Transition-Metal-Free Direct Arylation and Esterification Reaction of Unprotected Indolylcarboxylic Acid Derivatives: A New Entry to 2-(1 H -Indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazoles and Aryl 1 H -Indole-2-carboxylates

Article abstract of DOI:10.1055/s-0035-1562526

An efficient, ligand-free, transition-metal-free, direct arylation and esterification reaction of unprotected indolylcarboxylic acid derivatives with diaryliodonium salts was developed, thus providing a new entry to 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole and aryl 1H-indole-2-carboxylate derivatives with good yields.

Full text of DOI:10.1055/s-0035-1562526

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
D. Wang et al.
Letter
Synlett
Transition-Metal-Free Direct Arylation and Esterification Reaction
of Unprotected Indolylcarboxylic Acid Derivatives: A New Entry to
2-(1H-Indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazoles and Aryl 1H-In-
dole-2-carboxylates
Dawei Wang*a
O
Chenyang Gea
COOH
OAr
Ar2IX
Xin Yua
Huida Wana
Xiang Xu*b
N
H
N
H
transition-metal-free
ligand-free
no activating reagent
one step
N2H4-H2O
direct
CS2, MeOH
a The Key Laboratory of Food Colloids and Biotechnology, Ministry of
Education, School of Chemical and Material Engineering, Jiangnan
University, Wuxi 214122, Jiangsu Province, P. R. of China
wangdw@jiangnan.edu.cn
b College of Chemistry and Pharmaceutical Sciences, Qingdao Agri-
cultural University, Qingdao 266109, P. R. of China
rainerxu@163.com
N
N
N
N
Ar2IX
O
N
O
N
SH
SAr
H
H
Received: 06.05.2016
Accepted after revision: 04.07.2016
Published online: 01.08.2016
O
O
N
N
R
N
O
N
H
SAr
DOI: 10.1055/s-0035-1562526; Art ID: st-2016-w0323-l
H
O
Bn
O
inflammation inhibitor
NO2
antimicrobial form
Abstract An efficient, ligand-free, transition-metal-free, direct aryla-
tion and esterification reaction of unprotected indolylcarboxylic acid
derivatives with diaryliodonium salts was developed, thus providing a
new entry to 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole and aryl
1H-indole-2-carboxylate derivatives with good yields.
N
O
H
O
O
F
F
F
antiovulatory antagonist
O
N
F
H
N
O
Keywords arylation, diaryliodonium salts, indolylcarboxylic acids, es-
F
H
terification, transition-metal-free
AMPAR antagonism
O Cl
anti-lipid peroxidation
N
O
Cl
Aryl 1H-indole-2-carboxylate and 2-(1H-indol-2-yl)-5-
(phenylthio)-1,3,4-oxadiazole derivatives are important
types of natural products and intermediates for pharma-
ceuticals, medicines, and pesticides (Figure 1).1 Many scien-
tists have already proved that aryl 1H-indole-2-carboxylate
derivatives demonstrate benign biological activities, such
as antilipid peroxide (LP) activity, antisuperoxide forma-
tion, and have been shown to function as antioxidants, an-
tiovulatory antagonists of LHRH related to antide, X inhibi-
tors, Xa inhibitors, FXa inhibitors, cholecystokinin antago-
nists, antiovulatory antagonists and anticoagulants, etc.2
Moreover, these indole derivatives have also been applied
as antibiotic, antiparasitic, antitumoral, antimycobacterial,
and anti-inflammatory agents.1h,3
During the past ten years, some chemists have investi-
gated the synthesis of these aryl 1H-indole-2-carboxylate
derivatives.4 Generally, two main methods for the prepara-
tion of these indole derivatives have been developed: One is
the two-step method, which uses thionyl chloride and 1,1′-
carbonyldiimidazole (CDI) or oxalyl chloride as the activat-
H
Cl
cholecystokinin antagonist
Figure 1 Biologically active 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-
oxadiazole and aryl 1H-indole-2-carboxylate derivatives
ing reagent to elevate substrate activity;5 the other is the
transition-metal-catalyzed esterification reaction of indo-
lylcarboxylic acids and arylboronic acids.6 Recently, Mo-
chizuki et al. used the first method to synthesize indole acyl
chlorides under thionyl chloride conditions and converted
these acyl chlorides into the corresponding aryl 1H-indole-
2-carboxylate derivatives within two steps.7 In 2011, Rossi
and Martín converted carboxylic acids into amides using
1,1′-carbonyldiimidazole as activating reagent, then dealt
with phenols to obtain aryl 1H-indole-2-carboxylate deriv-
atives with a two-step method.5a In 2014, Baell and co-
workers developed the synthesis of 3-amino-1H-indole-2-
carboxylate through use of the cyclization method.8 Recent-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
D. Wang et al.
Letter
Synlett
ly, Liu and co-workers described copper-catalyzed direct O-
arylation reactions of carboxylic acids with arylboronic ac-
ids under silver carbonate and DMSO conditions with mod-
erate to good yields.9
Table 1 Screening of Reaction Conditionsa
O
(2a)
Ph2IOTf
COOH
N
H
N
H
OPh
1a
Concurrently, many scientists found that diaryliodoni-
um salts are very useful coupling reagents in the field of α-
arylation reactions and C–H activation reactions.10,11 Dia-
ryliodonium salts have many advantages, like easy synthe-
sis and high reactivity, among others.12 Very recently, sev-
eral groups have communicated their results in this area,
including Olofsson,13 Gaunt,14 Greaney,15 Shibata,16 Chen,17
Zhang and Yu,18 Nagorny,19 Chatani,20 Manetsch,21 Stuart,22
and others.23 Based on the coupling reactions of diaryliodo-
nium salts in our group,24a,b herein, we reported a simple,
efficient, ligand-free, transition-metal-free, direct esterifi-
cation reaction of unprotected indolylcarboxylic acids and
diaryliodonium salts, which provides a new method for the
synthesis of aryl 1H-indole-2-carboxylate derivatives with
good yields (Scheme 1).
3a
Entry
Base
KOH
Solvent
Yield (%)b
1
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
EtOH
60
71
59
84
<5
<5
37
<5
<5
<5
19
<5
21
<5
14
79c
Cs2CO3
K3PO4
K2CO3
Et3N
3
4
5
6
none
7
t-BuOK
AcOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
8
9
10
11
12
13
14
15
16
THF
CH2Cl2
DMF
previous work
O
[Cu], base
COOH
xylene
DMSO
DCE
N
N
PhB(OH)2
OAr
R'
R'
N-protected substrates
toluene
O
this work
COOH
a Reaction conditions: 1H-indole-2-carboxylic acid (0.5 mmol), di-
phenyliodonium triflate (0.6 mmol), base (0.75 mmol), solvent (2 mL), 130
°C or reflux, 12 h.
b Isolated yield.
c Under N2.
OAr
Ar2IX
R
R
N
N
H
H
no activating reagent
one step
transition-metal-free
ligand-free
N2H4-H2O
direct
CS2, MeOH
N
N
N
N
Ar2IX
R
R
acids were converted completely to produce the corre-
sponding products. The substrates having various electron-
donating or electron-withdrawing goups, such as MeO,
NO2, F, and Br, can be tolerated in order to obtain the corre-
sponding products with moderate to good yields in most
cases. The highest yield was obtained in the case of 5-fluo-
ro-1H-indole-2-carboxylic acid as substrate (Table 2, entry
3). When the anion was changed into BF4 or OTs, the reac-
tion gave slight lower yields (Table 2, entries 10–15). It
should be noted that the reaction could not happen when
bromide salt was used in this reaction (Table 2, entry 16).
Encouraged by such promising results, we applied the
above methods to different substrates. The results are sum-
marized in Figure 2. We were pleased to find that the corre-
sponding products were obtained with moderate to good
yields under the standard conditions. All kinds of the un-
protected acids were smoothly converted into aryl ester
derivatives.
O
N
O
N
SH
SAr
H
H
Scheme 1 The synthesis of 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-
oxadiazole and aryl 1H-indole-2-carboxylate derivatives
To begin this projection, the simple 1H-indole-2-car-
boxylic acid (1a) was chosen to react with diphenyliodoni-
um triflate (2a) under various reaction conditions (Table 1).
First, we selected toluene as the solvent and KOH as the
base. To our delight, we found that the desired product was
separated with moderate yield (Table 1, entry 1). Secondly,
the blank test showed that the base played an important
role in this reaction, so various bases were checked. The re-
sults showed that K2CO3 was better than other bases (Table
1, entry 4). Next, the evaluation of a range of solvents on
the activity was carried out (Table 1, entries 9–15). The re-
sults showed that toluene was superior to other solvents.
Having established the optimal conditions for the reac-
tion of 1H-indole-2-carboxylic acid with diphenyliodonium
triflate to obtain aryl 1H-indole-2-carboxylate derivatives,
we explored the reaction scope, and the results are summa-
rized in Table 2. In general, all the 1H-indole-2-carboxylic
It is well known that indole thiazoles belongs to a struc-
tural group in many natural products, which exhibit anti-
fungal, antibacterial, anticancer, and antithrombotic activi-
ties.1l,m The preparation and synthesis of indole thiazoles
have been a topic of special interest to medicinal and syn-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
D. Wang et al.
Letter
Synlett
Table 2 Substrate Scope Experimentsa,b
1) MeOH/H2SO4
N2H4⋅H2O
O
CS2, KOH
MeOH
COOH
N
CNHNH2
O
N
H
1a
N
Ar2IX 2
H
COOH
R
R
1a'
N
N
OAr
H
H
N
N
N
Ar2IOTf (1.2 equiv)
1
3
O
O
N
H
N
H
SH
Cs2CO3 (1.5 equiv)
toluene, 130 °C
SPh
Entry
R
Ar
Ph
X
Yield (%)
1a''
5: 78%
1
2
H
OTf
3a 84
3b 79
3c 77
3d 68
3e 72
3f 71
3g 64
3h 83
3i 68
3f 63
3b 71
3c 60
3i 66
3a 70
3g 62
<5
Scheme 2 The synthesis of 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-
oxadiazole (5)
5-OMe
5-F
Ph
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
BF4
BF4
BF4
BF4
OTs
OTs
Br
3
Ph
4
6-NO2
5-Br
H
Ph
To elaborate on this reaction, competition experiments
between 1H-indole-2-carboxylate derivatives with elec-
tron-withdrawing and electron-donating groups were set
up. The results revealed that the reaction for 1H-indole-2-
carboxylate with an electron-donating group is much bet-
ter than for that with an electron-withdrawing group
(Scheme 3).
5
Ph
6
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
Ph
7
5-OMe
5-F
8
9
5-Br
H
10
11
12
13
14
15
16
MeO
MeO
O
5-OMe
5-F
COOH
N
H
N
H
OPh
Ph
Ar2IOTf (2.2 equiv)
3b
0.5 mmol
5-Br
H
4-MeC6H4
Ph
K2CO3 (1.5 equiv)
toluene, 130 °C
O
COOH
N
H
O2N
N
H
OPh
O2N
5-OMe
H
4-MeC6H4
Ph
0.5 mmol
3d
3b/3d = 3.2:1
a Reaction conditions: 1 (0.5 mmol), Ar2IX (0.6 mmol), K2CO3 (0.75 mmol),
toluene (2 mL), 130 °C, 12 h.
Scheme 3 Intermolecular competition experiment of this esterifica-
tion reaction
b Isolated yield.
COOPh
COOPh
COOPh
Because these aryl 1H-indole-2-carboxylate and 2-(1H-
indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole
derivatives
N
N
N
demonstrated good biological activity, the evaluation of
this new method in a large-scale synthesis is necessary.
Next, we performed this direct esterification reaction of
unprotected indolylcarboxylic acids and diaryliodonium
salts in a gram scale, good yield of the desired product 3a
was separated in 78% yield, while only moderate yield of 2-
(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole (5) was
achieved (Scheme 4).
N
H
H
4a: 77%
PhOOC
4b: 69%
4c: 74%
COOPh
N
N
N
COOPh
N
H
4d: 68%
4e: 72%
4f: 71% (68%a)
Figure 2 Substrate scope expansion experiment. Reagents and condi-
tions: 1 (0.5 mmol), Ar2IX (0.6 mmol), K2CO3 (0.75 mmol), toluene (2
mL), 130 °C, 12 h. Isolated yields are reported. a Under N2.
O
Ph2IOTf (1.2 equiv)
COOH
N
K2CO3 (1.5 equiv), 12 h
toluene, 130 °C
N
OPh
H
H
thetic chemists. 5-(1H-indol-2-yl)-1,3,4-oxadiazole-2-thiol
and 2-(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole de-
rivatives are the typical examples. Here, we found that 2-
(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole (5) was
easily synthesized with diaryliodonium salts through this
methodology with moderate yield (Scheme 2).
1a, 1.000 g
3a, 1.147 g, 78% yield
N
N
Ph2IOTf (1.2 equiv)
N
N
O
N
SH
Cs2CO3 (1.5 equiv)
toluene, 130 °C, 12 h
O
N
H
SPh
H
1a'', 1.000 g
5, 0.916 g, 63% yield
Scheme 4 The synthesis of phenyl 1H-indole-2-carboxylate (3a) and 2-
(1H-indol-2-yl)-5-(phenylthio)-1,3,4-oxadiazole (5) on a gram scale
This methodology of direct esterification reaction of un-
protected indolylcarboxylic acids and diaryliodonium salts
provides a convenient route to synthesize aryl 1H-indole-2-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
D. Wang et al.
Letter
Synlett
carboxylate derivatives. Besides their own biological per-
formance and activity (Scheme 5), these compounds could
be easily converted into N-phenyl-1H-indole-2-carboxam-
ides,25 (1H-indol-2-yl)(pyrrolidin-1-yl)methanones,26 6H-
benzo[5,6][1,4]oxazino[4,3-a]indol-6-ones,5a and chrome-
no[3,4-b]indol-6(7H)-ones5b,27 through the use of one or
two steps.
Acknowledgment
We gratefully acknowledge financial support of this work by the Na-
tional Natural Science Foundation of China (21401080), the Natural
Science Foundation of Jiangsu Province of China (BK20130125), Jiang-
su Talents Project (2013-JNHB-027), the Fundamental Research Funds
for the Central Universities (JUSRP 51627B), and MOE & SAFEA for the
111 Project (B13025).
O
O
R
Supporting Information
R
N
N
HN R2
ref 25
N
H
H
ref 26
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562526.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
O
O
R
N
References and Notes
H
ref 5b, 27
R'
O
R
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N
O
O
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O
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H
Scheme 5 The transformations of aryl 1H-indole-2-carboxylate deriva-
tives
Finally, a possible mechanism for the esterification reac-
tion of unprotected indolylcarboxylic acids and diaryliodo-
nium salts was proposed (Scheme 6). Initially, we proposed
that 1H-indole-2-carboxylate was dealt with base and con-
verted into 1H-indole-2-carboxylate ion, and subsequent
exchange with diaryliodonium salts to form a T-shaped
Ar2IOOCAr D intermediate.28 Subsequent to the release of
ArI as a byproduct, the product E was obtained to finish the
reaction.
base
COO-
COOH
N
N
H
H
B
A
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20060211603, 2006.
–OTf
Ar
I
Ar
C
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O
O
O
I
ArI
+
Ar
HN
N
OAr
H
Ar
E
D
Scheme 6 The proposed possible mechanism
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nium salts with moderate to good yields. This provides a
new method for the synthesis of aryl 2-(1H-indol-2-yl)-5-
(phenylthio)-1,3,4-oxadiazole and aryl 1H-indole-2-car-
boxylate derivatives.29
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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Synlett
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(29) Typical Procedure for the Synthesis of 3a
Ph2IOTf (0.6 mmol) and toluene (2 mL) were stirred in a Schlenk
tube at room temperature for a moment. Subsequently, 1H-
indole-2-carboxylic acid (1a, 0.5 mmol) and K2CO3 (0.75 mmol)
was added. The mixture was heated under 130 °C for 12 h and
then cooled to room temperature. The resulting solution was
directly purified by column chromatography with PE–EtOAc
(5:1) as eluent to give phenyl 1H-indene-2-carboxylate (3a) as a
yellow solid in 84% yield. 1H NMR (600 MHz, CDCl3): δ = 9.42 (s,
1 H), 7.77 (d, J = 8.0 Hz, 1 H), 7.48 (d, J = 8.2 Hz, 3 H), 7.39–7.33
(m, 3 H), 7.31–7.26 (m, 2 H), 7.21 (m, 1 H). 13C NMR (151 MHz,
CDCl3): δ = 160.77, 150.45, 137.42, 129.56, 127.37, 126.31,
126.10, 125.88, 122.77, 121.71, 121.01, 112.12, 110.27.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E