have been reported. One of the most straightforward
methods for preparing unsymmetrical biaryls is a Pd-catalyzed
cross-coupling reaction such as Suzuki–Miyaura coupling.1 In
contrast, preparing symmetric biaryls, one of the simplest methods
for preparing symmetric biaryls is the palladium-catalyzed homo-
coupling of aryl halides,2 arylboronic acids,3 or arylboronic
esters.3b,d,h,j,4 We have been interested in electrochemical organic
syntheses,5 and recently reported both the electrooxidative syn-
thesis of cationic Pd complexes ([Pd(CH3CN)4][Y]2, Y¼BFꢀ4 , PF6ꢀ, and
ClOꢀ4 ) and the integration of this system into electrooxidative
Wacker-type reactions.5b We also reported the electrooxidative
homo-coupling reaction of arylboronic acids catalyzed by electro-
generated cationic PdII catalysts. This electrooxidation could be
applied to each arylboronic acids bearing electron-withdrawing or
-donating substituents.6 Under appropriate conditions, a wide va-
riety of arylboronic acids could be converted to the corresponding
biaryls at room temperature. We later discovered that the homo-
coupling of arylboronic esters could be achieved under similar
conditions. We described here the details of the electrooxidation of
a series of arylboronic acids as well as arylboronic esters, to give the
corresponding biaryls in moderate to high yields.
2.1. Optimization of the reaction conditions for the
electrooxidative homo-coupling of phenylboronic acid
The electrooxidative coupling of phenylboronic acid (1a) was
carried out under various conditions. Previously, we reported that
the electrooxidation of Pd(OAc)2 gave a cationic palladium complex
bearing a counter anion which came from the electrolyte.5b
Therefore, the effect of the supporting electrolyte was examined
first (Table 1). The electrooxidation of 1a in the presence of
Pd(OAc)2 (10 mol %), TEMPO (30 mol %), and Et4NBF4 (0.05 M)
afforded 2a in 64% yield (entry 1). With the use of Et4NPF6, 2a was
obtained in 76% yield (entry 2). With Et4NClO4, the yield of 2a
Table 1
Effect of electrolytesa
Pd(OAc)2/TEMPO
K2CO3, electrolyte
Ph–B(OH)2
Ph–Ph
CH3CN/H2O (7/1)
electrooxidation
2a
1a
Entry
Electrolyte
Yield (%)b
1
2
3
4
5
6
7
8
Et4NBF4
Et4NPF6
Et4NClO4
Et4NOTs
Et4NOAc
Bu4NClO4
LiClO4
64
76
88 (61)c
60
66
56
86
73
NaClO4
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), TEMPO (30 mol %),
K2CO3 (2 equiv), electrolyte (0.05 M), CH3CN/H2O (7/1, 10 mL), 5 mA, 3 F/mol.
b
* Corresponding authors. Tel.: þ81 86 251 8072; fax: þ81 86 251 8079.
E-mail address: tanaka95@cc.okayama-u.ac.jp (K. Mitsudo).
Isolated yield.
Pd(OAc)2 (5 mol %).
c
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.08.004

K. Mitsudo et al. / Tetrahedron 65 (2009) 8384–8388
8385
Table 4
drastically increased to 88% (entry 3). In contrast, with the elec-
Effect of solventa
trooxidation of 1a in the presence of Et4NOTs or Et4NOAc, the
product 2a was obtained in yields of only 60% and 66%, respectively
(entries 4 and 5). These results suggest that ammonium and/or
alkaline metal salts of strong acids may be favorable supporting
electrolytes for the reaction.
Pd(OAc)2/TEMPO
K2CO3
Ph–B(OH)2
1a
Ph–Ph
solvent
electrooxidation
2a
The proper choice of mediator plays a significant role in the
electrooxidative coupling of 1a (Table 2). Without any mediator,
the reaction did not proceed. Even with a stoichiometric amount
of Pd(OAc)2, 2a was obtained in only 51% yield (entry 1). While 2a
was obtained in 73% yield with TEMPO (entry 2), the yield of 2a
decreased to 36% with 4-benzoyl-TEMPO (entry 3). With hydro-
quinone, a frequently used mediator, 2a was obtained in only 49%
yield (entry 4). Triphenylamine (Ph3N) was also ineffective for
the reaction (entry 5). These results suggest that Pd0 species
generated in situ could not be oxidized to PdII directly on the
anode, and an efficient catalyst would be generated in situ only
with the use of TEMPO. When TEMPO was used as a mediator, it
would act not only as a mediator, but also as a ligand of the Pd
catalyst.
Entry
Solvent
Yield (%)b
86 (25)c (39)d
1
2
3
4
5
6
CH3CN/H2O (7/1)
Acetone/H2O (7/1)
THF/H2O (7/1)
DMA/H2O (7/1)
DMF/H2O (7/1)
H2O
39
36
14
13
78
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), TEMPO (30 mol %),
K2CO3 (2 equiv), LiClO4 (0.05 M), 5 mA, 3 F/mol.
b
Isolated yield.
CH3CN/H2O (5/1).
CH3CN/H2O (9/1).
c
d
obtained in 86% yield with a CH3CN/H2O ratio of 7/1, with a 5/1 or
9/1 solution of CH3CN/H2O, the reaction efficiency was quite low,
and 2a was obtained in respective yields of 25% and 39% (entry 1).
In these cases, the reaction media was apparently separated into
organic and aqueous phases, and this could help to explain why the
reaction did not proceed efficiently. Interestingly, water was also an
efficient solvent, in which the reaction proceeded smoothly to af-
ford 2a in 78% yield (entry 6).
Table 2
Effect of mediatorsa
Pd(OAc)2, mediator
K2CO3
Ph–B(OH)2
Ph–Ph
CH3CN/H2O (7/1)
electrooxidation
2a
1a
2.2. Scope and limitations of the Pd/TEMPO-catalyzed homo-
coupling of arylboronic acids
Entry
Mediator
Yield (%)b
n.d. (51)c
73
36
49
36
1
2
3
4
5
None
TEMPO
4-Benzoyl-TEMPO
Hydroquinone
Ph3N
As mentioned above, the homo-coupling of 1a gave the best
results with K2CO3 as a base, Et4NClO4 as an electrolyte, and TEMPO
as a mediator. The homo-coupling of several arylboronic acids was
carried out under the optimized conditions (Table 5). First, aryl-
boronic acids bearing electron-donating groups at the p-position
were examined (entries 2–6). The reaction of p-tolylboronic acid
(1b) and p-tert-butylphenylboronic acid (1c) proceeded smoothly
to give 4,40-dimethylbiphenyl (2b) and 4,40-di-tert-butylbiphenyl
(2c) in respective yields of 91% and 89% (entries 2 and 3). In con-
trast, the reaction of p-methoxyphenylboronic acid (1d) gave the
desired product in only 34% yield under the condition. Further
optimization revealed that aq NaClO4 was a suitable solvent for
substrate 1d, and the yield of 2d increased to 95% (entry 4). The
homo-coupling of p-phenoxyphenylboronic acid (1e) proceeded
smoothly under the typical conditions to give 2e in 95% yield (entry
5). Next, the Pd/TEMPO-catalyzed reaction of several arylboronic
acids bearing electron-withdrawing groups was examined (entries
7–10). The reactions of p-chlorophenylboronic acid (1g), p-acetyl-
phenylboronic acid (1h), and p-nitrophenylboronic acid (1i) pro-
ceeded smoothly to give biaryls 2g–i in respective yields of 93%,
95%, and 98%. Unfortunately, p-(ethoxycarbonyl)phenylboronic
acid (1j) suffered from hydrolysis under these conditions and 2j
was obtained in only 23% yield (entry 10). m- and o-Substituted
phenylboronic acids could also be used in the reaction (entries 11–
14). The reaction of m-tolylboronic acid (1k) and m-chlor-
ophenylboronic acid (1l) gave the corresponding biaryls (2k and 2l)
in respective yields of 85% and 87% (entries 11 and 12). Similarly, the
homo-couplingofo-tolylboronic acid(1m) proceeded to afford 2min
74% yield (entry 13). 1-Naphthylboronic acids could also be used for
the reaction, and 1,10-binaphthyl wasobtained in 97%yield(entry14).
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), mediator (30 mol %),
K2CO3 (2 equiv), NaClO4 (0.05 M), CH3CN/H2O (7/1, 10 mL), 5 mA, 3 F/mol.
b
Isolated yield.
Performed with Pd(OAc)2 (1 equiv).
c
The amount of TEMPO strongly dominated the reactivity of the
reaction (Table 3). TEMPO at 30 mol % was the best amount for the
reaction (86%, entry 2). With either more or less TEMPO, the yield of
2a decreased (entries 1 and 3).
Table 3
Effect of the amount of TEMPOa
Pd(OAc)2/TEMPO
K2CO3
Ph–B(OH)2
Ph–Ph
CH3CN/H2O (7/1)
electrooxidation
2a
1a
Entry
Amount of TEMPO (mol %)
Yield (%)b
1
2
3
20
30
40
55
86
57
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), K2CO3 (2 equiv), LiClO4
(0.05 M), CH3CN/H2O (7/1, 10 mL), 5 mA, 3 F/mol.
b
Isolated yield.
We next examined the effect of the solvent in the electro-
oxidative homo-coupling of 1a (Table 4). Mixtures of several or-
ganic solvents and water (7/1) were used for the reaction. When the
reaction was carried out in acetone/H2O, THF/H2O, DMA/H2O, or
DMF/H2O, the yield of the product 2a varied within a range of 13–
39% (entries 2–5). Among the solvents that have been examined
thus far, CH3CN/H2O was the most suitable combination for the
electrooxidative coupling of 1a (entry 1), which suggests that
CH3CN would coordinate to the Pd center to stabilize the catalyst.
The ratio of CH3CN and H2O was also important. While 2a was
2.3. Pd/TEMPO-catalyzed homo-coupling of arylboronic
esters
To elucidate the scope and limitations of the reaction condi-
tions, we next extended this procedure to the homo-coupling of

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K. Mitsudo et al. / Tetrahedron 65 (2009) 8384–8388
4,40-bitolyl (2b) in 72% yield, while the electrolysis of 4b gave 2b
Table 5
Electrooxidative homo-coupling of several arylboronic acidsa
in 99% yield (entry 4). The yield of 4-dimethylaminophenylbor-
onic ester was similar to that of 4-dimethylaminophenylboronic
acid (69%, entry 5). Next, the electrooxidation of arylboronic es-
ters bearing electron-withdrawing groups at the p-position (en-
tries 6–8) was examined. The electrooxidation of p-chloroboronic
esters 3g and 5g gave 4,40-dichlorobiphenyl (2g) in respective
yields of 92% and 96% (entries 6 and 7). p-Nitrophenylboronic
ester (3i) gave 2i in 92% yield (entry 8). o-Substituted arylboronic
esters could also be used in the reaction (entries 9 and 10). The
electrooxidation of 3k and 3l gave corresponding biaryl 2k and 2l in
the respective yields of 69% and 81% (entries 9 and 10). As with the
reaction of arylboronic acids, arylboronic esters bearing electron-
donating or withdrawing groups could be used in the reaction to
give the corresponding biaryls in moderate to high yields.
Pd(OAc)2/TEMPO
K2CO3, Et4NClO4
Ar–B(OH)2
Ar–Ar
CH3CN/H2O (7/1)
electrooxidation
2
1
Entry
Ar–B(OH)2
Ar–Ar
Yield
(%)b
1
2
B(OH)2
88
91
1a
2a
B(OH)2
1b
2b
3
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
89
95
95
69
2c
1c
1d
1e
1f
2.4. A plausible mechanism
4c
5
MeO
PhO
Me2N
Cl
MeO
PhO
Me2N
Cl
OMe
OPh
2d
The Pd/TEMPO-catalyzed electrooxidative homo-coupling of
arylboronic acids and esters exhibited high reactivity under mild
conditions in contrast to previously reported O2-oxidative re-
actions. This high reactivity was probably due to the generation of
oxygen-free active PdII species. Amatore and Jutand reported that
2e
6d
NMe2
the air-oxidation of Pd0 generates ( 2-O2)PdL2.3h They also found
h
2f
that a Pd/benzoquinone-catalyzed electrooxidation of arylboronic
acids and esters proceeded smoothly under anaerobic conditions to
afford biaryls.3b They noted that oxygen-free Pd species might play
a key role in the reaction. Since our electrooxidative reaction was
carried out under an argon atmosphere, oxygen-free PdII species
would be generated. Although the intermediate has not been
identified, it should be a PdII species bearing ClOꢀ4 or TEMPO as
a counteranion.7 A plausible mechanism is illustrated in Figure 1.
First, the electrooxidation of Pd(OAc)2 would proceed to generate
a PdII species (PdII[Y]2, Y¼ClO4ꢀ) through Kolbe oxidation.5b Trans-
metallation would then occur with an arylborate, which was gen-
erated by the reaction of an arylboronic acid/ester with base, to give
Ar–PdII[Y]. A second transmetallation would give Ar–PdII–Ar, and
subsequent reductive elimination would give biaryl and Pd0 species
(Pd0). The Pd0 species would be oxidized to PdII by the N-oxoam-
monium cation generated by the electrooxidation of TEMPO on the
anode, and the active PdII catalyst would be regenerated.
Cl
7
93
95
98
23
1g
2g
8
Ac
B(OH)2
Ac
O2N
Ac
1h
2h
9
O2N
B(OH)2
NO2
1i
2i
EtO2C
CO2Et
2j
EtO2C
B(OH)2
1j
10
11
12
85
87
B(OH)2
2k
Cl
1k
Cl
Cl
B(OH)2
1l
2l
3. Conclusion
13
74
97
The Pd/TEMPO-catalyzed electrooxidative homo-coupling of
arylboronic acids has been developed. Arylboronic acids 1a–n,
bearing electron-donating or -withdrawing groups, were success-
fully used for the reaction, and the corresponding biaryls 2a–n were
obtained in good to excellent yields under mild conditions. The
electrooxidative homo-coupling of arylboronic esters took place
under similar conditions. In the presence of a catalytic amount of
Pd/TEMPO, several arylboronic esters were oxidized to give the
corresponding biaryls.
B(OH)2
B(OH)2
1m
2m
14
1n
2n
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), TEMPO (30 mol %),
K2CO3 (2 equiv), Et4NClO4 (0.05 M), 5 mA, 3 F/mol.
b
Isolated yield.
Performed in aq NaClO4.
4-PhCO2-TEMPO was used instead of TEMPO.
c
d
4. Experimental
arylboronic esters (Table 6). First, the homo-coupling of phenyl-
boronic esters was carried out. Under the optimized conditions
for arylboronic acids, the electrooxidative homo-coupling of
phenylboronic esters 3a and 4a gave biphenyl (2a) in respective
yields of 67% and 64% (entries 1 and 2). Further optimization
revealed that the yield of 2a was highly influenced by the solvent,
and the reaction of 3a gave the product in up to 88% yield in H2O
(entry 1). We next examined arylboronic esters bearing electron-
donating groups at the p-position (entries 3–5). The electro-
oxidative homo-coupling of p-tolylboronic ester 3b afforded
4.1. General procedure of electrooxidative Pd-catalyzed
homo-coupling of arylboronic acids
The electrooxidation was carried out in an H-type divided cell
(separated by a glass filter) equipped with two platinum electrodes
(1.0ꢁ1.5 cm2). In the anodic chamber was placed a solution of
phenylboronic acid (1a, 25 mg, 0.21 mmol), Pd(OAc)2 (4.3 mg,
0.02 mmol), TEMPO (9.4 mg, 0.06 mmol), and K2CO3 (55 mg,
0.40 mmol) in a 0.05 M Et4NClO4 solution of CH3CN/H2O (7/1, 5 mL).
In the cathodic chamber was placed a 0.05 M Et4NClO4 solution of

K. Mitsudo et al. / Tetrahedron 65 (2009) 8384–8388
8387
Table 6
Electrooxidative homo-coupling of several arylboronic estersa
Pd(OAc)2/TEMPO
K2CO3, Et4NClO4
Ar–B
Ar–Ar
CH3CN/H2O
electrooxidation
2
Entry
1
Ar–B
Ratio of CH3CN/H2O
0/1
Ar–Ar
Yield (%)b
O
B
O
88 (67)c
64
2a
2a
3a
O
B
O
2
3
4
5
7/1
1/1
7/1
1/1
4a
O
B
72
99
69
2b
O
3b
O
B
2b
O
4b
O
O
Me2N
B
Me2N
NMe2
2f
3f
O
Cl
Cl
Cl
6
7
8
Cl
B
1/3
7/1
3/1
92
96
92
2g
O
3g
O
O
Cl
Cl
B
2g
5g
O
O
O2N
NO2
O2N
B
2i
3i
O
O
9
1/1
7/1
69
81
B
2k
Cl
3k
Cl
Cl
O
10
B
O
2l
3l
a
Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol %), TEMPO (30 mol %), K2CO3 (2 equiv), Et4NClO4 (0.05 M), 5 mA, 3 F/mol.
Isolated yield.
Performed in CH3CN/H2O (7/1).
b
c
CH3CN/H2O (7/1, 5 mL). Under argon, a constant current (5 mA, 3 F/
mol) was supplied at room temperature with vigorous stirring. To
the resulting mixture was added aq satd NaCl (10 mL) and extracted
with Et2O (3ꢁ10 mL). The combined organic phase was washed with
aq satd NaCl (15 mL), dried over MgSO4, and concentrated under
reduced pressure. The residue was purified by column chromatog-
raphy on silica gel (hexane) to afford biphenyl (2a, 88%).
Ar
Ar
B
Pd(OAc)2
-2e–
Et4NClO4
PdII[Y]2
B–
B
B
– = base
anode
B
2
2
N
N
O
O
4.1.1. Biphenyl (2a). Colorless solid; 1H NMR (600 MHz, CDCl3)
L = CH3CN or H2O
2e-
Y = ClO4–, or TEMPO
7.35–7.38 (m, 2H), 7.45–7.48 (m, 4H), 7.61–7.62 (m, 4H); 13C NMR
B
d
(150 MHz, CDCl3)
d 127.1, 127.2, 128.7, 141.2; IR (KBr) 3062, 3034,
1568, 1480, 1428, 729, 697 cmꢀ1
.
ArPdII[Y]
Pd0
4.1.2. 4,40-Dimethylbiphenyl (2b). Colorless solid; 1H NMR
(500 MHz, CDCl3) 2.38 (s, 6H), 7.22–7.24 (m, 4H), 7.46–7.48 (m,
4H); 13C NMR (150 MHz, CDCl3)
21.1, 126.8, 129.4, 136.7, 138.3; IR
(KBr) 2919, 1903, 1561, 1502, 1447, 1311, 1179, 1113, 1038 cmꢀ1
d
Ar
B
B
Ar Ar
d
ArPdIIAr
.
B
4.1.3. 4,40-Di-tert-butylbiphenyl (2c). Colorless solid; 1H NMR
(500 MHz, CDCl3)
1.36 (s,18H), 7.44 (dd, J¼6.8, 2.0 Hz, 4H), 7.52 (dd,
Figure 1. A plausible mechanism for the electrooxidative homo-coupling of arylboronic
acids and esters.
d

8388
K. Mitsudo et al. / Tetrahedron 65 (2009) 8384–8388
J¼6.8, 2.0 Hz, 4H); 13C NMR (75 MHz, CDCl3)
d
31.4, 34.5,125.6,126.6,
and Technology, Japan, by the Wesco Scientific Promotion Foun-
138.2,149.9; IR (KBr) 3087, 3053, 3029, 2959, 2901, 2867,1495 cmꢀ1
.
dation, and by Takahashi Industrial and Economic Research Foun-
dation. The authors are grateful to the SC-NMR Laboratory of
Okayama University for the NMR measurement.
4.1.4. 4,40-Dimethoxybiphenyl (2d). Colorless solid; 1H NMR
(500 MHz, CDCl3) 3.84 (s, 6H), 6.94–6.97 (m, 4H), 7.47–7.49 (m,
4H); 13C NMR (150 MHz, CDCl3)
55.2, 114.0, 127.6, 133.3, 158.5; IR
(KBr) 2957, 1500, 1438, 1275, 1248, 1041 cmꢀ1
d
d
.
Supplementary data
4.1.5. 4,40-Diphenoxybiphenyl (2e). Colorless solid; 1H NMR
(600 MHz, CDCl3) 7.05–7.08 (m, 8H), 7.11–7.14 (m, 2H), 7.34–7.38
(m, 4H), 7.51–7.54 (m, 4H); 13C NMR (150 MHz, CDCl3)
119.0, 119.1,
123.3, 128.2, 129.8, 135.6, 156.6, 157.1; IR (KBr) 3056, 3039, 2923,
2853, 1590, 1492, 1255 cmꢀ1
Spectroscopic and analytical data and selected experi-
mental procedure associated with this article can be found, in the
online version. Supplementary data associated with this arti-
cle can be found in the online version, at doi:10.1016/
j.tet.2009.08.004.
d
d
.
4.1.6. 4,40-Bis(dimethylamino)biphenyl (2f). Colorless solid; 1H
NMR (500 MHz, CDCl3)
d
2.97 (s, 12H), 6.80 (d, J¼8.4 Hz, 4H), 7.45
(d, J¼8.4 Hz, 4H); 13C NMR (75 MHz, CDCl3)
d 40.8, 113.1, 126.9,
129.8, 149.2; IR (KBr) 2799, 1611, 1352, 1227 cmꢀ1
.
References and notes
1. For a review, see: Tuji, J. Palladim Reagents and Catalysts; John Wiley & Sons:
Chichester, 2004.
4.1.7. 4,40-Dichlorobiphenyl (2g). Colorless solid; 1H NMR
(500 MHz, CDCl3)
(150 MHz, CDCl3)
d
7.40–7.42 (m, 4H), 7.47–7.49 (m, 4H); 13C NMR
128.2, 129.0, 133.7, 138.4; IR (KBr) 2962, 2356,
2. (a) Kuroboshi, M.; Kobayashi, R.; Nakagawa, T.; Tanaka, H. Synlett 2009, 85–
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d
1474, 1388, 1261, 1088, 1002 cmꢀ1
.
4.1.8. 4,40-Diacetylbiphenyl (2h). Colorless solid; 1H NMR
(600 MHz, CDCl3) 2.66 (s, 6H), 7.72–7.73 (m, 4H), 8.06–8.07 (m,
4H); 13C NMR (150 MHz, CDCl3)
26.7, 127.4, 128.9, 136.4, 144.2,
197.6; IR (KBr) 3343, 3044, 2920, 1681 cmꢀ1
d
d
.
4.1.9. 4,40-Dinitrobiphenyl (2i). Yellow solid; 1H NMR (600 MHz,
CDCl3)
d
7.79 (d, J¼9.0 Hz, 4H), 8.37 (d, J¼9.0 Hz, 4H); 13C NMR
(150 MHz, CDCl3)
d 124.4, 128.3, 145.0, 148.0; IR (KBr) 2935, 1598,
1511, 1477, 1375, 1343, 1108 cmꢀ1
.
4.1.10. 4,40-Bis(ethoxycarbonyl)biphenyl (2j). Colorless solid; 1H
NMR (500 MHz, CDCl3)
d
1.42 (t, J¼7.0 Hz, 6H), 4.41 (q, J¼7.0 Hz,
4H), 7.68 (dt, J¼8.5, 2.0 Hz, 4H), 8.13 (dt, J¼8.5, 2.0 Hz, 4H); 13C NMR
(75 MHz, CDCl3) d 14.3, 61.1, 127.2, 130.0, 130.1, 144.3, 166.3; IR (KBr)
2981, 2906, 1707, 1607,1279 cmꢀ1
.
4.1.11. 3,30-Dimethylbiphenyl (2k). Colorless liquid; 1H NMR
(500 MHz, CDCl3)
d
2.41 (s, 6H), 7.15 (d, J¼7.3 Hz, 2H), 7.24 (s, 2H),
7.31 (t, J¼7.3 Hz, 2H), 7.37–7.40 (m, 2H); 13C NMR (125 MHz, CDCl3)
d
21.5, 124.3, 127.9, 128.0, 128.6, 138.2, 141.3; IR (neat) 3028, 2949,
2919, 2859, 2732 cmꢀ1
.
4.1.12. 3,30-Dichlorobiphenyl (2l). Colorless liquid; 1H NMR
(500 MHz, CDCl3)
7.32–7.45 (m, 6H), 7.53–7.55 (m, 2H); 13C NMR
(75 MHz, CDCl3) 125.2, 127.2, 127.7, 130.1, 134.8, 141.6; IR (neat)
3064, 2924, 2853, 1591, 1561, 1462, 1395, 1101 cmꢀ1
d
d
.
4.1.13. 2,20-Dimethylbiphenyl (2m). Colorless liquid; 1H NMR
(500 MHz, CDCl3)
2.05 (s, 6H), 7.10–7.27 (m, 8H); 13C NMR
(75 MHz, CDCl3) 19.8, 125.5, 127.1, 129.3, 129.8, 135.8, 141.6; IR
(neat) 3059, 3017, 2922, 1599, 1477, 1453, 754, 728 cm–1
4. Yoshida, H.; Yamaryo, Y.; Ohshita, J.; Kunai, A. Tetrahedron Lett. 2003, 44,
1541–1544.
d
5. For recent works, see: (a) Mitsudo, K.; Ishii, T.; Tanaka, H. Electrochemistry
2008, 76, 859–861; (b) Mitsudo, K.; Kaide, T.; Nakamoto, E.; Yoshida, K.; Ta-
naka, H. J. Am. Chem. Soc. 2007, 129, 2246–2247; (c) Mitsudo, K.; Kumagai, H.;
Takabatake, F.; Kubota, J.; Tanaka, H. Tetrahedron Lett. 2007, 48, 8994–8997; (d)
Yoshida, T.; Kuroboshi, M.; Oshitani, J.; Goto, K.; Tanaka, H. Synlett 2007, 2691–
2694; (e) Tanaka, H.; Arai, S.; Ishitobi, Y.; Kuroboshi, M.; Torii, S. Electro-
chemistry 2006, 74, 656–658; (f) Mitsudo, K.; Matsuda, W.; Miyahara, S.; Ta-
naka, H. Tetrahedron Lett. 2006, 47, 5147–5150; (g) Kubota, J.; Ido, T.;
Kuroboshi, M.; Tanaka, H.; Uchida, T.; Shimamura, K. Tetrahedron 2006, 62,
4769–4773; (h) Kubota, J.; Shimizu, Y.; Mitsudo, K.; Tanaka, H. Tetrahedron Lett.
2005, 46, 8975–8979; (i) Tanaka, H.; Kubota, J.; Miyahara, S.; Kuroboshi, M.
Bull. Chem. Soc. Jpn. 2005, 78, 1677–1684; (j) Mitsudo, K.; Kawaguchi, T.;
Miyahara, S.; Matsuda, W.; Kuroboshi, M.; Tanaka, H. Org. Lett. 2005, 7, 4649–
4652.
d
.
4.1.14. 1,10-Binaphthyl (2n). Colorless solid; 1H NMR (300 MHz,
CDCl3) 7.22–7.30 (m, 2H), 7.38–7.60 (m, 8H), 7.92–7.95 (m, 4H);
13C NMR (125 MHz, CDCl3)
125.4, 125.8, 125.9, 127.0, 127.8, 127.9,
128.1, 132.8, 133.5, 138.4; IR (KBr) 3040, 1822, 1586, 1504,
1384 cmꢀ1
d
d
.
Acknowledgements
6. Mitsudo, K.; Shiraga, T.; Tanaka, H. Tetrahedron Lett. 2008, 49, 6593–6595.
7. In the presence of TEMPO, the electrooxidation of Pd(OAc)2 afforded cationic Pd
complexes containing TEMPO moieties.5b
This work was supported in part by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science