Solvent-free selective oxidation of primary and secondary alcohols catalyzed by ruthenium-bis(benzimidazole)pyridinedicarboxylate complex using hydrogen peroxide as an oxidant

Article abstract of DOI:10.1016/j.tetlet.2013.05.055

A convenient and selective oxidation of alcohols with aqueous hydrogen peroxide to give the corresponding carbonyl compounds under solvent-free conditions has been developed. By applying ruthenium-bis(benzimidazole) pyridinedicarboxylate complex [Ru(bbp)(pydic)] as catalyst, primary, and secondary alcohols were oxidized to aldehydes and ketones in good yield and excellent selectivity under mild conditions.

Full text of DOI:10.1016/j.tetlet.2013.05.055

Tetrahedron Letters 54 (2013) 3882–3885
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvent-free selective oxidation of primary and secondary alcohols
catalyzed by ruthenium-bis(benzimidazole)pyridinedicarboxylate
q
complex using hydrogen peroxide as an oxidant
Xian-Tai Zhou , Hong-Bing Ji , Sheng-Gui Liu b,
a
a,
⇑
⇑
a
School of Chemistry and Chemical Engineering, Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province,
Sun Yat-sen University, 510275 Guangzhou, PR China
School of Chemistry Science and Technology, Development Center for New Materials Engineering & Technology in Universities of Guangdong,
b
Zhanjiang Normal University, 524048 Zhanjiang, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient and selective oxidation of alcohols with aqueous hydrogen peroxide to give the correspond-
ing carbonyl compounds under solvent-free conditions has been developed. By applying ruthenium-
bis(benzimidazole)pyridinedicarboxylate complex [Ru(bbp)(pydic)] as catalyst, primary, and secondary
alcohols were oxidized to aldehydes and ketones in good yield and excellent selectivity under mild
conditions.
Received 17 February 2013
Revised 11 April 2013
Accepted 13 May 2013
Available online 21 May 2013
Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
Keywords:
Solvent-free
Oxidation
Alcohols
Hydrogen peroxide
Ruthenium complex
Introduction
Ruthenium complexes with nitrogen-based ligands have been
intensively investigated in order to develop catalysts for organic
6
Oxidation of alcohols to corresponding carbonyl compounds is
of great importance for both laboratory and synthetic industrial
applications. In traditional oxidation processes, large amounts of
oxidation processes. We ever reported an efficient oxidation pro-
cess of alcohols catalyzed ruthenium porphyrins in the presence of
1
7
molecular oxygen. Nishiyama first reported the asymmetric epox-
toxic and volatile organic solvents and metal oxidants were exten-
sively used. The need for environmentally benign and clean oxida-
tion reactions remains an important goal of chemical research.
idation by one kind of ruthenium complex based on bis(oxazoli-
nyl)pyridine,
that
is,
ruthenium-(pyridinebisoxazoline)
2
(pyridinedicarboxylate) complex [Ru(pybox)(pydic)].8 Through
modification of Nishiyama’s catalyst, Beller research group devel-
oped efficient asymmetric epoxidation processes with a greener
oxidant such as tert-butylhydroperoxide (TBHP) or hydrogen per-
Hence, developing green selective oxidation process of alcohols is
3
still a challenging task in catalysis. By comparing different oxida-
tion methods, it is apparent that the oxidant used in the respective
transformation defines the quality and applicability of the method.
In addition to molecular oxygen, hydrogen peroxide is an environ-
mentally benign oxidant, which theoretically generates only water
9
oxide in recent years.
We looked for ruthenium complexes that should be efficient for
the oxidation of alcohols. Inspired by the efficiency of Nishiyama’s
catalyst in the epoxidation, we adopted the introduction of 2,6-
bis(benzimidazole)pyridine as the counterpart to synthesize new
catalyst with dual closed meridional stereotopes around an active
metal. Hence, based on 2,6-bis(benzimidazole)pyridine and
pyridinedicarboxylate, the novel kind ruthenium complex
4
as a by-product. Therefore, the discovery of new protocol catalyst
2 2
using H O is gathering much attention. In this content, variety of
transition metal-based catalysts has been intensively investigated
toward the oxidation of alcohols so far.5
[
Ru(bbp)(pydic)] (Scheme 1) was successfully synthesized and
was applied as catalyst in the oxidation reactions. Interestingly to
find this ruthenium complex is efficient for the selective oxidation
of alcohols to corresponding carbonyl compounds with hydrogen
peroxide as oxidant. Meanwhile, the noteworthy feature for this
catalytic system could be that the selective oxidation of alcohols
can be achieved under solvent-free conditions.
q
This is an open-access article distributed under the terms of the Creative
Commons Attribution-NonCommercial-No Derivative Works License, which per-
mits non-commercial use, distribution, and reproduction in any medium, provided
the original author and source are credited.
⇑
Corresponding authors. Tel.: +86 20 84113658; fax: +86 20 84113654.
E-mail addresses: jihb@mail.sysu.edu.cn (H.-B. Ji), lsgui@sohu.com (S.-G. Liu).
0
040-4039/$ - see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.055

X.-T. Zhou et al. / Tetrahedron Letters 54 (2013) 3882–3885
3883
Results and discussion
H
N
H
N
N
To explore the reactivity and selectivity of the Ru(bbp)(pydic)
1
0
N
O
N
O
catalyst, benzyl alcohol was used as a model substrate. Various
solvents were examined in the presence of Ru(bbp)(pydic) catalyst
and H O (Table 1, entries 1–8). After much experimentation on
2 2
Ru
11
optimizing solvent, it was found that the use of a less-polar solvent
like toluene and ethyl acetate afforded benzaldehyde in low yields
(entries 1 and 2). Higher yield of benzaldehyde was obtained using
polar solvents like methanol and acetonitrile (entries 3 and 4).
Gratifyingly, it was found under solvent free conditions, the cata-
lyst gave excellent conversion to benzaldehyde with 96% yield (en-
try 5).
N
O
O
Scheme 1. The structure of Ru(bbp)(pydic).
The effect of reaction parameters was examined by performing
the reaction in solvent-free conditions, as listed in Table 2. Only
Table 1
Oxidation of benzyl alcohol catalyzed by Ru(bbp)(pydic) with aqueous H
a
2 2
O
3
5% yield of benzaldehyde was obtained when the reaction was
Conv.b (%)
Yieldb (%)
conducted at 40 °C, while the selectivity would become poor as
further rising the temperature to 80 °C (entries 1 and 2). Therefore,
the optimized temperature was proved to be 60 °C. The yield of
Entry
Solvent
1
2
3
4
5
Toluene
Ethyl acetate
5
12
66
78
97
4
10
63
74
96
CH
CH
3
OH
CN
benzaldehyde increased with increasing the molar ratio of H
benzyl alcohol (entries 3 and 4). The large excess amount of
could promote the over-oxidation of benzaldehyde, which re-
2 2
O /
3
Solvent free
2 2
H O
a
À3
Reaction condition: benzyl alcohol (2 mmol), catalyst (2 Â 10 mmol), 30%
(10 mmol), 60 °C, 60 min. solvent (2 mL).
Determined by GC.
sulted in the slight decreasing of selectivity toward benzaldehyde
(entry 5). The reaction almost did not occur in the absence of cat-
alyst (entry 6). Similarly, the yield of benzaldehyde increased with
the rising amount of catalyst. And the excess amount of catalyst
caused decrease of selectivity to benzaldehyde (entry 8).
H
2
b
O
2
2 2
To examine the scope of the alcohol reaction with H O –
Ru(bbp)(pydic) system, we extended our studies to various pri-
mary alcohols. The results are summarized in Table 3.
Table 2
a
Optimization of reaction conditions under solvent free conditions
b
Conv.c (%)
c
Entry
Substrate: H
2
O
2
T (°C)
Yield (%)
It was found that most primary alcohols were smoothly con-
verted to corresponding carbonyl compounds with high conversion
rate and excellent selectivity. Compared with the electron-with-
drawing groups at para-position for benzylic alcohols, the elec-
tron-donating groups seemed more favorable to the formation of
carbonyl compounds (entries 1–4, Table 3). For basic substrates
like 4-pyridinemethanol (entry 5, Table 3), additional amounts of
catalyst are needed to improve reaction rates and to ensure good
conversions. The catalytic system is also efficient for the oxidation
of saturated primary aliphatic alcohols such as 2-phenylethanol
and 1-octanol (entries 6 and 7, Table 3).
1
2
3
4
5
1:5
1:5
1:2
1:3
1:6
1:5
1:5
1:5
40
80
60
60
60
60
60
60
37
>99
43
64
98
5
35
87
40
62
93
3
d
6
7
8
e
83
>99
80
92
f
a
b
c
d
e
f
À3
Reaction condition: benzyl alcohol (2 mmol), catalyst (2 Â 10 mmol), 60 min.
Molar ratio.
Determined by GC.
In the absence of catalyst.
À4
Catalyst (2 Â 10 mmol).
As shown in Table 3, the H
2 2
O -Ru(bbp)(pydic) system was
À2
Catalyst (2 Â 10 mmol).
found to be selective, efficient for the oxidation of secondary
Table 3
2 2
Oxidation of various alcohols with H O
catalyzed by Ru(bbp)(pydic)a
Entry
Substrate
Product
Time (h)
1
Conv.b (%)
97
Yieldb (%)
96
OH
OH
O
1
O
2
3
1
1
98
96
96
93
OH
OH
O
MeO
MeO
O
4
2
3
82
76
81
O N
O N
2
2
OH
O
5c
74
N
N
(
continued on next page)

3
884
X.-T. Zhou et al. / Tetrahedron Letters 54 (2013) 3882–3885
Table 3 (continued)
Entry
Substrate
Product
Time (h)
Conv.b (%)
Yieldb (%)
OH
O
6
7
1
1
95
98
92
94
OH
O
OH
O
O
8
9
1
3
92
82
92
82
OH
OH
O
1
0
1
3
1
78
94
76
93
OH
O
1
OH
OH
O
1
1
2
3
1
1
89
97
87
95
O
a
b
c
À3
Reaction condition: substrate (2 mmol), catalyst (2 Â 10 mmol), 30% H
2 2
O (10 mmol), 60 °C.
Determined by GC.
À3
Catalyst (4 Â 10 mmol).
2 2
at 325 and 346 nm. Addition of H O resulted in the immediate con-
2
version of the original spectrum containing the two characterized
peaks into a new spectrum displaying absorption peak at 320 nm.
This spectrum as well as the spectroscopic features supports the
conclusion that the complex has been converted into a ruthe-
1
2
nium(III) species. As reported previously by Beller, ruthenium
oxo complex was the active catalyst in the asymmetric epoxidation
Ru(II)/alcohol
9
d
Ru(II)/alcohol/H O /0 min
system. As to Ru(bbp)(pydic)-catalyzed alcohols oxidation, the
reaction mechanism could also involve the participation of Ru-oxo
species generated from the reaction between Ru(bbp)(pydic) and
hydrogen peroxide. The formation of carbonyl compounds was
attributed to the reaction of alcohols with Ru-oxo species, followed
by the b-hydride elimination. Further mechanistic studies on the ac-
tive species are under investigation.
2
2
1
0
Ru(II)/alcohol/H O /30 min
2
2
In conclusion, the Ru(bbp)(pydic) catalyst system described
herein is efficiently oxidizing alcohols in solvent free conditions
when hydrogen peroxide is used as an oxidant. Both primary and
secondary alcohols were oxidized into their corresponding car-
bonyl compounds in good yield. Further studies to improve the
reaction rate through catalyst modifications and detailed mecha-
nistic studies are ongoing.
3
00
400
500
600
Wavelength/nm
Figure 1. UV–vis spectra of Ru(bbp)(pydic) catalyst in the solution of benzyl
alcohol oxidation in the presence of hydrogen peroxide, benzyl alcohol (2 mmol),
catalyst (2 Â 10 mmol), 30% H
À3
2 2
O (10 mmol), 60 °C.
Acknowledgments
alcohols. The reaction works well with sterically hindered alcohols
such as diphenylmethanol and 2-adamantanol (entries 9 and 10,
Table 3). Many other secondary cyclic alcohols were efficiently oxi-
dized in the catalytic system (entries 11 and 12, Table 3). The cat-
alytic oxidation can also be successfully performed with aliphatic
secondary alcohols (entry 13, Table 3).
The authors thank the National Natural Science Foundation of
China (Nos. 21036009 and 21176267), Guangdong Natural Science
Foundation (S2011040001776) and Research Fund for the Doctoral
Program of Higher Education of China (20110171120024) for pro-
viding financial support to this project.
Ru(bbp)(pydic) catalyst was monitored by in situ UV–vis spec-
troscopy during the reaction with H O (5 equiv) under solvent-free
2 2
References and notes
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the initial characteristic absorption peaks of Ru(bbp)(pydic) were
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disodium pyridine-2,6-dicarboxylate (0.48 mmol) in EtOH/H
2
]
2
(150 mg, 0.24 mmol) in EtOH (8 mL),
O (2:1, 10 mL)
2
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1
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H
16
N
6
O
4
Ru: C, 52.44;
À1
5
6
.
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3
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5
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À3
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2
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