Pd-NHC catalyzed Suzuki–Miyaura couplings on 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide

Article abstract of DOI:10.1080/00397911.2019.1614194

A series of novel α-carboline derivatives such as 3-aryl-9H-pyrido[2,3-b]indole-6-sulfonamides have been synthesized from the readily available low-cost raw material, benzotriazole which is subjected to nucleophilic aromatic substitution with 5-bromo-2-ch

Full text of DOI:10.1080/00397911.2019.1614194

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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Pd-NHC catalyzed Suzuki–Miyaura couplings on 3-
bromo-9H-pyrido[2,3-b]indole-6-sulfonamide
Sura Mallikarjun Reddy, Bijivemula N. Reddy, Venkata Krishna Reddy
Motakatla, Anusha Gokanapalli, Madhvesh Pathak & Peddiahgari Vasu
Govardhana Reddy
To cite this article: Sura Mallikarjun Reddy, Bijivemula N. Reddy, Venkata Krishna Reddy
Motakatla, Anusha Gokanapalli, Madhvesh Pathak & Peddiahgari Vasu Govardhana Reddy (2019):
Pd-NHC catalyzed Suzuki–Miyaura couplings on 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide,
Synthetic Communications, DOI: 10.1080/00397911.2019.1614194
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https://doi.org/10.1080/00397911.2019.1614194
Pd-NHC catalyzed Suzuki–Miyaura couplings on 3-bromo-
9H-pyrido[2,3-b]indole-6-sulfonamide
a
a
b
Sura Mallikarjun Reddy , Bijivemula N. Reddy , Venkata Krishna Reddy Motakatla ,
b
a
b
Anusha Gokanapalli , Madhvesh Pathak , and Peddiahgari Vasu Govardhana Reddy
a
b
Department of Chemistry, Vellore Institute of Technology, Vellore, India; Department of Chemistry,
Yogi Vemana University, Kadapa, India
ABSTRACT
ARTICLE HISTORY
A series of novel a-carboline derivatives such as 3-aryl-9H-pyrido[2,3-
b]indole-6-sulfonamides have been synthesized from the readily
available low-cost raw material, benzotriazole which is subjected to
nucleophilic aromatic substitution with 5-bromo-2-chloropyridine fol-
lowed by sequential steps of cyclization, sulfonation, amidation, and
finally Suzuki–Miyaura coupling reactions. The C–C bond formation
between 3-bromo-9H-pyrido[2.3-b]indole-6-sulfonamide and various
boronic acids was achieved with more accessible palladium pre-cata-
lyst, Pd-PEPPSI-IPr via Suzuki coupling under microwave condition in
a short reaction time with excellent yields.
Received 3 March 2019
KEYWORDS
3
-Aryl-9H-pyrido[2,3-
b]indole-6-sulfonamide
derivatives (a-carbolines);
benzotriazole; Pd-NHC
catalyst: PEPPSI-IPr;
Suzuki–Miyaura couplings
GRAPHICAL ABSTRACT
Introduction
Heterocycles with/without any substituents represent excellent biological activities,
which clearly indicate the importance of heterocyclic core as a key part of the pharma-
cophore. Many synthetic drugs, bioactive natural products, and agrochemicals encom-
[
1]
pass nitrogen-heterocycle scaffolds.
Among them, a-carboline derivatives are one
important class of nitrogen-containing heterocyclic motifs, besides well-known b-carbo-
lines and other heterocyclic keys. a-Carboline alkaloids are quite rare and only a few of
the natural products isolated to date possess this a-carboline core. Prominent examples
among them are grossularine-1 (A), grossularine-2 (B) (Figure 1) extracted from
Dendrodoagrossularia which work effectively on L1210 leukemia cell line and exhibit
[
2]
cytotoxicity against solid human tumor cell lines. Another a-carboline compound,
CONTACT Peddiahgari Vasu Govardhana Reddy
pvgr@yogivemanauniversity.ac.in
Department of Chemistry, Yogi
Vemana University, Kadapa, 516005 Andhra Pradesh, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
S. MALLIKARJUN REDDY ET AL.
Figure 1. Structures of three a-carboline natural products.
mescengricin (C) (Figure 1) shows the ability to completely prevent glutamate toxicity
[
3]
induced in chick primary mesencephalic neurons with an EC50 of 6.0 nM.
a-Carboline derivatives also find other biological and medicinal applications including
[
4–7]
anxiolytic, antiviral, antileishmanial and anti-inflammatory activities.
The most common synthetic approaches for this class of compounds have often been
modified from known routes to related carbazole compounds. The first synthesis of a
[
8]
simple a-carboline was reported by Robinson et al. a-Carbolines preparation was
[
9–11]
based on a modification of Garbee–Ulmann synthesis.
Various teams proposed the
[
12]
construction of a-carbolines following inter/intramolecular cyclization reactions.
A
different route involving Buchwald–Hartwig cross-coupling and intramolecular Heck-
type reaction from 2-halopyridine and aromatic amines proposed by Kumar and
[
13]
Nagarajan.
intramolecular amination of 3-(2-chlorophenyl)-2-amiinopyridines to a-carbolines.
Very recently Hachiya and his coworkers established palladium-catalyzed
[
14]
Song Ye et al. described the N-heteocyclic carbene catalyzed [3 þ 3] annulation of indo-
line-2-imines and bromenals for the synthesis of a-carbolines under microwave
[
15]
irradiation.
To continue our efforts on finding the applications of variously available NHCs and
[
16]
Pd-NHCs over C–C, C–N, and C–heteroatom bond formations,
we now report the
synthesis of unexisting a-carboline derivatives via Suzuki–Miyaura coupling reaction.
[
17]
Since the early work of Suzuki
and his coworkers, several reports are made avail-
able on Suzuki–Miyaura coupling reaction towards the construction of simple to com-
[
18]
plex organic frameworks.
Structurally distinct palladium complexes are being
[
19]
prepared by various researchers and been applied to diverse coupling reactions.
Recently, Priya and Ramaswamy reported bulky 2,6-dibenzhydryl-4-methylphenyl-b-dii-
minato derived complexes of Pd(II), Cu(II), and Co(II) catalysts for Suzuki coupling
[
20]
and alcohol oxidation.
Apart from the simple catalysts, phosphine based palladium
[
21]
catalysts are also known to execute Suzuki–Miyaura couplings.
However, the stability
of the catalyst, substrate scope, ease of handling, rate of reaction, reaction conditions,
etc., still encourage the associates to find even new reagents for the synthesis of potent
building blocks. In this perception, recently Pd-NHC catalysts have come into light to
show wide applicability with broad substrate scope and high stability. Ofele and
[
22a,b]
[22c]
Wanzlick identified NHCs
and later successfully isolated by Arduengo.
NHCs
were first time used for the Suzuki–Miyaura coupling reaction by Capretta and his

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SYNTHETIC COMMUNICATIONS
3
[
23]
coworkers.
C–N, and C–heteroatom bond formations.
new efficient catalysts for Suzuki–Miyaura coupling reaction.
bimetallic NHC complexes for C–C bond formation was presented by Majumder and
Later, several novel M-NHCs were synthesized and applied for C–C,
[
24]
Recently, Varma and his team reported
[
25]
Homo and hetero
[
26]
his coworkers.
Though there are several reports on NHC catalyzed C–C bond forma-
tion reactions, their application on building carbolines particularly sulfonamide bearing
a-carbolines is not yet reported. Earlier, we have synthesized a series of b-carbolines via
[
1a]
diastereoselective Pictet–Spengler domino method.
and now we explore the NHC
catalyzed the synthesis of 3-aryl-9H-pyrido[2,3-b]indole-6-sulfonamides from the simple
precursor benzotriazole using a mild and efficient pathway as shown in Scheme 1.
Results and discussions
Synthetic route for 3-aryl-9H-pyrido[2,3-b]indole-6-sulfonamides
Initially, 3-bromo-9H-pyrido[2,3-b]indole was prepared following the existing literature
report via nucleophilic aromatic substitution on benzotriazole (1) with 5-bromo-2-
chloropyridine (2) and subsequent cyclization with PPA under MW irradiation condi-
[
11]
tions.
3-Bromo-9H-pyrido[2,3-b]indole was then subjected to sequential chlorosulfo-
nylation and amidation steps to obtain 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide
[
27]
(5) from the work of Mahmoud and his coworkers.
Finally, compound 5 allowed to
react with various aryl/heteroaryl boronic acids in the presence of Pd-NHC catalyst via
Suzuki–Miyaura coupling reaction under microwave condition to obtain a series of 3-
aryl-9H-pyrido[2,3-b]indole-6-sulfonamides (6a–f) (Scheme 1) and the reaction condi-
tion is optimized in Table 1.
Reagents and conditions: (a) Benzotriazine (3 mmol), 3-bromo-5-chloropyridine
ꢀ
(
(
3.05 mmol), neat, 50 W/180 C, MW and 4 h; (b) Compound-3 (3 mmol), PPA
ꢀ
5 mol%), neat, 160 C, MW and 2 h; (c) Compound-4 (2.5 mmol), H SO Cl (3.2 mmol),
2
3
ꢀ
ꢀ
NH (3.8 mmol), THF, ꢁ10 C to RT (25 C); (d) Compound-5 (1 mmol), Ar-B(OH)
3
2
ꢀ
(
1.01 mmol), Pd Catalyst (5 mol%), K CO (3.0 mmol), 1,4-Dioxane:H O, 100 C, MW
2
3
2
irradiation.
Scheme 1. Synthesis of sulfonamide bearing a-carbolines.

4
S. MALLIKARJUN REDDY ET AL.
Table 1. Study on the effect of various palladium catalysts for C–C bond formation.
a
Entry
Catalyst
Ligand
Time (HH:MM)
Yield (%)
b
1
Pd(OAc)
PdCl
2
None
None
TPP
TPP
TPP
TPP
TPP
Sphos
Dppf
None
None
None
None
03:00
03:00
03:00
02:30
02:30
02:30
02:30
02:30
02:30
02:00
02:00
02:00
02:00
0
0
b
2
2
b
3
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
<11
26
32
39
38
32
41
47
47
51
77
c
4
d
5
e
6
f
7
8
e
e
9
1
1
1
1
e
e
e
e
0
1
2
3
3 4
Pd(PPh )
PdCl
PdCl
2
(Tpp)
(Dppf)2.DCM
2
2
1,3-Bis(2,6-diisopropylphenyl)-imi-
dazol-2-ylidene(1,4-naphtho
quinone)palladium(0)dimer
1,3-Bis(2,4,6-trimethylphenyl) imi-
dazol-2-ylidene(1,4-naphtho
quinone)palladium(0)dimer
Allyl [1,3-bis(mesityl)imidazol-2-
ylidene]chloropalladium(II)
[1,3-Bis(2,6-diisopropylphenyl)imi-
dazol-2-ylidene]chloro[3-
e
1
4
None
02:00
75
e
e
1
1
5
None
None
01:30
01:30
81
83
6
phenylallyl]palladium(II)
SingaCycle -A1
e
e
e
e
g
TM
17
18
19
20
21
None
None
None
None
None
01:30
00:30
00:45
00:45
00:45
88
94
98
95
92
Pd-PEPPSI-IPent
PEPPSI-IPr
PEPPSI-SONO-SP
2
PEPPSI-IPr
2 2 3
Reaction conditions: Compound 5 (1.0 mmol), Ph-B(OH) (1.01 mmol), catalyst (5 mol%), ligand (5 mol%), K CO (3eq),
b c
a
ꢀ
2
,4-dioxane:H O (10 mL), MW condition. Isolated yield after column purification. At room temperature (25 C). At 50
d e f g
1
˚
ꢀ
ꢀ
ꢀ
C. At 80 C. At 100 C. At 120 C. Compound 5 (1.0 mmol), Phenylboronic acid pinacol ester (1.01 mmol), PEPPSI-
IPr (5 mol%), K CO (3 equiv.), 1,4-Dioxane:H O (10 mL), MW condition.
2
3
2
Suzuki–Miyaura cross-coupling reaction
Different parameters such as catalyst, ligand, solvent, and temperature systems were
studied to develop optimum condition for the Suzuki–Miyaura coupling reaction on
compound-5. For this, a model reaction was carried out between 3-bromo-9H-pyr-
ido[2,3-b]indole-6-sulfonami-de and phenylboronic acid. In this connection, we have
optimized the reaction conditions with various PdNHC catalysts along with conven-
tional catalysts and ligands to identify the efficient catalyst for the C–C bond formation
reactions via Suzuki–Miyaura cross-coupling reaction (Table 1). At first, reaction was
conducted with simple palladium salts like Pd(OAc) and PdCl in the absence of any
2
2
added ligand at room temperature for 3 h, but no product was obtained in both the
cases (Table 1, entries 1 and 2). In continuation of our trials, reaction was performed
with Pd(OAc) catalyst and triphenyl phosphine (Tpp) ligand at room temperature and
2

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SYNTHETIC COMMUNICATIONS
5
was able to observe a small amount of product formation (Table 1, entry 3). The same
ꢀ
reaction was continued to stir at 50 and 80 and 100 C and identified a small progress
in yield of 26%, 32%, and 39%, respectively, in comparatively less reaction time (Table
ꢀ
1
, entries 4, 5, and 6). Temperature of 120 C was also tested, however, did not result in
ꢀ
improvement of product yield (Table 1, entry 7). Hence, 100 C was chosen as an opti-
mal temperature condition for carrying out supplementary reactions. In further studies,
experiment was repeated with Pd(OAc) in the presence of other ligands like 2-dicyclo-
2
0
0
hexylphosphino-2 ,6 -dimethoxybiphenyl(Sphos) and bis(diphenylphoshino)ferrocene
Dppf) and small increase in yields was observed (Table 1, entries 8 and 9).
Subsequently, catalysts like tetrakis (triphenylphosphine)palladium(0) (Pd(PPh ) ),
(
3
4
PdCl (Tpp) , and PdCl (Dppf) . DCM also tested without any ligand and
2
2
2
2
obtained moderate results (Table 1, entries 10–12). In our next examination, Pd-NHC
dimer catalysts like 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene(1,4-naphthoquino-
ne)palladium(0)dimer, 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene(1,4-naphthoqui-
none)palladium(0)dimer, monomer catalysts like allyl[1,3-bis(mesityl)imidazol-2-
ylidene]chloropalladium(II), [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]chloro[3-
TM
phenylallyl]palladium(II), palladacycles like SingaCycle -A1 and PEPPSI type catalysts
2
like Pd-PEPPSI-IPent, PEPPSI-IPr, PEPPSI-SONO-SP were tested and identified com-
paratively better results (Table 1, entries 13–20). However, among all the tested Pd-
NHC catalysts, the highest yield of 98% was obtained with Pd-PEPPSI-IPr in just
30 min (Table 1, entry 19) due to the increased steric crowding on the phenyl rings of
imidazolium cycle and the presence of pyridine ring system. The same reaction was
conducted between 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide and phenylboronic
acid pinacol ester at the same condition and able to obtain similar result (Table 1,
entry 20).
Effect of solvent
Further, we wish to identify the role of solvents on the above model reaction as they
play a key role in the C–C bond formation reactions. Recently, aqueous medium sup-
[
28]
ported Suzuki reactions also reported by various teams.
Hence, a thorough investiga-
tion on the effect of solvent also carried out, and the results are summarized in Table 2.
Among the different solvents and solvent systems tested, lower yields were obtained in
the case of reactions in dimethyl sulfoxide and water while the best yield of product
(98%) was gained in 1,4-dioxane:water (1:1) system as shown in Table 2.
Rate of reaction
In our next attempt, enquiry at finding the required load of catalyst for the effective
completion of reaction was carried out. Originally, reaction was started with 0.5 mol%
of catalyst and obtained a 27% of yield in 30 min. Next, the catalyst load is increased to
1
mol%, 3 mol%, and 5 mol% and obtained the product in 56%, 79%, and 98%, respect-
ively, in just 30 min (Figure 2). A further increase in catalyst load to 7 mol% was also
tested and identified no improvement in product formation. Hence, 5 mol% of catalyst
is more ample to complete the reaction in just 30 min.

6
S. MALLIKARJUN REDDY ET AL.
Table 2. Optimization of solvents.
a
Entry
Solvent
Time (HH:MM)
Yield (%)
1
2
3
4
5
6
7
8
9
Toluene
01:00
01:00
01:00
01:00
01:00
01:00
01:30
01:00
01:00
00:30
89
72
69
77
71
71
45
98
86
95
Dimethylformamide
Dimethylsulfoxide
1,4-Dioxane
THF
2-Methyl tetrahydrofuran
Water
1,4-Dioxane:Water (1:1)
Ethanol:Water (1:1)
Toluene:Ethanol:Water
10
Reaction conditions: 3-Bromo-9H-pyrido[2,3-b]indole-6-sulfonamide (1.0 mmol), phenylbor-
onic acid (1.01 mmol), PEPPSI-IPr (5 mol%), K CO (3eq), Solvent, MW condition and
2
3
ꢀ
00 C. Isolated yield after column purification.
a
1
Figure 2. Rate of reaction.
From the above all model reactions, it can be clearly evidenced that the best optimal
condition for the preparation of 3-aryl-9H-pyrido[2,3-b]indole-6-sulfonamides (6a–f) is
ꢀ
PEPPSI-IPr catalyst (5 mol%), K CO (3 mmol), 1,4-dioxane:water at 100 C in MW con-
2
3
dition (Table 3).
With the optimized reaction condition in hand, we then switched to monitor the
scope of the reaction and efficiency of the catalyst towards the construction of various
3-aryl-9H-pyrido[2,3-b]indole-6-sulfonamide derivatives. Coupling of simple and substi-
tuted phenylboronic acids took place more resourcefully in nearly same time with a
small variation in yields. (3-cyclohexylphenyl)boronic acid coupled very effectively in
contrast to those available in the early reports in 94% yield. Phenylboronic acid bearing
electron donating groups like –CH NH , C H , –OCH C H coupled and gave the
2
2
6
11
2
6
5
desired product in excellent yield with shorter reaction time (6b–d) while that contain-
ing withdrawing groups like –CN also gave good yield with extended reaction time (6e)

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SYNTHETIC COMMUNICATIONS
7
Table 3. Suzuki coupling reaction of 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide with various bor-
onic acids.
Reaction conditions: 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide (1.0 mmol), boronic acid (1.01 mmol), PEPPSI-IPr
ꢀ
a
(
5 mol%), K CO (3 equiv.), 1-4-dioxane:H O, and 100 C in MW condition. Isolated yield after column purification.
2 3 2
which clearly indicates that the catalyst works effectively in both the cases. On the other
hand, reaction with heteroarylboronic acids like pyridine-3-boronic acid and 3-thienyl-
boronic acid appears to be sluggish due to less solubility of the compounds.
Conclusion
In conclusion, we have reported the synthesis of various 3-aryl-9H-pyrido[2,3-b]indole-
6-sulfonamide derivatives (6a-f) from 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide
and various arylboronic acids by the use of various competent Pd-NHC complexes. We
observed that among all the catalysts, PEPPSI-IPr catalyst gave excellent yields at a short
reaction time of nearly 30 min. The result contributes to the steric factor from phenyl
rings of imidazolium system and pyridine ring which effectively catalyze the
Suzuki–Miyaura coupling. Also, to the delight, it is the first report of arylation of a-car-
boline derivatives with PEPPSI-IPr catalyst and moreover 5 mol% of the catalyst is
enough to carry out the reactions even in the absence of any ligand. Hence, the pro-
posed work could offer an important and beneficial alternative catalytic system toward
C–C bond forming reactions.

8
S. MALLIKARJUN REDDY ET AL.
Experimental
General procedure for the synthesis of 3-(substituted phenyl)-9H-pyrido[2,3-
b]indole-6-sulfonamide (6a–6f)
Into a microwave vial containing 3-bromo-9H-pyrido[2,3-b]indole-6-sulfonamide (com-
pound-5) (1 equiv.) and arylboronic acid (1.01 equiv.), 1,4-dioxane:water (1 mL) was
added K CO (3 equiv.) and allowed to stir for 10 min. The reaction mass was degassed
2
3
with argon for 5 min and then added PEPPSI-IPr pre catalyst (5 mol%) and allowed to
ꢀ
irradiate in a microwave reactor at 100 C for 30–45 min. After completion of the reac-
tion (analyzed by thin layer chromatography TLC), the reaction mass was quenched
with water and extracted with EtOAc (2 ꢂ 5 mL). The organic phase was dried over
sodium sulfate (Na SO ) and concentrated in rotary evaporator under reduced pressure
2
4
to yield crude compound. The crude material was purified by FCC method (flash col-
umn chromatography) and recrystallization method using methanol:CH Cl solvent sys-
2
2
tems to obtained pure title compounds (6a–6f). Yields: 82–99%.
3
-Phenyl-9H-pyrido[2,3-b]indole-6-sulfonamide (6a)
ꢀ
ꢁ1
Yield: 80%. Off white solid, m.p. 234–236 C. ꢀ
(KBr)/cm : 3238 (Ar-SO NH ),
2 2
max
3
114 (Ar–C–H), 1603 (Ar–C ¼ C), 1564 (C ¼ N), 1386 (Ar–C–N), 1151 (Ar–SO –NH );
2
2
1
H NMR (400 MHz, DMSO-d ): d(ppm) ¼ 12.30 (s, 1H), 9.04 (d, J ¼ 2.0 Hz, 1H), 8.82
6
(
(
d, J ¼ 2.0 Hz, 1H), 8.78 (d, J ¼ 1.6 Hz, 1H), 7.96 (dd, J ¼ 8.4, 1.6 Hz, 1H), 7.85
d, J ¼ 7.6 Hz, 2H), 7.67 (d, J ¼ 8.8 Hz, 1H), 7.54–7.50 (m, 2H), 7.42–7.38 (m, 1H), 7.25
þ
(s, 2H). Anal. calcd. for C H N O S : C, 63.14; H, 4.05; N, 12.99%; Found: C, 63.17;
1
7
13 3 2
H, 4.02; N, 13.01%.
1
13
Supplementary data (copies of H and C NMR spectra of all the synthesized com-
pounds) associated with this article can be found via the “Supplementary Content” sec-
tion of this article’s webpage.
Funding
The authors are thankful to the Vellore Institute of Technology, Vellore and Council of Scientific
and Industrial Research, Pusa, New Delhi, India (02(0248)/15/EMR-II) for providing finan-
cial support.
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