Calcium and magnesium chlorides-catalyzed oxidative esterification of aromatic aldehydes

Article abstract of DOI:10.1016/j.tetlet.2014.01.111

An interesting procedure for the oxidative esterification of aromatic aldehydes has been developed. By using catalytic amount of CaCl₂ or MgCl₂, various methyl benzoates were isolated in good yields with hydrogen peroxide as the terminal oxidant.

Full text of DOI:10.1016/j.tetlet.2014.01.111

Tetrahedron Letters 55 (2014) 1657–1659
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Calcium and magnesium chlorides-catalyzed oxidative esterification
of aromatic aldehydes
a
a
a
a,b,
⇑
Jian-Bo Feng , Jin-Long Gong , Qin Li , Xiao-Feng Wu
a
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha Campus, Hangzhou, Zhejiang Province 310018, People’s Republic of China
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
An interesting procedure for the oxidative esterification of aromatic aldehydes has been developed. By
Received 10 December 2013
Revised 22 January 2014
Accepted 24 January 2014
Available online 31 January 2014
using catalytic amount of CaCl
hydrogen peroxide as the terminal oxidant.
2
or MgCl
2
, various methyl benzoates were isolated in good yields with
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Aldehyde
Oxidative esterification
Ester
Magnesium chloride
Calcium chloride
Carboxylic acid derivatives are important building blocks in
industrial and organic synthesis. Among them, carboxylic esters
are representative examples with broad applications in polymers,
2 2
lent yields. During that study, CaCl and MgCl were tested as
catalysts and could give the desired methyl benzoate in moderate
1
perfumes, advanced materials, and so on. Regarding their impor-
tance, numerous methodologies have been developed for their
preparation. Traditionally, the reaction between carboxylic acids
and alcohols under the assistance of activating reagents or water
abstracting reagents are a well established procedure. Additionally,
palladium-catalyzed alkoxycarbonylation provides a promising
pathway for the synthesis of esters by applying aryl halides as
Table 1
Optimization of the oxidative esterification of benzaldehyde in methanola
O
O
[
Catal], H
2
O
o
2
(equiv.)
MeOH
2
PhCO H
Ph
H
65 C
Ph
OMe
2
substrates. More recently, the oxidation of activated alcohols to
3
the corresponding esters provide a more direct reaction pathway.
From a synthetic point of view, the oxidative transformation of
readily available aldehydes with alcohols offers a selective and
direct reaction manner for ester synthesis.
Yieldb (%)
Entry
Catal.
H
2
O
2
T (h)
Conv. (%)
1
2
3
4
5
6
7
8
9
10
CaCl
CaCl
CaCl
2
2
2
4
2
6
4
2
4
4
6
4
2
2
48
34
40
38
50
46
37
41
26
40
24
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
55
46
40
42
47
In the literature, stoichiometric amounts of heavy-metal
4
5
6
7
oxidants such as KMnO
4
,
CrO
3
,
hydrogen peroxide, ozone,
8
9
oxone, N-iodosuccinimide, or transition-metal catalysts such as
CaCl2
1
0
11
12
13
14
vanadium, rhenium, silver, palladium, ruthenium, rho-
CaCl
CaCl
2
c
dium, copper,16 titanium,17 iridium, iron, nickel,
1
5
18
19a
19b
2
53
etc. were
MgCl
MgCl
MgCl
MgCl
MgCl
2
2
2
2
2
42
38
36
68
38
usually used for the success of this transformation. More recently,
our group described a zinc-catalyzed oxidative transformation of
2
0
aromatic aldehydes. In the presence of catalytic amount of ZnBr
and H , various benzoates have been prepared in good to excel-
2
11
2 2
O
a
Catal. (10 mol %), H
5b °C.
Isolated yields.
2 2 2
O (30 w % in H O), MeOH (4 mL), aldehyde (1 mmol),
6
⇑
Corresponding author. Tel.: +49 3811281343.
E-mail address: xiao-feng.wu@catalysis.de (X.-F. Wu).
c
80 °C.
http://dx.doi.org/10.1016/j.tetlet.2014.01.111
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0

1
658
J.-B. Feng et al. / Tetrahedron Letters 55 (2014) 1657–1659
GC-yields. Regarding the abundance, environmental benign and
low toxicity of CaCl and MgCl , it will be interesting and necessary
to verify the generality of using CaCl and MgCl as catalysts in the
oxidative esterification of aldehydes.
Based on our previous results, further optimization was car-
ried out at the first stage (Table 1). 55% of methyl benzoate
Table 3
MgCl
-catalyzed oxidative esterificationa
2
2
2
2
2
O
O
o
MgCl2 (10 mol%), H O , 65 C
2
2
MeOH
Ar
H
Ar
OMe
was isolated by using CaCl
presence of 4 equiv. of H
was decreased either decreasing or increasing the amount of
(Table 1, entries 2 and 3). Further attempts on reaction
temperature and reaction time variation did not give improved
2
(10 mol %) as the catalyst, in the
O at 65 °C (Table 1, entry 1). The yield
2 2
2
H O
2
Entry
1
Products
Yieldb (%)
68
2
CO Me
yields (Table 1, entries 4–6). In the case of MgCl , 68% of methyl
2 2
benzoate was isolated with only 2 equiv of H O (Table 1, entry
2
1
0). Lower loading of catalyst resulted in non-completed conver-
2
CO Me
sion even after 60 h. In all the cases, benzoic acid was detected
as the only by-product.
2
3
4
5
65
53
78
68
CO
2
Me
BnO
CO
2
Me
Table 2
a
Calcium chloride-catalyzed oxidative esterification
2
CO Me
O
O
o
CaCl
2
(10 mol%), H
2
O
2
, 65 C
MeOH
Ar
H
Ar
OMe
3
F C
2
CO Me
6
7
78
75
NC
Entry
1
Product
Yieldb (%)
55
2
CO Me
2
CO Me
O
2
N
2
CO Me
2
CO Me
2
3
54
34
8
9
69
CO
2
Me
NO
2
2
CO Me
75
82
F3C
NC
CO
2
Me
Cl
Cl
2
CO Me
4
5
77
79
1
1
0
1
CO
2
Me
Cl
CO Me
2
2
O N
84
CO
2
Me
Cl
Cl
6
7
8
9
74
76
75
CO Me
2
1
1
2
3
80
51
NO
2
CO
2
Me
Br
S
CO Me
2
Cl
Cl
CO
2
Me
a
2 2 2
MgCl (10 mol %), H O (2 equiv), MeOH (4 mL), aldehyde (1 mmol), 65 °C, 40 h.
b
Isolated yields.
Cl
CO
2
Me
81
9
Br
O
CO
2
Me
MgCl2
OH
OMe MgCl2
1
0
O
H O
2
O
2
MeOH
Ar
Ar
OMe
Ar
H
a
CaCl
2
(10 mol %), H
2 2
O (4 equiv), MeOH (4 mL), aldehyde (1 mmol), 65 °C, 48 h.
b
Isolated yields.
Scheme 1. Proposed reaction mechanism.

J.-B. Feng et al. / Tetrahedron Letters 55 (2014) 1657–1659
1659
We then applied calcium chloride as the catalyst to test differ-
ent aromatic aldehydes in methanol under our standard conditions
Supplementary data
2
1
(Table 2). In general, electron withdrawing substituted methyl
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.
01.111.
benzoate derivatives can be isolated in moderate to good yields
from the corresponding aldehydes (Table 2, entries 3–9), while
lower yield of the desired ester was formed with electron-donating
substituted aldehyde (Table 2, entry 2). In the case of heterocyclic
aldehydes, low or no desired esters were formed. 9% of methyl
furan-2-carboxylate was isolated from the corresponding aldehyde
References and notes
1
2
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(
2
Table 2, entry 10). No desired ester was produced from thiophene-
-carbaldehyde, but only thiophene-2-carboxylic acid.
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should be the key intermediate, then followed by the oxidation
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1
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2
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1. Under air, in a 50 mL tube, CaCl or MgCl (10 mol%), and a stirring bar were
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2
2
2
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2
2
Acknowledgments
the tube after the addition of aldehyde (1 mmol) and MeOH (4 mL) by syringe.
Then close the tube and keep the final solution at 65 °C for 40 h. When the
reaction was finished, the solvent was removed in vacuo, and the residue was
purified by flash column (petroleum ether/ethyl acetate, 9:1). All the products
are commercially available.
The authors thank the financial support from Zhejiang Sci-Tech
University (1206838-Y). X.–F. Wu appreciate the general support
from Matthias Beller in LIKAT.