A simple, efficient and recyclable copper(II) acetylacetonate catalytic system for oxidation of sec-alcohols in ionic liquid

Article abstract of DOI:10.1055/s-2007-967969

A selective oxidation of secondary alcohols to the corresponding ketones in room temperature ionic liquid was achieved by using copper acetylacetonate/ tert-butylhydroperoxide system. The catalytic system can be recycled and reused for five runs without any significant loss of catalytic activity and products are obtained in excellent yield. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-967969

LETTER
643
A Simple, Efficient and Recyclable Copper(II) Acetylacetonate Catalytic
System for Oxidation of sec-Alcohols in Ionic Liquid
O
C
xidation of Sec
h
y
A
lcoh
o
ols byCu(a
n
)
in Ionic
g
Liquid Liu, Jinyu Han,* Juan Wang
2
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology,
Tianjin University, Tianjin 300072, P. R. of China
Fax +86(22)27890859; E-mail: hanjinyu@tju.edu.cn
Received 24 November 2006
the transformation of sec-alcohols is time-consuming for
Cu(I), with Cu(II) no reaction occurs, and both systems
are complex. Herein, we report a mild and efficient
procedure for the oxidation of sec-alcohols to ketones
Abstract: A selective oxidation of secondary alcohols to the corre-
sponding ketones in room temperature ionic liquid was achieved by
using copper acetylacetonate/tert-butylhydroperoxide system. The
catalytic system can be recycled and reused for five runs without
any significant loss of catalytic activity and products are obtained in with good to excellent conversion using 70% aqueous
excellent yield.
tert-butylhydroperoxide (t-BuOOH) as the oxygen
Key words: copper acetylacetonate, oxidation, sec-alcohols, ionic source, catalyzed by Cu(acac) in ionic liquid 1-butyl-3-
2
liquid, catalyst
methylimidazolium hexafluorophosphate {[bmim]PF6,
Scheme 1}.
OH
O
The oxidation of alcohols to carbonyl compounds is one
of the important reactions in organic chemistry both at the
laboratory and industrial synthetic chemistry. Tradition-
3
mol% Cu(acac)2
t-BuOOH
R1
R2
R1
R2
[
bmim]PF6, 5 h, r.t.
1
ally, this transformation is performed with stoichiometric Scheme 1 Oxidation of secondary alcohols with Cu(acac) in
2
amounts of toxic and expensive inorganic oxidizing [bmim]PF
agents [e.g. KMnO , Cr(VI) compounds, HNO ] and in
6
4
3
organic solvents which are of volatility, toxicity, and
flammability. In various metal-containing oxidation cat-
The initial study was carried out using 1-phenylpropan-1-
ol as the substrate to optimize the reaction conditions,
and the results are summarized in Table 1. At first, four
2
alysts, copper metal represents one of the most elegant
and effective members for these important transforma-
ionic liquids [bmim]PF , 1-butyl-3-methylimidazolium
6
3
tions. As a cheap, soluble, easy to prepare catalyst, cop-
tetrafluoroborate {[bmim]BF }, 1-ethyl-3-methylimida-
4
per acetylacetonate [Cu(acac) ] has been used in various
2
zolium trifluoroacetate {[emim]OOCCF } and N-butyl-
3
4
reactions, such as benzylation of alcohols, reduction of
aromatic nitro compounds, aziridination and epoxida-
tion. Immobilized catalysts have been of great interest
pyridinium hexafluorophosphate {[bpy]PF } were tested
6
5
6,7
with Cu(acac) for the oxidation. Generally, oxidation
2
7
proceeded rapidly in PF -type ionic liquids while a little
6
due to several advantages, such as the ease of product
slow rate occurred in BF -type ionic liquids. Furthermore,
4
work-up, separation, isolation and reuse of the catalyst.
imidazolium-type ionic liquids are better than the pyridin-
ium-type ionic liquids (Table 1, entries 1–3). It should be
noted that the oxidation in ionic liquid [emim]OOCCF3
8
To date, Cu(acac) has been both entrapped in zeolites
and used in combination with ionic liquids.
2
6
,9
Over the last several years, ionic liquids have generated
much excitement in the field of organic synthesis, because
of their potential as green solvents. Nowadays, they are
being used in a broad range of organic catalytic systems.
With the increasing environmental consciousness and
Table 1 Optimization of the Reaction Conditions for Oxidizing
1
-Phenylpropan-1-ol to Acetophenonea
Yield (%)b
Entry Ionic liquid
Conversion (%)
1
0
1
2
3
4
5
[bmim]BF4
90
98
95
–
84
91
90
–
improving economic efficiency, many transition metals
1
1
12
13
14
such as palladium, ruthenium, manganese, tungsten
[bmim]PF6
1
5
and rhenium have been used as catalysts for alcohol
oxidation with ionic liquids as the solvent. However, Cu
catalytic systems for oxidation of alcohols in ionic liquid
have received less attention, especially Cu(II). Recently,
aerobic oxidation of alcohols to the corresponding alde-
hydes or ketones using Cu–TEMPO systems in ionic liq-
[bpy]PF6
[emim]OOCCF3
[bmim]PF
96
49
88c
43d
6
6
[bmim]PF6
1
6
uid have been reported. However, the limitations are that
a
1
-Phenylpropan-1-ol (2 mmol), Cu(acac) (3 mol%), t-BuOOH
2
(
10 mmol), ionic liquid (1 mL), r.t. for 5 h.
Isolated yield by flash chromatography.
TEMPO (3 mol%) was added.
SYNLETT 2007, No. 4, pp 0643–0645
Advanced online publication: 21.02.2007
DOI: 10.1055/s-2007-967969; Art ID: W24306ST
0
1.
0
3.
2
0
0
7
b
c
d
H O (30% aq) as the oxygen source.
2
2
©
Georg Thieme Verlag Stuttgart · New York

6
44
C. Liu et al.
LETTER
was not satisfactory (Table 1, entry 4), and TEMPO has was obtained from the 2-substituted a-phenethyl alcohols
no efficiency in the oxidation reaction for the Cu(acac)2 (Table 2, entries 3 and 4). In addition, the conversion of
catalytic system (Table 1, entry 5). Due to the poor solu- benzyl alcohol was different compared other secondary
bility of H O , the yield of the product was just 43% alcohols and the oxidation occurred to generate the corre-
2
2
(
Table 1, entry 6).
sponding acid as the major product (Table 2, entry 9).
Having optimized the reaction conditions, the oxidation of Subsequently, the recyclability of Cu(acac) catalytic sys-
2
1
7
other alcohols were then examined (Table 2). It was tem was examined for the oxidation of 1-phenylpropan-1-
clear that both benzylic and aliphatic alcohols were oxi- ol in [bmim]PF (Table 3). Upon completion of the reac-
6
dized in excellent and moderate yield in the catalytic sys- tion, the ionic liquid phase that contained [bmim]PF and
6
tem, respectively. For benzylic alcohols, the 4-substituted catalyst Cu(acac) was washed with n-hexane to remove
2
a-phenethyl alcohols can be converted into the corre- the organic residues. After five times recycled, the
sponding ketones in high yield while the moderate yield catalyst activity did not reduce drastically.
Table 2 Oxidation of Secondary Alcohols Using Cu(acac) as Catalyst in [bmim]PF6a
2
Entry
Substrate
Product
Time (h)
Conversion (%)
98
Yield (%)b
91
OH
O
O
1
5
OH
2
3
4
5
5
5
97
99
48
91
93
41
OH
O
Cl
Cl
Cl
Cl
OH
O
OH
OH
O
O
5
5
99
99
94c
93
O
O
6
7
5
5
OH
OH
O
O
60
73
51
65
1
5
5
5
57
72
52
66
8
9
1
1
OH
O
5
5
36
64
31
58
O
OH
10
5
99
60d
OH
a
b
c
d
Secondary alcohol (2 mmol), Cu(acac) (3 mol%), t-BuOOH (10 mmol), ionic liquid (1 mL), r.t. for 5 h.
Isolated yield.
2
CH Cl (2 mL) was added to help dissolve the substrate.
2
2
The corresponding carboxylic acid was formed as the major product.
Synlett 2007, No. 4, 643–645 © Thieme Stuttgart · New York

LETTER
Oxidation of Secondary Alcohols by Cu(acac) in Ionic Liquid
645
2
Table 3 Recycling of the Catalytic System for the Oxidation of
-Phenylpropan-1-ol to Acetophenonea
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(
98
96
93
93
90
91
89
88
87
84
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2
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6
(
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The authors gratefully acknowledge that this work was financially
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bmim]PF (1 mL) was added to the mixture. After 10 min
[
6
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Synlett 2007, No. 4, 643–645 © Thieme Stuttgart · New York