COMMUNICATION
DOI: 10.1002/chem.201201105
A General and Efficient Zinc-Catalyzed Oxidation of Benzyl Alcohols to
Aldehydes and Esters
[
a]
Xiao-Feng Wu*
The development of environmental benign methodologies
[1]
has become one of the main targets for organic chemists.
To fulfill the needs of green chemistry, certain challenges
remain, such as the replacement of noble metals with cheap
catalysts, increasing of reaction efficiency, using green re-
agents etc. For example, H O , as one of the green oxidants
Scheme 1. Transition-metal-catalyzed oxidative esterification of alcohols.
Initially, the reaction was carried out with one equivalent
2
2
in oxidation reactions, generates water as the only by-prod-
uct: thus, it should be used to replace other toxic oxidants.
of ZnBr . Thereby, the corresponding ester and benzalde-
2
[2]
hyde were formed from benzyl alcohol (1 mmol) in 12 and
14% yield, respectively, with 64% conversion in MeOH
(2 mL) by using H O (4 mmol) as oxidant at room tempera-
Zinc catalysts are cheap, abundant, and nontoxic. Therefore,
the use of zinc catalysts instead of catalysts based on noble
metals (Pd, Ir, Ru, etc.) is interesting and important from
2
2
ture (Table 1, entry 1). The addition of AcOH (2 mmol) im-
[3]
academic, industrial, and sustainability aspects.
[
a]
Table 1. Zinc-catalyzed oxidation of benzyl alcohol.
Esters are an important moiety in organic synthe-
sis, which hold broad applications in bulk chemicals,
fine chemicals, natural products, and polymers.
[4]
Entry [Zn]
Additive
Conv. Yield of
Yield of
aldehyde [%]
Traditionally, esters are prepared by the reaction of
activated acid derivatives with alcohols. Alterna-
tively, the palladium-catalyzed carbonylation of aryl
halides with alcohols offers an interesting and con-
[
b]
[b]
[b]
[
%]
ester [%]
1
2
3
4
5
6
7
8
9
ZnBr
ZnBr
ZnBr
2
2
2
(1 mmol)
(1 mmol)
(10 mol%)
–
64
94
61
50
12
23
15
0
14
16
8
AcOH (2 mmol)
AcOH (2 mmol)
AcOH (2 mmol)
[5]
venient procedure for their preparation. Addition-
ally, the oxidative esterification of aldehydes with
alcohols is also reported for the synthesis of
ZnI (10 mol%)
7
2
ZnF
2
(10 mol%)
(OTf) (10 mol%) AcOH (2 mmol)
(OAc) (10 mol%) AcOH (2 mmol)
(TFA)
AcOH (2 mmol) 50
0
0
0
0
0
2
24
29
30
0
15
18
17
17
16
16
41
41
32
0
Zn
Zn
Zn
A
T
N
T
E
N
G
2
42
46
A
H
U
G
E
N
N
2
[6]
esters. More recently, the groups of Beller and Lei
reported a synthetically interesting oxidative cross-
A
H
U
G
R
N
U
G
2
(10 mol%) AcOH (2 mmol) 52
Zn(CN)
2
(10 mol%) AcOH (2 mmol)
56
esterification of benzylic and aliphatic alcohols, re- 10
ZnBr
ZnBr
ZnBr
ZnBr
–
2
2
2
2
(10 mol%)
(10 mol%)
(10 mol%)
(10 mol%)
PivOH (2 mmol) 53
[
7]
1
1
1
1
1
2
3
4
TFA (0.2 mL)
TFA (0.1 mL)
TFA (0.05 mL)
–
98
97
100
<3
spectively. In their methods, palladium was used
as catalyst and by using oxygen as the terminal oxi-
dant various esters were prepared in good yields.
Herein, we wish to report a general and efficient
methodology for the oxidation of benzyl alcohols to
[
a] Zinc catalyst, benzyl alcohol (1 mmol), MeOH (2 mL), H
RT, 16 h. [b] Conversion and yield were determined by GC using hexadecane as inter-
2 2
O (4 mmol), additive,
the corresponding aldehydes and esters by using nal standard, based on benzyl alcohol.
zinc bromide as catalyst. H O was used as a green
2
2
oxidant, and all the reactions were carried out at
room temperature under air. To the best of our knowledge,
this is the first zinc-catalyzed oxidative cross-esterification
of alcohols (Scheme 1).
proved the conversion to 94% with improved yield of the
ester (Table 2, entry 2). To our delight, similar results were
also observed with 10 mol% of ZnBr2 (Table 1, entry 3).
When using 10 mol% of CuBr , MnCl , or FeCl to replace
2
2
3
ZnBr2 under the conditions, only traces of benzaldehyde
were formed with less than 5% of benzyl alcohol converted.
Then, other zinc salts were tested, but, in all the cases, no
better results were observed and only moderate conversion
was achieved (Table 1, entries 4–9). After the testing of dif-
[
a] Dr. X.-F. Wu
Department of Chemistry, Zhejiang Sci-Tech University
Xiasha Campus, Hangzhou
ferent zinc salts, ZnBr was chosen as catalyst to examine
2
3
10018 Zhejiang Province (P. R. China)
some other acid additives. Pivalic acid (2 mmol) did not fur-
ther improve the yields of the desired products, and the best
results were obtained by using trifluoroacetic acid (TFA) as
additive (Table 1, entries 10–13). In all the experiments, ben-
and
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: xiao-feng.wu@catalysis.de
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1
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ÞÞ
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