Magnesium-Catalyzed Proficient Reduction of Oximes to Amines Using Ammonium Formate

Article abstract of DOI:10.1081/SCC-120027707

Various aldoximes and ketoximes were selectively reduced to the corresponding amines by catalytic transfer hydrogenation employing low cost magnesium powder and ammonium formate at room temperature. Many other functionalities such as halogens, -OH, -OCH₃, -COOH and -CH ₃ remained unaffected. The hydrogenation is fast, mild, clean, cost effective and high yielding.

Full text of DOI:10.1081/SCC-120027707

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Magnesium‐Catalyzed Proficient Reduction of Oximes
to Amines Using Ammonium Formate
K. Abiraj a & D. Channe Gowda a
a Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore,
Karnataka, India, 570 006
Available online: 16 Aug 2006
To cite this article: K. Abiraj & D. Channe Gowda (2004): Magnesium‐Catalyzed Proficient Reduction of Oximes to Amines
Using Ammonium Formate, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 34:4, 599-605
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 4, pp. 599–605, 2004
Magnesium-Catalyzed Proficient
Reduction of Oximes to Amines
Using Ammonium Formate
*
K. Abiraj and D. Channe Gowda
Department of Studies in Chemistry, University of Mysore,
Manasagangotri, Mysore, Karnataka, India
ABSTRACT
Various aldoximes and ketoximes were selectively reduced to the
corresponding amines by catalytic transfer hydrogenation employing low
cost magnesium powder and ammonium formate at room temperature.
Many other functionalities such as halogens, –OH, –OCH3, –COOH and
–CH3 remained unaffected. The hydrogenation is fast, mild, clean, cost
effective and high yielding.
Key Words: Oximes; Amines; Ammonium formate; Hydrogenation;
Magnesium powder.
*
Correspondence: D. Channe Gowda, Department of Studies in Chemistry, University
of Mysore, Manasagangotri, Mysore, Karnataka, India-570 006; Fax: 091-0821-
2421263/2518835; E-mail: dcgowda@yahoo.com.
599
DOI: 10.1081/SCC-120027707
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Abiraj and Gowda
The conversion of carbonyl derivatives to amines via oximes is a useful
transformation in the synthesis of numerous organic compounds and key
intermediates in the biosynthesis of many pharmacological important com-
pounds.[1,2] In comparison, the corresponding reactions of C55N bonds have
been studied much less. Although the addition to the C55N bond of imines,
hydrazones and oximes are well known,[3] the development of such reaction is
often limited by the poor electrophilicity of the C55N carbon atom. Numerous
new reagents have been developed for the reduction of oximes to amines
including NaBH4,[3,4] LiAlH4[5,6] and indium/acetic acid.[7] Though some of
these are widely used, still they have limitations based on chemo-selectivity
and economic considerations. Catalytic hydrogenation is also commonly
used,[8,9] although the success of reaction is sensitive towards catalyst, solvent
and substrate. Further, catalytic hydrogenation employs highly diffusible, low
molcular weight, flammable hydrogen gas and requires pressure equipment.
Electrolytic reduction of oximes to amines, in acid solution is also reported[10]
but this system offers very low yield.
There has been growing interest in the use of metals in synthetic
chemistry. Zinc,[11] indium,[12] tin,[13] magnesium[14–18] have been used in the
synthesis of many organic compounds. The utility of magnesium for the
synthesis of pinacols,[14] silaspiro compounds[15] has been demonstrated.
Further, the magnesium-mediated reduction of aromatic nitro compounds,[16]
Barbier reaction[17] and in situ Grignard reactions[18] has also been demon-
strated. The application of ammonium formate[19,20] in the field of catalytic
transfer hydrogenation for the reduction of variety of organic compounds and
in the synthesis of peptides is peer reviewed.
Nowadays, the heterogeneous catalytic transfer hydrogenation method
has proved to be a potent choice for reduction of organic compounds over
traditional hydrogenation or other methods of reduction as it involves mild
reaction condition, easy work-up and high degree of selectivity.[19–24] Earlier
reports reveal that catalytic transfer hydrogenation of oximes to amines had
been achieved with systems like ammonium formate/10% Pd/C,[23] cyclo-
hexene/10% Pd/C.[24] But these systems require long reaction times as long
as 5–10 hours at reflux temperature, expensive catalyst and also offer very low
yield. Moreover, stringent precautions must be taken while employing
palladium on carbon because of its flammable nature in presence of air.
Recently, we have explored the utility of magnesium powder and hydra-
zinium monoformate; a new hydrogen source for deblocking some commonly
used protecting groups in peptide synthesis.[25] Herein we report a rapid,
selective and simple reduction of oximes to the corresponding primary amines
by using low cost magnesium powder and ammonium formate at room
temperature as depicted in the Scheme 1. Various other functionalities like
halogens, –OH, –OCH3, –COOH and –CH3 are tolerated.

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Reduction of Oximes to Amines Using Ammonium Formate
601
Scheme 1. R1, R2 ¼ H or alkyl or phenyl or substituted phenyl group.
The results given in the Table 1 reveal the viability of using Mg/
HCOONH4 system for the reduction of oximes. The course of reaction was
monitored by TLC and IR spectra. The work-up and isolation of the products
were easy. Thus, the oximes reduced (few examples are listed in the Table 1)
by this system were obtained in good yield. The products were characterized
by comparison of their boiling points or melting points, TLC and IR spectra
with authentic samples. The disappearance of strong absorption bands
between 1690–1640 cm21 due to C55N stretching and between 3650–
3500 cm21 due to O–H stretching and appearance of two strong absorption
bands between 3500–3300 cm21 of –NH2 group clearly show that the oximes
were reduced to corresponding primary amines.
A control experiment was carried out using oximes with ammonium
formate, but without magnesium powder does not yield any reduced product
and the starting material is recovered in 100%. This confirms the role of
magnesium as catalyst. The reaction was also carried out for several hours
with ammonium chloride instead of ammonium formate expecting an efficient
reduction, but the yield is very low. Further, we observed that in the case of
nitro oximes, the nitro group at aryl residue and also oxime group underwent
reduction to yield respective diamine product at room temperature.
In summary, the reduction of oximes can be accomplished in a short time
with magnesium powder instead of expensive catalyst like palladium[23,24] at
room temperature and many other functionalities are tolerated. The yields are
almost quantitative and the compounds analytically pure. This magnesium-
catalyzed procedure provides a very efficient, inexpensive, mild, and general
methodology for reduction of oximes to amines.
EXPERIMENTAL
Materials
The oximes were either commercially available or prepared from the
corresponding carbonyl compound by standard methods. In cases where the
oxime was obtained as an E/Z-mixture, no attempts were made to separate
such mixtures and they are used as such for the reduction. Magnesium powder

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Abiraj and Gowda
Table 1. Magnesium-catalyzed reduction of oximes to amines using ammonium
formate.
Boiling point
Time
(min)
Yielda
(%)
1/2
R1
R2
Found
Literature Ref.
a
b
c
d
e
f
Ph
H
38
40
42
40
45
52
55
52
32
50
55
52
91
66b
64b
89
93
92
85
84
94
81
79
88
182–184 185
[26]
[26]
[26]
[27]
[26]
[27]
[26]
[26]
[26]
[26]
[26]
[26]
Me
Me
Ph
H
166c
152c
184
165
150
Me
Me
Ph
Me
H
184–186
Ph
295–296 295
p-(OH)C6H4CH2
p-(OCH3)3C6H4
3,4,5-(OCH3)C6H2
123c
236
122
125–126
236–237
121
g
h
i
H
p-(Cl)C6H4
d
H
d
212–214 215
157–159 158–160
j
e
f
e
f
k
l
135
100
134
101
aIsolated yields are based on single experiment and the yields were not optimized.
bLow yield is due to low boiling point of the product and are isolated as their
hydrochloride salts.
cMelting point.
d
e
f
was purchased from SISCO Research Laboratories Pvt. Ltd., Bombay (India)
and was treated with 0.01 N hydrochloric acid for about 2 min. It was filtered
through a sintered glass funnel and washed with water, dry methanol and dry
ether. Thus obtained magnesium was vacuum dried and stored. Ammonium
formate and 60–120-mesh silica gel (for column chromatography) were
purchased from E. Merck (India) Ltd. All of the solvents used were analytical
grade or were purified according to standard procedures. Thin layer
chromatography was carried out on silica gel plates obtained from Whatman

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Reduction of Oximes to Amines Using Ammonium Formate
603
Inc. The melting points were determined by a Thomas–Hoover melting point
apparatus and are uncorrected. IR spectra were recorded on a Shimadzu FTIR-
8300 spectrometer.
Typical Procedure
To a solution of the substrate (10 mmol) in methanol or in any other
suitable solvent (20 mmol) was added ammonium formate (30 mmol) and
magnesium powder (1 g, 0.041 mol). The mixture was stirred under nitrogen
atmosphere at room temperature. The reaction was exothermic and
effervescent. After the completion of reaction (monitored by TLC), the
reaction mixture was filtered through celite. The organic layer is evaporated
and the residue was dissolved in chloroform or dichloromethane or ether and
washed with saturated sodium chloride solution to remove excess ammonium
formate. The organic layer was dried over anhydrous sodium sulphate and
evaporation of the organic layer followed by purification either by
preparative TLC or by column chromatography to yield the desired product.
In order to obtain the volatile aliphatic amines in good yield, the reaction
was carried out using a condenser cooled with ice water and the reaction flask
immersed in a cold-water bath. After filtration, the reaction mixture was
neutralized with HCl. The solvent was evaporated under reduced pressure.
The residue was lyophilized or subjected to column chromatography by using
60–120 mesh silica gel and a suitable eluting system [50 : 50 chloroform : -
benzene (for entry 2e), 60 : 40 chloroform : benzene (for entries 2a & 2d),
80 : 20 chloroform : benzene (for entry 2j), 90 : 10 chloroform : benzene (for
entry 2k), 80 : 20 chloroform : methanol (for entries 2b & 2c), 85 : 15
chloroform : methanol (for entries 2f & 2g), 90 : 10 chloroform : methanol
(for entry 2i), 95 : 5 chloroform : methanol (for entries 2h & 2l)]. Aliphatic
amines were obtained as their hydrochloride salts in up to 65% yield.
ACKNOWLEDGMENT
The authors wish to thank University Grants Commission, New Delhi,
India for providing financial assistance.
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Received in the USA August 14, 2003

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