Mild and ligand-free palladium-catalyzed cross-couplings between aryl halides and arylboronic acids for the synthesis of biaryls and heterocycle-containing biaryls

Article abstract of DOI:10.1002/ejoc.200600879

Ligand-free palladium-catalyzed Suzuki-Miyaura cross-couplings between aryl halides and arylboronic acids performed at room temperature are presented. It was found that both solvent and base had a fundamental influence on the reaction, and the most effective system in terms of yield and rate was observed to be the combination of Pd(OAc)₂ with MeONa and alcohols. In the presence of Pd(OAc)₂ and MeONa, a variety of aryl halides reacted very rapidly with arylboronic acids in good to excellent yields at room ternperature in EtOH as the solvent. Moreover, the Pd(OAc)₂/MeONa catalytic system could also be applied in couplings between heteroaryl halides and arylboronic acids to provide satisfactory results in MeOH as the solvent after prolonged reaction times. It is noteworthy that the reactions were conducted under mild, aerobic, and ligand-free conditions. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200600879

FULL PAPER
DOI: 10.1002/ejoc.200600879
Mild and Ligand-Free Palladium-Catalyzed Cross-Couplings between Aryl
Halides and Arylboronic Acids for the Synthesis of Biaryls and Heterocycle-
Containing Biaryls
[a]
[a]
[a]
[a]
Chen-Liang Deng, Sheng-Mei Guo, Ye-Xiang Xie, and Jin-Heng Li*
Keywords: Palladium / Suzuki–Miyaura cross-coupling reaction / Aryl halide / Arylboronic acid
Ligand-free palladium-catalyzed Suzuki–Miyaura cross-
2
perature in EtOH as the solvent. Moreover, the Pd(OAc) /
couplings between aryl halides and arylboronic acids per- MeONa catalytic system could also be applied in couplings
formed at room temperature are presented. It was found that
both solvent and base had a fundamental influence on the
reaction, and the most effective system in terms of yield and
between heteroaryl halides and arylboronic acids to provide
satisfactory results in MeOH as the solvent after prolonged
reaction times. It is noteworthy that the reactions were con-
ducted under mild, aerobic, and ligand-free conditions.
rate was observed to be the combination of Pd(OAc)
MeONa and alcohols. In the presence of Pd(OAc)
2
with
and
2
MeONa, a variety of aryl halides reacted very rapidly with
arylboronic acids in good to excellent yields at room tem-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
fer catalysts (usually nBu
NBr), supramolecular reactors
4
[amphiphilic rod-coil molecules consisting of poly(ethylene
Biaryls are important intermediates broadly used in syn-
thetic organic chemistry and found in many naturally oc-
curring and biologically valuable molecules.[ Accordingly,
substantial attention has been paid to the development of
efficient methods for the synthesis of biaryls, one of the best
known methods for the construction of such biaryl com-
pounds being the Suzuki–Miyaura cross-coupling reac-
oxide)], and others, to provide the best results. Sajiki and
co-workers recently found that Pd/C-catalyzed Suzuki–
[6]
1]
Miyaura cross-couplings between aryl bromides and aryl-
boronic acids could be carried out smoothly without the
aid of any ligands and promoters at room temperature, but
long reaction times (24 h) were still necessary to ensure
complete conversion under oxygen-free conditions. To the
[
2–8]
tion.
The original and general Suzuki–Miyaura coup-
[7]
best of our knowledge, only one paper has demonstrated
ling procedure involves the use of palladium–ligand (often
a phosphane ligand) complexes as catalysts, and the reac-
tions are performed at high temperature and under oxygen-
free conditions to avoid side reactions. In addition, long
reaction times are usually required. Impressive progress in
the development of efficient catalytic systems to achieve this
reaction under mild conditions has been made in the last
that palladium-catalyzed Suzuki–Miyaura cross-couplings
between aryl bromides and arylboronic acids can be carried
out rapidly (20 min to 60 min) in aqueous acetone under
ligand-free and aerobic conditions, but the procedure was
performed with heating in order to complete the conversion
of aryl iodides and bromides rapidly (35 °C). In addition,
no reactions involving the synthesis of heterocycle ana-
logues were reported. The development of mild, rapid, li-
gand-free, and aerobic procedures for palladium-catalyzed
Suzuki–Miyaura cross-couplings is therefore still a topic of
unparalleled important in organic synthesis and industry.
Serendipitously, we found that couplings between various
aryl halides and arylboronic acids could be carried out very
rapidly (4–30 min) at room temperature in good to excellent
yields in the presence of Pd(OAc)2 (1 mol-%), NaOMe
[
3,4]
several years,
but there still exists considerable room for
further exploration, as only a few methods for palladium-
catalyzed Suzuki–Miyaura coupling at room temperature
[
5–7]
without the aid of any ligands have been developed.
Among those ligand-free and room temperature Suzuki–
Miyaura coupling transformations, many methods have still
required other additional promoters,[ such as phase-trans-
5]
[
a] Key Laboratory of Chemical Biology & Traditional Chinese (1 mmol), and EtOH (1 mL). In addition, the catalytic sys-
Medicine Research (Ministry of Education), College of Chemis-
tem could also be applied for couplings between heteroaryl
try and Chemical Engineering, Hunan Normal University,
Changsha 410081, China
halides and arylboronic acids, with satisfactory results still
being achieved at room temperature in MeOH as the sol-
vent after prolonged reaction times. Here we wish to report
our results in detail [Equation (1)].
Fax: +86-731-8872-531
E-mail: jhli@hunnu.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 1457–1462
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1457

C.-L. Deng, S.-M. Guo, Y.-X. Xie, J.-H. Li
FULL PAPER
Table 1. Screening conditions for palladium-catalyzed Suzuki–Mi-
yaura cross-coupling reactions between p-bromoanisole (1a) and
phenylboronic acid (2a).[
a]
(
1)
Results and Discussion
Coupling between p-bromoanisole (1a) and phenylbo-
ronic acid (2a) was chosen as a model reaction for screening
of the conditions, and the results are summarized in
Table 1. Initially, a number of solvents were examined, and
preliminary experiments showed that solvents had a funda-
mental influence on both rate and yield of the reaction:
treatment of substrate 1a with 2a, Pd(OAc) (1 mol-%), and Entry
[Pd]
Base
Time
Solvent Isolated
yield (%)
2
NaOMe (1 mmol) in MeOH had afforded the correspond-
ing product 3 in quantitative yield after 10 h (Entry 1),
whereas in EtOH substrate 1a had been completely con-
sumed after 6 min, to provide results otherwise identical to
those obtained in MeOH (Entry 2). Other solvents, includ-
ing nPrOH, iPrOH, nBuOH, iBuOH, tBuOH, and acetone,
however, decreased both rate and yield of the reaction (En-
tries 3–8). Encouraged by these results, we therefore per-
formed the reaction with other Pd catalysts, including
1
2
3
4
5
6
7
8
9
1
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
NaOMe 10 h
NaOMe 6 min
NaOMe 30 min nPrOH 69
NaOMe 30 min iPrOH 20
NaOMe 30 min nBuOH 59
NaOMe 30 min iBuOH trace
NaOMe 30 min tBuOH 57
NaOMe 10 h
NaOMe 30 min EtOH
NaOMe 8 min
MeOH 100
[
b]
EtOH
100
Pd(OAc)2
Pd(OAc)
Pd(OAc)
2
2
acetone 43
Pd
2
(dba)
3
trace
80
66
98
33
86
58
11
18
89
23
98
0
PdCl
2
EtOH
EtOH
EtOH
Pd (dba) , PdCl , and PdCl (MeCN) , in EtOH. Unfortu- 11[
PdCl (MeCN) NaOMe 8 min
2
3
2
2
2
2
2
b]
nately, though, they were less effective than Pd(OAc) in 12
Pd(OAc)
2
2
2
2
2
NaOEt 9 min
Na CO
Cs CO
PO
NaOH 30 min EtOH
Et N 30 min EtOH
NaOMe 30 min EtOH
NaOMe 22 h EtOH
NaOMe 30 min EtOH
2
1
1
1
1
3
4
5
6
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
3
30 min EtOH
7 min EtOH
30 min EtOH
terms of both yield and rate (Entries 2 and 9–11). A series
of other bases was also investigated, and it was found that
results identical to those seen with NaOMe was obtained
2
3
K
3
4
with NaOEt as the base (Entries 2 and 12), but that the 17
Pd(OAc)2
3
[
[
[
c]
d]
e]
18
19
20
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
yields of the target product 3 were reduced to some extent
when NaOMe was replaced by other bases, such as
Na CO , Cs CO , Na PO , NaOH, and Et N (Entries 13–
2
3
2
3
3
4
3
[
(
[
[
a] Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), Pd(OAc)
1 mol-%), base (1 mmol), and solvent (1 mL) at room temperature.
b] The reaction was monitored by GC-MS and TLC over two runs.
c] Pd(OAc) (0.1 mol-%). [d] Pd(OAc) (0.01 mol-%). [e] 1a
2
17). The yield of 3 was reduced to 86%, for example, when
Cs CO was used as the base (Entry 14). Et N, an organic
2
3
3
base, gave the corresponding product 3 in a rather low yield
(
2
2
Entry 17). Gratifyingly, we observed that satisfactory re- (5 mmol), 2a (6 mmol), base (10 mmol), and solvent (5 mL).
sults were still obtained at 0.1 mol-% loading of Pd (89%
yield in 30 min; Entry 18), although only a low yield was
isolated after 22 h when the loading of Pd(OAc) was re- NaOMe in EtOH at room temperature under ligand-free
2
duced to 0.01 mol-% (Entry 19). It is noteworthy that the and aerobic conditions to provide the target products in
reaction between 5 mmol of substrate 1a and 6 mmol of 2a good to excellent yields. The reactions between bromide 1c
in the presence of 1 mol-% of Pd(OAc) , and 10 mmol of and 2a–2d, for example, gave the corresponding products
2
NaOMe could also be conducted smoothly in EtOH to pro- 13–17 in good to excellent yields (Entries 4–7). However,
duce the desired product 3 in a 98% yield after 30 min (En- the reaction between 1c and 2e, another heteroarylboronic
try 20).
acid, was unsuccessful (Entry 8). Substrates 1d–1f, bearing
With the standard reaction conditions to hand, we next carbonyl and cyano groups, also underwent the reactions in
set out to examine the scope of the reaction. As shown in good yields under the standard conditions (Entries 9–11).
Table 2, substrate 1a had been consumed completely in the We were happy to observe that high yields were still
presence of boronic acids 2a or 2b, Pd(OAc) , and NaOMe achieved in EtOH after 30 min from the couplings of the
2
after 5 min, affording the corresponding products 4 and 5 bulky aryl bromide 1i in the presence of Pd(OAc) and
2
in 99% and 81% yields, respectively, in EtOH as the solvent NaOMe (Entry 15), although another bulky aryl bromide
(Entries 1 and 2). Substrate 1b, a deactivated aryl iodide, (1k) and the activated chloride 1l were not suitable sub-
also underwent the coupling with 2a, rapidly providing the strates for the reaction even in the presence of 3 mol-% of
biaryl product 3 in excellent yield under the same condi- Pd(OAc) (Entries 17 and 18).
2
tions (Entry 3). It was found that different substituents were
The application of the above optimized reaction condi-
tolerated well in the reaction, and a wide range of other tions to the corresponding treatment of heteroaryl halides
aryl bromides, whether electron-rich or electron-deficient, in order to construct heterocycle-containing biaryl systems
[
8]
all worked well with arylboronic acids, Pd(OAc) , and was also studied (Table 3). The results showed that EtOH
2
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Eur. J. Org. Chem. 2007, 1457–1462

Synthesis of Biaryls
Table 2. Rapid palladium-catalyzed Suzuki–Miyaura cross-couplings between aryl halides (1) and arylboronic acids (2) at room temperature.[a]
FULL PAPER
[
a] Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), Pd(OAc)
2
(1 mol-%), MeONa (1 mmol), and EtOH (1 mL) at room temperature. [b]
Isolated yield. [c] Pd(OAc) (3 mol-%).
2
Eur. J. Org. Chem. 2007, 1457–1462
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1459

C.-L. Deng, S.-M. Guo, Y.-X. Xie, J.-H. Li
FULL PAPER
was not an effective solvent for couplings of heteroaryl ha- 24 h when 2-bromo-5-methoxypyridine (1m) was treated
lides: in EtOH, for example, only a moderate yield of the with 2a, Pd(OAc) , and NaOMe at room temperature (En-
2
corresponding heteroaryl-aryl product 20 was isolated after try 1). To our delight, though, the yield of 20 was enhanced
Table 3. Pd(OAc)
2
-catalyzed Suzuki–Miyaura cross-couplings of heteroaryl halides.[a]
[
[
(
a] Reaction conditions: 1 (0.5 mmol), 2 (0.6 mmol), Pd(OAc)
2
(1 mol-%), MeONa (1 mmol), and MeOH (1 mL) at room temperature.
CO (1 mmol) and H O/acetone
b] Isolated yield. [c] EtOH (1 mL) instead of MeOH. [d] MeOH/DMF (2:1, 1 mL) as the solvent. [e] Na
2
3
2
[7]
3.5:3, 1 mL) instead of MeONa (1 mmol) and MeOH (1 mL), ref. . [f] Pd(OAc)
2
(3 mol-%).
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Eur. J. Org. Chem. 2007, 1457–1462

Synthesis of Biaryls
FULL PAPER
to 100% when MeOH was used as the solvent in place of NaOMe (1 mol) and MeOH or EtOH (1 mL) was stirred at room
temperature for the indicated time until complete consumption of
EtOH (Entry 2). However, two solvent mixtures, including
[
7]
starting material as monitored by TLC. After the mixture had been
filtered and concentrated, the residue was then purified by flash
column chromatography (hexane or hexane/ethyl acetate) to afford
the corresponding coupled product.
the previously reported effective aqueous acetone, were
not suitable solvents for the reaction. Subsequently, coup-
lings of the other heteroaryl bromides 1n–1s, including ni-
trogen-containing and sulfur-containing halides, with aryl-
boronic acids were conducted smoothly in the presence of
2
(
-Methoxy-5-phenylpyridine (17): Colorless oil. 1H NMR
400 MHz, CDCl ): δ = 8.39 (s, 1 H), 7.79 (d, J = 2.8 Hz, 1 H),
3
Pd(OAc) and NaOMe in MeOH at room temperature, af-
2
7
.53 (d, J = 8.8 Hz, 2 H), 7.46 (t, J = 7.2 Hz, 2 H), 7.35 (dd, J =
.6 Hz, 7.2 Hz, 1 H), 6.82 (d, J = 8.8 Hz, 1 H), 3.98 (s, 3 H) ppm.
fording the corresponding heteroaryl-aryl and heteroaryl-
heteroaryl products in moderate to excellent yields (En-
tries 5–13). The nitrogen-containing heteroaryl bromide 1q,
for example, coupled with arylboronic acid 2a to provide
7
13
C NMR (100 MHz, CDCl
3
): δ = 163.6, 145.0, 137.9, 137.4, 130.1,
1
1
28.9, 127.3, 126.7, 110.8, 53.5 ppm. LRMS (EI, 20 eV): m/z (%):
85 (100) [M] . HRMS (EI) for C12H11NO (M ): calcd. 185.0841;
+
+
the corresponding product 24 in a 98% yield (Entry 9). In found: 185.0840.
addition, the optimized reaction conditions were also effec-
tive for the reactions between substrate 1q and sulfur-con-
taining or nitrogen-containing boronic acids in good yields
2
-(Pyridin-4-yl)pyrazine (23): Light yellow solid, m.p. 87.6–88.4 °C
1
(
uncorrected). H NMR (400 MHz, CDCl
3
): δ = 9.11 (s, 1 H), 8.79
(d, J = 6.0 Hz, 2 H), 8.72 (s, 1 H), 8.64 (s, 1 H), 7.93 (d, J = 6.4 Hz,
(
Entries 10 and 11). The catalytic efficiency of the Pd- 2 H) ppm. 13C NMR (100 MHz, CDCl ): δ = 150.7, 150.1, 144.7,
3
(
OAc) /NaOMe/MeOH system was less, however, for coup- 144.6, 143.5, 142.3, 120.9; LRMS (EI, 20 eV) m/z (%): 157 (100)
2
+
+
9 7 3
lings of heteroaryl chlorides. Only 2-chloropyrazine (1u) [M ]. HRMS (EI) for C H N (M ): calcd. 157.0640, found
157.0640.
was able to undergo coupling with boronic acid 2a to gener-
ate the corresponding desired product smoothly in satisfac-
tory yield (Entry 15), but with heteroarylboronic acid 2d
this was unsuccessful even in the presence of 3 mol-% of
Acknowledgments
Pd(OAc) (Entry 16). Other chlorides 1t and 1v were also
2
The authors thank the Scientific Research Fund of Hunan Provin-
found to be less active for the reaction (Entries 14 and 18). cial Education Department (No. 05B038), the National Natural
Science Foundation of China (No. 20572020), the Key Project of
the Chinese Ministry of Education (No. 206102), Fok Ying Dong
Conclusions
Education Foundation (No. 101012), and Hunan Provincial Natu-
ral Science Foundation of China (No. 05JJ1002) for financial sup-
port.
In summary, we have demonstrated a simple, mild, and
rapid method for palladium-catalyzed cross-couplings be-
tween aryl halides and arylboronic acids for the synthesis
[
[
1] G. Bringmann, C. Gunther, M. Ochse, O. Schupp, S. Tasler, in:
Progress in the Chemistry of Organic Natural Products (Eds: W.
Herz, H. Falk, G. W. Kirby, R. E. Moore), Springer, New York,
of biaryls and heterocycle-containing biaryls. In relation to
previously reported results,[
3–7]
several interesting features
2
001, vol. 82, pp. 1–293.
2] For reviews, see: a) N. Miyaura, A. Suzuki, Chem. Rev. 1995,
5, 2457–2483; b) F. Diederich, P. J. Stang, Metal-Catalyzed
are obvious for this reaction. Firstly, the reaction is very
rapid with aryl iodides and aryl bromides. Secondly, several
functional groups – such as carbonyl, cyano, methoxy and
fluoro groups – could be tolerated in this system. In ad-
dition, the scope of the reactions was extended to heteroaryl
halides and heteroarylboronic acids. Finally, the reactions
could be conducted at room temperature under ligand-free
and aerobic conditions. Further applications of the system
in other coupling transformations are under investigation.
9
Cross-coupling Reactions, Wiley-VCH, Weinheim, 1998; c) N.
Miyaura, Cross-Coupling Reactions, Springer, Berlin, 2002; d)
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(
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General Remarks: 1H and 13C NMR spectra were recorded with
an INOVA-400 or INOVA-500 (Varian) spectrometer or a Bruker
3
AMX 300 spectrometer in CDCl as the solvent. All reagents were
directly used as obtained commercially. All products were deter-
mined by GC-MS (SHIMADZU GCMS-QP2010). All the solvents
were dried by standard procedures and the other reagents were used
4223–4229.
[
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1
directly from commercial sources. Analytical data and spectra ( H
and 13C NMR) for all products are available in the Supporting
Information. Supporting Information for this article is available on
the WWW under http://www.eurjoc.org. or from the author.
1
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Eur. J. Org. Chem. 2007, 1457–1462
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C.-L. Deng, S.-M. Guo, Y.-X. Xie, J.-H. Li
FULL PAPER
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4] For representative papers on room temperature palladium-cat-
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Received: October 7, 2006
Published Online: January 25, 2007
[
5] For representative papers on room temperature palladium-cat-
alyzed Suzuki–Miyaura cross-couplings with the aid of other
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