Palladium(II)-catalyzed oxidative ortho -arylation of 2-phenylpyridines

Article abstract of DOI:10.1055/s-0033-1339516

A palladium(II)-catalyzed oxidative ortho-arylation of 2-phenylpyridines via direct C-H activation with aryl boric acids is described. The reaction could be conducted under mild conditions in the presence of Pd(OAc) ₂/Cu(OTf)₂/TBHP system, and a variety of the azacyclic compounds bearing a biaryl unit were obtained with moderate to good yields. Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1339516

LETTER
▌2153
letter
Palladium(II)-Catalyzed Oxidative ortho-Arylation of 2-Phenylpyridines
Oxidative ortho-Arylation of 2-Phenylpyridines
Jie Feng, Guoping Lu, Meifang Lv, Chun Cai*
Chemical Engineering College, Nanjing University of Science & Technology, 200 Xiaolingwei, Nanjing 210094, P. R. of China
Fax +86(25)84315030; E-mail: c.cai@mail.njust.edu.cn
Received: 21.05.2013; Accepted after revision: 11.07.2013
few examples of the direct transition-metal-catalyzed
coupling of boronic acid derivatives with 2-phenylpyri-
dines and its analogues have been published to date. The
reaction was initially achieved in rhodium and ruthenium
Abstract: A palladium(II)-catalyzed oxidative ortho-arylation of
2-phenylpyridines via direct C–H activation with aryl boric acids is
described. The reaction could be conducted under mild conditions
in the presence of Pd(OAc)2/Cu(OTf)2/TBHP system, and a variety
of the azacyclic compounds bearing a biaryl unit were obtained with catalytic systems.5,6 Then, ortho-arylations of 2-phenyl-
moderate to good yields.
pyridines in Pd(OAc)2/Cu(OAc)2/BQ system and ortho-
arylations of 2-phenylpyridines in Pd(OAc)2/AcOH
system were reported, respectively.7 However, the former
system is only suitable for the ortho-arylations of 2-phe-
nylpyridines with potassium organotrifluoroborates in-
stead of boronic acid derivatives, and the reactivity of the
latter was poor and thus much more time was required to
finish this reaction (Scheme 1). More recently, the palla-
dium-catalyzed electrochemical iodination and one-pot
arylation of aryl pyridines have been described.8 Although
many efforts have been made, harsh reaction conditions,
low yields, and the limited scope of substrates are still the
main issues to be addressed in these protocols.
Key words: palladium, C–H functionalization, ortho arylation,
2-phenylpyridines, arylboronic acid
The biaryl structural unit is present in various drugs, na-
ture products, and key intermediates for organic synthe-
sis.1 Traditionally, these compounds are prepared by
transition-metal-catalyzed cross-coupling reactions of
aryl halides and aryl metal compounds.2 More recently,
oxidative metal-catalyzed C–H bond functionalization of
heteroatom-directed aromatics, avoiding the prefunction-
alization of substrates, has become a more significant and
challenging method to synthesize biaryl derivatives.3
To solve these problems, we herein developed a palladi-
um-catalyzed oxidative system with TBHP as oxidant and
Cu(OTf)2 as co-oxidant for the couplings between 2-phe-
nylpyridines and aryl boric acids with improved yields un-
der mild conditions [lower temperature (60 °C), lower
Transition-metal-catalyzed ortho-arylations of hetero-
atom directed aromatics with aryl electrophiles such as aryl
halides, arylboronic acids, and aryl pseudo halides have
been extensively studied.4 Among all these studies, only
Pd(OAc)2 (10 mol%)
Cu(OAc)2 (3 equiv), BQ (2 equiv)
B(OH)2
+
N
120 °C, 24 h
N
15%
Pd(OAc)2(10 mol%)
Me4-piperidoxyl, KF
+
B(OH)2
N
AcOH, 50 °C, 3 d
N
previous work
65%
R2
Pd(OAc)2 (5 mol%)
R1
Cu(OTf)2 (20 mol%),TBHP (2 equiv)
B(OH)2
+
R2
60 °C, 24 h
N
R1
N
this work
up to 94%
Scheme 1 Pd-catalyzed ortho-arylation of 2-phenylpyridines with arylboronic acid
SYNLETT 2013, 24, 2153–2159
Advanced online publication: 21.08.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339516; Art ID: ST-2013-W0473-L
© Georg Thieme Verlag Stuttgart · New York

2154
J. Feng et al.
LETTER
amount of additives], inhibiting the homocoupling of aryl (Table 1, entry 9). It is possible that TBHP works as the
boric acids (Scheme 1).
oxidant in this system, whereas Cu(OTf)2 works as a co-
oxidant which is used to accelerate the reoxidation of pal-
ladium(0).9 After that different protic and aprotic solvents
were tested, which revealed MeCN to be the reaction me-
dium of choice (Table 1, entry 9). The temperature proved
to be significant for the reaction selectivity. It was found
that higher temperature was conducive to the formation of
disubstituted products and the undesired biphenyl prod-
ucts (Table 1, entries 17 and 18), and only 3a was pro-
duced at room temperature with poor yield (13%, Table 1,
entry 16), Therefore, a slight heating (60 °C) was required
to improve the conversion and retain high selectivity.
We commenced our studies by exploring representative
oxidants and co-oxidants in the envisioned palladium-
catalyzed oxidative reaction of boronic acid and 2-phenyl-
pyridine. A variety of oxidants and additives were
screened, and the results are shown in Table 1. The com-
bination of TBHP and Cu(OTf)2 were found to be the best
selection for the reaction (Table 1, entry 6). Only a small
amount of substrates could be converted in the absence of
either of these two reagents (Table 1, entries 1 and 2). In-
terestingly, satisfactory yield was also obtained when the
dosages of Cu(OTf)2 were decreased to 0.2 equivalents
Table 1 Optimization Studies with 2-Phenylpyridine and Phenylboronic Acida
Pd(OAc)2
N
B(OH)2
+
+
N
solvent, oxidant, 24 h
N
H
3a
4a
Entry
Oxidant/co-oxidant
Solvent
Temp (°C)
Yield of 3a/4a (%)b
1c
2
Cu(OTf)2 (2 equiv)
TBHP
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
r.t.
80
110d
6/0
3/0
3
K2S2O8/Cu(OTf)2
K2S2O8/AgOAc
BQ/Cu(OTf)2
52/5
76/7
19/0
95/3
23/0
67/11
91/6
0/0
4
5
6c
7
TBHP/Cu(OTf)2
TBHP/Cu(OAc)2
TBHP/AgOAc
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
TBHP/Cu(OTf)2
8
9
10
11
12
13
14
15
16
17
18
AcOH
MeOH
DMSO
3/0
20/0
56/4
0/0
1,4-dioxane
DMF
45/2
13/0
43/35
32/36
MeCN
MeCN
MeCN
a Reaction conditions: 2-phenylpyridine(0.5 mmol), phenylboronic acid (1.0 mmol), Pd(OAc)2 (0.025 mmol), oxidants (1.0 mmol), co-oxidants
(0.1 mmol), solvent (2 mL).
b Based on GC–MS results.
c Cu(OTf)2 (1 mmol, 2 equiv) was added.
d In a sealed tube.
Synlett 2013, 24, 2153–2159
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative ortho-Arylation of 2-Phenylpyridines
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Table 2 Arylation of 2-Phenylpyridine with Arylboronic Acidsa
R
Pd(OAc)2 (5 mol%)
Cu(OTf)2 (20 mol%)
B(OH)2
+
TBHP (2 equiv)
MeCN, 60 °C, 24 h
N
R
H
N
1
2
3
Entry
Substrate 2
Product 3
Yield (%)b
N
1
2
56c
B(OH)2
2b
3b
N
B(OH)2
78 (84)c
69 (73)c
90 (94)c
77 (83)c
17c
2c
3c
O2N
N
B(OH)2
3
4
5
6
NO2
2d
3d
N
MeO
B(OH)2
2e
MeO
3e
N
Cl
B(OH)2
2f
Cl
3f
N
F3C
B(OH)2
2g
F3C
3g
a Reaction conditions: 2-phenylpyridine(1.0 mmol), aryl boric acids (2.0 mmol), Pd(OAc)2 (0.05 mmol), TBHP (2.0 mmol), Cu(OTf)2 (0.2
mmol), MeCN (2 mL), at 60 °C.
b Isolated yields.
c Based on GC–MS results.
© Georg Thieme Verlag Stuttgart · New York
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2156
J. Feng et al.
LETTER
With the optimized reaction conditions in hand, we firstly phenylboronic acid (Table 2, entries 1 and 2). As for 4-tri-
probed its applicability in the oxidative arylation of 2-phe- fluoromethylphenylboronic acid (2g), the homocoupling
nylpyridine and utilizing different phenylboronic acid de- suppressed the ortho-arylations affording more self-
rivatives. Most reactions proceeded smoothly, providing coupling products (Table 2, entry 6).
their corresponding mono ortho-arylated products in good
Substituted 2-phenylpyridines 1b–m compatibility stud-
yields (Table 2, entries 1–5). Comparatively, arylboronic
ies are then examined. As expected, a series of substitu-
acids bearing electron-donating groups show higher activ-
tions including methyl, methoxy, chloro, and
ity than electron-withdrawing ones. The lower yield ob-
trifluoromethyl on the phenyl ring were compatible under
served in the case of 2-methyl phenylboronic acid
the optimal reaction conditions (Table 3, entries 1–5). But
compared to 2-phenylpyridine, substituent in the phenyl
ring have negative effects on the product yields for both
compared to meta-substituted groups is possibly due to
the steric hindrance of the ortho-methyl group in 2-methyl-
Table 3 Arylation of 2-Phenylpyridine with Arylboronic Acidsa
Pd(OAc)2 (5 mol%)
Cu(OTf)2 (20 mol%)
R1
R2
B(OH)2
+
R2
N
TBHP (2 equiv)
MeCN, 60 °C, 24 h
H
R1
N
1
2
3
Entry
Substrate 1
Product 3
Yield (%)b
75 (81)c
N
1
N
1h
3h
N
2
34 (46)c
N
1i
3i
CF3
N
CF3
3
4
5
41 (54)c
56 (61)c
63 (69)c
N
1j
3j
Cl
N
Cl
N
1k
3k
OMe
N
OMe
N
1l
3l
a Reaction conditions: 2-phenylpridine(0.5 mmol), aryl boric acids (1.0 mmol), Pd(OAc)2 (0.025 mmol), TBHP (1.0 mmol), Cu(OTf)2 (0.1
mmol), MeCN (2 mL), at 60 °C.
b Isolated yields.
c Based on GC–MS results.
Synlett 2013, 24, 2153–2159
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative ortho-Arylation of 2-Phenylpyridines
2157
electron-donating groups and electron-withdrawing whereas three products 2-[(1,1′-biphenyl)-2-yl]-6-phenyl-
groups. It is probably because the substituent may have pyridine (3m, 11% yield), 2-[(1,1′:3′,1′′-terphenyl)-2′-yl]-
negative effects on the coordinating ability of the pyridine 6-phenylpyridine (3n 45% yield), and 2,6-di[(1,1′-biphe-
and the formation of the pyridine palladacycle.
nyl)-2-yl]pyridine (3o, 15% yield, Table 4, entries 2 and
3) were obtained when the dosage of phenylboronic acid
was increased to five equivalents (Table 4, entries 1 and
2). Probably due to the crowed structure, only trace
amount of trisubstituted or tetrasubstituted products could
be observed.
We also attempted to extend our catalytic system to more
phenyl-substituted heterocyclic compounds. 2,6-Diphe-
nylpyridine was firstly tried. 2-[(1,1′-Biphenyl)-2-yl]-6-
phenylpyridine (3m) was obtained with 82% yield using
two equivalents phenylboronic acid (Table 4, entry 1),
Table 4 Arylation of Heterocycle Derivatives with Phenylboronic Acida
Pd(OAc)2 (5 mol%)
Cu(OTf)2 (20 mol%)
Heterocycle
B(OH)2
+
TBHP (2 equiv)
Heterocycle
MeCN, 60 °C, 24 h
H
Entry
Substrate 1
Product 3
Yield (%)b
77 (82)c
N
1
N
1m
3m
N
N
2
40 (45)c
1m
3n
3o
15c
N
80 (84)c
N
3
N
H
H
1p
1p
3p
3q
N
N
N
81 (85)c
N
4
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2153–2159

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LETTER
Table 4 Arylation of Heterocycle Derivatives with Phenylboronic Acida (continued)
Pd(OAc)2 (5 mol%)
Cu(OTf)2 (20 mol%)
Heterocycle
B(OH)2
+
TBHP (2 equiv)
Heterocycle
MeCN, 60 °C, 24 h
H
Entry
Substrate 1
Product 3
Yield (%)b
S
5d
3c
S
1q
3r
3s
N
N
6d
13c
N
H
N
H
1r
a Reaction conditions: 2-phenylpyridine (1.0 mmol), aryl boric acids (2.0 mmol), Pd(OAc)2 (0.05 mmol), TBHP (2.0 mmol), Cu(OTf)2
(0.2 mmol), MeCN (2mL), at 60 °C.
b Isolated yields.
c Based on GC–MS results.
d The reaction was conducted at 110 °C in sealed tube.
After that, 2-phenylindole was tested which showed high acid bonds to the palladium center of the pyridine pallada-
activity under the optimized conditions, affording the di- cycle via ligand exchange. At last, the intermediate II is
substituted product with 84% yield instead of the mono- further converted into the ortho-arylated product. In addi-
substituted one (Table 4, entry 3). Good results were also tion, the released palladium(0) is reoxidized effciently by
obtained with phenylpyrazole in 85% yield (Table 4, entry Cu(OTf)2 and TBHP under mild conditions to regenerate
4). 2-Phenylbenzothiophene and 2-phenylbenzoimidazole palladium(II), which could continue the catalytic cycle.
were also examined; however, unfortunately, only small
amount of substrates converted (Table 4, entries 5 and 6).
In conclusion, we have described a palladium-catalyzed
oxidative ortho-arylation of 2-phenylpyridines with aryl-
Finally, a possible mechanism for the present palladi- boronic acids The reaction could be conducted in relative-
um(II)-catalyzed ortho-arylation of 2-phenylpyridine via ly mild conditions (60 °C, 24 h) in the presence of
C–H bond activation is proposed. As shown in Scheme 2, Pd(OAc)2 (5 mol%), Cu(OTf)2 (20 mol%), and TBHP.
the transformation was initiated from ortho-cyclopallada- Moreover, the reaction system could be extended to some
tion directed by pyridine to generate the cyclopalladium other azacyclics with good results.
species. Subsequently, the aryl group of phenylboronic
TBHP
Cu(OTf)2
Pd(0)
Pd(II)
N
N
H
cyclopalladation
H+
N
N
I
II
Pd
Pd
B(OH)2
AcO
2
Scheme 2 Proposed mechanism for ortho-arylation of 2-phenylpyridines
Synlett 2013, 24, 2153–2159
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative ortho-Arylation of 2-Phenylpyridines
2159
General Procedure for ortho-Arylated 2-Phenylpyridine Deriv-
atives
M. Chem. Rev. 2002, 102, 1359. (e) Bringmann, G.; Price
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A sealed tube was charged with 2-phenylpyridine (0.5 mmol),
phenylboronic acid (1 mmol), Cu(OTf)2 (0.1 mmol), TBHP (1 mmol),
and Pd(OAc)2 (0.025 mmol) in MeCN (5 mL) The mixture was
heated to 60 °C and stirred violently at this temperature for 24 h. Af-
ter being cooled to r.t., the mixture was filtered. The filtrate was
evaporated under vacuum. Subsequently, the residue was purified
by chromatography (silica gel; n-hexane–EtOAc, 10:1).
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2-[5-Chloro-(1,1′-biphenyl)-2-yl]pyridine (3k)
1H NMR (500 MHz, CDCl3): δ = 8.65 (d, J = 4.8 Hz, 1 H), 7.67 (d,
J = 8.9 Hz, 1 H), 7.46 (dd, J = 6.2, 2.1 Hz, 2 H), 7.40 (d, J = 1.7 Hz,
1 H), 7.27–7.25 (m, 2 H), 7.19–7.08 (m, 4 H), 6.86 (d, J = 7.9 Hz, 1
H).13C NMR (126 MHz, CDCl3): δ = 158.34, 149.72, 142.44,
140.30, 138.10, 135.56, 134.57, 132.12, 130.55, 129.74, 128.45,
127.89, 127.47, 125.53, 121.83. MS (EI+): m/z = 265 [M + H]+.
2-[(1,1′-Biphenyl)-2-yl]-6-phenylpyridine (3m)
1H NMR (500 MHz, CDCl3): δ = 7.84–7.80 (m, 3 H), 7.57 (t, J = 3.9
Hz, 2 H), 7.53–7.48 (m, 3 H), 7.45–7.39 (m, 3 H), 7.31–7.29 (m, 1
H), 7.28–7.23 (m, 4 H), 7.05–7.00 (m, 1 H). 13C NMR (126 MHz,
CDCl3): δ = 157.95, 155.79, 140.90, 139.98, 138.67, 138.56,
135.26, 129.72, 129.68, 128.76, 127.76, 127.55, 127.06, 126.59,
126.02, 125.58, 122.28, 117.06. MS (EI+): m/z = 308 [M + H]+.
2-[(1,1′:3′,1′′-Terphenyl)-2′-yl]-6-phenylpyridine (3n)
1H NMR (500 MHz, CDCl3): δ = 7.93 (dd, J = 9.7, 8.3 Hz, 5 H),
7.60–7.53 (m, 6 H), 7.50–7.45 (m, 4 H), 7.39 (d, J = 2.5 Hz, 2 H),
7.36 (t, J = 7.2 Hz, 2 H), 7.15 (d, J = 2.5 Hz, 2 H). 13C NMR (126
MHz, CDCl3): δ = 155.95, 150.77, 147.31, 135.04, 127.73, 127.71,
127.01, 126.36, 125.47, 124.78, 123.20, 121.52, 119.70, 113.82,
110.93, 104.30. MS (EI+): m/z = 384 [M + H]+.
2-[(1,1′:3′,1′′-Terphenyl)-2′-yl]-1H-indole (3p)
1H NMR (500 MHz, CDCl3): δ = 8.11 (s, 1 H), 7.52 (dd, J = 6.6, 3.0
Hz, 2 H), 7.49–7.42 (m, 2 H), 7.35 (d, J = 8.1 Hz, 1 H), 7.18 (m, 8
H), 7.02 (d, J = 8.1 Hz, 1 H), 6.95 (t, J = 7.6 Hz, 1 H), 6.82 (t,
J = 7.4 Hz, 1 H), 6.74 (d, J = 8.2 Hz, 1 H). 13C NMR (126 MHz,
CDCl3): δ = 158.27, 136.17, 134.55, 128.75, 127.29, 126.77,
126.77, 126.52, 125.80, 124.43, 121.41, 120.60, 119.04, 118.23,
111.30, 109.71. MS (EI+): m/z = 346 [M + H]+.
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Commun. 2011, 47, 1497.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
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