Palladium-catalyzed C-H bond activation for the assembly of: N -aryl carbazoles with aromatic amines as nitrogen sources

Article abstract of DOI:10.1039/c9cc09493h

A convenient and efficient palladium-catalyzed C-H bond activation for the assembly of N-aryl carbazole is reported, in which two C-N bonds were formed under one set of conditions. The desired carbazoles were achieved in decent yields with a wide substrate scope by utilizing readily available 2-iodo biphenyls and aromatic amines as starting materials.

Full text of DOI:10.1039/c9cc09493h

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DOI: 10.1039/C9CC09493H
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Palladium-Catalyzed C-H Bond Activation for the Assembly of N-
Aryl Carbazoles with Aromatic Amines as Nitrogen Sources
Xiaobing Liua, Heyun Shenga, Yao Zhouc and Qiuling Song*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
convenient and efficient palladium-catalyzed C-H bond carbazoles, in 2017, Mal and coworkers developed an
activation for the assembly of N-aryl carbazole is reported, in intermolecular dehydrogenative annulation of anilides for the
which two C-N bonds were formed under one set of conditions. construction of carbazoles under mild conditions (Scheme 1b).6
The desired carbazoles were achieved in decent yields with wide Recently, Zhang's group described an excellent Pd-catalyzed C-H
substrate scope by utilizing readily available 2-iodo biphenyls and amination for the synthesis of N-tert-butyl carbazole by employing
aromatic amines as starting materials.
diaziridinone as nitrogen source and oxidant (Scheme 1c).7 However,
nitrogen sources are limited to diaziridinone. Despite great progress
has been made for the assembly of carbazoles, the development of
concise route for tailor-made synthesis of N-aryl carbazoles in one-
pot under one set conditions is still in demand. Herein, we disclose
an expedient Pd-catalyzed C-H bond activation to streamline
synthesis of N-aryl carbazoles from easily available 2-iodide
biphenys and commercially available aromatic amines (Scheme 1d).
Carbazoles have emerged as one of the privileged core
structural frameworks in diverse array of drugs and natural
products,1 which also have immense applications in interdisciplinary
research fields. Due to their wide band gap, high electron density,
excellent electrical and optical properties, carbazole derivatives
have demonstrably loomed as prevailing molecular entities in
OLEDs materials, hole transport materials, electroluminescent
materials and photoconductors.2 Therefore, the development of an
expedient method for the synthesis of carbazoles is of remarkable
importance in the realm of both pharmaceutical chemistry and
materials science.
(1) Intramolecular C-H amination for the synthesis of carbazoles
R
Het
N
Pd, Cu, Mn, Ir, Ru, etc
(a)
Ar
H
X
X = RNH, N3
Given the broad-spectrum bioactivity and characteristic physical
property of carbazoles, there has been an extensive interest in the
synthesis of carbazoles.3-8 Transition-metal catalyzed Ulman-type C-
N coupling is a classical method for the assembly of N-aryl
carbazoles.3 Since transition-metal catalyzed C-H bond activation is
more economical and practical without further pre-
functionalization of the raw materials, direct C-H amination of
arenes has arguably represented as a promising platform to forge
these valuable carbazoles.4-5 In the past ten years, transition-metal
catalyzed intramolecular C-H amination of 2-amino biphenys4 and
2-azido biphenys5 have been dominated to access carbazoles
(Scheme 1a). With regard to intermolecular C-H amination to afford
(2) Intermolecular C-H amination for the synthesis of carbazoles
Me
R
Me
X
H
N
PhI(OAc)2
(b)
(c)
Me
+
R
N
H
Me
Me
X
H
Me
O
N
N
N
Pd
+
I
H
Het
N
NH2
Het
Het
+
Pd
(d)
Ar
Het
Ar
H
I
This work
aInstitute of Next Generation Matter Transformation, College of Materials Science
& Engineering and College of Chemical Engineering at Huaqiao University, 668
Jimei Blvd, Xiamen, Fujian, 361021, P. R. China
Scheme 1. Transition-metal-catalyzed C-H bond activation and
amination for the synthesis of carbazoles
.bFujian University Key Laboratory of Molecule Synthesis and Function Discovery,
College of Chemistry, Fuzhou University, Fuzhou, Fujian, 350108, P. R. China.
cCollege of Chemistry and Chemical Engineering, Hubei Normal University,
Huangshi 435002, Hubei, P. R China.
We commenced our study by using 2-iodobiphenyl (1a) and
aniline (2a) as benchmark substrates. Despondingly, when the
reaction was conducted in the presence of 5 mol% Pd(OAc)2 and 10
mol% PPh3 by employing AgOAc or AgTFA as oxidant, stagnant
Email: qsong@hqu.edu.cn; fax:86-592-6162990;
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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reaction was observed and no targeted product was detected halogen-substituted N-aryl carbazole derivatives (3i-3Vki)ewinAr4ti8cl%e O-9n2lin%e,
DOI: 10.1039/C9CC09493H
(Table 1, entries 1 and 2). To our delight, N-phenylcarbazole 3a was which could undergo further transformations. In addition to para-
obtained in 15% yield when Cu(TFA)2 was used as oxidant (Table 1, position substituted anilines, meta-position and ortho-position
entry 3). Replacement of Cu(TFA)2 with Cu(OAc)2 resulted in that substituted anilines also demonstrated good reactivity, furnishing
the yield of 3a was significantly increased to 76% (Table 1, entry 4). the exected products 3k-3o in good yields. The treatment of di-
Subsequently, various inorganic bases were inspected (Table 1, substituted aniline 2p in this reaction was also successful, in which
entries 5-8). NaHCO3, K3PO4 and KOAc have been shown to reduce the targeted carbazole 3p was isolated in 88% yield. To our delight,
the reaction efficiency, whereas Cs2CO3 could enhance the reactivity, 2-naphthylamine and 2-aminopyridine could also be engaged in this
in which the desired product 3a was isolated in 90% yield. We also Pd-catalyzed C-H bond activation and the expected 3q and 3r were
screened a sequence of solvents (Table 1, entry 9-11). Among the achieved in moderate yields.
solvent tested, N,N-dimethylformamide (DMF) still demonstrated
the optimal efficiency. The attempt to lessen the amount of Scheme 2. Substrate Scope with Respect to Aromatic amines
Cu(OAc)2 to 0.5 equiv was failed, in which the yield of 3a decreased
gravely (Table 1, entry 12). When the model reaction was
Het
NH2
Pd(OAc)2, PPh3
+
N
Het
2
Cu(OAc)2, Cs2CO3
DMF, Air, 12 h
H
conducted in the absence of PPh3, relatively low yield of the
targeted product 3a was obtained, which manifested that PPh3
played an important role in this system (Table 1, entry 13). When
the reaction was performed without Pd(OAc)2, no desired product
3a was obtained (Table 1, entry 14).
I
1
3
iPr
Me
N
N
N
Table 1. Screening of the Reaction Conditions
3a
3a
3b
3c
, 82%
, 90%
SMe
x-ray of
CN
,70%
NH2
Pd(OAc)2, PPh3
COMe
CO2Me
+
N
Oxidant, Base, Solvent
H
I
N
N
N
N
1a
Entrya
2a
3a
Yieldb
Solvent
Oxidant
Base
3d
3e
3f
3g
, 38%
F
, 67%
, 36%
Br
, 63%
F
Cl
1
2
3
4
5
AgOAc
AgTFA
DMF
DMF
DMF
DMF
DMF
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaHCO3
N.R
N.R
15
Cu(TFA)2
Cu(OAc)2
Cu(OAc)2
N
N
N
N
76
21
3h
3i
3j
3k
, 88%
, 92%
, 82%
, 48%
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
6
7
DMF
DMF
K3PO4
KOAc
34
47
90
14
45
CF3
iPr
Me
Cl
8
DMF
Cs2CO3
Cs2CO3
Cs2CO3
N
N
N
N
9
CH3CN
DMSO
10
3l
3m
3n
3o
, 75%
, 90%
Me
, 88%
, 55%
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
11
1.4-dioxane
DMF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
19
35
12c
13d
14e
Me
DMF
37
N
DMF
N.R
N
N
N
a
1a
2a
(1.2 equiv, 0.24 mmol), 5 mol%
Reaction conditions:
(0.2 mmol),
Pd(OAc)2, 10 mol% PPh3, 1.0 equiv oxidant, 1.0 equiv base, 2 mL solvent, 120
3p
3q
3r
, 41%
, 88%
, 44%
oC, Air, 12 h. b The isolated yield of
Without PPh3. e without Pd(OAc)2.
c 0.5 equiv of Cu(OAc)2 was used.
3a
.
d
1
2
Reaction conditions: (0.2 mmol), (1.2 equiv, 0.24 mmol), 5 mol% Pd(OAc)2, 10 mol% PPh3,
1.0 equiv Cu(OAc)2, 1.0 equiv Cs2CO3, 2 mL DMF, 120 oC, Air, 12 h.
Subsequently, we turned our attention toward the scope of 2-
iodobiphenyl (Scheme 3). Firstly, we investigated the compatibility
of various functional groups by using unsubstituted aniline as
coupling reagent. 2-Iodobiphenyls having either the electron-
withdrawing or the electron-donating groups could work smoothly
in this reaction, affording the desired carbazoles 4a-4f in 63%-90%
yields. To our delight and surprise, 2'-iodo-[1,1'-biphenyl]-4-
carbaldehyde was also amenable to Pd-catalyzed C-H bond
activation, giving rise to the formation of 4g in 88%, in which the
formyl group was intact. We also investigated a range of functional
With optimized conditions in hand, the substrate scope for this
Pd-catalyzed C-H bond activation was evaluated (Scheme 2). Overall,
all the tested aromatic amines were proved to be suitable nitrogen
sources, enabling the production of the corresponding carbazoles in
decent yields. The structure of 3a was unambiguously confirmed by
X-ray crystallographic analysis. Anilines bearing electron-donating
and electron-withdrawing groups were good candidates in this
transformation, delivering the products 3b-3g 36%-82% yields. It is
noteworthy that halogen-substituted anilines were well-tolerated
under the identified conditions as well, leading to the formation of
2 | J. Name., 2012, 00, 1-3
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groups on the aromatic ring of iodobenzene. The functional groups, scaled up to 5 mmol, the yield of 3a was readily obVtaieiwneArdticilenO6n5lin%e
such as methyl, fluorine, chlorine, trifluoromethyl, trifluoromethoxy yield (Scheme 4a). Carbazoles have comprDeOheI:n1s0i.v1e03a9p/Cp9liCcaCt0io9n49s3iHn
and ester groups were-well compatible in this transformation, material chemistry. The utility of this protocol is showcased by its
providing the corresponding N-aryl carbazoles 4h-4n in moderate to application to synthesize host molecules mCP.2j When
excellent yields. The fused 3-iodo-2-phenyldibenzo[b,d]furan and metaphenylene diamine was subjected to this reaction, compound
heterocyclic 4-(2-iodophenyl)pyridine were proved to be suitable 5 was achieved in 70% yield via two folds C-H activation (Scheme
substrates in this reaction, rendering the desired products 4o and 4b). The structure of
5 was definitely confirmed by X-ray
4p in 68% and 46% yields, respectively.
crystallographic analysis. In addition, based on previous work, 9,9'-
diphenyl-9H,9'H-3,3'-bicarbazoles could be readily gained in
qualitative yields via oxidative dehydrogenative coupling (Scheme
4c).9
Scheme 3. Substrate Scope with Respect to 2-iodobiphenyls
NH2
Het
Pd(OAc)2, PPh3
In order to further explore the mechanism of the reaction, we
conducted several control experiments. When stoichiometric
palladacycle B10 was subjected to this reaction, the desired N-
phenyl carbazole 3a could be obtained in 52% yield (Scheme 5a),
which indicated that the palladacycle B might be the intermediate.
However, at this point, we couldn’t absolutely eliminate the
pathway to construct carbazole via C-N coupling followed by
intramolecular C-H amination. To test this pathway, we prepared N-
phenyl-[1,1'-biphenyl]-2-amine 7.11 When N-phenyl-[1,1'-biphenyl]-
2-amine 7 was employed as starting material, the desired 3a was
also isolated in 56% yield (Scheme 5b). Control experiments
manifested that oxidant and palladium acetate were all obligatory
for transformation of compound 7 to carbazole 3a (Scheme 5c and
5d).
N
4
+
Ar
Cu(OAc)2, Cs2CO3
DMF, Air, 12 h
H
Ar
Het
I
1
2a
N
N
N
N
Me
tBu
OMe
Ph
4a
4b
4c
4d
, 83%
, 82%
, 63%
, 80%
N
N
N
N
F
NO2
CHO
Me
4e
4f
4g
4h
, 75%
, 90%
, 82%
, 88%
N
N
N
N
1.0 equiv. Cu(OAc)2
NH2
1.0 equiv. Cs2CO3
Cl
F3C
F3CO
MeO2C
2 mL DMF, 120 oC, Air
PPh3
PPh3
4i
4j
4k
4l
, 76%
, 93%
, 85%
, 76%
+
Pd
a)
N
N
N
N
3a,
52%
B,
2a,
0.24 mmol
0.2 mmol
O
N
5 mol% Pd(OAc)2
10 mol% PPh3
Me
4m
4n
2a
4o
, 46%
, 33%
, 68%
H
N
b)
N
1
Reaction conditions: (0.2 mmol),
equiv Cu(OAc)2, 1.0 equiv Cs2CO3, 2 mL DMF, 120 oC, Air, 12 h.
(1.2 equiv, 0.24 mmol), 5 mol% Pd(OAc)2, 10 mol% PPh3, 1.0
1.0 equiv. Cu(OAc)2
1.0 equiv. Cs2CO3
2 mL DMF, 120 oC, Air
7,
3a,
0.2 mmol
56%
We also studied the scalability of this Pd-catalyzed C-H bond
activation for the assembly of carbazoles. When the reaction is
5 mol% Pd(OAc)2
10 mol% PPh3
H
N
c)
N
1.0 equiv. Cs2CO3
2 mL DMF, 120 oC, Air
Scheme 4. Larger Synthesis and Synthetic Applications
without Cu(OAc)2
7,
3a
,10%
0.2 mmol
NH2
10 mol% PPh3
Standard Conditions
+
(a)
(b)
I
N
H
N
d)
N
1.0 equiv. Cu(OAc)2
1.0 equiv. Cs2CO3
2 mL DMF, 120 oC, Air
1a,
2a
, 6 mmol
5 mmol
3a,
65%
7,
3a,
trace
0.2 mmol
without Pd(OAc)2
NH2
Scheme 5. Mechanism studies
N
N
Standard Conditions
+
I
NH2
2u
Based on the above control experiments and previous reports,
we propose two possible mechanisms for this Pd-catalyzed C-H
bond activation. As for the pathway of palladacycle (Path A in
Scheme 6), firstly, oxidative addition of 2-iodobiphenyl with Pd(0)
forms Pd(II) species A, which undergoes intramolecular C-H
activation to afford palladacycle B. Immediately, B was oxidized to
palladacycle C by Cu(OAc)2. Then, the anilines coordinate with the
palladacycle C, releasing two molecules of acetic acid to give the
intermediate D. Finally, reductive elimination of intermediate D
leads to the formation of the targeted product 3 and regenerates
1a
5,
70%
R
R
ref [9]
N
N
(c)
N
R
3
4
or
6a
6b
6c
6d
: R = H; 97%
: R = Me; >99%
: R = F; >99%
: R = Br; >99%
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Mater. Chem., 2005, 15, 75; (e) P. Shih, C. Chiang, A. Dixit, C.
the Pd(0) species to continue the next catalytic cycle. As for the
pathway of C-N coupling followed by intramolecular C-H amination
(Path B in Scheme 6), compound 7 was formed via cross-coupling
reaction between A and aniline. Then, pre-association of the amino
moiety of 7 to Pd(OAc)2 facilitates the ortho-palladation process.
resulting in the formation of the C-H activation intermidate E via
releasing one molecule of acetic acid. Subsequently, with the
assistance of base, species F is produced via loss of another
molecule of acetic acid. Finally, reductive elimination of species F
delivers the carbazole 3 and regenerates Pd(0) species to continue
the next catalytic cycle.
View Article Online
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3
4
(a) F. Ullmann, 1903, 36, 2382. For a review, see: (b) S. Ley, A.
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N
N
Pd(OAc)2
PPh3
Pd(OAc)2
PPh3
Synthesis of carbazoles via intramolecular C-H amination of,
see: (a) L. Ackermann and A. Althammer, Angew. Chem., Int.
Ed., 2007, 46, 1627; (b) W. Tsang, N. Zheng and S. Buchwald,
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Chang, J. Am. Chem. Soc., 2011, 133, 5996; (g) C. Suzuki, K.
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Commun., 2014, 50, 9049; (j) S. Youn, J. Bihn and B. Kim, Org.
Lett., 2011, 13, 3738;4.
Synthesis of carbazoles via intramolecular C-H amination of
2-azido biphenys, see: (a) B. Stokes, B. Jovanović, H. Dong, K.
Richert, R. Riell and T. Driver, J. Org. Chem., 2009, 74, 3225;
(b) B. Stokes, K. Richert and T. Driver, J. Org. Chem., 2009, 74,
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2018, 20, 1362.
3a
3a
Pd0
Pd0
reductive
elimination
reductive
I
elimination
oxidative
addition
oxidative
addition
N
Pd Ph
N
AcOH
F
Pd
D
Pdll
Path A
Path B
H
A
AcOH
NHPh
Pd
C-H
activation
cross
coupling
E
OAc
C-H
NH2
2a
activation
AcOH
Ph3
P
PPh3
Pd
NH2
Pd
AcO OAc
NHPh
H
NHPh
H
C
B
Pd
Pd0
AcO
Cu(OAc)2
OAc
Pd(OAc)2
7
2a
Cu(OAc)2
Scheme 6. Proposed Reaction Mechanism
5
In summary, we have demonstrated a convenient and efficient
Pd-catalyzed C–H activation for the synthesis of N-aryl carbazoles
with easily accessed starting materials. The desired carbazoles were
achieved in good yields with a wide range of substrate scope. The
current protocol could also be applied to forge host molecules mCP.
To this end, we propose two possible mechanisms for this Pd-
catalyzed C–H activation/dual C–N bond formation. Further
investigations of the reaction mechanism synthetic applications of
this methods are still underway in our laboratory.
This research was financially supported the National Natural
Science Foundation (21772046) and the Natural Science Foundation
of Fujian Province (2016J01064) are gratefully acknowledged. We
also thank Instrumental Analysis Center of Huaqiao University for
analysis support. X. L. thanks for Subsidized Project for Cultivating
Postgraduates’ Innovative Ability in Scientific Research of Huaqiao
University.
6
7
S. Maiti and P. Mal, Org. Lett., 2017, 19, 2454.
C. Shao, B. Zhou, Z. Wu, X. Ji and Y. Zhang, Adv. Synth. Catal.,
2018, 360, 887.
8
Some other selected examples for synthesis of carbazoles,
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