Copper-catalyzed direct C-H arylation of pyridine N-oxides with arylboronic esters: One-pot synthesis of 2-arylpyridines

Article abstract of DOI:10.1039/c3cc48767a

The first example of the copper-catalyzed direct C-H arylation of pyridine N-oxides with arylboronic esters has been developed, leading to a wide range of 2-arylpyridines in a one-pot synthesis with moderate to good yields without an additional reductant. This transformation allows for rapid access to a variety of 2-arylpyridines using an inexpensive catalytic system that would be more difficult to access with traditional methods. Thus, this method represents a simple and practical procedure to access 2-arylpyridines.

Full text of DOI:10.1039/c3cc48767a

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Copper-catalyzed direct C–H arylation of pyridine
N-oxides with arylboronic esters: one-pot
synthesis of 2-arylpyridines†
Cite this:DOI: 10.1039/c3cc48767a
Received 17th November 2013,
Accepted 8th January 2014
Yan Shen, Jiuxi Chen,* Miaochang Liu, Jinchang Ding, Wenxia Gao, Xiaobo Huang
and Huayue Wu*
DOI: 10.1039/c3cc48767a
www.rsc.org/chemcomm
The first example of the copper-catalyzed direct C–H arylation of has rarely been reported,12 even though copper was the first transi-
pyridine N-oxides with arylboronic esters has been developed, tion metal shown to promote C–H arylation.13,14 We therefore
leading to a wide range of 2-arylpyridines in a one-pot synthesis considered that organoboron reagents15 might serve as both aryla-
with moderate to good yields without an additional reductant. This tion coupling reagents and as reductants in the copper-catalyzed
transformation allows for rapid access to a variety of 2-arylpyridines C–H activation reaction, achieving a one-pot synthesis of
using an inexpensive catalytic system that would be more difficult 2-arylpyridines. As part of the continuing efforts in our laboratory
to access with traditional methods. Thus, this method represents a toward the development of novel transition metal-catalyzed coupling
simple and practical procedure to access 2-arylpyridines.
reactions with organoboron reagents,16 herein we report copper-
catalyzed direct C–H arylation of pyridine N-oxides with arylboronic
esters for the one-pot synthesis of 2-arylpyridines.
Pyridines are among the most prevalent heterocyclic structural
motifs that occur in a wide variety of bioactive natural products,
pharmaceuticals, functional materials, ligands, organo-catalysts and
synthetic building blocks.1 The 2-arylpyridines, as a typical class of
C–H bond activation partners, have become increasingly important
in the past few years because they are good substrates for cyclo-
metallation.2 2-Arylpyridines are generally synthesized using 2-halo-
pyridines3 or 2-pyridyl organometallics4 as coupling partners. The
fundamental work of Fagnou and co-workers on the palladium-
catalyzed direct arylation of pyridine N-oxides with aryl bromides
provided attractive and valuable routes for the synthesis of
2-arylpyridines.5 Other improved methods have also been reported
for constructing 2-arylpyridines using pyridine N-oxides6 as coupling
partners, with palladium-catalyzed direct arylation via C–H activation
representing the major strategy.7 However, to realize the synthesis of
the desired 2-arylpyridines, reducing agents8 were required to reduce
the N-oxide products in these transformations. Several groups9 have
reported the use of Minisci-type reactions for the synthesis of
2-arylpyridines by a free-radical mechanism. Recently, Wu and
co-workers developed a novel synthetic route to 2-arylpyridines via
the palladium-catalyzed decarboxylative cross-coupling reactions.10
The arylation using Grignard reagents11 is also a powerful tool
for the construction of 2-arylpyridines.
Our preliminary studies focused on the reaction between
2-phenylpyridine N-oxide (1a) and phenylboronic acid to obtain the
optimal reaction conditions (Table 1). Through a screening process, no
target product was detected using Cs2CO3 as the base with a variety of
palladium or copper catalysts. To our delight, a trace amount of the
desired product 2,6-diphenylpyridine (3a) was observed by GC/MS (EI)
analysis using KOH as the base in the presence of CuI (entry 1).
However, we were pleased to find that the yield of the desired product
3a could be improved to 28% in toluene under an air atmosphere after
the base was changed to the stronger tBuOK base (entry 2). Encouraged
by this promising result, a series of trial experiments were performed in
the presence of copper catalysts and with adjustments to the reaction
parameters in order to obtain more satisfactory results. First, we
investigated copper catalysts. Among the copper sources used,
Cu(acac)2 exhibited the highest catalytic reactivity with 79% yield
t
(entries 2–14). Screening revealed that the use of BuOK as the base
produced the best result (entries 15–25). The best solvent is toluene,
and other solvents were ineffective for the reaction (see Table S1 in
ESI†). We also investigated the influences of material ratio, catalyst
amount, and reaction temperature on the arylation yield (see Table S1
in ESI†). Other organoboron reagents (see entries, 1–5 Table S2 in ESI†)
were used for this reaction under the same conditions and phenyl-
boronic ester (2a) showed the best result (87%, entry 26) whereas
alkylboronic acids (e.g. isobutylboronic acid and benzylboronic acid)
were unreactive (see entries 6 and 7, Table S2 in ESI†). Therefore, we
performed all the subsequent reactions using arylboronic esters.
Having the optimized reaction conditions in hand, we next
explored the scope and generality of the arylation reaction using
To the best of our knowledge, C–H functionalization of pyridine
N-oxides (or pyridine derivatives) using inexpensive copper catalysts
College of Chemistry & Materials Engineering, Wenzhou University, Wenzhou 325035,
P. R. China. E-mail: jiuxichen@wzu.edu.cn, huayuewu@wzu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details. See
DOI: 10.1039/c3cc48767a
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Table 1 Selected results of screening of the optimal conditionsa
standard conditions. The steric effects of substituents had an
obvious impact on the yield of the reaction. For example, arylation
of 1a with para-, meta- and ortho-tolylboronic ester (2b–2d) provided
84% of 3ab and 89% of 3ad, while the yield of 3ac was decreased to
41% (entries 2–4). The same phenomenon was observed in the
reaction of 1a with para- and ortho-methoxyphenylboronic ester
(entries 5 and 6). The electronic properties of the substituents on
the phenyl ring of the arylboronic esters had a little effect on the
reaction. In general, the arylboronic esters bearing an electron-
withdrawing substituent (e.g., –CF3) produced a slightly higher yield
of products than those analogues bearing an electron-donating
substituent (e.g., –OMe) (entries 5 and 7). However, substrate 2h,
bearing a cyano group, was treated with 1a to afford slightly lower
yields of 3ah (entry 8). It is noteworthy that the chloro, fluoro, bromo,
and iodo moieties (commonly used for cross-coupling reactions) in
arylboronic esters were all tolerated and afforded several halogen-
containing products 3i–3m in moderate to good yields, leading to a
useful handle for further cross-coupling reactions (entries 8–12). We
observed a moderate yield of 3an when 2n was used as the substrate
(entry 13). Moreover, (4-vinylphenyl)boronic ester (2o), possessing a
functionalized vinyl group, was tolerated well and afforded
the product 3ao in 62% yield (entry 14). In addition, several
2-arylpyridine N-oxides were examined. The electronic properties of
the groups on the phenyl ring moiety of the arylpyridine N-oxides
had a little effect on the reaction. For example, the reaction of 1a
with p-methylphenylpyridine N-oxide (1b), p-methoxyphenylpyridine
N-oxide (1c), and p-fluorophenylpyridine N-oxide (1d) resulted in the
formation of the corresponding desired products 3ba, 3ca, and 3da
in 79%, 83%, and 81% yields, respectively (entries 15–17).
Entry
Cu source
Base
Yieldb (%)
1
2
3
4
5
6
7
8
CuI
CuI
CuBr
CuCl
CuCN
CuSCN
CuOAc
CuOTf
CuCl2
KOH
Trace
28
34
32
41
39
37
35
32
37
Trace
51
49
79
41
19
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuONa
tBuOLi
EtOK
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
CuBr2
CuSO4
Cu(OAc)2
Cu(OTf)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Cu(acac)2
Trace
Trace
Trace
Trace
Trace
0
EtONa
CaH2
NaNH2
KOH
K3PO4
Cs2CO3
Et3N
0
0
DBACO
0
tBuOK
87c
a
Reaction conditions: 1a (0.5 mmol), phenylboronic acid (1.0 mmol), Cu
source (10 mol%), base (1.5 mmol), toluene (5 mL), 110 1C, 2 h, air.
b
c
Isolated yield. Using phenylboronic ester (2a) as the arylation reagent.
Next, we turned our attention to the effect of the reactions of
various 2-arylpyridine N-oxides (1a–1d) and arylboronic esters (2) various 2-unsubstituted pyridine N-oxides (1e–1i) and arylboronic
(Table 2). First, the reaction between 2-phenylpyridine N-oxide (1a) esters (2) under standard conditions (Table 3). The reactions pro-
and various arylboronic esters (2b–2o) was investigated under ceeded smoothly with high selectivity and tolerated a wide range of
Table 2 Copper-catalyzed reaction of 2-arylpyridine N-oxidesa
Table 3 Copper-catalyzed reaction of 2-unsubstituted pyridine N-oxidesa
Entry R (1)
Ar (2)
Product Yieldb (%)
Entry
R (1)
Ar (2)
Product
Yieldb (%)
1
2
3
4
5
6
7
8
Ph (1a)
Ph (2a)
3aa
3ab
3ac
3ad
3ae
3af
87
84
41
89
65
18
71
51
79
73
64
81
43
71
62
79
83
81
1
2
3
4
5
6
7
8
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
H (1e)
Ph (2a)
3ea
3eb
3ec
3ed
3ee
3eg
3eh
3ei
3ej
3ek
3el
3en
3eo
3fa
3ga
3ha
3ia
81
78
32
83
68
64
52
72
67
58
74
79
72
74
35
67
68
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
p-(Me)C6H4 (2b)
o-(Me)C6H4 (2c)
m-(Me)C6H4 (2d)
p-(OMe)C6H4 (2e)
o-(OMe)C6H4 (2f)
p-(CF3)C6H4 (2g)
p-(CN)C6H4 (2h)
p-(F)C6H4 (2i)
p-(Cl)C6H4 (2j)
m-(Cl)C6H4 (2k)
p-(Br)C6H4 (2l)
p-(I)C6H4 (2m)
p-(Ph)C6H4 (2n)
p-(Me)C6H4 (2b)
o-(Me)C6H4 (2c)
m-(Me)C6H4 (2d)
p-(OMe)C6H4 (2e)
p-(CF3)C6H4 (2g)
p-(CN)C6H4 (2h)
p-(F)C6H4 (2i)
p-(Cl)C6H4 (2j)
m-(Cl)C6H4 (2k)
p-(Br)C6H4 (2l)
p-(Ph)C6H4 (2n)
p-(CH2QCH)C6H4 (2o)
Ph (2a)
3ag
3ah
3ai
9
9
10
11
12
13
14
15
16
17
18
3aj
3ak
3al
10
11
12
13
14
15
16
17
3am
3an
H (1e)
4-Me (1f)
4-CN (1g)
4-Ph (1h)
3-Ph (1i)
p-(CH2QCH)C6H4 (2o) 3ao
Ph (2a)
Ph (2a)
Ph (2a)
p-(Me)C6H4 (1b) Ph (2a)
p-(OMe)C6H4 (1c) Ph (2a)
3ba
3ca
3da
p-(F)C6H4 (1d)
Ph (2a)
a
a
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), Cu(acac)2 (10 mol%),
Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), Cu(acac)2 (10 mol%),
tBuOK (1.5 mmol), toluene (5 mL), 110 1C, 2 h, air. Isolated yields.
b
tBuOK (1.5 mmol), toluene (5 mL), 110 1C, 2 h, air. Isolated yields.
b
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with arylboronic esters. Further efforts to explore the detailed
mechanism and extend the applications of the transformation
are currently underway in our laboratories.
Financial support was provided by the National Natural
Science Foundation of China (No. 21102105 and 21072153).
Notes and references
Scheme 1 Possible reaction pathway.
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