Electrooxidative homo-coupling of arylboronic acids catalyzed by electrogenerated cationic palladium catalysts

Article abstract of DOI:10.1016/j.tetlet.2008.09.022

An electrochemical method for generating cationic palladium complexes was integrated into an electrooxidative homo-coupling of arylboronic acids. In the presence of a catalytic amount of TEMPO, the homo-coupling reaction proceeded efficiently under argon to afford symmetric biaryls.

Full text of DOI:10.1016/j.tetlet.2008.09.022

Tetrahedron Letters 49 (2008) 6593–6595
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Tetrahedron Letters
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Electrooxidative homo-coupling of arylboronic acids catalyzed
by electrogenerated cationic palladium catalysts
*
*
Koichi Mitsudo , Takuya Shiraga, Hideo Tanaka
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An electrochemical method for generating cationic palladium complexes was integrated into an electro-
oxidative homo-coupling of arylboronic acids. In the presence of a catalytic amount of TEMPO, the homo-
coupling reaction proceeded efficiently under argon to afford symmetric biaryls.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 29 August 2008
Accepted 9 September 2008
Available online 11 September 2008
Biaryl compounds are significant building blocks for the materi-
als and pharmaceutical sciences. One of the most commonly used
methods to construct biaryls is the palladium-catalyzed coupling
of arylboronic acids or esters with aryl halides (Suzuki–Miyaura
Air oxidation
O2
O
O
Pd(0)L2
Pd(II)L2
1
coupling). Pd-catalyzed oxidative homo-coupling of arylboronic
Electrooxidation
acids has also received attention as a method for constructing biar-
yls. While there have been several reports on efficient Pd-catalyzed
oxidative homo-coupling of arylboronic acids, these methods have
usually required stoichiometric amounts of oxidants or an oxygen
-
2e-
[PdLn]2+
Pd(0)Ln
under argon
2
–4
Scheme 1. Key catalysts of Pd-catalyzed homo-coupling of arylboronic acids.
atmosphere.
Recently, Yamamoto and co-workers reported an
excellent air-oxidative Pd-catalyzed homo-coupling of arylboronic
acids.5 They found that several arylboronic acids can be trans-
formed to biaryls at room temperature without base or ligand in
MeOH. Mao and Zhang also reported an air-oxidative coupling
oxygen-free Pd catalyst would be so electrophilic as to exhibit a
2
similar catalytic activity to that of (
g
-O
2
)PdL
2
. According to this
hypothesis, we studied the electrooxidative homo-coupling of aryl-
boronic acids and found that electrochemically generated cationic
Pd(II) catalysts are efficient for the homo-coupling of arylboronic
acids. We report here, the electrogenerated cationic Pd-catalyzed
electrooxidative homo-coupling of arylboronic acids.
reaction using Pd(OAc)
2
as a catalyst and K
2 3
CO as a base in ace-
6
tone/H
2
O. In 2006, a mechanistic study of the palladium-cata-
lyzed homo-coupling of arylboronic acids was reported by
7
Amatore and Jutand. They documented that the peroxo complex
2
(g
-O
2
)PdL
2
(L = ligand), generated in situ by the reaction of
First, we chose phenylboronic acid as a representative aryl-
boronic acid, and attempted Pd-catalyzed electrooxidative
homo-coupling (Table 1). Electrooxidation was carried out in a di-
vided-type cell under argon. Passage of 3 F/mol of electricity
Pd(0) and O
2
, plays a key role in the reaction.
8
We have been interested in the electrochemical reactions, in
particular, in electrochemical generation of transition-metal com-
plexes, and reported the electrooxidative synthesis of cationic Pd
À
À
À
3 2
through CH CN/H O (7/1) solution of phenylboronic acid
catalysts ([Pd(CH
3
CN)
4
][Y]
2
, Y = BF4 , PF6 , and ClO4 ) and the
(
0.2 mmol), Pd(OAc)
2
(10 mol %), and TEMPO (30 mol %) in the
) afforded biphe-
integration of this system into electrooxidative Wacker-type reac-
9
presence of 0.05 M of sodium perchlorate (NaClO
4
tions. The advantages of these reactions are (i) high reactivity of
nyl (2a) in 65% yield (entry 1). While the reaction proceeded with-
out base, the yield of biphenyl 2a was unsatisfactory. To improve
the reaction efficiency, we next tried the reaction in the presence
of bases (entries 2–8). No appreciable change was observed with
the active cationic Pd species generated in situ, and (ii) electro-
chemical regeneration of the cationic Pd complexes in situ. We ex-
tended our interest to the homo-coupling reaction of arylboronic
acids catalyzed by the electrogenerated cationic Pd(II) catalysts
3 2
the addition of Et N or Cy NMe (entries 2 and 3). Interestingly,
(
EG Pd), which would be different species than peroxo Pd com-
2
the addition of i-Pr NEt, an electron-donating and sterically
plexes (Scheme 1). We thought that an electrogenerated cationic
hindered base (Hunig’s base), enhanced the reaction efficiency,
and biphenyl 2a was obtained in 78% yield (entry 4). In contrast,
the addition of N-methylpiperidine or N-methylmorpholine
(NMM) was ineffective for the reaction, and the yield of 2a was
*
Corresponding authors. Tel.: +81 86 251 8079; fax: +81 86 251 8074 (H.T.).
E-mail address: tanaka95@cc.okayama-u.ac.jp (H. Tanaka).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.022

6
594
K. Mitsudo et al. / Tetrahedron Letters 49 (2008) 6593–6595
Table 1
ionic Pd(II) catalyst, was optimized first (entries 1–3). Using
Et NBF or Et NPF , 2a was obtained in respective yields of 64%
and 76% (entries 1 and 2). In contrast, when Et NClO was used
Electrogenerated Pd-catalyzed electrooxidative homo-coupling of phenylboronic acid
1a) with several basesa
4
4
4
6
(
4
4
Pd(OAc) (10 mol %)
TEMPO (30 mol %)
base (2 equiv)
as an electrolyte, the yield of 2a increased drastically to 88% (entry
2
À
3
). Therefore, with the anionic part fixed at ClO4 , the effect of the
cationic part of the electrolytes was investigated (entries 4–6).
With Bu NClO , the efficiency of the reaction decreased, probably
due to the low solubility of the electrolyte in highly polar solvents.
LiClO was also an efficient electrolyte for the reaction. In the pres-
ence of LiClO , biphenyl 2a was obtained in 86% yield (entry 5).
These results indicated that K CO and i-Pr NEt were suitable
NClO and LiClO were appropriate
NaClO (0.05 M)
4
B(OH)2
4
4
CH CN/H O (7/1)
3
2
room temp.
1
a
2a
4
divided cell, (Pt)-(Pt)
4
5
mA, 3 F/mol
2
3
2
Entry
Base
None
Et
Yieldb (%)
bases for the reaction and Et
electrolytes for electrolysis.
4
4
4
1
2
3
4
5
6
7
8
65
66
66
78
14
13
61
73
3
N
To clarify the scope and limitations of the reaction, we per-
formed the electrooxidative homo-coupling of several kinds of
arylboronic acids under the optimized conditions (Table 4). First,
electron-donating arylboronic acids were examined. A solution of
Cy
i-Pr
2
NMe
NEt
2
N-methylpiperidine
N-methylmorpholine
2
Na CO
3
p-tolylboronic acid (1b), Pd(OAc) (10 mol %), TEMPO (30 mol %),
2
and K CO (2 equiv) in CH CN/H O was electrooxidized under a
2
2
K CO
3
2
3
3
a
4 4
constant current (5 mA, 3 F/mol) in the presence of Et NClO . The
The reactions were performed using 1a (0.20 mmol), Pd(OAc)
2
(10 mol %),
TEMPO (30 mol %), base (2 equiv), and NaClO
4
(0.05 M) in CH
3
CN/H
2
O (10 mL, 7/1).
reaction proceeded smoothly to afford bi(p-tolyl) (2b) in 91% yield
(entry 1). The reaction of p-tert-butylphenylboronic acid (1c) and
b
Isolated yield.
dramatically reduced, probably because these amines underwent
oxidation faster than TEMPO (entries 5 and 6). Inorganic bases
can also be used for the reaction (entries 7 and 8). In particular,
Table 4
a
Electrooxidative homo-coupling of several arylboronic acids
Pd(OAc) (10 mol %)
with potassium carbonate (K
3% (entry 8).
The addition of TEMPO as a mediator was indispensable for the
2
CO
3
), the yield of 2a increased to
2
TEMPO (30 mol %)
K CO (2 equiv)
7
2
3
Et NClO (0.05 M)
4
4
reaction. The effect of the mediator on the electrooxidative cou-
pling of 1a is summarized in Table 2. With hydroquinone, a typical
mediator, the yield of 2a decreased to 49% (entry 2). Ph N was also
3
ineffective for the reaction (36% yield, entry 3). Without any medi-
Ar B(OH)2
Ar Ar
CH CN/H O (7/1)
3
2
1
room temp.
2
divided cell, (Pt)-(Pt)
5
mA, 3 F/mol
ator, the reaction did not proceed, and 2a could not be detected
Entry
1
1
Product
Yieldb (%)
91
(
entry 4). These results suggest that Pd(0) species generated
in situ could not be oxidized to Pd(II) species directly on the anode.
A mediator would be oxidized on the anode, and the generated cat-
ionic species (N-oxoammonium) would oxidize Pd(0) complexes.
We next examined the electrolyte (Table 3).10 Since the anionic
part of the electrolyte would become the counter-anion of EG cat-
B(OH)2
1b
2b
2c
2d
2e
2
3
4
t-Bu
PhO
Cl
B(OH)2
B(OH)2
B(OH)2
1c
1d
1e
89
95
93
Table 2
Electrooxidative homo-coupling of 1a using several mediatorsa
Entry
Mediator
Yieldb (%)
1
2
3
4
TEMPO
Hydroquinone
73
49
36
N.D.
5
6
Ac
B(OH)2
1f
2f
95
98
Ph
3
N
None
a
The reactions were performed using 1a (0.20 mmol), Pd(OAc)
2
(10 mol %),
O(10 mL,7/1).
O N
B(OH)2
1g
2g
2
mediator(30mol%), K
2
CO
3
(2 equiv), andNaClO
4
(0.05 M)inCH
3
CN/H
2
b
Isolated yield.
7c
1h
2h
85
87
B(OH)2
Table 3
Cl
a
Electrooxidative homo-coupling of 1a in the presence of several electrolytes
8
1i
1j
2i
2j
Entry
Electrolyte
Yieldb (%)
B(OH)2
1
2
3
4
5
6
Et
Et
Et
4
4
4
NBF
NPF
NClO
NClO
LiClO
NaClO
4
64
76
88
56
86
73
6
9
74
4
B(OH)2
Bu
4
4
4
a
The reactions were performed using 1 (0.20 mmol), Pd(OAc)
2
(10 mol %),
4
TEMPO (30 mol %), K
1).
2 3 4 4 3 2
CO (2 equiv), and Et NClO (0.05 M) in CH CN/H O (10 mL, 7/
a
The reactions were performed using 1a (0.20 mmol), Pd(OAc)
30 mol %), K CO (2 equiv), and electrolyte (0.05 M) in CH CN/H
Isolated yield.
2
(10 mol %), TEMPO
O (10 mL, 7/1).
b
(
2
3
3
2
Isolated yield.
4 F/mol of electricity was passed.
b
c

K. Mitsudo et al. / Tetrahedron Letters 49 (2008) 6593–6595
6595
p-phenoxyphenylboronic acid (1d) smoothly afforded 2c and 2d in
9% and 95% yields, respectively (entries 2 and 3). We next exam-
Tetrahedron Lett. 2002, 43, 7899–7902; (f) Koza, D. J.; Carita, E. Synthesis 2002,
183–2186; (g) Yamamoto, Y. Synlett 2007, 1913–1916; (h) Zhou, L.; Xu, Q. X.;
Jiang, H. F. Chin. Chem. Lett. 2007, 18, 1043–1046; (i) Yoshida, H.; Yamaryo, Y.;
Ohshita, J.; Kunai, A. Tetrahedron Lett. 2003, 44, 1541–1544; (j) Smith, K. A.;
Campi, E. M.; Jackson, W. R.; Marcuccio, S.; Naeslund, C. G. M.; Deacon, G. B.
Synlett 1997, 131–132.
Previously reported Pd-catalyzed air-oxidative homo-coupling of arylboronic
acids: (a) Wong, M. S.; Zhang, X. L. Tetrahedron Lett. 2001, 42, 4087–4089; (b)
Punna, S.; Diaz, D. D.; Finn, M. G. Synlett 2004, 2351–2354; (c) Moreno-Manas,
M.; Perez, M.; Pleixats, R. J. Org. Chem. 1996, 61, 2346–2351.
Other transition metal-catalyzed homo-coupling of arylboronic acids: (a)
Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Langmuir
2004, 20, 11293–11296; (b) Sakurai, H.; Tsunoyama, H.; Tsukuda, T. J.
Organomet. Chem. 2007, 692, 368–374; (c) Gonzalez-Arellano, C.; Corma, A.;
Iglesias, M.; Sanchez, F. Chem. Commun. 2005, 1990–1992; (d) Carrettin, S.;
Guzmann, J.; Corma, A. Angew. Chem., Int. Ed. 2005, 44, 2242–2245; (e) Demir,
A. S.; Reis, O.; Emrullahhoglu, M. J. Org. Chem. 2003, 68, 10130–10134.
Yamamoto, Y.; Suzuki, R.; Hattori, K.; Nishiyama, H. Synlett 2006, 1027–1030.
Xu, Z.; Mao, J.; Zhang, Y. Catal. Commun. 2008, 9, 97–100.
2
8
ined the electrooxidative homo-coupling reaction of several boro-
nic acids bearing electron-withdrawing groups (entries 4–6). The
reaction of p-chlorophenylboronic acid (1e), p-acetylphenylboronic
acid (1f), and p-nitrophenylboronic acid (1g) proceeded smoothly
to give the corresponding biaryls in respective yields of 93%, 95%,
and 98%. The results showed that both electron-rich and elec-
tron-deficient arylboronic acids could be used for the electrooxida-
tive homo-coupling reaction. m-Substituted arylboronic acids
could also be applied to the reaction. The reaction of m-tolylbo-
ronic acid (1h) and m-chlorophenylboronic acid (1i) gave the cor-
responding biaryls (2h and 2i) in 85% and 87% yields, respectively
3
.
.
4
5.
6.
7.
(
entries 7 and 8). Similarly, the homo-coupling of o-tolylboronic
acid proceeded to afford 2j in 74% yield (entry 9).
Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am. Chem. Soc. 2006,
128, 6829–6836.
In summary, we have developed an electrooxidative coupling
reaction of arylboronic acids catalyzed by cationic Pd(II) catalysts
electrogenerated in situ. The scope of the use of EG transition metal
catalysts are being investigated further in our laboratory.
8. For recent works see: (a) Mitsudo, K.; Kumagai, H.; Takabatake, F.; Kubota, J.;
Tanaka, H. Tetrahedron Lett. 2007, 48, 8994–8997; (b) Yoshida, T.; Kuroboshi,
M.; Oshitani, J.; Goto, K.; Tanaka, H. Synlett 2007, 2691–2694; (c) Tanaka, H.;
Arai, S.; Ishitobi, Y.; Kuroboshi, M.; Torii, S. Electrochemistry 2006, 74, 656–658;
(
4
d) Mitsudo, K.; Matsuda, W.; Miyahara, S.; Tanaka, H. Tetrahedron Lett. 2006,
7, 5147–5150; (e) Kubota, J.; Ido, T.; Kuroboshi, M.; Tanaka, H.; Uchida, T.;
Acknowledgments
Shimamura, K. Tetrahedron 2006, 62, 4769–4773; (f) Kubota, J.; Shimizu, Y.;
Mitsudo, K.; Tanaka, H. Tetrahedron Lett. 2005, 46, 8975–8979; (g) Tanaka, H.;
Kubota, J.; Miyahara, S.; Kuroboshi, M. Bull. Chem. Soc. Jpn. 2005, 78, 1677–
This work was supported in part by a Grant-in-Aid for Young
Scientists (B) (No. 1530188228) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan and by the Wesco
Scientific Promotion Foundation. The authors are grateful to the
SC-NMR Laboratory of Okayama University for the NMR
measurement.
1684; (h) Mitsudo, K.; Kawaguchi, T.; Miyahara, S.; Matsuda, W.; Kuroboshi,
M.; Tanaka, H. Org. Lett. 2005, 7, 4649–4652.
Mitsudo, K.; Kaide, T.; Nakamoto, E.; Yoshida, K.; Tanaka, H. J. Am. Chem. Soc.
2007, 129, 2246–2247.
9
.
1
0. Typical procedure for the electrooxidative homo-coupling of boronic acids (Table
3
, entry 3): The electrooxidation was carried out in an H-type divided cell
2
(
glass filter) equipped with two platinum electrodes (1.0 Â 1.5 cm ). In the
anodic chamber was placed a solution of phenylboronic acid (1a, 25 mg,
.21 mmol), Pd(OAc) (4.3 mg, 0.02 mmol), TEMPO (9.4 mg, 0.06 mmol), and
(55 mg, 0.40 mmol) in a 0.05 M Et NClO solution of CH CN/H O (7/1,
mL). In the cathodic chamber was placed a 0.05 M Et NClO solution of
CN/H O (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 Et
0
K
5
2
References and notes
2
CO
3
4
4
3
2
4
4
1
.
.
For a review see: Tsuji, J. Palladium Reagents and Catalysts; John Wiley & Sons:
Chichester, 2004.
Pd-catalyzed oxidative homo-coupling of arylboronic acids: (a) Cravotto, G.;
Palmisano, G.; Tollari, S.; Nano, G. M.; Penoni, A. Ultrason. Sonochem. 2005, 12,
CH
3
2
2
2
O
(
3 Â 10 mL). The combined organic extracts were washed with aq satd
9
1–94; (b) Klingensmith, L. M.; Leadbeater, N. E. Tetrahedron Lett. 2003, 44,
4
NaCl (15 mL), dried over MgSO , and concentrated under reduced pressure.
7
65–768; (c) Falck, J. R.; Mohapatra, S.; Bondlela, M.; Venkataraman, S. K.
The residue was purified by column chromatography on silica gel (hexane)
to afford biphenyl (2a, 14 mg, 88%).
Tetrahedron Lett. 2002, 43, 8149–8151; (d) Kabalka, G. W.; Wang, L. Tetrahedron
Lett. 2002, 43, 3067–3068; (e) Parrish, J. P.; Jung, Y. C.; Floyd, R. J.; Jung, K. W.