Direct C-H bond activation: Palladium-on-carbon as a reusable heterogeneous catalyst for C-2 arylation of indoles with arylboronic acids

Article abstract of DOI:10.1039/d0nj00735h

Direct C(sp²)-H bond functionalization of indoles with arylboronic acids is achieved using palladium supported on carbon as a reusable heterogeneous catalyst in the presence of an oxidant under mild conditions. The current protocol formed exclusive C-2 selective products without the aid of any ligand or directing group. The catalyst is reusable for up to four catalytic cycles with the retention of catalytic efficiency.

Full text of DOI:10.1039/d0nj00735h

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Direct C-H bond activation: Palladium-on-carbon as a reusable
heterogeneous catalyst for C-2 arylation of indoles with arylboronic acids
Prantika Bhattacharjeea, Purna K Boruahb,c, Manash R Dasb,c and Utpal Boraa*
aDepartment of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam, India, Pin 784028
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bAdvanced Materials Group, Materials Sciences and Technology Division, CSIR-North East Institute of Science and
Technology, Jorhat 785006, Assam, India
cAcademy of Scientific and Innovative Research (AcSIR), CSIR-NEIST Campus, India
E-mail: ubora@tezu.ernet.in; utbora@yahoo.co.in
ABSTRACT
Direct C(sp2)-H bond functionalization of indoles with arylboronic acids is achieved using palladium
supported on carbon as a reusable heterogeneous catalyst in presence of an oxidant under mild conditions.
The current protocol formed exclusive C-2 selective product without the aid of any ligand or directing
group. The catalyst is recyclable up to four catalytic cycles with the retention of catalytic efficiency.
Introduction
Arylated indole moieties are fascinating class of heterocyclic compounds having pronounced interest for
their wide array of biological and pharmaceutical activities like antibacterial, cytotoxic, anti-oxidative,
anti-cancer, anti-inflammatory properties.1 Over the last several decades, immense efforts have been
devoted for the construction of arylated indoles based on traditional cross-coupling reactions which often
requires multi-step synthetic procedure.2 Hence, transition metal catalyzed C-H activation has emerged as
an influential means for the direct functionalization of unactivated C-H bonds forming new C(sp2)- C(sp2)
bond in an atom- and step- economic way and finds applications in modern drug research. It empowers
previously impossible transformations, new selectivity and shortened reaction paths.3
In this framework, various coupling species have been employed in the arena of C-H arylation of indoles
and pyrroles using aryl halides,4 hypervalent iodine electrophiles5 and arylsiloxanes.6 Despite significant
advantages, there occurs certain limitations such as high reaction temperature, acidic medium, long
reaction times, use of ligands and directing groups.7 Compared to other coupling partners,
organoboronates exhibits some unique advantages including non-toxicity, high stability, environment-
friendly and ease of availability.8 Therefore, development of mild and efficient methods for regioselective
direct C-H arylation of indoles demands extensive research.
1

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Scheme 1. Direct C-H arylation of indoles
Previous works
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ArB(OH)2
[RuCl2(p-cym)]2,
Cu(OAc)2, AgSbF6, 120oC11a
Ar
N
N
N
N
N
N
ArB(OH)2
9
Ar
[RuCl2(p-cym)]2,
Cu(OTf)2, Ag2O, 100oC11b
N
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N
N
N
ArB(OH)2
Ar
[Cp*RhCl2]2,
Cu(OAc)2, 60oC10b
N
N
NHOMe
NHOMe
O
O
Present work
ArB(OH)2
Pd/C, AgTFA, MeOH, 50oC
Ar
N
N
Heterogeneous
No directing group
Me
Me
In the recent times, many literatures have been reported for the metal-mediated selective C-2 arylation of
indole with organoborane reagents. Various transition metal complex based catalytic systems with Pd,9
Rh,10 Ru,11 Cu,12 Co13 have been employed in this regard. Shi and co-workers have reported direct
construction of C-C bonds through Pd(OAc)2-catalyzed cross coupling of electron-rich indoles with aryl
boronic acids in acetic acid under 1 atm of O2.14 Hong and co-workers have presented Pd(OAc)2-
catalyzed direct arylation of electron-deficient polyfluoroarenes with arylboronic acids in DMA at 110
°C.15 However, most of these reported methods, demand the use of acidic solvents14 and high reaction
temperatures15 and the expensive metal catalyst in homogeneous forms causes difficulty in separation and
recovery, which effects environment from heavy metallic ions and cost ineffective. In contrast
heterogeneous catalysts provide ease of separation and possibility of catalyst recyclability. Till date, only
few literatures have been available for direct C-H arylation of indoles using heterogeneous Pd catalysts.16
Palladium-on-carbon can offer as a beneficial alternative to conventional homogeneous catalysis for C-H
arylation of indoles. Compared to other expensive and air-sensitive Pd catalysts, Pd/C exhibits suitable
properties as ease of handling, availability, easy recovery and reusability.17 It has been successfully
employed as a catalyst in various coupling reactions including Suzuki-Miyaura coupling18 and
decarbonylation, hydrogenation and hydrogenolysis.19 Inspired by these reports, we anticipated to
determine its catalytic activeness in C-2 arylation of indoles with arylboronic acids in the absence of any
ligands or directing groups.
Results and discussion
C-H arylation of N-methylindole with phenylboronic acid was chosen as the model reaction and the
optimization is outlined in Table 1. In presence of 10% Pd/C (0.05 mmol, 5.3 mg) and AgTFA (0.4
mmol, 80 mg) obtained the desired product in 86% yield (Table 1, entry 2). No product formation was
observed in the absence of an oxidant (Table 1, entry 1). Encouraged by the results, a variety of copper
2

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and silver salts have been verified as oxidants in the catalytic system and the results are summarized.
Copper salts such as Cu(OAc)2.H2O and Cu(OTf)2 and silver salts such as Ag2O, Ag2CO3, AgBr, AgNO3
remain almost ineffective for the transformation (Table 1, entries 3-8) while AgOAc produced 40% of
the reaction product (Table 1, entry 9). Even combination of Cu-Ag salts remained inactive for the
current reaction (Table 1, entry 10). A variety of metal catalysts have been investigated such as
Pd(OAc)2 which obtained the desired product in 84% yield (Table 1, entry 11) whereas NiCl2.6H2O,
Co(NO3).6H2O, Ni(OAc)2.4H2O, Rh/C attained lower reaction yields (Table 1, entries 12-15). In order to
study the effect of temperature, reactions were performed at 40 °C and 50 °C and the better results were
observed at 50 °C (Table 1, entries 2&16). For studying the effectiveness of Pd/C as catalyst, reactions
were performed varying the amount of the catalyst. Use of 0.02 mmol of the catalyst gave lower yield
(Table 1, entry 17) and increased amount of catalyst 0.10 mmol gave no significant improvement in the
yield (Table 1, entry 18). The highest activity was observed using 0.05 mmol of the catalyst (Table 1,
entry 2). Performing under N2 atmosphere produced a relatively lower yield which specifies re-oxidation
of a catalytic amount of reduced Ag(0) species in air to attain complete substrate conversion (Table 1,
entry 19). Effect of base was studied on the current reaction condition but resulted in complete self-
coupling of arylboronic acids to corresponding biaryls in 90% yield and no trace of arylation at indole
was observed (Table 1, entry 20). A variety of solvent media have been investigated for the current
protocol. Protic solvents such as EtOH, iPrOH and tBuOH produced moderate yields of reaction (Table 1,
entries 21-23) whereas aprotic solvents such as CH3CN, THF, H2O, PEG-400 remained ineffective for
the transformation (Table 1, entries 24-27) except for DMSO and DMF which formed 60-65% yield of
products (Table 1, entry 28-29). The methodology did not necessitate the presence of any directing
groups or ligands to achieve C-2 selectivity.
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With the optimized condition in hand, the scope and limitations of the C-2 selective arylation of indoles
were explored based on electronically varied indole and arylboronic acid derivatives. The results are
summarized in Table 2 and 3. A range of phenylboronic acids participated in the regioselective arylation
of N-methylindole and produced selectively C-2 arylated products. Phenylboronic acid and phenylboronic
acid pinacol ester showed similar reactivity and produced the desired product in good yields (Table 2,
entries 1&2). The reaction tolerates both electron-donating substituents such as OMe, Me, tBu (Table 2,
entries 3,6,10&11) and electron-withdrawing substituents such as Cl, F, NO2, CF3, COMe (Table 2,
entries 4,5&7-9) irrespective of their positions whether ortho, meta or para, produced acceptable yields
of the arylated products (70-88%). These results indicate no correlation between the reaction yield and the
electronic nature of arylboronic acid substituents. However, the electronic nature of the indole nucleus
plays a vital role in the reaction. Both N-methylindole and N-benzylindole gave high yield and selectivity
of the C-2 arylated products (Table 2, entry 1 & Table 3, entry 5). Conversely, free N-H indole and N-
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benzoylindole remained unreactive for the arylation (Table 3, entries 12&13). Substituent groups on the
indole influenced the reactivity of direct arylation reaction. Electron-rich substituents on indole such as
OMe, Me showed better reactivity and afforded the selective arylation product in good yields (82-88%
yields) irrespective of position (Table 3, entries 1-4) compared to electron-poor substituents on indole
such as Cl, Br which furnished about 65-68% of desired product (Table 3, entries 6&7) and NO2
remained inert to the transformation (Table 3, entry 11). Interestingly, replacing the benzene ring of N-
methylindole with pyridine did not perturb the reactivity and afforded the direct arylation product in 71%
yield (Table 3, entry 8). Sterically demanding 3-methyl-N-methylindole reacted smoothly and afforded
82% of the desired product (Table 3, entry 3) whereas 2-methyl-N-methylindole resulted in no feasible
conversion (Table 3, entry 10). The probable reason for such behavior may be due to no possibility of
C3→C2 migration (Scheme 2) in this case which makes the C-3 intermediate revert back to starting
material as the stability of organometallic intermediates when metal is attached to an electron-deficient
centre (C-2) adjacent to nitrogen is higher.20 N-methylpyrrole produced 2,5-diarylated product arylating
both the positions adjacent to nitrogen (Table 3, entry 9).
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Table 1. Screening of reaction conditionsa
catalyst (0.05 mmol)
B(OH)2
oxidant (0.4 mmol)
solvent, 50 oC, 7h
+
N
N
2a
3a
1a
Entry
Catalyst
Oxidant
Solvent
Yieldb [%]
1
2
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
-
AgTFA
Cu(OAc)2.H2O
Cu(OTf)2
Ag2O
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
nr
86
nr
3
4
20
nr
5
6
Ag2CO3
AgBr
nr
7
nr
8
AgNO3
AgOAc
AgTFA
AgTFA
AgTFA
AgNO3
nr
9
40
nr
10
11
12
13
Cu(OAc)2.H2O
Pd(OAc)2
84
30
trace
NiCl2.6H2O
Co(NO3).6H2O
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14c
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Ni(OAc)2.4H2O
Rh/C
Pd/C
1,10-phen, AgTFA
AgTFA
DMF
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
50
nr
16d
17e
18f
19g
20h
21
AgTFA
72
66
86
63
trace
75
68
27
nr
Pd/C
AgTFA
Pd/C
AgTFA
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Pd/C
AgTFA
Pd/C
AgTFA, K2CO3
AgTFA
Pd/C
22
Pd/C
AgTFA
iPrOH
tBuOH
CH3CN
THF
23
Pd/C
AgTFA
24
Pd/C
AgTFA
25
Pd/C
AgTFA
10
nr
26
Pd/C
AgTFA
H2O
27
Pd/C
AgTFA
PEG-400
DMSO
DMF
nr
28
Pd/C
AgTFA
60
29
Pd/C
AgTFA
65
aReaction conditions: indole (1 mmol), boronic acid (1.2 mmol), catalyst (0.05 mmol), oxidant (0.4 mmol), Solvent
(1 ml); temperature (50 °C), time (7 h), in air; byield; ctemperature (100 °C); dtemperature (40 °C); ecatalyst (0.02
mmol); fcatalyst (0.10 mmol); gN2 atmosphere; hbase (3 mmol), 90% yield of biphenyl.
Table 2. Arylation of N-methylindole with various arylboronic acidsa
Pd/C (0.05 mmol)
B(OH)2
AgTFA (0.4 mmol)
MeOH, 50 oC, 7h
+
R
R
N
N
3a-j
1a
2
Entry
Arylboronic acid
Product
Yieldb [%]
B(OH)2
1
2
86 (3a)
80 (3a)
O
B
O
N
O
3
4
87 (3b)
81 (3c)
O
B(OH)2
N
N
Cl
Cl
B(OH)2
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F
B(OH)2
F
5
6
77 (3d)
83 (3e)
N
B(OH)2
B(OH)2
N
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NO2
CF3
7
8
88 (3f)
73 (3g)
O2N
N
B(OH)2
F3C
N
O
O
B(OH)2
9
70 (3h)
78 (3i)
85 (3j)
N
10
11
B(OH)2
B(OH)2
N
tBu
tBu
N
aReaction conditions: indole (1mmol), boronic acid (1.2 mmol), catalyst (0.05 mmol), oxidant (0.4 mmol), Solvent
(1 ml); temperature (50 °C), time (7 h), in air; byield.
Table 3. Arylation of various indole with phenylboronic acidsa
Pd/C (0.05 mmol)
B(OH)2
R1
AgTFA (0.4 mmol)
MeOH, 50 oC, 7h
R1
+
R3
R3
N
N
R2
3k-s
1
R2
2
Entry
Indoles
Product
Yieldb [%]
O
1
84 (3k)
O
N
N
O
2
87 (3l)
O
N
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3
82 (3m)
6
7
8
9
N
N
N
N
4
5
88 (3n)
90 (3o)
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N
N
Bn
Bn
Cl
Cl
Br
6
68 (3p)
N
N
Br
7
8
9
65 (3q)
71 (3r)
75 (3s)
N
N
N
N
N
N
N
N
10
11
trace
trace
N
N
O2N
O2N
N
N
12
13
trace
trace
N
H
N
H
N
N
Bz
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aReaction conditions: indole (1mmol), boronic acid (1.2 mmol), catalyst (0.05 mmol), oxidant (0.4 mmol), Solvent
(1 ml); temperature (50 °C), time (7 h), in air; byield.
6
7
8
9
In reactions involving immobilized palladium catalyst where the catalyst acts as reservoir of Pd species it
is important to elucidate the behavior of Pd/C catalyst in the current arylation reaction. Following
experiments were performed to recognize the behavior of palladium catalysis.
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Hot-filtration test
Hot-filtration test is performed to verify the heterogeneity of the catalyst. A round bottomed flask was
charged with 10% Pd/C (0.05 mmol, 5.3 mg) and AgTFA (0.4 mmol, 80 mg) in MeOH (1 ml) at 50 °C in
air. After 4h, the solid catalyst phase was filtered off using a Whatman filter paper (grade 41) at the
reaction temperature. The filtrate was collected in another round bottomed flask and indole (1 mmol) and
arylboronic acid (1.2 mmol) was added. After 24 h trace amount of product formation is detected. The
ICP-AES analysis of the liquid phase reveals residual Pd level of less than 0.01 ppm. This suggests the
heterogeneous nature of the active catalyst species.
Reusability test
Reusability is an important aspect for a catalyst. In the current protocol Pd catalyst was recycled for four
cycles with a little change in reactivity over the consecutive cycles as shown in Figure 1. A slight loss in
reactivity over the repeated use is probably due to physical loss of the catalyst.
Figure 1. Reusability of Pd/C over four cycles
From the experimental observations and literature findings, a plausible mechanism has been proposed as
shown in the Scheme 2. The first step is expected to be the generation of active PdII species 2 from Pd0
deposited on the surface of charcoal by the silver oxidant.21 Initially, arylboronic acid is activated by the
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PdII species forming the palladium intermediate 3. This step occurs first becomes evident from the
experimental observation that in presence of base complete self-coupling of arylboronic acids to
corresponding biaryls in 90% yield and no trace of arylation at indole was observed (Table 1, entry 20).
Then, in presence of strong π-nucleophile indole 4, electrophilic palladation takes place at the C-3
position of the indole ring (the most nucleophilic centre on the indole nucleus) to form an intermediate
species 5, which undergoes 1,2-migration. The driving force for this step comes from the presence of
TFA which is expected to provide an acidic environment which favors migration of a C3-Pd bond to
highly activated 2-position forming intermediate 6.22 The subsequent deprotonation forms intermediate 7
and reductive elimination of the later species forms 2-arylated indole 8 with the regeneration of Pd0
reabsorbed into the charcoal surface to complete the catalytic cycle. The mechanism makes the fact clear
why electron-rich indoles facilitate the arylation better than electron-poor indoles as the latter is less
prone to electrophilic attack.
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O
Ar
Ag
O
CF3
Pd/C
Pd/C
N
1
8
R
product
O
H
CF3
= Pd(TFA)2
ArB(OH)2
PdII
O
Pd Ar
2
2
N
R
7
-(TFA)B(OH)2
H
N
H
TFA Pd Ar
Pd Ar
3
R
6
H
1,2-migration
Pd Ar
H
N
N
4
R
5
R
Scheme 2. Plausible mechanism
The chemical bonding information and oxidation state of reused catalysts were examined by X-ray
photoelectron spectroscopy (XPS) analysis. The survey scans XPS spectrum of the reused sample is
shown in Figure 2a, which comprises five different sharp peaks at binding energies of 284.8, 336.9,
368.1, 532.2 and 688.7 eV signifying the presence of C, Pd, Ag, O and F, respectively (Figure 2a). The
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high-resolution deconvoluted Pd3d spectra show peaks at binding energies of 335.3 and 340.6 eV
indicating the presence of Pd(0) species (Figure 2b).23 Another two peaks of high-resolution Pd3d spectra
at binding energies of 337.2 and 342.6 eV corresponds to the Pd(II) species.23 The Pd(II) is attributed to
the formation of Pd(TFA)2 as shown in Scheme 2. The high-resolution Ag3d spectrum was obtained with
two binding energies peaks at 368.3 and 374.3 eV assigned as the spin-orbit splitting of Ag3d5/2 and
Ag3d3/2 respectively, which specify the metallic nature of Ag in the reused sample (Figure 2c).24 The
high-resolution C1s spectrum fitted with three different binding energies peaks at 284.7, 286.7 and 288.4
eV belong to the C-C, C-O and C=O bonds of TFA, respectively (Figure 2d).
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(b)
(a)
(c)
(d)
Figure 2. (a) Survey scans XPS spectrum and high resolution deconvoluted XPS spectra of (b) Pd3d, (c)
Ag3d and (d) C1s of reused sample.
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Conclusions
In summary, a new catalytic methodology has been demonstrated for the direct C-2 arylation of varied
indole derivatives with arylboronic acids using a relatively cheap and readily available form of palladium.
The experimental observation shows true heterogeneity of the catalyst. The arylation demonstrates
remarkable tolerance towards functional groups. The protocol offers substantial benefits as achieving high
C-2 selectivity without the use of any ligands or directing groups and reusability of the catalyst with the
retention of catalytic efficiency.
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Experimental section
General information
All reactions were carried out in Tarsons spinot digital magnetic stirrers under standard conditions. Rf
values were calculated by analytical thin layer chromatography (TLC) carried out on Merck silica gel
60F254 plates using short wave (254 nm) UV light. Column chromatography purifications were
performed over silica gel (100-200 mesh). 1H NMR and 13C NMR spectra were recorded on a JEOL JNM
ECS NMR spectrometer (400 MHz and 100 MHz respectively) using CDCl3 as solvent and TMS as
internal standard. Chemical shifts (δ) are reported in parts per million (ppm) relative to the central peak of
the solvent. And multiplicities are indicated as: s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet) and dd (doublet of doublet). Coupling constants (J values) are given in hertz (Hz). XPS
measurements were carried out using a Thermo-Scientific ESCALAB Xi+ spectrometer having a
monochromatic Al Kα X-ray source (1486.6 eV) and a spherical energy analyzer that operates in the CAE
(constant analyzer energy) mode. The CAE for the survey spectrum is 200 eV and that for high-resolution
spectra is 50 eV. ICP-AES analysis was performed on an ACROS ICP spectrometer. IR spectra are
recorded on a Perkin-Elmer Frontier MIR FIR spectrometer, the wavenumbers (n) of recorded IR-signals
are reported in cm−1. Melting points were recorded in a digital melting point apparatus and are
uncorrected. All chemicals used were purchased commercially from either Sigma Aldrich, Merck or Alfa
Aesar and used without further purification. Solvents used for extraction and chromatographic separations
were distilled prior use.
General procedures for the synthesis of N-substituted indoles: All of the N-substituted indoles were
prepared according to literatures.25
General procedure for C-2 arylation of indoles with arylboronic acids. A round bottomed flask was
charged with indole 1 (1 mmol) and arylboronic acid 2 (1.2 mmol) in presence of 10% Pd/C (0.05 mmol,
5.3 mg) and AgTFA (0.4 mmol, 80 mg) in MeOH (1 ml) at 50 °C for 7 h in air. After completion of
reaction (confirmed by TLC), reaction mixture was filtered on a Whatman filter paper (grade 41) to
remove the heterogeneous catalyst particles, filtrate poured into water and extracted with ethyl acetate.
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The organic layer dried over Na2SO4 and concentrate under reduced pressure. The residue was purified by
column chromatography (silica gel, hexane/ethyl acetate) to give the desired products 3a-3s.
For recycling experiments, the residue catalyst after centrifugation was washed with water and ethyl
acetate consecutively. The resultant catalyst was dried under vacuum and subjected to consequent run.
1-methyl-2-phenyl-1H-indole (3a). Following the general procedure, 3a was obtained as a white solid,
178 mg, 86% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 98-100 °C; 1H NMR (400 MHz, CDCl3): δ 7.64 (d,
J=8.0 Hz, 1H), 7.52-7.44 (m, 4H), 7.41-7.35 (m, 2H), 7.25 (t, J=8.2 Hz, 1H), 7.14 (t, J=7.4 Hz, 1H), 6.56
(s, 1H), 3.74 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 141.5, 138.3, 132.8, 129.3, 128.5, 127.9, 127.8,
121.6, 120.5, 119.8, 109.6, 101.6, 31.1. IR (KBr plate, cm-1): υ 3432, 3054, 1601, 1467, 1433, 1340,
1008, 845, 797, 768, 750, 734, 702.
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2-(4-methoxyphenyl)-1-methyl-1H-indole (3b). Following the general procedure, 3b was obtained as a
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white solid, 206 mg, 87% yield; Rf= 0.36 (PE/EtOAc 9.8:0.2); mp 118-120 °C; H NMR (400 MHz,
CDCl3): δ 7.62 (d, J=7.8 Hz, 1H), 7.48-7.41 (m, 2H), 7.34 (d, J=8.2 Hz, 1H), 7.23 (t, J=7.1 Hz, 1H), 7.13
13
(t, J=7.8 Hz, 1H), 7.00-6.94 (m, 2H), 6.50 (s, 1H), 3.86 (s, 3H), 3.71 (s, 3H); C{1H} NMR (100 MHz,
CDCl3): δ 159.4, 141.4, 138.1, 130.6, 127.9, 125.2, 121.3, 120.2, 119.7, 113.8, 109.5, 100.9, 55.3, 31.0.
IR (KBr plate, cm-1): υ 3437, 3049, 2927, 1609, 1496, 1464, 1246, 1176, 1035, 842, 784, 746, 736.
2-(4-chlorophenyl)-1-methyl-1H-indole (3c). Following the general procedure, 3c was obtained as a
1
colorless liquid, 196 mg, 81% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); H NMR (400 MHz, CDCl3): δ 7.64
(d, J=6.0 Hz, 1H), 7.44 (m, 4H), 7.36 (d, J=8.2 Hz, 1H), 7.28-7.24 (m, 1H), 7.15 (t, J=7.2 Hz, 1H), 6.55
(s, 1H), 3.73 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 140.2, 138.4, 134.0, 131.3, 130.5, 128.7, 127.8,
121.9, 120.5, 120.0, 109.6, 102.0, 31.2. IR (KBr plate, cm-1): υ 3437, 3059, 2923, 1604, 1471, 1383,
1328, 1068, 918, 827, 784, 698, 553.
2-(4-fluorophenyl)-1-methyl-1H-indole (3d). Following the general procedure, 3d was obtained as a
pale yellow solid, 174 mg, 77% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 118-120 °C; 1H NMR (400 MHz,
CDCl3): δ 7.63 (d, J=7.8 Hz, 1H), 7.48-7.44 (m, 2H), 7.35 (d, J=7.7 Hz, 1H), 7.27-7.23 (m, 1H), 7.17-
7.13 (m, 3H), 6.52 (s, 1H), 3.71 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 162.6 (d, J=248.3 Hz),
140.4, 138.2, 131.1 (d, J=8.6 Hz), 128.9, 127.8, 121.7, 120.5, 119.8, 115.5 (d, J=22.1 Hz), 109.6, 101.6,
31.1. IR (KBr plate, cm-1): υ 3430, 3045, 2946, 1603, 1548, 1497, 1465, 1338, 1223, 1156, 844, 797, 745,
549.
1-methyl-2-(m-tolyl)-1H-indole (3e). Following the general procedure, 3e was obtained as a colorless
1
liquid, 184 mg, 83% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); H NMR (400 MHz, CDCl3): δ 7.63 (d, J=8.8
Hz, 1H), 7.37-7.29 (m, 4H), 7.26-7.20 (m, 2H), 7.14 (t, J=7.9 Hz, 1H), 6.55 (s, 1H), 3.73 (s, 3H), 2.42 (s,
13
3H); C{1H} NMR (100 MHz, CDCl3): δ 141.7, 138.3, 138.2, 132.6, 130.1, 128.7, 128.3, 127.8, 126.4,
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121.6, 120.4, 119.8, 109.6, 101.5, 31.1, 21.5. IR (KBr plate, cm-1): υ 3437, 3051, 3031, 1627, 1605, 1496,
1465, 1349, 859, 766, 736, 702.
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1-methyl-2-(3-nitrophenyl)-1H-indole (3f). Following the general procedure, 3f was obtained as a
yellow solid, 222 mg, 88% yield; Rf= 0.32 (PE/EtOAc 9.8:0.2); mp 138-140 °C; 1H NMR (400 MHz,
CDCl3): δ 8.40 (s, 1H), 8.27 (dd, J=8.2, 3.3 Hz, 1H), 7.86 (d, J=7.7 Hz, 1H), 7.67 (t, J=7.9 Hz, 2H), 7.41
(d, J=8.3 Hz, 1H), 7.32 (t, J=8.2 Hz, 1H), 7.19 (t, J=7.4 Hz, 1H), 6.68 (s, 1H), 3.80 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): δ 148.4, 138.7, 138.6, 134.9, 134.5, 129.6, 127.7, 123.8, 122.6, 122.5, 120.9,
120.3, 109.8, 103.3, 31.3. IR (KBr plate, cm-1): υ 3425, 3082, 2924, 1611, 1576, 1546, 1518, 1485, 1463,
1428, 1353, 816, 785, 766, 753, 614.
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1-methyl-2-(3-(trifluoromethyl)phenyl)-1H-indole (3g). Following the general procedure, 3g was
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obtained as a Yellow liquid, 201 mg, 73% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); H NMR (400 MHz,
CDCl3): δ 7.78 (s, 1H), 7.70-7.64 (m, 3H), 7.59 (t, J=7.7 Hz, 1H), 7.38 (d, J=8.2 Hz, 1H), 7.28 (t, J=8.2
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Hz, 1H), 7.17 (t, J=7.4 Hz, 1H), 6.62 (s, 1H), 3.75 (s, 1H); C{1H} NMR (100 MHz, CDCl3): δ 139.8,
138.6, 133.6, 132.4 (q, J=32.0 Hz), 131.2, 130.9, 129.0, 127.8, 125.9 (q, J=3.9 Hz), 124.4 (q, J=270.6
Hz), 122.1, 120.7, 120.1, 109.7, 102.6, 31.2. IR (KBr plate, cm-1): υ 3429, 3059, 1615, 1467, 1411, 1344,
1173, 1118, 1107, 1069, 849, 804, 789, 733.
1-(4-(1-methyl-1H-indol-2-yl)phenyl)ethan-1-one (3h). Following the general procedure, 3h was
obtained as a yellow solid, 175 mg, 70% yield; Rf= 0.46 (PE/EtOAc 9:1); mp 121-124 °C; 1H NMR (400
MHz, CDCl3): δ 8.06 (d, J=8.6 Hz, 2H), 7.66-7.61 (m, 3H), 7.38 (d, J=8.3 Hz, 1H), 7.28 (t, J=7.0 Hz,
1H), 7.16 (t, J=7.3 Hz, 1H), 6.66 (s, 1H), 3.78 (s, 3H), 2.65 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ
197.6, 140.2, 138.8, 137.4, 136.0, 129.1, 128.6, 127.8, 122.4, 120.8, 120.1, 109.7, 102.9, 31.5, 26.6. IR
(KBr plate, cm-1): υ 3436, 3050, 2923, 2852, 1738, 1679, 1601, 1463, 1355, 1269, 1166, 837, 789, 742,
598.
1-methyl-2-(o-tolyl)-1H-indole (3i). Following the general procedure, 3i was obtained as a white solid,
173 mg, 78% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 88-90 °C; 1H NMR (400 MHz, CDCl3): δ 7.65 (d,
J=7.5 Hz, 1H), 7.38-7.25 (m, 6H), 7.16 (t, J=7.4 Hz, 1H), 6.43 (s, 1H), 3.52 (s, 3H), 2.21 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): δ 141.7, 138.3, 138.2, 132.7, 130.1, 128.6, 128.3, 127.9, 126.4, 121.5, 120.4,
119.8, 109.6, 101.5, 31.2, 21.5. IR (KBr plate, cm-1): υ 3433, 3051, 2962, 1601, 1584, 1496, 1452, 1383,
1349, 1076, 784, 737, 704.
2-(4-(tert-butyl)phenyl)-1-methyl-1H-indole (3j). Following the general procedure, 3j was obtained as
a white solid, 224 mg, 85% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 108-110 °C; 1H NMR (400 MHz,
CDCl3): δ 7.62 (d, J=7.8 Hz, 1H), 7.46 (q, J=8.6 Hz, 4H), 7.35 (d, J=8.2 Hz, 1H), 7.23 (t, J=7.0 Hz, 1H),
7.13 (t, J=7.0 Hz, 1H), 6.54 (s, 1H), 3.74 (s, 3H), 1.37 (s, 9H); 13C{1H} NMR (100 MHz, CDCl3): δ
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150.9, 141.6, 138.3, 129.9. 129.0, 128.0, 125.4, 121.4, 120.3, 119.7, 109.5, 101.3, 58.2, 34.7, 31.3. IR
(KBr plate, cm-1): υ 3423, 3046, 2960, 1540, 1465, 1383, 1344, 1308, 789, 776, 751, 740.
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5-methoxy-1-methyl-2-phenyl-1H-indole (3k). Following the general procedure, 3k was obtained as a
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white solid, 200 mg, 84% yield; Rf= 0.52 (PE/EtOAc 9.8:0.2); mp 115-117 °C; H NMR (400 MHz,
CDCl3): δ 7.51-7.37 (m, 5H), 7.25 (d, J=8.9 Hz, 1H), 7.10 (d, J=2.4 Hz, 1H), 6.91 (dd, J=8.9, 2.5 Hz,
1H), 6.49 (s, 1H), 3.86 (s, 3H), 3.71 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 154.3, 142.2, 133.8,
132.9, 129.2, 128.5, 128.2, 127.8, 111.9, 110.3, 102.1, 101.2, 56.0, 31.2. IR (KBr plate, cm-1): υ 3037,
2985, 1615, 1609, 1571, 1496, 1472, 1377, 1312, 1181,1034, 843, 822, 766, 732.
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5-methoxy-2-(4-methoxyphenyl)-1-methyl-1H-indole (3l). Following the general procedure, 3l was
obtained as a white solid, 233 mg, 87% yield; Rf= 0.40 (PE/EtOAc 9.5:0.5); mp 111-113 °C; 1H NMR
(400 MHz, CDCl3): δ 7.42 (d, J=8.9 Hz, 2H), 7.25 (s, 1H), 7.09 (d, J=2.5 Hz, 1H), 7.00 (d, J=8.7 Hz,
2H), 6.89 (dd, J=8.8, 2.5 Hz, 1H), 6.42 (s, 1H), 3.87 (s, 6H), 3.69 (s, 3H); 13C{1H} NMR (100 MHz,
CDCl3): δ 159.4, 154.3, 142.0, 133.6, 130.5, 128.2, 125.3, 113.9, 111.5, 110.2, 102.0, 100.6, 55.9, 55.3,
31.1. IR (KBr plate, cm-1): υ 3440, 3109 2955, 2921, 2837, 1905, 1612, 1480, 1248, 1219, 1176, 789.
1,3-dimethyl-2-phenyl-1H-indole (3m). Following the general procedure, 3m was obtained as a
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colorless liquid, 182 mg, 82% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); H NMR (400 MHz, CDCl3): δ 7.60
(d, J=7.8 Hz, 1H), 7.49 (t, J=6.6 Hz, 2H), 7.41 (t, J=6.8 Hz, 3H), 7.33 (d, J=8.2 Hz, 1H), 7.25 (t, J=7.6
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Hz, 1H), 7.15 (t, J=7.4 Hz, 1H), 3.61 (s, 3H), 2.28 (s, 3H); C{1H} NMR (100 MHz, CDCl3): δ 137.6,
137.2, 132.1, 130.6, 128.4, 128.3, 127.7, 121.7, 119.1, 118.8, 109.2, 108.6, 30.9, 9.1. IR (KBr plate, cm-
1): υ 3047, 3132, 2962, 1601, 1568, 1496, 1400, 1349, 1176, 782, 768, 737, 704.
1,7-dimethyl-2-phenyl-1H-indole (3n). Following the general procedure, 3n was obtained as a white
solid, 195 mg, 88% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 67-69 °C; 1H NMR (400 MHz, CDCl3): δ
7.50-7.43 (m, 5H), 7.41-7.37 (m, 1H), 7.00 (t, J=7.4 Hz, 1H), 6.94 (d, J=6.8 Hz, 1H), 6.51 (s, 1H), 3.92
(s, 3H), 2.81 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 142.7, 137.6, 133.0, 129.6, 128.8, 128.3, 124.6,
121.4, 120.0, 118.7, 102.4, 99.8, 34.4, 20.1. IR (KBr plate, cm-1): υ 3436, 3051, 3031, 2926, 1601, 1581,
1477, 1451, 1375, 1325, 1075, 789, 766, 736, 702.
1-benzyl-2-phenyl-1H-indole (3o). Following the general procedure, 3o was obtained as a colorless
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liquid, 255 mg, 90% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); H NMR (400 MHz, CDCl3): δ 7.68-7.66 (m,
1H), 7.45-7.42 (m, 2H), 7.40-7.36 (m, 3H), 7.29-7.22 (m, 3H), 7.17-7.13 (m, 3H), 7.03 (d, J=6.6 Hz, 2H),
6.65 (s, 1H), 5.36 (s, 2H); 13C{1H} NMR (100 MHz, CDCl3): δ 141.8, 138.2, 137.9, 132.7, 129.2, 128.7,
128.5, 128.3, 128.0, 127.1, 125.9, 121.9, 120.5, 120.1, 110.5, 102.3, 47.7. IR (KBr plate, cm-1): υ 3427,
3072, 3045, 2962, 1604, 1496, 1462, 1451, 1306, 1158, 781, 762, 749, 728, 614.
5-chloro-1-methyl-2-phenyl-1H-indole (3p). Following the general procedure, 3p was obtained as a
white solid, 164 mg, 68% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 87-88 °C; 1H NMR (400 MHz, CDCl3):
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δ 7.59 (d, J=2.0 Hz, 1H), 7.49-7.41 (m, 5H), 7.26 (t, J=8.7 Hz, 1H), 7.18 (dd, J=8.7, 2.0 Hz, 1H), 6.49 (s,
1H), 3.72 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 142.8, 136.7, 132.3, 129.3, 128.8, 128.6, 128.2,
125.4, 121.8, 119.8, 110.6, 101.1, 31.2. IR (KBr plate, cm-1): υ 3437, 3073, 3059, 2922, 1746, 1471,
1383, 1067, 881, 778, 767, 698, 553.
5-bromo-1-methyl-2-phenyl-1H-indole (3q). Following the general procedure, 3q was obtained as a
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white solid, 186 mg, 65% yield; Rf= 0.58 (PE/EtOAc 9.8:0.2); mp 108-110 °C; H NMR (400 MHz,
CDCl3): δ 7.75 (d, J=1.9 Hz, 1H), 7.50-7.40 (m, 5H), 7.31 (dd, J = 8.7, 1.9 Hz, 1H), 7.22 (t, J=8.7 Hz,
1H), 6.49 (s, 1H), 3.71 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 142.7, 137.0, 132.2, 129.5, 129.3,
128.5, 128.2, 124.2, 122.8, 113.0, 110.9, 100.9, 31.2. IR (KBr plate, cm-1): υ 3436, 2921, 1602, 1465,
1382, 1204, 1049, 866, 793. 758, 694, 548.
1-methyl-2-phenyl-1H-pyrrolo[2,3-b]pyridine (3r). Following the general procedure, 3r was obtained
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as a colorless liquid, 148 mg, 71% yield; Rf= 0.40 (PE/EtOAc 9.5:0.5); H NMR (400 MHz, CDCl3): δ
8.35 (dd, J=4.7, 1.5 Hz, 1H), 7.91 (dd, J=7.8, 1.6 Hz, 1H), 7.58–7.42 (m, 5H), 7.10 (dd, J=7.8, 4.7 Hz,
1H), 6.52 (s, 1H), 3.89 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 149.2, 142.6, 141.9, 132.3, 129.1,
128.6, 128.3, 128.2, 120.7, 116.1, 99.4, 29.9. IR (KBr plate, cm-1): υ 3429, 3056, 2924, 1722, 1594, 1485,
1404, 1370, 1318, 1226, 1108, 773, 755, 699, 538.
1-methyl-2,5-diphenyl-1H-pyrrole (3s). Following the general procedure, 3s was obtained as a white
1
solid, 175 mg, 75% yield; Rf= 0.58 Rf= 0.46 (PE/EtOAc 9:1); mp 198-200 °C; H NMR (400 MHz,
CDCl3): δ 7.48 (dd, J=8.3, 1.4 Hz, 4H), 7.44-7.40 (m, 4H), 7.33-7.29 (m, 2H), 6.32 (s, 2H), 3.61 (s, 3H);
13C{1H} NMR (100 MHz, CDCl3): δ 136.9, 133.5, 128.7, 128.4, 126.6, 108.7, 34.4. IR (KBr plate, cm-1):
υ 3437, 3061, 2993, 2920, 2852, 1960, 1599, 1465, 1331, 1077, 918, 754, 699, 506.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgments
Authors are grateful to SERB, New Delhi for providing financial support (No. EMR/2016/005944) and
CSIR, New Delhi for providing CSIR-Junior Research Fellowship to PB
Footnotes
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam, India, E-mail: ubora@tezu.ernet.in
& utbora@yahoo.co.in.
†Electronic Supplementary Information (ESI) available: Copies of H and 13C NMR spectra of the
1
compounds.
ORCID
Utpal Bora: 0000-0002-7403-0152
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Table of contents
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We report a methodology for exclusive C-2 arylated indoles without the aid of any ligand or directing
group in air.