Homocoupling of arylboronic acids catalyzed by simple gold salts

Article abstract of DOI:10.1016/j.tetlet.2011.07.030

A range of arylboronic acids undergo a homocoupling reaction in the presence of catalytic amount of gold salts to yield symmetrical biaryls. Alkenylboronic acids, arylboronic esters, and arylborates also participate in the gold-catalyzed homocoupling reaction.

Full text of DOI:10.1016/j.tetlet.2011.07.030

Tetrahedron Letters 52 (2011) 4779–4781
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Tetrahedron Letters
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Homocoupling of arylboronic acids catalyzed by simple gold salts
⇑
Takanori Matsuda , Taro Asai, Shigeru Shiose, Kotaro Kato
Department of Applied Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A range of arylboronic acids undergo a homocoupling reaction in the presence of catalytic amount of gold
salts to yield symmetrical biaryls. Alkenylboronic acids, arylboronic esters, and arylborates also partici-
pate in the gold-catalyzed homocoupling reaction.
Received 23 June 2011
Accepted 6 July 2011
Available online 19 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Arylboronic acid
Biaryl
Catalysis
Gold
Homocoupling
Biaryls are ubiquitous structural motifs in functional molecules,
natural products, and ligands for catalysis. Several synthetic meth-
ods have been developed for the construction of biaryl skeletons.
chloroaurate(III) dihydrate (NaAuCl
catalyst for the homocoupling of 1a. On the other hand, no homo-
coupling of 1a was observed with phosphine-ligated gold(I) com-
4
ꢀ2H
2
O) could also be used as a
1
3
1
Transition metal catalyzed cross-coupling reactions between aryl-
metal compounds and aryl halides are one of the most reliable and
efficient methods for preparing unsymmetrical biaryls. On the
3 3 2
plexes such as AuCl(PPh ) and (Ph P)AuNTf .
2
other hand, symmetrical biaryls are synthesized by homocoupling
reactions, which include reductive coupling of aryl halides and oxi-
dative coupling of arylmetal compounds. Oxidative homocouplings
ð1Þ
3
are mediated using various promoters including metal salts, io-
dine, and organic oxidants,5 which are used in stoichiometric
amounts. Homocoupling reactions using catalytic amounts of tran-
sition metals have also been developed. In particular, extensive
studies on the homocoupling reactions of arylboronic acids have
been conducted because of their ready availability and bench sta-
bility. There have been reports on homocoupling reactions of aryl-
4
Various arylboronic acids were then subjected to the present
reaction conditions. The two gold salts exhibited similar catalytic
activities toward a range of arylboronic acids shown in Table 1.
The gold-catalyzed homocoupling reaction of phenylboronic acids
b–d possessing alkyl groups at the 3- or 4-positions furnished the
corresponding biaryls 2b–d in good yields (entries 1–3). Using 1b,
we subsequently discovered that 1 mol % of NaAuCl O was suf-
ficient to produce the biaryl product in a comparable yield. Similarly,
we found that the reaction of 1d occurred at room temperature, al-
beit with a slight decrease in yield. The homocoupling reaction was
sensitive to steric effects associated with substituents on the aryl
6
7
8
9
boronic acids catalyzed by palladium, chromium, copper, gold,
1
1
0
and rhodium. The gold-catalyzed homocoupling reactions re-
9
a,b,d,e
ported so far have employed supported gold catalysts
and
4
ꢀ2H
2
gold(III) complexes bearing Schiff bases.9 In this Letter, we report
an operationally simple homocoupling reaction of arylboronic
acids catalyzed by gold. The reaction proceeds in the presence of
c
4
catalytic amounts of simple gold salts such as AuCl and NaAuCl .
Negligible homocoupling was observed when phenylboronic
acid (1a) was reacted in ethanol under air at 50 °C in the presence
of 5 mol % of gold(I) chloride. In contrast, 1.1 equiv of potassium
0
group. o-Tolylboronic acid (1f) gave 2,2 -dimethylbiphenyl (2f) in
3
5–39% yields (entry 5), and mesitylboronic acid (2,4,6-Me
3 6 2-
C H
B(OH) ) failed to react even in refluxing ethanol. Electronic effects
2
carbonate as an additive facilitated the reaction to afford biphenyl
were also found to be crucial. An electron-donating methoxy group
at the 4-position of the phenyl group had a negative effect on the
yield (entry 6), whereas 3-methoxyphenylboronic acid (1i) afforded
biaryl 2h in 70–75% yields (entry 7). The reactions of phenylboronic
acids 1i–k bearing electron-withdrawing groups at the 3- or
1,12
(
2a) in 73% isolated yield (Eq. 1).1
We found that sodium tetra-
⇑
Corresponding author. Fax: +81 3 5261 8437.
E-mail address: matta@rs.kagu.tus.ac.jp (T. Matsuda).
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.030

4
780
T. Matsuda et al. / Tetrahedron Letters 52 (2011) 4779–4781
Table 1
Gold-catalyzed homocoupling of aryl- and alkenylboronic acids 1
Table 2
a
a
Homocoupling of arylboronic esters and arylborates
Entry Arylboron compound
Biaryl
Yieldb with
2
AuCl
%)
NaAuCl
(%)
4
ꢀ2H
2
O
(
Yieldb with
NaAuCl
1
2
(3b)
2b
81
69
82
70
Entry
1 (R)
AuCl (%)
4
ꢀ2H
2
O (%)
(4b) 2b
1
2
3
4
5
6
7
8
9
1b (4-MeC
6
H
4
)
79
68
77
43
35
32
70
36
20
14
55
21
8
79 (75c)
72
1c (4-t-BuC
1d (3-MeC
1e (2-naphthyl)
1f (2-MeC
1g (4-MeOC
1h (3-MeOC
1i (4-F CC
1j (4-NCC
1k (3-AcC
1l (4-FC
1m (3-BrC
1n (4-IC
6
H
4
)
d
6
H
4
)
79 (61 )
47
39
49
75
50
23
3
(5b)
2b
59
61
6
4
H )
6
4
H )
6
H
4
)
4
5
6
PhBF
4-t-BuC
NaBPh
3
K (6a)
BF
(7a)
2a
2c
2a
50
22
34
64
28
44
H
4
3
K (6c)
3
6
H
4
)
6
b
b
6
H
4
)
4
e
10
11
12
13
14
15
6
H
4
)
11 (16 )
a
Arylboron compound was reacted in EtOH (0.125 M) under air at 50 °C for 24 h
in the presence of Au salt (5 mol %) and K CO (1.0–1.1 equiv).
6
H
4
)
56
24 (14 )
e
2
3
6
H
4
)
b
Isolated yield by preparative TLC. Yield based on the four phenyl groups of 7a.
6
H
4
)
13
70
26
1o (trans-b-styryl)
1p (1-phenylvinyl)
62
21
and (3) while the filtrate from the reaction mixture had virtually
no catalytic activity, the homocoupling could be successfully re-
peated using the filtration residue.
In summary, a facile method has been developed for the oxida-
tive homocoupling reaction of arylboronic acids catalyzed by gold.
The reaction conditions were also applied to the homocoupling of
alkenylboronic acids, arylboronic esters, and arylborates.
a
Unless otherwise noted, boronic acid 1 was reacted in EtOH (0.125 M) under air
at 50 °C for 24 h in the presence of Au salt (5 mol %) and K
2
CO
3
(1.0–1.1 equiv).
b
Isolated yield by preparative TLC.
Reaction was performed with 1 mol % of Au salt.
Reaction was performed at rt.
Reaction was performed in refluxing EtOH.
c
d
e
References and notes
4
-position were sluggish and gave the corresponding biaryls 2i–k in
14
yields below 50% (entries 8–10). In these cases, the remainder of
the mass balance comprised both unreacted 1 and a protodeboro-
nated arene. While the homocoupling of 4-fluorophenylboronic acid
1.
(a) Cepanec, I. Synthesis of Biaryls; Elsevier: Amsterdam, 2004; (b) Bringmann,
G.; Walter, R.; Weirich, R. Angew. Chem., Int. Ed. Engl. 1990, 29, 977; (c) Hassan,
J.; Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102, 1359;
(
d) Campeau, L.-C.; Fagnou, K. Chem. Commun. 2006, 1253; (e) Alberico, D.;
(1l) gave biaryl 2l in 55–56% yields (entry 11), the reaction of 3-bro-
Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174; (f) Ashenhurst, J. A. Chem.
Soc. Rev. 2010, 39, 540.
mo and 4-iodo derivatives (1m and 1n) resulted in the formation of
the corresponding biaryls with diminished yields (8–24%) (entries
1
acids also participated in the homocoupling reaction. For example,
trans-b-styrylboronic acid (1o) underwent gold-catalyzed homo-
coupling to afford 1,4-diphenylbuta-1,3-diene (2o) in 62–70% yields
2. (a) Stanforth, S. P. Tetrahedron 1998, 54, 263; (b) Suzuki, A. J. Organomet. Chem.
999, 576, 147; (c) Bellina, F.; Carpita, A.; Rossi, R. Synthesis 2004, 2419; (d)
2 and 13). In addition to arylboronic acids,15 the alkenylboronic
1
Espinet, P.; Echavarren, A. M. Angew. Chem., Int. Ed. 2004, 43, 4704; (e)Metal-
Catalyzed Cross-Coupling Reactions; de Meijere, A., Diederich, F., Eds., second ed;
Wiley-VCH: Weinheim, 2004.
3.
(a) Ghosal, S.; Luke, G. P.; Kyler, K. S. J. Org. Chem. 1987, 52, 4296; (b) Inoue, A.;
Kitagawa, K.; Shinokubo, H.; Oshima, K. Tetrahedron 2000, 56, 9601; (c) Piers,
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Emrullahoglu, M. J. Org. Chem. 2003, 68, 578.
(
entry 14). 2,3-Diphenylbuta-1,3-diene (2p) was obtained from 1p
in low yields, presumably because of steric hindrance from the phe-
nyl group at 1-position of the vinyl group of 1t (entry 15). The homo-
coupling reaction was also found to be amenable to the gram-scale
synthesis of biaryls. For example, 1.31 g (8.5 mmol, 68%) of 2a was
prepared from the reaction of 3.05 g (25 mmol) of 1a with 1 mol %
4. Mao, J.; Hua, Q.; Xie, G.; Yao, Z.; Shi, D. Eur. J. Org. Chem. 2009, 2262.
5.
(a) Miyake, Y.; Wu, M.; Rahman, M. J.; Iyoda, M. Chem. Commun. 2005, 411; (b)
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Lett. 2001, 42, 4087; (d) Lei, A.; Zhang, X. Tetrahedron Lett. 2002, 43, 2525;
4 2
NaAuCl ꢀ2H O.
Similar to arylboronic acids, arylboronic esters and arylborates
can also be used as substrates for the gold-catalyzed homocoupling
reaction (Table 2). Biaryl 2b was obtained in good yields from the
reaction of p-tolylboronic acid ethylene glycol ester (3b) (entry 1).
Neopentyl glycol and pinacol esters (4b and 5b, respectively) gave
(
e) Kabalka, G. W.; Wang, L. Tetrahedron Lett. 2002, 43, 3067; (f) Koza, D. J.;
Carita, E. Synthesis 2002, 2183; (g) Parrish, J. P.; Jung, Y. C.; Floyd, R. J.; Jung,
K. W. Tetrahedron Lett. 2002, 43, 7899; (h) Klingensmith, L. M.; Leadbeater,
N. E. Tetrahedron Lett. 2003, 44, 765; (i) Yoshida, H.; Yamaryo, Y.; Ohshita, J.;
Kunai, A. Tetrahedron Lett. 2003, 44, 1541; (j) Punna, S.; Díaz, D. D.; Finn, M.
G. Synlett 2004, 2351; (k) Cravotto, G.; Beggiato, M.; Penoni, A.; Palmisano,
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Yamamoto, Y.; Suzuki, R.; Hattori, K.; Nishiyama, H. Synlett 2006, 1027; (m)
Zhou, C.; Larock, R. C. J. Org. Chem. 2006, 71, 3184; (n) Adamo, C.; Amatore,
C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am. Chem. Soc. 2006, 128, 6829; (o)
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2007, 273, 240; (r) Mitsudo, K.; Shiraga, T.; Tanaka, H. Tetrahedron Lett. 2008,
2
b with yields ranging from 59% to 70% (entries 2 and 3). Under
similar conditions, aryltrifluoroborates 6a and 6c showed reduced
reactivity (entries 4 and 5).16 Even tetraphenylborate 7a was con-
verted to 2a in the presence of the gold salts (entry 6).17
Although the detailed mechanism of the gold-catalyzed homo-
coupling reaction has not been fully elucidated,1 on the basis of
8
the results described below, we assume that the present reaction
is catalyzed by heterogeneous gold19 and that molecular oxygen
4
9, 6593; (s) Xu, Z.; Mao, J.; Zhang, Y. Catal. Commun. 2008, 9, 97; (t) Chen,
acts as a reoxidant: (1) the efficiency of the reaction significantly
deteriorated (20% yield for the conversion of 1a–2a) when per-
formed under an argon atmosphere and otherwise identical condi-
tions; (2) the formation of phenol derivatives was observed in
some cases with arylboronic acids bearing electron-donating
J.-S.; Krogh-Jespersen, K.; Khinast, J. G. J. Mol. Catal. A: Chem. 2008, 285, 14;
(u) Burns, M. J.; Fairlamb, I. J. S.; Kapdi, A. R.; Sehnal, P.; Taylor, R. J. K. Org.
Lett. 2007, 9, 5397; (v) Jin, Z.; Guo, S.-X.; Gu, X.-P.; Qiu, L.-L.; Song, H.-B.;
Fang, J.-X. Adv. Synth. Catal. 2009, 351, 1575; (w) Mu, B.; Li, T.; Fu, Z.; Wu, Y.
Catal. Commun. 2009, 10, 1497; (x) Ciric, A.; Mathey, F. Organometallics 2010,
29, 4785; (y) Prastaro, A.; Ceci, P.; Chiancone, E.; Boffi, A.; Fabrizi, G.; Cacchi,
S. Tetrahedron Lett. 2010, 51, 2550.
2
0
groups (18% yield of 3-methoxyphenol for the reaction of 1h);

T. Matsuda et al. / Tetrahedron Letters 52 (2011) 4779–4781
4781
7
8
.
.
Falck, J. R.; Mohapatra, S.; Bondlela, M.; Venkataraman, S. K. Tetrahedron Lett.
002, 43, 8149.
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Kirai, N.; Yamamoto, Y. Eur. J. Org. Chem. 2009, 1864; (c) Cheng, G.; Luo, M. Eur.
J. Org. Chem. 2011, 2519.
13. Highly hygroscopic gold(III) chloride gave similar results.
6 4
14. When an equimolar mixture of 1h (Ar = 3-MeOC ) and 1k (Ar = 3-AcC H )
2
6
0
H
4
reacted, three possible products (2h, 3-acetyl-3 -methoxybiphenyl, and 2k)
were produced in the ratio of 52:40:8. These results indicate that electron-
richer 1h possesses higher reactivity than electron-poorer 1k under the
present reaction conditions.
9
.
(a) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Langmuir
2
2
004, 20, 11293; (b) Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem., Int. Ed.
005, 44, 2242; (c) González-Arellano, C.; Corma, A.; Iglesias, M.; Sánchez, F.
15. Heteroarylboronic acids like 4-pyridylboronic and 2-thienylboronic acids
exhibited no reactivity.
Chem. Commun. 2005, 1990; (d) Willis, N. G.; Guzman, J. Appl. Catal. A 2008, 339,
8; (e) Chaicharoenwimolkul, L.; Munmai, A.; Chairam, S.; Tewasekson, U.;
Sapudom, S.; Lakliang, Y.; Somsook, E. Tetrahedron Lett. 2008, 49, 7299.
0. Vogler, T.; Studer, A. Adv. Synth. Catal. 2008, 350, 1963.
1. Representative experimental procedure: mixture of AuCl (11.6 mg,
.050 mmol), phenylboronic acid (1a, 121.9 mg, 1.00 mmol), CO
152.0 mg, 1.10 mmol) in EtOH (8.0 mL) was heated at 50 °C under open air
16. For homocoupling of aryltrifluoroborates catalyzed by gold nanocluster, see:
Sakurai, H.; Tsunoyama, H.; Tsukuda, T. J. Organomet. Chem. 2007, 692, 368.
17. Vanadium-catalyzed oxidative homocoupling of sodiumtetraarylborates has
been reported: Mizuno, H.; Sakurai, H.; Amaya, T.; Hirao, T. Chem. Commun.
2006, 5042.
18. For discussions of the mechanism of homocoupling of arylboronic acids
catalyzed by gold, see Refs. 9a–c.
19. For reviews on heterogeneous and supported gold catalysis, see: (a) Corma, A.;
Garcia, H. Chem. Soc. Rev. 2008, 37, 2096; (b) Fierro-Gonzalez, J. C.; Gates, B. C.
Chem. Soc. Rev. 2008, 37, 2127; (c) Hashmi, A. S. K.; Hutchings, G. J. Angew.
Chem., Int. Ed. 2008, 37, 2127.
6
1
1
A
0
(
K
2
3
Ò
for 24 h. The reaction mixture was filtered through a plug of Florisil washing
with hexane–AcOEt (3:1). The filtrate was concentrated under reduced
pressure, and the resulting residue was subjected to preparative thin-layer
chromatography (hexane:AcOEt = 20:1) to afford biphenyl (2a, 58.6 mg, 76%).
1
2. Potassium bases such as K
3
PO
4
and KOH worked equally well (ꢁ80%), while
20. Formation of phenols was often observed in gold-catalyzed oxidative
homocoupling.
carbonate bases such as Na
2
CO
3
and Cs CO gave poor results (<40%). The
2
3
reaction was sluggish in aprotic solvents such as 1,4-dioxane (19%) and toluene
28%).
(