An Expedient Synthesis of Carbazoles through Potassium tert-Butoxide-Promoted Intramolecular Direct C–H Bond Arylation

Article abstract of DOI:10.1002/ejoc.201601293

Transition-metal-free access to carbazoles was achieved through base-mediated intramolecular C–C bond formation. Reactions of N-substituted o-halodiarylamines with potassium tert-butoxide in the presence of ethylene glycol or 1,10-phenanthroline provided carbazoles in moderate to excellent yields. This transformation may proceed through a radical pathway according to a control experiment with a radical scavenger.

Full text of DOI:10.1002/ejoc.201601293

A Journal of
Accepted Article
Title: An Expedient Synthesis of Carbazoles via Potassium tert-
Butoxide Promoted Intramolecular Direct C-H Bond Arylation
Authors: Songbo Lin, Xingrui He, Jinpeng Meng, Haining Gu, Peizhi
Zhang, and Jun Wu
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10.1002/ejoc.201601293
European Journal of Organic Chemistry
COMMUNICATION
An Expedient Synthesis of Carbazoles via Potassium tert-
Butoxide Promoted Intramolecular Direct C-H Bond Arylation
[a]
[a]
[a]
[a]
[b]
[a]
Songbo Lin, Xingrui He, Jinpeng Meng, Haining Gu, Peizhi Zhang, and Jun Wu*
Abstract: Transition metal-free access to carbazoles has been achieved on the basis of base-mediated intramolecular C−C bond formation.
Reactions of N-substituted o-halodiarylamines with potassium tert-butoxide in the presence of ethylene glycol or 1,10-phenanthroline provided
carbazoles in moderate to excellent yields. This transformation may proceed via the radical pathway according to the control experiment with a
radical scavenger.
bond cross-coupling for the efficient preparation of carbazoles is
highly desirable.
Introduction
Carbazoles are essential core structures found in natural
products,[1] drugs,[2] polymers[3] and photoelectric materials[4] and
have received considerable attention in recent years, especially
in material science[5] and biology[6] (Figure 1). For example, P7C3
is a kind of neuroprotective compound that can enhance
neurogenesis in the hippocampus, where it acts by blocking the
death of newborn cells.[2a, 2c]
Scheme 1. Previous studies and this work.
Figure 1. Molecules containing the carbazole structural motif.
In recent years, transition-metal-free cross-coupling reactions
have gained immense interest in chemical synthesis.[10] In 2008,
Consequently, intense efforts have been made to develop
Itami[10d] and co-workers first reported KOtBu promoted C–H bond
arylation of heterocycles using haloarenes. In 2010, Hayashi,[10e]
Lei[10f] and Shi[10g] independently moved forward this methodology
to unactivated arenes. Then Shi,[10h] Charette,[10i] Bisai,[10j]
Kumar[10k] and Wu[10l] applied this method into the construction of
various methods for the synthesis of carbazoles,[1a, 7] [8] (Scheme
1). Among the various routes to synthesize carbazoles, transition-
metal-catalyzed intramolecular C−C bond cross-coupling is
perhaps the most atom-economical and expedient route.
However, this transformation has limitations associated with the
requirements of metal catalysts, which has some disadvantages,
such as a high economic cost, and needing purification to remove
trace amounts of metals residues in the synthesis of drugs and
photoelectric materials.[9] Therefore, a transition-metal-free C-C
fused rings
amaryllidaceae
,
such as 6H-benzo[c]chromene derivatives,
alkaloids, dibenzoazepinones and
phenanthridinones. These fused rings were almost all six-
membered rings,[10h-l] and the reaction needed higher temperature
(100—160 oC),[10h-l] expensive additives (neocuproine,
DMEDA)[10h-j]
mesitylene).[10h,
and toxic
solvents
(benzene,
toluene,
10j-l]
We envisioned that N-substituted o-
[a]
[b]
Department of Chemistry, Zhejiang University, Hangzhou 310027,
P. R. of China
Fax +86(571)87951895
E-mail: wujunwu@zju.edu.cn
www.chem.zju.edu.cn/english
halodiarylamines would cyclize to the carbazoles under transition
metal-free conditions (Scheme 1).To the best of our knowledge,
such a transformation has not yet been achieved. In this paper,
we report the successful conversion of N-substituted o-
halodiarylamines to the carbazoles via the transition-metal-free C-
C bond cross-coupling reaction.
School of Biological and Chemical Engineering, Zhejiang University
of Science and Technology, Hangzhou 310012, P. R. of China
Supporting information for this article is given via a link at the end of the
document.
Results and Discussion
1
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10.1002/ejoc.201601293
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COMMUNICATION
Initially, we started our investigation using 2-iodo-N-(4-
methoxyphenyl)-N-methylaniline 1a as the model substrate. After
surveying the literature for the synthesis of five-membered
rings,[10m] we chose 0.2 mmol of 1a, 1,10-phenanthroline 6 (0.08
mmol) and potassium tert-butoxide (0.6 mmol) in mesitylene at
160 oC as the reaction conditions for the direct C-C bond coupling.
The reaction mixture was heated for 12 h to obtain the desired
product 2a and a dehalogenative byproduct 2ab, while 99%
conversion of the starting material 1a was observed (Table 1,
entry 1). When the reaction proceeded at 100 °C, the yield
dropped dramatically and only 39% conversion was obtained
(Table 1, entry 2). To increase yield and selectivity, other solvents
were also examined (Table 1, entries 3-5). DMSO was still best to
give 81% yield of 2a (Table 1, entry 3). Toluene and dioxane also
gave 99% conversion of starting material but a large amount of
byproduct 2ab were produced (Table 1, entries 4-5).
Consequently, we chose DMSO as solvent for use in the further
1, entries 10-12). It was noteworthy that 1,10-phenanthroline 6,
DMEDA 7 and ethylene glycol 8 all showed the good activity. In
view of application, the cheaper ethylene glycol was chose as the
additive in the further studies. Thus, 40 mol% ethylene glycol 8
3.0 equiv KOtBu in DMSO at 30 °C under N2 atmosphere for 12 h
was accepted as the optimized reaction conditions.
Table 2. Intramolecular direct C−H arylation of N-substituted iododiarylamines.[a]
Entry
1
Substrates
Products
Yield(%)[b]
81
Table 1. Screening of reaction conditions.[a]
1a
2a
2b
2
3
4
5
6
75
62
83
77
68
1b
1c
1d
1e
1f
2c
2d
Entry
Additive
Solvent
Tem
(oC)
Conv.
(%)
Yield(%)
2a 2ab
1
6
mesitylene
mesitylene
DMSO
160
100
100
100
100
50
>99
39
79
20[b]
7[b]
2e
2
6
31
81
69
76
83
93
89
86
43
62
57
3
6
>99
>99
>99
>99
>99
>99
97
19[b]
24[b]
21[c]
17[c]
8[c]
2fb
4
6
PhMe
5
6
dioxane
DMSO
81 [c]
m/o = 1:2
2gm
2go
7
1g
6
6
7
6
DMSO
30
8
7
DMSO
30
11[c]
8[c]
8
9
62
69
9
8
DMSO
30
1h
2h
10
11
12
9
DMSO
30
47
5[c]
10
11
DMSO
30
82
21[c]
31[c]
DMSO
30
92
1i
1j
2i
2j
[a] Reaction conditions:1 (0.2 mmol), additive (0.08 mmol), KOtBu (0.6 mmol),
and solvent (1.0 mL) were stirred at specified reaction temperature under N2
for 12 h.
10
0
[b] Isolated yields.
[c] Determined by 1H NMR of the crude reaction mixture.
[a] Reaction conditions: 1 (0.2 mmol), ethylene glycol (0.08 mmol), KOtBu (0.6
mmol) and DMSO (1.0 mL) were stirred at 30 oC under N2 for 12 h.
[b] Isolated yields.
optimization. Interestingly, lowering the temperature increased
both the yield and the selectivity (Table 1, entries 6-7). Thus, the
further studies were conducted at 30 oC. Then we tested other
additives in the transformation. We found that N,N'-dimethyl
[c] The ratio of two regioisomers determined by 1H NMR.
With the optimized reaction conditions in hand, a variety of N-
ethylenediamine (DMEDA) 7[10f] and
ethylene glycol 8[10l]
substituted iododiarylamines
1
were initially examined in
successfully promoted this coupling to afford 89% and 86% yields
of 2a, respectively (Table 1, entries 8-9). The use of other
additives , such as 2,2'-azoisobutyronitrile (AIBN) 9,[10k] n-butanol
10 and 1,3-pantanediol 11 resulted in the lower yields of 2a (Table
intramolecular direct C−C arylation (Table 2). We found that
electron-donating substituents on para-position of phenyl ring,
such as methoxyl and methyl groups were applicable, and the
desired cyclized products were isolated in good yields (Table 2,
2
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10.1002/ejoc.201601293
European Journal of Organic Chemistry
COMMUNICATION
entry 1, 3). The substrates with electron-withdrawing fluoro and
chloro groups were well tolerated and proceeded efficiently with
good yields (Table 2, entries 4-5). We were taken by surprise
when reaction of ortho-substituted starting material 1f afforded no
desired product 2f, while a dehalogenative byproduct 2fb was
abtained in a 68% isolated yield (Table 2, entry 6). This
transformation was sensitive to steric effect. However, the
coupling reaction of the meta-substituted substrate 1g proceeded
smoothly, and generated a mixture of compounds 2gm and 2go
in 81% overall yield (m/o = 1:2, Table 2, entry 7). The effects of N
substituents on the efficiencies of the reactions were also
explored (Table 2, entries 8-9). N-Ethyl- and N-n-butyl-containing
substrates reacted to produce the corresponding carbazoles 2h
and 2i in 62% and 69% yields, respectively. When the N-
substituent was removed, cyclization product was not obtained
(Table 2, entry 10).
Scheme 3. Control experiments in the presence of TEMPO.
Table
3.
Intramolecular
direct
C−H
arylation
of
N-
substituted bromodiarylamines
With the same optimized conditions (conditions A), bromide
substrates 3 were subsequently explored (Table 3). We found that
N-methyl o-bromodiphenylamine 3a went smoothly under
conditions A with 64% yield of 4a (2b) (Table 3, entry 1).
Unfortunately, the phenyl rings with some substituents gave
somewhat lower yields (Table 3, entries 2-7). In general, when the
halo group on the substrate was changed from iodo to bromo, the
yields of the reaction usually decreased. We attempted to restore
reactivity of bromide substrates 3 by increasing temperature. After
further investigation, 40 mol% 1,10-phenanthroline 6, 3.0 equiv
KOtBu in mesitylene at 160 °C under N2 atmosphere for 8 h was
accepted as optimized conditions B. In general, bromide
substrates 3 containing both electron-donating and electron-
withdrawing phenyl substituents reacted smoothly to form the
corresponding carbazoles 4 under conditions B (Table 3, entries
1-7). The substrates 3 with fluoro and chloro groups were well
tolerated (Table 3, entries 4-5). Specifically, starting material 3c
afforded the cyclized product 4c in 96% yield (Table 3, entry 3).
Interestingly, 11-methyl-11H-benzo[a]carbazole 4h was obtained
using this transformation (Table 3, entry 8).
Entry
Substrates
Products
Yield(%)[a]
Conditi
Conditi
ons A[b]
ons B[c]
64
67
1
2
4a (2b)
3a
3b
39
31
15
12
76
96
59
69
4b (2a)
3
4
5
3c
3d
3e
3f
4c
We also tested reactivity of chloride substrate 5 in two different
catalyst systems (Scheme 2). In contrast to iodide substrates 1
and bromide substrates 3, chloride substrate 5 exhibited lower
activity. In conditions A, chloride substrate 5 did not undergo the
intramolecular C-H arylation. Notably, conditions B afforded the
desired product 2b in 34% yield. Thus, a higher efficient catalyst
system in this transformation would be desirable.
4d
4e
6
7
35
8
67
67
4f
3g
3h
4g
8
–
90
Scheme 2. Intramolecular direct C−H arylation of N-methyl-o-
chlorodiphenylamine
4h
[a] Isolated yields.
Considering the previous reports,[10j-l] the reaction would
proceed through a radical process. To assume this hypothesis, we
[b] Reaction conditions A: 3 (0.2 mmol), ethylene glycol (0.08 mmol), KOtBu
(0.6 mmol) and DMSO (1.0 mL) were stirred at 30 oC under N2 for 12 h.
[c] Reaction conditions B: 3 (0.2 mmol), 1,10- phenanthroline (0.08 mmol), KOtBu
(0.6 mmol) and mesitylene (1.0 mL) were stirred at 160 oC under N2 for 8 h.
carried out the reaction in the presence of a radical scavenger
10f, 10i]
TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl].[10e,
The
reaction of 1a with 3.0 equiv of TEMPO under standard conditions
A afforded the corresponding carbazole 2a only in 6% yield and
66% of the starting material 1a was recovered (Scheme 3), which
indicated that this transformation would involve aryl radical in this
coupling reaction.
A plausible mechanism of homolytic aromatic substitution is
3
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10.1002/ejoc.201601293
European Journal of Organic Chemistry
COMMUNICATION
shown in Scheme 4. In the presence of additive and KOtBu,
compound A generates the radical anion B through SET.[7k, 11]
Radical anion B then can be transferred into aryl radical C via the
dehalogenation. This aryl radical C can add to the phenyl ring to
afford the intermediate D via 5-exo-trig cyclization. Radical D
gives E via KOtBu-assisted deprotonation. A radical chain transfer
reaction between E and A affords the desired product F along with
regeneration of radical anion B and continues the cycle.
KOtBu (67 mg, 0.6 mmol) and 1,10- phenanthroline (14 mg, 0.08 mmol)
were added into a dry reaction tube, which was filled by N2. 1 mL
mesitylene solution of 3 (0.2 mmol) was injected by syringe into the tube
under N2 at room temperature. The mixture was stirred 8 h at 160 oC in the
oil bath, then quenched with saturated NaCl (5 mL), extracted with ethyl
acetate (10 × 3 mL), dried over anhydrous Na2SO4. The residue was
purified by thin-layer chromatography to afford carbazoles 4.
Acknowledgements
Support of this work through the Special Fund for Agro-scientific
Research in the Public Interest （201403030）and the National
Natural Science Foundation of China (No.31471807).
Keywords: Carbazoles • C-H arylation • Metal-free • N-
Substituted o-halodiarylamines • Potassium tert-butoxide
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Conclusions
In conclusion, we have demonstrated facile access to
carbazoles via intramolecular C-H arylation from readily
accessible N-substituted o-halodiarylamines under transition
metal-free conditions. This methodology is efficient, operationally
simple and green. Especially, the intramolecular direct C−C
cross-coupling reactions of N-substituted o-iododiarylamines can
proceed in mild conditions (30 °C) using a cheap and nontoxic
additive ethylene glycol. The control experiment suggests that the
transformation takes place via the radical pathway. Further
development of transition metal-free C–C bond formation for the
synthesis of biologically natural products is being explored in our
lab.
Experimental Section
General procedure A for the synthesis of N-alkyl-carbazoles.
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KOtBu (67 mg, 0.6 mmol) was added into a dry reaction tube, which was
filled by N2. 1 mL DMSO solution of 1 (0.2 mmol) and ethylene glycol (5
mg, 0.08 mmol) were injected into the tube by syringe under N2 at room
temperature. The mixture was stirred 12 h at 30 oC in a calorstat, then
quenched with saturated NaCl (5 mL), extracted with ethyl acetate (10 × 3
mL), dried over anhydrous Na2SO4. The residue was purified by thin-layer
chromatography to afford carbazoles 2.
General procedure B for the synthesis of N-alkyl-carbazoles.
4
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10.1002/ejoc.201601293
European Journal of Organic Chemistry
COMMUNICATION
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5
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European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
Carbazoles
Songbo Lin, Xingrui He, Jinpeng Meng, Haining Gu, Peizhi
Zhang, Jun Wu*
Page 1– Page 6
A facile access to carbazoles via intramolecular C-H arylation from
readily accessible N-substituted o-halodiarylamines under transition
metal-free conditions is reported. Especially, the intramolecular direct
C−C cross-coupling reactions of N-substituted o-iododiarylamines
can proceed in mild conditions (30 °C) using a cheap and nontoxic
additive ethylene glycol.
An Expedient Synthesis of Carbazoles via Potassium
tert-Butoxide Promoted Intramolecular Direct C-H Bond
Arylation
6
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