Mild and efficient PtO2-catalyzed one-pot reductive mono-N-alkylation of nitroarenes

Article abstract of DOI:10.1080/00397911.2011.561397

(Chemical Equation Presented) A mild and efficient one-pot reductive monoalkylation of nitroarenes has been described using aldehydes as alkylating agents, molecular hydrogen as a reducing agent, and PtO₂ as a catalyst in methanol. This methodology is found to be applicable for both aliphatic and aromatic aldehydes and for a wide variety of nitroarenes. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Commuications to view the free supplemental file. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2011.561397

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Mild and Efficient PtO2-Catalyzed One-
Pot Reductive Mono-N-alkylation of
Nitroarenes
Bojja Sreedhar a & Vikas S. Rawat a
a Inorganic and Physical Chemistry Division, Indian Institute of
Chemical Technology, Hyderabad, India
Available online: 02 Jan 2012
To cite this article: Bojja Sreedhar & Vikas S. Rawat (2012): Mild and Efficient PtO2-Catalyzed One-
Pot Reductive Mono-N-alkylation of Nitroarenes, Synthetic Communications: An International Journal
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Synthetic Communications1, 42: 2490–2502, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.561397
MILD AND EFFICIENT PtO2-CATALYZED ONE-POT
REDUCTIVE MONO-N-ALKYLATION OF NITROARENES
Bojja Sreedhar and Vikas S. Rawat
Inorganic and Physical Chemistry Division, Indian Institute of Chemical
Technology, Hyderabad, India
GRAPHICAL ABSTRACT
Abstract A mild and efficient one-pot reductive monoalkylation of nitroarenes has been
described using aldehydes as alkylating agents, molecular hydrogen as a reducing agent,
and PtO2 as a catalyst in methanol. This methodology is found to be applicable for both
aliphatic and aromatic aldehydes and for a wide variety of nitroarenes.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Synthetic Commuications1 to view the free supplemental file.
Keywords Aldehyde; carbonyl compound; nitroarenes; platinum dioxide; protecting
group; reductive amination
INTRODUCTION
The amines and their derivatives are used as versatile building blocks for various
biologically active compounds, such as pharmaceuticals[1] and agrochemicals.[2]
These amines can be synthesized by various methods of which the most commonly
used include direct base-promoted N-alkylation,[3] direct reductive amination,[4]
and hydroamination of olefins.[5] The reductive amination of the carbonyl com-
pounds is among the most useful and important tool for the synthesis of a wide
variety of substituted amines. The reductive amination of carbonyl compounds is
usually carried out with hydride-based reagents such as NaBH4,[6] NaBH3CN,[7]
NaBH(OAc)3,[8]
pyridine–BH3,[9]
catalytic
hydrogenation,[10]
Zn-AcOH,[11]
Received October 4, 2010.
Address correspondence to Bojja Sreedhar, Inorganic and Physical Chemistry Division, Indian
Institute of Chemical Technology, Hyderabad 500007, India. E-mail: sreedharb@iict.res.in
2490

REDUCTIVE N-ALKYLATION OF NITROARENE
2491
Scheme 1. PtO2-catalyzed one-pot reductive mono-N-alkylation of nitrobenzene with benzaldehyde.
Et3SiH-CF3CO2H,[12] Bu3SnHDMF,[13] InCl3-Et3SiH,[14] triazole-derived iridium(I)
carbine complexes,[15] [Ir(cod)2]BF4,[16] Rh(I) catalysts, [17] and various modified borohy-
dride derivatives.[8,9,18] However, most of these reagents have certain limitations such as
need for a stoichiometric amount of reductant, poor functional group tolerance, side
reactions, overalkylation, toxic waste, thermal instability, handling risks, and extreme
reaction conditions.[19] Moreover, it is necessary to use an excess amount of amines to
limit or prevent the competitive reduction of carbonyl group. Consequently, there is a
requirement for a reducing agent that has the property to reduce imine over aldehyde.
Recently, attempts have been made to carry out reductive amination of car-
bonyl compounds directly from nitroarenes for the preparation of N-alkylated aryl
amines. Bae et al. have reported the reduction of nitroaromatics using decaborane–
Pd=C system followed by reductive amination with carbonyls using decaborane
and acetic acid as additives to prevent reductive etherification.[20] Another report
by Byun et al. describes the reductive N-alkylation of anilines and nitroarenes with
only aliphatic aldehydes using HCOO-NH4þ=Pd=C catalytic system.[21] More
recently, Sydnes et al. investigated monoalkylation of nitro aryls using hydrogen over
Pd=C (10%) in ethanol,[22] and we have reported the direct one-pot reductive amina-
tion of aldehydes with nitroarenes using gum-acacia-stabilized palladium nanoparti-
cles.[23] In continuation of our work on the synthesis of substituted amines,[24] herein
we report a direct reductive mono-N-alkylation of nitroarenes through reductive ami-
nation of aldehydes under atmospheric pressure of hydrogen using PtO2 as a catalyst
(Scheme 1).
RESULTS AND DISCUSSION
An initial evaluation of the proposed reductive mono-N-alkylation reaction
was performed with benzaldehyde and nitrobenzene under an atmospheric pressure
of hydrogen at room temperature, and the results are presented in Table 1. Among
the different solvents screened, it was observed that the desired product was obtained
in good yield when the reaction was carried out in hexane (72%) (Table 1, entry 1),
which can be attributed to the property of greater hydrogen solubility in hexane
when compared to other solvents. However, because of the poor solubility of the
substrates in hexane, the choice of this solvent was limited.[25] On the other hand,
the yields of the desired product in the cases of ethyl acetate (43%), dichloromethane
(64%), and methanol (50%) were moderate (Table 1, entries 2, 5, and 7 respectively).
However, no product was obtained in polyethylene glycol (PEG-400), and only
10% of the product was obtained in water (Table 1, entries 3 and 4 respectively). As
yields obtained by this protocol were very poor, further optimization of the reaction
conditions was required to obtain the desired products, N-alkylated aryl amines, in
good yields. When the reaction was conducted in a stepwise manner, instead of

2492
B. SREEDHAR AND V. S. RAWAT
Table 1. Optimization of reaction conditions in different solventsa
Entry
Solvent
Time
Yield (%)
1
2
3
4
5
6
7
8
Hexane
12 h
12 h
72
43
10
0
Ethylacetate
Water
PEG-400
12 h
12 h
Dichloromethane
Dichloromethane
Methanol
12 h
1 hþ 1 hþ 3 hb
64
82c
50
92c
12 h
1 hþ 1 hþ3 hb
Methanol
aReaction conditions: aldehyde (1.5 mmol), nitrobenzene (1 mmol),
solvent (5 ml), and PtO2 (8 mol%) at room temperature.
bThe times given correspond to the following reactions: reduction of
nitroarenes þ imine formation þ reduction of imine.
cReaction conditions: aldehyde (1.0 mmol), nitrobenzene (1.2 mmol),
solvent (2 ml), and PtO2 (3 mol%) at room temperature.
adding all the reactants and the catalyst at once, we were able to obtain N-alkylated
aryl amines in excellent yields, keeping the reaction still in one pot.
The reaction steps started with the initial prereduction of the catalyst with
hydrogen followed by the reduction of nitrobenzene. In the next step, benzaldehyde
was added once the aryl amine was formed, and the reaction mixture was then stirred
without hydrogen at room temperature until the imine formation reached com-
pletion. Finally, the resulting imine formed was reduced by stirring the reaction mix-
ture under an atmosphere of hydrogen for the required period of time, thus resulting
in the formation of the N-alkylated aryl amine in much greater amounts (Table 1,
entries 6 and 8), and it was observed that the yield obtained by using methanol
(Table 1, entry 8) as solvent was greater (92%) when compared to dichloromethane
(Table 1, entry 6) (82%). Therefore, for all the reactions, we used methanol as the ideal
solvent.
A plausible reaction mechanism is proposed as shown in Scheme 2. The reac-
tion involves the initial formation of platinum (0) in situ by the reduction of PtO2 by
molecular hydrogen, which acts as the active catalyst for the reduction of nitroarenes
to amines (I). The amine (I) formed the reaction with aldehydes gives the intermedi-
ate carbinol amine, which dehydrates under this reaction condition and which is
usually weakly acidic to neutral, forming an imine. The imine is protonated to form
an iminium ion (II). Subsequent reduction of this iminium ion (II) produces the
alkylated amine (III) product.
Under the optimized reaction conditions, various aromatic and aliphatic alde-
hydes were used as alkyl sources and reacted with nitrobenzene at ambient pressure
of molecular hydrogen. Results are presented in Table 2. As can be seen from
Table 2, aromatic as well as aliphatic aldehydes reacted smoothly to give the corre-
sponding products in excellent yields. On the other hand, aliphatic aldehydes, regard-
less of whether they are linear, branched, or cyclic, gave the products in good yield
(Table 2, entries 5–8) compared to aromatic aldehydes (Table 2, entries 1–4). There
are two plausible reasons for this: One is that the aliphatic aldehydes are more elec-
trophilic compared to the aromatic aldehydes, so the imine formation is favorable

REDUCTIVE N-ALKYLATION OF NITROARENE
2493
Scheme 2. Proposed mechanism for PtO2-catalyzed one-pot reductive mono-N-alkylation of nitroarene
with aldehyde.
with aliphatic aldehydes.[26] Second, the imine intermediates formed from aliphatic
aldehydes are unstable and readily get converted to the product by hydrogenation,
whereas in the case of aromatic aldehydes, the imine intermediate formed is stable
and undergoes a backward reaction to form aldehyde and amine, resulting in a com-
petition between the hydrogenation and backward reaction that leads to a decrease in
the overall yield.
Heteroaromatic aldehyde, thiophene-2-carboxaldehyde (Table 2, entry 4) also
underwent the reductive amination with nitrobenzene smoothly and gave the pro-
duct, phenyl-thiophen-2-ylmethyl-amine, in moderate yield. On the other hand, in
the case of heteroaromatic aldehydes containing N or O in the aromatic ring, these
desired products were not formed, as these compounds have a tendency to get reduced
to cyclic or aliphatic linear analogs.[27]
To show the generality and scope of this protocol, a variety of nitro aromatics
were subjected to the reductive N-alkylation with benzaldehyde, yielding the corre-
sponding secondary amines, and the results are summarized in Table 3. Unsubstituted
as well as substituted nitroarenes gave the desired products in excellent yields. N-
Alkylation of nitroarenes bearing electron-donating groups (Table 3, entries 1, 2, 5,
6, 8, and 9) at the aromatic ring proceeded smoothly and selectively to afford the
corresponding mono-N-alkylated aryl amine products in good yield. However,
nitroarenes having electron-withdrawing groups at the aromatic ring (Table 3, entries
3, 4, and 7) gave slightly lesser yields of the desired product. This may be attributed to
the presence of the electron-withdrawing group in the amines formed, which has low
nucleophilicity and leads to a sluggish rate of imine formation. This is also indicated

2494
B. SREEDHAR AND V. S. RAWAT
Table 2. PtO2-catalyzed reductive monobenzylation of nitrobenzene with different aldehydes under
molecular hydrogena
Entry
1
Aldehyde
Product
Timeb
Yield (%)
92
1 h þ 1 h þ 3 h
2
3
4
5
1 h þ 2 h þ 4 h
1 h þ2 h þ 4 h
1 h þ 3 h þ 5 h
1 h þ 3 h þ 6 h
95
94
95
52
6
7
1 h þ 1 h þ 3 h
1 h þ 1 h þ 3 h
98
99
8
9
1 h þ 1 h þ 3 h
1 h þ 1 h þ 3 h
97
97
aReaction conditions: aldehyde (1.0 mmol), nitrobenzene (1.2 mmol), methanol (2 ml), and PtO2
(3 mol%) at room temperature.
bThe times given correspond to the following reactions: reduction of nitroarenes þ imine formation þ
reduction of imine.
by the amount of unreacted primary amine present in the reaction mixture even after
running the reaction for a longer period of time without much improvement in the
yield of the desired product. In the case of 4-chloro and 4-bromo nitroarenes
(Table 3, entries 3 and 4), the desired products were formed in good yield without
the formation of any dehaloginated derivatives.
The selectivity of this protocol was observed when 1-(benzyloxy)-4-nitroben-
zene (Table 3, entry 9) and tert-butyl dimethyl (4-nitro benzyloxy)silane (Table 3,
entry 8) were subjected to reductive N-alkylation. Often the catalytic hydrogenations
lead to hydrogenolysis of C-O bond of benzyl ethers or benzyl alcohols and Si-O
bond in silyl alcohols.[28] However, the nitroarenes containing a benzyloxy group
(Table 3, entry 9) a siloxy group (Table 3, entry 8) reacted smoothly and selectively

REDUCTIVE N-ALKYLATION OF NITROARENE
2495
Table 3. PtO2-catalyzed reductive benzylation of different nitro arenes by benzaldehyde under molecular
hydrogen.a
Entry
1
Nitrobenzene
Product
Timeb
Yield (%)
94
1hþ1hþ3 h
2
3
4
1hþ1hþ3 h
2hþ3hþ5 h
2hþ3hþ5 h
97
86
82
5
6
7
8
9
1hþ1hþ3 h
1hþ1hþ3 h
2hþ2hþ4 h
1hþ1hþ3 h
1hþ1hþ3 h
94
98
87
99c
95
aReaction conditions: aldehyde (1.0 mmol), nitrobenzene (1.2 mmol), methanol (2 ml), and PtO2 (3 mol
%) at room temperature.
bThe times given correspond to the following reactions: reduction of nitroarenes þ imine forma-
tion þ reduction of imine.
c
R
=
tertiary butyl dimethyl silyl (TBDS).
to afford the corresponding N-alkylated arylamines without the cleavage of benzyl
and silyl groups, respectively, in excellent yields.
CONCLUSION
In conclusion, we have described a simple, mild, and efficient method for the
synthesis of mono-N-alkylated aryl amines via reductive amination using platinum
oxide as a catalyst under ambient pressure of hydrogen at room temperature without
using any additive. This protocol can be used to generate a diverse range of substi-
tuted amines in good yields.

2496
B. SREEDHAR AND V. S. RAWAT
EXPERIMENTAL
All chemicals were purchased from Sigma-Aldrich or S.D. Fine Chemicals Pvt.
Ltd., India, and used as received. ACME silica gel (100–200 mesh) was used for col-
umn chromatography, and reactions were monitored by thin-layer chromatography
(TLC) carried out on 0.25-mm silica-gel-coated glass plates 60 F254 using ultraviolet
(UV) light as visualizing agent and iodine or ninhydrin solution followed by heating
as developing agent. All the other chemicals and solvents were obtained from com-
mercial sources and purified using standard methods. The infrared (IR) spectra of all
compounds were recorded on a Perkin-Elmer Spectrum GX FTIR spectrometer
using KBr pellets. The IR values are reported in reciprocal centimeters (cmꢀ1).
1
The H and 13C NMR spectra were recorded on a Varian Gemini 200 MHz or a
Bruker Avance 300-MHz spectrometer. Chemical shifts (d) are reported in parts
per million (ppm), using tetramethyl silane (TMS; d ¼ 0 ppm) as an internal standard
in CDCl3 for 1H NMR. Electrospray ionization (ESI) mass spectra were recorded on
a Finnigan LCQ Advantagemax spectrometer. EI mass spectra were recorded on
GC-MS QP2010 plus (Shimadzu).
Typical Experimental Procedure for the Reductive Amination of
Benzaldehyde with Nitrobenzene to Give Benzyl-phenyl-amine
(2a) as a Product
The catalyst PtO2 (7 mg, 3 mol%) was suspended in methanol (2 mol), and a
hydrogen balloon was fitted to the flask and stirred under hydrogen atmosphere
for 15 min (brown suspension turns to black). Then nitrobenzene (1.2 mmol) was
added to it, and stirring continued under a hydrogen atmosphere at room tempera-
ture for 1 h. Before the addition of benzaldehyde (1.0 mmol) to the reaction mixture,
the hydrogen supply was stopped for 5 min. Then benzaldehyde (1.0 mmol) was
added, and the resultant solution was stirred without a hydrogen atmosphere at
room temperature for 1 h. After 1 h, hydrogen supply was started, and reaction
mixture continued to stir at room temperature for 3 h. The progress of the reaction
was monitored by TLC. The reaction mixture was filtered, and the solvent was
removed under reduced pressure to give the crude product. It was purified by column
chromatography on silica gel using hexane–ethyl acetate mixture as eluent. The reac-
tion procedure for rest of substituent remains the same, only there was a change in
different time intervals for completion of different stages of reaction as is mentioned
in Tables 2 and 3.
Spectral Data of Compounds
Characterization data for previously unknown compounds were determined,
and compounds described in the literature were characterized by comparing their
1H NMR and gas chromatography–mass spectra (GC-MS) to the previously
reported data (reference provided along with data).
Benzyl-phenyl-amine[21,23] (2a). Following the general experimental
procedure, product 2a was obtained as a light yellow oil; Rf ¼ 0.6 (in hexane=ethyl
acetate 95:5); IR (neat): 3418, 2923, 1735, 1601, 1505, 1252, 1069, 749 cmꢀ1
.

REDUCTIVE N-ALKYLATION OF NITROARENE
2497
1H NMR (300 MHz, CDCl3): d 3.93 (brs, 1H), 4.31 (brs, 2H), 6.56 (d, 2H,
J ¼ 7.8 Hz), 6.65 (t, 1H, J ¼ 7.2 Hz), 7.11 (d, 2H, J ¼ 7.5 Hz), 7.35–7.21 (m, 5H).
13C NMR (75 MHz, CDCl3): d 48.2, 112.7, 117.5, 127.2, 127.4, 128.6, 129.2, 139.4,
148. ESI MS (m=z): 184 (M þ H).
(4-Bromo-benzyl)-phenyl-amine[23] (2b). Following, the general experi-
mental procedure, product 2b was obtained as a yellow oil; Rf ¼ 0.6 (in hexane=ethyl
acetate 95:5); IR (neat): 3417, 1735, 1601, 1505, 1231, 750 cmꢀ1. 1H NMR (400 MHz,
CDCl3): d 4.05 (brs, 1H), 4.29 (s, 2H), 6.60 (d, 2H, J ¼ 8.7 Hz), 6.72 (t, 1H,
J ¼ 7.3 Hz), 7.17 (t, 2H, J ¼ 7.3, 8.1 Hz), 7.25 (d, 2H, J ¼ 8.1 Hz), 7.46 (d, 2H,
J ¼ 8.1 Hz). 13C NMR (75 MHz, CDCl3): d 47.8, 113.2, 118.1, 129.1, 129.3, 129.9,
138.2, 147.2. ESI MS (m=z): 262 (Mþ), 264 (M þ 2).
4-Phenylaminomethyl-phenol[23] (2c). Following the general experimental
procedure, product 2c was obtained as a light yellow oil; Rf ¼ 0.5 (in hexane=ethyl
acetate 90:10); IR (KBr): 3386, 1603, 1510, 1236 cmꢀ1. 1H NMR (300 MHz, CDCl3):
d 4.21 (s, 2H), 4.50 (brs, 1H), 5.28 (s, 1H), 6.56 (d, 2H, J ¼ 7.6 Hz), 6.67 (t, 1H,
J ¼ 7.2 Hz), 6.73 (d, 2H, J ¼ 8.6 Hz), 7.11 (t, 2H, J ¼ 7.3 Hz), 7.20 (d, 2H, J ¼ 8.5 Hz).
13C NMR (75 MHz, CDCl3): d 47.8, 113.1, 115.4, 117.7, 128.9, 129.2, 131.2, 148.0,
154.7. ESI MS (m=z): 200 (M þ H).
(4-Methoxy-benzyl)-phenyl-amine[21] (2d). Following the general experi-
mental procedure, product 2d was obtained as a yellow oil; Rf ¼ 0.7 (in hexane=ethyl
1
acetate 90:10); IR (neat): 3415, 2835, 1604, 1507, 1244, 1175, 1032, 749 cmꢀ1. H
NMR (300 MHz, CDCl3): d 3.78 (s, 3H), 3.83 (brs, 1H), 4.21 (s, 2H), 6.56 (d, 2H,
J ¼ 7.6 Hz), 6.64 (t, 1H, J ¼ 7.4 Hz), 6.81 (d, 2H, J ¼ 8.5 Hz), 7.11 (m, 2H), 7.24 (d,
2H, J ¼ 8.7 Hz). 13C NMR (75 MHz, CDCl3): d 47.7, 55.2, 112.7, 113.9, 117.4,
128.7, 129.2, 131.3, 148.1, 158.8. ESI MS (m=z): 214 (M þ H).
Phenyl-thiophen-2-ylmethyl-amine[29] (2e). Following the general experi-
mental procedure, product 2e was obtained as a yellow oil; Rf ¼ 0.4 (in hexane=ethyl
acetate 95:5); IR (KBr): 3409, 2923, 1601, 1504, 1315, 1258, 750, 696 cmꢀ1. 1H NMR
(300 MHz, CDCl3): d 3.38 (brs, 1H), 4.47 (s, 2H), 6.60 (d, 2H, J ¼ 7.5 Hz), 6.68 (t,
1H, J ¼ 7.5 Hz), 6.91 (t, 1H, J ¼ 3.0 Hz), 6.96 (s, 1H), 7.07–7.14 (m, 3H). 13C
NMR (75 MHz, CDCl3): d 49.4, 113.1, 114.2, 118.1, 124.5, 125.0, 126.7, 129.2. EI
MS (m=z): 189 (Mþ).
Cyclohexyl methyl-phenyl-amine[8,21] (2f). Following the general experi-
mental procedure, product was 2f was obtained as a colorless liquid; Rf ¼ 0.9 (in
1
hexane=ethyl acetate 90:10); IR (KBr): 3418, 2923, 1603, 1506, 747 cmꢀ1. H NMR
(300 MHz, CDCl3): d 1.88–0.90 (m, 11H), 2.93 (d, 2H, J ¼ 6.7 Hz), 3.58 (brs, 1H),
6.50 (d, 2H, J ¼ 7.5 Hz), 6.60 (t, 1H, J ¼ 7.5 Hz), 7.09 (m, 2H). 13C NMR (75 MHz,
CDCl3): d 25.9, 26.5, 31.2, 37.5, 50.5, 112.5, 116.8, 129.1, 148.5. EI MS (m=z): 189
(Mþ).
Phenyl-propyl-amine[21] (2g). Following the general experimental pro-
cedure, product 2 g was obtained as a colorless liquid; Rf ¼ 0.9 (in hexane=ethyl
1
acetate 95:5); IR (neat): 3410, 2960, 1603, 1507, 1320, 1178, 748 cmꢀ1. H NMR
(300 MHz, CDCl3): d 1.01 (t, 3H, J ¼ 7.5 Hz), 1.71–1.59 (m, 2H), 3.07 (t, 2H,
J ¼ 7.2 Hz), 3.58 (brs, 1H), 6.52 (d, 2H, J ¼ 8.5 Hz), 6.62 (t, 1H, J ¼ 7.2 Hz), 7.09

2498
B. SREEDHAR AND V. S. RAWAT
(t, 2H, J ¼ 8.5 Hz). 13C NMR (75 MHz, CDCl3): d 11.6, 22.6, 45.7, 112.6, 116.9, 129.1,
148.4. EI MS (m=z): 136 (Mþ).
Butyl-phenyl-amine[21] (2h). Following the general experimental procedure,
product 2h was obtained as a colorless liquid; Rf ¼ 0.7 (in hexane=ethyl acetate 95:5);
1
IR (neat): 3410, 2957, 1603, 1506, 1320, 747 cmꢀ1. H NMR (300 MHz, CDCl3): d
0.97 (t, 3H, J ¼ 7.1 Hz), 1.50–1.38 (m, 2H), 1.65–1.56 (m, 2H), 3.10 (t, 2H,
J ¼ 6.9 Hz), 3.47 (brs, 1H), 6.52 (d, 2H, J ¼ 7.7 Hz), 6.61 (t, 1H, J ¼ 7.4 Hz),
7.12–7.06 (m, 2H). 13C NMR (75 MHz, CDCl3): d 13.8, 20.3, 31.6, 43.6, 112.6,
116.9, 129.1, 148.5. ESI MS (m=z): 150 (M þ H).
Phenyl-(2-phenyl-propyl)-amine[23] (2i). Following the general experi-
mental procedure, product 2i was obtained as a yellow oil; Rf ¼ 0.8 (in hexane=ethyl
1
acetate 90:10); IR (neat): 3413, 2961, 1602, 1504, 1318, 1256, 750, 696 cmꢀ1. H
NMR (300 MHz, CDCl3): d 1.34 (d, 3H, J ¼ 6.9 Hz), 3.10–2.99 (m, 1H), 3.23–3.16
(m, 1H), 3.36–3.30 (m, 1H), 3.48 (brs, 1H), 6.50 (d, 2H, J ¼ 8.5 Hz), 6.62 (t, 1H,
J ¼ 7.3 Hz), 7.32–7.17(m, 5H), 7.09 (t, 2H, J ¼ 8.4 Hz). 13C NMR (75 MHz, CDCl3):
d 19.7, 39.1, 51.2, 113.3, 117.7, 126.6, 127.2, 128.6, 129.2, 144.4, 147.6. ESI MS (m=
z): 212 (M þ H).
(4-Methyl-benzyl)-phenyl-amine[23] (3a). Following the general experi-
mental procedure, product 3a was obtained as a yellow oil; Rf ¼ 0.6 (in hexane=ethyl
acetate 95:5); IR (neat): 3418, 2920, 1603, 1507, 1322, 1255, 1179, 749 cmꢀ1.1H NMR
(300 MHz, CDCl3): d 2.15 (s, 3H), 3.76 (brs, 1H), 4.34 (s, 2H), 6.53 (d, 2H,
J ¼ 7.6 Hz), 6.60 (t, 1H, J ¼ 7.4 Hz), 7.06–7.14 (m, 4H), 7.20 (d, 2H, J ¼ 7.9 Hz).
13C NMR (75 MHz, CDCl3): d 21.1, 48.1, 112.8, 117.5, 127.5, 129.2, 136.3, 136.8,
148.2. ESI MS (m=z): 198 (M þ H).
Benzyl-o-tolyl-amine[23] (3b). Following the general experimental pro-
cedure, product 3b was obtained as a yellow oil; Rf ¼ 0.6 (in hexane=ethyl acetate
95:5); IR (neat): 3068, 3019, 2831, 1612, 1512, 1430, 1279 cmꢀ1 1H NMR
.
(300 MHz, CDCl3): d 2.22 (s, 3H), 3.81 (brs, 1H), 4.27 (s, 2H), 6.58 (d, 1H, J ¼ 7.6 Hz),
6.63 (t, 1H, J ¼ 7.6 Hz), 7.07–7.01 (m, 2H), 7.38–7.25 (m, 5H). 13C NMR (75 MHz,
CDCl3): d 17.5, 48.2, 109.9, 117.1, 121.8, 127.1, 127.5, 128.6, 130.0, 139.4, 146.0.
ESI MS (m=z): 198 (M þ H).
Benzyl-(4-chloro-phenyl)-amine[23] (3c). Following the general experi-
mental procedure, product 3c was obtained as a yellow oil; Rf ¼ 0.6 (in hexane=ethyl
1
acetate 95:5); IR (KBr): 3412, 2831, 1618, 1506, 1419, 1321, 1072, 791 cmꢀ1. H
NMR (300 MHz, CDCl3): d 3.95 (brs, 1H), 4.28 (s, 2H), 6.49 (d, 2H, J ¼ 8.8 Hz),
7.3–7.23 (m, 5H), 7.06 (d, 2H, J ¼ 8.8 Hz). 13C NMR (75 MHz, CDCl3): d 48.3,
113.9, 122.1, 127.4, 128.7, 129.1, 138.9, 146.7. ESI MS (m=z): 218 (M þ H).
N-Benzyl-4-bromobenzenamine[30] (3d). Following the general experi-
mental procedure, product 3d was obtained as a white solid; Rf ¼ 0.8 (in hexane=ethyl
acetate 95:5); IR (KBr): 3423, 2925, 1596, 1497, 1466, 1376, 1071, 811, 730, 697 cmꢀ1
.
1H NMR (300 MHz, CDCl3): d 7.31 (m, 5 H), 7.19 (d, 2 H, J ¼ 8.6 Hz), 6.45 (d, 2H,
J ¼ 8.6 Hz) 4.27 (s, 2H), 3.97 (brs, 1 H). 13C NMR (75 MHz, CDCl3): d 48.2, 109.1,
114.3, 127.3, 128.6, 131.9, 138.8, 147.0. ESI MS (m=z): 262 (Mþ), 264 (M þ 2).

REDUCTIVE N-ALKYLATION OF NITROARENE
2499
4-Benzylamino-phenol[23] (3e). Following the general experimental pro-
cedure, product 3e was obtained as a obtained as a yellow oil; Rf ¼ 0.5 (in hexane=
1
ethyl acetate 70:30); IR (KBr): 3381, 2920, 1614, 1507, 1239, 1072 cmꢀ1. H NMR
(300 MHz, CDCl3): d 3.88 (brs, 2H), 4.24 (s, 2H), 6.48 (d, 2H, J ¼ 8.0 Hz), 6.62 (d,
2H, J ¼ 8.7 Hz), 7.33–7.20 (m, 5H). 13C NMR (75 MHz, CDCl3): d 49.5, 114.8,
116.3, 127.2, 127.7, 128.6, 139.4, 141.9, 148.2. ESI MS (m=z): 200 (M þ H).
4-Benzylamino-phenyl)-methanol[31] (3f). Following the General Experi-
mental procedure, product 3f was obtained as a white solid; Rf ¼ 0.4 (in hexane=
1
ethyl acetate 70:30); IR (KBr): 3258, 2920, 1611, 1520, 995, 727 cmꢀ1. H NMR
(300 MHz, CDCl3): d 2.99 (brs, 2H), 4.30 (s, 2H), 4.49 (s, 2H), 6.55 (d, 2H,
J ¼ 8.4 Hz), 7.12 (d, 2H, J ¼ 8.3 Hz), 7.31–7.29 (m, 5H). 13C NMR (75 MHz, CDCl3):
d 48.0, 64.9, 112.6, 126.4, 127.1, 128.4, 128.6, 139.1, 147.0. ESI MS (m=z): 214
(M þ H).
1-(4-Benzylamino-phenyl)-ethanone[32] (3g). Following the general experi-
mental procedure, product 3 g was obtained as a white solid; Rf ¼ 0.4 (in hexane=ethyl
1
acetate 70:30); IR (KBr): 3359, 2925, 1595, 1277, 732 cmꢀ1. H NMR (300 MHz,
CDCl3): d 2.45 (s, 3H), 4.38 (d, 2H, J ¼ 5.2 Hz), 4.55 (brs, 1H), 6.54 (d, 2H,
J ¼ 9.0 Hz), 7.33–7.25 (m, 5H), 7.77 (d, 2H, J ¼ 9.0 Hz). 13C NMR (75 MHz, CDCl3):
d 25.9, 47.4, 111.5, 126.2, 127.4, 130.6, 138.1, 151.9, 196.3. ESI MS (m=z): 226
(M þ H).
N-Benzyl-4-((tert-butyl dimethyl silyloxy)methyl)aniline (3h). Following
the general experimental procedure, product 3 h was obtained as a yellow oil;
Rf ¼ 0.7 (in hexane=ethyl acetate 90:10); IR (neat): 3419, 2927, 2854, 1616, 1521,
1
1468, 1253, 1079, 837, 775, 697, 541 cmꢀ1. H NMR (300 MHz, CDCl3): d 0.20 (s,
6H), 1.06 (s, 9H), 4.03 (brs, 1H), 4.44 (d, 2H), 4.72 (s, 2H), 6.68 (d, 2H, J ¼ 8.3 Hz),
7.20 (d, 2H, J ¼ 8.3 Hz), 7.30-7.50 (m, 5H). 13C NMR (75 MHz, CDCl3): d ꢀ5.1,
18.4, 25.9, 48.3, 65.0, 112.6, 127.1, 127.4, 127.6, 128.5, 139.4, 147.2. ESI MS (m=
z): 328 (M þ H). ESI-HRMS for C20H30NOSi (M þ H)þ requires 328.2083; found
328.2083.
N-Benzyl-4-(benzyloxy)aniline (3i). Following the general experimental
procedure, product 3i was obtained as a white solid; Rf ¼ 0.7 (in hexane=ethyl acetate
80:20); IR (KBr): 3392, 2922, 1723, 1511, 1452, 1234, 1020, 820, 736, 694, 584 cmꢀ1
.
1H NMR (300 MHz, CDCl3): d 3.65 (brs, 1H), 4.25 (s, 2H), 4.95 (s, 2H), 6.52 (d, 2H,
J ¼ 8.6 Hz), 6.76 (d, 2H, J ¼ 8.8 Hz), 7.15–7.44 (m, 10H). 13C NMR (75 MHz,
CDCl3): d 49.4, 70.7, 113.9, 116.0, 127.1, 127.4, 127.7, 128.5, 137.5, 139.6,
142.6.ESI MS (m=z): 290 (M þ H). ESI-HRMS for C20H19NO (M þ H)þ requires
290.1548; found 290.1548.
ACKNOWLEDGMENT
V. S. R. thanks the University Grant Commission (UGC), New Delhi, for the
award of a junior research fellowship.

2500
B. SREEDHAR AND V. S. RAWAT
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