Indian Journ al of Chemi stry
V ol. 43B , November 2004, pp. 2464-2466
Note
Sn/NH4 Br assisted selective reduction of
nitroarencs into anilines under neutral
conditions
M A Pasha'" & V P Jayashank ara
Dcpartmcnt of Studi cs in Chcmi stry, Ccntra l Coll cgc Campu s,
Bangalore Uni vcrsity, Bangalorc-560 00 I , Indi a
* E-ma il: m<lfpasha @vahoo.co. i n
Receil'cd 24 March 2004: accepled (rel'ised) 27 AlIglisl 200-1
A sim plc, cost-clTccti vc Illct hod for thc rcductio n or ary l nitro
compounds by tin in thc prescncc of amilloni um bromidc in
Illcthanolt o anili ncs is rcport cd.
IPC: Int.CI.? C 07 C 211146
Red ucti on of orga ni c co mpound s is widely practiced
both in laboratory and in industry. Hyd roge n tran sfer
red ucti on is one of th e easiest, safest and hi ghl y
selective method s of ca rrying out th e reduction.
Aro matic amines widely used as intermedi ate fo r
dyes, photog raphi c, pharmaceutical , agricultural
chem ica ls and anti ox idants can be eas il y prepared by
reduc ti on of aromatic nitroarenes usin g thi s method .
Recent investi ga ti ons on th e reduc ti on of different
fun cti onal groups indicate th at aro mati c nitro
co mpounds are red uced to an ili nes by a variety of
reagents which include metal-med iated reage nts like
In/FeC I:/ H20' , Te/near critical water temperature 2 ,
Raney Ni/NH4C1 3, Ru)(CO)' 2/c helat ing diimines 4 ,
FeS/NH 4C1/CH 30H/ H20 5 ,
SmINH.1Cl 6 ,
nanosized
7
acti vated metallic iron powder in water , and by
Zn/NH 4 C1 in ioni c liquids 8 . Howeve r, these meth ods
suffer from their ow n advantages and shortco mings.
Homogeneous catalytic hydroge nation requires
expensive metals such as ruth eniu m (i n the form of its
co mpl ex), tellurium, sa marium or indium , wh ere
workup and isolati on of the products are tedi ous, and
so me of th ese sys tems such as Raney-Ni/NH 4CI
require strin gent preca uti ons because of the
fl amma ble nature when exposed to air.
Recentl y, we reported selective reduction of
nitroarenes, nitrosoarenes and arylhydroxyl amines to
corresponding anilines by AIINH 4C1. In continuation
of our work on metal mediated reductions of di fferent
I gro ups9·1 4 , we have come up WIt
. I1 the use
.
f unctlOna
of tin in conjugation with ammonium salts for the
redu ction of di !'ferent fun ctional grou ps under proti c
and aprotic co nd iti ons. Herein we present an acco unt
of th e reducti on of nitroarenes by Sn/NH.1X (X = CI ,
Br) under different co nditi ons. It is found that the
reac ti on in the presence of ammonium chl orid e is
very slow at 25 DC , and at reflu x ta kes about 18-24 hI'
for co mpl etio n. However, the reaction in th e presence
of ammonium bromide goes to comp letion within 40
min at reflu x temperature of met han ol (Scheme I).
In a typical experiment nitroarene, tin metal and
ammonium bromi de were stirred in methanol at 65
DC, the progress of th e reacti on was monitored by
TLC. After completion of th e reactio n (35m in ), it 'was
wo rk ed up to get aniline. Our experiments in th e
absence of ei th er the ammonium bro mide or tin did
not yield the reduced co mpound. The red ucti on of
different substituted nitroarenes to correspo ndin g aryl
amin es in hi gh yields (85-95 %) indicates that it is a
very general reac ti on. The results of the reacti on are
tabul ated in Table I. Substituents like -Cl , -OCH 3 , COOH are not affected under th e above said reaction
conditions, hence it is co ncluded that the reagent
(S n/NH 4 Br) is selective in its act ion of redu ction .
Metals such as Zn, AI, Sn or Fe and aqueo us ac id,
have been used earli er to red uce nitroarenes. It is usually
presumed that, ni troso and hydrox ylamine are the
intermed iates through which the reaction takes place.
The metal ass isted reactions are ge nerally expected to
proceed by a single electron transfer from metal to the
substrate. From the above studi es, it is clear that,
reaction of nitroa renes wi th Sn in the presence of NH4Br
may also proceed by the transfer of electrons from the
metal to the nitro group to give anilines.
In conclusion, we have developed a mil d, conven ient
and an additional method for the preparati on of aryl
amines from nitroarenes using readily avail ab le,
abundant tin metal in presence of ammonium bromide in
high yields under neutra l conditions.
6
methanol
o
R
R
R =-H, -OH , -NH2 ,-CI ,-CH 3 ,-OCH 3 ,-COO H
Scheme I
NOTES
2465
Table I-Reduction of substituted nitroarenes to corresponding anilines by Sn/NH 4 Br in methanol
Entry
Substrate
Product"
N-H (str)
v 'cm"
6'
is'
Reaction
time (min)
Isolated
Yield (%)
mp/bp
°C
3359
35
95
I84-86 b
3446
40
90
69-71
3364
40
85
140-41
3325
35
88
223-25 b
3369
40
92
188-90
3381
20
86
185-88
3384
20
85
143-45
3376
40
90
9
3348
35
90
203-05 b
10
3338
30
92
44-46
2
Cl
Cl
¢'
3
4
5
~'
OH
OH
¢'
6
COOH
N0 2
NH2
A;COOH
7
V
N0 2
~
1'<:::
8
COOH
A;COOH
V
CH3
.&
" Characterized by IR spectral analysis.
b Boiling point at 690 torr.
Experimental Section
All nitro compounds and tin metal were of
commercial grade; all the solvents used were distilled
and dried before use. Reactions were monitored by TLC
by comparison with authentic samples. Yields refer to
the isolated products after purification by silica gel
chromatography. Melting points and boiling points were
taken in open capillaries using paraffin bath and are
uncorrected 15. The IR spectra of the products were
recorded on NICOLET 4000 Fr-IR Spectrophotometer.
2466
INDIA N J. CHEM., SEC B, NOVEMBER 2004
General procedure. A mixture of nitrobenze ne
( 1.23 g, 10 mmoles) and ammonium bromide (0.98 g,
20 mmol es) in methanol (10 mL) taken in a twonec ked SO mL round bottom flask was stirred on a
preheated oil-bath maintained at 65°C. The reacti on
was initiated by th e addition of tin (0 .60 mg, foil cut
into small pi eces). The progress of th e reaction was
monitored by TLC [eluant: 5% EtOAc-petro leum
ether (60-80°C)]. After co mpl eti on of th e reacti on (35
min ), th e reaction mixture was filtered throu gh a
celite pad , washed with a little eth er and qu ench ed by
add ing a little water. The orga ni c layer was then
extracted into diethyl eth er (2 x 25 mL) and th e
comb ined ethereal ex tract was dried over anhydrous
K2C0 3 and the solvent dist ill ed off on a rotavap. The
crude product thu s obtained was puri fied by si lica ge l
chromatog raphy using 5% acetone in light petroleum
as an eluant, and identifi ed to be aniline by
co mpari so n of the bp and IR spec trum with that of th e
authentic sa mpl e.
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