Chemoselective reduction of carbonyl compounds to alcohols with co-doped ammonia borane

Article abstract of DOI:10.14233/ajchem.2014.18504

Chemoselective reduction of various carbonyl compounds to alcohols with Co-doped ammonia borane was investigated in the present work. It was observed that Co-doped ammonia borane exhibited much better performance than ammonia borane. The Co-based catalysts could be reused up to four times with a slight decrease in activity. Thus, a mild and efficient method for chemoselective reduction of carbonyl compounds with Co-doped ammonia borane was established. The Co-doped ammonia borane sample was characterized by electron paramagnetic resonance. Electron paramagnetic resonance characterization revealed that Co element in a partially reduced state.

Full text of DOI:10.14233/ajchem.2014.18504

Asian Journal of Chemistry; Vol. 26, No. 23 (2014), 8248-8250
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.18504
Chemoselective Reduction of Carbonyl Compounds to Alcohols with Co-Doped Ammonia Borane
1
,2,3,*
3
1
1
1
2
2
PENGMIAN HUANG
, WENJUAN TANG , GUISHAN TAN , WENBIN ZENG , YUANJIAN LI , QINGHUA ZHANG and BO CHEN
1
2
3
School of Pharmaceutical Sciences, Central South University, Changsha 410000, Hunan Province, P.R. China
Fangsheng Pharmaceutical Co., Ltd. of Hunan, Changsha 410000, Hunan Province, P.R. China
School of Chemistry and Biology Engineering, Changsha University of Science & Technology, Changsha 410000, Hunan Province, P.R. China
Corresponding author: Fax: +86 731 85258733; Tel: +86 15973196958; E-mail: pengmianhuang@163.com
Received: 23 July 2014; Accepted: 1 September 2014; Published online: 15 November 2014;
*
AJC-16335
Chemoselective reduction of various carbonyl compounds to alcohols with Co-doped ammonia borane was investigated in the present
work. It was observed that Co-doped ammonia borane exhibited much better performance than ammonia borane. The Co-based catalysts
could be reused up to four times with a slight decrease in activity. Thus, a mild and efficient method for chemoselective reduction of
carbonyl compounds with Co-doped ammonia borane was established. The Co-doped ammonia borane sample was characterized by
electron paramagnetic resonance. Electron paramagnetic resonance characterization revealed that Co element in a partially reduced state.
Keywords: Co-doped ammonia boarne, Chemoselective reduction, Carbonyl compounds.
Co-doped ammonia borane could be preserved in solid
INTRODUCTION
form as well in methanol. When ammonia borane was added
Ammonia borane (NH
sive study since 2000, because of its abnormally high
3
BH
3
) has been a subject of inten-
into that methanol solution containing CoCl , immediately
2
1
after, the resulting solution or blending was distilled at room
temperature under a reduced pressure in a rotary evaporator
within 5 min and the samples were continuously vacuumed to
eliminate the rest solvent. After that, the samples were preser-
ved in refrigerator to avoid self-decomposition.
2
-11
hydrogen content (19.6 wt %) . In this study, 2 mol % nano-
sized Co-based catalysts were introduced to ammonia borane
through a "co-precipitation" method. We devoted to explore
the chemoselective reduction of carbonyl compounds to alcohols
with Co-doped ammonia borane. The reduction of carbonyl
compounds by Co-doped ammonia borane was found much
faster than by ammonia borane at room temperature. Therefore,
a facile method for the preparation of alcohols by chemo-
selective reduction of carbonyl compounds with Co-doped
ammonia borane was established. This is the first time to applied
Co-doped ammonia borane in synthetic organic chemistry. The
chemical state of Co element doped in ammonia borane was
identified by electron paramagnetic resonance. Furthermore,
the recyclability of Co-based catalysts was discussed.
Chemoselective reduction of carbonyl compounds with
ammonia borane or Co-doped ammonia borane: 10 mL of
5
mmol ammonia borane or 5 mmol Co-doped ammonia borane
methanol solution was added into 5 mL of 10 mmol aldehydes
or ketones solution in methanol in a sealed glass bottle and
these contents stirred for an appropriate time at ambient pressure
and temperature. The reaction was monitored by TLC. After
the reaction, methanol was evaporated and then 30 mL hexane
was added into the glass bottle to extract alcohol product three
times. Then, clear hexane solution was collected after centrifu-
gation. Next, hexane was evaporated by rotary evaporation,
the residue was purified by silica gel column chromatography
EXPERIMENTAL
(
elution by using petroleum ther : ethyl acetate = 5:1) to obtain
Synthesis of 2 mol % Co-doped ammonia borane:
1
the alcohol product and the product was characterized by H
NMR, C NMR.All of these mentioned reactions were carried
out at least twice to check for reproducibility.
NH
by Ramachandran and Gagare . 0.4 mmol of CoCl
3
BH was synthesized according to the procedure reported
3
13
17
2
and 10 mL
methanol were added into a sealed glass bottle at room tempe-
rature. The contents were sonicated for 20 min at room tempe-
rature, then, 20 mmol ammonia borane was added into that
methanol solution. The solution or blending could be directly
used in reduction reaction without further purification.
RESULTS AND DISCUSSION
Effect of solvent in reduction reaction: The reduction
of hypnone by Co-doped ammonia borane was studied in

Vol. 26, No. 23 (2014)
Chemoselective Reduction of Carbonyl Compounds to Alcohols with Co-Doped Ammonia Borane 8249
different solvents to figure out the solvent effect. The result
showed that reduction of carbonyl group was facilitated in
protic solvents (Table-1). When the reaction was in aprotic
solvents, precipitate was generated and the 1-phenylethanol
was achieved only after hydrolysis of the precipitate in aqueous
HCl solution or methanolysis of the precipitate in methanol.
According to Berke and co-workers , the generated precipitate
was borate ester. The reaction in alcohols solvents achieved
higher yield of 1-phenylethanol than in water, which was
probably due to the better solubility of reactant in alcohols.
And the methanol mediated reaction showed the best
performance.
Chemoselective reduction of carbonyl compounds by
ammonia borane and Co-doped ammonia borane: As
shown in Table-2, several structurally different carbonyl
compounds underwent reduction to corresponding alcohols
with ammonia borane and 2 mol % Co-doped ammonia borane
in methanol, respectively. The carbonyl group of carbonyl
compounds was selectively reduced by ammonia borane or
Co-doped ammonia borane while other unsaturated groups
such as double bond (1a-1d, 1g), nitro (1e) , ester group (1g)
and phenyl group (1h) were retained. It is noteworthy that
aromatic ketones reduced by Co-doped ammonia borane with
high yields at room temperature within several hours.
12
Berke and co-workers have reported the reaction route of
reduction of carbonyl compounds with ammonia borane in
TABLE-1
12
REDUCTION OF HYPNONE WITH Co-DOPED a
AMMONIA BORANE IN DIFFERENT SOLVENTS
methanol . In this reaction route, the alcoholysis of ammonia
borane was the rate-determined step. The addition of transition
metal catalysts could accelerate the alcoholysis of ammonia
O
Co-doped AB
OH
Ph
Me
Ph
Me
13
borane . Besides, cobalt element acted as a kind of Lewis acid
14
b
could activate the carbonyl group . Therefore, the doped Co-
based catalysts could accelerate the reduction reaction.
Chemical state of cobalt in Co-doped ammonia borane:
Noting that ammonia borane was a reducing agent and the
overall reaction environment was reductive, cobalt can hardly
Entry
Solvent
Neat water
Methanol
Yield (%)
1
2
3
4
5
6
7
8
81
90
85
82c
58c
76c
65c
Ethanol
Isopropanol
Acetonitrile
Tetrahydrofuran
Dichloromethane
Toluene
2+
keep the Co state. Electron paramagnetic resonance was
employed to identify the chemical state of cobalt doped in
ammonia borane. An absorption at g = 2.14 was observed
78
1
5
a
(Fig. 1), which was far from that of metallic cobalt (g = 1.41)
All reactions were carried out with Co-doped ammonia borane
5 mmol), hypnone (10 mmol) in solvent (15 mL) at room temperature
for 3 h; Isolated yields; Yield after hydrolysis
+
15,16
(
and close to that of Co (g = 2.16) . It was worth noting that
paramagnetic Co could not give a spectrum because of fast
b
c
2+
TABLE-2
CHEMOSELECTIVE REDUCTION OF CARBONYL COMPOUNDS TO
a
ALCOHOLS WITH AMMONIA BORANE AND Co-DOPED AMMONIA BORANE
O
CH OH
3
OH
R1
R2
R1
R2
1
2
b
Entry
Substrate
Product
Reductant
Time
Yield (%)
CHO
1
2
OH
AB
15 min
5 min
79
87
1a
2a
Co-doped AB
3
4
AB
15 min
5 min
90
98
1
b
OH
2b
2c
2d
2e
CHO
Co-doped AB
O
O
5
6
AB
15 min
5 min
80
89
1
c
CHO
CHO
OH
Co-doped AB
7
8
AB
15 min
5 min
82
92
1d
1e
Co-doped AB
OH
9
AB
15 min
5 min
87
95
O2N
CHO
O2N
1
0
OH
Co-doped AB
O
O
O
O
1
1
1
2
AB
15 min
5 min
90
97
CHO
1f
2f
Co-doped AB
OH
1
1
3
4
AB
2 h
2 h
77
95
1
g
h
2g
Co-doped AB
O
HO
O
OH
1
1
5
6
AB
16 h
3 h
86
90
1
2h
Co-doped AB
a
All reactions were carried out with ammonia borane (AB) or Co-doped ammonia borane (AB) (5 mmol), aldehyde or ketone (10 mmol) in
b
methanol (15 ml) at room temperature. Isolated yields

8
250 Huang et al.
Asian J. Chem.
95
90
85
4000000
3000000
2000000
1000000
0
g = 2.14
–1000000
–2000000
–3000000
–4000000
80
7
5
0
7
2000
2500
3000
3500
4000
4500
5000
1
2
3
4
Magnetic field (G)
Used times of co-catalysts in the reductio of hypnone
Fig. 1. Electron paramagnetic resonance curve of the Co-doped ammonia
borane sample
Fig. 2. Recyclability of Co-based catalysts
ACKNOWLEDGEMENTS
spin-lattice relaxation at room temperature. It was very likely
+
Financial support was provided by Science and Techno-
logy Bureau Foundation Project of Changsha (Grant No.
K1113005-31).
that part of Co element was in Co state.
2+
When the reaction completed, the cobalt was back to Co
state, according to electron paramagnetic resonance charac-
terization. Further investigations on the chemical state of Co
element and the reaction mechanism are needed.
REFERENCES
Recyclability of Co-based catalysts: The reusability of
Co-based catalysts was investigated in the model reaction of
reduction of hypnone. When the reduction reaction completed,
methanol was evaporated and hexane was used to extract the
alcohol. The residue was dried over anhydrous magnesium
sulfate and achieved red solid of which the main components
1. G. Wolf, J. Baumann, F. Baitalow and F.P. Hoffmann, Thermochim.
Acta, 343, 19 (2000).
2
.
A.D. Sutton, A.K. Burrell, D.A. Dixon, E.B. Garner, J.C. Gordon, T.
Nakagawa, K.C. Ott, J.P. Robinson and M. Vasiliu, Science, 331, 1426
(2011).
3. A. Staubitz,A.P.M. Robertson, M.E. Sloan and I. Manners, Chem. Rev.,
10, 4023 (2010).
Z. Tang, X. Chen, H. Chen, L. Wu and X. Yu, Angew. Chem. Int. Ed.,
2, 5832 (2013).
1
4
.
was CoCl . The solid was reused for the next reduction reaction.
2
5
As shown in Fig. 2, the Co-based catalysts could be reused up
to 4 times without significant loss in activity.
5. H. Inoue, T. Yamazaki, T. Kitamura, M. Shimada, M. Chiku and E.
Higuchi, Electrochim. Acta, 82, 392 (2012).
6
7
.
.
W. Li, G. Wu,Y. Chua,Y.P. Feng and P. Chen, Inorg. Chem., 51, 76 (2012).
S.K. Kim, H. Cho, M.J. Kim, H. Lee, J. Park, Y. Lee, H.C. Kim, C.W.
Yoon, S.W. Nam and S.O. Kang, J. Mater. Chem. A, 1, 1976 (2013).
R. Kumar and B.R. Jagirdar, Inorg. Chem., 52, 28 (2013).
S. Karahan, M. Zahmakiran and S. Özkar, Chem. Commun., 48, 1180
(2012).
Conclusion
Here we have reported a selective and new procedure for
chemoselective reduction of carbonyl compounds to corres-
ponding alcohols by Co-doped ammonia borane. The advan-
tages offered by this method include readily available and
stable reagent, excellent yields, mild conditions, operational
simplicity and applicability to a wide range of substrates.
Characterization results of Co-doped ammonia borane show
that the Co element in partially reduced chemical state, could
catalyze the reduction reaction.
8
9
.
.
10. D. Sun, V. Mazumder, O. Metin and S. Sun, ACS Nano, 5, 6458 (2011).
1
1. Z. Xiong,Y. Chua, G. Wu, L. Wang, M.W. Wong, Z.M. Kam, T. Autrey,
T. Kemmitt and P. Chen, Dalton Trans., 39, 720 (2009).
1
2. X. Yang, T. Fox and H. Berke, Tetrahedron, 67, 7121 (2011).
13. H. Dong and H. Berke, J. Organomet. Chem., 696, 1803 (2011).
14. S.E. Denmark and G.L. Beutner, Angew. Chem. Int. Ed., 47, 1560
(
2008).
1
5. E. Hayashi, E. Iwamatsu, M. Elias Biswas, Y. Sanada, S. Ahmed, H.
Hamid and T. Yoneda, Appl. Catal. A Gen., 179, 203 (1999).
1
1
6. M. Bhaduri and P.C.H. Mitchell, J. Catal., 77, 132 (1982).
7. P.V. Ramachandran and P.D. Gagare, Inorg. Chem., 46, 7810 (2007).