Synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes and their transformation into 6-nitro-4-(trifluoromethyl)- and 4-nitro-6-(trifluoromethyl)-1H-indoles

Article abstract of DOI:10.1016/j.jfluchem.2015.04.015

A facile two-step synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes is described. The first step is the nitration of otho- and para-methylbenzoic acids to furnish the corresponding dinitroacids. The dinitroacids then react with sulfur tetrafluoride to provide the corresponding trifluoromethylated products. The latter are easily transformed into 6-nitro-4-(trifluoromethyl)- and 4-nitro-6-(trifluoromethyl)-1H-indoles by applying the Batcho-Leimgruber synthetic protocol.

Full text of DOI:10.1016/j.jfluchem.2015.04.015

Accepted Manuscript
Title: Synthesis of 2-methyl-1,5-dinitro-3- and
2
-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes and their
transformation into 6-nitro-4-(trifluoromethyl)- and
-nitro-6-(trifluoromethyl)-1H-indoles.
4
Author: Oksana M. Petruk Yevhenii A. Kyriukha Andriy V.
Bezdudny Vladimir V. Rozhkov
PII:
S0022-1139(15)00121-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jfluchem.2015.04.015
FLUOR 8553
To appear in:
FLUOR
Received date:
Revised date:
Accepted date:
18-1-2015
26-4-2015
27-4-2015
Please cite this article as: O.M. Petruk, Y.A. Kyriukha, A.V. Bezdudny,
V.V. Rozhkov, Synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-
(trifluoromethyl)benzenes and their transformation into 6-nitro-4-(trifluoromethyl)-
and 4-nitro-6-(trifluoromethyl)-1H-indoles., Journal of Fluorine Chemistry (2015),
http://dx.doi.org/10.1016/j.jfluchem.2015.04.015
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Highlights


 A two-step synthesis of 2- and 4-trifluoromethylated dinitrotoluenes is developed.
 The method does not demand to carry out the fluorination reaction in HF media.

 The synthesized toluenes were converted into 4- and 6-trifluoromethylated nitroindoles.
Page 1 of 7

Synthesis
(
(
of
trifluoromethyl)benzenes and their transformation into 6-nitro-4-
trifluoromethyl)- and 4-nitro-6-(trifluoromethyl)-1H-indoles.
2-methyl-1,5-dinitro-3-
and
2-methyl-1,3-dinitro-5-
a
b
c
c,*
Oksana M. Petruk , Yevhenii A. Kyriukha , Andriy V. Bezdudny , Vladimir V. Rozhkov .
a
National Technical University of Ukraine “Kiev Polytechnic Institute”, 37 prospect Pobedy, Kiev, Ukraine, 03056.
b
Taras Shevchenko National University, Chemical Department, 62a Vladimirskaya st., Kiev, Ukraine, 01601.
c
Institute of Organic Chemistry National Academy of Sciences, 5 Murmanskaya st., Kiev, Ukraine, 02660.
*
Correspondence author: e-mail: vladimir_rozhkov@mail.ru, phone: +380-50-300-2502.
Abstract
A facile two-step synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes is described. The first step is the
nitration of otho- and para-methylbenzoic acids to furnish the corresponding dinitroacids. The dinitroacids then react with sulfur
tetrafluoride to provide the corresponding trifluoromethylated products. The latter are easily transformed into 6-nitro-4-(trifluoromethyl)- and
4
-nitro-6-(trifluoromethyl)-1H-indoles by applying the Batcho-Leimgruber synthetic protocol.
Key words: TNT surrogate, sulfur tetrafluoride, 2-methyl-1,5-dinitro-3-(trifluoromethyl)benzene, 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzene,
Batcho-Leimgruber synthetic protocol.
1
. Introduction
2
,4,6-Trinitrotoluene (TNT) is a powerful synthon for the synthesis of benzannelated five-membered ring heterocycles,
e.g., 6-nitroindole [1a,b,c], 4,6-dinitroindazoles [1d], and 6-nitrobenzofurane derivatives [1e,b]. However, manipulations with
TNT in a laboratory have two major disadvantages. First, TNT is an explosive and should be handled with a great care.
Second, a special permission should be obtained to work with TNT.
Given the above disadvantages, we decided to synthesize a compound with properties similar to those of TNT and not to
be explosive (so-called “TNT surrogate”). To do so, one nitro group should be replaced with another group having a close
3 2
electron withdrawing effect (e. g., CN, CF , CO Me). The trifluoromethyl group has been chosen because an introduction of
the group into organic molecules has become important in pharmaceutical and agrochemical research on account of its unique
properties.
In the present paper a facile synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes has
been developed. The method includes the nitration of ortho- and para-methylbenzoic acids and the transformation of the
3
carboxylic group into the CF group utilizing sulfur tetrafluoride. To show the similarity of the synthesized trifluoromethyl
derivatives to TNT 6-nitro-4-(trifluoromethyl)- and 4-nitro-6-(triflouromethyl)-1H-indoles have been synthesized by applying
the Bathco-Leimgruber synthetic protocol [2].
2
. Result and discussion
The most common method of the synthesis of aromatic trifluoromethylated compounds is a reaction of benzoic acids with
4
[3]. The yield of the reaction strongly depends on the substituents on the benzene ring. Electron withdrawing groups
SF
increase the yield of the desired product; on the contrary, electron donating groups decrease the yield of the
trifluoromethylated product. Yagupolsky and co-workers have shown that the presence of 40-50 fold excess of HF
dramatically increases yields of the trifluoromethylated products and allows carrying out the reaction under mild conditions
[4].
To prepare 2-triflouromethyl toluene ortho-methyl benzoic acid was used as a starting material. The reaction was carried
out with tenfold excess of SF at 100 °C in HF media for 24 h. Even under those conditions the yield was only 16%. Such
poor reactivity can be caused by the ortho-effect of methyl group and its electron donating properties. To improve the
4
Page 2 of 7

reactivity of the carboxylic group we prepared 4,6-dinitro-2-methylbenzoic acid by nitration of 2-methylbenzoic acid. The
reaction proceeded smoothly in a mixture of concentrated nitric and sulfuric acids at 10–20 °C. Product 1 was obtained in a
9
2% isolated yield (Scheme 1) [5].
4
Acid 1 readily reacted with SF in a stainless steel autoclave in the presence of a catalytic amount of water at 85–90 °C to
furnish compound 2 in 88% isolated yield. In the same manner we prepared compound 5 by nitration of para-methylbenzoic
o
acid [6]. Acid 5 also readily reacted with SF
4
with a catalytic amount of water at 85–90 C to furnish benzene 6. The products
were obtained in 75–78% overall isolated yield for two steps.
Benzenes 2 and 6 were synthesized with 95% purity. Analytically pure samples were obtained by either distillation in
i
vacuo (product 2) or re-crystallization from PrOH (product 6). Product 6 was already synthesized by nitration para-
trifluoromethyltoluene [7]. However, in the latter case, the synthesis of para-trifluoromethyltoluene demanded 50 fold excess
of rather expensive and dangerous HF [4] or an application of organometallic reagents [8].
N
CO
2
H
O
O
O
2
N
CO
2
H
O
2
N
CF
3
O2
N
CF3
HNO
3
N
SF
4
NO
2
NO
2
NO2
1
2
3
CF
3
2 2
SnCl 2H O
O
2
N
N
H
4
N
O
O
O
2
N
NO
2
O
2
N
NO
2
O2
N
NO2
HNO
3
N
SF
4
CO
2
H
CO
2
H
CF
3
CF3
6
5
7
NO
2
NH2
SnCl
2
2H
2
O
H
2
F
3
C
N
H
Pd/C
F3C
N
H
8
9
Scheme 1. Synthesis of 6-nitro-4-triflouromethyl- and 4-nitro-6-trifluoromethyl-1H-indoles.
o
Dinitrotoluenes 2 and 6 reacted with dimethylformamide dimethyl acetal in toluene at 80 C to furnish enamines 3 and 7
with 95% isolated yields (Scheme 1). The synthesized enamines are dark red air-stable solids readily soluble in toluene,
DCM, and alcohols. The purity (>95%) of the isolated products allowed their use in the next step without further purification.
Analytically pure samples were obtained by re-crystallization from CHCl
To transform the enamines into indoles 4 and 8 the Batcho-Liemgruber synthetic protocol was applied (Scheme 1). The
reaction was carried out in a mixture of CH Cl /MeOH (1:1) with SnCl ·2H O as a reducing agent. The tin salt reacted only
3
– hexane.
2
2
2
2
with the ortho-nitro group and did not affect the second one allowing the synthesis of the corresponding indoles with the good
yields.
Indoles 4 and 8 are stable compounds and can be stored under ambient conditions for a long time without any
decomposition. They are precursors of aminoindoles. 4-Amino-6-trifluoromethylindole was used as a coupling reagent for the
synthesis of pyrimidine derivatives as Pi3k inhibitors [5] but neither method of the synthesis nor its analytical data were
described in details. The product 8 was taken as an example to synthesize corresponding aminoindole 9. It has been found that
the catalytic hydrogenation on Pd/C as a catalyst is the best method. Another methods to reduce the nitro group (Fe – NH
or N ·H O – FeCl ·6H O – C) led to a complex mixture of products. The product 9 was stored under inert atmosphere
without visible changes at least for 3 months.
4
Cl
2
H
4
2
3
2
Page 3 of 7

3
. Conclusion
The convenient method of the synthesis of 2-methyl-1,5-dinitro-3-trifluoromethyl- and 2-methyl-1,3-dinitro-5-
(trifluoromethyl)benzenes was developed. To show their similarity to TNT the synthesized products were transformed into the
corresponding nitro-(trifluoromethyl)indoles by the Batcho-Leimgruber method. One of the nitroindole was transformed into
the corresponding aminoindole by the catalytic hydrogenation on Pd/C.
4
. Experimental
.1. General
H, F, and C NMR were recorded on a Bruker Avance DRX500 spectrometer (500 MHz, 470 MHz, and 125 MHz,
4
1
19
13
1
13
respectively). Chemical shifts are reported in  values from TMS as internal standard for H and C and from CFCl
3
as
19
internal standard for F (s, singlet; d, doublet; m, multiplet; q, quartet; t, triplet; br s, broad signal). Coupling constants (J) are
given in hertz. Melting points measured with a Buchi melting point apparatus. Benzoic acids and sulfur tetrafluoride were
2 2 2
used as purchased. Toluene, CH Cl , and dimethylformamide dimethyl acetal were distilled over CaH . Methanol was distilled
over Mg.
4.2. General procedure for the synthesis of trifluoromethyltoluenes 2 and 6.
A stainless steel autoclave (250 mL volume) was charged with acid 1 or 5 (22.6 g, 0.1 mol) and 1 mL of water. The
o
autoclave was evacuated and cooled down to -178 C with liquid nitrogen. Then, SF
4
(43.2 g, 0.4 mol) was condensed into it.
o
The autoclave was sealed and heated at 90 C for 20 h. After that, the autoclave was cooled and gaseous products were vented
2 2 2 4
off. The mixture was poured into ice water (500 mL), extracted with CH Cl (3 x 100 mL), and dried over Na SO . The
solvent was evaporated and the product was purified by distillation or re-crystallization.
o
1
4
.2.1. 2-Methyl-1,5-dinitro-3-(trifluoromethyl)benzene (2). Yield: 20.1 g, 80.4%; bp=51-53 C/0.05 torr. H NMR (500 MHz,
4
4
19
CDCl
3
3
)  (ppm): 8.78 (d, 1H, JHH = 1.9 Hz, Harom); 8.23 (d, 1H, JHH = 1.9 Hz, Harom); 2.70 (s, 3H, Me). F NMR (470 MHz,
13
2
CDCl
)  (ppm): -61.61 (s, 3F, CF
3
). C NMR (125 MHz, CDCl
3
)  (ppm): 152.0, 145.7, 138.6, 133.1 (q, JCF = 31.2 Hz),
3
1
1
24.2 (q, JCF = 5.9 Hz), 122.4 (q, JCF = 275.3 Hz), 122.2, 15.3. Anal. calcd for C
8
H
5
F
3
N
2
O
4
: C, 38.41; H, 2.01; N, 11.20;
found: C 38.46; H 2.07; N 11.28
o
i
1
4
.2.2. 2-Methyl-1,3-dinitro-5-(trifluoromethyl)benzene (6) [5]. Yield: 20.3 g, 81%; mp=65-67 C ( PrOH). H NMR (500
1
9
13
MHz, CDCl
3
)  (ppm): 8.65 (s, 2H, Harom); 2.65 (s, 3H, Me). F NMR (470 MHz, CDCl
3
3
)  (ppm): -59.89 (s, 3F, CF ). C
2
3
1
NMR (125 MHz, CDCl
3
)  (ppm): 152.6, 141.4, 127.4 (q, JCF = 31.5 Hz), 125.3 (q, JCF = 5.7 Hz), 122.1 (q, JCF = 275.5
Hz), 18.8. Anal. calcd for C
8
5 3 2 4
H F N O : C, 38.41; H, 2.01; N, 11.20; found: C 38.39; H 2.09; N 11.24.
4.3. General procedure for the synthesis of enamines 3 and 7.
To a solution of compound 2 or 6 (3.0 g, 0.012 mol) in 30 mL of toluene 1,1-dimethoxy-N,N-dimethylmethanamine (3.6 g,
o
0
.03 mol) was added. The mixture was stirred at 80 C for 72 h and the solvent was evaporated in vacuo. The product was
used in the next step without further purification. The analytically pure sample was prepared by re-crystallization from CHCl
3
-
hexane (1:1).
o
1
4
.3.1. (E)-2-(3,5-dinitro-2-(trifluoromethyl)phenyl)-N,N-dimethylethenamine (3). Yield: 3.5 g, 95%; mp=156-158 C. H
4
4
3
NMR (500 MHz, CDCl )  (ppm): 8.48 (d, 1H, JHH = 1.8 Hz, Harom), 8.41 (d, 1H, JHH = 1.8 Hz, Harom), 6.79 (d, 1H, JHH = 14
3
3
19
13
2 3
Hz, CH=CH), 5.27 (d, 1H, JHH = 14 Hz, CH=CH), 3.00 (s, 6H, NMe ). F NMR (470 MHz CDCl C
) (ppm): -62.56.
2
3
NMR (125 MHz, CDCl
3
)  (ppm): 149.2, 146.7, 139.6, 139.0, 126.7 (q, JCF = 30.9 Hz), 124.5 (q, JCF = 6.5 Hz), 122.9 (q,
: C, 43.29; H, 3.30; N, 13.77; found: C 43.32; H 3.34; N
1
J
CF = 273.7 Hz), 122.8, 86.0, 41.0. Anal. calcd for C11
3.78.
.3.2. (E)-2-(2,6-dinitro-4-(trifluoromethyl)phenyl)-N,N-dimethylethenamine (7). Yield: 3.4 g, 92%; mp=136-138 C.
10 3 3 4
H F N O
1
o
1
4
H
3
3
NMR (500 MHz, CDCl
3
)  (ppm): 7.93 (s, 2H), 6.65 (d, 1H, JHH = 13.4 Hz, CH=CH), 5.47, (d, 1H, JHH = 13.4 Hz, CH=CH)
19
13
2
.94 (s, 6H, NMe
2
). F NMR (470 MHz CDCl
3
) (ppm): -60.59. C NMR (125 MHz, CDCl
3
)  (ppm): 143.5, 142.5, 127.4,
Page 4 of 7

3
2
1
1
4
10 3 3 4
19.3 (q, JCF =3.6 Hz), 117.6 (q, JCF = 35.9 Hz), 117.2 (q, JCF = 272.1 Hz), 77.8, 35.5. Anal. calcd for C11H F N O : C,
3.29; H, 3.30; N, 13.77; found: C 43.31; H 3.28; N 13.81.
4
.4. General procedure for the synthesis of indoles 4 and 8.
To a solution of enamine 3 or 7 (1.5 g, 0.005 mol) in a mixture of MeOH – CH Cl (30 mL, 1:1 ratio) SnCl
.02 mol) was added in one portion. The mixture was stirred at room temperature overnight and poured into 500 mL of H
2
2
2
· 2H
2
O (4.5 g,
O.
0
2
The organic layer was separated, the water layer extracted with EtOAc (3  25 mL), the organic layers were combined and
dried over Na SO . Sodium sulfate was filtered off, the solvent was evaporated, and the crude product was purified on silica
gel using hexane – EtOAc (4:1) as an eluent to furnish pure indole 4 or 8.
.4.1. 6-Nitro-4-(trifluoromethyl)-1H-indole (4). Yield: 0.71 g, 62%; mp=156-158 C. H NMR (500 MHz, (CD
2
4
o
1
4
3
)
2
SO) 
1
9
(
(
ppm): 12.31 (br s, 1H, NH); 8.53 (br s, 1H, Harom); 8.15 (m, 2H, Harom and Hind), 6.61 (br s, 1H, Hind). F NMR (470 MHz,
1
3
1
CD
3
)
2
SO)  (ppm): -62.32 (s, 3F, CF
3
). C NMR (125 MHz, (CD
3
)
2
SO)  (ppm): 141.0, 133.6, 131.7, 123.6 (q, JCF = 270.8
2
3
Hz), 122.9, 120.0 (q, JCF = 35.0 Hz), 117.0 (q, JCF = 5.2 Hz), 109.2, 95.9. Anal. calcd for C
9
H
5
F
3
N
2
O
2
: C, 46.97; H, 2.19; N,
1
2.17; found: C 47.01; H 2.18; N 12.21.
o
1
4
.4.2. 4-Nitro-6-(trifluoromethyl)-1H-indole (8). Yield: 0.75 g, 65%; mp=142-145 C. H NMR (500 MHz, (CD ) SO) 
3 2
4
4
3
(
ppm): 12.25 (br s, 1H, NH); 8.24 (d, 1H, JHH = 1.9 Hz, Harom); 8.21 (d, 1H, JHH = 1.9 Hz, Harom); 8.16 (d, 1H, JHH = 3.0 Hz,
3
19
13
H
ind); 6.99 (d, 1H, JHH = 3.0 Hz, Hind). F NMR (470 MHz, (CD
3
)
2
SO)  (ppm): -60.08 (s, 3F, CF
3
). C NMR (125 MHz,
1
2
3
(
CD
3
)
2
SO)  (ppm): 139.1, 134.2, 133.5, 123.9 (q, JCF = 274.5 Hz), 120.9 (q, JCF =33.8 Hz), 118.7, 113.1 (q, JCF = 3.5 Hz),
3
1
12.6 (q, JCF = 3.8 Hz), 99.6. Anal. calcd for C
9
H
5
F
3
N
2
O
2
: C, 46.97; H, 2.19; N, 12.17; found: C 47.01; H 2.18; N 12.21.
4
.5. Synthesis of 4-amino-6-(triflouromethyl)-1H-indole 9.
To a solution of indole 8 (0.1 g, 0.43 mmol) in MeOH (5 mL) Pd/C (10% of Pd) (0.05 g) was added. The solution was
degassed and purged with H . The reaction mixture was hydrogenated overnight at rt and 1 atm pressure of H . The catalyst
2
2
was filtered off and the solvent was evaporated to furnish the desirable amine as a creamy solid. All manipulations were
carried out under Ar using the inert atmosphere technique.
o
1
4
.5.1. 4-Amino-6-(trifluoromethyl)-1H-indole (9). Yield: 0.075 g, 87%; mp=152-154 C. H NMR (500 MHz, CDCl
3
) 
3
4
4
(
ppm): 8.34 (br s, 1H, NH), 7.24 (d, 1H, JHH = 3.1 Hz, Hind); 7.15 (d, 1H, JHH = 1.9 Hz, Harom); 6.61 (d, 1H, JHH = 1.9 Hz,
3
19
13
H
arom); 6.51 (d, 1H, JHH = 3.1 Hz, Hind), 3.07 (br s, 2H, NH
2
). F NMR (470 MHz, CDCl
3
3
)  (ppm): -61.36 (s, 3F, CF ). C
2
1
NMR (125 MHz, CDCl
3
)  (ppm): 139.3, 135.2, 125.1 (q, JCF =31.9 Hz), 124.8 (q, JCF = 271 Hz), 124.3, 118.8, 100.0 (q,
3
3
J
9 7 3 2
CF =3.5 Hz), 99.4 (q, JCF =3.5 Hz), 99.0. Anal. calcd for C H F N : C, 54.00; H, 3.52; N, 14.00; found: C 54.03; H 3.59; N
1
4.06.
References
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(
(
(
(
b) V.V. Rozhkov, A.M. Kuvshinov, S.A. Shevelev. Synth. Commun. 32 (2002) 1465 - 1474.
c) V.V. Rozhkov. Tetrahedron. 70 (2014) 3595-3560.
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[
2] (a) A.D. Batcho, W. Leimgruber. US 3,976,639 (1976).
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(
[
3] (a) G.A. Boswell, W.C. Ripka, R.V. Skribner, C.W. Tullock. Fluorination by sulfur tetrafluoride. Organic reactions. Ed. by W.G. Dauben. John
Wiley & Son. New York ect. 21 (1974) 1-124.
(b) A.I. Burmakov, L.A. Alekseeva, L.M. Yagupolskii. Zh. Org. Chem. 8 (1972) 153-154. (in Russian). Chem. Absrt. 76 (1972) 11230z.
(c) L.M. Yagupolskii, A.I. Burmakov, L.A. Alekseeva, B.V. Kunshenko. Zh. Org. Chem. 9 (1973) 989-996. (in Russian). Chem. Abstr. 79
(1973)18610q.
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[
7] S.J. Baker, P.J. Goldshith, T.C. Hancox, N.A. Pegg, S. Price, S.J. Shuttleworh, S. Sohal. WO 122410 A1 (2007).
8] (a) M. Shenglof, G.A. Molander, J. Blum. Synthesis. (2007) 111-114.
[
(
(
(
b) B.R. Langlois, N. Roques. J. Fluorine Chem. 128 (2007) 1318-1325.
c) K.A. McReynolds, R.S. Lewis, L.K.G. Akerman, G.G. Dubinina, W.W. Brennssel, D.A. Vicic. J. Fluorine Chem. 131 (2010) 1108-1112.
d) T. Knauber, F. Arikan, G.-V. Roecshenthaler, L.G. Goosen. Chem. Eur. J. 17 (2011) 2689-2697.
Page 6 of 7

Graphical Abstract
Synthesis of 2-methyl-1,5-dinitro-3- and 2-methyl-1,3-dinitro-5-(trifluoromethyl)benzenes and
their transformation into 6-nitro-4-(trifluoromethyl)- and 4-nitro-6-(trifluoromethyl)-1H-
indoles.
a
b
c
c,*
Oksana M. Petruk , Yevhenii A. Kyriukha , Andriy V. Bezdudny , Vladimir V. Rozhkov .
a
National Technical University of Ukraine “Kiev Polytechnic Institute”, 37 prospect Pobedy, Kiev, Ukraine, 03056.
b
Taras Shevchenko National University, Chemical Department, 62a Vladimirskaya st., Kiev, Ukraine, 01601.
c
Institute of Organic Chemistry National Academy of Sciences, 5 Murmanskaya st., Kiev, Ukraine, 02660.
*
Correspondence author: e-mail: vladimir_rozhkov@mail.ru, phone: +380-50-300-2502.
CF
3
CO
2
H
O
O
N
CF
3
2
N
H
2
O N
NO
2
NO
2
2
N
NO
2
N
H
3
F C
CO
2
H
CF
3
Page 7 of 7