Chemoselective reduction of nitroaromatics to anilines using decaborane in methanol

Article abstract of DOI:10.1016/S0040-4039(99)02048-1

Nitrobenzenes were chemoselectively reduced to the corresponding anilines using decaborane (B₁₀H₁₄) in the presence of Pd/C and two drops of acetic acid at reflux under nitrogen atmosphere in high yields.

Full text of DOI:10.1016/S0040-4039(99)02048-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 175–177
Chemoselective reduction of nitroaromatics to anilines using
decaborane in methanol
a
b
a
a,∗
Jong Woo Bae, Young Jin Cho, Seung Hwan Lee and Cheol Min Yoon
a
Department of Life Science & Biotechnology, Graduate School of Biotechnology, Korea University, Seoul, South Korea
b
Department of Chemistry, Graduate School of Chemistry, Korea University, Seoul, South Korea
Received 1 September 1999; revised 20 October 1999; accepted 21 October 1999
Abstract
Nitrobenzenes were chemoselectively reduced to the corresponding anilines using decaborane (B10H14) in the
presence of Pd/C and two drops of acetic acid at reflux under nitrogen atmosphere in high yields. © 1999 Elsevier
Science Ltd. All rights reserved.
Aromatic amines are important intermediates for dyes, agricultural, and pharmaceutical chemicals.1
Most of these aromatic amines are prepared by the reduction of the corresponding nitro aromatics using a
2
metallo hydride reagent or catalytic hydrogenation. However, these reaction conditions are incompatible
with other sensitive functional groups such as nitriles, carbonyl groups, halides and the benzyl-protecting
group. Recently, selective reagents for this transformation were developed and reported. These are
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indium–NH Cl in ethanol, Sm metal–iodine in methanol, N,N-dimethylhydrazine/ferric chloride,
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hydrazine hydrate/Fe O –MgO, Diethyl chlorophosphite, and NaBH –NaOMe in methanol. These
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3
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methods still have limitations, such as long reaction time, nucleophilic attack (hydrazine), by-products,
and/or low yield.
Here, we report an efficient and chemoselective method for reducing nitroaromatics to the correspon-
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ding anilines (Scheme 1). The nitro groups of nitroaromatics were chemoselectively reduced in high
yields to the corresponding anilines using decaborane in the presence of 10% Pd/C and two drops of
acetic acid in methanol at reflux.
Scheme 1.
∗
Corresponding author.
0
040-4039/00/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02048-1
tetl 15976

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The reduction reaction was attempted under several reaction conditions (Table 1). When the reaction
was tried using decaborane at room temperature in methanol, the reaction was slow and in some cases
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required additional decaborane to reach completion. When the reaction mixture was heated to reflux, the
1
0
reaction was fast. However, the irreversible formation of the amine-decaborane adduct was observed
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and the reaction again required additional decaborane for complete reduction. To avoid the formation of
these adducts and to increase the rate of the reaction, two drops of acetic acid were added and the reaction
was heated at reflux. Under these conditions, the reaction was completed in several hours and gave
the corresponding anilines chemoselectively in high yields. Other functional groups such as carbonyls
(entries 1, 2, 3 and 4), esters (entries 2 and 3), halide (entry 6), nitrile (entry 8), or the benzyl groups
(entries 3 and 9) survived under the reaction condition.
Table 1
Reduction of nitroaromatics using decaborane in methanol
In conclusion, nitro groups of nitroaromatics in methanol were chemoselectively converted into the
corresponding amines using decaborane in the presence of 10% Pd/C and two drops of acetic acid at
reflux under nitrogen in high yields. We observed chemoselectivity in the presence of other functional
groups such as ester, bromo, nitrile, amide and benzyl group.
Acknowledgements
The authors wish to acknowledge the financial support of the Korea Research Foundation made in the
program year of 1997.
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. A representative run (entry 3) is as follows: To a solution of benzyl 4-nitrobenzoate (50 mg, 0.276 mmol) in methanol (5
ml) was added decaborane (10 mg), two drops of acetic acid and 10% Pd/C (15 mg). The resulting solution was stirred at
reflux under nitrogen for 0.5 h. The reaction was followed by TLC using a solution of ethyl acetate and n-hexane (1:2). The
mixture was concentrated under reduced pressure, chromatographed on a short pad of silica gel using a solution of ethyl
acetate and n-hexane (1:4) to give benzyl 4-aminobenzoate as a pale yellow solid.
1
1
f
0. The R value of the adduct was higher than that of the corresponding amine according to TLC when a mixture of ethyl
acetate and n-hexane (1:4) was used as the developing solvent.
1. The starting nitroaromatics remained even after stirring for 2 days or longer. After stirring for 2 days, all of the decaborane
was consumed.