Synthesis of Carbazoles from 2-Iodobiphenyls by Palladium-Catalyzed C?H Activation and Amination with Diaziridinone

Article abstract of DOI:10.1002/adsc.201701039

A facile and efficient approach has been developed for the synthesis of carbazoles from 2-iodobiphenyls and diaziridinone under palladium catalysis. A wide range of carbazoles were synthesized in good to excellent yields, and indole derivatives were obtained by using styrenes as the substrate. The palladacycles obtained from 2-iodobiphenyls acted as the key intermediate, and the reaction should proceed via a tandem Pd-catalyzed C?H activation/dual C?N bond formation sequence. (Figure presented.).

Full text of DOI:10.1002/adsc.201701039

Title: Synthesis of Carbazoles from 2-Iodobiphenyls by Palladium-
Catalyzed C–H Activation and Amination with Diaziridinone
Authors: Changdong Shao, Bo Zhou, Zhuo Wu, Xiaoming Ji, and
Yanghui Zhang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701039
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10.1002/adsc.201701039
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Carbazoles from 2-Iodobiphenyls by Palladium-
Catalyzed C–H Activation and Amination with Diaziridinone
Changdong Shao,a Bo Zhou,a Zhuo Wu,a Xiaoming Ji,a and Yanghui Zhanga,*
a School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability,
Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China
E-mail: zhangyanghui@tongji.edu.cn; Web: http://zhangyhgroup.tongji.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract: A facile and efficient approach has been
developed for the synthesis of carbazoles from 2-
iodobiphenyls and diaziridinone under palladium catalysis.
A wide range of carbazoles were synthesized in good to
excellent yields, and indole derivatives were obtained by
using styrenes as the substrate. The palladacycles obtained
from 2-iodobiphenyls acted as the key intermediate, and the
reaction should proceed via a tandem Pd-catalyzed C–H
activation/dual C–N bond formation sequence.
Keywords: Palladacycles; C–H activation; amination;
synthetic methods; carbazoles.
Figure 1. Natural products, bioactive compounds, dyes,
and photoelectric materials containing carbazoles.
Carbazoles are a class of important aromatic nitrogen
heterocycles. They are common structural motifs
prevalent in bioactive natural products and
therapeutic agents.[1] Moreover, carbazoles have also
found extensive applications in materials science due
to their unique thermal, electrical, and optical
properties (Figure 1).[2] It is not surprising that
continuous efforts have been devoted to exploring
new methods for the synthesis of carbazoles.
Currently, a variety of methods have been developed
to create carbazole molecular scaffolds.[3] These
methods generally follow two modes: 1) the
construction of a side benzene ring from indole
derivatives; 2) the construction of a middle pyrrole
ring from benzene derivatives.[3b] Among the current
methods, transition metal-catalyzed reactions play
significant roles and provide novel and powerful
synthetic means for carbazoles.[4]
The past few decades have witnessed noticeable
progress in C–H functionalization.[5] Direct C–H
functionalization reactions have great advantages
compared to traditional ones relying on the
transformation and conversion of active functional
groups, and they are emerging as novel and efficient
strategies in organic synthesis. The synthetic methods
of carbazoles via C–H activation have also been
reported. Carbazole skeletons could be constructed
through C–H amination,[6] C–H arylation,[7] and
coupling of two C–H bonds.[8]
Recently,
we
found
that
dibenzopalladacyclopentadienes obtained via C–H
activation of 2-iodobiphenyls exhibited novel
reactivities.[9] These palladacycles consist of two
carbon-Pd bonds, which can be functionalized and
therefore offer opportunities to develop novel organic
reactions. On the other hand, due to their unique and
versatile reactivities, diaziridinones are important
synthetic intermediates and have been utilized to
develop a variety of novel organic reactions.[10]
Recently, the Shi group reported that palladacycles
consisting of C(sp2)- and C(sp3)-Pd bonds could react
with diaziridinone to form indolines.[11] In these
reactions, the two carbon-Pd bonds were
functionalized and transformed into two C–N bonds.
Inspired by these excellent reactions, we envisioned
that dibenzopalladacyclopentadienes might react with
diaziridinone to generate carbazoles. Herein, we
report a facile protocol for the synthesis of carbazoles
starting from 2-iodobiphenyls and diaziridinone via
Pd-catalyzed C–H activation and dual C–N bond
formation. The protocol was also applicable to the
synthesis of indole derivatives by using styrenes as
the substrates.
1
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10.1002/adsc.201701039
Advanced Synthesis & Catalysis
Table 1. Optimization of the reaction conditions.[a]
(entries 11 and 12). The amount of Cs2CO3 could also
be reduced to 1.0 equivalent without affecting the
yield (entry 13). However, the use of 1.0 equivalent
of Cs2CO3 was necessary since the yield decreased
dramatically in the presence of 0.5 equivalent of
Cs2CO3 and only a trace amount of the desired
product was formed when Cs2CO3 was removed
(entries 14 and 15). Reducing KOAc to 0.5 equiv. led
to a slightly higher yield (entry 16). However, the
yield decreased when KOAc was removed (entry 17).
Although the roles of Cs2CO3 and KOAc in the
reaction remain to be investigated, some studies
suggested that bases should play an important role in
the generation of the active Pd(0) species and KOAc
could promote C–H activation.[12] A yield of 63% was
still obtained when the loading of Pd(OAc)2 was
reduced to 2 mol % (entry 18), and the reaction was
low-yielding at a lower temperature (entry 19).
Finally, the optimal yield was still obtained when the
reaction time was shortened to 6 hours (entry 20).
With the optimal conditions in hand, we then
explored the scope of the protocol for the synthesis of
Entry
base
solvent
(mL)
Pd
yield
(equiv.)
(mol%) (%)
1
2
3
4
K2CO3 (2.0)
DMF (1)
10
10
10
10
10
44
16
25
53
Li2CO3 (2.0) DMF (1)
Na2CO3 (2.0) DMF (1)
Cs2CO3 (2.0) DMF (1)
Cs2CO3 (2.0) DMA (1)
5
53
6
7
8
9
Cs2CO3 (2.0) dioxane (1) 10
Cs2CO3 (2.0) MeCN (1) 10
Cs2CO3 (2.0) toluene (1) 10
n.r.
trace
n.r.
90
90
90
78
91
36
6
Cs2CO3 (2.0) DMF (2)
Cs2CO3 (2.0) DMF (2)
Cs2CO3 (2.0) DMF (2)
Cs2CO3 (2.0) DMF (2)
Cs2CO3 (1.0) DMF (2)
Cs2CO3 (0.5) DMF (2)
10
5
5
5
5
5
5
5
5
2
5
5
10
11[b]
12[c]
13[b]
14[b]
15[b]
16[b,d]
17[b,e]
18[b,d]
carbazoles
towards
various
substituted
2-
iodobiphenyls. First, we examined the compatibility
of functional groups by investigating the reactions of
4’-substituted 2-iodobiphenyls. As shown in Table 2,
a range of electron-withdrawing groups, including
cyano, carbonyl, ester, and trifluoromethyl, were
compatible, and the corresponding carbazoles were
formed in good to excellent yields (entries 1-4). Both
fluoro and chloro groups were well-tolerated, and 2-
iodo-4’-phenylbiphenyl was also suitable (entries 5-
7). All the reactions were high-yielding. Electron-
donating groups, such as tert-butyl, methyl, and
methoxy, were also compatible, furnishing carbazoles
in good yields (entries 8-10). Next, we investigated
the reactivities of 2-iodobiphenyls possessing
substituents at other positions. A range of 2-
iodobiphenyls containing various functionalities at
3’-positions including electron-withdrawing and
electron-donating groups underwent the dual
amination reaction in excellent yields (entries 11-14).
3’, 4’-Disubstitued 2-iodobiphenyls were also
suitable (entries 15-17). For the majority of the
substrates containing 3’-substituents, the reactions
occurred at the less hindered positions selectively.
However, due to the small size of fluorine, two
regioisomers were formed for 1p (entry 15). 2’-
Substituted substrates, including those containing
/
DMF (2)
Cs2CO3 (1.0) DMF (2)
Cs2CO3 (1.0) DMF (2)
Cs2CO3 (1.0) DMF (2)
95
80
63
19[b,d,f] Cs2CO3 (1.0) DMF (2)
62
20[b,d,g] Cs2CO3 (1.0) DMF (2)
95 (91)[h]
[a]
1
The yields were determined by H NMR analysis of
crude reaction mixture using CH2Br2 as the internal
standard.
[b] 2.0 equiv. of 2.
[c] 1.0 equiv. of 2.
[d] 0.5 equiv. of KOAc.
[e] No KOAc.
[f] 100 oC.
[g] 6 h.
[h] Isolated yield.
We commenced our studies by investigating the
reaction of 2-iodobiphenyl 1a and diaziridinone 2. As
shown in Table 1, desired carbazole product 3a was
formed in 44% yield in the presence of 10 mol%
Pd(OAc)2, 2.0 equivalents of K2CO3, and 4.0
equivalents of KOAc (entry 1). Whereas replacing
K2CO3 with Li2CO3 or Na2CO3 resulted in lower
yields (entries 2 and 3), the yield was improved to
53% by using Cs2CO3 (entry 4). Solvent screening
showed that the reactions almost failed to generate
the desired product in other screened solvents except
DMA, which gave the same yield as DMF (entries 5-
8). Surprisingly, the yield increased to 90% when the
reaction mixture was diluted with 2 mL DMF (entry
9). The reason remains to be investigated. After
achieving the excellent yield, we turned to further
optimize the reaction conditions by trying to lower
the amount of the reagents in the reaction. First, we
reduced the catalyst loading to 5 mol %, and the yield
remained constant (entry 10). Two equivalents of 2
were sufficient and necessary to afford the high yield
methyl
or
methoxy
group
and
1-(2-
iodophenyl)naphthalene, were also suitable, but the
yields were comparatively lower (entries 18-20). The
lower yields should be due to the steric hindrance
caused by 2’-substituents, which could suppress the
formation of the five-membered palladacycle. The
reactivities of substrates containing functional groups
on the iodo-attached benzene rings were then
explored. A range of functionalities at the positions
para to the iodo groups, including ester, chloro,
fluoro, and methyl groups, were well-tolerated
(entries 21-24). Even sterically hindered 3-methyl-2-
2
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10.1002/adsc.201701039
Advanced Synthesis & Catalysis
Table 2. Substrate scope.
iodobiphenyl 1z could react with diaziridinone,
furnishing product 3z in 78% yield (entry 25).[13] The
substrates bearing substituents on both of the benzene
rings and 4-iodophenanthrene could also be
transformed into corresponding carbazoles in high
yields (entries 26-27). Interestingly, the rigid planar
substrate 1ac was also compatible (entry 28). It
should be mentioned that 2-bromobiphenyl was also
reactive, albeit in a lower yield (entry 29). Notably,
the vinyl C–H bond could also be activated under the
standard conditions, and substrate 1ae underwent the
C–H amination reaction to form indole derivative 3ae
(entry 30).[14]
3
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10.1002/adsc.201701039
Advanced Synthesis & Catalysis
Scheme 3. Removal of the tert-butyl group of product 3a.
Finally, the tert-butyl groups of the products could
be removed readily with trifluoroacetic acid (TFA),
providing unprotected carbazoles (scheme 3).
In conclusion, we have developed a facile and
efficient approach for the synthesis of carbazoles
starting from 2-iodobiphenyls and diaziridinone. A
wide range of carbazoles can be synthesized in good
to excellent yields. Indole derivatives could also be
obtained in the case of styrene as the substrate.
Mechanistic studies provided evidence to support the
intermediacy of a dibenzopalladacyclopentadiene in
the reaction, and the reaction proceeds via a tandem
Pd-catalyzed C–H activation/dual C–N bond
formation sequence.
Scheme 1. Mechanistic studies.
To get evidence to prove that the reaction
proceeded via a palladacycle as the intermediate, we
prepared complex dibenzopalladacyclopentadiene 4
(Scheme 1).[15] When palladacycle 4 was subjected to
the standard conditions, desired carbazole 3a was
formed. However, the yield was much lower than that
in the catalytic reaction of 2-iodobiphenyl. The lower
yield could be caused by the negative impact of the
bipyridine ligand on the reaction. To prove this
assumption, the reaction of 1a and 2 was carried out
in the presence of 5 mol % 2, 2’-bipyridine, and it
was found that the yield decreased dramatically to
56%. These experimental results support that the
palladacycle acted as the key intermediate in the
carbazole-forming reaction.
Experimental Section
A 35 mL Schlenk tube equipped with a stir bar was
charged with palladium acetate (2.3 mg, 0.01 mmol, 5
mol %), cesium carbonate (65 mg, 0.2 mmol, 1.0 equiv.),
potassium acetate (9.8 mg, 0.1 mmol, 0.5 equiv.), 2-
iodobiphenyls (0.2 mmol, 1.0 equiv.), di-tert-
butyldiaziridinone (68 mg, 0.4 mmol, 2.0 equiv.), and
DMF (2 mL). The tube was sealed with a Teflon® high
pressure valve, evacuated and backfilled with N2 (5 times).
After the reaction mixture was stirred in a preheated oil
bath (110 °C) for 6 h, it was allowed to cool down to room
temperature. The reaction mixture was diluted with ethyl
acetate (20 mL) and treated with brine (twice). The organic
layer was dried over anhydrous sodium sulfate and
concentrated in vacuo. The residue was purified on
preparative thin layer chromatography (PTLC) to give
substituted N-tert-butylcarbazoles.
Acknowledgements
The work was supported by the National Natural Science
Foundation of China (No. 21672162) and Shanghai Science and
Technology Commission (14DZ2261100).
Scheme 2. Proposed mechanism.
References
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proposed as shown in Scheme 2. The catalytic cycle
starts with the oxidative addition of 2-iodobiphenyls
to Pd(0) to form Pd(II) species A, which undergo
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Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201701039
Advanced Synthesis & Catalysis
COMMUNICATION
Synthesis of Carbazoles from 2-Iodobiphenyls by
Palladium-Catalyzed C–H Activation and
Amination with Diaziridinone
Adv. Synth. Catal. Year, Volume, Page – Page
Changdong Shao, Bo Zhou, Zhuo Wu, Xiaoming
Ji, and Yanghui Zhang*
6
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