Access to 2-Arylindoles via Decarboxylative C?C Coupling in Aqueous Medium and to Heteroaryl Carboxylates under Base-Free Conditions using Diaryliodonium Salts

Article abstract of DOI:10.1002/asia.201601290

Easily accessible heteroaromatic carboxylic acids and diaryliodonium salts were successfully employed to construct valuable 2-arylindoles and heteroaryl carboxylates in a regioselective fashion. C2-arylated indoles were produced using a Pd-catalyzed decarboxylative strategy in water without any base, oxidant, or ligand. Heteroaryl carboxylates were prepared under metal and base-free conditions. This protocol was successfully utilized to synthesize Paullone, a cyclin-dependent kinase (CDK) inhibitor.

Full text of DOI:10.1002/asia.201601290

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: An Access to 2-Arylindoles via Decarboxylative C-C Coupling in Aqueous Me-
dium and Heteroaryl Carboxylates under Base-Free Conditions using Diarylio-
donium Salts
Authors: Dalip Kumar; V Arun; Meenakshi Pilania
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An Access to 2-Arylindoles via Decarboxylative C-C Coupling in Aqueous
Medium and Heteroaryl Carboxylates under Base-Free Conditions using
Diaryliodonium Salts
V. Arun,[a] Meenakshi Pilania,[a] and Dalip Kumar*[a]
Abstract: Easily accessible heteroaromatic carboxylic acids
and diaryliodonium salts were successfully employed to
construct valuable 2-arylindoles and heteroaryl carboxylates in
a regioselective fashion. C2-arylated indoles were produced via
Pd-catalyzed decarboxylative strategy in water without any
base, oxidant and ligand. Heteroaryl carboxylates were
prepared under metal and base-free conditions. This protocol
(160 °C) dimethylacetamide (DMA) for 24 h. Despite the
availability of promising strategies, it is challenging to develop
decarboxylative direct arylation in aqueous medium without any
oxidant and base.
Diaryliodonium salts for being inherently stable solids and
easy to access, are frequently used in organic synthesis as
highly electrophilic arylating agents to prepare useful natural and
bioactive heterocycles.[9] Hence, utilities of diaryliodonium salts
got groundbreaking advancement for the arylation and assembly
of heterocyclic frameworks.[10] With ongoing research focus to
develop operationally simple protocols to assemble medicinally
important heteroaryl molecules by employing diaryliodonium
salts[11] herein we wish to disclose Pd-catalyzed decarboxylative
C2-arylation of heteroaryl acids in water, and base-free
synthesis of heteroaryl esters.
was successfully utilized to synthesize Paullone,
inhibitor.
a CDK
Heteroaromatic carboxylic acids are ubiquitous in naturally
occurring compounds that are frequently used as easily
available potential building blocks for the construction of many
drug-like molecules and pharmaceutical agents.[1],[2] In the recent
years, carboxylic acids have been employed as traceless
directing groups in transition metal-catalyzed arylation
reactions.[2] This strategy provides effective ways for the
selective functionalization of unreactive C-H bond by directing
the metal to activate the proximal C-H bond. Concurrent C-H
functionalization and removal of carboxylic acid group as non-
We initiated our investigation by using diphenyliodonium
triflate (1a) and indole-3-carboxylic acid (2a) as model
substrates. The decarboxylative arylation of 1a with 2a in the
presence of 1.0 mol% Pd(OAc)2 in acetic acid generated
anticipated 2-phenylindole (3a) in 80% yield. To improve the
yield of 3a, different solvents were screened. No product was
formed in DMF, whereas, other solvents including 1,4-dioxane,
toluene and 1,2-dichloroethane resulted in inferior yield.
Gratifyingly, the use of water as a solvent led to 3a in excellent
yield (91%). Water is a superior solvent with satisfactory
properties such as non-combustible non-toxic, cheap and can
be handled without any precaution.[12] Moreover, in present work,
after the reaction, crude product 3a was easily separated out
from the reaction medium (water) as thick oil.
toxic carbon dioxide,
are the significant features of this
strategy.[3] Many research groups have successfully utilized this
approach to develop new organic transformations for the
construction of therapeutic agents in fewer synthetic steps.[4]
Figure. 1 Selected drug-like 2-arylindoles and indole
carboxylate
Having the optimized reaction conditions in hand, scope of
diaryliodonium salts and heteroaromatic carboxylic acids was
explored. All the diaryliodonium salts were synthesized from the
corresponding arenes and iodoarenes.9a Unsymmetrical
iodonium salts possessing electron-withdrawing groups such as
ester and nitro, produced the corresponding 2-arylindoles 3b
and 3d in 85 and 81% yields, respectively. Halogen (chloro)
bearing iodonium salt 2c successfully produced 3c in 88% yield.
Similarly, sterically hindered ortho substituted (OMe, CO2Me and
NO2) iodonium salts 2e-f, 2h also delivered the anticipated
products 3e-f, 3h and 3j in high yields (68-75%). Heterocyclic
iodonium salts, indole(phenyl)iodonium tosylate (2j) and
mesityl(thienyl)iodonium triflate (2k) also worked well to afford
C2-heteroarene motifs 3l (60%) and 3n (70%) in good yields.
Medicinally important candidate, 2-indolyluracil 3o was easily
obtained from 1a and mesityl(uracil)iodonium triflate in 63%
yield. Biologically important 4/5-methoxy indoles were also
successfully arylated to prepare the corresponding 2-aryl-4/5-
methoxyindoles 3i and 3k in better yields (82%). 4/5-
Methoxyindoles are known to display interesting anticancer
2-Arylindoles are highly useful scaffolds in drug discovery
beside their utility as precursors in the synthesis of complex
synthetic molecules (Figure 1).[5],[6b] Consequently, preparation
of 2-arylindoles has been pursued with increasing interest by
organic and medicinal chemists. Recently, several elegant
directing groups[6] and direct arylation[7] approaches have been
developed to achieve 2-arylindoles and their analogues. In
literature, there is one report employing carboxylic acid as a
directing group[8] in the conversion of N-protected indole-3-
carboxylic acids into the corresponding 2-arylindoles.
Additionally, the reaction condition involves the use of Pd(0)
carbene complex (2.5 mol%) and base (KOAc) under refluxing
[a]Department of Chemistry, Birla Institute of Technology and Science,
Pilani-333031, Rajasthan, India Fax: +91-1596-244183; Tel: 91-1596-
515238; E-mail: dalipk@pilani.bits-pilani.ac.in.
http://universe.bits-pilani.ac.in/pilani/dalipk/profile
Supporting information for this article is given via a link at the end of the
document
.
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properties through the inhibition of tubulin.5b To further widen the
scope of this decarboxylative C2-arylation strategy, indole-3,5-
dicarboxylic acid 1g was subjected to optimized reaction
conditions.
accomplished heteroaryl carboxylates by employing carboxylic
with either boronic acids or arenes under metal-catalyzed[14] and
metal-free reaction conditions.[15] Recently, Olofsson et al.
reported the arylation of carboxylic acids using diaryliodonium
salts in the presence of KOt-Bu as a base in toluene.[16] The
protocol proceeds under mild conditions and useful to make
sterically hindered carboxylates in good yields. However,
arylation of heteroaryl carboxylic acids using variety of
diaryliodonium salts under base-free conditions is largely
unexplored. It is noteworthy to mention that, under both the
identified reaction conditions, C/O-arylated products were
obtained without any trace of N-arylated heteroaryl acids despite
the indole moiety having free -NH. Remarkably, our simple
protocol showed excellent control over regioselectivity by C2
and O-arylations.
Table 1. Solvents and catalyst screeninga
Entry
Catalyst
Solvent
Time (h)
Yield (%)c
3a
80
64
4a
1
2
Pd(OAc)2
Pd(OAc)2
AcOH
1,4-
2
2
-
-
dioxane
DMF
Toluene
water
water
DCE
water
DMSO
1,4-
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
12
2
1
1
2
24
24
24
NR
45
91
91
66
-
-
-
-
-
-
Table 2. Scope of 2-arylindolesa
5
6b
7
Pd(OAc)2
8
9
10
-
-
-
NR
30
NR
-
-
dioxane
11
-
DMF
12
-
80
aReaction conditions: 1a ( 100 mg, 0.62 mmol), 2a (267 mg, 0.62 mmol),
b
Pd(OAc)2 (1.0 mol%), 100 °C, used mesityl(phenyl)iodonium salt (2b, 0.62
mmol), cisolated yields, NR = no reaction
Interestingly, C-5 carboxylic acid remained unreactive and
selectively afforded C2-arylated product 3q in 72% yield.
Formation of 3q suggests activation of C2-position via co-
ordination of C3-carboxylic acid with palladium. In case of
pyrrole-3-carboxylic acid, we attained diarylated 3u rather than
monoarylated product. Diarylation of pyrrole-3-carboxylic acid
(1u) is likely to proceed via ortho-arylation followed by ipso-
arylation.[18e-f] Encouraged by the successful arylation of indole-
3-carboxylic acids and diarylation of pyrrole, next analogue
indole systems, indole-3-acetic acid, indole-3-butyric acid and
tryptamine were examined. Interestingly, carboxylic acid and
amine functionalities were successfully directed the incoming
iodonium salts to furnish C2-arylated products 3s-t and 3v-y in
excellent yields (85-90%). Compounds 3s-t and 3v-y with
carboxylic acid and amine moieties could be used for further
synthetic manipulation to generate complex bioactive molecules.
Subsequently, we also studied the O-arylation of indole-3-
carboxylic acids using substrates 1a and 2a for the model
reaction. Heating the reaction contents in DMF exclusively led to
O-arylated indole-3-phenylcarboxylate 4a (Table 1, entry 11)
which was confirmed by the characteristic peak of ester >C=O at
δ 163.6 ppm in 13C NMR, and band at 1710 cm-1 in IR spectral
aReaction Condition: 1(1.0 equiv), 2 (1.0 equiv) and Pd(OAc)2 (1.0 mol%)
in water (1 mL) at 100 °C for 1 h; isolated yields
Differently substituted indole-3-carboxylic acids 1 were treated
with substituted diaryliodonium triflates 2 under the optimized
reaction conditions. Compared to unsubstituted indole, electron
rich (4 and 5-methoxy) substituted indoles 1d-e showed better
reactivity towards diversely substituted diaryliodonium salts (NO2,
dimethyl, t-butyl & tolyl) to afford 4d-g in 75-85% yields. To
improve the significance of the developed strategy, we turned
our attention to arylate pyrrole and furan carboxylic acids. We
observed excellent reactivity of pyrrole-3-carboxyxlic acid and
furan-2-carboxylic acid towards chloro 2c and o-methoxy
substituted iodonium salts 2e to access the corresponding
esters 4j and 4l in 85-92% yields within 3h. Gratifyingly, the
developed esterification procedure was also amenable to six-
data
(
See the Supporting Information).14b
Heteroaryl
carboxylates serve as prospective precursors in organic
synthesis in addition to for being important subunits widely
present in pharmaceutical molecules, agroproducts and
polymers (Figure 1).[13] Generally, heteroaryl carboxylates are
prepared via esterification of appropriate carboxylic acids with
phenols in the presence of acid, base or coupling reagents.10
Alternatively, various impressive synthetic developments
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Chemistry - An Asian Journal
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membered pyridyl carboxylic acids to produce nicotinic (4o) and
isonicotinic acids (4p-q) in 82-88 % yields.
Table 3. Scope of hetero(aryl)carboxylatesa
Scheme 2. Evidence for Pd(II) carboxylate 9
Scheme 3. Reaction of 2a with 9
Subsequently, isolated Pd(II)carboxylate 9 was treated with
iodonium salt 2a (0.31 mmol) under identical conditions (See the
Supporting Information) and obtained 3a in 90% yield. Also,
during the reaction (after ten min), the crude mixture was
subjected to mass spectrometry analysis which showed a peak
at m/z 400.05, corresponds to the proposed Pd(IV) complex 10
(Scheme 3). These experimental observations are indicating that
the C2-arylation is likely to proceed via palladium intermediates
9 and 10. Literature reports[2c,18] and our findings (Schemes 2
and 3) in the formation of 3, suggest that the initial reaction of
indole-3-carboxylic acid (1a) with Pd(OAc)2 provides Pd(II)
carboxylate 10 with the concomitant release of acetic acid. Next,
oxidative addition of complex 9 with diphenyliodonium triflate
likely to furnish Pd(IV) 10 which upon reductive elimination and
decarboxylation believed to produce the desired C2-arylated
product 3a along with regeneration of the catalyst (Scheme 4).
aReaction Conditions: 1(1.0 equiv), 2 (1.0 equiv), DMF (0.5 mL) 100 °C, 12 h;
isolated product yields, breaction time 3h
To prove the synthetic utility of identified protocol, we
prepared Paullone (7), a well-known cyclin-dependent kinase
(CDK) inhibitor,[17] in 85% yield from the reaction of indole-3-
acetic acid (5) and iodonium salt 2h. Aryl intermediate 6 was
converted into Paullone 7 by reducing the nitro group and
followed by amide coupling as depicted in scheme 1. To
demonstrate the scalability of the reaction (Table 1), we
successfully isolated 3a (1.07g) in 90 % yield from the reaction
Scheme 1. Synthesis of CDK-Inhibitor, Paullone
of indole-3-carboxylic acid (1a, 1g) with mesityl(phenyl)iodonium
salt (2b). Generated iodomesitylene during the reaction, was
Scheme 4. Plausible mechanism for 3 and 4
recovered
and
reused
for
the
synthesis
of
mesityl(phenyl)iodonium salt (2b) indicating that the developed
protocol is highly economical.
In O-arylation of carboxylic acid 1, it is believed that carboxylic
acid 1 and iodonium salt 2 in DMF may form six-membered
transition state A which undergoes migration and reductive
elimination[18d] as depicted in scheme 4 to afford the anticipated
product 4.
To gain the mechanistic understanding of decarboxylative C2-
arylation, acid 1a (0.31 mmol) was refluxed with equimolar
quantity of Pd(OAc)2 (0.31 mmol) in water for ten min. The
resulting mixture was analyzed by NMR (1H & 13C), Mass and
IR spectral data (See the Supporting Information). Appearance
of characteristic peaks at δ 1.91 ppm (–CH3) in 1H NMR and at δ
172.6 ppm (>C=O) in 13C NMR (Scheme 2) indicates the
formation of Pd(II)carboxylate 9.
In summary, we have developed a simple approach to
prepare 2-arylindoles and heteroaryl carboxylates in
a
regioselective fashion using easily accessible heteroaryl
carboxylic acids and diaryliodonium salts. The C2-arylation of
indole derivatives proceed via decarboxylative coupling using
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10.1002/asia.201601290
COMMUNICATION
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only catalytic amount of Pd(OAc)2 (1.0 mol%) in water. Base-
free O-arylation of heteroaryl carboxylic acids occurred in neat
DMF. The developed protocol was successfully progressed
without any ligand, oxidant, base and acid to prepare a range of
heteroaryl carboxylic carboxylates in good to excellent yields.
The synthetic utility of the developed procedure was proved by
preparing CDK inhibitor, Paullone in good yield. Detailed
mechanistic investigation and application of this strategy to other
heterocyclic systems are ongoing and will be disclosed in due
course.
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Acknowledgements
We gratefully acknowledge DBT, New Delhi for financial
support. V.A and M.P are thankful to CSIR, New Delhi for
awarding SRF
Keywords: Diaryliodonium salt • Decarboxylative C-C coupling •
Pd-catalyst • Indole • Arylation
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COMMUNICATION
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conditions afforded a mixture of 2-phenylindole (3a, 70% yield) and
3-phenyl-indole (10% yield).
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Diaryliodonium salts
V. Arun, Meenakshi Pilania, Dalip
Kumar*
Page No1. – Page No5.
An Access to 2-Arylindoles via
Decarboxylative C-C Coupling in
Aqueous Medium and Heteroaryl
Carboxylates under Base-Free
Conditions using Diaryliodonium
Salts
Diaryliodonium salts has been successfully employed to construct 2-arylindoles using
decarboxylative C-C coupling strategy in water and heteroaryl carboxylates under base-free conditions
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