The sequential reactions of tetrazoles with bromoalkynes for the synthesis of (Z)-N-(2-bromo-1-vinyl)-N-arylcyanamides and 2-arylindoles

Article abstract of DOI:10.1039/c3ra40669e

2-Arylindoles were prepared by a sequential reaction of Ag-catalyzed α-addition-Pd-catalyzed C-H bond functionalization of tetrazoles with bromoalkynes. A stereocontrolled Ag-catalyzed α-addition reaction of tetrazoles with bromoalkynes underwent smoothly to generate (Z)-N-(2-bromo-1- vinyl)-N-arylcyanamides, which were subsequently converted into 2-arylindoles through an intramolecular cyclization by Pd-catalyzed direct C-H bond functionalizations.

Full text of DOI:10.1039/c3ra40669e

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The sequential reactions of tetrazoles with
bromoalkynes for the synthesis of (Z)-N-(2-bromo-1-
vinyl)-N-arylcyanamides and 2-arylindoles3
Jiajun Zhang,a Ling-Guo Meng,a Pinhua Lia and Lei Wang*ab
Cite this: DOI: 10.1039/c3ra40669e
Received 7th February 2013,
Accepted 15th March 2013
DOI: 10.1039/c3ra40669e
www.rsc.org/advances
2-Arylindoles were prepared by a sequential reaction of Ag-
catalyzed a-addition–Pd-catalyzed C–H bond functionalization of
tetrazoles with bromoalkynes. A stereocontrolled Ag-catalyzed
a-addition reaction of tetrazoles with bromoalkynes underwent
smoothly to generate (Z)-N-(2-bromo-1-vinyl)-N-arylcyanamides,
which were subsequently converted into 2-arylindoles through an
intramolecular cyclization by Pd-catalyzed direct C–H bond
functionalizations.
the above methods, such as the unstable starting materials or
harsh reaction conditions used in most of the cases.
1-Aryltetrazoles are stable multi-nitrogen compounds, which
are widely applied in rocket propellants and explosives.10 Apart
from that, they are also used for organic transformations via their
C–H bond functionalizations.11 Generally, they are easily converted
to N-arylcyanamides in the presence of a strong base. However, to
the best of our knowledge, there are few reports of tetrazoles being
used as synthetic equivalents of cyanamides.12
Nitrogen-containing heterocycles are of great importance in the
pharmaceutical industry and have attracted much attention from
organic chemists, as they exhibit favorable biological activity and
pharmaceutical significance. Among them, indoles are important
nitrogen heterocyclic compounds, which are key structural motifs
in many natural products and are present in a wide range of
pharmaceuticals with biologically relevant properties.1 For the
above reason, many useful and efficient synthetic methods have
been developed for the synthesis of indoles.2 To the best of our
knowledge, a powerful and classic route to 2-phenylindole is the
Fischer indole synthesis, which is based on the reaction of
phenylhydrazine with acetophenone in the presence of a Lewis
acid.3 The other routes to 2-arylindoles are based mainly on the
Bromoalkynes are obtained from terminal alkynes with NBS
and used as important intermediates in coupling reactions.13
Recently, a transition-metal-catalyzed cross-coupling and a cycliza-
tion reaction using bromoalkynes as one of the starting materials
have been reported by our group.14 In order to establish novel
organic transformations, we further investigated the possibility of
transition-metal-catalyzed reactions between 1-aryltetrazoles and
bromoalkynes. Herein, we wish to report an Ag2O-catalyzed
a-addition of 1-aryltetrazoles to bromoalkynes, which generated
(Z)-N-(2-bromo-1-vinyl)-N-arylcyanamides in good yields with
excellent stereoselectivity. Furthermore, the obtained (Z)-N-(2-
bromo-1-vinyl)-N-arylcyanamides undergo intramolecular cycliza-
tion to afford the corresponding 2-arylindoles in good yields via a
palladium-catalyzed direct C–H bond functionalization process
(Scheme 1).
transition-metal-catalyzed coupling of indoles with various aryl
2 5
compounds, such as halobenzenes,4 [Ar–I+–Ar]BF4
, arylsilox-
anes,6 aryltrifluoroborate salts,7 arylboronic acids,8 etc. In addition,
2-arylindoles also can be obtained through the following
methods,9 for example, Ir-catalyzed hydroamination of internal
alkynes,9a electrosynthesis from o-nitrostyrenes,9e Ni-catalyzed
cycloaddition of anthranilic acid derivatives to alkynes,9h and Pd-
catalyzed cascade reaction of imines with o-dihaloarenes or
o-chlorosulfonates,9i etc. However, there are some drawbacks in
aDepartment of Chemistry, Huaibei Normal University, Huaibei, Anhui 235000, P. R.
China. E-mail: leiwang@chnu.edu.cn; Fax: (+86) 561-380-2047
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032, P. R. China
Electronic supplementary information (ESI) available: General information,
3
detailed experimental procedures, and characterization data for all products.
CCDC reference numbers 920810 and 920811. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3ra40669e
Scheme 1
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In the initial exploration of the stereocontrolled a-addition
reaction of tetrazoles to bromoalkynes, 1-phenyl-1H-tetrazole (1a)
and phenylethynyl bromide (2a) were chosen as the model
substrates for the investigation and the results are shown in
Table 1. Firstly, the effect of the solvent on the model reaction was
examined. When the model reaction was performed in the
presence of Ag2CO3 (20 mol%) in DMSO at 130 uC for 12 h, 80%
yield of (Z)-N-(2-bromo-1-phenylvinyl)-N-phenylcyanamide (3a) was
isolated with the Z-isomer as the sole product, and it was
characterized by 1H and 13C NMR spectroscopy and HRMS
analysis (Table 1, entry 1). Slightly lower yields of 3a were obtained
when CH3CN, chlorobenzene or CH3NO2 was used instead of
DMSO (Table 1, entries 2–4). 1,2-Dichloroethane (DCE), dioxane,
N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA),
benzene, toluene, N-methyl-2-pyrrolidone (NMP), and C2H5OH
were inferior and afforded 31–63% yields of 3a (Table 1, entries 5–
12). On the other hand, the effect of the Ag source was also
examined. The results indicated that Ag2O was the most effective
among the tested Ag sources. AgOAc, Ag2SO4 and AgBF4 were
inferior (Table 1, entries 13–15). Only a trace amount of the
desired product 3a was observed when AgCl or AgNO3 was used in
the reaction (Table 1, entries 16 and 17). When 20 mol% of Ag2O
was used, the desired product 3a was isolated in 82% yield, but a
poor yield of 3a was obtained by decreasing the amount of Ag2O
(Table 1, entries 18–21).
generate the corresponding a-addition products in good yields
with excellent stereoselectivity, and the results are summarized in
Table 2. Clearly, 1-aryl-1H-tetrazoles bearing substituents on the
para-position of the benzene ring have no obvious effect on the
yields of the reactions. 1-Aryl-1H-tetrazoles with both electron-
donating and electron-withdrawing groups, such as Me, Et, F, Cl,
Br, I, NO2, CF3, and MeCO on the para-position of the benzene
ring reacted with phenylethynyl bromide (2a) to generate the
corresponding products (3b–j) in 72–81% yield of exclusively the
Z-isomer (Table 2, entries 2–10). Furthermore, treatment of
3-methylphenyl-, 3-iso-propylphenyl-, and 3-chlorophenyl-
1H-tetrazole with phenylethynyl bromide (2a) also gave the
corresponding products (3k–m) in 75–82% yields (Table 2, entries
11–13). The effect of substituents at the ortho-position of tetrazoles
was observed in the reaction of 2-methylphenyl- and 2-chlorophe-
nyl-1H-tetrazole with 2a, which generated the desired products (3n
and 3o) in 58% and 63% yields (Table 2, entries 14 and 15). The
multi-substituted 1-aryl-1H-tetrazoles reacted with 2a smoothly to
give the corresponding products 3p and 3q in 73% and 84% yields
(Table 2, entries 16 and 17). However, naphthalen-1-yl-tetrazole
reacted with 2a to afford the desired product 3r in 61% yield
(Table 2 entry 18). Meanwhile, the reactions of 1-phenyl-
1H-tetrazole with a series of arylethynyl bromides, such as (4-
methylphenyl)-, (4-tert-butylphenyl)-, (4-fluorophenyl)-, and (4-
chlorophenyl)ethynyl bromide proceeded well and generated the
corresponding products 3s–v in good yields (Table 2, entries 19–
22). It is important to note that aliphatic bromoalkynes, such as
1-bromohex-1-yne also could be converted to the corresponding
addition product 3w with 2a in good yield (Table 2, entry 23).
The structure of compounds 3e and 3f was determined to be in
the Z-configuration, confirmed unambiguously using single crystal
X-ray analysis. The corresponding CIF data are presented in the
Electronic Supplementary Information (CCDC 920810 and 920811,
Under the optimized reaction conditions,
a variety of
substituted tetrazoles reacted with bromoalkynes smoothly to
Table 1 Optimization of the reaction conditionsa
ESI ).15
3
With the obtained (Z)-N-(2-bromo-1-vinyl)-N-arylcyanamides
(3a–w) in hand, the transformation of 3a–w into the corresponding
2-arylindoles by palladium-catalyzed direct C–H bond activation
and intramolecular cyclization along with the loss of the CN group
was investigated. For optimization of the reaction conditions, a
variety of palladium sources were tested in the presence of K2CO3
at 140 uC in DMF, and the results indicated that the model
reaction could be catalyzed by PdII salts or Pd0 complexes.
Pd(OAc)2 was found to be the most active catalyst, and 3a was
converted into 2-phenylindole (4a) in 44% yield (Table 3, entry 1).
Other palladium sources, such as PdCl2, Pd(PPh3)4 or Pd(PPh3)2Cl2
were used instead of Pd(OAc)2, 20–40% yields of 4a were obtained
(Table 3, entries 2–4). The solvent also plays an important role in
the reaction; 80% yield of 4a was obtained when the reaction was
performed in NMP. DMA, C2H5OH, DMSO, toluene, CH3CN were
inferior and afforded lower yields of 4a (Table 3, entries 6–10).
When the solvent was switched to THF, dioxane, ClCH2CH2Cl or
CH3NO2, no product 4a was detected by TLC (Table 3, entries 11–
14). Further investigation on various bases showed that no other
bases performed better than K2CO3. Lower yields of 4a (12–71%)
were obtained when Na2CO3, KOAc, K3PO4, LiOtBu, KOtBu, or
Cs2CO3 was used as a base in the reaction (Table 3, entries 15–20).
Entry
Solvent
Ag source
Yield (%)b
1
2
3
4
5
6
7
8
DMSO
CH3CN
Chlorobenzene
CH3NO2
DCE
DMF
DMA
Toluene
Benzene
NMP
Dioxane
C2H5OH
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
Ag2SO4
AgBF4
80
74
73
71
60
56
53
51
45
44
38
31
40
17
12
9
10
11
12
13
14
15
16
17
18
19
20
21
AgCl
AgNO3
Ag2O
Ag2O
Ag2O
Trace
Trace
82
83c
82d
57e
Ag2O
a
Reaction conditions: 1a (0.50 mmol), 2a (0.75 mmol), Ag catalyst
b
(20 mol%), solvent (2.0 mL), 130 uC, sealed tube, air, 12 h. Isolated
yields. 50 mol% Ag2O was used. 30 mol% Ag2O was used. 10
mol% Ag2O was used.
c
d
e
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Table 2 Ag2O-catalyzed a-addition reactions of tetrazoles to bromoalkynesa
Table 2 (Continued)
R1
R2
Product, 3
Yield (%)b
63
Entry
1
R1
H
R2
Product, 3
Yield (%)b
82
Entry
15
C6H5
2-Cl
C6H5
2
3
4-Me
4-Et
C6H5
C6H5
80
81
16
17
2,4-(Me)2
3,4-(Me)2
C6H5
C6H5
73
84
18
19
C6H5
61
81
4
5
6
7
8
4-F
C6H5
C6H5
C6H5
C6H5
C6H5
72
74
76
75
70
H
H
H
H
H
4-MeC6H4
4-Cl
4-Br
4-I
20
21
22
23
a
4-tBuC6H4
4-FC6H4
4-ClC6H4
n-C4H9
75
85
82
75
4-NO2
9
4-CF3
C6H5
C6H5
C6H5
73
71
81
10
11
4-CH3CO
3-Me
Reaction conditions: 1 (0.50 mmol), 2 (0.75 mmol), Ag2O (0.10
b
mmol), DMSO (2.0 mL), 130 uC, sealed tube, air, 12 h. Isolated
yield.
Meanwhile, poor results were observed when Et3N or pyridine was
used as the base (Table 3, entries 21 and 22).
12
13
14
3-(iso-Pr)
3-Cl
C6H5
C6H5
C6H5
82
75
58
With the optimum reaction conditions for the cyclization of 3a
in hand, the prepared addition products (except 3f–h and 3w)
underwent palladium-catalyzed intramolecular cyclization
smoothly via the direct C–H bond functionalization to generate
the corresponding 2-arylindoles along with the loss of the CN
group. As can be seen from Scheme 2, substrates (Z)-N-(2-bromo-1-
vinyl)-N-arylcyanamides 3 with either electron-donating or elec-
tron-withdrawing groups attached to the benzene ring were able to
undergo the intramolecular cyclization reaction smoothly.
Electron-donating groups on the aromatic rings of 3 gave better
yields than electron-withdrawing groups on the aromatic rings of 3
2-Me
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Table 3 Optimization of the reaction conditions for the palladium-catalyzed
intramolecular cyclization of (Z)-N-(2-bromo-1-phenylvinyl)-N-phenylcyanamide
(3a)a
Entry
Pd source
Base
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
Pd(OAc)2
PdCl2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
KOAc
DMF
DMF
DMF
DMF
NMP
DMA
C2H5OH
DMSO
Toluene
CH3CN
THF
Dioxane
DCE
CH3NO2
NMP
NMP
NMP
NMP
NMP
NMP
44
40
29
20
80
62
17
13
Pd(PPh3)4
Pd(PPh3)2Cl2
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
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
9
10
7
10
11
12
13
14
15
16
17
18
19
20
21
22
NDc
NDc
NRd
NRd
71
58
48
15
13
12
9
Trace
K3PO4
LiOtBu
KOtBu
Cs2CO3
Et3N
NMP
NMP
Pyridine
a
Reaction conditions: 3a (0.50 mmol), base (2.0 equiv), Pd source
b
(5.0 mol%), solvent (2.0 mL), 140 uC, sealed tube, air, 8 h. Isolated
yield. ND = No desired product was detected. NR = No reaction.
c
d
Scheme 2 Palladium-catalyzed cyclization reactions of (Z)-N-(2-bromo-1-phenyl-
vinyl)-N-phenylcyanamides to 2-arylindoles.
(4b, 4c, 4k, 4l, 4n, 4p and 4q vs. 4d, 4e, 4i, 4j, 4m and 4o). It should
be noted that the cyclization reactions were complicated and no
desired products were obtained when (Z)-N-(4-bromophenyl)-, (Z)-
N-(4-iodophenyl)- and (Z)-N-(4-nitrophenyl)-N-(2-bromo-1-phenylvi-
nyl)cyanamide (3f–h) were used as substrates, which can be
ascribed to the more sensitive groups, Br, I and NO2 on the phenyl
rings in the presence of the Pd-catalyst. Meanwhile, it is important
to note that the cyclization products of 3k–m were 6-substituted-2-
phenylindoles (4k–m), and 3q was 5,6-disubstituted-2-phenylin-
dole (4q) with excellent regioselectivity. The ortho-position effect
was not observed in the reaction, which generated 4n and 4o in 84
and 73% yields, respectively. When reactions of the substrates
derived from the reactions of 1-phenyl-1H-tetrazole (1a) with
substituted phenylethynyl bromides, such as (4-methylphenyl)-, (4-
tert-butylphenyl)ethynyl-, (4-fluorophenyl)-, and (4-chlorophenyl)-
bromides were carried out under the standard reaction conditions,
the corresponding products were obtained in 71–87% yields
(Scheme 2, 4s–v). However, (Z)-N-(1-bromohex-1-en-2-yl)-
N-phenylcyanamide 3w could not give the corresponding cycliza-
tion product due to the lower reaction activity.
shown in Scheme 3. Firstly, N-phenylcyanamide (A) was formed by
a Ag-catalyzed decomposition of 1-phenyl-1H-tetrazole (1a) with
loss of N2 (it is highly recommended that an Ace pressure tube is
On the basis of our experimental results and our previous
reports,14a,16 a plausible mechanism for this sequential reaction of
the addition–Pd-catalyzed cyclization reaction is proposed, as
Scheme 3 Proposed reaction mechanism.
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Scheme 4 Controlled experiments.
employed for safety considerations),11,12 which underwent a-addi-
tion to phenylethynyl bromide (2a) to generate (Z)-N-(2-bromo-1-
phenylvinyl)-N-phenylcyanamide (3a) with excellent stereoselectiv-
ity. The obtained 3a reacted with Pd0 from its precursor Pd(OAc)2
to form an intermediate B via oxidative addition. Subsequently, B
underwent an intramolecular electrophilic aromatic palladation
through C–H activation of the aromatic hydrogen, and subsequent
proton abstraction, forming an intermediate C. This was followed
by a reductive elimination to afford intermediate D via carbon–
carbon bond formation and the Pd0 was regenerated for its
catalytic cycle. Finally, D could be transformed into the desired
product 2-phenylindole (4a) by losing a cyano group due to a little
water in the solvent.
In order to further understand the reaction process,
N-phenylcyanamide A was synthesized from the reaction of
1-phenyl-1H-tetrazole (1a) in the presence of Ag2O. Treatment of
A with 2a under the above reaction conditions, afforded product
3a, which was isolated in 71% yield. It should be noted that other
amines, such as aniline or N-methylaniline instead of A, could not
react with 2a to give addition product (Scheme 4).
In conclusion, we have developed a novel and efficient protocol
for the synthesis of 2-arylindoles from tetrazoles and alkynyl
bromides. The a-addition reactions of tetrazoles to bromoalkynes
generated (Z)-N-(2-bromo-1-vinyl)-N-arylcyanamides in good yields
with excellent stereoselectivity. The obtained (Z)-N-(2-bromo-1-
vinyl)-N-arylcyanamides underwent intramolecular cyclization well
to afford 2-arylindoles in good yields through palladium-catalyzed
direct C–H bond functionalizations, involving loss of the cyano
group.
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Acknowledgements
This work was financially supported by the National Science
Foundation of China (Nos. 21172092, 21072152) and the Anhui
Provincial Natural Science Foundation (1208085QB40).
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