Nitromethane as a reagent for the synthesis of 3-nitroindoles from 2-haloarylamine derivatives

Article abstract of DOI:10.1007/s11172-020-3028-8

A new approach to the synthesis of 3-nitroindoles using palladium-catalyzed arylation of nitromethane with N-(2-bromoaryl)imidates was developed. A convenient and rapid method for cyclization of ethyl N-(2-nitromethylaryl)acetimidates to 2-methyl-3-nitro-1H-indoles was proposed.

Full text of DOI:10.1007/s11172-020-3028-8

Russian Chemical Bulletin, International Edition, Vol. 69, No. 12, pp. 2370—2377, December, 2020
2370
Nitromethane as a reagent for the synthesis of 3-nitroindoles
from 2-haloarylamine derivatives
G. A. Chesnokov,a A. A. Ageshina,a A. V. Maryanova,a S. A. Rzhevskiy,a,b P. S. Gribanov,c
M. A. Topchiy,a,b M. S. Nechaev,a,b and A. F. Asachenkoa,b
аA. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,
29 Leninsky prosp., 119991 Moscow, Russian Federation
bLomonosov Moscow State University, Department of Chemistry,
Build. 3, 1 Leninskie Gory, 119991 Moscow, Russian Federation
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 647 5927, ext. 315. E-mail: aasachenko@ips.ac.ru
A new approach to the synthesis of 3-nitroindoles using palladium-catalyzed arylation of
nitromethane with N-(2-bromoaryl)imidates was developed. A convenient and rapid method
for cyclization of ethyl N-(2-nitromethylaryl)acetimidates to 2-methyl-3-nitro-1H-indoles was
proposed.
Key words: indoles, nitromethane, cross-coupling, palladium.
Indoles, being one of the most popular structural motifs
tization asymmetric reactions,22—26 cycloaddition reac-
tions,27—31 etc. Despite the fact that in recent years
a significant contribution has been made to the study of
transition metal-catalyzed reactions of regioselective C—H
nitration of heteroaromatic compounds32—36 under mild
conditions,37—40 the direct introduction of the NO2 group
into the indole core is still not fully resolved problem.41—43
3-Nitroindoles are commonly prepared by electrophilic
nitration with strong acids (Scheme 1).44—46 However,
this method has a number of significant disadvantages:
poor regioselectivity, low substrate stability, and sensitiv-
ity of various functional groups to the reaction condi-
tions.46—50 There are also several works on the preparation
of 3-nitroindoles by the Nenitzescu reaction (Scheme 2)51,52
and by the photocatalytic method (Scheme 3).53 Currently,
there is no general method for the synthesis of 1,2-disub-
stituted 3-nitroindoles. For this reason, the development
of alternative methods for the preparation of 1,2-disub-
stituted 3-nitroindoles is an important problem of modern
indole chemistry.
in the chemistry of natural compounds, occupy a special
place among the wide variety of heterocyclic systems. Over
the more than a century and a half history of the develop-
ment of indole chemistry, beginning with the works of
Adolf von Baeyer,1 Emil Fischer,2 and up to the present
time,3—8 a significant amount of preparative methods for
the preparation of the indole framework have been devel-
oped. The main reason for the continuing interest in in-
doles is the wide spectrum of their biological activity.9—11
Thus, the indole core is present in the structure of trypto-
phan and its metabolites, the plant growth hormone in-
dol-3-ylacetic acid, as well as of the key mammalian
neurotransmitter serotonin. The indole ring is also found
in many natural compounds: indole alkaloids, metabolites
of fungi and marine organisms.12 In addition, the more
than 50 years long search for specific receptor agonists for
certain indole-based metabolites has been one of the main
problems of pharmaceutical chemistry. The indole core is
present in various drugs, for example, indomethacin and
sumatriptan.13,14 Therefore, it is obvious that the develop-
ment and study of methods for the construction and
functionalization of the indole ring are the most important
issues of chemistry.15—18
Scheme 1
However, some indole derivatives are still poorly avail-
able. One such example is the most important family of
organic compounds, 3-nitroindoles,19—21 which are found
in a number of products of natural origin. One of the areas
of application of 3-nitroindoles is their use as universal
substrates in various reactions, such as catalytic dearoma-
Reagents: NO2+ or HNO2.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2370—2377, December, 2020.
1066-5285/20/6912-2370 © 2020 Springer Science+Business Media LLC

3-Nitroindoles synthesis
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
2371
Scheme 2
Scheme 5
Scheme 3
dba =
, XPhos =
The present work is devoted to a detailed study of the
previously unexplored cross-coupling reaction of ortho-
bromoanilide derivatives with nitromethane to form an
indole core resulting from a one-pot two-step reaction,
namely, arylation of nitromethane and cyclization to in-
dole, and the possibilities of further synthetic improvement
of this method (Scheme 4).
Cy is cyclohexyl.
Reagents and conditions: 2.5 mol.% of Pd2(dba)3•CHCl3,
6 mol.% of XPhos, K3PO4 (1.2 equiv.), MeNO2 (10 equiv.);
1,4-dioxane, 100 C.
this case, no formation of the product of subsequent
cyclization, 3-nitroindole 3a, was observed. It was sug-
gested that the cross-coupling proceeds inefficiently due
to the stabilization of the palladium intermediate by the
NH—CO fragment after the insertion of palladium into
the C—Br bond in substrate 1.
Scheme 4
In order to decrease the coordinating ability of the
palladium intermediate, we decided to methylate 1 at the
nitrogen atom and to use compound 4a for further catalytic
studies on tandem nitromethanation-cyclization of 2-bromo-
anilides to 3-nitroindoles54 (Scheme 6).
Scheme 6
R = Alk, Ar, Het; R´ = H, Alk, Ar, Het
Results and Discussion
To select the reaction conditions for the cross-coupling
of nitromethane with 2-bromoanilides followed by the
cyclization and formation of 2,3-disubstituted indoles, the
corresponding 2-bromoanilide 1 was obtained in two steps
through sequential bromination and acylation of para-
toluidine (Scheme 5).54
The primary screening of catalytic systems began with
the reproduction of the palladium-catalyzed nitrometha-
nation of aryl bromides for substrate 1 (see Scheme 5).55
For the chosen substrate, the nitromethanation pro-
ceeded in good yield at low conversion (99 and 20%, re-
spectively) and gave the nitromethanation product 2. In
Reagents and conditions: 2.5 mol.% of Pd2(dba)3•CHCl3,
6 mol.% of XPhos, K3PO4 (1.2 equiv.), MeNO2 (10 equiv.);
1,4-dioxane, 100 C.

Chesnokov et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
Table 1. Optimization of the conditions of obtaining 3-nitroindole 5a by a tandem cross-coupling/cyclization reaction
Entry
Conditions
Yield (%)
5a
6
1
2
3
Pd2(dba)3·CHCl3 (2.5 mol.%), XPhos (6 mol.%), MeNO2 (10 equiv.)
(6-Dipp)PdCl2-XPhos (2.5 mol.%), MeNO2 (10 equiv.)
Pd(OAc)2 (5 mol.%), XPhos (6 mol.%), MeNO2 (10 equiv.),
40
—
13
20
—
—
4
5
Pd2(dba)3·CHCl3 (2.5 mol.%), XPhos (6 mol.%), MeNO2 (10 equiv.), H2O (2.5 mol.%)
Pd2(dba)3·CHCl3 (2.5 mol.%), XPhos (6 mol.%), MeNO2 (20 equiv.), solvent-free
15
—
—
19
Scheme 7
As a result, we obtained 1-methyl-3-nitro-2-phenyl-
indole (5a) in 40% yield as the main product of one-pot
cross-coupling of nitromethane with compound 4a and
subsequent cyclization. The unexpected side formation of
N-methylphenanthridone (6) in 20% yield we explain by
the intramolecular palladium-catalyzed C—H activation
(Table 1, entry 1).
Thus, in the case of nitromethanation of 4a, we en-
countered a serious problem, namely, a competitive in-
tramolecular cyclization of the palladium intermediate to
phenanthridone (6). The use of a palladium complex
containing N-heterocyclic carbene and phosphine li-
gands,56 as well as the attempts to alter the reaction time,
the nature of the catalyst, or the polarity of the medium
did not lead to positive results (see Table 1, entries 3 and 4).
An attempt to carry out the cross-coupling under the
solvent-free conditions was also unsuccessful (see Table 1,
entry 5).18,57—58
To increase the yield of the desired product in the
tandem reaction of palladium-catalyzed nitromethanation
of ortho-bromoanilide 4a with subsequent base-catalyzed
cyclization of the cross-coupling product to 3-nitroindole
5a, we carried out experiments with different bases. It was
assumed that a change in the base nature can have
a positive effect on the step of nitromethanation of ortho-
bromoanilide and increase the yield of 3-nitroindole 5a.
However, all attempts to increase the yield of 3-nitroindole
5a by varying the base were unsuccessful (ButONa gave
5%; ButOK 10%; K2CO3 13%; NaOH, Cs2CO3, KOH,
Na2CO3 0%).
We assumed that if we take a benzanilide with ortho
substituents, this would eliminate the formation of phen-
anthridone and therefore would increase the yield of
3-nitroindole. For this purpose, the corresponding benz-
anilide 4b was obtained from 2,4,6-trimethylbenzoic acid
(Scheme 7).54
However, after the nitromethanation under standard
conditions, we isolated the unreacted starting anilide in-
stead of the expected 3-nitroindole 5b. In order to increase
the yield of the desired reaction product (indole), we
decided to replace the phenyl group in the benzoyl frag-
ment of the starting amide with a tert-butyl one (see
Scheme 7). We assumed that in this case the yield of the
4, 5: R = Mes (b), But (c)
Reagents and conditions: 2.5 mol.% of Pd2(dba)3•CHCl3,
6 mol.% of XPhos, K3PO4 (1.2 equiv.), MeNO2 (10 equiv.);
1,4-dioxane, 100 C.
main product should increase, since the intramolecular
C—H activation leading to phenanthridone cannot occur.
To test this hypothesis, we synthesized pivalic acid anilide
4c.14,54 However, neither under standard conditions, nor
with a change in the catalyst amount, nor under solvent-
free conditions at 80 C, nor with a change in the base
nature (ButONa, KOH, Cs2CO3), nor with the addition
of molecular sieves (3 Å), we observed no the formation
of reaction products.
This result allows us to suggest that the excessive steric
loading in the amide group of bromoanilide has a critical
effect on the nitromethanation reaction.
To solve this problem, we synthesized brominated
anilines 7a—g (Scheme 8), reflux of which in excess of
triethyl orthoacetate allowed us to obtain imidates 8a—g
in high yields (78—91%).
Nitromethanation of imidates 8a—g was carried out
under standard conditions using a Pd2(dba)3·CHCl3/
XPhos catalyst in 1,4-dioxane at 100 С. The nitrometha-
nation products 9a—g were isolated in 41—92% yields.
The conditions for cyclization were optimized using imid-
ate 9b (Table 2).
It was found that the full decomposition of the starting
imidate 9b occured in the presence of inorganic bases in
toluene or 1,4-dioxane, while DMSO was found to be
a more suitable solvent (see Table 2, entries 4—6). The best
result was obtained in the course of thermal cyclization of
imidate 9b applied to silica gel (see Table 2, entries 7—10).

3-Nitroindoles synthesis
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
2373
Scheme 8
Reagents and conditions: i. MeCH(OEt)3, CF3COOH (cat.); ii. Pd2(dba)3•CHCl3 (2.5 mol.%), XPhos (6 mol.%), K3PO4 (1.2 equiv.),
MeNO2 (10 equiv.), 1,4-dioxane (5 mL), 100 C; iii. silica gel (600 mg mmol–1), 150 C, 2 min.
Table 3. Nitromethanation of N-(2-bromoaryl)imidates
8a—g with subsequent cyclization to 3-nitroindoles 3a—g
Heating at 150 C for two minutes allowed us to obtain
the target N-unsubstituted 3-nitroindole 3b in 61% yield
(see Table 2, entry 9). To determine the scope of applica-
tion of these most efficient conditions, we involved a series
of different imidates into the nitromethanation reaction,
followed by cyclization of the resulting products to the
corresponding 3-nitroindoles (Table 3).
The cyclization of imidates 9a,c,e with aliphatic sub-
stituents on silica gel for 2 min at 150 C gave indoles 3a
and 3с,e in good yields (see Table 3). Heating the halogen-
containing substrates 9d,f,e also led to the formation of
the corresponding indoles in good yields (see Table 3).
Com-
pound
R1
R2
Yield (%)
8, 9
3
9
3
a
b
c
d
e
f
4-Me
H
5-Me
H
Ph
Me
Me
Me
Me
Me
Me
41
84
92
81
80
60
58
73
61
61
67
71
69
62
4-But
4-CF3
5-But
5-CF3
4,6-Me2 5,7-Me2
4-F
4-Cl
5-F
5-Cl
g
In conclusion, in the present work, we showed the
possibility of a two-step synthesis of 3-nitroindoles from
2-bromoanilides. We studied various conditions of cross-
coupling of bromoanilides with nitromethane, the effect
of the base, solvents, reaction time, temperature, and
substituents in 2-bromoanilides on the possibility to occur
and efficiency of the nitromethanation reaction followed
by cyclization to 3-nitroindole. We also developed efficient
conditions for the preparation of 2-substituted-3-nitro-
1H-indoles through the successive palladium-catalyzed
nitromethanation reactions (C—H activation of nitro-
methane) and cyclization of the cross-coupling product.
Table 2. Optimization of the conditions of cyclization of ethyl
N-2-(nitromethyl)phenylacetimidate (9b) to 3-nitro-2-methyl-
1H-indole (3b)
Entry
Conditions
Solvent
Yield
(%)
Inorganic base
T/С

1
2
3
4
5
6
ButOK
(1 equiv.)
Cs2CO3
(1 equiv.)
NaH
(1 equiv.)
ButOK
(2 equiv.)
KOH
PhCH3
PhCH3
110
1 h
0
0
110
25
1 h
12 h
2 h
1,4-Dioxane
DMSO
0
90
16
52
21
51
Experimental
DMSO
110
90
1 h
(2.2 equiv.)
NaH
1H and 13C spectra were recorded on Bruker Avance 600
DMSO
2 h
1
(600 MHz H, 151 MHz 13C) and Bruker Avance-400 spectro-
(2 equiv.)
SiO2*
meters (400 MHz 1H, 101 MHz 13C). Chemical shifts were
measured relative to the signals of solvents (1H: CDCl3 δ 7.27,
DMSO-d6 δ 2.50; 13C: CDCl3 δ 77.0, DMSO-d6 δ 39.5) and
recalculated to the δ-scale using standard formulas. IR spectra
were recorded on a Shimadzu IR Prestige 21 spectrometer.
High resolution mass spectra were recorded on an ABSciex
TripleTOF 5600+ instrument with TurboV and photospray ion
source instrument.
7
8
9
—
—
120 5 min
120 10 min 55
SiO2*
SiO2*
—
150
—
2 min
90 20 min 47
61
10
SiO2*
* Thermal cyclization of imidate 8b applied to silica gel
(600 mg mmol–1).

Chesnokov et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
The starting anilines 7a—g were synthesized according to the
procedures described earlier.59 The completion of the reaction
and the purity of chemical substances were monitored by TLC
(aluminum plates precoated with silica gel 60, F254), chromato-
grams were visualized under UV light (254 nm). Preparative
column chromatography was performed using silica gel 60
(230—400 mesh, Merck). All solvents we used were purified ac-
cording to standard procedures.
Ethyl N-(2-bromo-4,6-dimethylphenyl)acetimidate (8e). The
yield was 85%, a colorless oil, b.p. 90—92 C (0.8 Torr). 1H NMR
(600 МHz, CDCl3), δ: 7.22 (s, 1 H, Harom); 6.91 (s, 1 H, Harom);
4.42—4.26 (m, 2 H, CH2); 2.25 (s, 3 H, Me); 2.07 (s, 3 H, Me);
1.71 (s, 3 H, Me); 1.38 (t, 3 H, Me, J = 7.1 Hz). 13C NMR
(151 МHz, CDCl3), δ: 162.5, 143.6, 133.4, 130.7, 130.3, 130.0,
115.5, 62.1, 20.5, 18.9, 17.1, 14.5. MS: found m/z 270.0491,
272.0470 [M + H]+; calculated for C12H17BrNO+ 270.0493,
272.0473. IR, ν/cm–1: 816, 850, 1055, 1233, 1263, 1290, 1373,
1475, 1674, 2924, 2979.
Ethyl N-(2-bromo-4-fluorophenyl)acetimidate (8f). The yield
was 89%, a colorless oil, b.p. 68—70 C (0.7 Torr). 1H NMR
(600 МHz, CDCl3), δ: 7.30 (dd, 1 H, Harom, J = 8.1 Hz, J = 2.8 Hz);
6.96 (ddd, 1 H, Harom, J = 8.7 Hz, J = 7.9 Hz, J = 2.8 Hz); 6.73
(dd, 1 H, Harom, J = 8.7 Hz, J = 5.5 Hz); 4.28 (q, 2 H, CH2,
J = 7.1 Hz); 1.77 (s, 3 H, Me); 1.36 (t, 3 H, Me, J = 7.1 Hz).
13C NMR (151 МHz, CDCl3), δ: 163.3, 158.5 (d, J = 244.8 Hz);
144.2, 122.6 (d, J = 7.7 Hz); 119.8 (d, J = 25.0 Hz); 115.9
(d, J = 9.2 Hz); 115.1 (d, J = 21.9 Hz); 62.3, 16.9, 14.4. MS:
found m/z 260.0081, 262.0064 [M + H]+; calculated for
C10H12BrFNO+ 260.0086, 262.0066. IR, ν/cm–1: 764, 823, 856,
1055, 1192, 1257, 1294, 1374, 1483, 1670, 2980.
Methyl N-(2-bromo-4-methylphenyl)benzimidate (8a). The
product was synthesized according to the described procedure60
from N-(2-bromo-4-methylphenyl)benzamide, thionyl chloride,
methanol, and sodium hydride in 81% yield and purified by
chromatography (hexane—dichloromethane). 1H NMR (400 МHz,
CDCl3), δ: 7.31 (d, 4 H, Harom, J = 8.1 Hz); 7.23 (d, 2 H, Harom
,
J = 7.8 Hz); 6.85 (dd, 1 H, Harom, J = 8.1 Hz, J = 1.3 Hz); 6.48
(d, 1 H, Harom, J = 8.0 Hz); 4.02 (s, 3 H, Me); 2.23 (s, 3 H, Me).
13C NMR (101 МHz, CDCl3), δ: 160.1, 144.5, 133.7, 133.1,
131.5, 130.2, 128.8, 128.1, 122.3, 116.1, 110.1, 54.5, 20.5. MS:
found m/z 304.0339, 306.0317 [M + H]+; calculated for
C15H15BrNO+ 304.0337, 306.0317. IR, ν/cm–1: 697, 821, 1044,
1118, 1178, 1285, 1298, 1434, 1447, 1487, 1602, 1667, 2922, 2943.
Synthesis of compounds 8b—g (general procedure). The start-
ing aniline (50 mmol), triethyl orthoacetate (200 mmol), and
a drop of trifluoroacetic acid were mixed in a 50-mL round-
bottom flask. The reaction mixture was refluxed for 4 h and cooled
to room temperature. Ethanol formed during the reaction was
distilled off at atmospheric pressure, the residue was concen-
trated in vacuo to obtain compounds 8b—g as colorless oils.
Ethyl N-(2-bromophenyl)acetimidate (8b). The yield was 91%,
Ethyl N-(2-bromo-4-chlorophenyl)acetimidate (8g). The yield
was 85%, a colorless oil, b.p. 86 C (0.8 Torr). 1H NMR
(600 МHz, CDCl3), δ: 7.55 (d, 1 H, Harom, J = 2.3 Hz); 7.20
(dd, 1 H, Harom, J = 8.4 Hz, J = 2.3 Hz); 6.71 (d, 1 H, Harom
,
J = 8.4 Hz); 4.28 (q, 2 H, CH2, J = 7.1 Hz); 1.78 (s, 3 H, Me);
1.36 (t, 3 H,Me, J = 7.1 Hz). 13C NMR (151 МHz, CDCl3), δ:
163.0, 146.6, 132.4, 128.5, 128.3, 123.0, 116.5, 62.4, 17.0, 14.3.
MS: found m/z 275.9783, 277.9762 [M + H]+; calculated for
C10H12BrClNO+ 275.9791, 277.9770. IR, ν/cm–1: 692, 744, 828,
869, 953, 1053, 1095, 1234, 1258, 1294, 1374, 1468, 1670, 2980.
Synthesis of compounds 9a—g (general procedure). Alkyl
N-(2-bromoaryl)acetimidate 8a—g (1 mmol, 1 equiv.) was mixed
with Pd2(dba)3•CHCl3 (25.9 mg, 2.5 mol.%), XPhos (28.6 mg,
6 mol.%), nitromethane (610 mg, 10 equiv.), anhydrous 1,4-di-
oxane (5 mL), and anhydrous potassium phosphate (254 mg,
1.2 equiv.) in an 8-mL vial with a screw cap and a magnetic stir-
rer. The reaction mixture was stirred (900 rpm) at 70 C for
2 days, then cooled to room temperature, filtered through a short
layer of silica gel, and washed with CH2Cl2 (100 mL).
Concentration of the filtrate in vacuo and column chromatography
(eluent CH2Cl2) gave products 9a—g as yellow-orange or color-
less oils.
1
a colorless oil, b.p. 78—79 C (0.8 Torr). H NMR (600 МHz,
CDCl3), δ: 7.54 (dd, 1 H, Harom, J = 8.0 Hz, J = 1.4 Hz); 7.22
(td, 1 H, Harom, J = 7.6 Hz, J = 1.4 Hz); 6.89 (ddd, 1 H, Harom
J = 8.0 Hz, J = 7.3 Hz, J = 1.6 Hz); 6.78 (dd, 1 H, Harom
,
,
J = 7.8 Hz, J = 1.6 Hz); 4.33 (q, 2 H, Harom, J = 7.1 Hz);
1.78 (s, 3 H, Me); 1.37 (t, 3 H, Me, J = 7.1 Hz). 13C NMR
(151 МHz, CDCl3), δ: 162.6, 147.7, 132.9, 128.2, 124.2, 122.4,
116.2, 62.2, 17.0, 14.4. MS: found m/z 242.0172, 244.0152
[M + H]+; calculated for C10H13BrNO+ 242.0181, 244.0160.
IR, ν/cm–1: 749, 1055, 1237, 1260, 1296, 1374, 1469, 1586,
1674, 2981.
Ethyl N-(2-bromo-4-tert-butylphenyl)acetimidate (8с). The
yield was 87%, a colorless oil, b.p. 110—111 C (0.8 Torr).
1H NMR (600 МHz, CDCl3), δ: 7.53 (d, 1 H, Harom, J = 2.2 Hz);
7.22 (dd, 1 H, Harom, J = 8.2 Hz, J = 2.1 Hz); 6.70 (d, 1 H,
Harom, J = 8.3 Hz); 4.29 (q, 2 H, CH2, J = 7.1 Hz); 1.79 (s, 3 H,
Me); 1.37 (t, 3 H, Me, J = 7.1 Hz); 1.29 (s, 9 H, But). 13C NMR
(151 МHz, CDCl3), δ: 145.0, 129.8, 125.2, 121.8, 115.9, 34.4,
31.5, 17.0, 14.4. MS: found m/z 298.0804, 300.0784 [M + H]+;
Methyl N-[4-methyl-2-(nitromethyl)phenyl]benzimidate (9a).
The yield was 41%, a yellow-orange oil. 1H NMR (400 МHz,
CDCl3), δ: 7.39—7.31 (m, 3 H, Harom); 7.31—7.27 (m, 2 H,
Harom); 7.21—7.17 (m, 1 H, Harom); 6.95 (dd, 1 H, Harom
,
calculated for C14H21BrNO+ 298.0807, 300.0786. IR, ν/cm–1
833, 878, 1055, 1257, 1293, 1373, 1472, 1674, 2964.
:
J = 8.1 Hz, J = 1.4 Hz); 6.34 (d, 1 H, Harom, J = 8.1 Hz); 5.60
(s, 2 H, CH2NO2); 3.98 (s, 3 H, Me); 2.32 (s, 3 H, Me). 13C NMR
(101 МHz, CDCl3), δ: 145.3, 133.0, 132.1, 131.4, 130.8, 130.4,
129.1, 128.2, 122.2, 121.9, 77.0, 54.2, 20.8. MS: found m/z
285.1238 [M + H]+; calculated for C16H17N2O3+ 285.1239. IR,
ν/cm–1: 826, 873, 975, 1028, 1109, 1180, 1280, 1297, 1369, 1434,
1498, 1549, 1650, 1657.
Ethyl N-2-[bromo-4-(trifluoromethyl)phenyl]acetimidate (8d).
The yield was 78%, a colorless oil, b.p. 96—97 C (0.8 Torr).
1H NMR (600 МHz, CDCl3), δ: 7.81 (s, 1 H, Harom); 7.48
(d, 1 H, Harom, J = 7.4 Hz); 6.86 (d, 1 H, Harom, J = 8.2 Hz);
4.30 (q, 2 H, CH2, J = 7.1 Hz); 1.79 (s, 3 H, Me); 1.38 (t, 3 H,
Me, J = 7.1 Hz). 13C NMR (151 МHz, CDCl3), δ: 162.6, 151.2,
130.2 (d, J = 3.8 Hz); 125.3 (q, J = 3.6 Hz); 122.4, 116.2, 62.6,
17.2, 14.3. MS: found m/z 310.0049, 312.0030 [M + H]+; cal-
Ethyl N-[2-(nitromethyl)phenyl]acetimidate (9b). The yield
was 84%, a yellow-orange oil. 1H NMR (600 МHz, CDCl3), δ:
7.41—7.31 (m, 2 H, Harom); 7.10 (t, 1 H, Harom, J = 7.5 Hz);
6.76 (d, 1 H, Harom, J = 7.9 Hz); 5.42 (s, 2 H, CH2NO2); 4.23
(q, 2 H, CH2, J = 7.1 Hz); 1.84 (s, 3 H, Me); 1.34 (t, 3 H, Me,
J = 7.1 Hz). 13C NMR (151 МHz, CDCl3), δ: 163.0, 131.6,
culated for C11H12BrF3NO+ 310.0054, 312.0034. IR, ν/cm–1
:
690, 817, 840, 890, 956, 1051, 1077, 1127, 1168, 1258, 1323,
1376, 1397, 1605, 1674, 2984.

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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
2375
131.1, 123.5, 122.1, 121.6, 76.6, 62.0, 16.6, 14.4. IR, ν/cm–1
697, 747, 790, 880, 1055, 1248, 1290, 1373, 1455, 1489, 1556,
1602, 1666, 2982. MS: found m/z 223.1081 [M + H]+; calcu-
lated for C11H15N2O3+ 223.1083.
:
bath (150 C). The mixture was stirred for 2 min (900 rpm),
removed from the oil bath, and cooled to room temperature. The
silica gel with the applied substance was transferred onto a filter
with a short (1—2 cm) layer of pure silica gel and washed with
ethyl acetate. Concentration of the product-containing fractions
in vacuo gave 3-nitroindoles 3a—g.
Ethyl N-[4-tert-butyl-2-(nitromethyl)phenyl]acetimidate (9c).
The yield was 92%, a yellow-orange oil. 1H NMR (600 МHz,
CDCl3), δ: 7.39 (dd, 1 H, Harom, J = 8.2 Hz, J = 2.3 Hz); 7.33
(d, 1 H, Harom, J = 2.3 Hz); 6.69 (d, 1 H, Harom, J = 8.2 Hz);
5.42 (s, 2 H, CH2NO2); 4.22 (q, 2 H, CH2, J = 7.1 Hz); 1.85
(s, 3 H, Me); 1.32 (s, 9 H, But). 13C NMR (151 МHz, CDCl3), δ:
162.9, 146.5, 146.4, 128.5, 128.2, 121.7, 121.3, 77.5, 61.9, 34.5,
31.6, 16.7, 14.5. MS: found m/z 279.1703 [M + H]+; calculated
for C15H23N2O+ 279.1708. IR, ν/cm–1: 771, 840, 957, 1055,
1251, 1373, 1500, 1556, 1669, 2963.
5-Methyl-3-nitro-2-phenyl-1H-indole (3a). The yield was
73%. m.p. 222 C (from ethyl acetate). 1H NMR (600 МHz,
DMSO-d6), δ: 12.72 (s, 1 H, NH); 7.97 (d, 1 H, Harom, J = 1.6 Hz);
7.74 (dd, 2 H, Harom, J = 6.7 Hz, J = 2.9 Hz); 7.58—7.51 (m, 3 H,
Harom); 7.42 (d, 1 H, Harom, J = 8.3 Hz); 7.18 (dd, 1 H, Harom
,
J = 8.3 Hz, J = 1.7 Hz); 2.47 (s, 3 H, Me). 13C NMR (151 МHz,
DMSO-d6), δ: 141.6, 133.1, 132.0, 129.9, 129.9, 128.2, 125.9,
124.1, 121.8, 119.7, 112.5, 21.4. MS: found m/z 252.0891 [M]+;
calculated for C15H12N2O2 252.0898. IR, ν/cm–1: 879, 971,
+
Ethyl N-[2-nitromethyl-4-(trifluoromethyl)phenyl]acetimidate
(9d). The yield was 81%, a colorless oil. 1H NMR (600 МHz,
1025, 1127, 1182, 1191, 1209, 1299, 1347, 1366, 1445, 1485,
1544, 1595, 3212.
CDCl3), δ: 7.66—7.58 (m, 2 H, Harom); 6.86 (d, 1 H, Harom
,
J = 8.3 Hz); 5.44 (s, 2 H, CH2NO2); 4.23 (q, 2 H, CH2,
J = 7.1 Hz); 1.86 (s, 3 H, Me); 1.34 (t, 3 H, Me, J = 7.1 Hz).
13C NMR (151 МHz, CDCl3), δ: 163.5, 152.3, 128.8 (q, J = 3.8 Hz);
128.2 (q, J = 3.5 Hz); 125.7 (q, J = 33.1 Hz); 125.0, 123.2, 122.6,
122.0, 76.1, 62.4, 16.7, 14.3. MS: found m/z 291.0949 [M + H]+;
calculated for C12H14F3N2O3+ 291.0957. IR, ν/cm–1: 777, 788,
851, 886, 907, 947, 962, 1056, 1076, 1123, 1165, 1191, 1257,
1308, 1335, 1375, 1411, 1436, 1477, 1557, 1619, 1657, 2902,
2946, 2962, 2993.
2-Methyl-3-nitro-1H-indole (3b). The yield was 61%, m.p.
224 C (from ethyl acetate, with decomp.). 1H NMR (600 МHz,
DMSO-d6), δ: 12.58 (s, 1 H, NH); 8.04 (dd, 1 H, Harom, J = 7.2 Hz,
J = 1.2 Hz); 7.46 (d, 1 H, Harom, J = 6.9 Hz); 7.33—7.24 (m, 2 H,
Harom); 2.76 (m, 3 H, Me). 13C NMR (151 МHz, DMSO-d6),
δ: 143.5, 132.9, 132.2, 123.8, 123.3, 121.2, 119.4, 112.3, 14.6.
MS: found m/z 177.0660 [M + H]+; calculated for C9H9N2O2
+
177.0664. IR, ν/cm–1: 860, 1008, 1127, 1195, 1283, 1364, 1435,
1549, 1598, 3233.
Ethyl N-[2,4-dimethyl-6-(nitromethyl)phenyl]acetimidate
(9e). The yield was 80%, a yellow-orange oil. 1H NMR (600 МHz,
CDCl3), δ: 7.08 (s, 1 H, Harom); 7.02 (s, 1 H, Harom); 5.43 (d, 1 H,
CH2NO2, J = 13.3 Hz); 5.20 (d, 1 H, CH2NO2, J = 13.3 Hz);
4.27 (q, 2 H, CH2, J = 7.0 Hz); 2.30 (s, 3 H, Me); 2.04 (s, 3 H,
Me); 1.66 (s, 3 H, Me); 1.36 (t, 3 H, Me, J = 7.1 Hz). 13C NMR
(151 МHz, CDCl3), δ: 163.0, 145.1, 133.4, 132.5, 129.8, 128.5,
120.9, 77.1, 61.9, 20.7, 17.9, 16.9, 14.4. MS: found m/z 251.1392
5-tert-Butyl-2-methyl-3-nitro-1H-indole (3с). The yield was
61%, m.p. 255 C (from ethyl acetate, with decomp.). 1H NMR
(600 МHz, DMSO-d6), δ: 12.47 (s, 1 H, NH); 8.07 (s, 1 H,
Harom); 7.37 (m, 2 H, Harom); 2.75 (s, 3 H, Me); 1.34 (s, 9 H,
But). 13C NMR (151 МHz, DMSO-d6), δ: 145.9, 143.4, 131.0,
125.4, 121.8, 121.0, 115.2, 111.8, 34.6, 31.6, 14.6. MS: found
m/z 232.1202 [M]+; calculated for C15H12N2O2+ 232.1211. IR,
ν/cm–1: 880, 974, 1014, 1141, 1219, 1295, 1332, 1359, 1442,
1546, 1600, 2956, 3246.
[M + H]+; calculated for C13H19N2O3+ 251.1396. IR, ν/cm–1
:
801, 861, 1056, 1242, 1279, 1373, 1480, 1554, 1670, 2866,
2929, 2980.
2-Methyl-3-nitro-5-trifluoromethyl-1H-indole (3d). The
yield was 67%, m.p. 265 C (from ethyl acetate, with decomp.).
1H NMR (400 МHz, DMSO-d6), δ: 12.93 (s, 1 H, NH); 8.24
(s, 1 H, Harom); 7.61 (d, 1 H, Harom, J = 8.5 Hz); 7.54 (d, 1 H,
Harom, J = 7.2 Hz); 2.76 (s, 3 H, Me). 13C NMR (101 МHz,
DMSO-d6), δ: 145.5, 134.5, 123.8 (q, J = 31.4 Hz); 120.2 (q,
J = 3.6 Hz); 116.3 (q, J = 4.1 Hz); 113.2, 14.5. MS: found m/z
Ethyl N-[4-fluoro-2-(nitromethyl)phenyl]acetimidate (9f).
The yield was 60%, a yellow-orange oil. 1H NMR (600 МHz,
CDCl3), δ: 7.08 (ddd, 2 H, Harom, J = 8.2 Hz, J = 4.5 Hz,
J = 2.0 Hz); 6.71 (dd, 1 H, Harom, J = 9.5 Hz, J = 5.1 Hz); 5.37
(s, 2 H, CH2NO2); 4.20 (q, 2 H, CH2, J = 7.1 Hz); 1.82 (s, 3 H,
Me); 1.32 (t, 3 H, Me, J = 7.1 Hz). 13C NMR (151 МHz, CDCl3),
δ: 163.6, 158.8 (d, J = 242.1 Hz); 145.2, 123.2 (d, J = 8.0 Hz);
122.7 (d, J = 7.6 Hz); 118.1 (d, J = 23.1 Hz); 117.8 (d, J = 21.9 Hz);
76.1, 62.0, 16.5, 14.3. MS: found m/z 241.0983 [M + H]+; cal-
culated for C11H14FN2O3+ 241.0988. IR, ν/cm–1: 834, 866, 977,
1053, 1150, 1225, 1257, 1291, 1370, 1493, 1553, 1660.
Ethyl N-[4-chloro-2-(nitromethyl)phenyl]acetimidate (9g).
The yield was 58%, a yellow-orange oil. 1H NMR (600 МHz,
CDCl3), δ: 7.33 (dd, 2 H, Harom, J = 4.5 Hz, J = 2.3 Hz); 6.70 (d,
1 H, Harom, J = 9.1 Hz); 5.36 (s, 2 H, CH2NO2); 4.20 (q, 2 H, CH2,
J = 7.1 Hz); 1.83 (s, 3 H, Me); 1.32 (t, 3 H, Me, J = 7.1 Hz).
13C NMR (151 МHz, CDCl3), δ: 163.4, 147.6, 131.4, 131.0,
128.4, 123.5, 122.9, 76.0, 62.2, 16.6, 14.3. MS: found m/z
245.0529 [M + H]+; calculated for C10H8F3N2O2 245.0538.
+
IR, ν/cm–1: 818, 896, 970, 1016, 1053, 1106, 1156, 1207, 1266,
1323, 1367, 1446, 3191.
2,5,7-Trimethyl-3-nitro-1H-indole (3e). The yield was 71%,
m.p. 246 C (from ethyl acetate). 1H NMR (600 МHz, DMSO-d6),
δ: 12.24 (s, 1 H, NH); 7.66 (s, 1 H, Harom); 6.87 (s, 1 H, Harom);
2.74 (s, 3 H, Me); 2.42 (s, 3 H, Me); 2.37 (s, 3 H, Me). 13C NMR
(151 МHz, DMSO-d6), δ: 143.0, 132.5, 130.6, 125.9, 125.3,
121.3, 121.2, 116.7, 21.2, 16.2, 14.4. MS: found m/z 204.0891
[M]+; calculated for C11H12N2O2+ 204.0900. IR, ν/cm–1: 845,
998, 1093, 1124, 1167, 1210, 1245, 1294, 1321, 1345, 1370, 1415,
1557, 1614, 3092, 3154, 3205.
5-Fluoro-2-methyl-3-nitro-1H-indole (3f). The yield was
69%, m.p. 249 C (from ethyl acetate, with decomp.). 1H NMR
(400 МHz, DMSO-d6), δ: 12.71 (s, 1 H, NH); 7.68 (dd, 1 H,
Harom, J = 9.7 Hz, J = 2.6 Hz); 7.46 (dd, 1 H, Harom, J = 8.8 Hz,
J = 4.5 Hz); 7.17—7.07 (m, 1 H, Harom); 2.74 (s, 3 H, Me).
13C NMR (101 МHz, DMSO-d6), δ: 159.3 (d, J = 236.4 Hz);
144.8, 129.4, 125.3, 121.9 (d, J = 11.8 Hz); 113.8 (d, J = 9.9 Hz);
111.7 (d, J = 25.8 Hz); 104.8 (d, J = 26.6 Hz); 14.6. MS: found
257.0694 [M + H]+; calculated for C11H14ClN2O3 257.0693.
+
IR, ν/cm–1: 834, 952, 1052, 1092, 1192, 1246, 1292, 1373, 1482,
1553, 1660.
Synthesis of compounds 3a—g (general procedure). Alkyl
N-[2-(nitromethyl)aryl]acetimidate 9a—g was thoroughly
homogenized with silica gel (600 mg) in a 10-mL flask equipped
with an air condenser. The flask was placed in a preheated oil

Chesnokov et al.
2376
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 12, December, 2020
m/z 195.0563 [M + H]+; calculated for C9H8FN2O2+ 195.0570.
IR, ν/cm–1: 862, 980, 1010, 1100, 1119, 1175, 1201, 1279, 1345,
1359, 1404, 1438, 1603, 3193.
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Kirilenko, S. A. Rzhevskiy, A. A. Ageshina, M. A. Topchiy,
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5-Chloro-2-methyl-3-nitro-1H-indole (3g). The yield was
62%, m.p. 252 C (from ethyl acetate, with decomp.). 1H NMR
(400 МHz, DMSO-d6), δ: 12.76 (s, 1 H, NH); 7.95 (d, 1 H,
Harom, J = 2.1 Hz); 7.46 (d, 1 H, Harom, J = 8.6 Hz); 7.28 (dd,
1 H, Harom, J = 8.6 Hz, J = 2.1 Hz); 2.74 (s, 3 H, Me). 13C NMR
(101 МHz, DMSO-d6), δ: 144.7, 131.4, 128.0, 123.7, 122.2,
118.5, 114.0, 14.6. MS: found m/z 211.0278 [M + H]+; calcu-
lated for C9H8ClN2O2+ 211.0274. IR, ν/cm–1: 968, 1012, 1129,
1193, 1268, 1330, 1354, 1400, 1438, 1592, 3233.
This work was financially supported by the Russian
Science Foundation (Project No. 17-13-01076). Part
of the work (synthesis of ortho-bromoanilides) was carried
out by A. A. Ageshina and M. V. Maryanova within
the framework of the Russian state assignment of the
A. V. Topchiev Institute of Petrochemical Synthesis of
the Russian Academy of Sciences. Characterization of
some compounds obtained was financially supported by
the Ministry of Science and Higher Education of the
Russian Federation using scientific equipment of the
X-Ray Structural Center of the A. N. Nesmeyanov
Institute of Organoelement Compounds of the Russian
Academy of Sciences. The authors are grateful to the
Center for Shared Use of the Moscow State University
"Biospectrotomography" for the provision of an NMR
spectrometer.
24. Y. Li, F. Tur, R. P. Nielsen, H. Jiang, F. Jensen, K. A.
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Received December 11, 2019;
in revised form August 3, 2020;
accepted August 7, 2020