Ni-nanoparticles: an efficient green catalyst for chemoselective reduction of aldehydes

Article abstract of DOI:10.1016/j.tetlet.2006.04.048

A novel method for reduction of aromatic and heteroaromatic aldehydes with ammonium formate using Ni-nanoparticles is described. The Ni-nanoparticles act as a green catalyst for selective reduction of the aldehydic group in the presence of other functional groups, viz.: -NO₂, -CN and alkenes to give the corresponding alcohols in excellent yields.

Full text of DOI:10.1016/j.tetlet.2006.04.048

Tetrahedron Letters 47 (2006) 4161–4165
Ni-nanoparticles: an efficient green catalyst for
chemoselective reduction of aldehydes
Mazaahir Kidwai,a,* Vikas Bansal,a Amit Saxena,b Ravi Shankarb and Subho Mozumdarb
aGreen Chemistry Research Laboratory, Department of Chemistry, University of Delhi, Delhi 110 007, India
bLaboratory for Nanobiotechnology, Department of Chemistry, University of Delhi, Delhi 110 007, India
Received 19 October 2005; revised 4 April 2006; accepted 13 April 2006
Available online 8 May 2006
Abstract—A novel method for reduction of aromatic and heteroaromatic aldehydes with ammonium formate using Ni-nanoparti-
cles is described. The Ni-nanoparticles act as a green catalyst for selective reduction of the aldehydic group in the presence of other
functional groups, viz.: –NO2, –CN and alkenes to give the corresponding alcohols in excellent yields.
ꢀ 2006 Elsevier Ltd. All rights reserved.
The reduction of carbonyl compounds to the corre-
sponding alcohols is an important transformation in
organic synthesis1,2 and an important step in the synthe-
sis of biologically active compounds.3,4 A variety of
reducing systems are available to carry out reductions,
which include Raney nickel,5,6 Ni-MCm-41,7 nickel–
aluminium (Ni–Al) alloy,8 nickel–aluminium hydrotal-
cite,9 Al2O3–NaBH4,10,11 Cu/Ru complexes,12–14 HCO2-
Na/D/pressure,15,16 Meerwein–Pondorff–Verley (MPV)
reductions,17,18 Fe/HCl19 and Sn/HCl.20 The selective
reduction of aldehydes in the presence of other func-
tional groups vulnerable to reduction constitutes a
major task in organic synthesis, and in many such cases,
it is necessary to use the expensive reagents and/or work
at low temperatures. This can be circumvented by using
aminoborane,21,22 tributylstannane,23 low valency tita-
nium,24 tetrabutylammonium triacetoxyborohydride,25
potassium triphenylborohydride26 and lithium trialkoxy-
aluminium hydrides.27
However, there is strong need to develop new green
catalysts30,31 that should be efficient, involve an easy
work-up and afford greater yields in shorter reaction
times. In this regard, Raney-Ni has extensively been
used for catalytic reduction. However, reactions cataly-
sed by Raney-Ni are very slow owing to the large surface
area of the catalyst. It was therefore worthwhile to
develop a new green catalyst that could enhance the
reaction rate.
Work in the field of metal nanoparticles as catalysts in
synthetic organic chemistry has gained much atten-
tion.32–50 Pioneering work includes the Mizoroki–Heck
reaction using palladium nanoparticles,32–34 Suzuki
cross-coupling reactions using palladium nanoparti-
cles,32,34–41 Stille type reactions,42–44 Sonogashira
coupling reactions,39 Tsuji–Trost allylation and
Pauson–Khand reactions,45,46 aza-Michael reactions47
and nanoparticle catalysed reactions.48–50 Recent litera-
ture has shown that the application of nanoparticles as
catalysts in organic synthesis has been little explored.
Ni-nanoparticles, in particular being cheap, need mild
reaction conditions for high yields of products in short
reaction times as compared to the traditional Raney-
nickel catalysts.
All these methods have their own drawbacks which
include the use of high pressure/temperature or both, long
reaction times,7 low yields,7 toxic solvents28,29 and non-
selective stoichiometric amounts of reagent, that could
lead to the formation of quantitative amounts of unde-
sirable salts or require the use of a pyrophoric catalyst.
We report herein a novel protocol that employs Ni-
nanoparticles51–54 as an efficient and selective catalyst
for the reduction of aldehydes. The Ni-nanoparticles
selectively and catalytically reduce aldehydic groups in
the presence of other functional groups such as –NO2,
–CN, and double bonds at a,b positions. Reduction of
Keywords: Ni-nanoparticles; Chemoselective; Catalytic reduction;
Aldehydes.
*
Corresponding author. Tel./fax: +91 11 27666235; e-mail: Kidwai.
chemistry@gmail.com
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.048

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M. Kidwai et al. / Tetrahedron Letters 47 (2006) 4161–4165
Reverse micellar solution containing
Ni-nanoparticles
Reverse micelle-A containing
Ni(NO3)2(aq. sol.)
+
Reverse micelle-A containing
NaBH4(alk. sol.)
N2 atmosphere
extraction of
nanoparticles
with absolute
ethanol
25 ºC
Nanoparticles + surfactant molecules
washing of
nanoparticles
with absolute
ethanol
(4-5 times)
Ni-nanoparticles free from surfactant
Scheme 1. Preparation of Ni-nanoparticles.
Ni2+ ions to Ni(0) in a reverse micellar system was
employed to prepare the nickel nanoparticles (Scheme
1).55,56
, HCOONH
Ni-np
4
R
CH2OH
R
CHO
THF, 25 ºC, N2, 1 atm
Scheme 2. Catalytic reduction of aromatic aldehydes using Ni-
nanoparticles (18–62 nm) employing ammonium formate as a hydro-
gen donor.
The sizes of the Ni-nanoparticles prepared at W0 = 3–15
(the water content parameter W0 defined as the molar
ratio of water to surfactant, W0 = [H2O]/[surfactant])
were confirmed as 10 2 nm to 85 2 nm through
quasi-elastic light scattering data (QELS) (Fig. 1a) and
transmission electron microscopy (TEM) (Fig. 1b).
The Ni-nanoparticles prepared were round in shape
and black in color (colloidal state).
activity due to the sparingly soluble nature of ammo-
nium formate which acts as a hydrogen donor. Non-
polar solvents such as n-hexane were unsuitable for the
reactions. However, polar solvents such as methanol
gave promising results (Table 2).
Ni-nanoparticles (Ni-np), 10–85 nm in size have been
used efficiently for the catalytic reduction of aromatic
and heteroaromatic aldehydes (Scheme 2).57 A control
experiment was conducted in the absence of catalyst
and it was observed that the reduction of aromatic alde-
hydes using ammonium formate produced the required
products in 72 h in 15–20% yield. In contrast, our proto-
col reduces various aromatic and heteroaromatic alde-
hydes with significantly shorter reaction times and very
high yields (Table 1).
Ni-nanoparticles of different sizes from 10 nm to 85 nm
diameters were prepared in the aqueous cores of reverse
micellar droplets. The mechanism of the catalytic action
of the nanoparticles is dependent on the nanoparticle
size.
The maximum reaction rate has been observed for
an average particle diameter of about 20 nm. With a
decrease in particle size, a trend of decreasing reaction
rate has been found for particles less than a diameter
of 20 nm, while those above this diameter show a steady
decline of reaction rate with increasing size. It has been
postulated, that in the case of particles of average size
less than 20 nm, a downward shift of Fermi level takes
The solvent used plays an important role in deciding the
reaction path and the nature of the product. Changing
the solvent in the heterogeneous catalytic reduction
from methanol to n-hexane resulted in a steep drop in
Figure 1. (a) QELS data of‘ Ni-nanoparticles: plot of population distribution in percentile versus size distribution in nm, (b) TEM image of Ni-
nanoparticles. The scale bar corresponds to 20 nm in the TEM image.

M. Kidwai et al. / Tetrahedron Letters 47 (2006) 4161–4165
4163
Table 1. The heterogeneous catalytic reduction of aldehydes to alcohols using Ni-nanoparticles (10 mol %)a
Entry
Substrate
Time (h)
Product
Yield (%)b
1
1.5
85
72
CHO
CH2OH
2
3
3.0
3.5
O2N
O2N
CHO
O2N
O2N
CH2OH
78
89
90
76
93
97
92
CHO
CH2OH
4
5
6
7
8
9
4.0
3.6
2.8
1.7
2.2
6.0
Br
Br
CHO
Br
Br
CH2OH
CHO
CH2OH
NC
CHO
CHO
NC
CH2OH
CH2OH
HO
HO
MeO
CHO
MeO
CH2OH
CH2OH
CHO
CHO
MeO
MeO
MeO
MeO
MeO
10
1.4
95
CH2OH
MeO
11
12
2.3
4.6
88
60
S
CH2OH
S
CHO
O
CH2OH
O
CHO
a Reaction conditions: 1.0 equiv of aldehyde, 1.0 equiv of ammonium formate, 10 mol % of Ni-nanoparticles (45 nm) in THF at room temperature
stirred under an inert atmosphere.
b Isolated yields.
place, consequently, with an increase of band gap
energy. As a result, the particles require more energy
to pump electrons to the adsorbed ions for the electron
transfer reaction. This leads to a reduction in reaction
rate when catalysed by smaller particles. On the other
hand, for nanoparticles >20 nm in diameter, the change
of Fermi level is not appreciable. As these particles exhi-
bit less surface area for adsorption with increased parti-
cle size, a decrease in catalytic efficiency results (Table
3).
Table 2. Effect of solvents on the catalytic reduction of 4-methoxybenzaldehyde and 3,4,5-trimethoxybenzaldehyde using Ni-nanoparticles (18 nm)
Solvent
4-Methoxybenzaldehyde
3,4,5-Trimethoxybenzaldehyde
Yield (%)a
Time (h)
Yield (%)a
Time (h)
Methanol
97
97
93
81
1.5
2.2
2.8
6.2
96
95
89
68
1.2
1.4
2.2
5.7
Tetrahydrofuran
Acetonitrile
n-Hexane
a Isolated yields.

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M. Kidwai et al. / Tetrahedron Letters 47 (2006) 4161–4165
Table 3. Effect of size of the nanoparticles on catalytic reduction of 4-methoxybenzaldehyde and 3,4,5-trimethoxybenzaldehyde using 10 mol %
Ni-nanoparticles
Particle size ( 2 nm)
4-Methoxybenzaldehyde
3,4,5-Trimethoxybenzaldehyde
Yield (%)a
Time (h)
Yield (%)a
Time (h)
10
20
30
85
89
92
93
71
2.5
2.2
2.8
3.2
87
95
91
69
2.7
1.4
2.0
2.6
a Isolated yields.
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In summary, these results demonstrate that Ni-nanopar-
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57. General procedure for the reduction of aldehydes (Scheme
2): To a suspension of aldehyde (5 mmol), Ni-nanoparti-
cles (18–62 nm, 10 mol %), in tetrahydrofuran (THF,
3 ml), ammonium formate (5 mmol) was added at room
temperature and the resulting mixture stirred under a
nitrogen atmosphere. After completion of the reaction
(monitored by TLC), the mixture was filtered and THF
was removed in vacuo. The residue was extracted with
dichloromethane (2 · 15 ml). The organic layer was dried
over anhydrous Na2SO4 and after removal of the solvent
in vacuo, the residue was purified by column chromato-
graphy (silica gel 250–400 mesh size). The structure of the
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1
and H NMR data.