Efficient Copper-Catalysed Synthesis of Carbazoles by Double N -Arylation of Primary Amines with 2,2′-Dibromobiphenyl in the Presence of Air

Article abstract of DOI:10.1055/s-0040-1706641

An efficient Cu-catalyzed synthesis of carbazole derivatives is reported, which proceeds by double C-N coupling reactions of 2,2′-dibromobiphenyl and amines in the presence of air. The reaction is robust, proceeds in high yields, and tolerates a series of amines including neutral, electron-rich, electron-deficient aromatic amines and aliphatic amines.

Full text of DOI:10.1055/s-0040-1706641

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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2021, 31, 611–615
letter
611
en
H. N. Do et al.
Letter
Synlett
Efficient Copper-Catalysed Synthesis of Carbazoles by Double
N-Arylation of Primary Amines with 2,2′-Dibromobiphenyl in the
Presence of Air
Ha Nam Doa
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1
-6
9
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9
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X
Nguyen Minh Quanb
Ban Van Phucc
Dinh Van Tinhd
CuCl/L-proline
Br
N
R-NH2
+
Br
140 °C, DMF, 24 h
air
Nguyen Quyet Tienc
Truong Thi Thanh Ngac
Van Tuyen Nguyenb,c
Tran Quang Hung*b,c
Tuan Thanh Dang*a
Peter Langer*d,e
R
16 examples
up to 94% yield
a Faculty of Chemistry, Hanoi University of Science, Vietnam National University
(VNU), 19-Le Thanh Tong, Hanoi, Vietnam
dangthanhtuan@hus.edu.vn
b Graduate University of Science and Technology, Vietnam Academy of Science and
Technology, 18-Hoang Quoc Viet, Hanoi, Vietnam
c Institute of Chemistry, Vietnam Academy of Science and Technology, 18-Hoang
Quoc Viet, Hanoi, Vietnam
tqhung@ich.vast.vn
d Institut für Chemie, Universität Rostock, Albert-Einstein-Str. 3a, 18059 Rostock,
Germany
peter.langer@uni-rostock.de
e Leibniz Institut für Katalyse an der Universität Rostock e.V., Albert-Einstein-Str. 3a,
18059 Rostock, Germany
Corresponding Authors
Received: 24.09.2020
Accepted after revision: 13.11.2020
Published online: 08.01.2021
bazoles has been found to have biological activities,
including antitumor, anti-inflammatory, antihistamine, an-
tibiotic, anti-HIV, antiepileptic, anti-Alzheimer, psychotro-
pic, neuroprotection, and antioxidative functions.6 Due to
the excellent bioactivities of natural carbazoles, the carba-
zole core structure is also present in important synthetic
drugs, such as carprofen (a nonsteroidal anti-inflammatory
drug), carvedilol (a congestive heart failure drug), or poten-
tial agents, such as Go-6976, an anti-HCMV agent.7 Carba-
zole derivatives are not only important structures in phar-
maceutical compounds, but are also used in the develop-
ment of organic light-emitting devices (hole-transporting,
luminescent, and host materials).8 For example, Brunner et
al. have reported a variety of carbazole compounds as host
materials for triplet emitters in OLEDs.8 The excellent pho-
torefractive properties and fluorescent nature of the carba-
zole system were also explored in the development of OLED
materials, such as Bcz1 and CzCbPy (Figure 1).8
DOI: 10.1055/s-0040-1706641; Art ID: st-2020-d0525-l
Abstract An efficient Cu-catalyzed synthesis of carbazole derivatives
is reported, which proceeds by double C–N coupling reactions of 2,2′-
dibromobiphenyl and amines in the presence of air. The reaction is ro-
bust, proceeds in high yields, and tolerates a series of amines including
neutral, electron-rich, electron-deficient aromatic amines and aliphatic
amines.
Key words Cu catalysis, carbazole synthesis, C–N coupling, carbazole
alkaloids, sustainable process
Carbazole derivatives are present in the core structure
of many important natural products and pharmaceutically
active molecules.1 Carbazole was first isolated by Graebe
and Glaser from the anthracene fraction of coal tar in
1872.2 In 1987, nearly a century later, Bhattacharyya an-
nounced for the first time that carbazole was isolated from
a plant Glycosmis pentaphylla.3 During the past four de-
cades, a number of carbazole alkaloids has been isolated
from various natural sources (Figure 1).4 Most carbazole al-
kaloids are derived from higher plants of the genus Clause-
na, Murraya, and Glycosmis, all of which belong to the Ruta-
ceae family.5 Moreover, a significant number of natural car-
Due to the importance of carbazoles in medicinal and
material chemistry, a series of synthetic methods has been
reported for the preparation of the carbazole framework.9
Conventional methods to prepare carbazoles rely on Fischer–
Borsche, Graebe–Ullmann and Cadogan reactions.9 Car-
bazole derivatives can also be formed by Diels–Alder reac-
tions of pyrano[3,4-b]indoles with alkynes.10 Knölker et al.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 611–615
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

612
H. N. Do et al.
Letter
Synlett
Me
zole derivatives by activation of alkynes in the presence of
transition metals (or Lewis acids) as catalysts.14 Au, Pt, Ag
salts and Lewis acids (BF3, BiCl3, InCl3) have also been em-
ployed as efficient catalysts.14 However, these synthetic ap-
proaches are often complicated, low yielding, require ex-
pensive catalysts or need several steps to prepare the start-
ing materials.
R
R
N
N
OMe
OMe
N
H
OMe
N
Me
H
NH
OMe
Bismurrayafoline A (R = Me)
Bismurrayafolinol (R = CH2OH)
OMe
Murrastifoline A
Murrayafoline A (R = Me)
Mukonine (R = COOMe)
A straightforward approach to prepare carbazoles is
based on Pd-catalyzed double arylation of 2,2′-dihalobiphe-
nyls with amines.15 The groups of Nozaki and Chida inde-
pendently reported Pd-catalyzed twofold C–N coupling re-
actions of 2,2′-dihalobiphenyls with amines to form carba-
zoles.15a–c Due to the importance of this approach for the
preparation of carbazoles, the Verkade and Ageshina groups
designed highly efficient ligands for Pd-catalyzed double N-
arylation of primary amines with 2,2′-dihalobiphenyls
(Scheme 1).15d,e Recently, we successfully applied this strat-
egy as a key step in the synthesis of biscarbazoles in high
yields.16 In order to find cheaper catalysts for this transfor-
mation, Li et al. developed a convenient Cu-catalyzed dou-
ble C–N bond-forming reaction using diiodides and primary
amides (Scheme 1).14e However, this procedure only worked
with diiodides and primary amides and could not be ap-
plied to more challenging dibromide or dichloride sub-
strates. Therefore, the development of new procedures to
use cheaper dibromides in Cu-catalyzed C–N coupling reac-
tions is of considerable current interest. Herein, we wish to
report an efficient and practical method for the Cu-cata-
lyzed synthesis of carbazole derivatives with the tolerance
of many functional groups from readily available starting
materials such as 2,2′-dibromobiphenyl and amines.
H
Me
N
O
O
N
HO
N
OMe
Me
N
N
X
O
N
R
H
N
H
HO
Indocarbazostatin
OMe
COOEt
BCz1 (R = Et, X = CH)
CzBbPy (R = 2-pyridinyl, X = N)
Murrastifoline F
Figure 1 Several natural alkaloids and organic materials containing the
carbazole core
described an interesting method to access carbazoles via
Fe-mediated oxidative cyclization.11 In recent years, a series
of methods for the preparation of carbazoles relying on Pd-
catalyzed cyclizations has been developed.12 The Larock
group reported a one-pot Pd-catalyzed domino process to
approach carbazoles using cross-coupling reactions of iodo-
anilines with silylated aryl triflates.12a Langer et al. demon-
strated the synthesis of carbazoles by domino ‘two-fold
Heck/6-electrocyclization’ reactions of 2,3-dibromoin-
doles with alkenes.12b Carbazoles have also been successful-
ly synthesized by transition-metal catalyzed C–H activation
reactions.13 For example, Ackermann et al. developed an ef-
ficient access to carbazoles from aniline and 1,2-dihaloben-
zene derivatives via a domino Pd-catalyzed N–H/C–H acti-
vation process.13a,b Carbazoles can also be prepared by oxi-
dative Pd-catalyzed cyclizations of diaryl amines.13c Fagnou
et al. reported an intramolecular Pd(II)-catalyzed oxidative
C–C bond formation using pivalic acid as a solvent in the
presence of air.13d In 2014, we successfully applied this
method in a practical synthesis of biscarbazole deriva-
tives.13e The Xu group developed the synthesis of N-substi-
tuted carbazole derivatives by Rh-catalyzed direct amina-
tion of boronic acids with aryl azides via a domino C–H ac-
tivation and Rh–nitrene migratory insertion process.13f The
Verma group described a direct Pd-catalyzed transforma-
tion of indoles to form carbazoles using alkenes via domino
oxidative Heck coupling/cyclization reactions.13g Recently,
several cycloaddition reactions have been applied for the
synthesis of carbazoles derivatives.14 In 2019, Wang and co-
workers reported a Rh-catalyzed [4+2] cycloaddition of in-
doles with 1,3-dienes to form carbazole derivatives.14b In-
terestingly, carbazoles derivatives can also be prepared by a
domino process involving C–H activation and [2+2+2] cy-
clization of N-protected indoles with alkynes in the pres-
ence of Rh (or Mn) catalysts.14c,d Electrophilic cyclizations
can also be used as an efficient strategy to construct carba-
X
X
R-NH2
R-NH2
[Pd] cat.
[Cu] cat.
N
R
X
X
(X = I, Br, OTf)
(X = I) Li group
(X = Br) This work
Chida, Nozaki, Verkade groups
Scheme 1 Approaches to prepare carbazoles by double N-arylation of
primary amines with 2,2′-dihalobiphenyls
For the optimization we chose 2,2′-dibromodiphenyl
(1a) and benzylamine (2a) to give the corresponding carba-
zole 3a (Table 1). As a starting point we employed 1.5 equiv-
alents of 2a, 20 mol% of CuI, and 24 mol% of a ligand (vide
infra) at 150 °C. Several parameters, including the copper
source, ligand, solvent, and temperature, were investigated
in detail. First, we chose K2CO3 as the base and DMSO as the
solvent for optimizing ligands. Our initial optimizations
started with the use of phosphine and carbene ligands.
However, BINAP, Xantphos, and IPr only gave 3a in low
yields (Table 1, entries 1, 2). Subsequently, bipyridine li-
gands, frequently used in copper catalysis, were screened
which, however, also did not give satisfactory results. Re-
cently, amino acids were found to be useful bidentate li-
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 611–615

613
H. N. Do et al.
Letter
Synlett
Table 1 Optimization for the Synthesis of Carbazole
carried out in DMF. Then, stronger bases, such as KOH and
KOt-Bu, were examined (Table 1, entries 19, 20). Interest-
ingly, we achieved 81% yield of the desired product with
employment of KOH. When the reaction was performed at a
lower temperature (140 °C), we still obtained a satisfactory
yield (79%). The last screening with reduction of the CuCl
loading to 10 mol% was also examined under 140 °C. We re-
alized that this reaction still worked well and 3a was
formed in 78% yield (Table 1, entry 22).
With our optimized conditions in hand, we continued to
investigate the substrate scope using different amines. A va-
riety of carbazoles 3a–q were isolated in moderate to excel-
lent yields (Scheme 2). In general, both aromatic and ali-
phatic amines could be successfully employed. In the case
of benzylamine derivatives containing electron-donating
groups (methyl and methoxy) equally good yields were ob-
tained. In the presence of electron-withdrawing groups the
yields were lower, presumably due to the lower nucleo-
philicity of the amines. A carbazole bearing an indole sub-
stituent was effectively prepared in 67% isolated yield (3i).
Aliphatic amines were also used in double C-N coupling re-
actions, which gave moderate yields of corresponding car-
Br
H2N
[Cu] (20 mol%), ligand
+
N
base, 150 °C
solvent, 24 h
Ph
Br
Ph
Entry Catalyst
Ligand
BINAP
Base
Solvent Yield (%)a,b
1
2
CuI
K2CO3
K2CO3
K2CO3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NMP
25
5
CuI
Xantphos
IPr·HCl
3
CuI
8
4
CuI
1,10-phenanthroline K2CO3
15
14
30
40
14
70
72
34
45
52
40
55
74
5
CuI
bipyridine
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
L-proline
K2CO3
K2CO3
K3PO4
KOAc
6
CuI
7
CuI
8
CuI
9
CuI
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KOtBu
KOH
10
11
12
13
14
15
16
17
18
19
20
21
22
CuCl
CuBr
CuCl2
Cu(OAc)2
Cu(OTf)2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
Br
CuX/L-proline
(10 mol%)
Br
R-NH2
N
R
+
DMF
140 °C, DMF, 24 h
base
3a–q
toluene 56
dioxane 48
1
2a–q
DMF
DMF
DMF
DMF
51
N
N
N
N
81
KOH
79c
78c
MeO
OMe
OMe
Me
3a, 74%a
3b, 94%a
3c, 70%a
3d, 60%a
KOH
a Yields were calculated by 1H NMR analysis.
b 2a (1.5 equiv), base (3 equiv), [Cu] catalyst (20 mol%), ligand (24 mol%),
N
150 °C, 24 h.
N
N
N
c 2a (1.5 equiv), base (3 equiv), [Cu] catalyst (20 mol%), ligand (24 mol%),
CF3
F
F
2
140 °C, 24 h.
d 2a (1.5 equiv), base (3 equiv), [Cu] catalyst (10 mol%), ligand (12 mol%),
F
3g, 56%a
3f, 55%a
3e, 72%a
3h, 67%
140 °C, 24 h.
N
N
2
N
N
gands in combination with copper salts for C–N coupling
reactions.17 Basing on successful procedures using copper
catalysts in the combination with amino acids,16 thus, L-
proline was employed which gave 3a in 30% yield (Table 1,
entry 6). Then, a screening using different bases was carried
out, which gave a promising result of 70% yield when Cs2CO3
was employed (Table 1, entries 7–9). In order to examine
the effect of the copper source, we studied several copper
salts, such as CuBr, CuCl, CuCl2, Cu(OAc)2, Cu(OTf)2 (Table 1,
entries 10–14). In fact, the yield could be slightly improved
to 72% by the use of CuCl. This result led us to evaluate oth-
er solvents, for example, NMP, DMF, toluene, and dioxane
under the same conditions (Table 1, entries 15–18). The
product was formed in 74% yield when the reaction was
Me
N
EtO
H
3m, 54%a
3i, 67%a
3k, 70%a
3l, 62%a
N
N
N
N
OMe
nBu
F
3n, 40%b
3q, 57%b
3o, 53%b
3p, 60%b
Scheme 2 Synthesis of carbazoles 3a–q. Yields of the isolated products
are given. a Reaction conditions: 140 °C, CuCl/L-proline catalyst (10
mol%), KOH, DMF, 24 h. b Reaction conditions: 150 °C, CuI catalyst/L-
proline (15 mol%), Cs2CO3, DMSO, 24 h.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, 611–615

614
H. N. Do et al.
Letter
Synlett
bazole products (3k–m). The reaction of 1 with aniline gave
3n, albeit, in only 25% yield. The yield could be improved to
40% using 150 °C, 15 mol% of CuI/L-proline, Cs2CO3, and
DMSO (Table 1, entry 10). Likewise, carbazoles 3o–q, also
derived from anilines, were prepared under these condi-
tions.
(3) Bhattacharyya, P.; Chowdhury, B. K.; Mustapha, A.; Garba, M.
Phytochemistry 1987, 26, 2138.
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Structure Elucidation, Vol. 65; Cordell, G. A., Ed.; Academic Press:
London, 2008, Chap. 2, 3. (b) Chakraborty, D. P.; Roy, S. In Car-
bazole Alkaloids IV; Chakraborty, D. P.; Krohn, K.; Messner, P.;
Roy, S.; Schäffer, C., Ed.; Springer: Vienna, 2003, 125.
We were successful in finding an application of our pro-
cedure for the synthesis of commercially available 4,4′-
bis(N-carbazolyl)-1,1′-biphenyl (CBP), an OLED material, by
double Cu-catalyzed cyclization of 1 with diamine 2r
(Scheme 3). Indeed, CBP has been known as one of the most
popular host materials for efficient fluorescent and phos-
phorescent OLEDs with excellent hole mobility.18 Interest-
ingly, CBP OLED material was successfully prepared in 32%
yield using our one-pot procedure.
(5) (a) Bashir, M.; Bano, A.; Ijaz, A. S.; Chaudhary, B. A. Molecules
2015, 20, 13496. (b) Caruso, A.; Ceramella, J.; Iacopetta, D.;
Saturnino, C.; Mauro, M. V.; Bruno, R.; Aquaro, S.; Sinicropi, M.
S. Molecules 2019, 24, 1912.
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(b) Qvigstad, E.; Sjaastad, I.; Bøkenes, J.; Schiander, I.; Solberg, L.
M.; Sejersted, O.; Osnes, J.-B.; Skomedal, T. Eur. J. Pharm. 2005,
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S.; Ban, X. Opt. Mater. 2020, 101, 109781. (b) Gao, W.-J.; Yu, H.-J.;
Chen, J.; Xiao, J.; Fang, J.-K.; Jia, X.-R.; Peng, C.-F.; Shao, G.;
Kuang, D.-B. Chem. Eng. J. 2020, 126434. (c) Li, J.; Yin, X.; Xia, Y.;
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NH2
CuCl/L-proline
(20 mol%)
Br
+
N
N
Br
Cs2SO3, DMSO
150 °C, 48 h
(8) Jiang, H.; Sun, J.; Zhang, J. Curr. Org. Chem. 2012, 16, 2014.
(9) (a) Schmidt, A. W.; Reddy, K. R.; Knölker, H.-J. Chem. Rev. 2012,
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H2N
CBP, 32%
2r
1
Scheme 3 Synthesis of biscarbazole OLED material
(11) (a) Knölker, H.-J. Synlett 1992, 371. (b) Knölker, H.-J. Top. Curr.
Chem. 2005, 244, 115.
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Tùng, Đ. T.; Langer, P. Synlett 2009, 1822. (c) Hussain, M.;
Toguem, S.-M. T.; Ahmad, R.; Tùng, Đ. T.; Knepper, I.; Villinger,
A.; Langer, P. Tetrahedron 2011, 67, 5304.
In conclusion, we have developed a practical and conve-
nient one-pot synthesis of carbazole derivatives in the pres-
ence of air from commercially available chemicals. This
strategy is based on Cu-catalyzed C–N coupling reactions of
2,2′-dibromobiphenyl with amines and tolerates several
functional groups.
(13) (a) Ackermann, L.; Althammer, A. Angew. Chem. Int. Ed. 2007, 46,
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H.; Dang, T. T.; Villinger, A.; Langer, P. Org. Biomol. Chem. 2014,
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R. D.; Driver, T. G. J. Org. Chem. 2009, 74, 3225. (i) Bedford, R. B.;
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(14) (a) Aggarwal, T.; Verma, S.; Verma, A. K. Org. Biomol. Chem.
2019, 17, 8330. (b) Zhou, C.-J.; Gao, H.; Huang, S.-L.; Zhang, S.-S.;
Wu, J.-Q.; Li, B.; Jiang, X.; Wang, H. ACS Catal. 2019, 9, 556. (c) Yi,
W.; Jia, J.; Shi, J.; Zhou, J.; Liu, X.; Song, Y.; Xu, H. E. Chem.
Commun. 2015, 51, 2925. (d) Shi, L.; Zhong, X.; She, H.; Lei, Z.; Li,
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Funding Information
This research was supported by the Institute of Chemistry of the VAST
(Grant Number VHH.2020.1.01).
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Acknowledgment
Many thanks to BSc. Ngoc Khanh Nguyen for useful discussions
during the optimization of reactions.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0040-1706641.
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References and Notes
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demic Press: London, 2008, Chap. 4, 181. (b) Knölker, H.-J.;
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Reddy, K. R.; Knölker, H.-J. Chem. Rev. 2012, 112, 3193.
(15) (a) Nozaki, K.; Takahashi, K.; Nakano, K.; Hiyama, T.; Tang, H.-Z.;
Fujiki, M.; Yamaguchi, S.; Tamao, K. Angew. Chem. Int. Ed. 2003,
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Synth. Catal. 2010, 352, 616. (e) Ageshina, A. A.; Sterligov, G. K.;
(2) Graebe, C.; Glaser, C. Ber. Dtsch. Chem. Ges. 1872, 5. 12.
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