Palladium catalyzed oxidative Suzuki coupling reaction of indolizine at the 3-position using oxygen gas as the only oxidant

Article abstract of DOI:10.1039/c4ra03799e

Stoichiometric metal oxidant applied in the functionalization of indolizine at the 3-position through C-H activation in a previous report was found to increase the cost of the synthesis and worsen the environmental pollution. In this paper, we developed a Pd(OAc)₂/O₂ catalytic system with or without ligands for an oxidative Suzuki coupling reaction of indolizine at the 3-position through C-H activation. As reported in the literature, some indolizines dimerized when catalyzed by palladium acetate under ligand-free conditions. However, we found that the dimerization of 2,3-unsubstituted indolizines was inhibited by the addition of ligands. Based on this finding, the arylation of these indolizines can be successfully achieved via a Pd(OAc)/O₂ system using picolinic acid as a ligand. 1,2-disubstituted indolizines that do not easily dimerize can react smoothly with arylboronic acids under ligand-free conditions. Furthermore, broad group tolerance was shown in both indolizine and arylboronic acid. Finally, this method has the advantages of mild conditions, a broad array of starting materials and the use of a green oxidant. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4ra03799e

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Palladium catalyzed oxidative Suzuki coupling
reaction of indolizine at the 3-position using
oxygen gas as the only oxidant†
Cite this: RSC Adv., 2014, 4, 24389
ac
a
a
a
b
c
*
*
*
Huayou Hu,
Yong Liu, Juan Xu, Yuhe Kan, Chao Wang and Min Ji
Stoichiometric metal oxidant applied in the functionalization of indolizine at the 3-position through C–H
activation in a previous report was found to increase the cost of the synthesis and worsen the
environmental pollution. In this paper, we developed a Pd(OAc)2/O2 catalytic system with or without
ligands for an oxidative Suzuki coupling reaction of indolizine at the 3-position through C–H activation.
As reported in the literature, some indolizines dimerized when catalyzed by palladium acetate under
ligand-free conditions. However, we found that the dimerization of 2,3-unsubstituted indolizines was
inhibited by the addition of ligands. Based on this finding, the arylation of these indolizines can be
successfully achieved via a Pd(OAc)/O2 system using picolinic acid as a ligand. 1,2-disubstituted
indolizines that do not easily dimerize can react smoothly with arylboronic acids under ligand-free
conditions. Furthermore, broad group tolerance was shown in both indolizine and arylboronic acid.
Finally, this method has the advantages of mild conditions, a broad array of starting materials and the use
of a green oxidant.
Received 25th April 2014
Accepted 23rd May 2014
DOI: 10.1039/c4ra03799e
www.rsc.org/advances
Recently, indolizine derivatives attracted interests of many
chemists because they were an omnipresent component of a
Introduction
wide range of naturally occurring and biologically active mole-
cules.4 In addition, polycyclic indolizine derivatives were
observed to have long-wavelength absorption and uorescence
in the visible light spectrum. Thus, the successful synthesis of
this type of compounds will facilitate the development of novel
class of dyes and biological markers.5
Therefore, functionalization of indolizine through palladium
catalyzed C–H activation drew great interests of synthetic chem-
ists.6 Recently, several groups have reported palladium catalyzed
C–H activation reactions7,8 or traditional Suzuki cross-coupling
reactions9 of indolizine at the 3-position for the synthesis of
3-aryl-indolizines (Scheme 2). However, halogenated aromatic
Carbon–carbon bond formation reactions are among the most
important processes in chemistry because they occur in key
steps to build more complex molecules from simple precursors.
Among these reactions, sp2 C–sp2 C bond formation can be used
for the construction of biaryls, which are important structural
motifs in many organic molecules. Therefore, the development
of methods for new sp2 C–sp2 C bond formation remains an
ongoing challenge for organic chemists. Inspired by the need
for green and sustainable chemistry,1 synthetic chemists are
seeking more efficient ways to construct sp2 C–sp2 C bonds of
biheteroaryl compounds. Compared to the traditional palla-
dium catalyzed cross-coupling reaction, C–H activation reaction
uses a C–H bond instead of a C–X bond, thus making synthetic
routes shorter and more efficient.2 For example, oxidative
Suzuki coupling reaction can synthesize biheteroaryl
compounds directly from C–H bond (Scheme 1).3
aJiangsu Key Laboratory for Chemistry of Low-Dimensional materials, School of
Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300,
P.R. China. E-mail: njuhhy@hotmail.com; Fax: +86 (517)83525100-1
bMOE Engineering Research Center of Biomass Materials, School of Material Science
and Engineering, Southwest University of Science and Technology, Mianyang
621010, P.R. China
cState Key Laboratory of Bioelectronics, School of Biological Science and Medical
Engineering, Southeast University, Nanjing 210096, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03799e
Scheme 1 The comparison of traditional Suzuki coupling and oxida-
tive Suzuki coupling reaction.
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Table 1 Optimization of reaction conditions
Liganda
(%)
Base,
equiv.
Pd(OAc)2
(%)
Yieldd
(%)
Entry
Solventb
1
2
3
4
5
6
7
8
NAe
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
NAe
K2CO3, 3
HOAc, 3
KOAc, 3
Pyridine, 3
NEt3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
KHCO3, 3
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NMP
24
26
54
68
52
44
39
17
17
37
0
27
53
Trace
63
78
67
42
50
86
76
L1 (10)
L2 (10)
L3 (10)
L4 (10)
L5 (10)
L6 (10)
L7 (10)
L3 (10)
L3 (10)
L3 (10)
L3 (10)
L3 (10)
L3 (10)
L3 (30)
L3 (20)
L3 (20)
L3 (20)
L3 (20)
L3 (10)
L3 (10)
9
10
11
12
13
14
15
16
17
18
19
20
21
Scheme 2 The comparison of this paper with previous work.
hydrocarbons7,9 or stoichiometric metal oxidant8 was applied in
these reactions, which increased the cost of synthesis and envi-
ronmental pollution. Compared with halogenated aromatic
hydrocarbons, organic boronic acids were green coupling
partner, whose by-product was environmentally friendly boric
acid.8 Furthermore, oxygen gas is well known for a green oxidant
as it comes from air and its solo by-product is water.10 Naturally,
it's envisioned that the use of oxygen gas as oxidant and aryl-
boronic acid as coupling partner would greatly decrease the cost
of synthesis and environmental pollution.
As a follow-up of our reported methods,6e,11 we herein report
an oxidative Suzuki cross-coupling reaction of indolizines with
arylboronic acids to synthesize 3-aryl-substituted indolizines.
The reaction proceeded selectively at the 3-position through
palladium catalyzed C–H activation under mild conditions.
Notably oxygen gas was used as a solo and terminal oxidant to
make the reaction greener.
DMA
DMF
DMSOc
DMSO
5
a
L1 ¼ 2,20-bipyridine, L2 ¼ 1,10-phenanthroline, L3 ¼ picolinic acid, L4
¼ quinolin-8-ol, L5 ¼ 4,7-diphenyl-1,10-phenanthroline, L6 ¼ pyrazine-
b
2-carboxylic acid, L7 ¼ pyridine-2,6-dicarboxylic acid. 2.0 mL solvent
otherwise indicated, DMSO
¼
dimethyl sulfoxide, DMF
¼
dimethylformamide, DMA
¼
dimethylacetamide, NMP
¼
N-
c
d
e
methylpyrrolidone. 1.0 mL solvent. Isolated yield. NA ¼ no
addition.
When the amount of L3 was increased to 20%, the yield of
desired product 3aa also increased to 78% (entry 16). However,
when the amount of L3 was increased to 30%, the yield of 3aa
decreased to 67% (entry 15).
Dimethyl sulfoxide was the solvent of choice, for other media
provided a lower yield (entry 16–19). The best yield was achieved
under condition A, in which the catalytic loading was decreased
Results and discussion
In the preliminary study, indolizine 1a (0.20 mmol, 1.0 equiv.) to 5% with 10% picolinic acid added as a ligand and the
and phenylboronic acid 2a (2.0 equiv.) were chosen as our concentration of 1a was increased to 0.20 M as well (entry 20:
model system. Pd(OAc)2 was used as catalyst and oxygen gas reaction condition A).
(1 atm) as terminal oxidant (Scheme 2). As results shown in
To explore the substrate scope of arylboronic acids, an array
Table 1, the yield of the proposed product 3aa only reached 24% of arylboronic acids were reacted with indolizine 1a under
under ligand-free conditions (Table 1, entry 1). However, when condition A (Scheme 3). As shown in Scheme 3, either the
L3 (picolinic acid) was added (10%) as a ligand, the yield electronic effects (electron donating or electron withdrawing) or
increased to 68%. Other ligands were found less efficient than the position (ortho, meta or para) of substitution groups had
picolinic acid (entry 2–8). Notably, when potassium bicarbonate little effect on the yield of corresponding indolizine.
(3.0 equiv.) was added as a base, the result was superior to either
As regarding to the substrate scope of indolizine, an array of
other bases (such as potassium carbonate) added or weak acids indolizines with phenylboronic acid 2a as coupling partner were
(such as acetic acid) added (entry 9–14). This result was studied under reaction condition A (Scheme 4). As shown in
consistent with our previous work6e but different from other Scheme 4, several 2-unsubstituted indolizines generated corre-
groups' results.8
sponding 3-phenyl-indolizines smoothly. Notably, indolizines
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Table 2 Re-optimization of reaction conditions of indolizine 1j
Entry
Basea
Pd(OAc)2 (%)
Solventb
Yieldd (%)
1
2
3
4
5
6
7
8
9
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
5
DMSO
DMSO
NMP
DMF
40
59
82
70
68
90
81
91
85
10
10
10
10
10
5
DMA
NMP/DMSO ¼ 9/1
NMP/DMSO ¼ 9/1
NMP/DMSO ¼ 19/1
NMPc
5
5
a
b
c
3.0 equiv. base used. 1.0 mL solvent otherwise indicated. 0.10
mmol DMSO was added. d 1.0 mL solvent, Isolated yield.
Scheme 3 The results of indolizines 1a reacting with different aryl-
boronic acids under reaction condition A.
The scope of arylboronic acid was then checked under
re-optimized reaction condition B. The results were shown
in Scheme 5. As shown in Scheme 5, arylboronic acids gave
corresponding product in moderate to good yield. However,
the arylboronic acids bearing a strong electron donating group
or a group at ortho position did not gave corresponding
indolizines.
To explore the substrate scope of 1,2-disubstitutted
indolizine, several indolizines were reacted with phenyl-
boronic acid 2a under reaction condition B (Scheme 6).
Some indolizines previously reported not reactive8 were
able to react smoothly with phenylboronic acids to give cor-
responding product. However, the indolizines bearing a
phenyl group at 2-position gave desired product in poor yield
under either reaction condition A or reaction condition B
(Scheme 7).
Scheme 4 The results of indolizines reacting with phenylboronic acid
under reaction condition A.
with fused phenyl ring (1h and 1i) reacted with phenylboronic
ꢀ
acid under 120 C to give corresponding product in moderate
yields. However, the 1,2-disubstituted indolizines gave very low
yield or do not have any desired product under reaction condition A.
As reported previously, 1,2-disubstituted indolizines were
much less reactive than those without any substitution group at
2 or 3-position under palladium catalysis.6e,i Therefore, the way
to improve the reactivity of 1,2-disubstituted indolizines under
ligand free condition is being planned. As a result, indolizine 1j
(0.20 mmol, 1.0 equiv.) and phenylboronic acid 2a (2.0 equiv.)
were chosen as model system to re-optimize reaction condi-
tions. The results were shown in Table 2.
As shown in Table 2, when potassium acetate was used as a
base and NMP as a solvent, the yield of 3ja was increased to 82%
(Table 2, entry 3). The highest yield (91%) was achieved when
mixed solvent of NMP and DMSO was used (NMP/DMSO ¼ 19/1,
Table 2, entry 8: reaction condition B).
Scheme 5 The results of indolizine 1g reacting with different aryl-
boronic acids under reaction condition B.
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O2 gas. Then the mixture was stirred at 100 ꢀC for 24 h. The
reaction was monitored by TLC. When the reaction nished, the
mixture was cooled down to room temperature, poured into
water and extracted with CHCl3 (10 mL ꢁ 3). The organic layers
were combined and dried with Na2SO4 and ltered. Then the
mixture was evaporated under vacuum and the crude mixture
was puried by ash chromatography (silica gel, eluted by
petroleum ether–ethyl acetate).
General procedure for arylation of indolizines (1h–1i)
Scheme 6 The results of indolizines reacting with phenylboronic acid
under reaction condition B.
Indolizine (0.20 mmol), arylboronic acid (0.40 mmol), palla-
dium acetate (2.2 mg, 0.01 mmol), picolinic acid (2.5 mg, 0.02
mmol), KHCO3 (60.0 mg, 0.60 mmol) and 1.0 mL DMSO were
added to a reaction tube equipped with a balloon charged with
O2 gas. Then the mixture was stirred at 120 ꢀC for 24 h. The
reaction was monitored by TLC. When the reaction nished, the
mixture was cooled down to room temperature, poured into
water and extracted with CHCl3 (10 mL ꢁ 3). The organic layers
Scheme 7 The indolizines gave desired product in poor yield either were combined and dried with Na2SO4 and ltered. Then the
under reaction condition A or reaction condition B.
mixture was evaporated under vacuum and the crude mixture
was puried by ash chromatography (silica gel, eluted by
petroleum ether–ethyl acetate).
Conclusion
General procedure for arylation of indolizines (1j–1m)
An oxidative Suzuki coupling reaction of 3-unsubstituted
Indolizine (0.20 mmol), arylboronic acid (0.40 mmol), palla-
indolizines with arylboronic acids to produce 3-aryl-indolizines
dium acetate (0.01 mmol), KOAc (0.60 mmol) and 0.95 mL NMP
was reported. The reaction took place at the 3-position through
and 0.050 mL DMSO were added to a reaction tube equipped a
palladium catalyzed C–H activation, followed by a Suzuki type
balloon charged O2 gas. Then the mixture was heated at 100 ꢀC
cross-coupling reaction under mild conditions. Notably, the
under stirred for 4 h. The reaction was monitored by TLC. Aer
reaction used oxygen gas as solo and terminal oxidant and not
the reaction nished, the mixture was cooled down to room
sensitive to moistures. In addition, the use of cheap ligand or
temperature, poured into water and extracted with CHCl3 (10
ligand free condition was another notable point. Further study
mL ꢁ 3). The organic layers were combined and dried with
of replacement of expensive arylboronic acids by cheaper and
Na2SO4 and ltered. Then the mixture was evaporated under
greener partner such as arenes is being thoroughly investigated.
vacuum and the crude mixture was puried by ash chroma-
tography (silica gel, eluted by petroleum ether–ethyl acetate).
Experimental
General methods and materials
Acknowledgements
Unless otherwise noted, all commercial reagents and solvents
We are grateful to funding of NSFC (no.: 21202058), NSF of
were obtained from the commercial provider and used without
Jiangsu Province (no.: BK2011408), CPSF (no.: 2012M511645,
1
further purication. H NMR and 13C NMR spectra were recor-
2013T60483), Education Department of Jiangsu Province (no.
ded on Bruker 400 MHz spectrometers. Chemical shis were
13KJA150001), JSKLCLDM (no.: SKC11093), MOHRSS (no.
reported relative to internal tetramethylsilane (d 0.00 ppm) or
11zs0104), CSCSE (no. 13zs1102) and NBRPC (no.:
CDCl3 (d 7.26 ppm) for 1H NMR and CDCl3 (d 77.0 ppm) for
2011CB933503) for their nancial support.
13C NMR. Flash column chromatography was performed on
300–400 mesh silica gels. Analytical thin layer chromatography
was performed with pre-coated glass baked plates (250 m) and
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