Synthesis of dibenzo[: A, c] carbazoles from 2-(2-halophenyl)-indoles and iodobenzenes via palladium-catalyzed dual C-H functionalization

Article abstract of DOI:10.1039/c7ob01638g

An efficient palladium-catalyzed strategy for the synthesis of dibenzo[a,c]carbazole derivatives has been developed. In the presence of Pd(OAc)₂, 2-(2-halophenyl)-indoles and iodobenzenes proceeded smoothly to obtain the corresponding dibenzo[a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Synthesis of Dibenzo[a,c]carbazoles from 2-(2-Halophenyl)-indoles and
Iodobenzenes via Palladium-Catalyzed Dual C–H Functionalization
Lijun Wu, Guobo Deng* and Yun Liang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An efficient palladium-catalyzed strategy for the synthesis of
expeditious methods of synthesizing dibenzo[a,c]carbazoles are
dibenzo[a,c]carbazole derivatives has been developed. In the
still highly desirable and remain a challenge. Our previous work
presence
of
Pd(OAc)2,
2-(2-halophenyl)-indoles
and 50 indicated that benzo[a]carbazoles were able to be obtained from
iodobenzenes proceeded smoothly to obtain the corresponding
vinylindoles with arynes.13 As part of our ongoing studies on the
development of fused carbazoles chemistry, we continue to
explore the feasibility of synthesizing dibenzo[a,c]carbazoles.
Herein, we report an efficient and mild palladium-catalyzed
55 cross-coupling tandem reaction of 2-(2-halophenyl)-indoles with
10 dibenzo[a,c]carbazoles in moderate to good yields. This
methodology constructs two new C-C bonds via a palladium-
catalyzed dual C-H functionalization.
Carbazoles are an important class of heterocycle due to their
widespread application in many fields.1 Among these, fused
15 carbazoles have received much attention, since fused carbazoles
moieties are frequently found in natural products and
biologically active compounds. such as CDK inhibitors,2
antitumor,3 antiestrogenic properties4 and potential DNA
intercalating drugs.5 Therefore, great efforts have been made to
20 develop methods for the effective synthesis of this useful
structural skeleton in recent years.6 Traditional strategies for the
synthesis of fused carbazoles are as follows: (a) transition-metel-
catalyzed intermolecular cyclization reactions of indole
derivatives,7 (b) tandem cyclization of diynes,8 (c) intramolecular
25 cyclization of prefunctionalized indoles.9 These methods are
useful for the synthesis of some fused carbazoles. Regrettably,
there are few reports on the synthesis of dibenzo[a,c]carbazoles.
Recently, methods based on palladium catalysis have gained
great attention as they offer some strategies for the synthesis of
30 dibenzo[a,c]carbazoles (Scheme 1). For instance, Goggiamani
has recently reported the synthesis of dibenzo[a,c]carbazoles
from 2-(2-bromoaryl)-3-arylidole.10 Wu and co-workers have
reported a new route to synthesise dibenzo[a,c]carbazoles
through dual C-H functionalization of indoles with cyclic
35 diaryliodoniums.11 More recently, Jana and co-workers have also
demonstrated that 1,4-Pd migration can be a powerful tool for
the synthesis of dibenzo[a,c]carbazoles.12 Those methods are
iodobenzenes,
affording
the
corresponding
dibenzo[a,c]carbazole derivatives in moderate to good yields.
60
Scheme 1 Synthesis Approaches to Dibenzo[a,c]carbazoles
Our investigation began with the reaction of 2-(2-
iodophenyl)-1-methyl-1H-indole 1a with iodobenzene 2a to
65 determine the optimal reaction conditions, and the results were
summarized in Table 1. In the presence of Pd(OAc)2 as the
catalyst and NaOAc as base, substrate 1a and 2a were converted
into the desired product 9-methyl-9H-dibenzo[a,c]carbazole 3aa
in 22% yield. Inspired by this result, a series of base (including
70 NaOAc, CsF, K2CO3, Cs2CO3, Na2CO3) were examined (entries 1-
5). K2CO3 gave the best results, and the desired product of
dibenzo[a,c]carbazole was obtained in 81% yield. Subsequently,
a number of other Pd catalysts, such as PdCl2, Pd2(dba)3 and
Pd(PPh3)4 were tested (entries 3 and 6-8). The results indicated
very
effective
for
the
construction
of
some
dibenzo[a,c]carbazoles, however, the development of novel and
40
Key Laboratory of the Assembly and Application of Organic Functional
Molecules, Hunan Normal University, Changsha, Hunan 410081, China.
Fax: +86(0731)88872533; E-mail: gbdeng@hunnu.edu.cn. and
yliang@hunnu.edu.cn.
45 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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that Pd(OAc)2 was the best one for this cascade reaction.
Notably, control experiments showed that no formation of the
desired product 3aa occurred in the absence of a Pd catalysts
atom were screened. The results showed that a range of
electron-donating substituents including cyclopropylmethyl,
benzyl or substituted benzyl substrates could be smoothly
(entry 9). To improve the reaction performance, the effects of 25 transformed into the corresponding desired products (3ba–3ea)
5
ligands (including P(o-tol)3, PPh3, dppp, dppf, X-phos ) were also
examined (entries 10-14). PPh3 achieved the best result, and the
desired product 3aa was obtained in 86% yield (entry 11).
Solvents such as DMF, toluene and DMSO were also evaluated
in excellent yields under the optimized conditions. For example,
cyclopropylmethyl substituted substrate 1b regioselectively
formed 9-(cyclopropylmethyl)-9H-dibenzo[a,c]carbazole 3ba in
91% yield. Substrate 1d with an electron-donating methyl group
(entries 3 and 15-17), and a 92% yield of 3aa was afforded when 30 and substrate 1e with an electron-withdrawing CN group both
10 DMSO was used (entry 17). Finally, the temperature screening
indicated that 120 oC was the most suitable temperature for this
protocol (entries 18-19). Thus, the optimized reaction conditions
were as follows: 1a (0.3 mmol), 2a (0.36 mmol), Pd(OAc)2 (5 mol
showed high reactivity, giving 3da and 3ea in 93% and 75% yield
respectively. It is regrettable that group free and electron-
withdrawing substituted substrates are unfavourable to this
tandem reaction. Subsequently, our study focused on the
%), PPh3 (10 mol %), K2CO3 (0.9 mmol), DMSO (2 mL), under an 35 substitution effect on the aryl ring of indole 1 (products 3ia-
15 air atmosphere in sealed schlenk tube at 120 oC for 12 hours.
3ma). It is noteworthy that a halo group, such as Cl and Br, on
the aromatic ring is amenable to the optimal conditions, thereby
providing an opportunity for additional modifications at the
halo-genated position (3ja and 3ka).Substrate 1m with methyl at
40 the 4 and 6 positions were also viable for constructing 3ma in
75% yield.
Table 1 Optimization of reaction conditionsa
Table 2 Variation of 2-(2-iodophenyl)-indoles a,b
Catalyst
Ligand
Solvent
DMA
yield/3aab
Entry
1
Base
Pd(OAc)2
NaOAc
22
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
CsF
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
72
81
57
63
71
65
67
0
K2CO3
CsCO3
NaCO3
K2CO3
K2CO3
K2CO3
K2CO3
Pd2(dba)3
Pd(Ph3)4
10
11
12
13
14
15
16
17
18c
19d
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
P(o-tol)3
PPh3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
DMA
DMA
DMA
DMA
DMA
DMF
84
86
dppp
dppf
83
46
X-phos
PPh3
78
83
PPh3
Tol
Trace
92
PPh3
DMSO
DMSO
DMSO
PPh3
93
PPh3
42
a Reaction conditions: 1 (0.3 mmol), 2a (0.36 mmol), Pd(OAc)2 (5 mol %),
a Reaction conditions: 1a (0.3 mmol), 2a (0.36 mmol), Pd salt (5 mol %),
ligand (10 mol %), base (0.9 mmol), solvent (2 mL ), under air atmosphere
in sealed Schlenk tube, at 140 oC for 12 h. b Isolated yields. c120 oC. d 80 oC.
K2CO3 (0.9 mmol), DMSO (2 mL ), 120 oC and 12 h in air atmosphere.
b
Isolated yields.
45 We next explored the possibility of employing substituted
iodobenzenes as substrates to react with 1a, and the results are
With the optimized conditions in hand, a series of substituted
20 2-(2-iodophenyl)-indoles were examined, and the results are
summarized in Table 2. First, the substituents on the nitrogen
summarized
in
Table
3.
The
corresponding
dibenzo[a,c]carbazoles were obtained in good to perfect yields.
2
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Initially, the para-substituent of the aryl moiety was screened.
To our delight, substrates 2b-2f, bearing electron-donating or
indoles and iodobenzene were screened (Scheme 2). In the
presence of Pd(OAc)2 as the catalyst and K2CO3 as base, 2-(2-
electron-deficient groups on the aryl ring, were compatible 30 bromophenyl)-1-methyl-1H-indole 1n and 2a were converted
under the standard conditions. Substrate 2b with a methyl
group, for instance, underwent the reaction with 2-(2-
iodophenyl)-1-methyl-1H-indole 1a, affording 3ab in 98% yield.
Good yield was still achieved from substrate 2f with a CF3 group.
into the desired product 9-methyl-9H-dibenzo[a,c]carbazole 3aa
in 70% yield. Compared with 2-(2-iodophenyl)-1-methyl-1H-
indoles, the activities of 2-(2-bromophenyl)-1-methyl-1H-indoles
were slightly lower in terms of yield. The m-OMe-substituted
5
As expected, a series of the ortho-substituted (R = Me, F) or 35 and m-Cl-substituted substrates 1o and 1p formed the
meta-substituted (R = Me, F, CF3) iodobenzenes furnished the
corresponding dibenzo[a,c]-carbazoles 3na and 3oa in 51% and
10 transformation producing the desired dibenzo[a,c]carbazoles in
moderate to high yields. However, two regioisomers generated
from substrate 2i, in a 1.1:1 ratio. The results showed that the
electron-donating groups substrates displayed higher reactivity
than the electron-withdrawing groups substrates. Furthermore,
15 Substrates 2l and 2m with two substituents at the 3 and 5
positions were also viable for constructing 3al and 3am in 93%
and 78% yield respectively. Moreover, diphenyl and naphthyl
iodides could be successfully converted into poly-cyclicring
products (3an-3ao) in good yields. Thiophenyl iodides were
20 tolerated under the reaction conditions, although the reaction
yield was relatively low.
52% yield respectively.
40 Scheme 2 Variation of 2-(2-bromophenyl)-indoles
To shed light on the possible mechanism of the reaction,
several control experiments were carried out, as shown in
Scheme 3. First, using 2-([1,1'-biphenyl]-2-yl)-1-methyl-1H-indole
45 A was tested under standard condition and it can’t be converted
into the desired product 9-methyl-9H-dibenzo[a,c]carbazole 3aa.
Table 3 Synthesis of 2-aminobenzothiazolones from substituted o-
iodophenyl isocyanides a
Notably, 2-(2-bromophenyl)-1-methyl-3-phenyl-1H-indole
B
were tested under standard condition (Scheme 2). To our
delight, B was converted into the desired product 9-methyl-9H-
50 dibenzo[a,c]carbazole 3aa in 36% yield. The results implied that
B was the intermediate among this cross-coupling tandem
reaction.
R
R= Me, 3ab, 98%
R = OMe, 3ac, 82%
R = F, 3ad, 92%
R = Cl, 3ae, 89%
R = CF3, 3af, 75%
N
N
F
3ag, 74%
N
N
N
3ai : 3ai' = 1.1:1, 91%
3ah, 75%
F
R
F
N
N
N
55 Scheme 3 Control experiment
R = F, 3aj, 75%
R = CF3, 3ak, 35%
Ph
3al, 93%
3am, 78%
Based on the above observations and previous mechanistic
studies,12,15,16 a plausible mechanism is proposed in scheme 4.
First, the active Pd (0) is presumably generated in situ from the
60 palladium acetate during the reaction and formed Pd( Ⅱ)
complex D by oxidative addition of 1a.12,16 Then, intermediate E
is obtained by the electrophilic cyclization reaction of
intermediate D.15a,16 Subsequently, the five membered carbo-
palladacyle E undergoes a second oxidative addition with
65 iodobenzene to form intermediate F.15b,c,d Selective reductive
elimination of the intermediate F would afford the intermediate
S
N
N
N
3an, 85%
3ap, 23%
3ao, 72%
25
a
Reaction conditions: 1a (0.3 mmol), 2 (0.36 mmol), Pd(OAc)2 (5 mol
%), K2CO3 (0.9 mmol), DMSO (2 mL ), 120 oC and 12 h in air
atmosphere. b Isolated yields.
To extend the substrate scope of this cyclization methodology,
the different substituted 2-(2-bromophenyl)-1-methyl-1H-
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G. Another way to get intermediate G may be transmetalation-
type exchange of aryl ligands with the phenylpalladium(Ⅱ)
species. Finally, intramolecular C-H activation of G and reductive
elimination of H afford the desired product 3aa.12,15b,c,d,16
40
45
50
55
60
65
70
75
80
2
3
5
4
5
6
Scheme 4 Possible Mechanism
7
In summary, we have demonstrated a novel palladium-
10 catalyzed cross-coupling tandem reaction of N-substituted-(2-
halophenyl)indoles and iodobenzenes. This methodology allows
formation of two new C-C bonds via a palladium-catalyzed dual
C-H functionalization. Furthermore, this method provides an
efficient approach to synthesize dibenzo[a,c]carbazoles, which
15 are ubiquitous structural units in a number of biologically active
compounds.
8
9
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Acknowledgment
21, 16786.
This work was supported by the Natural Science Foundation of
China (21572051，21602057), Ministry of Education of the
20 People's Republic of China (213027A), Education Department of
Hunan Province (15A109) for financial support.
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Synthesis of Dibenzo[a,c]carbazoles from 2-(2-Halophenyl)-indoles
and Iodobenzenes via Palladium-Catalyzed Dual C–H
Functionalization
Lijun Wu, Guobo Deng* and Yun Liang*
We report an efficient approach to synthesize dibenzo[a,c]carbazoles via Palladium-Catalyzed
Cross-Coupling Tandem Reaction.