Rapid Construction of Fused Heteropolycyclic Aromatics via Palladium-Catalyzed Domino Arylations of Imidazopyridine Derivatives

Article abstract of DOI:10.1021/acs.orglett.9b00761

By using diaryliodonium salts, a novel approach of a palladium-catalyzed cascade of diarylation/intramolecular dehydrogenative coupling reaction has been developed in the synthesis of phenanthro-imidazopyridine fused heteropolycycles. The method can toler

Full text of DOI:10.1021/acs.orglett.9b00761

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rapid Construction of Fused Heteropolycyclic Aromatics via
Palladium-Catalyzed Domino Arylations of Imidazopyridine
Derivatives
Chenwei Xue,† Jianwei Han,*,†,‡ Min Zhao,† and Limin Wang*,†
†Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, Institute of Fine Chemicals,
School of Chemistry and Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai
200237, China
‡Shanghai−Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, The Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: By using diaryliodonium salts, a novel approach of a palladium-catalyzed cascade of diarylation/intramolecular
dehydrogenative coupling reaction has been developed in the synthesis of phenanthro-imidazopyridine fused heteropolycycles.
The method can tolerate various substrates, and the target products were rapidly constructed in one pot. Furthermore, studies
of the detailed reaction mechanism provide an insight into the C−H functionalization of 2-aryl-imidazopyridine derivatives and
the C−C bond formations in the presence of palladium catalyst.
used heteropolycyclic aromatics continue to stimulate
synthetic research in modern chemistry, as both syntheti-
synthesis of heterocyclic compounds.8 Recently, hypervalent
aryliodine reagents in the presence of palladium catalysts are
tremendously attractive to discover novel reaction pathways for
multiple C−H functionalization.9 In this regard, the Jana group
reported an elegant work in which the palladium-catalyzed
cascade of C-arylation/1,2-aryl shift/cross-dehydrogenative
coupling of 2-arylindoles with diaryliodonium salts afforded
dibenzo-fused carbazoles.10 The research group of Park and
Hong documented diarylation of 2-phenylquinazolin-4(3H)-
ones through a cascade of N-arylation/annulative π-extension
in a straightforward manner.11 In continuation of our interest
in exploring the activation of the vicinal C−H bond of
diaryliodonium salts for the efficient synthesis of polyaromatic
frameworks,12 in this context, we described herein a feasible
procedure of the palladium-catalyzed cascade of the diary-
lation/intramolecular cross-dehydrogenative coupling reaction
of 2-aryl imidazopyridine derivatives using diaryliodonium
salts. As a result, a family of phenanthro-imidazopyridine fused
heteropolycycles were rapidly constructed in one pot (Scheme
1II).
F
cally challenging targets and compounds that possess unique
optoelectrochemical properties.1 Imidazo[1,2-α]pyridine fused
π-extended heterocycles represent a prominent framework due
to their prevalence in bioactive substances and functional
materials.2 The development of an efficient synthetic method is
therefore highly desirable. As a consequence, synthetic efforts
were centered around direct functionalization of 2-aryl-
imidazo[1,2-α]pyridine derivatives.3 Among them, extensive
research focused on the introduction of functional groups on
the C3-position of the imidazo[1,2-α]pyridine motif (Scheme
1Ia),4 Of particular note, rhodium-catalyzed imidazopyridine-
directed ortho-amidation and ortho-cyanation were recently
reported, respectively (Scheme 1Ib).5 Nevertheless, the
catalytic domino/cascade reaction via multiple C−H bonds
activation, in which several new bonds were formed
sequentially in a single operation for the rapid construction
of polyaromatic hydrocarbons, is quite promising. For example,
annulations of 2-aryl-imidazo[1,2-α]pyridine with alkynes, o-
dihaloarenes, or α-diazo esters in the formation of two
carbon−carbon bonds were reported in the presence of
transition metal catalysts (Scheme 1Ic).6 On the other hand,
diaryliodonium salts were frequently employed in arylations.7
Seminal contributions from Chen and Mo groups showcased
the ability of diaryliodonium salts in cascade annulation for the
To begin our studies, we chose 2-phenylimidazo[1,2-
α]pyridine (1a) and diphenyliodonium triflate (2a) as the
Received: February 28, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00761
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Organic Letters
Letter
Scheme 1. Background of Functionalization of 2-
Phenylimidazopyridine and This Work
Table 1. Screening of Reaction Conditions for the Domino
Arylation of Imidazopyridine
a
b
entry
solvent
base
X
yield of 3aa (%) yield of 4 (%)
1
2
3
4
5
6
7
8
DMF
DMF
DCE
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
BF4
0
0
0
0
91
0
60
0
0
0
0
0
0
0
0
0
0
0
0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOAc
NaOAc
K2HPO4
K3PO4
EtOH
n-BuOH
AcOH
TFA
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
AcOH
0
trace
65
0
70
40
9
c
10
11
12
13
14
15
83 (80)
67
32
42
58
0
K2HPO4
K2HPO4
K2HPO4
K2HPO4
PF6
Br
OTf
d
16
67
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), base (2 equiv),
and Pd(OAc)2 as the catalyst (0.02 mmol, 10 mol %) in solvent (4
b
c
mL) at 100 °C for 24 h. Isolated yield. A 1 mmol scale reaction was
d
model substrates. Inspired by our previous work,12a the
reaction was performed in the presence of 3.0 equiv of 2a
and 10 mol % Pd(OAc)2 in DMF as the solvent at the
temperature of 100 °C (Table 1). However, no desired
product was observed with a total recovery of 1a. Upon the
addition of 2.0 equiv of bases of K2CO3, a monoarylated
product 4 was obtained in an excellent yield of 91% (Table 1,
entry 2).13 Then, we started to change the solvent; neither
product 3aa nor compound 4 was observed in DCE, while
carried out to afford 3aa in 80% yield. A 2.5 equiv portion of 2a was
used.
As shown in Table 2, we were pleased to find that a range of
symmetrical diaryliodonium triflates regardless of the elec-
tronic nature of the substituents could be successfully
employed in the reaction. The desired products 3ba−3ha
were furnished in yields of 57−78% (Table 2, entries 1−7).
Moreover, unsymmetrical diaryliodonium triflates bearing a
steric mesityl moiety were adopted to this protocol; as
expected, the aryl (Ar1) group with less steric hindrance was
selectively transferred to the target products while the formed
product containing the mesityl unit was not observed. Of note,
when one of the two aryl groups of diaryliodonium triflate is
3,4-dimethylaryl as the Ar1 unit, two possible products of 3ka
and 3ka′ would be formed as the methyl groups on the
different positions of the benzo-fused ring (Table 2, below).
However, it was found that only 3ka was obtained in 43%
yield, which indicated that the intramolecular dehydrogenative
coupling reaction favored the pathway in a less hindered
manner (Table 2, entry 14). Furthermore, 4-methoxyphenyl−
phenyliodonium triflate was attempted in the reaction and
underwent the exclusive transfer of the phenyl group to the
corresponding products in 48% yield (Table 2, entry 15).
Moreover, a single crystal of 3ca was obtained for X-ray
crystallographic analysis; the structure of 3ca was verified
unambiguously as presented in Table 2. The structure drawing
demonstrated that three carbon (sp2)−carbon (sp2) bonds are
formed in one pot, which suggested a diarylation with two
equivalent diaryliodonium salts and an intramolecular
aromatization by the formation of a benzene ring.
n
product 4 was obtained in 60% yield in EtOH. BuOH can
afford a trace amount of inseparable compounds which is
supposed to be 3aa later by a thin-layer chromatography
experiment (Table 1, entries 3−5). To our delight, a
polyarylated product 3aa was isolated in 65% yield after 24
h when AcOH was employed as the solvent (Table 1, entry 6).
While the solvent was changed to trifluoroacetic acid (TFA)
with a strong acidity (pKa of TFA is 0.23, compared to a pKa of
4.76 for AcOH), the target compound 3aa was not found
(Table 1, entry 7). Subsequently, various bases including
KOAc, NaOAc, K2HPO4, and K3PO4 were screened (Table 1,
entries 8, 9, 11, and 12). K2HPO4 was proven to be the best
choice, affording 3aa in an excellent yield of 83%. Notably,
when the reaction was conducted without base, the yield
decreased to 32% in AcOH as the solvent (Table 1, entry 10),
which suggested that AcOH played an important role in the
reaction (Table 1, entry 1 versus entry 12).14 We also
examined the counteranion effect; the diaryliodonium salt with
an anion of OTf gave the best yield of 83% (Table 1, entries
13−15). When we reduced the amounts of diphenyliodonium
salt to 2.5 equiv, the isolated yield decreased to 67% (Table 1,
entry 16).
With the optimized reaction conditions in hand, we then
started to investigate the diaryliodonium salts for this domino
arylation process with 2-phenyl-imidazo[1,2-α]pyridine (1a).
We subsequently examined the structural diversity of various
2-arylimidazo[1,2-α]pyridines by assessing the substitution
effect on both the heterocyclic framework and the 2-aryl
B
DOI: 10.1021/acs.orglett.9b00761
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Table 2. Scope of Diverse Iodonium Salts in the Domino
Arylation of 2-Phenylimidazopyridine
Scheme 2. Scope of Imidazopyridine Derivatives for
Domino Arylation
a
a
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Pd(OAc)2 (10
mol %), and K2HPO4 (0.4 mmol) in 4 mL of AcOH at 100 °C for 24
b
c
h. Isolated yield after column chromatography. X-ray-derived
structural views of 3ca (carbon atoms in this view are depicted with
ellipsoids at the 30% probability level).
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Pd(OAc)2 (10
mol %), and K2HPO4 (0.4 mmol) in 4 mL of AcOH at 100 °C for 24
h; isolated yield after column chromatography.
motifs. First, substrates (R2) with a broad range of substitution
of the 1-aryl group were examined. As shown in Scheme 2,
electron-neutral, electron-donating, or electron-withdrawing
substituents were generally well-tolerated, affording the desired
products 3ab−3ai bearing halogen (3ab−3ad), methyl (3ae),
methoxy (3af), phenyl (3ah), tert-butyl (3ag), and trifluor-
omethyl (3ai) functionalities in moderate to good yields of
41−78%. We next turned our attention to the substitution on
the heterocyclic framework of imidazopyridine (R1). Fortu-
nately, the reactions all went smoothly to afford the desired
products 3aj−3aq in 52−81% yields. Additionally, on the basis
of results as shown in Table 2, we attempted the cross-reaction
with substituted 2-arylimidazo[1,2-α]pyridine 1 and various
diaryliodonium salts 2 bearing a tert-butyl or methyl group; the
desired products 3ar−3at were afforded in 60−68% yields.
We next sought to investigate the mechanism of this
reaction. By the formation of three carbon (sp2)−carbon (sp2)
bonds (a, b, and c in Scheme 3), all of the five possible reaction
intermediates were assumed: the formation of single (1) a-
bond (4) or (2) b-bond (5) first; the simultaneous formation
of two bonds first such as (3) a-bond and c-bond (6), (4) a-
bond and b-bond (7), or (5) b-bond and c-bond (8). The
intermediates 4−7 were therefore prepared and attempted in
the reaction under the standard conditions, respectively. The
results are shown in Scheme 3; compounds 4 and 7 did not
transform into 3aa while 4 could form compound 7 in a good
yield of 89%. The reaction of 6 with 2a gave a low yield of 3aa
as well (Scheme 3, 1). It suggested that the reaction initiated
with the formation of b- and c-bonds but not the a-bond.
C
DOI: 10.1021/acs.orglett.9b00761
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Letter
might shield the highly reactive C−H bond at the 3-position of
2-arylimidazopyridine derivatives. We therefore proposed a
catalytic mechanism as shown in Figure 1. The reaction of 1a
Scheme 3. Controlled Reactions
Figure 1. Proposed mechanism for domino arylation.
a
with AcOH formed a salt (A), as shown in Figure 1, whose
positive cation diminished the reactivity of the C−H group at
the 3-position of A by electron-withdrawing inductive effects.
Furthermore, the associated anion of OAc might shield the
reactivity of the C−H group at the 3-position of A by steric
hindrance. A gave B bearing nucleophilic nitrogen with base.
Then, the reaction was initiated by electrophilic palladation
and C−H insertion to generate palladacycle C. Oxidation of C
afforded the Pd(IV) complex of D with Ph2IOTf as oxidants.
C−C bond formation of E by a reductive elimination from the
high-valent palladium center.16 The second C−C bond
formation proceeded via F by bond rotation of E in a similar
manner. Finally, an intramolecular dehydrogenative coupling
reaction by palladium-catalyzed C−H activations releases 3aa
from 8 which was an isolated intermediate, and the Pd species
is regenerated from the catalytic cycle.
In summary, we have developed a palladium-catalyzed
cascade of the diarylation/intramolecular dehydrogenative
coupling reaction of imidazopyridine derivatives using diary-
liodonium salts, which provides an access to a variety of ortho-
phenanthro-imidazopyridine fused heteropolycycles. The ad-
vantage of the current protocol is that it can tolerate various
substrates, and the target polyaromatic compounds were
rapidly constructed in one pot. Further investigation of the
detailed reaction mechanism and optimization of reaction
conditions to achieve a broader scope is ongoing in our
laboratory.
Standard conditions: 4, 5, 6, or 7 (0.2 mmol), 2 (0.6 mmol),
Pd(OAc)2 (10 mol %), and K2HPO4 (0.4 mmol) in 4 mL of AcOH,
100 °C, 24 h. Isolated yield after column chromatography. DDQ =
2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
b
c
Moreover, compounds 5 and 8 afforded 3aa in good yields.
Hence, it is reasonable to conclude that the a-bond was formed
lastly. Further experiments of the arylation of 9 bearing a
methyl group at the 3-position furnished diphenylated 10 as
well as monophenylated product 11 (Scheme 3, 2). More
importantly, a scale-up reaction (10 g) of the model reaction of
1a with 2a was carried out; a separable amount of 8 was
isolated and characterized by NMR and MS spectra.
To further understand the formation of the a-bond,
intramolecular aryl−aryl dehydrogenative coupling reactions
of 8 were conducted in the presence of a variety of oxidants
(Scheme 3, 3). When diphenyliodonium triflate was used as
the oxidant,15 the desired product of 3aa could be formed in
air. Moreover, we performed the oxidative aromatic couplings
(Scholl reaction) of 8 mediated by the strong Brønsted acid of
TfOH or Lewis acid of FeCl3 with additional oxidants of
DDQ; 3aa was isolated in 21% yield with TfOH. However, no
reaction occurred in the case of FeCl3. These results implied
that the formation of the a-bond of 3aa could proceed in acidic
conditions in the presence of oxidants. In comparison with the
palladium-catalyzed phenylation of 1a with 2a in DMF as the
solvent as described in Table 1, protic acetic acid as the solvent
D
DOI: 10.1021/acs.orglett.9b00761
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chen, L.; Liu, J.; Cai, H.; Qiu, S.; Tan, J. Chem. Commun. 2015, 51,
1823−1825.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00761.
■
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Accession Codes
CCDC 1900251 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
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*Phone: (+)-86-21-64253040. E-mail: jianweihan@ecust.edu.
cn.
*Phone/Fax: (+)-86-21-64253881. E-mail: wanglimin@ecust.
edu.cn.
ORCID
́
́
Toth, B. L.; Tolnai, G. L.; Novak, Z. Synlett 2016, 27, 1456−1485.
(8) (a) Cao, C. K.; Sheng, J.; Chen, C. Synthesis 2017, 49, 5081−
5092. (b) Su, X.; Chen, C.; Wang, Y.; Chen, J.; Lou, Z.; Li, M. Chem.
Commun. 2013, 49, 6752−6754. (c) Wang, Y.; Chen, C.; Peng, J.; Li,
M. Angew. Chem., Int. Ed. 2013, 52, 5323−5327. (d) Ma, X.-P.; Shi,
W.-L.; Mo, X.-L.; Li, X.-H.; Li, L.-G.; Pan, C.-X.; Chen, B.; Su, G.-F.;
Mo, D.-L. J. Org. Chem. 2015, 80, 10098−10107. (e) Ma, X.-P.; Li, K.;
Wu, S.-Y.; Liang, C.; Su, G.-F.; Mo, D.-L. Green Chem. 2017, 19,
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Jianwei Han: 0000-0002-8354-5684
Limin Wang: 0000-0002-4025-5361
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The work was supported by the National Key Research and
Development Program (2016YFA0200302), National Nature
Science Foundation of China (NSFC 21772039, 21472213),
as well as by Croucher Foundation (Hong Kong) in the form
of a CAS-Croucher Foundation Joint Laboratory Grant, the
Fundamental Research Funds for the Central Universities,
Shanghai Municipal Science and Technology Major Project
(Grant 2018SHZDZX03) and the Programme of Introducing
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