Metal-free nitrative cyclization of N-aryl imines with tert-butyl nitrite: Dehydrogenative access to 3-nitroindoles

Article abstract of DOI:10.1039/c4cc08498e

We describe here a new metal-free route for the synthesis of 3-nitroindoles by the nitrative cyclization of N-aryl imines with tert-butyl nitrite. The radical transformation allows the assembly of the indole framework through oxidative cleavage of multi C-H bonds, a nitration, cyclization and isomerization cascade.

Full text of DOI:10.1039/c4cc08498e

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Metal-Free Nitrative Cyclization of N-Aryl Imines with tert-Butyl Nitrite:
Dehydrogenative Access to 3-Nitroindoles
Guo-Bo Deng,a Jia-Ling Zhang,a Yan-Yun Liu,a Bang Liu,a Xu-Heng Yang,a and Jin-Heng Li*a
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
We here describe a new metal-free route to the synthesis of 3-
nitroindoles by the nitrative cyclization of N-aryl imines with tert-
butyl nitrite. The radical transformation allows the assembly of
10 the indole framework through oxidative cleavage of multi C-H
bonds, nitration, cyclization and isomerization cascade.
Indoles are an important motif that found in a wide range of
natural products, pharmaceuticals and organic dyes.1 Owing to
their remarkable biological and medicinal activities, much
15 attention has been attracted on the discovery of efficient
methods for the indole framework construction.2-6 Among the
elegant methods for indole synthesis, recent cyclization
approaches involving the C-H oxidative functionalization
process are particularly fascinating due to their efficiency,
20 highly atom-economy and sustainablity.3-6 For example, many
groups have developed efficient Rh-, Ru- or Pd-catalyzed
Scheme 1 Dehydrogenative Cyclization Routes to Indoles.
3-phenylquinoxaline 1-oxide (5a) (entry 1). Further screening
annulation of aryl C(sp2)-H bonds with 2π components to 55 revealed that the amount of tBuONO had a fundamental
assemble indoles.3 However, the majority of these approaches
are restricted to the requirement of noble transition metal
25 catalysts and limited substrate scope. In 2008, Glorius and co-
workers reported a new route to indoles by the PdII-catalyzed
influence on the reaction (entries 2 and 3). The presence of
1.5 equiv tBuONO gave products 3a and 4a in 38% and 40%
yield, respectively (entry 2). However, both products 3a and
5a were obtained in low yields from the reaction of substrate
oxidative cyclization of N-aryl enamines through 60 1a with 3 equiv tBuONO (entry 3). the reason is because
intramolecular dual C-H activation (Scheme 1a).4a-c Recently,
Yoshikai and co-workers have described a new strategy for
30 the synthesis of indoles by Pd-catalyzed oxidative cyclization
of N-aryl imines (Scheme 1a).4d Although these PdII-catalyzed
substrate 1a is readily decomposed in the presence of excess
tBuONO. Among the reaction temperature (entries 4 ad 5) and
solvents (entries 6 ad 7)examined, the reaction at 70 C in 1,4-
o
dioxane gave the best results (entry 1 vs. entries 4-7).
oxidative cyclization approaches possess operationally simple, 65 Subsequently, three other NO2 resources, including iBuONO
high atom-economy and broad substrate scope, they are
limited to the requirement of expensive Pd catalysts and
35 additives. Thus, developing a new metal-free oxidative C-H
functionalization alternative to the synthesis of indoles would
(2b), nAmONO (2c) and AgNO2 (2d), were tested (entries 8-
10). The use of iBuONO (2b) or nAmONO (2c) shifted the
chemoselectivity toward 4a as the major product with a trace
amount of 3a and 5a (entries 8 and 9). However, AgNO2 (2d)
be desirable and essential.5 Herein, we report a novel metal- 70 showed lower reactivity and lower selectivity (entry 10). In
free nitrative cyclization of N-aryl imines with tert-butyl
nitrite7 for the assembly of 3-nitroindoles6 (Scheme 1b); this
40 transformation achieves oxidative cleavage of multi C-H
bonds, nitration, cyclization and isomerization sequence, and
the previous report of Jiao group, nBu4NBr was employed to
improve the nitrogen incorporation into substrate 1a.
Interestingly, nBu4NBr could improve the reaction with
substrate 1a, but it shifted the chemoselectivity toward
represents a new shortcut for building 3-substituted indole 75 product 5a in 60% yield, not the desired indole 3a (entry 11).
skelectons with high functional group compatibility and
excellent selectivity control.
The reaction condition optimization for the nitrative
cyclization reaction was carried out by using 2-methyl-N-(1-
It should be noted that in argon the chemoselectivity of the
reaction is not desirable, and a mixture of three products 3a,
4a and 5a was observed in low yields (entry 12). The results
suggest that air plays an important role in the reaction.
45
phenylethylidene)aniline (1a) as the model substrate (Table 1). 80 As shown in Table 2, the substrate scope of this nitrative
In the presence of 2 equiv tBuONO, substrate 1a was
converted into the desired 7-methyl-3-nitro-2-phenyl-1H-
50 indole (3a) in 60% yield together with some by-products, (E)-
2-methyl-N-(2-nitro-1-phenylvinyl)aniline (4a) and 5-methyl-
cyclization reaction was investigated with a variety of N-aryl
imines 1 under the optimal reaction conditions. Initially, our
study focused on the substitution effect of the N-aryl moiety
of substrate 1 in the presence of tBuONO (2a) (Products 3b-j).
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DOI: 10.1039/C4CC08498E
Table 1 Screening of the Optimal Conditionsa
Table 2 Nitrative Cyclization of N-aryl imines (1) with tBuONO (2a)a
NO2
+
tBuONO
2a
dioxane, 70 o
air, 24 h
C
R2
N
R2
R1
N
R1
1
H
3
Entry
RONO
(equiv)
Solvent
T
Isolated yield (%)
(oC)
70
70
70
60
80
70
70
70
70
70
70
70
3a
60
38
15
55
50
4a
<5
40
5a
<5
trace
20
1
2
3
tBuONO 2a (2)
tBuONO 2a (1.5) 1,4-dioxane
1,4-dioxane
tBuONO 2a (3)
tBuONO 2a (2)
tBuONO 2a (2)
tBuONO 2a (2)
tBuONO 2a (2)
iBuONO 2b (2) 1,4-dioxane
nAmONO 2c (2) 1,4-dioxane
AgNO2 2d (2)
tBuONO 2a (2)
tBuONO 2a (2)
1,4-dioxane
1,4-dioxane
1,4-dioxane
c-hexane
trace
trace trace
trace trace
4
5c
6
trace trace trace
7
8
9
10
11b
12c
toluene
20
trace
<5
17
55
56
16
0
trace
<5
trace
61
25
1,4-dioxane
1,4-dioxane
1,4-dioxane
15
trace trace
10
8
a
Reaction conditions: 1a (0.2 mmol), RONO 2, and solvent (4 mL,
5
0.05% w/w of water in dioxane) in air for 24 h. Some other by-products
from decomposition of substrate 1a, particularly cleavage of the C=N
bond, were observed. b nBu4NOBr (5 mol%) was added. c In argon.
a Reaction conditions: 1 (0.2 mmol), 2a (2 equiv), and 1,4-dioxane (4 mL)
45 in air for 24 h.
The results indicated that a range of substituents, including
Me, MeO, MeS, Br and COMe, were well-tolerated. For
10 example, m-Me-substituted substrate 1b regioselectively
formed 6-methyl-3-nitro-2-phenyl-1H-indole 3b in 62% yield.
Substrates 1c-e with an electron-donating group, Me, MeO or
MeS, also showed high reactivity, giving 3c-e in moderate
yields. Gratifyingly, substrate 1g with an electron-
15 withdrawing COMe group was successfully converted into 3g
in 63% yield. Substrates 1h and 1i with two substituents at the
2 and 4 positions were also viable for constructing 3h and 3i
in good yields. Subsequently, the substitution effect of the
ethylimine moiety was evaluated (Indoles 3k-r). A variety of
20 aryl groups, either electron-rich groups (iPrC6H4 and
MeOC6H4) or electron-deficient aryl groups (ClC6H4, CNC6H4,
CF3C6H4 and NO2C6H4), at the 1 position of the ethylimine
moiety perfectly worked with tBuONO leading to 3k-r in
moderate yields, but an aliphatic group (Me) has no reactivity
25 for the reaction. For instance, substrate 1k with a iPrC6H4
group delivered 3k in 67% yield. Using substrates 1l and 1r
having a MeOC6H4 group or two MeOC6H4 groups to react
with tBuONO afforded the desired indoles 3l and 3r in high
yields. It is noteworthy that a halo group, such as Br and Cl,
30 on the aromatic ring is amendable to the optimal conditions,
thereby providing an opportunity for additional modifications
at the halogenated position (3f and 3m). Although substrates
1p and 1q with a m-NO2C6H4 group or a o-NO2C6H4 group
have lower reactivity, moderate yields were still achieved (3p
35 and 3q). However, N-(1-phenylpropylidene)aniline (1s) was
not asuitable susbtrate.
1,2-dihydrocinnoline 6t, a six-membered-ring, in 50% yield.
Notably, by-product 4a could be converted into product 3a in
40% yield along with another over-nitration product 7a in
25% yield (Eq 2). The results imply that by-product 4a is an
50 intermediate among the nitrative cyclization process.
Additionally, two radical inhibitors, TEMPO and BHT, were
added to the reaction of substrate 1a with tBuONO (2a),
which resulted in no detectable 3a (Eq 3). The results suggest
that the current reaction may include a radical process.
OMe
OMe
NO2
O2N
tBuONO 2a (2 equv)
OMe
OMe
(1)
(2)
(3)
dioxane, 70 o
C
N
NH
N
N
air, 24 h
1t
NO2
6t, 50%
NO2
NO2
O2N
BuONO 2a (2 equv)
t
+
Ph
Ph
dioxane, 70 o
C
N
H
4a
Ph
N
N
H
air, 24 h
H
3a, 40%
, 25%
7a
NO2
additive (2.2 equiv)
+
tBuONO
2a
dioxane, 70 o
air, 24 h
C
Ph
N
Ph
N
1a
H
additive = TEMPO, BHT
3a, trace
55
Scheme 2 Other Substrates and Control Experiments.
The possible mechanism outlined in Scheme
3
was
proposed for the nitrative cyclization reaction.7,8 Initially,
tBuONO is easily split into tBuO⋅ radical and ⋅NO radical
60 under heating.7,8 Reaction of ⋅NO radical with H2O occurs to
form HNO2, which is rapidly decomposed into NO2, NO and
H2O. The process is supported by the 18O-labeled experiment
using H218O (Figure S1 in the Supplementary Information). In
the presence of NO2, NO and air, substrate 1a is converted
65 into intermediate A, followed by hydrogen-abstraction of
Gratifyingly, this nitrative cyclization reaction could be
applicable
to
2-(1-(3,4-dimethoxyphenyl)ethylidene)-1-
methyl-1-phenylhydrazine (1t) (Eq 1 in Scheme 2). In the
40 presence of tBuONO (2a), substrate 1t assembled 4,6-dinitro-
2
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DOI: 10.1039/C4CC08498E
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Cyclization of intermediate B takes place to produce radical
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70
75
5
Scheme 3 Possible Mechanism.
80
In summary, we have developed the first nitrative
cyclization of N-aryl imines with tert-butyl nitrite under
metal-free conditions for the synthesis of 3-nitroindoles. This
10 method is realized through oxidative dehydrogenation,
nitration, cyclization and isomerization sequence, and
provides a operationally simple and atom-economical access
to indoles with high functional group compatibility and
excellent selectivity control.
This research was supported by the Hunan Provincial
Natural Science Foundation of China (No. 13JJ2018), Natural
Science Foundation of China (No. 21172060), and Specialized
Research Fund for the Doctoral Program of Higher Education
(No. 20120161110041). G.-B. Deng also thanks the
85
4
90
15
95
5
6
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105
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a State Key Laboratory of Chemo/Biosensing and Chemometrics, College
of Chemistry and Chemical Engineering, Hunan University, Changsha
410082, China. Fax: 0086731 8871 3642; Tel: 0086731 8882 2286; E-
25 mail: jhli@hnu.edu.cn
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
30 to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
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