One Pot Electrooxidative Conversion of Benzyl Type Alcohols into the Corresponding Methyl Carboxylates

Article abstract of DOI:10.1081/SCC-120027264

One-pot conversion of several benzyl type alcohols into the corresponding methyl carboxylates was carried out by indirect electrooxidation using iodide ion as the electron carrier.

Full text of DOI:10.1081/SCC-120027264

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One Pot Electrooxidative Conversion of Benzyl Type
Alcohols into the Corresponding Methyl Carboxylates
a
a
a
a
Mitsuhiro Okimoto , Yuji Nagata , Satoru Sueda & Yukio Takahashi
a
Department of Applied Chemistry and Circumstance Engineering, Kitami Institute of
Technology, Koen-cyo165, Kitami, 090-8507, Japan
Published online: 18 Oct 2011.
To cite this article: Mitsuhiro Okimoto , Yuji Nagata , Satoru Sueda & Yukio Takahashi (2004) One Pot Electrooxidative
Conversion of Benzyl Type Alcohols into the Corresponding Methyl Carboxylates, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 34:2, 281-285, DOI: 10.1081/
SCC-120027264
To link to this article: http://dx.doi.org/10.1081/SCC-120027264
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SYNTHETIC COMMUNICATIONS
Vol. 34, No. 2, pp. 281–285, 2004
One Pot Electrooxidative Conversion
of Benzyl Type Alcohols into the
Corresponding Methyl Carboxylates
*
Mitsuhiro Okimoto, Yuji Nagata, Satoru Sueda,
and Yukio Takahashi
Department of Applied Chemistry and Circumstance Engineering,
Kitami Institute of Technology, Kitami, Japan
ABSTRACT
One-pot conversion of several benzyl type alcohols into the correspond-
ing methyl carboxylates was carried out by indirect electrooxidation
using iodide ion as the electron carrier.
Key Words: Benzyl type alcohols; Electrooxidation; Methyl carboxy-
lates; Iodide ion.
Oxidation of organic compounds using molecular iodine as the oxidizing
agent has been limited due to the toxicity of iodine.
[1–4]
However, with the use
*
Correspondence: Mitsuhiro Okimoto, Department of Applied Chemistry and
Circumstance Engineering, Kitami Institute of Technology, Koen-cyo165, Kitami
90-8507, Japan; Fax: þ81-157-24-7719.
0
2
81
DOI: 10.1081/SCC-120027264
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

ORDER
REPRINTS
2
82
Okimoto et al.
of catalytic amounts of iodide ion instead of the toxic iodine, these oxidation
[5,6]
reactions have attracted renewed interest as useful synthetic methods.
Chemical oxidations of benzyl alcohols using various strong oxidizing
[
7,8]
[9,10]
agents, such as active manganese oxide,
chromium oxide,
are extensively reported in synthetic reactions. Although com-
paratively, a benzylic hydroxyl group is more readily oxidized to the corres-
and nitric
[
11,12]
acid,
[
13]
ponding carbonyl group than an aliphatic hydroxyl groups,
to the best of
our knowledge, application of the electrochemical technique to the one-pot
conversion of benzyl type alcohols (1a–l) into the corresponding methyl
carboxylates (2a–l) has not been reported. We have previously reported on the
[
14]
[15–17]
indirect electrooxidation of aldehydes,
[
ketones,
using halide ion as an electron carrier.
and hydra-
18,19]
zones
As a continuation in this series of studies, we have carried out one-pot
conversion of 1a–l to the corresponding 2a–l through indirect electro-
oxidation.
Benzyl type alcohols 1a–l were oxidized electrochemically to afford the
corresponding methyl carboxylates 2a–l in good to excellent isolated yields
(
Table 1). Electrooxidation products 2a–g and 2i–l were isolated with yields
of 80% or above, with the single exception for the electrooxidation of 4-
nitrobenzyl alcohol (1h), in which methyl 4-nitrobenzoate (2h)was obtained in
only 38% yield, along with unreacted 1h.
We have observed a tendency that, remarkably, the ortho-substituted
substrates (1c, 1e, and 1g) consumed significantly more quantities of
electricity to complete the electrooxidation reaction than the para-isomers
(1b, 1d, and 1f). We have also observed that several substrates that contain an
electron-donating group on the aromatic ring (1b, 1c, 1d, 1e, and 1i) oxidized
more readily than substrates bearing an electron-withdrawing group (1f, 1g,
and 1h). It is reasonable to assume that the electrooxidation of 1 proceeded via
formation of the corresponding aldehydes as the reaction intermediate since
detectable amounts of the aldehydes were usually confirmed by GLC analyses
of the anolyte during the course of the electrooxidation. Furthermore, the
conversion of 1 into the corresponding aldehydes appeared as the rate-
determining step, since several aromatic aldehydes were readily oxidized into
the corresponding methyl carboxylates 2 with yields of 78–88% under the
[14]
reaction conditions employed here.
The value of anode potential of the
electrolysis was not dependent on that of oxidation potential of the substrate.
In addition, the electrolysis could be achieved under 0.5 V or less vs. SCE of
anode potential. These observations suggest that the oxidation proceeded by
means of iodide cationic species which was formed from iodide ions.
As illustrated in Figure 1, the general reaction pathway of the electro-
oxidation involves a four-electron-oxidation reaction process with the iodide
ion functioning as the electron carrier. However, we have observed that large

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Conversion of Benzyl Type Alcohols
283
a
Table 1. Electrochemical indirect oxidation of benzyl type alcohols.
Bp(8C/mmHg) of (8C)
b
Current passed Yield of 2
2
1
c
Substrate R
(Fmol
)
(%)
Obs.
Ref.
1a
1b
1c
1d
1e
1f
Ph
4-CH C H
13
11
18
9
82
89
81
91
85
90
88–99 (19)
110–111 (23)
98–99 (16)
96–98 (24)
217
3
6
4
2-CH C H
97 (15)
256
3
3
3
6
4
4-CH
2-CH
OC
OC
6
H
4
4
130–131 (14)
128–129 (16)
40–42
6
H
16
15
245–246
44
4-ClC H
6
4
d
(
MeOH)
1
1
1
1
1
1
g
h
i
2-ClC
4-NO C H
6
H
4
27
15
12
18
11
22
83
38
83
73
80
93
114–117 (17)
96–97(EtOH)
120–122 (12)
104–106 (60)
103–105 (27)
160–162 (12)
234
96
d
2
6
4
4-iso-C
2-Furyl
3
H
7
C
6
H
4
126 (14)
181
j
k
l
2-Thienyl
1-Naphtyl
40 (0.5)
159–160 (11)
a
Substrate: 20 mmol; KI: 10 mmol; NaOMe: 5 mmol; MeOH: 80 mL; r.temp: ca 208C.
Isolated yield.
b
c
[20]
Ref. .
Solvent for recrystallization.
d
Figure 1. Electrooxidation general reaction pathway involving a 4-electron-
oxidation reaction with iodine ion as the electron carrier.

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2
84
Okimoto et al.
excesses in the quantity of electricity were consumed to complete the trans-
formation of substrates 1 into products 2. We can assume that the chemical
oxidation process of 1 to the aldehydes, through the action of iodonium ions
formed on the anode is less efficient than the subsequent oxidation of the
aldehydes to 2. In the case of the electrooxidation of 1a, the use of KBr or KCl
instead of KI lowered the yields of 2a to 28% or 7%, respectively, with
significant recovery of unreacted 1a. Higher reaction temperatures did not
improve the yields of 2a; for example, by raising the reaction temperature
from 158C to 508C, the yield of 2a decreased from 95% to 60%. However, in
the cases of 1l, elevating the temperature from 158C to 508C was favorable for
the formation of 2l. Furthermore, introduction of 0, 2.5, 5, and 10 mmol KI
afforded 2a with yields of 6%, 43%, 70%, and 95%, respectively (GLC yield
of 2a based on 1a). Addition of a stoichiometric (20 mmol) or excess amount
of KI did not improve either the current efficiencies or the yields of 2a.
In summary, we have developed a convenient one-pot indirect
electrooxidation method for the conversion of benzyl type alcohols into the
corresponding methyl carboxylates in good to excellent isolated yields. This
preparative electooxidation could be carried out under very mild conditions
and without the use of strong and/or special oxidizing agents.
EXPERIMENTAL
All electrooxidation products 2a–l were identified by comparison of their
physical and spectral data with those of authentic samples. Melting and
boiling points were uncorrected. Substrates and other reagents were obtained
from commercial suppliers, and used without further purifications. Prepara-
tive-scale electrooxidations were carried out in a tall 100-mL beaker equipped
with a fine frit cup as the cathode compartment, a cylindrical platinum net
anode (diameter, 33 mm; height, 40 mm), and a nickel coil cathode (diameter,
1
mm; length, 250 mm). Benzyl type alcohols 1a–l were oxidized under
conditions as follow: a solution of benzyl type alcohol 1a–l (20 mmol), KI
10 mmol), and NaOMe (5 mmol) in MeOH (80 mL) was electrooxidized
(
under a constant current (0.5 A). During the course of the electrooxidation,
while stirring the anolyte with a magnetic stirring bar, the temperature of
the cell was maintained at approximately 158C. After completion of the
electrooxidation, the reaction mixture was treated using a three-fold increase
in the amount of the anolyte. After concentrating the combined anolyte
solution in vacuo at approximately 308C to about one-fifth of its original
volume, the resulting residue was treated with water (60 mL), and the oily
layer was extracted with ether (3 Â 50 mL). The combined ether extracts were
washed with aqueous sodium thiosulfate solution (20 w%, 30 mL), and dried

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Conversion of Benzyl Type Alcohols
285
over anhydrous magnesium sulfate. After removal of the solvent, the residue
was purified by distillation in vacuo or by recrystallization from either MeOH
or EtOH.
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Received in Japan June 2, 2003

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