Transition-metal-free and organic solvent-free conversion of N-substituted 2-aminobiaryls into corresponding carbazoles via intramolecular oxidative radical cyclization induced by peroxodisulfate

Article abstract of DOI:10.1039/c7gc03130k

An atom-economical and environmentally benign approach for the synthesis of N-substituted carbazoles from analogous 2-aminobiaryls using peroxodisulfate in water is reported. The reactions proceeded through an intramolecular oxidative radical cyclization

Full text of DOI:10.1039/c7gc03130k

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Transition-metal-free and organic solvent-free conversion of N-substituted 2-
aminobiaryls into corresponding carbazoles via intramolecular oxidative
radical cyclization induced by peroxodisulfate
Palani Natarajan*, Priya and Deachen Chuskit
Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab University,
Chandigarh - 160 014, India
pnataraj@pu.ac.in
TOC
Abstract
An atomꢀeconomical and environmentally benign approach for the synthesis of Nꢀsubstituted
carbazoles from analogous 2ꢀaminobiaryls using peroxodisulfate in water is reported. The
reactions proceeded through an intramolecular oxidative radical cyclization of Nꢀsubstituted 2ꢀ
aminobiaryls with in situ reoxidation of the resulting radical species. While compared to known
methods for the synthesis of Nꢀsubstituted carbazoles from 2ꢀamidobiaryls, this protocol is
practical, efficient and do not require a transition metal catalyst and toxic organic solvents!
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Introduction
The carbazole and its derivatives are often found in a wide range of natural products1 and
pharmaceuticals2 exhibit interesting biological activities like antibacterial,3 antibiotic,4
antihistaminic,5 antiinflammatory,6 antimalarial,7 antioxidative,8 antitumor,9 antiviral10 and
psychotropics.11 They are also used as precursors for the synthesis of functional materials in
organic electronics.12 A number of synthetic methods have been developed for the preparation of
carbazoles and continues to be a topic of research interest. Among the various methods such as
BorscheꢀDrechsel cyclization,13 Pummerer cyclization,14 DielseꢀAlder reaction,15 Buchwaldꢀ
Hartwig amination,16,17 SuzukiꢀMiyaura coupling,18,19 dehydrogenative cyclization of
diarylamines,20 dehydrogenative CꢀH/NꢀH coupling of Nꢀsubstituted 2ꢀaminobiaryls,21,22 double
amination of 2,2′ꢀdihalobiaryls,23 nitrene insertion of 2ꢀazidoꢀbiphenyls,24 reductive amination of
nitroaryls,25 etc., thus far reported to synthesize carbazoles; the intramolecular CꢀH bond
amination of 2ꢀaminobiaryls is perhaps the most atomꢀeconomical route and being is used to
attain a myriad of carbazole derivatives rapidly (Scheme 1). In this synthetic path (Scheme 1),
generally, a variety of transition metal catalysts including Cu(II)21(a),22(a), Ir(I),22(c) Mn(IV),21(a)
Pd(0),21(b) Pd(II),21(c,e,f,g) Rh(II)22(d,e) and their combinations have been utilized. Despite the high
efficiency and wide tolerance of functionality, residual transitionꢀmetal contamination could
adversely affect the biological properties of the final products.1ꢀ3 Moreover, many transition
metal catalysts are expensive, requires harsh reaction conditions and unstable at ambient
conditions.12 As a consequence, the development of a transitionꢀmetalꢀfree and environmentally
benign synthetic protocol for carbazoles synthesis is highly desirable.
Recently, with the objective of developing environmentally benign conditions for organic
2−
synthesis26 and pollutants mineralization,27 peroxodisulfate (S2O8 , known also as
peroxydisulfate or persulfate) in aqueous solution has widely been utilized. For instances,
deoximation,28 oxidation,29 cyclization,30 polymerization,30 and so on are readily performed
using peroxodisulfate as a sole reagent. Since the peroxodisulfate is a white crystalline solid,
cheaper, chemically stable at ambient conditions, waterꢀsoluble, convenient to transport,
powerful oneꢀelectron oxidant with Eo = 2.01 V in aqueous solution and can be activated readily
2−
by heat/base/UV radiation.26,27 Moreover, peroxodisulfate provides biodegradable sulfate (SO4
)
as the only byproduct. Nevertheless, peroxodisulfate has not yet been employed for the synthesis
2

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of carbazole and its derivatives. In view of this and in continuation of our interest in the
development of green methodologies,31 herein, we disclose our finding on the synthesis of Nꢀ
substituted carbazoles from analogues 2ꢀaminobiaryls using peroxodisulfate in water as an
oxidizing agent, cf. Scheme 2. Moreover, a plausible mechanism for the reaction is reported,
vide infra.
Scheme 1. Various methods of synthesis of carbazoles from 2ꢀaminobiaryl and its derivatives yet
reported in the literatures.
2−
Scheme 2. S2O8 ꢀinduced synthesis of Nꢀsubstituted carbazoles from analogous 2ꢀaminobiaryls
in water reported here.
Results and Discussion
We began our investigation using 2ꢀacetamidobiphenyl (A1) as the model substrate. To our
delight, A1 (1 mmol) underwent intramolecular CꢀH bond amination rapidly upon the treatment
with Na2S2O8 (1 mmol) in water at reflux conditions, affording 9ꢀacetylcarbazole (C1) in 29%
yield (Entry 1, Table 1). The structure of C1 was confirmed by IR and NMR analysis, cf.
Supporting Information. The use of other green solvents such as glycols, ethyl alcohol and
3

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dimethyl sulfoxide was inadequate and gave lower product yields (Entries 2ꢀ4, Table 1). Next,
the effect of counterꢀion of peroxodisulfate was tested. When K2S2O8 or (NH4)2S2O8 was used as
oxidant instead of Na2S2O8, a lower yield of desired product was obtained (Entries 5ꢀ6, Table 1).
In contrast, when tetrabutylammonium peroxodisulfate {TBAPS, [(nꢀC4H9)4N]2S2O8} in situ
generated from the mixture of Na2S2O8 and tetrabutylammonium bromide was used instead of
Na2S2O8, a significant amount of C1 was obtained in short duration (Entry 7, Table 1). Thus,
TBAPS was taken as oxidant for further all optimizations and reactions. Later, we explored the
stoichiometry of TBAPS to A1. The best result was obtained with 1:2 ratios of A1 and TBAPS,
cf. Entry 11 in Table 1. Decreasing the amount of TBAPS from 1 equiv. to 0.5 equiv. led to a
lower yield of C1 (Entry 12, Table 1). However, increasing the amount of TBAPS to 2.0 equiv.
and above neither increased the yield nor lowered the reaction time drastically (Entries 13ꢀ14,
Table 1). Further optimizations revealed that the reaction atmosphere was crucial for the
outcome of the reaction. The reaction proceeded efficiently under nitrogen gas atmosphere while
the involvement of air brings down yield of the product, cf. Entries 15ꢀ16 in Table 1. Thus, the
optimized reaction conditions for the transformation of Nꢀsubstituted 2ꢀaminobiaryls into
corresponding carbazoles (Scheme 2) are substrate (1.0 mmol), TBAPS (2.0 mmol) and water (3
mL) under reflux conditions at nitrogen gas atmosphere.
Table 1. Selected results of screening the optimal conditions for the synthesis of Nꢀsubstituted
carbazoles from the analogous 2ꢀaminobiarylsa
4

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En Solventb
try
Oxidant (mmol)
Atmospꢀ Time Yield
here
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
N2
(min) (%)c
1
2
3
4
5
6
7
8
9
water
glycol
Na2S2O8 (1)
Na2S2O8 (1)
Na2S2O8 (1)
120
120
120
120
120
120
120
120
120
120
90
29
trace
<10
trace
24
13
58
58
66
76
93
ethyl alcohol
dimethyl sulfoxide Na2S2O8 (1)
water
water
water
water
water
K2S2O8 (1)
(NH4)2S2O8 (1)
[(nꢀC4H9)4N]2S2O8 (1)
[(nꢀC4H9)4N]2S2O8 (1.1)
[(nꢀC4H9)4N]2S2O8 (1.3)
[(nꢀC4H9)4N]2S2O8 (1.5)
[(nꢀC4H9)4N]2S2O8 (2)
[(nꢀC4H9)4N]2S2O8 (0.5)
[(nꢀC4H9)4N]2S2O8 (2.5)
[(nꢀC4H9)4N]2S2O8 (3)
[(nꢀC4H9)4N]2S2O8 (2)
[(nꢀC4H9)4N]2S2O8 (2)
10 water
11 water
12 water
13 water
14 water
15 water
16 water
180
90
90
17
93
95
open air 90
air 90
47
38
a
General conditions: Unless stated otherwise all reactions were performed in a Schlenk tube
with 1.0 mmol of A1, oxidant and solvent (3 mL).
b All solvents and water were doubleꢀdistilled and deꢀaerated prior to use.
c Isolated yields.
Having optimized the reaction conditions, the scope and limitations of the methodology (Scheme
2) was investigated and the results obtained are listed in Table 2. Initially, substrates A2-A7 with
different substitution patterns on the aryl part of the 2ꢀamidobiaryls were screened. Functional
groups including CH3, OCH3, F and Cl were well compatible with the oxidation system, and the
desired products C2-C7 were obtained in 64ꢀ91% yields. Afterwards, substrates A8-A11 bearing
various electronꢀdonating and electronꢀwithdrawing functional groups on the acetanilide ring
were employed. Unsurprisingly, the reaction proceeded smoothly and afforded the desired
products in moderate to good yields (Table 3) under the optimized reaction conditions (Table 1).
Subsequently, the substrates A12-A13 bearing both electronꢀdonating and electronꢀwithdrawing
groups on the aromatic rings were employed. Indeed, regardless of the functional groups;
expected products C12-C13 were obtained in 72ꢀ83% yields. Notably, steric hindered substrates
A5-A7 and A11 showed good reactivity, which is not a common feature with transition metalꢀ
catalyzed methodologies reported for synthesis of carbazole derivatives, cf. Scheme 1.19,20
5

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Next, the effects of Nꢀsubstituents on the efficiencies of the reaction were also explored (Table
2). Nꢀsulfonylꢀcontaining (arylsulfonyl (A14-A18) and methylsulfonyl (A19-A22) substrates
reacted to produce the required carbazoles (C14-C22, Entries 13ꢀ21, Table 2) in good yield.
However, tertꢀbutyl [1,1'ꢀbiphenyl]ꢀ2ꢀylcarbamate (A23), ethyl [1,1'ꢀbiphenyl]ꢀ2ꢀylcarbamate
(A24), Nꢀmethylꢀ[1,1'ꢀbiphenyl]ꢀ2ꢀamine (A25), Nꢀbenzylꢀ[1,1'ꢀbiphenyl]ꢀ2ꢀamine (A26) and 2ꢀ
aminobiphenyl (A27) did not undergo the intramolecular CꢀH amination effectively and afforded
expected carbazole derivatives in very poor yield (< 30%), cf. Table 2. Thus, the peroxodisulfate
may be used as oxidizing agent for the synthesis of carbazoles functionalized by electron with
drawing groups at Nꢀatom. Nevertheless, this transformation is compatible to various functional
groups including chloro, fluoro, methoxy, aceyl and sulfonyl groups that generally advantageous
for further modifications.
Table 2. Substrate scope for the transformation of Nꢀsubstituted 2ꢀaminobiaryls into analogous
carbazoles.a
Entry
1
Substrate
Product
Yield b, c
(%)
86
2
81
6

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3
4
89
Cl
91
86
NH
Cl
O
N
CH3
A5
O
CH3
C5
F
F
5
6
7
N
NH
O
O
CH3
A6
H3CO
O
CH3
C6
64
OCH3
NH
CH3
A7
88
87
8
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F
F
9
89
N
NH
CH3
O
O
CH3
C10
A10
H3C
10
85
72
N
H3C
O
CH3
C11
11
12
F
OCH3
83
86
91
N
O
CH3
C13
13
14
O
O
N
S
NH
S
O
C6H5
A14
C6H5
O
C14
H3C
H3C
O
N
S
NH
S
O
O
C6H5
A15
C6H5
O
C15
8

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F
15
87
O
NH
S
O
C6H5
A16
16
17
18
19
79
92
89
94
O
O
N
NH
S
O
S
CH3
A19
CH3
O
C19
H3C
H3C
O
N
NH
S
O
O
S
CH3
A20
CH3
C20
O
Cl
Cl
20
21
85
81
O
N
NH
S
O
O
S
CH3
A21
CH3
C21
O
OCH3
N
S
O
CH3
O
C22
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22
23
17
NH
O
O
O
O
t-Bu
A23
28
NH
C2H5
A24
24
25
NDd
NDd
N
NH
C6H5
A26
C6H5
C26
26
NDd
a
Unless stated otherwise all reactions were performed in a Schlenk tube with 2.0 mmol of 2ꢀ
amidobiaryls, 4.0 mmol of Na2S2O8 and 10.0 mmol of TBAB in water at reflux temperature in
the nitrogen gas atmosphere for 90ꢀ120 min.
b Isolated yield.
c
All products were characterized by IR and NMR spectroscopy (Supporting Information) in
comparison with known compounds from literature data.
d ND: not detected (common organic solventꢀinsoluble brown color materials were detected)
Preliminary experiments were carried out to probe the reaction mechanism. When 1 equiv. of the
radical scavenger 2,2,6,6ꢀtetramethylpiperidineꢀ1ꢀoxyl or ascorbic acid was added in the reaction
of A1 under the standard conditions, no desired product C1 observed. It indicated that the
reaction follows a radical pathway as in line with the previously reported transformations
initiated by S2O82− ion.26ꢀ30 On these bases, a plausible radical mechanism (Scheme 3) is outlined
2−
for the reaction (Scheme 2). Initially, the thermal decomposition of S2O8 afforded sulfate
10

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−•
radical anions (I, SO4 ); which abstract the hydrogen atoms from the substrates (A) to afford a
nitrogenꢀcentered amidyl radical (II). Then, Radical II undergoes intramolecular CꢀH amination
to form a new radical intermediate (III), which was then oxidized by I followed by
deprotonation by SO42− afforded the desired carbazoles (C), cf. Scheme 3.
Scheme 3. A plausible mechanism for the formation of Nꢀsubstituted carbazoles from 2ꢀ
amidobiaryls in the presence of peroxodisulfate.
As mentioned at the outset, peroxodisulfate initiated transformations have several advantages
over other reported protocols in view of both economic and environmental standpoints.
Moreover, the reaction employs inexpensive starting materials and water as a solvent. Thus, we
believe that the method reported here will find a bright future in industry for the synthesis of
carbazoles and other related heterocyclic compounds. It is worth nothing that the price of 1.0 g of
9ꢀacetylcarbazole with a purity of 95ꢀ98% is 100ꢀ150 USD from recognized suppliers that
indicates the importance of development of a costꢀeffective protocol for it.
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Conclusion
In summary, we have developed an atomꢀeconomical and environmentally benign method for
synthesis of carbazoles from 2ꢀamidobiaryls. This transformation proceeded in water employing
2−
the inexpensive S2O8 as an oxidant. Moreover, desired products in pure form are obtained in
moderate to good yields by the easy workup consisting of simple filtration. Further utilization of
the method to develop other heterocyclic compounds is currently under investigation and will be
disclosed in due course.
Acknowledgments
The authors are pleased to acknowledge the financial support from the Council of Scientific and
Industrial Research (CSIR), New Delhi, vide project no.: 02(0246)/15/EMRꢀII. Priya and D. C.,
respectively, thank the Department of Science and Technology (DST) and University Grant
Commission (UGC), New Delhi, for their junior research fellowships (JRF).
Electronic supplementary information
Electronic supplementary information (general aspects, procedure and experimental
characterization data) associated with this article can be found in the online version at www.
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