Silver-catalyzed 2-pyridyl arylation of pyridine N-oxides with arylboronic acids at room temperature

Article abstract of DOI:10.1055/s-0031-1290088

A novel direct arylation of pyridine N-oxides with arylboronic acids through C-H functionalization has been developed. This new reaction is performed at room temperature using catalytic silver(I) nitrate in the presence of potassium persulfate and give 2-

Full text of DOI:10.1055/s-0031-1290088

LETTER
145
Silver-Catalyzed 2-Pyridyl Arylation of Pyridine N-Oxides with Arylboronic
Acids at Room Temperature
2-Pyridyl
Arylation of
P
e
yridine
N
-O
n
xides peng Mai,* Jinwei Yuan, Zhicheng Li,* Gangchun Sun, Lingbo Qu
Chemistry and Chemical Engineering School, Henan University of Technology, Zhengzhou Henan 450001, P. R. of China
Fax +86(371)67756715; E-mail: wpmai@haut.edu.cn; E-mail: lizhicheng65@sina.com
Received 28 August 2011
Though Baran and co-workers have reported that the cat-
Abstract: A novel direct arylation of pyridine N-oxides with aryl-
alyst system AgNO3–K2S2O8 was effective for the cou-
boronic acids through C–H functionalization has been developed.
pling between electron-deficient heterocycles and
This new reaction is performed at room temperature using catalytic
arylboronic acids,10 one equivalent of trifluoroacetic acid
silver(I) nitrate in the presence of potassium persulfate and give 2-
pyridyl arylation derivatives of pyridine N-oxides.
Key words: silver, pyridine N-oxides, arylation
must be needed to block the nitrogen atom of pyridine to
reduce the electron density and to increase the nucleophi-
licity of the 2-position of the pyridine. In addition, the ter-
tiary butyl group must be blocked at the 4-position of
pyridine, otherwise two or three regioisomers were ob-
tained. Thereby, we studied the C–H bond activation us-
ing pyridine N-oxides as platforms for the 2-arylpyridines.
Described herein is a new approach for the C–C bond
formation of pyridine N-oxides – a regioselective direct
cross-coupling with arylboronic acids catalyzed by silver
salt at room temperature (Scheme 1, equation 4).
Palladium-catalyzed C–H bond activation for the C–C
cross-coupling reaction has emerged as a powerful meth-
od for the preparation of biaryls.1 In particular, reactions
involving Pd-catalyzed aryl halides with aromatic C–H
bond activation have been extensively investigated.2 In
this area of research, various functional groups containing
heteroatoms such as acetyl, pyridyl, and imino groups
were needed as directing groups (DG) for C–H activation
chemistry.3 Using aryl boronic acids as coupling partners
with the direct C-arylation of aromatic rings under very
mild, room-temperature conditions remain rare.4 Most of
the successful reactions employ a combination of a palla-
dium catalyst with silver or copper salts as oxidants and
require high reaction temperature.
previous work:
R1
R1
R1
Ar X, X = Br, OTs
(1)
(2)
(3)
Pd(II), R23P, 110 °C
N
O
Ar
Ar
R1
Ar
H
Pd(II)/Ag, 130 °C
N
O
N
O
Pyridine moiety is an important component of natural
products, materials, and medicinal chemistry.5 Substitut-
ed pyridines are usually synthesized from metalated or ha-
lopyridyl compounds. However, this route needs extra
preparation steps and is inevitably accompanied with
problems such as expensive reagents and more waste.
Since Fagnou reported the palladium-catalyzed regiose-
lective direct arylation of pyridine N-oxides with aryl bro-
mides,6 this method has been studied and developed for
the preparation of substituted pyridines and other hetero-
cycles. Recently, palladium-catalyzed C–H functionaliza-
tion of pyridine N-oxides with unactivated arenes was
reported by Chang’s group.7 More recently, Ackermann
and co-worker reported direct arylations of pridine N-
oxides with aryl or alkenyl tosylates and mesylates,8
Tzschucke reported the synthesis of unsymmetrically sub-
stitued bipyridines by a palladium-catalyzed direct C–H
arylation of pyridine N-oxides.9 However, these processes
suffer either from high reaction temperatures or expensive
phosphine ligands and co-oxidants (Scheme 1, equations
1–3), which limit their potential application.
R3
Br
N
Pd(II), R23P, 120 °C
N
O
R3
N
this work:
R
R
Ar B(OH)2
Ag(I), .r.t
(4)
N
O
Ar
N
O
Scheme 1 Comparison of previous works with this work
We initiated an investigation of the C–H coupling using
pyridine N-oxides (1a) with phenylboronic acid (2a) in
the presence of 10 mol% of AgNO3 and three equivalents
of K2S2O8 at room temperature, and product 3aa was ob-
tained in 41% yield (Table 1, entry 1). The yield was in-
14
creased to 69% when 20 mol% of AgNO3 were loaded
(Table 1, entry 2). Other metals such as Cu(OAc)2,14
14
CuCl2,14 and FeCl3 were ineffective for this reaction
even at 90 °C (Table 1, entries 3–6). When BPO was used
as oxidant, no desired product was obtained (Table 1, en-
try 10). For other solvent mixtures, THF–H2O (1:1, v/v)
and DMSO–H2O (1:1, v/v) were found not to be better
SYNLETT 2012, 23, 145–149
Advanced online publication: 05.12.2011
DOI: 10.1055/s-0031-1290088; Art ID: W18711ST
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x.
2
0
1
1

146
W. Mai et al.
LETTER
than CH2Cl2/H2O, maybe due to solvability and extract- As shown in Scheme 2, based on Baran’s study,10 if there
ability. The yield was slightly reduced when the is no oxide on pyridine ring, a mixture of C2 and C4 cou-
(NH4)2S2O8 was used as an oxidant (Table 1, entry 11).
pling products were obtained in a 2:1 ratio together with
some bisarylation product. In our work, only the C2 aryl-
ation product was found (Table 2, entry 1). Similarly, only
30% yield of product was obtained when pyrazine cou-
pled with p-methyphenylboronic acid directly.
Table 1 Screening of Reaction Conditionsa
B(OH)2
catalyst/oxidant (3 equiv)
+
N
O
N
O
solvent, r.t.
Table 2 Scope of the Coupling of Arylboronic Acids to Substituted
Pyridine N-Oxide 1a
1a
3aa
2a
R1
R1
B(OH)2
R2
Entry Catalyst (mol%) Oxidant
Solvent (1:1) Yield (%)b
CH2Cl2/H2O 41
AgNO3 (0.2 equiv)
K2S2O8 (3.0 equiv)
+
1
AgNO3 (10)
AgNO3 (20)
K2S2O8
K2S2O8
N
O
N
O
R2
CH2Cl2/H2O (1:1)
r.t., 18 h
2
CH2Cl2/H2O 69
1a–e
3aa–3ef
2a–f
3c
4d
5e
6
Cu(OAc)2 (10) K2S2O8
CH2Cl2/H2O trace
CH2Cl2/H2O trace
CH2Cl2/H2O 11
Entry
Product
Yield (%)b
69
CuCl2 (10)
CuCl2 (100)
FeCl3 (20)
–
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
BPO
1
2
3
4
5
6
7
N
O
CH2Cl2/H2O
CH2Cl2/H2O
THF–H2O
0
0
7
3aa
8
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
AgNO3 (20)
43
OMe
9
DMSO–H2O 37
CH2Cl2/H2O
62
58
56
64
42
80
N
O
10
11
0
3bb
(NH4)2S2O8 CH2Cl2/H2O 60
a Conditions: 1a (1.5 mmol), 2a (1.0 mmol), catalyst (10–100%), ox-
N
O
idant (3.0 mmol), solvent (30 mL, 1:1) at r.t. for 18 h.
b Isolated yield based on 2a.
c At 70 °C.
3bc
d At 70 °C.
e At 90 °C.
N
O
The standard conditions were applied to a series of substi-
tuted pyridine N-oxides and arylboronic acids as shown in
Table 2. Pyridine N-oxides 1a with 4-methoxyphenyl-
boronic acid (2d) gave the coupling product in 56% yield
(Table 2, entry 4), while a moderate yield (69%) was ob-
tained when phenylboronic acid (2a) was reacted with 1a
(Table 2, entry 1). Pyridine N-oxide with an electron-
withdrawing group, for example, cyano in the 4-position,
gave slightly higher yield (Table 2, entries 11 and 12) than
those without electron-withdrawing substituents (Table 2,
entries 1 and 4). Products 3ab and 3bb were obtained in
moderate to good yields though 2-methoxyphenylboronic
acid (2b) has an ortho substituent (Table 2, entries 2 and
9). When pyrazine N1-oxide 3c was used as substrate, the
reaction proceeded smoothly and products 3ca and 3cf
were obtained in high yields (Table 2, entries 7 and 8). 1-
Naphthaleneboronic acid (2f) coupled with 3-cyanopyri-
dine N-oxide (3e) gave 2-substituted and 6-substituted
products 3ef and 3ef¢ with a ratio of 1:2, and the overall
yield was 77% (Table 2, entry 13). It should be mentioned
that this process for the direct cross-coupling through C–
H activation was operationally very easy.13
OMe
O
3ad
N
O
3ae
N
O
3af
N
N
O
3ca
N
8
87
N
O
3cf
Synlett 2012, 23, 145–149
© Thieme Stuttgart · New York

LETTER
2-Pyridyl Arylation of Pyridine N-Oxides
147
Table 2 Scope of the Coupling of Arylboronic Acids to Substituted
Pyridine N-Oxide 1a (continued)
N
N
no regioselectivity
R1
N
30%
N
R1
B(OH)2
R2
AgNO3 (0.2 equiv)
K2S2O8 (3.0 equiv)
O
>80%
+
N
O
N
O
R2
CH2Cl2/H2O (1:1)
r.t., 18 h
regioselectivity
1a–e
3aa–3ef
2a–f
N
N
Entry
Product
Yield (%)b
75
two regioisomers
O
controlled 2-pyridyl product
OMe
Scheme 2 Comparison of previous studies with this work
9
N
O
into sulfate radical anion in the presence of silver(I) salts.
This radical could induce the arylboronic acid to provide
an aryl radical which is an important intermediate in this
reaction. This was indeed supported by experiments per-
formed in the presence of radical scavenger. When two
equivalents of tetramethylpiperidine N-oxide (TEMPO)
was added under the same conditions, a trace of the de-
sired product was observed. As shown in Scheme 3, the
reaction was completely shut off when 2.5 equivalents of
TEMPO were added, which indicated that a radical inter-
mediate was involved in this transformation.
3ab
10
55
76
N
O
3ba
CN
11
12
N
O
AgNO3 (0.2 equiv)
K2S2O8 (3.0 equiv)
B(OH)2
3da
CN
+
N
O
N
O
CH2Cl2/H2O (1:1)
r.t., 18 h
1a
2a
3aa
66
N
O
no TEMPO
2.0 equiv TEMPO
2.5 equiv TEMPO
69%
trace
0
OMe
3dd
NC
Scheme 3 A radical pathway test
N
To study the mechanism of the reaction deeply, we did an-
other reaction to testify if an aryl radical was produced
here. It is well known that phenyl iodide or phenyl bro-
mide could produce a phenyl radical in the presence of po-
tassium tert-butoxide.12 As shown in Scheme 4, it is
interesting to note that product 3aa was also obtained in
34% yield when phenyl iodide was used as a substrate in
the presence of potassium tert-butoxide. It could provide
some evidence for the mechanism of this reaction.
O
3ef
and
77
(3ef/3ef¢ = 1:2)
13
CN
N
O
I
t-BuOK (3 equiv)
1,10-phenanthroline (20 mol%)
3ef¢
N
O
+
a Conditions: 1.5 mmol 1 and 1.0 mmol 2 were used.
N
O
DMF, 100 °C
b Isolated yields after column chromatography were based on 2.
3aa 34%
In our work, using pyrazine N1-oxide as substrate, product
3ca could be obtained in 80% isolated yield (Table 2, en-
try 7). These observations led us to conclude that the in-
troduction of oxide to pyridine could increase the
reactivity and selectivity of the reaction.
Scheme 4
In summary, a novel, room-temperature approach to a sil-
ver-catalyzed direct 2-pyridyl arylation of pyridine N-ox-
ides with arylboronic acids has been developed. In view of
the importance of pyridine N-oxide derivatives in medical
chemistry as well as their easy deoxygenation to the more
important pyridine derivatives, this novel method will
The mechanism of the reaction was also tested by us,
based on Baran’s and Li’s work,11 and a radical pathway
was postulated here. Persulfate anion disproportionates
© Thieme Stuttgart · New York
Synlett 2012, 23, 145–149

148
W. Mai et al.
LETTER
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Acknowledgment
We gratefully acknowledge Henan University of Technology for fi-
nancial support.
References and Notes
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(13) General Procedure for this Reaction
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A 50 mL vial was charged with a magnetic stir bar, pyridine
N-oxide (1a, 1.5 mmol), phenylboronic acid (2a, 1.0 mmol),
AgNO3 (0.2 mmol), K2S2O8 (3.0 mmol), followed by
CH2Cl2 and deionized H2O (1:1, v/v, 30 mL in total). After
stirring at r.t. for 18 h, the reaction mixture was filtered
through Celite (washed with MeOH and CH2Cl2), extracted
with CH2Cl2 (3 × 10 mL). The combined organic phase was
dried over Na2SO4, then evaporated under reduced pressure,
and the isolated yield was obtained by flash chromatography
column on silica gel (gradient eluent of MeOH in CH2Cl2:
1–5%, v/v).
(14) The metal sources used here [AgNO3, Cu(OAc)2, CuCl2,
FeCl3] were all purchased from Aladdin Company in
Shanghai. AR grade of AgNO3 (>99.8%), AR grade of
Cu(OAc)2 (anhyd, >99.0%), AR grade of CuCl2 (anhyd,
>98.0%), AR grade of FeCl3 (anhyd, >97.5%).
2-Phenylpyridine N-Oxide (3aa)6
1H NMR (400 MHz, CDCl3, 293 K): d = 7.26–7.33 (m, 1 H),
7.41–7.45 (m, 1 H), 7.47–7.53 (m, 4 H), 7.80–7.83 (m, 2 H),
8.48 (d, J = 6.4 Hz, 1 H). 13C NMR (100 MHz, CDCl3, 293
K): d = 149.4, 140.6, 132.6, 129.7, 129.3, 128.3, 127.5,
126.0, 124.6. Mp 144–146 °C (CH2Cl2).
2-(2-Methoxyphenyl)-6-methylpyridine N-Oxide (3bb)
1H NMR (400 MHz, CDCl3, 293 K): d = 7.40–7.45 (t, J = 8.4
Hz, 1 H), 7.35–7.37 (d, J = 7.6 Hz, 1 H), 7.16–7.25 (m, 3 H),
6.99–7.06 (m, 2 H), 3.80 (s, 3 H), 2.57 (s, 3 H). 13C NMR
(100 MHz, CDCl3, 293 K): d = 157.3, 149.5, 147.9, 130.7,
125.9, 125.2, 124.1, 122.9, 120.5, 111.2, 55.8, 18.4. ESI-
MS: m/z = 216.0 [M + 1]+. ESI-HRMS: m/z [M + H]+ calcd
(m) Engle, K. M.; Wang, D. H.; Yu, J. Q. J. Am. Chem. Soc.
2010, 132, 14137. (n) Mousseau, J. J.; Vallée, F.; Lorion, M.
M.; Charette, A. B. J. Am. Chem. Soc. 2010, 132, 14412.
(o) Ng, K. H.; Chan, A. S. C.; Yu, W. Y. J. Am. Chem. Soc.
2010, 132, 12862.
+
for C13H14NO2 : 216.1025; found: 216.1021.
2-(2-Methylphenyl)pyridine N-Oxide (3bc)6
1H NMR (400 MHz, CDCl3, 293 K): d = 8.37 (d, J = 4.8 Hz,
1 H), 7.36–7.40 (m, 1 H), 7.24–7.32 (m, 6 H), 2.25 (s, 3 H).
13C NMR (100 MHz, CDCl3, 293 K): d = 150.8, 140.1,
137.8, 132.9, 130.1, 129.6, 129.3, 128.0, 125.9, 125.4,
125.0, 19.5. Mp 107–109 °C (CH2Cl2).
(4) (a) Shi, B. S.; Zhang, Y. H.; Lam, J. K.; Wang, D. H.; Yu,
J. Q. J. Am. Chem. Soc. 2010, 132, 460. (b) Chu, J. H.; Tsai,
S. L.; Wu, M. J. Synthesis 2009, 3757. (c) Nishikata, T.;
Abela, A. R.; Huang, S. L.; Lipshutz, B. H. J. Am. Chem.
Soc. 2010, 132, 4978. (d) Wang, D. H.; Wasa, M.; Giri, R.;
Yu, J. Q. J. Am. Chem. Soc. 2008, 130, 7190. (e) Wen, J.;
Zhang, J.; Chen, S. Y.; Li, J.; Yu, X. Q. Angew. Chem. Int.
Ed. 2008, 47, 8897. (f) Nishikata, T.; Abela, A. R.;
2-(4-Methoxyphenyl)pyridine N-Oxide (3ad)6
1H NMR (400 MHz, CDCl3, 293 K): d = 8.35 (d, J = 6.4 Hz,
1 H), 7.80 (d, J = 8.8 Hz, 2 H), 7.43 (d, J = 7.6 Hz, 1 H), 7.32
(t, J = 7.6 Hz, 1 H), 7.21 (t, J = 6.4 Hz, 1 H), 7.02 (d, J = 8.8
Hz, 2 H), 3.87 (s, 3 H). 13C NMR (100 MHz, CDCl3, 293 K):
Synlett 2012, 23, 145–149
© Thieme Stuttgart · New York

LETTER
2-Pyridyl Arylation of Pyridine N-Oxides
149
d = 160.7, 149.2, 140.6, 130.9, 127.1, 126.7, 124.6, 123.9,
H), 2.65 (s, 3 H). 13C NMR (100 MHz, CDCl3, 293 K): d =
197.6, 148.3, 140.5, 137.1, 133.9, 133.0, 129.3, 128.6,
127.4, 126.2, 125.2, 26.7. ESI-MS: m/z [M + 1]+ = 214.1.
113.8, 55.4. Mp 120–122 °C (CH2Cl2).
2-(3-Acetylphenyl)pyridine N-Oxide (3ae)
1H NMR (400 MHz, CDCl3, 293 K): d = 8.35–8.39 (m, 2 H),
8.07 (t, J = 8.0 Hz, 2 H), 7.60 (t, J = 8.0 Hz, 1 H), 7.50 (t,
J = 6.0 Hz, 1 H), 7.36 (t, J = 7.2 Hz, 1 H), 7.26–7.31 (m, 1
ESI-HRMS: m/z [M + H]+ calcd for C13H12NO2 : 214.0868;
+
found: 214.0866.
© Thieme Stuttgart · New York
Synlett 2012, 23, 145–149

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