Homocoupling of arylboronic acids catalyzed by CuCl in air at room temperature

Article abstract of DOI:10.1002/ejoc.201001729

Homocoupling of arylboronic acids has been successfully carried out by using the inexpensive simple copper salt CuCl as the catalyst in methanol to obtain symmetric biaryls in good to excellent yields. The reaction proceeds with a CuCl loading of 2 mol-% under extremely mild conditions: in air, at room temperature, without the need of any additives such as base, oxidant, or ligand. A simple and efficient CuCl-catalyzed homocoupling of arylboronic acids for the synthesis of symmetrical biaryls is described. The homocoupling reaction takes place with a catalyst loading of at 2 mol-% in economical and easily handled methanol and under extremely mild conditions: in air, at room temperature, without any additives such as base, oxidant, or ligand. Copyright

Full text of DOI:10.1002/ejoc.201001729

FULL PAPER
DOI: 10.1002/ejoc.201001729
Homocoupling of Arylboronic Acids Catalyzed by CuCl in Air at Room
Temperature
[a]
[a]
Guanjun Cheng and Meiming Luo*
Keywords: Boron / Biaryls / Copper / C–C coupling / Homogeneous catalysis
Homocoupling of arylboronic acids has been successfully
carried out by using the inexpensive simple copper salt CuCl
as the catalyst in methanol to obtain symmetric biaryls in
good to excellent yields. The reaction proceeds with a CuCl
loading of 2 mol-% under extremely mild conditions: in air,
at room temperature, without the need of any additives such
as base, oxidant, or ligand.
aryls.[
6a,6c–6g,7b–7e,8a,8c–8f,9,11]
(4) Some reactions even require
Other metals such as Au,
Introduction
[
6b,6c,6e,7a,8b,8c,9b,10c]
[12]
heating.
Rh, Ni,[14] Cr,[15] and Cu[16–18] have recently been used to
promote the homocoupling of arylboronic acids. Au and
Rh are expensive and some additives are necessary for catal-
[13]
Biaryls synthesized by C–C bond-forming reactions are
important building blocks for functional materials and nat-
[
1]
ural products. Developing straightforward and environ-
mentally friendly reactions for the preparation of biaryls
continues to be the subject of intense investigation. Great
progress in the synthesis of symmetrical and unsymmetrical
biaryls has been achieved by transition-metal-catalyzed
[
12,13]
ysis.
The Ni catalytic system employed malodorous
[
14]
pyridine as the solvent and required heating. In the chro-
mium-mediated homocoupling of arylboronic acids, Ag O
2
(
3 equiv.) is required as an expensive oxidant to restore the
[15]
catalyst, and the reaction was performed with heating.
[2]
couplings such as the Suzuki reaction, modified Ullmann
Three papers report that certain copper species can be used
to mediate the homocoupling of arylboronic acids. Demir
and co-workers have shown that homocoupling of aryl-
boronic acids can be carried out in DMF in the presence
[
3]
[4]
reaction, Kumada–Corriu–Tamao reaction, and so on.
For the synthesis of symmetrical biaryls, the Ullmann cou-
pling reaction, which has been known for a century, has
gained much attention as an efficient process.[ On the
other hand, oxidative dimerization of organometallic spe-
cies has emerged as another choice. Because arylboronic ac-
ids prepared from the corresponding aryl halides are more
stable and less toxic than many other organometallic rea-
gents, it is not surprising that many studies have been re-
ported recently on the oxidative homocoupling of aryl-
boronic acids as an excellent method to obtain symmetrical
5]
of Cu(OAc) without any additive such as a base or a li-
2
[
16]
gand.
However, this reaction requires a substoichi-
ometric amount of Cu(OAc) (0.5 equiv.) and a relatively
2
high reaction temperature (100 °C). When the amount of
Cu(OAc) was decreased to 10 mol-% in several examples,
2
an oxygen atmosphere and 4-Å molecular sieves were re-
quired. Yamamoto and co-workers reported that the homo-
coupling reaction of arylboronic acids was catalyzed by a
copper complex employing 1,10-phenanthroline as li-
biaryls. Palladium-based catalysts are most frequently used
for the homocoupling of arylboronic acids.[
6–11]
However,
[
17]
gand. During the preparation of this manuscript, Kabou-
din and co-workers reported that inexpensive copper(II)
sulfate could mediate the homocoupling of arylboronic ac-
ids. This method requires a stoichiometric amount of
there are limitations and room for improvement in these
catalytic methods exists: (1) Palladium is expensive and ad-
ditional ligands are usually required.[
6,10a,11]
(2) Certain oxi-
dants are employed to restore the catalyst.[
8b–8g,9a]
(3) Ad-
[
18]
CuSO and heating in uneasily handled DMF.
In ad-
4
dition of a base is necessary to acquire high yields of bi-
dition to metal-mediated homocoupling, Shi and co-
workers reported the iodine-promoted homocoupling of ar-
ylboronic acids in PEG-400 under aerobic conditions.[
19]
[
a] Key Laboratory of Green Chemistry & Technology of Ministry This reaction required a relatively high temperature (100–
of Education at Sichuan University, College of Chemistry,
Sichuan University,
140 °C) and long reaction time (48 h). In this paper, we re-
Chengdu 610064, People’s Republic of China
Fax: +86-28-85462021
port an easy and efficient copper-catalyzed homocoupling
reaction of arylboronic acids for the synthesis of symmetri-
cal biaryls under mild conditions without the need of any
additives.
E-mail: luomm@scu.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001729.
Eur. J. Org. Chem. 2011, 2519–2523
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2519

G. Cheng, M. Luo
FULL PAPER
Results and Discussion
Table 1. Copper-catalyzed homocoupling of arylboronic acid: opti-
mization of the reaction conditions.[
a]
Initially, the symmetric biaryl byproducts encountered in
some Cu-catalyzed C–N bond-forming reactions of aryl-
[
20–22]
boronic acids
attracted our attention. We surmised
that certain simple copper salts may be used as efficient
catalysts for the homocoupling of arylboronic acids under
mild conditions. During optimization studies, 3-nitrophen-
ylboronic acid was chosen as the model substrate, and all
the reactions were conducted at room temperature in air.
Solvents, catalysts, and bases were investigated. As shown
in Table 1, various copper salts (2 mol-%) were tested with-
out any additives such as base, oxidant, or ligand (Table 1,
Entries 1–6). The best activity was shown with CuCl, which
gave not only the best yield, but also the shortest reaction
time (Table 1, Entry 4). Other copper salts were less active
than CuCl, though they could afford symmetric biaryls.
The influence of the amount of CuCl on the catalytic reac-
Entry
Catalyst
Solvent
Base
Time
[h]
Yield
[%]
[b]
(mol-%)
1
2
3
4
5
6
7
8
9
Cu (2)
Cu O (2)
2
CuI (2)
CuCl (2)
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
H O
2
iPrOH
THF
DMF
acetone
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
15
20
20
4
15
15
2
73
57
55
93
86
77
86
89
91
78
0
59
57
15
8
67
trace
78
48
82
14
4
CuCl (2)
2
2
Cu(OAc) (2)
CuCl (10)
CuCl (5)
CuCl (3)
CuCl (1)
– (0)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
CuCl (2)
2.5
3
10
15
20
24
24
24
24
24
24
16
16
4
tions was also investigated (Table 1, Entries 4 and 7–11). 11
12
13
14
15
The results show that a CuCl loading of 2 mol-% gave the
best homocoupling product yield (Table 1, Entry 4). The
homocoupling reaction did not occur in the absence of a
copper catalyst (Table 1, Entry 11). We next examined the 16
effect of the solvent (Table 1, Entries 4 and 12–19), and 17
O (1:1)[
c]
18
19
20
21
MeOH/H
EtOH/H
2
methanol was found to be the best of choice (Table 1, En-
try 4). It is well known that the addition of a base can im-
prove the reaction efficiency for some catalytic sys-
O (1:1)[
c]
2
MeOH
MeOH
MeOH
MeOH
NEt
Na CO
3
2
3
20
20
3
[
6a,6c–6g,7b–7e,8a,8c–8f,9,11]
tem.
investigated in the presence of different bases (Table 1, En-
Subsequently, the reaction was 22
2
K CO
–
3
[d]
23
90
tries 20–22). Interestingly, it was found that some inorganic [a] Reaction conditions: 3-nitrophenylboronic acid (0.75 mmol),
solvent (2.0 mL), room temperature, air. [b] Yield of isolated prod-
uct. [c] v/v. [d] Air was bubbled through the reaction mixture.
bases, such as Na CO and K CO , inhibited rather than
2
3
2
3
promoted the homocoupling reaction, giving very poor
yields (Table 1, Entries 21 and 22). On the other hand, NEt3
as an organic base had little effect on the reaction (Table 1,
Entry 20). It was not necessary to bubble air through the catalytic system, it is noteworthy that the reaction of p-bro-
reaction mixture, though it could shorten the reaction time mophenyl boronic acid (1m) furnished the desired product
(Table 1, Entries 4 and 23). Thus, the optimized reaction 4,4Ј-dibromobiphenyl (2m) in 74% isolated yield (Table 2,
conditions for the homocoupling of arylboronic acids in- Entry 13), demonstrating good selectivity. Careful inspec-
volve the use of CuCl (2 mol-%) as the catalyst, methanol tion of the results revealed that para- and meta-substituted
as the solvent, in air, at ambient temperature, and without arylboronic acids gave biaryls in good to excellent yields.
any additives such as base, oxidant, or ligand.
Diverse results were obtained for ortho-substituted aryl-
Having defined the optimized reaction conditions, we in- boronic acids. o-Tolylboronic acid (1n) produced the corre-
vestigated the scope of the CuCl-catalyzed homocoupling sponding product 2,2Ј-dimethylbiphenyl (2n) in a lower
reaction with respect to the boronic acids. As shown in yield of 43% (Table 2, Entry 14). With anthracene-9-yl-
Table 2, most of the substrates with a variety of substituents boronic acid (1o), only a trace amount of self-coupling
afforded the products in good to excellent yields under the product 2o was formed and the major product was the pro-
optimum reaction conditions. Phenylboronic acids bearing todeboronation product anthracene (Table 2, Entry 15). Re-
electron-withdrawing nitro, cyano, fluoro, trifluoromethyl, action with 2-naphthylboronic acid (1p) afforded corre-
and methoxycarbonyl groups gave corresponding homo- sponding dimer 2p in 90% yield (Table 2, Entry 16), and
coupling products 2a–f in good yields (73–93%; Table 2, phenanthren-9-ylboronic acid (1q) gave 9,9Ј-biphen-
Entries 1–6). We next examined the homocoupling reaction anthrene (2q) in 83% yield (Table 2, Entry 17). The proto-
of aromatic boronic acids bearing electron-donating groups. col was also applied to the homocoupling of styrylboronic
Phenylboronic acids bearing methyl, tert-butyl, methoxy, acid (1r), giving desired product 2r in 77% yield (Table 2,
[
17]
dimethoxy, and propylsulfanyl (strongly coordinating)
groups gave desired product 2g–k in 73–92% yields acids 2-thienylboronic acid (1s) and 3-pypridylboronic acids
Table 2, Entries 7–11). It has been reported that some cop- (1t) gave corresponding products 2s and 2t in 21 and 63%
Entry 18). Finally, the homocoupling of heteroarylboronic
(
per species can mediate the Suzuki–Miyaura cross-coupling yield, respectively, under the same reaction conditions
of arylboronic acids with arylbromides.[ With the present (Table 2, Entries 19 and 20).
23]
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Eur. J. Org. Chem. 2011, 2519–2523

Homocoupling of Arylboronic Acids Catalyzed by CuCl
Table 2. Copper-catalyzed homocoupling of various arylboronic acids.[a]
[a] Reaction conditions: 1 (0.75 mmol), CuCl (0.015 mmol, 1.5 mg), MeOH (2.0 mL), room temperature (ca. 25 °C), air. [b] Isolated yield.
[c] Major product was anthracene. [d] MeOH (5.0 mL).
The coupling between two different arylboronic acids (Table 1). (4) Solvent affects the catalyst activity signifi-
was also tested under the optimum reaction conditions. As cantly. On the basis of these observations and the proposed
[
16–18]
shown in Scheme 1, the cross-experiments were performed mechanisms in the literature,
possible catalytic cycles
0
I
II
by combining two arylboronic acids with different and sim- for Cu , Cu , and Cu are depicted in Scheme 1. It seems
ilar electron density. In addition to the homocoupling prod- that there are three possible reaction pathways. The first
ucts of the individual arylboronic acid components, GC– one is the Cu /Cu catalytic cycle. Transmetalation of aryl-
MS analysis showed that the unsymmetrical biaryls from boronic acid with Cu produces Cu –Ar, which reacts with
the cross-coupling of two different arylboronic acids were another arylboronic acid in the presence of oxygen to gen-
erate Ar–Cu –Ar; this species releases Cu by reductive eli-
Demir, Yamamoto, and Kaboudin inspected the mination of homocoupling product Ar–Ar. The second one
Cu-mediated homocoupling of arylboronic acids and pro- is the Cu /Cu cycle. Double transmetalation of Cu with
posed putative mechanisms for the transformation. How- two molecules of arylboronic acid affords Ar–Cu –Ar,
ever, the mechanism of the copper-catalyzed homocoupling which releases Cu by reductive elimination of product Ar–
reaction is not yet obvious at the present stage. The follow- Ar. Air oxidation of Cu reproduces Cu . The third pos-
ing observations may be useful to speculate on the mecha- sibility is the Cu /Cu /Cu cycle, in which the Ar–Cu –Ar
nism: (1) Copper of any oxidation state, Cu , Cu , or Cu , intermediate may be produced either by double transmet-
is catalytically active in the homocoupling (Table 1). (2) Air alation of Cu with two molecules of arylboronic acid or
is critical to the reaction. (3) The negative counterion plays by disproportion of Ar–Cu . In view of the fact that Cu Cl
I
III
I
I
III
I
also formed. No significant selectivity was observed.
[
16]
[17]
[18]
0
II
II
II
0
0
II
0
I
II
II
0
I
II
II
I
I
–
I
II
an important role, and Cl is the best for Cu catalysts is more effective than Cu Cl under the same reaction con-
2
Eur. J. Org. Chem. 2011, 2519–2523
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2521

G. Cheng, M. Luo
FULL PAPER
biaryls. This method allows the use of the readily available,
inexpensive copper salt CuCl as the catalyst and offers very
good results for most arylboronic acids. The homocoupling
reaction proceeds in economical and easily handled meth-
anol under extremely mild conditions: in air, at room tem-
perature, and without any additives such as base, oxidant,
or ligand. With these novel features, the present simple cop-
per salt based catalytic system is much superior to the
known protocols documented in the literature. That some
inorganic bases such as Na CO and K CO can inhibit the
2
3
2
3
homocoupling reaction of arylboronic acids, which is often
encountered as a serious side reaction in some arylboronic
acid involved reactions, is an interesting finding. The good
selectivity of homocoupling over the Suzuki cross-coupling
shows potential application in synthesis and merits further
investigation.
Experimental Section
General Remarks: All reagents were purchased from commercial
suppliers and used without further purification. Both copper(I)
chlorides of 97.0+% and 99.999% purity were tested and showed
almost the same reactivity. CuCl (97.0+%) was also examined by
ICP-MS, and no palladium was detected. Column chromatography
was performed with silica gel (200–300 mesh). Thin layer
chromatography was carried out by using Merck silica gel GF254
1
13
plates. H NMR (400 MHz) and C NMR (100 MHz) spectra
were recorded in CDCl . Chemical shifts are reported in ppm using
3
TMS as internal standard. Splitting patterns are designated as fol-
lows: s, singlet; d, doublet; t, triplet; m, multiplet. Products were
1
13
characterized by comparison of H and C NMR spectroscopic
data with those in the literatures.
Representative Procedure for the CuCl-Catalyzed Homocoupling of
Scheme 1. Coupling of different arylboronic acids.
Arylboronic Acid 1a To Afford Biaryl 2a: A solution of CuCl
(
0
1.5 mg, 0.015 mmol) and 3-nitrophenylboronic acid (125 mg,
.75 mmol) in MeOH (2 mL) was stirred at 25 °C for 4 h in air.
ditions (Table 1, Entries 4 and 5), the homocoupling of ar-
ylboronic acids may take place most probably through the
Cu /Cu catalytic cycle (Scheme 2). Considering that there
The reaction mixture was filtered and washed with ethyl acetate.
The combined filtrate was concentrated in vacuo. The residue was
purified by flash chromatography on silica gel column (ethyl acet-
I
III
is no information currently available on the active species, ate/petroleum ether, 1:5) to afford 2a (85.0 mg, 93%) as a light
it is hard to tell the real reaction pathway.
yellow solid.
Representative Procedure for the CuCl-Catalyzed Coupling of 1a and
1g To Afford Biaryls 2a, 3a, and 2g: A solution of 3-nitrophenylbo-
ronic acid (50 mg, 0.3 mmol), 4-methoxyphenylboronic acid
(
(
45.6 mg, 0.3 mmol), and CuCl (1.1 mg, 0.012 mmol) in MeOH
1.5 mL) was stirred at 25 °C for 4 h in air. The reaction mixture
was filtered and washed with ethyl acetate. The combined filtrate
was analyzed by GC–MS.
Supporting Information (see footnote on the first page of this arti-
1
13
cle): Characterization data and copies of the H and C NMR
spectra of the coupling products.
Scheme 2. Plausible mechanism for the homocoupling of aryl-
boronic acids catalyzed by copper species.
Acknowledgments
Conclusions
This work was supported by the National Natural Science Founda-
tion of China (21021001, 21072134) and Program for Changjiang
Scholars and Innovative Research Team in University (IRT0846).
In conclusion, we have successfully developed a simple
and efficient copper-catalyzed method for the homocou- We thank the Analytical & Testing Centre of Sichuan University
pling of arylboronic acids for the synthesis of symmetrical for NMR measurements.
2522
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 2519–2523

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Received: December 27, 2010
Published Online: March 14, 2011
Eur. J. Org. Chem. 2011, 2519–2523
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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