Studies on the reduction of the nitro group in 4-nitroindazoles by anhydrous SnCl2in different alcohols

Article abstract of DOI:10.1080/00397911003707212

(Chemical Equation Presented) The synthesis of new 7-alkoxy-4-amino- protected indazole and 4-amino-protected indazole derivatives by the reduction of the nitro group of 4-nitroindazoles using anhydrous stannous chloride in different alcohols is described

Full text of DOI:10.1080/00397911003707212

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Studies on the Reduction of the Nitro
Group in 4-Nitroindazoles by Anhydrous
SnCl2 in Different Alcohols
N. Abbassi a , E. M. Rakib a , L. Bouissane a , A. Hannioui a , M.
Khouili a , A. El Malki a , M. Benchidmi b & E. M. Essassi b
a Laboratory of Organic and Analytical Chemistry, Faculty of
Sciences and Technology , Béni-Mellal , Morocco
b Laboratory of Heterocyclic Chemistry, Faculty of Sciences , Rabat ,
Morocco
Published online: 03 Mar 2011.
To cite this article: N. Abbassi , E. M. Rakib , L. Bouissane , A. Hannioui , M. Khouili , A. El Malki , M.
Benchidmi & E. M. Essassi (2011) Studies on the Reduction of the Nitro Group in 4-Nitroindazoles by
Anhydrous SnCl2 in Different Alcohols, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 41:7, 999-1005, DOI: 10.1080/00397911003707212
To link to this article: http://dx.doi.org/10.1080/00397911003707212
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Synthetic Communications1, 41: 999–1005, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911003707212
STUDIES ON THE REDUCTION OF THE NITRO GROUP
IN 4-NITROINDAZOLES BY ANHYDROUS SnCl2 IN
DIFFERENT ALCOHOLS
N. Abbassi,1 E. M. Rakib,1 L. Bouissane,1 A. Hannioui,1
M. Khouili,1 A. El Malki,1 M. Benchidmi,2 and E. M. Essassi2
1Laboratory of Organic and Analytical Chemistry, Faculty of Sciences and
´
Technology, Beni-Mellal, Morocco
2Laboratory of Heterocyclic Chemistry, Faculty of Sciences, Rabat, Morocco
GRAPHICAL ABSTRACT
Abstract The synthesis of new 7-alkoxy-4-amino-protected indazole and 4-amino-protected
indazole derivatives by the reduction of the nitro group of 4-nitroindazoles using anhydrous
stannous chloride in different alcohols is described.
Keywords 7-Alkoxy-4-aminoindazoles; 4-nitroindazoles; SnCl2=alcohol
INTRODUCTION
Reduction of aromatic nitro compounds to the corresponding amines is an
important chemical transformation in synthetic organic chemistry. This is because
the nitro group is often used to activate the aromatic nucleus for nucleophilic substi-
tution reactions, but the amino group often serves as a site for further derivatization
to final products. A variety of methods for the direct reduction of aromatic nitro
compounds to the corresponding amines has been well documented.[1–3] Common
methods for the reduction of nitro groups to amines include Zn,[4] Sn,[5] and Fe[6]
in the presence of acid, TiCl3,[7] Raney Ni–hydrazine,[8] and catalytic hydrogen-
ation.[9] Reduction of aromatic nitro compounds with anhydrous SnCl2 in alcohol
was reported recently by our research group.[10] However, reduction of 7-nitroinda-
zoles with SnCl2 in ethanol produced a mixture of two products: the desired amine
Received January 26, 2010.
Address correspondence to E. M. Rakib, Laboratory of Organic and Analytical Chemistry, Faculty
´
of Sciences and Technology, B.P. 523, Beni-Mellal, Morocco. E-mail: elmostapha1@ymail.com
999

1000
N. ABBASSI ET AL.
Scheme 1. Reduction of 3-halogeno-7-nitroindazoles with SnCl2 in ethanol.
and the amine substituted with ethoxy group in the 4-position of indazole. We
observed a new kind of transformation of 7-nitroindazoles under the action of stan-
nous chloride in ethanol leading to 4-ethoxy-7-aminoindazoles accompanied by the
desired amine (Scheme 1). It is noteworthy that significant degradation of aromatic
primary amine was observed. Consequently, we immediately protected the amine by
using 4-methoxylbenzenesulfonyl chloride in pyridine. We selected arylsulfonyl
chloride as a protective agent of aminoindazole because in our previous study, we
found that N-(7-indazolyl)arylsulfonamides[11] showed an important antiprolifera-
tive activity against human (colon and prostate) and murine (leukemia) cell lines.
As an extension of these preliminary results, in the present work, we continue our
studies on reactions of the reduction of aromatic nitro compounds with anhydrous
SnCl2 in alcohol. Here, we report the results of our more detailed study on the reduction
of the nitro group in 4-nitroindazoles by anhydrous SnCl2 in different alcohols.
RESULTS AND DISCUSSION
Treatment of 4-nitroindazoles 4a and 4b with anhydrous SnCl2 in ethanol pro-
vided a mixture of two products, 4-aminoindazoles and 7-ethoxy-4-aminoindazoles,
which were immediately coupled by 4-methybenzenesulfonamide chloride in pyridine
(Scheme 2). 4-Nitroindazole 4a was prepared by addition of aqueous sodium nitrite
Scheme 2. Reduction of 3-halogeno-4-nitroindazoles with SnCl2 in ethanol.

REDUCTION OF THE NITRO GROUP
1001
Scheme 3. Reduction of 4-nitroindazole with SnCl2 in alcohol.
solution on 2-methyl-3-nitroaniline in acetic acid and was obtained with 85%
yield.[12] Compound 4b was obtained by brominating 4-nitroindazole 4b with
N-bromosuccinimide in refluxing acetonitrile.
The yields of isolated products 5a, 5b, 6a, and 6b were determined after flash
chromatography. The assignment of the structure of 7-ethoxy-N-(4-indazolyl)-4-
1
methylbenzenesulfonamides 5a and 5b was unambiguously supported by the H
and 13C NMR spectra, in particular by the evaluation of the multiplet pattern of
the C-ring proton signals: two doublets were observed at d 6.24–6.64 ppm and
6.38–6.78 ppm due to 6-H and 5-H protons of indazole.
We further studied the reduction of the 4-nitroindazole 4a with SnCl2 in differ-
ent alcohols to introduce an alkoxy group in the 7-position of indazole. The
7-alkoxy-N-(4-indazolyl)-4-methylbenzenesulfonamides 7a and 7b were formed in
35% and 37% yields, respectively, after separation by flash chromatography
(Scheme 3).
These results could be related to the important role played by nitro groups of
4-nitro-indazole with regards to the introduction of the substituent alkoxy in 7-
position. Because of its electron-withdrawing effect, the nitro group in nitroarene
activates in ortho and para for addition of nucleophilic agents.[13–15] Thus, the forma-
tion of the compound substituted by an alkoxy group in the 7-position could be
explained by the presence of the ROꢀ anion in the reaction mixture, followed by
the SNH on 4-nitroindazole.
In conclusion, we reported a new synthesis of N-(4-indazolyl)-4-methylbenze-
nesulfonamides and 7-alkoxy-N-(4-indazolyl)-4-methylbenzenesulfonamides using
anhydrous stannous chloride in different alcohols as a method of reduction. This
methodology is a valuable and general method for the preparation of new functio-
nalized indazoles such as 7-alkoxy-4-aminoprotectedindazoles.
EXPERIMENTAL
Melting points were determined using a Buchi-Tottoli apparatus and are
¨
uncorrected. Infrared (IR) spectra were recorded on a Perkin-Elmer 577 spec-
1
trometer using KBr disks; only noteworthy IR absorptions are listed (cmꢀ1). H
and 13C NMR spectra were recorded in dimethylsulfoxide (DMSO-d6) and solution
(unless otherwise specified) with textramethylsilane (TMS) as an internal reference

1002
N. ABBASSI ET AL.
using a Bruker AC 300 (1H) or 75 MHz (13C) instruments. Chemical shifts are given
in d parts per million (ppm) downfield from tetramethylsilance (TMS). Multiplicities
of 13C NMR resources were assigned by distortionless enhancement by polarization
transfer (DEPT) experiments. Low-resolution mass spectra (MS) were recorded on a
Perkin-Elmer Sciex API 3000 spectrometer. Column chromatography was carried
out on SiO2 (silica gel 60 Merck 0.063–0.200 mm). Thin-layer chromatography
(TLC) was carried out on SiO2 (silica gel 60, F 254 Merck 0.063–0.200 mm), and
the spots were located with ultraviolet light. Commercial reagents were used without
further purification unless stated.
4-Nitroindazole 4a
In a 500-mL, round-bottomed flask were introduced 2-methyl-3-nitroaniline
(5 g, 33 mmol) and 200 mL of AcOH. The solution was warmed under stirring until
completed dissolution. Addition drop by drop of a solution of NaNO2 (2.3 g,
37.70 mmol) in 5 ml of water led to diazonium salt precipitation. The solution was
stirred until this precipitate redisolved, and the mixture was concentrated to the third
of its initial volume. Then hot water (250 mL) was added to yield an orange-yellow
product. The mixture was warmed and filtered hot. After cooling, the obtained pre-
cipitate was filtered, washed with cold water, and dried to yield 4a: 85%; mp
198–200 ꢀC; 1H NMR (DMSO-d6): d 7.54 (t, 1H, J ¼ 7.9 Hz), 7.87 (d, 1H,
J ¼ 7.5 Hz), 7.94 (d, 1H, J ¼ 7.9 Hz), 8.46 (s, 1H), 13.98 (s, 1H, NH); 13C NMR
(DMSO-d6): d 118.1, 119.3; 125.9, 127.1 (4CH), 110.6, 141.4, 143.4 (3C).
3-Bromo-4-nitro-1H-indazole 4b
N-Bromosuccinimide 1.2 g, in acetonitrile was added to a solution of 4-nitroin-
dazole (1 g, 6.13 mmol) and heated at reflux for 1 h 30 min. The solvent was removed in
vacuo, and the residue was taken up in 50 mL of ethyl acetate, and washed with
2 ꢁ 100 ml of water, 100 mL of 10% sodium thiosulfate, and brine and then dried.
The solvent was removed in vacuo, and the residue was recrystallized from methanol.
Yield: 84%; mp: 200–202 ꢀC; IR (KBr, cmꢂ1): 3130 (NH), 1630 (CN), 1520,
1330 (NO2); 1H NMR (DMSO-d6): d 7.57 (t, 1H, J ¼ 7.7 Hz), 7.88 (d, 1H,
J ¼ 7.5 Hz), 7.98 (d, 1H, J ¼ 7.7 Hz), 14.23 (s, 1H, NH); 13C NMR (DMSO-d6): d
117.7, 119.0, 127.1 (3CH), 112.7, 117.7, 141.7, 143.2 (4C). MS m=z ¼ 242 (79Br)
[M þ 1]þ, 244 (81Br) [M þ 3]þ.
General Method for the Synthesis of 7-Ethoxy-N-(4-indazolyl)-
4-methylbenzene Sulfonamides and N-(4-Indazolyl)-4-
methylbenzenesulfonamides
A mixture of 3-halogeno-4-nitroindazole 4a and 4b (1.22 mmol) and anhydrous
SnCl2 (1.1 g, 6.1 mmol) in 25 mL of absolute ethanol was heated at 60 ꢀC. After
reduction, the starting material disappeared, and the solution was allowed to cool
down. The pH was made slightly basic (pH 7–8) by addition of 5% aqueous potass-
ium bicarbonate before extraction with ethyl acetate. The organic phase was washed
with brine and dried over magnesium sulfate. The solvent was removed to afford the

REDUCTION OF THE NITRO GROUP
1003
amine, which was immediately dissolved in pyridine (5 mL) and then reacted with
4-methylbenzenesulfonyl chloride (0.26 g, 1.25 mmol) at room temperature for
24 h. After the reaction mixture was concentrated in vacuo, the resulting residue
was purified by flash chromatography (eluted with EtOAc=hexane 1=9).
N-(7-Ethoxy-1H-4-indazolyl)-4-methyl-benzenesulfonamide 5a. Yield:
33%, mp: 210–212 ꢁC; IR (KBr, cmꢀ1): 3340, 3220 (NH), 1585 (CN), 1338, 1165
1
(SO2); H NMR (DMSO-d6): d 1.37 (t, 3H, CH3, J ¼ 6.9 Hz), 2.28 (s, 3H, CH3),
4.12 (q, 2H, OCH2, J ¼ 6.9 Hz), 6.64 (d, 1H, J ¼ 8.1 Hz), 6.72 (d, 1H, J ¼ 8.1 Hz),
7.25 (d, 2H, J ¼ 8.1 Hz), 7.58 (d, 2H, J ¼ 8.1 Hz), 7.97 (s, 1H), 10.06 (s, 1H, NH),
13.21 (s, 1H, NH); 13C NMR (DMSO-d6): (15.1 (CH3), 21.3 (CH3), 64.2 (CH2O),
106.1, 114.9, 127.2, 129.9, 132.7 (5 CH), 120.6, 122.3, 132.9, 137.4, 142.4, 143.4 (6
C); MS m=z ¼ 332 [M þ 1]þ.
N-(1H-4-Indazolyl)-4-methyl-benzenesulfonamide 6a. Yield: 35%; mp:
1
170–172 ꢁC; IR (KBr, cmꢀ1): 3340, 3235 (NH), 1595 (CN), 1335, 1160 (SO2); H
NMR (DMSO-d6): d 2.27 (s, 3H, CH3), 6.92 (dd, 1H, J ¼ 2.1 Hz and 6.1 Hz), 7.16
(d, 2H, J ¼ 6.2 Hz), 7.28 (d, 2H, J ¼ 8.3 Hz), 7.68 (d, 2H, J ¼ 8.3 Hz), 8.22 (s, 1H),
10.51 (s, 1H, NH), 13.05 (s, 1H, NH); 13C NMR (DMSO-d6): (21.3 (CH3), 106.7,
111.0, 126.9, 127.2, 130.1, 132.4 (6 CH), 117.3, 130.5, 137.3, 141.4, 143.7 (5C); MS
m=z ¼ 288 [M þ 1]þ.
N-(3-Bromo-7-ethoxy-1H-4-indazolyl)-4-methyl-benzenesulfonamide 5b.
Yield: 30%; mp: 118–120 ꢁC; IR (KBr, cmꢀ1): 3345, 3231 (NH), 1600 (CN), 1338,
1
1162 (SO2); H NMR (DMSO-d6): d 1.39 (t, 3H, CH3, J ¼ 7.0 Hz), 2.36 (s, 3H,
CH3), 4.15 (q, 2H, OCH2, J ¼ 7.0 Hz), 6.24 (d, 1H, J ¼ 8.1 Hz), 6.38 (d, 1H,
J ¼ 8.1 Hz), 7.35 (d, 2H, J ¼ 8.0 Hz), 7.57 (d, 2H, J ¼ 8.0 Hz), 9.49 (s, 1H, NH),
13.69 (s, 1H, NH); 13C NMR (DMSO-d6): d (15.0 (CH3), 21.4 (CH3), 64.5
(CH2O), 106.6, 121.0, 127.4, 130.0 (4 CH), 118.0, 119.2, 120.8, 134.4, 138.4, 142.1,
143.5 (7 C); MS m=z ¼ 410 (79Br) [M þ 1]þ, 412 (81Br) [M þ 3]þ.
N-(3-Bromo-1H-4-indazolyl)-4-methyl-benzenesulfonamide 6b. Yield:
25%; mp: 150–152 ꢁC; IR (KBr, cmꢀ1): 3335, 3225 (NH), 1595 (CN), 1340, 1156
1
(SO2); H NMR (DMSO-d6): d 2.36 (s, 3H, CH3), 6.57 (d, 1H, J ¼ 7.4 Hz), 7.25 (t,
1H, J ¼ 7.4 Hz), 7.35 (d, 2H, J ¼ 8.1 Hz), 7.41 (d, 1H, J ¼ 7.3 Hz), 7.63 (d, 2H,
J ¼ 8.1 Hz), 9.69 (s, 1H, NH), 13.50 (s, 1H, NH); 13C NMR (DMSO-d6): d 21.4
(CH3), 109.8, 118.3, 127.4, 127.9, 130.0 (5 CH), 117.9, 118.7, 129.3, 138.1, 142.8,
143.5 (6C); MS m=z ¼ 366 (79Br) [M þ 1]þ, 368 (81Br) [M þ 3]þ.
Synthesis of Compounds 7a and 7b
These compounds were synthesized as described for 4a and 4b by using the
appropriate alcohol (methanol or propanol).
N-(7-Methoxy-1H-4-indazolyl)-4-methyl-benzenesulfonamide 7a. Yield:
35%; mp: 230–232 ꢁC; IR (KBr, cmꢀ1): 3320, 3200 (NH), 1590 (CN), 1340, 1158
(SO2); 1H NMR (DMSO-d6): d 2.27 (s, 3H, CH3), 3.84 (s, 3H, OCH3), 6.68
(d, 1H, J ¼ 6.9 Hz), 6.87 (d, 1H, J ¼ 6.9 Hz), 7.24 (d, 2H, J ¼ 7.9 Hz), 7.55 (d, 2H,
J ¼ 7.9 Hz), 7.94 (s, 1H), 10.05 (s, 1H, NH), 13.24 (s, 1H, NH); 13C NMR

1004
N. ABBASSI ET AL.
(DMSO-d6): d 21.4 (CH3), 55.9 (CH3O), 105.3, 114.8, 127.2, 129.9, 132.4 (5 CH),
120.5, 122.4, 132.7, 137.3, 141.4, 143.4 (6 C); MS m=z ¼ 318 [M þ 1]þ.
N-(7-Propoxy-1H-4-indazolyl)-4-methyl-benzenesulfonamide 7b. Yield:
37%; mp: 135–137 ꢁC; IR (KBr, cmꢀ1): 3350, 3260 (NH), 1585 (CN), 1328, 1155
1
(SO2); H NMR (DMSO-d6): d 1.00 (t, 3H, CH3, J ¼ 7.4 Hz,), 1.74 (m, 2H, CH2),
2.26 (s, 3H, CH3), 3.99 (t, 2H, OCH2, J ¼ 6.8 Hz), 6.63 (d, 1H, J ¼ 8.0 Hz), 6.69
(d, 1H, J ¼ 8.0 Hz), 7.24 (d, 2H, J ¼ 8.1 Hz), 7.53 (d, 2H, J ¼ 8.1 Hz), 7.93 (s,
1H), 10.02 (s, 1H, NH), 13.20 (s, 1H, NH); 13C NMR (DMSO-d6): d 11.0 (CH3),
21.4 (CH3), 22.5 (CH2), 69.9 (CH2O), 105.9, 114.9, 127.2, 129.9, 132.7 (5 CH),
120.6, 122.2, 132.9, 137.4, 142.7, 143.4 (6 C). MS m=z ¼ 346 [M þ 1]þ.
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