Metal-free transfer hydrogenation of nitroarenes in water with vasicine: Revelation of organocatalytic facet of an abundant alkaloid

Article abstract of DOI:10.1021/jo5019415

Vasicine, an abundantly available quinazoline alkaloid from the leaves of Adhatoda vasica, has been successfully employed for metal- and base-free reduction of nitroarenes to the corresponding anilines in water. The method is chemoselective and tolerates a wide range of reducible functional groups, such as ketones, nitriles, esters, halogens, and heterocyclic rings. Dinitroarenes reduced selectively to the corresponding nitroanilines under the present reaction conditions.

Full text of DOI:10.1021/jo5019415

Note
pubs.acs.org/joc
Metal-Free Transfer Hydrogenation of Nitroarenes in Water with
Vasicine: Revelation of Organocatalytic Facet of an Abundant
Alkaloid
Sushila Sharma,†,‡ Manoranjan Kumar,†,‡ Vishal Kumar,‡ and Neeraj Kumar*,†,‡
†Academy of Scientific and Innovative Research, CSIR-Institute of Himalayan Bioresource Technology, Palampur, Himachal Pradesh
176 061, India
‡Natural Plant Products Division, CSIR-Institute of Himalayan Bioresource Technology, Palampur, Himachal Pradesh 176 061, India
S
* Supporting Information
ABSTRACT: Vasicine, an abundantly available quinazoline alkaloid
from the leaves of Adhatoda vasica, has been successfully employed for
metal- and base-free reduction of nitroarenes to the corresponding
anilines in water. The method is chemoselective and tolerates a wide
range of reducible functional groups, such as ketones, nitriles, esters,
halogens, and heterocyclic rings. Dinitroarenes reduced selectively to
the corresponding nitroanilines under the present reaction conditions.
Scheme 1. Strategies for Hydrogenation of Nitroarenes
lant-derived natural products are attractive alternatives to
Pmetal-based catalysts in terms of sustainability, stability,
selectivity, and economy, therefore emerged as promising
organocatalysts for various organic transformations.1 In this
regard, vasicine, a quinazoline alkaloid abundantly available in
Adhatoda vasica leaves, has been developed as an efficient and
renewable organocatalyst for C−C bond formation2 and is still
in its infancy. Vasicine is a privileged structure having both
Lewis basic and acidic sites and hence is able to activate both a
nucleophile and an electrophile, so it has great potential to be
explored as an organocatalyst for various organic trans-
formations.
Metal-free approaches for chemoselective hydrogenation of
unsaturated compounds has emerged as an environmentally
benign and cost-effective alternative to hydrogenation employ-
ing H2 or other reducing agents based on transition-metal
catalysts.3 Reduction of nitroarenes to anilines is an important
organic transformation as anilines are important precursors for
bioactive natural products, pharmaceuticals, agrochemicals,
dyes, and photographic materials.4 Different strategies for
reduction of nitroarenes encompass catalytic hydrogenation,
electrocatalytic hydrogenation, and hydrogenation with stoi-
chiometric amounts of reducing agents in the presence of
various metal-based catalysts (Scheme 1a).5−11 Despite the
advantages of high yield and high chemo- and regioselectivity,
the use of high pressure equipment, flammable hydrogen gas,
hazardous reagents, relatively expensive, and the presence of
inherent toxicity with these metal complexes demand an
alternative approach.
Concerning metal-free hydrogenation of nitroarenes, very
few reports, including 9,10-dihydroanthracene,12 1,4-dihydro-
pyridines,13 fullerenes/H2,14 reduced graphene oxide/hydrazine
hydrate,15 mesoporous carbon/hydrazine hydrate,16 and (2-
pyridyl)phenyl methanol,17 are available (Scheme 1b);
however, the chemoselective reduction of a nitro moiety in
the presence of other reducible functional groups is a
challenging task.
Furthermore, approaches avoiding the use of organic solvents
in the sense of toxicity and cost have attracted much attention.
Water, which is also known as a universal solvent, is an
economic, nontoxic, and environmentally benign alternative to
Received: August 21, 2014
© XXXX American Chemical Society
A
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organic solvents.18 Therefore, it is requisite to develop selective
and metal-free approaches utilizing environmentally benign
reagents and solvents. Recently, our group has reported a
transition-metal-free approach for reduction of nitroarenes in
DMSO:H2O employing glucose as a hydrogen source and
KOH as base.19 Use of glucose as reducing agent for transition-
metal-free reduction was inspiring, so carrying out the reaction
in water without employing any base could make the present
method highly attractive.
Herein, we report a metal-free approach for the reduction of
nitroarenes to the corresponding anilines in water employing
vasicine as an organocatalyst and reducing agent (Scheme 1c).
Vasicine is mainly isolated from Adhatoda vasica leaves (present
up to 1.3%),20 a renewable source, which is a prime
requirement of sustainability. Moreover, easy synthetic
protocols for the synthesis of vasicine are also available.21 To
the best of our knowledge, this is the first report on metal-free
transfer hydrogenation of nitroarenes with vasicine in water.
In our previous report on vasicine-catalyzed direct arylation
of unactivated arenes with aryl halides,2a we serendipitously
observed the reduction of 1-bromo-4-nitrobenzene to the
corresponding aniline in 94% yield. Encouraged by this result,
we planned to explore the potential of vasicine and related
alkaloids for reduction of nitroarenes. To optimize the reaction
conditions, the reduction of 1,3-dinitrobenzene was carried out
in the presence of vasicine and related alkaloids (Figure 1,
(L1−L5)) in different solvents under varying temperature
conditions (Table 1).
Table 1. Optimization of Reaction Conditions for Reduction
of 1,3-Dinitrobenzene
a
b
entry
reducing agent
solvent
H2O
conv./yield (%)
1
2
L1
L1
L1
L1
L1
L1
>99/95
>99/92
>99/94
>99/90
>99/50
50/30
nr
ethylene glycol
PEG-400
DMF
3
4
5
toluene
DMSO
H2O
6
7
8
L2
H2O
15/13
50/43
76/70
13/10
68/62
>99/95
9
L3
H2O
10
11
12
13
14
15
L4
H2O
L5
H2O
L1 (1 equiv)
H2O
L1 (2 equiv)
H2O
c
L1
L1
H2O
50/45
d
H2O
85/81
a
Reaction conditions: 1,3-dinitrobenzene (0.5 mmol), reducing agent
b
(0.75 mmol), H2O (4 mL) at 120 °C for 24 h. Isolated yield.
c
d
Reaction carried out at 80 °C. Reaction carried out for 12 h.
Next, the scope of the method was evaluated for reduction of
various substituted nitroarenes. Generally, nitroarenes sub-
stituted with electron-withdrawing substituents lead to
accumulation of N-hydroxylamine and azo or azoxy inter-
mediates;22 however, under the present reaction conditions,
reduction of nitroarenes bearing electron-withdrawing sub-
stituents proceeded smoothly and the desired products were
obtained in good to excellent yield (Table 2) without the
accumulation of intermediates.
Nitroarenes substituted with easily reducible functional
groups such as ketones and nitriles selectively reduced to the
corresponding anilines with excellent yield (Table 2, 2a−2b
and 2c−2d). Nitroarenes containing other susceptible func-
tional groups, such as sulphonamide, esters, acid, and amide,
were reduced selectively in good to excellent yields (Table 2,
2e−2j). The nitroarene containing the N-phenyl moiety
provided the corresponding aniline in 93% yield (Table 2,
2k). In the case of dinitroarenes, only one nitro group reduced
selectively, leaving the other nitro group unaffected (Table 2,
2l−2n). Many metal-based approaches failed to show such high
chemoselectivity.23 4-Nitrochalcone reduced selectively to 4-
aminochalcone with 81% yield without affecting the carbonyl
and conjugated CC bond (Table 2, 2o). In the case of 1-
bromo-4-nitrobenzene, though poor reduction was observed in
water, a moderate yield corresponding to the desired product
was obtained in toluene (Table 2, 2p). 1-Nitronaphthalene and
3-nitrobiphenyl reduced selectively, providing products in 90%
yield (Table 2, 2q and 2r).
In the case of nitrobenzene and its derivatives having
electron-donating groups such as p-methoxy and p-methyl
(Table 2, 2s−2u), poor yields of the products were observed.
Reduction of polysubstituted nitroarenes having labile
functionalities such as chloro and ester groups also proceeded
smoothly, providing the corresponding aniline in good yield. 2-
Chloro-5-nitroaniline provided the corresponding aniline with
85% yield without dehalogenation (Table 2, 2v). In the case of
Figure 1. Structures of vasicine and related alkaloids.
Among different alkaloids, vasicine (L1) was found to be the
most active in water at 120 °C and gave the product 3-
nitroaniline chemoselectively in 95% yield (Table 1, entry 1).
Although comparable yields were obtained with other
solvents, i.e., ethylene glycol, PEG-400, and DMF (Table 1,
entries 2−4), water was chosen as the solvent of choice,
considering the cost, availability, and environment. In the case
of toluene, although full conversion of the starting substrate was
observed, only 50% of the desired product was obtained along
with azo and azoxy derivatives as side products, while, in
DMSO, a poor yield of product was observed (Table 1, entries
5 and 6). As expected, no reaction occurred in the absence of
vasicine (Table 1, entry 7). Vasicinone and other alkaloids
provided the desired product in lower yield under the same
reaction conditions (Table 1, entries 8−11). While 1 equiv of
vasicine gave lower yield, no significant change in the yield of
product was observed using 2 equiv of vasicine (Table 1, entries
12 and 13). A decrease in reaction time and temperature gave a
lower yield of the product (Table 1, entries 14 and 15).
B
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a
hydrogen evolved via degradation of the ring C of vasicine are
responsible for reduction of nitroarenes (Scheme 2). This fact
Table 2. Reduction of Nitroarenes to Anilines
Scheme 2. Degradation of Vasicine
is supported by the observation that a poor yield of the product
was observed when reduction of 1,3-dinitrobenzene was carried
out in the presence of vasicinone (Table 1, entry 8).
Further, to investigate the role of vasicine, the model reaction
was monitored for different time intervals using a UV−visible
spectrophotometer (Figure 2).
a
Reaction conditions: nitroarene (0.5 mmol), vasicine (0.75 mmol),
b
H2O (4 mL), 120 °C, 24 h. Isolated yield. Reaction carried out in
toluene (4 mL). Reaction carried out in the presence of K2CO3 (0.5
mmol), vasicine (0.5 mmol). Mixture of corresponding azo (50%)
and azoxy (35%) products was observed. Mixture of corresponding
c
d
Figure 2. UV−vis absorption spectral change in the course of reaction
time of the reaction mixture containing 1,3-dinitrobenzene and
vasicine in water: (a) vasicine; (b) vasicinone; (c) reaction after 15
min; (d) reaction after 30 min; (e) reaction after 1 h; (f) reaction after
2 h; (g) reaction after 4 h; (h) reaction after 24 h.
e
azo (46%) and azoxy (43%) products was observed.
ethyl-3,5-dinitrobenzoate, selective reduction was observed
with 65% yield (Table 2, 2w). Different substituted heterocyclic
nitroarenes were reduced to the corresponding anilines in
excellent yield without affecting the heterocyclic ring (Table 2,
2x−2aa).
A blue shift in λmax of vasicine was observed from 286 to 249
nm during 30 min of reaction, which indicated some kind of
interaction between vasicine and nitroarene. However, the band
at λmax 249 nm further returned to 282 nm after 1 h of reaction.
At the end of reaction (after 24 h), the λmax was observed at 275
nm, which further confirmed the formation of vasicinone.
To understand the importance of basicity and rigidity of
vasicine (L1) in the reduction of nitroarenes, reduction of 1,3-
dinitrobenzene was carried out with different derivatives of
vasicine (Table 3). L6 was found to be less effective (65%
Further, to investigate the reaction mechanism and role of
vasicine, different experiments have been carried out. To
determine whether the nitro group is reduced via transfer
hydrogenation or by molecular hydrogen generated by
dehydrogenation of vasicine, reduction of 1,3-dinitrobenzene
was carried out under a H2 atmosphere in the presence of a
catalytic amount of vasicine. In this experiment, no reaction
occurred, which demonstrates that the nitro moiety is not
reduced by molecular hydrogen. Furthermore, to rule out the
possibility of a radical mechanism, as proposed by Coellen and
Reuchardt,12 the model reaction was carried out in the presence
of TEMPO, a radical scavanger, and no suppression in the yield
of product was observed. Therefore, it concludes that this is a
case of transfer hydrogenation in which vasicine is working
both as a hydrogen source as well as an organocatalyst.
Further, to determine which hydrogen from vasicine was
transferred to nitroarene, the byproducts of vasicine from the
model reaction were analyzed.
Table 3. Evaluation of Vasicine Derivatives for the
ab
,
Reduction of Nitroarenes
a
Vasicinone and quinazoline were observed as major by-
products, which shows that benzylic hydrogen (C9) and
Reaction conditions: 1,3-dinitrobenzene (0.5 mmol), reducing agent
b
(0.75 mmol), H2O (4 mL), 120 °C, 24 h. Isolated yield
C
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respectively. The identification and characterization of compounds
yield) when compared with vasicine, while reduction was not
observed with L7. Thus, reaction with ligands L6 and L7 clearly
demonstrated the requirement for basicity and rigidity, which
decreases from vasicine > L6 > L7. The importance of the
hydroxyl group was proved by the reaction of 1,3-
dinitrobenzene with L8, which showed no reactivity.
1
were carried out using H NMR and 13C NMR.
Vasicine (L1).20c L1 was obtained by following the general
procedure described above and purified by column chromatography
(ethyl acetate:MeOH (9:1)) as a white amorphous solid. Yield 12.5 g,
1
0.25%. mp 196−197 °C; H NMR (600 MHz, CDCl3): δ 7.13−6.83
(m, 4H), 4.75−4.73 (m, 1H), 4.55−4.50 (m, 2H), 3.35−3.19 (m, 2H),
2.37−2.36 (m, 1H) 2.12−2.09 (m, 1H); 13C NMR (150 MHz,
MeOD): δ 163.9, 142.4, 128.4, 125.8, 124.1, 123.7, 119.0, 70.2, 48.1,
47.1, 28.9. MS (ESI-TOF) m/z: [M + H]+ for C11H13N2O 189.26.
Vasicinone (L2).20c L2 was obtained by following the general
procedure described above and purified by column chromatography
(100% ethyl acetate) as a white amorphous solid. Yield 1 g. mp 198−
199 °C; 1H NMR (600 MHz, MeOD): δ 8.32−8.29 (m, 1H), 7.75 (m,
1H), 7.53−7.49 (m, 2H) 5.31−5.26 (m, 1H), 4.40−4.35 (m, 1H),
4.12−4.02 (m, 1H), 2.70−2.66 (m, 1H), 2.37−2.33 (m, 1H); 13C
NMR (150 MHz, MeOD): δ 160.6, 130.1, 148.6, 134.4, 126.9, 126.7,
126.6, 121.0, 71.9, 43.4, 29.3. MS (ESI-TOF) m/z: [M + H]+ for
C11H11N2O2 203.09.
Further, to investigate the reduction pathway of nitroarenes,
the reduction of 3-nitroacetophenone was monitored at
different time intervals by mass spectroscopy (ESI-MS). The
formations of N-(3-acetylphenyl)hydroxylamine and 3,3′-
diacetylazoxybenzene as intermediates were observed in mass
spectra, suggesting both types of routes in the reduction of
nitroarenes, i.e., the direct and condensation pathways.
Furthermore, when the reductions of N-(3-acetylphenyl)-
hydroxylamine and 3,3′-diacetylazobenzene were carried out
under the present reaction conditions, no reaction occurred
with 3,3′-diacetylazobenzene, but N-(3-acetylphenyl)-
hydroxylamine provided 3-aminoacetophenone as the major
product. Therefore, these results clearly suggested that the
direct route is predominant in the present reaction conditions
(Scheme 3).
Vasicol (L3).20c L3 was obtained by following the general procedure
above and purified by column chromatography (ethyl acetate) as a
1
yellow viscous oil. Yield 1 g. H NMR (600 MHz, CDCl3): δ 7.08 (t,
1H), 7.00 (d, J = 6.8 Hz, 1H), 6.63 (t, 2H) 4.42−4.27 (m, 4H), 3.27−
3.24 (m, 1H), 3.16−3.13 (m, 1H), 2.34−2.33 (m, 1H) 1.90−1.87 (m,
1H); 13C NMR (150 MHz, CDCl3): δ 175.3, 145.7, 131.3, 129.5,
118.6, 117.3, 115.7, 69.9, 44.7, 43.1, 27.6, 142.4, 128.4, 125.8, 124.1,
123.7, 119.0, 70.2, 48.1, 47.1, 28.9. MS (ESI-TOF) m/z: [M + H]+ for
C11H15N2O2 207.00.
Scheme 3. Possible Pathways for Reduction of Nitroarenes
7-Hydroxyvasicine (L4).24 L4 was obtained by following the general
procedure described above and purified by column chromatography
(ethyl acetate:MeOH (9:1)) as a white amorphous solid. Yield 100
1
mg. H NMR (600 MHz, MeOD): δ 6.96−6.94 (m, 1H), 6.69−6.68
(m, 1H), 6.54 (s, 1H), 4.90 (merged with solvent peak, 1H), 4.72−
4.64 (m, 2H), 3.63−3.60 (m, 1H), 3.53−3.49 (m, 1H) 2.53 (m, 1H),
2.06−2.03 (m, 1H); 13C NMR (150 MHz, MeOD): δ 161.0, 156.1,
124.2, 120.0, 118.8, 115.2, 112.8, 70.8, 49.4, 46.2, 29.2. MS (ESI-TOF)
m/z: [M + H]+ for C11H13N2O2 205.22.
In conclusion, the dual role of vasicine has been explored, i.e., a
hydrogen source as well as an organocatalyst, for efficient
chemoselective reduction of nitroarenes to the corresponding
anilines under metal-free conditions. In the presence of
vasicine, reduction of different substituted nitroarenes to the
corresponding anilines occurred in good to excellent yield.
Reactions under metal- and base-free conditions and water as a
solvent are a few advantages of this method.
3-Hydroxyanisotine (L5).20c L5 was obtained by following the
general procedure described above and purified by column
chromatography (hexane:ethyl acetate (1:1)) as an off-white
1
amorphous solid. H NMR (600 MHz, CDCl3): δ 8.32 (d, J = 8.1
Hz, 1H), 7.98 (d, J = 1.9 Hz, 1H), 7.75−7.71 (m, 3H), 7.50−7.48 (m,
1H), 7.39 (dd, 1H), 6.62 (d, J = 8.8 Hz, 1H), 4.33−4.29 (m, 4H),
3.27−3.24 (m, 1H), 4.01−3.97 (m, 1H), 3.78 (s, 1H), 2.88 (d, J = 7.8
Hz, 3H), 2.65−2.61 (m, 2H); 13C NMR (150 MHz, CDCl3): δ 168.6,
161.0, 160.8, 151.8, 149.0, 134.2, 131.9, 128.5, 127.5, 127.0, 126.8,
126.5, 121.1, 111.1, 109.6, 81.1, 51.5, 42.8, 37.7, 29.5. MS (ESI-TOF)
m/z: [M + H]+ for C17H17N3O4 365.96.
EXPERIMENTAL SECTION
■
General Information. High grade solvents were used for all
reactions. Column chromatography was carried out with 60−120 mesh
silica gel and monitored with TLC on silica gel 60 F254 plates using UV
Method for Synthesis of Vasicine Derivatives. (L6).25 To a
solution of vasicine (200 mg, 1.06 mmol) in ethanol (5 mL) was
added sodium borohydride (40 mg, 1.06 mmol) at room temperature,
and the mixture was stirred for 2 h. Upon completion of reaction (as
monitored by TLC), the reaction was quenched with ethyl acetate and
concentrated in vacuo, and the residue was chromatographed over
basic alumina to afford the desired product (mixture of diaster-
eoisomers). 1H NMR (300 MHz, CDCl3): δ 7.10−6.91(m, 2H),
6.78−6.48 (2H), 4.49 (s, 2H), 4.36−4.32 (1H), 4.14−4.01 (m, 1H)
3.97−3.72 (m, 2H), 2.93−2.89 (m, 1H), 2.47−2.30 (m, 1H), 1.85−
1.74 (m, 1H) 13C NMR (75 MHz, CDCl3): δ 142.8, 142.7, 127.9,
127.7, 120.3, 119.2, 118.7, 118.6, 117.0, 115.1, 74.4, 71.3, 52.5, 49.9,
49.4, 32.7, 32.2. MS (ESI-TOF) m/z: [M + H]+ for C11H15N2O
191.26 [M + H]+.
1
light as visualizing agent. H NMR and 13C NMR experiments were
recorded on 300 and 600 MHz instruments. Chemical shifts are
reported in parts per million (ppm) downfield from an internal
standard. Mass spectra were recorded on electrospray ionization
quadrupole time-of-flight (ESI-QTOF-MS) mass spectrometry.
Isolation of Vasicine and Related Alkaloids. Dried leaves of
Adhatoda vasica (5 kg) were powdered and extracted with
MeOH:H2O (80:20). The extracts were collected and concentrated
using a rotary evaporator. The extract was then treated with 2 N HCl
and stirred at room temperature for a few hours. It was then filtered,
and the filtrate was extracted with ethyl acetate. The aqueous acidic
layer was basified with ammonium hydroxide to pH 9 and then
extracted with CHCl3. The chloroform extract was concentrated using
a rotary evaporator to give the crude alkaloid extract. The crude
alkaloidal extract was then subjected to column chromatography on
basic alumina and eluted with hexane:ethyl acetate:MeOH in
increasing polarity. Fractions eluted with hexane:ethyl acetate (1:1)
afforded 3-hydroxyanisotine (100 mg). Fractions eluted with ethyl
acetate afforded vasicinone (1 g) and vasicol (1 g). Fractions eluted
with ethyl acetate:MeOH (9:1) and ethyl acetate:MeOH (7:3)
provided vasicine (12.5 g) and 7-hydroxyvasicine (100 mg),
(L7).25 To a solution of vasicine (200 mg, 1.06 mmol) in
methanol:water (1:1, 4 mL) was added sodium borohydride (101.7
mg, 3.17 mmol) in small portions at regular intervals at room
temperature, and stirring was continued for 10 h. The reaction was
quenched with ethyl acetate, and the product was extracted with
chloroform. The chloroform layer was evaporated at reduced pressure
on a rotary evaporator, and chromatography over basic alumina and
elution with hexane−ethyl acetate gave a semisolid light yellow
D
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1
compound in 90% (183 mg) yield. H NMR (600 MHz, CDCl3): δ
Ethyl-4-aminobenzoate (2f).7c 2f was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a pale yellow
solid. Yield 54.4 mg, 66%; mp 90−91 °C; 1H NMR (300 MHz,
CD3COCD3): δ 7.76−7.72 (m, 2H), 6.71−6.66 (m, 2H), 5.38 (brs,
2H), 4.28−4.21 (q, 2H), 1.31 (t, 3H); 13C NMR (75 MHz,
CD3COCD3): δ 166.3, 153.4, 131.5, 118.4, 113.3, 59.8, 14.2. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C9H12NO2 166.0868; Found
166.0884.
7.18−7.12 (m, 1H), 7.04 (d, J = 6.3 Hz, 1H), 6.71(t, 2H), 5.20 (brs,
2H), 4.45−4.42 (m, 1H), 3.87 (dd, J = 12.9, 14.2 Hz, 2H), 3.25−3.17
(m, 1H), 3.01 (d, J = 11.1 Hz, 1H), 2.82−2.77 (m, 1H), 2.68−2.59
(m, 1H), 2.32−2.22 (m, 1H), 1.97−1.87 (m, 1H). 13C NMR (75
MHz, CDCl3): δ147.1, 131.7, 130.0, 119.8, 118.5, 116.9, 70.8, 62.0,
57.5, 52.3, 34.7. MS (ESI-TOF) m/z: [M + H]+ for C11H17N2O
192.95.
3-Chlorovasicine (L8).26 To a solution of vasicine (1 mmol) in
dichloromethane was added thionyl chloride (216 μL, 3 mmol) and
pyridine (8 μL, 0.1 mmol) at 0 °C. The mixture was allowed to stir for
5 h at room temperature. The excess of thionyl chloride was removed
in vacuo, and the residue was chromatographed over basic alumina to
Methyl-3-aminobenzoate (2g).7c 2g was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a white solid.
1
Yield 51.5 mg, 67%; mp 48−49 °C; H NMR (600 MHz, CDCl3): δ
1
7.39 (d, J = 7.5 Hz, 1H), 7.33 (s, 1H), 7.17 (t, J = 7.7 Hz, 1H), 6.82 (d,
J = 7.8 Hz, 1H), 3.85 (s, 3H); 13C NMR (150 MHz, CDCl3): δ 167.3,
146.6, 131.0, 129.2, 119.5, 119.4, 115.7, 52.0.
afford the product as a brown solid. H NMR (600 MHz, CDCl3): δ
7.16−7.12 (m, 2H), 7.01(t, 1H), 6.88 (d, J = 7.2 Hz, 1H), 4.75 (d, J =
5.8 Hz, 1H), 4.64 (dd, 2H), 3.58−3.54 (m, 1H), 3.32−3.29 (m, 1H),
2.54−2.48 (m, 1H), 2.29−2.26 (m, 1H) 13C NMR (150 MHz,
CDCl3): δ 160.0, 142.2, 128.6, 125.8, 125.0, 124.9, 119.0, 56.9, 48.8,
47.4, 31.1. MS (ESI-TOF) m/z: [M + H]+ for C11H12ClN 207.30.
Representative Experimental Procedure for Reduction of
Nitro Compounds. A mixture of the nitro compound (0.5 mmol)
and vasicine (0.75 mmol) in H2O (4 mL) was stirred at 120 °C for 24
h. Time was not optimized separately for all substrates. After
completion of reaction, as monitored by TLC, the reaction mixture
was cooled to ambient temperature and extracted with ethyl acetate.
The ethyl acetate layer was dried under reduced pressure using a
rotatory evaporator. The crude was chromatographed over silica gel to
afford the desired product.
Methyl-4-aminobenzoate (2h).7c 2h was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a white solid.
Yield 58.8 mg, 78%; mp 111−113 °C; 1H NMR (300 MHz,
CD3COCD3): δ 7.74 (d, J = 8.7 Hz, 2H), 6.69 (d, J = 8.7 Hz, 2H),
5.42 (brs, 2H), 3.78 (s, 3H); 13C NMR (75 MHz, CD3COCD3): δ
166.8, 153.5, 131.5, 118.0, 113.3, 50.9. HRMS (ESI-TOF) m/z: [M +
H]+ calcd for C8H10NO2 152.0712; Found 152.0741.
4-Aminobenzoic Acid (2i).7c 2i was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (1:1)) as a white solid.
Yield 41 mg, 60%; mp 166−167 °C; 1H NMR (300 MHz, CD3OD): δ
7.75 (d, J = 8.6 Hz, 2H), 6.64 (d, J = 8.6 Hz, 2H); 13C NMR (75 MHz,
CD3OD): δ 171.2, 155.0, 133.2, 119.6, 114.8. HRMS (ESI-TOF) m/z:
[M + H]+ calcd for C7H8NO2 is 138.0555; Found 138.0576.
4-Aminobenzamide (2j).7c 2j was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (1:4)) as a white solid. Yield 62.5 mg, 92%; mp
Characterization of Nitro Reduction Products. All compounds
were identified by spectral comparison with literature data.
3-Aminoacetophenone (2a, CAS Number 99-03-6). 2a was
obtained by following the general procedure described above and
purified by column chromatography (n-hexane:ethyl acetate (17:3)) as
a white solid. Yield 62 mg, 92%; mp 94−95 °C; 1H NMR (600 MHz,
CDCl3): δ 7.31 (d, J = 7.4 Hz, 1H), 7.25−7.21 (m, 2H), 6.86 (d, J =
7.8 Hz, 1H), 3.83 (s, 2H), 2.54 (s, 3H); 13C NMR (150 MHz,
CDCl3): δ 198.5, 146.7, 138.2, 129.4, 119.6, 118.8, 114.0, 26.6. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C8H10NO 136.0762; Found
136.0772.
1
178−179 °C; H NMR (600 MHz, CD3OD): δ 7.63 (d, J = 8.3 Hz,
2H), 6.65 (d, J = 8.3 Hz, 2H); 13C NMR (150 MHz, CD3OD): δ
171.3, 152.1, 129.0, 120.8, 113.2. HRMS (ESI-TOF) m/z: [M + H]+
calcd for C7H9N2O 137.0715; Found 137.0722.
2-Aminodiphenylamine (2k).7c 2k was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (19:1)) as a brown solid.
4-Aminoacetophenone (2b).7c 2b was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a white solid.
1
Yield 85.5 mg, 93%; mp 66−67 °C; H NMR (600 MHz, CDCl3): δ
7.21 (t, 2H), 7.13 (d, J = 7.6 Hz, 1H), 7.02 (t, 1H), 6.83−6.80 (m,
2H), 6.77−6.74 (m, 3H), 5.19 (s, 2H); 13C NMR (150 MHz, CDCl3):
δ 145.3, 141.8, 129.3, 128.6, 125.7, 124.8, 119.3, 119.2, 116.2, 115.2.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C12H13N2 185.1079;
Found 185.1068.
1
Yield 64 mg, 95%; mp 100−101 °C; H NMR (300 MHz, CDCl3): δ
7.83−7.80 (m, 2H), 6.67−6.64 (m, 2H), 4.19 (s, 2H), 2.52 (s, 3H);
13C NMR (75 MHz, CDCl3): δ 196.9, 151.5, 131.2, 128.2, 114.1, 26.5.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C8H10NO 136.0762;
Found 136.0772.
2-Nitroaniline (2l).7c 2l was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (9:1)) as a yellow solid. Yield 64.6 mg, 93%;
3-Aminobenzonitrile (2c).7c 2c was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (9:1)) as a brown solid.
Yield 53 mg, 90%; mp 51−52; °C; 1H NMR (300 MHz,
CD3COCD3): δ 7.26−7.21 (m, 1H), 6.98−6.90 (m, 3H), 5.16 (s,
2H); 13C NMR (75 MHz, CD3COCD3): δ 149.1, 130.4, 120.0, 119.4,
118.9, 116.7, 112.9. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C7H7N2 119.0609; Found 119.0612.
1
mp 73−74 °C; H NMR (600 MHz, CDCl3): δ 8.12−8.10 (m, 1H),
7.37−7.3 (m, 1H), 6.81−6.79 (m, 1H), 6.71−6.69 (m, 1H); 13C NMR
(150 MHz, CDCl3): δ 144.6, 135.65, 135.62, 126.2, 118.7, 116.9.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C6H7N2O2 139.0508;
Found 139.0525.
3-Nitroaniline (2m).7c 2m was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (4:1)) as a yellow solid. Yield 65.5 mg, 95%;
mp 111−112 °C; 1H NMR (300 MHz, CDCl3): δ 7.60−7.57 (m, 1H),
7.51−7.50 (m, 1H), 7.32−7.26 (m, 1H), 6.98−6.94 (m, 1H), 4.02 (s,
2H); 13C NMR (75 MHz, CDCl3): δ 149.6, 147.8, 130.3, 121.0, 113.5,
109.4. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C6H7N2O2
139.0508; Found 139.0552.
4-Aminobenzonitrile (2d).7c 2d was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a brown solid.
Yield 50 mg, 85%; mp 82−83 °C; 1H NMR (300 MHz, CD3COCD3):
δ 7.39 (d, J = 8.4 Hz, 2H), 6.75 (d, J = 8.4 Hz, 2H), 5.61 (s, 2H); 13C
NMR (75 MHz, CD3COCD3): δ 153.0, 133.7, 120.3, 114.2, 98.2.
4-Aminobenzenesulphonamide (2e).7c 2e was obtained by
following the general procedure described above and purified by
column chromatography (n-hexane:ethyl acetate (3:7)) as a yellow
solid. Yield 83 mg, 96%; mp 161−162 °C; 1H NMR (300 MHz,
CD3OD): δ 7.59 (d, J = 8.7 Hz, 2H) 6.70 (d, J = 8.7 Hz, 2H); 13C
NMR (75 MHz, CD3OD): δ 152.5, 130.3, 127.9, 113.4. HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C6H9N2O2S 173.0385; Found
173.0389.
4-Nitroaniline (2n).7c 2n was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (4:1)) as a yellow solid. Yield 65.5 mg, 95%;
mp 147−148 °C; 1H NMR (300 MHz, CDCl3): δ 8.10−8.06 (m, 2H),
6.67−6.62 (m, 2H), 4.43 (s, 2H); 13C NMR (75 MHz, CDCl3): δ
152.9, 126.7, 113.7. HRMS (ESI-TOF) m/z: [M + H]+ calcd for
C6H7N2O2 139.0508; Found 1389.0441.
E
dx.doi.org/10.1021/jo5019415 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
4-Aminochalcone (2o).19 2o was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (9:1)) as a yellow solid. Yield 90.7 mg, 81%;
mp 140−142 °C; 1H NMR (600 MHz, CDCl3): δ 8.00−7.98 (m, 2H),
7.75 (d, J = 15.5 Hz, 1H), 7.55−7.54 (m, 1H), 7.49−7.47 (m, 4H),
7.34 (d, J = 15.5 Hz, 1H), 6.68 (d, J = 7.8 Hz, 2H), 4.03 (s, 2H); 13C
NMR (150 MHz, CDCl3): δ 190.7, 149.1, 145.5, 138.8, 132.3, 130.5,
128.5, 128.3, 125.1, 118.0, 114.8. HRMS (ESI-TOF) m/z: [M + H]+
calcd for C15H14NO 224.1075; Found 224.1089.
(600 MHz, CDCl3): δ 7.62 (s, 1H), 6.86 (s, 1H), 4.00 (brs, 2H), 2.22
(s, 3H); 13C NMR (150 MHz, CDCl3): δ 139.0, 138.9, 134.4, 133.3,
123.2, 17.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C6H8ClN2
143.0376; Found 143.0354.
6-Aminobenzothiazole (2aa).7c 2aa was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (7:3)) as a brown solid.
Yield 45 mg, 50%; mp 105−108 °C; 1H NMR (600 MHz, CDCl3): δ
8.68 (s, 1H), 7.88−7.86 (m, 1H), 7.14 (s, 1H), 6.86−6.84 (m, 1H),
3.46 (brs, 2H); 13C NMR (150 MHz, CDCl3): δ 149.8, 146.7, 144.8,
135.4, 123.9, 115.8, 105.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd
for C7H7N2S 151.0330; Found 151.0314.
4-Bromoaniline (2p).7c 2p was obtained by following the general
procedure described above and purified by column chromatography
(n-hexane:ethyl acetate (9:1)) as a brown solid. Yield 49 mg, 57%; 1H
NMR (600 MHz, CDCl3): δ 7.23 (d, J = 8.4 Hz 2H), 6.55 (d, J = 8.4
Hz 2H), 3.67 (brs, 2H) 13C NMR (150 MHz, CDCl3): δ 145.5, 132.0,
116.7, 110.1. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C6H7BrN
171.9762; Found 171.9741.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra. This material is available
1-Aminonaphthalene (2q).7c 2q was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (19:1)) as a brown solid.
Yield 64 mg, 90%; 1H NMR (600 MHz, CDCl3): δ 7.83 (d, J = 7.2 Hz,
2H), 7.48−7.47 (m, 2H), 7.34−7.31 (m, 2H), 6.79 (d, J = 7.02 Hz,
1H), 3.98 (brs, 2H); 13C NMR (150 MHz, CDCl3): δ 142.1, 134.4,
128.5, 126.3, 125.8, 124.9, 123.7, 120.8, 119.0, 109.7. HRMS (ESI-
TOF) m/z: [M + H]+ calcd for C10H10N 144.0813; Found 144.0834.
3-Aminobiphenyl (2r, CAS Number 2243-47-2). 2r was obtained
by following the general procedure described above and purified by
column chromatography (n-hexane:ethyl acetate (19:1)) as a brown
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: neerajnpp@rediffmail.com, neeraj@ihbt.res.in (N.K.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
The authors are grateful to Dr. P. S. Ahuja, Director of the
CSIR-Institute of Himalayan Bioresource Technology, India,
for encouragement and support. S.S., M.K., and V.K. also thank
UGC for granting research fellowship. Financial support from
CSIR (ORIGIN, CSC-0108) is also acknowledged.
solid. Yield 76 mg, 90%; H NMR (600 MHz, CDCl3): δ 7.57 (d, J =
7.5 Hz 2H), 7.44−7.41 (m, 2H), 7.35−7.32 (m, 1H), 7.26−7.22 (m,
1H), 7.00 (d, J = 7.5 Hz 1H), 6.91 (s, 1H), 6.68 (d, J = 7.3 Hz 1H),
3.61 (brs, 2H); 13C NMR (150 MHz, CDCl3): δ 146.7, 142.4, 141.4,
129.6, 128.6, 127.2, 127.1, 117.7, 114.1, 113.9. HRMS (ESI-TOF) m/
z: [M + H]+ calcd for C12H12N 170.0970; Found 170.0984.
2-Chloro-4-aminoaniline (2v). 2v was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (4:1)) as a brown solid.
REFERENCES
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1
Yield 60.3 mg, 85%; mp 71−73 °C; H NMR (300 MHz, CDCl3): δ
7.00 (d, J = 8.3 Hz 1H), 6.12−6.06 (m, 2H), 3.67 (brs, 4H); 13C NMR
(75 MHz, CDCl3): δ 146.5, 143.8, 130.2, 109.7, 107.1, 102.6. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C6H8ClN2 143.0376; Found
143.0387.
Ethyl-3-amino-5-nitrobenzoate (2w).7c 2w was obtained by
following the general procedure described above and purified by
column chromatography (n-hexane:ethyl acetate (3:1)) as a yellow
solid. Yield 68 mg, 65%; mp 155−156 °C; 1H NMR (600 MHz,
CD3OD): δ 8.19 (m, 1H), 7.64−7.61 (m, 2H), 4.39 (q, J = 7.1 Hz
2H), 1.40 (t, J = 7.1 Hz, 3H); 13C NMR (150 MHz, CDCl3): δ 164.8,
149.2, 147.5, 132.7, 120.9, 113.9, 112.4, 61.7, 14.2. HRMS (ESI-TOF)
m/z: [M + H]+ calcd for C9H11N2O4 211.0719; Found 211.0701.
5-Aminoisoquinoline (2x).7c 2x was obtained by following the
general procedure described above and purified by column
chromatography (n-hexane:ethyl acetate (6:4)) as a brown solid.
1
Yield 64 mg, 90%; mp 128−129 °C; H NMR (600 MHz, CDCl3): δ
9.20 (s, 1H), 8.50 (d, J = 5.8 Hz, 2H), 7.59 (d, J = 5.9 Hz, 1H), 7.42(d,
J = 4.0 Hz, 2H), 6.97 (t, 1H), 4.25 (brs, 2H); 13C NMR (150 MHz,
CDCl3): δ 153.3, 142.4, 141.6, 126.8, 1281, 126.3, 118.3, 114.4, 113.4.
HRMS (ESI-TOF) m/z: [M + H]+ calcd for C9H9N2 145.0766;
Found 145.0798.
2-Amino-5-chloropyridine (2y, CAS Number 1072-98-6). 2y was
obtained by following the general procedure described above and
purified by column chromatography (n-hexane:ethyl acetate (7:3)) as
1
a yellow solid. Yield 59 mg, 92%; mp 115−116 °C; H NMR (600
MHz, CDCl3): δ 7.78 (s, 1H), 7.02−6.91 (m, 2H), 3.89 (brs, 2H); 13
C
NMR (150 MHz, CDCl3): δ 142.0, 139.8, 136.2, 124.8, 124.1. HRMS
(ESI-TOF) m/z: [M + H]+ calcd for C5H6ClN2 129.0220; Found
129.0250.
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1). 2z was obtained by following the general procedure described
above and purified by column chromatography (n-hexane:ethyl acetate
1
(7:3)) as a yellow solid. Yield 64 mg, 90%; mp 84−85 °C; H NMR
F
dx.doi.org/10.1021/jo5019415 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
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G
dx.doi.org/10.1021/jo5019415 | J. Org. Chem. XXXX, XXX, XXX−XXX