Direct C-H bond arylation of (Hetero)arenes with Aryl and heteroarylboronic acids

Article abstract of DOI:10.1055/s-0029-1219234

A novel microwave-promoted protocol mediated by Mn(OAc)³ was developed for the single-electron-transfer oxidative direct C-H bond arylation of (hetero)arenes with aryl and hetero-arylboronic acids. Various electron-deactivated and electron-rich heteroarenes and benzene underwent successfully the direct arylation in this general process. The use of slight excess of heteroarene or benzene in this method was found to be sufficient. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0029-1219234

1
166
PAPER
Direct C–H Bond Arylation of (Hetero)arenes with Aryl and
Heteroarylboronic Acids
D
S
irec
t
C–HBo
a
ndArylati
n
on of (Hetero
k
)arenes ar K. Guchhait,* Maneesh Kashyap, Shuddham Saraf
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER),
Sector 67, S. A. S. Nagar, Punjab 160 062, India
Fax +91(172)2214692; E-mail: skguchhait@niper.ac.in
Received 9 November 2009; revised 7 December 2009
Although the direct arylation of sufficiently nucleophilic
electron-rich heteroarenes have been well documented, p-
electron-deficient nitrogen heterocycles remain still as
Abstract: A novel microwave-promoted protocol mediated by
Mn(OAc) was developed for the single-electron-transfer oxidative
3
direct C–H bond arylation of (hetero)arenes with aryl and hetero-
arylboronic acids. Various electron-deactivated and electron-rich
8
challenging substrates. In addition, many of the direct
heteroarenes and benzene underwent successfully the direct aryla- arylation processes suffer from their inflexibility for ver-
tion in this general process. The use of slight excess of heteroarene satile heteroarenes, complication in regioselectivity, and
or benzene in this method was found to be sufficient.
susceptibility towards the formation of unwanted homo-
Key words: cross-coupling, direct arylation, biaryls, single- coupling and polyarylation products.
electron transfer, microwave
Herein, we report a novel and general microwave-promot-
ed protocol of SET oxidative direct C–H bond arylation of
structurally versatile, electron-deactivated and -rich het-
Biaryls are common scaffolds in biologically active com-
eroarenes, and benzene (Scheme 1).
pounds, natural products, organic materials, dyes, and
1
ligands for metal catalysts. The most widely used proto-
Mn(OAc)3
Ar/
Ar/
HetAr
Ar/
Ar/
HetAr
C
C
HetAr
cols to access biaryls are the palladium-catalyzed cross-
C H
+
(HO) B C
2
HetAr
EtOH, μW
2
coupling reactions (such as Suzuki coupling) of aryl
170 °C
7
–10 min
(
pseudo)halides with aryl organometallics. These reac-
tions are dependent on preactivation of arenes with (pseu-
do)halides and electropositive groups. Thus, they require
several extra synthetic steps and generate waste. In the
perspective of development of sustainable chemistry, an
increased attention is being focused on the discovery of
direct C–H bond arylation processes that avoid preactiva-
Scheme 1 SET oxidative direct C–H bond arylation
We envisaged that the SET oxidative radical cross-cou-
pling might be a favorable approach to achieve a general
process of direct arylation of electron-deactivated, -unac-
tivated, and -activated (hetero)arenes. In SET approach,
3
,4
tion of either arene coupling partner or both. Recently,
transition-metal-catalyzed methods for the direct aryla-
9
we speculated to use microwave irradiation that could
bring sufficient activation of electron-deficient nitrogen
heterocycles, which are troublesome coupling partners.
Arylboronic acids as radical precursors were chosen, as
they are thermally stable. For initial studies, three model
5
a
tion of (hetero)arenes with aryltin reagents, arylboronic
5
b–e
5f
acids,
aryltrifluoroborate salts and diaryliodoni-
5
g
um(III) salts have been reported.
An approach of single-electron-transfer (SET) oxidative substrates, pyridine, thiophene, and benzene in respective
radical coupling for the direct arylation is also known.6 categories of electron-deficient, -rich and -neutral het-
1
0
The severe drawback of this approach is that these reac- eroarenes, and Mn(OAc) as single-electron oxidant
3
tions are mostly limited to electron-rich heteroarenes and were used. The reaction of phenylboronic acid was carried
require heteroarenes as solvent in large excess. Recently, out with pyridine (10 equiv) and Mn(OAc) ·2H O (3
3
2
as a strategic improvement, a radical chain reaction for the equiv) in EtOH under microwave irradiation (Biotage Ini-
TM
direct arylation of phenols and anilines with arenediazoni- tiator EXP) at 170 °C for 10 minutes. To our delight, 2-
um salts as radical precursors has been explored.7a Re- phenylpyridine was formed in 78% yield and the compet-
markably, Kita et al. demonstrated a direct arylation ing homocoupling reaction product, biphenyl was isolated
process that involved the direct generation of an aromatic in 4% yield only. In contrast, the same reaction performed
7
b
cation radical from an arene. This method is highly se- with a conventional method of heating at 85 °C required
lective, but allows only the generation of a-naphthalene nine hours to complete and provided 2-phenylpyridine
radicals with mesitylene as ultimate nucleophilic coupling and biphenyl in 28% and 26% yield, respectively. To ver-
partner.
ify the flexibility of this microwave-promoted protocol
for electron-rich and unactivated (hetero)arenes, two sep-
arate reactions of thiophene and benzene with phenylbo-
ronic acid were then performed. Interestingly, the
reactions afforded 2-phenylthiophene and biphenyl in
SYNTHESIS 2010, No. 7, pp 1166–1170
Advanced online publication: 20.01.2010
DOI: 10.1055/s-0029-1219234; Art ID: Z24009SS
x
x
.
x
x
.2
0
1
0
8
2% and 86% yield, respectively. We anticipated that
©
Georg Thieme Verlag Stuttgart · New York

PAPER
Direct C–H Bond Arylation of (Hetero)arenes
1167
arylboronic acid in the presence of a base, due to its the C-H bonds ortho to the ring heteroatoms, due to their
known increased aryl nucleophilic character, could under- relatively higher acidity, exhibited higher reactivity.
go easier single-electron oxidation to form an aryl radical
Table 1 Direct Arylation of Versatile Electron-Activated
and -Deactivated Heteroarenes and Benzene
and might enhance the efficiency of the process. Unfortu-
nately, the addition of KF (2 equiv) in the reaction of py-
ridine with phenylboronic acid lowered the yield of the
cross-coupling product 2-phenylpyridine (from 78 to
Mn(OAc)3
Ar/
HetAr
Ar/
HetAr
C
H
+
(HO)2B
C
EtOH, μW,
1
7
70 °C,
–10 min
4
6%), favoring the homocoupling of phenylboronic acid
(
increasing the yield of biphenyl from 4% to 18%). This
Product
Yield (%)a
82
plausibly implies that aryl radicals generated by single Entry (Hetero)arene
electron oxidation of boronic acids in this process are suf-
ficiently reactive to undergo the direct C–H arylation with
heteroarenes, and the reaction progress of the cross-cou-
1
S
S
H
pling as well as the yields are dependent on the activation
of heteroarenes, as speculated. On comparing the conven-
tional heating with microwave irradiation, it can be seen
that the conventional heating caused an enhanced homo-
coupling reaction of phenylboronic acid. These results in-
dicated that the microwave irradiation was essential to
influence the required activation of arenes, especially p-
electron-deficient nitrogen heterocycles for promoting the
cross-coupling. Several experimentations for optimiza-
tion of the protocol were then carried out for further pos-
sible enhancement in the yield of the reaction. Mn(OAc)3
was found to be the superior single-electron oxidant to the
2
3
4
70
65
54
O
O
H
H
N
N
Me
Me
N
N
S
S
H
O
S
H
H
1
1
oxidants screened; Ce(IV) [cerium(IV) ammonium ni-
trate], Cu(II)Cl/OAc, Fe(III)Cl, Ag(I)BF , and V(V)
4
(
V O ). Mn(OAc) (50 mol%) and Co(OAc) (30 mol%)
5
6
7
8
62
52
86
78
2
5
3
2
O
S
as co-oxidant under oxygen atmosphere, which are known
to act as an efficient catalyst cascades for SET oxida-
H
H
1
0d,e
tion,
resulted in a complex mixture of products besides
N
the desired biaryl. Use of AcOH as solvent, which is com-
monly used in SET oxidations mediated by Mn(OAc)3,
caused usual protodeboronation of arylboronic acids as
competing reaction producing parent arenes. EtOH was
the most effective solvent among the tested: MeCN,
N
DMF, DMSO, THF, and H O. The investigations in
2
which the ratio between the reactants and reagents have
been changed revealed that as few as 10 equivalents of the
arenes and 3 equivalents of Mn(OAc) with respect to bo-
3
ronic acids were necessary to afford good yields of prod-
ucts. The experimental procedure is simple and
straightforward.
N
N
CN
CN
Electron-deficient and -rich heteroarenes, and electron-
unactivated arene underwent successfully the direct aryla-
tion in this general process of SET oxidative approach
9
46
N
(
Table 1). Pyridine, cyanopyridine, chloropyrimidine,
N
H
isoquinoline, and quinoline (Table 1, entries 8–12) de-
spite their strong electron-deficient nature underwent the
direct arylation well. As these heterocycles are privileged 10
scaffolds in medicinally relevant compounds, the estab-
lishment of these heterocycles as viable substrates in di-
rect arylation is highly significant. The process was found
to be flexible as well for the direct arylation of versatile, 11
N
N
48
50
Cl
N
Cl
N
H
N
N
electron-rich heteroarenes and an electron-unactivated
arene (Table 1, entries 1–7). Remarkably, heteroarenes
H
Ph
(
except quinoline, Table 1, entry 12) in this process
formed the regioselective coupled products indicating that
Synthesis 2010, No. 7, 1166–1170 © Thieme Stuttgart · New York

1
168
S. K. Guchhait et al.
PAPER
Table 1 Direct Arylation of Versatile Electron-Activated
and -Deactivated Heteroarenes and Benzene (continued)
small amounts of homocoupling products of arylboronic
acids was observed, the homocoupling products of het-
eroarenes did not form. This implies that arylboronic ac-
ids undergo the single electron oxidative decomposition
Mn(OAc)3
Ar/
HetAr
Ar/
HetAr
C
H
+
(HO)2B
C
EtOH, μW,
1
7
70 °C,
–10 min
by Mn(OAc) to generate the aryl radicals, but heteroar-
3
enes do not undergo the oxidation. Aryl radicals produced
from arylboronic acids, almost without experiencing the
homocoupling, undergo the cross-coupling with het-
eroarenes. Conventional heating in place of microwave ir-
radiation did not provide sufficient activation for
heteroarenes.
Yield (%)a
Entry (Hetero)arene
Product
H
1
2
38
N
Ph
N
H
In conclusion, we have developed a novel and synthetical-
ly useful microwave-promoted single electron transfer
oxidative direct arylation of heteroarenes and benzene.
The synergistic effect of microwave-promoted activation
of heteroarenes and the single electron transfer oxidative
approach has enabled the establishment of the general
process for the direct arylation of versatile, electron-defi-
cient and -rich heteroarenes and benzene. This micro-
wave-assisted SET oxidative cross-coupling represents an
alternative to the known transition-metal-catalyzed (Pd,
Ru, and Rh) and t-BuOK-promoted approaches in the area
of C–H bond arylation of p-electron-deficient heteroare-
nes.
Ph
3
0
N
a
Isolated yield.
7
The reported oxidative radical methods and other
3
h,8b,c
approaches
required the use of a large excess of arene
coupling partner as solvent. Interestingly, this protocol
needed only 10 equivalents of the heteroarene or benzene.
Various boronic acids were found to be feasible radical
precursors to undergo the cross-coupling with arenes
(
Table 2). This method tolerates halogens on aromatic
rings and performance of this reaction is compatible with The commercially available starting materials and solvents were
used as received without further purification. Melting points were
water and air.
determined in a capillary tube and are not corrected. The IR spectra
were taken on Perkin-Elmer spectrometer as KBr pellets for solid
Table 2 Flexibility of Boronic Acids as Radical Precursors in
Direct Arylation
1
13
and neat for liquid samples. The H and C NMR spectra were re-
corded on Bruker Avance III 400 and DPX 300 spectrometers using
TMS as internal standard. Mass spectra were analyzed on Finnigan
MAT LCQ (APCI) and Bruker Daltonics Ultraflex (MALDI) spec-
trometers.
Mn(OAc)3 (3 equiv)
(
HO)2B
R
EtOH, μW
R
O
O
1
70 °C, 7–10 min
Entry
Boronic acid
Yield (%)a
51
2
-Phenylpyridine; Typical Procedure (Table 1, Entry 8)
To a mixture of phenylboronic acid (0.122 g, 1 mmol) and
Mn(OAc) ·2H O (0.804 g, 3 mmol) in EtOH (1 mL) taken in a mi-
crowave vial was added pyridine (0.806 g, 10 mmol). The vial was
B(OH)2
3
2
1
2
3
4
F
sealed and the mixture was irradiated in a microwave synthesizer
TM
(
Biotage Initiator EXP) at 170 °C for 10 min. The resultant mix-
B(OH)2
B(OH)2
53
65
45
43
ture was, without workup, directly subjected as dry pack to the col-
umn chromatographic purification over silica gel (mesh size: 60–
1
Cl
Br
20) and eluted with EtOAc–hexane (1:20) to afford 2-phenylpyri-
dine (0.121 g, 78%) as a colorless oil.
–
1
IR (neat):1722, 1587, 1450, 1261, 1074 cm .
1
H NMR (400 MHz, CDCl ): d = 7.23–7.26 (m, 1 H), 7.41–7.51 (m,
3
B(OH)2
3
H), 7.73–7.79 (m, 2 H), 7.98–8.01 (m, 2 H), 8.71 (d, J = 5.0 Hz, 1
H).
MeO
N
1
3
C NMR (100 MHz, CDCl ): d = 120.6, 122.1, 126.9 (2 CH), 128.7
3
B(OH)2
(
2 CH), 128.9, 136.7, 139.3, 149.6, 157.5.
5
MS (APCI): m/z = 156 (M + 1).
All reactions (Tables 1 and 2) were carried out following this pro-
cedure. The spectroscopic data of all known compounds were iden-
tical to those reported in the literature. The data of new
compounds synthesized are given below.
a
Isolated yield.
1
2
Addition of TEMPO (20 mol%) as a radical scavenger to
the separate reactions of thiophene and furan with phenyl-
boronic acid decreased abruptly the reaction conversions.
This indicates that the reaction proceeded through a radi-
cal pathway. While in some cases the formation of only
4
-Cyano-2-phenypyridine (Table 1, Entry 9)
Yellowish viscous liquid.
–
1
IR (neat): 2926, 2851, 1556, 1435, 1260, 1025, 800, 759 cm .
Synthesis 2010, No. 7, 1166–1170 © Thieme Stuttgart · New York

PAPER
Direct C–H Bond Arylation of (Hetero)arenes
1169
1
H NMR (400 MHz, CDCl ): d = 7.47 (dd, J = 4.9, 1.4 Hz, 1 H),
Organic Synthesis via Boranes, Vol. 3; Aldrich Chemical
Company, Inc.: Milwaukee / Wisconsin, 2003.
3
7
.51–7.55 (m, 3 H), 7.95 (t, J = 1.1 Hz, 1 H), 8.01 (dd, J = 5.9, 1.9
Hz, 2 H), 8.87 (dd, J = 4.9, 0.9 Hz, 1 H).
(3) For intermolecular C–H arylation, see: (a) Alberico, D.;
Scott, M. E.; Lautens, M. Chem. Rev. 2007, 107, 174.
1
3
C NMR (100 MHz, CDCl ): d = 116.7, 121.2, 122.0, 123.1, 126.9
3
(
b) Seregin, I. V.; Gevorgyan, V. Chem. Soc. Rev. 2007, 36,
(
2 CH), 129.1 (2 CH), 130.2, 137.3, 150.6, 158.7.
1173. (c) Phipps, R. J.; Gaunt, M. J. Science 2009, 323,
MS (APCI): m/z = 181 (M + 1).
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Lett. 2009, 11, 1733. (e) Liégault, B.; Lapointe, D.; Caron,
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2
-Chloro 4-phenylpyrimidine (Table 1, Entry 10)
White solid; mp 84–86 °C.
(
f) Ackermann, L.; Mulzer, M. Org. Lett. 2008, 10, 5043.
–
1
(g) Brasche, G.; Garca-Fortanet, J.; Buchwald, S. L. Org.
Lett. 2008, 10, 2207. (h) Qin, C.; Lu, W. J. Org. Chem.
IR (KBr): 2923, 2853, 1570, 1421, 1350, 1181, 750 cm .
1
H NMR (400 MHz, CDCl ): d = 7.50–7.57 (m, 3 H), 7.66 (d,
3
2008, 73, 7424. (i) Yanagisawa, S.; Sudo, T.; Noyori, R.;
J = 5.2 Hz, 1 H), 8.10 (dd, J = 8.2, 1.6 Hz, 2 H), 8.65 (d, J = 5.2 Hz,
Itami, K. Tetrahedron 2008, 64, 6073. (j) Stuart, D. R.;
Fagnou, K. Science 2007, 316, 1172.
1
H).
1
3
C NMR (100 MHz, CDCl ): d = 115.1, 127.4 (2 CH), 129.1 (2
(4) For intramolecular C–H arylation, see: (a) Shimizu, M.;
Mochida, K.; Hiyama, T. Angew. Chem. Int. Ed. 2008, 47,
9760. (b) Hostyn, S.; Baelen, G. V.; Lemière, G. L. F.; Maes,
B. U. W. Adv. Synth. Catal. 2008, 350, 2653. (c) Bedford,
R. B.; Betham, M.; Charmant, J. P. H.; Weeks, A. L.
Tetrahedron 2008, 64, 6038.
5) (a) Kawai, H.; Kobayashi, Y.; Oi, S.; Inoue, Y. Chem.
Commun. 2008, 1464. (b) Wen, J.; Zhang, J.; Chen, S.; Li,
J.; Yu, X. Angew. Chem. Int. Ed. 2008, 47, 8897. (c) Yang,
S.; Sun, C.; Fang, Z.; Li, B.; Li, Y.; Shi, Z. Angew. Chem.
Int. Ed. 2008, 47, 1473. (d) Ban, I.; Sudo, T.; Taniguchi, T.;
Itami, K. Org. Lett. 2008, 10, 3607. (e) Shi, Z.; Li, B.; Wan,
X.; Cheng, J.; Fang, Z.; Cao, B.; Qin, C.; Wang, Y. Angew.
Chem. Int. Ed. 2007, 46, 5554. (f) Zhao, J.; Zhang, Y.;
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Kalyani, D.; Krause, A.; Sanford, M. S. J. Am. Chem. Soc.
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3
CH), 131.9, 135.0, 159.8, 161.8, 167.2.
3
5
37
MS (APCI): m/z (%) = 191 [M ( Cl) + 1, 100], 193 [M ( Cl) + 1,
3
2].
2
-(4-Fluorophenyl)furan (Table 2, Entry 1)
(
White solid; mp 86–88 °C.
–
1
IR (KBr): 2928, 2857, 1634, 1404, 1012,802, 734 cm .
1
H NMR (400 MHz, CDCl ): d = 6.48 (dd, J = 3.4, 1.8 Hz, 1 H),
3
6
0
.59 (d, J = 3.4, 0.4 Hz, 1 H), 7.07–7.12 (m, 2 H), 7.47 (dd, J = 1.8,
.4 Hz, 1 H), 7.65–7.67 (m, 2 H).
1
3
C NMR (100 MHz, CDCl ): d = 104.6, 111.6, 115.6 (d, J = 11 Hz,
3
1
1
CH), 115.8 (d, J = 11 Hz, 1 CH) 125.4, 125.5, 127.2, 142.0, 153.0,
62.1 (d, J = 245 Hz, 1 C).
MS (APCI): m/z = 163 (M + 1).
(
6) (a) Demir, A. S.; Reis, Ö.; Emrullahoglu, M. J. Org. Chem.
2003, 68, 578. (b) Demir, A. S.; Findik, H. Tetrahedron
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Iwata, M.; Kita, Y. Angew. Chem. Int. Ed. 2008, 47, 1301.
2
-(4-Bromophenyl)furan (Table 2, Entry 3)
White solid; mp 80–82 °C.
–
1
IR (KBr): 2925, 2859, 1634, 1558, 1471, 1012, 734, 685 cm .
1
H NMR (400 MHz, CDCl ): d = 6.49 (dd, J = 3.4, 1.8 Hz, 1 H),
3
6
.66 (dd, J = 3.4, 0.6 Hz, 1 H), 7.48 (dd, J = 1.8, 0.6 Hz, 1 H), 7.51
(
(
d, J = 8.4 Hz, 2 H), 7.55 (d, J = 8.4 Hz, 2 H).
1
3
C NMR (100 MHz, CDCl ): d = 105.5, 111.8, 121.0, 125.5 (2
3
CH), 129.7, 131.1 (2 CH), 142.3, 152.9.
(8) (a) Bermen, A. M.; Lewis, J. C.; Bergman, R. G.; Ellman, J.
A. J. Am. Chem. Soc. 2008, 130, 14926. (b) Yanagisawa,
S.; Ueda, K.; Taniguchi, T.; Itami, K. Org. Lett. 2008, 10,
8
0
82
MS (APCI): m/z (%) = 223 [M ( Br) + 1, 100], 225 [M ( Br) + 1,
6].
9
4673. (c) Campeau, L.; Schipper, D. J.; Fagnou, K. J. Am.
Chem. Soc. 2008, 130, 3266. (d) Larivée, A.; Mousseau, J.
J.; Charette, A. B. J. Am. Chem. Soc. 2008, 130, 52.
Acknowledgment
(
e) Leclerc, J.; Fagnou, K. Angew. Chem. Int. Ed. 2006, 45,
We gratefully acknowledge financial support from CSIR, New
Delhi for this investigation. M.K. is also thankful to CSIR for a fel-
lowship.
7781. (f) Godula, K.; Sezen, B.; Sames, D. J. Am. Chem.
Soc. 2005, 127, 3648.
(9) For microwave-promoted C–H activation in transition-
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Berman, A. M.; Bergman, R. G.; Ellman, J. A. J. Am. Chem.
Soc. 2008, 130, 2493. (b) Franck, P.; Hostyn, S.; Halász-
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