Chemoselective reduction of aldehydes using decaborane in aqueous solution

Article abstract of DOI:10.1080/00397910600781224

Reduction of aldehydes using decaborane (B₁₀H₁₄) in an aqueous solution gave the corresponding alcohol chemoselectively in good to high yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600781224

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Chemoselective Reduction of Aldehydes using
Decaborane in Aqueous Solution
a
a
a
b
Seung Hwan Lee , Mi Hye Nam , Min Young Cho , Byung Woo Yoo , Hak
c
b
June Rhee & Cheol Min Yoon
a
Graduate School of Biotechnology , Korea University , South Korea
Department of New Material Chemistry , Korea University , South Korea
College of Science and Technology , Hanyang University , South Korea
b
c
Published online: 27 Oct 2006.
To cite this article: Seung Hwan Lee , Mi Hye Nam , Min Young Cho , Byung Woo Yoo , Hak June Rhee & Cheol
Min Yoon (2006) Chemoselective Reduction of Aldehydes using Decaborane in Aqueous Solution, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:17,
2469-2474, DOI: 10.1080/00397910600781224
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Synthetic Communications , 36: 2469–2474, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600781224
Chemoselective Reduction of Aldehydes
using Decaborane in Aqueous Solution
Seung Hwan Lee, Mi Hye Nam, and Min Young Cho
Graduate School of Biotechnology, Korea University, South Korea
Byung Woo Yoo
Department of New Material Chemistry, Korea University, South Korea
Hak June Rhee
College of Science and Technology, Hanyang University, South Korea
Cheol Min Yoon
Department of New Material Chemistry, Korea University, South Korea
Abstract: Reduction of aldehydes using decaborane (B H ) in an aqueous solution
1
0 14
gave the corresponding alcohol chemoselectively in good to high yields.
Keywords: Aldehyde, decaborane, reduction, water
Reduction of carbonyls to alcohols is one of the most simple and important
[1]
transformations for the preparation of alcohol in organic synthesis.
[
Reducing agents such as NaBH and Bu SnH have been employed in
2]
[3]
3
aqueous solution as well as in organic solvent for this transformation. The
reaction medium has recently changed from organic to aqueous medium
[
4]
because of environmental benefits. More hydrophobic reducing agents
such as LiRBH (R ¼ C F , Ph, Bn, Naphthyl) have been introduced and
3
6 5
Received in Japan November 24, 2005
Address correspondence to Cheol Min Yoon, Department of Advanced Material
Chemistry, Korea University, Jochiwon, Chungnam 339-700, Korea. E-mail:
cmyoon@korea.ac.kr
2
469

2
470
S. H. Lee et al.
investigated recently in aqueous solution for increasing reactivity and
[
5]
selectivity.
Because several reduction reactions such as reductive amination and
[6]
reductive etherification have been developed in alcohol. One of our efforts
to explore the utility of decaborane as a hydrophobic reducing agent led us
to find that aldehydes using decaborane in aqueous solution were efficiently
reduced to the corresponding alcohols. Herein we report the chemoselective
reduction of various aldehydes to the corresponding alcohols using decaborane
in aqueous medium at room temperature (Scheme 1).
In the beginning of the research, the reduction of p-nitrobenzaldehyde,
chosen as a model substrate of p-nitrobenzyl alcohol, was examined in
water, THF, and in various ratios of aqueous THF solution using a various
amount of decaborane at room temperature. The optimum amount of decabor-
ane used in all cases was 30 mol% of the substrates. If less than 30 mol% of
decaborane was used, the reaction resulted in an incomplete reaction. Water
was necessary for the reaction; the reaction in THF did not proceed at all.
The proper solvent system turned out to be a solution of water and THF
(
9:1) (entry 1 in Table 1). A drop of THF as a cosolvent seemed to be
necessary to increase the reaction rate by increasing the water solubility of
decaborane. The reaction in only water proceeded slowly to give an alcohol
in 24 h in 95% yield, depending on the slow dissolving rate in water,
probably due to hydrophobicity of decaborane. Increasing amounts of THF
in water decrease the rate of reaction as well. Our method is easy to use,
and the reaction condition is environmentally benign.
Both aliphatic and aromatic aldehydes under the reaction conditions were
smoothly reduced to afford the corresponding alcohols in high yields as shown
in Table 1. Aromatic aldehydes possessing a electron-withdrawing group
(
entries 1–4, 9, 10) and electron-donating group (entries 5–8) were
converted into the corresponding alcohols in good to high yields. Acidic
sensitive groups such as methylenedioxy (entry 5) and ester (entries 4 and
11) remained intact. In particular, cinnamaldehyde (entries 12) was regiose-
lectively converted to the cinnamyl alcohol as 1,2-adduct. Heterocyclic
aldehydes (entries 14–16) were also transformed into the correspondmg
alcohols chemoselectively with heterocycles remained intact.
Various aldehydes were readily reduced as shown in Table 1, but conver-
[7]
sion of ketone was incomplete even after extended reaction times. These
data showed a possibility that aldehydes are reduced chemoselectively in
the presence of ketone. 4-Acetylbenzaldehyde using 30 mol% of decaborane
Scheme 1. R ¼ Aliphatic or aromatic.

Reduction of Aldehydes using Decaborane
2471
Table 1. Reduction of aldehydes using decaborane
a,b
Entry
1
Substrate
Time (h)
1
Yield (%)
90
c
(95 )
2
4
2
0.5
99
3
4
5
0.5
0.5
0.5
88
97
91
6
7
1
82
91
0.5
8
9
1
6
84
d
84
1
0
1
0.5
6
85
d
95
1
1
2
3
0.5
0.5
84
94
1
(
continued)

2
472
S. H. Lee et al.
Table 1. Continued
a,b
Entry
Substrate
Time (h)
1
Yield (%)
1
4
72
1
1
5
6
0.5
0.5
72
76
a
b
Isolated yields.
A solution of THF and water (0.1 mL:0.9 mL, 0.2 M) was used as a reaction
medium.
c
Water as a solvent was used at room temperature.
d
Reflux condition in water.
at rt was chemoselectively reduced into 4-acetylbenzylalcohol in 83% yield
entry 1 in Table 2). Electron-deficient and sufficient aldehydes in the
(
presence of 3-nitroacetophenone were also converted into the primary
alcohol under the reaction condition, and the unreacted ketone was
recovered (entries 2 and 3). Numerous methods for the chemoselective
reduction of an aldehyde in the presence of a ketone have been known, but
each has its limitations. (Reduction of 5-bromo-2-hydroxyacetophenone
using decaborane in THF and water (0.1 mL:0.9 mL, 0.2 M) for 24 h at rt
gave the corresponding alcohol in 50.4% yield, and the unreacted ketone
was recovered in 34.4% yield.) Our new methodology should be of interest
and complementary.
In conclusion, the reduction of aliphatic and aromatic aldehydes using
0 mol% of decaborane in aqueous solvent (water and 1 drop of THF) at
3
room temperature chemoselectively gave the corresponding alcohols in
good to high yields. Especially an aldehyde in the presence of a ketone was
chemoselectively converted into the primary alcohol. Decaborane was ident-
ified as a mild, efficient, and chemoselective reducing agent for the reduction
of aldehyde group.
EXPERIMENTAL
The following experimental procedure is representative. To a solution of
aldehydes (0.2 mmol) in 0.9 mL of H O and 0.1 mL of THF was added
2
30 mol% of decaborane at room temperature under nitrogen. After the

Reduction of Aldehydes using Decaborane
2473
Table 2. Chemoselective reduction of aldehyde in the presence of ketone
Yield
a,b
(%)
Entry
1
Substrate
Product
Time (h)
1
83
2
1
100
c
0
8
3
1.5
90
c
7
9
a
Isolated yields.
b
A solution of THF and water (0.1 mL:0.9 mL, 0.2 M) was used as a reaction
medium.
c
The recovered ketone.
reaction progress was complete (monitored by TLC using a solution of EA and
hexane), the reaction mixture was evaporated under reduced pressure and the
concentrate was chromatographed on a short silica-gel pad using a solution of
ethyl acetate and hexane (1 : 4) to give the corresponding alcohol in high yield,
1
which was characterized by H NMR spectroscopy.
ACKNOWLEDGMENTS
The authors acknowledge the financial support of the Korea Science and
Engineering Foundation in the program in 2005.
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